Method for operating a heat pump and heat pump system

Abstract

The invention relates to a method (100) for operating a heat pump (12) for heating tap water in a hot water tank (32), wherein the heat pump (12) has a refrigeration circuit (20) with a compressor (22), a condenser (24), an expansion valve (26) and an evaporator (28) and a controller (16) for controlling the refrigeration circuit (20), wherein the condenser (24) is connected to the hot water tank (32) by means of a secondary circuit (30) in a heat-transferring manner, wherein a condenser pump (14) for the controlled circulation of a heat transfer medium of the secondary circuit (30) is arranged within the secondary circuit (30), comprising the steps of: detecting (102) a heat energy requirement in the hot water tank (32), controlling (104) the refrigeration circuit (20) and generating heat energy by means of the refrigeration circuit (20), transferring (106) the heat energy from the refrigeration circuit (20) to the secondary circuit (30),Detecting (108) a pressure (p) on the high-pressure side in the refrigeration circuit (20) by means of a pressure sensor (34), and operating (110) the condenser pump (14) in the secondary circuit (30) at a variable speed as a function of the pressure (p) detected on the high-pressure side in the refrigeration circuit (20), wherein the pressure (p) on the high-pressure side is adjusted to a pressure setpoint by changing the speed of the condenser pump (14).

Description

[0001] The present invention relates to a method for operating a heat pump for heating tap water in a hot water tank, wherein the heat pump has a refrigeration circuit and a controller for controlling the refrigeration circuit. Furthermore, the invention relates to a heat pump system with a heat pump, comprising a refrigeration circuit, a condenser pump arranged in a secondary circuit, and a controller for controlling the refrigeration circuit.

[0002] Heat pumps are well-known in the art and are increasingly being used to supply buildings with the required thermal energy (hot water and space heating). Heat pumps are devices that use energy to transfer heat from a cooler space to a warmer space using a refrigeration cycle. Heat pumps are more energy-efficient than conventional heating systems, which use primary energy sources such as natural gas or petroleum to generate heat. For practical and economic reasons, heat pumps are designed and operated in such a way that they require a longer period of time than conventional heating systems, for example, to bring the contents of a hot water tank to a desired temperature level.

[0003] Situations may arise in which the current demand for heated tap water is greater than the heat pump is able to produce. It is therefore important to store the heat energy generated by the heat pump, particularly in a layered manner, in the hot water tank. The temperature at which a heat transfer medium conveyed in a secondary circuit is introduced into the hot water tank is also crucial; this temperature in turn depends on the conveying speed of the heat transfer medium in the secondary circuit. The conveying speed of the heat transfer medium in the secondary circuit is usually controlled as a function of the temperature difference between the heat transfer medium at the condenser outlet and the condenser inlet. A change in the temperature measurement is often recorded very slowly, which means that the change in the conveying speed of the heat transfer medium in the secondary circuit occurs with a delay.Within the refrigeration circuit of the heat pump, this can lead to an undesirable change in the process parameters and possibly to a forced stop of the heat pump.

[0004] Therefore, there is a need for adapted control in the operation of heat pumps, in particular on a condenser pump coupled to the heat pump, in order to optimise the process of heating the tap water in the hot water tank so that, as far as possible, an almost unlimited amount of tap water with a predetermined minimum temperature, in particular greater than 40 °C, can be drawn from the hot water tank.

[0005] The invention was therefore based on the object of demonstrating a method for operating a heat pump for heating tap water in a hot water tank and a heat pump system by means of which an almost unlimited amount of tap water with a predetermined minimum temperature can be safely provided for withdrawal from the hot water tank.

[0006] The invention solves the underlying problem in a method for operating a heat pump for heating tap water in a hot water tank, wherein the heat pump has a refrigeration circuit with a compressor, an evaporator, an expansion valve and a condenser and a controller for controlling the refrigeration circuit, wherein the condenser is connected to the hot water tank in a heat-transferring manner by means of a secondary circuit, wherein a condenser pump for the controlled circulation of a heat transfer medium of the secondary circuit is arranged within the secondary circuit, with the features of claim 1.In particular, the method comprises the steps of: detecting a heat energy requirement in the hot water tank, controlling the refrigeration circuit and generating heat energy by means of the refrigeration circuit, transferring the heat energy from the refrigeration circuit to the secondary circuit, detecting a pressure on the high-pressure side in the refrigeration circuit by means of a pressure sensor, and variable-speed operation of the condenser pump in the secondary circuit depending on the pressure detected on the high-pressure side in the refrigeration circuit, wherein the pressure on the high-pressure side is adjusted to a pressure setpoint by changing the speed of the condenser pump.

