Heat pump

Abstract

A heat pump system 100 comprises a refrigerant circuit 132 containing a transcritical refrigerant, a compressor 108, and a heat exchanger 104 downstream of the compressor which transfers heat from the

Description

The present disclosure relates to heat pump systems and, more specifically, to the use of a transcritical refrigerant in heat pump systems. Background Many different configurations of heat pump systems are available for different purposes. Heat pump systems are often used for heating houses or other types of buildings. Conventionally, heat pump systems operate by allowing heat exchange between a refrigerant and a process fluid (e.g. water). The refrigerant often goes through a series of operations which may involve the refrigerant absorbing heat from air or the ground so that its temperature can increase and, in turn, increase the temperature of the process fluid. The refrigerant normally remains in a subcritical state. The refrigerants used in conventional heat pump systems are problematic because they often have a high global warming potential (GWP). If refrigerants with high GWPs are released into the environment, for example due to a fault or when the heat pump system is decommissioned, then there could be a negative impact on the environment. Summary In accordance with a first aspect of the present disclosure, a heat pump system is provided. The heat pump system may comprise a refrigerant circuit which may contain a transcritical refrigerant. The heat pump system may comprise a compressor which may be configured to increase the pressure and temperature of the transcritical refrigerant. The heat pump system may also comprise a heat exchanger which may be downstream of the compressor and connected thereto by the refrigerant circuit. The heat exchanger may be configured to transfer heat from the transcritical refrigerant to a process fluid flowing through the heat exchanger. The heat pump system may also comprise an evaporator which is configured to transfer heat from a source fluid to a working fluid flowing through the evaporator. The heat pump system may also comprise a gas cooler which may be positioned adjacent to the evaporator and downstream of the heat exchanger and connected thereto by the refrigerant circuit. The gas cooler may be configured to transfer heat from the transcritical refrigerant to the working fluid flowing through the evaporator. The heat pump system may also comprise an expansion valve which is located downstream of the gas cooler and connected thereto by the refrigerant circuit. The expansion valve may also be located upstream of the compressor and connected thereto by the refrigerant circuit. The expansion valve may be configured to reduce the pressure and temperature of the transcritical refrigerant exiting the gas cooler. The primary purpose of the heat pump system disclosed herein is to heat the process fluid. The process fluid may be water, or may comprise water and one or more additives. The process fluid may be used to heat a building. For example, the process fluid heated by the heat pump system may be caused to flow through one or more radiators or underfloor heating pipes within the building. Alternatively or in addition, the process fluid may be used for washing or in any other situation in which hot process fluid is required. For example, the process fluid heated by the heat pump system may be supplied to a tap, faucet, shower or other outlet. The applicability of the heat pump system disclosed herein is not limited solely to heating water. The heat pump system could be used to heat any other suitable process fluid, such as a process fluid used in an industrial process. The source of thermal energy that is used to heat the process fluid is referred to herein as "the source fluid”. The source fluid may be any fluid is suitable for transferring heat from outside the heat pump system (e.g., from the environment) to the working fluid. Non-limiting examples of source fluids include ambient air or water. The source fluid possesses an inherent amount of thermal energy arising from the environment outside the heat pump system. The evaporator transfers the thermal energy of the source fluid to the working fluid flowing through the evaporator, thus making using of renewable energy present in the environment outside the heat pump system. When the source fluid is ambient air, the heat pump system receives heat from the environment outside the heat pump system via the ambient air, and the heat pump system may thus be regarded as an air source heat pump. The evaporator and gas cooler may be located outside a building or other structure, such that both are exposed to the ambient air or other source fluid. Alternatively, if the evaporator and gas cooler are located inside a building or other structure, they may receive ambient air or another source fluid from outside the building via a conduit (e.g., an air duct when the source fluid is ambient air). The thermal energy of the source fluid is supplemented by reclaiming otherwise unutilised thermal energy from within the heat pump system itself. By positioning the gas cooler and evaporator adjacent to each other, unutilised thermal energy from the transcritical refrigerant flowing through the gas cooler can be used to heat the working fluid flowing through the evaporator. The overall efficiency of the heat pump system is increased by reclaiming unutilised thermal energy from the transcritical refrigerant for the next heat pump cycle, thus reducing the amount of heat that needs to be added to the system in each heat pump cycle. Reclaiming the unutilised thermal energy from the transcritical refrigerant also has the advantage of cooling the transcritical refrigerant. Thus, when the transcritical refrigerant exits the gas cooler, the transcritical refrigerant is at a lower temperature at which subsequent steps in the heat pump cycle are more thermodynamically efficient. The unutilised thermal energy of the transcritical refrigerant flowing through the gas cooler is also used to defrost the evaporator. In this context, defrosting refers to removing ice from the external surface of the evaporator and also to preventing ice forming on the external surface of the evaporator. In more detail, when the source fluid is ambient air, ice can form on the outside of the evaporator when the temperature of the ambient air around the evaporator decreases to a point at which water vapour in the ambient air condenses and then freezes on the surface of the evaporator. The temperature of the ambient air around the evaporator decreases due to the thermal energy of the ambient air being used to heat the working fluid in the evaporator. Positioning the gas cooler adjacent to the evaporator allows for heat transfer from the transcritical refrigerant to the working fluid. This increases the temperature of the working fluid, which in turn increases the temperature of the external surface of the evaporator. The evaporator is thereby defrosted, which increases the efficiency of the evaporator. Additionally, using the unutilised thermal energy of the transcritical refrigerant in this manner eliminates the need for a “reverse cycle” of the evaporator. Conventionally, a reverse cycle is performed to remove ice from the outside of the evaporator. A reverse cycle typically involves involved using heat from the process fluid to melt ice which has formed on the outside of the evaporator. The use of a reverse cycle is disadvantageous because it causes a loss of thermal energy from the process fluid, and because the heat pump system is unable to perform its primary purpose of heating the process fluid while the cycle is reversed. Using the unutilised thermal energy of the transcritical refrigerant to defrost the evaporator avoids wasting thermal energy from the process fluid. When the transcritical refrigerant has insufficient unutilised thermal energy to defrost the evaporator, the evaporator may be defrosted by using a further heat exchanger or a heat storage system as described in more detail below. The inventors have also realised that transcritical refrigerants can advantageously be used in a heat pump system. Carbon dioxide is currently the only known example of a transcritical refrigerant and, advantageously, has a low GWP. The GWP of carbon dioxide is one. This means that the transcritical refrigerant, if released into the atmosphere, would have a lower impact on global warming relative to many other refrigerants commonly used in heat pump systems. Another advantage of using transcritical refrigerants is their ability to transfer heat efficiently at high temperatures and pressures (supercritical state) and low temperatures or pressures (subcritical state). Resultantly, the transcritical refrigerant can be used to heat the process fluid when in a supercritical state, where the transcritical refrigerant is able to operate more efficiently; when the temperature and pressure of the transcritical refrigerant have decreased, the remaining unutilised thermal energy can be used to increase the temperature of the working fluid flowing through the evaporator. In accordance with its usual meaning in the art, the term “working fluid” refers to a gas or liquid that converts thermal energy into mechanical energy, or vice versa. The working fluid may be compressed to convert mechanical energy into thermal energy. The working fluid may expand to convert thermal energy into mechanical energy. The working fluid may change phase when it is heated or cooled. The gas cooler may be further configured to transfer heat from the transcritical refrigerant to the working fluid flowing through the evaporator via the source fluid. As noted above, the heat pump system makes use of thermal energy present in the environment outside the heat pump system, as well as reclaiming otherwise unutilised thermal energy from the transcritical refrigerant flowing through the gas cooler. By using the source fluid as a medium to transfer heat, a single flow of the source fluid can transfer heat to the working fluid in the evaporator from both the environment outside the heat pump system and the transcritical refrigerant in the gas cooler. This, in turn, simplifies the structure of the heat pump system. The gas cooler and evaporator may each comprise heat exchange elements. The heat exchange elements of the gas cooler may be interleaved with the heat exchange elements of the evaporator. Interleaving the heat exchange elements of the gas cooler with the heat exchange elements of the evaporator is beneficial because it allows for the heat transfer to be distributed evenly across the entirety of the evaporator. A heat exchange element of the evaporator may be located between adjacent heat elements of the gas cooler. This configuration is beneficial because the heat exchange element of the evaporator is exposed to the gas cooler on both sides, which promotes heat transfer from the gas cooler to the evaporator. The heat exchange elements of the gas cooler and / or the evaporator may further comprise finned tubes through which the working fluid flows. The finned tubes allow for an increased surface area for heat transfer and thus increased efficiency of energy transfer between the transcritical refrigerant and the working fluid flowing through the evaporator. The finned tubes of the heat exchange elements of the gas cooler and / or evaporator may be further configured so that at least one portion of the finned tubes of the evaporator is located between adjacent finned tubes of the gas cooler. The finned tubes of the gas cooler and / or evaporator may be further configured so that they are angled with respect to a vertical plane. By angling the finned tubes with respect to the vertical plane, the hotter working fluid and / or transcritical refrigerant can rise. Angling the finned tubes with respect to the vertical plane also promotes run-off of condensate which may form on the exterior surface of the gas cooler and / or evaporator, thereby improving efficiency. The finned tubes may be angled at forty-five degrees to the vertical, or at any other suitable angle that promotes run-off of condensate. The gas cooler may have at least one additional heat exchange element located at the lowest point of the evaporator. When the source fluid is ambient air, if the surface of evaporator is