GB2592992A

GB2592992A - Heat-pump load shifting

Info

Publication number: GB2592992A
Application number: GB2003706.5A
Authority: GB
Inventors: Gibb Russell
Original assignee: Planet Heat Ltd
Current assignee: Planet Heat Ltd
Priority date: 2020-03-13
Filing date: 2020-03-13
Publication date: 2021-09-15
Also published as: GB202003706D0

Abstract

A load-shifting heat pump system in which a heat pump 402 is operable to charge a latent heat thermal store 406 in series with a sensible heat load 404, 408, with the heat pump’s fluid output flow directed firstly via the latent heat thermal store then secondly via the sensible heat load prior to returning to the heat pump. The sensible heat load may be configured to reduce the return temperature of the flow to the heat pump by greater than 10°C from its temperature exiting the latent heat thermal store. The system may further charge the latent heat thermal store using heat from the same sensible heat load that it was in series with for its charging. The latent heat store may have a bypass 412 to direct fluid flow from the heat pump via the sensible heat load while bypassing the latent heat thermal store.

Description

HEAT-PUMP LOAD SHIFTING

The present invention relates to a load-shifting heat pump system, a method of heat-pump load shifting, a controller for operation with a load-shifting heat pump system, and an associated computer program product.

Background Art

A typical electric domestic heating system has a heat source, storage for hot water, and radiators for space heating.

When electricity is used to generate thermal energy as the heat source, it is preferable to generate the thermal energy when the electricity costs less, such as during cheap-rate electricity periods, for example overnight. The thermal energy generated with cheap-rate electricity is then kept in a thermal store and released as heat at different times, for example during the day.

Production of stored domestic hot water (DHW) can easily be timed to occur during cheap-rate electricity periods, such as at night. Thermal energy in the hot water is routed from a hot water thermal store to taps and showers when required.

In this case the thermal store is often a DHW hot water cylinder using resistive "immersion" heating as the heat source.

Conventional night storage heaters generate thermal energy using resistive heating with cheap-rate electricity overnight and store it in solid blocks, so the stored thermal energy is released as heat transfer to the house from the heater during the day, by radiation and convention. In this case the thermal store is the solid blocks.

In both of these cases, the thermal store is a sensible heat store in which the exchange of heat with the thermal store causes the temperature of the thermal store to change. The hot water cylinder and night storage heater are thus referred to as sensible heat thermal stores (SHTS).

In contrast to sensible heat thermal stores (SHTS), latent heat thermal stores (LHTS), use the phase transition of a material. Upon melting, heat is transferred to the material, storing large amounts of heat at constant temperature; the heat is released when the material solidifies. The material of the latent heat thermal store is commonly a phase change material (PCM).

Adding thermal energy to a thermal store (SHTS or LHTS) can be referred to as charging the thermal store. Charging a thermal store at one point in time and subsequently discharging it later by transferring heat to a thermal load can be described as load shifting, demand side management, or agile load control.

Nowadays using a heat pump is the most efficient method for generating heat using electricity, providing heat at a typical energy efficiency of 280% compared to the 95% from electric night storage heaters. A heat pump is a device that transfers thermal energy from a source of heat (e.g. environmental air, ground or water heat) to a thermal load. In a heat pump, external power, normally electricity, is used to perform the work of transferring the thermal energy from the source to the thermal load.

A thermal store (SHTS or LHTS) can be charged using a heat pump.

Thermal loads which also receive thermal energy from a heat pump in a domestic system may be a space-heating thermal emitter, such as a radiator, fan coil and/or a domestic hot water (DHW) cylinder.

Electrically-powered heat pumps can be used for charging thermal stores such as DHW cylinders using off-peak electricity. However, for space heating typical heat pump systems consume energy during peak electricity price periods, with only a relatively small proportion of the electricity consumption occurring during-cheap rate hours. The price of electricity for space heating is therefore subject to the tariff imposed by the electricity provider at the time of use.

Load shifting using a thermal store charged by a heat pump combines the ability to maximise use of cheap-rate electricity with the efficiencies provided by a heat pump. It is possible to perform load shifting conventionally using sensible heat thermal stores (such as large water tanks), but in order to store sufficient energy to be effective they need to be very large.

