GB2621309A

GB2621309A - Heat pump

Info

Publication number: GB2621309A
Application number: GB2209721.6A
Authority: GB
Inventors: Atkinson Michael
Original assignee: Clade Eng Systems Ltd
Current assignee: Clade Eng Systems Ltd
Priority date: 2022-07-01
Filing date: 2022-07-01
Publication date: 2024-02-14
Also published as: GB202209721D0

Abstract

A heat pump circuit 10 for a transcritical CO2 heat pump comprises a main circulation circuit communicating a heat transfer medium 50 to a first discharge heat exchanger 20 (condenser) suitable for heating a first external supply line 14. The main circulation circuit comprises a heat transfer line 57 for the heat transfer medium to pass through a receiver vessel 30 for separation into a liquid phase 51 and a gas phase 41. An evaporator arrangement 71 – 78 and a suction line arrangement processes the heat transfer medium to provide a compressible heat transfer medium for subsequent compression. A compressor arrangement 36, 46, 48 compresses the compressible heat transfer medium before it is passed as a heated medium through the first discharge heat exchanger. The main circulation circuit comprises an off-take 80 (bypass) to supply a defrost circuit to divert at least a portion of the heat transfer medium for heat exchange with one or more evaporators of the evaporator arrangement. The heat pump circuit allows individual evaporators to be defrosted for limited periods of time and reduces downtime for defrosting.

Description

Heat Pump

Field of the Invention

The present invention relates to a heat pump (HP) system and a method of operation thereof. More specifically, the invention relates to a transcritical CO2 heat pump, and more specifically to a circuit arrangement that is thought to result in better operating efficiency at least in certain application scenarios.

Background

Heat pumps operate by controlled use of compression and expansion of a heat transfer medium, and may be used for warming or cooling in households, commercial sites and in industry. A general principle of operation is to allow a heat transfer medium to expand and/or to compress it within a recirculation circuit, in a controlled sequence, to allow it to absorb ambient heat (e.g. for cooling) and to release energy to regenerate cooling capacity of the heat transfer medium and/or to warm fluids such as household water or service water. Transcrifical carbon dioxide (CO2) heat pumps may be operated in a transcritical mode, or supercritical mode, particularly during heat rejection, avoiding condensation issues during that phase of a cycle, however there may be challenges with regard to temperature and pressure conditions that are suitable for absorbing heat from ambient air.

Depending on ambient temperatures and required supply water temperatures, different performance characteristics may be achieved by positioning and dimensioning heat exchangers along a heat pump circuit.

The present invention seeks to provide an improved heat pump circuit design.

Summary of the Invention

In accordance with a first aspect of the invention, there is disclosed a heat pump circuit as defined by claim 1, operable as a transcritical CO2 heat pump. The circuit comprises a main circulation circuit comprising a first discharge heat exchanger, the main circulation circuit being of the type supplying a heat transfer medium to the a first discharge heat exchanger for heating a first external supply line, the main circulation circuit comprising a heat transfer line for the heat transfer medium to pass through a receiver vessel for separation into a liquid phase and a gas phase, an evaporator arrangement and a suction line arrangement for processing the heat transfer medium to provide a compressible heat transfer medium for subsequent compression, and a compressor arrangement for compressing the compressible heat transfer medium before it is passed as heated medium through the first discharge heat exchanger, wherein the main circulation circuit comprises an off-take to supply a defrost circuit to divert at least a portion of the heat transfer medium for heat exchange with one or more evaporators of the evaporator arrangement The first discharge heat exchanger may be understood to correspond to a gas cooler to remove heat from the heat transfer medium. Depending on the application scenario, a distinction may be made whether the first external supply line is intended primarily to warm external process water for subsequent use, or primarily as a discharge line to remove heat energy from the main circulation circuit, although it will be understood that, depending on its installation, the heat pump may be used for heating and/or cooling. The heat transfer medium may also be referred to as a refrigerant.

The evaporator arrangement is understood, typically, to evaporate or process the heat transfer medium to provide a gas phase, or a medium with sufficient gas content to provide a compressible heat transfer medium. Depending on the arrangement and operating conditions, heat transfer medium may be blended in from the suction line. The compressor is understood to compress the heat transfer medium, thereby increasing its temperature that may be used for heating water or other secondary fluid as may be fed via the first external supply line.

