A method for controlling the high side pressure of a transcritical vapor compression heat pump system, and a heat pump using the method

Abstract

A method for controlling the high side pressure of a trans critical vapor compression heat pump system is disclosed, where the method comprises the steps of calculating a first pinch temperature difference at a pinch point location of the heat rejecting heat exchanger and of adjusting the high side pressure in dependence of a predetermined second pinch temperature difference and / or the pinch point location. The method allows operation of a trans critical heat pump with the lowest possible high side pressure, while satisfying the demand of heating power at all time.

Description

Field of the invention

[0001] The invention relates to the field of vapor compression systems, especially transcritical heat pump systems, where carbon dioxide (CO 2 ) is used as refrigerant.

[0002] This natural refrigerant is environmentally very attractive due to the absence of ozon depletion potential and a very low global warming potential.

[0003] Such heat pump systems are widely used in space- and hot water heating applications, as they are capable of delivering the inlet water supply temperatures required.Background of the invention

[0004] While low temperature heating systems, which can be achieved with traditional heat pumps, are applicable in newer buildings, there is in older buildings a need for high temperature systems, which can be achieved by CO 2 heat pumps.

[0005] To provide the required supply temperature, especially in colder climates with high heat demand, the heat pump system has to be operated with relatively high pressure values on the high pressure side of the transcritical CO 2 refrigerant cycle.

[0006] In known transcritical CO 2 vapor compression systems for refrigeration application, the high pressure side is optimized for obtaining a maximum cooling COP (Coefficient of performance).

[0007] When used as a heat pump, the high side pressure of such a vapor compression system will have to be different from the high side pressure in a cooling cycle. It is known, to operate such heat pump systems with a constant high side pressure for always ensuring sufficient supply temperatures levels of the heat pump system. Hence, the system will be operated with an excessive high side pressure in situations of lower heating power demand. This leads to increased power consumption of the compressor.Summary of the invention

[0008] It is an object of the present invention to provide an improved method for controlling the high side pressure of a transcritical heat pump system optimized for high heating COP, by finding and subsequently operating the heat pump with the lowest possible high side pressure, while satisfying the demand of heating power delivered by the heat rejecting heat exchanger at all time.

[0009] It is also an object of the invention to provide a transcritical CO 2 heat pump system using such optimized high pressure control method.

[0010] According to a first aspect the present invention provides a method for controlling the high side pressure of a transcritical vapor compression heat pump system, comprising the steps of defining a set point temperature (TfSP) for the outlet of a cooling fluid passing through a heat rejecting heat exchanger of the transcritical vapor compression heat pump system, determining a pressure (Pgc) in the heat rejecting heat exchanger of the vapor compression system, determining an inlet temperature (Th) and an outlet temperature (Tgc) of a refrigerant gas passing through the heat rejecting heat exchanger, determining an inlet temperature (Tr) and an outlet temperature (Tf) of the cooling fluid passing through the heat rejecting heat exchanger in a counterflow direction in relation to the refrigerant gas, calculating a first pinch temperature difference between the refrigerant gas and the cooling fluid at a pinch point location of the heat rejecting heat exchanger, adjusting the high side pressure in dependence of a predetermined second pinch temperature difference and / or the pinch point location.

[0011] The set point temperature (TfSP) may be chosen to satisfy the required supply temperature of heating water in a space heating or hot water system.

[0012] The high side pressure of the transcritical heat pump system is defined as the pressure (Pgc) of the refrigerant gas in the heat rejecting heat exchanger.

[0013] Determining pressure and temperatures may be performed by measuring these fluid properties with suitable sensors arrangements. Sensor signals may be received and processed in a system controller, which further performs the calculating steps of the inventive method.

