Transcritical Heat Pump System

Abstract

A transcritical heating system for providing heat to a heating medium includes a controller and a heat pump. The heat pump includes a working fluid characterized by a critical pressure and a critical temperature, a compressor configured to compress the working fluid above its critical pressure and temperature, a first heat exchanger (HX) in fluid communication with the compressor and with a closed medium flow circuit, a high-pressure valve, and an evaporator for cooling the working fluid. The controller is in communication with the heat pump in order to regulate a temperature of the heating medium in the closed medium flow circuit. The system may also include a second heat exchanger in fluid communication with the first HX and with an open medium flow circuit.

Description

TECHNICAL FIELD

[0001] The present invention relates to central heating systems using heat pumps and specifically to a transcritical heat pump system.BACKGROUND OF THE INVENTION

[0002] Two working fluids that have been used extensively in heat pumps for central hot water heating systems are freon (R22) and puron (R410A). R22 contains a hydrochlorofluorocarbon (HCFC) that has been found to deplete the Earth's ozone layer, and its manufacture has been banned since 2020 in all countries belonging to the United Nations. R410A is in the process of being phased out starting in the year 2025 in the U.S. and the European Union, because of its high global warming potential (GWP). Whereas R410 has a GWP greater than two thousand, many other working fluids such as ammonia, ethane, propane, butane, and carbon dioxide (CO2) have GWP values less than five.

[0003] In particular, CO2 has a GWP equal to one, and a critical point at a temperature of 30.98° C. and a pressure of 73.8 bar. Above the critical point, CO2 is a supercritical fluid, which is neither a true liquid nor a true gas. Furthermore, based on thermodynamic heat-cycle considerations, a transcritical CO2 heat pump can supply hot water at 90° C., whereas conventional hot-water heat pumps are frequently limited to 60° C.

[0004] Chinese Patent number CN101576283B to J. Lu et al., dated Feb. 15, 2012, and entitled “Water Heater with Heating-Supply from Transcritical Carbon Dioxide Heat-Pump” (hereinafter referred to as “Lu”) teaches a water heater with heating-supply from a transcritical carbon dioxide heat-pump comprising a transcritical carbon dioxide heat-pump system, a hot-water supply system and a heating circulation system. The transcritical carbon dioxide heat-pump system comprises a compressor, a first gas cooler, a second gas cooler, a heat regenerator, an expansion valve, an evaporator and the other heat regenerator which are sequentially positioned on a first circulation pipeline; the hot-water supply system comprises a water tank, a first pump and a hot water outlet valve, wherein the water tank comprises a high-temperature zone A, an intermediate-temperature zone B and a low-temperature zone C; and the heating circulation system comprises a second gas cooler, a second pump and end equipments which are positioned on a second circulation pipeline. The invention reduces the temperature of dioxide carbon at the inlet of the expansion valve and respectively raises the superheat degree of dioxide carbon at the inlet of the compressor and EER of heat-pump circulation so as to ensure the lower temperature of water entering the gas cooler from the water tank; the reduction of the temperature of the first gas cooler enhances the efficiency of the heat-pump system; and both high-temperature water and intermediate-temperature water can be provided according to requirement of users.SUMMARY OF THE INVENTION

[0005] For the purposes of this disclosure, the term “coefficient of performance (COP)” of a heat pump with a compressor is defined as the ratio of the output heating power to the input power supplied to the compressor. The term “working fluid” refers to a refrigerant or coolant fluid used in a compressor. R744 is an example of a CO2-based refrigerant, which typically is blended with an oil lubricant. The term “heating medium”, or simply “medium”, denotes a substance undergoing heating by the heat pump, and is for example liquid water. The heating medium typically circulates in a closed medium flow circuit and / or in an open medium flow circuit.