The provision of a defrost circuit allows heat transfer medium to be used to transfer heat energy to an evaporator, which may assist in reducing the amount of built-up ice, or "defrosting". It will be appreciated the process may not necessarily result in a complete de-icing of evaporator components. Defrosting is of interest because evaporators experience a temperature reduction during the evaporation/expansion process. The temperature reduction may be sufficient to cause a build-up of frost, from humidity in ambient air, on evaporator equipment. The system may be used with a plurality of individual evaporators operating in parallel. The evaporators may be individually supplied by heat transfer medium.

It will be appreciated that the defrost circuit is arranged to direct heat transfer medium into proximity with the evaporator equipment, sufficiently close for a heat transfer to the evaporator equipment, resulting in a defrosting effect, frost-reducing effect, frost-preventing effect, or frost-slowing effect in which the formation of frost is slower than in would otherwise be.

In some embodiments, the defrost circuit comprises an arrangement of defrost radiators, the defrost radiators arranged to permit heat exchange with the evaporators.

Conveniently, the defrost circuit comprises one or more radiator arrangements to improve the heat transfer to the evaporators. The radiator arrangements may comprise a larger surface compared to a straight, cylindrical pipe section.

In some embodiments, the defrost radiators are of coiled form.

The coils may, for practical purposes, be understood as constituting heat exchange surfaces. It will be understood that energy of the heat transfer medium in the form of heat passes to the evaporators, assisting with a phase transition (melting) of ice and/or defrosting the evaporators.

In some embodiments, at least one or more defrost radiators are operable independently of other defrost radiators.

The defrost radiators may be operable, for instance by way of defrost line flow control valves that stop or permit flow of heat transfer medium through the defrost radiators. In this manner, one or more individual defrost radiators may be in an open condition, as open defrost lines, that may be used to transfer energy for defrosting some of the evaporators, while other defrost radiators may be in a closed condition, as closed defrost lines, that are not in use to transfer energy for defrosting evaporators. In embodiments, the mass flow through a defrost radiator may be controllable by a flow control valve although in a simple embodiment the defrost radiators may operate either fully open or fully shut.

In this manner, the degree of defrosting can be modulated differently for one or more evaporators. As such, the defrosting can be controlled individually for each evaporator, or for groups of evaporators.

In some embodiments, one or more defrost radiators are operable sequentially to thereby defrost evaporators sequentially.

This allows a plurality of evaporators, or each evaporator, to be supplied by the defrost circuit, while avoiding that an excessive amount of heat transfer medium is diverted from the main circulation circuit. It was an appreciation by the applicants that an intermittent, low mass (low volume) diversion of heat transfer medium may suffice for practical purposes to achieve an effective evaporator defrosting. This is believed to be the case because the formation of frost takes time, and usually takes place over a longer time frame than intermittent defrosting, and so an intermittent defrosting of individual evaporators, the individual evaporators being defrosted in turn, enables for practical purposes a defrosted operation of all evaporators.

In some embodiments, two or more defrosters of the defroster arrangement are controllable by a common flow controller.

As may be imagined, two or more defrosters (or defrost lines), may be grouped, for instance via a manifold, to be controllable by a single actuator, e.g. by a single flow controller such as a zone valve or flow control valve. This may reduce the amount of components required for installation and servicing.

In some embodiments, the off-take is located in the main discharge circuit downstream of the first discharge heat exchanger.

Thereby, the off-take is supplied with heat transfer medium at discharge pressure, i.e. in a condition after leaving the discharge heat exchanger. As such, the off-take may be supplied directly from the discharge line, after/downstream of the discharge heat exchanger and upstream of the receiver vessel. The off-take may be supplied upstream of a suction line heat exchanger.

In some embodiments, the heat pump circuit comprises a configuration allowing it to increase the compression work of the compression arrangement to compensate for a loss in suction pressure.

The diversion of some of the heat transfer medium into the defrost circuit may result in correspondingly less heat transfer medium in circulation in the remaining main circulation circuit, relative to a circuit without defrost line, and/or may result a correspondingly lower suction pressure. However, provided the diverted mass flow per time unit is not too high, for instance in the region of 10% to 15% of the outflow downstream of the first discharge heat exchanger, the drop in suction pressure (at a given constant compressor performance) has been found to be relatively modest, such that it can be mitigated by an increase in compressor performance. The increased compressor performance may be achieved by increasing the compressor operating frequency, e.g. form 50 Hz to 60 Hz. The increased compressor performance may be achieved by switching on one or more additional compressors and/or supply from the receiver vessel. The compressor performance may be increased for short periods of time.

