GB2532439A

GB2532439A - Improved air-source heat pump

Info

Publication number: GB2532439A
Application number: GB1420490.3A
Authority: GB
Inventors: Forbes Pearson Stephen
Original assignee: Star Refrigeration Ltd
Current assignee: Star Refrigeration Ltd
Priority date: 2014-11-18
Filing date: 2014-11-18
Publication date: 2016-05-25
Also published as: GB201420490D0

Abstract

A heat pump system comprises an air source heat pump 100 with an evaporator 102 in heat exchange with ambient air 104 and a condenser 106 in heat exchange with a target fluid 116 to heat a target such as water for space heating or domestic hot water, a defrost system to defrost the evaporator 102 using the target fluid 116 as a heat source for defrosting, and an alternative heat source 128 to heat the target fluid when the target fluid is used as a heat source. The system may comprise a valve 130 to operate the heat pump in reverse to defrost the evaporator 102. The alternative heat source 128 may be a gas or oil fired boiler and may compensate for the reduction in temperature of the target fluid when the target fluid 116 is being used to defrost the evaporator 102, or to supply additional heat to the target fluid 116 when the heat pump 100 cannot provide the required heating duty for the target fluid to heat the target. The defrost system may be initiated in response to a sensor measuring air temperature or a pressure drop across the evaporator.

Description

Improved Air-Source Heat Pump

FIELD OF DISCLOSURE

The field of disclosure relates to a heat pump system, and in particular to an air-source heat pump system.

BACKGROUND TO THE DISCLOSURE

A typical heat pump operates by permitting a working fluid, such as a refrigerant, to vaporise in an evaporator thus absorbing heat from a heat source, where-after the vaporised working fluid is then compressed and delivered as a hot vapour to a condenser where the fluid is condensed, thus releasing heat at a target location. Following this the condensed working fluid is passed through an expansion valve and delivered at reduced pressure to the evaporator for the thermodynamic cycle to be repeated. In this way, heat may be transferred from the heat source to the target location to be used in, for example, space heating, water heating, providing heat to a process or the like. The available heat produced at the target location is a quantity that equals the heat absorbed from the heat source plus the energy required to drive the compressor. Although energy required to drive the compressor may be significantly less than the energy released from the condensing working fluid, it is still recognised that this energy driving the compressor is high grade energy. It is therefore desirable to maximise the efficiency and use of such high grade energy in any heat pump design. Air source heat pumps, in which the evaporator component of the heat pump is provided as an air cooler and arranged in heat exchange with ambient air, are superficially attractive because air is universally available at zero cost. However, air-source heat pumps suffer from the disadvantage that their efficiency decreases as ambient temperature falls, just when more heat is required. Also, some form of defrosting will eventually be required as ambient temperature falls. As frost forms on the air cooler, the air flow through it becomes restricted. The heat pump will cease to function when the air cooler becomes blocked. Therefore, in order to keep the heat pump functioning at low ambient temperatures, it is necessary to provide some form of defrosting to the air cooler.

A form of defrosting known as reverse cycle defrosting is currently employed, in which the flow of hot working fluid normally sent to the condenser is instead redirected to the evaporator where the hot fluid condenses and provides large quantities of heat, thus raising the evaporator to temperatures at which frost is rapidly melted. Thus, during the reverse cycle defrosting, the evaporator temporarily functions as the condenser, and as such the condenser at the target heating location temporarily functions as an evaporator or heat source. This has the undesirable effect of cooling the target location.

Legislators have expressed the effectiveness of a heat pump in terms of a coefficient of performance (COP). Coefficient of Performance is defined as the ratio of the heat output to the electricity used, at defined source and heat flow temperatures. The COP of an air source heat pump will vary throughout the year as ambient temperature varies. Legislators have further expressed the effectiveness of a heat pump in terms of a seasonal performance factor (SPF). Seasonal performance factor is defined as the ratio of heat delivered to the total electrical energy supplied over the year.

