DESCRIPTION

The present invention relates to energy storage systems and methods of operating energy storage systems.

A number of thermo-mechanical energy storage technologies have been developed or proposed to date including compressed air energy storage (CAES), adiabatic compressed air energy storage (ACAES), pumped hydro energy storage (PHS), pumped heat energy storage (PHES) and cryogenic energy storage (CES) including liquid air energy storage (LAES).

PHES type energy storage systems can be split into:

i. those based around Brayton cycles where the working fluid does not change phase and heat exchange is via sensible heat for both hot and cold thermal storage (with the terms "hot" and "cold" used to denote the fact that one of the stores is at a higher temperature than the other).

ii. those based around sub-critical Rankine cycles where heat exchange is pre-dominantly latent for both hot and cold stores

iii. those based around supercritical Rankine cycles where heat exchange is pre-dominantly latent for the cold store and sensible for the hot store Examples of Brayton PHES systems in the prior art include:

Published PCT Application No. WO 2009/044139 which describes a PHES system where two packed bed thermal stores are placed within a thermal heat pump cycle to produce a hot and cold thermal store respectively during charging. Energy is then recoverable in a discharging mode by passing gas through the cold thermal store, compressing the gas cooled by the cold thermal store, further heating the compressed gas by passing it through the hot thermal store and generating power by expanding the hot pressurized gas back to near the initial condition. The difference in work between the compressor and expander means that in the energy recovery mode a proportion of the work input during storage mode can be recovered and used to drive an electrical generator.

The system proposed in WO 2009/044139 uses reciprocating compressors and expanders as well as crushed rock as the storage medium within the thermal stores. Heat exchange is direct to the storage medium, which means that the storage vessels must be designed for operation at the working pressure.

A similar PHES system is also proposed in published PCT Application No WO2008/148962, but with the use of turbo-compressors and expanders proposed and where the storage medium is a refractory material.

The problem with these proposed PHES systems is that the use of pressurized thermal stores is expensive and the packing density and heat capacity of solids is low relative to liquids. This leads to relatively low energy densities and high cost.

Examples of sub-critical Rankine PHES systems in the prior art include:

Published PCT Application No. WO2014/162129 describes an energy storage and recovery system based upon a closed system in which a working fluid is transferred, during charging and discharging, between a first pressure vessel and a second pressure vessel, via power machinery, and where the working fluid is stored in each tank as a saturated liquid/vapour mixture under equilibrium pressure and temperature conditions which match in the sense that condensation of the vapour in a tank causes a progressive increase in the equilibrium vapour pressure and temperature of the saturated mixture, and evaporation of the vapour in a tank causes a progressive decrease in the equilibrium vapour pressure and temperature of the saturated mixture.

The problem with the proposed system of WO2014/162129 is that the pressures within the two vessels are constantly changing and, secondly, the quantity of 'ballast' liquid required in each vessel to slow the rate of change down versus the quantity evaporated is high. Having a large quantity of liquid in both vessels leads to an expensive system with low energy density.

Examples of a supercritical Rankine PHES systems in the prior art include:

Published PCT Application No. WO2012/ 168472 describes a thermoelectric energy storage system with an evaporative ice storage arrangement and method for storing thermoelectric energy. The evaporative ice storage arrangement comprises a heat exchanger, an ice slurry storage tank, a vacuum evaporation chamber and a slurry heat exchanger. The evaporative ice storage arrangement functions as a dedicated cold storage for the thermoelectric energy storage system. The cold storage is realized by producing an ice-water mixture during charging of the storage, and using the stored ice-water mixture to condense the working fluid during the discharge cycle.

The problem with the proposed system of WO2012/168472 is that the working fluid is not the same as the cold storage fluid (water) that is at or near its triple point. Therefore, heat exchange to or from this cold storage is indirect via a heat exchanger. Using water means that this occurs at very low vapour pressures, which makes heat exchange expensive and inefficient.

Examples of a Brayton PHES combined with a LAES system in the prior art include: UK patent GB2537126 describes a hybrid energy storage system where two different storage technologies are synergistically combined with a gas turbine. In this case a liquid air energy storage (LAES) system is combined with a pumped heat energy storage (PHES) system. The combination of the two different systems allows the removal of the cold store from both systems, thereby improving energy density and lowering cost.

This avoidance of the requirement for a sensible heat cold store is very helpful, however there are significant irreversibility issues with conventional LAES systems that tend to lead to low overall round-trip efficiencies.

A variant of PHES is a system called adiabatic compressed air energy storage (Adiabatic CAES). This involves the compression of air to a much higher temperature and pressure than is typical in a conventional PHES. The process involves the storage of some or all of this heat of compression in one or more hot thermal stores and the separate storage of the compressed air, normally in an underground salt cavern. Multiple proposals have been made for the thermal storage medium, however the main obstacle is either the requirement for very specific geological features or the requirement to use expensive fabricated pressure vessels.