sufficiently cold, water vapour in the ambient air may condense and then freeze on the evaporator. The bottom of the evaporator is particularly at risk of ice formation, because its temperature tends to be lower than that at the top of the evaporator due to convection within the evaporator. The presence of additional heat exchange elements on the gas cooler, located towards the bottom of the evaporator, melts ice which may form on the evaporator and thereby increases the overall efficiency of the evaporator. The heat exchanger may further comprise a gas cooler. The transcritical refrigerant may be carbon dioxide. As noted above, carbon dioxide is currently the only known example of a transcritical refrigerant. The use of carbon dioxide as a refrigerant is advantageous because carbon dioxide has a low GWP, is abundant and inexpensive. However, the present disclosure could also be carried out using another suitable transcritical refrigerant, if one were to be discovered. The heat pump system may also comprise a further heat exchanger. The further heat exchanger may be configured to transfer heat extracted from a surrounding environment to the transcritical refrigerant. The further heat exchanger may be located upstream of the compressor and downstream of the expansion valve and connected thereto by the refrigerant circuit. The further heat exchanger may be a ground source heat exchanger configured to transfer heat from below ground to the transcritical refrigerant. Using the further heat exchanger to transfer heat extracted from the surrounding environment to the transcritical refrigerant is advantageous because the heat extracted from the surrounding environment may be from a renewable source, such as a ground source heat exchanger or a solar heat exchanger. The further heat exchanger can be used to provide heat to the evaporator, via the transcritical refrigerant, in order to defrost the evaporator. Alternatively or additionally, the further heat exchanger can be used to supplement the energy input into the heat pump system when there is a high demand for heated process fluid or when the source fluid is too cold to provide enough heat. The further heat exchanger is used to heat the transcritical refrigerant, which in turn can provide more heat to the process fluid. The heat pump system may further comprise a heat storage system which is configured to extract heat from the process fluid and store the heat extracted from the process fluid in a second working fluid. Using the heat storage system to store heat from the heated process fluid and transfer it to the evaporator is advantageous because it allows the evaporator to be defrosted the evaporator without the need for the ground source heat exchanger described elsewhere herein. The heat pump system comprising the heat storage system may be located anywhere it is required. For instance, the heat storage system allows defrosting of a heat pump system located on the roof of a building or in an area where it is impractical to install a ground loop The heat storage system may comprise a working fluid store (e.g., a vessel) configured to store the second working fluid. The heat storage system may further comprise a heat exchanger configured to transfer heat from the process fluid to the second working fluid. The heat exchanger may be located within the working fluid store. The heat storage system may further comprise a defrosting heat exchanger configured to transfer heat from the second working fluid to the evaporator. Using the defrosting heat exchanger is advantageous as it allows the ice which may form on the evaporator to be melted and thus allowing the evaporator to be defrosted. The name “defrosting heat exchanger” is used to distinguish this heat exchanger from the other heat exchangers described herein. The defrosting heat exchanger may be positioned proximal to the evaporator. In a more specific example, the defrosting heat exchanger may be in contact with an external surface of the evaporator. The defrosting heat exchanger may be positioned between the gas cooler and evaporator. It is advantageous to position the defrosting heat exchanger between the gas cooler and evaporator in order to optimise the efficiency of the defrosting of the evaporator. The heat pump system may further comprise a refrigerant receiver and a flash-gas valve. The refrigerant receiver may be a pressure vessel which acts as a reservoir for the transcritical refrigerant. The refrigerant receiver may advantageously be configured to separate the higher temperature transcritical refrigerant from lower temperature transcritical refrigerant. The higher temperature transcritical refrigerant, which may be a flash-gas in a gaseous state may advantageously go through the flash-gas valve which relieves the pressure generated by the flash-gas. The evaporator may optionally form part of the refrigerant circuit, such that the working fluid flowing through the evaporator is the transcritical refrigerant. Having only one refrigerant in the heat pump system allows for reduced complexity and fewer parts. This results in a heat pump system which is easier to maintain, and cheaper to manufacture and install. The evaporator may be located downstream of the expansion valve and upstream of the compressor. The evaporator may also be located downstream of the further heat exchanger and upstream of the compressor. The heat pump system may further comprise a second refrigerant circuit containing a second refrigerant. The heat pump system may also comprise a second compressor which may be configured to increase the pressure and temperature of the second refrigerant. The second compressor may be located upstream of the heat exchanger and connect thereto by the second refrigerant circuit, such that the heat exchanger transfers heat from the second refrigerant to the process fluid. The heat pump system may also comprise a second expansion valve, which may be located downstream of the heat exchanger and connected thereto by the second refrigerant circuit. The second expansion valve may be configured to reduce the pressure and temperature of the second refrigerant exiting the heat exchanger. The evaporator may be located downstream of the second expansion valve and connected thereto by the second refrigerant circuit, such that the working fluid flowing through the evaporator is the second refrigerant. The use of two different refrigerants may be advantageous as these refrigerants could be selected for their different optimum operating ranges. For example, one refrigerant may be selected for its ability to operate efficiently at a lower temperature and the other refrigerant may be selected for its ability to operate efficiently at a higher temperature. This consequently allows the heat exchanger to utilise the two refrigerants to heat the process fluid in stages, using both the refrigerants in their optimum temperature range, thereby improving the overall efficiency of heat transfer between the two refrigerants and the process fluid. The heat exchanger may comprise at least one gas cooler and a condenser. The gas cooler of the heat exchanger may form part of the first refrigerant circuit, such that the gas cooler of the heat exchanger is located downstream of the first compressor and connected thereto by the first refrigerant circuit. The condenser may form part of the second refrigerant circuit, such that the condenser is located downstream of the second compressor and connected thereto by the second refrigerant circuit. The heat exchanger may optionally comprise a first gas cooler and a second gas cooler. The second gas cooler may be located downstream of the first gas cooler of the heat exchanger and connected thereto by the refrigerant circuit. The gas cooler may be located downstream of the second gas cooler of the heat exchanger and connected thereto by the first refrigerant circuit. The first gas cooler of the heat exchanger, the second gas cooler of the heat exchanger, and the condenser may be arranged so that the process fluid flowing through the heat exchanger flows sequentially through the second gas cooler of the heat exchanger, then the condenser, and then the first gas cooler of the heat exchanger. The inventors have realised that using a transcritical refrigerant along with a different second refrigerant allows a multiple stage heat exchanger (e.g., a three-stage heat exchanger) to be used to improve the overall efficiency of the heat pump system. A multiple stage heat exchanger allows the process fluid’s thermal energy to be progressively increased by the two refrigerants when they are in their optimum temperature ranges. Consequently, the efficiency of the heat transfer is increased. Additionally, having multiple heat exchangers allows more of the thermal energy available in the refrigerants to be extracted and transferred to the process fluid. The transcritical refrigerant may be carbon dioxide and the second refrigerant may be propane. As mentioned previously, carbon dioxide is advantageous to use due to its transcritical properties as well as its low global warming potential. Propane is also advantageous to use due to its low global warming potential as well as its high heat transfer efficiency. Propane and carbon dioxide are also abundant and inexpensive. The source fluid may be ambient air. Ambient air is advantageous as a source fluid as it is abundant and easy to manipulate. The heat pump system may comprise a plate heat exchanger configured to transfer heat from the transcritical refrigerant to the working fluid. In some implementations, the plate heat exchanger may be a distinct component that is provided in addition to the gas cooler. Alternatively, the functionality of the gas cooler may be provided by the plate heat exchanger (in other words, the gas cooler is constituted by the plate heat exchanger, such that the gas cooler and plate heat exchanger are not separate components). Plate heat exchangers allow for high heat transfer efficiency, and are compact and easy to maintain. The plate heat exchanger may comprise at least one first fluid passage through which the transcritical refrigerant flows, and at least one second fluid passage through which the working fluid flows. The first fluid passage(s) may be upstream of the pressure reducing valve and connected thereto by the refrigerant circuit. The second fluid passage(s) may be downstream of the expansion valve. The plate exchanger comprises at least one plate, and preferably comprises a plurality of plates. The first and second fluid passages may each pass through the at least one plate. The at least one plate allows conductive heat transfer from the transcritical refrigerant to the working fluid. The first fluid passage(s) may be a component of the gas cooler, or may be downstream of the gas cooler. The second fluid passage(s) may be a component of the evaporator, or may be upstream of the evaporator. The plate(s) of the plate heat exchanger may be located adjacent to each other. There may be a physical connection between the plates of the plate heat exchanger. Brief Description of the Drawings Embodiments will now be described purely by way of example, with reference to the following drawings, in which: Figure 1 is a schematic diagram of a first example of a heat pump system in accordance with the present disclosure, in which only one refrigerant is used; Figure 2 is a schematic diagram of a second example of a heat pump system in accordance with the present disclosure, in which two refrigerants are used; Figure 3 is a schematic diagram of the gas cooler and evaporator of Figure 1 and / or Figure 2 in accordance with the present disclosure; Figure 4 is a phase diagram for carbon dioxide; Figure 5 is a phase diagram for propane; and Figure 6 is a schematic diagram of a third example of a heat pump system in accordance with the present disclosure, in which only one refrigerant is used and which includes a heat storage system. Detailed Description Single Refrigerant Heat Pump System Figure 1 is a schematic diagram of a first example of a heat pump system 100 in accordance with the present disclosure. The heat pump system 100 comprises a heat exchanger 104, a compressor 108, a gas cooler 128, an evaporator 130, an expansion valve 120 and a refrigerant circuit 132. The heat pump system 100 may optionally include a pressure reducing valve 