Heat pump system

Abstract

A heat pump system 100 for heating or cooling a building, the heat pump 110 comprising a DC compressor (2, fig. 1). A DC battery 120 is arranged to power the DC compressor, and the system further comp

Description

Field This invention relates to a heat pump system that is designed to efficiently provide heating and / or cooling for residential and / or commercial buildings. Heat pumps are used for their ability to efficiently transfer heat from one location to another. Typically, heat pumps use electricity to power compressors and refrigerants to transfer heat from outdoor to indoor spaces, and vice versa. Background A typical high-efficiency heat pump system is capable of providing heating and cooling to a building. The heat pump system has a compressor, refrigerant, and heat exchangers to transfer heat energy from one location to another. The heat pump system is usually configured as either a ground source or air source heat pump (other configurations are also available, e.g. water source heat pump). The compressor compresses refrigerant to increase its temperature and pressure. The refrigerant then flows through a heat exchanger where it absorbs or releases heat energy depending on whether the system is being used for heating or cooling. Examples of heat exchanger configurations include: tube and fin; plate and fin; or microchannel. Other configurations are known to the skilled person. In the ground source heat pump configuration, a ground loop is installed underground to exchange heat between the heat pump system and the earth. In some examples, the ground loop typically consists of a series of polyethylene pipes containing a water and antifreeze solution. Depending on the amount of available space, the ground loop pipes are buried in a desired configuration, such as a horizontal or vertical configuration. A heat exchanger transfers heat energy between the heat pump’s refrigerant and the ground loop’s water / antifreeze solution, allowing the heat pump system to heat or cool the building as necessary. In the air source heat pump configuration, the heat pump system typically uses outdoor air as a heat source in the winter and a heat sink in the summer. Typically, the heat pump is used to heat hot potable water all year round. In summer, the heat pump thereby has a mixed use. Environmental factors (e.g. being located at a hot part of the earth) affects how the heat pump is used. The outdoor air is drawn, e.g. by a fan, into the heat pump system and passes over a first heat exchanger where it transfers heat energy to or from the refrigerant. The heat pump system then circulates the refrigerant to a second heat exchanger which transfers the energy from or to the refrigerant. The second heat exchanger transfers energy to or from air or water which is circulated through the building, e.g. using a series of ducts and vents or pipes and emitters. The heat pump system can be equipped with a variety of controls and sensors to optimise its performance. For example, the system can include a variable-speed compressor that adjusts its speed based on the building's heating and cooling demands. The system can also include a smart thermostat that learns the building's occupancy patterns and adjusts the temperature accordingly. Heat pumps provide several advantages over traditional heating and cooling systems. They are highly efficient and typically move up to four times (sometimes 6 or 7 times) the amount of energy consumed. Heat pump efficiency is likely to improve even more in future. They can further reduce a building's carbon footprint, e.g. by using renewable energy sources. They can provide consistent heating and cooling throughout a building without the need for separate systems. Key components of a typical heat pump system include an evaporator, a compressor, a condenser and an expansion valve. The evaporator is the heat exchanger that is responsible for absorbing heat from the air, ground or water source. It contains a refrigerant, typically a type of fluid with low boiling point and high latent heat of vaporisation. As the refrigerant absorbs heat, it evaporates and turns into a gas. The compressor is a mechanical device that is responsible for compressing the refrigerant gas and increasing its temperature. This is done by reducing the volume of the gas, which causes its pressure to increase. As the pressure increases, the temperature of the gas also increases. The compressor typically runs on mains AC power that is converted to DC via an AC-DC power rectifier. The condenser is another heat exchanger that is responsible for releasing the heat into the heating or cooling system, such as a room or hot water tank. The hot refrigerant gas enters the condenser, and as it flows through the condenser, it releases heat to the surrounding environment, which causes the gas to condense back into a liquid. The expansion valve is a metering device that is responsible for regulating the flow of refrigerant and lowering its temperature before it enters the evaporator again. The valve reduces the pressure of the refrigerant, which causes it to expand and cool down before it re-enters the evaporator to absorb more heat. In addition to these components, a heat pump system may also include other components such as a reversing valve (to switch the direction of flow of refrigerant and thereby provide both heating and cooling functions via the same heat pump system), an external backup heating system (such as electric resistance heating or a gas furnace), and a controller to regulate the system's operation. The reversing valve is a component that allows the heat pump to switch between heating and cooling modes. In heating mode, the valve directs the refrigerant to flow in one direction, while in cooling mode, it directs the flow in the opposite direction. In colder temperatures or when the heat pump is unable to meet the heating demand, a backup heating system may be used. This could be an electric resistance heater, a gas furnace, or another type of heating system. The controller is responsible for regulating the operation of the heat pump system, including the temperature set point, the switching between heating and cooling modes, and other settings. It may be a simple thermostat or a more sophisticated electronic controller of a known type. The power consumption of each component in a heat pump system varies depending on the specific capacity of the system. Usually, the compressor and the evaporator fan (if present, e.g. in an air source system) are the two main components that consume the most electrical power. The compressor is the largest consumer of electrical power in a heat pump system. The power consumption of a compressor will depend on the size and capacity of the unit, as well as the ambient temperature and operating conditions. For example, a typical 2-ton (24,000 BTU) residential heat pump compressor may consume between 1200 and 2400 watts of electrical power during normal operation. The evaporator fan is responsible for circulating air over the evaporator coil and transferring heat to the refrigerant. The power consumption of the evaporator fan will depend on the size and capacity of the unit, as well as the speed of the fan. A typical residential heat pump evaporator fan may consume between 100 and 500 watts (maybe more) of electrical power during normal operation. The controls for a heat pump system typically consume a relatively small amount of electrical power, usually less than 10 watts. However, the controls are responsible for managing the operation of the heat pump and ensuring that all of the components are operating efficiently. If the heat pump system includes a backup heating system, such as electric resistance heating or a gas furnace, the power consumption of these components will depend on the type of heating system and the capacity of the unit. For example, an electric resistance heating system may consume several kilowatts of electrical power during operation, while a gas furnace may consume between 500 and 2000 watts of electrical power for the blower motor and other components. It's worth noting that the power consumption of a heat pump system will depend on a variety of factors, including the efficiency of the system, the size and capacity of the unit, the local climate, and the operating conditions. The inventors have realised that a better heat pump can be produced and have created the claimed solution. Summary According to a first aspect of the present invention, there is provided a heat pump system as claimed in claim 1. Advantageously, a heat pump that can be powered directly from a DC battery (i.e. need not rely on AC input) is provided. This type of heat