Heat pump system

Abstract

A heat pump system for controlling the temperature of a building interior. The system includes a compressor, a first heat exchanger, an expansion device, and a second heat exchanger fluidly connected together by a flow of refrigerant to define a refrigerant circuit, and a thermal energy storage device thermally connectable to the refrigerant circuit to exchange thermal energy with the refrigerant. The heat pump system is configured to operate in a normal heating mode and a defrost mode. In the normal heating mode, thermal energy is transferred from the second heat exchanger into the refrigerant and through the first heat exchanger from the refrigerant to heat the building. In the defrost mode, thermal energy is transferred from the thermal energy storage device into the refrigerant and through the first heat exchanger from the refrigerant to heat the building, and through the second heat exchanger from the refrigerant to defrost the second heat exchanger. The heat pump system includes a switching assembly configured to switch between the normal heating mode and the defrost mode, and the switching assembly is configured to direct the flow of refrigerant exiting the first heat exchanger through the second heat exchanger to defrost the second heat exchanger with residual heat in the refrigerant when operating the heat pump system in the defrost mode.

Description

TECHNICAL FIELD

[0001] The present invention relates to a heat pump system and a method of operating a heat pump system. BACKGROUND

[0003] It is known to provide a heat pump to extract thermal energy (i.e. heat) from a heat source, such as the outside air, and then release the extracted thermal energy to the interior of an enclosed space, such as a building. Any heat source at a temperature above absolute zero contains some thermal energy which can be utilised to increase the temperature of the interior of the enclosed space.

[0004] An example of a known heat pump system is an air source heat pump (ASHP) which typically comprises an evaporator, a compressor, a condenser and an expansion device. The components of the ASHP are fluidly connected by fluid conduits to form a refrigerant circuit. The evaporator and the condenser each comprise a heat exchanger configured to allow heat to be transferred into and / or out of a refrigerant flowing through the refrigerant circuit. The evaporator is arranged in an external location such that it can transfer heat from the surrounding outside air, whilst the condenser is typically connected to a central heating system of a building.

[0005] The refrigeration circuit starts with the compressor, where the evaporated refrigerant is compressed to form a hot vapour. The hot refrigerant vapour is then directed to the condenser, which transfers some heat from the refrigerant, thereby condensing the vapour into a liquid. The liquid refrigerant then flows to the expansion valve, where expansion occurs, thereby reducing the pressure and temperature. The cold refrigerant mixture is directed through the evaporator, whereafter it transfers heat from the outside air, causing the refrigerant to evaporate. The refrigerant vapour is then directed back to the compressor to start the refrigerant circuit again.

[0006] One known problem with ASHPs is that the heat capacity and coefficient of performance (COP) drops dramatically when the surrounding outside air temperature drops. This means that when heat flow is most needed to increase the interior temperature of a building, the performance of the ASHP drops to a minimum.

[0007] Another problem with known ASHPs is that frost and ice can form on the coils or fins of the evaporator when the outside temperature drops below about 6°C. The ice formation reduces the operating efficiency of the evaporator, thereby causing the ASHP to stop working. It is necessary to defrost the evaporator on a regular basis to prevent ice formation, especially in cold and humid climates.

[0008] A typical method of evaporator defrosting involves reversing the flow direction of the refrigerant through the circuit, directing the refrigerant to flow from the compressor to the evaporator. This so-called “reverse cycle method” is performed by configuring the ASHP to extract heat from the condenser to melt the ice accumulated in the evaporator.

[0009] One alternative is to provide the refrigerant circuit with a bypass conduit or passage configured to fluidly connect the output of the compressor to the input of the evaporator while bypassing the condenser. In this "hot gas bypass method" the compressor is configured to produce hot vaporized refrigerant which is then directed to the evaporator to melt ice formed thereon.

[0010] Another alternative defrosting method uses a separate electric heater configured to directly heat the outer surface of the evaporator to melt accumulated frost and ice under cold and humid conditions. This defrosting method uses additional electricity and also requires the ASHP device to be shut down during the defrosting process, which results in an interruption of the heating of the building interior.

[0011] During the defrosting operation, each of these defrosting methods consumes electricity without providing heat to the building interior. For example, in the hot gas bypass method, the compressor operates to provide hot refrigerant vapor, but no heat is provided to the building's central heating system because the hot vapor is diverted from the condenser. Alternatively, the reverse cycle method causes cold refrigerant to flow through the condenser on its way to the evaporator. This results in the extraction of heat from the condenser, thereby lowering the temperature of the building interior.

[0012] Each of these defrosting methods requires a backup heat source, such as an electric heater or a gas boiler, to be provided to heat the building interior while the evaporator is defrosting during operation. Therefore, each defrosting method significantly reduces the overall coefficient of performance (COP) of the heat pump system.

[0013] The present disclosure aims to address one or more of the above-mentioned problems of existing heat pump systems. SUMMARY

[0014] In the broadest terms, aspects of the present invention provide a heat pump system configured to direct residual heat energy from a condenser to an evaporator defrost when operating in a defrost mode, while also directing stored heat energy from a heat energy storage device to the condenser in order to heat the interior of a building during the defrost operation. Aspects of the present invention also provide a method of operating a heat pump system in a defrost mode, the method comprising directing residual heat energy from a condenser to an evaporator defrost, and directing stored heat energy from a heat energy storage device to the condenser.

[0015] A first aspect of the present invention provides a heat pump system for a building temperature control system, the system comprising: a compressor, a first heat exchanger, an expansion device and a second heat exchanger fluidly connected together by a flow of refrigerant to define a refrigerant circuit, and a thermal energy storage device thermally connectable to the refrigerant circuit to exchange thermal energy with the refrigerant. The heat pump system is configured to be operable in a normal heating mode and a defrost mode. In the normal heating mode, thermal energy is transferred from the second heat exchanger into the refrigerant and from the refrigerant through the first heat exchanger to heat the building; in the defrost mode, thermal energy is transferred from the thermal energy storage device into the refrigerant and from the refrigerant through the first heat exchanger to heat the building, and from the refrigerant through the second heat exchanger to defrost the second heat exchanger. The heat pump system comprises a switching assembly configured to switch between the normal heating mode and the defrost mode, and the switching assembly is configured to direct the flow of refrigerant exiting the first heat exchanger through the second heat exchanger when operating the heat pump system in the defrost mode, such that the residual heat in the refrigerant defrosts the second heat exchanger.

[0016] The refrigerant circuit can be configured by the switching assembly such that residual thermal energy in the refrigerant flowing from the first heat exchanger is directed to the second heat exchanger in order to melt ice that has accumulated on an outer surface thereof.

[0017] The residual heat from the first heat exchanger represents excess heat that is not transferred by the first heat exchanger into the central heating system of the building. In known heat pump systems, this "residual heat" is typically wasted. During the defrost operation, the thermal energy storage device is configured to transfer stored thermal energy into the refrigerant circuit, such that the first heat exchanger is able to continue to provide heat to the central heating system of the building.

[0018] Accordingly, the defrost mode of the heat pump system is able to combine defrosting of the second heat exchanger with continuous (i.e. uninterrupted) heating of the building. This functionality eliminates the need for a back-up heater, thereby significantly improving the coefficient of performance (COP) of the heat pump system, and thereby reducing its operating costs.

[0019] Optional features will now be set out. These features can be applied individually or in any combination with any aspect.

[0020] It will be appreciated that the term "refrigerant" in this context refers to a heat transfer fluid, for example, a fluid that is capable of transitioning between a gas phase and a liquid phase and that can be used in a heat transfer process. For example, the refrigerant can comprise any heat transfer fluid that is capable of acting as a medium to cool on one side of a heat transfer process, to heat on the other side of the heat transfer process, and to transfer thermal energy from one side of the heat transfer process to the other. Accordingly, in accordance with the present disclosure, each refrigerant circuit described herein can define a heat transfer fluid circuit.

[0021] The first and second heat exchangers can define an internal heat exchanger and an external heat exchanger, respectively. That is, the first heat exchanger can be configured to transfer thermal energy between the heat pump system and an interior space of the building, while the second heat exchanger can be configured to transfer thermal energy between the heat pump system and an external environment. Thus, the first and second heat exchangers can be defined as indoor and outdoor heat exchangers (or units) of the heat pump system, respectively. The first heat exchanger can be a condenser. The second heat exchanger can be an evaporator.

[0022] It will be appreciated that the expansion device can comprise any expansion device suitably configured to cause expansion of the refrigerant. For example, the expansion device can comprise an expansion valve. The expansion device can be configured to operate in either flow direction. Thus, the expansion device can be a capillary tube. Alternatively, the expansion device can comprise two or more one-way expansion valves arranged in a switchable circuit in order to cause expansion of the refrigerant flow in either direction, as will be appreciated by the skilled person. Another expansion device can comprise two asymmetric expansion devices connected together with two one-way valves. Or, the expansion device can comprise a single asymmetric expansion valve connected to the refrigerant circuit by four control valves, as will be appreciated by the skilled person.

[0023] The switching assembly can be configured to direct the refrigerant exiting the first heat exchanger through the second heat exchanger, the expansion device, and the compressor in sequence when operating the heat pump system in the defrost mode. In this way, the heat pump system can be configured to cause residual thermal energy from the first heat exchanger to be carried by the refrigerant to defrost the evaporator. Thus, the switching assembly can be arranged downstream of the first heat exchanger and upstream of the compressor.

[0024] The switching assembly can be configured to direct the refrigerant from the first heat exchanger through the thermal energy storage device, the expansion device, the second heat exchanger, and the compressor in sequence. In this way, the thermal energy storage device can be configured such that, in the normal heating mode, at least some residual thermal energy present in the hot refrigerant exiting the condenser is transferred into the thermal energy storage device. Thus, in the normal heating mode, the thermal energy storage device can be configured to recover residual thermal energy from the hot refrigerant exiting the condenser.

[0025] The thermal energy storage device can be connected to the refrigerant circuit between the expansion device and the compressor to recover and store residual thermal energy carried by the refrigerant.

[0026] The switching assembly can comprise a four-way valve, which can be configured to connect the first heat exchanger directly to the second heat exchanger when operating the heat pump system in the defrost mode. The four-way valve provides a convenient way to redirect the flow of refrigerant through the heat pump system.

[0027] Alternatively, it will be appreciated that the switching assembly can comprise any suitably configured "four-way" switching device (e.g. an assembly of four interconnected one-way valves, or an assembly of two interconnected two-way valves).

[0028] The switching assembly can be configured to, when operating the heat pump system in the defrost mode, bypass the expansion device (i.e. the first expansion device) and direct the refrigerant exiting the first heat exchanger through the second expansion device, the second heat exchanger and the compressor in turn. This enables heat from the thermal energy storage device to be transferred by the refrigerant to defrost the second heat exchanger.

[0029] The switching assembly can comprise a first bypass assembly which can be configured to, when operating the heat pump system in the defrost mode, isolate the expansion device (i.e. the first expansion device) from the refrigerant circuit. This arrangement allows hot refrigerant from the thermal energy storage device to reach the second heat exchanger to melt ice that has built up on it.

[0030] In embodiments, the thermal energy storage device can be connected to the refrigerant circuit between the second expansion device and the second heat exchanger.

[0031] The switching assembly can comprise a second bypass assembly which can be configured to, when operating the heat pump system in the defrost mode, fluidly connect the second expansion device to the refrigerant circuit between the first heat exchanger and the thermal energy storage device. The second expansion device is thereby configured to reduce the pressure of the refrigerant so that it can more effectively absorb heat stored in the thermal energy storage device as the refrigerant passes through.

[0032] The heat pump system can be operated in a charging mode in which thermal energy can be transferred from the refrigerant to the thermal energy storage device, wherein the switching assembly can be configured to, when operating the heat pump system in the charging mode, direct the refrigerant exiting the compressor to bypass the second expansion device and the first heat exchanger. In this mode, when heat is not required by the occupants of the building, thermal energy from the compressor is directed to be stored in the thermal energy storage device.

[0033] The heat pump system can be operated in an auxiliary heating mode in which thermal energy can be transferred from the thermal energy storage device into the refrigerant, wherein the switching assembly can be configured to, when operating the heat pump system in the auxiliary heating mode, bypass the expansion device and the second heat exchanger. Thus, when defrosting of the second heat exchanger (e.g. an evaporator) is not required, thermal energy stored in the thermal energy storage device can be used to heat the interior of the building.

[0034] Thermal energy storage devices may include phase change materials (PCMs). PCMs can be configured to be in direct thermal contact with the piping of the refrigerant circuit. PCMs can be configured to provide (and store) thermal energy at a substantially constant temperature, which improves control of the heat pump system. PCMs can also be configured to store large amounts of thermal energy in a relatively small volume, which increases the encapsulation efficiency of the heat pump system.

[0035] Thermal energy storage devices can be thermally connected to the piping of a refrigerant circuit via a separate fluid loop that includes a heat transfer fluid (such as water). This separate fluid loop allows for large-capacity thermal energy storage devices that can be located externally to the rest of the heat pump system.

[0036] The heat pump system may also include a refrigerant storage device, which may be fluidly connected to the output of the first heat exchanger. The refrigerant storage device may be configured to accommodate fluctuations in the volume of refrigerant flowing through the refrigerant circuit during operation of the heat pump system.

[0037] The heat pump system may also include a phase separator, which can be fluidly connected to the inlet of the compressor to prevent unwanted fluid from entering the compressor.

[0038] A refrigerant circuit may include a high-pressure stage and a low-pressure stage, which are fluidly connected together by a phase separator. The high-pressure stage may include (i.e., fluidly connected to) a first heat exchanger, and the low-pressure stage may include (i.e., fluidly connected to) a second heat exchanger. Therefore, a heat pump system including a high-pressure stage and a low-pressure stage can define a two-stage heat pump system.