124, a flash-gas valve 118, a refrigerant receiver 122, and / or a further heat exchanger 114. The refrigerant circuit 132 contains a transcritical refrigerant. The purpose of the refrigerant circuit 132 is to convey the transcritical refrigerant between the various components mentioned above. The refrigerant circuit 132 may comprise pipes, connectors and / or one or more pumps to convey the transcritical refrigerant. The transcritical refrigerant may be carbon dioxide (CO2), which is otherwise known in the art of heat pump systems as R744. Although carbon dioxide is the only known transcritical refrigerant at the present time, other transcritical refrigerants may become available in the future. Therefore, the scope of the present disclosure is not limited solely to heat pump systems in which the transcritical refrigerant is carbon dioxide. The compressor 108 is configured to compress the transcritical refrigerant. When compressed by the compressor 108, the temperature of the transcritical refrigerant increases. The transcritical refrigerant remains in a gaseous state when it is compressed by the compressor 108. The compressor 108 is located upstream of the heat exchanger 104, and downstream of both the further heat exchanger 114 and the refrigerant receiver 122. The heat exchanger 104 is configured to transfer heat from the transcritical refrigerant to a process fluid 102. The term “process fluid” encompasses any fluid that passes through the heat exchanger 104 and receives heat from the transcritical refrigerant. In the present example, the process fluid 102 comprises water (H2O). The process fluid may optionally further comprise one or more additives, such as antifreeze and / or a corrosion inhibitor. The heat exchanger 104 is configured to heat the process fluid entering the heat exchanger, such that the temperature of the process fluid at an outlet of the heat exchanger is greater than the temperature of the process fluid at an inlet of the heat exchanger 104. Conversely, the temperature of the transcritical refrigerant decreases as the transcritical refrigerant passes through the heat exchanger 104. In this example, the heat exchanger 104 comprises a single gas cooler. The gas cooler allows for heat transfer between the transcritical refrigerant and the process fluid 102, whilst keeping the transcritical refrigerant in a gaseous state. In other arrangements, the heat exchanger 104 may comprise a plurality of gas coolers. Although heat is removed from the transcritical refrigerant by the heat exchanger 104, the transcritical refrigerant still has unutilised thermal energy when it exits the heat exchanger 104. The inventors have realised that this unutilised thermal energy can be exploited to improve the overall efficiency of the heat pump system 100. Consequently, the gas cooler 128 is positioned downstream of the heat exchanger 104 to transfer the unutilised thermal energy of the transcritical refrigerant to a working fluid flowing through the evaporator 130. In the heat pump system 100, the working fluid in the evaporator 130 is the transcritical refrigerant. The gas cooler 128 is positioned adjacent to the evaporator 130, so as to allow heat transfer from the transcritical refrigerant in the gas cooler 128 to the working fluid (i.e., the transcritical refrigerant) in the evaporator 130. The gas cooler 128 is located downstream of the heat exchanger 104. In the implementation shown in Figure 1, heat transfer from the gas cooler 128 to the evaporator 130 is mediated by ambient air 126. In more detail, ambient air 126 may be caused to flow over the gas cooler 128 and then over the evaporator 130, such that heat transfer occurs via the ambient air 126. In other implementations, heat transfer from the gas cooler 128 to the evaporator 130 may be mediated by another source fluid and / or by conduction. Whichever implementation is used, the heat transfer causes the temperature of the working fluid (i.e., the transcritical refrigerant) in the evaporator 130 to increase, and the temperature of the transcritical refrigerant in the gas cooler 128 to decrease accordingly. The evaporator 130 is configured to heat the working fluid (i.e., the transcritical refrigerant) flowing through the evaporator 130. The evaporator 130 is located upstream of the compressor 108. The thermal energy of the working fluid in the evaporator 130 may be increased by means of the ambient air 126. Even in the absence of the gas cooler 128, the ambient air 126 possesses an inherent amount of thermal energy arising from the environment outside the heat pump system 100, which can be transferred to the working fluid flowing through the evaporator 130. When the ambient air passes over the gas cooler 128, the inherent thermal energy of the ambient air 126 is supplemented by the unutilised thermal energy of the transcritical refrigerant exiting the heat exchanger 104. Some of the thermal energy of the transcritical refrigerant flowing though the gas cooler 128 may thus be transferred to the ambient air 126, so that the total heat being transferred to the working fluid flowing through the evaporator 130 by the ambient air 126 is increased. The unutilised thermal energy of the transcritical refrigerant in the gas cooler 128 may advantageously be used to melt any ice which forms on the outside of the evaporator 130. Defrosting the evaporator 130 in this manner can improve the efficiency of the evaporator 130, without the need to reverse the cycle. The heat pump system 100 may optionally comprise a fan (not shown in Figure 1). The fan may promote a flow of ambient air 126 over the evaporator 130 and gas cooler 128. The fan may cause the ambient air 126 to flow over the gas cooler 128 before flowing over the evaporator 130. The pressure reducing valve 124 is positioned downstream of the gas cooler 128. The pressure reducing valve 124 is configured to reduce the pressure of the transcritical refrigerant exiting the gas cooler 128. In other words, the transcritical refrigerant expands as it passes through the pressure reducing valve 124. The transcritical refrigerant may be in either a gaseous or liquid state after exiting the pressure reducing valve 124. The refrigerant receiver 122 is located downstream of the pressure reducing valve 124. The refrigerant receiver 122 is configured to separate transcritical refrigerant in a gaseous state from transcritical refrigerant in a liquid state. The refrigerant receiver 122 has at least one outlet on its top surface which allows transcritical refrigerant in a gaseous state to join the refrigerant circuit 132 atone location, and at least one outlet on its bottom surface to allow transcritical refrigerant in a liquid state to join the refrigerant circuit 132 at a different location. The transcritical refrigerant may exit the refrigerant receiver 122 in a gaseous state when it becomes a flash-gas (i.e., when the transcritical refrigerant spontaneously turns from a liquid state to a gaseous state). The expansion valve 120 is configured to reduce the pressure of the transcritical refrigerant when it is in a liquid state downstream of the refrigerant receiver 122. In other words, the transcritical refrigerant expands as it passes through the expansion valve 120. The expansion valve 120 lowers the pressure of the transcritical refrigerant to a point at which the transcritical refrigerant is capable of changing state from a liquid to a gas. The flash-gas valve 118 is configured to relieve the pressure of gaseous transcritical refrigerant exiting the refrigerant receiver 122, so that the transcritical refrigerant exiting the evaporator 130 and the transcritical refrigerant exiting the flash-gas valve 118 may be recombined. The flash-gas valve 118 is located downstream of the refrigerant receiver 122 and upstream of the compressor 108. The further heat exchanger 114 is configured to increase the temperature of transcritical refrigerant. In general, the further heat exchanger 114 transfers heat from the surrounding environment to the transcritical refrigerant. The transcritical refrigerant enters the further heat exchanger 114 with a lower temperature and exits with a higher temperature. The transcritical refrigerant remains in a gaseous state in the further heat exchanger 114. The further heat exchanger 114 is located upstream of the evaporator 130 and downstream of the expansion valve 120. The transcritical refrigerant flows through the evaporator 130, and when the further heat exchanger 114 is in operation, the transcritical refrigerant with an increased temperature inhibits the formation of ice on the evaporator 130 and / or melt the ice that has formed on the evaporator 130. The further heat exchanger 114 may be a ground source heat exchanger, which is configured to transfer heat extracted from the ground to the transcritical refrigerant. The ground source heat exchanger may include a ground loop 112 and a pump 110. In more detail, the ground loop 112 consists of at least one loop of piping positioned underground. The depth, size and number of the ground loop(s) can vary. As an example, in a hot climate with large amounts of thermal energy stored in the ground, there may be a need for very few ground loops which may be small and located near to the surface of the ground. As a contrasting example, in a cold climate, there may be a need for many ground loops 112 which may be large and located far below the surface of the ground. The ground loop 112 contains a working fluid, which will be referred to as a “ground loop working fluid” to distinguish it from the other working fluids mentioned herein. The ground loop 112 comprises a fluid circuit, separate from the refrigerant circuit 132, and contains the ground loop working fluid. The ground loop working fluid may comprise a mixture of glycol and water. The pump 110 may be located between the ground loop 112 and the further heat exchanger 114. When the pump 110 is in operation, the ground loop working fluid is pumped around the ground loop 112. Heat from the ground surrounding the ground loop 112 is then transferred to the ground loop working fluid flowing through the ground loop. The ground loop working fluid then flows through the further heat exchanger 114. The further heat exchanger 114 transfers heat from the ground loop working fluid to the transcritical refrigerant. The thermal energy of the ground loop working fluid is reduced. The thermal energy of the transcritical refrigerant is commensurately increased, and the temperature of the transcritical refrigerant thereby increases. When the transcritical refrigerant flows through the evaporator 130, the thermal energy of the transcritical refrigerant is transferred to the evaporator 130. This increases the temperature of the evaporator 130 and thus causes any ice formed on the surface of the evaporator 130 to melt and inhibits ice from forming. Although Figure 1 shows an implementation in which the further heat exchanger 114 is coupled to a ground loop 112, the present disclosure is not limited thereto. Other suitable heat sources could be used in addition to, or in place of, the ground loop 112. For example, a solar and / or a hydrothermal heat source could be used. Example Single Refrigerant Heat Pump System Cycle An example cycle of the heat pump system 100 shown in Figure 1 will now be described. Reference to Figure 4 will be made. The description of the cycle will begin at the heat exchanger 104. High pressure, high temperature gaseous transcritical refrigerant flows through the heat exchanger 104. When the transcritical refrigerant is at these pressures and temperatures it is in a supercritical state 410, as shown in Figure 4. As the transcritical refrigerant is in its supercritical state 410, the heat transfer from the transcritical refrigerant to the process fluid 102 is at a higher efficiency as opposed to if the transcritical refrigerant was in any other state. The heat exchanger 104 allows for transfer of heat from the transcritical refrigerant to the process fluid 102. This increases the temperature of the process fluid 102 and reduces the temperature of the transcritical refrigerant. The temperature of the transcritical refrigerant decreases in the heat exchanger 104, but the pressure of the transcritical refrigerant remains constant, which causes the transcritical refrigerant to be in a subcritical