[0007] The invention utilizes the finding that, instead of the temperature difference of the heat transfer medium between the condenser outlet and the condenser inlet, the pressure measured on the high-pressure side of the refrigeration circuit is now used as the control variable for the flow rate of the heat transfer medium in the secondary circuit. By measuring the pressure on the high-pressure side of the refrigeration circuit and adjusting the pressure to a predetermined pressure setpoint, the speed of the condenser pump and the resulting flow rate of the heat transfer medium in the secondary circuit can be effectively adjusted, which in turn directly influences the measured pressure on the high-pressure side of the refrigeration circuit.The speed of the condenser pump is thus simply matched to the measured pressure and adjusted so that the pressure approximates the pressure setpoint, thus avoiding a possible forced shutdown of the compressor in the refrigeration circuit due to an inadmissibly high pressure on the high-pressure side. The lowest possible speed of the condenser pump results in optimal temperature stratification within the hot water tank. In addition, a low speed of the condenser pump enables the control of the heat pump's refrigeration circuit to compensate for the heat energy demand in the hot water tank at a comparatively low temperature threshold. This can advantageously reduce average daily heat losses. The pressure on the high-pressure side is preferably set within a permissible range around the pressure setpoint.In one embodiment of the method according to the invention, the pressure setpoint is a predetermined pressure value, whereby the pressure of the refrigerant on the high-pressure side can fluctuate within specified limits around the predetermined pressure value during operation of the refrigeration circuit. The measured pressure can thus deviate from the pressure setpoint by a predetermined amount without initiating a change in the speed of the condenser pump. The speed of the condenser pump is only adjusted when a permissible deviation is exceeded, thus minimizing the control effort.

[0008] According to a preferred development, the method provides that the pressure (p) on the high-pressure side is detected by means of a pressure sensor or estimated based on detected temperature measurements. The direct detection of the pressure on the high-pressure side by means of a sensor enables an accurate and precise determination of the pressure as an initial value for adjusting the condenser pump speed, which defines the conveying speed of the heat transfer medium in the secondary circuit. A change in the speed of the condenser pump has an almost direct effect on the detected pressure without a significant time delay. Alternatively, the pressure on the high-pressure side can be estimated by measuring the temperature of the heat transfer medium in the secondary circuit, preferably at the inlet and outlet of the condenser.

[0009] According to a preferred development, the method comprises the step of specifying the pressure setpoint for the pressure to be detected in the refrigeration circuit on the high-pressure side, and increasing a speed of the condenser pump if the detected pressure exceeds the pressure setpoint or reducing the speed of the condenser pump if the detected pressure falls below the pressure setpoint. Preferably, the pressure setpoint to which the pressure of the refrigerant circulating in the refrigeration circuit is to be adjusted is specified by the control system of the heat pump. In order to adapt the pressure of the refrigerant in the refrigeration circuit on the high-pressure side to the pressure setpoint, instead of changing the process parameters in the refrigeration circuit, a simple adjustment of the speed of the condenser pump and thus the conveying speed of the heat transfer medium in the secondary circuit partially flowing through the condenser of the refrigeration circuit is carried out.Specifically, if the detected pressure is above the setpoint pressure or above a pressure range around the setpoint pressure, the speed of the condenser pump is increased, effectively reducing the pressure in the refrigeration circuit on the high-pressure side. If the detected pressure in the refrigeration circuit is below the specified setpoint pressure or below the pressure range around the setpoint pressure, the speed of the condenser pump in the secondary circuit is reduced, causing the pressure in the refrigeration circuit to rise again.

[0010] According to a preferred development of the method, this comprises the step of detecting a pressure change on the high-pressure side in the refrigeration circuit, in particular by means of the pressure sensor, and increasing or decreasing the speed of the condenser pump depending on the detected pressure change. Preferably, instead of only detecting when the pressure setpoint or the pressure range around the pressure setpoint is exceeded, in particular by means of the heat pump control, the change in pressure on the high-pressure side in the refrigeration circuit is already detected and the speed of the condenser pump is increased or decreased in accordance with the detected increase in the pressure change. This enables the control to react more quickly and efficiently to a possible pressure change occurring in the refrigeration circuit.

