An energy storage apparatus and method of operation

Abstract

The present invention relates to an energy storage apparatus (101) and a method of operation. The energy storage apparatus (101) comprising: a hot reservoir (102) containing a first storage medium; a cold reservoir (103) containing a second storage medium; and an intermediary reservoir (104) containing a third storage medium. The energy storage apparatus (101) further comprising a first heat engine (105) thermally couplable to the hot reservoir (102) and to the cold reservoir (103), wherein the first storage medium is a heat source for the first heat engine (105) and the second storage medium is a heat sink for the first heat engine (105). The energy storage apparatus (101) further comprising a first heat pump (125) having a cold side thermally couplable to the cold reservoir (103) for cooling the second storage medium and a hot side thermally couplable to the intermediary reservoir (104) for heating the third storage medium. The energy storage apparatus (101) is operable in: a first hot reservoir discharge mode in which the first heat engine (105) generates work from a temperature difference between the hot reservoir (102) and cold reservoir (103) and a first cold reservoir charge mode in which the first heat pump (125) is energised to cool the second storage medium and heat the third storage medium.

Description

AN ENERGY STORAGE APPARATUS AND METHOD OF OPERATION

[0001] The invention relates to an energy storage apparatus and method of operation. In particular, the energy storage apparatus is for storing heat.BACKGROUND

[0002] The need to move to zero carbon forms of electricity generation is well known. However, many zero carbon forms of electricity generation are variable and intermittent which can be problematic for local grids where the supply and demand for electricity needs to be balanced.

[0003] For example, in some regions where electricity is generated from solar energy at solar farms there can be an oversupply of electricity generated at certain points of the day. This has led to solar farms being asked to disconnect from the local grid at times of oversupply, negative electrical pricing at times of oversupply, and a restriction on planning permits for new solar generation applications. Additionally, the same regions experience increased demand for electricity in the early evening as solar generation and the supply for electricity is declining. Rolling blackouts are sometimes employed to cope with the imbalance between supply and demand. A solution is to store energy that is generated in periods of oversupply in batteries, such as lithium-ion batteries. However, large scale energy storage in this way has several drawbacks. Supply constraints can be placed on lithium due to competing demand for lithium-ion batteries for transportation. Lithium-ion batteries degrade with use as they cannot be discharged to 100% depth without compromising battery life. Therefore, the lifetime of storage facilities based on lithium-ion batteries is limited. Lithium-ion batteries can also present a serious fire risk.

[0004] There exists a need to manage the variable and intermittent supply of energy from zero carbon sources and improve storage of energy generated from such sources.

[0005] The present invention seeks to address this need. The present invention provides a low-temperature and battery free energy storage apparatus and a method of operation.SUMMARY OF INVENTION

[0006] According to an aspect of the invention, there is provided an energy storage apparatus. The energy storage apparatus comprising:a hot reservoir containing a first storage medium;a cold reservoir containing a second storage medium;an intermediary reservoir containing a third storage medium;a first heat engine thermally couplable to the hot reservoir and to the cold reservoir, wherein the first storage medium is a heat source for the first heat engine and the second storage medium is a heat sink for the first heat engine; anda first heat pump having a cold side thermally couplable to the cold reservoir for cooling the second storage medium and a hot side thermally couplable to the intermediary reservoir for heating the third storage medium;wherein the energy storage apparatus is operable in:a first hot reservoir discharge mode in which the first heat engine generates work from a temperature difference between the hot reservoir and cold reservoir; anda first cold reservoir charge mode in which the first heat pump is energised to cool the second storage medium and heat the third storage medium.

[0007] The energy storage apparatus may comprise a thermal collector configured to heat to the first storage medium. The energy storage apparatus may be operable in a first hot reservoir charge mode in which heat is transferred from the thermal collector to the first storage medium to heat the first storage medium.

[0008] In some embodiments, the thermal collector may comprise a solar thermal collector.

[0009] In some embodiments, the energy storage apparatus may comprise one or more solar cells electrically connected to the first heat pump, and wherein in the first cold reservoir charge mode the first heat pump is energised by electricity from the one or more solar cells.

[0010] In some embodiments, the thermal collector may comprise a photovoltaic thermal collector.

[0011] The photovoltaic thermal collector may be electrically connected to the first heat pump, and wherein in the first cold reservoir charge mode reservoir charge mode the first heat pump may be energised by electricity from the photovoltaic thermal collector.

[0012] The thermal collector may be thermally couplable to the intermediary reservoir. The energy storage apparatus may be operable in a cooling mode in which the thermal collector is cooled by the third storage medium.

[0013] The thermal collector may be thermally couplable to the cold reservoir. The energy storage apparatus may be operable in a cooling mode in which the thermal collector is cooled by the second storage medium.

[0014] In some embodiments, the energy storage apparatus may comprise a variable speed drive connected to the first heat pump, wherein the variable speed drive is configured to drive the first heat pump.

[0015] The energy storage apparatus may comprise a controller configured to:receive a generation signal from a source of electrical power indicative of a rate of generation of electricity by the source of electrical power; andcontrol the variable speed drive such that the rate consumption of electricity by the first heat pump corresponds to the rate of generation of electricity to operate the energy storage apparatus in the first cold reservoir charge mode.

[0016] Optionally, the source of electrical power may be the above described one or more solar cells or the photovoltaic thermal collector.

[0017] In some embodiments, the energy storage apparatus may comprise a heat rejection means for rejecting heat to a first heat sink, wherein the heat rejection means is thermally couplable to the intermediary reservoir for cooling the third storage medium. The energy storage apparatus may be operable in an intermediary reservoir discharge mode in which the heat rejection means is used to cool the third storage medium.

[0018] The hot side of the first heat pump may be thermally couplable to the heat rejection means. The energy storage apparatus may be operable in a second cold reservoir charge mode in which the first heat pump is energised to cool the second storage medium and reject heat via the heat rejection means.

[0019] The thermal collector may be thermally couplable to the heat rejection means. The energy storage apparatus may be operable in a cooling mode in which the thermal collector is cooled by the heat rejection means.

[0020] The first heat engine may be thermally couplable to the heat rejection means such that the heat rejection means provides a heat sink for the first heat engine. The energy storage apparatus may be operable in a second hot reservoir discharge mode in which the first heat engine generates work from a temperature difference between the hot reservoir and the first heat sink.

[0021] In some embodiments, the first heat engine may be thermally couplable to a first auxiliary heat rejection means for rejecting heat to a second heat sink. The energy storage apparatus may be operable in a second hot reservoir discharge mode in which the first heat engine generates work from a temperature difference between the hot reservoir and the second heat sink. The energy storage apparatus may comprise the first auxiliary heat rejection means.

[0022] In some embodiments, the energy storage apparatus may comprise a second auxiliary heat rejection means for rejecting heat to a third heat sink. The hot side of the first heat pump may comprise a first section and a second section. The intermediary reservoir may be thermallycouplable to the first section and the second auxiliary heat rejection means may be thermally couplable to the second section. The energy storage apparatus may be operable in a third cold reservoir charge mode in which the first heat pump is energised to cool the second storage medium and reject heat via the second auxiliary heat rejection means.

[0023] In some embodiments, the first heat engine may comprise a pre-heating heat exchanger thermally couplable to the intermediary reservoir and wherein in the first hot reservoir discharge mode heat is transferred from the third storage medium to a working fluid of the first heat engine prior to a transfer of heat from the first storage medium to the working fluid.

[0024] In some embodiments, the first heat engine may comprise a pre-heating heat exchanger and the method may comprise operating the energy storage apparatus in first hot reservoir discharge mode and / or the second hot reservoir discharge mode by thermally coupling the preheating heat exchanger to the intermediary reservoir and transferring heat from the third storage medium to a working fluid of the first heat engine prior to transferring heat from the first storage medium to the working fluid.

[0025] In some embodiments, the energy storage apparatus may comprise a second heat pump having a cold side thermally couplable to the intermediary reservoir for cooling the third storage medium and a hot side thermally couplable to the hot reservoir for heating the first storage medium. The energy storage apparatus may be operable in a second hot reservoir charge mode in which the second heat pump is energised to cool the third storage medium and heat the first storage medium.

[0026] The energy storage apparatus may be operable in the first hot reservoir charge mode and second hot reservoir charge mode concurrently.

[0027] In some embodiments, the energy storage apparatus may comprise a cold reservoir heat exchanger, wherein the cold reservoir heat exchanger is thermally couplable to the cold reservoir and / or the cold side of the first heat pump.

[0028] In some embodiments, the energy storage apparatus may comprise a hot reservoir heat exchanger thermally couplable to the hot reservoir and / or to the thermal collector.

[0029] In some embodiments, the energy storage apparatus may comprise a second heat engine thermally couplable to the cold reservoir and to an external heat source. The energy storage apparatus may be operable in an external cooling mode in which the second heat engine generates work from a temperature difference between the external heat source and the cold reservoir thereby cooling the external heat source.

[0030] In some embodiments, the energy storage apparatus may comprise a third heat engine thermally couplable to the hot reservoir or the thermal collector and to a fourth heat sink, whereinthe hot reservoir or the thermal collector is a heat source for the third heat engine and the fourth heat sink is a heat sink for the third heat engine. The energy storage apparatus may be operable in an external heating mode in which the third heat engine generates work from a temperature difference between either the hot reservoir or the thermal collector and the fourth heat sink thereby providing heat to the fourth heat sink.

[0031] According to an aspect of the invention, there is provided a method of operating an energy storage apparatus. The method comprising:(i) providing an energy storage apparatus comprising:a hot reservoir containing a first storage medium;a cold reservoir containing a second storage medium;an intermediary reservoir containing a third storage medium;a first heat engine thermally couplable to the hot reservoir and to the cold reservoir, wherein the first storage medium is a heat source for the first heat engine and the second storage medium is a heat sink for the first heat engine; anda first heat pump having a cold side thermally couplable to the cold reservoir for cooling the second storage medium and a hot side thermally couplable to the intermediary reservoir for heating the third storage medium;(ii) operating the energy storage apparatus in a first hot reservoir discharge mode by generating work using the first heat engine from a temperature difference between the hot reservoir and cold reservoir; and(iii) operating the energy storage apparatus in a first cold reservoir charge mode by energising the first heat pump to cool the second storage medium and heat the third storage medium.

[0032] The method may comprise storing the cooled second storage medium in the cold reservoir.

[0033] In some embodiments, the energy storage apparatus may comprise a thermal collector configured to heat to the first storage medium. The method may comprise operating the energy storage apparatus in a first hot reservoir charge mode by transferring heat from the thermal collector to the first storage medium thereby heating the first storage medium.

[0034] In some embodiments, the method may comprise thermally coupling the first heat engine to a heat rejection means for rejecting heat from the energy storage apparatus into an external heat sink and operating the energy storage apparatus by generating work using the firstheat engine from a temperature difference between the thermal collector and the external heat sink

[0035] The method may comprise storing the heated first storage medium in the hot reservoir.

[0036] In some embodiments, the thermal collector may be a photovoltaic thermal collector, and the method may comprise operating the energy storage apparatus in the first cold reservoir charge mode by energising the first heat pump with electricity from the photovoltaic thermal collector.

[0037] In some embodiments, the method may comprise comprising operating the energy storage apparatus in a cooling mode by thermally coupling the thermal collector to the cold reservoir or the intermediary reservoir and cooling the thermal collector with the second storage medium or the third storage medium.

[0038] In some embodiments, the apparatus may comprise one or more solar cells electrically connected to the first heat pump and the method may comprise operating the energy storage apparatus in the first cold reservoir charge mode by energising the first heat pump with electricity from the one or more solar cells.

[0039] In some embodiments, operating the energy storage apparatus in a first cold reservoir charge mode may comprise driving the first heat pump using a variable speed drive.

[0040] In some embodiments, operating the energy storage apparatus in a first cold reservoir charge mode may comprise receiving a generation signal from a source of electrical power indicative of a rate of generation of electricity by the source of electrical power; and controlling the variable speed drive such that the rate consumption of electricity by the first heat pump corresponds to the rate of generation of electricity.

[0041] The source of electrical power may be the one or more solar cells or the photovoltaic thermal collector.

[0042] In some embodiments, the method may comprise thermally coupling the intermediary reservoir to a heat rejection means for rejecting heat to a first heat sink and operating the energy storage apparatus in an intermediary reservoir discharge mode by cooling the third storage medium via the heat rejection means.

[0043] The method may comprise storing the cooled third storage medium in the intermediary reservoir.

[0044] In some embodiments, the method may comprise thermally coupling the hot side of the first heat pump to the heat rejection means and operating the energy storage apparatus in asecond cold reservoir charge mode by energising the first heat pump to cool the second storage medium and reject heat to the first heat sink via the heat rejection means.

[0045] In some embodiments, the method may comprise thermally coupling the first heat engine to the heat rejection means and operating the energy storage apparatus in a second hot reservoir discharge mode by generating work using the first heat engine from a temperature difference between the hot reservoir and the first heat sink.

[0046] In some embodiments, the method may comprise thermally coupling the first heat engine to a first auxiliary heat rejection means for rejecting heat to a second heat sink, and operating the energy storage apparatus in a second hot reservoir discharge mode in which the first heat engine generates work from a temperature difference between the hot reservoir and the second heat sink.

