Heat pump system

Abstract

A heat pump system includes 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 including determining a temperature indicative of frosting conditions on an exterior surface of the heat absorbing heat exchanger 16; operating the heat pump system in a first mode if the temperature indicative of frosting conditions is above a threshold value, and operating the heat pump system in a second mode if the temperature indicative of frosting conditions is within a range of temperatures that is below the threshold value.

Claims

1. A method for operating a heat pump system, the heat pump system comprising: a compression device, a heat rejecting heat exchanger, an expansion device and a heat absorbing heat exchanger; wherein the expansion device provides a controllable degree of expansion; the method comprising: determining a temperature indicative of frosting conditions on an exterior surface of the heat absorbing heat exchanger; operating the heat pump system in a first mode if the temperature indicative of frosting conditions is above a threshold value; and operating the heat pump system in a second mode if the temperature indicative of frosting conditions is within a range of temperatures that is 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 absorbing heat exchanger compared to the superheat when operating in the first mode to thereby increase an external temperature of the heat absorbing heat exchanger; wherein the range of temperatures below the threshold value is a range having a lower bound where the heat pump system is switched back to the first mode of operation.

2. A method as claimed in claim 1, wherein the step of determining a temperature indicative of frosting conditions comprises determining the outside air temperature.

3. A method as claimed in claim 1, wherein the step of determining a temperature indicative of frosting conditions comprises determining a temperature linked to the outside air temperature and/or to the temperature of the exterior surface of the heat absorbing heat exchanger.

4. A method as claimed in claim 1, wherein when the temperature indicative of frosting conditions is within the range of temperatures below the threshold value the expansion device is controlled in order that the level of superheat is sufficient to prevent frost formation on the heat absorbing heat exchanger without any additional heating.

5. A method as claimed in claim 1, wherein the degree of expansion at the expansion device is actively controlled, with the degree of expansion varying as the temperature indicative of frosting conditions varies.

6. A method as claimed in claim 1, wherein the first mode of operation comprises control of superheat for minimum superheat in the heat absorbing heat exchanger; and wherein the second mode of operation comprises increasing superheat sufficient to prevent frost without significantly exceeding that increase.

7. A method as claimed in claim 1, wherein the threshold value is a first threshold value, and the range of temperatures below the first threshold value is a range between the first threshold value, and a second threshold value that is lower than the first threshold value; and wherein the heat pump system is switched from the first mode of operation to the second mode of operation at the first threshold value, in order to delay frost formation, and switched from the second mode of operation to the first mode of operation at the second threshold value.

8. A method as claimed in claim 7, wherein the first threshold value is a temperature indicative of an outside air temperature in the range 6-13° C.

9. A method as claimed in claim 7, wherein the second threshold value is a temperature indicative of an outside air temperature in the range 0-6° C.

10. A method as claimed in claim 1, comprising using the second mode of operation when the temperature indicative of frosting conditions is indicative of an outside air temperature in the range 2-10° C.

11. A method as claimed in claim 1, comprising determining superheat of the refrigerant at the outlet of the heat absorbing heat exchanger via measurements of refrigerant temperature and pressure at the outlet of the heat absorbing heat exchanger and/or at the compressor suction inlet.

12. A method as claimed in claim 1, wherein the heat absorbing heat exchanger is an evaporator of the heat pump system and the evaporator has multiple rows of heat absorbing elements.

13. A computer programme product comprising instructions for execution on a controller for a heat pump system comprising: a compression device, a heat rejecting heat exchanger, an expansion device and a heat absorbing heat exchanger; wherein the expansion device provides a controllable degree of expansion; wherein the instructions, when executed will configure the controller to operate the heat pump system in accordance with a method as claimed in claim 1.

14. A heat pump system comprising: a compression device, a heat rejecting heat exchanger, an expansion device and a heat absorbing heat exchanger; wherein the expansion device provides a controllable degree of expansion; the heat pump system being arranged to: receive measurements for a temperature indicative of frosting conditions on an exterior surface of the heat absorbing heat exchanger, operate in a first mode if the temperature indicative of frosting conditions is above a threshold value, and operate in a second mode if the temperature indicative of frosting conditions is within a range of temperatures that is 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 absorbing heat exchanger compared to the superheat when operating in the first mode to thereby increase an external temperature of the heat absorbing heat exchanger; wherein the range of temperatures below the threshold value is a range having a lower bound where the heat pump system is switched back to the first mode of operation.

