Air source heat pump and method of defrosting an air source heat pump

Abstract

Provided is a method of defrosting an air heat exchanger of an air source heat pump, the method comprising: receiving in a refrigerant reservoir of the heat pump refrigerant from the air heat exchanger; passing refrigerant from the refrigerant reservoir through an expansion valve to expand the refrigerant before passing it through a refrigerant heat exchanger within liquid refrigerant within the reservoir; supplying refrigerant from the refrigerant heat exchanger to a heat exchanger of a heating system; supplying refrigerant from the heat exchanger of the heating system to the compressor of the heat pump and thereafter supplying refrigerant from the compressor to the air heat exchanger; and controlling the expansion valve to control the amount of energy extracted from the heat exchanger of the heating system and via the refrigerant heat exchanger from refrigerant in the reservoir. Also provided is a corresponding heat pump device.

Description

[0001] AIR SOURCE HEAT PUMP AND METHOD OF DEFROSTING AN AIR SOURCE

[0002] HEAT PUMP

[0003] Technical field

[0004] The present invention relates to an air source heat pump and to a method of defrosting an air source heat pump.

[0005] Background

[0006] According to Directive 2012 / 27 / EU buildings represent 40 % of the final energy consumption and 36% of CO2 emissions. The EU Commission report of 2016 “Mapping and analyses of the current and future (2020 - 2030) heating / cooling fuel deployment (fossil / renewables)” concluded that in EU households, heating and hot water alone account for 79% of total final energy use (192.5Mtoe). The EU Commission also report that, “according to 2019 figures from Eurostat, approximately 75% of heating and cooling is still generated from fossil fuels while only 22% is generated from renewable energy. To fulfil the EU’s climate and energy goals, the heating and cooling sector must sharply reduce its energy consumption and cut its use of fossil fuels. Heat pumps, with energy drawn from the air, the ground or water, have been identified as potentially significant contributors in addressing this problem. Because only a small percentage of domestic households have access to a body of water to support the use of a heat pump that draws energy from water, and because the cost and space requirements for installing a ground source heat exchanger are very significant, it is generally cheaper and more convenient to install an air source heat pump. It is also generally recognised that an air source heat pump is the better replacement for an existing gas central heating boiler.

[0007] Air source heat pumps do however have one disadvantage not shared by heat pumps that extract energy from a body of water or from the ground, and that is the need in temperate and cooler climates periodically to defrost the heat pump’s air heat exchanger when the heat pump has been run to extract energy from ambient air (“heating mode”). The air heat exchanger is a heat exchanger in which in heating mode heat from ambient air is transferred to liquid refrigerant. A matrix of, typically finned, tubes (more generally conduits) receive cold liquid refrigerant, and ambient air is moved over the tubes by the action of a fan or impeller. In heating mode, energy is extracted from the ambient air because the liquid refrigerant is colder than the ambient air, and the liquid refrigerant vaporises as a result of its increased temperature, and the temperature of the ambient air drops. Water vapour carried by the ambient air will consequently tend to condense on the surface of the air heat exchanger. If the ambient air temperature is sufficiently low, the water that condenses on the surface of the air heat exchanger will freeze to form ice. A build-up of ice reduces the heat transfer efficiency of the air heat exchanger, and a significant build-up of ice can block the gaps between adjacent conduits of the air heat exchanger leading to a loss of air flow and hence further reduction in efficiency. Air source heat pumps are consequently configured to perform from time to time what is known as a defrost cycle (which in effect involves briefly running the heat pump in cooling mode, in which heat is given up from the refrigerant to ambient via the air heat exchanger) during which relatively warm refrigerant is fed into the air heat exchanger to melt the ice and hence restore the efficiency of the air heat exchanger. The frequency of defrost cycle events may be as often as every 35 minutes, for perhaps 10 minutes a time, but depends upon the ambient conditions, and the amount of heat load the system is trying to deliver, among other things. The defrost cycle thus involves a reversal of the usual energy extraction cycle used in the heating mode: the heat pump operates in reverse, with energy being transferred from the home (e.g. from the heated space within the home or from the hot water of the space heating system or of the domestic hot water supply) to the air heat exchanger and hence to ambient.

[0008] The present invention seeks to provide an air source heat pump in which at least some of the drawbacks of existing air source heat pumps are mitigated in whole or in part.

[0009] Summary

[0010] According to a first aspect there is provided a method of defrosting an air heat exchanger of an air source heat pump, the method comprising: supplying refrigerant from a compressor of the heat pump to the air heat exchanger; receiving in a refrigerant reservoir refrigerant from the air heat exchanger; passing refrigerant from the refrigerant reservoir through an expansion valve to expand the refrigerant before passing the expanded refrigerant through a refrigerant heat exchanger within refrigerant within the reservoir, supplying the refrigerant from the refrigerant heat exchanger to a heat exchanger of a heating system, and returning refrigerant from the heat exchanger of the heating system to the compressor.

[0011] According to a second aspect there is provided a method of defrosting an air heat exchanger of an air source heat pump, the method comprising: supplying refrigerant from a compressor of the heat pump to the air heat exchanger; receiving in a refrigerant reservoir refrigerant from the air heat exchanger; passing refrigerant from the refrigerant reservoir through an expansion valve to expand the refrigerant before passing the expanded refrigerant through a refrigerant heat exchanger within refrigerant within the reservoir and thence to a heat exchanger of a heating system; and returning refrigerant from the heat exchanger of the heating system to the compressor.

[0012] Although the refrigerant heat exchanger and heating system heat exchanger are connected in series they essentially act as one. Heat may be sucked out of both the refrigerant heat exchanger and heating system heat exchanger together, with essentially only one control mechanism (the cooling / defrost expansion valve) affecting both heat exchangers.

[0013] It will be appreciated that passage of refrigerant through an expansion valve reduces the effective boiling point of the refrigerant, which is beneficial upstream (during defrost) of the air heat exchanger, and under many circumstances will also lead to cooling of the refrigerant.

[0014] According to a third aspect there is provided a method of defrosting an air heat exchanger of an air source heat pump, the method comprising: receiving in a refrigerant reservoir of the heat pump refrigerant from the air heat exchanger; passing refrigerant from the refrigerant reservoir through an expansion valve to expand the refrigerant before passing it through a refrigerant heat exchanger within liquid refrigerant within the reservoir; supplying refrigerant from the refrigerant heat exchanger to a heat exchanger of a heating system; supplying refrigerant from the heat exchanger of the heating system to the compressor of the heat pump and thereafter supplying refrigerant from the compressor to the air heat exchanger; and controlling the expansion valve to control the amount of energy extracted from the heat exchanger of the heating system and via the refrigerant heat exchanger from refrigerant in the reservoir.

[0015] In the expansion valve the refrigerant passes through a small opening, resulting in a reduction in pressure which in turn reduces the boiling point of the refrigerant. Depending on the temperature of the refrigerant at the expansion valve inlet and the pressure of the refrigerant at the expansion valve outlet, a portion of the refrigerant may spontaneously vaporise, resulting in a reduction in temperature of the refrigerant, (such that the refrigerant leaves the expansion valve as a mixture of liquid and vapour).