[0047] In some embodiments, the method may comprise thermally coupling the hot side of the first heat pump to a second auxiliary heat rejection means for rejecting heat to a third heat sink and operating the energy storage apparatus in a third cold reservoir charge mode by energising the first heat pump to cool the second storage medium and reject heat via the second auxiliary heat rejection means.

[0048] In some embodiments, the first heat engine may comprise a pre-heating heat exchanger and the method may comprise operating the energy storage apparatus in the first hot reservoir discharge mode by thermally coupling the pre-heating heat exchanger to the intermediary reservoir and transferring heat from the third storage medium to a working fluid of the first heat engine prior to transferring heat from the first storage medium to the working fluid.

[0049] In some embodiments, the energy storage apparatus may comprise a second heat pump having a cold side thermally couplable to the intermediary reservoir for cooling the third storage medium and a hot side thermally couplable to the hot reservoir for heating the first storage medium. The method may comprise operating the energy storage apparatus in a second hot reservoir charge mode by energising the second heat pump to cool the third storage medium and heat the first storage medium.

[0050] In some embodiments, the method may comprise operating the energy storage apparatus in the first and second hot reservoir charge modes concurrently.

[0051] In some embodiments, the method may comprise cooling an external heat source via a cold reservoir heat exchanger thermally coupled to the cold reservoir or the cold side of the first heat pump.

[0052] In some embodiments, the method may comprise heating a fourth heat sink via a hot reservoir heat exchanger thermally coupled to the hot reservoir and / or the thermal collector.

[0053] In some embodiments, the energy storage apparatus may comprise a second heat engine thermally couplable to the cold reservoir and to an external heat source; and the method may comprise operating the energy storage apparatus in an external cooling mode by generating work with the second heat engine from a temperature difference between the external heat source and the cold reservoir thereby cooling the external heat source.

[0054] In some embodiments, the energy storage apparatus may comprise a third heat engine thermally couplable to the hot reservoir or the thermal collector and to a fourth heat sink, wherein the hot reservoir or the thermal collector is a heat source for the third heat engine and the fourth heat sink is a heat sink for the third heat engine. The method may comprise operating the energy storage apparatus in an external heating mode by generating work with the second heat engine from a temperature difference between either the hot reservoir or the thermal collector and the fourth heat sink thereby providing heat to the fourth heat sink.BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:FIG. 1 schematically shows an energy storage apparatus in accordance with an embodiment of the invention;FIG. 2 illustrates a method in accordance with an embodiment of the invention;FIG. 3 schematically shows an energy storage apparatus in accordance with another embodiment of the invention;FIG. 4 schematically shows an energy storage apparatus in accordance with another embodiment of the invention;FIG. 5 schematically shows an energy storage apparatus in accordance with another embodiment of the invention;FIG. 6 schematically shows an energy storage apparatus in accordance with another embodiment of the invention;FIG. 7 schematically shows an energy storage apparatus in accordance with another embodiment of the invention;FIG. 8 schematically shows an energy storage apparatus in accordance with another embodiment of the invention;FIG. 9 schematically shows an energy storage apparatus in accordance with another embodiment of the invention;FIG. 10 schematically shows an energy storage apparatus in accordance with another embodiment of the invention; andFIG. 11 schematically shows an energy storage apparatus in accordance with another embodiment of the invention.DETAILED DESCRIPTION

[0056] FIG. 1 shows an energy storage apparatus 101 according to an embodiment of the invention. An energy storage apparatus 101 comprises a hot reservoir 102, a cold reservoir 103 and an intermediary reservoir 104. The reservoirs 102, 103, 104 may be provided by any suitable store or vessel. For example, one or more of the hot reservoir 102, the cold reservoir 103 and the intermediary reservoir 104 may comprise a tank or pit store. The reservoirs 102, 103, 104 may each be at atmospheric pressure.

[0057] Each of the reservoirs 102, 103, 104 contains a storage medium for storing heat. The hot reservoir 102 contains a first storage medium, the cold reservoir 103 contains a second storage medium and the intermediary reservoir 104 contains a third storage medium. Optionally, each of the reservoirs 102, 103, 104 may also contain a heat transfer fluid to facilitate heat transfer into or out of the reservoir. The hot reservoir 102 may contain a first heat transfer fluid, the cold reservoir 103 may contain a second heat transfer fluid, and the intermediary reservoir 104 may contain a third heat transfer fluid.

[0058] Any suitable storage medium and, if present, heat transfer fluid may be used in the reservoirs 102, 103, 104. For example, the storage medium in one or more of the reservoirs 102, 103, 104 may be liquid, such as water, that does not change phase when heated or cooled during operation of the energy storage apparatus 101. In such embodiments, there is no need for there to also be a heat transfer fluid in the reservoir. In an alternative example, the storage medium in one or more of the reservoirs 102, 103, 104 may comprise an encapsulated phase change material whereby the material solidifies within the encapsulation when cooled and the material is returned to a liquid or sublimes when heated. The storage medium may therefore be retained within the reservoir 102, 103, 104. One or more of the reservoirs 102, 103, 104 may then comprise a heat transfer fluid which may be a brine, an aqueous solution, or pure fluid. In a further example, the storage medium in one or more of the reservoirs 102, 103, 104 may comprise an unencapsulated phase change material whereby it is a fragmented solid or a slurry when cooled and the material is returned to an unencapsulated liquid or sublimes when heated.The storage medium may therefore be retained within the reservoir 102, 103, 104. One or more of the reservoirs 102, 103, 104 may then comprise a heat transfer fluid which may have a different specific gravity to the unencapsulated phase change material in its solid and liquid states and is not miscible with the unencapsulated phase change material. The same storage medium and, where present, same heat transfer fluid may be used in each of the reservoirs 102, 103, 104. In certain embodiments, the first, second and third storage media may each be water.

[0059] The energy storage apparatus 101 comprises a first heat engine 105 thermally couplable to the hot reservoir 102 and to the cold reservoir 103. The first storage medium provides a heat source for the first heat engine 105 and the second storage medium provides a heat sink for the first heat engine 105. The first heat engine 105 may generate work based on a temperature difference between the first storage medium in the hot reservoir 102 and the second storage medium in the cold reservoir 103. FIG. 1 shows an example of the first heat engine 105, however, the skilled person will appreciate that any suitable heat engine may be used. The first heat engine 105 may contain a working fluid. As shown in FIG. 1, the first heat engine 105 may comprise a first heat engine condenser 106 and a first heat engine evaporator 108. The cold reservoir 103 may be thermally couplable to the first heat engine condenser 106 so that the second storage medium provides a heat sink for the first heat engine 105. That is, heat may be transferred from the working fluid in the first heat engine 105 to the second storage medium. The hot reservoir 102 may be thermally couplable to the first heat engine evaporator 108 so that heat from the first storage medium may be used to evaporate the working fluid of the first heat engine 105. That is, the first storage medium provides a heat source for the first heat engine 105. The first heat engine 105 may comprise an expander 110. The expander 110 may be a turbine or a positive displacement device such as a screw, scroll or reciprocating expander. The expander 110 may be disposed between the first heat engine condenser 106 and the first heat engine evaporator 108 such that the working fluid in the first heat engine 105 passes from the first heat engine evaporator 108 to the first heat engine condenser 106 via the expander 110. The first heat engine 105 may comprise a fluid pump 112 disposed between the first heat engine condenser 106 and the first heat engine evaporator 108 such that the working fluid of the first heat engine 105 passes from the first heat engine condenser 106 to the first heat engine evaporator 108 via the fluid pump 112. The first heat engine 105 may be configured to operate according to any suitable thermodynamic cycle. For example, the first heat engine 105 may be configured to operate in accordance with an organic Rankine cycle. The expander 110 may be coupled to a generator (not shown) such that work generated by the expander 110 causes electrical energy to be generated in the generator. The generator may be, but is not limited to, a permanent magnet generator. During use, the output of the generator may be conditionedbefore it is exported to a local grid. The conditioning of output of the generator may include a rectification stage whereby alternating current from the generator is converted to direct current, a DC-DC stage where voltage of the direct current is changed to a predetermined required value, and then an inverter stage whereby the direct current is converted back to alternating current at a desired frequency, voltage and number of phases. The inverter stage may include a grid-tied inverter such that the AC output is exported to the local grid with the grid-tied inverter ensuring export is compliant with local regulations regarding quality and connection / disconnection protocols. Alternatively, the electrical energy may be utilised directly by electrical loads without export to the local grid.

[0060] To thermally couple the hot reservoir 102 to the first heat engine 105, fluid from the hot reservoir 102 may be directed to the first heat engine 105 so that the first storage medium in the hot reservoir 102 may provide a heat source for the first heat engine 105. The hot reservoir 102 may be fluidly couplable to the first heat engine 105 by any suitable means. In the embodiment shown in FIG. 1, the energy storage apparatus 101 comprises a plurality of pipes, valves and manifolds to connect the different components. For example, as shown in FIG. 1, the hot reservoir 102 may comprise an outlet connected to the first heat engine 105 by a fluid pipe. A diverter valve 114 may be disposed on the fluid pipe. The diverter valve 114 is configured to control the flow of a fluid, either the first storage medium or a first heat transfer fluid, from the outlet of the hot reservoir 102 to the first heat engine evaporator 108 of the first heat engine 105. The first heat engine 105 may be connected to an inlet to the hot reservoir 102 by another fluid pipe. A diverter valve 116 may be arranged to control the flow of fluid from the first heat engine 105 to the inlet of the hot reservoir 102. A pump 118 may be disposed between the first heat engine 105 and the diverter valve 116 to drive fluid toward the inlet of the hot reservoir 102. In FIG. 1, the arrows show the direction of fluid flow around the energy storage apparatus 101. As shown in FIG. 1, the first storage medium or first heat transfer fluid is in a separate fluid circuit to the working fluid of the first heat engine 105.

[0061] To thermally couple the cold reservoir 103 to the first heat engine 105, fluid from the cold reservoir 103 may be directed to the first heat engine 105 so that the second storage medium in the cold reservoir 103 may provide a heat sink for the first heat engine 105. The cold reservoir 103 may be fluidly couplable to the first heat engine 105 by any suitable means. For example, as shown in FIG. 1, the cold reservoir 103 may comprise an outlet connected to the first heat engine 105 by a fluid pipe. A diverter valve 120 may be disposed on the fluid pipe to control the flow of a fluid, either the second storage medium or a second heat transfer fluid, from the outlet to the first heat engine 105. A pump 122 may also be disposed on the fluid pipe to drive fluid from the cold reservoir 103 to the first heat engine 105. The first heat engine 105 maybe connected to an inlet of the cold reservoir 103 by another fluid pipe to return fluid from the first heat engine 105 to the cold reservoir 103. A diverter valve 124 may be disposed on the fluid pipe to control the flow of fluid into the cold reservoir 103. The second storage medium or second heat transfer fluid is in a separate fluid circuit to the working fluid of the first heat engine 105.

[0062] The energy storage apparatus 101 comprises a first heat pump 125. The first heat pump 125 may be a chiller or a refrigeration apparatus. The first heat pump 125 has a cold side thermally couplable to the cold reservoir 103 for cooling the second storage medium and a hot side thermally couplable to the intermediary reservoir 104 for heating the third storage medium. As such, the first heat pump 125 may be operated to transfer heat from the cold reservoir 103 to the intermediary reservoir 104. FIG. 1 shows an example of the first heat pump 125, however, the skilled person will appreciate that any suitable heat engine may be used. As shown in FIG.1, the first heat pump 125 may comprise a first heat pump condenser or gas cooler 128 and a first heat pump evaporator 126. The cold reservoir 103 may be thermally couplable to the first heat pump evaporator 126 so that the second storage medium may be cooled. The intermediary reservoir 104 is thermally couplable to the first heat pump condenser 128 so that heat may be transferred to the third storage medium . The first heat pump 125 may comprise a compressor 130 disposed between the first heat pump evaporator 126 and the first heat pump condenser 128 such that a working fluid of the first heat pump 125 passes from the first heat pump evaporator 126 to the first heat pump condenser 128 via the compressor 130. The first heat pump 125 may comprise an expansion valve 132 disposed between the first heat pump condenser 128 and the first heat pump evaporator 126 so that the working fluid passes from the first heat pump condenser 128 to the first heat pump evaporator 126 via the expansion valve 132. The expansion valve 132 may cause the working fluid in the first heat pump 125 to undergo isenthalpic expansion. Whilst the first heat pump 125 is shown as having a single compressor 130, the first heat pump 125 may comprise multiple compressors 130 such that the working fluid may be compressed in stages.

[0063] To thermally couple the cold reservoir 103 to the first heat pump 125, fluid from the cold reservoir 103 may be directed to the first heat pump 125. The cold reservoir 103 may be fluidly couplable to the first heat pump evaporator 126 of the first heat pump 125. In the embodiment in FIG. 1, in addition to directing fluid from the cold reservoir 103 to the first heat engine 105, the diverter valve 120 near the outlet of the cold reservoir 103 may control the flow of fluid from the cold reservoir 103 to the first heat pump 125 via a fluid pipe. The pump 122 may be disposed between the outlet of the cold reservoir 103 and the diverter valve 120. Similarly, diverter valve 124 near the inlet of the cold reservoir 103 may be configured to control the flow of fluid fromthe first heat pump 125 back to the cold reservoir 103. Therefore, the two diverter valves 120, 124 may direct fluid from the cold reservoir 103 to the first heat engine 105 or the first heat pump 125. As shown in FIG. 1, the second storage medium or second heat transfer fluid is in a separate fluid circuit to the working fluid of the first heat pump 125.