15. A method for operating a heat pump system, the heat pump system comprising: a compression device, a heat rejecting heat exchanger, an expansion device and a heat absorbing heat exchanger; wherein the expansion device provides a controllable degree of expansion; the method comprising: determining a temperature indicative of frosting conditions on an exterior surface of the heat absorbing heat exchanger; operating the heat pump system in a first mode if the temperature indicative of frosting conditions is above a threshold value; and operating the heat pump system in a second mode if the temperature indicative of frosting conditions is within a range of temperatures that is 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 absorbing heat exchanger compared to the superheat when operating in the first mode to thereby increase an external temperature of the heat absorbing heat exchanger; wherein the threshold value is a first threshold value, and the range of temperatures below the first threshold value is a range between the first threshold value, and a second threshold value that is lower than the first threshold value; and wherein the heat pump system is switched from the first mode of operation to the second mode of operation at the first threshold value, in order to delay frost formation, and switched from the second mode of operation to the first mode of operation at the second threshold value.

16. A heat pump system comprising: a compression device, a heat rejecting heat exchanger, an expansion device and a heat absorbing heat exchanger; wherein the expansion device provides a controllable degree of expansion; the heat pump system being arranged to: receive measurements for a temperature indicative of frosting conditions on an exterior surface of the heat absorbing heat exchanger, operate in a first mode if the temperature indicative of frosting conditions is above a threshold value, and operate in a second mode if the temperature indicative of frosting conditions is within a range of temperatures that is 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 absorbing heat exchanger compared to the superheat when operating in the first mode to thereby increase an external temperature of the heat absorbing heat exchanger; wherein the threshold value is a first threshold value, and the range of temperatures below the first threshold value is a range between the first threshold value, and a second threshold value that is lower than the first threshold value; and wherein the heat pump system is switched from the first mode of operation to the second mode of operation at the first threshold value, in order to delay frost formation, and switched from the second mode of operation to the first mode of operation at the second threshold value.

Description

DRAWING DESCRIPTION

(1) Certain preferred embodiments will now be described by way of example only and with reference to the accompanying drawings in which:

(2) FIG. 1 shows a heat pump system;

(3) FIG. 2 is a graph showing parameters at a heat absorbing heat exchanger of the heat pump system with a risk of frosting; and

(4) FIG. 3 shows similar parameters after implementation of a modified, second, mode of operation of the heat pump system to delay formation of frost.

DETAILED DESCRIPTION

(5) As seen in FIG. 1, a heat pump system includes a compression device 12, a heat rejecting heat exchanger 14, an expansion device 18 and a heat absorbing heat exchanger 16 that operate together in a refrigeration/heat pump cycle. The heat pump system contains a refrigerant fluid and circulation of the refrigerant fluid via the compression device 12 enables the refrigeration system to utilise a refrigeration cycle (heat pump cycle) to satisfy a heating load. In this example the compression device 12 is a compressor 12 for compression of gaseous refrigerant fluid, the heat rejecting 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 with a controllable degree of expansion, and the heat absorbing heat exchanger 16 is an evaporator for at least partially evaporating the refrigerant fluid. The heat pump system may advantageously be arranged so that the fluid is fully condensed at the condenser 14, and fully evaporated at the evaporator 16.

(6) The heat pump system is controlled by a controller 26, which in this example controls the expansion device 18 based on input from a superheat sensor 28 and outside air temperature sensor 30 as discussed below. The controller 26 can also be used for control and/or monitoring of other parts of the refrigeration system, such as the compressor 12.

(7) A set of typical operating parameters for the heat absorbing heat exchanger 16 are shown in FIG. 2, for an example in which the heat absorbing heat exchanger 16 is a evaporator 16 with three rows of fins. The graph of FIG. 2 illustrates the air temperature 101 of air passing over the fins, the fin wall temperature 102, and the refrigerant temperature 103, i.e. the temperature of the working fluid within the evaporator 16. This graph relates to an outside air temperature of about 7° C., which is the outside air temperature prior to heat absorption and prior to flow of air over the evaporator 16, as shown at the left hand end of the plot of fin air temperature 101.

(8) As a result of the heat exchange process the air temperature 101 close to the evaporator 16 fin wall decreases across the rows of fins, and the fin wall temperature 102 likewise decreases. The refrigerant temperature 103 is below 0° C. at the point of evaporation, and in this example it the evaporation temperature is −3° C. When the ambient outside air temperature is below a threshold value, which may typically be a value between 6-13° C. depending on the nature of the evaporator, then it is possible for the fin wall temperature to drop below 0° C., with frost forming on the evaporator exterior as a consequence. If frost forms then the efficiency of the system is reduced. FIG. 2 shows a situation in which frost will form on the third row of fins, as indicated by the arrow F, when the fin wall temperature drops below 0° C.