In some embodiments, the heat pump circuit comprises a sensor arrangement configured to provide an evaporator condition output indicative of a frost condition for one or more of the evaporators of the evaporator arrangement.

Alternatively, the evaporator condition output may be indicative of a defrost demand of the evaporators.

In some embodiments, the sensor arrangement comprises individual sensors for each evaporator.

Alternatively, the sensor functionality may be provided by a sensor arrangement capable of monitoring multiple evaporators.

The sensor arrangement may be a temperature sensor or other suitable sensor for determining the temperature or degree of frost formation on the evaporators. The sensors may be configured as part of a control system to provide an output, in the form of an evaporator condition output value. The output (such as an output value or output indicator) may be used to determine whether or not the evaporators require defrosting and/or require a supply of heat energy. The temperature sensor may be arranged to measure the surface temperature of evaporator equipment. The temperature sensor may measure the ambient (air) temperature, in a location near the evaporator equipment that allows a determination to be made about frost conditions and/or defrost demand of the evaporator equipment.

In some embodiments, the heat pump circuit comprises a configuration to process the evaporator condition output (from the sensors) and to operate the defrost circuit to divert heat transfer medium towards one or more evaporators.

In some installations, some evaporators may be more prone to frost formation than others. In that case, the defrost circuit may comprise a configuration allowing it to be operated to increase a supply of defrosting medium (in the form of heat transfer medium diverted from the main circulation circuit) to evaporators with a higher defrost demand.

Likewise, the sensor arrangement may be used to monitor a defrost progress, or to monitor a defrosted condition, for each one or groups of evaporators. In this manner, the defrost circuit may be operated (e.g., by closing some defrost lines and opening other defrost lines) to stop defrosting one evaporator as soon as there is no defrost demand for it, and to start defrosting another evaporator.

The sensor arrangement may, therefore, be operated as a closed feedback control loop in which defrosting is carried out for evaporators requiring defrosting, as long as there is a defrost demand for an evaporator. Once there is no defrost demand for an evaporator, the system may supply defrost heat transfer medium to another evaporator.

As such, the heat pump circuit may supply different amounts of heat transfer medium to different evaporators.

In some embodiments, the heat pump circuit comprises a configuration to increase the compression work of the compression arrangement depending on the amount of heat transfer medium diverted to the defrost circuit.

In some embodiments, the heat pump circuit comprises a configuration to increase the compression work of the compression arrangement depending on a suction line pressure of the main circulation circuit.

The configuration may be provided in the form of pressure level sensor arrangement. The pressure level sensor arrangement may obtain pressure level values indicative of the line pressure in the main circulation circuit, and/or in the suction line. The pressure level value maybe compared, by a control system, to a target pressure level. If the pressure level value is below the target pressure level, the compression arrangement may be controlled to increase the compression power. The configuration may be active while a defrost line is open. To provide an illustrative example of the type of pressure sensor used, a suitable sensor may be a Danfoss AKS 3000 sensor capable for sensing pressure levels of up to 60 bar suction pressure. The sensor may be of a type provided in a sealed enclosure to reduced influence from atmospheric pressure fluctuations. However, other sensors may be used.

In some embodiments, the heat pump circuit comprises a second discharge heat exchanger for heating a secondary external supply line, the second heat exchanger being located in the main circulation circuit.

An appreciation following the development of the invention was that the heat transfer medium may be used to heat a secondary external supply, and still be suitable for defrosting. Depending on the mass flow and heating requirements, individual defrost lines may need to be modulated (e.g. by opening and closing), in combination with appropriately controlling compressor performance, to maintain a desired suction pressure.

In some embodiments, the second discharge heat exchanger is located in the main circulation circuit downstream of the first discharge heat exchanger.

In some embodiments, the second discharge heat exchanger is located in the main circulation circuit upstream of the off-take.

In some embodiments, the heat pump circuit comprises a suction line heat exchanger for heat exchange between heat transfer medium downstream of the off-take and the heat transfer medium supplied to the compressor arrangement.

Description of the Figures

Exemplary embodiments of the invention will now be described with reference to the Figures, in which: Figure 1 shows a schematic diagram of a heat pump circuit; and Figure 2 shows a schematic diagram of another heat pump circuit.