[0039] At least one or each of the high-pressure stage and the low-pressure stage may be fluidly connected to the phase separator. The high-pressure stage may be fluidly connected to the gas-containing section or segment of the phase separator. For example, the high-pressure stage may be configured to receive gaseous refrigerant from the gas-containing section of the phase separator. The low-pressure stage may be fluidly connected to the liquid-containing section of the phase separator. For example, the low-pressure stage may be configured to receive liquid refrigerant from the liquid-containing section of the phase separator. The high-pressure stage may be configured to deliver a mixture of liquid and gaseous refrigerant to the phase separator. Therefore, the high-pressure stage circuit may be configured to deliver a liquid-gas refrigerant mixture to the liquid-containing section of the phase separator.

[0040] A refrigerant circuit can define a refrigerant circuit assembly including a high-pressure stage and a low-pressure stage. The high-pressure stage and the low-pressure stage can be fluidly connected to each other because they can share the same refrigerant circulating around both stages. The refrigerant in the high-pressure stage can be maintained at a higher pressure on average than the equivalent portion of the low-pressure stage. The low-pressure stage can be configured to raise the temperature of the refrigerant from a low temperature (e.g., at the evaporator) to an intermediate temperature (e.g., in the phase separator). The high-pressure stage can be configured to raise the temperature of the refrigerant from an intermediate temperature (e.g., in the phase separator) to a high temperature (e.g., in the condenser).

[0041] The heat pump system can be operable in a normal heating mode (e.g. by configuring the switching assembly accordingly) such that heat energy is transferred from the refrigerant to the thermal energy storage device. For example, this can involve directing a flow of refrigerant from a first heat exchanger (e.g. a condenser) to a location in the refrigerant circuit which is thermally connectable to the thermal energy storage device to transfer at least some of the heat in the refrigerant to the phase change material, which can be stored for later use. In this way, the heat pump system can be configured to provide heat to the interior of the building (i.e. via the condenser) whilst charging the thermal energy storage device. This mode of operation can define a continuous heating and charging mode.

[0042] The thermal energy storage device can be thermally connected to the phase separator. The thermal energy storage device can be thermally connected to a liquid containing portion or phase of the phase separator. The phase change material can be thermally connected to the phase separator of the refrigerant circuit by a separate circuit comprising a heat transfer fluid or refrigerant. The phase change material can be arranged within the phase separator. The separate refrigerant circuit can comprise a switching assembly and / or a pump configured to control the flow of refrigerant, and thus the thermal energy between the phase separator and the thermal energy storage device. The phase change material can be encapsulated in an enclosure formed of a thermally conductive material to enable thermal conduction between the phase change material and the refrigerant in the phase separator.

[0043] The compressor can define a compressor assembly comprising a first compressor fluidly connected to the high pressure stage. The compressor assembly can comprise a second compressor fluidly connected to the low pressure stage.

[0044] The compressor can comprise a steam injection compressor fluidly connected to the high pressure stage and the low pressure stage of the refrigerant circuit. The low pressure stage can be fluidly connected to a low pressure inlet of the compressor, and a high pressure output of the compressor can be fluidly connected to the high pressure stage. The intermediate inlet can be fluidly connected to the phase separator.

[0045] The expansion device can define an expansion device assembly comprising a first expansion device fluidly connected to the high pressure stage. The expansion device assembly can comprise a second expansion device fluidly connected to the low pressure stage.

[0046] The switching assembly can be configured to, when operating the heat pump system in a defrost mode, bypass the first expansion device and direct refrigerant exiting the first heat exchanger through the second expansion device, the second heat exchanger and the phase separator in turn.

[0047] A second aspect of the present invention provides a building comprising a heat pump system as described in any of the preceding paragraphs. The second heat exchanger is thermally connected to an external heat source, and the first heat exchanger is thermally connected to a central heating system of the building.

[0048] A third aspect of the invention provides a method for operating a heat pump system according to any of the preceding paragraphs. The method is for operating the heat pump system to control the internal temperature of a building. The method includes: when operating the heat pump system in defrost mode, directing refrigerant leaving a first heat exchanger through a second heat exchanger, so that residual heat in the refrigerant defrosts the second heat exchanger.

[0049] According to the present invention, the method may include switching the heat pump system between a normal heating mode and at least one of an auxiliary heating mode and a defrosting mode.

[0050] In embodiments, the method may include: switching the heat pump system between a normal heating mode and a defrost mode when there is no ice on the evaporator. The method may also include: switching the heat pump system between a normal heating mode and an auxiliary heating mode. Furthermore, the method may include: periodically switching between different operating modes to improve the COP of the heat pump system.

[0051] According to each of the switching strategies described above, the heat pump system can be configured to operate as a quasi-two-stage heat pump system by using only a single refrigerant loop (i.e., a single-stage heat pump device). In this way, the heat pump system according to the invention can continuously supply heat to the interior of a building without requiring a complex and expensive two-stage heat pump system.

[0052] In one embodiment, the method may include operating the heat pump system during off-peak electricity consumption periods (i.e., when grid electricity prices are cheaper) according to at least one of a normal heating mode and a charging mode to store thermal energy in a thermal energy storage device for backup. In this way, the COP of the heat pump system can be improved.

[0053] A fourth aspect of the invention provides a method for operating a heat pump system to control the internal temperature of a refrigeration unit. The system includes: a compressor, a condenser, an expansion device, and an evaporator, fluidly connected together via a refrigerant flow to define a refrigerant circuit; and a thermal energy storage device thermally connected to the refrigerant circuit to exchange heat energy with the refrigerant, wherein the refrigerant circuit includes a high-pressure stage and a low-pressure stage, fluidly connected together via a phase separator, wherein the high-pressure stage includes a condenser and the low-pressure stage includes an evaporator. The heat pump system is configured to operate in a charging cooling mode and an auxiliary cooling mode, wherein:

[0054] In cooling mode, heat energy is transferred from the thermal energy storage device to the refrigerant, and then transferred from the refrigerant through the condenser to heat the ambient air.

[0055] In auxiliary refrigeration mode, heat energy is transferred from the evaporator to the refrigerant to cool the internal area of ​​the refrigeration unit and from the refrigerant to the heat energy storage device;

[0056] wherein the method comprises isolating the high pressure stage when the heat pump system is operated in the charge cooling mode, and isolating the low pressure stage when the heat pump system is operated in the auxiliary cooling mode.

[0057] According to a fourth aspect of the application, the method enables the two-stage heat pump system to operate as a refrigeration system. In particular, the system can be configured to cool a region requiring cooling, such as a refrigeration unit or an interior space of a building. It will be appreciated that for a refrigeration (i.e. cooling) application, the heat pump system can be configured in reverse arrangement to a heating application. For example, the condenser can be exposed to ambient air (e.g. outside the refrigeration zone) and the evaporator can be positioned at a location requiring a load (e.g. inside the refrigeration zone). In this case, the required load can be a negative thermal load (i.e. cold) for reducing the temperature of the refrigeration region. Accordingly, when the heat pump system is operated in the charge cooling mode, the switching assembly can be configured to remove or extract thermal energy (i.e. heat) from the thermal energy storage device. This can be considered as "charging" the thermal energy storage device with "cool thermal energy". When the heat pump system is operated in the auxiliary cooling mode, the stored "cool thermal energy" can be used to reduce the temperature of the refrigerant passing to the evaporator (i.e. by extracting thermal energy from the refrigerant and storing it in the thermal energy storage device), thereby cooling the refrigeration region.

[0058] The method can comprise operating the heat pump system in accordance with the charge cooling mode during periods when the external ambient air temperature is low (e.g. at night) to store cool thermal energy in the thermal energy storage device for later use. The cool thermal energy stored in the thermal energy storage device can then be directed to cool the interior space of the refrigeration unit when the heat pump system is operated in the auxiliary cooling mode. In this way, the COP of the heat pump system can be improved.

[0059] A fifth aspect of the application provides a controller or control system for controlling a heat pump system as described in any of the preceding paragraphs. The controller can be configured to perform the method as described in any of the preceding paragraphs. In particular, the controller can be configured to control the switching device to operate the heat pump system in at least one of a plurality of operating modes.

[0060] Each of the example heat pump systems can be incorporated within an air source heat pump system, i.e. configured to extract thermal energy from ambient air, as will be appreciated by the skilled person. Alternatively, each of the above heat pump systems can be configured for use in a water source heat pump system and / or a ground source heat pump system.

[0061] The skilled person will appreciate that features or parameters described in relation to any of the above aspects can be applied to any other aspect, except where mutually exclusive. Furthermore, any feature or parameter described herein can be combined with any other feature or parameter described herein, except where mutually exclusive.

[0062] Brief description of the attached figures

[0063] Various aspects and embodiments of this disclosure will now be described by way of example with reference to the accompanying drawings, in which:

[0064] Figure 1 This is a schematic diagram of a single-stage heat pump system according to a first arrangement of the present invention, the heat pump system being configured to operate in normal heating mode;

[0065] Figure 2 yes Figure 1 The pressure-enthalpy curves corresponding to the normal heating mode of the heat pump system are shown below.

[0066] Figure 3 yes Figure 1 A schematic diagram of a heat pump configured to operate in defrost mode;

[0067] Figure 4 yes Figure 3 The pressure-enthalpy curves corresponding to the defrosting modes of the heat pump system are shown below.

[0068] Figure 5 This is a schematic diagram of a single-stage heat pump system according to a second arrangement of the present invention, which can be configured to operate in a variety of different operating modes;

[0069] Figure 6 yes Figure 5 The pressure-enthalpy curves corresponding to different operating modes of the heat pump system are shown below;

[0070] Figure 7 yes Figure 1 , Figure 3 or Figure 5 A schematic diagram of the thermal energy storage components of the heat pump system shown.

[0071] Figure 8 yes Figure 1 , Figure 3 or Figure 5 The graph shows the relationship between the coefficient of performance and time for the operating method of the heat pump system.

[0072] Figure 9 yes Figure 5 A schematic diagram of alternative configurations for a single-stage heat pump system is shown.

[0073] Figure 10 This is a schematic diagram of a two-stage heat pump system according to a third arrangement of the present invention, which can be configured to operate in a variety of different operating modes;

[0074] Figure 11 This is a schematic diagram of a two-stage heat pump system according to a fourth arrangement of the present invention, which can be configured to operate in a variety of different operating modes;

[0075] Figures 12 to 16 is Figure 11 a pressure-enthalpy diagram corresponding to different operating modes of the heat pump system shown in Figure 1 ;

[0076] Figure 17 is Figure 11 a schematic diagram of an alternative configuration of the two-stage heat pump system shown in Figure 2;

[0077] Figure 18 is Figure 10 a schematic diagram of an alternative configuration of the two-stage heat pump system shown in Figure 2;

[0078] Figure 19 is Figure 11 a schematic diagram of an alternative configuration of the two-stage heat pump system shown in Figure 2; and

[0079] Figure 20 is Figure 10 , Figure 11 and Figures 17 to 19 schematic diagrams of alternative thermal energy storage assemblies for a heat pump system.

[0080] DETAILED DESCRIPTION

[0081] Aspects and embodiments of the present application will now be discussed with reference to the drawings. Further aspects and embodiments will be apparent to those skilled in the art.

[0082] Single-stage heat pump system

[0083] A heat pump system 10 according to a first arrangement of the present application will now be described with reference to Figures 1 to 4 The heat pump system 10 forms part of an air source heat pump (ASHP) configured to transfer thermal energy (i.e. heat) from outside a building to inside the building (not shown).

[0084] The heat pump system 10 comprises a compressor 12, a condenser 14, a thermal energy storage device 16, an expansion device 18, an evaporator 20 and an optional phase separator 22, which are fluidly connected by fluid conduits 24 to define a refrigerant circuit 26. The fluid conduits 24 define fluid paths through which refrigerant is directed between the components of the heat pump system 10, as will be readily understood by the skilled person. Thus, the heat pump system 10 is defined as a single-stage heat pump system because it comprises only a single refrigerant circuit 26.

[0085] The heat pump system 10 is configured to be operable in a normal heating mode in which thermal energy is transferred from an external heat source into the refrigerant via the evaporator 20 and from the refrigerant to the inside of the building via the condenser 14.

[0086] The heat pump system 10 also includes a switching assembly 40 configured to switch between a normal heating mode (as shown in Figure 1 and a defrost mode (as shown in Figure 3 ). In the normal heating mode, heat energy is transferred from the evaporator 20 into the refrigerant, which is then directed to the condenser 14 to heat the building, while the residual heat from the condenser is also directed to heat the thermal energy storage device 16.

[0087] In the defrost mode, heat energy is transferred from the thermal energy storage device 16 into the refrigerant, which is then directed to heat the condenser 14 and the evaporator 20. In this way, in the defrost mode, the heat stored in the thermal energy storage device 16 can be used as a heat source. Thus, the heat pump system 10 is able to provide continuous heating during defrosting of the evaporator 20, which eliminates the need for a backup heater.

[0088] The components of the heat pump system 10 will now be described with particular reference to Figure 1 and Figure 3 . The refrigerant is a heat transfer fluid capable of absorbing, retaining, and releasing heat energy, such that it can be transferred between different components of the refrigerant circuit 26. In embodiments, the refrigerant comprises 1,1,1,2-tetrafluoroethane (134a), which is a hydrofluorocarbon and a haloalkane material with little ozone depletion potential. Its chemical formula is CF3CH2F, and its boiling point at atmospheric pressure is -26.3°C. In alternative embodiments, the refrigerant can be one of a variety of suitably configured heat transfer fluids, as will be understood by those skilled in the art.

[0089] The refrigerant, in a gaseous state, is pressurized by the compressor 12 and circulated through the refrigerant circuit 26. The compressor 12 is an electrically powered mechanical device configured to increase the pressure of the refrigerant in the refrigerant circuit 26. During use, the compressor 12 typically receives low pressure refrigerant through an inlet, which is then pressurized by the compressor 12 and discharged into the refrigerant circuit 26 through an outlet. By increasing the pressure of the refrigerant, the compressor 12 also increases the temperature of the refrigerant that subsequently circulates around the refrigerant circuit 26.

[0090] The evaporator 20 and the condenser 14 each comprise a heat exchanger configured to be in direct thermal contact with the tubing 24 of the refrigerant circuit 26. Thus, each of the evaporator 20 and the condenser 14 is configured such that heat energy can be transferred into and out of the refrigerant flowing through the refrigerant circuit 26.