A very space-efficient (and low-loss) method for load shifting is to use PCM latent heat thermal stores (LHTS), mentioned above. However, these LHTS devices have charging characteristics very different to water-based sensible heat thermal stores (SHTS). In particular latent heat thermal stores (LHTS) require a high temperature to charge them, not easily provided by "standard" heat pumps based on (for example) R407c, R41 Oa or R32 refrigerants.

A known example is illustrated in Figures lA and 1B, that combines a high temperature single-stage or cascade heat pump 102 (using R41 Oa, R407c and/or R134a refrigerants) with a PCM latent heat thermal store (LHTS) 106 and a timeclock synchronised to low-rate electricity. At off-peak times the system charges the PCM LHTS 106 with thermal energy generated using cheap-rate electricity for later consumption. The output from the heat pump 102 must be over 58°C, to properly charge this particular phase-change material in the LHTS. The return temperature exceeding 50°C (for this example load) means there is typically a range of 3°C to 10°C outlet-to-return temperature difference AT. A second stage in a cascaded heat pump system could readily charge a PCM58 LHTS at higher temperatures. With reference to Figure 1B, during peak times the stored thermal energy is consumed by discharging heat from the PCM LHTS 106 into a sensible heat load 108, such as a central heating (CH) emitter or fan coil. The discharging can be performed by pumping a process fluid around a circuit via the LHTS 106 and sensible heat load 108. A process fluid can be a liquid or gas, or a multi-phase fluid.

A problem with such systems is that using cascaded heat pumps can be bulky, noisy and expensive, and using a single-stage heat pump can require the heat pump to run very hard at the limit of its temperature and pressure operational envelope. As a result, reliability could be lower than desired for such systems.

As illustrated in Figure 2, a CO2 heat pump 202, which is environmentally friendly because it does not use refrigerants with high Global Warming Potential (GWP), can be used with a thermal load 204 such as a Domestic Hot Water sensible heat thermal store (DHW SHTS), or space-heating thermal emitters, such as central heating (CH) emitters also known as radiators. The output from the CO2 heat pump in this example is 65°C. The return temperature of 40°C (for this example sensible heat load) means there is a 25°C outlet-to-return temperature difference AT. Charging of the DHW can be timed to occur during cheap-rate tariff periods to take advantage of cheaper electricity, but space heating remains on-demand even at peak-rate times, and is subject to the price of electricity at the time of consumption.

It would be advantageous to be able to store space heating energy in order to provide load-shifting, so that the heat pump is running less during peak-rate times and more during cheap-rate times. Using a PCM thermal store would provide the storage in an efficient, compact form. However, it is difficult to use high temperature CO2 heat pumps with PCM LHTS.

EP3121522A1 discloses a latent heat thermal store (latent heat storage material 1 in Figure 1 of EP3121522A1) charged by a heat pump (heating means 8), where the latent heat thermal store is arranged inside a sensible heat thermal store (heat storage means 2). Carbon dioxide (CO2) and HFC refrigerants are used as the refrigerant. Water flowing through a heat storage pipe (15) exchanges heat with the latent heat storage material (1) in the heat storage means (2).

A problem with this arrangement is that it requires a specially-constructed thermal store, so cannot be used with off-the-shelf latent heat thermal stores, and cannot be retrofitted to an existing sensible heat thermal store. Furthermore, the heat exchange between the latent heat thermal store and sensible heat thermal store is achieved via conduction limited to the surface area of the latent heat thermal store.

Summary of invention

It is desirable to provide a load-shifting heat pump system that can use a simple arrangement of off-the-shelf heat pumps and thermal stores, that may be retrofitted to existing systems.

According to a first aspect of the present invention, there is provided a load-shifting heat pump system comprising: a heat pump; a latent heat thermal store; and a sensible heat load, wherein the heat pump is operable to charge the latent heat thermal store in series with the sensible heat load, with the heat pump's fluid output flow directed firstly via the latent heat thermal store then secondly via the sensible heat load, prior to returning to the heat pump.

Preferably, the sensible heat load is configured to reduce the return temperature of the flow to the heat pump by greater than 10°C from its temperature exiting the latent heat thermal store.

Preferably, the load-shifting heat pump system is operable to subsequently further charge the latent heat thermal store using heat from the same sensible heat load that was in series with the latent heat thermal store for its charging.

Preferably, the load-shifting heat pump system is operable to discharge the latent heat thermal store to a sensible heat load that was in series with a latent heat thermal store for its charging.