(9) For a “normal” mode of operation, without taking account of frosting, the most effective control of the heat pump system would be for a constant refrigerant temperature in the evaporator 16, with heat absorption occurring via evaporation of the refrigerant fluid (in this case at −3° C.). This may be a first mode of operation for the heat pump system described herein, providing maximum heating capacity by avoiding unnecessary superheat.

(10) In the example plots of FIG. 2 there is some slight superheat 104 in the third row as shown in the plot of refrigerant temperature 103, but it is not sufficient to prevent frost formation. The effect of the superheat is to increase the refrigerant temperature 103 and consequently increasing the fin wall temperature 102 as shown. The heat pump system can be controlled to provide a required degree of superheat by control of the expansion valve 18. The example plots of FIG. 2 do not show an effective use of such superheat, since the fin wall temperature 102 still drops below 0° C. allowing frosting to occur.

(11) The superheat within the outlet end of the evaporator 16 can be further increased when the outside air temperature drops sufficiently for there to be a risk of frost formation, and an example of this is shown in FIG. 3. This illustrates a possible second mode of operation for the heat pump system, with this second mode being adopted when the outside air temperature is within a set range below a threshold value, as discussed further herein. A measure of the outside air temperature can be done directly, such as via an outside air temperature sensor 30 as in FIG. 1. The superheat 104 at the outlet of the evaporator 16 is increased to a level sufficient to keep the fin wall temperature 102 above 0° C. via control of the expansion device 18. The fin air temperature 101 increases accordingly. The increased superheat 104 means that the refrigerant temperature 103 increases above the evaporation temperature, leading to a drop in efficiency, but this drop in efficiency is balanced by the increased effectiveness of the heat transfer when there is no frost formed on the exterior surfaces of the evaporator 16. Thus, there are gains in performance by delaying frost formation, i.e. by reducing the outside air temperature at which the evaporator 16 would be operated in a frosted state.

(12) As a basic example, noting that the temperature ranges and so on may be adjusted dependent on the nature of the heat absorbing heat exchanger and on external conditions, such as taking account of outside air humidity, the heat pump system may be arranged to operate in a first mode with minimal superheat until the outside air temperature drops below a first threshold value, such as being below 7° C. as in FIGS. 2 and 3. In the first mode the heat pump system may be controlled to provide a refrigerant temperature 103 that remains constant at all points within the heat absorbing heat exchanger 16, as with an evaporator 16 operating at the evaporation temperature of the refrigerant. This may involve a refrigerant temperature of −3° C. as noted above.

(13) When the outside air temperature drops below the threshold then the heat pump system is instead operated in a second mode, which can be similar to that shown in FIG. 3. In the second mode the superheat 104 is increased at the outlet of the heat absorbing heat exchanger with the increase in refrigerant temperature 103 acting to increase the fin wall temperature 102 to above 0° C. and hence prevent frost formation. The second mode is used within a range of outside air temperatures until the temperature drops so far that the second mode does not provide any increase in performance over that of a frosted heat exchanger. For a typical heat exchanger this can be 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 may vary depending on the parameters linked to the heat pump system, such as the drop in efficiency of heat exchange that arises from frosted operation and the drop in heating capacity that arises due to the added superheat 104. Below the lower threshold temperature, i.e. an outside air temperature of 2° C. in the example above, the heat pump system is again operated in the first mode.

(14) Referring again to FIG. 1, the level of superheat 104 at the outlet of the heat exchanger 16 can be measured via a suitable superheat sensor 28. This superheat sensor 28 might be arranged to determine refrigerant temperature and pressure at the outlet of the heat exchanger 16, or alternatively may be at the suction inlet of the compressor 12, as shown. The superheat 104 is adjusted via use of the expansion valve 18, which is controlled via the control system 26 of the heat pump system. This control can be done in any suitable fashion. In this example the control system 26 also receives a measure of outside air temperature from an outside air temperature sensor 30, as shown. This provides a simple way to determine temperatures with a risk of frosting when the heat pump system should switch to the second mode of operation, as well as utilising sensors 28, 30 that are often already present in the heat pump system for other reasons.