Heat Pump Comprising a Heating Circuit and a Buffer Circuit

Description

The present invention relates to a heat pump comprising a heating circuit and a buffer circuit.

A heat pump is capable of transferring thermal energy from a heat source to a heat sink, thereby forcing the thermal energy to flow opposite to its natural flow direction from hot to cold. In this way, heat may be extracted from air (such as ambient air or air of an indoor ventilation system), from water, or from the earth, allowing heat to be extracted from sustainable sources or from waste streams. Such heat pumps may be used to either heat or cool an indoor area or heat a water storage tank that stores water for (domestic) use.

A heating circuit of a basic heat pump comprises an evaporator, a compressor, a condenser, and an expansion valve. In the evaporator, a liquid refrigerant absorbs heat at a first pressure level by evaporation. The compressor increases the pressure of the refrigerant from the first pressure level to a second pressure level, thereby also causing the temperature of the refrigerant to increase significantly. Still at the second pressure level, the refrigerant successively condenses in the condenser, thereby releasing heat. The condenser is a heat exchanger, and the heat released from the refrigerant may be used to act as a heat source for a further heating circuit, e.g. for indoor heating. In the expansion valve, the refrigerant expanses and the pressure drops from the second pressure level to the first pressure level. Due to this pressure drop, the refrigerant cools down, preparing it to absorb heat when it successively evaporates in the evaporator.

Before a heat pump comes into operation, a refrigerant charge is introduced into the heating circuit. The optimal amount of refrigerant charge in the heating circuit is determined based on the expected average operating conditions, and aims to optimize the amount of refrigerant in the condenser and in the evaporator. The efficiency of the heat pump is dependent on the amount of refrigerant in said condenser and evaporator. For example, if there is too much refrigerant in the condenser, the refrigerant takes up too much space in the condenser and may consequently leave too limited space left for optimal condensation purposes, thereby also reducing the efficiency of the condenser. To complicate the situation even further, the density of the refrigerant is temperature dependent. Thus, for a certain fixed amount of refrigerant, the volume it occupies will differ when the operating temperature of the heat pump changes. As the actual operating conditions will most of the time differ from the expected average operating conditions the charge of the refrigerant was based on, the efficiency of a heat pump is almost always suboptimal.

International patent application WO 2019/073870 A1 is considered the closest prior art. It discloses a refrigeration device comprising a heat pump having two indoor heat exchangers and one outdoor heat exchanger. During normal use, the outdoor heat exchanger functions as an evaporator, and the two indoor heat exchangers both function as a condenser in the heat pump cycle. The refrigeration device comprises a refrigerant sensor that is configured to output a detection signal when the concentration of refrigerant in the air exceeds a predetermined reference concentration. The detection signal of the refrigerant sensor is a leakage signal indicating that the refrigerant has leaked from the indoor circuit. When a signal indicating that refrigerant has leaked from the indoor circuit, i.e. one of the two condensers, is received, a controller performs refrigerant recovery control operations. Specifically, the controller opens a bypass line that is closed during normal operating conditions. The controller further operates a compressor, thereby causing the remaining refrigerant in the indoor circuit to be guided towards an accumulator that is arranged in a bypass line. In this way, further leakage of refrigerant from the indoor circuit can be prevented. A leakage and resulting safety measures to guide the remaining refrigerant to the accumulator results in a safety mode, wherein the refrigeration device cannot be used anymore until a mechanic has found and solved the leakage. The configuration does not allow the leaking condenser to be bypassed while the non-leaking condenser could remain working. Furthermore, WO 2019/073870 A1 also mentions that it is necessary to perform processing for removing moisture and impurities contained in recovered refrigerant before reusing the refrigerant that has been stored in the accumulator during the safety operation. For removing moisture and impurities contained in recovered refrigerant, an appropriate device such as a filter dryer or oil separator, may be used. In this way, it is possible to prevent deterioration of performance of the refrigerant due to mixing with other types of refrigerants or contamination with impurities. As such, the heat pump of the refrigeration apparatus of the refrigeration device of WO 2019/073870 A1 is in fact a normal heat pump as described above, facing the same challenges as described above. After all, it will also comprises a certain fixed amount of refrigerant. The optimal amount of refrigerant charge in the heating circuit is determined based on the expected average operating conditions. As described above, the actual operating conditions will most of the time differ from the expected average operating conditions the charge of the refrigerant was based on, and consequently also the efficiency of the heat pump of the refrigeration apparatus disclosed in WO 2019/073870 A1 is almost always suboptimal.