[0064] To thermally couple the intermediary reservoir 104 to the first heat pump 125, fluid from the intermediary reservoir 104 may be directed to the first heat pump 125. The intermediary reservoir 104 may be fluidly couplable to the first heat pump condenser 128 of the first heat pump 125. For example, as shown in FIG. 1, the intermediary reservoir 104 may comprise a first opening connected to the first heat pump 125 by a fluid pipe. A diverter valve 134 may be disposed on the fluid pipe. The diverter valve 134 is configured to control the flow of a fluid, either the third storage medium or a third heat transfer fluid, from the first opening of the intermediary reservoir 104 to the first heat pump condenser 128 of the first heat pump 125. The first heat pump 125 may be connected to a second opening of the intermediary reservoir 104 by another fluid pipe. A diverter valve 136 may be arranged to control the flow of fluid from the first heat pump 125 to the second opening of the intermediary reservoir 104. A pump 138 may be disposed between the diverter valve 134 and the first heat pump 125 to drive fluid from the first opening of the intermediary reservoir 104 to the first heat pump 125. The third storage medium or third heat transfer fluid is in a separate fluid circuit to the working fluid of the first heat pump 125.

[0065] The energy storage apparatus 101 may comprise a thermal collector 139. The thermal collector 139 is for collecting heat from a thermal source 140. The thermal collector 139 may be configured to transfer heat to the first storage medium. In certain embodiments, the thermal collector 139 may transfer heat to the first storage medium via the first heat transfer fluid. Any suitable thermal source 140 and collector may be used. In certain embodiments, the thermal source 140 may comprise a waste heat source or a geothermal heat source. The waste heat sources may include but are not limited to combined heat and power (CHP) plants, nuclear power stations, fossil fuel power stations and biomass power stations. In such embodiments, the thermal collector 139 may comprise a heat exchanger. In alternative embodiments, the thermal source 140 may be solar. In such embodiments, the thermal collector 139 may comprise a solar thermal collector or a photovoltaic thermal collector. The solar thermal collector may convert solar irradiance into heat. Examples of solar collectors include a plate, evacuated flat plate, evacuated tube, parabolic collector or a mirror concentrator. The photovoltaic thermal collector may convert solar irradiance into both electrical energy and heat at the same time. The energy storage apparatus 101 may be incorporated into a solar farm or power station. Providing the thermal collector 139 as a photovoltaic thermal collector may be advanteagous as theelectricity generated can be used within the energy storage apparatus 101 and / or exported to the local grid. Additionally, providing the thermal collector 139 as a photovoltaic thermal collector may reduce the size or footprint of the energy storage apparatus 101 compared to, for example, embodiments that use separate solar collectors to collect heat and to generate electricity.

[0066] As shown in the embodiment in FIG. 1, the thermal collector 139 may be fluidly couplable to the hot reservoir 102 to heat the first storage medium. Fluid from the hot reservoir 102, either the first storage medium or a first heat transfer fluid, may be directed to the thermal collector 139. For example, as shown in FIG. 1, the thermal collector 139 may be fluidly couplable to the hot reservoir 102. The diverter valve 116 near the outlet of the hot reservoir 102 may be configured to direct fluid from the hot reservoir 102 to the thermal collector 139 along a fluid pipe. Heat may be transferred to the fluid from the thermal collector 139. Once the fluid has passed through the thermal collector 139, it may be returned to the hot reservoir 102. The diverter valve 114 near the inlet of the hot reservoir 102 may be configured to control the flow of fluid into the hot reservoir 102. The energy storage apparatus 101 may comprise a pump 142 to drive fluid from the hot reservoir 102 to the thermal collector 139. The pump 142 may be disposed on the fluid line between the outlet of the hot reservoir 102 and the thermal collector 139. The two diverter valves 116, 114, near the inlet and outlet of the hot reservoir 102 may direct fluid from the hot reservoir 102 to either the first heat engine 105 or the thermal collector 139.

[0067] As described above, in certain embodiments, the thermal collector 139 may comprise a photovoltaic thermal collector. Additionally or alternatively, the photovoltaic thermal collector may export electricity to the local grid. In such embodiments, the thermal collector 139 may be electrically connected to the first heat pump 125, specifically to the compressor 130 of the first heat pump 125, such that the first heat pump 125 may be energised by electricity generated by the thermal collector 139. The electricity produced by a photovoltaic thermal collector is variable and depends on the solar irradiance. The energy storage apparatus 101 may comprise a variable speed drive 144. The variable speed drive 144 may be connected to the first heat pump 125 to drive the compressor 130 of the first heat pump 125. As shown in FIG. 1, the variable speed drive 144 may be operably connected to the compressor 130 of the first heat pump 125. The energy storage apparatus 101 may comprise a controller 146 comprising one or more processors. The one or more processors of the controller 146 may be collectively configured to a receive a generation signal from the photovoltaic thermal collector 139 indicative of a rate of generation of electricity by the photovoltaic thermal collector 139. The one or more processors may be collectively configured to control the variable speed drive 144 such that the rate consumption of electricity by the first heat pump 125 corresponds to the rate of generation ofelectricity. Therefore, the speed of the first heat pump 125 may be matched to the rate of generation of electricity by the thermal collector 139.

[0068] In an alternative embodiment, the thermal collector 139 may comprise a solar thermal collector which is not a photovoltaic thermal collector. In such embodiments, the energy storage apparatus 101 may comprise one or more solar cells (not shown) to generate electricity. The one or more solar cells may be electrically connected to the first heat pump 125 in the same manner as the photovoltaic thermal collector described above. The energy storage apparatus 101 may comprise the variable speed drive 144 connected to the first heat pump 125 and the controller 146. The one or more processors of the controller 146 may be collectively configured to a receive a generation signal from the one or more solar cells indicative of a rate of generation of electricity. The one or more processors may be collectively configured to control the variable speed drive 144 such that the rate consumption of electricity by the first heat pump 125 corresponds to the rate of generation of electricity.

[0069] As shown in the embodiment in FIG. 1, the energy storage apparatus 101 may comprise a heat rejection means 147. The heat rejection means 147 is for rejecting heat into a first heat sink 148. The first heat sink 148 may be external to the energy storage apparatus 101. The first heat sink 148 may be the ambient surroundings of the energy storage apparatus 101. The heat rejection means 147 may be selected dependent on the first heat sink 148. The heat rejection means 147 may be passive, i.e., requiring no energy input, or active i.e., requiring energy input. For example, if the first heat sink 148 is a body of naturally occurring water (e.g., a river, ground water, pond, lake or seawater) or a ground / subterranean heat sink, the heat rejection means 147 may comprise of a heat exchanger. Alternatively, if for example the first heat sink 148 is ambient air, an active heat rejection means 147 may be used such as a fan cooled heat exchanger such as a dry-cooler, or an evaporative or adiabatic cooler. Alternatively, a passive heat rejection means 147 may be used such as a fan-less heat exchanger relying on the natural movement of air or heat pipes utilising phase change fluids to transfer heat to the first heat sink 148.

[0070] The heat rejection means 147 is configured to reject heat from the intermediary reservoir 104. As such the heat rejection means 147 is thermally couplable to the intermediary reservoir 104. As shown in the embodiment in FIG. 1, the heat rejection means 147 may be thermally coupled to the intermediary reservoir 104 by fluidly coupling the heat rejection means 147 to the intermediary reservoir 104. For example, the diverter valve 136 near the second opening of the intermediary reservoir 104 may be configured to direct fluid, either the third storage medium or the third heat transfer fluid, from the intermediary reservoir 104 to the heatrejection means 147 via a fluid pipe. A pump 150 may be disposed on the fluid pipe to drive fluid to the heat rejection means 147. The diverter valve 134 near the first opening of the intermediary reservoir 104 may be configured to control the flow of fluid from the heat rejection means 147 to the intermediary reservoir 104. Therefore, the diverter valves 136, 146 near the first and second openings of the intermediary reservoir 104 may be configured to direct fluid between the intermediary reservoir 104 and each of the first heat pump 125 and heat rejection means 147.

[0071] The hot side of the first heat pump 125 may also be thermally couplable to the heat rejection means 147. That is, the first heat pump condenser 128 of the first heat pump 125 may be thermally coupled to the heat rejection means 147. The energy storage apparatus 101 may be configured such that fluid may be circulated between the hot side of the first heat pump 125 and the heat rejection means 147. As shown in the embodiment in FIG. 1, the diverter valves 136, 134 near the openings of the intermediary reservoir 104 may be configured to bypass the intermediary reservoir 104. Therefore, fluid can be circulated between the first heat pump 125 and the heat rejection means 147 so that heat entering the fluid at the first heat pump condenser 128 of the first heat pump 125 may be rejected into the first heat sink 148 by the heat rejection means 147 without first being stored in the intermediary reservoir 104. As the intermediary reservoir 104 and heat rejection means 147 are in the same fluid circuit, the fluid that is circulated between the hot side of the first heat pump 125 and the heat rejection means 147 may be the third storage medium or third heat transfer.

[0072] The energy storage apparatus 101 of the embodiment shown in FIG. 1 is operable in multiple different modes.

[0073] The energy storage apparatus 101 may be operable in a first hot reservoir charge mode. In the first hot reservoir charge mode, heat is transferred from the thermal collector 139 to the first storage medium to heat the first storage medium. Once heated, the first storage medium may be stored in the hot reservoir 102. Fluid may be directed from the hot reservoir 102 to the thermal collector 139 to heat the first storage medium.

[0074] The energy storage apparatus 101 is operable in a first cold reservoir charge mode. In the first cold reservoir charge mode, the first heat pump 125 is energised to cool the second storage medium and heat the third storage medium. That is, the first heat pump 125 is used to transfer heat from the second storage medium in the cold reservoir 103 to the third storage medium in the intermediary reservoir 104 thereby cooling the second storage medium and heating the third storage medium. The efficiency of the first heat pump 125 in the first cold reservoir charge mode depends on the temperature difference between the cold reservoir 103 and the intermediary reservoir 104.

[0075] The energy storage apparatus 101 may be operable in a second cold reservoir charge mode. In the second cold reservoir charge mode, the first heat pump 125 is energised to cool the second storage medium and reject heat via the heat rejection means. As such, the second storage medium in the cold reservoir 103 may be cooled and the heat may be rejected into the first heat sink 148. The efficiency of the first heat pump 125 in the second cold reservoir charge mode depends on the temperature difference between the cold reservoir 103 and the first heat sink 148. The energy storage apparatus 101 may be operated in the second cold reservoir charge mode rather than the first cold reservoir charge mode so that heat from the first heat pump 125 is rejected directly in to the first heat sink 148 if doing so would achieve greater efficiency for the first heat pump 125 than rejecting heat to the intermediary reservoir 104. This may occur if the present temperature of the first heat sink 148 is lower than that experienced during the previous intermediary reservoir discharge mode.

[0076] Once cooled by the first and / or second cold reservoir charge mode, the second storage medium may be stored in the cold reservoir 103. Similarly, the heated third storage medium may be stored in the intermediary reservoir 104.

[0077] In either the first or second cold reservoir charge mode, the first heat pump 125 may be energised using electricity generated by the thermal collector 139, when the thermal collector 139 is a photovoltaic thermal collector, or by electricity generated by the one or more solar cells. The energy storage apparatus 101 may be operated in the first hot reservoir charge mode and the first or second cold reservoir charge mode at the same time. Alternatively, the first heat pump 125 may be energised using electricity from any suitable source such as, for example, the local grid.

[0078] The energy storage apparatus 101 is operable in a first hot reservoir discharge mode. In the first hot reservoir discharge mode, the first heat engine 105 generates work from a temperature difference between the hot reservoir 102 and cold reservoir 103. That is, work is generated due to the temperature difference between the first storage medium in the hot reservoir 102 and the second storage medium in the cold reservoir 103. During the first hot reservoir charge mode, residual heat is rejected into the cold reservoir 103 causing the second storage medium to be heated. The efficiency at which work can be extracted during the first hot reservoir discharge mode depends on the temperature difference between the hot reservoir 102 and the cold reservoir 103, the greater the temperature difference the more efficiently work can be extracted. As the energy storage apparatus 101 is operated in the first hot reservoir discharge mode the heat is extracted from the first storage medium in the hot reservoir 102.

[0079] The energy storage apparatus 101 may be operable in an intermediary reservoir discharge mode. In the intermediary reservoir discharge mode, the heat rejection means 147 may be used to cool the third storage medium. Therefore, heat from the third storage medium in the intermediary reservoir 104 may be transferred to the first heat sink 148 thereby removing heat from the third storage medium in the intermediary reservoir 104. Once cooled (i.e., heat has been removed), the third storage medium may be stored in the intermediary reservoir 104. The amount of heat which can be removed from the third storage medium is dependent on the temperature of the first heat sink 148. The energy storage apparatus 101 may be operated in the intermediary reservoir discharge mode at the same time as the energy storage apparatus 101 is operated in one or more of the first hot reservoir discharge mode, the first cold reservoir charge mode and the first hot reservoir charge mode. Alternatively, the energy storage apparatus 101 may be operated only in the intermediary reservoir discharge mode at one time.