[0014] In this aspect, the step of calculating a first pinch temperature difference between the refrigerant gas and the cooling fluid at the pinch point of the heat rejecting heat exchanger comprises the steps of: determining enthalpy values for the refrigerant gas corresponding to the inlet temperature and the outlet temperature of the refrigerant gas, defining a plurality of locations along the length of the heat exchanger, determining the temperatures of the refrigerant gas and the cooling fluid at the plurality of locations, calculating a temperature difference between the refrigerant gas and the cooling fluid at the plurality of locations, defining the location with the minimum temperature difference as the pinch point and the calculated minimum temperature difference as the first pinch temperature difference.

[0015] Determining the enthalpy values for the refrigerant at the given high side pressure may be performed by calculating or looking up in tables available in the system controller.

[0016] Defining the plurality of locations may be performed by choosing a number of positions along along at least a part the flow path of the refrigerant gas within the heat exchanger between inlet and outlet of the refrigerant gas. The locations may be equidistantly distributed along the flow path, or have different distance to each other.

[0017] Determining the temperatures of the refrigerant gas and the cooling fluid at the plurality of locations may be performed by calculating in the controller based on the known thermodynamic properties of the fluids. Alternatively, temperature sensors may be arranged at these locations, and sending measuring signals to the system controller.

[0018] In this aspect, the high side pressure is increased, when the first pinch temperature difference is equal to or lower than the predetermined second pinch temperature difference. This leads to an increase of heat transfer within the heat exchanger and an increase of the outlet temperature of the cooling fluid, necessary to move towards the set point temperature.

[0019] In this aspect, the high side pressure is decreased, when the first pinch temperature difference is higher than the predetermined second pinch temperature difference.

[0020] Reducing the high side pressure leads to reduced energy consumption and hereby optimized heating COP.

[0021] Preferably, the high side pressure is decreased, when the pinch point location is at a distance d < 0.1 x L from the outlet of the refrigerant gas, and advantageously at a distance d < 0.05 x L from the outlet of the refrigerant gas, where L is the total length of the heat rejecting heat exchanger.

[0022] Preferably, the step of adjusting the high side pressure is performed by changing the refrigerant flow through a high pressure valve of the vapor compression heat pump system or by changing the compressor capacity. The system controller is connected with actuators or controls of the high pressure pressure valve and / or the compressor to initiate changes of the refrigerant flow through the heat exchanger.

[0023] Preferably, the predetermined second pinch temperature difference is 2 to 6K. A pinch difference within this range results in good heat transfer between the refrigerant gas and the cooling fluid.

[0024] Preferably, the refrigerant circulating in the vapor compression heat pump system is carbon dioxide.

[0025] Preferably, the cooling fluid is heating water circulating in a space- or hot water heating system.

[0026] According to a second aspect the present invention provides a heat pump system comprising an evaporating heat exchanger, a compressor, a gas cooler, a high pressure valve and a receiver coupled to form a transcritical carbon dioxide refrigerant cycle, where the system is using the method according to the first aspect of the present invention.

[0027] Preferably, the system further comprises an expansion valve connected between the receiver and the evaporating heat exchanger and a receiver valve connected between the receiver and a suction line.

[0028] Preferably, the heat rejecting heat exchanger is one of a plate heat exchanger or a micro channel heat exchanger.Brief description of the drawings

[0029] Fig. 1 discloses a heat pump system according to the invention, Fig. 2 discloses a second embodiment of a heat pump system according to the invention, Fig. 3 shows a log P - h diagram including the transcritical heat pump cycles of the systems of Fig. 1 and Fig.2, Fig. 4 shows the temperature courses for refrigerant gas and cooling media flowing through the heat rejecting heat exchanger of the heat pump systems of Fig. 1 and Fig. 2, Fig. 5 shows a flow chart of the method according to the invention. Detailed description of the drawings

[0030] Fig. 1 discloses a transcritical heat pump system with a circuit comprising an evaporating heat exchanger, also known as evaporator, a compressor, a gas cooler, a high pressure valve, a receiver and an expansion valve (Vexp) fluidly connected in series. A further expansion device (Vrec) is arranged between the receiver and the suction line of the circuit.

