WO2023225706A1

WO2023225706A1 - Co 2 hvac system

Info

Publication number: WO2023225706A1
Application number: PCT/AU2023/050431
Authority: WO
Inventors: Julian Hudson; Hesam Semsarilar
Original assignee: Glaciem Cooling Technologies Pty Ltd
Priority date: 2022-05-23
Filing date: 2023-05-23
Publication date: 2023-11-30
Also published as: AU2023277994A1

Abstract

Disclosed is a method of retrofitting a synthetic refrigerant-based HVAC system 10. The HVAC system comprises one or more air handling units 20. The one or more air handling units comprise at least one coil, and a series of pipes, valves and ducts. The method comprises removing those parts of the existing system that 5 are not compatible with CO2, for example the synthetic refrigerant compressor unit, the existing compressors, condenser, existing coils, pipes and valves. The method further comprises installing components that are compatible with CO2, for example a CO2 refrigeration unit 12, a CO2 gas cooler/condenser 60, coils 18, 42, 76, pipes and valves. Also disclosed herein are CO2 based HVAC systems and 0 methods of operating the same.

Description

CO2 HVAC SYSTEM

TECHNICAL FIELD

This disclosure relates to HVAC systems and, in particular, to refrigeration systems in which carbon dioxide is the refrigerant.

BACKGROUND ART

The vapour compression cycle has been used in the refrigeration industry for many years. The cycle employs the continuous flow of refrigerant between four primary components: a metering device, evaporator, compressor, and condenser.

The refrigerant employed in this cycle varies depending on the application and the refrigeration temperature required. Synthetic refrigerants such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and hydrofluoroolefms (HFOs) have been used. However, CFCs and HCFCs have now been banned in many countries under the Montreal Protocol due to the ozone depleting potential (ODP) of such refrigerants. Similarly, HFCs are being phased out in those same countries under the Kigali Amendment to the Montreal Protocol due to their high global warming potential (GWP). This phase out in the use of synthetic refrigerants due to their environmental impact has resulted in increased interest in the use of natural refrigerants such as carbon dioxide (CO2), ammonia and hydrocarbons.

Variable Refrigerant Flow (VRF) systems (equivalent to the Variable Refrigerant Volume (VRV) systems developed by Daikin Industries) are an example of refrigeration units in which CO2 is the refrigerant and are increasingly available due to the phase out of other refrigerants. They are termed “variable” as they comprise variable motor speed devices which enable a variable volume of CO2 to flow in the system.

It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms a part of the common general knowledge in the art, in Australia or any other country. SUMMARY

Disclosed herein in a first aspect is a method of retrofitting a synthetic refrigerant based HVAC system. The HVAC system can comprise one or more air handling units. Each air handling unit can comprise at least one coil, and a series of pipes, valves and ducts (as is known). Surprisingly and advantageously, the method can exploit existing components associated with refrigerant based HVAC systems in that, as part of the retrofitting, it is not necessary to replace all the components of the existing HVAC system. This can help reduce the cost of altering the refrigerant used by an existing HVAC system from a synthetic refrigerant or synthetic “drop in” to CO2.

Following the phase out of CFCs in the 1990s, the HCFC R22 has been used in both residential and commercial air conditioning systems. With the phasing out of HCFCs in many developing countries (i.e. due to commitments made under the Montreal Protocol), a number of systems still operating have had the R22 refrigerant replaced with an R22 compatible synthetic refrigerant “drop in”. Yet, “drop in” refrigerants are only a temporary fix, and a long-term solution is required. Systems still operating with R22 (or other HCFCs) also require replacement.

One particularly suitable refrigerant is CO2 (also known as R744) due to both its low global warming potential and zero ozone depleting potential when compared to other refrigerants. CO2 is currently used in VRF systems as a refrigerant. However, known VRF systems, such as the Daikin VRV, have several disadvantages. For example, installation of these systems requires replacing the entire existing synthetic based refrigeration system, which can represent an appreciable cost. The cost of replacing an entire HVAC system can be prohibitively expensive. It can be seen as advantageous to provide a cost-effective means by which an existing synthetic based refrigeration system may be upgraded to be compatible with CO2, especially where the equipment is still functional. By exploiting components of the old synthetic refrigerant based HVAC system in the new CO2 HVAC system, the cost of the upgrade may be minimised. The method as disclosed herein can comprise removing a synthetic refrigerant compressor unit and installing a CO2 refrigeration unit. The CO2 refrigeration unit can comprise at least one compressor, one or more valves, a series of pipes and electrical controls. The at least one compressor can be configured to compress CO2 vapour into a superheated vapour or a supercritical vapour. It should be understood that the CO2 refrigeration unit need not be a stand-alone unit per se. Rather, as used herein, the term ‘CO2 refrigeration unit’ refers to the part of the HVAC system that comprises the one or more CO2 compressors, series of pipes and valves and electrical controls. Thus, the CO2 refrigeration unit may be configured from disparate parts. However, the CO2 refrigeration unit may also comprise a stand-alone CO2 refrigeration unit preconfigured to comprise the at least one compressor, one or more valves, a series of pipes and electrical controls.

The method can also comprise installing a CO2 condenser/gas cooler. The CO2 condenser/gas cooler may be configured to cool CO2 vapour at a pressure above atmospheric. Further, the CO2 condenser/gas cooler may be configured to condense CO2 vapour at a pressure above atmospheric.

The method can further comprise removing the at least one coil in each of the at least one air handling units.

The method can still further comprise installing a cooling coil in each of the at least one air handling units. The cooling coil may be suitable to comprise CO2 at a pressure above atmospheric.

The method can yet further comprise installing a heating coil in each of the one or more air handling units. The heating coil may be suitable to comprise CO2 at a pressure above atmospheric. That is, in the system disclosed herein, the one or more air handling units each comprise a heating coil and a cooling coil. This is advantageous because it enables the retrofitted HVAC system to operate in a dehumidification mode, using CO2 as the only source of energy. The installed cooling and heating coils can replace the at least one removed coil.

The method can still further comprise removing the series of pipes and valves and installing a new series of pipes and valves. Said new series of pipes and valves may be configured to comprise CO2 at a pressure above atmospheric. Said new series of pipes and valves may be configured such that four pipes are used to connect to the evaporator and heating coil, allowing for both the cooling coil and the heating coil to be used simultaneously within the same air handling unit. In systems comprising more than one air handling unit, this configuration further advantageously allows for heating to be performed in one air handling unit whilst cooling is performed in another (separate) air handling unit.

A primary advantage of the method as disclosed herein is that only those components that are not CO2 compatible require replacement; however other existing components can still be used.

In some embodiments, the method may further comprise installing an additional cooling coil. The additional cooling coil may be coupled to the CO2 condenser/gas cooler and may be suitable to heat CO2 at a pressure above atmospheric. The additional cooling coil may be coupled with the CO2 condenser/gas cooler in such a way that, when both components are in-use, ambient air is forced to pass from the CO2 condenser/gas cooler to the additional cooling coil.

In some embodiments, the method may further comprise removing one or more of the one or more air handling units and installing respective new air handling unit(s) therein. One or more of the existing air handling units may be replaced if they are no longer in good condition. For example, where the main supply fan or other components have visible signs of wear and/or rust. Advantageously, only those air handling units that are no longer usable need to be replaced, whilst the other air handling units that are still in good condition do not need replacing. This may reduce the cost of the retrofit, i.e. because only some air handling units need to be replaced. Furthermore, if all of the one or more air handling units are still usable, then no air handling units need to be replaced.

In some embodiments, the method may further comprise removing one or more electric heater elements (when present) from the ducts. Typically, electric heater elements are present in direct expansion units and systems that are able to heat, but that do not have a boiler - e.g. R22 chilled water systems without boilers. An advantage of the system disclosed herein is that it enables heating of an air-conditioned space without the need for further electrical components - i.e. because CO2 vapour is passed through the heating coil to achieve heating. Therefore, when electric heater elements are present in the system to be retrofitted, these may be removed as they are not required in the retrofitted system. In the event the existing system comprises other elements that are redundant in the retrofitted HVAC system, e.g. chiller unit, boiler etc., these elements may also be removed as a step of the retrofit.

In some embodiments, the cooling coil in each of the at least one air handling units may be suitable to comprise CO2 at a maximum pressure of about 80 bar. In some embodiments, the additional cooling coil may be suitable to comprise CO2 at a maximum pressure of about 80 bar. In some embodiments, the heating coil in each of the at least one air handling units may be suitable to comprise CO2 at a maximum pressure of about 120 bar. In some embodiments, the CO2 condenser/gas cooler may be suitable to operate with CO2 at a maximum pressure of about 120 bar.

In some embodiments, the cooling coil in each of the at least one air handling units may comprise a fin coil. In some embodiments, the additional cooling coil may comprise a fin coil. In some embodiments, the heating coil in each of the at least one air handling units may comprise a fin coil. In some of these or in each of these embodiments, the fin coil may be comprised of copper and/or aluminium and/or other conductive material with or without protective coatings, e.g. brass. In some embodiments, the CO2 refrigeration unit, the CO2 condenser/gas cooler, the at least one air handling unit comprising the cooling coil and the heating coil, the additional cooling coil (when present) and the series of pipes and valves may be configured to allow the HVAC system to cool air passing through the one or more air handling units. The CO2 refrigeration unit, the CO2 condenser/gas cooler, the at least one air handling unit comprising the cooling coil and the heating coil, the additional cooling coil (when present) and the series of pipes and valves may also be configured to allow the HVAC system to heat air passing through the one or more air handling units. That is, the HVAC system can provide either heating or cooling of ambient air passing through the one or more air handling units.

In some embodiments, the CO2 refrigeration unit, the CO2 condenser/gas cooler, the at least one air handling unit comprising the cooling coil and the heating coil, the additional cooling coil (when present) and the series of pipes and valves, may be configured to allow the HVAC system to dehumidify air passing through the one or more air handling units. In the HVAC system, the cooling coil and the heating coil in the at least one air handling unit can each have individual inlet and outlet pipes. This results in a total of four pipes connecting each of the at least one air handling units to the CO2 refrigeration unit - i.e. each of the cooling/heating coils has a designated inlet and a designated outlet. Such a configuration allows both the cooling coil and the heating coil to be used simultaneously. Advantageously, this enables dehumidification to be achieved using CO2 as the only source of energy. In contradistinction, VRF systems of the prior art typically have only one coil and employ two pipes when the VRF system is configured to provide only heating or only cooling (or three pipes when the VRF system is configured to provide heating and cooling) to connect the coil with the CO2 refrigeration unit. Such configurations do not allow for dehumidification to be performed using only CO2 - i.e. a chiller or heater is also required.

In some embodiments, the CO2 refrigeration unit, the CO2 condenser/gas cooler, the at least one air handling unit comprising the cooling coil and the heating coil, the additional cooling coil (when present) and the series of pipes and valves may be configured to allow the HVAC system to operate in a transcritical CO2 cycle. The CO2 refrigeration unit, the CO2 condenser/gas cooler, the at least one air handling unit comprising the cooling coil and the heating coil, the additional cooling coil (when present) and the series of pipes and valves may also be configured to allow the HVAC system to operate in a subcritical CO2 cycle. The HVAC system may operate in a transcritical CO2 cycle when the ambient air is warm and/or humid. Alternatively, when the ambient air is cool and/or dry, the HVAC system may operate in a subcritical CO2 cycle. Advantageously, by enabling the HVAC system to operate in either a transcritical or a subcritical cycle, the efficiency of the system may be increased.

In some embodiments, the synthetic refrigerant in the synthetic refrigerant based HVAC system may be R22. The synthetic refrigerant may also be a synthetic drop in HCFC replacement. Further, the synthetic refrigerant may be any refrigerant from the group of CFCs, HFCs, HCFCs or HFOs. The synthetic refrigerant based HVAC system may also comprise chilled water. The method as disclosed herein is such as to enable each such refrigerant to be replaced by CO2 using components adapted for use with CO2.

Also disclosed herein in a second aspect is a method for dehumidifying air using an HVAC system comprising CO2 as a refrigerant.

The method can comprise opening a pressure-reducing valve between a CO2 refrigeration unit and a cooling coil inside an air handling unit. Opening said valve can allow liquid CO2 to flow into the cooling coil. The pressure of the liquid CO2 may be reduced as the liquid CO2 flows into the cooling coil at a set pressure via the valve. In the cooling coil, energy may be transferred between the liquid CO2 within the cooling coil and air passing over the cooling coil. The transfer of energy between the liquid CO2 within the cooling coil and air passing over the cooling coil can cause the liquid CO2 to evaporate at that set pressure and said air to be cooled. Typically, the liquid CO2 is a saturated liquid at the operating pressure of the cooling coil, such that the energy transfer in the cooling coil results in only a latent enthalpy change to the liquid CO2, causing the CO2 to evaporate.

Typically, the set evaporating (i.e. saturated) temperature of the liquid CO2 will be below the dew point of the air passing over the cooling coil, causing moisture to condense from the air as the air passes over the cooling coil and is cooled. This condensation reduces the moisture content of the air passing over the cooling coil, thereby dehumidifying said air. The dehumidified air can then be passed to a heating coil inside the same air handling unit. The heating coil can allow the vapour to be cooled and the air to be heated. Typically, the air that is passed over the cooling coil is a combination of fresh air (i.e. external to the air- conditioned space) and return air (i.e. internal to the air-conditioned space). Fresh air is required to ensure adequate oxygen levels are maintained within the air- conditioned space.

The method can also comprise passing the resultant CO2 vapour to a CO2 compressor. In the compressor, the CO2 vapour may be compressed. It will be understood that when the HVAC system is operating in a subcritical cycle, the compressed CO2 vapour will be a superheated vapour, whilst when the HVAC system is operating in a transcritical state, the compressed CO2 vapour will be a supercritical fluid. Typically, dehumidification is required when the ambient air is hot and/or when there is a latent load generated in the air-conditioned space, e.g. when the air-conditioned space is a gymnasium or a swimming pool or an industrial kitchen.

The method can further comprise allowing the compressed CO2 vapour to flow through the heating coil. As above, in the heating coil, energy may be transferred between the compressed CO2 vapour within the heating coil and air passing over the heating coil, causing the vapour to be cooled and said air to be heated. The cooling coil and heating coil may both be located within an air handling unit such that the cooled air from the cooling coil is forced to pass over the heating coil. A heating stage is typically required because the air that has passed from the cooling coil can be too cold to be directly passed to the air- conditioned space. As the air passes over the heating coil, the air may be heated to a suitable temperature.

Advantageously, the dehumidification method herein disclosed provides a method for dehumidification in which both the heating and cooling stages are achieved using CO2. This stands in contradistinction to other HVAC methods in which the heating stage is achieved using either electrical heating coils and/or a boiler unit and/or the cooling stage is achieved using a chiller. By using CO2 as the only energy-transfer medium, the HVAC system herein disclosed requires no additional equipment, such as a chiller and/or boiler. The HVAC system is thereby simplified and is less costly.

In some embodiments, the method may further comprise selectively passing at least some of the resultant cooled CO2 vapour from the heating coil to a gas cooler. In the gas cooler, said at least some of the cooled CO2 vapour selectively passed thereto may be further cooled. The further cooled CO2 vapour may be passed back to the CO2 refrigeration unit along with remaining cooled CO2 vapour not selectively passed to the gas cooler. It will be appreciated that, because the HVAC system typically operates in a transcritical state, the further cooled CO2 vapour from the gas cooler is still a supercritical fluid. When the HVAC system operates in a subcritical state, the gas cooler cools and condenses the CO2 vapour typically to a saturated liquid or a subcooled liquid, i.e. the gas cooler acts as a condenser.

In some embodiments, the cooled vapour may be selectively passed to the gas cooler so as to achieve a predetermined temperature at an inlet to the CO2 refrigeration unit. The predetermined temperature at the inlet to the CO2 refrigeration unit may be selected so as to reduce the amount of flash gas generated by a high-pressure expansion valve located at the inlet to the CO2 refrigeration unit. By reducing the amount of flash gas generated, the overall efficiency of the system may be increased. In some embodiments, the flow of liquid CO2 from the CO2 refrigeration unit to the cooling coil may be increased when a pressure of CO2 vapour leaving the cooling coil and entering the compressor is higher than a predetermined operating pressure of the compressor. Conversely, the flow of liquid CO2 may be decreased when the pressure of CO2 vapour leaving the cooling coil and entering the compressor is lower than the compressor predetermined operating pressure. The flow of liquid CO2 may be increased/decreased by increasing/decreasing the number of operating compressors and/or by increasing/decreasing the operating frequency of the one or more compressors respectively.

In some embodiments, the flow of compressed CO2 vapour to the heating coil may be increased when a temperature of air leaving the heating coil is lower than a predetermined temperature of heated air leaving the heating coil. Conversely, the flow of compressed CO2 vapour may be decreased when the temperature of air leaving is higher than the predetermined heated air temperature.

In some embodiments, the method may be performed using the retrofitted HVAC system as disclosed in the first aspect.

Also disclosed herein in a third aspect is an HVAC system for dehumidifying air. The HVAC system of the third aspect can be used to deploy the method for dehumidifying air of the second aspect.

The HVAC system can comprise a CO2 refrigeration unit comprising one or more CO2 compressors, one or more valves, a series of pipes and electrical controls. The at least one compressor can be configured to compress CO2 vapour into a superheated vapour or a supercritical vapour. Again, it should be understood that the CO2 refrigeration unit need not be a stand-alone unit per se. Rather, as used herein, the term ‘CO2 refrigeration unit’ refers to the part of the HVAC system that comprises the one or more CO2 compressors, one or more valves, series of pipes and electrical controls. Thus, the CO2 refrigeration unit may be configured from disparate parts. However, the CO2 refrigeration unit may also comprise a stand-alone CO2 refrigeration unit pre-configured to comprise the one or more compressors, one or more valves, a series of pipes and electrical controls.

The HVAC system can also comprise a cooling coil connected to the CO2 refrigeration unit by a pressure-reducing valve. Said valve may be configured such that, when the valve is in an open position, liquid CO2 is caused to flow from the CO2 refrigeration unit to the cooling coil. Typically, the liquid CO2 is a saturated liquid at the set pressure of the cooling coil. In the cooling coil, the (saturated) liquid CO2 can evaporate (at the set pressure) as air passes over the cooling coil, the air being cooled thereby. The cooling coil may be located within an air handling unit.

The HVAC system can further comprise an outlet of the cooling coil configured to allow the resultant CO2 vapour to be passed to the one or more CO2 compressors of the CO2 refrigeration unit. In the one or more CO2 compressors, the CO2 vapour may be compressed to form a compressed CO2 vapour. It will be understood that when the HVAC system is operating in a subcritical cycle, the compressed CO2 vapour will be a superheated vapour, whilst when the HVAC system is operating in a transcritical state, the compressed CO2 vapour will be a supercritical fluid. Typically, during dehumidification, the HVAC system operates in a transcritical cycle, as the ambient air is hot and humid.

The HVAC system can still further comprise a heating coil connected to the one or more CO2 compressors within the CO2 refrigeration unit. The connection can be configured to allow the compressed CO2 vapour to pass from the one or more CO2 compressors and into the heating coil. In the heating coil, the compressed CO2 vapour can be cooled as air passes over the heating coil, the air being heated thereby. The heating coil may be located within the same air handling unit as the cooling coil.

The HVAC system can yet further comprise an outlet of the heating coil configured to allow the resultant cooled CO2 vapour to pass to the refrigeration unit. In the HVAC system for dehumidifying air, the cooling coil and heating coil can be located within the same air handling unit and can be configured such that cooled air leaving the cooling coil is forced to pass over the heating coil.

Typically, the temperature of the liquid CO2 will be below the dew point of the air passing over the cooling coil, causing moisture to condense from the air as it passes over the cooling coil and is cooled. This condensation reduces the moisture content of the air passing over the cooling coil, thereby dehumidifying the air. The dehumidified air is then passed to a heating coil inside the same air handling unit.

Typically, the air that is passed over the cooling coil is a combination of fresh air (i.e. external to the air-conditioned space) and return air (i.e. internal to the air-conditioned space). As above, fresh air is required to ensure adequate oxygen levels within the air-conditioned space.

Advantageously, the dehumidification system disclosed herein provides a system for dehumidification in which CO2 is used for both heating and cooling. A particular advantage of the system is that part of the energy that is transferred from the air to the CO2 in the cooling coil (i.e. to dehumidify the air) is transferred back to the air in the heating coil (i.e. to heat the air). This is in contradistinction to other HVAC systems which require electrical heating coils and/or a boiler unit to perform the heating stage. The use of CO2 can thereby reduce the complexity of the HVAC system and can increase the efficiency of the system - e.g. by making use of at least some of the heat energy transferred from the air in the air handling unit to the cooling coil, which would otherwise be wasted.

In some embodiments, the system may further comprise a gas cooler configured to selectively pass at least some of the cooled CO2 vapour from the heating coil to the gas cooler. The at least some of the cooled CO2 vapour may be further cooled as it is selectively passed to the gas cooler. An outlet of the gas cooler may be configured to allow the further cooled CO2 vapour to pass to the CO2 refrigeration unit, along with remaining cooled CO2 vapour not selectively passed to the gas cooler. In some embodiments, the system may further comprise a temperature sensor at the inlet to the CO2 refrigeration unit and a three-way valve. The three- way valve may be located and configured to selectively pass the cooled CO2 vapour from the heating coil to the gas cooler so as to achieve a predetermined temperature at the inlet to the CO2 refrigeration unit, as measured by the temperature sensor.

In some embodiments, the one or more CO2 compressors may be suitable to compress CO2 vapour to thereby form a superheated CO2 vapour. The CO2 vapour may be compressed to a superheated CO2 vapour when the HVAC system operates in a subcritical cycle.

In some embodiments, the one or more CO2 compressors may be suitable to compress CO2 vapour to thereby form a supercritical CO2 fluid. The CO2 vapour may be compressed to a supercritical CO2 fluid when the HVAC system operates in a transcritical cycle.

In some embodiments, the system may further comprise a pressure sensor configured to measure a pressure of CO2 vapour leaving the cooling coil and entering the one or more CO2 compressors. The pressure sensor may be further configured to cause an increased flow of liquid CO2 to the cooling coil when the CO2 vapour pressure is above a predetermined CO2 vapour pressure and to cause a decreased flow of liquid CO2 when the CO2 vapour pressure is below the predetermined CO2 vapour pressure.

In some embodiments, the system may further comprise a temperature sensor configured to measure the temperature of heated air leaving the heating coil. This temperature sensor may be further configured to adjust the position of a three-way valve located at an outlet of the CO2 refrigeration unit. This three-way valve may be configured to selectively pass compressed CO2 vapour to the heating coil to allow an increased flow of compressed CO2 vapour to the heating coil when a temperature of air leaving the heating coil is below a predetermined air temperature. This three-way valve may be further configured to allow a decreased flow of compressed CO2 vapour to the heating coil when the heated air temperature is above the predetermined heated air temperature.

In some embodiments, the system may further comprise a temperature sensor configured to measure the temperature of ambient air leaving the gas cooler. This temperature sensor may be further configured to open a second pressure-reducing valve connecting the CO2 refrigeration unit and a third cooling coil, thereby allowing liquid CO2 to selectively pass to the third cooling coil, when the ambient air temperature is below a predetermined ambient air temperature.

In some embodiments, the cooling, heating and/or additional cooling coils may each comprise a fin coil. Each such fin coil may be comprised of copper and/or aluminium and/or other conductive material with or without protective coatings, e.g. brass.

In some embodiments, the HVAC system for dehumidifying air of the third aspect may employ the retrofitted HVAC system of the first aspect.

Disclosed herein in a fourth aspect is an HVAC system that is operable in either a subcritical or a transcritical CO2 cycle.

The HVAC system of the fourth aspect can comprise a CO2 refrigeration unit comprising one or more CO2 compressors, one or more valves, a series of pipes and electrical controls. The one or more CO2 compressors can be configured to compress CO2 vapour into a superheated vapour or a supercritical vapour.

The HVAC system of the fourth aspect can also comprise a cooling coil connected to the CO2 refrigeration unit by a pressure-reducing valve. The pressure-reducing valve can be configured such that, when the valve is in an open position, liquid CO2 is caused to flow from the CO2 refrigeration unit to the cooling coil. Typically, the liquid CO2 is a saturated liquid at the pressure of the cooling coil. In the cooling coil, the (saturated) liquid CO2 can evaporate (at the set pressure of the cooling coil) as a stream of ambient air passes over the cooling coil, the air being cooled thereby. Typically, the ambient air that is passed over the cooling coil is a combination of fresh air (i.e. external to the air-conditioned space) and return air (i.e. recycled air from the air-conditioned space). As above, fresh air is required to ensure adequate oxygen levels within the air-conditioned space.

The HVAC system of the fourth aspect can further comprise an outlet of the cooling coil being configured to allow the resultant CO2 vapour to be passed to the one or more CO2 compressors of the CO2 refrigeration unit. The one or more CO2 compressors may be configured to compress the CO2 vapour to thereby form superheated CO2 vapour or to form a supercritical CO2 fluid.

The HVAC system of the fourth aspect can still further comprise a gas cooler configured to cool the resultant compressed CO2 vapour to form the substantially liquid CO2. The substantially liquid CO2 can be passed to an inlet of the CO2 refrigeration unit.

