Heat pump system

Abstract

A heat pump system comprises a compression device 12, a heat rejecting heat exchanger 14, an expansion device 18 and a heat absorbing heat exchanger 16; wherein the expansion device 18 provides a controllable degree of expansion. The heat pump system is operated in accordance with a method comprising: determining a temperature indicative of a frost condition on an external surface of the heat absorbing heat exchanger 16; operating the heat pump system in a first mode if the temperature indicative of the frost condition is above a threshold value, and operating the heat pump system in a second mode if the temperature indicative of the frost condition is within a range of temperatures below the threshold value, wherein in the second mode the heat pump system is arranged to adjust the degree of expansion at the expansion device 18 to increase a degree of superheat at an outlet of the heat absorbing heat exchanger 16 compared to when operating in the first mode to thereby increase an external temperature of the heat absorbing heat exchanger.

Description

Technical Field

[0001] This invention relates to a method for operating a heat pump system, and a corresponding heat pump system. Background Technology

[0002] As is well known, cooling or heating can be provided by a refrigeration system that uses a refrigeration cycle in which a refrigerant fluid is compressed, cooled, expanded, and then heated. In a common application (where such a refrigeration cycle is used to meet a heating load), cooling of the refrigerant fluid is achieved via a heat exchanger that dissipates heat to the space inside the building, and heating of the refrigerant fluid is achieved via a heat exchanger that absorbs heat from the exterior of the building to be occupied. In this way, the refrigeration cycle can transfer heat from the exterior of the building to the interior, even when the interior is cooler than the atmosphere. A complete or partial phase change of the refrigerant fluid can be used to increase the possible temperature difference between the heat dissipation and heat absorption phases.

[0003] In such a heat pump system, the heat-absorbing exchanger (typically an evaporator) carries a low-temperature refrigerant fluid so that it absorbs heat even when the outside air temperature is low. Under certain conditions, this can lead to the risk of frost forming on the external surfaces of the heat-absorbing exchanger. Summary of the Invention

[0004] From a first aspect, the present invention provides a method for operating a heat pump system, the heat pump system comprising: a compression device, a heat dissipation heat exchanger, an expansion device, and a heat absorption heat exchanger; wherein the expansion device provides a controllable degree of expansion; the method comprising: determining a temperature indicating a frosting condition on the outer surface of the heat absorption heat exchanger; operating the heat pump system in a first mode if the temperature indicating the frosting condition is above a threshold value; and operating the heat pump system in a second mode if the temperature indicating the frosting condition is within a temperature range below the threshold value; wherein, in the second mode, the heat pump system is arranged to adjust the degree of expansion at the expansion device to increase the superheat at the outlet of the heat absorption heat exchanger compared to the superheat (or superheat) when operating in the first mode, thereby increasing the external temperature of the heat absorption heat exchanger.

[0005] Traditionally, such heat pump systems can be configured to operate with minimal superheat at the outlet of the absorber heat exchanger in order to maximize capacity. This can be similar to the operation in the first mode of the method described above. The inventors have recognized that when the outside air temperature is within a certain range, as determined based on a temperature indicating frosting conditions, benefits can be derived from operating with enhanced superheat in the second mode. In this arrangement, the heat pump system can operate at an enhanced outside temperature to the absorber heat exchanger, and this allows for an extended temperature range (where the absorber heat exchanger can be operated without frost formation).

[0006] When frost is present on the external surface of the heat exchanger, the operating efficiency of the heat pump system can typically decrease by up to 20%. Therefore, it is advantageous, as stated above, to use a mode with enhanced superheating to delay (or postpone) frost formation because, although enhanced superheating will reduce the system capacity compared to normal frost-free operation, the avoidance of frost provides a greater benefit than that capacity reduction. This may be particularly valuable in areas where the outside air temperature typically drops to a range where frost can initially form (e.g., within the range of 1-9°C or 2-7°C) without remaining below freezing for an extended period. These conditions commonly occur in inhabited areas of the world, such as across much of Europe.