[0031] In the evaporator, thermal energy absorbed from an external media evaporates the refrigerant passing through the heat exchanger. The external media delivering the thermal energy may typically be a liquid, e.g. brine or water, or air surrounding the evaporator. The refrigerant used in the transcritical cycle is preferably carbon dioxide (CO 2 ).

[0032] Gaseous refrigerant coming from the evaporator outlet is flowing through the suction line to a refrigerant compressor. Here, the gaseous refrigerant is compressed and heated, before it is discharged into a gas cooler. At the refrigerant inlet of the gas cooler the refrigerant gas has a temperature Th and a high side pressure Pgc. The gas cooler is working as the heat rejecting heat exchanger of the system by exchanging thermal energy between the hot refrigerant gas and a cooling fluid also passing through the gas cooler, preferably in a counterflow direction in relation to the refrigerant gas. The cooling fluid is preferably a liquid, e.g. water used in space- or hot water heating systems, entering the gas cooler at a cooling liquid inlet with a temperature Tr and leaving the gas cooler at a cooling liquid outlet with a temperature Tf.

[0033] The refrigerant gas is leaving the gas cooler at a refrigerant outlet with a reduced temperature Tgc in comparison to inlet temperature Th. After passing through a high pressure valve, the refrigerant enters a receiver, containing refrigerant in both liquid and gaseous phase. Liquid refrigerant from the liquid volume of the receiver leaves the receiver at a bottom outlet and expands to a low side pressure Plow when passing through expansion valve Vexp and reaches again the inlet of the evaporator.

[0034] The receiver valve Vrec may deliver gaseous refrigerant at an intermediate pressure, which is between the low side pressure Plow and the high side pressure Pgc, from the receiver to the suction line.

[0035] Also shown is a controller connected to the high pressure valve, receiver valve, expansion valve and the compressor and adapted to perform the steps of the inventive method, e.g. to adjust the refrigerant flow through the high pressure valve or the capacity of the compressor. The controller is further adapted to receive signals from pressure and / or temperature sensors (not shown) arranged to measure fluid properties at the inlets and outlets of the gas cooler.

[0036] Fig. 2 shows an alternative heat pump system with selfcirculating evaporator, comprising similar components than the system of Fig. 1. However, expansion valve and receiver valve are omitted and the suction line passes through the gaseous volume of the receiver. Expansion from high side pressure Pgc to low side pressure Plow occurs when passing the high side pressure valve. The refrigerant in the receiver is at low side pressure Plow. Hereby, a less complex and cheaper system is achieved due to reduced number of components and pipings.

[0037] Fig. 3 discloses the transcritical heat pump cycle for both heat pump systems shown in Figures 1 and 2 within a simplified logarithmic pressure vs. specific enthalpy diagram for the applied refrigerant.

[0038] The cycle starts at point (1) in the suction line, before the refrigerant is sucked into the compressor. Point (2) represents the state after compression and before entrance into the gas cooler. At this point, the refrigerant is at high side pressure Ph / Pgc and at temperature Th, as indicated by its isothermal line.

[0039] After heat transfer within the gas cooler, point (3) represents the refrigerant state at the gas cooler outlet with temperature Tgc, again indicated by the corresponding isothermal line. As can be seen, the entire cooling process between point (2) and point (3) is well above the critical point, where bubble line and dew line meet each other.

[0040] In the cycle according to Fig. 2, the refrigerant is expanded by passing the high pressure valve and reaches directly the low side pressure Plow at point (4) below the bubble line. This point represents the state of the refrigerant at the entrance of the evaporator. The evaporation of liquid refrigerant by heat absorption within the evaporator leads back to point (1).