[0006] According to one aspect of the presently disclosed subject matter, there is provided a transcritical heating system for providing heat to a heating medium, the system including a controller and a heat pump. The heat pump includes a working fluid characterized by a critical pressure and temperature, a compressor configured to compress the working fluid above its critical pressure and temperature, a first heat exchanger in fluid communication with the compressor and with a closed medium flow circuit, a high-pressure valve, and an evaporator for cooling the working fluid. The controller is in communication with the heat pump in order to regulate a temperature of the heating medium in the closed medium flow circuit.

[0007] According to some aspects, the working fluid comprises carbon dioxide (CO2).

[0008] According to some aspects, the heating medium is water.

[0009] According to some aspects, a difference in temperature between the medium entering and exiting the closed medium flow circuit is in a range extending from 1.5° C. to 40° C.

[0010] According to some aspects, the controller is a variable speed drive (VSD) controller.

[0011] According to some aspects, the compressor is a semi-hermetic reciprocating compressor.

[0012] According to some aspects, a mass flow rate of the compressor is controlled by the controller.

[0013] According to some aspects, the first heat exchanger is a plate heat exchanger.

[0014] According to some aspects, the working fluid in the first heat exchanger is a gas.

[0015] According to some aspects, the system further includes a second heat exchanger in fluid communication with the first heat exchanger and with an open medium flow circuit.

[0016] According to some aspects, the working fluid in the second heat exchanger is a mixture of a gas and a liquid.

[0017] According to some aspects, the evaporator is a variable speed fan type evaporator.

[0018] According to some aspects, a setpoint temperature at an outlet of the evaporator is adjusted to be less than an ambient external temperature by a predetermined temperature difference.

[0019] According to some aspects, the system further includes an electrical expansion valve.

[0020] According to some aspects, the system further includes a defroster.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Some embodiments of the present invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced.

[0022] FIG. 1: A schematic diagram of a prior art transcritical CO2 heat pump system.

[0023] FIG. 2: A schematic diagram of an exemplary transcritical CO2 heat pump system, according to the invention.

[0024] FIG. 3: A process-instrumentation (PI) design diagram of an exemplary transcritical CO2 heat pump system, according to the invention.

[0025] FIG. 4: A temperature-enthalpy (t,h) diagram corresponding to FIG. 3.

[0026] FIG. 5: A log pressure-enthalpy (log p,h) diagram corresponding to FIG. 3.

[0027] FIG. 6: A perspective image of a portion of a transcritical CO2 heat pump, according to the invention.DETAILED DESCRIPTION OF THE INVENTION

[0028] FIG. 1 shows a schematic diagram of a prior art transcritical CO2 heat pump system, which appears in the abovementioned patent by Lu et al. The dashed box shows the outdoor unit portion of the system, containing a compressor 1, a regenerator 3, an expansion valve 4, an evaporator 5, water pumps 7 and 11, and air coolers 21 and 22. The inside portion of the system contains a storage tank 6, a drainpipe 8, a water inlet pipe 9, a hot water outlet valve 10, and a floor heating pipeline 12. Compressor 1 compresses the CO2 into high-pressure and high-temperature gas. After passing through an air cooler and a regenerator 3, the CO2 is changed to a low-temperature and high-pressure gas. In the expansion valve 4, the CO2 expands to become a low-pressure low-temperature liquid. Heat from the outdoor air is absorbed and evaporated in the evaporator 5. The heat is further absorbed in the regenerator 3, and finally returns to compressor 1 to complete the cycle.

[0029] The operation of this prior art transcritical CO2 heat pump system depends critically on the expansion valve 4, whose primary purpose is CO2 subcooling. In contrast, the present invention achieves subcooling by means of fan speed control, and therefore does not require an expansion valve in series with the evaporator. This aspect of the invention is explained in detail below.