[0080] Each of the hot reservoir 102, the cold reservoir 103 and the intermediary reservoir 104 may be insulated to either prevent heat loss or heat ingress and improve the efficiency of operating the energy storage apparatus 101. One or more of the reservoirs 102, 103, 104 may be insulated to reduce heat loss. If the reservoir 102, 103, 104 is a pit store, it may be sufficient to provide a cover (not shown) for the reservoir 102, 103, 104. The cover may be impermeable to prevent heat loss by evaporation and / or insulated to prevent heat loss or ingress by conduction. Additionally or alternatively, the cover may be reflective to prevent heat loss or heat ingress by radiation. The cover may be configured to float on the reservoir 102, 103, 104 on top of the storage medium. Alternatively, the cover may be supported in the reservoir 102, 103, 104 by alternative means. In certain embodiments, the sides of the reservoir 102, 103, 104 may also be insulated. In certain embodiments, the cover may be provided by a loose floating media deposited directly onto the storage medium in the reservoir 102, 103, 104. The floating media may comprise hollow, or part water filled, polypropylene spheres deposited onto storage medium. Partially filling the spheres with water may provide the spheres with sufficient mass to avoid being displaced by high winds. The spheres may be black to prevent UV degradation and prevent light transmittance into the storage medium thereby helping to prevent biological growth. In certain embodiments, the intermediary reservoir 104 may be covered by a retractable cover. As such, the cover may be extended to cover the intermediary reservoir 104 when the third storage medium is being or has been cooled such as during the intermediary reservoir discharge mode. The cover may be retracted during or after the first cold reservoir charge mode so allow heat to dissipate from the third storage medium.

[0081] During operation, the energy storage apparatus 101 may be operated in the first hot reservoir charge mode and the first cold reservoir charge mode prior to operating in the first hotreservoir discharge mode. Alternatively, the energy storage apparatus 101 may be operated in the first hot reservoir charge mode and first hot reservoir discharge mode at the same time. For example, heat from a solar thermal collector may be transferred to the first storage medium whilst the first heat engine 105 is being used to generate work.

[0082] The energy storage apparatus 101 allows energy to be stored in the first storage medium in the hot reservoir 102 as heat. This energy can then be used by the first heat engine 105 to generate work when the energy storage apparatus 101 is operated in the first hot reservoir discharge mode. This is particularly advantageous when the source 140 is an intermittent heat source. The energy storage apparatus 101 provides battery-free and low-cost energy storage.

[0083] A heat engine’s efficiency is governed by the temperature difference between the heat source providing it with thermal energy and the heat sink to which it rejects it. The greater the difference between these two, the greater the heat engine efficiency. Therefore, the efficiency of the energy storage apparatus 101 at generating work using the first heat engine 106 may be improved by minimising the temperature of the cold reservoir 103.

[0084] The intermediary reservoir 104 of the energy storage apparatus 101 allows the performance and efficiency of the energy storage apparatus 101 to be enhanced. The intermediary reservoir 104 and its use enables the temperature of the cold reservoir 103 to reach a lower temperature than would be achievable in an energy storage apparatus without the intermediary reservoir 104. This is because the temperature difference across the first heat pump 125 may be kept as small as possible at the start of charging the cold reservoir 103 and the intermediary reservoir 104 temperature can be lower than the current ambient temperature. The intermediary reservoir 104 may be cooled to its lowest possible temperature during the intermediary reservoir discharge mode to maximise the efficiency or / coefficient of performance of cooling of the first heat engine 105 when charging the cold store.

[0085] Additionally, in the energy storage apparatus 101 heating of the hot reservoir 102 is provided from the thermal source 140 and is independent of the first heat pump 125 and the cold reservoir 103 i.e. , in the energy storage apparatus 101 the cold reservoir 103 is not charged by rejecting heat from the cold reservoir 103 into the hot reservoir 102. This enables the first storage medium in the hot reservoir 102 to reach a higher temperature without negatively impacting heat pump efficiency during the first cold reservoir charging mode.

[0086] The energy storage apparatus 101 therefore allows the temperature difference between the hot reservoir 102 and cold reservoir 103 at the start of the first hot reservoir discharge mode to be maximised resulting in improved thermodynamic efficiency in the first hot reservoir discharge mode. The energy storage apparatus 101 also does not require heating the first heatengine evaporator to be above the boiling point of water at atmospheric pressure, hence avoiding the need for expensive pressurised thermal storage systems or complex thermal storage systems.

[0087] The energy storage apparatus 101 may attain electrical round-trip efficiencies comparable to lithium-ion battery storage systems but using thermal stores. The storage medium in the energy storage apparatus 101 may utilise low cost fluids, such as water, which are kept below their boiling point at atmospheric during operation of the energy storage apparatus 101 resulting in low capital cost and a levelized cost of storage lower than that of batteries or other storage technologies.

[0088] The skilled person will appreciate that FIG. 1 shows one example of how the components of the energy storage apparatus 101 may be coupled using a plurality of pipes, manifolds, diverter valves and pumps. The skilled person will appreciate that the components of the energy storage apparatus 101 may be coupled in one or more different configurations to that shown in FIG. 1 whilst still operating in substantially the same way and achieving the same effect.

[0089] FIG. 2 shows a method 200 of operating the energy storage apparatus 101 according to an embodiment of the invention. The method comprises the step 202 of providing the energy storage apparatus 101 of the embodiment shown in FIG. 1.

[0090] The method 200 may comprise step 204 of operating the energy storage apparatus in the first hot reservoir charge mode by transferring heat from the thermal collector to the first storage medium thereby heating heat the first storage medium. The step 204 of the method may comprise thermally coupling the hot reservoir 102 to the thermal collector 139. The diverter valves 114, 116 near the inlet and outlet of the hot reservoir 102 may be controlled to direct fluid between the hot reservoir 102 to the thermal collector 139. Once heated, the method 200 may comprise storing the heated first storage medium in the hot reservoir 102. This may be done by closing the diverter valves 114, 116 near the inlet and outlet of the hot reservoir 102.

[0091] The method 200 comprises the step 206 of operating the energy storage apparatus 101 in the first cold reservoir charge mode by energising the first heat pump 125 to cool the second storage medium and heat the third storage medium. This step 206 of the method may comprise thermally coupling each of the cold reservoir 103 and intermediary reservoir 104 to the first heat pump 125. The diverter valves 124, 122 near the inlet and outlet of the cold reservoir 103 may be used to control the flow of fluid between the cold reservoir 103 and the first heat pump 125.The diverter valves 134, 136 may be used to control the flow of fluid between the intermediary reservoir 104 and first heat pump 125.

[0092] In certain embodiments, the step 206 of operating the energy storage apparatus 101 in the first cold reservoir charge mode may comprise energising the first heat pump with electricity from the photovoltaic thermal collector or the one or more solar cells. In such embodiments, the step 206 of operating the energy storage apparatus 101 in the first cold reservoir charge mode may comprise receiving a generation signal from the one or more solar cells or photovoltaic thermal collector indicative of a rate of generation of electricity by the one or more solar cells or photovoltaic thermal collector. The step 206 of operating the energy storage apparatus in the first cold reservoir charge mode may comprise driving the first heat pump 125 using the variable speed drive 144. The step 206 may comprise controlling the variable speed drive such that the rate consumption of electricity by the first heat pump 125 corresponds to the rate of generation of electricity to operate the energy storage apparatus 101 in the first cold reservoir charge mode.

[0093] The method 200 may comprise the step 208 of operating the energy storage apparatus 101 in the second cold reservoir charge mode by thermally coupling the hot side of the first heat pump 125 to the heat rejection means 147 and energising the first heat pump 125 to cool the second storage medium and reject heat via the heat rejection means 147. In a similar manner as described above for the first cold reservoir charge mode, the step 208 of operating the energy storage apparatus 101 in the second cold reservoir charge mode may comprise energising the first heat pump with electricity from the photovoltaic thermal collector or the one or more solar cells. The step 208 of operating the energy storage apparatus 101 in the second cold reservoir charge mode may comprise driving the first heat pump using a variable speed drive.

[0094] The method 200 may comprise operating the energy storage apparatus 101 in the first hot reservoir charge mode in accordance with step 204 and one of the first and second cold reservoir charge modes in accordance with steps 206 and 208 at the same time.

[0095] Once cooled (i.e., after step 206 or 208 has been performed), the method 200 may comprise storing the cooled second storage medium in the cold reservoir 103. This may be done by controlling the diverter valves 124, 120 near the inlet and outlet of the cold reservoir 103 to prevent fluid flow from or into the cold reservoir 103. Whilst the cooled second storage medium is stored in the cold reservoir 103, the first heat pump 125 is idle. In a similar manner, the method 200 may comprise storing the heated third storage medium in the intermediary reservoir 104. This may be done by controlling the diverter valves 136, 134 near the first and second openings of the intermediary reservoir 104 to prevent fluid flow from or into the intermediary reservoir 104.

[0096] The method 200 comprises the step 210 of operating the energy storage apparatus 101 in the first hot reservoir discharge mode by generating work using the first heat engine 105 from a temperature difference between the hot reservoir and cold reservoir. This step 210 may comprise thermally coupling the hot reservoir 102 and the cold reservoir 103 to the first heat engine 105. The diverter valves 116, 114 near the inlet and outlet of the hot reservoir 102 may be used to control the flow of fluid to the first heat engine 105. The method 200 may comprise performing the step 210 of operating the energy storage apparatus 101 in the first hot reservoir discharge mode once the first storage medium has been heated using the thermal collector 139 and the cold reservoir 103 has been charged using either the first or second cold reservoir charge mode. That is, after step block 204 and one of steps 206 and 208 have been performed. Work generated by the first heat engine 105 may be used for the production of electricity. In certain embodiments, the method 200 may comprise the steps operating the energy storage apparatus 101 in the first hot reservoir charge mode and first hot reservoir discharge mode at the same time.

[0097] The method 200 may comprise operating the step 212 of operating the energy storage apparatus 101 in the intermediary reservoir discharge mode by cooling the third storage medium via the heat rejection means 147. This step 212 may comprise thermally coupling the intermediary reservoir 104 to the heat rejection means 147. The diverter valves 136, 134 may be used to control the flow of fluid between the intermediary reservoir 104 and the heat rejection means 147. As such, heat from the third storage medium may be rejected into the first heat sink 148. The method 200 may comprise operating the energy storage apparatus 101 in the intermediary reservoir discharge mode at the same time as one or more of the first hot reservoir discharge mode, the first cold reservoir charge mode and the first hot reservoir charge mode.

[0098] The method 200 may comprise the step of operating the first heat engine 105 using the thermal collector 139 as the heat source and the cold reservoir 103 as the heat sink. That is, the first heat engine 105 is operated directly between the thermal collector 139 and the cold reservoir 103 bypassing the hot reservoir 102. The first heat engine 105 may therefore generate work based on a temperature difference between the thermal collector 139 and the second storage medium in the cold reservoir 103.

[0099] The energy storage apparatus 101 may comprise additional features as set out in the embodiments shown in Figures 3 to 11. Except where explicitly described as alternatives, the additional features included in the embodiments shown in Figures 3 to 11 may be combined in an energy storage apparatus and used in the method 200 of operation. The skilled person willappreciate that the components of each energy storage apparatus may be fluidly couplable to one another in many different ways via a plurality of pipes, manifolds, diverter valves and pumps and achieve the same effect. As such, the fluid connection between each of the components will not be described in detail for every feature in the following embodiments. Additionally, for simplicity not all reference numerals included in FIG. 1 are included in Figures 3 to 11.

[0100] FIG. 3 and FIG. 4 show two alternative embodiments of the invention which enable heat to be utilised by the first heat engine 105 from the hot reservoir 102 or thermal collector 139 without rejecting residual heat into the cold reservoir 103.

[0101] In each of the embodiments in FIG. 3 and FIG. 4, the energy storage apparatuses includes all features of the energy storage apparatus 101 of the embodiment shown in FIG. 1. The reference numerals used in FIG. 1 are also used in these Figures for the same features. The energy storage apparatuses of FIG. 3 and FIG. 4 may comprise the variable speed drive 144 and controller 146 but these features are not shown in FIG. 3 and FIG. 4 for simplicity.

[0102] As shown in FIG. 3, the first heat engine 105 of the energy storage apparatus 302 may comprise two condensers. As shown in the embodiment in the Figures, the two condensers may be connected in parallel. The first heat engine 105 may comprise a first section having the first heat engine condenser 106 and a second section having an auxiliary first heat engine condenser 304. The first heat engine 105 may comprise diverter valves 306, 308 to control the flow of working fluid along the first and second sections. The auxiliary first heat engine condenser 304 is thermally couplable to the heat rejection means 147. As such, the first heat sink 148 provides a heat sink for the first heat engine 105. The energy storage apparatus 302 may comprise diverter valves 310, 312 to control the flow of fluid between the heat rejection means 147 and the auxiliary first heat engine condenser 304.

[0103] The energy storage apparatus 302 may therefore be operable in a second hot reservoir discharge mode in which the first heat engine 105 generates work from a temperature difference between the hot reservoir 102 and the first heat sink 148 using the auxiliary first heat engine condenser 304. The work generated by the first heat engine 105 may be used to generate electricity. The second hot reservoir discharge mode enables heat from the hot reservoir 102 to be used to produce work with rejecting heat into the cold reservoir 103.