The international patent application WO 2019/158318 A1 and US 2020/064031 A1 define further prior art.

An objective of the present invention is to provide a heat pump, that is improved relative to the prior art and wherein at least one of the above stated problems is obviated or alleviated.

Said objective is achieved with the heat pump according to claim 1, that according to the present invention comprises:

• • a heating circuit that comprises:
    • an evaporator;
    • one or more than one expansion valve that is arranged upstream of the evaporator;
    • a compressor that is arranged downstream of the evaporator; and
    • a condenser that is arranged downstream of the compressor and upstream of the one ore more than one expansion valve; and
  • a buffer circuit that comprises or defines a refrigerant accumulator;
  • wherein the heating circuit and the buffer circuit are arranged in a parallel connection and share a common line passing through the evaporator and the compressor; and
  • wherein the heat pump further comprises a distributor that comprises at least two valves, wherein the distributor is configured to redistribute a refrigerant charge from the heating circuit to the buffer circuit or vice versa by controlling the at least two valves; and
  • wherein the distributor comprises a controller that is configured to selectively redistribute the refrigerant charge from the heating circuit to the buffer circuit or vice versa, by controlling the at least two valves of the distributor in dependency of actual operating conditions, to thereby actively optimize the amount of refrigerant in the condenser and in the evaporator for the actual operating conditions.

The skilled person will acknowledge that heat pumps may be reversible if a reversing valve is applied, allowing the heat pump to work in either direction and thereby use it for heating or cooling. In order to prevent unnecessary repetition, the invention is described with reference to the heat pump having a “heating” circuit, simply because the heating mode will be the most used mode for such a system. In such a heating mode, the common line passes through the evaporator, while this evaporator may temporarily function as a condenser in a reverse operating condition, e.g. for defrosting the evaporator. It is also emphasized that whether a cycle may be interpreted as a heating cycle or a cooling cycle is also dependent on the position of the observer. After all, indoor heating may be interpreted as outdoor cooling, wherein heat is withdrawn from the environment. Thus, it is explicitly mentioned that, although the invention is primarily described in relation to a heating cycle, the invention is also applicable to a cooling cycle.

The heat pump proposed by the present invention comprises a heating circuit and a buffer circuit, that share a common line passing through the evaporator and the compressor. The buffer circuit comprises or defines a refrigerant accumulator. Thus, the buffer circuit may comprise a dedicated accumulator, e.g. in the form of a vessel, but the buffer circuit itself may alternatively or additionally also function as the refrigerant accumulator, e.g. by storing refrigerant in the conduits thereof.

During normal operation, the heating circuit is involved in an active heating cycle, and the distributor allows refrigerant charge to be moved from the heating circuit to the buffer circuit, or vice versa. In this way, the refrigerant charge of the heating circuit may be optimized, wherein it is even possible to adjust the refrigerant charge during operation, thereby increasing the efficiency of the heat pump. For example, when the heating circuit is used heating a (not shown) water storage tank, the average water temperature in said water storage tank may increase from e.g. 30° C. starting temperature to 50° C. or even higher. Instead of setting a fixed predetermined refrigerant charge for the expected mean temperature of 40° C., the distributor allows the refrigerant charge to be actively adjusted during the heating. In this way, it is possible to gradually adjust the refrigerant charge to take the differing operation conditions into account and thereby optimize the efficiency of the heat pump. This efficiency may be selectively set to correspond to one of: the optimum coefficient of performance, and thus the minimum energy use on the one hand, and the maximum heating capacity obtainable on the other hand. In this way, it is possible to selectively choose for energy efficiency most of the time, while having the ability to temporarily prioritize for setting the heat pump to obtain maximum heating capacity. Moreover, it is not required anymore that the amount of refrigerant charge is precisely set during installation. After all, adjustments may be made—even automatically by the controller—once the heat pump is in operation. Consequently, the level of expertise required from an installer is less critical, even allowing layman to install the improved heat pump according to the invention.

Due to the distributor being able to redistribute a refrigerant charge from the heating circuit to the buffer circuit or vice versa, the amount of total refrigerant charge in the heating circuit of the heat pump may be adjusted while performing a heating (or cooling) operation, thereby allowing the heat pump to operate with the optimal quantity of refrigerant charge at all times.

Conversely, refrigerant charge that is not needed in the heating circuit may be temporarily stored in the buffer circuit. The controlled migration of refrigerant charge from the heating circuit to the buffer circuit and vice versa enables operation of the heat pump with an optimum refrigerant charge at each operating. The ability to adjust the refrigerant charge in the heating circuit enables operation at a preset optimum level of subcooling for each operating condition.