[0091] The condenser 14 defines the primary heat sink of the heat pump system 10. The condenser 14 is thermally coupled to the interior of the building. In particular, the condenser 14 is configured to transfer thermal energy between the refrigerant in the refrigerant circuit 26 and the ambient air within the building. Thus, the condenser 14 defines an interior heat exchanger 14 of the heat pump system 10. The condenser 14 is thermally coupled to the central heating system of the building and is configured to transfer thermal energy between the refrigerant in the refrigerant circuit 26 and a separate heat transfer fluid (e.g., water) that flows through the central heating system of the building. Such a central heating system can be configured to distribute the thermal energy it receives from the condenser 14 to different areas of the building. The circulating thermal energy is then discharged from multiple radiators or underfloor heating assemblies, as will be readily appreciated by those skilled in the art.

[0092] The condenser 14 is configured to transfer thermal energy into the building at a heat production temperature (e.g., 65°C). In other words, the condenser 14 is configured to receive pressurized vapor refrigerant at a higher temperature (e.g., approximately 91°C).

[0093] In an alternative exemplary arrangement of the heat pump system 10, the central heating system of the building includes an interior fan for directing a flow of air across the surface (e.g., coil or fin) of the condenser 14 that is exposed to the interior of the building. In this way, the interior fan is operable to increase the thermal energy exchange efficiency of the condenser 14.

[0094] The evaporator 20 defines the primary heat source of the heat pump system 10. When the heat pump system 10 is installed within a building, the evaporator 20 is arranged in an exterior location so that it can absorb heat from the surrounding outside air and transfer it into the refrigerant flowing through the refrigerant circuit 26. In this way, the evaporator 20 defines an exterior heat exchanger 20 of the heat pump system 10. An exterior fan 34 is used to direct a flow of air across the surface (e.g., coil or fin) of the evaporator 20 that is exposed to the outside ambient air to increase its thermal energy exchange efficiency. The evaporator 20 is configured to absorb thermal energy from the outside environment when the surrounding outside air temperature is approximately 0°C.

[0095] The thermal energy storage device 16 includes a thermal energy storage medium that is configured to retain thermal energy over an extended period of time. According to an exemplary arrangement of the heat pump system 10, the thermal energy storage medium includes a phase change material that is contained within an enclosure 30 or periphery that is thermally insulated so as to retain the heat stored within the phase change material.

[0096] Phase change materials are substances configured to release and absorb sufficient energy during a phase change to provide useful heating and / or cooling to the refrigerant in the heat pump system 10. In alternative arrangements of the heat pump system 10, the thermal energy storage medium may include a heat transfer fluid, such as water, which may be held in a tank, as will be readily understood by those skilled in the art. Alternatively, the thermal energy storage medium may include multiple heated stones or pebbles configured to transfer heat directly to and from the refrigerant circuit (e.g., via conductive elements) or via a separate heat transfer fluid circuit, as will be readily understood by those skilled in the art.

[0097] The phase change material is configured to undergo a solid / liquid phase change at a temperature between 25°C and 30°C. Accordingly, the phase change material is configured to absorb heat into the refrigerant and release heat from the refrigerant at a temperature between 25°C and 30°C. Thus, the phase change material is configured with substantially constant heat storage and release temperatures, thereby improving the control of heat recovery and release into the refrigerant loop 26.

[0098] It should be understood that the optimal operating temperature range of the thermal energy storage device 16 will depend at least in part on the operating temperatures of the heat source (e.g., evaporator) and the radiator (e.g., condenser). In embodiments, the thermal energy storage device is configured to operate at an intermediate temperature below the operating temperature of the condenser and above the operating temperature of the evaporator (e.g., transferring heat energy into and out of the refrigerant circuit).

[0099] The phase change material is configured to be in direct thermal contact with the piping portion 28 of the refrigerant circuit 26. The piping portion 28 is configured to extend through the inner cavity of the housing 30, such that it is in direct thermal contact with the phase change material, as... Figure 1 As shown. The conduit portion 28 is configured to have a spiral shape to increase its contact area with the phase change material. The conduit portion 28 is at least partially formed of a thermally conductive material and is configured to allow heat energy to be conducted between the refrigerant and the phase change material in the thermal energy storage device 16. Therefore, the thermal energy storage device 16 is configured to transfer (i.e., absorb and release) heat energy into the refrigerant flowing through the conduit portion 28.

[0100] Although in the embodiments described herein, the thermal energy storage device 16 is shown (e.g., in...) Figure 1 The heat pump system 10 has a spiral conduit section 28 through which the phase change material passes; however, it should be understood that this arrangement is only one possible configuration among many. For example, in an alternative exemplary arrangement, the phase change material may be thermally connected to the refrigerant in the refrigerant circuit via a finned heat exchanger. Without departing from the scope of this disclosure, the heat pump system 10 may include other arrangements of the thermal energy storage device 16.

[0101] The thermal energy storage device 16 is configured to passively transfer thermal energy between the phase change material and the refrigerant in the refrigerant circuit 26. In particular, the thermal energy storage device 16 is configured such that when the temperature of the refrigerant is lower than the temperature of the phase change material, thermal energy stored in the phase change material is released into the refrigerant. Alternatively, if the refrigerant temperature is higher than the phase change material temperature, latent heat energy in the refrigerant is discharged into and absorbed by the phase change material in the thermal energy storage device 16.

[0102] With particular reference to Figure 1 and Figure 3 The thermal energy storage device 16 is located downstream of the condenser 14 such that it can recover heat from the warm liquid refrigerant discharged by the condenser 14 in a normal heating mode of operation. In this way, the thermal energy storage device 16 is configured to operate as a sub-cooler for the heat pump system 10. In a defrost mode of operation of the heat pump system 10, the recovered heat stored in the phase change material can be discharged back into the refrigerant during subsequent operation. Thus, the thermal energy storage device 16 can also be configured to operate as a secondary or auxiliary heat source for the heat pump system 10.

[0103] The expansion device 18 is configured to reduce the pressure of the refrigerant so as to cause a pressure and temperature drop, and subsequently the refrigerant to evaporate as it passes through the evaporator. Thus, the expansion device 18 is configured to control the amount of refrigerant released into the evaporator 20. In this way, the expansion valve 18 is intended to regulate the degree of superheat of the vapour leaving the evaporator 20.

[0104] The expansion device 18 comprises an orifice through which the refrigerant is directed. The orifice is configured to reduce the pressure of the refrigerant flowing through it, which also cools due to the associated pressure drop. Thus, the expansion device 18 is configured to not extract thermal energy from the refrigerant. In this way, the refrigerant flowing through the expansion device expands due to a substantially isenthalpic process.

[0105] The expansion device 18 comprises a capillary tube configured to cause the refrigerant to expand when it flows through the refrigerant circuit in either flow direction. According to an alternative arrangement, the expansion device 18 can comprise two one-way expansion valve arrangements in a bypass circuit, as will be readily appreciated by the skilled person.

[0106] The phase separator 22 is a gas-liquid phase separator configured to collect liquid refrigerant condensed in an upstream component of the refrigerant circuit 26. This can be caused by cooling or depressurisation of the refrigerant, for example. The phase separator 22 is arranged upstream of the compressor 12 and is thereby configured to prevent liquid refrigerant from entering the compressor 12, which can otherwise cause damage and / or render it inoperative.

[0107] The switching assembly 40 includes a four-way valve 42, which is configured to switch the operation of the heat pump system 10 between a normal heating mode and a defrost mode. This is achieved by adjusting the flow direction of the refrigerant through the refrigerant circuit 26.

[0108] The operating modes of the heat pump system 10 will now be described in more detail. As described above, the normal heating mode is configured to heat the interior of the building while simultaneously charging the heat storage device 16 (i.e., storing heat energy within the phase change material). The defrosting mode is configured to heat the building while simultaneously defrosting the evaporator 20.

[0109] When the heat pump system 10 is operating in normal heating mode, the four-way valve 42 is configured to directly connect the condenser 14 to the thermal energy storage device 16, such as Figure 1 As shown. The four-way valve 42 is also configured to connect the evaporator 20 to the compressor 12. In particular, the refrigerant flow leaving the condenser 14 is directed through the four-way valve 42, then to the thermal energy storage device 16, through the expansion device 18, through the evaporator 20, and then through the four-way valve 42 to the compressor 12 via the phase separator 22.

[0110] At the start of the thermodynamic cycle in normal heating mode, compressor 12 draws vapor from separator 22, increasing its pressure and temperature. The superheated refrigerant vapor (e.g., 91°C) is directed to condenser 14, where it transfers heat into the building at a heat-generating temperature (e.g., 65°C). This causes the gaseous refrigerant to condense into a warm liquid at a higher temperature (e.g., 70°C).

[0111] After leaving the condenser 14, the warm liquid refrigerant is directed to the thermal energy storage device 16, where it transfers heat to the phase change material housed therein. This causes the liquid refrigerant to cool to a subcooled temperature (e.g., approximately 35°C, which is slightly higher than the melting temperature of the phase change material, i.e., 30°C).

[0112] Upon exiting the thermal energy storage unit 16, the subcooled liquid refrigerant then passes through the expansion unit 18 and is guided through the evaporator 20. As the refrigerant passes through the evaporator 20, it absorbs heat energy from the outside air (e.g., 0°C). The refrigerant then exits the evaporator 20 as a cold, low-pressure vapor. Finally, the refrigerant flows to the liquid / vapor separator 22 to begin the next cycle of the refrigerant circuit 26. During normal heating mode, each of the internal and external fans 34 is energized to improve the heat transfer efficiency through the corresponding condenser 14 and evaporator 20.

[0113] Now refer to, as follows Figure 2The illustrated pressure-enthalpy (p-h) curve describes the thermodynamic characteristics of the normal heating mode. The presently described p-h curve is based on an outdoor air temperature of approximately 0°C, a heat production temperature of approximately 65°C (i.e., the operating temperature of the central heating system), and a phase change material having a melting temperature of approximately 30°C.

[0114] At the beginning of the normal heating mode cycle, hot refrigerant vapor exits the compressor 12 (as indicated by point A on the p-h curve) and flows to the condenser 14 at a temperature of approximately 70°C. The refrigerant is condensed by the condenser 14 into a liquid, which is then directed to the thermal energy storage device 16 (B) at a temperature of approximately 70°C. The thermal energy storage device 16 recovers heat from the refrigerant and cools it into a subcooled liquid at a temperature of approximately 35°C (C). The refrigerant, after expansion in the expansion device 18, assumes the form of a gas-liquid saturated mixture at a temperature of approximately -10°C (D). The refrigerant then enters the evaporator 20, where it absorbs heat from outdoor air at a temperature of approximately 0°C. The refrigerant then becomes saturated vapor at approximately -10°C, and then enters the separator 22, followed by the compressor 12 (E).

[0115] In known single refrigerant circuit heat pump systems, when the refrigerant is directed through the expansion device 18 and is“throttled” by the expansion device 18, the“surplus” thermal energy from the condenser is wasted. The present invention utilizes the heat recovered from the refrigerant exiting the condenser 14 to defrost the evaporator 20, and therefore defrosting does not require an additional electric heater.

[0116] Reference will now be made to the drawings Figure 3 and Figure 4 The defrost mode of the heat pump system 10 will now be described. The four-way valve 42 is configured to switch the heat pump system to operate in the defrost mode upon determining that ice is forming on the surface of the evaporator 20. The switching assembly 40 is configured to directly connect the condenser 14 to the evaporator 20, and to connect the thermal energy storage device 16 to the compressor 12. According to this configuration, the refrigerant flow exiting the condenser 14 is directed through the four-way valve 42, the evaporator 20, the expansion device 18, the thermal energy storage device 16, and then back through the four-way valve 42, through the phase separator 22 to the compressor 12. In this manner, the switching assembly 40 is configured to reverse the flow direction of the refrigerant through the thermal energy storage device 16, the expansion device 18, and the evaporator 20. It will therefore be appreciated that the inlet and outlet of each relevant component of the refrigerant circuit are reversed when switching between the normal heating and defrost modes of the heat pump system 10.

[0117] As with the normal heating mode, at the beginning of the defrost mode, the compressor 12 draws vapor from the liquid / vapor separator 22 and increases its pressure and temperature. The superheated refrigerant vapor is directed to the condenser 14 to transfer heat to the building at a temperature of, for example, 65°C. This causes the gaseous refrigerant to condense into a warm liquid at a higher residual temperature, for example, about 70°C.

[0118] After leaving the condenser 14, the warm liquid refrigerant is directed to the evaporator 20, causing the ice on the outer surface of the evaporator 20 to melt. As a result, the warm liquid refrigerant is subcooled by the evaporator 20. The subcooled liquid refrigerant then passes to the expansion device 18, which further reduces its temperature and pressure. The refrigerant is then directed to the thermal energy storage device 16, absorbs thermal energy from the phase change material, and becomes superheated vapor. This superheated vapor refrigerant is finally directed to the separator 22, and the cycle begins again.

[0119] In the exemplary arrangement of the heat pump system 10, an internal fan can be provided to direct air through the condenser 14. For example, when operating the heat pump system 10 in the defrost mode, the internal fan can be energized to increase the efficiency of heat transfer from the condenser 14 to the interior of the building.

[0120] Reference is made to Figure 4 The illustrated p-h curve describes the thermodynamic characteristics of the defrost mode. At the beginning of the defrost mode cycle, the hot refrigerant vapor from the compressor 12 (as indicated by point A on the p-h curve) condenses in the condenser 14 at a temperature of about 70°C. The refrigerant leaves the condenser 14 and enters the evaporator 20 at a temperature of about 70°C (B). In this case, the evaporator 20 acts as a subcooler for the refrigerant circuit, absorbing heat from the warm liquid refrigerant and cooling it to a subcooled liquid at a temperature slightly higher than the temperature of the evaporator 20 (for example, about 40°C), which varies during the defrost operation as the ice melts.