[0104] Additionally, the energy storage apparatus 302 may be used to generate work using the first heat engine 105 operating between the thermal collector 139 and the heat rejection means 147. That is, using heat directly from the thermal collector 139.

[0105] To operate the energy storage apparatus 302, the method 200 may comprise the step of thermally coupling the auxiliary first heat engine condenser 304 to the heat rejection means 147 and operating the energy storage apparatus 302 in the second hot reservoir discharge mode.

[0106] FIG. 4 shows an alternative embodiment to FIG. 3. The energy storage apparatus 402 comprises a first auxiliary heat rejection means 404. In the embodiment in FIG. 4, the auxiliary first heat engine condenser 304 is thermally couplable to a first auxiliary heat rejection means 404 rather than the heat rejection means 147. The remaining features of the energy storage apparatus 402 are the same as those of the energy storage apparatus 302 in FIG. 3. The first auxiliary heat rejection means 404 is arranged to reject heat into a second heat sink 406.

[0107] The first auxiliary heat rejection means 404 may reject heat into the ambient surroundings of the energy storage apparatus 402. The second heat sink 406 may be the same as the first heat sink 148. The second heat sink 406 may be an external heat sink (i.e. , external to the energy storage apparatus 101). In the same manner as described above, the first auxiliary heat rejection means 404 may be selected dependent on the second heat sink 406. The first auxiliary heat rejection means 404 may be active or passive. The first auxiliary heat rejection means 404 may be a fan cooled heat exchanger such as a dry-cooler, an evaporative or adiabatic cooler, a fan-less heat exchanger relying on the natural movement of air or heat pipes utilising phase change fluids to transfer heat to the second heat sink 406.

[0108] The energy storage apparatus 402 may be operated in substantially the same way as the energy storage apparatus 302 of the embodiment shown in FIG. 3. The energy storage apparatus 402 may therefore be operable in a second hot reservoir discharge mode in which the first heat engine 105 generates work from a temperature difference between the hot reservoir 102 and the second heat sink 406 using the auxiliary first heat engine condenser 304. The work generated by the first heat engine 105 may be used to generate electricity. The second hot reservoir discharge mode enables heat from the hot reservoir 102 to be used to produce work with rejecting heat into the cold reservoir 103.

[0109] Additionally, the energy storage apparatus 402 may be used to generate work using the first heat engine 105 operating between the thermal collector 139 and the first auxiliary heat rejection means 404. That is, using heat directly from the thermal collector 139.

[0110] To operate the energy storage apparatus 402, the method 200 may comprise the step of thermally coupling the auxiliary first heat engine condenser 304 to the first auxiliary heat rejection means 404 and operating the energy storage apparatus 302 in the second hot reservoir discharge mode.

[0111] FIG. 5 shows an energy storage apparatus 502 according to an embodiment of the invention. The energy storage apparatus 502 in FIG. 5 includes all features of the energy storage apparatus 101 of the embodiment shown in FIG. 1. The reference numerals used in FIG. 1 are also used in FIG. 5 for the same features. The energy storage apparatus 502 may comprise the variable speed drive 144 and controller 146 but these features are not shown in FIG. 5 for simplicity.

[0112] As shown in the embodiment in FIG. 5, the first heat engine 105 may comprise a preheating heat exchanger 504. The pre-heating heat exchanger 504 may be thermally couplable to the intermediary reservoir 104. To thermally couple the pre-heating heat exchanger 504 and the intermediary reservoir 104, the energy storage apparatus 502 may comprise diverter valves 506, 508 configured to control the flow of fluid, either the third storage medium or the third heat transfer fluid, between the intermediary reservoir 104 and the pre-heating heat exchanger 504.

[0113] The pre-heating heat exchanger 504 may be included in the first heat engine 105 between the fluid pump 112 and the first heat engine evaporator 108. As such, the working fluid in the first heat engine 105 passes from through the pre-heating heat exchanger 504 prior to entering the first heat engine evaporator 108. Therefore, when the energy storage apparatus 502 is operated in the first hot reservoir discharge mode heat may be transferred from the third storage medium to the working fluid of the first heat engine 105 in the pre-heating heat exchanger 504 prior to a transfer of heat from the first storage medium to the working fluid. As such, the third storage medium can be cooled and operating the first heat engine 105 causes less heat to be drawn from the first storage medium in the hot reservoir 102. This may reduce the amount of heat that needs to be removed from the third storage medium by the heat rejection means 147 in the intermediary reservoir discharge mode or further reduce the temperature of the third storage medium if performed after the intermediary reservoir discharge mode. Additionally, pre-heating the working fluid in the first heat engine 105 extends the energy output of the energy storage apparatus 502 as the working fluid in the first heat engine 105 is not solely heated from the hot reservoir 102. As shown in FIG. 5, the third storage medium or third heat transfer fluid is in a separate fluid circuit to the working fluid of the first heat engine 105.

[0114] As shown in the embodiment in FIG. 5, the energy storage apparatus 502 may comprise a second auxiliary heat rejection means 510 for rejecting heat to a third heat sink 512. In the same manner as described above, the third heat sink 512 may be external to the energy storage apparatus 502. The third heat sink 512 may be the ambient surroundings of the energy storage apparatus 502. The third heat sink 512 may be the same as the first heat sink 148. In the samemanner as described above, the second auxiliary heat rejection means 510 may be selected dependent on the third heat sink 512. The second auxiliary heat rejection means 510 may be active or passive. The second auxiliary heat rejection means 510 may be a fan-cooled heat exchanger such as a dry-cooler, an evaporative or adiabatic cooler, a fan-less heat exchanger relying on the natural movement of air or heat pipes utilising phase change fluids to transfer heat to the third heat sink 512.

[0115] As shown in FIG. 5, the hot side of the first heat pump 125 may comprise a first section 520 and a second section 522. The first section 520 and the second section 522 may be connected in parallel to one another between the compressor 130 and the expansion valve 132. The first section 520 may comprise the first heat pump condenser 128 and may be thermally couplable to the intermediary reservoir 104. The second section 522 may comprise an auxiliary first heat pump condenser 514. The auxiliary first heat pump condenser 514 in the second section 522 may be thermally couplable to the second auxiliary heat rejection means 510. The first heat pump 125 may comprise diverter valves 516, 518 to control the flow of working fluid along the first and second sections 520, 522.

[0116] The second auxiliary heat rejection means 510 enables the first heat pump 125 to reject heat to third heat sink 512 rather than, for example, to the intermediary reservoir 104. The energy storage apparatus 502 may therefore be operable in a third cold reservoir charge mode in which the first heat pump is energised to cool the second storage medium and reject heat via the second auxiliary heat rejection means 510 to the third heat sink 512. The third cold reservoir charge mode may improve the efficiency of operating the first heat pump 125 compared to operating the energy storage apparatus 502 in the first or second cold reservoir charge mode if the temperature of the third heat sink 512 is lower than that of the third storage medium. Heat rejection to the third heat sink 512 in the third cold reservoir charge mode involves one less heat transfer stage than heat rejection in the first or second cold reservoir charge modes. In the third cold reservoir charge mode, heat is transferred from the first heat pump 125 directly a heat sink whereas in the first or second cold reservoir charge mode heat from the first heat pump 125 is first transferred to the third storage medium or third heat transfer fluid then to the cold reservoir 103 or a heat sink. The third cold reservoir charge mode may improve the efficiency of operating the first heat pump 125 compared to operating the energy storage apparatus 502 in the first or second cold reservoir charge mode if the temperature of the third heat sink 512 is lower than the sum of the temperature of the third heat sink and the minimum temperature difference required to facilitate heat transfer in the first heat pump condenser 128.

[0117] To operate the energy storage apparatus 502, the method 200 of the embodiment shown in FIG. 2 may be modified to include additional steps. The method may comprise the step of operating the energy storage apparatus 502 in the first hot reservoir discharge mode by thermally coupling the pre-heating heat exchanger 504 to the intermediary reservoir and transferring heat from the third storage medium to the working fluid of the first heat engine 105 prior to transferring heat from the first storage medium of the hot reservoir 102 to the working fluid.

[0118] To operate the energy storage apparatus 502, the method 200 may comprise additional steps of thermally coupling the hot side of the first heat pump 125 to the second auxiliary heat rejection means 510 for rejecting heat to the third heat sink 512 and operating the energy storage apparatus 502 in the third cold reservoir charge mode by energising the first heat pump 125 to cool the second storage medium and reject heat via the second auxiliary heat rejection means 510. In the third cold reservoir charge mode may be energised by electricity generated by the photovoltaic thermal collector or the one or more solar cells.

[0119] Whilst the energy storage apparatus 502 is operated in the third cold reservoir charge mode, the first heat pump 125 is not transferring heat into the intermediary reservoir 104. Therefore, the method of operating the energy storage apparatus 502 may comprise transferring heat from the third storage medium to the working fluid of the first heat engine 105 at the same time as operating the energy storage apparatus 502 in the third cold reservoir charge mode.

[0120] FIG. 6 shows an energy storage apparatus 602 according to an embodiment of the invention. The energy storage apparatus 602 in FIG. 6 includes all features of the energy storage apparatus 101 of the embodiment shown in FIG. 1. The reference numerals used in FIG. 1 are also used in FIG. 6 for the same features. The energy storage apparatus 602 may comprise the variable speed drive 144 and controller 146 but these features are not shown in FIG. 6 for simplicity.

[0121] As shown in the embodiment in FIG. 6, the energy storage apparatus 602 may comprise a second heat pump 604. The second heat pump 604 has a cold side thermally couplable to the intermediary reservoir 104 for cooling the third storage medium. The second heat pump 604 has a hot side thermally couplable to the hot reservoir 102 for heating the first storage medium. In the same manner as the first heat pump 125, the second heat pump 604 may comprise a second heat pump condenser 606, second heat pump evaporator 608, a compressor and an expansion valve. As shown in FIG. 6, the second heat pump evaporator 608 is thermally coupled to theintermediary reservoir 104. The second heat pump condenser 606 is thermally coupled to the hot reservoir 102.

[0122] To thermally couple the intermediary reservoir 104 to the second heat pump 604, fluid from the intermediary reservoir 104 may be directed to the second heat pump 604. For example, the third storage medium or the third heat transfer fluid can be directed to the second heat pump evaporator 608. The energy storage apparatus 602 may comprise diverter valves 610, 612 configured to control the flow of fluid between the intermediary reservoir 104 and the second heat pump evaporator 608. As shown in FIG. 4, the third storage medium or third heat transfer fluid is in a separate fluid circuit to the working fluid of the second heat pump 604.

[0123] As shown in FIG. 6, the second heat pump 604 may be connected to the intermediary reservoir 104 in parallel with the heat rejection means 147 and the first heat pump condenser 128 of the first heat pump 125. Therefore, the heat rejection means 147 and / or the first heat pump 125 may be thermally couplable to the cold side (i.e., the second heat pump evaporator 608) of the second heat pump 604. During operation, fluid may be circulated between the intermediary reservoir 104 and the second heat pump 604, between the heat rejection means 147 and the second heat pump 604, and / or between the hot side of the first heat pump 125 and the cold side of the second heat pump 604 so that heat is removed from either the third storage medium, the first heat sink 148 or the first heat pump 125. The diverter valves 610, 612 may be configured to control the flow of fluid between the second heat pump 604, the intermediary reservoir 104 and the heat rejection means 147.

[0124] To thermally couple the second heat pump 604 to the hot reservoir 102, the first storage medium or first heat transfer fluid may be directed from the hot reservoir 102 to the second heat pump 604. The second heat pump 604 may be fluidly couplable to the hot reservoir 102 in parallel or series with the thermal collector 139. The energy storage apparatus 602 comprises diverter valves 614, 616, 618, 620, 622 to control the flow of fluid, either the first storage medium or first heat transfer fluid, between the hot reservoir 102 and the second heat pump 604. As shown in FIG. 6, the first storage medium or first heat transfer fluid is in a separate fluid circuit to the working fluid of the second heat pump 604. The diverter valves 614, 616, 618, 620, 622 can be operated so that fluid can be circulated between the hot reservoir 102 and the second heat pump 604 without passing through the thermal collector 139. As such, the second heat pump 604 may directly heat the third storage medium in the hot reservoir 102. The diverter valves 614, 616, 618, 620, 622 can be operated so that fluid from the second heat pump 604 is directed through the thermal collector 139 prior to being directed to the hot reservoir 102 (i.e., the fluid flows from the second heat pump 604, to the thermal collector 139, to the hot reservoir102 then back to the second heat pump 604). The second heat pump 604 may therefore be used to pre-heat the third storage medium or third heat transfer fluid prior to it being heated in the thermal collector 139. Finally, the diverter valves 614, 616, 618, 620, 622 can be operated so that fluid from the thermal collector 139 is directed to the second heat pump 604 prior to being directed to the hot reservoir 102 (i.e. , the fluid flows from the thermal collector 139, to the second heat pump 604 then to the hot reservoir 102). The second heat pump 604 may therefore be used to heat the third storage medium or third heat transfer fluid after it has been heated in the thermal collector 139.