[0030] FIG. 2 shows a schematic of a transcritical CO2 heat pump system 100, according to the invention. The system includes a heat pump 110 which provides a heating medium, such as hot water, to a closed medium flow circuit 210 and to an open medium flow circuit 310. The components of heat pump 110 and flow circuits 210 and 310 are under the control of a variable speed drive (VSD) controller 380. The reference symbols for FIG. 2 are explained in Table 1 below.TABLE 1Reference Symbols for Transcritical CO2 Heat Pump System110Transcritical CO2 heat pump120Compressor141First heat exchanger (HX)142Second heat exchanger (HX)143Subcooling heat exchanger150Receiver160High pressure valve (HPV)170Evaporator190Circulation pump210Closed medium flow circuit220Storage tank230Circulation pump240Heat exchanger250Closed circuit consumer (e.g. radiators)310Open medium flow circuit320Storage tank330Circulation pump350Open circuit consumer (e.g. showers)360Medium drainpipe370Medium source inlet380Variable speed drive (VSD) controller

[0031] All of the temperature values in FIG. 1, such as the external ambient temperature (T_ext=3° C.), the input temperature (140°) and output temperature (85°) of HX 141, the input temperature (85°) and output temperature (46°) of HX 142, the input temperature (80°) and output temperature (70°) of the closed medium flow circuit 210, the input temperature (45°) and output temperature (70°) of the open medium flow circuit 310, and the temperature (15°) at the medium inlet source 390, are in units of degrees Celsius (° C.). The temperature values in FIG. 1 are meant to be illustrative, and are not to be construed as limiting features.

[0032] A user of system 100 typically enters setpoints for the input temperatures of the open and closed flow circuits, by inputting their values via a digital interface in communication with controller 380. The communication may be wired or wireless. The controller 380 receives input of the ambient external temperature, T_ext, via an external temperature sensor (not shown in FIG. 1).

[0033] In compressor 120, the CO2 working fluid remains a gas and does not undergo a phase change to liquid. The heat generated is therefore sensible (i.e. not latent) heat. Because the density of the CO2 gas is variable, it is preferable to specify the CO2 flow rate in terms of mass flow, using for example units of kilograms per hour (kg / hr).

[0034] The compressor 120 is for example a semi-hermetic, reciprocating compressor. To maintain the temperature setpoints established by the user, compressor 120 operates with variable output, by means of speed regulators or internal discharges of the compressor.

[0035] For example, if the set point in the closed medium flow circuit is 80° and the measured temperature is 81°, the compressor 120, under the control of controller 380, lowers its output using speed regulation or internal discharges of the compressor, thereby lowering the mass flow rate of the working fluid inside heat pump 110.

[0036] The compressor outlet, having an exemplary temperature of 140° C. is connected to the input of the first heat exchanger, HX 141. The latter supplies hot water, for example at a temperature of 80° C. to the closed medium flow circuit 210, which provides the hot water needed for consumer 250, e.g. space radiators or underfloor heating pipes.

[0037] The working fluid leaves HX 141 and enters HX 142, with an exemplary temperature of 85° C. HX 142 supplies hot water to an open medium flow circuit 310, which provides the hot water needed for consumer 350, e.g. a hot shower connected to a drainpipe 360. The water flowing out of the drainpipe is replenished by intake water from medium intake 370 which is connected to HX 142. The medium intake 370 is connected for example to a network water source, in which the water temperature typically may fluctuate from 5 to 20° C. As a result, some or all of the working fluid undergoes a phase change from gas to liquid and there is a transfer of latent heat in HX 142.

[0038] When only part of the working fluid undergoes a phase change, a liquid-gas mixture enters HX 143 for additional “subcooling”, and then continues to a high-pressure valve (HPV) 160, which serves to maintain consistent compression pressure.

[0039] Because there is no consistent water flow at medium source inlet 370, there can be a change in the amount of liquid coolant in HX 142. To counteract this, an electrical expansion valve may be installed in the subcooling heat exchanger 143, whose purpose is to keep a stable temperature before the HPV 160, thus providing a constant supply of coolant inside the cooling system. In an embodiment where the set points are known in advance (or predetermined), there is generally no need for inserting the electrical expansion valve.

[0040] The heat exchangers 141, 142, and 143 may be of various types, such as plate, shell and tube, or fin heat exchangers.