[0121] The temperature of the refrigerant at the evaporator 20 increases as the amount of ice decreases until all of the ice has melted (i.e., until the evaporator is fully defrosted). The subcooled refrigerant is then directed through the expansion device 18. After expansion in the expansion device 18, the refrigerant is in the form of a saturated gas and liquid mixture at a temperature below 30°C (for example, about 25°C) (D). The refrigerant then enters the thermal energy storage device 16, is evaporated, and superheated to a temperature of about 25°C. Finally, the refrigerant is directed through the separator 22 and toward the compressor 12, whereupon the cycle begins again (E).

[0122] It should be appreciated that in the defrost mode, the thermal energy recovered by the thermal energy storage device 16 during the normal heating mode is used as a heat source to provide heat to the building. The use of the thermal energy storage device 16 as an auxiliary heat source improves the COP of the heat pump system 10.

[0123] In particular, when the heat pump system 10 is operated in the defrost mode, the temperature lift of the heat pump (i.e. the increase in the temperature of the refrigerant provided by the compressor 12) is less than the corresponding temperature lift in the normal heating mode. This is because the temperature of the phase change material is about 30°C, which is much higher than the outdoor air temperature of about 0°C. Therefore, the COP of the heat pump system 10 in the defrost mode is significantly higher than the COP in the normal heating mode. This means that once the thermal energy storage device 16 is fully charged, even if there is no ice on the evaporator 20, the heat pump system 10 can switch from the normal heating mode to the defrost mode in order to achieve a higher COP.

[0124] To demonstrate the performance difference between the normal heating mode and the defrost mode, a representative simulation was performed for each mode based on data obtained from a commercial simulation software.

[0125] For the normal heating mode simulation, the parameters were determined as follows:

[0126] • Outdoor air temperature: 0°C

[0127] • Condenser outlet temperature: 65°C

[0128] • Compressor isentropic efficiency: 75%

[0129] • Melting temperature of phase change material: 30°C

[0130] • Approach temperature difference in condenser 5°C (e.g. minimum temperature difference between refrigerant in refrigerant circuit and heat transfer fluid in building central heating system).

[0131] • Approach temperature difference in thermal energy storage device 10°C (e.g. when using the thermal energy storage device as a heat source).

[0132] The results of the normal heating mode simulation are summarized as follows:

[0133] • Refrigerant mass flow rate: 0.027 kg / s

[0134] • Evaporation pressure: 2 bar

[0135] • Condensation pressure: 21.28 bar

[0136] • Compressor power consumption: 1.486 kW (5348 kJ / h)

[0137] • Thermal power output: 4 kW (= 14500 kJ / h)

[0138] • Heat extracted from outdoor air: 2.514 kW (8182 kJ / h)

[0139] • COP = 14500 / 5348 = 2.71 2.7

[0140] • Enthalpy of refrigerant at state point B, hB= 295.51 kJ / kg

[0141] • Enthalpy of refrigerant at state point C, hC= 248.77 kJ / kg

[0142] • Enthalpy difference: Ah = hB - hC = 47 kJ / kg

[0143] • Heat recovered by thermal storage device: m * (hB - hC) = 0.027 * 47 = 1.26 kW

[0144] For the defrost mode simulation, the parameters were determined as follows:

[0145] • Thermal storage device temperature: 30°C

[0146] • Condenser output temperature: 65°C

[0147] • Compressor isentropic efficiency: 75%

[0148] • Approach temperature in condenser 5°C

[0149] • Approach temperature of thermal storage device 10°C (e.g. difference when thermal storage device is used as heat source)

[0150] The results of the defrost mode simulation are summarized as follows:

[0151] • Refrigerant mass flow: 0.029 kg / s

[0152] • Evaporation pressure: 6.6 bar

[0153] • Condensation pressure: 21.28 bar

[0154] • Compressor power: 0.92 kW (3334 kJ / h)

[0155] • Heat output of condenser: 4 kW (= 14400 kJ / h)

[0156] • Heat extracted from thermal storage device: 3.1 kW (11200 kJ / h)

[0157] • COP = 14500 / 3334 = 4.36 4.35

[0158] From these simulation results, it is clear that the defrost mode COP (4.35) is significantly higher than the normal heating mode COP (2.7). The higher COP in the defrost mode means that the heat pump system 10 can preferentially switch from the normal heating mode to the defrost mode once the thermal energy storage device 16 is fully charged. It is contemplated that in some cases, configuring the heat pump system 10 to periodically and repeatedly operate in the defrost mode can advantageously prevent frost or ice buildup on the exterior surface of the evaporator 20.

[0159] In view of this, it is necessary to determine how long and how often the heat pump system 10 should operate in the normal heating mode and / or the defrost mode in order to optimize the performance of the system over time. From the above simulation, the thermal energy storage device 16 is able to recover heat from the hot refrigerant at a rate of 1.26 kW in the normal heating mode. In the defrost mode, the heat recovered from the thermal energy storage device 16 can be transferred back into the refrigerant at a rate of 3.1 kW.

[0160] Consider a scenario in which the heat pump system 10 operates in the normal heating mode until the thermal energy storage device 16 is fully charged, and then operates in the defrost mode to fully discharge the recovered heat into the refrigerant circuit 26. Assuming an energy balance in the system, then all of the heat transferred into the thermal energy storage device in the normal heating mode can be discharged in the defrost mode, such that:

[0161] Charge time * Charge power = Discharge time * Discharge power (1)

[0162] Rearranging equation (1) then gives:

[0163] Charge time / Discharge time = Discharge power / Charge power (2)

[0164] = 3.1 / 1.26 = 2.46

[0165] From equation (2), it can be seen that the heat pump system 10 must operate in the normal heating mode for 2.46 time units in order to fully charge the thermal energy storage device. The thermal energy storage device 16 will then take 1 time unit to fully discharge.

[0166] It should be appreciated that the specific time units spent in each of the normal and defrost modes will depend on the capacity of the thermal energy storage device 16, and the time ratio will also depend on the melting temperature of the phase change material, which determines the thermal energy that can be recovered while operating in the normal heating mode. The time spent in each operating mode will also depend on the temperature at which the thermal energy can be released from the thermal energy storage device during the defrost mode. Thus, the optimal operating conditions for the heat pump system will be determined by an understanding of the trade-off between these two configurations of the thermal energy storage device (i.e., the transfer of thermal energy into and out of the refrigerant circuit).

[0167] As Figure 8 shown, periodically switching between normal heating mode and defrost mode is believed to more effectively utilize the thermal energy recovered by the thermal energy storage device 16, and in this way also utilizes the higher COP defrost mode. The time ratio calculated from equation (2) is used to calculate the percentage of time the system can operate in defrost mode to benefit from its higher COP:

[0168] 1 / (1 + 2.46) * 100% = 28.9% (3)

[0169] Figure 8 The average COP over the period of operation shown is calculated as follows:

[0170] Average COP = (2.8 * 2.46 + 4.35 * 1) / (2.46 + 1) = 3.24 (4)

[0171] By comparing the result of equation (4) to the alternative of using only normal heating mode (i.e., no defrost mode), the percentage COP improvement can be calculated as:

[0172] (3.24 - 2.8) / 2.8 * 100% = 15.7% (5)

[0173] Thus, by periodically switching between normal heating mode and defrost mode (as Figure 8 shown), the COP of the heat pump system 10 can be improved by 15.7% compared to a heat pump system that does not recover condenser waste heat.

[0174] In summary, the heat pump system 10 exhibits a significantly higher COP when operating in defrost mode. Furthermore, it has been shown that by periodically switching between normal heating mode and defrost mode, a higher system COP can be obtained for a greater proportion of the operating life of the heat pump system (even when there is no ice on the evaporator 20).

[0175] During normal heating mode, the supercooling of the refrigerant by the thermal energy storage device 16 reduces the irreversible throttling losses caused by the refrigerant flowing through the expansion device 18. In this way, transferring heat into the energy storage device 16 increases the COP of the heat pump system 10.

[0176] Furthermore, since additional electrically powered heating does not have to be provided to defrost the evaporator, the present invention achieves a 5-10% efficiency gain over known heat pump systems.

[0177] Furthermore, in the present case study, the COP of the system can be further improved by 15.7% compared to a heat pump that does not recover such waste heat from the condenser by periodically switching to a defrost mode to utilize the waste heat recovered from the supercooled heat. It will be appreciated that the change in COP of the system will depend on the particular configuration of the system and the operating conditions.

[0178] Reference is now made to Figures 5 to 8 a second alternative heat pump system according to the present application is described.

[0179] For the sake of brevity, Figure 5 the features and functions thereof described with reference to Figure 1 and Figure 3 are identical or similar to those described with reference to the first arrangement and are assigned similar reference numerals, but increased by 100 and, where appropriate, appended by the suffix "a" to distinguish from the latter arrangement, and are not repeated here.

[0180] The heat pump system 110a is configured to provide a flexible, multi-mode air source heat pump that utilizes a novel defrost mechanism and is capable of continuously heating the building in which it is installed.

[0181] Similar to the system described previously, as shown in Figure 1 and Figure 3 , the heat pump system 110a includes a compressor 112, a condenser 114, a thermal energy storage device 116, a first expansion device EV1 (e.g., a first expansion valve), an evaporator 120, and a phase separator 122, which are fluidly connected in series by fluid conduits 124 to define a refrigerant circuit 126. An external fan 134 is used to direct airflow across the coils or fins of the evaporator 120 to improve its operational efficiency.

[0182] The heat pump system 110a further includes a refrigerant storage device 108 arranged between the condenser 114 and the thermal energy storage device 116. The refrigerant storage device 108 includes a container or tank configured to contain a volume of refrigerant, as will be readily understood by those skilled in the art. The refrigerant storage device 108 is configured to compensate for any fluctuations in the volume of refrigerant flowing within the fluid circuit 124. For example, the heat pump system 110a is configured to maintain a volume of refrigerant in the evaporator 120 during a defrost mode (as described below). In this case, the refrigerant storage device 108 is configured to compensate for the reduction in the volume of refrigerant circulating within the circuit.

[0183] The thermal energy storage device 116 includes a phase change material housed within an enclosure 130 configured to allow direct thermal contact with a conduit portion 128 of the refrigerant circuit 126. The thermal energy storage device 116 is configured to passively transfer thermal energy between the phase change material in the refrigerant circuit 126 and the refrigerant.

[0184] According to an alternative exemplary arrangement, the thermal energy storage device 116a is configured such that the phase change material is thermally connected to the pipe section 128 of the refrigerant circuit 126 by a heat transfer fluid circuit 136, as shown in Figure 7 The heat transfer fluid (e.g. water) is guided through the secondary circuit 136 by a mechanical fluid pump 146 and is thermally connected to the first refrigerant in the first refrigerant circuit 126 by a heat exchanger 138. It will be appreciated that the thermal energy storage device 116a can be connected to the refrigerant circuit 126 between the points AA-BB, as shown in Figure 1 and Figure 5

[0185] The thermal energy storage device 116a is configured such that the transfer of heat into the refrigerant circuit 126 can be actively controlled by operation of the pump 146, as will be appreciated by the skilled person.

[0186] The heat transfer fluid circuit 136 can be arranged outside the heat pump system. It can thus be configured with a large thermal storage capacity (compared to a phase change material thermal storage unit arranged internally). The heat transfer fluid circuit 136 is configured to operate at atmospheric pressure, which means that it can be made of components that are easy to manufacture and control (e.g. less costly).

[0187] The heat pump system 110a has four different modes of operation, namely a normal heating mode, a defrosting mode, an auxiliary heating mode and a charging mode. These four modes correspond to modes 1, 2, 3 and 4, respectively, outlined in Table 1 below.

[0188] Table 1: Four modes of operation (valves - open / close, fans - on / off)

[0189]

[0190] The heat pump system 110a comprises a switching assembly 140a configured to control the flow of refrigerant around the refrigerant circuit 126 in order to switch between the different modes of operation of the heat pump system 110a.

[0191] In particular, the switching assembly comprises six control valves VI, V2, V3, V4, V5, V6, which are independently configurable to achieve the required mode of operation. The switching assembly 140a further comprises four bypass assemblies or fluid conduits 144a, 144b, 144c, 144d, which are configured to bypass different components and sections of the refrigerant circuit 126 depending on the required mode of operation.

[0192] According to an alternative exemplary arrangement of the heat pump system 110b, the six two-way control valves VI-6 are replaced by four three-way valves V10, V12, V14, V16, as shown in Figure 9 which defines the switching assembly 140b of the system. It will be appreciated that, as Figure 9 ​The operation of the illustrated heat pump system 110b is substantially the same as Figure 5

[0193] Table 1 also outlines the different valve configuration combinations (i.e. open / close) required to configure the heat pump system 110a in the four operating modes. The required operating state (i.e. on / off) of the external fan 134 is also included in the table, corresponding to each of the four system operating modes.

[0194] When the heat pump system 110a is operating in the normal heating mode (i.e. mode 1), refrigerant from the compressor 112 is directed through the second control valve V2, the thermal energy storage device 116, the first expansion valve EV1, the sixth control valve V6, the evaporator 120, the phase separator 122, and back to the compressor 112 to begin a new cycle. In this mode, the heat pump system 110a operates as a normal single-stage heat pump, such that heat extracted from the outdoor air is released into the building through the condenser 114. The thermal energy storage device 116 acts as a subcooler to recover heat from the hot liquid refrigerant discharged by the condenser 114. As a by-product, the liquid refrigerant is subcooled by transferring thermal energy from the refrigerant to the phase change material within the thermal energy storage device 116. The recovered heat is stored in the phase change material, such that it can be used in subsequent active operating modes of the heat pump system 110a. The thermodynamic cycle corresponding to the normal heating mode is described by the points “A-B-C-D-E-A” as shown in the pressure-enthalpy (p-h) curve shown in Figure 6

[0195] When the heat pump system 110a is operating in the defrost mode (i.e. mode 2), the switching assembly 140a is configured to direct thermal energy to the evaporator and condenser, as described in relation to the previous embodiment. This is achieved by the second bypass assembly 144b directing refrigerant through a second expansion device EV2 (e.g. expansion valve) fluidly connected to the refrigerant circuit 126 between the condenser 114 and the thermal energy storage device 116. The switching assembly 140a is also configured to bypass the first expansion valve EV1 by diverting refrigerant through the first bypass assembly 144a, which is configured to isolate the first expansion valve EV1 from the refrigerant circuit 126.