According to a preferred embodiment, the controller is configured to redistribute the refrigerant charge from the heating circuit to the buffer circuit or vice versa, in dependency of the actual operating conditions that are defined by a level of subcooling of the refrigerant in the heating circuit. In this context, subcooling is defined as a temperature difference between the condensing temperature and a temperature of the refrigerant. The level of subcooling is directly related to the volume of liquid refrigerant present in the condenser, and is as such a reliable parameter in determining the actual operating conditions. For example, if the condenser is filled for 50% with liquid refrigerant, its active capacity is also 50%, but the level of subcooling increases. Instead of the level of subcooling, also the level of superheating in the evaporator may be used, but this is more complex because the flow rate of the expansion valve also influences the level of superheating.

According to a further preferred embodiment, the controller is configured to determine a level of subcooling in the heating circuit by calculating a temperature difference between a condensing temperature at the condenser and a temperature of the refrigerant leaving said condenser.

According to an even further preferred embodiment, the controller is configured to at least one of:

• • redistribute refrigerant charge from the heating circuit to the buffer circuit if the subcooling in the heating circuit is above a pre-determined upper threshold temperature difference; and
  • redistribute refrigerant charge from the buffer circuit to the heating circuit if the subcooling in the heating circuit is below a pre-determined lower threshold temperature difference.

Further preferred embodiments are the subject of the dependent claims.

The various aspects and features described and shown in the specification can be applied, individually, wherever possible. These individual aspects, and in particular the aspects and features described in the attached dependent claims, may be an invention in its own right that is related to a different problem relative to the prior art.

In the following description preferred embodiments of the present invention are further elucidated with reference to the drawing, in which:

FIG. 1A is a schematic view of a prior art heat pump in a first mode, e.g. a heating mode;

FIG. 1B is a schematic view of a prior art heat pump in a second mode, e.g. a cooling mode;

FIG. 2A is a schematic view of a heat pump according to a first preferred embodiment of the invention, wherein a heating circuit is active in a heating mode;

FIG. 2B is a schematic view of the heat pump according to the first preferred embodiment of the invention, wherein the heating circuit is active in a cooling mode;

FIG. 3A is a schematic view of a heat pump according to a second preferred embodiment of the invention, wherein the heating circuit is active in a heating mode;

FIG. 3B is a schematic view of the heat pump according to the second preferred embodiment of the invention, wherein the refrigerant charge in the heating circuit is increased by migrating refrigerant charge from the buffer circuit to the heating circuit;

FIG. 3C is a schematic view of the heat pump according to the second preferred embodiment of the invention, wherein the refrigerant charge in the heating circuit is reduced by migrating refrigerant charge from the heating circuit to the buffer circuit;

FIG. 4 is a schematic view of a heat pump according to a third preferred embodiment of the invention, comprising two heating circuits and one buffer circuit; and

FIG. 5 is a schematic view of a heat pump according to a fourth preferred embodiment of the invention, wherein the first and the second heating circuit each comprise a dedicated expansion valve that also functions as the shutoff-valve in said respective heating circuit.

The prior art reversible heat pump 1 shown in FIGS. 1A and 1B comprises an evaporator 2, a compressor 3, a condenser 4, and an expansion valve 5. A controller 6 may control the setting of a four-way reversing valve 7 to allow the flow direction of a refrigerant inside the heating circuit to be reversed between a heating mode (FIG. 1A) and a cooling mode (FIG. 1B), as well as other components, such as the compressor 3.

In the heating mode of FIG. 1A, the evaporator 2 is capable of extracting heat from air (such as ambient air or air of an indoor ventilation system) or from the earth. The temperature of the refrigerant may be increased by the compressor 3 compressing the refrigerant, after which this heat may be extracted from the refrigerant in the condenser 4, where it may be used for e.g. indoor heating.

In the cooling mode of FIG. 1B, the flow direction of the refrigerant is reversed relative to the flow direction of FIG. 1A. The heat exchanging element that functioned as the evaporator 2 in FIG. 1A now acts as the condenser 4, and the heat exchanging element that functioned as the condenser 4 in FIG. 1A now acts as the evaporator 2.