[0041] Since compressors generally are designed (and measured) with a constant volume flow rate, the coolant flow in heat pump 110 before the HPV 160 has been designed specifically to maintain a stable mass flow rate through the compressor and to the heat exchangers, and thereby to achieve a stable flow of heat to the medium.

[0042] The Coefficient of Performance (COP) of system 100 depends on the work of the compressor 120 and on the flow rates in both the open and closed medium flow circuits. Customer usage of the open medium flow circuit 310 is typically intermittent, whereas that of the closed medium flow circuit 210 tends to be continuous. When there is no medium flow in the open circuit 310, system 100 maintains a consistent mass flow rate inside the closed circuit 210. In this case the CoP is in direct proportion to the work of the compressor 120.

[0043] A secondary role of an electrical expansion valve in subcooling is to prevent high temperature in the coolant entrance line to the compressor, thus avoiding overheating at the compressor entrance. When set points are known in advance, and there is no need for an electrical expansion valve, there is also no need to maintain differences in water temperature inside the closed medium flow circuit 210. In this case, the overall difference in water temperature between the water entering and exiting the closed medium flow circuit 210 can be in a range extending from about 1.5° C. to 40° C.

[0044] CO2 coolant enters the evaporator 170 in free flow from receiver 150. Evaporator 170 is for example a variable speed fan-type evaporator, without an expansion valve. Temperature sensors (not shown) are mounted for example on the evaporator inlet and outlet pipes, and their readings are relayed to controller 380, by wired or wireless transmission.

[0045] The controller 380 adjusts a setpoint of the exit temperature of the evaporator by subtracting a predetermined temperature difference of, say, 5° C. from the ambient external temperature, T_ext. If the exit temperature of the evaporator 170 is lower than the setpoint, the controller activates variable-speed fans in the evaporator in order to raise the evaporator exit temperature until it reaches (or just slightly overshoots) the setpoint. In this way, the controller adapts the operation of heat pump 110 to changes in the ambient external temperature.

[0046] In cold environments, when the ambient external temperature falls below −5° C., an additional plate heat exchange may be installed in order to defrost the evaporator 170. During defrosting, the variable-speed fans in the evaporator do not operate.

[0047] As shown in FIG. 1, closed medium flow circuit 210 includes, for example, a storage tank 220, a circulation pump 230, an HX 240 and a closed-circuit customer 250, such as space radiators or underfloor heating pipes. Open medium flow circuit 310 includes, for example, a storage tank 320, a circulation pump 330, an open-circuit customer 350, a medium drainpipe 360, and a medium source inlet 370.

[0048] Other embodiments of the invention may be obtained, for example, by using a shutoff valve to shut the medium flow through HX 142 and through the open medium flow circuit 310. In this case, heat transfer into the heating medium only occurs in HX 141.

[0049] Furthermore, in applications which do not require an open medium flow circuit, a “reduced” system 100′ may be implemented in which HX 142 and the circuit 310 are totally absent.

[0050] FIG. 3 shows a process-instrumentation (PI) design diagram of an exemplary transcritical heat pump system, according to the invention. The labels 5, 6, and 7 (in red) designate control points in a software simulation of the system. In this particular example, HX 141 and HX 142 transfer heating powers of 68.7 kW and 115.1 kW, respectively, to a water medium. The evaporation fan 170 and the compressor (not shown) consume 1.9 kW and 60.7 kW of electrical power, respectively. The calculated COP for this example is thus (68.7+115.1) / (1.9+60.7)=2.9.

[0051] FIG. 4 shows a temperature-enthalpy (t,h) diagram corresponding to the heat pump system of FIG. 3. The labels 4, 5, 7, 7 / 1, 12, 13 and 14 (in red) designate control points in a software simulation of the system. The medium temperature (MT) refrigeration process is indicated by the dashed red line. The CO2 critical point is indicated by the red dot 430. The black curve 440 is the boundary between liquid and gas CO2. Line segments 441 and 442 indicate the thermodynamic transitions of the working gas inside HX 141 and HX 142, respectively. Note that line segment 442 crosses the liquid-gas boundary 440, giving rise to a liquid-gas CO2 mixture.