[0196] Specifically, refrigerant exiting the compressor 112 is directed to flow through the first control valve V1, the second expansion valve EV2, where expansion occurs, thereby reducing the pressure and temperature. Accordingly, the expansion valve EV2 is configured to reduce the temperature of the refrigerant below the operating temperature of the thermal energy storage device 116, which ensures that thermal energy will be transferred from the storage device 116 into the refrigerant. It is also configured to equalise the refrigerant pressure within the refrigerant circuit.

[0197] ​​After the second expansion valve EV2, the refrigerant is then directed through the thermal energy storage device 116, becoming superheated refrigerant vapor. In this way, the thermal energy storage device 116 functions as an evaporator and superheater. Upon exiting the thermal energy storage device 116, the refrigerant is split into two streams, the first stream is directed through the fourth control valve V4, the third bypass assembly 144c, the phase separator 122, and finally the compressor 112, in that order.

[0198] The second stream is directed through the fifth control valve V5, the second bypass assembly 144b, and then into the evaporator 120. Depending on the defrost mode configuration of the heat pump system 110a, the thermal energy storage device 116 is thermally connected to the refrigerant circuit 126 between the second expansion valve EV2 and the evaporator 120. In this way, the refrigerant vapor flowing through the thermal energy storage device 116 is released of its latent heat to melt ice on the outer surface of the evaporator 120. The liquid refrigerant formed during this condensation process is temporarily retained in the evaporator 120 until the system switches to the normal heating mode.

[0199] The first refrigerant stream exiting the thermal energy storage device 116 is directed to the condenser 114 to provide heat to the building, while the second refrigerant stream is used to defrost the evaporator 120. Thus, the system provides continuous heating to the building interior during the evaporator defrost process.

[0200] Reference is made to Figure 6 , the continuous heating cycle of the first refrigerant stream is represented by the sequence “G-B-H-F-G”, and the second refrigerant stream is represented by the sequence “G-B-H-F-I”. In particular, the sequence “F-I” represents the heat released when the superheated refrigerant vapor flows through the evaporator 120 via the first bypass assembly 144a.

[0201] When operating in the defrost mode, the switching assembly 140a recovers the heat from the refrigerant exiting the condenser (typically wasted during the throttling process) as a heat source for defrosting the evaporator 120, thereby eliminating the need for an input of additional thermal energy for evaporator defrosting.

[0202] The heat pump system can operate in an auxiliary heating mode, in which thermal energy is transferred from the thermal energy storage device 116 to the refrigerant without any input from the evaporator 120. To this end, the switching assembly 140a is configured to bypass the expansion valve EV1 and the evaporator 120.

[0203] Specifically, the refrigerant from the compressor 112 is directed to flow through the condenser 114, the first control valve V1, the first expansion valve EV1, the thermal energy storage device 116, the fourth control valve V4, the third bypass assembly 144c, the phase separator 122, and finally back to the compressor 112 to start a new cycle. In this mode, the thermal energy storage device 116 operates as an evaporator of the system, and the heat stored in the thermal energy storage device 116 is used as an auxiliary heat source. The corresponding thermodynamic cycle is denoted as “F-G-B-H-F”, as shown in Figure 6 .

[0204] Similar to the defrosting mode in the first embodiment (as shown in Figures 1 to 4 ), the auxiliary heating mode of the present embodiment is configured to exhibit a higher COP than the normal heating mode. For example, in the exemplary arrangement of the heat pump system 110a, the COP values of the normal heating and auxiliary heating modes are 2.8 and 4.35, respectively. This is because, in the auxiliary heating mode, the thermal energy storage device 116 replaces the evaporator 120 as the primary heat source of the system. Compared to the outdoor air (~0°C), the phase change material transfers heat to the refrigerant at a significantly higher temperature (~30°C), which results in a higher COP rating (reasons refer to the first embodiment).

[0205] The exemplary operating scheme of the heat pump system 110a involves periodically switching between the two heating modes, as shown in Figure 8 . The time and duration of each switch are determined such that 28.9% of the time is in the auxiliary heating mode, which results in a 15.7% increase in COP compared to using only the normal heating mode. Another advantage of the heat pump system 110a is that the switching assembly 140a enables the refrigerant circuit 126 to easily switch between the normal heating mode (e.g., to charge the thermal energy storage device 116) and the auxiliary heating mode (e.g., to utilize the stored thermal energy), thereby increasing the flexibility of the system.

[0206] The heat pump system 110a can also operate in a charging mode (i.e., mode 4), in which thermal energy is transferred from the refrigerant to the thermal energy storage device 116. In this mode, the switching assembly 140a is configured to direct the refrigerant leaving the compressor 112 to bypass the second expansion valve EV2 and the condenser 114.

[0207] When operating in the charging mode, the refrigerant from the compressor is directed to flow through the third control valve V3, the fourth bypass assembly 144d, the thermal energy storage device 116, the first expansion valve EV1, the evaporator 120, the phase separator 122, and then back to the compressor 112 to start a new cycle.

[0208] In this charging mode, the condenser 114 can be turned off and the thermal energy storage device 116 used as a secondary condenser. This mode of operation is particularly advantageous in situations where there is no heat demand in the building but the outdoor temperature is high, so that heat can be efficiently extracted from the outside environment and stored in the thermal energy storage device 116 for later use. This can occur, for example, when there is no one in the building during the day. The corresponding thermodynamic cycle is denoted by "E-J-K-L-E", as shown in Figure 6

[0209] In known single-stage heat pump systems, heat is stored at the operating temperature of the condenser (i.e. heat can be extracted from the refrigerant as it flows between the compressor and the condenser). The charging mode according to the present application causes heat to be absorbed by the thermal energy storage device 116 at an intermediate temperature (i.e. heat is transferred from the refrigerant as it flows out of the condenser). Configuring and operating the heat pump system in this way, the charging mode reduces the thermal energy storage capacity by a factor of 1 / COP while maintaining the operating temperature of the condenser. Thus, the heat pump system 110a can be configured to have a relatively small thermal energy storage capacity, which reduces the complexity and cost of the system. According to the above example case study, the COP of the charging mode is 4.35. Thus, the thermal energy storage capacity can be reduced by a factor of 1 / 4.35 = 23%.

[0210] According to an alternative example arrangement, the thermal energy storage device 116a is configured so that the phase change material is thermally connected to the conduit 128 of the refrigerant circuit 126 by a heat transfer fluid circuit 136, as shown in Figure 7 The heat transfer fluid (e.g. water) is directed through the second circuit 136 by a fluid pump 146. The heat transfer fluid in the second refrigerant circuit 136 is thermally connected to the refrigerant in the first refrigerant circuit 126 by a heat exchanger 138.

[0211] The thermal energy storage device 116a is configured so that the transfer of heat into the refrigerant circuit 126 can be actively controlled by operation of the pump 146, as will be appreciated by the skilled person. This contrasts with arrangements such as that shown in Figure 5 where the thermal energy storage device 116 is configured to pass energy into and out of the refrigerant circuit 126 passively.

[0212] Advantageously, the thermal energy storage device 116a can be provided externally of the heat pump system 110a, and so a larger thermal capacity (i.e. a larger volume of phase change material) can be configured. The larger capacity thermal energy storage device 116a can be used to store heat when off-peak electricity and / or warm air is more freely available but there is low demand for heat in the building.

[0213] Two-stage heat pump system

[0214] ​The previously described heat pump systems 10, 110a, 110b are all single-stage heat pump systems as they each include only a single refrigerant circuit 26, 126. A plurality of dual-stage heat pump systems 210a, 210b, 210c according to another aspect of the present disclosure will now be described with reference to Figures 10 to 17

[0215] For the sake of brevity, Figures 10 to 17 the features and their functions in the drawings of Figs. 10a, 10b, 10c are the same or similar to those already described with reference to Figures 1 to 9 the drawings of Figs. 1, 2, 3, 4, 5, 6, 7, 8, 9, and are assigned similar reference numerals increased by 100, but will not be described again here.

[0216] With reference to Figure 10 the heat pump system 210a includes a first compressor 212a, a condenser 214, and a first expansion device EV21 (e.g., a first expansion valve) fluidly connected in series by fluid conduits 224 to define a first refrigerant circuit 226a. The heat pump system 210a further includes a second compressor 212b, a second expansion device EV22, and an evaporator 220 fluidly connected in series by fluid conduits 224 to define a second refrigerant circuit 226b.

[0217] A two-phase gas-liquid separator 222 (e.g., a flash tank) is fluidly connected between the first refrigerant circuit 226a and the second refrigerant circuit 226b. The first refrigerant circuit 226a is connected to a gas-containing portion (e.g., an upper portion) of the phase separator 222, while the second refrigerant circuit 226b is connected to a liquid-containing portion (e.g., a lower portion). The phase separator 222 is disposed upstream of the first compressor 212b, downstream of the first expansion device EV21, downstream of the second compressor 212b, and upstream of the second expansion device EV22, as shown in Figure 10 During operation of the heat pump system 210a, the conduit downstream of the expansion device EV21 contains a liquid-gas mixture of refrigerant. Accordingly, the output of the high-pressure stage can be fluidly connected to either the liquid-containing portion or the gas-containing portion of the phase separator 222. The input of the high-pressure stage is fluidly connected to the gas-containing portion, as shown in Figure 10

[0218] During operation of the heat pump system 210a, the refrigerant in the first circuit 226a circulates at a higher pressure and temperature than the refrigerant in the second circuit 226b. Accordingly, the first refrigerant circuit 226a and the second refrigerant circuit 226b represent a high-pressure / temperature stage and a low-pressure / temperature stage, respectively, of the heat pump system 210a. The phase separator 222 is fluidly connected at a location between the refrigerant circuits 226a, 226b that represents an intermediate pressure / temperature state within the heat pump system 210a.

[0219] ​​Each of the high-stage and low-stage is configured to transfer thermal energy from one location to another within their respective refrigerant circuits 226a, 226b. For example, when the heat pump system 210a is operating in a heating mode, the high-stage transfers heat from the phase separator 222 to the condenser 214, while the low-stage transfers heat from the evaporator 220 to the phase separator 222. Thus, the first refrigerant circuit 226a and the second refrigerant circuit 226b are effectively two single-stage heat pump systems connected together by the phase separator 222.

[0220] The two-stage heat pump system 210a is suitable for situations where the temperature difference (i.e. temperature rise) between the outdoor and indoor environments is large, for example between 50°C to 70°C. According to exemplary operating conditions of the heat pump system 210a, the ambient air temperature can be in the range of -10°C to 10°C (i.e. at the evaporator 220). The central heating system of a building can require heat to be generated at a temperature between 50°C and 60°C (i.e. at the condenser 214). In this case, the phase separator 222 is arranged within the heat pump system 210a at an intermediate temperature between 20°C to 30°C. Thus, each of the low-stage and high-stage provides a temperature rise of about 25°C to 35°C.

[0221] The two-stage heat pump system 216a is also configured to operate at a higher COP than an equivalent single-stage heat pump system operating at the same temperature rise. For example, the two-stage heat pump system 210a has two compressors, and thus the pressure difference (i.e. pressure ratio) across each compressor can be reduced compared to a single compressor in a single-stage heat pump system. The phase separator 222 is configured to remove “flash” gas at an intermediate pressure, such that the gas is not throttled to the evaporating pressure but only recompressed, thereby saving compression power. The phase separator 222 also provides intermediate cooling for the gas discharged from the low-stage compressor 212b, further reducing the compression power required by the high-stage compressor 212a.

[0222] At least some of the problems associated with operating known single-stage heat pump systems also apply to known two-stage heat pump systems. For example, known two-stage heat pump systems are configured to defrost their outdoor heat exchanger by extracting heat from the indoor heat exchanger (i.e. reverse cycle method) or by extracting the hot vapour produced by the compressor (i.e. hot gas bypass method).

[0223] Each of these defrosting processes can take several minutes to fully defrost the outdoor heat exchanger. During defrosting, the indoor heat exchanger is effectively shut down, and thus the heat pump does not continuously provide heat. To this end, a backup electric heater is often required to provide heat during the outdoor heat exchanger defrosting process, which can reduce the annual average COP of the heat pump system by 5-10%.

[0224] As Figure 10As shown, the two-stage heat pump system 210a according to the present disclosure comprises a thermal energy storage device 216a thermally connected to the phase separator 222. By thermally connecting the thermal energy storage device 216a to the phase separator 222, this enables the heat pump system 210a to operate more efficiently, thereby addressing the problems of known two-stage heat pump systems.

[0225] The thermal energy storage device 216 comprises a phase change material housed within an enclosure 230. The phase change material is thermally connected to the phase separator 222 by a heat transfer fluid circuit 236. A heat transfer fluid (e.g. water) is directed through the heat transfer fluid circuit 236 by a mechanical fluid pump 246 and is thermally connected to the liquid refrigerant housed in the liquid containing portion of the phase separator 222 by a heat exchanger 238.

[0226] The thermal energy storage device 216 is arranged externally to the heat pump system 216a. As such, it can be configured to have a large thermal storage capacity (compared to a phase change material thermal storage unit arranged internally). Furthermore, by arranging the thermal energy storage device 216 externally to the other components of the heat pump system provides the possibility of recovering waste heat from other external sources. For example, warm waste water from a bath or shower can be directed through a pipe 500 thermally connected to the thermal energy storage device, as Figure 10 shown.

[0227] A switching assembly 240a can be provided to control the flow of refrigerant within the heat pump system 210a, thereby determining the operating mode of the system. The switching assembly 240a comprises a set of valves V21, V22, V23, V24 which are configured to switch the operation of the heat pump system 210a between different operating modes, as will be explained in more detail below.

[0228] Figure 11 An alternative two-stage heat pump system 210b according to the present disclosure is shown. The heat pump system 210b comprises substantially the same components as the heat pump system 210a shown in Figure 10 However, the switching assembly 240b is configured differently to allow the heat pump system 210b to operate in a continuous heating and defrost mode, as will be explained in more detail below.