state 408 when it leaves the heat exchanger 104. The transcritical refrigerant then enters the gas cooler 128. As explained above, the gas cooler 128 transfers unutilised thermal energy from the transcritical refrigerant to the working fluid flowing through the evaporator 130. This heat transfer may be mediated by ambient air 126 or other source fluid. The temperature of the transcritical refrigerant decreases as it passes through the gas cooler 128, while the pressure of the transcritical refrigerant remains constant. The transcritical refrigerant remains in a subcritical state 408 as it leaves the gas cooler 128. The transcritical refrigerant, which is at a low temperature and a high pressure, then flows through the pressure reducing valve 124. The pressure reducing valve 124 reduces the pressure of the transcritical refrigerant. This causes the transcritical refrigerant to change from a subcritical state 408 to either a liquid state 406 or a gaseous state 414 depending on its temperature. As external factors such as ambient air temperature and process fluid 102 usage can alter the demand for heated process fluid, the amount of unutilised thermal energy when the transcritical refrigerant exits the heat exchanger 104 may vary. The temperature of the transcritical refrigerant exiting the gas cooler 128 and the pressure reducing valve 124 may thus also vary. The transcritical refrigerant, which could be either a liquid or a gas, then flows into the refrigerant receiver 122. As explained above, the refrigerant receiver 122 separates liquid transcritical refrigerant from gaseous transcritical refrigerant. In some cases, when the transcritical refrigerant inside the refrigerant receiver 122 mixes with the transcritical refrigerant entering the refrigerant receiver 122, the transcritical refrigerant may start to boil. This may cause some of transcritical refrigerant to spontaneously vaporise, i.e., to change from a liquid state to a gaseous state. The gaseous transcritical refrigerant is called a “flash-gas”. Consequently, inside the refrigerant receiver 122, the transcritical refrigerant may be in either a liquid or gaseous state. The refrigerant receiver 122 is also capable of storing excess transcritical refrigerant in order to regulate the amount of transcritical refrigerant passing through the compressor 108 and the heat exchanger 104. The transcritical refrigerant entering the refrigerant receiver 122 mixes with any transcritical refrigerant already inside. This may increase or decrease the temperature of the transcritical refrigerant inside the refrigerant receiver 122, and may cause the transcritical refrigerant to vaporise and be in either a liquid or gaseous state. In order to be used inside the compressor 108, the transcritical refrigerant should generally be in a gaseous state. If the transcritical refrigerant is already in a gaseous state, due to being a flash-gas, it exits the refrigerant receiver 122 via an outlet located on the top of the refrigerant receiver 122. If the transcritical refrigerant is in a liquid state, its state is changed from liquid to gas before entering the compressor 108. Any liquid transcritical refrigerant exits the refrigerant receiver 122 via an outlet located on the bottom surface of the refrigerant receiver 122. The transcritical refrigerant which exited the refrigerant receiver 122 as a gas then flows through a flash-gas valve 118. The flash-gas valve 118 matches the pressure of the transcritical refrigerant which exited the refrigerant receiver 122 as a gas to the transcritical refrigerant which has exited the evaporator 130. The flash-gas valve 118 prevents over-pressurisation caused by the flash-gas. The transcritical refrigerant which exited the refrigerant receiver 122 as a liquid passes through the expansion valve 120, which decreases the pressure of the transcritical refrigerant. The temperature of the transcritical refrigerant remains constant, causing it to change from a liquid state 406 to a gaseous state 414. The transcritical refrigerant exiting the expansion valve 120 then passes through the further heat exchanger 114. The further heat exchanger 114 can, at times, increase the temperature of the transcritical refrigerant by transferring heat from the surrounding environment to the transcritical refrigerant. Heat from the surrounding environment may be transferred to the transcritical refrigerant using a ground source heat exchanger, which optionally comprises a ground loop 112 and a pump 110. When the pump 110 is in operation (e.g., to defrost the evaporator 130 or when there is a high demand for heated process fluid), the transcritical refrigerant leaves the further heat exchanger 114 with a higher temperature, but remains in a gaseous state 414. When the pump 110 is not in operation, the further heat exchanger 114 does not affect the temperature or state of the transcritical refrigerant. The transcritical refrigerant exiting the further heat exchanger 114 then passes through the evaporator 130. The evaporator 130 transfers heat from the transcritical refrigerant in the gas cooler 128 and the ambient air 126, as explained above, to the transcritical refrigerant flowing through the evaporator 130. The temperature of the transcritical refrigerant increases while in the evaporator 130. This causes the transcritical refrigerant to be in a subcritical state 412 when it leaves the evaporator 130. The transcritical refrigerant exiting the flash-gas valve 118 is then combined with the transcritical refrigerant exiting the evaporator 130. The transcritical refrigerant then flows through the compressor 108. The compressor 108 causes the pressure of the transcritical refrigerant to increase. The increase in pressure of the transcritical refrigerant is accompanied by an increase in temperature. This causes the transcritical refrigerant to change to a supercritical state 410, in which the transcritical refrigerant has a high temperature and high pressure. The transcritical refrigerant exits the compressor 108 in a supercritical state 410, then flows into the heat exchanger 104 and the cycle repeats. The following indicative values of temperatures and pressures of fluids at various points are provided to illustrate the foregoing description of the heat pump system 100. These values should not be regarded as limiting the scope of the present disclosure. The process fluid 102 may enter the heat exchanger 104 with a temperature in the range of 15°C to 35°C, and may exit the heat exchanger 104 with a temperature of approximately 60°C. The transcritical refrigerant may enter the heat exchanger 104 with a temperature in the range of 60°C to 100°C, and may exit the heat exchanger 104 with a temperature in the range of 50°C to 70°C. The transcritical refrigerant may enter the further heat exchanger 114 with a temperature in the range of -15°C to +10°C. The transcritical refrigerant may enter the compressor 108 with a pressure in the range of 25 to 40 bar, and may exit the compressor 108 with a pressure in the range of 70 to 120 bar. To defrost the evaporator 130, the pump 110 is operated. The further heat exchanger 114 increases the temperature of the transcritical refrigerant by transferring heat from the surrounding environment (e.g., the ground loop 112) to the transcritical refrigerant. The transcritical refrigerant leaves the further heat exchanger 114 with a higher temperature, and then flows through the evaporator 130. This increases the temperature of the evaporator 130, melting any ice on the surface of the evaporator 130 and inhibiting ice formation. Multi-refrigerant Heat Pump System Figure 2 is a schematic diagram of a second example of a heat pump system 200 in accordance with the present disclosure. In contrast to the first example heat pump system 100, which uses a single refrigerant, the second example heat pump system 200 uses two refrigerants. The heat pump system 200 comprises a heat exchanger 216, a compressor 220, a gas cooler 238, an evaporator 242, an expansion valve 228 and a refrigerant circuit 204. The heat pump system 200 may optionally include a second refrigerant circuit 202, a second compressor 218, a second expansion valve 244, a refrigerant receiver 232, a flash-gas valve 230, a pressure reducing valve 234, and / or a further heat exchanger 222. The refrigerant circuit 204 contains a transcritical refrigerant. The purpose of the refrigerant circuit 204 is to convey the transcritical refrigerant between some of the components mentioned above. The refrigerant circuit 204 may comprise pipes, connectors and / or one or more pumps to convey the transcritical refrigerant. The transcritical refrigerant may be carbon dioxide (CO2), or any other transcritical refrigerant which may become available in the future. The second refrigerant circuit 202 contains a second refrigerant. The purpose of the second refrigerant circuit 202 is to convey the second refrigerant between at least some of the various components mentioned above. The second refrigerant circuit 202 may comprise pipes, connectors and / or one or more pumps to convey the second refrigerant. The second refrigerant is different from the transcritical refrigerant. The second refrigerant may be propane (CsHs), which is otherwise known in the art of heat pump systems as R290. Although propane is used in the heat pump system 200 as the second refrigerant, other suitable refrigerants such as hydrofluorocarbons (HFCs) could be used. The compressor 220 is configured to compress the transcritical refrigerant. When compressed by the compressor 220, the temperature of the transcritical refrigerant increases. The transcritical refrigerant remains in a gaseous state when it is compressed by the compressor 220. The compressor 220 is located upstream of the heat exchanger 216, and downstream of both the further heat exchanger 222 and the refrigerant receiver 232. The second compressor 218 is configured to compress the second refrigerant. When compressed by the second compressor 218, the temperature of the second refrigerant increases. The second refrigerant remains in a gaseous state when it is compressed by the second compressor 218. The second compressor 218 is located upstream of the heat exchanger 216, and downstream of the evaporator 242. The heat exchanger 216 is configured to transfer heat from the transcritical refrigerant and the second refrigerant to a process fluid 206. The term “process fluid” encompasses any fluid that passes through the heat exchanger 216 and receives heat from the transcritical refrigerant and the second refrigerant. In the present example, the process fluid 206 comprises water (H2O). The process fluid may optionally further comprise one or more additives, such as antifreeze and / or a corrosion inhibitor. The heat exchanger 216 is configured to heat the process fluid entering the heat exchanger, such that the temperature of the process fluid at an outlet of the heat exchanger is greater than the temperature of the process fluid at an inlet of the heat exchanger 216. Conversely, the temperature of the transcritical refrigerant decreases as the transcritical refrigerant passes through the heat exchanger 216. Likewise, the temperature of the second refrigerant decreases as the second refrigerant passes through the heat exchanger 216. In this example, the heat exchanger 216 comprises two gas coolers 208, 212 and a condenser 210. The two gas coolers 208, 212 are connected in series via the first refrigerant circuit 204, such that one gas cooler 212 is immediately downstream of the compressor 220 and the other gas cooler 208 is immediately upstream of gas cooler 238. The condenser 210 is connected to the second refrigerant circuit 202, downstream of the second compressor 218 and upstream of the second expansion valve 244. The gas coolers 208, 212 allow heat transfer between the transcritical refrigerant and the process fluid 206, whilst the transcritical refrigerant remains in a gaseous state. The condenser 210 allows heat transfer between the second refrigerant and the process fluid 206, whilst the second refrigerant changes from a gaseous state to a liquid state. When the process fluid 206 flows through the heat exchanger 216, it first enters a first gas cooler 208, then flows through the condenser 210, and then enters a second gas cooler 212. The process fluid 206 then exits the heat