The competitiveness of a heat pump in terms of running cost depends on the ratio of cost of electricity to cheapest available fuel for heating. By way of example, currently in most parts of the United Kingdom the cheapest available fuel is piped natural gas. Cost ratios indicate that, without subsidy, the break-even SPF would be around 3 in the United Kingdom. A seasonal performance factor of over 3 is readily available for most of the year in the United Kingdom but it is difficult to achieve when ambient temperatures fall to about +4°C. The United Kingdom currently operates a Renewable Heat Incentive scheme which subsidises the cost of heat produced by renewal heat sources. In 2014, the effect of the UK subsidy of 2.5p/kWh of heat provided by the heat pump contributes to make the operation at ambient temperatures below +4°C economic but at the cost of lower efficiency and a requirement for some form of defrost system.

In order to avoid subsidising inefficient air-source heat pump systems, it has been specified that the SPF should be 2.5 or over, to be eligible for the subsidy through the Renewable Heat Incentive scheme. However, the instantaneous COP is allowed to fall below 2.5 in very cold weather provided the SPF does not fall below 2.5.

SUMMARY

According to the present disclosure, there is provided a heat pump system, comprising: a heat pump having an evaporator to be arranged in heat exchange with a primary heat source and a condenser to be arranged in heat exchange with a target fluid; and a defrost system configured to defrost the evaporators using the target fluid as a defrost heat source.

The primary heat source may comprise air, such that the heat pump may comprise an air source heat pump. In such an air-source heat pump the evaporators may comprise an air cooler. The heat pump may be arranged to cool ambient air through a temperature range sufficient to provide a mass flow of working fluid that, when cooled and condensed, will provide the amount of heat required by a target.

The target may comprise a fluid such as oil, water, air, or the like.

The heat pump may include a working fluid configured to be communicated between the evaporator and the condenser to transfer heat there between. The heat pump may comprise a compressor configured to compress the working fluid to establish pressure and temperature conditions suitable to achieve condensing within the condenser. The heat pump may comprise a pressure reducing arrangement, such as an expansion valve, configured to permit a reduction in pressure of the working fluid to establish pressure and temperature conditions suitable to achieve evaporation within the evaporators.

During operation of the heat pump, the working fluid may be vaporised within the evaporator thus absorbing heat from the primary heat source, and delivered to the condenser to transfer the absorbed heat to the target. The vaporised working fluid from the evaporator may be compressed and delivered to the condenser at high temperature and pressure; this compressed fluid is condensed within the condenser to heat the target fluid. Condensed working fluid from the condenser may be reduced in pressure and returned to the evaporator.

The working fluid may comprise any appropriate refrigerant. The working fluid may comprise carbon dioxide, ammonia, hydrocarbons, fluorocarbons or the like. The heat pump system may comprise a target fluid flow circuit defining an inlet and an outlet, wherein the target fluid to be heated is received via the inlet and the heated target fluid is delivered via the outlet. Heated target fluid delivered from the outlet may be utilised in a process, such as a space heating process, and then be returned for reheating via the inlet.

The target fluid flow circuit may be configured to establish heat exchange between the target fluid and the condenser of the heat pump.

Heat source conditions falling below a specified temperature, for example 4°C, may cause a build-up of frost on the evaporators, for example, upon the evaporator fins.

The defrost system may comprise a means for reversing the heat pump cycle, for example a reverse flow valve. Said defrost system may be a reverse cycle defrost system.

The heat pump system may further comprise an alternative heat source.

The alternative heat source may be, for example, a gas fired boiler, an oil fired boiler or a wood or coal fired boiler.

The alternative heat source may provide an additional source of heat to the target fluid when required by climatic conditions. For example, the alternative heat source may operate when the heat pump cannot provide the required heat duty for the target fluid because the ambient temperature is too low.

The alternative heat source may operate to heat the target during the operation of the reverse cycle defrost.

The alternative heat source may be sized to have a larger capacity than the heat pump.

The reverse cycle defrost system may be configured to defrost the evaporators in the event that frost has formed on the evaporator.

During normal operation, the evaporators may draw air from the surroundings, for example through the use of circulating fans. Heat from circulating fans may be added to the air stream prior to heat exchange with the evaporators.

When the ambient air temperature falls below temperatures which may cause frost to form on the evaporators, the defrost system may begin to operate to defrost the evaporator.