[0229] In particular, the switching assembly 240b comprises a first bypass assembly 244a which fluidly connects the first refrigerant circuit 226a (at a location immediately downstream of the condenser 214) to the second refrigerant circuit 226b (at a location immediately upstream of the second expansion valve EV22), as Figure 11 shown. As such, the first bypass assembly 244a is configured to bypass the valves V21, V23, the first expansion valve EV21 and the phase separator 222.

[0230] The switching assembly 240b also includes a second bypass assembly 244b that fluidly connects the second refrigerant circuit 226b at a location immediately downstream of the evaporator 220 to the first refrigerant circuit 226a at a location immediately upstream of the first expansion valve EV21, as shown. Figure 11 The second bypass assembly 244b is configured to bypass the second compressor 212b and the switching valve V24. The switching valves V25, V26 are used to control the flow of refrigerant through the first bypass assembly 244a and the second bypass assembly 244b, respectively.

[0231] The operation of the heat pump systems 210a, 210b will now be described with reference to the pressure-enthalpy (p-h) curves shown in Figures 12 to 16 FIG. 3. The heat pump system 210a has four modes of operation, namely normal heating mode, heat charging mode, auxiliary heating mode, and heating enhancement mode. These four modes of operation correspond to modes 1, 4, 3, and 5, respectively, outlined in Table 2 below. The heat pump system 210b has the same four modes as the heat pump system 210a (i.e., modes 1, 4, 3, and 5), and it also has an additional defrost mode, which corresponds to mode 2 in Table 2 below.

[0232] Table 2: Modes of operation (valves - open / close; compressors and pumps - on / off)

[0233]

[0234] Table 2 outlines the different configurations of the valves V21-V26 (i.e., open / close), the pump 246 (i.e., on / off), and the compressors 212a, 212b (i.e., on / off) required to operate the heat pump systems 210a, 210b in the different modes.

[0235] When the heat pump systems 210a, 210b are operated in the normal heating mode (i.e., mode 1), the compressors 212a, 212b are on, the switching valves V21-V24 are open, and the pump 246 is off. Additionally, for the heat pump system 212b, the valves V25, V26 are closed. Accordingly, the high-pressure stage of refrigerant is directed by the first compressor 212a through the condenser 214, the first valve V21, the first expansion valve EV21, the phase separator 222, the second valve V22, and back to the compressor 212 to start a new cycle. At the same time, the low-pressure stage of refrigerant is directed by the second compressor 212b through the phase separator 222, the third valve V23, the second expansion valve EV22, the evaporator 220, the fourth valve V24, and back to the second compressor 212b to start a new cycle. In this mode, the heat pump system 110a operates as a normal two-stage heat pump such that the heat extracted from the outdoor air is released into the building through the condenser 214. The corresponding thermodynamic cycles for the high-pressure stage and the low-pressure stage in the normal heating mode are shown in Figure 12 FIG. 4.

[0236] During the charging mode (i.e. mode 4), valves V21, V22, V25, V26 are closed and the first compressor 212a is closed. Thus, the high pressure stage is deactivated. At the same time, valves V23 and V24 remain open, the second compressor 212b is open. In addition, the pump 246 is open to direct heat between the phase separator 222 and the thermal energy storage 216a, 216b. Thus, the phase separator 222 in combination with the TES 216a, 216b acts as a condenser, effectively for the low pressure stage of the heat pump system 210a, 210b. This can be achieved by controlling the pressure within the phase separator 222. Thereby, heat from the outdoor air is transferred through the low pressure stage to an intermediate temperature range, which is subsequently used to charge the thermal energy storage 216a, 216b. The corresponding thermodynamic cycle for the charging mode is described by the points "H-I-J-G-H" as shown in Figure 13 .

[0237] This mode of operation is particularly suitable for situations where heat can not be required, for example during the day when the occupants of the building are not present but the outdoor temperature is warm (i.e. high COP conditions). The thermal energy collected in the thermal energy storage 216a, 216b is stored for later use, for example when the occupants of the building return in the evening, the outdoor temperature is lower and the heat demand is high. Once the phase change material in the thermal energy storage 216a, 216b is fully charged, the low pressure stage can also be deactivated by switching the second compressor 212b and the pump 246.

[0238] During the auxiliary heating mode (i.e. mode 3), valves V21 and V22 are open, valves V23, V24, V25 and V26 are closed. The first compressor 212a and the pump 246 are both open, while the second compressor 212b is closed. Thus, the low pressure stage is deactivated. By reducing the pressure in the phase separator 222, it effectively acts as an evaporator for the high pressure stage of the heat pump system 210a, 210b. Heat is extracted from the thermal energy storage 216a, 216b and transferred to the refrigerant in the phase separator 222, which is then circulated by the high pressure stage to the condenser 214. The corresponding thermodynamic cycle for the auxiliary heating mode is described by the points "K-A-B-L-K" as shown in Figure 14 .

[0239] The heat pump system 210a, 210b effectively transforms into a single stage heat pump system using the thermal energy storage 216a, 261b as a heat source to provide heat. It will be appreciated that the thermal energy storage 216a, 216b is preferably fully charged when the auxiliary heating mode is operated. The auxiliary heating mode is particularly suitable for operation during cold nights following a warm day when the occupants of the building have left, and thus the system has been operated in the charging mode.

[0240] During the heating enhancement mode (i.e. mode 5), valves V21-V24 are open, and valves V25 and V26 (in system 216b) are closed. Both compressors 212a, 212b are on, and pump 246 is also on. In this mode of operation, the pressure in phase separator 222 is reduced, such that its temperature is slightly lower than the melting temperature of the thermal energy storage devices 216a, 216b. As a result, thermal energy from thermal energy storage devices 216a, 216b is directed into the refrigerant in phase separator 222. In this way, phase separator 222 acts as both a phase separator for the high pressure stage and an evaporator, and the system effectively transforms into a dual evaporator and dual compressor heat pump system. The corresponding thermodynamic cycle for the heating enhancement mode is shown as Figure 15 .

[0241] When operating in the heating enhancement mode, both the heat from thermal energy storage devices 216a, 216b and the heat from the outdoor air (i.e. the heat absorbed by evaporator 220) are used as heat sources to increase the thermal capacity of heat pump systems 210a, 210b. This mode is therefore particularly suitable for situations where there is a high demand for heating at night but the outdoor temperature is low.

[0242] During the defrosting mode (i.e. mode 2) which is only applicable to heat pump system 216b, valves V21, V22, V25 and V26 are open while valves V23 and V24 are closed. First compressor 212a and pump 246 are on, while second compressor 212b is off. As a result, the low pressure stage is deactivated. The warm liquid refrigerant leaving condenser 214 is directed through evaporator 220, releasing its remaining heat to melt the ice formed on it. The refrigerant is then throttled by second bypass component 244b into phase separator 222. As a result, heat pump system 210b uses the excess heat carried by the warm liquid refrigerant leaving condenser 214 to defrost evaporator 220. The unused heat from condenser 214 is a waste heat source, and therefore does not need to be used to heat the interior of the building. The corresponding thermodynamic cycle for the defrosting mode is described by points “K-A-N-L-K” as shown in Figure 16 .

[0243] During the defrosting mode, phase separator 222 transforms into an evaporator for the high pressure stage. The heat stored in thermal energy storage device 216b is extracted and used to heat the refrigerant in phase separator 222 to the temperature required for effectively operating the high pressure stage of heat pump 210a. In this way, heat pump system 216b is converted into a single stage system using thermal energy storage device 216b as a heat source to provide heat. In this way, evaporator 220 can be defrosted without interrupting the continuous supply of heat to condenser 214. Furthermore, unlike the conventional defrosting mode of a dual stage heat pump system, the defrosting of evaporator 220 does not require additional power. The defrosting mode is particularly suitable for situations where the outdoor ambient temperature is low and evaporator 220 starts to frost.

[0244] Figure 17 An alternative two-stage heat pump system 210c according to the present disclosure is shown. The heat pump system 210c includes substantially the same components as the heat pump systems 210a and 210b, as shown in Figure 10 and Figure 11 However, its switching assembly 240c is configured to provide an alternative way of defrosting the evaporator 220.

[0245] The switching assembly 240c includes a first bypass assembly 244c fluidly connected to the second refrigerant circuit 226b at a first location immediately downstream of the phase separator 222 and at a second location immediately upstream of the evaporator 220, as shown in Figure 17 Accordingly, the first bypass assembly 244c is configured to bypass the third valve V23 and the second expansion valve EV22. A second bypass assembly 244d is fluidly connected between the second refrigerant circuit 226b at a first location immediately downstream of the evaporator 220 and the liquid- containing portion of the phase separator 222. Accordingly, the second bypass assembly 244d is configured to bypass the second compressor 212b and the fourth switching valve V24. Switching valves V27, V28 are used to control the flow of refrigerant through the first bypass assembly 244c and the second bypass assembly 244d, respectively. The heat transfer fluid circuit 236 is equipped with a first pump 246a to control the flow of refrigerant between the phase separator 222 and the thermal energy storage device 216b, and the second bypass assembly 244d is equipped with a second pump 246b located downstream of the valve V28.

[0246] During the operating modes 1, 4, 3, and 5, the heat pump system 216c operates in a similar manner to the previously described two-stage heat pump systems 216a, 261b. During these operating modes, the valves V27, V28 are closed and the pump 246b is closed. When operating in defrost mode, the valves V23 and V24 are closed while the valves V27, V28 are open and the second pump 246b is on. Accordingly, the warm refrigerant from the phase separator 222 is pumped through the bypass assemblies 244c, 244d and directed through the evaporator 220 to melt ice thereon. It should be appreciated that this configuration of the heat pump 216c requires an additional fluid pump 246b, which increases the complexity and cost of the system. Defrost time can also be longer than the heat pump system 216b, as shown in Figure 11

[0247] ​The two-stage heat pump systems 210a, 210b, 210c improve operational flexibility compared to single-stage heat pump systems and known two-stage heat pump systems. For example, they provide flexibility to charge the thermal energy storage devices 216a, 216b with off-peak electricity and / or warm outdoor air during the day, when heat is not needed but the outdoor temperature is warm (e.g., during the day). The two-stage heat pump systems of the present disclosure also maximize heat production during heat demand peaks by using the thermal energy storage devices and outdoor air as heat sources. In addition, the heat pump systems can store heat at an intermediate temperature (i.e., at the phase separator 222), which requires smaller storage sizes than storing heat at the production temperature (i.e., at the condenser 214). For example, a medium-pressure thermal energy storage device (according to the present disclosure) can be 20-40% smaller than an equivalent high-pressure thermal energy storage device while maintaining the same performance. The size reduction is related to the COP of the high-pressure stage (e.g., 1 / COP). For example, if the COP of the high-pressure stage is 3, the size reduction of the intermediate thermal energy storage device can be 33%.

[0248] In addition, the heat pump systems 216b, 216c can continuously provide heat to the condenser 214 while defrosting the evaporator 220. The system does not require reversing the refrigerant cycle to defrost the evaporator 220. In addition, the system does not consume additional electricity at the low-pressure stage compressor 212b during defrosting.

[0249] In addition, defrosting does not require a backup heater because the high-pressure stage continuously provides heat during the defrosting operating mode. The switching assemblies 240a, 240b, 240c rely on low-cost valves and bypass assemblies that are easy to install and control.

[0250] The previously described heat pump systems 210a, 210b, 210c each include two independent compressors 212a, 212b (i.e., one compressor for each of the high-pressure stage and the low-pressure stage). Now referring to Figure 18 and Figure 19 a pair of alternative two-stage heat pump systems 310a, 310b will be described, each system including a vapor-injected compressor 312 according to another aspect of the present disclosure.

[0251] For the sake of brevity, Figure 18 and Figure 19 features and their functions are the same as or similar to those features already described with reference to Figures 10 to 17 are assigned similar numerical designations but incremented by 100, and will not be described again here.

[0252] Referring to Figure 18 , the heat pump system 310a includes a vapor-injected compressor 312 fluidly connected to the system’s condenser 314, evaporator 320, and phase separator 322 by separate fluid conduits 324.

[0253] The vapor-injected compressor 312 has a low-pressure input for receiving refrigerant from the evaporator 320, a high-pressure output for outputting high-pressure refrigerant to the condenser 314, and a medium-pressure input for receiving refrigerant from the gas-containing portion of the phase separator 322. In operation, the pressure of the vaporized refrigerant received from the phase separator 322 is higher than the refrigerant leaving the evaporator 320, but lower than the pressure of the refrigerant leaving the compressor 312. The refrigerant from the phase separator 322 is “injected” into the compressor 312, thereby being compressed to the normal output pressure (i.e. high pressure output to the condenser 314), while only passing through a portion of the compressor 312.

[0254] The heat pump system 310a further comprises a first expansion device EV31 fluidly connected in sequence between the condenser 314 and the phase separator 322. A second expansion device EV32 is provided in sequence between the phase separator 322 and the evaporator 320, as shown. Figure 18 At least one of the first expansion device EV31 and the second expansion device EV32 is configured to control the downstream pressure of the refrigerant in the circuit 326, as will be appreciated by the skilled person. For example, the at least one expansion device is a modulating expansion valve.

[0255] The thermal energy storage device 316 is fluidly connected to the phase separator 322 by a separate refrigerant circuit 336 in the same manner as described above in relation to the heat pump systems 210a, 210b, 210c. The heat pump system 310a is further provided with a switching assembly 340a comprising valves V31, V32. The switching assembly 340a is configured to switch the operation of the heat pump system 310a between different operating modes, which will be explained in more detail below.

[0256] The arrangement of the phase separator 322 and the compressor 312 means that the heat pump 310a does not comprise two distinct refrigerant circuits, as was the case for the dual-stage heat pump systems 210a, 210b, 210c described previously. However, the low-pressure stage of the heat pump 310a is defined by the refrigerant flowing in sequence from the phase separator 322, to the second expansion device EV32, through the evaporator 320 and to the low-pressure input of the compressor 312. Similarly, the high-pressure stage of the heat pump system 310a is defined by the refrigerant flow in the sequence from the compressor 312, through the condenser 314, through the first expansion device EV31 and to the phase separator 322. The intermediate pressure stage of the heat pump system is defined, for example, by the refrigerant flow from the phase separator 322 to the medium-pressure input of the compressor 312.