An adiabatic CAES system can also be regarded as being half of a PHES system, where the storage of the high pressure compressed air store avoids the need for a cold store. This system has a further advantage in that all of the mechanical processes are either work input (compressing) when charging or work output (expanding) when discharging. One of the main disadvantages of the PHES patents based upon Brayton cycles is that the power input and output involves a difference between a compression and expansion process. This means that the total of the compression and expansion work is always much higher than the power input and output. Each compression and expansion process has associated losses and the overall result is a lower energy density and a lower round trip efficiency than a storage process where there is just compression on charge and expansion on discharge.

The present applicant has identified the need for an improved energy storage system that overcomes or alleviates problems with the prior art.

In accordance with a first aspect of the present invention, there is provided an energy storage system, comprising: a three-phase store comprising: a heat transfer fluid; and a working fluid that is immiscible with the heat transfer fluid, the three-phase store being operative during a charging mode to: maintain the heat transfer fluid simultaneously in three phases in a manner that allows each of the three phases of the heat transfer fluid to exchange heat direct with at least one other of the three phases of the heat transfer fluid; and with the working fluid being present in at least two phases, mix the working fluid with the heat transfer fluid to allow direct heat exchange between the working fluid and the heat transfer fluid; compressor/expander apparatus; and a thermal energy store; wherein the energy storage system is configured: to store energy in the charging mode by: evaporating a liquid phase of the working fluid in the three-phase phase store to form a gaseous flow of the working fluid, whereby a portion of the liquid phase of the heat transfer fluid is converted to solid phase as the liquid phase of the working fluid evaporates to form the gaseous flow; passing the gaseous flow of the working fluid from the three-phase store to the compressor/expander apparatus for compression by the compressor/expander apparatus; and transferring thermal energy from the working fluid compressed by the compressor/expander apparatus to the thermal energy store; and to generate power in a discharging mode by: transferring thermal energy from the thermal energy store to the or a further working fluid; and passing the or the further working fluid to the compressor/expander apparatus for expansion to generate power.

In this way, a system is provided in which only a small quantity of one of the fluids is needed to achieve a certain vapour pressure. For example, a heat transfer fluid with a relatively low vapour pressure at ambient or below ambient temperature can be used in combination with a working fluid having a substantially higher vapour pressure at the same temperature. Advantageously, this allows a reduced quantity of working fluid to be used in the system. In one embodiment, in the three-phase store the heat transfer fluid has a vapour pressure substantially lower than the vapour pressure of the working fluid (e.g. at least 5 times smaller, e.g. at least 10 times smaller, e.g. at least 20 times smaller). In this way, the gas phase of the three-phase store may be substantially the gas phase of the working fluid (e.g. with only a small amount of heat transfer fluid present in the gas phase).

The heat transfer fluid may be a substantially pure fluid or a mixture (e.g. solution). In the case of a solution, the heat transfer fluid may be a working fluid may be a liquid solution or a solid solution.

Where the heat transfer fluid is a substantially pure fluid, the three phases of the heat transfer fluid will be three phases of the pure fluid (i.e. substantially the same substance in three different phases). Where the heat transfer fluid is a mixture, the solid phase may be a solid phase of only a component of the mixture (e.g. solid phase of the solvent in the case of a solution). Similarly, the gas phase may be a gas phase of only a component of the mixture (e.g. gas phase of the solvent in the case of a solution).

In one embodiment, the solid/liquid component of the heat transfer fluid is maintained within 15°C (e.g. within 10°C, e.g. within 5°C) of the triple point temperature or the freezing point temperature of the heat transfer fluid. In the case that the heat transfer fluid is a substantially pure fluid, the solid/liquid component of the heat transfer fluid may be maintained in the three-phase store within 15°C (e.g. within 10°C, e.g. within 5°C) of the triple point temperature or freezing point temperature of the heat transfer fluid. In the case that the heat transfer fluid is a mixture, the solid/liquid component of the heat transfer fluid may be maintained within 15°C (e.g. within 10°C, e.g. within 5°C) of the freezing point temperature of the heat transfer fluid.

Typically the direct heat exchange will be any of (or any combination of): between the gas phase of the working fluid and the gas phase of the heat transfer fluid; between the gas phase of the working fluid and the liquid phase of the heat transfer fluid; between the gas phase of the working fluid and the solid phase of the heat transfer fluid; between the liquid phase of the working fluid and the liquid phase of the heat transfer fluid; and/or between the liquid phase of the working fluid and the solid phase of the heat transfer fluid.

In one embodiment, during the charging mode at least 10% by mass of the liquid phase of the heat transfer fluid is converted to solid phase (e.g. at least 20%, e.g. at least 30%).

In one embodiment, the mass fraction of the solid phase of the heat transfer fluid relative to the combined liquid and solid phase of the heat transfer fluid increases by over 30% during the charging mode (e.g. increases by over 40% during the charging mode, e.g. increases by over 50% during the charging mode).

In one embodiment, the three-phase store stores the heat transfer fluid at a different pressure (either higher or lower) to atmospheric pressure.