Preferably, the load-shifting heat pump system further comprises a controller configured to control the operation of the heat pump to charge the latent heat thermal store in series with the sensible heat load, by directing the heat pump's fluid output flow via the latent heat thermal store and via the sensible heat load, prior to returning to the heat pump.

Preferably, the controller is configured to control a sequence of: directing fluid flow from the heat pump to charge the latent heat thermal store in series with the sensible heat load, and directing fluid flow through the latent heat thermal store to discharge the latent heat thermal store.

Preferably, the controller is configured to control the sequence synchronised with different rate electricity tariff periods.

Preferably, the sensible heat load comprises a hot water thermal store.

Preferably, the sensible heat load comprises one or more space-heating thermal emitters.

Alternatively, the sensible heat thermal load comprises a sensible heat thermal store, and the heat pump is operable to pre-charge the latent heat thermal store in series with a sensible heat load before charging the latent heat thermal store in series with the sensible heat thermal store.

Preferably, the load-shifting heat pump system further comprises a latent heat thermal store bypass operable to direct fluid flow from the heat pump via the sensible heat load while bypassing the latent heat thermal store.

Preferably, the heat pump comprises a CO2 heat pump.

Preferably, the heat pump is operable to produce fluid output flow with a temperature greater than or equal to 60°C.

According to a second aspect of the present invention, there is provided a method of heat-pump load shifting, comprising operating a heat pump to charge a latent heat thermal store in series with a sensible heat load, by directing the heat pump's fluid output flow firstly via the latent heat thermal store and secondly via the sensible heat load, prior to returning to the heat pump.

Preferably, the method comprises using the sensible heat load to reduce the return temperature of the flow to the heat pump by greater than 10°C from its temperature exiting the latent heat thermal store.

Preferably, the method comprises subsequently further charging the latent heat thermal store using heat from a sensible heat load that was in series with a latent heat thermal store for its charging.

Preferably, the method comprises discharging the latent heat thermal store to the same sensible heat load that was in series with the latent heat thermal store for its charging.

Preferably, the method comprises a sequence of: directing fluid flow from the heat pump to charge the latent heat thermal store in series with the sensible heat load, and directing fluid flow through the latent heat thermal store to discharge the latent heat thermal store.

Preferably, the method comprises synchronising the sequence with different rate electricity tariff periods.

Preferably, the method comprises pre-charging the latent heat thermal store in series with a sensible heat load before charging the latent heat thermal store in series with the sensible heat thermal load, which comprises a sensible heat thermal store.

Preferably, the method further comprises the step of directing fluid flow from the heat pump via the sensible heat load while bypassing the latent heat thermal store.

According to a third aspect of the present invention, there is provided a controller for operation with a load-shifting heat pump system comprising a heat pump, a latent heat thermal store and a sensible heat load, wherein the controller is configured to output control signals to implement the method of the second aspect.

According to a fourth aspect of the present invention, there is provided a computer program product comprising machine readable instructions, which when executed on a controller, cause the controller to output control signals to perform the steps of a method of the second aspect.

Brief description of drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the drawings, in which: Figures lA and 1B illustrate, in schematic form, a conventional load-shifting heat pump system using a latent heat thermal store charged by a heat pump.

Figure 2 illustrates, in schematic form, conventional use of a CO2 heat pump with a sensible heat load.

Figure 3 illustrates, in schematic form, a CO2 heat pump charging a latent heat thermal store (LHTS).

Figure 4 illustrates, in schematic form, a load-shifting heat pump system, in accordance with an embodiment of the present invention.

Figure 5 illustrates, in schematic form, directing fluid flow from the heat pump to charge the latent heat thermal store in series with space-heating thermal emitters, in accordance with an embodiment of the present invention.

Figure 6 illustrates, in schematic form, directing fluid flow from the heat pump to charge the latent heat thermal store in series with a hot water thermal store, in accordance with an embodiment of the present invention.

Figure 7 illustrates, in schematic form, directing fluid flow through the latent heat thermal store to discharge it.

Figure 8 illustrates, in schematic form, further charging the latent heat thermal store using heat from the same sensible heat load as used in series for the latent heat thermal store's charging, in accordance with an embodiment of the present invention.

Figure 9 illustrates, in schematic form, directing fluid flow from the heat pump via the sensible heat load while bypassing the latent heat thermal store to meet a space heating demand.