[0125] The energy storage apparatus 602 of the embodiment shown in FIG. 6 may therefore be operable in a second hot reservoir charge mode in which the second heat pump 604 is energised to cool the third storage medium and heat the first storage medium. As such, heat may be transferred from the intermediary reservoir 104 to the hot reservoir 102. The energy storage apparatus 602 may heat the first storage medium in combination with the thermal collector 139 as fluid may be directed through both the second heat pump 604 and the thermal collector 139. The energy storage apparatus 602 may be operable in the first hot reservoir charge mode and second hot reservoir charge mode at the same time (i.e., concurrently).

[0126] The energy storage apparatus 602 may be further operable in a third hot reservoir charge mode in which the second heat pump 604 is energised to transfer heat from the first heat sink 148 to the first storage medium in the hot reservoir 102. The energy storage apparatus 602 may be operable in a fourth hot reservoir charge mode in which the second heat pump 604 is energised to transfer heat from the first heat pump 125 to the first storage medium in the hot reservoir 102.

[0127] To operate the energy storage apparatus 602, the method 200 of the embodiment shown in FIG. 2 may be modified to include additional steps. The method 200 may comprise thermally coupling the cold side of the second heat pump 604 to the intermediary reservoir 104 and the hot side of the hot reservoir 102 and operating the energy storage apparatus 602 in the second hot reservoir charge mode by energising the second heat pump 604 to cool the third storage medium and heat the first storage medium. In the second hot reservoir charge mode, the method 200 may comprise circulating fluid between the hot reservoir 102 and second heat pump 604 or between the hot reservoir 102, the second heat pump 604 and the thermal collector 139 to heat the first storage medium in the hot reservoir 102. Fluid may be directed to the second heat pump 604 either before or after the fluid has passed through the thermal collector 139. Alternatively, fluid may be directed to the second heat pump 604 and the thermal collector 139in parallel (i.e. , fluid may pass through the second heat pump 604 and the thermal collector 139 concurrently).

[0128] The method 200 may comprise thermally coupling the first heat sink 148 to the cold side of the second heat pump 604 and the hot reservoir 102 to the hot side of the second heat pump 604, and operating the energy storage apparatus 602 in the third hot reservoir charge mode in which the second heat pump 604 is energised to transfer heat from the first heat sink 148 to the first storage medium in the hot reservoir 102. The method 200 may comprise thermally coupling the hot side of the first heat engine 105 to the cold side of the first heat pump 125 and the hot reservoir 102 to the hot side of the second heat pump 604, and operating the energy storage apparatus 602 in the fourth hot reservoir charge mode in which the second heat pump 604 is energised to transfer heat from the first heat pump 125 to the first storage medium in the hot reservoir 102. When operating in either the third or fourth hot reservoir charge modes, the method 200 may comprise circulating fluid between the hot reservoir 102 and second heat pump 604 or between the hot reservoir 102, the second heat pump 604 and the thermal collector 139 to heat the first storage medium in the hot reservoir 102. Fluid may be directed to the second heat pump 604 before the fluid has passes through the thermal collector 139, after the fluid has passed through the thermal collector 139 or in parallel with the fluid passing through the thermal collector 139. The energy storage apparatus 602 may be operated in one of the second, third or fourth hot reservoir charge mode at one time. The second, third or fourth hot reservoir charge mode may be selected depending on which mode would result in the greatest efficiency in heat delivery to the hot reservoir 102.

[0129] T ransferring heat from the intermediary reservoir 104, first heat sink 148 or the first heat pump 125 to the first storage medium in the hot reservoir 102 may be advantageous to increase the heat stored in the hot reservoir 102. It may be particularly useful when the thermal source 140 is intermittent or subject to seasonal variation. For example, in embodiments where the thermal collector 139 is a solar thermal collector or photovoltaic collector, the energy storage apparatus 602 can be used to mitigate days of unusually low solar irradiance, or to boost the efficiency of the thermal collector 139. Additionally, removing heat from the intermediary reservoir 104 lowers its temperature and therefore would allow greater efficiency of the first heat pump 125 to be reached if the energy storage apparatus 602 is operated in the second hot reservoir charge mode prior to being operated in the first cold reservoir charge mode. Removing heat from the intermediary reservoir 104 also reduces the amount of work required from the heat rejection means 147 in the first intermediary reservoir discharge mode.

[0130] FIG. 7 shows an energy storage apparatus 702 according to an embodiment of the invention. The energy storage apparatus 702 in FIG. 7 includes all features of the energy storage apparatus 101 of the embodiment shown in FIG. 1. The reference numerals used in FIG.1 are also used in FIG. 7 for the same features. The energy storage apparatus 702 may comprise the variable speed drive 144 and controller 146 but these features are not shown in FIG. 7 for simplicity.

[0131] The energy storage apparatus 702 may comprise a cold reservoir heat exchanger 704. The cold reservoir heat exchanger 704 may be thermally couplable to the cold reservoir 103 and / or the cold side of the first heat pump 125. The cold reservoir heat exchanger 704 may be arranged to provide cooling to an external heat source or process 706. For example, the cold reservoir heat exchanger 704 may provide cooling for air-conditioning, data centre cooling, HVAC or cold-chain applications.

[0132] As shown in FIG. 7, fluid may be directed from the cold reservoir 103 to the cold reservoir heat exchanger 704 so that the cold reservoir heat exchanger 704 may provide cooling using the second storage medium. The energy storage apparatus 702 may comprise diverter valves 708, 710 configured to control the flow of fluid between the cold reservoir heat exchanger 704 and the cold reservoir 103. Fluid may be directed from the cold side (i.e. first heat pump evaporator 126) of the first heat pump 125 to the cold reservoir heat exchanger 704 so that the cold reservoir heat exchanger 704 may be cooled directly by the first heat pump 125. The energy storage apparatus 702 may comprise diverter valves 708, 710, 744, 746, 748, 750 configured to control the flow of fluid between the cold reservoir heat exchanger 704 and the cold side of the first heat pump 125. The energy storage apparatus 702 may comprise a pump 752 to circulate fluid between the cold reservoir heat exchanger 704 and the cold side of the first heat pump 125.

[0133] The cold reservoir heat exchanger 704, cold reservoir 103, and first heat pump 125 may be fluidly coupled in parallel with one another.

[0134] To operate the energy storage apparatus 702, the method 200 may comprise the step of thermally coupling the cold reservoir heat exchanger 704 to the cold reservoir 103 and cooling the external heat source or process 706 via the cold reservoir heat exchanger 704. The method 200 may comprise thermally coupling the cold reservoir heat exchanger 704 to the cold side of the first heat pump 125 and cooling the external heat source or process 706 via the cold reservoir heat exchanger 704.

[0135] As an addition or alternative to the cold reservoir heat exchanger 704, the energy storage apparatus 702 may comprise a second heat engine 712 thermally couplable to anexternal heat source or process 714 and one or both of the cold reservoir 103 and the cold side of the first heat pump 125. The second heat engine 712 may provide cooling to the external heat source or process 714 whilst generating work based on the temperature difference between the external heat source or process 714 and the cold reservoir 103 or cold side of the first heat pump 125. The generated work may be used to generate electricity. The second heat engine 712 may, for example, provide cooling for air-conditioning, data centre cooling, HVAC or coldchain applications.

[0136] In the same manner as the first heat engine 105, the second heat engine 712 may comprise a second heat engine evaporator 716, a second heat engine condenser 718 a fluid pump and an expander. The second heat engine evaporator 716 may be thermally coupled to the external heat source or process 714. The second heat engine condenser 718 may be thermally couplable to the cold reservoir 103 or the cold side of the first heat pump 125. The energy storage apparatus 702 may comprise diverter valves 720, 722 to direct fluid, either the second storage medium or the second heat transfer fluid, from the cold reservoir 103 or first heat pump 125 to the second heat engine condenser 718 of the second heat engine 712. As shown in FIG. 7, the second storage medium and the second heat transfer fluid are in a separate fluid circuit to the working fluid of the second heat engine 712.

[0137] The energy storage apparatus 702 may be operable in an external cooling mode in which the second heat engine 712 generates work from a temperature difference between the external heat source 714 and the cold reservoir 103 or between the external heat source 714 and the cold side of the first heat pump 125 thereby cooling the external heat source 714.

[0138] To operate the energy storage apparatus 702, the method 200 may comprise the step of thermally coupling the second heat engine thermally to the cold reservoir 103 and to the external heat source 714. The method 200 may comprise the step of operating the energy storage apparatus 702 in the external cooling mode by generating work with the second heat engine 712 from the temperature difference between the external heat source 714 and the cold reservoir 103 thereby cooling the external heat source 714. Optionally, the method 200 may comprise the step of thermally coupling the second heat engine thermally to the cold side of the first heat pump 125 and to the external heat source 714. The method 200 may comprise the step of operating the energy storage apparatus 702 in the external cooling mode by generating work with the second heat engine 712 from a temperature difference between the external heat source 714 and the cold side of the first heat pump 125 thereby cooling the external heat source 714.

[0139] The energy storage apparatus 702 may comprise a third heat engine 724 thermally couplable to a fourth heat sink 726 and at least one of the hot reservoir 102 or the thermal collector 139. The hot reservoir 102 or the thermal collector 139 may be a heat source for the third heat engine 724 and the fourth heat sink 726 may be a heat sink for the third heat engine 724. The third heat engine 724 may therefore generate work based on the temperature difference between the hot reservoir 102 or the thermal collector 139 and the fourth heat sink 726. The generated work may be used to generate electricity. Residual heat from the thermal collector 139 may be rejected into the fourth heat sink 726. As such, the fourth heat sink 726 may be heated. The fourth heat sink 726 may be an external heat sink. The fourth heat sink 726 may, for example, be an industrial process or a district heating system. The third heat engine 724 therefore enables heat to be provided to an external heat sink and work to be generated.

[0140] The energy storage apparatus 702 may comprise diverter valves 728, 730, 740, 742 to direct fluid, either the first storage medium or the first heat transfer fluid, from the hot reservoir 102 or thermal collector 139 to the third heat engine 724. In the same manner as the first heat engine 105, the third heat engine 724 may comprise a third heat engine evaporator 732, a third heat engine condenser 734, a fluid pump and an expander. The third heat engine evaporator 732 may be thermally couplable to the hot reservoir 102 or thermal collector 139. The third heat engine condenser 734 may be thermally couplable to the fourth heat sink 726. Heat may be transferred from the hot reservoir 102 or directly from the thermal collector 139 to the third heat engine evaporator 732 of the third heat engine 724. For example, if the hot reservoir 102 is fully charged, heat from the thermal collector 139 can be directed to the third heat engine 724 so that the thermal collector 139 may continue to collect useful heat.

[0141] The energy storage apparatus 702 may be operable in an external heating mode in which the second heat engine 712 generates work from a temperature difference between either the hot reservoir 102 or the thermal collector 139 and the fourth heat sink 726 thereby providing heat to the fourth heat sink 726.

[0142] To operate the energy storage apparatus 702, the method 200 may comprise the step of thermally coupling the third heat engine 724 to one of the hot reservoir 102 and the thermal collector 139 and the fourth heat sink 726 and operating the energy storage apparatus 702 in the external heating mode by generating work with the second heat engine from a temperature difference between either the hot reservoir 102 or the thermal collector 139 and the external heat sink thereby providing heat to the fourth heat sink 726 .

[0143] As an addition or alternative, the energy storage apparatus 702 may comprise a hot reservoir heat exchanger 736 thermally couplable to the hot reservoir 102 and / or the thermalcollector 139. The hot reservoir heat exchanger 736 may be thermally couplable to the hot reservoir 102 and / or the thermal collector 139 in parallel with the first heat engine 105. The hot reservoir heat exchanger 736 enables heat to be provided from the hot reservoir 102 or thermal collector 139 to an external heat sink e.g., a fifth heat sink 738. The fifth heat sink 738 may be an external heat sink. The fifth heat sink 738 may, for example, be an industrial process or a district heating system. The energy storage apparatus 702 may comprise diverter valves 740, 742 configured to control the flow of fluid, either the first storage medium or first heat transfer fluid, to the hot reservoir heat exchanger 736. Heat may be transferred from the hot reservoir 102 or directly from the thermal collector 139 to the hot reservoir heat exchanger 736. For example, if the hot reservoir 102 is fully charged, heat from the thermal collector 139 can be directed to the hot reservoir heat exchanger 736 so that the thermal collector 139 may continue to collect useful heat.

[0144] To operate the energy storage apparatus 702, the method 200 may comprise the step of thermally coupling the hot reservoir heat exchanger 736 to one of the hot reservoir 102 and / or the thermal collector 139 and heating the fifth heat sink 738 via the hot reservoir heat exchanger 736.

[0145] In embodiments where the thermal collector 139 is a photovoltaic thermal collector, it may be advantageous to cool the thermal collector 139 to improve its electrical output because photovoltaic thermal collectors have a negative temperature coefficient and, therefore, the electrical efficiency of photovoltaic thermal collectors improves as they are cooled. This may be particularly beneficial when the photovoltaic thermal collector is part of a solar farm or power station. The thermal collector 139 may be cooled by thermally coupling the thermal collector 139 to the hot reservoir 102 or the intermediary reservoir 104 as shown in the embodiments in FIG.8 to FIG. 11. Cooling of the thermal collector 139 may occur concurrently or sequentially with operating the charging the hot reservoir 102.