A first preferred embodiment of the invention is shown in FIGS. 2A and 2B, wherein the heat pump 1 comprises a heating circuit H and a buffer circuit B. The heat pump 1 comprises an evaporator 2, an expansion valve 5 that is arranged upstream of the evaporator 2, and a compressor 3 that is arranged downstream of the evaporator 2. A condenser 4 is arranged in the heating circuit H, and a refrigerant accumulator 22 is arranged in the buffer circuit B. The heating circuit H and the buffer circuit B share a common line 21 passing through the evaporator 2 and the compressor 3. In this first preferred embodiment, the common line 21 also passes through the expansion valve 5. The heat pump 1 further comprises a distributor 12 comprising at least two valves V, wherein the distributor is configured to redistribute a refrigerant charge from the heating circuit H to the buffer circuit B or vice versa by controlling the at least two valves V. In the first preferred embodiment, the two valves V comprise two three-way valves T₁ and T₂. It is remarked that the distributor 12 comprises the controller 6 and the valves V. For clarity, only the control lines between the controller 6 and the valves V that belong to the distributor 12 are shown with dashed lines. In addition, the controller 6 may comprise further (not shown) control lines to other components, such as the compressor 3 and the optional four-way reversing valve 7.

The four-way reversing valve 7 is optional and only necessary to make the heat pump 1 reversible, thereby also allowing a cooling mode as shown in FIG. 2B. FIG. 2B may be considered a defrost mode, wherein the evaporator 2 is defrosted.

The condenser 4 is arranged in the heating circuit H, which in FIGS. 2A, 2B is connected to a space heating circuit 8 that is configured to provide a continuous heated water flow for indoor heating when it is active. The active state of the heating circuit H is represented by the thick lines in FIGS. 2A and 2B.

Heat pump 1 comprises a branch 13 that is arranged downstream of the compressor 3 and configured to branch the common line 21 off into a first line L₁ associated with the heating circuit H and a second line L₂ associated with the buffer circuit B.

Heat pump 1 further comprises a combiner 14 that is arranged upstream of the evaporator 2 and configured to re-combine the line L₁ of the heating circuit H and the line L₂ of the buffer circuit B into the common line 21. For this first preferred embodiment, as well as for the second preferred embodiment (FIGS. 3A-3C) and the third preferred embodiment (FIG. 4), the combiner 14 is also arranged upstream of the expansion valve 5. This is however not essential, as becomes apparent from the fourth preferred embodiment that is shown in FIG. 5.

In the first preferred embodiment shown in FIGS. 2A, 2B, the distributor 12 comprises a three-way valve T₁ that defines the combiner 14, and a further three-way valve T₂ that defines the branch 13. By selectively setting the two three-way valves T₁ and T₂, the controller 6 may activate the heating circuit H in a closed cycle, but also control the distributor 12 to redistribute the refrigerant charge from the heating circuit H to the buffer circuit B or vice versa. In order to cause this redistribution of refrigerant charge, the heating circuit H and the buffer circuit B are temporarily connected to each other. Thus, the closed cycle of the heating circuit H is temporarily opened, resulting in the heating circuit H being connected to the buffer circuit B to allow refrigerant charge to be redistributed. How this works, will be discussed in more detail when discussing the next, even more preferred, second embodiment.

A second and even more preferred embodiment of the invention is shown in FIGS. 3A-3C. This second preferred embodiment is closely related to the first embodiment shown in FIGS. 2A, 2B, except that the distributor 12 now comprises at least two shut-off valves S₁, S₂.

Also for this second preferred embodiment, the heat pump 1 comprises a heating circuit H and a buffer circuit B. The heat pump 1 comprises an evaporator 2, an expansion valve 5 that is arranged upstream of the evaporator 2, and a compressor 3 that is arranged downstream of the evaporator 2. A condenser 4 is arranged in the heating circuit H and a refrigerant accumulator 22 is arranged in the buffer circuit B. The heating circuit H and the buffer circuit B share a common line 21 passing through the evaporator 2 and the compressor 3. In this embodiment, the common line 21 also passes through the expansion valve 5. The heat pump 1 further comprises a distributor 12 comprising at least two valves V, wherein the distributor 12 is configured to redistribute a refrigerant charge from the heating circuit H to the buffer circuit B or vice versa by controlling the at least two valves V.

Instead of the two three-way valves T₁, T₂ of the first preferred embodiment, the at least two valves V of the distributor 12 now comprises at least two shut-off valves S₁, S₂, wherein a first shut-off valve S₁ is arranged downstream of the refrigerant accumulator 22 and upstream of the combiner 14, and a second shut-off valve S₂ is arranged downstream of the condenser 4 and upstream of the combiner 14. The two shut-off valves S₁ and S₂ are a preferred advantageous alternative for a three-way valve at the combiner 14. The first and second shutoff-valves S₁, S₂ may be solenoid operated valves, that allow for easy control by the controller 6.