[0011] Preferably, according to a further development of the method according to the invention, the pressure setpoint is limited downwards by a minimum pressure predetermined by a subcooling condition of the refrigerant used in the refrigeration circuit. This prevents, in particular, critical subcooling of the refrigerant in the condenser and counteracts the undesired entry of flash gas into the expansion valve downstream of the condenser. The provision of a minimum pressure predetermined by the subcooling condition of the refrigerant enables permanently stable and efficient operation of the heat pump. The pressure setpoint predetermined by the controller and the pressure detected within the refrigeration circuit are thus always above the minimum pressure defined by the refrigerant in the refrigeration circuit.

[0012] According to a preferred embodiment of the method, the minimum pressure is determined at the condenser outlet as a function of a reference temperature of the refrigerant, in particular as a function of a reference pressure linked to the reference temperature. The minimum pressure to be maintained is defined in particular by the refrigerant used and the process parameters that arise during operation of the refrigeration circuit together with the refrigerant in the refrigeration circuit. Propane is preferably used as the refrigerant for operating the heat pump. The reference temperature and the resulting reference pressure preferably correspond to a minimum temperature value to be maintained at the condenser outlet during operation of the heat pump in order to be able to guarantee the required minimum pressure for trouble-free operation of the refrigeration circuit.

[0013] According to a preferred development, the minimum pressure is above a reference pressure linked to the reference temperature, wherein the reference temperature, depending on the required subcooling of the refrigerant, is a predetermined temperature value below a bubble point temperature of the refrigerant in the condenser. Subcooling of the refrigerant takes place within the condenser, which is preferred, and thus the refrigerant has completely converted into its liquid phase before leaving the condenser. By maintaining the minimum pressure, which is above the reference pressure defined by the reference temperature, the refrigerant is prevented from becoming excessively subcooled, thereby counteracting the formation of flash gas in the refrigerant and its entry into the expansion valve downstream of the condenser. This ensures a consistently stable refrigeration cycle.The minimum pressure for the pressure setpoint is preferably between the reference pressure defined by the reference temperature and a pressure value linked to the bubble point temperature of the refrigerant. The bubble point temperature of the refrigerant corresponds to the temperature in the condenser at which the refrigerant is completely liquefied. Preferably, the refrigerant is subcooled within the condenser by 2–3 K below the bubble point temperature of the refrigerant. The bubble point temperature always depends on the type of refrigerant used and its resulting physical properties. Propane is preferably used as the refrigerant in this case.

[0014] According to a possible development of the method according to the invention, the pressure setpoint is limited upwards by a condenser pressure defined by the manufacturer as the maximum permissible for the compressor. The maximum permissible condenser pressure is therefore a fixed system parameter determined by the compressor arranged in the refrigeration circuit. According to the present invention, both the minimum pressure in the refrigeration circuit defined by the refrigerant and the maximum permissible condenser pressure determined by the compressor are stored in the control system as limit values ​​for regulating the heat pump. This enables safe and, above all, trouble-free operation for the almost unlimited production of tap water at a predetermined temperature level of, for example, greater than 40 °C.

[0015] According to a preferred embodiment of the method, this comprises the step of specifying the pressure setpoint in a range between the minimum pressure and the maximum permissible condenser pressure depending on the electricity currently available for operating the heat pump or the electricity price. The pressure setpoint is preferably specified by the heat pump control system depending on the availability of the electricity required to operate the heat pump, whereby in the event of a surplus of electricity or a comparatively low electricity price, a pressure setpoint approximating the maximum permissible condenser pressure is specified. If the availability of electricity is limited or the electricity price is comparatively high, the heat pump control system specifies a pressure setpoint adjacent to the minimum pressure to be maintained.A low pressure in the refrigeration circuit has a particularly beneficial effect on the efficiency of the compressor, which is accompanied by an increase in the speed of the condenser pump and thus an increased flow rate in the secondary circuit.