In contradistinction to CO2 HVAC systems of the prior art, such as the Daikin VRV, the HVAC system of the fourth aspect is able to operate in either a transcritical or subcritical cycle depending on the mode of operation and/or ambient conditions (e.g. temperature, humidity). The Daikin VRV (and to the best of the inventors’ knowledge analogous VRF systems) are configured to only operate in transcritical cycles. The advantage of a system which can also operate in a subcritical cycle is that, under certain conditions, subcritical cycles are more energy efficient (e.g. in winter when the ambient air temperature is cold). Thus, the HVAC system of the fourth aspect may provide increased versatility and efficiency when compared to prior art systems.

In some embodiments, the HVAC system of the fourth aspect may further comprise a heating coil connected to the one or more CO2 compressors within the CO2 refrigeration unit. The connection may be configured to allow the compressed CO2 vapour to pass from the one or more CO2 compressors and into the heating coil. In the heating coil, the compressed CO2 vapour may be cooled as the cooled air from the cooling coil passes over the heating coil, the air being heated thereby. The HVAC system may also comprise an outlet of the heating coil configured to allow the resultant cooled CO2 vapour to pass to the CO2 refrigeration unit.

In some embodiments, the HVAC system of the fourth aspect may comprise a temperature sensor at the inlet to the CO2 refrigeration unit and a three-way valve. The three-way valve may be located and configured to selectively pass the cooled CO2 vapour from the heating coil to the gas cooler so as to achieve a predetermined temperature at the inlet to the CO2 refrigeration unit, as measured by the temperature sensor.

In some embodiments, the HVAC system of the fourth aspect may comprise an additional cooling coil located adjacent to the gas cooler and connected to the CO2 refrigeration unit by a second pressure-reducing valve. The second pressure-reducing valve may be configured to selectively pass liquid CO2 leaving the CO2 refrigeration unit to the additional cooling coil. The liquid CO2 passed to the additional cooling coil may evaporate as ambient air passes over the additional cooling coil, the ambient air being cooled thereby.

In some embodiments, the HVAC system of the fourth aspect may comprise a pressure sensor configured to measure a pressure of CO2 vapour entering the one or more CO2 compressors. The pressure sensor may be further configured to cause an increased flow of liquid CO2 to the cooling coil when the CO2 vapour pressure is above a predetermined CO2 vapour pressure. The pressure sensor may also be configured to cause a decreased flow of liquid CO2 to the cooling coil when the CO2 vapour pressure is below the predetermined CO2 vapour pressure.

In some embodiments, the pressure sensor may be further configured to open the second pressure-reducing valve connecting the CO2 refrigeration unit and the additional cooling coil, thereby allowing liquid CO2 to selectively pass to the additional cooling coil, when the compressed CO2 pressure is above the predetermined compressed CO2 pressure and the position of the pressure-reducing valve is above a predetermined position. In some embodiments, the HVAC system of the fourth aspect may comprise a temperature sensor configured to measure the temperature of heated air leaving the heating coil. The temperature sensor may be further configured to adjust the position of a three-way valve located at an outlet of the CO2 refrigeration unit. This three-way valve may be configured to selectively pass compressed CO2 vapour to the heating coil to allow an increased flow of compressed CO2 vapour when a temperature of air leaving the heating coil is below a predetermined air temperature and to allow a decreased flow of compressed CO2 vapour when the heated air temperature is above the predetermined air temperature.

In some embodiments, the cooling, heating and/or additional cooling coils may each comprise a fin coil. Each fin coil may be comprised of copper and/or aluminium and/or other conductive material with or without protective coatings, e.g. brass.

In some embodiments, the HVAC system of the fourth aspect may comprise the retrofitted HVAC system of the first aspect.

Also disclosed herein in a fifth aspect is a method for operating the HVAC system of the fourth aspect. The method can comprise operating the HVAC system of the fourth aspect in either a subcritical or a transcritical CO2 cycle. As above, the subcritical cycle for operating the HVAC system of the fourth aspect may be selected when, under certain conditions (e.g. for cold ambient air temperatures in winter), the subcritical cycle is more energy efficient.

Also disclosed herein in a sixth aspect is a method for controlling the heat load in an HVAC system comprising CO2 as a refrigerant and at least one air handling unit. The method of the sixth aspect may be used when the HVAC system is in a heating mode. The method of the sixth aspect may also be used when the HVAC system is providing heating and cooling to different air handling units, but the heating load is greater than the cooling load.

The method of the sixth aspect can comprise opening a pressure-reducing valve between a CO2 refrigeration unit and a cooling coil, thereby allowing liquid CO2 to flow into the cooling coil. Typically, the liquid CO2 is a saturated liquid at the operating pressure of the cooling coil. In the cooling coil, energy can be transferred between the (saturated) liquid CO2 within the cooling coil and air passing over the cooling coil, causing the liquid CO2 to boil (at the set pressure) and said air to be cooled. The cooling coil may be located externally to the at least one air handling unit.

The method of the sixth aspect can also comprise passing the resultant CO2 vapour to one or more compressors, wherein the CO2 vapour is compressed to form a compressed CO2 vapour. Typically, when the HVAC system is providing heating, it will operate using a transcritical cycle. Thus, the compressed CO2 vapour will typically be in the form of a supercritical fluid.

The method of the sixth aspect can further comprise allowing the compressed supercritical CO2 vapour to flow through a heating coil in each of the at least one air handling units. In the heating coil, energy can be transferred between the compressed supercritical CO2 vapour within the heating coil and air passing over the heating coil, causing the vapour to be cooled and said air to be heated.

The method of the sixth aspect can still further comprise passing the cooled supercritical CO2 vapour to the CO2 refrigeration unit. A temperature of the cooled supercritical CO2 vapour can be measured prior to it reaching the CO2 refrigeration unit. When the temperature of the cooled supercritical CO2 vapour is lower than a predetermined temperature, the flow of CO2 vapour from the CO2 refrigeration unit into the heating coil can be increased. Alternatively, when the temperature of the cooled supercritical CO2 vapour is greater than the predetermined temperature, the flow of CO2 vapour can be decreased. In contradistinction to prior art methods of controlling the heat load in heating coils, the method of the sixth aspect can use the temperature of the cooled supercritical CO2 vapour to adjust the flow of CO2 vapour passing to the heating coil. Typical prior art methods comprise measuring the temperature of heated air leaving the heating coils and altering the flow of CO2 vapour to the heating coil based on the measured temperature of the heated air. For example, when the measured temperature of heated air is below a predetermined temperature, the flow of CO2 vapour to the heating coil is increased (e.g. by using a three-way modulating valve), thereby providing a greater heating load.

Advantageously, by controlling the heating load using the temperature of the cooled supercritical CO2 vapour rather than the heated air, the efficiency of the system may be increased. This is because a cooled supercritical CO2 vapour temperature may be selected that allows the cooled supercritical CO2 vapour to be passed directly to the CO2 refrigeration unit, i.e. without the need for further cooling in a gas cooler. Such a gas cooler can represent a loss of energy from the system, as energy is transferred from the CO2 vapour to surrounding ambient air.

In some embodiments of the method of the sixth aspect, the HVAC system may comprise two or more air handling units, and the method may be performed such that the resultant cooled supercritical CO2 vapour from each of the two or more air handling units is combined. A temperature of the combined cooled supercritical CO2 vapour may be measured prior to reaching the CO2 refrigeration unit.

In some embodiments of the method of the sixth aspect, the HVAC system may comprise one compressor. In these embodiments, the method may be performed such that the flow of CO2 vapour is increased by increasing a motor speed of the one compressor and decreased by decreasing the motor speed of the one compressor.

In some embodiments of the method of the sixth aspect, the HVAC system may comprise more than one compressor. In these embodiments, the method may be performed such that the flow of CO2 vapour may be increased by increasing a motor speed of the more than one compressors and/or by increasing the number of operating compressors. Conversely, the flow of CO2 vapour may be decreased by decreasing the motor speed of the more than one compressors and/or by decreasing the number of operating compressors.

In some embodiments, the HVAC system may comprise up to fourteen compressors, with the method of the sixth aspect making various use of such compressors.

Notwithstanding that the need for a gas cooler can be avoided, in some embodiments, the method of the sixth aspect may further comprise selectively passing at least some of the cooled supercritical CO2 vapour leaving the heating coil into a gas cooler located adjacent to the cooling coil. Such a gas cooler may be employed only in certain circumstances. For example, a sufficient amount of cooled supercritical CO2 vapour may be selectively passed from the heating coil to the gas cooler to achieve a predetermined temperature at an inlet to the CO2 refrigeration unit. In the gas cooler, the at least some of the cooled supercritical CO2 vapour may be further cooled. The further cooled supercritical CO2 vapour may be passed from an outlet of the gas cooler to the CO2 refrigeration unit, along with remaining cooled supercritical CO2 vapour not passed to the gas cooler.

In some embodiments, one or more gas cooler fans may be located adjacent to the gas cooler to direct a flow of ambient air through the gas cooler. The method may further comprise measuring a suction pressure of the CO2 refrigeration unit such that, when the measured suction pressure is above a predetermined suction pressure, a speed of the one or more gas cooler fans is increased, thereby increasing the flow of ambient air across the gas cooler.

In some embodiments, the gas cooler and cooling coil may be configured such that the ambient air directed through the gas cooler by the one or more gas cooler fans thereafter passes over the cooling coil. In some embodiments, the HVAC system comprising CO2 that is employed in the method of the sixth aspect, may comprise the retrofitted HVAC system disclosed herein in a first aspect.

Also disclosed herein in a seventh aspect is a system for controlling the heat load in an HVAC system comprising CO2 as a refrigerant. The system of the seventh aspect may be used to control the heat load during a heating mode.

The system of the seventh aspect can comprise a CO2 refrigeration unit comprising one or more compressors, one or more valves, a series of pipes and electrical controls. Yet again, it should be understood that the CO2 refrigeration unit need not be a stand-alone unit per se. Rather, as used herein, the term ‘CO2 refrigeration unit’ refers to the part of the HVAC system that comprises the one or more CO2 compressors, one or more valves, series of pipes and electrical controls. Thus, the CO2 refrigeration unit may be configured from disparate parts. However, the CO2 refrigeration unit may also comprise a stand-alone CO2 refrigeration unit pre-configured to comprise the one or more compressors, one or more valves, a series of pipes and electrical controls.

The system of the seventh aspect can also comprise at least one air handling unit.

The system of the seventh aspect can further comprise a cooling coil connected to the CO2 refrigeration unit by a pressure-reducing valve. Said valve can be configured such that, when the valve is in an open position, liquid CO2 is caused to flow from the CO2 refrigeration unit to the cooling coil. In the cooling coil, the liquid CO2 can evaporate as air passes over the cooling coil, the air being cooled thereby. The cooling coil may be located externally to the at least one air handling unit.

The system of the seventh aspect can still further comprise an outlet of the cooling coil being configured such that the resultant CO2 vapour passes to the one or more compressors of the CO2 refrigeration unit, with the CO2 vapour being compressed to a supercritical state.

The system of the seventh aspect can yet further comprise a heating coil located within each of the at least one air handling units. Each heating coil can be connected to the compressor within the CO2 refrigeration unit such that the compressed supercritical CO2 vapour passes from the compressor and into the heating coil, wherein the compressed supercritical CO2 vapour is cooled as the air passes over the heating coil, the air being heated thereby.

The system of the seventh aspect can also comprise an outlet of the heating coil being configured such that the resultant cooled supercritical CO2 vapour is passed from the heating coil to the CO2 refrigeration unit.

The system of the seventh aspect can also comprise a temperature sensor located and configured to measure a temperature of the cooled supercritical CO2 vapour prior to it reaching the CO2 refrigeration unit. The system can be configured such that, when the temperature of the cooled supercritical CO2 vapour is lower than a predetermined temperature, the flow of CO2 vapour from the CO2 refrigeration unit into the heating coil can be increased. The system can also be configured such that, when the temperature of the combined cooled supercritical CO2 vapour leaving the heating coil is greater than the predetermined temperature, the flow of CO2 vapour can be decreased.

Advantageously, by using the temperature of the cooled CO2 vapour to control the flow of CO2 vapour in the system, the efficiency of the system may be increased. This is because a cooled CO2 vapour temperature may be selected that allows the cooled CO2 vapour to be passed directly to the CO2 refrigeration unit, e.g. without the need for further cooling in a gas cooler. The gas cooler represents a loss of energy from the system, as energy that is transferred from the CO2 vapour to surrounding ambient air is effectively lost.

In some embodiments, the HVAC system of the seventh aspect may comprise two or more air handling units. In these embodiments, the system may be configured such that the resultant cooled supercritical CO2 vapour from each of the two or more air handling units is combined, and a temperature of the combined cooled supercritical CO2 vapour is measured prior to it reaching the CO2 refrigeration unit. This is in contradistinction to HVAC systems of the prior art in which the temperature of the combined cooled supercritical CO2 vapour cannot be measured, because prior art systems do not comprise a dedicated means (e.g. a passage) by which the cooled supercritical CO2 vapour from individual heating coils can be combined and through which the combined cooled supercritical CO2 vapour can be passed to the CO2 refrigeration unit.

In some embodiments, the HVAC system of the seventh aspect may comprise one compressor. The one compressor may be configured to increase the mass flow of CO2 vapour by increasing a motor speed and to decrease the mass flow of CO2 vapour by decreasing a motor speed.

In some embodiments, the HVAC system of the seventh aspect may comprise more than one compressor. Such compressors may be configured to increase the mass flow of CO2 vapour by increasing a motor speed of the more than one compressors and/or by increasing the number of operating compressors. Conversely, such compressors may be configured to decrease the mass flow of CO2 vapour by decreasing the motor speed of the more than one compressors and/or by decreasing the number of operating compressors.

In some embodiments, the HVAC system of the seventh aspect may comprise up to fourteen compressors.

Notwithstanding that the need for a gas cooler can be avoided, in some embodiments (e.g. for certain conditions) the HVAC system of the seventh aspect may further comprise a gas cooler located adjacent to the cooling coil and a three- way valve. The three-way valve may be located and connected to selectively pass at least some of the combined cooled supercritical CO2 vapour from the heating coil to the gas cooler, wherein the at least some of the combined cooled supercritical CO2 vapour is further cooled. An outlet of the gas cooler may be configured to pass the further cooled supercritical CO2 vapour to the CO2 refrigeration unit, along with remaining combined cooled supercritical CO2 vapour not selectively passed to the gas cooler.

In some embodiments, the HVAC system of the seventh aspect may further comprise a temperature sensor located at an inlet to the CO2 refrigeration unit. The system may be configured such that at least some of the combined cooled supercritical CO2 vapour from the heating coils is selectively passed to the gas cooler to achieve a predetermined temperature at the inlet to the CO2 refrigeration unit, as measured by the temperature sensor.

In some embodiments, the HVAC system of the seventh aspect may further comprise one or more gas cooler fans located adjacent to the gas cooler and a pressure sensor. The system may be configured such that, when the suction pressure is below a predetermined suction pressure, a speed of the one or more gas cooler fans is increased.

In some embodiments, the gas cooler and cooling coil may be configured such that the ambient air caused to flow through the gas cooler by the one or more gas cooler fans thereafter passes over the cooling coil.

In some embodiments, the HVAC system of the seventh aspect may further comprise an internal cooling coil in each of the at least one air handling units. Each internal cooling coil may have a corresponding pressure-reducing valve configured such that, when the valve is in an open position, liquid CO2 may be caused to flow between the CO2 refrigeration unit and the internal cooling coil. The system may be configured such that, when an air handling unit is in heating mode, the corresponding pressure-reducing valve of each cooling coil remains in the closed position.

In some embodiments, the HVAC system of the seventh aspect can employ the retrofitted HVAC system of the first aspect.

Also disclosed herein in an eighth aspect is an HVAC system for providing simultaneous heating and cooling. The system of the eighth aspect can comprise a CO2 refrigeration unit. The CO2 refrigeration unit can comprise at least one compressor, one or more valves, a series of pipes and electrical controls. Once again, it should be understood that the CO2 refrigeration unit need not be a stand-alone unit per se. Rather, as used herein, the term ‘CO2 refrigeration unit’ refers to the part of the HVAC system that comprises the one or more CO2 compressors, one or more valves, series of pipes and electrical controls. Thus, the CO2 refrigeration unit may be configured from disparate parts. However, the CO2 refrigeration unit may also comprise a stand-alone CO2 refrigeration unit pre-configured to comprise the one or more compressors, one or more valves, a series of pipes and electrical controls.

The system of the eighth aspect can also comprise at least two air handling units. Each air handling unit can comprise a cooling coil and a heating coil, and such coils may be separate from, and may comprise separate pipework to, the heating/cooling coils of each other air handling unit.

An inlet of each cooling coil of the at least two air handling units can be connected to the CO2 refrigeration unit by a pressure-reducing valve. Each cooling coil can have a corresponding pressure-reducing valve. This valve can be configured such that, when the valve is in an open position, liquid CO2 is caused to flow between the CO2 refrigeration unit and the cooling coil, wherein the liquid CO2 evaporates as air passes over the cooling coil, the air being cooled thereby.

An outlet of each cooling coil can be configured to combine the resultant CO2 vapour from the at least two air handling units, to thereby form a combined CO2 vapour that is passed to the at least one compressor of the CO2 refrigeration unit. In the at least one compressor, the combined CO2 vapour can be compressed to a supercritical state.

An inlet of each heating coil can be connected to the one or more compressors within the CO2 refrigeration unit by at least one three-way valve. This valve can be configured such that, when the valve is in an open position, compressed supercritical CO2 vapour leaving the CO2 refrigeration unit is able to flow to each heating coil. In the heating coil, the compressed supercritical CO2 can be cooled as air passes over the heating coil, the air being heated thereby.

An outlet of each heating coil can be configured to combine the resultant cooled supercritical CO2 vapour from the at least two air handling units, to thereby form a combined cooled supercritical CO2 vapour which is passed to the CO2 refrigeration unit.

Advantageously, each air handling unit can comprise a dedicated heating coil and a dedicated cooling coil. Of further advantage is that each heating coil can comprise a dedicated inlet and a dedicated outlet for incoming and outgoing CO2 respectively, and each cooling coil can comprise a dedicated inlet and a dedicated outlet for the incoming and outgoing CO2 respectively. That is, each air handling unit can comprise four pipes - an inlet and an outlet for each coil. The heating coils can be connected to one piping system and the cooling coils can be connected to a separate piping system. This enables the temperature of the combined cooled supercritical CO2 vapour passing from the heating coils to be measured, and the temperature of the combined supercritical CO2 vapour passing from the cooling coils to be measured.

In contradistinction, in many CO2 HVAC systems of the prior art, the air handling unit only comprises one coil which can operate to provide either heating or cooling. Alternatively, prior art CO2 HVAC systems with two coils do not comprise dedicated inlet/outlet pipes. Instead, these systems only have three pipes - one for supplying liquid CO2 to the cooling coil, one for supplying CO2 vapour to the heating coil, and a common pipe for transporting CO2 vapour and cooled CO2 back to the CO2 refrigeration unit.

In some embodiments, the HVAC system of the eighth aspect may comprise a first temperature sensor configured to measure a temperature of the combined cooled supercritical CO2 vapour. The temperature of the combined cooled supercritical CO2 vapour may be measured before the combined cooled CO2 vapour is passed to the CO2 refrigeration unit. The first temperature sensor may be configured such that, when the temperature of the combined supercritical CO₂ vapour is lower than a predetermined temperature, the operating number and/or speed of the one or more compressors may be increased. Conversely, when the temperature of the combined cooled supercritical CO₂ vapour leaving the heating coil is greater than the predetermined temperature the operating number and/or speed may be decreased.

In some embodiments, the HVAC system of the eighth aspect may further comprise a gas cooler and a second three-way valve. The second three-way valve may be located and connected to selectively pass at least some of the combined cooled supercritical CO2 vapour from the heating coils to the gas cooler such that the at least some of the combined cooled supercritical CO2 vapour is further cooled. An outlet of the gas cooler may be configured to pass the further cooled supercritical CO2 vapour to the CO2 refrigeration unit, along with remaining combined cooled supercritical CO2 vapour not selectively passed to the gas cooler.

In some embodiments, the HVAC system of the eighth aspect may further comprise one or more gas cooler fans located adjacent to the gas cooler. The one or more gas cooler fans may be configured to cause a flow of ambient air over the gas cooler.

In some embodiments, the HVAC system of the eighth aspect may further comprise a second temperature sensor located at an inlet to the CO2 refrigeration unit. The system may be configured such that at least some of the combined cooled supercritical CO2 vapour from the heating coils is selectively passed to the gas cooler to achieve a predetermined temperature at the inlet to the CO2 refrigeration unit, as measured by the second temperature sensor.

In some embodiments, the HVAC system of the eighth aspect may further comprise a third temperature sensor located at an outlet of the gas cooler and prior to where the further cooled supercritical CO2 vapour from the gas cooler and the remaining combined cooled supercritical CO2 vapour are combined. The system may be configured to adjust a speed of the one or more gas cooler fans so as to achieve a predetermined temperature at the outlet of the gas cooler, as measured by the third temperature sensor.

In some embodiments, the HVAC system of the eighth aspect may further comprise an additional cooling coil arranged adjacent to the gas cooler such that the ambient air that is caused to flow over the gas cooler by the one or more gas cooler fans is thereafter caused to pass over the additional cooling coil. An inlet of the additional cooling coil may be connected to the CO2 refrigeration unit by an additional pressure-reducing valve that is configured such that, when the additional valve is in an open position, liquid CO2 is caused to flow between the CO2 refrigeration unit and the additional cooling coil. In the additional cooling coil, the liquid CO2 may evaporate as air passes over the additional cooling coil, the air being cooled thereby. An outlet of the additional cooling coil may be connected to the one or more compressors within the CO2 refrigeration unit to enable the CO2 vapour to pass thereto.

In some embodiments, the HVAC system of the eighth aspect may be configured such that, when a cooling load on the cooling coil(s) is less than a heating load on the heating coil(s) the additional pressure-reducing valve is opened to thereby cause liquid CO2 to flow between the CO2 refrigeration unit and the additional cooling coil.

In some embodiments, the HVAC system of the eighth aspect may comprise the retrofitted HVAC system of the first aspect.

Also disclosed herein in a ninth aspect is a method for operating the HVAC system of the eighth aspect. The method can comprise operating at least one of the air handling units in a cooling mode and at least one of the other air handling units in a heating mode. Alternatively, the method can comprise operating all of the at least two air handling units in a heating mode. Also disclosed herein in a tenth aspect is a system for defrosting a cooling coil. The cooling coil can be located externally to at least one air handling unit.

The system of the tenth aspect can comprise a CO2 refrigeration unit comprising one or more compressors, one or more valves, a series of pipes and electrical controls. As above, the CO2 refrigeration unit can be unitary or formed from disparate parts.

The system of the tenth aspect can also comprise at least one air handling unit.

The system of the tenth aspect can further comprise the cooling coil connected to the CO2 refrigeration unit by a pressure-reducing valve. This valve may be configured such that, when the valve is in an open position, liquid CO2 can be caused to flow from the CO2 refrigeration unit to the cooling coil whereby the liquid CO2 evaporates as air passes over the cooling coil, the air being cooled thereby.

The system of the tenth aspect can yet further comprise an outlet of the cooling coil arranged such that the resultant CO2 vapour can pass to the one or more compressors of the CO2 refrigeration unit, wherein the CO2 vapour is compressed.

The system of the tenth aspect can still further comprise a heating coil located within each of the at least one air handling units and connected to the compressor within the CO2 refrigeration unit. Each air handling unit may be connected to the compressor such that the compressed CO2 vapour passes from the compressor and into the heating coil, wherein the compressed CO2 vapour is cooled as air passes over the heating coil, the air being heated thereby.

The system of the tenth aspect can also comprise a gas cooler and a three- way valve. The three-way valve may be located and connected to selectively pass at least some of the cooled CO2 vapour from the heating coils to the gas cooler such that the at least some of the cooled CO2 vapour is further cooled. An outlet of the gas cooler can be arranged to pass the further cooled CO2 vapour to the CO2 refrigeration unit, along with remaining cooled CO2 vapour not selectively passed to the gas cooler via the three-way valve.

The system of the tenth aspect can still also comprise one or more gas cooler fans located adjacent to the gas cooler and configured to cause a flow of ambient air over the gas cooler. The cooling coil can be located adjacent to the gas cooler such that the ambient air that is caused to flow over the gas cooler by the one or more gas cooler fans is thereafter caused to pass over the cooling coil.

At temperatures around 0 °C, frost/ice starts to form on the cooling coil, reducing the efficiency of the cooling coil. The frost/ice thus needs to be removed to ensure the cooling coil can operate effectively. In prior art systems, the flow to the cooling coil must be stopped while defrosting occurs, thus requiring the at least one air handling unit to stop providing heating. Alternatively, to minimise disruption to the heating process, an electric heater can be used to provide the heat energy to defrost the cooling coil. Both prior art systems have disadvantages, e.g. disruption to heating or the need for additional equipment and electricity consumption.

In contradistinction, the system of the tenth aspect enables heating to be provided by the at least one air handling unit whilst the cooling coil is being defrosted, with no additional heating elements (e.g. heater, boiler) required. This is because the cooling coil is adjacent to the gas cooler, with ambient air caused to flow over the gas cooler and then the cooling coil. As the ambient air passes over the gas cooler, the ambient air is heated as the cooled CO2 vapour from the heating coil is further cooled within the gas cooler. Typically, the heated ambient air will have a temperature greater than 0 °C, even in winter when ambient air temperatures may be below zero. This is because the temperature of the cooled CO2 vapour is typically above 30 °C.