[0146] In each of the embodiments in FIG. 8 to FIG. 11, the energy storage apparatuses includes all features of the energy storage apparatus 101 of the embodiment shown in FIG. 1. The reference numerals used in FIG. 1 are also used in these Figures for the same features. The energy storage apparatuses of FIG. 8 to FIG. 11 may comprise the variable speed drive 144 and controller 146 but these features are not shown in for simplicity.

[0147] As shown in the embodiment in FIG. 8, the thermal collector 139 may be thermally couplable to the cold reservoir 103. The energy storage apparatus 802 may comprise a cooler 804. The cooler 804 may be a heat exchanger. Fluid from the cold reservoir 103, either thesecond storage medium or second heat transfer fluid, may be directed to from the cold reservoir 103 to a first side of the cooler 804. The energy storage apparatus 802 may comprise diverter valves 806, 808 configured to control the flow of fluid between the cold reservoir 103 to the first side of cooler 804. The diverter valves 806, 808 may be configured to divert some or all of the second storage medium or second heat transfer fluid to the cooler 804 whilst the rest of the second storage medium or second heat transfer fluid is used in a different mode. Once the fluid has passed through the cooler 804, it may be returned to the cold reservoir 103. Fluid from the thermal collector 139 may be directed to a second side of the cooler 804. The energy storage apparatus 802 may comprise diverter valves 810, 812 configured to control the flow of fluid between the thermal collector 139 and the second side of the cooler 804.

[0148] The energy storage apparatus 802 may be operable in a cooling mode in which the thermal collector 139 is cooled by the second storage medium. Heat may be transferred from the thermal collector 139 to the second storage medium via the cooler 804. In the cooling mode, fluid, either the first storage medium or the first heat transfer fluid, may be circulated through the thermal collector 139 and the second side of the cooler 804 but not through the hot reservoir 102. At the same time the second storage medium or second heat transfer fluid is circulated through the first side of the cooler. The energy storage apparatus 802 may be operated in the cooling mode concurrently or sequentially with the first or second cold reservoir charge mode.

[0149] To operate the energy storage apparatus 802, the method 200 may comprise the step of thermally coupling the thermal collector to the cold reservoir via the cooler 804 and operating the energy storage apparatus 802 in the cooling mode to transfer heat from the thermal collector 139 to the second storage medium.

[0150] FIG. 9 shows an alternative embodiment of the invention to FIG. 8 where heat is transferred from the thermal collector 139 to the second storage medium. The energy storage apparatus 902 may be used when the first and second storage media or the first and second heat transfer fluids are the same as one another. As shown in FIG. 9, the fluid from the cold reservoir 103, either the second storage medium or second heat transfer fluid, may be pumped directly through the thermal collector 139 to cool the thermal collector 139. The energy storage apparatus 902 may comprise diverter valves 904, 906 to control the flow of fluid between the cold reservoir 103 and the thermal collector 139.

[0151] The energy storage apparatus 902 is also operable in a cooling mode in which the thermal collector 139 is cooled by the second storage medium. Heat may be transferred from the thermal collector 139 directly to the second storage medium or second heat transfer fluid.Whilst the energy storage apparatus 902 is operated in the cooling mode, the fluid is not directed from the hot reservoir 102 to the thermal collector 139. The energy storage apparatus 902 may be operated in the cooling mode concurrently or sequentially with the first or second cold reservoir charge mode.

[0152] To operate the energy storage apparatus 902, the method 200 may comprise the step of thermally coupling the thermal collector directly to the cold reservoir and operating the energy storage apparatus 902 in the cooling mode to transfer heat from the thermal collector 139 to the second storage medium.

[0153] In the embodiments in FIG. 8 and FIG. 9, the thermal collector 139 is cooled using the second storage medium. The second storage medium may be used to provide rapid cooling of the thermal collector 139 and may be done over short periods. Alternatively, the thermal collector 139 may be cooled using the third storage medium to provide longer term cooling of the thermal collector 139. This may be done in embodiments where the thermal collector 139 is a photovoltaic thermal collector or when the thermal collector 139 is an alternative type collector as described above with reference to FIG. 1. Cooling the thermal collector 139 using the third storage medium may improve the electrical output if the thermal collector 139 is a photovoltaic thermal collector and / or may prevent accelerated aging or damage to the thermal collector 139. FIG. 10 and FIG. 11 show embodiments of the invention where the first heat engine 105 may be cooled using the third storage medium.

[0154] As shown in FIG. 10, in the energy storage apparatus 1002 the thermal collector 139 may be thermally couplable to the intermediary reservoir 104 and / or to the heat rejection means 147 so that the thermal collector 139 may be cooled by the third storage medium.

[0155] The energy storage apparatus 1002 may comprise a cooler 1004. The cooler 1004 may be a heat exchanger. Fluid from the intermediary reservoir 104, either the third storage medium or third heat transfer fluid, may be directed to from the intermediary reservoir 104 to a first side of the cooler 1004. The energy storage apparatus 1002 may comprise diverter valves 1006, 1008 configured to control the flow of fluid between the intermediary reservoir 104, the heat rejection means 147 and the first side of cooler 1004. Once the fluid has passed through the cooler 1004, it may be returned to the intermediary reservoir 104 for storage or may be directed to the heat rejection means 147. Fluid from the thermal collector 139 may be directed to a second side of the cooler 1004. The energy storage apparatus 1002 may comprise diverter valves 1010, 1012 configured to control the flow of fluid between the thermal collector 139 and the second side of the cooler 1004. In certain embodiments, fluid may circulate between: the cooler 1004and the intermediary reservoir 104; the cooler 1004 and the heat rejection means 147 or the cooler 1004, the intermediary reservoir 104 and the heat rejection means 147.

[0156] The energy storage apparatus 1002 may be operable in a cooling mode in which the thermal collector 139 is cooled by the third storage medium. Heat may be transferred from the thermal collector 139 to the third storage medium via the cooler 1004. In the cooling mode, fluid, either the first storage medium or the first heat transfer fluid, may be circulated through the thermal collector 139 and the second side of the cooler 1004 but not through the hot reservoir 102. At the same time the third storage medium or third heat transfer fluid is circulated through the first side of the cooler 1004. Heat transferred to the third storage medium from the thermal collector 139 may be stored in the intermediary reservoir 104 or directed to the heat rejection means 147 to be rejected into the first heat sink 148. Therefore, fluid may be directed from the first side of the cooler 1004 to either the intermediary reservoir 104 or the heat rejection means 147. The energy storage apparatus 1002 may be operated in the cooling mode concurrently with the first or second cold reservoir charge mode.

[0157] To operate the energy storage apparatus 1002, the method 200 may comprise the step of thermally coupling the thermal collector 139 to the intermediary reservoir 104 via the cooler 1004 and operating the energy storage apparatus 1002 in the cooling mode to transfer heat from the thermal collector 139 to the third storage medium. The method 200 may comprise the step of thermally coupling the thermal collector 139 to the heat rejection means 147 via the cooler 1004 and operating the energy storage apparatus 1002 in the cooling mode to transfer heat from the thermal collector 139 to the third storage medium.

[0158] FIG. 11 shows an alternative embodiment to FIG. 10 where heat is transferred from the thermal collector 139 to the third storage medium. The energy storage apparatus 1102 may be used when the first and third storage media or the first and third heat transfer fluids are the same as one another. As shown in FIG. 11 , the third storage medium or third heat transfer fluid, may be pumped directly through the thermal collector 139 to cool the thermal collector 139. The energy storage apparatus 1102 may comprise diverter valves 1104, 1106 to control the flow of fluid between the intermediary reservoir 104 and / or heat rejection means 147 and the thermal collector 139.

[0159] The energy storage apparatus 1102 may also be operable in a cooling mode in which the thermal collector 139 is cooled by the third storage medium. Heat may be transferred from the thermal collector 139 to the third storage medium. In the cooling mode, fluid, either the third storage medium or third heat transfer fluid, is circulated from the intermediary reservoir 104 orheat rejection means 147 through the thermal collector 139. Heat transferred to the third storage medium from the thermal collector 139 may be stored in the intermediary reservoir 104 or directed to the heat rejection means 147 to be rejected into the first heat sink 148. Therefore, fluid may be directed from the thermal collector 139 to either the intermediary reservoir 104 or the heat rejection means 147. The energy storage apparatus 1102 may be operated in the cooling mode concurrently or sequentially with the first or second cold reservoir charge mode.

[0160] To operate the energy storage apparatus 1102 , the method 200 may comprise the step of thermally coupling the thermal collector 139 directly to the intermediary reservoir 104 and operating the energy storage apparatus 1102 in the cooling mode to transfer heat from the thermal collector 139 to the third storage medium. The method 200 may comprise the step of thermally coupling the thermal collector 139 directly to the heat rejection means 147 and operating the energy storage apparatus 1102 in the cooling mode to transfer heat from the thermal collector 139 to the third storage medium.

[0161] The skilled person will appreciate that various modifications can be made to the abovedescribed embodiments.

[0162] For example, in the above-described embodiments, each heat engine and heat pump is described and illustrated as a single heat engine or pump. However, in each embodiment, each heat engine may be replaced by multiple heat engines. The multiple heat engines may be connected in parallel or in series with one another or in any other suitable arrangement. Similarly, each heat pump may be replaced by multiple heat pumps. The multiple heat pumps may be connected in parallel or series with one another or in any other suitable arrangement.

[0163] In the above-described embodiments, the hot reservoir 102, the cold reservoir 103 and the intermediary reservoir 104 are single thermal stores (i.e., single volumes). However, each reservoir may instead comprise multiple stores or a single store divided into separate volumes using baffles or insulated baffles. Each of the multiple stores or separate volumes may have a different capacity. Using multiple stores or divided stores may enable the run time for the charging and discharging modes of the apparatus to be varied.

[0164] The above-described embodiments include first, second, third, fourth and fifth heat sinks. Whilst each of the first, second, third, fourth and fifth heat sinks are separately described, one or more of the heat sinks may be the same.

[0165] The skilled person will appreciate that the components of each energy storage apparatus described herein may be connected together in multiple different ways and that theinvention is not limited to specific embodiment of pipes, manifolds, pumps and valves shown in the Figures. For example, in the embodiments shown in FIG. 3 and FIG. 4 the two condensers in the first heat engine 105 are connected in parallel with one another. However, in alternative embodiments, the two condensers may be connected in series or in any other suitable arrangement. Additionally, in the embodiment in FIG. 5, the first section 520 and second section 522 of the first heat engine 105 are connected in parallel with one another. However, in alternative embodiments, the first section 520 and second section 522 may be connected in series or in any other suitable arrangement.

[0166] In the energy storage apparatus 502 of FIG. 5, the pre-heating heat exchanger 504 is thermally coupled to the intermediary reservoir 104 to pre-heat the working fluid in the first heat engine 105 using heat from the intermediary reservoir 104. However, the pre-heating heat exchanger 504 could be coupled to alternative sources of heat. For example, the pre-heating heat exchanger 504 may be thermally couplable to any component of the energy storage apparatus 502 which generates heat. For example, the pre-heating heat exchanger 504 may be thermally couplable to the variable speed drive 144 of the first heat pump 125, the generator coupled to the expander 110 of the first heat engine 105 or any other source of waste heat in the energy storage apparatus 502.

[0167] As described above, the energy storage apparatus 101 may comprise a variable speed drive 144 connected to the first heat pump 125 to drive the compressor 130 of the first heat pump 125. The variable speed drive 144 is not limited to embodiments where the first heat pump 125 is electrically connected to a thermal collector 139 that is a photovoltaic thermal collector or electrically connected to one or more solar cells. Rather, the first heat pump 125 may be electrically connected to an alternative source of electrical power. The source of electrical power may have a variable output. The source of electrical power may be the same as the thermal source 140, for example, a combined heat and power (CHP) plant, nuclear power station, fossil fuel power station or biomass power station. The source of electrical power may be used to drive the first heat pump 125 in any of the cold reservoir charge modes. As such, the controller 146 may be configured to receive a generation signal from a source of electrical power indicative of a rate of generation of electricity by the source of electrical power. The controller may be configured to control the variable speed drive such that the rate consumption of electricity by the first heat pump corresponds to the rate of generation of electricity.

[0168] In an embodiment, the features of the energy storage apparatus 502 of the embodiment shown in FIG. 5 may be combined with those of the embodiments shown in FIG 3. or FIG. 4, so that operating the energy storage apparatuses 302, 402 of FIG. 3 and FIG. 4 comprise the pre-heating heat exchanger 504. Operating the energy storage apparatus in the second hot reservoir discharge mode, as described in relation to FIG. 3 and FIG. 4, may then comprise thermally coupling the pre-heating heat exchanger 504 to the intermediary reservoir 104 and transferring heat from the third storage medium to a working fluid of the first heat engine 105 prior to transferring heat from the first storage medium in the hot reservoir 102 to the working fluid.

[0169] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0170] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0171] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

CLAIMS1. An energy storage apparatus comprising:a hot reservoir containing a first storage medium;a cold reservoir containing a second storage medium;an intermediary reservoir containing a third storage medium;a first heat engine thermally couplable to the hot reservoir and to the cold reservoir, wherein the first storage medium is a heat source for the first heat engine and the second storage medium is a heat sink for the first heat engine; anda first heat pump having a cold side thermally couplable to the cold reservoir for cooling the second storage medium and a hot side thermally couplable to the intermediary reservoir for heating the third storage medium;wherein the energy storage apparatus is operable in:a first hot reservoir discharge mode in which the first heat engine generates work from a temperature difference between the hot reservoir and cold reservoir; anda first cold reservoir charge mode in which the first heat pump is energised to cool the second storage medium and heat the third storage medium.