[0052] FIG. 5 is a log pressure-enthalpy (log p,h) diagram corresponding to the heat pump system of FIG. 3. Here too, the labels 4, 5, 6, 7, 7 / 1, 12 and 14 (in red) designate control points in a software simulation of the system. The CO2 critical point is indicated by the red dot 530. The black curve 540 is the boundary between liquid and gas CO2. Line segment 541 represents the compression of the working fluid inside the compressor and line segment 542 represents the heat exchange occurring at constant pressure.

[0053] FIG. 6 is a perspective image of a portion of a transcritical CO2 heat pump 110, according to the invention. The figure shows the semi-hermetic compressor 120 connected to a plate heat exchanger 142 which provides hot water to an open water flow circuit (for, say 5 showers) at a flow rate of 15 liters per minute. VSD controller 380 is in communication with a digital interface 390, through which a user can input temperature setpoints and other control information. The dimensions L, W, and H are equal to 90, 50, and 50 cm., respectively.

[0054] HX 142 has an exemplary input water temperature of 75° C. and an exemplary output water temperature of 90° C. The following table shows the water heating performance, in terms of output power in kW and the COP, as measured in accordance with Standard 1660 of the Air-Conditioning, Heating, & Refrigeration Institute (AHRI), for the case of an output water temperature of 60° C. and a network water source of 15° C.TABLE 2Measured Water Heating PerformanceAmbientHeatingExternalPowerTemp. (° C.)(kW)COP5805.810856.320876.8

[0055] In general, the descriptions of the various embodiments of the present disclosure have been presented for purposes of illustration but are not intended to be exhaustive or limited to the embodiments disclosed. For example, in the above explanation of the invention, the use of CO2 as the working fluid and of liquid water as the heating medium are illustrative and are not intended to be limiting features of the invention. The principles of the invention may be applied by those skilled in the art to heating systems involving a variety of other working fluids and heating mediums. Furthermore, the specific thermodynamic parameters shown in the figures are exemplary and not limiting. Their inclusion is for the sake of clarity of the detailed explanation.

[0056] Many other modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

1. A transcritical heating system for providing heat to a heating medium, the system comprising a controller and a heat pump, the heat pump further comprising:a) a working fluid characterized by a critical pressure and temperature,b) a compressor configured to compress the working fluid above its critical pressure and temperature,c) a first heat exchanger in fluid communication with the compressor and with a closed medium flow circuit,d) a high-pressure valve, ande) an evaporator for cooling the working fluid;wherein the controller is in communication with the heat pump in order to regulate a temperature of the heating medium in the closed medium flow circuit.

2. The system of claim 1 wherein the working fluid comprises carbon dioxide (CO2).

3. The system of claim 1 wherein the heating medium is water.

4. The system of claim 1 wherein a difference in temperature between the medium entering and exiting the closed medium flow circuit is in a range extending from 1.5° C. to 40° C.

5. The system of claim 1 wherein the controller is a variable speed drive (VSD) controller.

6. The system of claim 1 wherein the compressor is a semi-hermetic reciprocating compressor.

7. The system of claim 1 wherein a mass flow rate of the compressor is controlled by the controller.

8. The system of claim 1 wherein the first heat exchanger is a plate heat exchanger.

9. The system of claim 1 wherein the working fluid in the first heat exchanger is a gas.

10. The system of claim 1 further comprising a second heat exchanger in fluid communication with the first heat exchanger and with an open medium flow circuit.

11. The system of claim 10 wherein the working fluid in the second heat exchanger is a mixture of a gas and a liquid.

12. The system of claim 1 wherein the evaporator is a variable speed fan type evaporator.

13. The system of claim 1 wherein a setpoint temperature at an outlet of the evaporator is adjusted to be less than an ambient external temperature by a predetermined temperature difference.

14. The system of claim 1 further comprising an electrical expansion valve.

15. The system of claim 1 further comprising a defroster.

Images