As the heated ambient air is caused to pass over the cooling coil (i.e. by the gas cooler fans), the heated ambient air contacts the frost/ice. Energy is transferred from the heated ambient air to the frost/ice, thereby causing the frost/ice to melt. Meanwhile, the at least one air handling unit can continue to provide heating. This is because the liquid CO2 can continue to flow to the cooling coil, producing the CO2 vapour required by the heating coil.

In some embodiments, the system of the tenth aspect may further comprise one or more pressure and/or electronic sensors located adjacent to the cooling coil. The one or more pressure and/or electronic sensors may be configured to measure a presence of frost and/or ice at the cooling coil. Further, the one or more pressure and/or electronic sensors may be configured such that, when frost and/or ice is present, the three-way valve may selectively pass all of the combined cooled CO2 vapour from the heating coils to the gas cooler, and a speed of the one or more gas cooler fans may be increased to a predetermined fan speed.

In some embodiments, the system of the tenth aspect may further comprise a temperature sensor. The temperature sensor may be configured to measure a temperature of the cooled CO2 vapour before the cooled CO2 vapour is passed to the three-way valve. Further, the temperature sensor may be configured such that, when the temperature of the CO2 vapour is lower than a predetermined temperature, the operating number and/or speed of the one or more compressors may be increased. Conversely, the temperature sensor may be configured such that, when the temperature of the cooled CO2 vapour leaving the heating coil is greater than the predetermined temperature, the operating number and/or speed of the one or more compressors may be decreased.

In some embodiments, the system of the tenth aspect may comprise two or more air handling units. In these embodiments, the system may be configured such that the resultant cooled CO2 vapour from each of the two or more air handling units is combined, and a temperature of the combined cooled CO2 vapour is measured prior to it reaching the three-way valve.

In some embodiments, the system of the tenth aspect may employ the retrofitted HVAC system of the first aspect. Also disclosed herein in an eleventh aspect is a method for defrosting a cooling coil in an HVAC system comprising CO2 as a refrigerant and at least one air handling unit, the cooling coil located externally to the at least one air handling unit.

The method of the eleventh aspect can comprise opening a pressurereducing valve between a CO2 refrigeration unit and the cooling coil, thereby allowing liquid CO2 to flow into the cooling coil. Typically, the liquid CO2 is a saturated liquid at the set pressure of the cooling coil. In the cooling coil, energy can be transferred between the (saturated) liquid CO2 within the cooling coil and air passing over the cooling coil, causing the liquid CO2 to boil (at the set pressure) and said air to be cooled.

The method of the eleventh aspect can also comprise passing the resultant CO2 vapour to one or more compressors, wherein the CO2 vapour is compressed.

The method of the eleventh aspect can further comprise allowing the compressed CO2 vapour to flow through a heating coil in each of the at least one air handling units, wherein energy is transferred between the compressed CO2 vapour within the heating coil and air passing over the heating coil, causing the vapour to be cooled and said air to be heated.

The method of the eleventh aspect can yet further comprise selectively passing at least some of the resultant cooled CO2 vapour from the heating coil(s) to a gas cooler located adjacent to the cooling coil.

The method of the eleventh aspect can still further comprise increasing a flow of ambient air across the gas cooler, and wherein the gas cooler and the cooling coil are arranged such that the ambient air thereafter passes over the cooling coil.

The method of the eleventh aspect can also comprise combining the further cooled CO2 vapour from the gas cooler with remaining cool CO2 vapour not selectively passed to the gas cooler and passing said combined CO2 vapour to the CO2 refrigeration unit. The method of the eleventh aspect has the same advantages as outlined above for the system of the tenth aspect.

In some embodiments of the method of the eleventh aspect, the presence of frost and/or ice at the cooling coil may be measured such that, when frost and/or ice is present, all of the cooled CO2 vapour is passed from the heating coil(s) to the gas cooler and the flow of ambient air is increased to a predetermined level.

In some embodiments of the method of the eleventh aspect, the HVAC system may comprise two or more air handling units. In these embodiments, the method may be performed such that the resultant cooled CO2 vapour from each of the two or more air handling units is combined, and a temperature of the combined cooled CO2 vapour is measured prior to reaching the CO2 refrigeration unit.

In some embodiments of the method of the eleventh aspect, a temperature of the further cooled CO2 vapour may be measured prior to it reaching the CO2 refrigeration unit. Further, when the temperature of the further cooled CO2 vapour is lower than a predetermined temperature, the flow of CO2 vapour from the CO2 refrigeration unit into the heating coil may be increased. Conversely, when the temperature of the further cooled CO2 vapour is greater than the predetermined temperature, the flow of CO2 vapour may be decreased.

In some embodiments of the method of the eleventh aspect, the HVAC system may comprise one compressor. In these embodiments, the method may be performed such that the flow of CO2 vapour is increased by increasing a motor speed of the compressor and decreased by decreasing the motor speed of the compressor.

In some embodiments of the method of the eleventh aspect, the HVAC system may comprise more than one compressor. In these embodiments, the method may be performed such that, the flow of CO2 vapour is increased by increasing a motor speed of the more than one compressors and/or by increasing the number of operating compressors. Conversely, the flow of CO2 vapour may be decreased by decreasing the motor speed of the more than one compressors and/or by decreasing the number of operating compressors.

In some embodiments of the method of the eleventh aspect, the HVAC system may comprise up to fourteen compressors.

In some embodiments of the method of the eleventh aspect, the HVAC system using CO2 may comprise the retrofitted HVAC system of the first aspect and the method of the eleventh aspect may be deployed on such a system.

In some embodiments of the method of the eleventh aspect, the HVAC system using CO2 may comprise the HVAC system disclosed in the tenth aspect herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with reference to the accompanying drawings in which

Fig- 1 is a schematic representation of an embodiment of a CO2 HVAC system with one air handling unit.

Fig- 2 is a schematic representation of an embodiment of a CO2 HVAC system with two air handling units.

Fig- 3 is a schematic representation of the CO2 HVAC system of Fig. 1 highlighting the flow of CO2 in the system when the system is in cooling mode.

Fig. 4 is a schematic representation of the CO2 HVAC system of Fig. 1 highlighting the flow of CO2 in the system when the system is in heating mode.

Fig. 5 is a schematic representation of the CO2 HVAC system of Fig. 1 highlighting the flow of CO2 in the system when the system is in dehumidification mode.

Fig. 6 is a schematic representation of the CO2 HVAC system of Fig. 2 highlighting the flow of CO2 in the system when one air handling unit is in heating mode and another air handling unit is in cooling mode. Fig- 7 is a schematic representation of the CO2 HVAC system of Fig. 2 highlighting the flow of CO2 in the system when both air handling units are in heating mode and an additional cooling coil is being defrosted employed in the CO2 HVAC system of Figs. 1 and 2.

It should be understood that the same reference numeral denotes the same feature in each of the Figures.

DETAILED DESCRIPTION

In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

Disclosed is an HVAC system in which CO2 is the refrigerant. The HVAC system may be of particular use where large-scale heating/cooling is required and especially where simultaneous heating and cooling is required, for example in commercial buildings. The HVAC system may also be of particular use where dehumidification of air from an air-conditioned space is required, for example in certain areas of Australia, such as Queensland and New South Wales, which have a high relative humidity. Dehumidification may also be required where there is a latent load generated in the air-conditioned space, for example, when the air- conditioned space is a gymnasium, swimming pool or an industrial kitchen - i.e. spaces that generate humidity.

In one aspect of this disclosure, the HVAC system may comprise preexisting components from a synthetic refrigerant based HVAC system which has been retrofitted to use CO2 as the refrigerant. This method of retrofitting is advantageous in that, by using components that are already present in an existing HVAC system, the system can be retrofitted to a CO2 refrigerant system at minimal cost. The method of retrofitting an existing refrigerant based HVAC system to be CO2 compatible is described in more detail in Example 1. Alternatively, in another aspect of this disclosure, the HVAC system may comprise entirely new components - e.g. where there is no prior existing HVAC system that can be retrofitted, or where the existing HVAC system is deemed to be in such poor condition that no components can be repurposed.

Referring now to Fig. 1, a schematic representation of an HVAC system 10 comprising a single air handling unit 20 is provided. The HVAC system 10 is able to operate using either a subcritical CO2 cycle or a transcritical CO2 cycle. Particularly advantageously, the HVAC system can provide cooling, heating or humidification using only CO2 (i.e. a separate boiler or electrical heating is not required). A humidification mode is possible as the HVAC system comprises both a cooling coil and a heating coil within the air handling unit and is configured such that CO2 can flow through each coil simultaneously, i.e. due to the separate cooling and heating coils and the number and arrangement of the pipes and valves. The different operational modes of the HVAC system comprising a single air handling unit are explained in more detail with reference to Figs. 3 to 5 and Examples 2 to 4.

Referring now to Fig. 2, a schematic representation of an HVAC system 10a comprising two air handling units (20 and 20a) is provided. The HVAC system 10a is able to operate using either a subcritical CO2 cycle or a transcritical CO2 cycle.

The HVAC system 10 of Fig. 1 comprises three main components: air handling unit 20, CO2 refrigeration unit 12 and an outdoor unit 11. The HVAC system 10a of Fig. 2 comprises the same air handling unit 20, the same CO2 refrigeration unit 12 and the same outdoor unit 11. However, the HVAC system 10a further comprises a second air handling unit 20a. In the embodiment shown, the second air handling unit 20a is configured identically to the air handling unit 20.

The air handling units 20, 20a are typically located in an air duct within a building, whilst the CO2 refrigeration unit 12 is normally located in a plantroom and the outdoor unit 11 is located in a well-ventilated outdoor area (e.g. rooftop). However, the inventors noted that, in some circumstances, the CO2 refrigeration unit 12 can be located in the outdoor area, along with the outdoor unit 11. For example, if there is a space constraint in the plantroom. Alternatively, in other circumstances, the outdoor unit 11 can be located in the plantroom along with the CO2 refrigeration unit 12, as long as there is sufficient outside air intake into the plantroom and the exhaust air of the outdoor unit 11 is ducted to the outside. For example, if there are structural issues with the roof or noise issues associated with having the outdoor unit 11 located outside. When the CO2 refrigeration unit 12 and the outdoor unit 11 are in the same location, the inventors noted that the components can be mounted on a common frame, thereby forming a singular unit.

It should be understood that although Figs. 1 and 2 show only one air handling unit and two air handling units respectively, the HVAC system is not limited to either one or two such air handling units. That is, the HVAC system of either Figs. 1 or 2 can comprise many more such air handling units 20b, 20c, 20d etc. The addition of additional air handling units would be within the normal skill set of a person of skill in the art.

Advantageously, the same CO2 refrigeration unit 12 and the same outdoor unit 11 may be connected to several such air handling units 20, 20a, etc. as in Fig. 2, thereby providing heating/cooling/dehumidification to several locations 28, 28a, etc. within the building. Further advantageously, each air handling unit 20, 20a, etc. connected to the outdoor unit 11 can operate independently. For example, one such air handling unit 20 located in one section of the building may be providing cooling, whilst another such air handling unit 20a located in another section of the building may be providing heating. Furthermore, there is no maximum limit on the number of air handling units that can be associated with the outdoor unit 11, as long as the outdoor unit 11 is capable of providing sufficient cooling/heating load to each air handling unit 20, 20a, etc. This stands in contradistinction to HVAC systems comprising CO2 of the prior art which typically have a maximum number of air handling units per outdoor unit.

The HVAC system 10, 10a of Figs. 1 and 2 finds particular application in buildings where cooling is required in certain areas, but heating is required in other areas. For example, a cinema complex where cooling may be required in the projector room, but heating may be required in the cinema(s) themselves, in the foyer, etc. - i.e. during winter. As another example, office buildings may require heating in certain places within the office (e.g. in the middle of an open plan office) but cooling in other places (e.g. near windows where it tends to be warmer, even in winter). The different operational modes of the HVAC system comprising multiple air handling units is explained with reference to Figs. 2, 6 and 7 and Examples 5 to 6.

The HVAC system 10, 10a comprises a programmable logic controller (PLC) and a series of electrical components which enable the system 10, 10a to be controlled.

Each air-conditioned space 28, 28a comprises a dedicated temperature probe which measures the temperature therein. The measured temperature is transmitted back to the PLC, which then determines what mode a particular air handling unit should be operating in. For example, when the temperature probe of an air-conditioned space 28 measures a temperature greater than the target temperature, the air handling unit is placed in cooling mode. Conversely, when the temperature probe measures a temperature less than the target temperature, the air handling unit is placed in heating mode. Each air-conditioned space 28, 28a can also comprise a humidity sensor, which enables the PLC to deduce when an air handling unit should be placed in dehumidification mode, i.e. when the measured humidity is above a predetermined threshold.

The target temperature for each air-conditioned space 28, 28a can be set independently. For example, the target temperature(s) can be present internally, such as in older buildings which have thermostats. When the target temperature(s) are set using thermostats, the target temperatures cannot be manually adjusted, i.e. they are hard-wired into the system. More modem buildings, however, typically comprise a Building Management System (BMS) which allows target temperatures to be manually set for each air-conditioned space controlled by the PLC system.

Depending on the number of air handling units, the temperature probes in the air-conditioned spaces 28, 28a and the PLC can either be connected through direct digital inputs (e.g. when there are fewer air handling units) or through a communications protocol such as Modbus (e.g. when there are many air handling units).

A schematic of a CO2 refrigeration unit 12 is provided in Fig. 8 and will be described in more detail throughout this section. The shaded area of Fig. 8 shows the components that are part of the CO2 refrigeration unit 12. The CO2 refrigeration unit 12 itself comprises one or more CO2 compressors 14, a series of pipes and valves and electrical controls. The one or more compressors 14 are suitable to operate with CO2 in either a supercritical cycle or a transcritical cycle. The series of pipes and valves are configured to allow two-phase CO2 77 to pass into the receiver 12-1 of the CO2 refrigeration unit 12. The liquid component of the two-phase CO2 is collected at the bottom 12-4 of the receiver 12-1. The receiver 12-1 is configured to allow the liquid CO2 to pass from the CO2 refrigeration unit 12 through one or more pressure-reducing valves 22, 80 to one or more cooling coils 18, 76 (as explained in more detail below). The vapour component of the two-phase CO2 is collected at the top 12-3 of the receiver 12-1. When valve 12-5 is open, the receiver 12-1 is configured such that the CO2 vapour passes to the one or more CO2 compressors 14. The valve 12-5 is typically a pressure regulator and acts to maintain a pressure of about 50 bar in the receiver 12-1. The series of pipes and valves within the CO2 refrigeration unit 12 are also configured to allow superheated (or supercritical) CO2 vapour 30, 31, 88 to pass from the one or more cooling coils 18, 76 into the one or more CO2 compressors 14, wherein the CO2 vapour is compressed. The one or more CO2 compressors 14 are configured to pass the resultant CO2 vapour when valve 12-5 is open to a three-way valve 40, along with the compressed CO2 (compressed at compressor 14-1) exiting the receiver 12-1, typically as a combined stream 38.

In this regard, in the arrangement of Fig. 8, the CO2 refrigeration unit 12 is configured such that the CO2 vapour 12-3 from the receiver 12-1 is compressed in a separate and respective compressor 14-1 to that of the returning superheated CO2 vapour 30, 31, 88. The compressed CO2 vapour from each of the compressor(s) 14-1, 14-2 is then combined to form the compressed CO2 vapour stream 38. However, it will be understood that this need not be the case and the vapour 12-3 can be likewise compressed by compressors 14-2 (i.e. along with the CO2 vapour 30, 31, 88). In addition, although the CO2 refrigeration unit 12 shows only three compressors, up to fourteen compressors can be present in the unit. The number of compressors is typically selected based on the application of the system 10.

The flow of CO2 within the CO2 refrigeration unit 12 and the other components of the HVAC system 10, is set forth in more detail below.

The components comprising the CO2 refrigeration unit 12 are suitable to operate with CO2 at pressures above atmospheric. In particular, the CO2 refrigeration unit 12 is suitable to operate at a maximum pressure of 120 bar. Typically, the one or more compressors 14 will compress CO2 to a pressure of between about 60 bar to 90 bar, depending on whether the HVAC system 10 is operating in a subcritical cycle or a transcritical cycle.

It should be understood that the CO2 refrigeration unit 12 refers to the part of the HVAC system that comprises the one or more CO2 compressors 14, series of pipes and valves and electrical controls. The CO2 refrigeration unit 12 may therefore comprise a stand-alone unit, e.g. such as may be purchased from a supplier in its entirety and in an already preconfigured state. However, the CO2 refrigeration unit 12 may also be configured from disparate parts, i.e. not all purchased from the same supplier and/or not already configured. The CO2 refrigeration unit 12 comprises the receiver 12-1 in which liquid CO2 is stored. When required, liquid CO2 16 passes from the receiver 12-1 via a pipe located at the bottom 12-4 of the receiver 12-1 to a cooling coil 18 located in the air handling unit 20. The liquid CO2 in the receiver 12-1 is typically a saturated liquid.

The air handling unit 20 comprises air filters, coils (e.g. cooling and heating), fans, instruments and associated ductwork, ductwork, and louvres. The air handling unit 20 enables the circulation of air between the air-conditioned space 28 and the air handling unit 20 - i.e. in order to achieve the desired cooling, heating, dehumidification etc. of the air in the air-conditioned space 28. The air handling unit 20a of Figs. 2, 6 & 7 (and any other additional air handling units) is typically configured identically to air handling unit 20.

The cooling coil 18 typically comprises a fin cooling coil and is suitable to operate at a maximum pressure of 80 bar. The (saturated) liquid CO2 16 from the CO2 refrigeration unit 12 passes through a pressure-reducing valve 22 positioned between the CO2 refrigeration unit 12 and the cooling coil 18. The pressurereducing valve 22 is typically an Electronic Expansion Valve (EEV) Metering device as this enables the volume flow of CO2 through the valve to be controlled (i.e. to control the cooling load of the cooling coil 18).

The pressure-reducing valve 22 is operable between a closed position and an open position. The pressure-reducing valve 22 is operable in a fully open position or a partially open position. When the pressure-reducing valve 22 is open (either fully or partially), the liquid CO2 16 can pass from the CO2 refrigeration unit 12 to the cooling coil 18. The pressure-reducing valve 22 can be incrementally opened or closed so as to control the flow of liquid CO2 16 passing from the CO2 refrigeration unit 12 to the cooling coil 18. For example, the flow of liquid CO2 16 is increased by further opening the pressure-reducing valve 22 and is decreased by partially closing the pressure-reducing valve 22. When the pressure-reducing valve 22 is fully closed, no liquid CO2 16 can pass from the refrigeration unit 12. This enables the cooling coil 18 to be isolated from the CO2 refrigeration unit 12, e.g. when no cooling is required in the air handling unit 20, the pressure-reducing valve 22 is fully closed.

As the liquid CO2 16 is passed through the open (or partially open) pressure-reducing valve 22, the pressure of the liquid CO2 16 decreases. The pressure-reducing valve 22 is designed to operate at a maximum (standstill) pressure of 80 bar. Typically, the pressure of the liquid CO2 after the pressurereducing valve 22 passing into the cooling coil 18 is about 40 bar, which corresponds to a liquid CO2 temperature of about 6 °C. The pressure rarely rises above 45 bar. Because the pressure of the liquid CO2 is reduced and the liquid CO2 is a saturated liquid, a portion of the liquid CO2 may boil, thereby producing a two-phase (liquid and vapour) CO2 24 which flows to the cooling coil 18. However, the two-phase CO2 24 comprises primarily liquid phase CO2, i.e. typically the portion of liquid CO2 that boils due to the flow of liquid CO2 16 through the pressure-reducing valve 22 is minimised.

The two-phase CO2 24 is passed to the inside of the cooling coil 18 in the air handling unit 20. As the two-phase CO2 24 passes through the cooling coil 18, air 26 is passed over the outside of the cooling coil 18. Usually, the air 26 is a mixture of air from the air-conditioned space 28 and fresh, ambient air 27 from outside the air-conditioned space 28. The addition of fresh, ambient air 27 is required to ensure that the levels of oxygen in the air-conditioned space 28 remain at safe levels for people occupying the air-conditioned space 28.

As the air 26 passes over the cooling coil 18, energy is transferred from the air 26 passing over the cooling coil 18 to the two-phase CO2 24 within the cooling coil 18. Due to the reduced pressure of the two-phase CO2 24 (i.e. compared with the liquid CO2 16 prior to the pressure-reducing valve 22) and because the liquid CO2 in the two-phase CO2 24 is a saturated liquid, the energy transfer from the air 26 to the two-phase CO2 24 causes any remaining liquid in the two-phase CO2 to evaporate in the cooling coil 18 at the set pressure of the cooling coil 18, thereby producing a CO2 vapour 30. At the same time, the air 26 leaving the cooling coil 18 is cooled air 32. When the HVAC system 10 is in cooling mode (i.e. when the temperature of the air-conditioned space 28 is high), the set evaporating (saturated) temperature of the saturated liquid CO2 in the two-phase CO2 24 is typically below the dew-point of the air 26 that passes over the cooling coil 18. As a result, moisture is condensed from the air 26 as it passes over the cooling coil 18, as the temperature of the cooled air 32 is also below the dew-point thereof. The condensed moisture collects on the cooling coil. In this way, the absolute humidity of the air 32 leaving the cooling coil 18 is lower than the absolute humidity of the air passing over the cooling coil 26 - i.e. the cooling coil 18 acts to dehumidify the air 26 passing over the cooling coil 18.

When the system 10a comprises more than one air handling unit, typically each air handling unit 20, 20a, etc. comprises a corresponding pressure-reducing valve 22, 22a. This arrangement enables liquid CO2 from the refrigeration unit to be selectively passed to some cooling coils 18, 18a, etc., but not other cooling coils. For example, when cooling is required in only cooling coil 18, pressurereducing valve 22 is opened allowing liquid CO2 16 to pass into cooling coil 18, but pressure-reducing valve 22a is closed. Alternatively, when cooling is required in only cooling coil 18a, pressure-reducing valve 22a is opened allowing liquid CO2 16a to pass into cooling coil 18a, but pressure-reducing valve 22 is closed. If cooling is required in both cooling coils 18 and 18a, then both pressure-reducing valves 22 and 22a are opened, allowing liquid CO2 16 and 16a respectively to pass into cooling coil 18 and 18a respectively. If cooling is not required in either cooling coil 18 or 18a, then both pressure-reducing valves 22 and 22a are closed.

It will be appreciated that each of the cooling coils (18, 18a) in each of the air handling units (20, 20a) further comprises a corresponding inlet pipe through which liquid CO2 from the refrigeration unit 12 passes to the cooling coil 18, 18a (when the corresponding pressure-reducing valve is open) and a corresponding outlet pipe through which CO2 vapour passes from the cooling coil 18, 18a to the CO2 refrigeration unit 12 (when the cooling coil is in-use). Furthermore, each of the corresponding inlet pipes 16, 16a of the cooling coils 18, 18a is derived from a single outlet 17 of the CO2 refrigeration unit 12 (Fig. 2). Likewise, each of the corresponding outlet pipes 30, 30a of the cooling coils 18, 18a merges to form a single inlet 31 to the CO2 refrigeration unit 12.

A temperature sensor 34 is placed after the cooling coil 18 and measures the temperature of cooled air 32 leaving the cooling coil 18. The temperature sensor 34 is typically a 4-20Ma PT 1000 sensor. A like temperature sensor 34a is placed after the cooling coil 18a.

A temperature sensor 36 is placed at the entrance to the air handling unit 20. The temperature sensor 36 is typically a 4-20Ma PT1000 sensor. A like temperature sensor 36a is placed at the entrance to the air handling unit 20a. The temperature sensor 36 measures the temperature of the air in the air-conditioned space 28. The temperature of the air in the air-conditioned space 28 is also the temperature of the air 26 that is passed over the first cooling coil 18.

The CO2 vapour 30 (or 31 when there is more than one air handling unit) passes from the cooling coil 18 back to the compressor 14 in the CO2 refrigeration unit 12. In the compressor 14, the CO2 vapour 30 is compressed. The compressed CO2 vapour forms either a supercritical fluid or a superheated CO2 vapour, depending on whether the HVAC system is operating in a transcritical cycle or a subcritical cycle respectively. The compressed CO2 vapour 38 has a maximum pressure of 120 bar. Typically, the compressed CO2 vapour 38 has a pressure of between 60 bar to 90 bar. A pressure sensor (12-6 in Fig. 8) installed at the outlet of the CO2 refrigeration unit 12 measures the pressure of the compressed CO2 vapour 38. In certain operating modes of the system 10, the pressure of the compressed CO2 vapour 38 can be used to control the speed of fans 66 located adjacent to the gas cooler 60. This is explained more fully by way of Examples 2 to 6, with reference to Figs. 3 to 7.

It will be appreciated that the exact pressure of the compressed CO2 vapour 38 will depend on whether the HVAC system 10, 10a is operating in a subcritical cycle or a transcritical cycle. For example, when the HVAC system 10, 10a operates in a subcritical cycle, the compressed CO2 vapour 38 can have a pressure of about 65 bar. However, when the HVAC system 10 operates in a transcritical cycle, the compressed CO2 vapour 38 can have a pressure of about 84 bar.