Figure 10 illustrates, in schematic form, meeting a space heating demand as illustrated in Figure 9 at the same time as discharging the thermal store into the same space heating load as illustrated in Figure 7.

Figure 11 is a graph, illustrating the power absorption versus time of latent heat and sensible heat thermal stores.

Description of embodiments

Figure 3 illustrates a problem of using a CO2 heat pump with a latent heat thermal store (LHTS). If it did work properly, such an arrangement would gain the benefits of a CO2 heat pump and also the benefits of load shifting by charging the LHTS using low-cost electricity at an off-peak time. However, it does not work properly, for the reasons given below.

CO2 based heat pumps can readily achieve temperatures sufficiently high to charge a PCM LHTS, but, when connected as shown in Figure 3, high temperature CO2 heat pumps do not readily charge PCM LHTS. As shown by the temperatures indicated in Figure 3, latent heat loads are not readily charged using CO2 heat pumps because they exhibit high thermal resistance, resulting in a temperature difference across them that is too small, producing a return temperature that is too high for the heat pump to cope with. CO2 heat pumps need a large difference (AT) between output flow and return flow temperatures of the process fluid in order to work effectively and efficiently. The smaller the difference the less efficient the heat pump becomes and the lower the output it produces.

Therefore, in conventional practice, a low-carbon CO2 heat pump 302 cannot be used with a LHTS 306 in such an arrangement, because the temperature difference AT is too small, typically in the range 3°C to 8°C (e.g. as shown in Figure 3, AT= 65°C -57°C = 8°C) resulting from the return temperature being too high (e.g. 57°C), for efficient operation of the CO2 heat pump.

Embodiments achieve a large enough temperature difference for efficient CO2 heat pump operation by charging the PCM thermal store in series with a "normal" (sensible heat) load, thus lowering the return temperature. In these examples, a sensible heat wet load is used. Wet loads include the DHW hot water cylinder supplied as part of a standard heat pump system, or the central heating (CH) heat emitters. It is possible to add additional wet loads to the system (e.g. a buffer tank or an air handling unit) to provide an additional sensible heat load. These all function to keep the return temperature low enough for effective CO2 heat pump operation.

In an example load-shifting heat pump system, during each charging cycle a DHW cylinder is series-connected with the LHTS (as shown in Figure 6), thus providing stored space heating and DHW thermal energy in one charging period. By combining the PCM LHTS and DHW charging the return temperature can be kept low and the CO2 heat pump run at peak output and flow temperature needed to effectively charge PCM material during the charging period. If there is insufficient demand for DHW the proposed system may instead combine LHTS charging with space heating (as shown in Figure 5) to ensure the return temperature is kept as low as possible. When using series-connected latent heat thermal store and sensible heat loads, because a latent heat thermal store requires the highest flow temperatures in order to change phase, the PCM LHTS is charged first by the heat pump, as shown in Figures 5 and 6.

Thus the inventor has found a way to reduce the return temperature, while integrating the solution in conventional components of a heating system. Advantages are found in the way that the CO2 heat pump system and a phase-change material (PCM) thermal store (LHTS) are integrated and the way that the entire system synchronises with a cheap-rate electricity tariff to perform load shifting.

With reference to Figure 4, the load-shifting heat pump system 400 has a heat pump 402 and a latent heat thermal store 406.

In order to provide load shifting, in this example an off-the-shelf heat pump 402 is used with a phase-change material (PCM) latent heat thermal store (LHTS) 406 that offers very high energy density in small rectangular form.

In this example, the heat pump is a CO2 heat pump, that is operable to produce process fluid output flow with a temperature generally greater than 60°C. For the selected phase change material in the latent heat thermal store 406, it doesn't necessarily have to be charged using a CO2 heat pump. Another heat pump capable of efficiently and reliably producing temperatures greater than or equal to 60°C would be suitable. Embodiments of the present invention are not limited to using to use CO2 as a refrigerant, as other refrigerants may be used that can readily provide temperatures in excess of 60°C. The heat pump 204 is powered by an electrical supply 424. Load shifting allows the use of cheap-rate electricity from the supply 424.