[0016] By taking from the refrigerant reservoir some energy for defrosting the air heat exchanger we can reduce the amount of energy for defrosting which must be extracted from the heating circuit, reducing the adverse effect of defrosting on the heating performance of the system. In retrofit scenarios (e.g. where a heat pump is being installed in place of a gas boiler) there may be a limitation on flow rate within the heating system, and the method allows for improved control of the temperature of refrigerant flowing to the heating system heat exchanger to reduce the risk of freezing of the water within the heating system heat exchanger in heating systems using water as the heat transfer medium, which is a very significant step forward. It will be appreciated that once the excess heat has been extracted from the refrigerant reservoir further passage of refrigerant through the heat exchanger in the reservoir will not result in further warming of refrigerant passing through that heat exchanger until the next defrost event, but the harvesting of excess energy from refrigerant in the refrigerant reservoir may still make a useful contribution to improving the overall energy efficiency of the heat pump. In any event, this approach should help to achieve successful defrosting.

[0017] According to a fourth aspect there is provided a method of defrosting an air heat exchanger of an air source heat pump, the method comprising: supplying warm refrigerant from a compressor of the heat pump to the air heat exchanger; supplying refrigerant from the air heat exchanger to a refrigerant reservoir; passing refrigerant from the refrigerant reservoir through an expansion valve to expand the refrigerant before using the expanded refrigerant to absorb energy from refrigerant in the refrigerant reservoir by passing the expanded refrigerant through a refrigerant heat exchanger within refrigerant within the reservoir; supplying refrigerant from the refrigerant heat exchanger to a heat exchanger of a heating system; and supplying refrigerant from the heat exchanger of the heating system to the compressor.

[0018] According to a fifth aspect there is provided a method of defrosting an air heat exchanger of an air source heat pump, the heat pump being configured to flow refrigerant through conduits of the air heat exchanger in a first sense during a heating mode of the heat pump, method comprising: reversing the sense of the flow of refrigerant through the air heat exchanger and passing refrigerant received from the air heat exchanger along a flow path through a reservoir of liquid refrigerant, the flow path including a refrigerant heat exchanger that is at least partially submerged in the liquid refrigerant, supplying refrigerant from the flow path via a heating heat exchanger to a compressor of the heat pump; and supplying refrigerant from the compressor to the air heat exchanger.

[0019] The method of the fifth aspect may further comprise: passing refrigerant received from the air heat exchanger through the reservoir before passing the refrigerant through an expansion valve; and passing refrigerant received from the expansion valve through the refrigerant heat exchanger before supplying the refrigerant to the compressor via the heating heat exchanger.

[0020] The method according to any variant of the first through fifth aspects may further comprise determining a need to defrost the air heat exchanger based on a detected change in an electrical characteristic or behaviour of a motor of a fan of the heat pump, optionally an increase in current drawn by the motor.

[0021] The method according to any variant of the first through fifth aspects may further comprise determining successful defrosting of the air heat exchanger based on a detected change in an electrical characteristic or behaviour of a motor of a fan of the heat pump, optionally a decrease in current drawn by the fan motor.

[0022] The method according to any variant of the first through fifth aspects may further comprise storing one or more reference values for power (or alternatively current) drawn by the motor, optionally acquired during installation of the heat pump. According to a sixth aspect there is provided an air source heat pump comprising an air heat exchanger; a heating system heat exchanger; a compressor; and a refrigerant reservoir for liquid refrigerant; the heat pump having a defrost mode in which refrigerant is supplied from the compressor to the air heat exchanger, thence to the refrigerant reservoir, and then to the heating system heat exchanger before returning to the compressor; the heat pump further comprising a refrigerant heat exchanger and an expansion valve, the refrigerant heat exchanger being located within the refrigerant reservoir; wherein in the defrost mode the expansion valve supplies the refrigerant heat exchanger with refrigerant from the reservoir, the arrangement being such that refrigerant is first expanded by passage through the expansion valve and then heated by the liquid refrigerant within the refrigerant reservoir before being supplied to the heating system heat exchanger.

[0023] According to a seventh aspect there is provided an air source heat pump including a compressor for compressing refrigerant, a refrigerant air heat exchanger for extracting energy from ambient air during a heating mode of operation, and a reservoir for liquid refrigerant through which, during defrosting of the refrigerant air heat exchanger, refrigerant flows from an input, at which refrigerant is received from the air heat exchanger, to an output and thence via an expansion valve through a refrigerant heat exchanger, the refrigerant heat exchanger being at least partially submerged in liquid refrigerant within the reservoir, to the compressor.

[0024] According to a eighth aspect there is provided an air source heat pump including a compressor for compressing refrigerant, a refrigerant air heat exchanger for extracting energy from ambient air during a heating mode of operation, and a reservoir for liquid refrigerant through which, during defrosting of the refrigerant air heat exchanger, refrigerant flows from an input at which refrigerant is received from the air heat exchanger, to an output and thence via an expansion valve through a refrigerant heat exchanger to the compressor via a heating system heat exchanger, the refrigerant heat exchanger being at least partially submerged in liquid refrigerant within the reservoir.

[0025] According to a nineth aspect there is provided an air source heat pump including a fluid reservoir to hold liquid refrigerant, the fluid reservoir including a heat exchanger that in use is at least partially submerged in the liquid refrigerant, an expansion valve being provided on what in the cooling mode of operation is the input side of the heat exchanger.

[0026] The air source heat pump according to any variant of the fifth through eighth aspects may further comprise a processor which is configured to determine a need to defrost the air heat exchanger based on current drawn by a motor of a fan of the heat pump. Optionally the processor is further configured to determine successful defrosting of the air heat exchanger based on a detected decrease in current drawn by the motor of the fan of the heat pump. Optionally the processor is configured to determine and store, when the heat pump is commissioned, the current drawn by the motor of the fan.

[0027] The air source heat pump according to any variant of the fifth through eighth aspects may further comprise a processor that is operatively connected to a camera and / or another optical sensor to detect the build-up of ice on the air heat exchanger matrix.

[0028] In an air source heat pump according to any variant of the fifth through eighth aspects the air heat exchanger may include in a lower portion a liquid sub-cooler which is configured, in a heating mode of operation, to receive refrigerant and to supply the refrigerant to an expansion valve through which the refrigerant is arranged to pass before passing through an upper portion of the air heat exchanger. Optionally such a heat pump may include a fan and the liquid subcooler may be so arranged that it receives little or no air flow occasioned by the fan thereby reducing the likelihood of ice build-up on the lower portion of the air heat exchanger.

[0029] Brief description of the Figures

[0030] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying Figures, in which:

[0031] Figure 1 shows schematically an air source heat pump operating in heating mode;

[0032] Figure 2 shows schematically the air source heat pump of Figure 1 operating in defrost mode; Figure 3 shows schematically an air source heat pump according to an aspect of the invention operating in heating mode;

[0033] Figure 4 shows schematically the air source heat pump of Figure 3 operating in defrost mode; and

[0034] Figure 5 illustrates schematically a heat pump according to an aspect of the invention.