A pressure sensor (12-2 in Fig. 8) measures the pressure of the CO2 vapour 31 at an inlet of the CO2 refrigeration unit 12, i.e. pressure sensor 12-2 measures a suction pressure of the one or more compressors 14. In certain operating modes of the system 10, the pressure measured by the pressure sensor 12-2 can be used to control the flow of liquid CO2 16 to the cooling coil 18. This is explained more fully by way of Examples 2 to 6, with reference to Figs. 3 to 7. The compressed CO2 vapour 38 passes from the CO2 refrigeration unit 12 to a three-way valve 40. The three-way valve 40 is typically a motorised ball valve. The three-way valve 40 is configured to selectively pass the compressed CO2 vapour 38 to a heating coil 42 of the air handling unit 20. The heating coil 42 is located within the air handling unit 20 adjacent to the cooling coil 18. Typically, the heating coil 42 also comprises a fin coil and is suitable to operate at a maximum pressure of 120 bar. It will be appreciated that the actual operating pressure of the heating coil 42 will be the pressure of the compressed CO2 vapour 38, i.e. between about 60 bar to 90 bar.

Depending on the mode of operation of the HVAC system, a portion, none, or all of the compressed CO2 vapour 38 is selectively passed to the heating coil 42. For example, when the air handling unit 20 is in heating mode or dehumidification mode, the three-way valve 40 is positioned such that compressed CO2 vapour 38 is allowed to pass to the heating coil 42. However, when the air handling unit 20 is in a cooling mode or is offline, the three-way valve 40 is positioned to cause the compressed CO2 vapour 38 to bypass the heating coil 42.

When the heating coil 42 is in-use, the three-way valve 40 can be configured to selectively pass some or all of the compressed CO2 vapour 38 to the heating coil 42. The compressed CO2 vapour 44 is passed through the inside of the heating coil 42. Whilst the compressed CO2 vapour 44 is passing through the inside of the heating coil 42, air 32 leaving the cooling coil 18 passes over the outside of the heating coil 42. Energy is transferred from the compressed CO2 vapour 44 inside the heating coil 42 to the air 32 passing over the outside of the heating coil 42. This energy transfer causes the compressed CO2 vapour 44 to cool and the air 32 to be concurrently heated, thereby producing a cooled CO2 vapour 46 and heated air stream 48. The heated air 48 is blown by fans 52 from the air handling unit 20 back into the air-conditioned space 28.

When the HVAC system 10 comprises more than one air handling unit, each air handling unit 20, 20a, etc. can have a corresponding two-way ball valve 45, 45a, etc. associated therewith at an inlet of the heating coil 42, 42a, etc. (see Fig. 2). Each air handling unit 20, 20a etc. can also have a corresponding two-way ball valve 47, 47a, etc. associated therewith at an outlet of the heating coil 42, 42a, etc. The two-way ball valves are employed because, when the system 10 comprises multiple air handling units, the three-way valve 40 is configured such that all the compressed CO2 vapour 38 is directed through pipe 44 when any one of the air handling units is in a heating mode. The two-way ball valves enable the compressed CO2 vapour 38 from the one or more compressors to be selectively passed to some heating coils 42, 42a, etc., but not other heating coils.

For example, when heating is required in only heating coil 42, ball valves 45 and 47 are opened allowing CO2 vapour 44 to pass into heating coil 42, but ball valves 45a and 47a are closed. Alternatively, when heating is required in only heating coil 42a, ball valves 45a and 47a are opened, but ball valves 45 and 47 are closed, allowing CO2 vapour 44a to pass only into heating coil 42a, but not into heating coil 42. If heating is required in both heating coils 42 and 42a, then all ball valves 45, 45a, 47 and 47a are opened, allowing CO2 vapour to pass into both heating coil 42 and 42a. If heating is not required in either heating coil 42 or 42a, then three-way valve 40 is positioned so as to cause the CO2 vapour to bypass the heating coils entirely, i.e. all the compressed CO2 vapour 38 is directed to stream 54. It will be appreciated that each of the heating coils 42, 42a in each of the air handling units 20, 20a further comprises a corresponding inlet pipe through which CO2 vapour from the one or more compressors 14 passes to the heating coil (when the three-way valve(s) is open) and a corresponding outlet pipe through which cooled CO2 vapour passes from the heating coil. Furthermore, each of the corresponding inlet pipes of the heating coils 42, 42a is derived from a single outlet 38 of the one or more compressors 14 in the CO2 refrigeration unit 12. Likewise, each of the corresponding outlet pipes 46, 46a of the heating coils 42, 42a merges to form a single combined CO2 vapour stream 47. That is, the HVAC system 10, 10a, comprises a dedicated series of pipes through which the CO2 vapour is passed from the heating coil(s).

A temperature sensor 50 (or 50a) is placed after the heating coil 42 which measures the temperature of the heated air 48 leaving the heating coil 42. Typically, the temperature sensor 50 is a 4-20Ma PT1000 temperature sensor.

In certain operating modes and when the system 10 has only one air handling unit, the temperature sensor 50 is configured to facilitate adjustment of the portion of compressed CO2 vapour 44 that is selectively passed from the three- way valve 40 to the heating coil 42. For example, when the temperature of the heated air 48 measured by the temperature sensor 50 is above a predetermined heated air temperature, the portion of compressed CO2 vapour 44 that is selectively passed to the heating coil 42 is decreased. Conversely, when the temperature of the heated air 48 is below the predetermined heated air temperature, the portion of compressed CO2 vapour that is selectively passed to the heating coil 42 is increased. The portion of compressed CO2 vapour that is selectively passed to the heating coil 42 is controlled using the three-way valve 40 - i.e. by changing the position of three-way valve 40, the portion of compressed CO2 vapour 38 that is selectively passed 44 to the heating coil 42 is changed. This method of control is primarily used when the air handling unit 20 is set to a dehumidification mode. When the system 10 has more than one air handling unit (e.g. Fig. 2), the temperature sensors 50, 50a are configured to facilitate the opening and closing of the ball valves 45, 47 and 45a, 47a respectively. For example, when the temperature of the heated air 48 measured by the temperature sensor 50 is above a predetermined heated air temperature, the ball valves 45 and 47 are closed, i.e. to stop the air 32 from being heated. Conversely, when the temperature of the heated air 48 is below the predetermined heated air temperature, the ball valves 45 and 47 are opened, i.e. to allow the air 32 to be heated. This is because, when the system 10 has more than one air handling unit, the three-way valve 40 is configured so as to pass all the compressed CO2 vapour 38 to the stream 44. Another temperature sensor 51 is placed after the heating coil 42 which measures the temperature of the cooled CO2 vapour 46 leaving the heating coil 42 (and after the combined CO2 vapour stream 47 is formed). Typically, the temperature sensor 51 is a 4-20Ma PT 1000 temperature sensor.

In HVAC systems comprising multiple air handling units (e.g. Fig. 2), the temperature sensor 51 is located such that it measures the temperature of the combined cooled CO2 vapour 47. The combined cooled CO2 vapour 47 comprises the returning cooled CO2 vapour 46 and 46a from the heating coils 42 and 42a respectively of all air handling units 20 and 20a respectively. This stands in contradistinction to CO2 HVAC systems of the prior art which do not comprise a dedicated return for the CO2 vapour. Instead, cooled CO2 vapour from the heating coil passes from the heating coil via the same pipe(s) through which CO2 vapour from the cooling coil is passed.

The remaining compressed CO2 vapour 54 that is not selectively passed to the heating coil 42 (i.e. when the system 10 comprises only a single air handling unit) is mixed with the cooled CO2 vapour 46 from the heating coil 42, thereby forming a further combined CO2 vapour 56. The further combined CO2 vapour 56 is passed to another three-way valve 58. The three-way valve 58 is typically a 3- way motorised ball valve. The three-way valve 58 is configured to selectively pass the combined CO2 vapour 56 to a CO2 condenser/gas cooler 60. As above the CO2 condenser/gas cooler 60 comprises part of the outdoor unit 11. The CO2 condenser/gas cooler 60 operates to further cool the combined CO2 vapour 56. Further cooling of the combined CO2 vapour 56 can be needed when the HVAC system 10, 10a operates in a cooling, heating or dehumidification mode, as will be described in more detail below. The combined CO2 vapour 56 is also passed to the CO2 condenser/gas cooler 60 when the HVAC system 10, 10a is in a defrosting mode. However, when the combined CO2 vapour 56 does not require further cooling or when the HVAC system 10, 10a is not in defrosting mode, the three- way valve 58 can be positioned to cause the combined CO2 vapour 56 to bypass the CO2 condenser/gas cooler and flow directly to the CO2 refrigeration unit 12.

The CO2 condenser/gas cooler 60 is configured to operate at a maximum constant pressure of 120 bar. Typically, the CO2 condenser/gas cooler 60 operates at a pressure between about 60 bar to 90 bar, i.e. the same pressure as the heating coil 42 and the compressor 14. The CO2 condenser/gas cooler 60 typically provides isobaric (constant pressure) cooling. It will be appreciated that the exact operating pressure of the CO2 condenser/gas cooler 60 will depend on whether the CO2 HVAC system 10 is operating in a subcritical cycle or a transcritical cycle. When the CO2 HVAC system 10 operates in a subcritical cycle, the CO2 condenser/gas cooler 60 can operate at a pressure of about 65 bar. However, when the HVAC system 10 operates in a transcritical cycle, the CO2 condenser/gas cooler 60 can operate at a pressure of about 84 bar.

Typically, the CO2 condenser/gas cooler 60 comprises fin coils comprised of either copper or stainless-steel coils with aluminium fins. The portion 62 of the combined CO2 vapour selectively passed to the CO2 condenser/gas cooler 60 is passed into the tubes of the CO2 condenser/gas cooler 60. Ambient air 64 surrounding the CO2 condenser/gas cooler 60 is forced to flow over the coil and the outside of the tubes of the CO2 condenser/gas cooler 60 by means of fans 66 located adjacent to the CO2 condenser/gas cooler 60. As the ambient air 64 is forced to flow around the outside of the tubes of the CO2 condenser/gas cooler 60, energy is transferred from the CO2 vapour 62 within the tubes to the ambient air 64. This energy transfer causes the CO2 vapour 62 to be further cooled and the ambient air 64 to be heated.

A temperature sensor 78 is located adjacent to the CO2 condenser/gas cooler 60 and measures the temperature and humidity of the ambient air 64. Typically, the temperature sensor 78 is a 4-20Ma PT1000 temperature sensor.

When the HVAC system 10, 10a is operating in a subcritical cycle, the CO2 vapour 62 is a superheated CO2 vapour. In the CO2 condenser/gas cooler 60, the superheated CO2 vapour is condensed into liquid CO2. The liquid CO2 can either be saturated or may be further cooled thereby forming a subcooled liquid CO2.

When the HVAC system 10 is operating in a transcritical cycle, the CO2 vapour 62 is a supercritical CO2 fluid. In the CO2 condenser/gas cooler 60, the supercritical CO2 fluid is cooled, but remains a supercritical fluid.

The cooled CO2 68 from the CO2 condenser/gas cooler 60 is combined with the remaining combined CO2 vapour 70 that is not selectively passed by the three-way valve 58 to the CO2 condenser/gas cooler 60, forming a combined cooled CO2 stream 72. As the cooled CO2 68 and the remaining combined CO2 vapour 70 mix, energy is transferred from the combined CO2 vapour 70 (which has a higher enthalpy than the cooled CO2 68) to the cooled CO2 68, and the enthalpy of the two streams thereby equilibrates. The combined cooled CO2 stream 72 passes to a high-pressure electronic expansion valve 75.

As the combined cooled CO2 stream 72 passes to the high-pressure electronic expansion valve 75, a temperature sensor 74 located adjacent to the inlet of the high-pressure electronic expansion valve 75 measures the temperature of the combined cooled CO2 stream 72. Typically, the temperature sensor 74 is a 4-20Ma PT 1000 temperature sensor. In some operating modes, the temperature sensor 74 is configured in the system to facilitate adjustment of the portion of the combined CO2 vapour 62 that is selectively passed by the three-way valve 58 to the CO2 condenser/gas cooler 60. When the temperature at the inlet to the high-pressure electronic expansion valve 75 measured by the temperature sensor 74 is above a predetermined inlet temperature, the portion 62 of the combined CO2 vapour selectively passed to the CO2 condenser/gas cooler 60 by the three-way valve 58 is increased. Conversely, when the temperature at the high-pressure electronic expansion valve inlet is below the predetermined inlet temperature, the portion 62 of the combined CO2 vapour selectively passed to the CO2 condenser/gas cooler 60 is decreased. The predetermined inlet temperature is typically around 15 °C to 20 °C.

As the combined cooled CO2 stream 72 passes through the high-pressure electronic expansion valve 75, the pressure of the stream is reduced. The pressure of the expanded CO2 77 is typically around 50 bar, regardless of whether the HVAC system is operating in a transcritical cycle or a subcritical cycle. As the pressure of the combined cooled CO2 stream 72 is reduced to about 50 bar, the CO2 forms a two-phase (vapour and liquid) CO2 stream 77 that is passed into an inlet of the CO2 refrigeration unit 12. The liquid CO2 in the two-phase CO2 stream 77 is a saturated liquid. It will be appreciated that the ratio of vapour and liquid in the two-phase CO2 stream 77 is dependent on the temperature at the inlet to the high-pressure electronic expansion valve (i.e. since the pressure is constant at about 50 bar).

The two-phase CO2 stream 77 passes into the liquid receiver 12-1 (see Fig. 8) within the CO2 refrigeration unit 12. Because the vapour-phase CO2 is less dense than the liquid-phase CO2, it moves to the top 12-3 of the receiver 12-1. The vapour-phase CO2 collected from the top 12-3 of the receiver passes to one of the one or more CO2 compressors 14 when valve 12-5 is open, thereby forming part of the compressed CO2 vapour 38 passed to the heating coil(s). When valve 12-5 is closed, the vapour-phase CO2 remains in the receiver 12-1. The liquidphase CO2, being denser than the vapour-phase CO2, collects at the bottom 12-4 of the receiver 12-1. The liquid-phase CO2 that collects in the receiver 12-1 forms the liquid CO2 16, 82 that is passed to the in-use cooling coils 18, 76.

The HVAC system 10, 10a can advantageously comprise an additional cooling coil 76 located adjacent to and coupled to the CO2 condenser/gas cooler 60 within the outdoor unit 11. Typically, the additional cooling coil 76 is a fin coil. The CO2 condenser/gas cooler 60 and the additional cooling coil 76 are configured such that the ambient air 64 is caused to first pass over the CO2 condenser/gas cooler 60 by the fans 66 and thereafter passes over the additional cooling coil 76. That is, the heated ambient air 90 from the CO2 condenser/gas cooler 60 passes over the additional cooling coil 76.

The additional cooling coil 76 is only used in certain operating modes. For example, when the air handling unit(s) of system 10, 10a are all set to a heating mode, the additional cooling coil 76 is used to provide for the heat load to the heating coils 42, 42a etc. As another example, the additional cooling coil 76 advantageously allows the system 10, 10a to be operated in a manner that defrosts the additional cooling coil 76 whilst providing heating to the one or more air handling units 20, 20a etc. As a further example, the additional cooling coil 76 is used when one air handling unit 20 requires cooling, another air handling unit 20a requires heating, but the heating load is greater than the cooling load.

When the additional cooling coil 76 is required, for example when the system 10, 10a is operating in any of the three modes aforementioned, the pressure-reducing valve 80 between the CO2 refrigeration unit 12 and the additional cooling coil 76 is opened.

The pressure-reducing valve 80 is operable between a closed position and an open position. The pressure-reducing valve 80 can also be partially open. When the pressure-reducing valve 80 is opened (partially or fully), a stream of liquid CO2 82 is enabled to pass from the CO2 refrigeration unit 12 to the additional cooling coil 76 - i.e. the additional cooling coil 76 is turned “on”. When the pressure-reducing valve 80 is fully closed, no liquid CO2 can pass from the CO2 refrigeration unit 12 to the additional cooling coil 76 - i.e. the additional cooling coil 76 is turned “off”. Typically, the pressure-reducing valve 80 is an Electronic Expansion Valve (EEV) Metering device. The pressure-reducing valve 80 can be incrementally opened or closed so as to control the flow of liquid CO2 82 passing from the CO2 refrigeration unit 14 to the additional cooling coil 76. For example, the flow of liquid CO2 82 is increased by further opening the pressure-reducing valve 80 and decreased by partially closing the pressurereducing valve 80. Typically, the liquid CO2 82 is a saturated liquid, i.e. the same as the liquid CO2 16 that flows to the cooling coil 18, because it is also passed from the receiver 12-1 of the CO2 refrigeration unit 12.

As the (saturated) liquid CO2 82 passes through the open (or partially open) pressure-reducing valve 80, the pressure of the liquid CO2 82 decreases to a set pressure. Typically, the pressure of the liquid CO2 after the pressure-reducing valve is about 35 bar, which corresponds to a CO2 temperature of about 1 °C. Because the pressure of the liquid CO2 is reduced and the liquid CO2 82 is a saturated liquid, a portion of the liquid CO2 boils, thereby producing a two-phase (liquid and vapour) CO2 84 which flows to the additional cooling coil 76. However, the two-phase CO2 84 comprises primarily liquid phase CO2, i.e. the portion of liquid CO2 that boils due to the flow of liquid CO2 82 through the pressure-reducing valve 80 is minimised. The liquid CO2 in the two-phase CO2 84 is also a saturated liquid.

The two-phase CO2 84 is passed to the inside of the additional cooling coil 76. As the two-phase CO2 84 passes through the additional cooling coil 76, ambient air 90 is passed over the outside of the additional cooling coil 76. The ambient air 90 passing over the outside of the additional cooling coil 76 is the air leaving the CO2 gas cooler/condenser 60, as afore-described. When the CO2 gas cooler/condenser 60 is in-use, the ambient air 90 will be heated air.

As the ambient air 90 passes over the additional cooling coil 76, energy is transferred from the ambient air 90 passing over the additional cooling coil 76 to the two-phase CO2 84 within the additional cooling coil 76. Due to the reduced pressure of the two-phase CO2 94 (i.e. compared with the liquid CO2 82 prior to the pressure-reducing valve 80) and because the liquid CO2 is saturated, the energy transfer from the ambient air 90 to the two-phase CO2 84 causes remaining liquid in the two-phase CO2 to substantially evaporate in the additional cooling coil 76 at the operating pressure thereof, thereby producing CO2 vapour 88. At the same time, the air leaving the additional cooling coil is cooled air 86.

Frost/ice can develop on the additional cooling coil 76, for example in colder months when the ambient air temperature is below 0 °C. The presence of frost/ice can reduce the efficiency of the additional cooling coil 76. Advantageously, when the CO2 gas cooler/condenser 60 is in-use, the ambient air 90 passing over the additional cooling coil 76 is heated (i.e. because of the energy transfer between the CO2 vapour 62 and the ambient air in the CO2 gas cooler/condenser 60). As the (heated) ambient air 90 passes over the additional cooling coil 76, it causes ice/frost on the additional cooling coil 76 to melt. The HVAC system 10, 10a can thereby operate in a defrosting mode, i.e. by making use of the CO2 gas cooler/condenser 60 to provide heated ambient air 90 to the additional cooling coil 76. The defrosting mode of operation is described in more detail below with reference to Fig. 7 and Example 6.

The CO2 vapour stream 88 passes back to the CO2 refrigeration unit 12 where it is combined with CO2 vapour passing from the cooling coil 18 of the air handling unit 20 and compressed 14, forming a portion of the compressed CO2 vapour 38.

Variations and modifications may be made to the parts previously described without departing from the spirit or ambit of the disclosure. For example, the system may comprise further valves which allow parts of the system to be isolated for maintenance etc.

Examples

Non-limiting Examples of the method and system will now be described. Example 1 - Retrofitting

In this Example, an existing R22 HVAC system was retrofitted to enable the existing system to be compatible with CO2 refrigerant. The HVAC system was retrofitted to determine whether, by replacing certain parts of an existing synthetic refrigerant based HVAC system with CO2 compatible components, the HVAC system would still be capable of providing sufficient cooling and heating capacity to operate on an industrial scale. To properly test the system, this exercise was performed using an industrial scale system (such as may be used at a cinema complex). After the retrofit was completed, the configuration of the retrofitted CO2 HVAC system was the same as the HVAC system 10 of Figs. 1 and 2, with several air handling units 20, 20a etc. attached to the one outdoor unit 11.

The (pre-existing) synthetic refrigerant based HVAC system comprised multiple air handling units. Each air handling unit comprised only one coil, and a series of pipes, valves and ducts.

In a first step, components of the (pre-existing) synthetic refrigerant based HVAC system that were not compatible with a CO2 refrigerant system were identified, removed and replaced. The components identified as non-CCh compatible were: the synthetic refrigerant compressor unit; the coil in each air handling unit; and pipes and valves through which the synthetic refrigerant circulated.

In place of these components which were not compatible with CO2, new components which were CCh-compatible were installed. A CO2 refrigeration unit 12 was installed. The CO2 refrigeration unit comprised four CO2 compressors 14 each of which was suitable to operate in either a subcritical CO2 cycle or a transcritical CO2 cycle. This allowed the HVAC system 10 to operate using either a subcritical CO2 cycle or a transcritical CO2 cycle.

The inventors noted that the number of CO2 compressors 14 did not need to be four. For example, the CO2 refrigeration unit 12 could simply comprise one CO2 compressor or six CO2 compressors or fifteen CO2 compressors, etc., i.e. any number of CO2 compressors that would be practical in a given space. The inventors further noted that the number of CO2 compressors 14 suitable in a particular HVAC system to be retrofitted would be dependent on the expected heating/cooling load of the HVAC system 10. For example, where the air- conditioned space is large and/or there are many air handling units resulting in higher loads, more CO2 compressors 14 are required in the CO2 refrigeration unit 12. However, where the air-conditioned space is small and/or there are not many air handling units, one CO2 compressor may suffice.

A CO2 condenser/gas cooler 60 was installed within the outdoor unit 11. The CO2 condenser/gas cooler 60 was configured to condense and/or cool CO2 vapour at a maximum pressure of about 120 bar.

A cooling coil 18 was installed in each of the at least one air handling units 20. The cooling coil 18 was a fin coil and was suitable to comprise CO2 at a maximum pressure of about 80 bar. The inventors noted that fin coils allowed for better heat transfer compared with other types of heat exchanger coils because the presence of fins increased the surface area over which heat transfer occurred. In addition, the fin coils acted to ‘straighten’ the airflow out as it contacted the coils.

A further part of the retrofit involved the inlet of each cooling coil 18 being connected to the CO2 refrigeration unit 12 via an Electronic Expansion Valve 22. The Electronic Expansion Valve 22 allowed liquid CO2 to pass into the cooling coil 18 when the valve 22 was in an (fully or partially) open position. Each cooling coil 18 had a dedicated inlet/Electronic Expansion Valve through which liquid CO2 was able to be passed into the cooling coil 18, when the dedicated valve was in an (fully or partially) open position. Each cooling coil 18 also had a dedicated outlet through which CO2 vapour 30 could be passed back to the CO2 compressor(s) 14 in the CO2 refrigeration unit 12.

A further part of the retrofit involved a heating coil 42 being installed in each of the air handling units 20. The heating coil 42 was a fin coil and was suitable to comprise CO2 at a maximum pressure of about 120 bar. An inlet of each heating coil 42, 42a was connected to a two-way ball valve 45, 45a. An outlet of each of the two-way ball valves 45, 45a was then connected to a common motorised three-way ball valve 40. The three-way valve 40 was located and configured to direct CO2 vapour 38 from the CO2 compressor(s) 14 to the heating coils 42, 42a when the heating coils 42, 42a were in-use. An outlet of each heating coil 42, 42a was connected to another set of two-way ball valves 47, 47a. The outlet 46, 46a of each two-way ball valve 47, 47a was combined and connected to another motorised three-way ball valve 58 such that the three-way valve 58 only had one inlet 56 which comprised cooled CO2 vapour 46, 46a, etc. from each of the heating coils 42, 42a, etc. A 4-20Ma PT 1000 temperature sensor was installed before the three-way valve 58 to enable the temperature of the combined cooled CO2 vapour to be measured. The three-way valve 58 was located and configured to selectively pass some or all of the combined cooled CO2 vapour to the CO2 condenser/gas cooler 60, in which the CO2 vapour was further cooled. The remaining combined cooled CO2 vapour 70 that was not selectively passed to the CO2 condenser/gas cooler 60 and the further cooled CO2 68 was able to be combined and passed to the CO2 refrigeration unit 12. Another 4-20Ma PT1000 temperature sensor was installed to measure the temperature of the CO2 being passed to the CO2 refrigeration unit.

The inventors noted that because the system being retrofitted had multiple air handling units, the two-way ball valves 45, 45a, 47, 47a were required, but if the retrofit was being performed on a system with only one air handling unit, the two-way ball valves 45, 45a, 47, 47a need not be installed. However, if there were plans to expand the system to more than one air handling unit, then the two-way ball valves 45, 45a, 47, 47a can be installed during the retrofit. The inventors noted that, since the two-way ball valves were required in systems with more than one air handling unit, installation during the retrofit can save costs in the future during expansion of the system.