pump is efficient and resilient (e.g. can be operated when AC input is unavailable or undesirable). Direct DC power (without transforming voltage to a different level) is most efficient. Transforming one voltage to another is very efficient and can be at least 99% efficient, or at least 99.5% efficient. In contrast, power conversion (e.g. via an rectifier taking AC to DC or inverter taking DC to AC) has to do more work, and such systems are generally less than 95% efficient and occasionally very slightly above 95% if very high power and expensive. For every watt ‘lost’ before the compressor (e.g. due to passing through the relatively inefficient inverter compared to the relatively efficient direct DC to DC battery), the system loses approximately 3 to 7 times that wattage in heat output for the heat pump (due to typical heat pumps being able to operate at a coefficient of performance of 3 to 7). Therefore, as the inventor has realised, the relative efficiency is amplified by the current, inventive setup. Optionally, the heat pump system comprises a battery charger arranged to charge the DC battery. The battery charger may comprise any one or more of: • an AC-DC charger arranged to charger the DC battery from an AC source, such as mains AC; • a DC-DC charger arranged to charger the DC battery from a DC source, such as a solar power source, optionally via a DC-DC converter; • a two-way inverter-rectifier in the form of AC-DC and DC-AC in a single unit; and • a three-phase AC-DC charger arranged to charger the DC battery from a three-phase AC source. Optionally, the DC battery can be charged at an easy voltage (e.g. 320V, 165V, 72V), but then switch the batteries for output to the compressor at lower voltage, e.g. 24V, 48V, 120V, 230V, 400V. Furthermore, the battery can be charged at any suitable time (e.g. when there is no, or little, heating (or cooling) demand on the system. A controller may be arranged to control when to charge the battery. In contrast, when the heating demand is significant, then the compressor requires significant power input (and less is available for charging). In some examples, for charging DC battery cells, one option is to charge at lower power or at a maximum, or other desired, efficiency of the charging circuitry when the heat pump system is otherwise idle. Optionally, any one or more of the voltage inputs and outputs of the: compressor; DC battery; and AC-DC rectifier are matched, i.e. they have substantially the same value. This allows for very efficient operation without undue electronic processing. In some examples, the system may never run the compressor from the AC powered inverter, and always use only DC battery power. Optional features of the invention are as claimed in the dependent claims - various advantages are thereby provided as discussed in the detailed description. These optional features add efficiency and intelligence to the inventive heater setup. Any of these optional features may be combined with any other of the optional features as will be appreciated by those skilled in this art. Brief Description of Drawings Embodiments will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a known schematic heat pump circuit for a building heat pump; Figure 2 shows a schematic view of a heat pump system according to an aspect of the invention; Figure 3 shows a schematic view of some components, including power electronics components, of a heat pump system according to aspects of the invention; Figures 4, 5 and 6 show schematic views of some components, including power electronics components, of further heat pump systems according to further aspects of the invention; Figures 7a and 7b, 8, 9, 10a and 10b, 11, 12, and 13 show schematic views of further heat pump systems according to further aspects of the invention; Description of Embodiments The exemplary embodiments described in the detailed description and claims are not meant to be limiting. Other embodiments may be used, and other changes may be made, without departing from the scope of the invention. Various embodiments are described. The specific embodiments are not intended as an exhaustive description or as a limitation to the broader discussed and claimed aspects. Features described in conjunction with a particular embodiment are not necessarily limited to that embodiment and can be incorporated into any other embodiment(s). Protection afforded by any applicable doctrine of equivalents is retained to its fullest extent. Terms such as up, down, top, bottom, left, right, Inner, outer, vertical, upstanding etc. have been used to simply and clearly describe the invention. These terms are not to be interpreted in a manner that would be limiting. The person skilled in the art will envisage other suitable embodiments within the scope of the invention. Figure 1 shows a known schematic heat pump circuit for a building heat pump comprising: an evaporator 1 configured to absorb heat from a source (e.g. ambient air in an outdoor environment); a compressor 2 configured to compress a refrigerant and increase its temperature; a condenser 3 configured to release heat from the refrigerant into a sink (e.g. via pipes in a radiator circuit or to surrounding air in an air conditioner circuit); and an expansion valve 4 configured to regulate the flow of refrigerant and lower its temperature before it re-enters the evaporator. The refrigerant is arranged to flow continuously through the circuit, from the evaporator to the compressor, to the condenser, to the expansion valve, and back to the evaporator, to provide heating or cooling to a building. Many standard options exist for configuring each of the evaporator, compressor, condenser, and expansion valve, and these will be apparent to the skilled person. One or more pipes may be configured to contain the refrigerant as it flows continuously through the circuit, from the evaporator to the compressor, to the condenser, to the expansion valve, and back to the evaporator. For different examples, the same circuit can be applied to a ground, air, water or other heat pump system, e.g. via a swimming pool or loft. Referring to figure 2, there is schematically shown a first example of an inventive heat pump system 100 used to heat a building via a typical heating fluid circuit, which includes radiator heating and potable hot water circuits. In this specification, references to heating or cooling a building are intended to cover: heating or cooling water in a wet heating or cooling circuit; or heating or cooling air in an air conditioning system; or any other suitable known heating or cooling assembly. The skilled person will understand that heat pumps can be operated in reverse, in a known manner, so that a building heating circuit can become a cooling circuit and vice versa. Various aspects of the heat pump and heat pump system will be described in detail with reference to non-limiting examples. Other details will be apparent to the skilled person. In particular, aspects (including undescribed aspects) of known heat pump systems can be incorporated and used with this invention by the skilled person. As seen in figure 2, the system 100 includes several standard components that are located inside a building and outside a building, separated by an exterior wall 102. In other suitable examples, components may be distributed differently, such that different system components are inside / outside the building dependent on needs of the specific system. Inside the building, there is a wet heating network including at least one radiator 104 arranged to heat a space (e.g. a room) in the building, and at least one water cylinder 106 (or other suitable water tank) arranged to store and supply hot water to potable water units, such as a taps in sinks, baths etc. Components of the wet heating network are connected by pipework in a known manner. Heat pump pipework 105 carrying warmed water passes internally through the cylinder 106 and is arranged to transfer heat to potable water stored therein in a known manner. Separate pipework 107 dedicated to the delivery of potable water to potable water units from the water cylinder is provided in order to ensure that potable water does not become contaminated with radiator water or other heated fluid in the heat pump pipework. The system 100 includes a heat pump, for a wet heating system, comprising two main components: an outdoor unit 110 and an indoor unit (not shown). In this example, the heat pump is an air source heat pump. The outdoor unit comprises a housing 112, which contains a compressor, a fan 114, and a heat exchanger or coil, which transfers heat between the refrigerant and the outside air to heat, in a known manner, water or a water-based