[0035] Specific description

[0036] Figure 1 shows schematically an air source heat pump 10 operating in heating mode. The main mechanical components of the heat pump are shown, but for the sake of clarity the electrical, sensing, and control elements and connections, including the controller or processor of the heat pump, have been omitted. Starting in the bottom right-hand corner of the Figure, cold liquid refrigerant is forced from a liquid receiver 12, under the action of the compressor 32, along a conduit 14, which passes through the lower portion 23 of an air heat exchanger 20, the lower portion 23 functioning as a sub-cooler arrangement during heating mode, to a heating expansion valve 16. The upper portion of the air heat exchanger 20, labelled as 21, serves as a refrigerant evaporator in the heating mode.

[0037] The conduit 14 has an open end 15 which in use is submerged within liquid refrigerant - so that refrigerant liquid rather than refrigerant vapour is supplied to the conduit 14 from the reservoir 12. During the heating mode, passing the warm liquid refrigerant from the liquid receiver or reservoir 12 through the sub-cooler arrangement 23 provides a degree of refrigerant subcooling to ensure that the refrigerant is fully condensed prior to being expanded through the heating expansion valve 16. A small amount of subcooling is beneficial to the operation of the refrigerant cycle. An excessive amount of subcooling is detrimental to efficiency, as the evaporator 21 has to provide significant heat input to the refrigerant before the process of evaporating the refrigerant can begin.

[0038] In the heating expansion valve 16 the refrigerant passes through a small opening, resulting in a reduction in pressure which in turn reduces the boiling point of the refrigerant. Depending on the temperature of the refrigerant at the expansion valve inlet and the pressure of the refrigerant at the expansion valve outlet, a portion of the refrigerant may spontaneously vaporise, resulting in a reduction in temperature of the refrigerant, such that the refrigerant leaves the expansion valve as a mixture of liquid and vapour. From the expansion valve 16 the refrigerant passes into a distributor 18 in which a single input conduit is coupled to a plurality of output conduits each of which supplies a conduit of the evaporator 21.

[0039] A one-way valve 50 is provided in parallel with the heating expansion valve 16, but this is oriented to permit flow from the distributor 18 to the conduit 14 (the direction in which refrigerant flows during the cooling mode of operation) and to prevent flow of refrigerant from the conduit 14 to the distributor - so that in heating mode the refrigerant must pass through the heating expansion valve 16 to reach the distributor 18. As the skilled person will understand, the heating expansion valve 16 (like other expansion valves in the heat pump) is controlled by the heat pump’s processor to achieve a target value of refrigerant superheat as appropriate to the prevailing operating conditions.

[0040] A filter-drier 51 may be provided in the flow path to the heating expansion valve 16, and if present this too is bypassed by flow through the one-way valve 50 during cooling and defrosting.

[0041] In one variant, the distributor 18 may include three flow paths - a single input conduit 17 with a small internal fixed orifice (not shown), a plurality of output conduits (indicated with reference numeral 19) and an internal bypass conduit. In heating mode, the refrigerant flows through the heating expansion valve 16 to the single input conduit 17 of the distributor 18, through the internal fixed orifice and out of the plurality of output conduits 19. In heating mode there is no flow via the bypass conduit. Expansion of the refrigerant is achieved through the heating expansion valve and fixed orifice. In cooling mode, the refrigerant is able to flow from the plurality of output conduits 19 and out of the internal bypass conduit and through the oneway valve 50, bypassing the internal fixed orifice and heating expansion valve 16 such that the heating expansion valve 16 and small internal fixed orifice of the distributor 18 have negligible effect in cooling mode. In another variant, the distributor may be formed without the small internal fixed orifice and the bypass conduit.

[0042] The air heat exchanger 20 is shown schematically as comprising just four conduits traversing the air heat exchanger, but in reality, many more such conduits are provided, and each is typically provided with thermally conductive fins to improve heat transfer from the air to the refrigerant inside the conduits.

[0043] The heat pump includes a motor driven fan 24 which causes ambient air to move over the evaporator 21.

[0044] In heating mode, refrigerant inside the air heat exchanger 20 is warmed by energy absorbed from the ambient air and this causes it to change state into vapour, and the vapour may itself be further heated by absorbing energy from the air so that the vapour may be superheated. The vapourised refrigerant leaves the air heat exchanger 20 via a collector (or manifold) 26 which feeds a return conduit 28 which supplies a first port, port 1, of a 4-way valve 30. The vapourised refrigerant exits the 4-way valve 30 via port 2 from where it flows to a compressor 32. In the compressor the warmed vapourised refrigerant, which is received at low pressure, is compressed to become high pressure vapour, and in the process of being compressed the refrigerant further increases in temperature. From the compressor 32 the refrigerant passes back through the 4-way valve 30, entering via port 3 and exiting via port 4 which feeds a heat exchanger matrix, not shown, in heating system heat exchanger 34. The heating system heat exchanger 34 may be, as shown here, a heating system heat exchanger in which energy from the hot refrigerant is transferred to water (or some other suitable operating fluid in the case of space heating), e.g., for space heating via a central heating system, or to provide hot water for domestic or industrial use. Hot refrigerant enters the heat exchanger 34 at port 36, condenses and then exits, as cooled refrigerant at port 38. Water for heating enters the heat exchanger 34 at port 40, on the cooler side of the heat exchanger 34, and exits at port 42 on the hotter side of the heat exchanger 34.

[0045] Within the heating system heat exchanger 34 the refrigerant condenses from a vapour to a liquid, giving up heat to water in the water circuit of the heat exchanger 34. From port 38 the cooled liquid refrigerant passes to a one-way valve 54 that is oriented to permit flow from the heating system heat exchanger 34 to the liquid reservoir 12. This one way valve 54 therefore serves as a bypass for another expansion valve 44 which is provided to expand refrigerant as it flows from the liquid reservoir 12 towards the heating system heat exchanger 34 in the cooling mode of operation (and hence also during defrosting of the air heat exchanger). In heating mode, the refrigerant is able to flow through the one-way valve, 54 bypassing the cooling expansion valve 44 with minimal restriction such that the cooling expansion valve 44 has negligible effect in heating mode. The liquid refrigerant then passes back into the liquid reservoir 12 at which we started the process. It will be appreciated that the heating system heat exchanger 34 may, instead of using the refrigerant to heat water, use the refrigerant to heat air - for example for warm air space heating, but the principal remains the same.

[0046] Figure 1 also includes a further heat exchanger 46 which is fed from the refrigerant conduit 14 via a further expansion valve 48. This further heat exchanger 46 is optional but is preferably provided as a means of cooling (and controlling the temperature of) an electrical inverter (not shown) that powers the compressor and various other of the heat pump’s electrical components and systems. The further heat exchanger 46 may include a heat sink that is thermally coupled to the inverter and optionally other of the heat pump’s electronics and power components. The cooling provided by the further heat exchanger 46 helps to ensure correct operation of the inverter and other electronics, extending their lifetimes, and also captures what would otherwise be waste heat and feeds this into refrigerant which is then fed into the compressor 32 - thus further contributing to the efficiency of the heat pump. The function of, and flow direction through, the inverter cooling expansion valve and inverter cooler heat exchanger is independent of and regardless of operating mode (i.e., heating, cooling, defrost). The inverter cooling expansion valve is controlled by the processor of the heat pump to regulate the refrigerant conditions through the inverter cooler heat exchanger to achieve a desired cooling effect, e.g. a target value of inverter drive temperature.