Installing both the cooling coil 18 and the heating coil 42 in each air handling unit 20 with the configuration of inlet pipes and outlet pipes described enabled the HVAC system 10 to operate in a number of different modes. For example, by opening the Electronic Expansion Valve 22 and positioning three- way valve 40 such that CO2 vapour bypassed the heating coil 42, air passing through the air handling unit 20 was cooled. Alternatively, by opening Electronic Expansion Valve 80 and allowing CO2 vapour 44 to pass to the heating coil 42, air passing through the air handling unit 20 was heated. By both opening Electronic Expansion Valve 22 and positioning three-way valve 40 such that CO2 vapour 44 passed to the heating coil 42, air passing through the air handling unit 20 was dehumidified. The previous air handling units comprised only a singular coil and could therefore not operate in a dehumidification mode.

A further part of the retrofit involved pipes and valves being installed, i.e. as shown in Figs. 1 and 2, which were configured to comprise CO2 at a pressure above atmospheric.

A further part of the retrofit involved an additional cooling coil 76 being installed adjacent to the CO2 condenser/gas cooler 60. The additional cooling coil 76 was a fin coil and was suitable to heat CO2 at a pressure up to a maximum of about 80 bar. An inlet of the additional cooling coil 76 was connected to the CO2 refrigeration unit 12 via an Electronic Expansion Valve 80. The valve 80 allowed liquid CO2 to pass from the CO2 refrigeration unit 12 to the additional cooling coil 76 when the valve 80 was in an open position. An outlet of the additional cooling coil 76 was connected to the CO2 compressor(s), to allow CO2 vapour 88 to be passed from the additional cooling coil 76 to the CO2 compressor(s).

During the retrofit, the air handling units were closely inspected to determine their condition. Specifically, the filters were inspected to see if they were clogged and the supply air fan/fresh intake louvres were inspected for signs of rust, wear and tear, etc. It was determined that the air handling units were in good condition and were still usable. Thus, the air handling units were not replaced. However, the inventors noted that if the conditions of some or all of the air handling units were not satisfactory, the air handling units which were not in satisfactory condition could be replaced with a new unit during the retrofit. The inventors also noted that some existing HVAC systems will comprise additional components such as electric heater elements. Typically, electric heater elements are present in refrigerant-based HVAC systems that do not have a boiler and offer heating and/or dehumidification modes. When present, these electric heater elements were also removed during step 1 of the retrofit. This was because the CO2 HVAC system of the present application comprised a heating coil wherein compressed CO2 vapour was used to heat the surrounding air.

Similarly, some existing HVAC systems may comprise a boiler. Typically, a boiler is present in refrigerant-based HVAC systems that do not have an electric heater and offer heating and/or dehumidification modes. When present, these boilers were also removed during step 1 of the retrofit. This was because the CO2 HVAC system of the present application comprised a heating coil wherein compressed CO2 vapour is used to heat the surrounding air.

The inventors noted that, although the pre-existing HVAC system used R22 as a refrigerant, the method of retrofitting was able to be performed on other pre-existing HVAC systems which used other synthetic refrigerants, e.g. such as a synthetic drop in HCFC replacement or HFOs.

A further part of the retrofit involved configuring a Modbus communications protocol to allow data to be transmitted from temperature probes within each air-conditioned space 28, 28a to a PLC installed in the CO2 refrigeration unit 12. The inventors noted that a Modbus was suitable for this retrofit because the building was modem and had a BMS. The BMS allowed target temperatures to be manually adjusted for each air-conditioned space 28, 28a. However, for retrofits performed in older buildings, the existing thermostats with hard-wired set points could be used.

Example 2 - Cooling Mode (Fig, 3)

The PLC of HVAC system 10 detected that the temperature in the air- conditioned space 28 was above the target temperature and placed air handling unit 20 in a cooling mode. Fig. 3 highlights the pipes, valves and components as thick lines of the HVAC system 10 that were always used when the HVAC system 10 was in the cooling mode. Fig. 3 also highlights the pipes, valves and components as dotted lines that were optionally used when the HVAC system 10 was in the cooling mode.

When the cooling mode was selected, the pressure-reducing valve 22 was opened. As the pressure-reducing valve 22 was opened, liquid CO2 16 was able to pass from the CO2 refrigeration unit 12 to the cooling coil 18, via the pressurereducing valve 22. The CO2 refrigeration unit 12 provided liquid CO2 16 at a maximum pressure of between about 45 bar to 60 bar. The pressure of the liquid CO2 16 from the CO2 refrigeration unit 12 was observed to always be around 50 bar, i.e. because the pressure regulating valve 12-5 maintained the pressure in the receiver 12-1 to about 50 bar. The liquid CO2 16 was observed to be a saturated liquid.

As the (saturated) liquid CO2 16 passed through the pressure-reducing valve 22, the pressure of the liquid CO2 was decreased - i.e. the pressure of the CO2 after the pressure-reducing valve 22 was lower. The pressure of the CO2 after the pressure-reducing valve 22 was observed to be about 40 bar. It was noted that this pressure value was not dependent on whether the compressor was operating in a subcritical cycle or a transcritical cycle.

As the pressure of the liquid CO2 16 decreased, a portion of the liquid CO2 evaporated (boiled) because the liquid CO2 was a saturated liquid. Thus, the CO2 after the pressure-reducing valve 22 formed a two-phase (liquid and vapour) CO2 stream 24. It was observed, however, that most of the CO2 was still in the (saturated) liquid phase.

The two-phase CO2 stream 24 then passed into the inside of the cooling coil 18, which operated at about 40 bar, i.e. the same pressure as the CO2 after the pressure-reducing valve. As the two-phase CO2 24 passed through the cooling coil 18, air 26 was caused by fan 52 to pass over the outside of the cooling coil 18. The air 26 was noted to be a mixture of air from the air-conditioned space 28 and fresh ambient air 27.

As the air 26 was passed over the cooling coil 18, energy was transferred from the air 26 passing over the cooling coil 18 to the two-phase CO2 stream 24 flowing within the cooling coil 18. The liquid-phase CO2 of the two-phase CO2 stream 24 was still a saturated liquid. Thus, the energy transfer from the air 26 to the two-phase CO2 caused the remaining (saturated) liquid CO2 to evaporate in the cooling coil 18, at the operating pressure thereof. This resulted in a CO2 vapour 30 passing from the exit of the cooling coil 18. At the same time, the air 26 leaving the cooling coil 18 was cooled to form cooled air stream 32.

The temperature sensor 34 placed after the cooling coil 18 measured the temperature of the cooled air 32 leaving the cooling coil 18. The cooled air 32 leaving the cooling coil 18 passed over the (not in-use) heating coil 42 and was direct into the air-conditioned space 28 by fans 52. In this regard, three-way valve 40 was configured to not pass the compressed CO2 vapour 38 to the heating coil 42 of the air handling unit 20.

The selected pressure-reducing valve 22 was an electronic expansion valve metering device that regulated the flow of liquid CO2 16 into the cooling coil 18. To ensure that all liquid CO2 evaporated in the cooling coil 18, the pressurereducing valve 22 was fitted with sensors to monitor the superheat of the CO2 passing through the pressure-reducing valve 22 (i.e. a measurement of a temperature reading minus a saturated pressure reading). When the superheat was above a predetermined value, typically 6K, the flow of liquid CO2 16 was increased by further opening the pressure-reducing valve 22. Conversely, when the superheat was below a predetermined value, the flow of liquid CO2 16 was decreased by partially closing the pressure-reducing valve 22. Such control ensured that all the CO2 exiting the cooling coil 18 was in the form of a vapour. Here, it was noted that the presence of liquid CO2 could detrimentally affect the CO2 compressor(s) 14. The CO2 vapour 30 was passed from the cooling coil 18 back to CO2 compressor(s) 14 in the CO2 refrigeration unit 12. The inventors noted that the exact number of CO2 compressor(s) 14 in the CO2 refrigeration unit 12 was able to be varied, depending on the application. For example, where the HVAC system 10 only needed to provide cooling to a small air-conditioned space 28, a single CO2 compressor 14 was able to be used. However, where the HVAC system 10 needed to provide cooling to a larger air-conditioned space 28, multiple CO2 compressors 14 were typically used.

In the CO2 compressor(s) 14, the CO2 vapour 30 was compressed therein. The pressure and phase of the compressed CO2 38 passing from the compressor 14 was observed to be dependent on whether the compressor was operating in a subcritical cycle or a transcritical cycle. For example, in the subcritical cycle, the compressed CO2 38 formed a superheated vapour at a pressure of about 65 bar. In the transcritical cycle, the compressed CO2 38 formed a supercritical fluid at a pressure of about 84 bar. However, the maximum upper pressure limit of compressed CO2 vapour was always controlled to be 120 bar, because this was the maximum operating pressure of the CO2 compressor 14.

A pressure sensor (12-2 in Fig. 8) measured the pressure of CO2 at an inlet 30 of the one or more CO2 compressors 14. The pressure sensor had a set pressure of about 40 bar, equivalent to a CO2 gas temperature of ~6°C. When the pressure of the CO2 at the inlet to the CO2 compressor(s) was above 40 bar, the flow rate of liquid CO2 16(i.e. the flow of liquid CO2 from the receiver 12-1 of the CO2 refrigeration unit 12 ) was increased. This corresponded to an increase in the amount of refrigerant recirculating in the system 10. Conversely, when the pressure of the CO2 at the inlet to the CO2 compressor(s) was below 40 bar, the flow rate of CO2 was decreased.

It was observed that an increase in pressure at the inlet to the CO2 compressor(s) was typically correlated with the pressure-reducing valve 22 opening more. This was because, as the superheat of the CO2 passing through the pressure-reducing valve 22 increased, the pressure-reducing valve 22 was further opened to increase the flow of liquid CO2 16 passing through the pressurereducing valve 22. This resulted in a higher amount of refrigerant boiling and therefore an increased pressure in the system 10. The pressure sensor at the inlet of the CO2 compressor(s) 14 sensed this increased pressure and acted to increase the mass flow rate of recirculating CO2 by increasing the number of operating compressors (i.e. switch-on more compressors) and/or by increasing the frequency of the compressors.

It was noted that, when the system 10 comprised one compressor, the flow of liquid CO2 16 to the cooling coil 18 was increased by increasing a motor speed of this one compressor whereas the flow of liquid CO2 16 was decreased by decreasing a motor speed of the one compressor. This was accomplished by fitting the compressor with a Variable Speed Drive (VSD) that could increase or decrease the speed of the motor by adjusting the frequency of the electrical power supply to the compressor.

Alternatively, when the system 10 comprised multiple compressors, the flow of liquid CO2 16 was increased by increasing a motor speed of the compressors and/or increasing the number of operating compressors, and the flow of liquid CO2 16 was decreased by decreasing a motor speed of the compressors and/or by decreasing the number of operating compressors.

Typically, the system 10 comprised up to fourteen compressors with at least one fitted with a VSD that could increase or decrease the speed of the motor by adjusting the frequency of the electrical power supply to the compressor. When the flow of liquid CO2 16 had to be increased, the first compressor fitted with a VSD would act to increase the frequency of the compressor. Once the maximum frequency was reached, then a second compressor was started. If fitted with a VSD, and once the second compressor was operating at the maximum frequency, the third compressor was started and so on. In the cooling cycle of Fig. 3, the heating coil 42 was not used and thus the three-way valve 40 was configured to pass compressed CO2 vapour 38 directly to the second three-way valve 58 located prior to the CO2 condenser/gas cooler 60. The second three-way valve 58 was configured to selectively pass the CO2 vapour to the CO2 condenser/gas cooler 60. The CO2 condenser/gas cooler 60 was operated to further cool the CO2 vapour 62 that was selectively passed to the CO2 condenser/gas cooler 60 by the second three-way valve 58. The CO2 condenser/gas cooler 60 was operated to further cool the CO2 vapour 62 at a constant pressure - i.e. the cooling was isobaric. The inventors noted that when the system 10 was operating in a subcritical cycle, the second three-way valve 58 was configured to selectively pass all the compressed CO2 vapour 56 to the CO2 condenser/gas cooler 60. However, when the system 10 was operating in a transcritical cycle, the second three-way valve 58 was configured such that some of the compressed CO2 vapour 56 bypassed the CO2 condenser/gas cooler 60.

That portion of the CO2 vapour that was selectively passed to the gas cooler was passed into the tubes of the CO2 condenser/gas cooler 60. Ambient air 64 surrounding the CO2 condenser/gas cooler 60 was forced to flow through the gas cooler 60 by means of fans 66 that were located adjacent to the CO2 condenser/gas cooler 60, the air flowing around the tubes. As the ambient air 64 was forced to flow around the outside of the tubes of the CO2 condenser/gas cooler 60, energy was transferred from the CO2 vapour within the tubes to the ambient air 64. This energy transfer caused the CO2 vapour to be further cooled, thereby producing further cooled CO2 vapour 68, concurrently causing the ambient air to be heated.

It was observed that when the system 10 was in a subcritical cycle, the incoming CO2 vapour 62 to the CO2 condenser/gas cooler 60 was a superheated CO2 vapour. In the CO2 condenser/gas cooler 60, the energy transfer between the superheated CO2 vapour and the ambient air 64 caused the superheated CO2 vapour to condense into liquid CO2. The liquid CO2 was either saturated or subcooled. The pressure of the liquid CO2 was observed to be about 65 bar. On the other hand, it was observed that when the system 10 was in a transcritical cycle, the incoming CO2 vapour 62 to the CO2 condenser/gas cooler 60 was a supercritical CO2 fluid. In the CO2 condenser/gas cooler 60, the energy transfer between the supercritical CO2 fluid and the ambient air 64 caused the temperature of the supercritical CO2 fluid to decrease, however the CO2 remained in the supercritical state. The pressure of the supercritical CO2 was observed to be about 84 bar.

The inventors noted that, when the system 10 was in cooling mode, the speed of fans 66 was increased or decreased, depending on the pressure of the compressed CO2 vapour 38 exiting the compressor(s) 14, as measured by the pressure sensor 12-6 (see Fig. 8). The pressure sensor 12-6 was electrically connected to the PLC. The pressure sensor 12-6 measured the pressure of the compressed CO2 vapour 38 and signalled this to the PLC. The PLC compared the measured outlet pressure to a target outlet pressure. When the measured outlet pressure was higher than the target outlet pressure, the PLC signalled to the fans 66 to increase in speed. Conversely, when the measured outlet pressure was lower than the target outlet pressure, the PLC signalled to the fans 66 to decrease in speed.

The PLC used a look-up table hard-coded in to determine the target outlet pressure, based on maintaining a fixed temperature difference between the compressed CO2 vapour 38 entering the gas cooler 60 and ambient air. The inventors noted that a typical look-up table uses a fixed temperature difference of about 5°C to determine what the target outlet pressure should be, until the critical temperature of CO2 (i.e. ~31 °C). Above the critical point of CO2, a fixed target outlet pressure is used.

The further cooled CO2 vapour 68 from the CO2 condenser/gas cooler 60 was combined with the remaining CO2 vapour 70 that was not selectively passed by the three-way valve 58 into the CO2 condenser/gas cooler 60, forming a combined cooled CO2 stream 72. As the cooled CO2 68 and the remaining CO2 vapour 70 were mixed, energy was transferred from the remaining CO2 vapour 70 (which had a higher enthalpy) to the cooled CO2 68, and the enthalpy of the two streams equilibrated. The combined cooled CO2 stream 72 was then passed to an inlet of the high-pressure electronic expansion valve 75.

As the combined cooled CO2 stream 72 passed to the inlet of the high- pressure electronic expansion valve 75, the temperature sensor 74 located adjacent to the inlet of the high-pressure electronic expansion valve 75 measured the temperature of the combined cooled CO2 stream 72. When the system 10 was operating in a transcritical cycle, the temperature measured by temperature sensor 74 was used to adjust the portion 62 of the combined cool CO2 vapour that was selectively passed to the CO2 condenser/gas cooler 60. Specifically, when the measured temperature of the combined cooled CO2 stream 72 was above a predetermined inlet temperature, the temperature sensor 74 signalled the PLC situated inside the CO2 refrigeration unit 12. The PLC was electrically connected to the three-way valve 58 (and other components) and sent a signal to the three- way valve 58, causing the three-way valve 58 to open further. As the three-way valve 58 opened further, a greater portion of the CO2 vapour 62 was observed to selectively pass to the CO2 condenser/gas cooler 60. This was observed to cause the temperature of the combined cooled CO2 stream 72 to decrease.

Conversely, when the PLC received a signal from the temperature sensor 74 that the temperature at the high-pressure electronic expansion valve inlet was below the predetermined inlet temperature, the three-way valve 58 was caused to partially close, reducing the portion of the CO2 vapour 62 selectively passed to the CO2 gas cooler/condenser 60. This was observed to increase the temperature of the combined cooled CO2 stream 72.

The portion of the CO2 vapour 62 selectively passed to the CO2 condenser/gas cooler 60 was increased by changing the position of the three-way valve 58 such that a greater proportion of the incoming CO2 vapour was directed to the CO2 condenser/gas cooler 60.

As the combined cooled CO2 stream 72 passed to the inlet of the high- pressure electronic expansion valve 75, the pressure of the stream 72 was reduced to about 50 bar. As the pressure of the stream 72 was reduced, this caused most of the CO2 vapour to condense, forming a two-phase CO2 stream 77. The inventors noted that the high-pressure electronic expansion valve 75 operated adiabatically. The two-phase CO2 stream 77 was passed to an inlet of the CO2 refrigeration unit 12.

The predetermined inlet temperature was between about 15°C to 20°C. The inventors noted that it was advantageous to maintain an inlet temperature in this range because, when the temperature at the inlet was higher, more vapour CO2 was present in the two-phase CO2 stream 77 which caused the compressors to have to do more work during compression. This made the system less efficient.

Because of the arrangement of pipes and valves inside the CO2 refrigeration unit 12, the liquid component of the stream 74 formed at least a part of the (liquid) CO2 16 stream which passed from the CO2 refrigeration unit 12 to the cooling coil 18, when the pressure-reducing valve 22 was opened. That is, the CO2 in the HVAC system 10 flowed in a continuous cycle between the cooling coil 18, the CO2 compressor(s) 14 and the CO2 gas cooler/condenser 16. Furthermore, the vapour component of the stream 74 formed at least a part of the (vapour) CO2 which passed to the compressor(s) 14 in the CO2 refrigeration unit 12 to form the compressed CO2 vapour 38.

It was observed that the system 10 was able to change between operating in a subcritical cycle to operating in a transcritical cycle depending on the ambient conditions at the time. The ambient conditions (temperature and humidity) were measured by the temperature sensor 78 configured to measure ambient air outside the gas cooler 60. When the measured ambient air temperature was above a predetermined transcritical temperature, the temperature sensor 78 signalled the PLC, situated inside the CO2 refrigeration unit 12. The PLC was electrically connected to the compressor 14 (and other components) and signalled to the compressor 14 that a transcritical CO2 cycle was required. The predetermined transcritical temperature (as measured by the temperature sensor 78) was set to ~31 °C.

Conversely, when the measured ambient air temperature was below the predetermined transcritical temperature of ~31 °C, the temperature sensor 78 signalled the PLC to signal to the compressor 14 that a subcritical CO2 cycle was required.

During the cooling mode, a temperature sensor 36 that was located in the air-conditioned space 28 measured the temperature of air in the air-conditioned space 28. The temperature sensor 36 was a 4-20Ma PT 1000 temperature sensor. The temperature sensor 36 compared the measured temperature of the air- conditioned space 28 to the target temperature. When the measured temperature was the same as the target temperature, the pressure-reducing valve 22 was closed and cooling was no longer provided to the air-conditioned space 28.

In the system of Fig. 2, when air handling unit 20 was in a cooling mode, air handling unit 20a (see Fig. 2) was also placed in a cooling mode by selecting the appropriate setting on an electronic panel connected to the programmable logic control (PLC) system. The electronic panel allowed the user to select different settings.

When the cooling mode was selected, the pressure-reducing valve 22a was opened. As the pressure-reducing valve 22a was opened, liquid CO2 16a was able to pass from the CO2 refrigeration unit 12a to the cooling coil 18a, via the pressure-reducing valve 22a. It was noted that air handling unit 20a operated in the same way as previously described for air handling unit 20.

The inventors noted that many more such parallel air handling units 20, 20a etc. could be placed in a cooling mode at the same time, e.g. to increase the cooling capacity of the system or when cooling was required in multiple locations. Example 3 - Heating Mode (Fig, 4)

The PLC in the system 10 detected that the measured temperature in the air-conditioned space 28 was below the set temperature and placed air handling unit 20 in a heating mode. Fig. 4 highlights in thick lines the pipes, valves and components of the HVAC system 10 that were always used when the HVAC system 10 was in the heating mode. Fig. 4 also highlights the pipes, valves and components as dotted lines that were optionally used when the HVAC system 10 was in the heating mode. In the heating mode, the CO2 HVAC system 10 was deliberately caused to operate in a transcritical cycle. That is, when the HVAC system 10 was in the heating mode, it operated using a transcritical cycle, regardless of ambient conditions.

The inventors noted that a transcritical cycle was advantageous because it provided more efficient heat exchange. Additionally, because the CO2 refrigeration unit was located above the heating coil, by operating in a transcritical cycle, the chance of CO2 vapour condensing into liquid in the heating coil was reduced. Any condensed CO2 liquid would be trapped within the heating coil, i.e. because the liquid CO2 cannot flow back up to the CO2 refrigeration unit. On the other hand, when the CO2 leaving the heating coil is a supercritical fluid, it can travel upwards back to the CO2 refrigeration unit.

Referring now to Fig. 4, when the system 10 (with only one air handling unit 20) was in heating mode, this caused the pressure-reducing valve 80 located between the CO2 refrigeration unit 12 and a cooling coil 76 to open. As the pressure-reducing valve 80 was opened, fluid CO2 82 was able to pass from the receiver 12-1 in the CO2 refrigeration unit 12 to the cooling coil 76, via the pressure-reducing valve 80. The CO2 refrigeration unit 12 provided liquid CO2 82 at a pressure of about 50 bar. The inventors noted that the liquid CO2 82 was a saturated liquid.

As the (saturated) liquid CO2 82 passed through the pressure-reducing valve 80, the pressure of the liquid CO2 was decreased - i.e. the pressure of the CO2 after the pressure-reducing valve 84 was lower than the fluid CO2 82 passing from the CO2 refrigeration unit 12. The pressure of the fluid CO2 84 after the pressure-reducing valve 80 was observed to be about 35 bar.

As the pressure of the liquid CO2 82 was decreased a portion of the liquid CO2 evaporated (boiled), because the liquid CO2 82 was a saturated liquid. Thus, the CO2 after the pressure-reducing valve 80 formed a two-phase (liquid and vapour) CO2 stream 84. It was observed, however, that most of the CO2 was still in the (saturated) liquid phase.

The two-phase CO2 84 then passed into the inside of the cooling coil 76, which operated at a pressure of about 35 bar, i.e. the same pressure as the pressure of the liquid CO2 84 passing from the pressure-reducing valve. As the two-phase CO2 84 was passed through the cooling coil 76, air 90 was caused to pass over the outside of the cooling coil 76. The air 90 was directed over the outside of the cooling coil 76 via the use of fans 66. As the air 90 was passed over the cooling coil 76, energy was transferred from the air 90 passing over the cooling coil 76 to the two-phase CO2 84 within the cooling coil 76. The energy transfer from the air 90 to the two-phase CO2 caused the remaining saturated liquid CO2 to evaporate in the cooling coil 76, at the operating pressure of the cooling coil 76. This resulted in a CO2 vapour 88 exiting the cooling coil 76. At the same time, the air leaving the cooling coil was cooled air 86.

The selected pressure-reducing valve 80 was an electronic expansion valve metering device that regulated the flow of liquid CO2 82 into the cooling coil 76. To ensure that all liquid CO2 evaporated in the cooling coil 18, the pressurereducing valve 80 was fitted with sensors to monitor the superheat of the CO2 passing through the pressure-reducing valve 80. When the superheat was above a predetermined value, typically about 6K, the flow of liquid CO2 82 was increased by further opening the pressure-reducing valve 80. Conversely, when the superheat was below the predetermined value of about 6K, the flow of liquid CO2 16 was decreased by partially closing the pressure-reducing valve 80. This ensured that all the CO2 exiting the cooling coil 76 was in the form of a vapour. As above, the presence of liquid was noted to detrimentally affect the CO2 compressor(s) 14.

The CO2 vapour 88 was passed back to the CO2 compressor(s) 14 of the CO2 refrigeration unit 12. As the CO2 vapour 88 was passed back to the compressor(s) 14, the pressure of the vapour stream was measured by a pressure sensor 12-2. The PLC was electrically connected to the pressure sensor 12-2 and the fans 66 (as well as other components of the system 10). The pressure sensor 12-2 signalled to the PLC when the measured pressure was higher than a target inlet pressure. When the measured outlet pressure was higher than the target outlet pressure, the PLC signalled to the fans 66 to increase in speed. Conversely, when the measured outlet pressure was below the target outlet pressure, the PLC signalled to the fans 66 to decrease in speed.

The CO2 vapour 88 was compressed in the CO2 compressor(s) 14, thereby forming the compressed CO2 vapour stream 38. The compressed CO2 vapour 38 was at a pressure of about 84 bar. Because the system 10 was operating in a transcritical CO2 cycle, the compressed CO2 vapour 38 was a supercritical fluid.

Again, the inventors noted that the exact number of CO2 compressor(s) 14 in the CO2 refrigeration unit 12 was able to be varied, depending on the application. For example, where the HVAC system 10 only needed to provide heating to a small air-conditioned space 28, a single CO2 compressor 14 was able to be used. However, where the HVAC system 10 needed to provide heating to a larger air-conditioned space 28, multiple CO2 compressors 14 were typically used.

