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

Abstract

An air source heat pump (10, fig 1) comprises a compressor (32) for compressing refrigerant, an air heat exchanger 20 extracting energy from ambient air during a heating mode of operation, a drip tray 300 collecting condensate that drops from the heat exchanger, and a fan (24) producing an air flow through the heat exchanger. The heat exchanger includes in a first (lower) portion 23 defining a liquid sub-cooler and in the heating mode receives liquid refrigerant and supplies the refrigerant to an expansion valve (16) before passing through an evaporator 21 in a second (upper) portion of the heat exchanger. The liquid sub-cooler is at least partially received within the drip tray and at least partially shielded 302 from the air flow. The shielding may comprises of one or more structures such as flanges and define a cavity 301 within which the sub-cooler is received. The sub-cooler may comprise a plural tubes 323A-D and the flanges partially shield the tubes from the air flow and the tubes in use defrost or prevent freezing of the collected condensate. The heat exchanger may be a finned structure with fins serving both the sub-cooler and evaporator whereby condensate runs down the fins.

Description

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. 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 (also referred to as the evaporator) when the heat pump has been run to extract energy from ambient air (“heating mode” or “normal operation”). 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. During normal operation, condensation forms on the evaporator surface of the outdoor heat exchanger and runs down the evaporator exterior. Similarly, during the defrost cycle, accumulated frozen condensate on the evaporator melts and runs down the external surface of the evaporator. For this reason, a drip tray is provided as part of the heat pump assembly (optionally within the body of the heat pump) below the air heat exchanger to capture the condensate and route this away to a suitable discharge point in order to avoid adverse effects on either the product or the surrounding environment. For example, if not suitably managed, there may be a risk of ingress of condensate to parts of the heat pump not suitable for contact with condensate, such as electrical components. Also, accumulation of pools of condensate around or beneath the heat pump on areas such as patios or footpaths may freeze, giving rise to a safety hazard to persons in the vicinity. Because the evaporator surface can develop a coating of frost and ice during normal operation in certain conditions, it is possible for condensate inside and in proximity to the drip tray beneath the evaporator to become frozen and block the flow of condensate into, through or out of the drip tray, with consequent problems caused by disruption to the management of the condensate. The present invention seeks at least in part to address this issue and to reduce the likelihood of problems caused by condensate. Summary According to a first aspect there is provided an air source heat pump including a compressor for compressing refrigerant, an air heat exchanger for extracting energy from ambient air during a heating mode of operation, a drip tray to collect condensate that drops from the air heat exchanger, and a fan to produce an air flow through the air heat exchanger; wherein the air heat exchanger includes in a lower portion a liquid sub-cooler which is configured, in the 2 heating mode of operation, to receive liquid refrigerant and to supply the refrigerant to an expansion valve through which the refrigerant is arranged to pass before passing through an evaporator within an upper portion of the air heat exchanger, the liquid sub-cooler being at least partially received within the drip tray and at least partially shielded from the air flow. According to a second aspect there is provided an air source heat pump including a compressor for compressing refrigerant, an air heat exchanger for extracting energy from ambient air during a heating mode of operation, a drip tray to collect condensate that drops from the air heat exchanger, and a fan to produce an air flow through the air heat exchanger; wherein the air heat exchanger includes in a lower portion a liquid sub-cooler which is configured, in the heating mode of operation, to receive liquid refrigerant and to supply the refrigerant to an expansion valve through which the refrigerant is arranged to pass before passing through an evaporator within an upper portion of the air heat exchanger, the liquid sub-cooler being at least partially received within the drip tray and shielded from the air flow by being contained in a trough or well. According to a third aspect there is provided an air source heat pump including a compressor for compressing refrigerant, an air heat exchanger for extracting energy from ambient air during a heating mode of operation, wherein the air heat exchanger includes in a lower portion a liquid sub-cooler which is configured, in the heating mode of operation, to receive liquid refrigerant and to supply the refrigerant to an expansion valve through which the refrigerant is arranged to pass before passing through an evaporator within an upper portion of the air heat exchanger, wherein the heat pump includes: a drip tray to collect condensate that drops from the air heat exchanger; and a fan to produce an air flow through the air heat exchanger; and wherein at least a portion of the liquid sub-cooler is received within the drip tray, the drip tray shielding the portion of the subcooler from the air flow. With arrangements according to any of the first through third aspects condensate in the drip tray is less likely to freeze and the flow of condensate is less likely to be disrupted. In an air source heat pump according to any of the first through third aspects, one or more structures or formations may be provided to shield the sub-cooler from the air flow. The one or more structures or formations may together define a cavity within which the sub-cooler is received, the cavity communicating with the drip