[0047] If the heat pump of Figure 1 is run in a temperate climate on a cool day (e.g. 1 to 10 C) water vapour in the ambient air may condense on the air heat exchanger 20, and then turn to ice. The build-up of ice on the air heat exchanger 20 both restricts the flow of air through the air heat exchanger and also reduces the efficiency of energy transfer from the ambient air to the refrigerant. The heat pump will typically include either a mechanism to detect the build-up of ice, or to detect climatic conditions when ice build-up is to be expected, or have some other mechanism to ensure timely defrosting, and have a processor configured (e.g. programmed) to cause the heat pump to enter a defrost mode as appropriate. Throughout this specification, references to the processor are references to a processor or controller (e.g. a microcontroller or microprocessor) configured or programmed to perform the relevant functions and operations. The processor or controller will typically include, and or be coupled to, a memory arrangement that stores relevant code, instructions, programs, algorithms, and data to enable the processor to perform the relevant operations, functions, methods, and procedures. Although not shown in Figure 1, a drip tray is preferably provided beneath the air heat exchanger 20 to collect condensate when the heat pump is running in defrost or heating mode, for the reasons given above.

[0048] At the inlet of the compressor 32 sensors 31 may be provided to measure the temperature and pressure of the refrigerant. Based on values obtained from these sensors 31 the processor or controller of the heat pump is able to determine the state of the refrigerant, and in particular the degree of superheating, at or just upstream of the compressor inlet.

[0049] Figure 2 corresponds to Figure 1, but shows the heat pump operating in a cooling mode, which is also used as the defrost mode, rather than in a heating mode. In the cooling mode the flow of refrigerant is reversed compared to that of the heating mode, and this reversal is achieved by controlling the 4-way valve 30 so that the output from the compressor 32, which is received on port 3, flows to port 1 rather than port 4. So instead of refrigerant heated by the compressor 32 passing next to the heat exchanger 34, it passes to the collector (or manifold) 26 and thence to the air heat exchanger 20. By supplying hot refrigerant to the air heat exchanger, ice build-up can be thawed. Cooled refrigerant leaves the air heat exchanger 20 via the distributor 18 and then passes via one-way bypass valve 50 back to the liquid receiver 12.

[0050] From the liquid receiver 12 the expanded refrigerant flows out along conduit 52. The conduit 52 has an open end 53 which in use is submerged within liquid refrigerant - so that refrigerant liquid rather than refrigerant vapour is supplied to conduit 52 from the reservoir 12. Refrigerant from the reservoir 12 flows along conduit 52 through the cooling expansion valve 44, and thence to port 38 of the heating system heat exchanger 34. In cooling mode, there is no flow via the bypass conduit due to the position and orientation of the one-way valve 54. Expansion of the refrigerant is achieved through the cooling expansion valve 44. In the expansion valve the refrigerant passes through a small opening, resulting in a reduction in pressure which in turn reduces the boiling point of the refrigerant. Depending on the temperature of the refrigerant at the expansion valve inlet and the pressure of the refrigerant at the expansion valve outlet, a portion of the refrigerant may spontaneously vaporise, resulting in a reduction in temperature of the refrigerant, (such that the refrigerant leaves the expansion valve as a mixture of liquid and vapour). The expansion vale may be controlled by the heat pump’s processor based on data from the pressure and temperature sensors 31 at the input side of the compressor 32.

[0051] Flowing through the heating system heat exchanger 34 the expanded refrigerant absorbs heat from the hot water in the heat exchanger (water flowing, in use, from port 40 to port 42) and is vaporised. Warmed vaporised refrigerant leaves the heat exchanger by port 36 and passes to port 4 of the 4-way valve 30 and exits via port 2. From port 2 of the 4-way valve 30 the refrigerant vapour passes to the compressor 32. It is important that the refrigerant arrives at the compressor in the form of a vapour, because the compressor can be damaged if it receives liquid refrigerant. The compressor compresses the refrigerant, further increasing its temperature, and supplies the refrigerant to port 3 of the 4-way switch 30. The refrigerant leaves the 4-way switch via port 1. Because the input side of the compressor 32 is always at a lower pressure when the compressor is running, in cooling mode refrigerant continues to flow through the heat exchanger 46 of the inverter towards the compressor 32 as it did in heating mode, under the control of the processor of the heat pump.

[0052] The purpose of the distributor 18 is to distribute the refrigerant as evenly as possible into the multiple parallel circuits of the evaporator 21 when operating in heating mode. During reverse operation, in defrost mode, the hot refrigerant gas is instead fed into the collector 26, on what is usually the downstream side of the evaporator 21, as opposed to being fed in to the distributor 18 as in normal ‘forward’ operation in heating mode. In reverse operation, the collector 26 now functions to distribute the hot refrigerant gas into the multiple parallel circuits of the evaporator 21. Because of its geometry, the collector 26 typically achieves relatively poor distribution - the collector 26 is generally just a simple, large bore pipe running the length of the end of the evaporator 21 into which the parallel circuits connect.

[0053] Due to the relatively poor distribution performance of the collector 26 and the natural buoyancy effect of the hot refrigerant gas, an imbalance in defrosting performance is typically witnessed, whereby the upper circuits of the evaporator assembly defrost more quickly than the lower circuits. As a result, melted condensate from the upper portions of the evaporator 21 may run down over ice on the lower portions of the evaporator 21 which has yet to melt, and may refreeze. This gives rise to a risk of the melted condensate refreezing condensate blocking or partially blocking the drip tray beneath the evaporator 21 thereby preventing further melted condensate running into, along and draining from the drip tray as intended - and hence the risk that melted condensate will cause problems to the reliable operation of the heat pump and / or pose a health and safety risk in the vicinity of the heat pump.

[0054] It is proposed that the outdoor heat exchanger 20 be configured to include the sub-cooler 23 as a sub-cooler section at the base of the heat exchanger 20 with an evaporator section 21 above. The sub-cooler section 23 preferably comprising one or more tubes or circuits of tubes at the base of the outdoor heat exchanger 20 which are separate from the circuits of tubes of the evaporator section 21. The connection pipework of the outdoor heat exchanger assembly is then configured such that condensed liquid refrigerant from the condenser or liquid receiver 12 is routed through the sub-cooler 23 at the base of the outdoor heat exchanger 20 assembly prior to entering the heating expansion valve 16 where it is expanded and subsequently distributed via the distributor 18 to the circuits of tubes of the evaporator section 21. The sub-cooler and drip tray may be so configured that the sub-cooler is effective to prevent freezing of condensate in the drip tray.

[0055] The heat pump may be so configured that the sub-cooler section is shielded from air flow occasioned by operation of the fan, for example by being positioned behind or in front of a flange or baffle, optionally provided as part of or associated with the drip tray located beneath the air heat exchanger.