solution that circulates through the system. The indoor unit contains a heat exchanger or coil, which transfers heat, in a known manner, between the water or solution and radiator fluid / water as it travels through the radiator 104 or water cylinder 106. This heat transfer may occur directly (e.g. if the same refrigerant flows through the heat pump circuit and the radiator, or indirectly (e.g. if a different, separate circuit is provided for the radiator and the heat pump - in this case, standard heat exchange techniques between the circuits will be known to the skilled person). Overall, in a first heating configuration, the outdoor unit is responsible for heating the water or solution, while the indoor unit is responsible for transferring that heat to the indoor air. Relative to a building, the outdoor unit 110 is typically installed outside close to an exterior wall (but not so close that air flow around the unit becomes undesirably restricted), while the indoor unit may be located inside the building, e.g. in a closet, attic, or utility room. Heat pump piping runs internally through the heat pump circuit within the outdoor unit and also connects the outdoor and indoor units and distributes the heated or cooled fluid through the heat pump pipework in the building. The evaporator, compressor, condenser, and expansion valve are not shown separately in figure 2; they are distributed in a known manner through the system. As previously described, refrigerant is arranged to flow continuously through the heat pump circuit, from the evaporator to the compressor, to the condenser, to the expansion valve, and back to the evaporator, to provide said heating or cooling to the building, The system 100 also comprises a diverter valve 108, which is arranged to send warmed fluid to a desired part or parts (e.g. to the radiator 104 or water cylinder 106) of the wet heating network. The diverter valve may be arranged to send warmed fluid according to demand and supply factors. A controller (not shown) is arranged to control the diverter based on a number of control factors, e.g. based on typical supply and demand factors, some of which may be obtained by making measurements, e.g. via sensors in the system; and some of which may not require sensors, e.g. based on time or date. As per this invention, the heat pump system 100 comprises a DC battery 120 arranged to at least partially power the compressor. In this example, the compressor is a DC compressor, i.e. it runs on DC power in a known manner. By “at least partially power”, we mean that the compressor can: sometimes operate using only power from the DC battery 120; sometimes operate using only power from an AC supply (see below); and sometimes operate using power from a combination of the AC supply and DC battery 120. The controller (not shown) may be arranged to control balancing the powering operation (via-DC only or AC-only or DC-AC combination) of the compressor. Such control might be based on a number of supply or demand control factors, including DC battery charge level, anticipated demand or supply (e.g. via time of day or sensed temperature or weather forecast, e.g. if it is very cold and potable hot water demand is anticipated imminently, then the controller may conserve battery charge level for anticipated demand). The controller (not shown) is arranged to control driving the compressor or switching of the diverter valve 108 or both based on a number of control factors, including, but not limited to, any combination of: • capacity of the compressor; • capacity of the or each of the battery or AC power supply; • instantaneous demand for potable water or heating water or heated air; • forecasted demand for potable water or heating water or heated air; • instantaneous or forecasted available supply type. Some known DC heat pump compressors operate using AC power, e.g. mains AC power. Such known compressors comprise a rectifier to convert incoming AC power to DC power, before feeding the (converted) DC power to the DC compressor. In the example of this invention, the compressor is also able to operate using incoming AC power in this known manner (as described below with reference to figure 3, in addition to being able to operate directly via power from the DC power supply as per the invention. In other examples, this may not be the case, i.e. the compressor may be arranged to be powered only by the DC battery. Referring to figure 3, the heat pump system 100 includes power electronics to efficiently power the compressor. The arrows represent possible power flow paths according to the power electronics. In this example, the compressor is a DC compressor and can be most efficiently powered directly from the DC power supply, i.e. the DC battery 120. Further, in this example the DC output from the battery 120 is at a voltage matched to the input voltage of the compressor. Therefore no extraneous processing is required between the DC battery and the compressor, resulting in very efficient operation. In other examples, there may be a mismatch between the DC battery output voltage and the compressor input voltage. In that case (not shown), the system 100 can include further power electronics between the DC battery output and the compressor input in order to suitably step the voltage therebetween. This will still result in more efficient operation than converting external AC (e.g. from mains AC) power to DC power before feeding the compressor. In such examples, this invention provides efficient driving of the compressor by directly powering the DC compressor from the DC battery. Such direct powering involves driving the compressor’s motor directly by DC switched inputs, i.e. DC power powers the DC compressor motor (whether a voltage step change is required or not). E.g. a brushless DC electric motor can be used between the power supply 120 and the compressor. There is no need to convert from DC to AC (e.g. via an inverter) or from AC to DC (e.g. via a rectifier). Unnecessary or extraneous power electronics (and associated reduction in efficiency) is thereby reduced in implementing such examples of the invention. In this example, the power electronics of the system 100 also comprises an AC-DC rectifier 122 arranged to convert incoming AC power (in this case AC mains power) to a DC power output level suitable for driving the DC compressor in a known manner. Notably, the compressor can be efficiently powered by DC battery 120 even when the AC supply is not available, e.g. during a power cut. In some other examples, a heat pump system according to the invention may not include an AC connection for powering the compressor. However, in most cases, the invention does include an AC connection. The DC battery in this example comprises one or more rechargeable cells. The system 100 comprises a charging interface arranged to charge the DC battery. In the present example, the charging interface comprises a rectifier that is arranged to receive and convert external AC (e.g. mains AC) to a suitable DC power supply for charging the battery 120. In another example, the charging interface may alternatively or additionally comprise a DC-DC charging interface, e.g. for receiving DC power from a solar panel assembly or other such arrangement. In another example, the charging interface may alternatively or additionally comprise a three phase AC-DC charger. Advantageously, in the present example and similar examples, the same rectifier 122 that is used to power the compressor from the AC power supply can be used for charging the battery 120 via the external AC supply. In this example, the rectifier 122 is arranged to supply DC power at a suitable voltage for charging the DC battery 120 without the need for any intermediate voltage stepping. In other examples, where this is not the case, the power electronics may further comprise a voltage stepper arranged to convert the DC signal output by the rectifier 122 to a suitable voltage DC signal for charging the battery 120. Another example configuration of the invention is schematically shown in figure 4. Many of the components are similar to the configuration of figure 3 and so the same reference numerals are used where appropriate. For conciseness, similar features will not be described again. In this example, the system 100 includes a further DC output interface arranged to allow the DC battery 120 to provide power to other electronic devices (i.e. other than the heat pump compressor). The DC output interface may be used to provide DC power (direct unstepped voltage or stepped voltage) or AC power (e.g. via an inverter) to external components arranged to interface therewith. The external components may include further components of the