tray. The one or more structures may comprise at least one flange, optionally at least one pair of opposed flanges. The one or more structures may comprise a groove or recess formed in a floor of a housing of the heat exchanger. The one or more structures or formations may form part of the drip tray structure for collecting / managing condensate run off. In an air source heat pump according to any variant of the first through third aspects, the heat pump may be housed in a housing made of plastics material, and the drip tray is formed integrally with the housing, and optionally the one or more structures are formed integrally with the housing. The housing of plastics material may comprise a plastics moulding, and the drip tray may be formed as part of the moulding, and optionally the one or more structures may be formed as part of the moulding. The plastics moulding may house the air heat exchanger and optionally the compressor. Such arrangements may help reduce manufacturing costs while providing advantages in the handling of condensate. In an air source heat pump according to any variant of the first through third aspects the sub-cooler may comprise a plurality of lengths of tube that together form one or more circuits for refrigerant flow. The one or more structures or formations may provide all of the plurality of lengths of tube with at least partial shielding from the air flow. Alternatively, the one or more structures or formations may shield only a subset of the plurality of lengths of tube from the air flow. The subset may comprise at least half of the plurality, optionally a majority of the plurality. In an air source heat pump according to any variant of the first through third aspects, the air heat exchanger may be a finned structure in which individual fins serve both the liquid subcooler and the upper portion. The individual fins may be so arranged that when condensate forms from atmospheric water vapour condensate runs down the fins from the upper portion towards the liquid sub-cooler. Brief description of Figures Embodiments of the invention will now be described, by way of example only, with reference to the accompanying Figures, in which: Figure 1 shows schematically an air source heat pump operating in heating mode; Figure 2 shows schematically the air source heat pump of Figure 1 operating in defrost mode; Figure 3a to 3d are schematic partial vertical sections through the lower end of an air heat exchanger, and part of a support structure, of a heat pump according to an aspect of the invention; and Figure 4 illustrates schematically a heat pump according to an aspect of the invention. Specific description 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 4 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. The conduit 14 has an open end 15 which in use is submerged within liquid refrigerant in the liquid receiver 12, 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. 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. 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 direct 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 current operating conditions. 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. 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 small internal fixed orifice and out of the plurality of output conduits 19. In heating mode there is no flow via the internal 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 one-way valve 50, bypassing the internal fixed orifice and heating expansion valve 16 such that the heating expansion valve 16 and the 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 internal bypass conduit. The evaporator 21 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. The heat pump includes a motor driven fan 24 which causes ambient air to move over the evaporator 21. In heating mode, refrigerant inside the evaporator 21 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 evaporator 21 via a manifold or collector 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 6 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. 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. 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. 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 evaporator 21, and then turn to ice. The build-up of ice on the evaporator 21 both restricts the flow of air through the evaporator 21 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 7 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 evaporator 21 to collect condensate when the heat pump is running in heating or defrost modes, for the reasons given above. 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. 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 26 and thence to the evaporator 21. By supplying hot refrigerant to the evaporator 21, ice build-up can be thawed. Cooled refrigerant leaves the evaporator 21 via the distributor 18 and then passes via one-way bypass valve 50 and the sub-cooler back to the liquid receiver 12. 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 8 temperature of the refrigerant, (such that the refrigerant leaves the expansion valve as a mixture of liquid and vapour). The expansion valve 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. 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. 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. 