Two sensible heat loads 404, 408 are illustrated in this example. One sensible heat load is a hot water thermal store 404. The hot water thermal store 404 may be charged by the heat pump 402 via the latent heat thermal store 406. This is illustrated in Figure 6. The hot water thermal store 404 is discharged through normal domestic consumption of hot water. In the case where hot water thermal store 404 (that is used to reduce the return temperature) comprises a buffer vessel then it may be discharged through normal central heating consumption.

Another sensible heat load comprises one or more space-heating thermal emitters 408. The space-heating thermal emitters 408 may be charged by the heat pump 402 via the latent heat thermal store 406. This is illustrated in Figure 5, with the hot water thermal store 404 being bypassed by the controller directing the flow through bypass pipe 410.

In both of those cases, the heat pump 402 is operable to charge the latent heat thermal store 406 in series with the sensible heat load 404, 408, with the heat pump's fluid output flow directed firstly via the latent heat thermal store 406 then secondly via the sensible heat load 404, 408, prior to returning to the heat pump 402.

Another way of describing the arrangement is that the heat pump's fluid output flow charges the latent heat thermal store and returns to the heat pump via the sensible heat load.

The sensible heat load is configured to reduce the return temperature of the flow to the heat pump by greater than 10°C from its temperature exiting the latent heat thermal store.

The skilled person will appreciate that the pipework leading back from the LHTS 306 to the heat pump 302 in Figure 3 is not a sensible heat load as such. The pipework and its environment may reduce the return temperature of the flow to the heat pump by a few degrees, or less (well insulated pipework will present a load of less than 10 Watts per meter), from its temperature exiting the latent heat thermal store. The sensible heat load in embodiments may be configured in different ways. For example, the sensible heat load may be configured for thermal storage. It may be configured for thermal emission, such as by radiation and convention in the case of a radiator. It may be configured for discharge, such as for DHW cylinder.

As described with reference to Figure 8, the load-shifting heat pump system 400 is operable to subsequently further charge (top-off) the latent heat thermal store 406 using heat from the same sensible heat load 404 that was in series with the latent heat thermal store for its charging (as shown in Figure 6). Alternatively (not shown), the latent heat thermal store 406 may be further charged using heat from a different sensible heat load from the one that that was in series with the latent heat thermal store for its charging. At times of particularly cheap (or negatively-priced) electricity any thermal store could also or alternatively be charged using a built-in electric immersion element.

The load-shifting heat pump system may be operable to discharge the latent heat thermal store to the same sensible heat load that was in series with the latent heat thermal store for its charging. These steps are illustrated by Figure 5 then Figure 7, with the sensible heat load being the space-heating thermal emitters 408. In a two-zone heating system, with one LHTS per zone but discharging into the other, the load-shifting heat pump system may be operable to discharge the latent heat thermal store to a different sensible heat load (zone) that was in series with a different [HIS for its charging.

A controller 420 is configured to control the operation of the heat pump 402 to charge the latent heat thermal store 406 in series with the sensible heat load 404, 408, by directing the heat pump's fluid output flow via the latent heat thermal store 406 and via the sensible heat load 404, 406, prior to returning to the heat pump 402. The controller 420 decides the timing of operations of the system and how the process fluid is directed between the latent heat thermal store 406 and the sensible heat thermal stores 404, 408.

The controller 420 interfaces between various valves 428, pumps 430, 432, sensors (not shown) and a built-in heat pump controller (not shown) within heat pump 402. By communicating with the built-in heat pump controller the controller 402 can control the heat pump's functionality as required. It also communicates with a cheap-rate tariff electricity provider via the internet and/or a smart meter 422 feeding the heat pump (or house in which the heat pump is located).

The controller 420 is configured to output control signals to operate the heat pump system as described with reference to Figure 4 and to implement the method described with reference to Figures 5 to 9.

A computer program product, such as memory card 426 has stored on it machine readable instructions, which when executed on the controller, cause the controller to output the control signals to operate the heat pump system as described with reference to Figure 4 and to implement the method described with reference to Figures 5 to 9.

The controller 420 is configured to control a sequence, directing fluid flow from the heat pump 402 to charge the latent heat thermal store 406 in series with the sensible heat load 408, 404 (as shown in Figure 5 or Figure 6 respectively) then directing fluid flow through the latent heat thermal store 406 to discharge it (as shown in Figure 7). The controller is configured to control the sequence synchronised with different rate electricity tariff periods, using information from the smart meter 422. The controller receives input from a variety of sensors. The system thus responds to internal timings, control from a smart meter or communication via the internet (e.g. an API, data service update or other protocol).