building heating or cooling system that require power, such as a second heat pump or an air conditioning unit or a boiler, e.g. an electric boiler or a hybrid combustion fuel-electric boiler, e.g. of the type described in the applicant’s co-pending GB2115477.8, GB2115478.6 or GB2115479.4. In some examples, the external components may not be further components of the building heating or cooling system; e.g. the external component may be a vehicle battery that can be charged efficiently from the DC power supply 120, e.g. via a DC-DC converter. Another example configuration of the invention is schematically shown in figure 5. Many of the components are similar to the configuration of figure 4 and so the same reference numerals are used where appropriate. For conciseness, similar features will not be described again. In this example, the system 100 includes a further AC output interface arranged to allow the DC battery 120 to provide power to other electronic devices. The AC output interface may be used to provide AC power or DC power (e.g. via a further rectifier, e.g. a further external rectifier) to external components arranged to interface therewith. The external components may include further components of the building heating or cooling system that require power, such as a second heat pump or an air conditioning unit or a boiler, e.g. an electric boiler or a hybrid combustion fuel-electric boiler, e.g. of the type described in the applicant’s copending GB2115477.8, GB2115478.6 or GB2115479.4. The external components may not be further components of the building heating or cooling system. In some examples, a single combined rectifier-inverter is used to both a) charge the battery and b) provide the AC output external appliance interface. Referring to figure 5, the power electronics of the system 100 further comprises a DC-AC inverter 124 arranged to convert incoming DC power from the DC battery 120 to an AC power output level suitable for being used as a household AC power supply or supplying a mains AC grid in a known manner. Notably, the AC output interface is available to provide AC power (from the DC power supply 120) even when mains AC may not be available, e.g. during a power cut. Another example configuration of the invention is schematically shown in figure 6. Many of the components are similar to the configuration of figure 5 and so the same reference numerals are used where appropriate. For conciseness, similar features will not be described again. In this example, the separate rectifier 122 and inverter 124 of the figure 5 example are replaced with a single combined power converter (rectifier-inverter) 126 that is arranged to carry out the same functions as described in relation to figure 5. In some examples, the single combined power converter comprises a single circuit that is operated in reverse direction to provide the rectifier and inverter functions (and so both the inverter and rectifier functions can not be carried out simultaneously). In such examples, a compact setup that can use shared electronic components is envisaged. In other examples, the single combined power converter comprises separate paths for the rectifier and inverter functions (and so both the inverter and rectifier functions can be carried out simultaneously) In such examples, assembly may be easier and electronic component cooling may be easier. In some examples, the rectifier output voltage is matched to the DC battery (instead of, or as well as, the compressor) - this allows for efficient charging. In some cases, all components are matched (i.e. the compressor, the DC battery and the rectifier (or inverter and rectifier). Another example of a heat pump system 200 according to the invention is shown schematically in figures 7a and 7b. Some features similar to those shown in figure 2 and described with reference to the system 100 are labelled with similar reference numbers (with the first digit changed from ‘1 ’ to ‘2’) and these features will not be described again, for conciseness. The heat pump system 200 comprises an air source heat pump with components outside the building identical or similar to those previously described. The components inside and outside the building do differ in that there is no water tank 106. In this example, the external all-in-one heat pump unit 210 comprises an integral mechanism (not shown) for heating potable water on demand (e.g. when a tap is opened) using power from the DC battery. The mechanism for heating potable water on demand may include one or more heating elements arranged to be powered by the DC battery or a combination of DC (from the battery) and AC (e.g. from mains AC) as described in the applicant’s co-pending GB2115477.8, GB2115478.6 or GB2115479.4. The one or more heating elements may be placed in, on, around, embedded (completely or partially) in walls of pipework or other components of any of the previously-described heat pump systems (this includes components such as boilers, air conditioning units, pipes, fans or vents). The heat pump system is arranged to heat fluid (in this case radiator fluid solution and potable water) to supply heated potable water in a first pipe network 207 and heated radiator fluid in an second, independent pipe network 205 (so that the potable water does not get contaminated). A mains water supply pipe 209 feeds into the system 200 (into the housing 212 in this example) in a known manner to supply water to be heated. In this example, the potable water heating occurs in the housing 212. In other examples, potable water heating may instead or also occur outside the housing 212, e.g. at another part of the potable pipe network 207 via heated elements as previously described. The heat pump system 200 is arranged to provide heated fluid as previously described in relation to system 100 for the radiator pipe network 205. In some examples, the heat pump pipework and the radiator circuit pipework 205 are the same pipework; they are unitary. The fluid to be circulated through the radiators is the same refrigerant that travels through the heat pump. The potable water pipework 205 is separate and carries separate potable water that is heated in a similar manner to that previously described in relation to system 100. Water in the potable network is kept separate in a well-known manner, e.g. via separate heat exchangers, so that fluids between networks cannot mix, but can transfer heat therebetween. In other examples, the heat pump pipework and the radiator circuit pipework 205 are not the same pipework; they are not unitary - they are independent. The radiator fluid carried in the radiator circuit pipework 205 can be heated in a similar manner to that previously described for potable water in relation to system 100. The configuration of system 200 allows (for only a small resource cost) a very responsive system without detracting from heat pump efficiency or taking heat away from radiator heating whilst allowing on-demand potable hot water. Advantageously, the footprint of the inventive heat pump system is relatively small - in this example, there is no indoor tank, no indoor wall unit; only a box / unit outside. Transport and installation is made easier and building floor area used (inside and out) is minimal. Another example of a heat pump system 300 according to the invention is shown schematically in figure 8. Some features similar to those shown in figures 2, 7a and 7b and described with reference to the system 100, 200 are labelled with similar reference numbers (with the first digit changed to ‘3’) and these features will not be described again, for conciseness. The heat pump system 300 comprises an air source heat pump with components outside the building identical or similar to those previously described. The components inside and outside the building do differ (from the example of figures 7a and 7b) in that there is a water tank 306 arranged to supply domestic hot potable to the potable water units, as previously described in relation to the figure 2 example. The heat pump system 300 also includes a partially or wholly electric fluid heater 330 arranged to provide a further source for heated fluid in the system 300. The fluid heater 330 is of the type described in the applicant’s co-pending GB2115477.8, GB2115478.6 or GB2115479.4. The fluid heater 330, in this example, includes a DC battery 320b arranged to, partially or wholly, power heating of fluid passing therethrough, e.g. via heating elements as previously described. The fluid heater may be an electric boiler, such as a DC electric boiler, or a may be a hybrid gas-electric boiler. In this example, the heat pump system 300 also includes a hydraulic unit 340 arranged to regulate fluid