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 21 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 freezing and blocking or partially blocking the drip tray beneath the evaporator 21 thereby preventing further melted condensate running into the drip tray - and 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. It is proposed that the air 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 air heat exchanger 20 which are separate from the circuits of tubes of the evaporator section 21. The connection pipework of the air 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 air 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 23 serves to improve the quality of the refrigerant that is fed into the evaporator part of the air heat exchanger. Turning now to Figure 3a we see a proposed arrangement that is intended to reduce the risk that the drip tray will ice up during defrosting, between defrosting cycles, or as the result of cumulative heat-defrost-heat cycles. Figure 3 shows schematically a vertical cross section of the lower part of the heat exchanger 20 with the lowermost part received an associated drip tray structure 300. As before, the heat exchanger 20 comprises an upper part that functions as an evaporator 21 in heating mode, and a lower part 23 that functions as a sub-cooler in heating mode. The evaporator part 21 is made up of circuits of tubes or conduits 321, and likewise the sub-cooler 23 is made up of circuits of tubes or conduits 323 A to D. The tubes / conduits of the heat exchanger are typically coupled to fins that extend generally vertically (rather than horizontally) so that condensate can readily run down the fins into the drip tray structure 300. Because the figure only shows the lower portion of the heat exchanger 20 only nine rows of tubes / conduits are shown, but in practice many more rows (e.g. 40 to 70, 45 to 65, 50 to 60) will be provided. By way of contrast, the sub-cooler, will typically comprise only a few rows of tubes / conduits, so that the three rows of tubes shown is more typical of the number of rows used in practice (typically, fewer than 10 rows will be used, e.g. 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 rows may be used). Depending upon the number of rows of tubes / conduits to be accommodated, the fins of the heat exchanger may be in the region of 600 to 900 mm in length, and between 50 to 75 mm (e.g. 55 to 70, 60 to 66 mm) wide. The conduits and the fins are preferably made of a material or materials with high thermal conductivity, such as copper (or a copper alloy) or aluminium (or an aluminium alloy). In use, under the action of the heat pump fan 24 air flows through the heat exchanger 20, as shown by the arrows, from one face of the heat exchanger to the other. Figure 3a shows four tube portions 323 A to D that make up a sub-cooler as viewed in cross-section. In this example these four tube portions may provide two flow paths and two return paths, the flow paths feeding the return paths, so that in practice the four tube portions shown in cross section may simply be cross-sections through a single continuous conduit that 10 provides the full flow path of the sub-cooler. So, for example 323A may be a cross-section through a flow path that runs to the remote end of the sub-cooler where it connects to return path 323B. Return path 323B may connect at the near end of the sub-cooler to flow path 323C which in turn may connect to return path 323D. Return path 323D may then connect at the near end of the heat exchanger 20 to a first of the flow paths of the evaporator 21. This arrangement is used because the illustrated heat exchanger is assumed to have all the connections at one side of the assembly, so all circuits of sub-cooler and evaporator must go along in one direction then turn around and come back again. But it is of course equally possible to have the refrigerant inlet(s) at one side of the heat exchanger with the refrigerant outlet(s) at the opposite side, so that the flow paths could simply be from one end to the other, without coming back. Or it could be from end A to end B then to end A then back to B (three passes). Depending how one connects the tubes at each end many possible arrangements are possible - for example, one could also split the flow at either end as one wishes, so for example have 3 tubes with the flow all going down one, then splitting at the far end and coming back along the other 2 tubes. It can be seen that in this example with four tube portions, the three lowermost portions (A, C, and D) are shielded from air flow occasioned by the fan - here all being located fully within a cavity or recess 301 which in this case is defined by opposed flanges 302 and 302’: that is, these three portions are all shielded from air flow occasioned by operation of the fan of the heat pump. These three tube portions are present in the form of two rows, both rows being shielded by the flanges. The fourth tube portion 323D, present as a third row, is itself also partly shielded by the flanges 302, as can be seen from the position of the dashed line 303 that indicates the level of the upper edge of the flanges 302. In this example the conduit or tube sections that make up the sub-cooler are substantially shielded by the pair of flanges 302 from air flow occasioned by the fan - in that 3 of the 4 sections are fully shielded and the 4th is partially shielded. Of course, those portions of the fins that make up the heat exchanger 20 and that are adjacent to the relevant conduit sections are also shielded by the flanges, but when considering the extent to which the sub-cooler is shielded from air flow it is easier to consider just the conduit sections. Substantially shielded should be taken to mean at least half of the tube portions (and / or at least half of the rows) making up the sub-cooler are shielded from air flow occasioned by the fan. The figure shows the sub-cooled 23 received in a groove or recess 301. Here a recess 301 is here formed between a pair of upstanding flanges 302 and 302’ to either side of the drip tray structure 300, but it could equally well simply be formed as a groove in a surface without the use of flanges that are upstanding with respect to the surface. The recess 301 communicates with a channel 304 which further communicates with a drip tray outlet, not shown. In use, 11 condensate that forms on the evaporator 21 is collected in the channel 304 before passing out from the drip tray 300 via the drip tray outlet. The recess 301 is so formed that walls of the recess (that may be defined by the pair of flanges) shield the tubes / conduits of the sub-cooler 23 from the air flow that passes through the heat exchanger, whereas the tubes / conduits of the evaporator 21 are not so shielded. The figure shows the sub-cooler circuit 23 at the base of the evaporator 21 significantly enclosed by the surrounding structure of the drip tray, limiting the air flow over sub-cooler so