The compressed (supercritical) CO2 vapour 38 was then passed to the three-way valve 40, located at the exit of the CO2 refrigeration unit 12. Because the system 10 comprised only one air handling unit, this valve was configured to selectively pass compressed supercritical CO2 vapour 38 to the inside of the heating coil 42, when the system 10 was in the heating mode. As the compressed supercritical CO2 vapour 44 passed through the inside of the heating coil 42, air 32 was simultaneously caused by fan 52 to pass over the outside of the heating coil 42. Typically, the air 32 was a mixture of air from the air-conditioned space 28 and fresh ambient air 27.

Energy was transferred from the compressed supercritical CO2 vapour 44 inside the heating coil 42 to the air 32 that was passed over the outside of the heating coil 42. This energy transfer caused the compressed supercritical CO2 vapour 44 to cool and the air 32 to be simultaneously heated, thereby producing a cooled CO2 stream 46 and heated air 48. The heated air 48 was blown by fans 52 from the air handling unit 20 back into the air-conditioned space 28. It was observed that the cooled CO2 46 was still in a supercritical state. However, the enthalpy of the cooled supercritical CO2 46 was lower than the enthalpy of the compressed supercritical CO2 44, i.e. because energy had been transferred from the compressed supercritical CO2 44 to the air 32 in the heating coil 42.

The temperature sensor 50 measured the temperature of the heated air 48 leaving the heating coil 42. The temperature sensor 50 was electrically connected to the PLC. The temperature sensor 50 sent a signal to the PLC when the measured temperature of the heated air 48 was above a predetermined heated air temperature. When the PLC received the signal that the temperature of the heated air 48 was above the predetermined heated air temperature, it sent a signal to the three-way valve 40 to cause the three-way valve 40 to decrease the portion of compressed CO2 vapour 38 that was selectively passed to the heating coil 42.

Conversely, the temperature sensor 50 sent a signal to the PLC when the measured temperature of the heated air 48 was below a predetermined heated air temperature. When the PLC received the signal that the temperature of the heated air 48 was below the predetermined heated air temperature, it sent a signal to the three-way valve 40 to cause the three-way valve 40 to increase the portion of compressed CO2 vapour 38 that was selectively passed to the heating coil 42.

The inventors noted that when the system 10 had more than one air handling unit 20 (i.e. such as in Fig. 2), the three-way valve 40 was always configured to pass all the compressed CO2 vapour 38 to the heating coils 42, 42a. In this case, the PLC was configured to close ball valves 45, 45a and 47, 47a located at the inlet and outlet respectively of the heating coils 42, 42a when the measured temperature of the heated air 48 was above the predetermined heated air temperature. Conversely, the PLC was configured to open ball valves 45, 45a and 47, 47a when the measured temperature of the heated air 48 was below the predetermined heated air temperature.

The temperature of the cooled supercritical CO2 46 from the heating coil 42 was measured by temperature sensor 51. When the measured temperature of the cooled supercritical CO2 46 was above a predetermined cooled supercritical CO2 temperature, the temperature sensor 51 signalled to the PLC situated inside the CO2 refrigeration unit 12. The PLC was electrically connected to the one or more CO2 compressors 14 (and other components). The PLC sent a signal to the one or more CO2 compressors 14 to increase the flow of CO2 vapour 38 from the refrigeration unit 12 to the three-way valve 40.

Conversely, when the measured temperature of the cooled supercritical CO2 46 was below a predetermined temperature, the temperature sensor 51 signalled to the PLC. The PLC sent a signal to the one or more CO2 compressors 14 to decrease the flow of CO2 vapour 38 from the refrigeration unit 12.

The inventors noted that the control for the staging of the compressor(s) in heating mode was thus different to the control for the staging of the compressor(s) in cooling mode.

As discussed above, the inventors also noted that by using the temperature of the cooled supercritical CO2 46 to control the staging of the compressor(s), the efficiency of the system 10 was increased.

As above, it was noted that when the HVAC system 10 comprised a single compressor, the flow of compressed CO2 vapour 38 was increased (or decreased) by increasing (or decreasing) a motor speed of this one compressor, i.e. by fitting the compressor with a VSD. Alternatively, when the HVAC system 10 comprised multiple compressors, the flow of compressed supercritical CO2 vapour 38 was increased (or decreased) by increasing (or decreasing) a motor speed of the compressors and/or increasing (or decreasing) the number of operating compressors.

As above, typically, the HVAC system 10 comprised up to fourteen compressors with at least one fitted with a VSD, i.e. allowing the speed of the motor to be increased or decreased. When the flow of CO2 vapour 38 had to be increased, the first compressor fitted with a VSD would act to increase the frequency of the compressor. Once the maximum frequency was reached, then a second (optionally VSD-fitted) compressor was started. Once the second compressor was operating at the maximum frequency, the third compressor was started and so on.

The cooled supercritical CO2 46 exiting the heating coil 42 was combined with the remaining compressed supercritical CO2 vapour 54 that was not selectively passed by the three-way valve 40 to the heating coil 42 (when present). It was observed that, during heating only mode, there was typically no remaining compressed supercritical CO2 vapour 54 because all the compressed supercritical CO2 vapour 38 was passed to the heating coil 42. This increased the efficiency of the HVAC system 10 by minimising the need to divert the combined CO2 fluid 56 to the CO2 gas cooler/condenser for further cooling.

The inventors noted that when the HVAC system 10 comprised multiple air handling units 20, 20a etc. (i.e. see Fig. 2) that were all in heating mode, the cooled supercritical CO2 46, 46a, etc. from each air handling unit 20, 20a, etc. was combined to form a combined cooled supercritical CO2 stream 47. The temperature of the combined cooled supercritical CO2 vapour 47 was then measured using temperature sensor 51. The flow of fluid CO2 from the CO2 refrigeration unit 12 was adjusted based on the temperature of the combined cooled supercritical CO2 vapour 47, as described. Because all the compressed CO2 vapour 38 was passed to the heating coils 42, 42a, there was no remaining compressed supercritical CO2 vapour 54. The combined CO2 fluid 56 was passed to the second three-way valve 58. The combined CO2 fluid 56 was a supercritical fluid.

The second three-way valve 58 was configured to selectively pass the combined supercritical CO2 56 to the gas cooler 60, if required. The gas cooler 60 was operated to further cool the combined supercritical CO2 56 selectively passed thereto as required. The cooling in the gas cooler 60 was performed at a constant pressure of about 84 bar, i.e. the pressure of the compressed CO2. However, it was observed that when no further cooling of the combined supercritical CO2 56 was required, the second three-way valve 58 passed all the combined supercritical CO2 56 directly to the CO2 refrigeration unit 12, i.e. the gas cooler 60 was by-passed entirely. Thus, in the heating mode, the use of the gas cooler 60 was optional - as indicated by the dotted lines in Fig. 4.

The portion of the combined CO2 fluid was selectively passed into the inside of the tubes of the gas cooler 60. Ambient air 64 was caused to flow over the gas cooler 90 by means of the fans 66 located adjacent to the gas cooler 60. As the ambient air 64 was forced to flow over the gas cooler 60, energy was transferred from the CO2 fluid 62 within the tubes of the gas cooler 60 to the ambient air 64. This energy transfer caused the CO2 fluid 62 to be further cooled (whilst remaining as a supercritical fluid) and the ambient air 64 to be heated.

In the heating mode of the system 10, a suction pressure of the CO2 refrigeration unit 12 was measured. When the suction pressure of the CO2 refrigeration unit 12 was above a predetermined suction pressure, the speed of the fans 66 was increased. When the suction pressure of the CO2 refrigeration unit 12 was below the predetermined suction pressure, the speed of the fans 66 was decreased. Such increase and decrease of fans 66 also affected the flow of ambient air 90 over the adjacent cooling coil 76.

The further cooled supercritical CO2 68 from the in-use gas cooler 60 was combined with the remaining supercritical CO2 70 that was not selectively passed by the three-way valve 58 to the gas cooler 60 to forma combined cooled supercritical CO2 stream 72. As the further cooled supercritical CO2 fluid 68 and the remaining cooled supercritical CO2 70 mixed, energy was transferred from the remaining cooled supercritical CO2 70 (with a higher enthalpy) to the further cooled supercritical CO2 68. The enthalpy of the two streams equilibrated and the combined further cooled supercritical CO2 fluid 72 was thereby produced. The combined further cooled supercritical CO2 fluid 72 was passed to the inlet of the high-pressure expansion valve 75.

When the gas cooler 60 was not in-use, the combined supercritical CO2 56 by-passed the gas cooler 60 and was directly passed to the inlet of the CO2 refrigeration unit 12. That is, none of the combined supercritical CO2 56 was selectively passed by the three-way valve 58 to the gas cooler 60.

As above, the temperature sensor 74 measured the temperature of the combined supercritical CO2 56 as it passed to the inlet of the high-pressure expansion valve 75. The temperature sensor 74 signalled to the PLC when the measured temperature was above (or below) a predetermined inlet temperature. The PLC then sent a signal to the three-way valve 58, causing the three-way valve to open (or close) further. This caused a greater (or lesser) portion of the combined supercritical CO2 56 to selectively pass to the gas cooler 60, thereby causing the inlet temperature to decrease (or increase) respectively. As above, the predetermined inlet temperature was between about 15°C to 20°C, i.e. to increase the efficiency of the system.

It was observed that, typically, none of the flow 62 of combined supercritical CO2 56 was selectively passed into the gas cooler 60. This was as a result of the predetermined temperature of temperature sensor 51 being selected such that the cooled supercritical CO2 vapour 46 was at a suitable temperature to be returned directly to the CO2 refrigeration unit 12, i.e. thereby bypassing the gas cooler 60. This configuration was noted to be advantageous because any energy transferred from the supercritical CO2 62 to the ambient air 64 within the gas cooler 60 represented an energy loss from the system 10.

However, it was observed that, sometimes due to fluctuations in the temperature of the combined supercritical CO2 56, a portion of stream 56 still needed to be selectively passed into the gas cooler 60 to achieve the predetermined temperature at the inlet to the CO2 refrigeration unit 12.

As above, as the combined cooled CO2 stream 72 passed to the inlet of the high-pressure electronic expansion valve 75, the pressure of the stream 72 was reduced to about 50 bar causing most of the CO2 vapour to condense. The two- phase CO2 stream 77 was passed to an inlet of the CO2 refrigeration unit 12 and to the receiver 12-1. The liquid component of the two-phase CO2 77 entering the receiver 12-1 then formed the (saturated) liquid CO2 82 which passed from the CO2 refrigeration unit 12 to the cooling coil 76, when the pressure-reducing valve 80 was opened.

In the heating mode of the system 10, a temperature sensor 36 was located in the air-conditioned space 28 and measured the temperature of air in the air- conditioned space 28. The temperature sensor 36 was a 4-20Ma PT 1000 temperature sensor. The temperature sensor 36 compared the measured temperature of the air-conditioned space 28 to a target temperature. When the measured temperature was the same as the target temperature, the three-way valve 40 was positioned to stop flow of CO2 vapour to the heating coil 42 and heating was no longer provided to the air-conditioned space 28. Alternatively, when the system 10 comprised multiple air handling units, the flow of CO2 vapour to each heating coil 42, 42a was stopped by closing the two-way valves 45, 47 and 45a, 47a located at the inlet and outlet respectively of the heating coils 42.

In a system 10 comprising multiple air handing units (see Fig. 2), whilst air handling 20 was in a heating mode, air handling unit 20a was also placed in a heating mode, because the PLC detected that the temperature in the air- conditioned space 28a was below the target temperature.

In the embodiment of Fig. 2, each air handling unit 20, 20a was able to operate in a heating mode by fully opening the three-way valve 40 so as to allow compressed CO2 vapour 38 to pass to the heating coils 42, 42a, and by opening each of the two-way ball valves 45, 45a, 47, 47a. The temperature of the combined cooled CO2 vapour 47 was used to control the flow of CO2 vapour 38 (i.e. by controlling the staging of the compressor(s)). It was noted that air handling unit 20a operated in the same way as previously described for air handling unit 20, when the system 10 comprised multiple air handling units.

The inventors noted that many more such air handling units 20, 20a etc. could be placed in a heating mode at the same time, e.g. to increase the heating capacity of the system or when different spaces needed to be heated.

Example 4 - Dehumidification Mode (Fig, 5)

The PLC in the system 10 detected that dehumidification was required in the air-conditioned space 28 and placed the air handling unit 20 in a dehumidification mode. Fig. 5 highlights the pipes, valves and components of the HVAC system 10 in thick lines that were always used when the HVAC system 10 was set to the dehumidification mode. Fig. 5 also highlights the pipes, valves and components as dotted lines that were optionally used when the HVAC system 10 was set to the dehumidification mode.

The inventors noted that the HVAC system 10 typically used a transcritical cycle when in the dehumidification mode, but that the HVAC system 10 could still provide dehumidification when in a subcritical cycle.

Dehumidification was required when the ambient air was hot and humid or when there was a latent load generated in the air-conditioned space. For example, when the air-conditioned space was a gymnasium or swimming pool or an industrial kitchen (i.e. in which humid air is a common occurrence).

When the dehumidification mode was selected, the pressure-reducing valve 22 was opened. As the pressure-reducing valve 22 was opened, liquid CO2 16 was able to pass from the CO2 refrigeration unit 12 to the cooling coil 18, via the pressure-reducing valve 22. The CO2 refrigeration unit 12 provided saturated liquid CO2 16 at a pressure of about 50 bar. As above for the cooling mode, as the (saturated) liquid CO2 16 passed through the pressure-reducing valve 22, the pressure of the liquid CO2 was decreased and was observed to be about 40 bar. As the pressure of the liquid CO2 16 was decreased, a portion of the liquid CO2 evaporated (boiled) at the set pressure of the valve because the liquid was a saturated liquid. However, most of the CO2 was still in the (saturated) liquid phase.

As above, the two-phase CO2 24 then passed into the inside of the cooling coil 18, which operated at a pressure of about 40 bar. As the two-phase CO2 24 passed through the cooling coil 18, air 26 was caused by fan 52 to pass over the outside of the cooling coil 18. Typically, the air 26 was a mixture of recycled air from the air-conditioned space 28 and fresh ambient air 27. As the air 26 passed over the cooling coil 18, the energy transfer from the air 26 to the two-phase CO2 caused the saturated liquid CO2 to evaporate in the cooling coil 18, at the operating pressure of the cooling coil, resulting in a CO2 vapour 30 being passed from the exit of the cooling coil 18. At the same time, the air leaving the cooling coil 18 was cooled air 32.

The selected pressure-reducing valve 22 was an electronic expansion valve metering device that regulated the flow of liquid CO2 16 into the cooling coil 18. As above, the pressure-reducing valve 22 was fitted with sensors to monitor the superheat of the CO2 passing through the pressure-reducing valve 22. When the superheat was above a predetermined value, typically 6K, the flow of liquid CO2 16 was increased by further opening the pressure-reducing valve 22. Conversely, when the superheat was below a predetermined value, the flow of liquid CO2 16 was decreased by partially closing the pressure-reducing valve 22. Such control ensured that all the CO2 exiting the cooling coil 18 was in the form of a vapour (i.e. because the presence of liquid CO2 could detrimentally affect the CO2 compressor(s) 14).

It was observed that when the air 26 was warm and humid, condensation formed on the cooling coil 18 as the air 26 passed over the cooling coil 18 and was cooled. This was because, as the two-phase CO2 24 passed through the cooling coil 18, it also acted to cool the surface of the cooling coil 18 to a temperature below the dew-point of the air 26 and the set (evaporating) temperature of CO2 in the cooling coil 18 was below the dew-point of the air 26. Because the temperature of the surface of the cooling coil 18 was below the dewpoint of the air 26, water was caused to condense from the air 26. Therefore, the cooled air 32 leaving the cooling coil 18 had a decreased humidity compared to the air entering the cooling coil 26, i.e. because some of the water had condensed onto the cooling coil 18.

The temperature sensor 34 placed after the cooling coil 18 measured the temperature of the cooled air 32 leaving the cooling coil 18. The cooled (dehumidified) air 32 leaving the cooling coil 18 was then caused to pass over the operating heating coil 42 by the fans 52, as described below. The air 32 needed to be heated because the cooled (dehumidified) air 32 was too cold to be passed directly back into the air-conditioned space 28.

The CO2 vapour 30 was passed from the cooling coil 18 back to the CO2 compressor(s) 14 in the CO2 refrigeration unit 12. As above, a pressure sensor 12- 2 measured the pressure of CO2 30 at an inlet of the one or more CO2 compressors 14. The pressure sensor had a set pressure of about 40 bar, equivalent to a CO2 gas temperature of 6°C. When the pressure of the CO2 at the inlet to the CO2 compressor(s) was above 40 bar, the flow rate of liquid CO2 16 to the cooling coil 18. Conversely, when the pressure of the CO2 at the inlet to the CO2 compressor(s) was below 40 bar, the flow rate of liquid CO2 was decreased. It was observed that an increase in pressure at the inlet to the CO2 compressor(s) typically correlated with the pressure-reducing valve 22 opening more (i.e. for the same reasons as provided in Example 2).

As in the cooling mode, the pressure measured by pressure sensor 12-2 is used to control the staging of the compressor(s) and hence the flow of liquid CO2 16. When the system 10 comprised one compressor, the flow of liquid CO2 16 was increased (or decreased) by increasing (or decreasing) a motor speed of this one compressor, i.e. by fitting the compressor with a VSD. Alternatively, when the system 10 comprised multiple compressors, the flow of liquid CO2 16 was increased (or decreased) by increasing (or decreasing) a motor speed of the compressors and/or increasing (or decreasing) the number of operating compressors.

As above, typically, the system 10 comprised up to fourteen compressors with at least one fitted with a VSD, i.e. allowing the speed of the motor to be increased or decreased. When the flow of liquid CO2 16 had to be increased, the first compressor fitted with a VSD would act to increase the frequency of the compressor. Once the maximum frequency was reached, then a second (optionally VSD-fitted) compressor was started. Once the second compressor was operating at the maximum frequency, the third compressor was started and so on.

As above, in the CO2 compressor(s) 14, the CO2 vapour 30 was compressed therein to a pressure of about 84 bar, thereby forming compressed CO2 38. The compressed CO2 38 passing from the compressor was observed to typically be a supercritical fluid (i.e. because the HVAC system 10 typically operated in a transcritical cycle whilst in the dehumidification mode). Again, the inventors noted that the exact number of CO2 compressor(s) 14 in the CO2 refrigeration unit 12 was able to be varied depending on the application.

The compressed CO2 vapour 38 was then passed to the three-way valve 40, located at the exit of the CO2 refrigeration unit. When the system 10 was in dehumidification mode, the three-way valve 40 was configured to selectively pass at least some of the compressed CO2 vapour 38 to the heating coil 42 as stream 44. As the compressed CO2 vapour 44 was passed through the inside of the heating coil 42, the cooled air 32 leaving the cooling coil 18 simultaneously passed over the outside of the heating coil 42.

As above, energy was transferred from the compressed CO2 vapour 44 inside the heating coil 42 to the cooled air 32 passing over the outside of the heating coil 42. This energy transfer caused the compressed CO2 vapour 44 to cool and the cooled air 32 to be simultaneously heated, thereby producing cooled CO2 vapour 46 and heated air 48. The heated air 48 was blown by fans 52 from the air handling unit 20 back into the air-conditioned space 28, as it was now sufficiently warmed to be passed back into the air-conditioned space 28.

As above, the temperature sensor 50 located after the heating coil 42 measured the temperature of the heated air 48 leaving the heating coil 42. When the measured temperature of the heated air 48 was below a predetermined heated air temperature, the temperature sensor 50 signalled the PLC situated inside the CO2 refrigeration unit 12. The PLC was electrically connected to the three-way valve 40 (and other components) and sent a signal to the three-way valve 40, causing the three-way valve 40 to direct more of the compressed CO2 vapour 38 to the heating coil 42 (i.e. the flow of stream 44 was increased). This was observed to cause the temperature of the heated air 48 to increase.

When the temperature of the heated air 48 was above a predetermined heated air temperature, the temperature sensor 50 signalled the PLC. The PLC sent a signal to the three-way valve 40 to cause the three-way valve 40 to direct less of the compressed CO2 vapour 38 to the heating coil 42 (i.e. the flow of stream 44 was decreased). This was observed to cause the temperature of the heated air 48 to decrease.

The cooled CO2 vapour 46 exiting the heating coil 42 was combined with the remaining compressed CO2 vapour 54 that was not selectively passed by the three-way valve 40 to the heating coil 42. The combined CO2 stream 56 was passed to the second three-way valve 58.

As above, the inventors noted that when the system 10 comprised multiple air handling units and dehumidification was required in at least one, the three-way valve 40 was configured to be fully open such that all of the compressed CO2 vapour 38 was directed toward the heating coils 42, 42a, i.e. bypass stream 54 was not used. When this occurred, the PLC acted to cause the two-way ball valves 45, 45a, 47, 47a to close when the temperature of the heated air 48, 48a (i.e. as measured by temperature sensors 50, 50a) was above the predetermined heated air temperature. Conversely, the PLC acted to cause the two-way ball valves 45, 45a, 47, 47a to open when the temperature of the heated air 48, 48a was below the predetermined heated air temperature.

The second three-way valve 58 was configured to selectively pass the combined CO2 vapour 56 to the gas cooler 60, when required. The gas cooler 60 operated to further cool of the combined CO2 vapour 56. The cooling in the gas cooler 60 was performed at a constant pressure of about 84 bar, i.e. the compressed CO2 vapour remained at approximately the same pressure through the heating coil 42 and the gas cooler 60. As in the heating mode, it was observed that when no further cooling of the combined CO2 vapour 56 was required, the gas cooler 60 was by-passed. Thus, in the dehumidification mode, the use of the gas cooler 60 was optional - as indicated by the dotted lines in Fig. 5.

As above, the portion 62 of the combined CO2 vapour was selectively passed into the inside of the tubes of the gas cooler 60. Ambient air 64 was caused to flow over the tubes of the gas cooler 60 by means of fans 66 located adjacent to the gas cooler 60. Energy transferred from the CO2 vapour 62 within the tubes to the ambient air 64, caused the CO2 vapour to be further cooled and the ambient air 64 to be heated. The further cooled CO2 vapour 68 from the in-use gas cooler 60 was combined with the remaining cooled CO2 vapour 70 that was not selectively passed by the three-way valve 58 to the gas cooler 60 to form a combined cooled CO2 stream 72. As the further cooled CO2 vapour 68 and the remaining cooled CO2 vapour 70 mixed, the enthalpy of the two streams equilibrated. The combined cooled CO2 vapour 72 was passed to the inlet of the high-pressure expansion valve 75.

Alternatively, when the gas cooler 60 was not in-use, none of the combined supercritical CO2 56 was passed to the gas cooler 60. For example, when the combined supercritical CO2 56 was at a suitable temperature to pass directly to the inlet of the high-pressure expansion valve 75. However, the inventors noted that during dehumidification mode the gas cooler 60 was typically used because the cooling load on the cooling coil 18 was generally much greater than the heating load on the heating coil 42. This was because the cooling load required latent energy (i.e. condensing water from air), whereas the heating load required only sensible energy (i.e. heating air from a lower temperature to a higher temperature with no phase change).

As the combined cooled CO2 vapour 72 was passed to the inlet of the high-pressure expansion valve 75, the temperature sensor 74 located adjacent to the inlet of the CO2 refrigeration unit 12 measured the temperature of the combined cooled CO2 stream 72. As above, the temperature sensor 74 measured the temperature of the combined supercritical CO2 56 as it passed to the inlet of the high-pressure expansion valve 75. The temperature sensor 74 signalled to the PLC when the measured temperature was above (or below) a predetermined inlet temperature. The PLC then sent a signal to the three-way valve 58, causing the three-way valve to open (or close) further. This caused a greater (or lesser) portion of the combined supercritical CO2 56 to selectively pass to the gas cooler 60, thereby causing the inlet temperature to decrease (or increase) respectively.

As above, as the combined cooled CO2 stream 72 passed to the inlet of the high-pressure electronic expansion valve 75, the pressure of the stream 72 was reduced to about 50 bar causing most of the CO2 vapour to condense. The two- phase CO2 stream 77 was passed to an inlet of the CO2 refrigeration unit 12 and to the receiver 12-1. The liquid component of the two-phase CO2 77 entering the receiver 12-1 then formed the (saturated) liquid CO2 82 which passed from the CO2 refrigeration unit 12 to the cooling coil 18, when the pressure-reducing valve 22 was opened.

The inventors observed that, during dehumidification, the load on the cooling coil was always greater than the load on the heating coil, meaning that there was sufficient energy available to heat the air passing over the heating coil.

Whilst air handling unit 20 was in a dehumidification mode, air handling unit 20a (see Fig. 2) was also placed in a dehumidification mode by selecting the appropriate setting on an electronic panel connected to the PLC system. The electronic panel allowed the user to select different settings. In the embodiment of Fig. 2, when the dehumidification mode was selected, the pressure-reducing valve 22a was opened. As the pressure-reducing valve 22a was opened, liquid CO2 16a was able to pass from the CO2 refrigeration unit 12a to the cooling coil 18a, via the pressure-reducing valve 22a. In addition, three-way valve 40 was configured so all the compressed CO2 vapour 38 was directed to the heating coils 42, 42a. It was noted that air handling unit 20a operated in the same way as previously described for air handling unit 20 in a system 10 with multiple air handing units.