[0056] Turning now to Figure 3, this corresponds generally to Figure 1, again showing the main components of an air source heat pump 310 operating in heating mode, but in this case the heat pump 310 is according to an aspect of the invention and includes a modified refrigerant reservoir arrangement 300 according to an aspect of the invention. The modification comprises the inclusion of a heat exchanger 301 within the reservoir 312 in such a position that the heat exchanger 301 is at least largely (but preferably wholly) submerged within liquid refrigerant within the reservoir 312 at least when the heat pump is in use, together with a modified refrigerant flow circuit. The new heat exchanger 301 is provided for use during the defrosting of the air heat exchanger 320 as will be explained with reference to Figure 4. But first we will describe the flow of refrigerant during the heating phase, with reference to Figure 3.

[0057] As with the arrangement illustrated in Figures 1 and 2, the refrigerant liquid reservoir is provided between the output 338 of the heating system heat exchanger 334 and the heating expansion valve 316. In Figure 3 we see that refrigerant coming from the heating system heat exchanger 334 first encounters a one-way valve (or check valve) 354 which permits refrigerant to flow into conduit 352 and then into the liquid reservoir 312. Downstream of the one-way valve 354 is an expansion valve 344 which is used in the cooling mode but not in the heating mode. A heat exchanger coil 301 is within the lower part of the liquid reservoir 312, where in use the coil 301 will be submerged (or largely submerged) in liquid refrigerant, but in the heating mode there is no refrigerant flow through this coil 301.

[0058] The operation of the heat pump of Figure 3 in heating mode is very similar to the operation of the heat pump of Figure 1 in heating mode. Starting in the bottom right-hand corner of the Figure, cold liquid refrigerant is forced from the liquid receiver 312, under the action of the compressor, along the conduit 314, through the lower portion of the air heat exchanger to a heating expansion valve 316 (optionally passing through a filter-drier shown as 351). As with the reservoir shown in Figures 1 and 2, the conduit 314 has an open end 315 which in use is submerged within liquid refrigerant - so that refrigerant liquid rather than refrigerant vapour is supplied to conduit 314 from the reservoir 312.

[0059] In the heating expansion valve 316 the refrigerant passes through a small opening, resulting in a reduction in pressure which in turn reduces the boiling point of the refrigerant. Depending on the temperature of the refrigerant at the expansion valve inlet and the pressure of the refrigerant at the expansion valve outlet, a portion of the refrigerant may spontaneously vaporise, resulting in a reduction in temperature of the refrigerant, such that the refrigerant leaves the expansion valve as a mixture of liquid and vapour. From the expansion valve 316 the refrigerant passes into a distributor 318 which is as described with reference to Figure 1. in which a single input conduit is coupled to a plurality of output conduits each of which supplies a conduit of the air heat exchanger 320. The air heat exchanger 320 is again shown schematically as comprising just four conduits traversing the air heat exchanger, but in reality, many more such conduits may be provided, and each is typically provided with thermally conductive fins to improve heat transfer from the air to the refrigerant inside the conduits. The air heat exchanger 320 is associated with a fan 324 which causes ambient air to pass over the matrix of the air heat exchanger 320.

[0060] Refrigerant inside the air heat exchanger 320 is warmed by energy absorbed from the ambient air and this causes it to change state into vapour, and the vapour may itself be further heated by absorbing energy from the air so that the vapour may be superheated. The vapourised refrigerant leaves the air heat exchanger 320 via a manifold 326 which feeds a return conduit 328 which supplies a first port, port 1, of a 4-way valve 330. The vapourised refrigerant exits the 4- way valve 330 via port 2 from where it flows to the compressor 332. In the compressor 332 the warmed vapourised refrigerant, which is received at low pressure, is converted into high pressure vapour and in the process of being compressed the refrigerant further increases in temperature - becoming superheated or more superheated. From the compressor 332 the refrigerant passes back through the 4-way valve 330, entering via port 3 and exiting via port 4 which feeds the heating system heat exchanger 334. Hot refrigerant enters the heat exchanger 334 at port 336 and exits, as cooled refrigerant at port 338. Water for heating (e.g. for space heating or for use in a hot water supply system) enters the heat exchanger 334 at port 340, on the cooler side of the heat exchanger 334, and exits at port 342 on the hotter side of the heat exchanger 334. Within the heating system heat exchanger 334 the refrigerant condenses from a vapour to a liquid, giving up heat to water in the water circuit of the heat exchanger 334. From port 338 the cooled liquid refrigerant passes to the one-way valve 354 and into the liquid reservoir 312 via conduit 352 and entry port 357.

[0061] As with Figure 1, Figure 3 also includes a further heat exchanger 346 which is fed from the refrigerant conduit 314 via a further expansion valve 348. This further heat exchanger 346 is optional but is preferably provided as a means of cooling (and controlling the temperature of) an electrical inverter (not shown) that powers the compressor and various other of the heat pump’s electrical components and systems. The further heat exchanger 346 may include a heat sink that is thermally coupled to the inverter and optionally other of the heat pump’s electronics and power components. The cooling provided by the further heat exchanger 346 helps to ensure correct operation of the inverter and other electronics, extending their lifetimes, and also captures what would otherwise be waste heat and feeds this into refrigerant which is then fed into the compressor 332.

[0062] The heat exchanger assembly 320 preferably also includes a liquid sub-cooler 23 which receives refrigerant from conduit 314 and supplies it to heating expansion valve 316. The purpose of the liquid sub-cooler 323 is to ensure that the refrigerant arrives at the heating expansion valve as liquid cooled to below the liquid’s boiling point and additionally is preferably so arranged that it receives little or no fan-induced air flow such that the lower portion of the air heat exchanger may generally be maintained at a temperature above freezing thus preventing ice build-up on the lower portion of the air heat exchanger. Preferably the sub-cooler 323 is at least partly contained in a drip tray that is arranged to catch condensation (and water from melted ice) that falls from the air heat exchanger, to reduce the likelihood that condensate in the drip tray will freeze. Operation of the fan leads to a pressure difference across the air heat exchanger, and the pressure difference gives rise to air flow over the air heat exchanger. The sub-cooler may be shielded from this air flow by means of an upstanding flange on either face (front and back) of the heat exchanger, or the sub-cooler may be received in a groove or trough. Only minimal air flow is required across the sub-cooler for sub-cooling to be effective, so that the sub-cooler may be significantly shielded from the air flow occasioned by the fan.