Description

Referring to Figure 1, a heat pump system comprises a heat pump circuit 10 constituting a main circulation circuit. The circuit 10 comprises a first supply-discharge circuit 12 comprising an external supply line 14 for an external medium such as, for example, household water, to be heated. Alternatively or concurrently, the first supply-discharge circuit 12 may be considered a discharge circuit for regenerating cooling capacity of heat transfer medium in the circuit 10. The external supply line 14 comprises a pump 16 to supply a fluid provided to absorb heat, such as water, in a condition P12 towards a main heat exchanger 20, constituting a first discharge heat exchanger of the invention. Fluid passing through the main heat exchanger 20 leaves the supply-discharge circuit 12 via the discharge line 18 in a condition P11 and is discharged for subsequent use. The main heat exchanger is understood to be constituted by a gas cooler if the circuit 10 is operated as a transcritical CO2 heat pump.

In the description below, certain temperature and pressure values will be omitted and it will be understood that some lines operate a liquid phase medium, other lines operate a gas phase medium, and other lines may operate in a supercritical fluid phase. Depending on compression or expansion work, a heat transfer medium may transition between the phases, and/or a mixture of gas and liquid phase may be present. The heat pump circuit 10 is an example of a transcritical CO2 heat pump (or transcritical heat pump system) and may as such be operated in a manner in which CO2 as heat transfer medium remains in a transcritical condition when passing through the main heat exchanger 20. The advantage of using transcritical CO2 as heat transfer medium is that condensation can be avoided while operating in a transcritical condition and therefore higher temperatures can be achieved compared to subcrifical systems.

The heat pump circuit 10 operates by generating a supply of heated medium 50 constituting a discharge supply of a heat transfer medium for the main heat exchanger 20, provided via a compression arrangement of compressors 36, 46, 48 (described below) that are operable to bring the heat transfer medium into a On the case of CO2: transcritical) compressed condition P3, to be supplied into the main heat exchanger 20 for heat exchange with the external medium, and to exit the main heat exchanger 20 in a cooled condition P5. To provide illustrative values, the temperature of the external medium (e.g. water) in the condition P12 may be in the region of 29°C and the temperature of the warmed external medium in the condition P11 may be in the region of 60 °C. The heated medium 50 (e.g. CO2) in the compressed condition P3 may have a temperature in the region of 114°C, and may have a temperature in the region of 32°C in the condition P5, downstream of the main heat exchanger 20 (wherein a temperature above about 31°C of the cooled medium is expected for a CO2 medium to operate in a transcritical range). Generally, the circuit 10 may be operated such that the condition P11 is at least 30°C above, and preferably at least 40°C above, the condition P12.

After leaving the main heat exchanger 20, the heat exchange medium is circulated as cooled medium 56 back into a liquid receiver 30 (constituting a receiver vessel), however its caloric properties may be used for pre-warming or pre-cooling other parts of the circuit 10. After the main heat exchanger, the cooled medium 56 has a discharge pressure. It will be appreciated that the cooled medium 56, particularly in the case of CO2 as refrigerant, may be considered a high pressure fluid. The cooled medium 56 may be supercritical. In some situations the cooled medium 56 may be liquid. Downstream of the main heat exchanger 20, the heat exchange medium, now constituted by cooled medium 56, passes an off-take point 80 that will be described below. The circuit 10 may be operated such that most of the cooled medium 56 bypasses the off-take point 80. For instance, about 85-90% of the cooled medium 56 may bypass the off-take point 80. The circuit may be operated such that during some periods of time all of the cooled medium 56 bypasses the off-take point 80, and during other periods of time most or all of the cooled medium 56 is diverted via the off-take point 80.

The remainder of the cooled medium 56 downstream of the off-take point 80, in an area indicated by line 57, may have residual heat (in the region of 32°C, corresponding to the condition P5) and supplies a suction line heat exchanger 44. The heat transfer medium exits the suction line heat exchanger 44 in a condition P8, having about 25 °C, as a refrigerant 58 for a internal heat exchanger 60, leaving the internal heat exchanger 60 as return medium 59 a condition P6, having about 22 °C to 24°C, before being returned into the liquid receiver 30. The liquid receiver 30 constitutes a receiver vessel and is understood to be a gas/liquid separator of the type used in heat pump systems, in which the heat transfer medium may absorb ambient heat and may in the process transition between a gas phase 41 and a liquid phase 51 before being drawn as heat transfer medium supply for another heat pump cycle.