flow through the system 300. The hydraulic unit 340 receives heated fluid from the outdoor heat pump unit 312 and from the fluid heater 330. The hydraulic unit 340, in this example, includes a DC battery 320c arranged to, partially or wholly, power heating of fluid passing therethrough, e.g. via heating elements as previously described. The hydraulic unit 340 communicates with a controller (not shown) that is arranged to take into account any combination of demand, environmental and supply factors (as described previously) to control efficient generation and transfer of heated fluid from any one or more of: the air source heat pump unit 312; fluid heater 330; and hydraulic unit 340 to any one or both of the radiator pipe network 305 or water tank 306 (which transfers heat to the potable water pipework 307), in this example. The system 300 also includes a diverter valve 308, which is arranged to receive heated fluid output from the hydraulic unit 340 and to send warmed fluid to a desired part or parts (e.g. to the radiator pipe network 305 or water cylinder 306) of the wet heating network under instruction of the controller (not shown) as previously described. In some examples the diverter valve 308 is located within the hydraulic unit 340. In the heat pump unit 312, there is provided a DC battery 320a as described in relation to other examples. Advantageously, in some examples, DC battery power supply within the heat pump system 300 can be distributed such that the required DC power supply can be located wholly or partly in any one or any combination of: the unit 312; heater 330; and hydraulic unit 340. So, in some examples, the (outdoor) unit 312 may have no batteries and all DC battery power supply is located in other parts of the system. A controller (not shown) may be arranged to control sourcing of power from the or each DC battery within the distributed system based on a number of factors as explained previously, e.g. based on battery charge level, or outdoor temperature (e.g. useful if part of the DC battery is located outdoors). Another example of a heat pump system 400 according to the invention is shown schematically in figure 9. Some features similar to those shown in earlier figures, e.g. figure 8, and described with reference to the earlier-described systems, e.g. heat pump system 300 are labelled with similar reference numbers (with the first digit changed to ‘4’) and these features will not be described again, for conciseness. The heat pump system 400 comprises an air source heat pump with components outside the building identical or similar to those previously described. The components inside and outside the building do differ (from the example of figure 8) in that there is no water tank or diverter valve. Instead, in this example, fluid heated by the heat pump unit 410 and fluid heated by the fluid heater 430 is input to the hydraulic unit 440. The hydraulic unit 440 supplies heated fluid into the radiator pipe network 405 as previously described. Options for configuration of the unit outdoor unit 410, hydraulic unit 440 and fluid heater 430 are similar to those previously described. The hydraulic unit does not supply potable water in this example. Instead, only the fluid heater 430 is arranged to output heated potable water to the potable water circuit 407. The system 400 includes a mains water input (not shown) into the fluid heater 430. The fluid heater 430 is also arranged to receive fluid from the hydraulic unit (which has been heated by the outdoor unit 410) and to heat it further before returning it to the hydraulic unit 440 for distribution to the radiator pipework 405. Due to its internal DC battery 420b, in this example, the fluid heater 430 is able to provide a quick, sharp boost of heated fluid to the radiator network if and when it is needed, e.g. when radiators first turned on or in particularly cold conditions. This can be useful because typical heat pump outputs may not be very hot. In the system 400, a DC battery 420a is provided in the housing 412, as described in relation to other examples. Advantageously, in some examples, DC battery power supply within the heat pump system 400 can be distributed such that the required DC power supply can be located wholly or partly in any one or any combination of: the unit 410; heater 430; and hydraulic unit 440. A controller (not shown) may be arranged to control sourcing of power from the or each DC battery within the distributed system based on a number of factors as explained previously, e.g. based on battery charge level, or outdoor temperature (e.g. useful if part of the DC battery is located outdoors). As described previously, the partially or wholly electric fluid heater 430 is arranged to provide a further source for heated fluid in the system 400. The fluid heater 430 is of the type described in the applicant’s co-pending GB2115477.8, GB2115478.6 or GB2115479.4. The fluid heater 430, in this example, includes a DC battery 420b arranged to, partially or wholly, power heating of fluid passing therethrough, e.g. via heating elements as previously described. The fluid heater may be an electric boiler, such as a DC electric boiler, or a may be a hybrid gas-electric boiler. Another example of a heat pump system 500 according to the invention is shown schematically in figure 10. Some features similar to those shown in earlier figures, e.g. figures 7a and 7b, and described with reference to the earlier-described systems, e.g. heat pump system 200 are labelled with similar reference numbers (with the first digit changed to ‘5’) and these features will not be described again, for conciseness. The heat pump system 500 comprises an air source heat pump with components outside the building identical or similar to those previously described. The components inside and outside the building do differ (from the example of figures 7a and 7b) in that the system 500 additionally includes a supplemental heater 550 housed within the outdoor unit 510, within its housing 512. The supplemental heater, in this example, comprises a gas-fired heat exchanger, such as a H2-ready gas-fired heat exchanger. In other examples, the gas-fired heat exchanger may be a natural gas-fired heat exchanger. The supplemental heater 550 is arranged to provide additional heating power to the system setup shown and described with reference to figures 7a and 7b. In this example, the DC battery 520 is arranged to power the compressor (as in all other embodiments) and also to provide additional direct heating via heating elements (as described previously). In other examples with a supplementary heater, the DC battery may not provide additional direct heating via heating elements since the supplementary heater is sufficient to meet additional heating demand. Advantageously, in this example, the heat pump system 500 provides an all-in-one unit 510 that carries out all necessary fluid heating functions and that is located conveniently entirely outside a building. The all-in-one unit is easy to set up, install, and is compact for transit. In this example, supplementary heating is provided by the integrated H2-ready gas-fired heat exchanger. Other alternatives for supplementary heating will be apparent to the skilled person. Supplementary heating is particularly useful when needing to supply both domestic heating and on-demand hot (potable) water since a traditional heat pump alone may not be powerful enough. The partially or wholly electric DC power supply of this invention advantageously provides for more powerful operation to provide a useful, working system. Furthermore, in some cases, housing sites do not get permission because an electricity grid is not capable of providing enough electricity for heat pumps - the present invention can mitigate this. This can be both a national and local peak demand limitation electricity grid issue. In some examples, the external heat pump unit 510 effectively comprises an integral fluid heater (powered by the gas-fired heat exchanger in this example). In this scenario, the unit 510 is similar to a typical combi boiler. The unit 510 is arranged to heat fluid (in this case radiator fluid solution and potable water) to supply heated potable water in a first pipe network 507 and heated radiator fluid in a distinct pipe network 505 (so that the potable water does not get contaminated). A mains water supply pipe 509 feeds into the unit 510 in a known manner to supply water to be heated. In this example, the gas-fired heat exchanger is housed in the housing 512. The heat pump system 500 is arranged to provide heated fluid as previously described in relation to system 200. In some examples, the heat pump pipework and the radiator circuit