[0007] The step of determining the temperature indicating frosting conditions may include determining the outside air temperature. The outside air temperature is the temperature of the air outside the heat exchanger. Alternatively, determining the temperature indicating frosting conditions may include determining some other temperature associated with the outside air temperature and / or the temperature of the outer surface of the heat exchanger. This may include using a temperature sensor for some other indirect measurement of one of those temperatures. Alternatively or additionally, the method may use a more direct measurement of the temperature of the outer surface of the heat exchanger, such as via a temperature sensor in thermal contact with the outer surface. In one example, the method may use a combination of determining the outside air temperature and the refrigerant fluid temperature at the outlet of the heat exchanger to assess the likelihood of frosting conditions on the outer surface of the heat exchanger.

[0008] This method controls the expansion device so that the level of overheating is sufficient to prevent frost formation on the heat exchanger when the temperature indicating frost conditions (e.g., outside air temperature) is below a threshold value. Therefore, controlling the expansion during second-mode operation ensures that the minimum external temperature of the heat exchanger is above a minimum defrost value, for example, above 0°C. The external temperature of the heat exchanger can be the temperature of the external surface (such as fins, etc.), with the minimum external temperature located at the cold end (outlet end) of the heat exchanger.

[0009] An expansion device provides a controllable degree of expansion, which is used to control superheat at the outlet of the heat exchanger, as discussed above. The expansion device can be any suitable controllable expansion device used to reduce the pressure of the refrigerant fluid, such as, for example, an electronic expansion valve.

[0010] The degree of expansion at the expansion device can be actively controlled, wherein the degree of expansion (e.g., the degree to which the expansion valve is open) varies with the temperature indicating the frosting condition (e.g., the outside air temperature). This can be done so that an increase in superheat is used to prevent frost, without excessive superheat that could unnecessarily reduce capacity. As noted above, the first operating mode may involve conventional control of superheat for minimum superheat in the absorber heat exchanger. The second operating mode may involve increasing the superheat sufficiently to prevent frost, for example, by increasing the outside temperature of the absorber heat exchanger as described above, without significantly exceeding the required increase.

[0011] This method controls superheating at the outlet of the heat exchanger based on the difference between a threshold value and the outside air temperature (e.g., proportional to that difference or based on some other function determined for frost prevention). Such a function can vary for different types of heat exchangers. The required function can be determined empirically and / or through modeling. The method can use a table of outside air temperature and superheat or a table of outside air temperature and expansion requirements. Thus, since the outside temperature varies within a range below the threshold value, the expansion device can be actively controlled to impart the required superheat. It will be appreciated that by using superheating in this way, such as by actively controlling the expansion device based on the outside air temperature, it becomes possible to operate frost-free without any other modifications to the heat pump system.

[0012] A heat pump system may not require additional defrosting equipment for the absorber heat exchanger, and therefore one or more additional defrosting devices may not be present. Advantageously, a heat pump system does not include a separate heater for defrosting the external surfaces of the absorber heat exchanger; for example, no form of electric heater may be present. Therefore, the heat pump system can use control of the expansion device against overheating to prevent frost within a temperature range below a threshold value, without requiring any other heat source. Thus, overheating may be the sole cause of increased external temperature of the absorber heat exchanger when operating in the second mode.

[0013] The temperature range below the threshold value can be a range with a lower bound (here, the heat pump system is switched back to the first operating mode). This will then allow frost to form, where the result is a decrease in efficiency, but it will be realized that as the temperature gets lower, the cost of increasing superheating efficiency increases, so that at some point, operation in the "normal" mode (i.e., the first operating mode, where frost is allowed) becomes optimal. Therefore, the second operating mode can be considered a frost-preventing mode, which uses increased superheating to lower the outside air temperature (where frost can form).

[0014] The temperature range below the threshold value can be a range between a first threshold value (which is the threshold value discussed above) and a second threshold value below the first threshold value. The heat pump system can switch from a first operating mode to a second operating mode at the first threshold value to delay frost formation, and switch from the second operating mode to the first operating mode at the second threshold value, thus allowing frost to be tolerated if the outside air temperature becomes too low for efficient use. The first threshold value can be a temperature indicating an outside air temperature within the range of 6-13°C, optionally within the range of 7-11°C (e.g., a temperature value of approximately 9°C or approximately 10°C). As noted above, the method can include directly measuring the outside air temperature using an outside air temperature sensor. The second threshold value can be a temperature indicating an outside air temperature within the range of 0-6°C, optionally within the range of 1-4°C (e.g., a temperature value of approximately 2°C or approximately 3°C). Thus, for example, when the outside air temperature is determined to be within the range of 2-10°C or 3-7°C, the heat pump system can use the second operating mode.