[0041] In the cycle according to Fig. 1, points (3A) and (3B) represent the refrigerant state at the intermediate pressure in the liquid phase and the gaseous phase of the receiver, respectively. Point (4) is only reached after passage of liquid refrigerant from the receiver towards the evaporator through expansion valve Vexp.

[0042] All cycle points are also shown in the Figures 1 and 2.

[0043] Fig. 4 discloses the temperature courses for a CO 2 refrigerant gas and a liquid cooling fluid along the length of the gas cooler of the heat pump systems of Fig. 1 and Fig. 2 to illustrate the inventive control method for the high side pressure.

[0044] The extension of the gas cooler is displayed on the x-axis by ratios d / L representing locations along the length of the gas cooler, where d is the distance from the outlet of the refrigerant gas and L is the total length of the gas cooler. Due to the counterflow of refrigerant gas and cooling liquid, gas outlet temperature Tgc and liquid inlet temperature Tr are given at d=0. Accordingly, gas inlet temperature Th and liquid outlet temperature Tf are given at d=1. At the latter location, also set point temperature TfSP for the cooling liquid is shown.

[0045] The temperature course for the cooling liquid between Tr and Tf is a straight line, whereas the temperature course for the refrigerant gas is a curved line due to the thermodynamical properties of CO 2 in the transcritical state.

[0046] Both lines are found by defining a plurality of locations along the the length of the gas cooler. The number N of locations may be 10 to 50.

[0047] For each location d / L, the temperatures of CO 2 and cooling liquid can be calculated for a given high side pressure based on the known inlet and outlet temperatures and the thermodynamic properties of both fluids. Hence, for each location d / L, a temperature difference between the CO 2 gas and the cooling liquid can be calculated. The found minimum temperature difference is the first pinch temperature difference, and the corresponding location is the pinch point.

[0048] In Fig. 4, two curved lines for CO 2 gas are shown. The full line is calculated for a high side pressure P1. The found first pinch temperature difference deltaT(P1) at the pinch point (P1) is lower than the predetermined second pinch temperature difference.

[0049] According to the present invention, the system controller will increase the high side pressure to a pressure P2 > P1, which results in a new CO 2 line, illustrated as dotted line. This line calculated for high side pressure P2 has a less curved shape, due to the thermodynamical properties of CO 2 , and results in a new pinch point (P2) and a new first pinch temperature difference deltaT(P2). The increase of the high side pressure hereby leads to a moving of the pinch point towards the gas outlet of the gas cooler and to an increase of the pinch temperature difference. For clarity reasons, a corresponding new line at P2 for the cooling liquid, which would have a slight increase in its slope, is not shown.

[0050] Fig. 5 shows a flow diagram for the inventive control method.

[0051] In a first step, a setpoint for the temperature TfSP of the cooling fluid outlet of the heat rejecting heat exchanger, e.g. of a gas cooler in a CO 2 system, is defined. Temperature TfSP might equal the required supply temperature of heating water in a heating system.

[0052] In a second step, values for inlet temperature Th and outlet temperature Tgc of the CO 2 refrigerant gas, inlet temperature Tr and outlet temperature Tf of the cooling fluid and the pressure Pgc in the gas cooler are determined., e.g. by receiving measuring signals from corresponding sensors arranged at suitable positions in the heat pump system.

[0053] In a third step, the specific enthalpy of the refrigerant gas at the inlet and the outlet are determined based on the measured pressure and temperatures, as well as the known thermodynamic properties of the refrigerant gas.

[0054] In a forth step, a number of locations are defined along the length of the heat exchanger.

[0055] In a fifth step, for each location, temperatures for both refrigerant gas and cooling fluid are calculated, based on the known inlet and outlet temperatures and the thermodynamic properties of both fluids.

[0056] In a sixth step, for each location, a temperature difference between the refrigerant gas and the cooling fluid is calculated. The location with the minimum temperature difference is the pinch point and the found minimum temperature difference at the pinch point is defined as at the first pinch temperature difference.