[0063] We will now consider the operation of the heat pump of Figure 3 when operated in the cooling or defrost mode, as illustrated in Figure 4. In defrost mode the flow of refrigerant through the air heat exchanger is reversed, and energy is supplied to the air heat exchanger to melt ice that has built up on the air heat exchanger. This reversal of flow can be achieved by changing the setting of the 4-port valve, e.g. under the control of a controller of the heat pump, so that refrigerant arriving at port 3, from the compressor 332, exits from port 1 towards the air heat exchanger 320 rather than through port 4 towards the heating system heat exchanger 334. Because of this change of flow path, refrigerant arrives at the air heat exchanger as warm rather than cold refrigerant, and the warm refrigerant gives up heat to the air heat exchanger matrix, and hence emerges from the air heat exchanger cooler than it entered. The refrigerant passes through the one way valve 350, bypassing both the previously mentioned small orifice of the distributor and the expansion valve 316, and then passes along conduit 314 into the liquid reservoir 312 through orifice 315 and what in the heating mode was exit port 355. Under pressure from the compressor 332 refrigerant is forced out of the liquid reservoir 312 via submerged outlet 353 and then through what in the heating mode was entry port 357 along conduit 352. Again, as with the reservoir shown in Figures 1 and 2, the open end 353 of the conduit 352 is in use submerged within liquid refrigerant - so that refrigerant liquid rather than refrigerant vapour is supplied to conduit 352 from the reservoir 312. Refrigerant passes along conduit 352 and then through expansion valve 344 - because the one-way valve 354 is configured to block the passage of refrigerant arriving from the reservoir 312 on conduit 352. In the expansion valve 344 the refrigerant passes through a small opening, resulting in a reduction in pressure which in turn reduces the boiling point of the refrigerant. Depending on the temperature of the refrigerant at the expansion valve inlet and the pressure of the refrigerant at the expansion valve outlet, a portion of the refrigerant may spontaneously vaporise, resulting in a reduction in temperature of the refrigerant, (such that the refrigerant leaves the expansion valve as a mixture of liquid and vapour). After passing through the expansion valve 344, the refrigerant passes through the heat exchanger 301 where, if the refrigerant arriving from the expansion valve 344 is cooler than the refrigerant in the reservoir, it absorbs heat from the refrigerant surrounding the heat exchanger 301. At the start of a defrost cycle it is likely that there is thermal energy to be recovered from the reservoir, but as the defrost cycle continues it is likely that the excess energy will have been extracted.

[0064] Refrigerant leaving the heat exchanger 301 passes through optional one-way valve 360 and then enters the heating system heat exchanger at port 338. At least at the start of a defrost cycle the refrigerant is likely to arrive at port 338 hotter than it would have done in the absence of the heat exchanger arrangement 301. Consequently the temperature difference between the refrigerant and the fluid in the heating heat exchanger will have been reduced, thereby reducing the amount of energy absorbed by the refrigerant from the fluid in the heating heat exchanger. The processor of the heat pump will generally be programmed to control the expansion valve 344 to achieve a certain degree or range of superheat at the input of the compressor (the degree of superheat being determined based on pressure and temperature readings from the sensors 331 at the input side of the compressor.

[0065] The one-way valve 360 is optional because its function can be achieved by fully closing expansion valve 344 when operating in heating mode so that there is no refrigerant flow through the heat exchanger within the liquid receiver.

[0066] The heat exchanger 301 may be in the form of a coil (e.g., formed of copper or aluminium tubing) or another configuration that facilitates efficient heat transfer from the refrigerant liquid within the liquid reservoir 312 to the refrigerant coming from the expansion valve 344.

[0067] As previously noted, by taking energy for defrosting from the refrigerant reservoir we can reduce the amount of energy for defrosting which is extracted from the heating circuit, reducing the adverse effect of defrosting on the heating performance of the system. In retrofit scenarios (e.g. where a heat pump is being installed in place of a gas boiler) there may be a limitation on flow rate within the heating system, and the method allows for improved control of the temperature of refrigerant flowing to the heating system heat exchanger to reduce the risk of freezing of the water within the heating system heat exchanger in heating systems using water as the heat transfer medium, which is a very significant step forward.

[0068] Although embodiments have been shown and described as using a 4-way valve, it will be appreciated that the relevant functionality can be achieved by using several simpler valves, appropriately arranged and controlled by the heat pump controller. For example, using the valve port numbers given above, a 2-way valve could be arranged to switch port 3 to either port 1 or port 4, a first on / off valve could be arranged to connect port 4 to port 2, and a second on-off valve could be arranged to connect port 1 to port 2. Other arrangements of valves, suitably controlled by the heat pump controller could also be used.

[0069] Figure 5 illustrates schematically a heat pump 510 according to an aspect of the invention, and in particular an arrangement of elements to support the heat pump’s processor 500 in deciding that a defrost cycle is needed, and also in determining for how long to perform a defrost cycle. The heat pump has a housing 511 within which are located the air heat exchanger matrix 20 (of which only the evaporator part 21 can be seen), one or more fans 524 with an associated drive motor 504, the processor 500, and (optionally) the heating system heat exchanger 534 - although this may be mounted remotely, for example in place of a gas boiler that has been replaced by the heat pump. The processor 500, which has an associated memory 502 (storing software instructions to control operation of the processor, etc.), is operatively connected to a valve arrangement 530 for use in controlling the direction of flow of refrigerant within the heat pump, and in particular to enable the direction of flow through the air heat exchanger matrix to be reversed between a heating mode of operation and a defrosting mode of operation. As previously noted the valve arrangement 530 may comprise a 4-way valve or some suitable combination of simpler valves 530A, 530B, and 530C. The processor 500 is also operatively connected to the motor 504 that drives the fan 524 that rotates to drive (pull or push) ambient air over the air heat exchanger matrix 522, and may be configured both to control the fan motor and to receive information from appropriate circuitry on the motor’s power consumption, for example based on the current drawn by the motor 504.

[0070] Figure 5 shows the evaporator part 21 of the air heat exchanger 20, with the lower part of the air heat exchanger which comprises the sub-cooler arrangement 23 at least partly concealed within the drip tray structure 590. The drip tray structure may for example comprise a pair of upstanding flanges one for each of the front and back of the heat exchanger. Alternatively, a groove may be formed, for example in a floor of the housing 511, to receive and at least partially mask the sub-cooler at the lower end of the heat exchanger.

[0071] The heat exchanger 20 is preferably arranged with its fins extending generally vertically so that condensate can easily run down the fins past the sub-cooler to the drip tray.

[0072] Figure 5 also shows the air heat exchanger 20 located upstream of the fan 524 and its motor 504, with the fan arranged to draw air through the air heat exchanger 20, which is the preferred configuration, the fan may alternatively be arranged to push air through the air heat exchanger 20.

[0073] The fan operates to produce a difference in air pressure across the air heat exchanger 20. The air heat exchanger 20 preferably extends vertically or substantially vertically within a housing of the heat pump, For example, fins of the air heat exchanger 20 may be disposed vertically or substantially vertically with pipework of the air heat exchanger 20 arranged generally transverse to the fins.