2. The energy storage apparatus of claim 1, comprising a thermal collector configured to heat to the first storage medium; and wherein the energy storage apparatus is operable in a first hot reservoir charge mode in which heat is transferred from the thermal collector to the first storage medium to heat the first storage medium.

3. The energy storage apparatus of claim 2, wherein the thermal collector comprises a solar thermal collector.

4. The energy storage apparatus of any one of claims 1 to 3, comprising one or more solar cells electrically connected to the first heat pump, and wherein in the first cold reservoir charge mode the first heat pump is energised by electricity from the one or more solar cells.

5. The energy storage apparatus of claim 2 or 3, wherein the thermal collector comprises a photovoltaic thermal collector.

6. The energy storage apparatus of claim 5, wherein the photovoltaic thermal collector is electrically connected to the first heat pump, and wherein in the first cold reservoir charge mode reservoir charge mode the first heat pump is energised by electricity from the photovoltaic thermal collector.

7. The energy storage apparatus of claim 5 or 6, wherein the thermal collector is thermally couplable to the intermediary reservoir; and wherein the energy storage apparatus is operable in a cooling mode in which the thermal collector is cooled by the third storage medium.

8. The energy storage apparatus of any one of claims 5 to 7, wherein the thermal collector is thermally couplable to the cold reservoir; and wherein the energy storage apparatus is operable in a cooling mode in which the thermal collector is cooled by the second storage medium.

9. The energy storage apparatus of any one of the preceding claims, comprising a variable speed drive connected to the first heat pump, wherein the variable speed drive is configured to drive the first heat pump.

10. The energy storage apparatus of claim 9, comprising a controller configured to:receive a generation signal from a source of electrical power indicative of a rate of generation of electricity by the source of electrical power; andcontrol the variable speed drive such that the rate consumption of electricity by the first heat pump corresponds to the rate of generation of electricity to operate the energy storage apparatus in the first cold reservoir charge mode.

11. The energy storage apparatus of any one of claims 1 to 9, comprising a heat rejection means for rejecting heat to a first heat sink, wherein the heat rejection means is thermally couplable to the intermediary reservoir for cooling the third storage medium and wherein the energy storage apparatus is operable in an intermediary reservoir discharge mode in which the heat rejection means is used to cool the third storage medium.

12. The energy storage apparatus of claim 11 , wherein the hot side of the first heat pump is thermally couplable to the heat rejection means, and wherein the energy storage apparatus is operable in a second cold reservoir charge mode in which the first heat pump is energised to cool the second storage medium and reject heat via the heat rejection means.

13. The energy storage apparatus of claim 11 or 12, when dependent on claim 2, wherein the thermal collector is thermally couplable to the heat rejection means; and wherein the energy storage apparatus is operable in a cooling mode in which the thermal collector is cooled by the heat rejection means.

14. The energy storage apparatus of any one of claims 11 to 13, wherein the first heat engine is thermally couplable to the heat rejection means such that the heat rejection means provides a heat sink for the first heat engine, and wherein the energy storage apparatus is operable in a second hot reservoir discharge mode in which the first heat engine generates work from a temperature difference between the hot reservoir and the first heat sink.

15. The energy storage apparatus of any one of claims 1 to 13, wherein the first heat engine is thermally couplable to a first auxiliary heat rejection means for rejecting heat to a second heat sink, and wherein the energy storage apparatus is operable in a second hot reservoir discharge mode in which the first heat engine generates work from a temperature difference between the hot reservoir and the second heat sink.

16. The energy storage apparatus of any one of claims 1 to 15, comprising a second auxiliary heat rejection means for rejecting heat to a third heat sink, wherein the hot side of the first heat pump comprises a first section and a second section; wherein the intermediary reservoir is thermally couplable to the first section and the second auxiliary heat rejection means is thermally couplable to the second section; and wherein the energy storage apparatus is operable in a third cold reservoir charge mode in which the first heat pump is energised to cool the second storage medium and reject heat via the second auxiliary heat rejection means.

17. The energy storage apparatus of any one of claims 1 to 16, wherein the first heat engine comprises a pre-heating heat exchanger thermally couplable to the intermediary reservoir and wherein in the first hot reservoir discharge mode heat is transferred from the third storage medium to a working fluid of the first heat engine prior to a transfer of heat from the first storage medium to the working fluid.

18. The energy storage apparatus of claim 17, when dependent on claim 14 or 15, wherein in the second hot reservoir discharge mode heat is transferred from the third storage medium to a working fluid of the first heat engine prior to a transfer of heat from the first storage medium to the working fluid.

19. The energy storage apparatus of any one of claims 1 to 18, comprising a second heat pump having a cold side thermally couplable to the intermediary reservoir for cooling the third storage medium and a hot side thermally couplable to the hot reservoir for heating the first storage medium; and wherein the energy storage apparatus is operable in a second hot reservoir charge mode in which the second heat pump is energised to cool the third storage medium and heat the first storage medium.

20. The energy storage apparatus of claim 19, wherein the energy storage apparatus is operable in the first hot reservoir charge mode and second hot reservoir charge mode concurrently.

21. The energy storage apparatus of any one of claims 1 to 20, comprising a cold reservoir heat exchanger, wherein the cold reservoir heat exchanger is thermally couplable to the cold reservoir and / or the cold side of the first heat pump.

22. The energy storage apparatus of any one of claims 1 to 21 , comprising a hot reservoir heat exchanger thermally couplable to the hot reservoir.

23. The energy storage apparatus of claim 22, when dependent on claim 2, wherein the hot reservoir heat exchanger is thermally couplable to the thermal collector.

24. The energy storage apparatus of any one of claims 1 to 23, comprising a second heat engine thermally couplable to the cold reservoir and to an external heat source; wherein the energy storage apparatus is operable in an external cooling mode in which the second heat engine generates work from a temperature difference between the external heat source and the cold reservoir thereby cooling the external heat source.

25. The energy storage apparatus of any one of claims 1 to 24, comprising a third heat engine thermally couplable to the hot reservoir or the thermal collector and to a fourth heat sink, wherein the hot reservoir or the thermal collector is a heat source for the third heat engine and the fourth heat sink is a heat sink for the third heat engine, wherein the energy storage apparatus is operable in an external heating mode in which the third heat engine generates work from a temperature difference between either the hot reservoir or the thermal collector and the fourth heat sink thereby providing heat to the fourth heat sink.

26. A method of operating an energy storage apparatus comprising:(i) providing an energy storage apparatus comprising:a hot reservoir containing a first storage medium;a cold reservoir containing a second storage medium;an intermediary reservoir containing a third storage medium;a first heat engine thermally couplable to the hot reservoir and to the cold reservoir, wherein the first storage medium is a heat source for the first heat engine and the second storage medium is a heat sink for the first heat engine; anda first heat pump having a cold side thermally couplable to the cold reservoir for cooling the second storage medium and a hot side thermally couplable to the intermediary reservoir for heating the third storage medium;(ii) operating the energy storage apparatus in a first hot reservoir discharge mode by generating work using the first heat engine from a temperature difference between the hot reservoir and cold reservoir; and(iii) operating the energy storage apparatus in a first cold reservoir charge mode by energising the first heat pump to cool the second storage medium and heat the third storage medium.

27. The method of claim 26, comprising storing the cooled second storage medium in the cold reservoir.

28. The method of claim 26 or 27, wherein the energy storage apparatus comprises a thermal collector configured to heat to the first storage medium; and comprising operating the energy storage apparatus in a first hot reservoir charge mode by transferring heat from the thermal collector to the first storage medium thereby heating the first storage medium.

29. The method of claim 28, comprising thermally coupling the first heat engine to a heat rejection means for rejecting heat from the energy storage apparatus into an external heat sink and operating the energy storage apparatus by generating work using the first heat engine from a temperature difference between the thermal collector and the external heat sink30. The method of claim 28 or 29, comprising storing the heated first storage medium in the hot reservoir.

31. The method of any one of claims 28 to 30, wherein the thermal collector is a photovoltaic thermal collector and the method comprises operating the energy storage apparatus in the first cold reservoir charge mode by energising the first heat pump with electricity from the photovoltaic thermal collector.

32. The method of any one of claims 28 to 31, comprising operating the energy storage apparatus in a cooling mode by thermally coupling the thermal collector to the cold reservoir or the intermediary reservoir and cooling the thermal collector with the second storage medium or the third storage medium.

33. The method of any one of claims 26 to 30 or claim 32, wherein the apparatus comprises one or more solar cells electrically connected to the first heat pump, and the method comprises operating the energy storage apparatus in the first cold reservoir charge mode by energising the first heat pump with electricity from the one or more solar cells.

34. The method of any one of claims 26 to 33, wherein operating the energy storage apparatus in a first cold reservoir charge mode comprises driving the first heat pump using a variable speed drive.

35. The method of claim 34, wherein operating the energy storage apparatus in a first cold reservoir charge mode comprises receiving a generation signal from a source of electrical power indicative of a rate of generation of electricity by the source of electrical power; and controlling the variable speed drive such that the rate consumption of electricity by the first heat pump corresponds to the rate of generation of electricity .

36. The method of any one of claims 26 to 35, comprising thermally coupling the intermediary reservoir to a heat rejection means for rejecting heat to a first heat sink and operating the energy storage apparatus in an intermediary reservoir discharge mode by cooling the third storage medium via the heat rejection means.

37. The method of claim 36, comprising storing the cooled third storage medium in the intermediary reservoir.

38. The method of claim 36 or 37, comprising thermally coupling the hot side of the first heat pump to the heat rejection means and operating the energy storage apparatus in a second cold reservoir charge mode by energising the first heat pump to cool the second storage medium and reject heat to the first heat sink via the heat rejection means.

39. The method of any one of claims 36 to 38, comprising thermally coupling the first heat engine to the heat rejection means and operating the energy storage apparatus in a second hot reservoir discharge mode by generating work using the first heat engine from a temperature difference between the hot reservoir and the first heat sink.

40. The method of any one of claims 26 to 38, comprising thermally coupling the first heat engine to a first auxiliary heat rejection means for rejecting heat to a second heat sink, and operating the energy storage apparatus in a second hot reservoir discharge mode in which thefirst heat engine generates work from a temperature difference between the hot reservoir and the second heat sink.

41. The method of any one of claims 26 to 40, comprising thermally coupling the hot side of the first heat pump to a second auxiliary heat rejection means for rejecting heat to a third heat sink and operating the energy storage apparatus in a third cold reservoir charge mode by energising the first heat pump to cool the second storage medium and reject heat via the second auxiliary heat rejection means.

42. The method of any one of claims 26 to 41, wherein the first heat engine comprises a preheating heat exchanger and the method comprises operating the energy storage apparatus in the first hot reservoir discharge mode by thermally coupling the pre-heating heat exchanger to the intermediary reservoir and transferring heat from the third storage medium to a working fluid of the first heat engine prior to transferring heat from the first storage medium to the working fluid.

43. The method of any claim 39 or 40, wherein the first heat engine comprises a pre-heating heat exchanger and the method comprises operating the energy storage apparatus in the first hot reservoir discharge mode and / or the second hot reservoir discharge mode by thermally coupling the pre-heating heat exchanger to the intermediary reservoir and transferring heat from the third storage medium to a working fluid of the first heat engine prior to transferring heat from the first storage medium to the working fluid.

44. The method of any one of claims 26 to 43, wherein the energy storage apparatus comprises a second heat pump having a cold side thermally couplable to the intermediary reservoir for cooling the third storage medium and a hot side thermally couplable to the hot reservoir for heating the first storage medium; and the method comprises operating the energy storage apparatus in a second hot reservoir charge mode by energising the second heat pump to cool the third storage medium and heat the first storage medium.

45. The method of claim 44, comprising operating the energy storage apparatus in the first and second hot reservoir charge modes concurrently.

46. The method of any one of claims 26 to 45, comprising cooling an external heat source via a cold reservoir heat exchanger thermally coupled to the cold reservoir or the cold side of the first heat pump.

47. The method of any one of claims 26 to 46, comprising heating a fourth heat sink via a hot reservoir heat exchanger thermally coupled to the hot reservoir and / or the thermal collector.

48. The method of any one of claims 26 to 47, wherein the energy storage apparatus comprises a second heat engine thermally couplable to the cold reservoir and to an external heat source; and the method comprises operating the energy storage apparatus in an external cooling mode by generating work with the second heat engine from a temperature difference between the external heat source and the cold reservoir thereby cooling the external heat source.

49. The method of any one of claims 26 to 48, wherein the energy storage apparatus comprises a third heat engine thermally couplable to the hot reservoir or the thermal collector and to a fourth heat sink, wherein the hot reservoir or the thermal collector is a heat source for the third heat engine and the fourth heat sink is a heat sink for the third heat engine; and comprising operating the energy storage apparatus in an external heating mode by generating work with the second heat engine from a temperature difference between either the hot reservoir or the thermal collector and the fourth heat sink thereby providing heat to the fourth heat sink.

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