exchanger 216. Although the gas coolers 208, 212 and condenser 210 are configured in this order in the implementation shown in Figure 2, the gas coolers 208, 212 and condenser 210 could be configured in a different order. Advantageously, the order shown in Figure 2 allows for the most efficient heat transfer from the transcritical refrigerant and second refrigerant to the process fluid 206. In other arrangements, the heat exchanger 216 may comprise a plurality of gas coolers and a plurality of condensers. In general, the heat exchanger 216 comprises at least one gas cooler and at least one condenser. Although heat is removed from the transcritical refrigerant by the heat exchanger 216, the transcritical refrigerant still has unutilised thermal energy when it exits the heat exchanger 216. This unutilised thermal energy is exploited to improve the overall efficiency of the heat pump system 200. Consequently, the gas cooler 238 is positioned downstream of the heat exchanger 216 to transfer the unutilised thermal energy of the transcritical refrigerant to a working fluid flowing through the evaporator 242. In the heat pump system 200, the working fluid in the evaporator 242 is the second refrigerant. The gas cooler 238 is positioned adjacent to the evaporator 242, so as to allow heat transfer from transcritical refrigerant in the gas cooler 238 to the working fluid (i.e., the second refrigerant) in the evaporator 242. The gas cooler 238 is located downstream of the heat exchanger 216. In the implementation shown in Figure 2, heat transfer from the gas cooler 238 to the evaporator 242 is mediated by ambient air 236. In more detail, ambient air 236 may be caused to flow over the gas cooler 238 and then over the evaporator 242, such that the heat transfer occurs via the ambient air 236. In other implementations, heat transfer from the gas cooler 238 to the evaporator 242 may be mediated by another source fluid and / or by conduction. Whichever implementation is used, the heat transfer causes the temperature of the working fluid (i.e. the second refrigerant) in the evaporator 242 to increase, and the temperature of the transcritical refrigerant in the gas cooler 238 to decrease accordingly. The evaporator 242 is configured to heat the working fluid. The evaporator 242 is located upstream of the second compressor 218. The thermal energy of the working fluid may be increased by means of the ambient air 236. Even in the absence of the gas cooler 238, the ambient air 236 possesses an inherent amount of thermal energy arising from the environment outside the heat pump system 200, which can be transferred to the working fluid flowing through the evaporator 242. When the ambient air passes over the gas cooler 238, the inherent thermal energy of the ambient air 236 is supplemented by the unutilised thermal energy of the transcritical refrigerant exiting the heat exchanger 216. Some of the thermal energy of the transcritical refrigerant flowing through the gas cooler 238 may thus be transferred to the ambient air 236, so that the total heat being transferred to the working fluid in the evaporator 242 by the ambient air 236 is increased. The unutilised thermal energy of the transcritical refrigerant in the gas cooler 238 is also advantageously used to melt any ice which forms on the outside of the evaporator 242. Defrosting the evaporator 242 in this manner can improve the efficiency of the evaporator 242, without the need to reverse the cycle. The heat pump system 200 may optionally comprise a fan (not shown in Figure 2). The fan may promote a flow of ambient air 236 over the evaporator 242 and the gas cooler 238. The fan may cause the ambient air 236 to flow over the gas cooler 238 before flowing over the evaporator 242. The second expansion valve 244 is located downstream of the heat exchanger 216 and upstream of the evaporator 242. The second expansion valve 244 is configured to reduce the pressure of the second refrigerant. In other words, the second refrigerant expands as it passes through the second expansion valve 244. The second refrigerant may change state from a liquid to a gas after exiting the second expansion valve 244. The pressure reducing valve 234 is located downstream of the gas cooler 238. The pressure reducing valve 234 is configured to reduce the pressure of the transcritical refrigerant exiting the gas cooler. In other words, the transcritical refrigerant expands as it passes through the pressure reducing valve 234. The transcritical refrigerant may be in either a gaseous or liquid state after exiting the pressure reducing valve 234. The refrigerant receiver 232 is located downstream of the pressure reducing valve 234. The refrigerant receiver 232 is configured to separate transcritical refrigerant in a gaseous state from transcritical refrigerant in a liquid state. The refrigerant receiver 232 has at least one outlet on its top surface which allows transcritical refrigerant in a gaseous state to join the refrigerant circuit 204 at one location, and at least one outlet on its bottom surface to allow transcritical refrigerant in a liquid state to join the refrigerant circuit 204 at a different location. The transcritical refrigerant may exit the refrigerant receiver 232 in a gaseous state when it becomes a flash-gas and spontaneously turns from a liquid state to a gaseous state. The expansion valve 228 is configured to reduce the pressure of the transcritical refrigerant when it is in a liquid state downstream of the refrigerant receiver 232. In other words, the transcritical refrigerant expands as it passes through the expansion valve 228. The expansion valve 228 lowers the pressure of the transcritical refrigerant to a point at which the transcritical refrigerant is capable of changing state from a liquid to a gas. The flash-gas valve 230 is configured to relieve the pressure of gaseous transcritical refrigerant exiting the refrigerant receiver 232 so that the transcritical refrigerant exiting the expansion valve 228 and the transcritical refrigerant exiting the flash-gas valve 230 may be recombined. The flash-gas valve 230 is located downstream of the refrigerant receiver 232 and upstream of the compressor 220. The further heat exchanger 222 is configured to increase the temperature of the transcritical refrigerant. The further heat exchanger 222 transfers heat from the surrounding environment to the transcritical refrigerant. The transcritical refrigerant enters the further heat exchanger 222 with a lower temperature and exits with a higher temperature. The transcritical refrigerant remains in a gaseous state in the further heat exchanger 222. The further heat exchanger 222 is located upstream of the expansion valve 228 and downstream of the compressor 220. The further heat exchanger 222 may be a ground source heat exchanger which is configured to transfer heat extracted from the ground to the transcritical refrigerant. The ground source heat exchanger may include a ground loop 224 and a pump 226. In more detail, the ground loop 224 consists of at least one loop of piping positioned underground. The depth, size and quantity of the ground loop(s) can vary, as discussed above in relation to the ground loop 112 shown in Figure 1. The ground loop 224 contains a working fluid, which will be referred to as a “ground loop working fluid”. The ground loop 224 comprises a fluid circuit, separate from the refrigerant circuit 204 and second refrigerant circuit 202, containing the ground loop working fluid. The ground loop working fluid may be glycol-water mixture. Other fluids, or combinations of fluids could also be used. The ratio of glycol to water may vary. For instance the mixture could be 50% glycol and 50% water, however the present disclosure is not limited thereto. The pump 226 may be located between the ground loop 224 and the further heat exchanger 222. When the pump 226 is in operation, the ground loop working fluid is pumped around the ground loop 224. Heat from the ground surrounding the ground loop 224 is then transferred to the ground loop working fluid flowing through the ground loop. The ground loop working fluid then flows through the further heat exchanger 222. The further heat exchanger 222 transfers heat from the ground loop working fluid to the transcritical refrigerant. The thermal energy of the ground loop working fluid is reduced. The thermal energy of the transcritical refrigerant is commensurately increased, and the temperature of the transcritical refrigerant thereby increases. Although Figure 2 shows an implementation in which the further heat exchanger 222 is coupled to a ground loop 224, the present disclosure is not limited thereto. Other suitable heat sources could be used in addition to, or in place of, the ground loop 224. For example, a solar and / or hydrothermal heat source could be used. Example Multi-refrigerant Heat Pump System Cycle An example cycle of the heat pump system 200 shown in Figure 2 will now be described. Reference to Figures 4 and 5 will be made. The description of the cycle will begin at the heat exchanger 216. In the heat pump system 200, the heat exchanger 216 comprises two gas coolers 208, 212 and a condenser 210. To begin, the process fluid 206 flows through the first gas cooler 208. Heat is transferred from the transcritical refrigerant in the first gas cooler 208 to the process fluid 206. The process fluid 206 then flows through the condenser 210, where heat is transferred from the second refrigerant to the process fluid 206. The process fluid 206 then flows through the second gas cooler 212, where more heat is transferred from the transcritical refrigerant to the process fluid 206. Overall, the transcritical refrigerant enters the heat exchanger 216 with a high temperature and exits the heat exchanger with a lower temperature, whilst remaining at a high pressure. The transcritical refrigerant enters the heat exchanger 216 as a supercritical fluid 410, and exits as a subcritical fluid 408. The second refrigerant enters the heat exchanger as a high temperature gas 510 and exits as a liquid 512. The gas coolers 208, 312 and condenser 210 are configured in the order explained above to optimise the efficiencies of the transcritical refrigerant’s and second refrigerant’s respective heat transfers to the process fluid 206. Focusing on the first refrigerant circuit 204, the transcritical refrigerant then enters the gas cooler 238. As explained above, the gas cooler 238 transfers unutilised thermal energy from the transcritical refrigerant to the working fluid flowing through the evaporator 242. This heat transfer may be mediated by ambient air 236. The temperature of the transcritical refrigerant decreases as it passes through the gas cooler 238, while the pressure of the transcritical refrigerant remains constant and the transcritical refrigerant remains in a subcritical state 408 as it leaves the gas cooler 238. The transcritical refrigerant, which is at a low temperature and a high pressure, then flows through the pressure reducing valve 234. The pressure reducing valve 234 reduces the pressure of the transcritical refrigerant, causing the transcritical refrigerant to change from a subcritical state 408 to either a liquid state 406 or a gaseous state 414 depending on its temperature. As external factors, such as ambient air temperature and process fluid 206 usage, can alter the demand for heated process fluid, the amount of unutilised thermal energy when the transcritical refrigerant exits the heat exchanger 216 may vary. The temperature of the transcritical refrigerant exiting the gas cooler 238 and the pressure reducing valve 234 may thus also vary. The transcritical refrigerant, which could be either a liquid or a gas, then flows into the refrigerant receiver 232. As explained above, the refrigerant receiver 232 separates liquid transcritical refrigerant from gaseous transcritical refrigerant. In some cases, when the transcritical refrigerant inside the refrigerant receiver 232 mixes with the transcritical refrigerant entering it, the transcritical refrigerant may start to boil. This may cause some of transcritical refrigerant to spontaneously vaporise. Consequently, inside the refrigerant receiver 232, the transcritical refrigerant may be in either a liquid or gaseous state. The refrigerant receiver 232 is also capable of storing excess transcritical