As illustrated in Figure 1, the liquid receiver 30 comprises three receiver offtake lines 32, 42 and 52. The first receiver offtake line 32 draws fluid from the gas phase 41 via the internal heat exchanger 60 after heat exchange with the refrigerant 58, as cooled refrigerant 34 in a condition P9, having about 27 °C, for supply to the parallel compressor 36. The second receiver offtake line 42 may be used for drawing fluid (flash gas) from the gas phase to a two-phase condition P7, at about -4 °C that may subsequently be mixed with the gas flow exiting the evaporator. The third receiver offtake line 52 draws heat transfer medium in liquid form in a condition P10, having about 3°C, as supply to an evaporator array comprising a plurality of (here: eight) evaporators 71, 72, 73, 74, 75, 76, 77, 78, the supply of heat transfer medium from the offtake line 52 to each one of the evaporators 71-78 being controllable by a respective evaporator flow controller 61, 62, 63, 64, 65, 66, 67, 68. It will be appreciated that, in the case of CO2 as medium, expansion in the evaporators 71-78 is of dissipative nature, i.e. the expansion occurs without requiring work input or external energy supply. After the evaporator stage, as illustrated in Figure 1, the heat transfer medium has evaporated and is present as a gas phase medium, in a condition to be supplied to a compressor downstream. It will be appreciated that, depending on operating conditions and type of installation, some liquid content may remain in the form of droplets. It would not be unusual to observe in the region 10-20% liquid content.

Furthermore, the arrangement may be used in so-called flooded systems in which liquid phase exists the evaporator, for combination with gas phase heat transfer medium from the suction line to provide a compressible heat transfer medium for subsequent compression at the compressor.

As shown in Figure 1, the evaporators 71-78 are arranged in parallel such that each one of the evaporators 71-78 is provided with a supply of heat transfer medium directly from the third receiver offtake line 52, the supply from the third offtake line 52 to the evaporators 71-78 being individually controllable by their respective evaporator flow controller 61-68. By way of the controllers 61-68, supply of heat transfer medium to selected ones of the evaporators 71-78 may be stopped temporarily, for instance to allow one or more evaporators to defrost by exposure to ambient air (air temperature).

The outlet of each evaporator 71-78, having a condition P14 (i.e. the output of the evaporator 71 has a condition P14-1, of the evaporator 72 has a condition P14-2, etc.), in gas phase merges with the second receiver offtake line 42 which is in a two-phase condition. The heat transfer medium exiting the evaporators 71-78 constitutes an evaporated medium having turned into a gas phase On the case of 002, forming a superheated gas phase), having a condition P14, typically in a region of about 3 'C. It will be understood that the condition downstream of each evaporator 71-78 may differ depending on evaporator performance, frost condition, application scenario and other conditions. To provide illustrative values of typical temperatures, the condition P14-1 after the evaporator 71 may be about 3.9 °C, P14-2 after evaporator 72 may be about 3.5 °C, P14-3 after evaporator 73 may be about 4.3 °C, P14-4 after evaporator 74 may be about 2.7 °C, P14-5 after evaporator 75 may be about 2.7 °C, P14-6 after evaporator 76 may be about 2.3 °C, P14-7 after evaporator 77 may be about 2.8 °C, and P14-8 after evaporator 78 may be about 2.9 °C, i.e. at a similar level with some variation between evaporators.

After the evaporator stage and after the suction line heat exchanger stage, the (evaporated, gas phase and/or blended) heat transfer medium is passed through, depending on the capacity requirements, one or more compressors, which may be operating in parallel, the one or more compressors including (here) a second compressor 46 and/or third compressor 48, to be compressed to heat it to a temperature condition P3 suitable as heated medium 50 as a discharge supply, having a temperature sufficiently high above the temperature of the external medium supplied via the first (and/or second, if applicable) external supply line, for supply to the main heat exchanger 20. In a transcritical CO2 circuit, the heat transfer medium is understood to be in a supercritical phase such that it remains supercritical after leaving the main heat exchanger 20 as a cooled medium 56.

The circuit 10 comprises a defrost circuit 22 supplied via the off-take point 80. Cooled medium 56 may be diverted from the main circulation circuit into the defrost circuit 22. To provide an illustrative value, about 10 % to 15 % of the cooled medium 56 may be diverted into the defrost circuit 22, the remaining 85 % to 90 % bypassing the off-take point 80 via the line 57. The heat transfer medium diverted into the defrost circuit 22 may have a condition P16 that may be practically identical to the condition P5.