[0016] According to an alternative embodiment of the method according to the invention, this comprises the step of specifying the pressure setpoint in a range between the minimum pressure and the maximum permissible condenser pressure depending on a user-adjustable preference, in particular a minimum energy consumption to be achieved with the heat pump or the best possible comfort level with the heat pump. The pressure setpoint specified by the controller can also be individually set or adjusted by a user. Depending on personal preferences, the user can choose, for example, between economical operation of the heat pump and a high level of comfort in hot water preparation by the heat pump.When selecting economical operation, the control system preferably sets a higher pressure setpoint, whereas for a high level of comfort in hot water production, the control system sets a lower pressure setpoint.

[0017] According to a preferred embodiment of the method, the pressure on the high-pressure side is measured immediately downstream of or at the outlet of the condenser. By measuring the pressure downstream of or at the outlet of the condenser, reliable pressure measurement is possible, which allows for rapid and, above all, efficient control of the speed of the condenser pump in the secondary circuit and the resulting flow rate of the heat transfer medium in the secondary circuit. To measure the pressure downstream of or at the outlet of the condenser, at least one pressure sensor is provided at the fluid outlet of the condenser or in a section of the fluid line connected to the condenser outlet.

[0018] According to a preferred development of the method according to the invention, the temperature in the hot water tank is measured to determine the heat energy requirement in the hot water tank, with the refrigeration circuit being operated by the controller when the temperature measured, in particular, in the hot water tank falls below a predetermined threshold value. Detecting the temperature as an initial value for determining the heat energy requirement in the hot water tank provides a simple way to trigger the method according to the invention. Alternatively, instead of detecting the temperature in the hot water tank, the discharge of the hot water stream from the hot water tank can be quantitatively determined, and the resulting heat energy requirement can be determined based on this quantitative determination.

[0019] In a further aspect, the invention relates to a heat pump system with a heat pump, wherein the heat pump has a refrigeration circuit with a compressor, an evaporator, an expansion valve, and a condenser; a condenser pump which is arranged within a secondary circuit that connects the refrigeration circuit of the heat pump to a hot water tank in a heat-transferring manner, and a controller which is configured at least to control the refrigeration circuit. The heat pump system achieves the underlying problem in that the controller is configured to carry out a method for operating a heat pump for heating tap water in a hot water tank according to one of the preferred embodiments described above. In particular, the heat pump system has a pressure sensor arranged on the high-pressure side of the refrigeration circuit, which pressure sensor detects the pressure in the refrigerant on the high-pressure side of the refrigeration circuit.In addition to controlling the refrigeration circuit, the control of the heat pump system is configured to operate the condenser pump in the secondary circuit at a variable speed depending on the pressure detected on the high-pressure side of the refrigeration circuit. With the aid of a heat pump system designed in accordance with the invention in this way, the tap water in a hot water tank can be heated to a predetermined temperature in a safe and, above all, efficient manner. By detecting the pressure value on the high-pressure side and changing the speed of the condenser pump based on this, unstable conditions within the refrigeration circuit or a forced shutdown of the heat pump due to exceeding a permissible condenser pressure in the refrigeration circuit can preferably be avoided.

[0020] The preferred embodiments and further developments described for the method according to the invention are also preferred embodiments of the heat pump system according to the invention. The control of the heat pump system is configured to adjust the pressure on the high-pressure side, in particular to a pressure setpoint, by changing the speed of the condenser pump.

[0021] Further advantages and embodiments are described in more detail with reference to the attached figures. Fig. 1: a schematic representation of a heat pump system according to the invention; Fig. 2: a detailed illustration of a flow diagram of a heat pump system in a building; Fig. 3: a schematic representation of a method according to the invention according to a preferred embodiment; and Fig. 4: a diagram showing the course of various process variables as a function of the detected pressure and the subsequently adjusted speed of the condenser pump.

[0022] Fig. 1 shows a schematic representation of a heat pump system 10, which has a heat pump 12, a condenser pump 14, and a controller 16. In one possible embodiment, the controller 16 of the heat pump system 10 is also the controller for the heat pump 12.

[0023] The heat pump 12 comprises a refrigeration circuit 20 with a compressor 22, a condenser 24, an expansion valve 26 and an evaporator 28. The refrigeration circuit 20 contains a refrigerant (not shown in detail), such as propane, which is raised from a low energy level to a higher energy level by means of the compressor 22 in order to transfer the heat energy contained therein.