[0015] This method may include determining the refrigerant superheat at the outlet of the heat exchanger. This may involve measuring the refrigerant temperature and pressure at one or more points within the heat pump system, such as by taking measurements at the outlet of the heat exchanger and / or at the compressor suction inlet. Those skilled in the art will be aware of various techniques for determining suitable measurements that may be used in this context.

[0016] As noted above, the method may include determining the outside air temperature directly or indirectly. For example, the method may include using a temperature sensor to measure the air temperature outside the heat exchanger. It is relatively common for the external portion of a heat pump system to include an outside air temperature sensor, and conveniently, current methods can use existing sensors of this type. Alternatively, the method may determine a measurement reflecting changes in the outside air temperature, and thereby indirectly determine the outside air temperature. It will be appreciated that determining the outside air temperature may include any measurement equivalent to determining when the temperature, as discussed above, drops below a threshold where there is a risk of frost.

[0017] A heat exchanger is typically an evaporator in a heat pump system. The outer surface of the heat exchanger can be the outer surface of the heat-absorbing elements (such as the fins of the heat exchanger). An exemplary arrangement has two, three, or more rows of heat-absorbing elements, for example, three rows of fins, connected to a multi-row heat exchanger tube that can be coupled to carry the working fluid of the heat pump system for heat exchange with outside air. It will be appreciated that the greatest risk of frosting exists in the last row of such a multi-row heat exchanger, closest to the outlet for the working fluid in the heat pump system, where the outside air across the outer surface will be at its coldest, and the fin temperature will also be at its coldest. Therefore, the proposed operating method may involve increased superheating in the last row of fins of the heat exchanger during operation in a second mode to prevent frosting on that row. Advantageously, superheating can be avoided in other rows to maximize the capacity of the heat pump system.

[0018] The compressor can be any suitable device for increasing the pressure of the refrigerant fluid, and therefore can be any suitable type of compressor. The compressor can be arranged to operate using a single-phase refrigerant (i.e., a completely gaseous refrigerant) or a two-phase refrigerant having a mixture of liquid and gas phases. The compressor can have an inlet connected to a fluid passage leading to a self-absorbing heat exchanger and an outlet connected to a fluid passage leading to a dissipating heat exchanger. In some examples, the fluid passage provides a direct connection without any other refrigeration system components that would modify the state of the refrigerant fluid. The compressor can have intermediate inlets, such as those for connecting to a heat-saving device.

[0019] A heat pump system may include refrigerant piping. Refrigerant piping may be connected to or interact with an expansion unit. Refrigerant piping may extend from a branch point in the heat pump system located after the exhaust heat exchanger and before the expansion unit, or at the expansion unit, to the intermediate inlet of the compressor. There may be refrigerant valves in the refrigerant piping for refrigerant expansion and for controlling the degree of refrigerant flow, as well as refrigerant heat exchangers for heat exchange between the refrigerant fluid in the refrigerant piping located after the refrigerant valve and the refrigerant fluid in the heat pump system located after the branch point and before the expansion unit.

[0020] The heat exchanger for heat dissipation can be a condenser.

[0021] The method may include using a heat pump system for heating a building, and in this case, the heat-absorbing heat exchanger may be located outside the building, wherein the outside air temperature is therefore the temperature outside the building and near the heat-absorbing heat exchanger.

[0022] It will be recognized that the main components of the heat pump system are the same as those of existing heat pump systems, with the main modification relating to the control of the expansion valve in response to increased superheat. The methods described above can therefore be implemented on existing heat pump systems, for example, by modifying the control system and / or its software. Advantageously, such modifications / upgrades can utilize existing outside air temperature sensors.

[0023] From a second aspect, the present invention provides a computer program product comprising instructions executable on a controller for a heat pump system, the heat pump system comprising: a compression device, a heat exhaust heat exchanger, an expansion device, and a heat absorption heat exchanger; wherein the expansion device provides a controllable degree of expansion; wherein, when executed, the instructions configure the controller to operate the heat pump system according to the methods discussed above with respect to the first aspect or optional features thereof.