[0257] Figure 19 An alternative dual-stage heat pump system 310b is shown, which also comprises a vapor-injected compressor according to the present disclosure. The heat pump system 310b comprises a compressor 312, a condenser 314, an expansion device EV31, an evaporator 320 and a phase separator 322, as shown. Figure 18The heat pump system 310a shown has substantially the same components. However, the switching assembly 340b is configured differently to allow the heat pump system 310b to operate in a continuous heating and defrost mode, which will be explained in more detail below.

[0258] In particular, the switching assembly 340b includes an additional valve V33 disposed sequentially between the condenser 314 and the first expansion valve EV31. The assembly also includes a first bypass assembly 344a fluidly connecting the refrigerant circuit 326 at a location immediately downstream of the condenser 314 to a location on the refrigerant circuit 326 immediately upstream of the second expansion valve EV32, as shown in the figure 19. Thus, the first bypass assembly 344a is configured to bypass the valves V31, V33, the first expansion valve EV31 and the phase separator 322. The switching assembly 240b also includes a second bypass assembly 344b fluidly connecting the refrigerant circuit 326 at a location immediately downstream of the evaporator 320 to a location immediately upstream of the first expansion valve EV31, as shown in the figure 20. Thus, the second bypass assembly 344b bypasses the compressor 312 and the switching valves V32, V33 and the condenser 314. The switching valves V34, V35 are used to control the flow of refrigerant through the first bypass assembly 344a and the second bypass assembly 344b, respectively. Figure 19

[0259] The operation of the heat pump systems 310a, 310b will now be described with reference to Table 3 below. The heat pump system 310a has four different modes of operation, namely a normal heating mode, a heat charging mode, a heating boost mode and an auxiliary heating mode. These four modes of operation correspond to modes 1, 4, 5 and 3, respectively, as outlined in Table 3. The heat pump system 310b has an additional defrost mode corresponding to mode 2, as outlined in Table 3.

[0260] Table 3: Modes of operation (valves - open / close; compressor and pump - on / off)

[0261]

[0262] Table 3 outlines the different configurations of the valves V31-V36 (i.e. open / close) and the pump 346 (i.e. on / off) required to operate the heat pump systems 310a, 310b in the different modes. The compressor 312 is activated in each mode of operation. The table also indicates the relative temperature of the refrigerant in the phase separator 322 relative to the temperature of the phase change material in the thermal energy storage device 316.

[0263] It will be appreciated that each numbered mode of operation (i.e. 1, 4, 5, 3 and 2) described in Table 3 corresponds to the modes of operation of the heat pump systems 210a, 210b outlined above in Table 2.

[0264] ​Thus, during normal heating mode (i.e. mode 1), the pump 346 is turned off so that the thermal energy storage device 316 is isolated from the phase separator 322. In addition, valves V31, V32, V33 are opened to enable the heat pump system 310a, 310b to operate as a conventional two-stage heat pump.

[0265] During charging mode (i.e. mode 4), valves V31, V32 and V33 are open and valves V34, V35 are closed. The pump 346 is turned on to connect the thermal energy storage system 316 to the phase separator 322. During charging mode, the high pressure stage is operated to supply heat to the condenser 314 (as in normal heating mode). However, in contrast to normal heating mode, during charging mode, the pump 346 is turned on, thereby connecting the thermal energy storage device 316 to the phase separator 322. At the same time, the expansion device EV31 is adjusted to control the pressure within the phase separator 322 to ensure that the temperature of the refrigerant is slightly above the melting temperature of the phase change material within the thermal energy storage device 316. This causes heat from the refrigerant to be extracted and stored in the thermal energy storage device 316.

[0266] The heat pump systems 310a, 310b differ from the heat pump systems 210a, 210b, 210c in that, during charging mode (mode 4), the heat pump continues to supply heat to the condenser 314 (i.e. the “charging mode” of the heat pump systems 310a, 310b is effectively a “continuous heating and charging mode”), whereas the heat pump systems 210a, 210b, 210c can be operated to charge the thermal energy storage device without also supplying heat to the condenser 214 (i.e. the “charging mode” of the heat pump systems 210a, 210b, 210c is effectively a “pure charging mode”).

[0267] During heating enhancement mode (i.e. mode 5), valves V31, V32 and V33 are open, while valves V34-V35 are closed. The pump 346 is activated so that the thermal energy storage device 316 and the evaporator 320 are used simultaneously as heat sources to increase the heating capacity of the heat pump systems 316a, 316b. During this mode of operation, the refrigerant pressure in the phase separator 322 is adjusted so that the temperature of the refrigerant is below the melting point of the phase change material in the thermal energy storage device 316. This causes heat from the thermal energy storage device 316 to be absorbed by the refrigerant in the phase separator 322.

[0268] During auxiliary heating mode (i.e. mode 3), valves V31, V32, V34, V35 are closed and valve V33 is open. The pump 346 is turned on to transfer heat from the thermal energy storage device 316 to the refrigerant in the phase separator 322. The low pressure stage is deactivated, so the phase separator 315 is used as an evaporator for the high pressure stage together with the thermal energy storage device 316. As in mode 5, the pressure in the phase separator 322 is controlled so that the temperature of the refrigerant is below the melting temperature of the phase change material in the thermal energy storage device 316.

[0269] During defrost and continuous heating mode (i.e. mode 2) applicable to heat pump system 310b, valves V31, V32 and V33 are closed while valves V34 and V35 are open. The warm liquid refrigerant leaving the condenser 312 is directed through the evaporator 320 to release heat to melt the ice formed thereon. The cooled refrigerant then throttles in expansion valve EV31 before re-entering the phase separator 322. In this mode, the phase separator 322 together with the thermal energy storage device 316 is converted into an evaporator at high pressure stage such that the heat stored in the thermal energy storage device 316 can be extracted and boosted to a heat production temperature.

[0270] It should be appreciated that each of the operating modes 1, 4, 5, 3 and 2 are preferably applicable to substantially the same conditions as described above in relation to the dual-compressor dual-stage heat pump systems 210a, 210b, 210c. Furthermore, the vapour injection compressor heat pump systems 310a, 310b have the additional advantage of being more easily packaged in a building's central heating system as they require fewer components (e.g. fewer compressors and fewer fluid piping sections and valves, etc.).

[0271] The heat pump systems 210a, 210b, 210c, 310a, 310b, as shown in Figure 10 , Figure 11 and Figures 17 to 19 , can also be applied to refrigeration applications (e.g. in a refrigerator, freezer or air conditioning system). It should be appreciated that for refrigeration applications, the dual-stage heat pump systems are configured in reverse, with the condenser exposed to ambient air (i.e. the heat sink) and the evaporator located at the location requiring the load (e.g. inside the refrigerator). In this case, the required load is a negative heat load (i.e. a cold load) whereby the temperature of the refrigeration area is reduced.

[0272] The dual-stage heat pump systems 210a, 210b, 210c, 310a, 310b are particularly beneficial in situations where the heat sink temperature (i.e. the outdoor ambient air temperature) is too high and / or the required temperature drop is too large. The operation of two dual-stage heat pump systems 210a, 310a for refrigeration applications is described below.

[0273] According to exemplary operating conditions of the refrigeration heat pump system, the ambient air temperature (i.e. at the condenser) is between 30°C and 40°C and the cold load temperature is around -20°C. The thermal energy storage device is thermally coupled to the phase separator which operates at an intermediate temperature of 5°C to 10°C. Thus, the total temperature drop is approximately 55°C, with the low pressure stage and the high pressure stage each providing a temperature drop of approximately 25°C to 30°C.

[0274] As shown in Figure 10As shown, the heat pump system 210a can operate in four different refrigeration modes, which are referred to as normal refrigeration mode, auxiliary refrigeration mode, charge refrigeration mode, and refrigeration boost mode. The four modes correspond to modes 6, 7, 8, and 9, respectively, outlined in Table 4 below.

[0275] Table 4: Modes of operation (valves - open / close; compressors and pumps - on / off)

[0276]

[0277] During normal refrigeration mode (i.e., mode 6), the pump 246 is off, each of the valves V21-V24 is open, and the heat pump system 210a operates as a traditional two-stage refrigeration system by absorbing heat from the evaporator 220 and transferring it to the condenser 214. In this way, the heat pump system 210a uses outside air (i.e., at the evaporator 220) as a heat sink.

[0278] The auxiliary refrigeration mode is particularly useful when the outdoor air temperature is high, making the normal operation of the refrigeration system less efficient. When operating in the auxiliary refrigeration mode (i.e., mode 7), the pump 246 is on, valves V21 and V22 are closed, and the first compressor 212a is closed. As a result, the high-pressure stage is deactivated, and the low-pressure stage is configured to direct heat into the thermal energy storage 216a. This is equivalent to extracting cooling thermal energy (i.e., the “cooling” of energy) from the thermal energy storage 216a and directing it into the refrigerant in the phase separator 222. In this way, the heat pump uses the thermal energy storage 216a as a heat sink to reject heat from the evaporator 220 into it.

[0279] During the charge refrigeration mode (i.e., mode 8), the pump 246 is on and valves V21, V22 are open, while valves V23, V24 are closed. This arrangement effectively turns the heat pump into a single-stage system, using ambient air (i.e., at the condenser 214) as a heat sink. At intermediate temperatures between 5°C and 10°C, the heat pump is operated to extract heat from the thermal energy storage 216a into the refrigerant in the phase separator 222. The charge refrigeration mode is particularly advantageous when refrigeration demand is low, particularly when the outdoor ambient temperature is also low.

[0280] The refrigeration boost mode of operation is particularly advantageous when cooling demand is high. When operating in the refrigeration boost mode (i.e., mode 9), each of the valves V21-V24 is open, and the compressors 212a, 212b and the pump 246 are on. Both the high-pressure stage and the low-pressure stage are activated to direct thermal energy into the thermal energy storage 216a and ambient air (i.e., at the condenser 214). By using the thermal energy storage 216a as an additional heat sink, this improves the refrigeration performance of the heat pump.

[0281] As Figure 18As shown, the vapor injection compressor two-stage heat pump system 310a can also operate as a refrigeration system. Specifically, the heat pump system 310a can operate in four different refrigeration modes, which are referred to as normal refrigeration mode, refrigeration enhancement mode, refrigeration and charging combination mode, and charging refrigeration mode. These four modes correspond to modes 10, 11, 12, and 13, respectively, as shown in Table 5 below. The compressor 312 is activated during each mode of operation.

[0282] Table 5: Modes of operation (valves - open / close; pump - on / off)

[0283]

[0284] When operating in normal refrigeration mode (i.e., mode 10), the valves V31, 32 are open and the pump 346 is off. Thus, the heat pump system 310a operates as a conventional two-stage refrigeration heat pump, i.e., by extracting heat from the evaporator 320 and directing it to the condenser 314 and wherein the thermal energy storage device 316 is deactivated.

[0285] During refrigeration enhancement mode (i.e., mode 11), the pump 346 is on and the valves V31, V32 are open. The pressure in the phase separator 322 is adjusted such that the temperature of the refrigerant is above the melting temperature of the phase change material in the thermal energy storage device 316a. Thus, the heat pump operates as a two-stage refrigeration system that uses the thermal energy storage device 316a as a sub-cooler for the refrigerant and the ambient air at the condenser 214 as a heat sink.

[0286] When operating in refrigeration and charging combination mode (i.e., mode 12), the valves V31 and V32 are open and the pump 346 is on. The pressure of the refrigerant in the phase separator 322 is adjusted such that the temperature of the refrigerant is below the melting point of the phase change material in the thermal energy storage device 316. This results in heat from the thermal energy storage device 316 being absorbed by the refrigerant in the phase separator 322, which effectively cools the thermal energy stored in the thermal energy storage device 316. This mode of operation is particularly useful when the ambient temperature is relatively low, as it allows the heat pump to operate as a two-stage system with the ambient air acting as a heat sink while extracting heat from the thermal energy storage device 316.

[0287] When cooling is not required, it is advantageous to use the charging refrigeration mode (i.e., mode 13). During operation, the valves V31 and V32 are closed, thereby converting the heat pump to a single-stage system. Likewise, the pressure of the refrigerant in the phase separator 322 is adjusted such that the temperature of the refrigerant is below the melting point of the phase change material. Thus, heat is extracted from the thermal energy storage device 316 at a temperature of approximately 5-10°C for later use (e.g., during mode 11) and is directed to the condenser 314, which acts as a heat sink.

[0288] When heating and cooling demands do not match, heat pump systems 210a, 210b, 210c, 310a, 310b, as shown in Figure 10 and Figure 18 shown, can also be applied to combined heating and cooling operation. In this case, thermal energy can be stored at intermediate temperatures within the thermal energy storage device, enabling the system to match production and demand as required.

[0289] In an alternative exemplary arrangement of any two-stage heat pump system, the thermal energy storage device can be arranged within the phase separator 422, as shown in Figure 20 The thermal energy storage device 416 comprises a plurality of individual thermal energy storage elements (e.g. spheres or spheroids). Each energy storage element comprises a volume of phase change material encapsulated within an outer shell 430. In contrast to the thermal energy storage devices 216, 316 described above, the internal thermal energy storage device 416 does not comprise a pump or heat transfer fluid circuit. Instead, each outer shell 430 of the plurality of energy storage elements is configured to allow thermal energy to flow passively (e.g. by thermal conduction) between the phase change material contained therein and the refrigerant contained within the phase separator 422. For example, the outer shell 430 can be formed of a metallic material. By providing a plurality of smaller energy storage elements, this increases the surface area of the phase change material in contact with the refrigerant in the phase separator 422.