The distributor 12 shown in FIGS. 3A-C comprises at least two further shut-off valves S₃, S₄, wherein a third shut-off valve S₃ is arranged downstream of the branch 13 and upstream of the condenser 4, and a fourth shut-off valve S₄ is arranged downstream of the branch 13 and upstream of the refrigerant accumulator 22. The two shut-off valves S₃, S₄ are a preferred advantageous alternative for the three-way valve T₂ at the branch 13 of the first embodiment. The third and fourth shutoff-valves S₃, S₄ are preferably solenoid operated valves, that allow for easy control by the controller 6.

The distributor 12 is configured to selectively activate the heating circuit H (as shown in FIG. 3A). The heating circuit H is activated by opening the third shut-off valve S₃ and the second shut-off valve S₂ and closing the fourth shut-off valve S₄ and the first shut-off valve S₁.

The arrangement of the heating circuit H and the buffer circuit B with the distributor 12 comprising at least two valves V allows the distributor 12 to selectively redistribute the refrigerant charge from the heating circuit H to the buffer circuit B or vice versa. For example, refrigerant may be distributed from the heating circuit H to the buffer circuit B by opening the second shut-off valve S₂ and the fourth shut-off valve S₄ while the first shut-off valve S₁ and the third shut-off valve S₃ are closed (FIG. 3C). Alternatively, refrigerant may be distributed from the buffer circuit B to the heating circuit H by opening the first shut-off valve S₁ and the third shut-off valve S₃ while the second shut-off valve S₂ and the fourth shut-off valve S₄ are closed (FIG. 3B). By redistributing refrigerant charge between the heating circuit H and the buffer circuit B, the amount of refrigerant charge may be continuously adapted, thereby improving the efficiency of the heat pump 1. The skilled person will understand that applying three-way valves T₁, T₂ according to the first preferred embodiment also allows the distributor 12 to redistribute refrigerant charge. Nevertheless, the second embodiment having a plurality of shut-off valves S₁, S₂, S₃, S₄ is preferred. Firstly, shut-off valves are less susceptible to leaking than three-way valves. Secondly, shut-off valves may be easily controllable, especially when they are embodied as solenoid valves. Thirdly, it provides the option to combine more than two lines, as will be elucidated in more detail in the third preferred embodiment shown in FIG. 4.

In the first, second and third embodiment according to the invention, the distributor 12 preferably comprises a controller 6 that is configured to redistribute the refrigerant charge from the heating circuit H to the buffer circuit B or vice versa, in dependency of a level of subcooling of the refrigerant in the heating circuit H. In this context, subcooling is defined as a temperature difference between the condensing temperature and a temperature of the refrigerant.

The controller 6 may be configured to adjust the refrigerant charge in the heating circuit H at a preset optimum level of subcooling. This optimum subcooling may be selectively set to correspond to one of: the optimum coefficient of performance, and thus the minimum energy use on the one hand, and the maximum heating capacity obtainable on the other hand.

Preferably, in an embodiment having multiple heating circuits (such as FIG. 4 showing two heating circuits H, H₁), the subcooling of at least the most critical heating circuit, which is typically the heating circuit comprising the condenser with the lowest volume, is taken into account by the controller 6. After all, the skilled person will acknowledge that the volume of refrigerant is far more critical in a relatively small heat exchanger, relative to a larger heat exchanger.

The controller 6 is preferably configured to determine at least one of:

• • a level of subcooling in the heating circuit H by calculating a temperature difference between a condensing temperature at the condenser 4 and a temperature of the refrigerant leaving said condenser 4; and
  • a level of subcooling in a further heating circuit H₁, by calculating a temperature difference between a condensing temperature at the further condenser 4₁ and a temperature of the refrigerant leaving said further condenser 4₁.

The skilled person will acknowledge that the condensing temperature at one of the condenser 4 or the further condenser 4₁ may be determined directly or indirectly. On the one hand, a direct measurement may be provided by a first temperature sensor 15, 19 arranged near an entrance in or shortly upstream of the condenser 4, 4₁, and a second temperature sensor 16, 20 arranged near an output in or shortly downstream of the condenser 4, 4₁. On the other hand, in a direct measurement, said condensing temperature may be derived from a condensing pressure, that may be determined by a pressure sensor in a vapour line leading to said condenser, or even via a temperature obtained in an even further heating circuit that is heated by the condenser.