The controller 420 can charge the latent heat thermal store 406 when there is a timed, predicted or sensed demand for heating. The aim is to charge the LHTS fully during times of low energy prices.

The sensible heat thermal load for the series-charging comprises a sensible heat thermal store (in this case a hot water thermal store) 404, and in a mode of operation, the heat pump 402 is operable to pre-charge the latent heat thermal store 406 in series with a different sensible heat load (space-heating thermal emitters 408 as shown in Figure 5) before charging the latent heat thermal store 406 in series with the sensible heat thermal store 404 (as shown in Figure 6).

A latent heat thermal store bypass 412 pipe directs fluid flow from the heat pump 402 via the sensible heat load 408 while bypassing the latent heat thermal store 406. It's use is illustrated in Figure 9.

Figure 5 illustrates, in schematic form, directing fluid flow from the heat pump to charge the latent heat thermal store in series with space-heating thermal emitters, in accordance with an embodiment of the present invention. The method of heat-pump load shifting involves operating the heat pump 402 to charge the latent heat thermal store 406 in series with space-heating thermal emitters 408, by directing the heat pump's fluid output flow firstly via the latent heat thermal store 406 and secondly via the space-heating thermal emitters 408, prior to returning to the heat pump 402.

Using the sensible heat load (space-heating thermal emitters) 408 reduces the return temperature of the flow to the heat pump by greater than 10°C from its temperature exiting the latent heat thermal store. In this example, the return temperature is -10 to 40°C, which with a heat pump output temperature of 65°C gives an output-to-return temperature difference AT of -55°C to 25°C, which is suitable for efficient CO2 heat pump operation, in contrast to the inferior AT of 10°C in the arrangement shown in Figure 3.

When the heat pump is charging the series arrangement, at the end of a charging period the return temperature of the flow to the heat pump may be only about 5°C lower than the flow temperature exiting the latent heat thermal store. In practice for approximately the last two hours of charging the AT may be less than 10°C. However, performance is very poor when the heat pump output-to-return temperature difference AT is < 10°C but embodiments may still be operated in this mode.

Figure 6 illustrates, in schematic form, directing fluid flow from the heat pump to charge the latent heat thermal store in series with a hot water thermal store, in accordance with an embodiment of the present invention. This method of heat-pump load shifting includes the steps of operating the heat pump 402 to charge the latent heat thermal store 406 in series with a hot water thermal store 404, by directing the heat pump's fluid output flow firstly via the latent heat thermal store 406 and secondly via the hot water thermal store 404, prior to returning to the heat pump 402. It is also possible to partially charge the LHTS when the system is performing space heating, rather than a defined period. This gives the defined period a greater chance of obtaining a full charge.

The steps described with reference to Figures 5 and 6 may be combined. This involves pre-charging the latent heat thermal store 406 in series with a sensible heat thermal load 408 (as per Figure 5) before (in the temporal sense) charging the latent heat thermal store 406 in series with a different sensible heat thermal load, which in this case comprises a sensible heat thermal store 404 (as per Figure 5).

Figure 7 illustrates, in schematic form, directing fluid flow through the latent heat thermal store to discharge it to the same sensible heat load that was in series with the latent heat thermal store for its charging (as illustrated in Figure 5). In this example, the sensible heat load is the space-heating thermal emitters 408. In another arrangement (not shown), the sensible heat load used for series charging and then discharging of the latent heat thermal store 406 is a sensible heat thermal store, such as the hot water thermal store 404.

Figure 7 is thus just one example of discharging the latent heat thermal store. The charge and discharge sequence is performed by the controller 420 outputting signals to perform a sequence of directing fluid flow from the heat pump 402 to charge the latent heat thermal store 406 in series with the sensible heat load 408, and directing fluid flow through the latent heat thermal store 406 to discharge it. The charge and discharge sequence is synchronised with different rate electricity tariff periods.

Figure 8 illustrates, in schematic form, further charging the latent heat thermal store using heat from the same sensible heat load as for its charging, in accordance with an embodiment of the present invention.