[0290] It will be appreciated that the "internal" configured thermal energy storage device 416 can have a smaller capacity than an "external" mounted thermal energy storage device (such as the device 216 shown in Figure 10 However, the internal thermal energy storage device 416 is easier to accommodate within the heat pump system, as it can be encapsulated within the footprint of the phase separator 422. Furthermore, the passive internal thermal energy storage device 416 has no moving parts, making operation and maintenance easier and cheaper.

[0291] Case 1—Two-stage heating system

[0292] To illustrate the difference in performance between normal heating mode and the combination of charging and auxiliary heating mode, we have performed a representative simulation of the heat pump system based on data obtained from commercial simulation software.

[0293] The operating conditions were determined to be:

[0294] • The outdoor air temperature varies between 0°C and 10°C throughout the day

[0295] • Condenser outlet temperature: 65°C

[0296] • Isentropic efficiency of the compressor: 70%

[0297] • Melting temperature of the phase change material: 30°C

[0298] • Approach temperature of the evaporator: 10°C

[0299] • Condenser approach temperature 3°C

[0300] • Thermal storage approach temperature 3°C

[0301] • Refrigerant: R134a

[0302] A two-stage heat pump system is used as a benchmark for the following analysis. The system undergoes the following steps when operating in normal heating mode (i.e. Mode 1):

[0303] 80.3°C high temperature refrigerant vapour enters the condenser, releasing heat to water of the central heating system, which is then condensed and cooled to 70°C. Water from the central heating system returns at 40°C and is heated to 65°C within the condenser. Hot liquid refrigerant at 70°C, 21.28 bar is throttled through the first expansion device to 33°C, 8.39 bar and then enters the phase separator. Vapour is removed by the high pressure stage compressor, while liquid refrigerant is further throttled through the second expansion device to -20°C, 1.33 bar. The mixture then enters the evaporator, absorbing heat from ambient air at 0°C and is cooled to -10°C. The mixture then fully evaporates to low pressure saturated vapour at -20°C. The low pressure stage compressor extracts the low pressure vapour and compresses it to the intermediate pressure of the phase separator. After compression, the superheated vapour is bubbled through the liquid refrigerant in the phase separator, cooling to saturated vapour. Together with the vapour produced by the first throttling process, it is extracted and compressed by the high pressure stage compressor and directed to the condenser pressure, to start a new cycle.

[0304] The parameters and results of the normal heating mode (i.e. Mode 1) simulation are summarised as follows:

[0305] • Outdoor air temperature: 0°C

[0306] • Evaporation pressure: 1.33 bar

[0307] • Condensation pressure: 21.28 bar

[0308] • Low pressure stage compressor power consumption: 0.905 kW

[0309] • Low pressure stage compressor mass flow rate: 0.017 kg / s

[0310] • High pressure stage compressor power consumption: 0.781 kW

[0311] • High pressure stage compressor mass flow rate: 0.029 kg / s

[0312] • Heat power output of the condenser at 65°C: 4 kW (= 14400 kJ / h)

[0313] • Heat extracted from outdoor air at 0°C: 2.314 kW (= 8330.4 kJ / h)

[0314] • COP of the system in normal heating mode is 4 / (0.905+0.781) = 2.57 2.37

[0315] The results of the simulation of the charge heating mode (i.e. mode 4) are summarized as follows:

[0316] When the heat demand is low during the day and the outdoor air temperature is high (e.g. 10°C), the charge mode is used. During this mode, the high pressure stage is deactivated and the low pressure stage is isolated to charge the thermal energy storage. The evaporator extracts 2.51 kW of heat from the ambient air and cools it from 10°C to 0°C. The temperature of the phase separator is kept at 33°C and the temperature of the phase change material in the thermal energy storage is 30°C. The phase separator and the thermal energy storage effectively act as condensers for the low pressure stage. The heat is stored in the thermal energy storage at a power of 3.25 kW. The low pressure stage compressor power consumption is 0.73 kW, so the calculated COP of the heat pump L is 4.45 .

[0317] The results of the simulation of the auxiliary heating mode (i.e. mode 3) are summarized as follows:

[0318] In the auxiliary heating mode, the high pressure stage is isolated to run, taking heat from the thermal energy storage through the phase separator and boosting it for heating. The heat stored in the thermal energy storage is determined to be 30°C with a temperature approach of 3°C, so the temperature of the phase separator is kept at 27°C. The phase separator and the thermal energy storage effectively act as evaporators for the high pressure stage, producing heat at the condenser at a power of 4 kW. The high pressure stage compressor power consumption is 0.92 kW and the thermal energy storage releases 3.08 kW at 30°C. Therefore, the calculated COP of the heat pump with the isolated high pressure stage H is 4.35 .

[0319] By using the combination of charge heating and auxiliary heating modes to run the heat pump system 216a, the system is enabled to deliver heat more efficiently under conditions of varying ambient temperature throughout the day. These two modes of operation are possible because each of the high pressure stage and the low pressure stage of the heat pump system can be run independently and isolated from each other. To illustrate the effect, the overall COP of the heat pump system can be calculated from the COP values corresponding to the charge heating and auxiliary heating modes, as determined above.

[0320]

[0321] The calculated overall COP is 2.48 5% higher than the baseline case of extracting heat directly from air at 0°C and producing heat at 65°C during the night.

[0322] Case Study 2—Two-Stage Refrigeration System

[0323] To illustrate the performance difference between the normal refrigeration mode and the combined charge and auxiliary refrigeration mode, we have performed a representative simulation of the heat pump system based on data obtained from a commercial simulation software.

[0324] The operating conditions were determined as:

[0325] • The outdoor air temperature varies between 20°C and 35°C throughout the day

[0326] • Evaporator inlet temperature: -25°C

[0327] • Isentropic efficiency of the compressor: 70%

[0328] • Melting temperature of the phase change material: 5°C

[0329] • Approach temperature of the evaporator 10°C

[0330] • Approach temperature of the condenser 10°C

[0331] • Approach temperature of the thermal energy storage device 3°C

[0332] • Refrigerant: R134a.

[0333] A two-stage heat pump system 210a was used as a reference for the following analysis. The system undergoes the following steps when operating in the normal refrigeration mode (i.e. mode 6):

[0334] A high temperature refrigerant vapor of 67.2°C enters the condenser, rejecting heat to the 35°C ambient air. The hot liquid refrigerant of 55°C, 14.96 bar is then throttled through a first expansion device to 8°C, 3.87 bar before entering the phase separator. The vapor is removed by the high pressure stage compressor while the liquid refrigerant is further throttled through a second expansion device to -25°C, 1.06 bar. The mixture then enters the evaporator to absorb heat from the cooling load. The mixture finally evaporates completely to a low pressure saturated vapor at -25°C. The low pressure vapor is extracted by the low pressure stage compressor and compressed to an intermediate pressure, which is also the pressure of the phase separator. After compression, the superheated vapor is bubbled through the liquid refrigerant in the phase separator, cooling to a saturated vapor. Together with the vapor produced by the first throttling process, it is then extracted and compressed by the high pressure stage compressor to the condenser pressure.

[0335] The parameters and results of the normal refrigeration mode (i.e. mode 6) simulation are summarized as follows:

[0336] • Outdoor air temperature: 35°C

[0337] • Evaporator pressure: 1.06 bar

[0338] • Condensation pressure: 14.96 bar

[0339] • Low-pressure stage compressor power consumption: 0.821 kW

[0340] • Low-pressure stage compressor mass flow: 0.023 g / s

[0341] • High-pressure stage compressor power consumption: 1.479 kW

[0342] • High-pressure stage compressor mass flow: 0.039 g / s

[0343] • Heat output of condenser at 55 °C: 6.34 kW (= 22840 kJ / h)

[0344] • Cooling capacity extracted from evaporator at -25 °C: 4.04 kW (= 14560 kJ / h)

[0345] • COP of the system in normal heating mode is 4.04 / (0.821+1.479) = 2.04 1.76

[0346] According to the exemplary operating method of the refrigeration system, the isolated high-pressure stage can be used to first extract heat from the thermal energy storage device when the ambient temperature is low at night, thus the COP will be higher, and then the depleted phase change material absorbs thermal energy when the outdoor ambient temperature is higher the next day, thus further improving the COP of the system. To evaluate the potential energy-saving effect of this flexible operating strategy, a simulated case is considered, in which the heat pump is operated in the charging cooling mode (i.e., mode 8) at night, and then in the auxiliary cooling mode (i.e., mode 7) the next day.

[0347] The charging cooling mode (i.e., mode 8) simulation results are summarized as follows: during the charging cooling mode, the cooling demand is low and the outdoor air temperature is also low (e.g., 20 °C), so the high-pressure stage is isolated to extract heat from the thermal energy storage device. The temperature of the thermal energy storage device is 5 °C, and the charge-discharge temperature difference is 3 °C. Correspondingly, the phase separator temperature is maintained at 2 °C. The phase separator and the thermal energy storage device are used as the evaporator of the high-pressure stage. The refrigeration capacity provided by the heat pump system is 4.627 kW, which is stored in the thermal energy storage device, while the high-pressure stage compressor power consumption is 1.128 kW. The calculated refrigeration COP of the isolated high-pressure stage is H 2.04 4.1 .

[0348] The results of the simulation for the auxiliary cooling mode (i.e., mode 7) are summarized as follows: During the auxiliary cooling mode, the low side of the system is isolated and heat is rejected through the phase separator into the thermal energy storage device. The thermal energy storage device acts as a heat sink in place of the ambient air, which is warmer during the day. For this simulation, it was assumed that the thermal energy storage device stored cooling energy at 5°C, the approach temperature for heat transfer was 3°C, and the phase separator was maintained at 8°C. The phase separator and thermal energy storage device acted as the condenser for the low-pressure stage, which was cooled at a power of 4 kW. The low-pressure stage compressor power consumption was 0.87 kW. Thus, the calculated COP for the low-pressure stage cooling L For 4.6 .

[0349] Based on the calculated COPs for the two isolated stages of the cooling system, the energy flows follow this decoupled and interleaved operating strategy. The overall COP for this operation can then be calculated:

[0350]

[0351] The calculated overall COP is 1.94 10.5% higher than the baseline case of extracting heat directly at -25°C during the day and rejecting heat to air at 35°C.

[0352] The several example heat pump systems described herein with reference to Figures 1 to 20 share one or more features having similar structure and / or function. Moreover, it should be understood that each of the example heat pump systems is not limited to the particular arrangements described and illustrated. For example, each of the example heat pump systems can incorporate additional features or can be incorporated within other systems (e.g., a building heating system) without departing from the scope of the present disclosure. Moreover, it should be understood that any one feature of an example heat pump system can be combined with any feature of another example heat pump system without departing from the scope of the present disclosure.

Claims

1. A method of operating a heat pump system to control the temperature within a building, the system comprising: a compressor, a first heat exchanger, an expansion device and a second heat exchanger fluidly connected together by a flow of refrigerant to define a refrigerant circuit, and a thermal energy storage device thermally connectable to the refrigerant circuit to exchange thermal energy with the flow of refrigerant; wherein the heat pump system is operated in a normal heating mode and a defrost mode, wherein: in the normal heating mode, thermal energy is transferred from the second heat exchanger into the flow of refrigerant and from the flow of refrigerant through the first heat exchanger to heat the building, in the defrost mode, thermal energy is transferred from the thermal energy storage device into the flow of refrigerant and from the flow of refrigerant through the first heat exchanger to heat the building and through the second heat exchanger to defrost the second heat exchanger; wherein the method comprises, when operating the heat pump system in the defrost mode, directing the flow of refrigerant exiting the first heat exchanger through the second heat exchanger, the residual heat in the flow of refrigerant defrosting the second heat exchanger.

2. The method of claim 1, wherein the heat pump system comprises a switching assembly configured to switch between the normal heating mode and the defrost mode.

3. The method of claim 2, wherein the switching assembly is configured to, when operating the heat pump system in the defrost mode, direct the flow of refrigerant exiting the first heat exchanger through the second heat exchanger, the expansion device and the compressor in sequence.

4. The method of claim 2 or claim 3, wherein the thermal energy storage device is connected to the refrigerant circuit between the expansion device and the compressor.

5. The method of claim 3 or claim 4, wherein the switching assembly comprises a four-way valve configured to, when operating the heat pump system in the defrost mode, connect the first heat exchanger directly to the second heat exchanger.

6. The method of claim 2, wherein the switching assembly is configured to, when operating the heat pump system in the defrost mode, bypass the expansion device and direct the flow of refrigerant exiting the first heat exchanger through a second expansion device, the thermal energy storage device, the second heat exchanger and the compressor in sequence.

7. The method of claim 6, wherein the switching assembly comprises a first bypass assembly configured to, when operating the heat pump system in the defrost mode, isolate the expansion device from the refrigerant circuit.

8. The method of claim 6 or claim 7, wherein the thermal energy storage device is connected to the refrigerant circuit between the second expansion device and the second heat exchanger.

9. The method of claim 8, wherein the switching assembly comprises a second bypass assembly configured to fluidly connect the second expansion device to the refrigerant circuit between the first heat exchanger and the thermal energy storage device when the heat pump system is operated in a defrost mode.

10. The method of any one of claims 6-9, wherein the heat pump system is operable in a charging mode in which thermal energy is transferred from the refrigerant to the thermal energy storage device, wherein the switching assembly is configured to direct refrigerant exiting the compressor to bypass the second expansion device and the first heat exchanger when the heat pump system is operated in the charging mode.

11. The method of any one of claims 6-10, wherein the heat pump system is operable in an auxiliary heating mode in which thermal energy is transferred from the thermal energy storage device into the refrigerant, wherein the switching assembly is configured to bypass the expansion device and the second heat exchanger when the heat pump system is operated in the auxiliary heating mode.

12. The method of any one of the preceding claims, wherein the thermal energy storage device comprises a phase change material.

13. The method of claim 12, wherein the phase change material is configured to be in direct thermal contact with a pipe of the refrigerant circuit.

14. The method of claim 12, wherein the phase change material is thermally connected to a pipe of the refrigerant circuit by a circuit comprising a heat transfer fluid.

15. The method of any one of claims 1-14, wherein the second heat exchanger is thermally connected to an external heat source, and wherein the first heat exchanger is thermally connected to a central heating system of the building.

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