as to reduce the amount of sub-cooling which would otherwise occur if the sub-cooler were fully exposed. The sub-cooler is shown here as comprising three rows of conduits, with the lower two rows of the rows set at a level beneath the level of the top of the recess. By shielding the sub-cooler 23 from significant airflow the temperature of the bottom section of the outdoor heat exchanger remains warmer than if the sub-cooler were exposed to the full airflow, thereby reducing the risk that water in the drip tray will freeze. By shielding the sub-cooler within the drip tray, heat from the sub-cooler is to some extent retained within the drip tray (rather than being blown away under the action of the fan), reducing the risk that the contents of the drip tray will freeze. The heat from the sub-cooler may also pass by conduction (through the fins) and / or convection to the lower part of the evaporator above the sub-cooler helping defrosting (if that is relevant) or providing heat to that part of the evaporator where the refrigerant is colder (increasing the temperature differential and thereby improving the efficiency of heat transfer to the refrigerant. The drip tray structure 300 may be formed directly in the casing of the heat pump. Optionally the casing of the heat pump may be formed of plastics material, for example by moulding (e.g., roto-moulded, blow-moulded, or injection moulded) or by 3D printing, with the drip tray structure 300 provided by integral features of the casing. The plastics moulding may be made of MDPE, HDPE, or polypropylene for example. The plastics moulding may be formed as a hollow body. Wholly or partially filling the hollow moulding may serve to increase its structural stability, improve thermal insulation properties of the housing - possibly improving the energy efficiency of the heat pump, and / or improve the sound-deadening performance of the housing (potentially reducing noise pollution caused by the heat pump’s compressor). Thus the hollow body may subsequently be wholly or partially filled with a material that is thermally insulating (such as a polyurethane foam, foamed plastics beads) and / or sound deadening (as described, but optionally a liquid or a material such as sand). Figure 3b corresponds to Figure 3a but shows a possible arrangement in which the fan 350 is set at an angle above and to the right of the heat exchanger 20 drawing air through the heat exchanger 20 from left to right (the air being drawn in through an air inlet indicated as 360, and expelled through an outlet 352). The approximate direction of air flow through the heat 12 exchanger is indicated by the bold arrows and it is approximately parallel to the axes defined by the rows of tubing in the heat exchanger (which are indicated by the feint lines 355). While a pair of flanges is generally effective to trap heat in the void between the flanges, air shielding (that is shielding the sub-cooler from the fan-induced air flow) can readily be achieved without resorting to the use of a pair of flanges. Figures 3c and 3d correspond generally to Figures 3a and 3b, but illustrate alternative arrangements in which the sub cooler is only shielded from fan-induced air flow from one rather than from both sides. Again, only the lower portion of the heat exchanger 20 is shown, so that the arrangement details around the sub-cooler can be represented at a larger scale. In Figure 3c the sub-cooler is shielded by a flange 302 only on the upstream side of the heat exchanger 20. On the downstream side, where air exits the heat exchanger 20, the sub-cooler is unshielded. In the arrangement shown, - instead, the floor 360 of the compartment housing the heat exchanger 20 slopes generally upwards to guide towards the outlet 352 air that has passed through the heat exchanger 20. The flange 302 and the floor 360 may together still define a cavity within which the sub-cooler is received. Conversely, Figure 3d illustrates an arrangement in which the sub-cooler is shielded only on the downstream side, by flange 302’. In place of the upstanding upstream flange 302, a lip or other shallow feature (or series of features, s there would be no need for a continuous single feature) 362 may be provided to prevent the lower end of the heat exchanger 20 sliding forward or otherwise becoming dislodged. It will be appreciated however that the same effect could equally well be achieved by providing restraining features in the heat pump case (possibly as part of the case or as part of an internal chassis) at the edges of the heat exchanger 20 - in which case the lip 362 or alternative features at the lower edge of the heat exchanger 20 could be omitted. Depending upon whether the lip 362 or equivalent feature(s) is provided, the sub-cooler may or may not be received within a cavity. The angled floor feature 360 of the Figure 3c variant could optionally also be used with the downstream flange arrangement of Figure 3d, as shown by the broken line 360. The arrangements illustrated in Figures 3c and 3d may be slightly less effective at shielding the sub-cooler than the arrangements illustrated in Figures 3a and 3b, but satisfactory performance may still be obtained. Figure 4 illustrates schematically a heat pump 410 according to an aspect of the invention, and in particular an arrangement of elements to support the heat pump’s processor 400 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 411 within which are located the air heat exchanger matrix 20, one or more fans 424 with an associated drive motor 404, the processor 400, and 13 (optionally) the heating system heat exchanger 434 - although this may be mounted remotely, for example in place of a gas boiler that has been replaced by the heat pump. The processor 400, which has an associated memory 402 (storing software instructions to control operation of the processor, etc.), is operatively connected to a valve arrangement 430 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 430 may comprise a 4-way valve or some suitable combination of simpler valves 430A, 430B, and 430C. The processor 400 is also operatively connected to the motor 404 that drives the fan 