The defrost circuit 22 comprises an array of (here: eight) defrost coils 81, 82, 83, 84, 85, 86, 87, 88, each defrost coil constituting a defrost radiator. In the example configuration shown herein, the defrost coils 81-88 are arranged in pairs, such that one defrost line of the defrost circuit 22 supplies two defrost coils (and four defrost lines supply eight defrost coils). The defrost lines are each controllable by an individual defrost line valve, which may be considered a secondary high pressure valve, in the form of a defrost line flow controller 90, 92, 94, 96, each operable to open a line to permit flow of heat transfer medium through the respective defrost coils 81-88, or to be closed to stop flow of a heat transfer medium through the defrost coils 81-88.

The defrost lines are arranged for parallel operation, such that each defrost line may be provided with supply of heat transfer medium, independently of a supply to other defrost lines, directly from the off-take point 80. A first pair of defrost coils 81 and 82 is controllable by a first defrost line flow controller 90. A second pair of defrost coils 83 and 84 is controllable by a second defrost line flow controller 92. A third pair of defrost coils 85 and 86 is controllable by a third defrost line flow controller 94. A fourth pair of defrost coils 87 and 88 is controllable by a fourth defrost line flow controller 96. The defrost line flow controllers 90-96 allow the pressure at the defroster outlet to be reduced close to and above the pressure present in the liquid receiver 30. For instance, the pressure at the defroster outlet may be no more than 1, 3, 5, 7 or 10 bar above the pressure in the liquid receiver 30.

Although illustrated in Figure 1 as being spatially separated from the evaporators 7178, it will be appreciated that the defrost coils 81-88 are arranged sufficiently close, and potentially along and/or around, a respective evaporator 71-78 to allow a heat transfer between a defrost coil and an evaporator to take place. The defrost line flow controllers 90 to 96 may be provided in any suitable form such as a flow control valve or otherwise.

It will be appreciated that upon opening of one or more of the defrost line flow controllers 90, 92, 94 or 96, heat transfer medium may pass via the off-take point 80 through some or all of the defrost coils 81-88 that are open, and via a return line 98 into the liquid receiver 30. As such, heat transfer medium diverted into a defrost line is returned into circulation via the liquid receiver 30. Cooled medium 56 that is not diverted into the defrost circuit 22 is understood to circulate downstream of the off-take point 80 via line 57 towards the suction line heat exchanger 44 and other components downstream before being returned to the liquid receiver 30. While all defrost line flow controllers 90-96 are closed, all cooled medium 56 bypasses the off-take point 80.

The evaporator flow controllers 61-68, upstream of the evaporators 71-78, allow the flow of heat exchange medium to each individual evaporator 71-78 to be modulated and/or blocked. As such, the circuit 10 may be operated to reduce or stop flow through one or more evaporators 71-78 while they are being defrosted. Defrosting may, therefore, be effected by 'ambient' defrosting, relying on the atmosphere (air) temperature at the evaporators, or via a supply of heat transfer medium from the defrost circuit 22. As such, the system may be operated in different defrost modes.

For instance, the defrost line flow controller 90 may be open such that flow is permitted through the defrost coils 81 and 82 (here: supplied as a pair), while the defrost line flow controllers 92,94,96 are closed, resulting in no flow in the defrost coils 83-88. The evaporator flow controllers 61 and 62 are closed to stop flow through the evaporators 71 and 72 while they are being defrosted, while the evaporator flow controllers 63-68 remain open to continue evaporation via the evaporators 73-78.

In this manner, operation of the defrost line flow controllers 90-96 and of the evaporator flow controllers 61-68 can be coordinated to defrost each evaporator in turn, or pairs or groups of evaporators in turn, without affecting operation of one or more other evaporators. A control system may operate such that at any given time at least 20%, 25%, 30%, 40%, 50%, 60%, 70%, 75%, 80%, 85% or 90%, of the number of evaporators, or of the evaporator capacity of a multi-evaporator array, is operating to expand and/or evaporate, respectively, heat transfer medium.

A diversion of heat transfer fluid via the defrost coils 81-88 may affect the mass flow and pre-heating performance at the suction line heat exchanger 44 and downstream components. Therefore, the circuit 10, and/or the defrost circuit 22 within it, may be operated to open only one or few of the defrost lines via their respective defrost line flow controllers 90-96 at any time. For instance, the defrost line flow controllers 90-96 may be opened sequentially to ensure that only one, or only two, controllers of the defrost line flow controllers 90-96 is/are open at a time.