pipework 505 are the same pipework; they are unitary. The fluid to be circulated through the radiators is the same refrigerant that travels through the heat pump. The potable water pipework 507 is separate and carries separate potable water that is heated in a similar manner to that previously described in relation to system 200. In other examples, the heat pump pipework and the radiator circuit pipework 505 are not the same pipework; they are not unitary - they are independent. The radiator fluid carried in the radiator circuit pipework 505 can be heated in a similar manner to that previously described for potable water in relation to system 100. Another example of a heat pump system 600 according to the invention is shown schematically in figure 11. Some features similar to those shown in earlier figures, e.g. figure 2, and described with reference to the earlier-described systems, e.g. heat pump system 100 are labelled with similar reference numbers (with the first digit changed to ‘6’) and these features will not be described again, for conciseness. The heat pump system 600 comprises an ground source heat pump with some components identical or similar to those previously described. The components inside and outside the building do differ (from the example of figure 2) in that the system 600 is a ground source system and so comprises a ground heat exchanger loop 614 of a known type and as previously described. The ground heat exchanger loop 614 is located underground in a known manner and is arranged to exchange heat between the heat pump system and the earth. In some examples, the ground loop consists of a series of polyethylene pipes containing a water and antifreeze solution. The ground loop 614 is arranged to carry heated fluid to the heat pump unit 612. In this example, the heat pump unit 612 is not necessarily located outdoors; it may be located indoors or outdoors - in this example, it is located indoors. The DC battery 620 is located inside the housing 612 of the unit 610. A heat exchanger (not shown) within the unit 610 transfers heat energy between the solution in the heat pump pipe network (that is output from the unit 610) and the water / antifreeze solution in the ground loop 614, allowing the heat pump system to heat or cool the building as necessary. A diverter valve 608 is arranged to send heated fluid from the unit 610 selectively to either the water tank 606 (to heat potable water in the potable pipework 607) or the radiator pipework 605, as previously described. In other examples, the ground loop may be the same as the heat pump pipe network (that is output from the unit 610); they are unitary and the water / antifreeze solution of the ground loop is the same solution that flows through the radiators. The skilled person will understand that any of the examples can be used with any of (or any combination of) air source, ground source, water source, or other suitable heat pumps. The DC battery powering the compressor, especially If powering directly and further if located in the same housing as the compressor, provides significant redundancy and efficiency advantages. In some examples of the invention, the heat pump system supplies heating or cooling to other types of heating system (not necessarily wet heating systems), such as to air conditioning units. Some example system setups will be briefly described. Referring to figure 12, another example of a heat pump system 700 comprises an outdoor air source heat pump unit 710 supplying heated air to an air conditioning unit 704 inside a building. The system 700 includes similar components to those described previously. The system 700 includes an outdoor heat pump unit 710 having a housing 712 containing a DC battery 720a therein. The air conditioning unit 704 has a housing containing a DC battery 720b therein. As described in relation to earlier examples, the DC batteries 720a and 720b can work together to supply continuous and on-demand heating as required. In this example, the air conditioning unit 704 is wall mounted and can also be powered by an AC power supply, e.g. mains AC. The power from DC supplies is available in both the heat pump circuit and the wall-mounted air conditioning unit to heat or to run off DC cells rather than an AC supply, e.g. at high-tariff periods or during a power cut. In some examples, the DC battery may be located only in the unit 710 or 704 or distributed between the two (as in this example). Referring to figure 13, another example of a heat pump system 800 comprises a ground source heat pump unit 810 supplying heated air to an air conditioning unit 804 inside a building. The system 800 includes similar components to those described previously. The system 800 includes an indoor heat pump unit 810 having a housing 812 containing a DC battery 820a therein. The system 800 includes a ground loop 814 similar to that previously described to exchange heat with the surrounding earth. The air conditioning unit 804 has a housing containing a DC battery 820b therein. As described in relation to earlier examples, the DC batteries 820a and 820b can work together to supply continuous and on-demand heating as required. In this example, the air conditioning unit 804 is wall-mounted and can also be powered by an AC power supply, e.g. mains AC. The power from DC supplies is available in both the heat pump circuit and the wall-mounted air conditioning unit to heat or to run off DC cells rather than an AC supply, e.g. at high-tariff periods or during a power cut. In some examples, the DC battery may be located only in the unit 810 or 804 or distributed between the two (as in this example). Boost mode In conjunction with any of the above-described examples, the heat pump system may optionally be arranged to operate in a boost or overrun mode, e.g. under the instruction of a controller. The compressor usually operates at a first power level for continuous operation (e.g. when providing its typical coefficient of performance). In the boost mode, the compressor is operated at a second power level that is higher than the first power level, and is arranged to deliver more aggressive heating than in its continuous operation. This boost mode may be used in relatively short bursts. In one example, boost mode may comprise operating for a short period of time, e.g. about 5 mins, at the second, higher power level, e.g. for a 5kWh battery pack, running at 15kW instead of 2kW. Other specific operating parameters will be apparent to the skilled person dependent on the use scenario. In some examples, in boost mode, the compressor is arranged to be powered only by the DC battery (since it can provide quick, aggressive, powerful heating compared to normal operation). In some examples, the DC battery is arranged to heat up heating elements located at / in or near components of the heating circuit (e.g. pipework, water tank etc.) to directly heat said components without the heat pump circuit. Battery size In conjunction with any of the above-described examples, the DC battery (whether a single battery or a distributed battery across multiple components) may have a capacity of at least 1,5kWh, and optionally at least 3.5kWh. Significant heating power can thereby be provided. The DC battery may comprises multiple cells, e.g. arranged in series. In some cases, the battery pack may be 2kWh to 5kWh (for a flat), 7kWh to 15kWh (for a typical house), 15kWh to 20kWh (for a large house). The skilled person will understand that these values may vary, e.g. based on insulation type, size of dwelling, environmental factors etc. Battery protection In conjunction with any of the above-described examples, the heat pump system may optionally comprise one or more battery protection feature(s). For example, any of the above-described heat pump systems may comprise battery protection means arranged to provide: heat protection for the DC battery; or cold protection for the DC battery; or both. The battery protection means may comprise a battery jacket (not shown) arranged to keep the battery warm or to keep the battery cool. The precise form of the battery protection means will depend on the expected conditions of use. Batteries might need to be protected outside from heat (sun etc.) and from the cold, e.g. to guard against infrared radiation from sun when batteries might overheat, or retain infrared radiation within the battery pack (e.g. via a reflective casing with reflective internal surface(s)) when batteries might require warming. The battery protection means is arranged to ensure that the DC power supply remains in suitable conditions (e.g. at a suitable temperature) to allow charging of the DC power supply, and ideally to allow efficient charging of the