refrigerant in order to regulate the amount of transcritical refrigerant passing through the compressor 220 and into the heat exchanger 216. The transcritical refrigerant entering the refrigerant receiver 232 mixes with any transcritical refrigerant already inside. This may further or decrease the temperature of the transcritical refrigerant inside the refrigerant receiver 232. In order to be used inside the compressor 220, the transcritical refrigerant should generally be in a gaseous state. If the transcritical refrigerant is already in a gaseous state, due to being a flash-gas, it exits the refrigerant receiver 232 via an outlet located on the top of the refrigerant receiver 232. If the transcritical refrigerant is in a liquid state its state is changed from liquid to gas, before entering the compressor 220. Any liquid transcritical refrigerant exits the refrigerant receiver 232 via an outlet located on the bottom surface of the refrigerant receiver 232. The transcritical refrigerant which exited the refrigerant receiver 232 as a gas then passes through a flash-gas valve 230. The flash-gas valve 230 matches the pressure of the transcritical refrigerant which exited the refrigerant receiver 232 as a gas to the transcritical refrigerant which has exited the further heat exchanger 222. The flash-gas valve 230 prevents over-pressurisation caused by the flash-gas. The transcritical refrigerant which exited the refrigerant receiver 232 as a liquid passes through the expansion valve 228, which decreases the pressure of the transcritical refrigerant. The temperature of the transcritical refrigerant remains constant, causing it to change from a liquid state 406 to a gaseous state 414. The transcritical refrigerant exiting the expansion valve 228 then passes through the further heat exchanger 222. The further heat exchanger 222 can, at times, increase the temperature of the transcritical refrigerant by transferring heat from the surrounding environment to the transcritical refrigerant. Heat from the surrounding environment may be transferred to the transcritical refrigerant using a ground source heat exchanger, which optionally comprises a ground loop 224 and a pump 226.. When the pump 226 is in operation (e.g., when there is a high demand for heated process fluid), the transcritical refrigerant leaves the further heat exchanger 222 with a higher temperature, but remains in a gaseous state 414. When the pump 126 is not in operation, the further heat exchanger 222 does not affect the temperature or state of the transcritical refrigerant. The transcritical refrigerant exiting the flash-gas valve 230 is then combined with the transcritical refrigerant exiting the further heat exchanger 222. The transcritical refrigerant then flows through the compressor 220. The compressor 220 causes the pressure of the transcritical refrigerant to increase. The increase in pressure of the transcritical refrigerant is accompanied by an increase in temperature. This causes the transcritical refrigerant to change to a supercritical state 410, in which the transcritical refrigerant has a high temperature and high pressure. The transcritical refrigerant exits the compressor 220 in a supercritical state 410, then flows into the heat exchanger 216 and the cycle repeats. Now focusing on the second refrigerant circuit 202, after exiting the heat exchanger 216 the second refrigerant is a low temperature liquid 512. The second refrigerant then flows to the second expansion valve 244, in which the pressure of the second refrigerant is reduced. This causes the second refrigerant to vaporise and enter the gaseous state 510. The second refrigerant then passes through the evaporator 242. The evaporator 242 transfers heat from the transcritical refrigerant in the gas cooler 238 and the ambient air 236 to the second refrigerant flowing through the evaporator 242. The temperature of the second refrigerant increases while in the evaporator 242. The second refrigerant exits the evaporator 242 in a gaseous state 510. The second refrigerant then enters the second compressor 218. The second compressor 218 causes the pressure and temperature of the second refrigerant to increase. The second refrigerant remains in a gaseous state 510 as it exits the second compressor 218. The second refrigerant then flows into the heat exchanger 216, and the cycle repeats. To defrost the evaporator 242, the pump 226 is in operated. The further heat exchanger 222 increases the temperature of the transcritical refrigerant by transferring heat from the surrounding environment (e.g., the ground loop 224) to the transcritical refrigerant. The transcritical refrigerant leaves the further heat exchanger 222 with a higher temperature, and then flows through the transcritical refrigerant circuit 204, eventually flowing through the gas cooler 238. This increases the temperature of the evaporator 242, melting any ice on the surface of the evaporator 242 and inhibiting ice formation. The following indicative values of temperatures and pressures of fluids at various points are provided to illustrate the foregoing description of the heat pump system 100. These values should not be regarded as limiting the scope of the present disclosure. The process fluid 206 may enter the heat exchanger 216 with a temperature around 30°C, and may exit the heat exchanger 216 with a temperature of up to 70°C. The first gas cooler 208 may raise the temperature of the process fluid 206 from 30°C to 40°C. The condenser 210 may raise the temperature of the process fluid 206 from 40°C to 55°C. The second gas cooler 212 may raise the temperature of the process fluid 206 from 55°C to 70°C. The transcritical refrigerant may enter the heat exchanger 216 with a temperature in the range of 70°C to 100°C, and may exit the heat exchanger 216 with a temperature in the range of 25°C to 45°C. The transcritical refrigerant may have a temperature in the range of 45°C to 55°C when it exits the second gas cooler 212 and enters the first gas cooler 208. The transcritical refrigerant may enter the gas cooler 238 with a temperature in the range of 25°C to 45°C, and may exit the gas cooler 238 with a temperature in the range of 0°C to 25°C. The transcritical refrigerant may enter the pressure reducing valve 234 with a pressure in the range of 80 to 120 bar, and may exit the pressure reducing valve 234 with a pressure in the range of 20 to 30 bar. The transcritical refrigerant may exit the further heat exchanger 222 with a temperature in the range of -15°C to +10°C The transcritical refrigerant may enter the compressor 220 with a pressure in the range of 20 to 30 bar. Gas Cooler and Evaporator Figure 3 is a schematic diagram of a gas cooler 306 and evaporator 302 in accordance with the present disclosure. The gas cooler 306 of Figure 3 may correspond to the gas cooler 128 of Figure 1, while the evaporator 302 of Figure 3 may correspond to the evaporator 130 of Figure 1. Alternatively, the gas cooler 306 of Figure 3 may correspond to the gas cooler 238 of Figure 2, while the evaporator 302 of Figure 3 may correspond to the evaporator 242 of Figure 2. As mentioned previously, the gas cooler 306 and evaporator 302 are positioned adjacent to each other. The gas cooler 306 comprises a plurality of (in this case, two) heat exchange elements 316a, 316b. The evaporator 302 comprises a plurality of (in this case, three) heat exchange elements 312a, 312b, 312c. The heat exchange elements 316a, 316b of the gas cooler 306 are interleaved with the heat exchange elements 312a, 312b, 312c of the evaporator 302. A first heat exchange element 312a of the evaporator 302 is disposed between a first heat exchange element 316a of the gas cooler 306 and a second heat exchange element 316b of the gas cooler 306. A second heat exchange element 312b of the evaporator 302 is positioned adjacent the second heat exchange element 316b of the gas cooler 306. A third heat exchange element 312c of the evaporator 302 is positioned adjacent the second heat exchange element 312b of the evaporator 302. Ambient air 304 flows over the heat exchange elements 316, 312. More specifically, the ambient air 304 first flows over the first heat exchange element 316a of the gas cooler 306, then over the first heat exchange element 312a of the evaporator 302, then over the second heat exchange element 316b of the gas cooler 306, then over the second heat exchange element 316b of the gas cooler 306 and finally over the third heat exchange element 312c of the evaporator 302. The ambient air acts as a medium to transfer heat from the transcritical refrigerant in the gas cooler 306 to the working fluid in the evaporator 302. The ambient air 304 possesses an inherent amount of thermal energy, which it has gained from its surrounding environment. Ambient temperature varies in accordance with the amount of thermal energy the ambient air 304 possesses. The ambient temperature is “high” when the ambient air 304 possesses a “high” amount of thermal energy, and vice versa. The gas cooler 306 transfers heat from the transcritical refrigerant to the ambient air 304, which increases the total thermal energy of the ambient air 304. When the ambient air 304 then passes over the evaporator 302, some of the thermal energy of the ambient air 304 is transferred to the working fluid in the evaporator 302. As explained previously, the working fluid may be the transcritical refrigerant in the case of the single refrigerant heat pump system 100 shown in Figure 1, or another refrigerant such as propane in the case of the multi-refrigerant heat pump system 200 shown in Figure 2. The rate of heat transfer from the gas cooler 306 to the ambient air 304, or from the ambient air 304 to the evaporator 302, is related to the velocity of the ambient air 304. In order to maximise the heat transfer, the velocity of the ambient air 304 may be increased. One or more fans (not shown in Figure 3) may be provided, so as to increase the flow of ambient air 304 over the gas cooler 306 and the evaporator 302. In more detail, the one or more fans may comprise axial fans, backward curved fans, inline duct fans or any other suitable type of fan. The one or more fans may be positioned upstream of the gas cooler 306 and evaporator 302 with respect to the flow of ambient air 304, such that the fan(s) push the ambient air 304 over the gas cooler 306 and evaporator 302. Alternatively, the one or more fans 304 may be positioned downstream of the gas cooler 306 and evaporator 302 with respect to the flow of ambient air 304, such that the fan(s) pull the ambient air 304 over the gas cooler 306 and evaporator 302. Alternatively, one or more fans may be positioned upstream of the gas cooler 306 and evaporator 302, and one or more fans may be positioned downstream of the gas cooler 306 and evaporator 302 with respect to the flow of ambient air 304. As noted above, the heat exchange elements 312a, 312b, 312c of the evaporator 302 are interleaved with the heat exchange elements 316a, 316b of the gas cooler 306. Interleaving the heat exchange elements maximises heat transfer from the gas cooler 306 to the evaporator 302. The heat exchange elements of the evaporator 302 or gas cooler 306 may each comprise a plurality of tubes, which may be connected in series or parallel to form a heat exchange element. The tubes that compose the heat exchange elements of the gas cooler 306 are positioned close to the tubes that compose the heat exchange elements of the evaporator 302 in order to maximise heat transfer therebetween. The heat exchange elements of the evaporator 302 and gas cooler 306 may be positioned in an alternating fashion, such that the ambient air 304 will first pass over the first heat exchange element 316a of the gas cooler 306, then the first heat exchange element 312a of the evaporator 302, then the second heat exchange element 316b of the gas cooler 306, and so on. This configuration allows for increased heat transfer as the temperature gradient is more uniform across the entire evaporator 302 and gas cooler 306. The present disclosure is not limited to the specific ordering of heat exchange elements shown in Figure 3, and other suitable configurations are possible within the scope of the present disclosure. Heat transfer between the gas cooler 306 and evaporator 302 may be further increased through the use of finned tubes. As