[0074] The processor 500 may also be operatively connected to a temperature and humidity sensing arrangement 506 that is configured to sense the ambient temperature and humidity at the site of the air heat exchanger 520, as well as being connected to sensors 31 at the input side of the compressor (as previously explained). The processor 500 and the air heat exchanger matrix may be contained in a unitary structure, or they may be housed in separate structures / housings, for example with the processor positioned within a housing within the premises heated by the heat pump, and the air heat exchanger 520 in a separate housing outside the premises. In the latter case, the housing for the air heat exchanger 520 may also contain a processor or controller (e.g. a microcontroller) that is controlled by or works in tandem with the processor within the premises. Also in the latter case, the processor within the premises may communicate with the components and systems of, or associated with, the external housing by wireline or wirelessly (for example using Matter, Bluetooth, Zigbee, Wi-Fi, 5G. etc.). The processor 500 may be operatively connected to a camera (e.g. a video camera) and / or another optical sensor 508 to detect the build-up of ice on the air heat exchanger matrix 522, optionally using image processing in the event that a camera is used, and also optionally using machine learning to detect the build-up of ice, and the need or otherwise for defrosting, based on analysis of signals / images. The processor 500 may also be operatively connected to the Internet 550 and hence to a source 560 of (local or hyper local) weather forecasts and also to a system back-end 512 managed by the manufacturer / supplier or installer of the heat pump. Using data from the source 560 of weather forecasts, the processor 500 may be configured to prepare for adverse weather situations, such as those in which the build-up of frost or ice on the air heat exchanger is likely to be quicker and or more severe than usual, for example by limiting (reducing) the maximum output of the heat pump (i.e. reducing the amount of heat extracted from the air, thereby reducing ice build-up - for example rather than extracting 16kW, the controller of the heat pump may be configured to reduce the amount of energy extracted to 10-12kW - e.g. a 25- 50% reduction in energy extraction), or reducing the superheat setting so that refrigerant is supplied to the air heat exchanger at a higher temperature to reduce ice build-up. In this way the heat pump can prepare for forthcoming adverse weather conditions and possibly enable a reduction in the frequency of defrost events. Alternatively, particularly adverse weather may lead to the heat pump increasing the frequency of defrost cycles. The system back end 512 may also be configured to supply advance notification of weather events, for example based on reports from other heat pumps installed in the same neighbourhood or installed in locations experiencing the arrival of a weather front that is on its way to the location of the heat pump in question. The system back end 512 may also of course provide any necessary firmware upgrades, respond to “incident alerts’ for example by detecting system or component failure, or responding to reports of system or component failure, by for example arranging for the despatch of an engineer or service agent together with any necessary replacement parts or service items.

[0075] The motor 504 may be an electronically commutated motor (EC motor) also known as brushless DC electric motor (BLDC motor) using a direct current (DC) electric power supply. It may use an electronic controller (typically situated within the fan motor housing) to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller may adjust the phase and amplitude of the DC current pulses, and / or use some form of pulse width modulation and / or voltage control, to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors. The fan motor controller is itself controlled by the heat pump controller which provides a fan speed demand signal based on the heat pump's operating condition. There are many means of communication between the fan controller and the heat pump controller (analogue, PWM, MODBUS, etc.) of which we prefer to use MODBUS as it allows 2-way communication such that information from the fan controller, such as fan motor current, power, voltage, pulse amplitude and / or duration and / or frequency, can be taken from the fan controller and used by the heat pump controller.

[0076] The processor 500 may be programmed or configured to detect blockages such as a build-up of frost on the air heat exchanger based on a characteristic change in a power consumption characteristic of a fan motor, for example the power consumption characteristic may be the amount of current consumed or a rate of change of current consumption. An increase in the power consumed by the fan motor, for example detected based on an increased current consumption (or an increase in the effective duty cycle in a pulse width modulation drive arrangement) may indicate that the fan motor is having to work harder to drive air through the air heat exchanger possibly as a result of the air heat exchanger being partially or wholly blocked. A detected increase in power consumption may also be correlated with certain environmental factors - typically temperature and humidity (e.g. relative humidity), and the rate of increase may have significance. Thus the processor may usefully be configured to apply machine learning based on these factors in order to determine whether a detected increase in power consumption is likely to be in consequence of a build-up of ice or snow on the air heat exchanger, or possibly some other cause - such as a build-up of leaves or other debris at the air inlet of the housing containing the air heat exchanger. In the latter case, the processor may be configured to generate an alert for the owner / occupier of the premises (and also sent to the system back-end 512) of the need to clean the air inlet - the alert optionally being sent by the system back-end 512, for example as a push notification to a user device or even as an automated phone call or email or letter. The processor 500 may be configured to detect and record the power consumption of the fan motor 504 at the time of initial installation, and use this as a datum against which later power consumption figures may be compared, or may be pre-programmed before installation - for example based on machine learning or empirical testing performed during development of the heat pump. Continued monitoring (on a continuous or periodic basis) of fan power consumption characteristic may also reveal the need for maintenance (e.g. the clearing of air inlet(s), the removal of pest infestation - such as a wasp or bee nest, or the need to replace a damaged fan or fan motor.

[0077] Similarly, the processor may be able to detect successful removal of a blockage such as the defrosting of the air heat exchanger based on a characteristic change in a power consumption characteristic of a fan motor - e.g. a reduction in current consumption or a reduction in the effective duty cycle in a PWM drive arrangement. It should be appreciated however that the effective unblocking of the heat exchanger may actually lead to an increase in power consumption - for example if the fan has in effect been starved of a supply of air. For this reason it is important to observe the behaviour of the fan motor under varied test conditions so that the processor can be suitably programmed to best detect the condition of the air heat exchanger based on motor drive characteristics and hence optimise control.

[0078] If the fan motor has an associated fan motor controller, the processor 300 may receive information on fan motor behaviour from the fan motor controller, rather than directly detecting fan motor behaviour. The fan motor controller may be configured (e.g. programmed) to update, periodically or continuously, the processor 300 with information on fan motor power consumption and / or other performance characteristics. The fan motor controller may optionally report to the processor 300 based on consideration of the operating behaviour of the fan motor and all (or a sub-set of) the motor drive factors.

[0079] The controller of the fan motor (which may or may not be an electronically commutated motor) may be programmed or configured to control the rotational speed of the fan motor, for example to hold a certain speed (X rpm) or to hold a speed within a certain range (Y to Z rpm). and may additionally or alternatively be programmed or configured to control the fan motor to achieve a certain rate of increase of fan speed or a rate of increase in fan speed that is within a certain range, for example when accelerating the fan from a stationary or zero rpm (or low rpm state) to a desired or target fan speed or range of fan speeds. The controller may be programmed or configured to achieve and hold certain fan speeds or ranges of speeds that differ according to different control states of the heat pump, for example as signalled by the processor of the heat pump, such as heat pump power setting and / or heat pump operation mode (heating, cooling, defrosting, for example). In order to control fan speed and / or rate of increase in fan speed, the controller may adjust one or more of multiple variables such as pulse width and / or effective duty cycle in a pulse width modulation drive arrangement, current and / or voltage supplied to the motor, the amplitude and / or phase of current pulses applied to the motor, among others. The fan motor controller may receive information on speed of rotation of the motor from one or more sensors, and / or may be able to determine speed of rotation, and hence rate of change of speed of rotation based on information derived from electrical characteristics / demands and / or behaviour of the motor. In the cases where there is no separate fan motor controller, a (the) processor of the heat pump may be programmed or configured to behave as just described with reference to the fan motor controller. Where a dedicated fan motor controller is provided, the fan motor controller may be programmed or configured to provide a (the) processor of the heat pump with information about speed of rotation, rate of increase in speed of rotation, and / or electrical characteristics / demands and / or behaviour of the motor. The supply of such information may be continuous or periodic, and may be automatic or may be upon request from the processor of the heat pump.