The inventors noted that many more such air handling units 20, 20a etc. could be placed in a dehumidification mode at the same time to thereby increase the dehumidification capacity of the system or to provide dehumidification to separate rooms.

Example 5 - Simultaneous Heating and Cooling (Figs. 2 & 6)

During winter, it was observed that, although cooling was still required in certain rooms/spaces (e.g. a projector room, computer room, etc.), typically heating was required in main (e.g. occupied) rooms/spaces, because of the low ambient temperature. As also previously described, the configuration of coils, pipes and valves in the HVAC system 10a allowed individual air handling units 20, 20a, etc. to operate in different modes.

In HVAC system 10a (see Fig. 2), the PLC detected that the measured temperature in the air-conditioned space 28 was above the target temperature and placed air handling unit 20 in a cooling mode. The air handling unit 20 operated in the same was as previously described in Example 2. At the same time, the PLC detected that the measured temperature in the air-conditioned space 28a was below the target temperature and placed air handling unit 20a in a heating mode. The air handling unit 20a operated in the same way as previously described in Example 3. Fig. 6 highlights, in thick lines, the components of the HVAC system 10a that were always used when air handling unit 20 was placed in cooling mode and air handling unit 20a was placed in heating mode. Fig. 6 also highlights, in dotted lines, the components of the HVAC system 10a that were optionally used when air handling unit 20 was placed in cooling mode and air handling unit 20a was placed in heating mode.

When the HVAC system 10a was providing simultaneous heating and cooling, the CO2 compressors 14 were signalled to operate in a transcritical cycle. Typically, the transcritical cycle was observed to operate between 41 bar to 84 bar. As above, the inventors noted that a transcritical cycle was advantageous because it provided more efficient heat exchange and ensured CO2 leaving the heating coil was a supercritical fluid, i.e. so could flow back up to the CO2 refrigeration unit 12.

When the cooling mode was selected in air handling unit 20, the air handling unit 20 operated as describe in detail above in Example 2. The pressurereducing valve 22 was opened, allowing (saturated) liquid CO2 16 to pass from the receiver 12-1 within the CO2 refrigeration unit 12 to the cooling coil 18, via the pressure-reducing valve 22. The CO2 refrigeration unit 12 provided liquid CO2 16 at a pressure of about 50 bar. As the liquid CO2 16 passed through the pressure-reducing valve 22, the pressure of the liquid CO2 was observed to decrease to be about 40 bar.

As above, as the pressure of the liquid CO2 16 was decreased, a portion of the liquid CO2 evaporated (boiled) because it was a saturated liquid, however, most of the CO2 was still in the (saturated) liquid phase.

The two-phase CO2 stream 24 then passed into the inside of the cooling coil 18, which operated at about 40 bar. As the two-phase CO2 24 passed through the cooling coil 18, air 26 was caused by fan 52 to pass over the outside of the cooling coil 18. Again, to maintain suitable oxygen levels, the air 26 was a mixture of air from the air-conditioned space 28 and fresh ambient air 27. As above, energy was transferred from the air 26 passing over the cooling coil 18 to the two-phase CO2 24 within the cooling coil 18. This energy transfer caused the remaining saturated liquid CO2 to evaporate in the cooling coil 18 at the operating pressure of the cooling coil, resulting in a CO2 vapour 30 being passed from the exit of the cooling coil 18. The CO2 vapour 30 was observed to be a superheated vapour. At the same time, the air 26 leaving the cooling coil 18 was cooled air 32.

Again, typically the pressure-reducing valve 22 was an electronic expansion valve metering device that regulated the flow of liquid CO2 16 into the cooling coil 18. The pressure-reducing valve 22 was fitted with sensors to monitor the superheat of the CO2 passing through the pressure-reducing valve 22. When the superheat was above (or below) a predetermined value, typically 6K, the flow of liquid CO2 16 was increased (or decreased) by further opening (or by further closing) the pressure-reducing valve 22. Such control ensured that all the CO2 exiting the cooling coil 18 was in the form of a vapour.

The temperature sensor 34 placed after the cooling coil 18 measured the temperature of the cooled air 32 leaving the cooling coil 18. The cooled air 32 leaving the cooling coil 18 passed over the (not in-use) heating coil 42 and was directed into the air-conditioned space 28 by fans 52. In this regard, two-way ball valves 45 and 47 were closed such that the compressed CO2 vapour 38 could not pass to the heating coil 42 of the air handling unit 20.

The CO2 vapour 30 was passed from the cooling coil 18 back to CO2 compressor(s) 14 in the CO2 refrigeration unit 12. Again, it was noted that the exact number of CO2 compressor(s) 14 in the CO2 refrigeration unit 12 was able to be varied, depending on the application.

In the CO2 compressors 14, the CO2 vapour 30 was compressed to a pressure of about 84 bar. The compressed CO2 vapour 38 was observed to be a supercritical fluid, since the CO2 compressors 14 were operating in a transcritical cycle. The compressed CO2 vapour 38 was then passed to the three-way valve 40, located at the exit of the CO2 refrigeration unit 12. The three-way valve 40 was configured to pass all the compressed CO2 vapour 38 to the heating coils 42, 42a, because air handling unit 20a was in heating mode. Because air handling unit 20 was in cooling mode, ball valves 45 and 47 remained closed, i.e. to ensure no compressed CO2 vapour 38 could pass to heating coil 42. However, because air handling unit 20a was in heating mode, ball valves 45a and 47a opened to allow compressed CO2 vapour 38 to pass to heating coil 42a, via stream 44a.

As the compressed CO2 vapour 44a passed through the inside of the heating coil 42a, air 32a simultaneously passed over the outside of the heating coil 42a, drawn by the fan 52a. As above, typically the air 32a was a mixture of air from the air-conditioned space 28a and fresh ambient air 27.

As above, energy was transferred from the compressed CO2 vapour 44a inside the heating coil 42a to the air 32a that was passed over the outside of the heating coil 42a. This energy transfer caused the compressed CO2 vapour 44a to cool and the air 32a to be simultaneously heated, thereby producing cooled CO2 vapour 46a and heated air 48a. The heated air 48a was blown by fans 52a from the air handling unit 20a and back into the air-conditioned space 28a. Because the HVAC system 10a was operating in a transcritical cycle, the cooled CO2 vapour 46a was still a supercritical fluid.

The temperature of the cooled supercritical CO2 46a was measured by temperature sensor 51. Because air handling unit 20 was in cooling mode, there was no cooled supercritical CO2 46 returning from the heating coil 42. Therefore, the temperature sensor 51 only measured the temperature of supercritical CO2 46a.

When the system 10 comprised multiple air handling units 20a, 20b etc. (i.e. Fig. 2) that were all in heating mode, the cooled supercritical CO2 46a, 46b, etc. from each air handling unit 20a, 20b, etc. that was in heating mode were able to be combined to form a combined cooled supercritical CO2 47. The temperature of the combined cooled supercritical CO2 47 was then measured using temperature sensor 51.

As above for the heating mode, the staging of the CO2 compressors 14 was controlled by the temperature measured by the temperature sensor 51. Again, the PLC was signalled when the temperature of the cooled supercritical CO2 46a was above a predetermined temperature. The PLC was electrically connected to the one or more CO2 compressor 14. The PLC sent a signal to the one or more CO2 compressor 14 to increase the flow of compressed CO2 vapour 38. When the temperature of the cooled CO2 vapour 46a was below a predetermined temperature, the temperature sensor 51 signalled the PLC. The PLC sent a signal to the one or more CO2 compressor 14 to decrease the flow of compressed CO2 vapour 38.

As above, it was noted that the flow of compressed CO2 vapour 38 was increased by increasing a motor speed of at least one compressor and/or increasing the number of operating compressors, and the flow of compressed CO2 vapour 38 was decreased by decreasing a motor speed of at least one of the compressors and/or by decreasing the number of operating compressors. Again, increasing the motor speed was accomplished by fitting the compressor(s) with a VSD.

The combined cooled supercritical CO2 47 was passed to a second three- way valve 58 which was configured to selectively pass the combined supercritical CO2 56 to the gas cooler 60, if required. The gas cooler 60 operated to further cool the combined supercritical CO2 62 passed thereto. As above, it was observed that when no further cooling of the combined supercritical CO2 56 was required, the second three-way valve 58 passed all the combined supercritical CO2 56 directly to the CO2 refrigeration unit 12. That is, use of the gas cooler 60 was optional in this mode, as indicated by the dotted lines in Fig. 5.

As above, the portion of the combined supercritical CO2 was selectively passed into the inside of the tubes of the gas cooler 60. Ambient air 64 was forced to flow over (i.e. around) the tubes of the gas cooler 60 by means of fans 66 located adjacent to the gas cooler 60. Energy was transferred from the supercritical CO2 62 within the tubes of the gas cooler 60 to the ambient air 64. This energy transfer caused the supercritical CO2 62 to be further cooled (but remaining as a supercritical fluid) and the ambient air 64 to be heated to form a heated air stream 90.

The further cooled supercritical CO2 68 from the in-use gas cooler 60 was combined with the remaining supercritical CO2 70 that was not selectively passed by the three-way valve 58 to the gas cooler 60 and formed a combined cooled CO2 stream 72. As the cooled CO2 fluid 68 and the remaining CO2 fluid 70 mixed, the enthalpy of the two streams equilibrated. The combined cooled CO2 fluid 72 was passed to the inlet of the high-pressure electronic expansion valve 75.

As above, the temperature sensor 74 measured the temperature of the combined supercritical CO2 56 as it passed to the inlet of the high-pressure electronic expansion valve 75. The temperature sensor 74 signalled to the PLC when the measured temperature was above (or below) a predetermined inlet temperature. The PLC then sent a signal to the three-way valve 58, causing the three-way valve to open (or close) further. This caused a greater (or lesser) portion of the combined supercritical CO2 56 to selectively pass to the gas cooler 60, thereby causing the inlet temperature to decrease (or increase) respectively.

The PLC was also electrically connected to the gas cooler fans 66. When the measured temperature was above the predetermined inlet temperature, the PLC sent a signal to increase the speed of the gas cooler fans 66. Conversely, when the measured temperature was below the predetermined inlet temperature, the PLC sent a signal to decrease the speed of the gas cooler fans 66.

As above, as the combined cooled CO2 stream 72 passed to the inlet of the high-pressure electronic expansion valve 75, the pressure of the stream 72 was reduced to about 50 bar causing most of the CO2 vapour to condense. The resultant two-phase CO2 stream 77 was passed to an inlet of the CO2 refrigeration unit 12 and to the receiver 12-1. The liquid component of the two-phase CO2 77 entering the receiver 12-1 then formed the (saturated) liquid CO2 16 which passed from the CO2 refrigeration unit 12 to the cooling coil 18, when the pressurereducing valve 22 was opened.

It will be understood that in the cooling coil 18, energy was transferred from the air 26 in the air handling unit 20, to the two-phase CO2 24 entering the cooling coil 18. Energy was then transferred to the CO2 vapour 30 in the CO2 compressors 14. The energy of the CO2 fluid 38 leaving the CO2 compressors 14 was therefore a combination of the energy transferred in the cooling coil 18 plus the energy from the compressor(s) (i.e. the electrical energy used to run the compressor(s)). In the heating coil 42a, energy was transferred from the CO2 vapour 44a to the air 32a in the air handling unit 20a, i.e. to provide heated air 48a. In some circumstances, the total energy of the CO2 vapour 44a was insufficient to adequately heat the air 32, e.g. when the heating required by the heating coil 42a was significantly greater than the cooling required by the cooling coil 18.

Advantageously, the HVAC system 10a also comprised an additional cooling coil 76 located adjacent to the gas cooler 60. The additional cooling coil 76 was arranged such that the ambient air 64 that was caused to flow over the gas cooler 60 by the gas cooler fans 66 was thereafter caused to pass over the additional cooling coil 76 as airstream 90.

The PLC was electrically connected to the pressure-reducing valve 80 and sent a signal thereto, causing the pressure-reducing valve 80 to open when the PLC determined that further heating was required. As the pressure-reducing valve 80 was opened, liquid CO2 82 was able to pass from the receiver 12-1 of the CO2 refrigeration unit 12 to the additional cooling coil 76, via the pressure-reducing valve 80.

It was noted that the liquid CO2 82 that passed to the pressure-reducing valve 80 was from the same part of the CO2 refrigeration unit 12 as the liquid CO2 16 that passed to pressure-reducing valve 22, i.e. the receiver 12-1. Thus, the liquid CO2 82 also had a pressure of about 50 bar and was also a saturated liquid. As above, as the liquid CO2 82 passed through the pressure-reducing valve 80, the pressure of the liquid CO2 was decreased and a portion of the liquid CO2 evaporated (boiled) at the set pressure. It was observed, however, that most of the CO2 was still in the (saturated) liquid phase. The pressure of the CO2 after the pressure-reducing valve 84 was observed to be about 35 bar.

As above, the two-phase CO2 stream 84 then passed into the inside of the additional cooling coil 76, which operated at a pressure of about 35 bar, i.e. the same pressure as the CO2 passing from the pressure-reducing valve 80. Airstream 90 (from the gas cooler 60) was simultaneously caused to pass over the outside of the additional cooling coil 76 via the use of fans 66. As the air 90 was passed over the additional cooling coil 76, energy was transferred from the air 90 passing over the additional cooling coil 76 to the two-phase CO2 84 within the additional cooling coil 76. The energy transfer from the air 90 to the two-phase CO2 caused the liquid CO2 to evaporate in the additional cooling coil 76 at the operating pressure thereof, because the liquid was saturated. This resulted in a CO2 vapour being passed from the exit of the cooling coil 88. At the same time, the air leaving the cooling coil was cooled air 86.

The CO2 vapour 88 was passed back to the CO2 refrigeration unit 12 where it was compressed in CO2 compressor(s) 14 and formed a portion of the compressed CO2 vapour 38.

Example 6 - Defrosting Cooling Coil (Figs. 2 & 7)

When one or more air handling units 20, 20a, etc. were set to the heating mode (i.e. the HVAC system 10 was operating as described in Example 3 using a transcritical cycle) for an extended period of time, it was observed that frost/ice accumulated on the cooling coil 76. This was because, in the heating mode, the cooling coil 76 located outside the air handling units 20, 20a, etc was used to generate the CO2 vapour that was passed through the heating coils 42, 42a, etc. However, during cold months when the ambient air temperature was ~1 °C, the transfer of energy from the ambient air to the CO2 inside the cooling coil 76 caused the ambient air temperature to drop below 0 °C. This caused frost/ice to accumulate on the cooling coil 76 (i.e. because the ambient air was below the freezing point of water). As the frost/ice accumulated on the cooling coil 76, the heat transfer between the CO2 inside the cooling coil 76 and the ambient air passing over the cooling coil was restricted. This reduced the ability of the system to generate heat.

The HVAC system 10 (Fig. 7) comprised pressure and electronic sensors located and configured to measure a presence of frost/ice at the cooling coil 76. When the pressure and electronic sensors detected a presence of frost/ice, the three-way valve 58 was configured so as to pass all the combined cooled supercritical CO2 vapour 56 from the heating coils 42, 42a to the gas cooler 60. That is, the control for the three-way valve 58 based on the temperature sensor 74 described in Example 3 was over-ridden to allow all the combined cooled supercritical CO2 vapour 56 to pass to the gas cooler 60.

At the same time the three-way valve 58 was positioned so as to selectively pass all the combined cooled supercritical CO2 vapour 56 to the gas cooler 60, the speed of the gas cooler fans 66 was increased to a predetermined set value. This increased the volume flow rate of ambient air 64 forced to pass around the tubes of the gas cooler 60.

The combined cooled supercritical CO2 62 passed into the tubes of the gas cooler 60. As the ambient air 64 was forced to flow around the tubes of the gas cooler 60, energy was transferred from the supercritical CO2 within the tubes of the gas cooler 60 to the ambient air 64 flowing around the tubes of the gas cooler 60. This energy transfer caused the supercritical CO2 to be further cooled, thereby producing further cooled supercritical CO2 68, and concurrently caused the ambient air to be heated, producing heated air 90.

It was observed that since the system 10 was in a transcritical cycle (i.e. because it is operating in the heating mode as described in Example 3), the incoming CO2 62 to the gas cooler 60 was a supercritical CO2 fluid. In the gas cooler 60, the energy transfer between the supercritical CO2 and the ambient air 64 caused the temperature of the supercritical CO2 to decrease, however the CO2 remained in the supercritical state.

The further cooled supercritical CO2 68 from the gas cooler 60 was passed back to an inlet of the CO2 refrigeration unit 12.

The heated air 90 was caused to pass over the cooling coil 76 by the gas cooler fans 66. Because the heated air 90 was at a temperature above 1 °C (i.e. because it was heated in the gas cooler by the CO2 vapour), as the heated air 90 then passed over the cooling coil 76, energy was transferred from the heated air 90 to the accumulated frost/ice, causing the accumulated frost/ice to melt (i.e. defrost mode).

Once the pressure and electronic sensors no longer sensed the accumulation of frost/ice on the cooling coil 76 (i.e. because it had melted away), the three-way valve 58 was configured so as to selectively pass only some of the combined cooled supercritical CO2 vapour 56 to the gas cooler 60, the position of the three-way valve 58 once again being controlled by the temperature sensor 74 at the inlet to the CO2 refrigeration unit 12. The speed of the gas cooler fans 66 was also decreased.

Advantageously, this allowed accumulated frost/ice to be removed from the cooling coil 76 without interrupting heating of the air-conditioned spaces 28, 28a. This was only possible because the cooling coil 76 was located adjacent to the gas cooler 60, with the gas cooler fans 66 causing ambient air to first pass over the gas cooler 60 and thereafter pass over the cooling coil 76. In contradistinction, typical HVAC systems require heating to be stopped whilst the cooling coil is defrosted, i.e. because the cooling coil is not positioned adjacent to a gas cooler.

Of further advantage, the method of defrosting used heat recovered directly from the CO2 to defrost the ice - i.e. because the cooled supercritical CO2 vapour 36 leaving the heating coil 42 contained sufficient energy to heat the ambient air 64. Again, this is in contrast to prior art systems which typically comprise an electric heater located within the cooling coil. To defrost the cooling coil, the electric heater is turned on, thereby producing heat which melts the accumulated frost/ice. Thus, in the prior art systems, electricity is necessarily consumed during defrosting.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the method and system as disclosed herein.

Claims

1. A method of retrofitting a synthetic refrigerant based HVAC system, said HVAC system comprising one or more air handling units, the one or more air handling units comprising at least one coil, and a series of pipes, valves and ducts, the method comprising: a. removing a synthetic refrigerant compressor unit and installing a CO2 refrigeration unit; b. installing a CO2 condenser/gas cooler configured to condense and/or cool CO2 vapour at a pressure above atmospheric; c. removing the at least one coil in each of the at least one air handling units; d. installing a cooling coil in each of the at least one air handling units, the cooling coil suitable to comprise CO2 at a pressure above atmospheric; e. installing a heating coil in each of the at least one air handling units, said heating coil suitable to comprise CO2 at a pressure above atmospheric; f. removing the series of pipes and valves and installing a new series of pipes and valves configured to comprise CO2 at a pressure above atmospheric.

2. A method of retrofitting as claimed in claim 1, further comprising installing an additional cooling coil, the additional cooling coil coupled to the CO2 condenser/gas cooler and suitable to heat CO2 at a pressure above atmospheric.

3. A method of retrofitting as claimed in claim 1 or 2, comprising removing one or more of the one or more air handling units and installing respective new air handling unit(s) therein.

4. A method of retrofitting as claimed in any one of the preceding claims, comprising removing one or more electric heater elements (when present) from the ducts.

5. A method of retrofitting as claimed in any one of the preceding claims, wherein the cooling coil in each of the at least one air handling units is suitable to operate with CO2 at a maximum pressure of about 80 bar.

6. A method of retrofitting as claimed in claim 2, or any one of claims 3 to 5 when dependent on claim 2, wherein the additional cooling coil is suitable to operate with CO2 at a maximum pressure of about 80 bar.

7. A method of retrofitting as claimed in any one of the preceding claims, wherein the heating coil in each of the at least one air handling units is suitable to operate with CO2 at a maximum pressure of about 120 bar.

8. A method of retrofitting as claimed in any one of the preceding claims, wherein the CO2 condenser/gas cooler is suitable to operate with CO2 at a maximum pressure of about 120 bar.

9. A method of retrofitting as claimed in any one of the preceding claims, wherein the cooling coil in each of the at least one air handling units comprises a fin coil.

10. A method of retrofitting as claimed in claim 2, or any one of claims 3 to 9 when dependent on claim 2, wherein the additional cooling coil comprises a fin coil.

11. A method of retrofitting as claimed in any one of the preceding claims, wherein the heating coil in each of the at least one air handling units comprises a fin coil.

12. A method of retrofitting as claimed in any one of the preceding claims, wherein the CO2 refrigeration unit, the CO2 condenser/gas cooler, the at least one air handling unit comprising the cooling coil and the heating coil, the additional cooling coil (when present) and the series of pipes and valves are configured to allow the HVAC system to cool and/or heat air passing through the one or more air handling units.

13. A method of retrofitting as claimed in any one of the preceding claims, wherein the CO2 refrigeration unit, the CO2 condenser/gas cooler, the at least one air handling unit comprising the cooling coil and the heating coil, the additional cooling coil (when present) and the series of pipes and valves are configured to allow the HVAC system to dehumidify air passing through the one or more air handling units.

14. A method of retrofitting as claimed in any one of the preceding claims, wherein the CO2 refrigeration unit, the CO2 condenser/gas cooler, the at least one air handling unit comprising the cooling coil and the heating coil, the additional cooling coil (when present) and the series of pipes and valves are configured to allow the HVAC system to operate in a transcritical and/or subcritical cycle.

15. A method of retrofitting as claimed in any one of the preceding claims, wherein the synthetic refrigerant in the synthetic refrigerant based HVAC system comprises: HCFC R22, HFC, HFO or a synthetic drop in HCFC replacement.

16. A method for dehumidifying air using an HVAC system comprising CO2 as a refrigerant, the method comprising: a) opening a pressure-reducing valve between a CO2 refrigeration unit and a cooling coil, thereby allowing liquid CO2 to flow into the cooling coil, wherein energy is transferred between the liquid CO2 within the cooling coil and air passing over the cooling coil, causing the liquid CO2 to boil and said air to be cooled; b) passing the resultant CO2 vapour to one or more CO2 compressors within the CO2 refrigeration unit, wherein the CO2 vapour is compressed; and c) allowing the compressed CO2 vapour to flow through a heating coil, wherein energy is transferred between the compressed CO2 vapour within the heating coil and air passing over the heating coil, causing the vapour to be cooled and said air to be heated; wherein the cooling coil and the heating coil are located within an air handling unit such that the cooled air from step a) is forced to pass over the heating coil in step c).

17. A method as claimed in claim 16, further comprising selectively passing at least some of the resultant cooled CO2 vapour of step c) to a gas cooler, wherein at least some of the cooled CO2 vapour is further cooled, with the further cooled CO2 vapour passing back to the CO2 refrigeration unit with remaining cooled CO2 vapour not selectively passed to the gas cooler.

18. A method as claimed in claim 17, wherein the cooled CO2 vapour is selectively passed to the gas cooler so as to achieve a predetermined temperature at an inlet to the CO2 refrigeration unit.

19. A method as claimed in any one of claims 16 to 18, wherein in step a) the flow of liquid CO2 from the CO2 refrigeration unit to the cooling coil is increased when a pressure of CO2 vapour leaving the cooling coil and entering the compressor is higher than a predetermined operating pressure of the compressor, and the flow of liquid CO2 is decreased when the pressure of CO2 vapour leaving the cooling coil and entering the compressor is lower than the predetermined operating pressure.

20. A method as claimed in any one of claims 16 to 19, wherein in step c) the flow of compressed CO2 vapour to the heating coil is increased when a temperature of air leaving the heating coil is lower than a predetermined temperature of heated air leaving the heating coil, and the flow of compressed CO2 vapour is decreased when the temperature of air leaving is higher than the predetermined heated air temperature.

21. A method as claimed in any one of claims 16 to 20, wherein the HVAC system using CO2 comprises the retrofitted HVAC system of any of claims 1 to 15.

22. An HVAC system for dehumidifying air, the HVAC system comprising: i. a CO2 refrigeration unit comprising one or more CO2 compressors, one or more valves, a series of pipes and electrical controls; ii. a cooling coil connected to the CO2 refrigeration unit by a pressure-reducing valve, said valve configured such that when the valve is in an open position, liquid CO2 is caused to flow between the CO2 refrigeration unit and the cooling coil, wherein the liquid CO2 evaporates as air passes over the cooling coil, the air being cooled thereby; iii. an outlet of the cooling coil configured to allow the resultant CO2 vapour to be passed to the one or more CO2 compressors of the CO2 refrigeration unit wherein the CO2 vapour is compressed to form a compressed CO2 vapour; iv. a heating coil connected to the one or more CO2 compressors within the CO2 refrigeration unit, the connection configured to allow the compressed CO2 vapour to pass from the one or more CO2 compressors and into the heating coil, wherein the compressed CO2 vapour is cooled as air passes over the heating coil, the air being heated thereby; and v. an outlet of the heating coil configured to allow the resultant cooled CO2 vapour to pass to the CO2 refrigeration unit; wherein the cooling coil and heating coil are located within an air handling unit and are configured such that the cooled air from step ii. is forced to pass over the heating coil in step iv..