noted above, the heat exchange elements of the evaporator 302 or gas cooler 306 may each comprise a plurality of tubes. Each tube may have a plurality of elongated fins attached to its external surface. The fins increase the surface area available for heat exchange, and therefore allow a greater heat transfer between the ambient air 304 and the gas cooler 306 or evaporator 306. The finned tubes of the evaporator 302 may be separate from the finned tubes of the gas cooler 306. Alternatively, the finned tubes may be formed by passing the tubes constituting the heat exchange elements of the evaporator 302 and the gas cooler 306 through a series of metal sheets, such that the metal sheets act as fins. In this manner, the finned tubes may constitute a plate heat exchanger configured to transfer heat from the transcritical refrigerant flowing through the gas cooler to the working fluid flowing through the evaporator. This construction allows for further heat transfer by conduction through the metal sheets. The heat exchange elements of the gas cooler 306 and / or evaporator 302, may also be angled with respect to the vertical plane. The heat exchange elements may be at an angle of 45°, although other angles are possible. Advantageously, this allows for the hotter transcritical refrigerant or working fluid to rise in the gas cooler 306 or evaporator 302 respectively. This ensures that all of the transcritical refrigerant or working fluid is able to transfer heat, making the heat transfer more even through the heat exchange elements, preventing cold spots from forming in the evaporator 302 or gas cooler 306. Additionally, by having the heat exchange elements of the gas cooler 306 and / or evaporator 302 angled with respect to the vertical plane, condensate can run off the heat exchange elements of the gas cooler 306 and / or evaporator 302. When the evaporator 302 is in use, ice may form around the external surface of its heat exchange elements 312a, 312b, 312c. In more detail, when the evaporator 302 is in use, the temperature of the working fluid increases, and the temperature of the ambient air 304 decreases. When the ambient air 304 is at the dew point, water vapour in the ambient air condenses on the external surface of the evaporator 302. If the external surface of the evaporator 302 is sufficiently cold, the condensate freezes causing ice to form on the evaporator 302. The formation of ice may be concentrated towards the lower surfaces of the evaporator 302, which will typically be colder than the higher surfaces of the evaporator 302 due to upwards convection of working fluid. Condensate forming on the higher surfaces of the evaporator 302 falls towards the lower surfaces of the evaporator 302, where it may freeze. To overcome this issue, the gas cooler 306 may comprise additional heat exchange elements (shown generally by reference numeral 308) positioned at the lowest point of the evaporator 302. The additional heat exchange elements 308 of the gas cooler 306 increase the temperature of the lower surfaces of the evaporator 302, and thereby reduce or prevent the formation of ice. Efficiency is improved by reducing the amount of ice which forms on the evaporator 302, since ice can inhibit heat exchange from the ambient air 304 to the working fluid inside the evaporator 302. Single Refrigerant Heat Pump System with Heat Storage System Figure 6 is a schematic diagram of a further example of a heat pump system 600 in accordance with the present disclosure. The heat pump system 600 shown in Figure 6 is a variation of the heat pump system 100 shown in Figure 1, but comprises a heat storage system 620 instead of a ground loop 112. The purpose of the heat storage system 620 is to store heat for use in defrosting the evaporator 130. The heat pump system 600 shown in Figure 6 has many components in common with the heat pump system 100 shown in Figure 1; those components are indicated using the same reference numerals as Figure 1, and are not described again. The heat storage system 620 is configured to extract heat transferred from the heated process fluid 102, and to store that heat in a second working fluid. The heat stored in the second working fluid can be transferred to the evaporator 130 when needed, to melt ice on the evaporator 130 and defrost it. The heat storage system 620 comprises a working fluid store 606 for storing the second working fluid, a first pump 608, a second pump 610, a defrosting heat exchanger 614, and a second working fluid circuit 612. The working fluid store 606 is configured to store the heated second working fluid whilst the heat pump system 600 is operating in a process fluid heating cycle. The working fluid store 606 is a vessel, which may be insulated. The working fluid store 606 comprises a chamber with an inlet and outlet to allow second working fluid to enter and exit the working fluid store 606, respectively. The working fluid store 606 also comprises an internal heat exchanger to transfer heat from the heated process fluid 102 to the second working fluid. The internal heat exchanger may be a liquid-to-liquid heat exchanger, and may comprise one or more pipes located within the working fluid store 606. When the process fluid passes through the internal heat exchanger, heat from the process fluid 102 is transferred to the second working fluid in the working fluid store 606. The first pump 608 is configured to induce a flow of second working fluid through the second working fluid circuit 612 when in operation. The first pump 608 may be located upstream or downstream of the working fluid store 606. The second pump 610 is used to allow heated process fluid 102 to flow through the working fluid store 606 when in operation. The heated process fluid 102 is extracted from downstream of the heat exchanger 104. The process fluid 102 which has passed through the working fluid store 606 re-enters the process fluid 102 flow upstream of the heat exchanger 104. The second pump 610 may be located upstream or downstream of the working fluid store 606. As shown in Figure 6, the defrosting heat exchanger 614 is located between the gas cooler 128 and evaporator 130. The defrosting heat exchanger 614 is configured to transfer heat from the second working fluid in the second working fluid circuit 612 to the evaporator 130. The defrosting heat exchanger 614 is advantageously located in the flow stream of the ambient air 126 to transfer the heat from the second working fluid to the external surface of the evaporator 130. The defrosting heat exchanger 614 may be located elsewhere, for example before the gas cooler 128 and evaporator 130 in the flow stream of the ambient air 126. With regard to the heat storage system 620, the defrosting heat exchanger 614 is located downstream from the working fluid store 606. The second working fluid circuit 612 contains the second working fluid. The purpose of the second working fluid circuit 612 is to convey the second working fluid between the various components mentioned above. The second working fluid circuit 612 may comprise pipes, connectors and / or further pumps to convey the second working fluid. The second working fluid may be a glycol-water mixture. Other fluids, or combinations of fluids could also be used. The ratio of glycol to water may vary. For instance the mixture could be 50% glycol and 50% water, however the present disclosure is not limited thereto. The mixture of glycol and water is used to prevent the second working fluid from freezing during prolonged periods of no use in cold weather. In the heat pump system 600 of Figure 6, the transcritical refrigerant exiting the expansion valve 120 passes through to the evaporator 130. Unlike the heat pump system 100 of Figure 1, the transcritical refrigerant exiting the expansion valve 120 does not pass through a further heat exchanger 114. Example Single Refrigerant Heat Pump System with Heat Storage System Cycles The heat pump system 600 shown in Figure 6 has two modes of operation: a process fluid heating mode; and a defrosting fluid heating mode. The thermodynamic cycles for each mode of operation will be discussed in turn. Process Fluid Heating Cycle During the process fluid heating cycle, the first pump 608 is not in operation. The second working fluid does not flow through the heat storage system 620. The heat storage system 620 and the defrosting heat exchanger 614 do not transfer heat to the external surface of the evaporator 130. The second pump 610 is occasionally turned on during process fluid heating cycles. For example, the second pump 610 may be turned on when the temperature of the second working fluid stored inside the working fluid store 606 is below a threshold temperature, and turned off when the temperature of the second working fluid is above a second threshold temperature. As another example, the second pump 610 may be turned on and off periodically. The operation of the second pump 610 causes heated process fluid 102 to flow into the working fluid store 606. The heated process fluid 102 then transfers its heat to the working fluid stored inside the working fluid store 606. This increases the temperature of the second working fluid inside the working fluid store 606, while the temperature of the process fluid 102 decreases commensurately. The process fluid 102 then re-joins the process fluid circuit upstream of the heat exchanger 104. The working fluid store 606 then retains the heat in the second working fluid, ready for use during the defrosting cycle. Defrosting Cycle During the defrosting cycle, the first pump 608 is in operation. The first pump 608 causes the second working fluid to flow around the second working fluid circuit 612. The second working fluid flows from the working fluid store 606 into the defrosting heat exchanger 614. When the second working fluid is inside the defrosting heat exchanger 614, thermal energy is transferred from the second working fluid to the external surface of the evaporator 130. This causes the temperature of the second working fluid to decrease and the temperature of the evaporator 130 to increase. As the temperature of the evaporator 130 increases, any ice that has formed on the surface of the evaporator 130 melts and / or the formation of ice on the surface of the evaporator 130 is inhibited. The second working fluid exits the defrosting heat exchanger 614, flows through the second working fluid circuit 612 and returns to the working fluid store 606. The cycle then repeats. When the second working fluid is inside the working fluid store 606, the second working fluid which flows into the working fluid store, with a lower temperature, and the second working fluid already inside the store, with a higher temperature, will mix together to cause the second working fluid to have an overall lower temperature than before the defrosting cycle began. Eventually, the temperature of the second working fluid already inside the working fluid store 606 will be equal to the temperature of the second working fluid which flows into the working fluid store 606. At this point no further defrosting can occur, and the heat pump system 600 will have to run a process fluid heating cycle in order to increase the temperature of the second working fluid stored within the working fluid store 606. Multi-refrigerant Heat Pump System with Heat Storage System A heat storage system 620 as described above and shown in the implementation of Figure 6 may be used as an alternative to the further heat exchanger 222 shown in Figure 5 2. That is, the further heat exchanger 222 may be replaced by the heat storage system 620 shown in the implementation of Figure 6. In this case, the transcritical refrigerant which has exited the expansion valve 228 is combined with the transcritical refrigerant which has exited the flash-gas valve 230, without passing through the further heat exchanger 222. A defrosting heat exchanger 614, as described above in connection with 10 Figure 6, may be located between the gas cooler 238 and evaporator 242. The defrosting heat exchanger may be configured to transfer heat from a second working fluid in the heat storage system to the evaporator 242. The heat pump systems described above are intended as examples, and should not be 15 regarded as limiting the scope of the claims. Various modifications of detail may be made within the scope of the claims.