424 that rotates to drive (pull or push) ambient air over the air heat exchanger matrix 20, and may be configured both to control the fan motor and to receive information from appropriate circuitry on an electrical characteristic or behaviour of the fan’s motor, such as the motor’s power consumption, for example based on the current drawn by the motor 404. Figure 4 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 largely concealed within the drip tray structure 300. The heat exchanger 20 is preferably arranged with its fins extending generally vertically. Figure 4 also shows the air heat exchanger 20 located upstream of the fan 424 and its motor 404, 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 (the air heat exchanger being located downstream of the fan). The processor 400 may also be operatively connected to a temperature and humidity sensing arrangement 406 that is configured to sense the ambient temperature and humidity at the site of the air heat exchanger 420, as well as being connected to sensors 31 at the input side of the compressor (as previously explained). The processor 400 and the air heat exchanger 20 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 20 in a separate housing outside the premises. In the latter case, the housing for the air heat exchanger 20 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 400 may be operatively connected to a camera (e.g., a video camera) and / or another optical sensor 408 to detect the build-up of ice on the air heat exchanger 20, optionally using image processing in the event that 14 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 400 may also be operatively connected to the Internet 450 and hence to a source 460 of (local or hyper local) weather forecasts and also to a system back-end 412 managed by the manufacturer / supplier or installer of the heat pump. Using data from the source 460 of weather forecasts, the processor 400 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 412 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 412 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. The processor 400 may be configured to distinguish between a blockage or obstruction caused by a build-up of frost on the air heat exchanger, and a blockage or obstruction due to some other cause (such as a build-up of leaves, or the partial or complete blockage of an air inlet or outlet) based on information from the temperature and humidity sensing arrangement 306 or from an external source of weather information (such as an online meteorology resource), and knowledge of the heat pump’s recent operational history. These information sources may be supplemented with information from system camera 408, and information from system back end (optionally following a request for relevant information) for example information on the likelihood of frost build-up and / or on the behaviour of other air heat pumps in the vicinity of the installation that are also supported by the system back-end 412. The processor 400 may be programmed or configured to detect a blockage, such as a build-up of frost on the air heat exchanger based on a characteristic change in power 15 consumption characteristic of a fan motor, for example the power consumption characteristic may be the amount of current consumed, a rate of change of current consumption, a voltage applied, an effective duty cycle in a PWM drive arrangement, or the like. 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 412) of the need to clean the air inlet - the alert optionally being sent by the system back-end 412, for example as a push notification to a user device or even as an automated phone call or email or letter. The processor 400 may be configured to detect and record the power consumption of the fan motor 404 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. It is possible that the heat pump’s air inlet(s) and / or air outlet(s) may be blocked by an external obstruction (such as a towel or clothing draped over the inlet, the placing of equipment such as a paddle board or surfboard, or building materials over the inlet(s) or outlet(s)), or an internal obstruction (such as an insect or rodent nest or other result of insect or rodent activity, or a build-up of leaves or vegetation) may obstruct the usual path of air flow between the air inlet(s) and air outlet(s). Any of these obstructions should, if significant enough, be detectable by the processor of a heat pump according to aspects of the invention. Similarly, the processor may be able to detect successful 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 16 exchanger may actually lead to an increase in current 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. 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. 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. Additionally or alternatively, the processor 400 may be configured to detect a blockage, such as 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 470, and a pressure measured outside the housing e.g. by a second pressure sensor 472. The fan associated with an air heat exchanger may be arranged to suck air into the housing through one or more air inlets 480 remote from the air heat exchanger, the air being expelled from the housing over / through the air heat exchanger and out an exit 482 of the housing - that is, the fan is arranged upstream of the air heat exchanger, the fan effectively pushing air over the downstream air heat exchanger. Alternatively, the air heat exchanger may be located on the inlet side of the fan, with the fan downstream of the air heat exchanger, so that the fan sucks air into the housing through the air heat exchanger. Accuracy of the pressure differential technique may be influenced by whether the fan is upstream or downstream of the air heat exchanger, so that under some circumstances this may not prove to be a reliable means of detecting a blockage such as a build-up of ice - the accuracy potentially depending on the precise configuration chosen. 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 electrical behaviour or an electrical 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