While the latent heat thermal store 406 is charged as illustrated in Figure 6, as the return temperature from the series-connected sensible heat thermal store 404 to the CO2 heat pump 402 increases, the efficiency of the heat pump 402 decreases. A point may come where it is more efficient to cease using the heat pump 402 to charge the latent heat thermal store 406 but instead use pump 432 to reverse flow from the sensible heat thermal store 404 back through pipe 434 into the latent heat thermal store 406 to complete the charge. Thus, after the latent heat thermal store 406 is charged as illustrated in Figure 6, it is possible to subsequently further charge (i.e. top-off) the latent heat thermal store 406 using heat from the same sensible heat load 404 that was in series with the latent heat thermal store 406 for its charging.

In this example, water flows from the heat pump through the LHTS then into the top of the SHTS. To get access to the hottest water it has to be drawn from the top of the SHTS to be put into the LHTS. This is achieved using reverse flow. Energy can be taken out of or injected into either end of the LHTS, unlike the SHTS which is stratified to keep the return temp as cold as possible. Charging fluid flow for the SHTS is injected into the top of it and is drawn from the top of it in reverse mode. 10 Valves can be used to achieve the reverse flow, as illustrated, although other arrangements are possible, for example having water flow the wrong way through a pump when it's not energised yet turning it on to reverse the flow in the pipework.

If electricity is cheap enough direct electric immersion could instead be used to perform the topping off.

The benefit of load shifting is achieved by avoiding running the heat pump when electricity is expensive, which on some tariffs typically lasts at least six hours in each day. The charge within the latent heat thermal store 406 may be used to skip periods when electricity is expensive. When not charging the latent heat thermal store 406 or sensible heat thermal store 404 the heat pump 402 can operate as normal to meet central heating demand. This can be implemented by the controller 420 directing fluid flow from the heat pump 402 via the sensible heat load 408 while bypassing the latent heat thermal store 406.

Figure 10 illustrates, in schematic form, meeting a space heating demand as shown in Figure 9 at the same time as discharging the thermal store into the same space heating load as illustrated in Figure 7.

As described with reference to Figure 9, the heat pump 402 operates as normal to meet central heating demand. This can be implemented by the controller 420 directing fluid flow from the heat pump 402 via the sensible heat load 408 while bypassing the latent heat thermal store 406. At the same time the latent heat thermal store 406 is discharged into the same space heating load 408 as illustrated in Figure 7.

Figure 11 is a graph, illustrating the power absorption versus time of series-connected latent heat and sensible heat thermal stores.

The LHTS requires a consistently high flow temperature in order to melt the phase-change material. In order for a heat pump to consistently produce that high temperature efficiently it needs to run at as high a capacity as possible. As the return temperature to the heat pump increases, the capacity of the heat pump reduces and the efficiency drops. Due to its high thermal resistance the heat pump flow temperature is only slightly reduced as it passes through the LHTS, with a fairly constant charge profile after the initial inrush. The function of the series-connected sensible heat load is to keep the return temperature to the heat pump as low as possible to maximise efficiency and heat pump capacity.

Fig. 11 illustrates the impact of the LHTS output temperature and increasing return temperature on the power transfer into both the LHTS and SHTS. The sensible and latent heat stores have different charging profiles, with the SHTS absorbing more energy at the start of the charge cycle due to its cold output temperature, diminishing as the charge cycle progresses. When the LHTS is fully charged no more energy is absorbed by it and the full flow temperature is passed through to the SHTS, resulting in a top-off charge of the SHTS occurring after the LHTS has completed charging, bringing the SHTS charge up towards the heat pump flow temperature.

Advantages of embodiments are as follows: Embodiments can provide load shifting by storing heat in a PCM thermal store ([HIS) that offers very high energy density in small rectangular form. By combining charging of the LHTS with a standard sensible heat load such as Domestic Hot Water (DHW) or space heating (SH) using central heating (CH) emitters (radiators) it is possible to maximise the efficiency of the CO2 heat pump charging of the PCM.

By combining the ability to load-shift with smart metering or internet control it is possible to charge the TS and DHW at the cheapest periods, supporting hourly charging rates and dynamic "agile" tariffs that are becoming increasingly available.

By load-shifting space heating and DHW loads to avoid peak rate electricity prices it is possible to reduce the running costs by at least one third in comparison with an equivalent "standard" heat pump system. Furthermore, by using a CO2 heat pump the Global Warming Potential of the refrigerant used in the system is just 1, instead of the many hundreds to thousands for the refrigerants (R32, R407C, R410a, R134a) used in typical heat pump systems.