The controller 6 may be configured to at least one of:

• • redistribute refrigerant charge from the heating circuit H to the buffer circuit B if the subcooling in the heating circuit H is above a pre-determined upper threshold temperature difference; and
  • redistribute refrigerant charge from the buffer circuit B to the heating circuit H if the subcooling in the heating circuit H is below a pre-determined lower threshold temperature difference.

The temperature difference between the lower threshold temperature difference and the upper threshold temperature difference may, for a heat pump for domestic use having a subcooling of about 5K, be in the range of 0.2-3° C., preferably in the range of 0.5-2° C., and more preferably in the range of 1-1.5° C. The lower threshold temperature difference is in the range of 0.5-1.2° C., and/or the upper threshold temperature difference is in the range of 0.3-1.2° C. The skilled person will acknowledge that industrial heat pump may have a significantly larger subcooling than 5 K, e.g. a subcooling of 10 K and above, and consequently the temperature ranges may differ.

The heat pump 1 may, according to a third preferred embodiment as shown in FIG. 4, comprise at least one further condenser 4₁ arranged in a further heating circuit H₁, wherein the heating circuit H, the buffer circuit B and the at least one further heating circuit H₁ share the common line 21 passing through the evaporator 2 and the compressor 3. In FIG. 4, the common line 21 also passes through the expansion valve 5. The distributor 12 is configured to redistribute a refrigerant charge from at least one of the heating circuit H, the buffer circuit B and the at least one further heating circuit H₁ to at least one other of the heating circuit H, the buffer circuit B and the at least one further heating circuit H₁. In FIG. 4, only one further heating circuit, i.e. a second heating circuit H₁, is shown, but the skilled person will understand that applying shut-off valves S₁, S₂, S₃, etc, instead of three-way valves T₁, T₂ provides the opportunity to apply multiple further heating circuits in addition to the heating circuit H, and the further heating circuit H₁.

FIG. 5 shows a schematic view of a heat pump 1 according to a fourth preferred embodiment of the invention. This fourth embodiment is closely related to the second preferred embodiment shown in FIGS. 3A-3C but differs relative to this second preferred embodiment in that the heating circuit H and the buffer circuit B now each comprise a dedicated expansion valve 5.

The second shut-off valve S₂ is defined by a first expansion valve 5_H of the one or more than one expansion valve, wherein this first expansion valve 5_H is arranged in the heating circuit H, more in particular downstream of the condenser 4 and upstream of the combiner 14.

The first shut-off valve S₁ is defined by a second expansion valve 5_B of the one or more than one expansion valve, wherein this second expansion valve 5_B is arranged in the buffer circuit B, more in particular upstream of the combiner 14 and downstream of the refrigerant accumulator 22. Refrigerant charge is stored under high pressure in the refrigerant accumulator 22. The second expansion valve 5_B in the buffer circuit B may release some pressure of the refrigerant charge before the refrigerant reaches the evaporator 2.

The common line 21 that extends between the combiner 14 and the branch 13 now lacks an expansion valve 5 as applied in the second preferred embodiment. Instead, the common line 21 now comprises the evaporator 2 and the compressor 3. A four-way reversing valve 7 is optionally also arranged in the common line 21. Replacing the single expansion valve 5 in the common line 21 of the second preferred embodiment for dedicated expansion valves 5_H, 5_B in the heating circuit H and the buffer circuit B, respectively, allows the expansion valves 5_H, 5_B to also fulfil the functionality of the shutoff-valve S₂, S₁ in said respective circuit H, B. In other words, the expansion valve 5₁, 5₂ may also serve as valves V that are controllable by the controller 6 of the distributor 12 to thereby redistribute refrigerant charge from the heating circuit H to the buffer circuit B or vice versa.

Although they show preferred embodiments of the invention, the above-described embodiments are intended only to illustrate the invention and not to limit in any way the scope of the invention. Accordingly, it should be understood that where features mentioned in the appended claims are followed by reference signs, such signs are included solely for the purpose of enhancing the intelligibility of the claims and are in no way limiting on the scope of the claims. Furthermore, it is particularly noted that the skilled person can combine technical measures of the different embodiments. For example, the fourth embodiment of FIG. 4 having two heating circuits H and H₁ may alternatively apply two (not shown) dedicated expansion valves 5 and 5₁. The scope of protection is defined solely by the following claims.