DC power supply. For example, some lithium batteries can not charge below OdegC. The battery protection means may include a mechanism arranged to warm cells. One way of protecting the DC battery from elements, especially outdoors (excess sun, cold, wind, rain etc.) is to locate the battery underground. The battery protection means may comprise a suitable chamber or housing arrangement arranged to safely locate the DC battery underground (fully or at least partially). The chamber or housing arrangement may be arranged to facilitate easy access for repair or replacement of the DC battery or component cells thereof. Even if the DC battery is located underground, its charging interface may be located above ground for easy access, e.g. via suitable wiring. Locating the DC battery underground may be particularly advantageous in conjunction with ground source heat pump systems of this invention since a common chamber or chambers may be used for locating the DC battery and the ground loop. This feature would be particularly efficient with a ground source heat pump, where some underground digging / disruption is already likely to be needed for most assemblies - it could also be used with other types of heat pump though (e.g. water source, or air source). In any of the described examples with a battery protection means, the heat pump system may comprise one or more thermal sensors arranged to sense temperature at at least one location in the heat pump system. The battery protection means is being arranged to operate based on sensed temperature at the at least one location. In this way the battery protection means can operate efficiently as required. This can be important in a heat pump system with a big DC battery as in this invention. Cooling system In conjunction with any of the above-described examples, the heat pump system may optionally comprise a cooling system for other components, such as for the power electronics, e.g. for any one or more of the or each rectifier, inverter, combined rectifier-inverter. In particular, in examples in which the compressor can be powered by a combination of AC and DC power supplies, the power electronics may be at risk of overheating (this is not a problem that would be envisaged in a simple AC-only heat pump or a simple DC-only heat pump). This specific problem arises due to power conversion and power switching demands caused by the controller switching between AC and DC power supplies due to the above-mentioned control factors. In such examples, the heat pump system of this invention may include a dedicated cooling system for the power electronics. The cooling mechanism may be arranged to direct cooled refrigerant close to any one or more of: the DC battery; the battery charger; the AC-DC rectifier; and the DC-DC converter, to transfer heat therebetween and to thereby cool the electronic component. Waste heat capture mechanism In conjunction with any of the above-described examples, the heat pump system may optionally comprise a waste heat capture mechanism arranged to capture waste heat from any one or more of: the DC battery; the battery charger; the AC-DC rectifier; and the DC-DC converter, and to transmit the captured heat to the evaporator. The waste heat capture mechanism may be arranged to transmit the captured heat to the evaporator via a waste heat fluid circuit, such as an air flow circuit or liquid circuit. The waste heat fluid circuit may, in some examples, comprise a waste heat fluid pipe containing the waste heat transfer fluid, which is arranged to transmit the captured heat to the evaporator. Further advantageous scenarios Water temp and disease: Water stored in hot water tanks, especially in a heat pump heating system can be at relatively low temperature (they never / hardly ever get very hot). Harmful bacteria can grow and proliferate in such systems (since water does not get hot enough to kill the harmful bacteria). In any of the described examples, the heat pump system of this invention may include a pasteurization feature that occasionally, e.g. once a week, via direct electric heating (using DC power to power electric heating elements located at an appropriate part of the fluid circuit, e.g. a) associated with circuit pipework near, or elements provided directly within, a water tank and / or b) pipework anywhere else in the system) gets the water in the tank hot enough to kill harmful bacteria. In some examples, the heat pump system is arranged to boost temperatures on demand within the fluid network (radiator or potable or both), e.g. in a water tank or if supplying direct to taps. In examples that comprise air source heat pumps, especially if the air is moist and outside temperature is cold, traditional heat pump systems often have condensation freeze issues. This means they have to be run in reverse to melt / defrost their heat exchanger(s) and allow efficient operation. In any of the examples of this invention, the DC battery can be used to power embedded heating elements to directly defrost frozen components (e.g. vents, fan, pipework). In addition, or instead, the DC battery can be used to take over powering the heating demand while the heat pump carries out the defrosting activity -advantageously, the inventive heat pump system is able to operate with no break or lull in heating performance, as would be the case in a traditional heat pump system. Optionally, in some examples, the heat pump system uses the heat wasted in electronics being able to feed into the evaporative process so that lost heat can be extracted and used. The whole system is thereby made more efficient. Optionally, waste heat from the fan motor can be used in a similar manner. As described above, heat pumps can be run in reverse to provide cooling instead of heating, or vice versa. In some examples, the cold-creating-ability of operating the inventive heat pump system in reverse can be used to cool power electronics or the DC battery cells. In this invention, this feature works particularly well since the electronics burden is higher than for a typical heat pump system due to the large DC battery. In particular, as previously described, in examples in which AC and DC is combined to power the compressor, this feature may be particularly useful. Various modifications will be apparent to the skilled person without departing from the scope f this invention. Any of the power electronics configurations (described in relation to figures 3, 4, 5 and 6) can be used with any of the described heat pump system examples. Other power electronics configurations may also be apparent to the skilled person within the scope of the claims. In any of the described examples, heating may be directly provided by the heat pump circuit as described, or / and may be supplemented (in addition to heating via the compressor and heat pump cycle) or boosted by direct heating via heating elements powered by the DC battery. Such heating elements may be placed in, on, around, embedded (completely or partially) in walls of pipework or other components of any of the previously-described heat pump systems (this includes components such as boilers, air conditioning units, pipes, fans or vents). On-demand hot water, e.g. potable water requires relatively short bursts of high power. The DC battery of this invention is particularly useful in some examples to provide extra bursts of power to the compressor. Some examples of the invention include a controller or controllers. In any of these examples, a single controller having global oversight of supply, demand and environmental factors may replace multiple distributed controllers. In examples with a hydraulic unit, the hydraulic unit may be wall-mounted. Fluid heaters of the type described in the applicant’s co-pending GB2115477.8, GB2115478.6 or GB2115479.4 are mentioned in this specification. The disclosures of these published applications are incorporated by reference. Any reference to radiators includes wall- or ceiling-mounted or underfloor heating radiators or any other known radiator types. In any of the examples, the DC battery may also be arranged to power (in addition to powering the compressor) any one or more of the other heat pump system components, such as the heat pump fan 5 motor (if present), AC / DC control system, any heat pump system valves, main electronics control system, any claves or pumps used for flow of fluid around the system. In any of the described examples, the compressor may be a reciprocating compressor, a scroll compressor, a screw compressor, a rotary compressor, a centrifugal compressor, or any other suitable, known type of compressor. 10 Any reference to buildings includes parts of buildings (e.g. a flat in a block of flats).