[0057] In an seventh step, the location of the pinch point is checked. In case that the pinch point is closer to the outlet of the refrigerant gas than 10% of the total length of the gas cooler, the high side pressure will be decreased, as a tenth step.

[0058] In a eighth step, the calculated first pinch temperature difference is compared with the predetermined second pinch temperature difference. If the first pinch temperature difference exceeds the predetermined second pinch temperature difference, the high side pressure will be decreased, as a ninth step.

[0059] If the first pinch temperature difference equals or is less than the predetermined second pinch temperature difference, the high side pressure will be increased, as an eleventh step.

[0060] In a twelfth step, the control system waits for a predetermined amount of time to allow temperature and pressure changes in the heat pump system, before a new sequence is started.

Claims

1. A method for controlling the high side pressure of a transcritical vapor compression heat pump system, comprising the steps of - defining a set point temperature (TfSP) for the outlet of a cooling fluid passing through the heat rejecting heat exchanger, - determining a pressure (Pgc) in the heat rejecting heat exchanger of the vapor compression system, - determining an inlet temperature (Th) and an outlet temperature (Tgc) of a refrigerant gas passing through the heat rejecting heat exchanger, - determining an inlet temperature (Tr) and an outlet temperature (Tf) of the cooling fluid passing through the heat rejecting heat exchanger in a counterflow direction in relation to the refrigerant gas, - calculating a first pinch temperature difference between the refrigerant gas and the cooling fluid at a pinch point location of the heat rejecting heat exchanger, - adjusting the high side pressure in dependence of a predetermined second pinch temperature difference and / or the pinch point location.

2. The method according to claim 1, where the step of calculating a first pinch temperature difference between the refrigerant gas and the cooling fluid at the pinch point of the heat rejecting heat exchanger comprises the steps of: - determining enthalpy values for the refrigerant gas corresponding to the inlet temperature and the outlet temperature of the refrigerant gas, - defining a plurality of locations along the length of the heat exchanger, - determining the temperatures of the refrigerant gas and the cooling fluid at the plurality of locations, - calculating a temperature difference between the refrigerant gas and the cooling fluid at the plurality of locations, - defining the location with the minimum temperature difference as the pinch point and the calculated minimum temperature difference as the first pinch temperature difference.

3. The method according to claim 1 or 2, where the high side pressure is increased, when the first pinch temperature difference is equal to or lower than the predetermined second pinch temperature difference.

4. The method according to claim 1 or 2, where the high side pressure is decreased, when the first pinch temperature difference is higher than the predetermined second pinch temperature difference.

5. The method according to claim 1 or 2, where the high side pressure is decreased, when the pinch point location is at a distance d < 0.1x L from the outlet of the refrigerant gas, where L is the total length of the heat rejecting heat exchanger.

6. The method according to any of claims 1 to 5, where the step of adjusting the high side pressure is performed by changing the refrigerant flow through a high pressure valve of the vapor compression heat pump system or by changing the compressor capacity.

7. The method according to any of claims 1 to 6, where the predetermined second pinch temperature difference is 2 to 6K.

8. The method according to any of claims 1 to 7, where the refrigerant circulating in the vapor compression heat pump system is carbon dioxide.

9. The method according to any of claims 1 to 8, where the cooling fluid is heating water circulating in a space- or hot water heating system.

10. A heat pump system comprising an evaporating heat exchanger, a compressor, a gas cooler, a high pressure valve and a receiver coupled to form a transcritical carbon dioxide refrigerant cycle, where the system is using the method according to any of claims 1 to 9.

11. A heat pump system according to claim 10, where the system further comprises an expansion valve connected between the receiver and the evaporating heat exchanger and a receiver valve connected between the receiver and a suction line.

12. A heat pump system according to claim 10 or 11, where the heat rejecting heat exchanger is one of a plate heat exchanger or a micro channel heat exchanger.

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