The operation of the defrost system may be initiated by any suitable indication, for example, by temperature measurement or by the measurement of an air pressure drop across an evaporator. Such pressure drop measurement may indicate the presence of frost on evaporator fins or other surfaces.

The operation of the defrost system may be automated, or manual, and may be initiated only as required.

The operation of the defrost system may be such that the defrost system is operational and the evaporators are subject to defrosting whenever the ambient temperature drops below a specified value.

The operation of the defrost system may be such that an evaporator is defrosted when a pressure drop measurement across the evaporator is above a specified value.

The operation of the defrost system may be initiated by a combination of suitable indicators. For example, the ambient temperature falling below a specified value and a pressure drop measurement across the evaporators, wherein the defrost system is in operation at ambient temperatures below a specified value but the evaporators will be taken offline as and when the pressure drop measurement exceeds a specified value indicating defrosting is required.

During operation of the defrost system, the cycle of the heat pump is reversed such that the heat is removed from the target fluid in the condenser to evaporate the working fluid. The working fluid is then condensed within the evaporators providing heat to defrost the evaporator fins.

During operation of the defrost system, heat is removed from the target fluid. To compensate for the cooling of the target fluid, the alternative heat source will operate to heat the target fluid to the temperature required by the target.

The combination of a reverse cycle defrost and an alternative heat source will result in quicker and more efficient defrosting of the evaporator and more efficient operation of the heat pump.

The defrosting time may be a small fraction of the refrigeration time, for example less than one quarter of the refrigeration time. The defrost time may, for example, be ten minutes within every hour of refrigeration time.

According to a second aspect of the present disclosure there is provided a method of operation of a heat pump, comprising: arranging an evaporator of a heat pump in heat exchange with air, and arranging a condenser of the heat pump in heat exchange with a target fluid; arranging a defrost system utilising the target fluid as a heat source and configured to defrost the evaporator; arranging an alternative heat source to heat the target fluid when the target fluid is being utilised as a heat source; and establishing a defrost operation in which the evaporator is defrosted and heat is supplied to the target fluid to heat a target.

The method may comprise arranging the heat pump to operate in reverse such that heat from the target fluid is removed in the condenser of the heat pump to evaporate a working fluid, wherein the working fluid is then condensed in the evaporator providing heat to defrost the evaporator.

The method may further comprise operating the gas boiler to supply additional heat to the target during normal operation of the heat pump when required by the target.

Establishing a defrost operation may comprise initiating the defrost system by any suitable means which indicate defrosting is required, for example ambient air temperature or air pressure drop across an evaporator.

Features and aspects of operating the heat pump defined in accordance with the first aspect may be applied to the second aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present disclosure will now be described, by way of example only, with reference to the accompanying drawing which shows an air source heat pump system with a gas boiler and four port reversing valve.

DETAILED DESCRIPTION OF THE DRAWINGS

A heat pump system, in accordance with an embodiment of the present disclosure is schematically illustrated in Figure 1. The system is composed of an air-source heat pump 100. The heat pump utilises ambient air 104 as a heat source and functions to heat water 116 flowing within a water circuit 110 to a desired output temperature. The water circuit 110 comprises an inlet 112 for receiving water to be heated, and an outlet 114 for delivering heated water. In some embodiments the heated water may be consumed, for example as domestic hot water. In other embodiments, the heated water is utilised in a process, such as a space heating process, and returned to the inlet 112 for reheating.

The heat pump 100 comprises evaporators 102, arranged in parallel, and arranged in heat exchange with ambient air 104. The embodiment illustrated in Figure 1 shows two evaporators 102, although in practice the number and capacity of evaporators will be determined by the heating load required from the heat pump. The heat pump also comprises a condenser 106 arranged in heat transfer with the water flowing 116 in the water circuit 110. The heat pump further comprises a compressor 108. An expansion valve 103 is provided for each evaporator 102.