Furthermore, by operating (i.e. maintaining open) only some of the defrost lines at a time, some -and usually a majority of -the evaporators are not being defrosted, and are therefore operable to continuously evaporate or process heat transfer medium for supply to one or more compressors. Thereby, the circuit 10 can tolerate the temporary inactivity of some evaporators for defrosting without this affecting the continuous operation of the circuit, practically eliminating the need for defrosting downtime of the entire system.

The circuit 10 may be operated to increase the power, or compression work, of one or more of the first compressor 36, the second compressor 46 and/or the third compressor, to increase the transfer medium mass flow rate and thus achievable capacity. While defrosting a pair of evaporators 71-72, 73-74, 75-76 or 77-78 may decrease the suction pressure, this may be counterbalanced by increasing the compressor performance. This may be achieved by activating one or more additional compressors of the available compressors 46, 48 and 36 and/or by increasing individual compressor power (speed). In that manner, the desired heating capacity can be delivered at the main heat exchanger 20.

The arrangement of the circuit 10 achieves that the evaporators 71-78 of the evaporator arrangement can be defrosted using a portion of the heat transfer fluid after it had been used to supply the main heat exchanger 20. To provide an illustrative example, about 10% to 15% of the mass flow may be diverted, although it will be appreciated the amount may depend on several factors including heat demand (or cooling capacity regeneration demand), ambient humidity and frost build-up, as well as size, surface area and number of evaporators and other factors. The energy transferred from the defrost coils 81-88 to the evaporators 71-78 may remove frost that may have formed on the evaporator structures. While defrosting a pair of evaporators 71-78 may slightly decrease the suction pressure, it is believed that this also reduces the temperature of the return medium 59, reducing an overall efficiency loss. If required, the desired heating capacity can also be delivered by controlling the compressors 46, 48 and 36.

For these reasons, after taking into account efficiency gains in other parts of the system, it is believed that efficiency losses caused by diverting heat transfer medium via the off-take point 80 are not much, indeed it is believed that efficiency losses are difficult to quantify. In addition, the arrangement avoids the need for auxiliary external defrosting systems and avoids the need for defrosting system downtime for reasons set out above. Furthermore, the arrangement is suitable for a closed-loop control using temperature sensors and/or frost sensors to control operation (i.e. fluid passage through) the defrost coils 81-88. For instance, it may be imagined that depending on the installation and site characteristics, one or more of the evaporators may be more prone to frost formation than others. In that case, a feedback loop such as the temperature sensor based feedback loop suggested herein may be used to activate the defrost coils for the particular frost-affected evaporators, to achieve an even degree of defrosting across all evaporators, to be able to address different levels of defrost demand. As may be imagined, the use of a closed loop feedback facilitates approximating an optimised amount of defrost medium supply and exposure time for each evaporator.

The illustrative arrangement of Figure 1 uses an array of eight evaporators and eight defrosters, the defrost lines being controllable in pairs. An array of eight evaporators is believed to strike a good balance between the complexity of an installation and complexity of its control, and the granularity of individual defrosting operation for a quarter, or eighth, respectively, of the total evaporator capacity. However, embodiments may use any number of evaporators, including fewer or more evaporators. Likewise, the number of defrosters (defrost lines) may be selected to match the number of evaporators. Alternatively, the circuit 10 may use fewer defrosters, as may be imagined, in an arrangement in which a defroster is used for heat transfer to multiple evaporators. This may be a practical arrangement for installations that permit the location of defrost coils sufficiently close to the evaporators for the required heat transfer Conversely, a circuit 10 may use more defrosters, for instance to provide one or more evaporators with multiple defrosters. However, it has been found that a larger number of defrosters provides a greater degree of control freedom for modulating the fluid medium supply, and to control the mass flow diverted from the main circulation circuit. Likewise, multiple defrost lines may be controlled by a common flow controller (as illustrated in exemplary paired embodiments) or each defrost line may be controlled by an individual defrost line flow controller.

As a further alternative, the system may be used with a single evaporator, or may be configured to defrost all evaporators simultaneously. In that case, to avoid a system downtime due to evaporator inactivity during a defrosting operation, the heat pump circuit 10 may comprise a buffer tank (not shown in the Figures) to feed the compressors during evaporator inactivity. By using a sequential defrosting method, the need for a buffer tank is avoided.