[0024] The heat pump 12 is connected with its refrigeration circuit 20 to a hot water tank 32 via a closed secondary circuit 30. Within the secondary circuit 30 is a heat transfer medium for transferring the heat energy generated by the heat pump 12 from the condenser 24 to the hot water tank 32. Fig. 1 The heat pump 12 shown is an air-to-water heat pump. A brine heat pump can, of course, also be used instead of the air-to-water heat pump.

[0025] If a heat energy requirement is detected within the hot water tank 32, which can also be done, for example, using the controller 16 of the heat pump system, the refrigeration circuit 20 is controlled by the controller 16 and the required heat energy is generated. With the operation of the heat pump 12 and the simultaneous operation of the condenser pump 14 in the secondary circuit 30, the heat energy generated by the refrigeration circuit 20 is transported via the secondary circuit 30 into the hot water tank 32.

[0026] In order to ensure efficient heating of the tap water, the pressure p acting in the refrigerant is detected on the high-pressure side in the refrigeration circuit 20 by means of a pressure sensor 34, in the present embodiment upstream of the condenser 24, and the condenser pump 14 in the secondary circuit 30 is operated at a variable speed depending on the pressure p detected on the pressure side in the refrigeration circuit 20.

[0027] In the refrigeration circuit 20, a temperature sensor 50 is arranged downstream of the condenser 24, which detects the temperature of the refrigerant after exiting the condenser 24. In addition, temperature sensors 52, 54 are provided in the secondary circuit 30, by means of which the temperature of the heat transfer medium in the secondary circuit 30 is measured before entering and after exiting the condenser 24.

[0028] Fig. 2 shows a flow diagram of the heat flows to be distributed within a building 80 by means of the heat pump system 10 to various consumers therein. In addition to the hot water tank 32, the heat pump 12 of the heat pump system 10 can also supply heat to a heating system 82 of the building 80, which is connected to the secondary circuit 30 in parallel with the hot water tank 32.

[0029] The controller 16 of the heat pump 12 is connected via a signal line 36 to the condenser pump 14 arranged within the building 80. The condenser pump 14 can be part of a fluid unit 38 arranged in the building, which in one possible embodiment is also a component of the heat pump system 10. To detect the heat energy demand in the hot water tank 32, a temperature sensor 40 is arranged in the hot water tank 32. The temperature sensor 40 transmits signals to the fluid unit 38 via a signal line 42 and from there to the controller 16 of the heat pump system 10 via the signal line 36.

[0030] The fluid unit 38 comprises a distribution unit 44, in particular a distribution valve, by means of which the heat transfer medium is distributed via lines 46, 48 of the secondary circuit 30 to the component to be supplied with heat energy (hot water tank 32, heating system 82).

[0031] Fig. 3shows a block diagram of a preferred embodiment of a method 100 according to the invention for operating a heat pump 12, as shown for example in Fig. 1 shown and described in more detail above.

[0032] The method 100 comprises a first method step 102 relating to the detection of a heat energy requirement in the hot water tank 32. In a preferred embodiment, the temperature in the hot water tank 32 is detected by means of a temperature sensor 40 in order to detect the heat energy requirement.

[0033] In a next step 104, the refrigeration circuit 20 is controlled by the controller 16 of the heat pump system 10 to cover the heat energy demand and the required heat energy is generated by means of the refrigeration circuit 20, as described above. Fig. 1 described.

[0034] In a subsequent step 106, the thermal energy is transferred from the refrigeration circuit 20 to the secondary circuit 30. In particular, the thermal energy contained in the refrigerant is transferred in the condenser 24 to the heat transfer medium of the secondary circuit 30, which also flows through the condenser 24. The heat transfer medium is pumped by means of a condenser pump 14, which is arranged in the secondary circuit 30.

[0035] In the next method step, the detection 108 of a pressure p in the refrigerant on the high-pressure side in the refrigeration circuit 30 takes place by means of a pressure sensor 34. According to one embodiment, the pressure sensor 34 is arranged on the high-pressure side immediately behind or at the outlet of the condenser 24.