Claims

1. A heat pump system arranged to heat or cool a building, wherein the heat pump system comprises a heat pump circuit including:an evaporator configured to absorb heat from a source;a DC compressor configured to compress a refrigerant and increase its temperature;a condenser configured to release heat from the refrigerant into a sink; andan expansion valve configured to regulate the flow of refrigerant and lower its temperature before it re-enters the evaporator; andwherein the refrigerant is arranged to flow continuously through the circuit, from the evaporator to the DC compressor, to the condenser, to the expansion valve, and back to the evaporator, to provide said heating or cooling to the building,the heat pump system further comprising a DC battery arranged to at least partially power the DC compressor,the heat pump system further comprising one or both of:a)• DC-AC inverter arranged to convert DC power from the DC battery to AC;• a first appliance interface connected to the DC-AC inverter and arranged to power external appliances interfaced therewith, and• a two-way inverter-rectifier in the form of AC-DC and DC-AC in a single unit;such that external appliances, such as household appliances, can be powered by the DC battery; andb) a DC appliance interface connected to the DC battery, optionally via a DC-DC converter, and arranged to power external appliances interfaced therewith, such that external appliances, such as household appliances, can be powered by the DC battery.

2. The heat pump system of claim 1, wherein the DC battery has a capacity of at least 1.5kWh, and optionally at least 3.5kWh.

3. The heat pump system of any preceding claim comprising battery protection means arranged to provide: heat protection for the DC battery; or cold protection for the DC battery; or both.

4. The heat pump system of claim 3, wherein the battery protection means comprises any one or more of:• a thermal jacket arranged to protect the DC battery from a cold environment;• warm refrigerant flow past the battery, e.g. pre- or post- compressor, arranged to protect the DC battery from a cold environment;• an electric heater arranged to protect the DC battery from a cold environment;• double or triple wall insulation arranged to protect the DC battery from a cold environment;• a Peltier device arranged to protect the DC battery from a cold environment;• a thermal jacket arranged to protect the DC battery from a hot environment;• coolant arranged to flow past the battery, e.g. via a coolant pipe network, arranged to protect the DC battery from a hot environment;• cold water return, e.g. from a radiator circuit, arranged to flow past the battery;• double or triple wall insulation arranged to protect the DC battery from a hot environment;• a Peltier device arranged to protect the DC battery from a hot environment.

5. The heat pump system of claim 4 comprising thermal sensors arranged to sense temperature at at least one location in the heat pump system and the battery protection means being arranged to operate based on sensed temperature at the at least one location.

6. The heat pump system of any preceding claim comprising a heat pump system, optionally a ground source heat pump system, wherein the DC battery is arranged to be located at least partially underground.

7. The heat pump system of any preceding claim, wherein the DC battery is arranged to directly power the DC compressor.

8. The heat pump system of any preceding claim further comprising an AC-DC rectifier arranged to convert an AC power supply, such as a mains AC power supply, to DC and arranged to thereby power the DC compressor.

9. The heat pump system of any preceding claim comprising a battery charger arranged to charge the DC battery.

10. The heat pump system of claim 9, wherein the battery charger comprises any one or more of:• an AC-DC charger arranged to charge the DC battery from an AC source, such as mains AC;• a DC-DC charger arranged to charge the DC battery from a DC source, such as a solar power source, optionally via a DC-DC converter;• a two-way inverter-rectifier in the form of AC-DC and DC-AC in a single unit; and• a three-phase AC-DC charger arranged to charge the DC battery from a three-phase AC source.

11. The heat pump system of claim 10 when dependent on any of claims 9 to 12, wherein the AC-DC rectifier is arranged to charge the DC battery.

12. The heat pump system of claim 11, wherein the AC-DC rectifier is arranged to charge the DC battery via a DC-DC converter arranged to change the voltage to a desired charging voltage.

13. The heat pump system of any of claims 8 to 12, wherein the AC-DC rectifier and the DC-AC inverter are located on the same electronics module.

14. The heat pump system of any preceding claim, wherein any one or more of the voltage inputs and / or outputs of the: DC compressor; DC battery; AC-DC rectifier; DC-AC inverter; and battery charger are matched.

15. The heat pump system of any preceding claim comprising a cooling mechanism arranged to cool any one or more of: the DC battery; the battery charger; the AC-DC rectifier; and the DC-DC converter, wherein the cooling mechanism is arranged to direct cooled refrigerant close to any one or more of: the DC battery; the battery charger; the AC-DC rectifier; and the DC-DC converter, to transfer heat therebetween.

16. The heat pump system of any preceding claim comprising a waste heat capture mechanism arranged to capture waste heat from any one or more of: the DC battery; the battery charger; the AC-DC rectifier; and the DC-DC converter, and to transmit the captured heat to the evaporator.

17. The heat pump system of any preceding claim comprising a controller arranged to control one or more of:• DC battery charging;• DC battery discharging;• switching of the DC battery;• the cooling mechanism;• distribution of power to the DC compressor from the DC battery and the AC power supply; and• combining of the outputs from the AC power supply and the DC power supply.

18. The heat pump system of any of claim 17 wherein the system can be operated in a boost mode in which the DC compressor runs at higher-than-average power, and wherein the controller is arranged to power the DC compressor using only the DC battery in the boost mode.

19. The heat pump system of any preceding claim, wherein the controller is arranged to control activities taking into account any one or more of:• capacity of the DC compressor;• capacity of the or each of the battery or AC power supply;• instantaneous demand for tap water or heating water or heated air;• forecasted demand for tap water or heating water or heated air;• instantaneous or forecasted available supply type; and• weather patterns.

20. The heat pump system of any preceding claim further comprises a heat pump housing arranged to house the DC compressor, and optionally any one or more of: the DC battery; the expansion valve; the evaporator; and the condenser.

21. The heat pump system of any claim 20 comprising one or more electric heating elements located on, or near, or embedded at partially in, the heat pump housing, wherein the one or more electric heating elements are arranged to be powered by the DC battery to provide heat for frost protection of components of the heat pump system or to otherwise provide heat for components of the heat pump system to facilitate operation.

22. A combined heat pump and water tank system comprising the heat pump system of any preceding claim in fluid communication with a water tank, wherein the heat pump system is arranged to heat water in the water tank, and wherein the boost mode is arranged to heat water in the tank above a threshold pasteurisation temperature.

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