[0024] From a third aspect, the present invention provides a heat pump system comprising: a compression device, a heat dissipation heat exchanger, an expansion device, and a heat absorption heat exchanger; wherein the expansion device provides a controllable degree of expansion; the heat pump system is arranged to: receive a measurement of a temperature indicating the condition of frost formation on the outer surface of the heat absorption heat exchanger; if the temperature indicating the condition of frost formation is higher than a threshold value, operate in a first mode; and if the temperature indicating the condition of frost formation is within a temperature range below the threshold value, operate in a second mode, wherein in the second mode, the heat pump system is arranged to adjust the degree of expansion at the expansion device to increase the superheat at the outlet of the heat absorption heat exchanger compared to the superheat when operating in the first mode, thereby increasing the external temperature of the heat absorption heat exchanger.

[0025] A heat pump system may include a controller for receiving temperature measurements and for controlling the operating mode of the heat pump system. Therefore, the controller may be configured to control the expansion valve to enhance superheat, as described above. A heat pump system of the second aspect may be arranged to operate according to the methods discussed above with respect to the first aspect or its optional features. It may include features of a heat pump system as mentioned above, such as those relating to one or more of an expansion device, heat exchanger, compressor, temperature sensor, superheat sensor, etc. Attached Figure Description

[0026] Some preferred embodiments will now be described by way of example only and with reference to the accompanying drawings, wherein:

[0027] Figure 1 This illustrates a heat pump system;

[0028] Figure 2 It is a graph showing the parameters at the heat exchanger of a heat pump system with the risk of frost formation; and

[0029] Figure 3 Similar parameters are shown after implementing a modified second operating mode of the heat pump system to delay the formation of frost. Detailed Implementation

[0030] like Figure 1 As seen in the diagram, the heat pump system includes a compressor 12, a heat dissipation heat exchanger 14, an expansion device 18, and a heat absorption heat exchanger 16, all operating together in a refrigeration / heat pump cycle. The heat pump system contains a refrigerant fluid, and the circulation of the refrigerant fluid via the compressor 12 enables the refrigeration system to meet heating loads using a refrigeration cycle (heat pump cycle). In this example, the compressor 12 is a compressor 12 for compressing the gaseous refrigerant fluid, the heat dissipation heat exchanger 14 is a condenser for at least partially condensing the refrigerant fluid, the expansion device 18 is an expansion valve for expanding the refrigerant fluid to a controllable extent, and the heat absorption heat exchanger 16 is an evaporator for at least partially evaporating the refrigerant fluid. The heat pump system can be advantageously arranged such that the fluid is completely condensed at the condenser 14 and completely evaporated at the evaporator 16.

[0031] The heat pump system is controlled by controller 26, which in this example controls the expansion device 18 based on inputs from the superheat sensor 28 and the outside air temperature sensor 30, as discussed below. Controller 26 can also be used to control and / or monitor other parts of the refrigeration system, such as compressor 12.

[0032] exist Figure 2 The diagram shows a set of typical operating parameters for a heat exchanger 16, for example, in which the heat exchanger 16 is an evaporator 16 with three rows of fins. Figure 2 The graph illustrates the air temperature 101 across the fins, the fin wall temperature 102, and the refrigerant temperature 103 (i.e., the temperature of the working fluid within the evaporator 16). The graph relates to an outside air temperature of approximately 7°C, which is the outside air temperature before heat absorption and before the airflow over the evaporator 16, as shown at the left-hand end of the graph of the fin air temperature 101.

[0033] As a result of the heat exchange process, the air temperature 101 near the fin wall of the evaporator 16 decreases across the fin row, and the fin wall temperature 102 decreases similarly. The refrigerant temperature 103 is below 0°C at the evaporation point, and in this example, its evaporation temperature is -3°C. When the ambient air temperature is below a threshold value (which can typically be between 6 and 13°C depending on the nature of the evaporator), the fin wall temperature may drop below 0°C, resulting in frost formation on the exterior of the evaporator. If frost forms, the system efficiency decreases. Figure 2 The following scenario illustrates that, as indicated by arrow F, frost will form on the third row of fins when the fin wall temperature drops below 0°C.

[0034] In the "normal" operating mode, without considering frosting, the most efficient control of the heat pump system will be aimed at a constant refrigerant temperature in the evaporator 16, where heat absorption occurs via the evaporation of the refrigerant fluid (in this case, at -3°C). This can be the first operating mode for the heat pump system described herein, providing maximum heating capacity by avoiding unnecessary overheating.