[0036] In a further step 110, the condenser pump 14 in the secondary circuit 30 is operated at a variable speed depending on the pressure p detected on the high-pressure side in the refrigeration circuit 30. In one embodiment, the pressure p is detected in step 108 in the form of a pressure change 108' on the high-pressure side in the refrigeration circuit 30 by means of the pressure sensor 34, whereupon the speed of the condenser pump 14 is adjusted, in particular increased or decreased, depending on the detected pressure changes.

[0037] In one possible embodiment of the method 100, in an intermediate step 112, the controller 16 of the heat pump system 10 specifies a pressure setpoint for the pressure p to be detected on the high-pressure side in the refrigeration circuit 20. The operation 110 of the condenser pump 14 is carried out in particular by adjusting 110' the pressure p detected upstream or downstream of the condenser 24 to the specified pressure setpoint by changing the speed of the condenser pump 14.

[0038] In a preferred embodiment, if the detected pressure p (detected pressure change) exceeds the pressure setpoint or deviates upwards therefrom, the speed of the condenser pump 14 is increased and if the detected pressure p (detected pressure change) falls below the pressure setpoint or deviates downwards therefrom, the speed of the condenser pump 14 is reduced.

[0039] In Fig. 4is a diagram showing the control of certain process parameters that change during operation of the heat pump system. Time is shown on the horizontal axis and a unitless numerical scale is plotted on the vertical axis. The uppermost characteristic curve A plots the frequency of an inverter that controls, for example, the compressor 22 of the refrigeration circuit 20. At the start of operation of the refrigeration circuit 20 at time t 0 , the compressor is driven at a frequency of approximately 50 Hz. The frequency of the inverter, not described in more detail, increases during a start-up phase of the heat pump system 10 to approximately 110 to 115 Hz up to time t 1 . Over the remaining time period, the inverter is then controlled at a virtually constant frequency.

[0040] Characteristic curve B represents the recorded pressure p on the high-pressure side in the refrigeration circuit 20. When the refrigeration circuit 20 is activated at time t 0 , a pressure p of approximately 52 bar is present on the high-pressure side. This pressure initially drops and initially fluctuates around a value of approximately 58 bar. Characteristic curve C represents the speed of the condenser pump 14 in the secondary circuit 30. When the heat pump system starts operating, the speed of the condenser pump 14 (characteristic curve C) starts at approximately 50 revolutions per minute. As can be seen from the diagram, the speed (characteristic curve C) is adjusted to the changing pressure p (characteristic curve B) on the high-pressure side of the refrigeration circuit 30 until a virtually constant pressure p is established in the refrigerant at approximately time t 2 . When the pressure is set at an approximately constant 57 bar, the speed of the condenser pump remains regulated at approximately 30 revolutions per minute.

[0041] The characteristic curve D, also shown in the diagram, indicates the temperature of the refrigerant on the high-pressure side after exiting the condenser 24, which is approximately 44 °C. The characteristic curve E represents the temperature of the heat transfer medium in the secondary circuit 30 as the heat transfer medium exits the condenser 24 of the heat pump 12. The heat transfer medium is directed towards the hot water tank 32 at a temperature of approximately 70 °C. The characteristic curve F indicates the temperature of the heat transfer medium in the secondary circuit 30 before entering the condenser 24, which is approximately 24 °C. List of reference symbols

[0042] 10Heat pump system 12Heat pump 14Condenser pump 16Control 20Refrigerant circuit 22Compressor 24Condenser 26Expansion valve 28Evaporator 30Secondary circuit 32Hot water tank 34Pressure sensor 36Signal line 38Fluid unit 40Temperature sensor 42Signal line 44Distribution unit 46, 48Lines 50Refrigerant temperature sensor 52Heat transfer medium temperature sensor 54Heat transfer medium temperature sensor 80Building 82Heating system 100Procedure 102Step: Determine heat energy demand 104Step: Generate 106Step: Transfer 108Step: Determine pressure 110Step: Operate Condenser pump pPressure Refrigeration circuit t 1 , t 2 , t 3 Time ACharacteristic curve Frequency of the inverter BCharacteristic curve of pressure p high-pressure side of refrigeration circuit CCharacteristic curve of condenser pump speed DCharacteristic curve of heat transfer medium temperature after condenser outlet ECharacteristic curve of refrigerant temperature before condenser inlet FCharacteristic curve of refrigerant temperature after condenser outlet