Claims

1. A heat pump system comprising:a refrigerant circuit containing a transcritical refrigerant;a compressor configured to increase the pressure and temperature of the transcritical refrigerant;a heat exchanger, downstream of the compressor and connected thereto by the refrigerant circuit, configured to transfer heat from the transcritical refrigerant to a process fluid flowing through the heat exchanger;an evaporator configured to transfer heat from a source fluid to a working fluid flowing through the evaporator;a gas cooler, positioned adjacent to the evaporator, downstream of the heat exchanger and connected thereto by the refrigerant circuit, configured to transfer heat from the transcritical refrigerant to the working fluid flowing through the evaporator; andan expansion valve, downstream of the gas cooler and connected thereto by the refrigerant circuit, and configured to reduce the pressure and temperature of the transcritical refrigerant exiting the gas cooler, wherein the expansion valve is upstream of the compressor and connected thereto by the refrigerant circuit.

2. A heat pump system according to claim 1, wherein the gas cooler is configured to transfer heat from the transcritical refrigerant to the working fluid flowing through the evaporator via the source fluid.

3. A heat pump system according to claim 1 or 2, wherein the gas cooler and the evaporator each comprise heat exchange elements, and wherein the heat exchange elements of the gas cooler are interleaved with the heat exchange elements of the evaporator.

4. A heat pump system according to claim 3, wherein a heat exchange element of the evaporator is located between adjacent heat exchange elements of the gas cooler.

5. A heat pump system according to claim 3 or claim 4, wherein the heat exchange elements of the gas cooler comprise finned tubes through which the transcritical refrigerant flows and / or the heat exchange elements of the evaporator comprise finned tubes through which the working fluid flows.

6. A heat pump system according to claim 5, wherein the finned tubes of the gas cooler and / or the finned tubes of the evaporator are angled with respect to a vertical plane.

7. A heat pump system according to any of claims 3 to 6, wherein the gas cooler comprises at least one additional heat exchange element located at the lowest point of the evaporator.

8. A heat pump system according to any preceding claim, wherein the heat exchanger comprises a gas cooler.

9. A heat pump system according to any preceding claim, wherein the transcritical refrigerant is carbon dioxide.

10. A heat pump system according to any preceding claim, comprising a further heat exchanger configured to transfer heat extracted from a surrounding environment to the transcritical refrigerant, the further heat exchanger being located upstream of the compressor and downstream of the expansion valve and connected thereto by the refrigerant circuit.

11. A heat pump system according to claim 10, wherein the further heat exchanger is a ground source heat exchanger configured to transfer heat extracted from below ground to the transcritical refrigerant.

12. A heat pump system according to any preceding claim, further comprising a heat storage system configured to extract heat from the process fluid and to store the heat extracted from the process fluid in a second working fluid.

13. A heat pump system according to claim 12, further comprising a defrosting heat exchanger configured to transfer heat from the second working fluid to the evaporator.

14. A heat pump system according to claim 13, wherein the defrosting heat exchanger is positioned between the gas cooler and evaporator.

15. A heat pump system according to any preceding claim, further comprising a refrigerant receiver and a flash-gas valve.

16. A heat pump system according to any preceding claim, wherein the evaporator forms part of the refrigerant circuit, such that the working fluid flowing through the evaporator is the transcritical refrigerant.

17. A heat pump system according to claim 16, wherein the evaporator is located downstream of the expansion valve and upstream of the compressor.

18. A heat pump system according to claim 16 or 17, when dependent directly or indirectly on claim 10, wherein the evaporator is located downstream of the further heat exchanger and upstream of the compressor.

19. A heat pump system according to any of claims 1 to 15, wherein the compressor is a first compressor, the expansion valve is a first expansion valve, and the refrigerant circuit is a first refrigerant circuit configured to circulate the transcritical refrigerant between the first compressor, the heat exchanger, the gas cooler and the first expansion valve, and wherein the heat pump system further comprises:a second refrigerant circuit containing a second refrigerant;a second compressor configured to increase the pressure and temperature of the second refrigerant, and located upstream of the heat exchanger and connected thereto by the second refrigerant circuit such that the heat exchanger transfers heat from the second refrigerant to the process fluid; anda second expansion valve, located downstream of the heat exchanger and connected thereto by the second refrigerant circuit, and configured to reduce the pressure and temperature of the second refrigerant exiting the heat exchanger, andwherein the evaporator is located downstream of the second expansion valve and connected thereto by the second refrigerant circuit, such that the working fluid flowing through the evaporator is the second refrigerant.

20. A heat pump system according to claim 19, wherein the heat exchanger comprises at least one gas cooler and a condenser,wherein the at least one gas cooler of the heat exchanger forms part of the first refrigerant circuit, such that the at least one gas cooler of the heat exchanger is locateddownstream of the first compressor and connected thereto by the first refrigerant circuit, andwherein the condenser forms part of the second refrigerant circuit, such that the condenser is located downstream of the second compressor and connected thereto by the second refrigerant circuit.

21. A heat pump system according to claim 20, wherein the heat exchanger comprises a first gas cooler and a second gas cooler, the second gas cooler of the heat exchanger being located downstream of the first gas cooler of the heat exchanger and connected thereto by the first refrigerant circuit.

22. A heat pump system according to claim 21, wherein the gas cooler is located downstream of the second gas cooler of the heat exchanger and connected thereto by the first refrigerant circuit.

23. A heat pump system according to claim 21 or 22, wherein the first gas cooler of the heat exchanger, the second gas cooler of the heat exchanger and the condenser are arranged such that the process fluid flowing through the heat exchanger flows sequentially through the second gas cooler of the heat exchanger, then the condenser and then the first gas cooler of the heat exchanger.

24. A heat pump system according to any of claims 19 to 23, wherein the second refrigerant is propane.

25. A heat pump system according to any of the preceding claims, wherein the source fluid is ambient air.

26. A heat pump system according to any preceding claim, further comprising a plate heat exchanger configured to transfer heat from the transcritical refrigerant flowing through the gas cooler to the working fluid flowing through the evaporator.IntellectualPropertyOfficeApplication GB2500311.2Search report under Section 17 of the Patents Act 1977Date search completed: 10 June 2025Claims searched: 1-26International classificationSubclass and subgroup Valid from F24D3 / 18 01 / 01 / 2006 F25B30 / 02 01 / 01 / 2006 F25B47 / 02 01 / 01 / 2006 F25B9 / 00 01 / 01 / 2006Field of searchWorldwide search of patent documents classified in the following areas of the IPC:F25B, F24DDatabases used in the preparation of this search report:SEARCH-NPL; SEARCH-PATENTDocuments considered to be relevantPatent literatureCategory Relevant Document of relevanceclaimsA - US 2010032133A1 CARRIER CORP, See Fig. 1 &related text. A - US 11913687 B1 DTP THERMOELECTRICS LLC, See Figs. 1,7 &related text. A - US 2008302118A1 CHEN Y, ZHANG L, See Fig. 1 &related text.Non-patent literatureCategory Relevant claims Document of relevanceCategoriesLetter or DescriptionsymbolX Document indicating lack of novelty or inventive step.Y Document indicating lack of inventive step, if combined with anotherdocument of the same category.& Member of the same patent family. A Document indicating technological background. P Document published on or after the priority date but before the fling date of the present application. E Earlier application published on or after the filing date of the present application.