Figure 2 shows another heat pump circuit 110 that is a variant of the Figure 1 heat pump circuit 10. Elements that have been described in relation to Figure 1 are reproduced with the same numerals in Figure 2 without repeating the description thereof. The circuit 110 comprises a second discharge heat exchanger 120 in the main circulation line, downstream of the first discharge heat exchanger 20, upstream of the take-off point 80. The second discharge heat exchanger 120 may be used to heat an external supply medium of a secondary external supply line 112. The Figure 2 arrangement is thought to be useful where more than one sources of an external supply require heating. Similarly to Figure 1, the heat pump circuit 110 provides an off-take point 80 for supply of cooled medium 56 to a defrost circuit, which allows the cooled medium 56 to be supplied at discharge pressure and after having passed the first heat exchanger 20 and the second heat exchanger 120 The elements described in relation to the heat pump circuit, such as flow controllers, valves, pumps/compressors and evaporators, as well as sensors, may be of any suitable form and a range of suitable components will be known to a skilled person.

The elements may be controlled and/or controllable remotely and/or by wire command. Some of the control elements may be manually actuatable.

The operation of the heat pump circuit may be controlled by an arrangement of one or more processors, located locally or remotely, implementing functionality of the system in the form of software instructions.

Whilst the principle of the invention has been illustrated using exemplary embodiments, it will be understood that the invention is not limited to exemplary embodiments and that the invention may be embodied by other variants defined within the scope of the appended claims.

Claims

CLAIMS: 1. A heat pump circuit operable as a transcrifical CO2 heat pump, the circuit comprising a main circulation circuit comprising a first discharge heat exchanger, the main circulation circuit being of the type supplying a heat transfer medium to the first discharge heat exchanger for heating a first external supply line, the main circulation circuit comprising a heat transfer line for the heat transfer medium to pass through a receiver vessel for separation into a liquid phase and a gas phase, an evaporator arrangement and a suction line arrangement for processing the heat transfer medium to provide a compressible heat transfer medium for subsequent compression, and a compressor arrangement for compressing the compressible heat transfer medium before it is passed as heated medium through the first discharge heat exchanger, wherein the main circulation circuit comprises an off-take to supply a defrost circuit to divert at least a portion of the heat transfer medium for heat exchange with one or more evaporators of the evaporator arrangement.
2. The heat pump circuit according to claim 1, wherein the defrost circuit comprises an arrangement of defrost radiators, the defrost radiators arranged to permit heat exchange with the evaporators.
3. The heat pump circuit according to claim 2, wherein the defrost radiators are of coiled form.
4. The heat pump circuit according to claim 2 or 3, wherein at least one or more defrost radiators are operable independently of other defrost radiators.
5. The heat pump circuit according to claim 4, wherein one or more defrost radiators are operable sequentially to thereby defrost evaporators sequentially.
6. The heat pump arrangement according to any one of claims 2 to 5, wherein two or more defrosters of the defroster arrangement are controllable by a common flow controller.
7. The heat pump circuit according to any one of the preceding claims, wherein the off-take is located in the main circulation circuit downstream of the first discharge heat exchanger.
8. The heat pump circuit according to any one of the preceding claims, comprising a configuration allowing it to increase the compression work of the compression arrangement to compensate for a loss in suction pressure.
9. The heat pump circuit according to any one of the preceding claims, comprising a sensor arrangement configured to provide an evaporator condition output indicative of a frost condition for one or more of the evaporators of the evaporator arrangement.
10. The heat pump circuit according to claim 9, wherein the sensor arrangement comprises individual sensors for each evaporator.
11. The heat pump circuit according to claim 9 or 10, comprising a configuration to process the evaporator condition output and to operate the defrost circuit to divert heat transfer medium towards one or more evaporators.
12. The heat pump circuit according to claim 11, comprising a configuration to increase the compression work of the compression arrangement depending on the amount of heat transfer medium diverted to the defrost circuit.
13. The heat pump circuit according to claim 11 or 12, comprising a configuration to increase the compression work of the compression arrangement depending on a suction line pressure of the main circulation circuit.
14. The heat pump circuit according to any one of the preceding claims, comprising a second discharge heat exchanger for heating a secondary external supply line, the second heat exchanger being located in the main circulation circuit.
15. The heat pump circuit according to claim 14, wherein the second discharge heat exchanger is located in the main circulation circuit downstream of the first discharge heat exchanger.
16. The heat pump circuit according to claim 14 or 15, wherein the second discharge heat exchanger is located in the main circulation circuit upstream of the off-take.
17. The heat pump circuit according to any one of the preceding claims, comprising a suction line heat exchanger for heat exchange between heat transfer medium downstream of the off-take and the heat transfer medium supplied to the compressor arrangement.