[0035] exist Figure 2 In the example plot, as shown in the plot of refrigerant temperature 103, there is a slight superheat 104 in the third row, but this is insufficient to prevent frost formation. As shown, the effect of superheat is to increase the refrigerant temperature 103 and consequently increase the fin wall temperature 102. The heat pump system can be controlled to provide the required degree of superheat via the control of the expansion valve 18. Figure 2 The example diagram does not show the effective use of such overheating because the fin wall temperature 102 still drops below 0°C, thus allowing frost to occur.

[0036] When the outside air temperature drops sufficiently to create a risk of frost formation, the superheat within the outlet end of the evaporator 16 can be further increased, and an example of this situation is shown in... Figure 3 This is illustrated in the diagram. This diagram shows a feasible second operating mode for the heat pump system, wherein the second mode is employed when the outside air temperature is within a set range below a threshold value, as discussed further herein. The outside air temperature can be measured, for example via... Figure 1The outside air temperature sensor 30 is used directly. The superheat 104 located at the outlet of the evaporator 16 is increased to a level sufficient to maintain the fin wall temperature 102 above 0°C via the control of the expansion device 18. The fin air temperature 101 is increased accordingly. The increased superheat 104 means that the refrigerant temperature 103 increases above the evaporation temperature, resulting in a decrease in efficiency, but this decrease in efficiency is offset by the increased effectiveness of heat transfer when there is no frost forming on the outer surface of the evaporator 16. Thus, there is a performance benefit by delaying frost formation (i.e., by reducing the outside air temperature (in which case the evaporator 16 will operate in a frosted state)).

[0037] As a basic example, note that the temperature range, etc., can be adjusted depending on the properties of the heat exchanger and on external conditions, such as, taking into account the external air humidity, the heat pump system can be arranged to operate with minimal superheat in a first mode until the outside air temperature drops below a first threshold value (e.g., ...). Figure 2 and Figure 3 Until it is below 7°C. In the first mode, with the evaporator 16 operating at the refrigerant evaporation temperature, the heat pump system can be controlled to provide a refrigerant temperature 103 that remains constant at all points within the heat exchanger 16. This can involve a refrigerant temperature of -3°C as noted above.

[0038] When the outside air temperature drops below a threshold, the heat pump system switches to operating in the second mode, which is similar to... Figure 3 The situation is illustrated. In the second mode, the superheat 104 is increased at the outlet of the absorber heat exchanger, wherein the increase in refrigerant temperature 103 is used to raise the fin wall temperature 102 above 0°C and thus prevent frost formation. The second mode is used within the following range of outside air temperatures until the temperature drops to such a point that the second mode does not provide any performance improvement over the performance of the frost-covered heat exchanger. For a typical heat exchanger, this is possible at outside air temperatures below 2°C, so that the second mode is used for outside air temperatures below 7°C and above 2°C. It will be appreciated that this lower threshold can vary depending on parameters associated with the heat pump system, such as the decrease in heat exchange efficiency due to frost-covered operation and the decrease in heating capacity due to the increased superheat 104. Below the lower threshold temperature (i.e., the outside air temperature of 2°C in the example above), the heat pump system operates again in the first mode.

[0039] Refer again Figure 1The level of superheat 104 at the outlet of heat exchanger 16 can be measured via a suitable superheat sensor 28. This superheat sensor 28 can be arranged to determine the refrigerant temperature and pressure at the outlet of heat exchanger 16, or alternatively, it can be located at the suction inlet of compressor 12, as shown. Superheat 104 is adjusted using an expansion valve 18, which is controlled via the control system 26 of the heat pump system. This control can be performed in any suitable manner. In this example, as shown, the control system 26 also receives measurements of the outside air temperature from an outside air temperature sensor 30. This provides a simple way to determine temperatures at risk of frosting using sensors 28, 30 that are typically present in the heat pump system when the heat pump system should switch to a second operating mode, and for other reasons.