[0080] Additionally or alternatively, the processor 500 may be configured to detect the build-up of ice on the air heat exchanger based on a detected pressure difference across the air heat exchanger - that is between a pressure measured within the housing of the air heat exchanger (on the “upstream” side of the air heat exchanger as it were), e.g. by a first pressure sensor 570, and a pressure measured outside the housing e.g. by a second pressure sensor 572. The fan associated with an air heat exchanger may be provided upstream of the air heat exchanger, sucking air into the housing through one or more air inlets 580 remote from or adjacent to the air heat exchanger, the air being expelled from the housing over / through the air heat exchanger and out an exit 582 of the housing 511. Alternatively, the fan associated with an air heat exchanger may be provided downstream of the air heat exchanger, to suck air into the housing 511 through one or more air inlets and pulling the air through the air heat exchanger. In the latter case, the pressure differential technique may not always be a reliable means of detecting the build-up of ice - depending very much on the precise configuration chosen.

[0081] Under some circumstances it may be preferred to provide more than one motor driven fan in an air source heat pump. The preceding discussion has generally, for the sake of clarity and simplicity, focussed on the control of a single motor driven fan, but it will be appreciated that the principles of the previously described methods and systems apply equally to multi-motor set ups. While the pressure differential method of detecting blockage of the air heat exchanger is useable, we have found that detecting air heat exchanger blockage based on changes in the power consumption characteristic of the fan motor is generally more reliable and avoids the need to provide sensitive air pressure sensors (whose reliability is not always good).

Claims

Claims1. A method of defrosting an air heat exchanger of an air source heat pump, the method comprising: receiving in a refrigerant reservoir of the heat pump refrigerant from the air heat exchanger; passing refrigerant from the refrigerant reservoir through an expansion valve to expand the refrigerant before passing it through a refrigerant heat exchanger within liquid refrigerant within the reservoir; supplying refrigerant from the refrigerant heat exchanger to a heat exchanger of a heating system; and supplying refrigerant from the heat exchanger of the heating system to the compressor of the heat pump and thereafter supplying refrigerant from the compressor to the air heat exchanger.

2. A method of defrosting according to claim 1, further comprising . controlling the expansion valve to control the amount of energy extracted from the heat exchanger of the heating system and via the refrigerant heat exchanger from refrigerant in the reservoir3. A method of defrosting according to claim 1 or claim 2, further comprising determining a need to defrost the air heat exchanger based on a detected change in an electrical characteristic or electrical behaviour of a motor of a fan of the heat pump.

4. A method of defrosting according to claim 3, wherein the detected change in an electrical characteristic is an increase in current or power drawn by the motor.

5. A method of defrosting according to claim 3, wherein the detected change in an electrical characteristic is an increase in the effective duty cycle of a pulse width modulation drive to the motor.

6. A method of defrosting according to any one of the preceding claims, further comprising determining successful defrosting of the air heat exchanger based on a detected change in an electrical characteristic of a motor of a fan of the heat pump.

7. A method of defrosting according to claim 6, wherein successful defrosting of the air heat exchanger is detected based on a detected decrease in current drawn by the motor of the fan of the heat pump.

8. A method of defrosting according to claim 6, wherein successful defrosting of the air heat exchanger is detected based on a detected decrease in the effective duty cycle of a pulse width modulation drive to the motor of the fan of the heat pump.

9. A method of defrosting according to any one of claims 3 to 8, further comprising storing one or more reference values for current or power drawn by the motor, and / or one or more reference values for pulse width modulation duty cycle for the motor, optionally acquired during installation of the heat pump.

10. An air source heat pump including a fluid reservoir (312) to hold liquid refrigerant, the fluid reservoir (312) including a heat exchanger (301) that in use is at least partially submerged in the liquid refrigerant, an expansion valve (344) being provided in a refrigerant flow path on what in the cooling mode of operation is the input side of the heat exchanger (301).

11. An air source heat pump as claimed in claim 10, further comprising an air heat exchanger (320); a heating system heat exchanger (334); a compressor (332); and a refrigerant reservoir (312) for liquid refrigerant; the heat pump having a defrost mode in which refrigerant is supplied from the compressor (332) to the air heat exchanger (320), thence to the refrigerant reservoir (312), and then to the heating system heat exchanger (334) before returning to the compressor (332); the heat pump further comprising a refrigerant heat exchanger (301) and an expansion valve (344), the refrigerant heat exchanger (301) being located within the refrigerant reservoir (312); wherein in the defrost mode the expansion valve (344) supplies the refrigerant heat exchanger (301) with refrigerant from the reservoir (312), the arrangement being such that refrigerant is first expanded by passage through the expansion valve (344) and then passes through the heat exchanger in the liquid refrigerant within the refrigerant reservoir (312) before being supplied to the heating system heat exchanger (334).

12. An air source heat pump as claimed in claim 10, the heat pump including a compressor (332) for compressing refrigerant, an air heat exchanger (320) for extracting energy from ambient air during a heating mode of operation, and a reservoir (312) for liquid refrigerant through which, during defrosting of the air heat exchanger (320), refrigerant flows from an input (315, 355) at which refrigerant is received from the air heat exchanger, to an output (353, 357) and thence via an expansion valve (344) through a refrigerant heat exchanger (301) to the compressor (332) via a heating system heat exchanger (334), the refrigerant heat exchanger (301) in use being at least partially submerged in liquid refrigerant within the reservoir (312).

13. An air source heat pump according to any one of claims 10 to 12, further comprising a processor which is programmed to determine a need to defrost the air heat exchanger based on a detected change in an electrical characteristic or electrical behaviour of a motor of a fan (324) of the heat pump.

14. An air source heat pump according to claim 13, wherein the electrical characteristic is an increase in drive current or an increase in the rate of change of drive current.

15. An air source heat pump according to claim 13, wherein the electrical characteristic is an increase in the effective duty cycle of a pulse width modulation drive to the motor.

16. An air source heat pump according to any one of claims 13 to 15, wherein the processor is further programmed to determine successful defrosting of the air heat exchanger based on a detected change in an electrical characteristic or electrical behaviour of the motor of the fan of the heat pump.

17. An air source heat pump according to claim 16, wherein the electrical characteristic used to determine successful defrosting is a decrease in drive current or a decrease in the rate of change of drive current.

18. An air source heat pump according to claim 17, wherein the electrical characteristic used to determine successful defrosting is a decrease in the effective duty cycle of a pulse width modulation drive to the motor.

19. An air source heat pump according to any of claims 14 to 18, wherein the processor is programmed to determine and store, when the heat pump is commissioned, the current drawn by the motor of the fan and / or the duty cycle of a pulse width modulation motor drive.

20. An air source heat pump according to any one of claims 10 to 19, further comprising a processor that is operatively connected to a camera and / or another optical sensor to detect the build-up of ice on the air heat exchanger matrix.

21. An air source heat pump according to any one of claims 10 to 20, wherein the heat pump includes a drip tray to collect condensate that drops from the air heat exchanger, and a fan (324) to produce an air flow through the air heat exchanger; air heat exchanger (320) includes in a lower portion a liquid sub-cooler (23) which is configured, in a heating mode of operation, to receive refrigerant and to supply the refrigerant to an expansion valve (316) through which the refrigerant is arranged to pass before passing through an evaporator in an upper portion of the air heat exchanger (320), the liquid sub-cooler (23) being at least partially received within the drip tray and shielded from the air flow.

22. An air source heat pump according to claim 21, wherein the drip tray includes one or more upstanding walls or flanges that in use serve to shield the sub-cooler from air flow occasioned by operation of the fan.

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