Claims

Claims 1. A load-shifting heat pump system comprising: a heat pump; a latent heat thermal store; and a sensible heat load, wherein the heat pump is operable to charge the latent heat thermal store in series with the sensible heat load, with the heat pump's fluid output flow directed firstly via the latent heat thermal store then secondly via the sensible heat load, prior to returning to the heat pump.
2. The load-shifting heat pump system of claim 1 wherein the sensible heat load is configured to reduce the return temperature of the flow to the heat pump by greater than 10°C from its temperature exiting the latent heat thermal store.
3. The load-shifting heat pump system of claim 1 or claim 2, operable to subsequently further charge the latent heat thermal store using heat from the same sensible heat load that was in series with the latent heat thermal store for its charging.
4. The load-shifting heat pump system of any preceding claim, operable to discharge the latent heat thermal store to a sensible heat load that was in series with a latent heat thermal store for its charging.
5. The load-shifting heat pump system of any preceding claim, further comprising a controller configured to control the operation of the heat pump to charge the latent heat thermal store in series with the sensible heat load, by directing the heat pump's fluid output flow via the latent heat thermal store and via the sensible heat load, prior to returning to the heat pump.
6. The load-shifting heat pump system of any preceding claim, wherein the controller is configured to control a sequence of: directing fluid flow from the heat pump to charge the latent heat thermal store in series with the sensible heat load, and directing fluid flow through the latent heat thermal store to discharge the latent heat thermal store.
7. The load-shifting heat pump system of claim 6, the controller is configured to control the sequence synchronised with different rate electricity tariff periods.
8. The load-shifting heat pump system of any preceding claim, wherein the sensible heat load comprises a hot water thermal store.
9. The load-shifting heat pump system of any preceding claim, wherein the sensible heat load comprises one or more space-heating thermal emitters.
10. The load-shifting heat pump system of any of claims 1 to 8, wherein the sensible heat thermal load comprises a sensible heat thermal store, and the heat pump is operable to pre-charge the latent heat thermal store in series with a sensible heat load before charging the latent heat thermal store in series with the sensible heat thermal store.
11. The load-shifting heat pump system of any preceding claim, further comprising a latent heat thermal store bypass operable to direct fluid flow from the heat pump via the sensible heat load while bypassing the latent heat thermal store.
12. The load-shifting heat pump system of any preceding claim, wherein the heat pump comprises a CO2 heat pump.
13. The load-shifting heat pump system of any preceding claim, wherein the heat pump is operable to produce fluid output flow with a temperature greater than or equal to 60°C.
14. A method of heat-pump load shifting, comprising operating a heat pump to charge a latent heat thermal store in series with a sensible heat load, by directing the heat pump's fluid output flow firstly via the latent heat thermal store and secondly via the sensible heat load, prior to returning to the heat pump.
15. The method of claim 14 comprising using the sensible heat load to reduce the return temperature of the flow to the heat pump by greater than 10°C from its temperature exiting the latent heat thermal store.
16. The method of claim 14 or claim 15, comprising subsequently further charging the latent heat thermal store using heat from a sensible heat load that was in series with a latent heat thermal store for its charging.
17. The method of any of claims 14 to 16, comprising discharging the latent heat thermal store to the same sensible heat load that was in series with the latent heat thermal store for its charging.
18. The method of any of claims 14 to 17, comprising a sequence of: directing fluid flow from the heat pump to charge the latent heat thermal store in series with the sensible heat load, and directing fluid flow through the latent heat thermal store to discharge the latent heat thermal store.
19. The method of claim 18, comprising synchronising the sequence with different rate electricity tariff periods.
20. The method of any of claims 14 to 19, comprising pre-charging the latent heat thermal store in series with a sensible heat load before charging the latent heat thermal store in series with the sensible heat thermal load, which comprises a sensible heat thermal store.
21. The method of any of claims 14 to 20, further comprising the step of directing fluid flow from the heat pump via the sensible heat load while bypassing the latent heat thermal store.
22. A controller for operation with a load-shifting heat pump system comprising a heat pump, a latent heat thermal store and a sensible heat load, wherein the controller is configured to output control signals to implement the method of any of claims 14 to 21.
23. A computer program product comprising machine readable instructions, which when executed on a controller, cause the controller to output control signals to perform the steps of a method as claimed in any of claims 14 to 21.