Claims

1. A method for operating a heat pump system, the heat pump system comprising: The method comprises: a compression device, a heat dissipation heat exchanger, an expansion device, and a heat absorption heat exchanger; wherein the expansion device provides a controllable degree of expansion; the method includes: Determine the temperature indicating the frosting condition on the outer surface of the heat exchanger; If the temperature indicating frosting is higher than a threshold value, the heat pump system operates in a first operating mode; and If the temperature indicating the frosting condition is within a range below the threshold value, the heat pump system is operated in the second operating mode. In the second operating mode, the heat pump system is arranged to adjust the degree of expansion at the expansion device to increase the superheat compared to the superheat at the outlet of the heat exchanger when operating in the first operating mode, thereby increasing the external temperature of the heat exchanger. The range of temperatures below the threshold value is the range with a lower bound where the heat pump system is switched back to the first operating mode.

2. The method according to claim 1, wherein, The steps for determining the temperature that indicates frost conditions include determining the outside air temperature.

3. The method according to claim 1, wherein, The step of determining the temperature indicating frost conditions includes determining a temperature associated with the outside air temperature and / or the temperature of the external surface of the heat exchanger.

4. The method according to any one of claims 1-3, wherein, When the temperature indicating frost conditions is below the threshold value, the expansion device is controlled so that the level of overheating is sufficient to prevent frost formation on the heat exchanger without the use of any additional heating.

5. The method according to any one of claims 1-3, wherein, The degree of expansion at the expansion device is actively controlled, wherein the degree of expansion varies with the temperature indicating the frosting condition.

6. The method according to any one of claims 1-3, wherein, The first operating mode includes control of superheat for minimum superheat in the heat exchanger; and the second operating mode includes increasing the superheat sufficiently to prevent frost without significantly exceeding that increase.

7. The method according to any one of claims 1-3, wherein, The threshold value is a first threshold value, and the range of temperatures below the first threshold value is the range between the first threshold value and a second threshold value below the first threshold value; and wherein the heat pump system switches from the first operating mode to the second operating mode at the first threshold value to delay frost formation, and switches from the second operating mode to the first operating mode at the second threshold value.

8. The method according to claim 7, wherein, The first threshold value is a temperature indicating the outside air temperature within the range of 6-13°C.

9. The method according to claim 7, wherein, The first threshold value is a temperature indicating the outside air temperature within the range of 7-11°C.

10. The method according to claim 7, wherein, The second threshold value is a temperature indicating the outside air temperature within the range of 0-6°C.

11. The method according to claim 7, wherein, The second threshold value is a temperature indicating the outside air temperature within the range of 1-4°C.

12. The method according to any one of claims 1-3, comprising using the second operating mode when the temperature indicator indicating the frosting condition is in the range of 2-10°C for the outside air temperature.

13. The method according to any one of claims 1-3, comprising using the second operating mode when the temperature indicator indicating the frosting condition is in the range of 3-7°C for the outside air temperature.

14. The method according to any one of claims 1-3, comprising determining the superheat of the refrigerant at the outlet of the heat exchanger by measuring the refrigerant temperature and pressure at the outlet of the heat exchanger and / or at the suction inlet of the compression device.

15. The method according to any one of claims 1-3, wherein, The heat exchanger is the evaporator of the heat pump system, and the evaporator has multiple rows of heat-absorbing elements.

16. A computer program product comprising instructions for execution on a controller for a heat pump system, the heat pump system comprising: A compression device, a heat dissipation heat exchanger, an expansion device, and a heat absorption heat exchanger; wherein the expansion device provides a controllable degree of expansion; wherein the instruction, when executed, will configure the controller to operate the heat pump system according to the method of any one of claims 1-15.

17. A heat pump system, comprising: A compression device, a heat dissipation heat exchanger, an expansion device, and a heat absorption heat exchanger; wherein the expansion device provides a controllable degree of expansion; The heat pump system is arranged as follows: Receive a measurement of the temperature indicating the frost condition on the outer surface of the heat exchanger. If the temperature indicating frosting is higher than a threshold value, then the system operates in the first operating mode, and If the temperature indicating frosting is below the threshold value, then operate in the second operating mode. In the second operating mode, the heat pump system is arranged to adjust the degree of expansion at the expansion device to increase the superheat compared to the superheat at the outlet of the heat exchanger when operating in the first operating mode, thereby increasing the external temperature of the heat exchanger. The range of temperatures below the threshold value is the range with a lower bound where the heat pump system is switched back to the first operating mode.

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