Claims

1. A method (100) for operating a heat pump (12) for heating tap water in a hot water tank (32), wherein the heat pump (12) has a refrigeration circuit (20) with a compressor (22), a condenser (24), an expansion valve (26), and an evaporator (28), and a controller (16) for controlling the refrigeration circuit (20), wherein the condenser (24) is connected to the hot water tank (32) in a heat-transferring manner by means of a secondary circuit (30), wherein a condenser pump (14) for the controlled circulation of a heat transfer medium of the secondary circuit (30) is arranged within the secondary circuit (30), comprising the steps of: - detecting (102) a heat energy requirement in the hot water tank (32), - controlling (104) the refrigeration circuit (20) and generating heat energy by means of the refrigeration circuit (20), - transferring (106) the heat energy from the refrigeration circuit (20) to the secondary circuit (30),- detecting (108) a pressure (p) on the high-pressure side in the refrigeration circuit (20), and - operating (110) the condenser pump (14) in the secondary circuit (30) at a variable speed as a function of the pressure (p) detected on the high-pressure side in the refrigeration circuit (20), wherein the pressure (p) on the high-pressure side is adjusted to a pressure setpoint by changing the speed of the condenser pump (14).

2. The method according to claim 1, wherein the pressure (p) on the high-pressure side is detected by means of a pressure sensor or estimated on the basis of detected temperature measurements.

3. The method according to claim 2, further comprising the step of: - specifying the pressure setpoint (112) for the pressure (p) to be detected in the refrigeration circuit (20) on the high-pressure side, and increasing a speed of the condenser pump (14) when the detected pressure (p) exceeds the pressure setpoint or reducing the speed of the condenser pump (14) when the detected pressure (p) falls below the pressure setpoint.

4. Method according to one of claims 1 to 3, further comprising the step: - detecting a pressure change (108') on the high-pressure side in the refrigeration circuit (20), in particular by means of the pressure sensor (34), and increasing or decreasing the speed of the condenser pump (14) as a function of the detected pressure change.

5. Method according to one of the preceding claims, wherein the pressure setpoint is limited downwards by a minimum pressure defined by a subcooling condition of the refrigerant used in the refrigeration circuit (20).

6. The method according to claim 5, wherein the minimum pressure is determined as a function of a reference temperature of the refrigerant at the outlet of the condenser (24).

7. The method according to claim 6, wherein the minimum pressure is above a reference pressure associated with the reference temperature, the reference temperature being below a bubble point temperature of the refrigerant in the condenser (24) by a predetermined temperature value.

8. Method according to one of the preceding claims, wherein the pressure setpoint is limited upwards by a condenser pressure defined by the manufacturer as the maximum permissible for the compressor (22).

9. The method according to claim 5 and 8, further comprising the step of: specifying the pressure setpoint in a range between the minimum pressure and the maximum permissible condenser pressure as a function of the electricity currently available for the operation of the heat pump (12) or the electricity price.

10. The method according to claim 5 and 8, further comprising the step of: specifying the pressure setpoint in a range between the minimum pressure and the maximum permissible condenser pressure depending on a preference that can be set by a user, in particular a minimum energy consumption to be achieved at the heat pump (12) or a comfort that can be best achieved with the heat pump (12).

11. Method according to one of the preceding claims, wherein the pressure (p) on the high pressure side is preferably detected behind or at the outlet of the condenser (24).

12. Method according to one of the preceding claims, wherein the temperature in the hot water tank (32) is measured to detect the heat energy requirement in the hot water tank (32), and the refrigeration circuit (20) is operated when the temperature in the hot water tank (32) falls below a predetermined threshold value.

13. Heat pump system (10) with - a heat pump (12), wherein the heat pump (12) has a refrigeration circuit (20) with a compressor (22), a condenser (24), an expansion valve (26) and an evaporator (28), - a condenser pump (12) which is arranged within a secondary circuit (30) which connects the refrigeration circuit (20) of the heat pump (12) to a hot water tank (32) in a heat-transferring manner, and - a controller (16) which is designed at least to control the refrigeration circuit (20), characterized in that the controller (16) is configured to carry out a method for operating a heat pump (12) for heating tap water in a hot water tank (32) according to one of the features of claims 1 to 12.

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