1. An air source heat pump including a compressor (332) for compressing refrigerant, an air 5 heat exchanger (320) for extracting energy from ambient air during a heating mode of operation, 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; wherein the air heat exchanger (320) includes in a lower portion of the air heat exchanger (320) a liquid sub-cooler (23) which is configured, in the heating mode of operation, to receive liquid refrigerant and to supply the10 refrigerant to an expansion valve (316) through which the refrigerant is arranged to pass before passing through an evaporator within an upper portion of the air heat exchanger (320), the liquid sub-cooler (23) being at least partially received within the drip tray and at least partially shielded from the air flow.15 2. An air source heat pump as claimed in claim 1, wherein one or more structures orformations are provided to shield the sub-cooler from the air flow.

3. An air source heat pump as claimed in claim 2, wherein the one or more structures or formations together define a cavity within which the sub-cooler is received, the cavity20 communicating with the drip tray.

4. An air source heat pump as claimed in claim 2 or claim 3, wherein the one or more structures comprise at least one flange.25 5. An air source heat pump as claimed in claim 4, wherein the one or more structurescomprise at least one pair of opposed flanges.

6. An air source heat pump as claimed in any of claims 2 to 5, wherein the one or more structures comprise a groove or recess formed in a floor of a housing of the heat exchanger.

307. An air source heat pump as claimed in any one of claims 2 to 6, wherein the one or morestructures or formations form part of the drip tray structure for collecting / managing condensate run off.15 01 248. An air source heat pump as claimed in any one of the preceding claims, wherein the heat pump is housed in a housing made of plastics material, and the drip tray is formed integrally with the housing, and optionally the one or more structures are formed integrally with the housing.5 9. An air source heat pump as claimed in claim 8, wherein the housing of plastics materialcomprises a plastics moulding and the drip tray is formed as part of the moulding, and optionally the one or more structures are formed as part of the moulding.

10. An air source heat pump as claimed in claim 9, wherein the plastics moulding houses the 10 air heat exchanger and optionally the compressor.

11. An air source heat pump as claimed in any one of claims 2 to 10, wherein the sub-cooler comprises a plurality of lengths of tube that together form one or more circuits for refrigerant flow.1512. An air source heat pump as claimed in claim 11, wherein the one or more structures or formations provide all of the plurality of lengths of tube with at least partial shielding from the air flow.20 13. An air source heat pump as claimed in claim 11, wherein the one or more structures orformations shield only a subset of the plurality of lengths of tube from the air flow.

14. An air source heat pump as claimed in claim 13, wherein the subset comprises at least half of the plurality, optionally a majority of the plurality.2515. An air source heat pump as claimed in any one of the preceding claims wherein the air heat exchanger is a finned structure in which individual fins serve both the liquid sub-cooler and the upper portion.30 16. An air source heat pump as claimed in claim 15, wherein the individual fins are soarranged that when condensate forms from atmospheric water vapour condensate runs down the fins from the upper portion towards the liquid sub-cooler.

Images