23. A system as claimed in claim 22, further comprising a gas cooler configured to selectively pass at least some of the cooled CO2 vapour from the heating coil to the gas cooler, wherein the at least some of the cooled CO2 vapour is further cooled, and an outlet of the gas cooler is configured to allow the further cooled CO2 vapour to pass to the CO2 refrigeration unit, along with remaining cooled CO2 vapour not selectively passed to the gas cooler.

24. A system as claimed in claim 23, comprising a temperature sensor at the inlet to the CO2 refrigeration unit and a three-way valve, the three-way valve being located and configured to selectively pass the cooled CO2 vapour from the heating coil to the gas cooler so as to achieve a predetermined temperature at the inlet to the CO2 refrigeration unit, as measured by the temperature sensor.

25. A system as claimed in any one of claims 22 to 24, wherein the one or more CO2 compressors are suitable to compress CO2 vapour to thereby form a superheated CO2 vapour.

26. A system as claimed in any one of claims 22 to 24, wherein the one or more CO2 compressors are suitable to compress CO2 vapour to thereby form a transcritical CO2 vapour.

27. A system as claimed in any one of claims 22 to 26, further comprising a pressure sensor configured to measure a pressure of CO2 vapour leaving the cooling coil and entering the compressor, wherein the pressure sensor is further configured to cause an increased flow of liquid CO2 to the cooling coil when the CO2 vapour pressure is above a predetermined CO2 vapour pressure and to cause a decreased flow of liquid CO2 when the CO2 vapour pressure is below the predetermined CO2 vapour pressure.

28. A system as claimed in any one of claims 22 to 27, further comprising a temperature sensor configured to measure the temperature of heated air leaving the heating coil, wherein the temperature sensor is further configured to adjust the position of a three-way valve located at an outlet of the CO2 refrigeration unit, the three-way valve configured to selectively pass compressed CO2 vapour to the heating coil to allow an increased flow of compressed CO2 vapour when a temperature of air leaving the heating coil is below a predetermined air temperature and to allow a decreased flow of compressed CO2 vapour when the heated air temperature is above the predetermined heated air temperature.

29. A system as claimed in any one of claims 22 to 28, wherein the cooling coil comprises a fin coil.

30. A system as claimed in any one of claims 22 to 29, wherein the heating coil comprises a fin coil.

31. A system as claimed in any one of claims 22 to 30, comprising the retrofitted HVAC system one of any of claims 1 to 15.

32. An HVAC system for operating in either a subcritical or transcritical CO2 cycle, the system comprising: a. A CO2 refrigeration unit comprising one or more CO2 compressors, one or more valves, a series of pipes and electrical controls, the one or more CO2 compressors configured to compress CO2 vapour into a superheated vapour or a transcritical vapour; b. a cooling coil located within an air handling unit connected to the CO2 refrigeration unit by a pressure-reducing valve, said valve configured such that when the valve is in an open position, liquid CO2 is caused to flow from the CO2 refrigeration unit to the cooling coil, wherein the liquid CO2 evaporates as a stream of ambient air passes over the cooling coil, the air being cooled thereby; c. an outlet of the cooling coil configured to allow the resultant CO2 vapour to be passed to the compressor of the CO2 refrigeration unit wherein the CO2 vapour is compressed to form a compressed CO2 superheated vapour or a supercritical CO2 fluid; d. a gas cooler configured to cool the resultant compressed CO2 vapour to form the substantially liquid CO2 and pass this substantially liquid CO2 to an inlet to the CO2 refrigeration unit.

33. A system as claimed in claim 32, further comprising: a heating coil located within the air handling unit connected to the one or more compressors within the CO2 refrigeration unit, the connection configured to allow the compressed CO2 vapour to pass from the compressor(s) and into the heating coil, wherein the compressed CO2 vapour is cooled as the cooled air from the cooling coil passes over the heating coil, the air being heated thereby; and an outlet of the heating coil configured to allow the resultant cooled CO2 vapour to pass to the CO2 refrigeration unit.

34. A system as claimed in claim 32 or 33, further comprising a temperature sensor at the inlet to the CO2 refrigeration unit and a three-way valve, the three- way valve being located and configured to selectively pass the cooled CO2 vapour from the heating coil to the gas cooler so as to achieve a predetermined temperature at the inlet to the CO2 refrigeration unit, as measured by the temperature sensor.

35. A system as claimed in any one of claims 32 to 34, further comprising an additional cooling coil located adjacent to the gas cooler and connected to the CO2 refrigeration unit by a second pressure-reducing valve, said valve configured to selectively pass liquid CO2 leaving the CO2 refrigeration unit to the additional cooling coil, wherein the liquid CO2 passed to the additional cooling coil evaporates as ambient air passes over the additional cooling coil, the ambient air being cooled thereby.

36. A system as claimed in any one of claims 32 to 35, further comprising a pressure sensor configured to measure a pressure of CO2 vapour entering the compressor, wherein the pressure sensor is further configured to cause an increased flow of liquid CO2 to the cooling coil when the CO2 vapour pressure is above a predetermined CO2 vapour pressure and to cause a decreased flow of liquid CO2 to the cooling coil when the CO2 vapour pressure is below the predetermined CO2 vapour pressure.

37. A system as claimed in claim 36, wherein the pressure sensor is further configured to open the second pressure-reducing valve connecting the CO2 refrigeration unit and the additional cooling coil, thereby allowing liquid CO2 to selectively pass to the additional cooling coil, when the CO2 vapour pressure is above the predetermined CO2 vapour pressure and the position of the pressurereducing valve is above a predetermined position.

38. A system as claimed in any one of claims 32 to 37, further comprising a temperature sensor configured to measure the temperature of heated air leaving the heating coil, wherein the temperature sensor is further configured to adjust the position of a three-way valve located at an outlet of the CO2 refrigeration unit, the three-way valve configured to selectively pass compressed CO2 vapour to the heating coil to allow an increased flow of compressed CO2 vapour when a temperature of air leaving the heating coil is below a predetermined air temperature and to allow a decreased flow of compressed CO2 vapour when the heated air temperature is above the predetermined air temperature.

39. A system as claimed in any one of claims 32 to 38, wherein the cooling coil comprises fin coil.

40. A system as claimed in claim 33, or any one of claims 34 to 39 when dependent on claim 33, wherein the heating coil comprises fin coil.

41. A system as claimed in claim 35, or any one of claims 36 to 40 when dependent on claim 35, wherein the additional cooling coil comprises fin coil.

42. A system as claimed in any one of claims 32 to 41, comprising the retrofitted HVAC system of any one of claims 1 to 15.

43. A method for operating the HVAC system as claimed in any one of claims 32 to 42, the method comprising operating the HVAC system in either a transcritical or subcritical CO2 cycle.

44. A method for controlling the heat load in an HVAC system comprising CO2 as a refrigerant and at least one air handling unit, the method comprising: a- opening a pressure-reducing valve between a CO2 refrigeration unit and a cooling coil, thereby allowing liquid CO2 to flow into the cooling coil, wherein energy is transferred between the liquid CO2 within the cooling coil and air passing over the cooling coil, causing the liquid CO2 to boil and said air to be cooled, and wherein said cooling coil is located externally to the at least one air handling unit; b- passing the resultant CO2 vapour to one or more compressors, wherein the CO2 vapour is compressed to form a compressed supercritical CO2 vapour; c- allowing the compressed supercritical CO2 vapour to flow through a heating coil in each of the at least one air handling units, wherein energy is transferred between the compressed supercritical CO2 vapour within the heating coil and air passing over the heating coil, causing the vapour to be cooled and said air to be heated; d- passing the cooled supercritical CO2 vapour to the CO2 refrigeration unit, wherein a temperature of the cooled supercritical CO2 vapour is measured prior to it reaching the CO2 refrigeration unit, such that, when the temperature of the cooled supercritical CO2 vapour is lower than a predetermined temperature, the flow of CO2 vapour from the CO2 refrigeration unit into the heating coil is increased, and such that when the temperature of the cooled supercritical CO2 vapour is greater than the predetermined temperature, the flow of CO2 vapour is decreased.

45. A method as claim in claim 44, wherein the HVAC system comprises two or more air handling units, and wherein the method is performed such that the resultant cooled supercritical CO2 vapour from each of the two or more air handling units is combined, and a temperature of the combined cooled supercritical CO2 vapour is measured prior to reaching the CO2 refrigeration unit.

46. A method as claimed in claim 44 or 45, wherein when the HVAC system comprises one compressor, the method is performed such that the flow of CO2 vapour is increased by increasing a motor speed of the one compressor and decreased by decreasing the motor speed of the one compressor.

47. A method as claimed in claim 44 or 45, wherein when the HVAC system comprises more than one compressor, the method is performed such that the flow of CO2 vapour is increased by increasing a motor speed of the more than one compressors and/or by increasing the number of operating compressors and the flow of liquid CO2 is decreased by decreasing the motor speed of the more than one compressors and/or by decreasing the number of operating compressors.

48. A method as claimed in claim 47, wherein the HVAC system comprises up to fourteen compressors.

49. A method as claimed in any one of claims 44 to 48, further comprising selectively passing at least some of the cooled supercritical CO2 vapour leaving the heating coil in step c. into a gas cooler located adjacent to the cooling coil, wherein the at least some of the cooled supercritical CO2 vapour is further cooled, the further cooled supercritical CO2 vapour being passed from an outlet of the gas cooler to the CO2 refrigeration unit, along with remaining cooled supercritical CO2 vapour not passed to the gas cooler.

50. A method as claimed in claim 49, wherein a sufficient amount of cooled supercritical CO2 vapour is selectively passed from the heating coil to the gas cooler to achieve a predetermined temperature at an inlet to the CO2 refrigeration unit.

51. A method as claimed in claim 49 or 50, wherein one or more gas cooler fans are located adjacent to the gas cooler to direct a flow of ambient air over the gas cooler, the method further comprising measuring a suction pressure of the CO2 refrigeration unit such that, when the measured suction pressure is above a predetermined suction pressure, a speed of the one or more gas cooler fans is increased, thereby increasing the flow of ambient air across the gas cooler.

52. A method as claimed in claim 51, wherein the gas cooler and cooling coil are configured such that the ambient air directed over the gas cooler by the one or more gas cooler fans thereafter passes over the cooling coil to thereby comprise the ambient air of step a-.

53. A method as claimed in any one of claims 44 to 52, wherein the HVAC system using CO2 comprises the retrofitted HVAC system of any one of claims 1 to 15.

54. An HVAC system for controlling the heat load during heating mode, the system comprising: a. a CO2 refrigeration unit comprising one or more compressors, one or more valves, a series of pipes and electrical controls; b. at least one air handling unit; c. a cooling coil connected to the CO2 refrigeration unit by a pressure-reducing valve, said valve configured such that, when the valve is in an open position, liquid CO2 is caused to flow from the CO2 refrigeration unit to the cooling coil, wherein the liquid CO2 evaporates as air passes over the cooling coil, the air being cooled thereby, said cooling coil being located externally to the at least one air handling unit; d. an outlet of the cooling coil configured such that, the resultant CO2 vapour passes to the one or more compressors of the CO2 refrigeration unit, wherein the CO2 vapour is compressed to a supercritical state; e. a heating coil located within each of the at least one air handling units and connected to the compressor within the CO2 refrigeration unit such that, the compressed supercritical CO2 vapour passes from the compressor and into the heating coil, wherein the compressed supercritical CO2 vapour is cooled as the air passes over the heating coil, the air being heated thereby; f. an outlet of the heating coil configured such that, the resultant cooled supercritical CO2 vapour is passed from the heating coil to the CO2 refrigeration unit, g. a temperature sensor located and configured to measure a temperature of the cooled supercritical CO2 vapour prior to it reaching the CO2 refrigeration unit, the system configured such that, when the temperature of the cooled supercritical CO2 vapour is lower than a predetermined temperature, the flow of CO2 vapour from the CO2 refrigeration unit into the heating coil is increased, and when the temperature of the combined cooled supercritical CO2 vapour leaving the heating coil is greater than the predetermined temperature, the flow of CO2 vapour is decreased.

55. A system as claimed in claim 54, comprising two or more air handling units, and wherein the system is configured such that, the resultant cooled supercritical CO2 vapour from each of the two or more air handling units is combined, and a temperature of the combined cooled supercritical CO2 vapour is measured prior to it reaching the CO2 refrigeration unit.

56. A system as claimed in claim 54 or 55, comprising one compressor, the compressor configured to increase the mass flow of CO2 vapour in step g. by increasing a motor speed and to decrease the mass flow of CO2 vapour by decreasing a motor speed.

57. A system as claimed in claim 54 or 55, comprising more than one compressor, the more than one compressor configured to increase the mass flow of CO2 vapour by increasing a motor speed of the more than one compressors and/or by increasing the number of operating compressors and to decrease the mass flow of CO2 vapour by decreasing the motor speed of the more than one compressors and/or by decreasing the number of operating compressors.

58. A system as claimed in claim 57, comprising up to fourteen compressors.

59. A system as claimed in any one of claims 54 to 58, further comprising a gas cooler located adjacent to the cooling coil and a three-way valve, the three- way valve being located and connected to selectively pass at least some of the combined cooled supercritical CO2 vapour from the heating coil to the gas cooler, wherein the at least some of the combined cooled supercritical CO2 vapour is further cooled, an outlet of the gas cooler being configured to pass the further cooled supercritical CO2 vapour to the CO2 refrigeration unit, along with remaining combined cooled supercritical CO2 vapour not selectively passed to the gas cooler.

60. A system as claimed in claim 59, further comprising a temperature sensor located at an inlet to the CO2 refrigeration unit, the system being configured such that at least some of the combined cooled supercritical CO2 vapour from the heating coils is selectively passed to the gas cooler to achieve a predetermined temperature at the inlet to the CO2 refrigeration unit, as measured by the temperature sensor.

61. A system as claimed in claim 59 or 60, further comprising one or more gas cooler fans located adjacent to the gas cooler and a pressure sensor, the system configured such that, when the suction pressure is below a predetermined suction pressure a speed of the one or more gas cooler fans is increased.

62. A system as claimed in claim 61, wherein the gas cooler and cooling coil are configured such that, the ambient air caused to flow over the gas cooler by the one or more gas cooler fans thereafter passes over the cooling coil.

63. A system as claimed in any one of claims 54 to 62, further comprising an internal cooling coil in each of the at least one air handling units, each internal cooling coil having a corresponding pressure-reducing valve, said valve configured such that, when the valve is in an open position, liquid CO2 is caused to flow between the CO2 refrigeration unit and the internal cooling coil, the system configured such that, when an air handling unit is in heating mode, the corresponding pressure-reducing valve remains in the closed position.

64. A system as claimed in any one of claims 54 to 63, wherein the HVAC system using CO2 comprises the retrofitted HVAC system of any one of claims 1 to 15.

65. An HVAC system for providing simultaneous heating and cooling, the system comprising: a CO2 refrigeration unit comprising at least one compressor, one or more valves, a series of pipes and electrical controls; at least two air handling units, each air handling unit comprising a cooling coil and a heating coil; wherein: an inlet of each cooling coil of the at least two air handling units is connected to the CO2 refrigeration unit by a pressure-reducing valve, each cooling coil having a corresponding pressure-reducing valve, said valve configured such that, when the valve is in an open position, liquid CO2 is caused to flow between the CO2 refrigeration unit and the cooling coil, wherein the liquid CO2 evaporates as air passes over the cooling coil, the air being cooled thereby; an outlet of each cooling coil is configured to combine the resultant CO2 vapour from the at least two air handling units, to thereby form a combined C0₂ vapour that is passed to the at least one compressor of the CO₂ refrigeration unit, wherein the combined CO2 vapour is compressed to a supercritical state; an inlet of each heating coil is connected to the one or more compressors within the CO2 refrigeration unit by at least one three-way valve, said valve configured such that when the valve is in an open position, compressed supercritical CO2 vapour leaving the CO2 refrigeration unit is able to flow to each heating coil wherein the compressed supercritical CO2 is cooled as air passes over the heating coil, the air being heated thereby; an outlet of each heating coil is configured to combine the resultant cooled supercritical CO2 vapour from the at least two air handling units, to thereby form a combined cooled supercritical CO2 vapour which is passed to the CO2 refrigeration unit.

66. A system as claimed in claim 65, comprising a first temperature sensor configured to measure a temperature of the combined cooled supercritical CO2 vapour before the combined cooled supercritical CO2 vapour is passed to the CO2 refrigeration unit, such that, when the temperature of the combined supercritical CO2 vapour is lower than a predetermined temperature the operating number and/or speed of the one or more compressors is increased, and when the temperature of the combined cooled supercritical CO2 vapour leaving the heating coil is greater than the predetermined temperature the operating number and/or speed is decreased.

67. A system as claimed in claim 66, further comprising a gas cooler and a second three-way valve, the second three-way valve being located and connected to selectively pass at least some of the combined cooled supercritical CO2 vapour from the heating coils to the gas cooler such that the at least some of the combined cooled CO2 vapour is further cooled, an outlet of the gas cooler being configured to pass the further cooled CO2 vapour to the CO2 refrigeration unit, along with remaining combined cooled supercritical CO2 vapour not selectively passed to the gas cooler. - I l l -

68. A system as claimed in claim 67, comprising one or more gas cooler fans located adjacent to the gas cooler and configured to cause a flow of ambient air over the gas cooler.

69. A system as claimed in claim 67 or 68, comprising a second temperature sensor located at an inlet to the CO2 refrigeration unit, the system being configured such that at least some of the combined cooled supercritical CO2 vapour from the heating coils is selectively passed to the gas cooler to achieve a predetermined temperature at the inlet to the CO2 refrigeration unit, as measured by the second temperature sensor.

70. A system as claimed in claim 68 or 69, comprising a third temperature sensor located at an outlet of the gas cooler and prior to where the further cooled CO2 vapour from the gas cooler and the remaining combined cooled supercritical CO2 vapour are combined, the system being configured to adjust a speed of the one or more gas cooler fans so as to achieve a predetermined temperature at the outlet of the gas cooler, as measured by the third temperature sensor.

71. A system as claimed in any one of claims 67 to 70, further comprising an additional cooling coil arranged adjacent to the gas cooler such that the ambient air that is caused to flow over the gas cooler by the one or more gas cooler fans is thereafter caused to pass over the additional cooling coil, and wherein an inlet of the additional cooling coil is connected to the CO2 refrigeration unit by an additional pressure-reducing valve that is configured such that, when the additional valve is in an open position, liquid CO2 is caused to flow between the CO2 refrigeration unit and the additional cooling coil whereby the liquid CO2 evaporates as air passes over the additional cooling coil, the air being cooled thereby, and wherein an outlet of the additional cooling coil is connected to the one or more compressors within the CO2 refrigeration unit to enable the CO2 vapour to pass thereto.

72. A system as claimed in claim 71, wherein the system is configured such that, when a cooling load on the cooling coil(s) is less than a heating load on the heating coil(s) the additional pressure-reducing valve is opened to thereby cause liquid CO2 to flow between the CO2 refrigeration unit and the additional cooling coil.

73. A system as claimed in any one of claims 65 to 72, comprising the retrofitted HVAC system of any one of claims 1 to 15.

74. A method for operating the HVAC system as claimed in any one of claims 65 to 73, the method comprising operating at least one of the air handling units in a cooling mode and at least one of the other air handling units in a heating mode.

75. A method for operating the HVAC system as claimed in any one of claims 65 to 71, the method comprising operating each of the at least two air handling units in a heating mode.

76. An HVAC system for defrosting a cooling coil, the cooling coil located externally to at least one air handling unit, the HVAC system comprising: a. a CO2 refrigeration unit comprising one or more compressors, one or more valves, a series of pipes and electrical controls; b. at least one air handling unit; c. the cooling coil connected to the CO2 refrigeration unit by a pressure-reducing valve, said valve configured such that, when the valve is in an open position, liquid CO2 is caused to flow from the CO2 refrigeration unit to the cooling coil whereby the liquid CO2 evaporates as air passes over the cooling coil, the air being cooled thereby; d. an outlet of the cooling coil arranged such that, the resultant CO2 vapour passes to the one or more compressors of the CO2 refrigeration unit, wherein the CO2 vapour is compressed; e. a heating coil located within each of the at least one air handling units and connected to the compressor within the CO2 refrigeration unit such that the compressed CO2 vapour passes from the compressor and into the heating coil, wherein the compressed CO2 vapour is cooled as air passes over the heating coil, the air being heated thereby; f. a gas cooler and a three-way valve, the three-way valve being located and connected to selectively pass at least some of the cooled CO2 vapour from the heating coils to the gas cooler such that the at least some of the cooled CO2 vapour is further cooled, an outlet of the gas cooler being arranged to pass the further cooled CO2 vapour to the CO2 refrigeration unit, along with remaining cooled CO2 vapour not selectively passed to the gas cooler via the three-way valve; g. one or more gas cooler fans located adjacent to the gas cooler and configured to cause a flow of ambient air over the gas cooler, wherein the cooling coil is located adjacent to the gas cooler such that, the ambient air that is caused to flow over the gas cooler by the one or more gas cooler fans is thereafter caused to pass over the cooling coil.

77. A system as claimed in claim 76, further comprising one or more pressure and/or electronic sensors located adjacent to the cooling coil configured to measure a presence of frost and/or ice at the cooling coil such that, when frost and/or ice is present, the three-way valve selectively passes all of the combined cooled CO2 vapour from the heating coils to the gas cooler, and a speed of the one or more gas cooler fans is increased to a predetermined fan speed.

78. A system as claimed in claim 76 or 77, further comprising a temperature sensor configured to measure a temperature of the cooled CO2 vapour before the cooled CO2 vapour is passed to the three-way valve such that, when the temperature of the CO2 vapour leaving the heating coil is lower than a predetermined temperature the operating number and/or speed of the one or more compressors is increased, and when the temperature of the cooled CO2 vapour is greater than the predetermined temperature the operating number and/or speed of the one or more compressors is decreased.

79. A system as claimed in any one of claims 76 to 78, comprising two or more air handling units, and wherein the system is configured such that the resultant cooled CO2 vapour from each of the two or more air handling units is combined, and a temperature of the combined cooled CO2 vapour is measured prior to it reaching the three-way valve.

80. A system as claimed in any one of claims 76 to 79, comprising the retrofitted HVAC system of any one of claims 2 to 15.

81. A method for defrosting a cooling coil in an HVAC system comprising CO2 as a refrigerant and at least one air handling unit, the cooling coil located externally to the at least one air handling unit, the method comprising: a- opening a pressure-reducing valve between a CO2 refrigeration unit and the cooling coil, thereby allowing liquid CO2 to flow into the cooling coil, wherein energy is transferred between the liquid CO2 within the cooling coil and air passing over the cooling coil, causing the liquid CO2 to boil and said air to be cooled; b- passing the resultant CO2 vapour to one or more compressors, wherein the CO2 vapour is compressed; c- allowing the compressed CO2 vapour to flow through a heating coil in each of the at least one air handling units, wherein energy is transferred between the compressed CO2 vapour within the heating coil and air passing over the heating coil, causing the vapour to be cooled and said air to be heated; d- selectively passing at least some of the resultant cooled CO2 vapour from the heating coil(s) to a gas cooler located adjacent to the cooling coil; e- increasing a flow of ambient air across the gas cooler, and wherein the gas cooler and the cooling coil are arranged such that the ambient air thereafter passes over the cooling coil; f- combining the further cooled CO2 vapour from the gas cooler with remaining cool CO2 vapour not selectively passed to the gas cooler and passing said combined CO2 vapour to the CO2 refrigeration unit.

82. A method as claimed in claim 81, wherein the presence of frost and/or ice at the cooling coil is measured such that, when frost and/or ice is present, all of the cooled CO2 vapour is passed from the heating coil(s) to the gas cooler and the flow of ambient air is increased to a predetermined level.

83. A method as claim in claim 81 or 82, wherein the HVAC system comprises two or more air handling units, and wherein the method is performed such that the resultant cooled CO2 vapour from each of the two or more air handling units is combined, and a temperature of the combined cooled CO2 vapour is measured prior to reaching the CO2 refrigeration unit.

84. A method as claimed in any one of claims 81 to 83, wherein a temperature of the further cooled CO2 vapour is measured prior to it reaching the CO2 refrigeration unit such that, when the temperature of the further cooled CO2 vapour is lower than a predetermined temperature, the flow of CO2 vapour from the CO2 refrigeration unit into the heating coil is increased and such that, when the temperature of the further cooled CO2 vapour is greater than the predetermined temperature, the flow of CO2 vapour is decreased.

85. A method as claimed in claim 84, wherein when the HVAC system comprises one compressor, the method is performed such that the flow of CO2 vapour is increased by increasing a motor speed of the compressor and decreased by decreasing the motor speed of the compressor.

86. A method as claimed in any one of claims 81 to 84, wherein when the HVAC system comprises more than one compressor, the method is performed such that, the flow of CO2 vapour is increased by increasing a motor speed of the more than one compressors and/or by increasing the number of operating compressors and the flow of CO2 vapour is decreased by decreasing the motor speed of the more than one compressors and/or by decreasing the number of operating compressors.

87. A method as claimed in claim 86, wherein the HVAC system comprises up to fourteen compressors.

88. A method as claimed in any one of claims 81 to 87, wherein the HVAC system using CO2 comprises the retrofitted HVAC system of any of claims 2 to 15.

89. A method as claimed in any one of claims 81 to 87, wherein the HVAC system using CO2 comprises the HVAC system of any one of claims 75 to 78.