During normal operation at ambient temperatures above, for example 4°C, the evaporators 102 will be operational to share the heating load required to heat the target fluid 114 to the required temperature. Vaporised working fluid (refrigerant) from the evaporators 102 is compressed by the compressor 108 and delivered to the condenser 106 at a pressure and temperature which permits condensing of the working fluid to achieve heating of the water 116 flowing within the water circuit 110. The water may be heated to a desired output temperature, which may be assisted by the use of appropriate thermostatic controls. The condensed working fluid is then passed through the expansion valves 103 to be reduced in pressure and temperature to the evaporating temperature, and then passed through the evaporator 102. Cyclical operation of the heat pump in this manner may permit continuous heating of the water 116 within the water circuit 110.

The heat pump system optionally comprises an oil cooler 122 which is arranged in heat exchange with the water 116 flowing within the water circuit 110. The oil cooler operates to cool the oil required by the compressor and in doing so provides additional heat to the water 116 prior to the water reaching the condenser 106. Bypass valve 124 is also included to allow the oil in the compressor 106 to bypass the oil cooler 122 if desired.

The heat pump further comprises a four port reversing valve 130. During normal operation the four port reversing valve 130 routes refrigerant vapour at high pressure from the compressor 108 to the condenser 106. A crankcase pressure regulator 132 is also present to aid in control of the pressure of the refrigerant.

During normal operation of the heat pump 100, the four port reversing valve 130 also routes wet refrigerant vapour from the evaporators 102 to a low pressure receiver (LPR) 140. The low pressure receiver 140 acts to evaporate the refrigerant vapour to dryness. Dry refrigerant vapour exits the LPR 140 and returns to the compressor 108.

The heat pump system 100 further comprises a gas boiler 128 which will operate to provide additional heat to the target fluid when the output of the heat pump cannot meet demand. This will occur when the ambient air temperature is low and the required heat from the target fluid is high, for example during winter months.

The heat pump 100 optionally comprises an economiser 118 to increase sub-cooling, in which a portion of refrigerant flow from the compressor 108 is evaporated to a pressure intermediate between discharge pressure and suction pressure and arranged to extract heat from the main flow of refrigerant. The effect of this is to increase sub-cooling in the main flow of refrigerant. Solenoid valve 126 is provided to regulate the flow of refrigerant to the economiser.

Therefore, the typical flow of refrigerant during normal operation of the heat pump is as follows. Air 104 is drawn across the evaporators 102 transferring heat to liquid refrigerant within the evaporators 102. The refrigerant is evaporated to a vapour. The flow of vapour refrigerant from the evaporators 102 is routed to the LPR 140 by the reversing valve 130 where the vapour is evaporated to dryness before entering the compressor 108. Compressed refrigerant vapour leaves the compressor 108 and is routed to the condenser 106 by the reversing valve 130 where the compressed vapour is condensed supplying heat to the water 116 in the water circuit 110. The condensed refrigerant is then passed through the economiser 118 where it is sub-cooled as described above before passing through the LPR 140 and returning to the evaporators 102.

When the ambient air temperature falls to a temperature at which frost may form on the evaporators 102, for example below or close to 4°C, this will adversely affect the functionality of the heat pump 100. In severe cases of frost, the heat pump may cease to operate. In order to address any frost formation on one or more of the evaporators 102, the heat pump may be operated in a reverse cycle.

Frost formed on the evaporators 102 can be detected by measuring the air pressure drop across each individual evaporator 102 using a sensor 145. An increased air pressure drop reading may indicate that some defrosting of the evaporator 102 is required. As frost forms on the evaporators 102, the flow of air 104 through them is restricted, thus the pressure drop across the evaporator 102 will increase. The air pressure drop may also be sensed by the current drawn by the fan motors of the evaporators 102.

When the air pressure drop reaches a pre-determined value, the four port reversing valve 130 is rotated 90° such that hot refrigerant vapour from the compressor 108 is fed to the frosted evaporators 102 in order to melt said frost. When the reversing valve 130 is rotated 90°, refrigerant is drawn from the condenser 106 where heat is extracted from the target fluid to evaporate the refrigerant. The condenser 106 operates as an evaporator. This is known as reverse cycle defrosting; the flow of refrigerant is reversed when compared with the typical flow during normal operation as described above.

The heat pump 100 further comprises a reverse trap 120. The reverse trap is a liquid store for liquid refrigerant such that when the reversing valve 130 is rotated, there is an immediate supply of refrigerant available for defrosting the evaporators 102.

During defrosting, the condenser 106 operates as an evaporator and cools the target fluid instead of providing heat to the target fluid, as is the case during normal operation. In order to maintain the heat output of the heat pump system during defrost of the evaporators, the gas boiler 128 is operational to supply heat to the target fluid such that the temperature of the outlet 114 is maintained at its required value.

The combination of a reverse cycle defrost and a gas boiler results allows for quick defrost of the evaporators and allows for more efficient operation of the heat pump 100.

The evaporator fins of the evaporators 102 may be designed and optimised to maximise heat duty of the heat pump and to minimise the time required for defrosting.

The time required to defrost an evaporator 102 may be a small fraction of the refrigeration time, for example less than one quarter of the refrigeration time. Or, for example, an air source heat pump comprising 12 fins/inch may require 10 minutes of defrosting time once an hour.

It should be understood that the embodiments and possible variations identified above are merely exemplary and that various or further modifications may be made thereto.

Claims

CLAIMS: 1. A heat pump system, comprising: an air source heat pump having an evaporator to be arranged in heat exchange with air and a condenser to be arranged in heat exchange with a target fluid to heat a target; and a defrost system configured to defrost the evaporators using the target fluid as a defrost heat source; and an alternative heat source configured to heat the target fluid when the target fluid is being utilised as a heat source..
2. The heat pump system of claim 1, further comprising a valve arrangement for changing the flow of working fluid within the heat pump such that the condenser operates as an evaporator and the evaporator operates as a condenser to defrost the evaporator.
3. The heat pump system of claim 1 or 2, wherein the alternative heat source is a gas fired boiler or an oil fired boiler.
4. The heat pump system of claim 1, 2 or 3, wherein the alternative heat source has larger heating duty than the heat pump.
5. The heat pump system of claims 1 to 4 wherein the alternative heat source is configured to compensate for the reduction in temperature of the target fluid when the target fluid is being used a defrost heat source.
6. The heat pump system of claims 1 to 5 wherein the alternative heat source is configured to supply additional heat to the target fluid when the heat pump cannot provide the required heating duty for the target fluid to heat the target.
7. A method of operating a heat pump system comprising: arranging an evaporator of a heat pump in heat exchange with air, and arranging a condenser of the heat pump in heat exchange with a target fluid; arranging a defrost system utilising the target fluid as a heat source and configured to defrost the evaporators; arranging an alternative heat source to heat the target fluid when the target fluid is being utilised as a heat source; and establishing a defrost operation in which the evaporator is defrosted and heat is supplied from the boiler to the target fluid to heat a target.
8. The method of claim 7, wherein the alternative heat source is a gas fired boiler or an oil fired boiler.
9. The method of claims 7 and 8, wherein the method further comprises arranging the heat pump to operate in reverse such that heat from the target fluid is removed in the condenser of the heat pump to evaporate a working fluid, wherein the working fluid is then condensed in the evaporator providing heat to defrost the evaporator.
10. The method of claims 7, 8 and 9, further comprising operating the alternative heat source to supply additional heat to the target during normal operation of the heat pump when required by the target.
11. The method of claims 7 to 10, further comprising initiating the defrost system by a sensor which indicates defrosting is required.
12. The method of claim 11 wherein the sensor measures the ambient air temperature or an air pressure drop across the evaporator or both the ambient air temperature and the air pressure drop.
13. The method of claims 7 to 12, wherein the alternative heat source provides an additional source of heat to the target fluid when required by climatic conditions.
14. The method of claim 13 wherein the alternative heat source operates when the heat pump cannot provide the required heat duty for the target fluid to heat the target.
15. The method of claims 7 to 14 wherein the alternative heat source operates to heat the target fluid during the defrosting operation and compensates for the reduction in temperature of the target fluid when the target fluid is being used to defrost the evaporators.
16. The method of claims 7 to 15 wherein the defrosting time is less than one quarter of the refrigeration time.
17. The method of claims 7 to 16 wherein the defrosting time is ten minutes within every hour of refrigeration time.