In one embodiment, the thermal energy store and the three-phase store act as hot and cold stores respectively during the discharging mode.

In one embodiment, the three-phase store is supplied by a supply store of the heat transfer fluid (e.g. operating at a different pressure to the three-phase store, e.g. at or above atmospheric pressure).

Typically the supply store stores the heat transfer fluid at a temperature substantially below that of the thermal energy store. Accordingly, the thermal energy store and the supply store act as hot and cold stores respectively during the discharging mode.

In one embodiment, the supply store stores the heat transfer fluid in one or two phases (e.g. liquid phase or mixture of liquid/solid phases).

In one embodiment, during charging the heat transfer fluid supplied by the supply store has a higher liquid to solid ratio than the heat transfer fluid being returned from the three-phase store to the supply store.

In one embodiment, during discharging the heat transfer fluid supplied by the supply store of has a higher solid to liquid ratio than the heat transfer fluid being returned from the three-phase store to the pressurised supply store.

In one embodiment, the supply store is operative to store heat transfer fluid at a lower pressure (e.g. but still at or above atmospheric pressure) than the fluid pressure in the three- phase store.

In one embodiment, the supply store comprises a pressurisation device for increasing the pressure of heat transfer fluid as the heat transfer fluid passes from the supply store to the three-phase store.

In one embodiment, the supply store comprises a pressure reduction device for reducing the pressure of heat transfer fluid as the heat transfer fluid passes from the three-phase store to the supply store.

In one embodiment, the supply store further comprises a separator operative to separate

(e.g. under gravity) the working fluid (e.g. liquid phase of the working fluid) from the heat transfer fluid (e.g. mixture of liquid/solid phases of the heat transfer fluid) and supply substantially only the heat transfer fluid to the supply store. In one embodiment the three-phase store is operative to evaporate working fluid during the charging mode and operative to condense working fluid during the discharging mode (e.g. by spraying heat transfer fluid into a flow of expanded working fluid returned to the three-phase store).

In the case that the three-phase store is supplied by a supply store, the three-phase store may be operative to evaporate working fluid and the system may further comprise a discharge heat exchanger operative during the discharging mode to transfer thermal energy from working fluid expanded by the compressor/expander apparatus to the supply store (e.g. to a flow of heat transfer fluid circulating between the supply store and the discharge heat exchanger during the discharging mode). In this way, the three-phase store may operate as an evaporator only.

In the case that the three-phase store is supplied by a supply store, the supply store may be configured to store a predetermined volume of the mixture of liquid/solid phases associated with a predetermined liquid/solid phase level inside the supply store.

In one embodiment, at least one (e.g. both) of the three-phase store and the separator define a chamber for receiving heat transfer fluid from the supply store having an upper end that is provided at a height below the height of the predetermined liquid/solid phase level (e.g. at a height at least lm below the height of the predetermined liquid/solid phase level, e.g. e.g. at a height at least 5m below the height of the predetermined liquid/solid phase level, e.g. at a height at least 10m below the height of the predetermined liquid/solid phase level). In this way, the static head of liquid/solid phase may be used to reduce the load on the pressurisation/pressure reduction devices.

In one embodiment, the system further comprises cooling apparatus to cool the three- phase store or supply store, e.g. to generate additional solid phase of the heat transfer fluid in the three-phase store. In this way, a minimum ratio of solid to liquid phase of the heat transfer fluid may be maintained in the three-phase store during the discharging mode. The cooling apparatus may be operated to provide cooling to the three-phase store/supply store during the charging mode, during the discharging mode, or even when the system is not operating in either mode.

In one embodiment, the cooling apparatus is operative to cool a flow of working fluid returning to the three-phase store.

In one embodiment, the cooling apparatus comprises a refrigeration circuit operative to condense and cool a component of the gaseous flow of the working fluid before returning the component of the gaseous flow of the working fluid to the three-phase store. In one embodiment, the component of the gaseous flow of the working fluid is taken from a point in the gaseous flow between the three-phase store and the thermal energy storage.

In one embodiment, the component of the gaseous flow of the working fluid cooled by the refrigeration circuit is returned in liquid phase to the three-phase store (e.g. via an expander or throttle valve).

In one embodiment, the system further comprises a heat rejection stage (e.g. operative to reject heat to an external process or to atmosphere). In one embodiment, the refrigeration circuit comprises the heat rejection system. In this way, the heat rejection and three-phase store cooling processes required in a closed system may be provided by a single device.

In one embodiment, the refrigeration circuit is operative to heat working fluid evaporated from the three-phase store prior to compression of the working fluid by the compressor/expander apparatus. In this way, a small amount of super heat may be added to the working fluid prior to compression in order to reduce liquid drop formation during compression.

In one embodiment, the working fluid is a substantially pure fluid or a mixture (e.g. solution). In the case of a solution, the working fluid may be a liquid solution or a solid solution.

In one embodiment, the heat transfer fluid is a substantially pure fluid or a mixture (e.g. solution). In the case of a solution, the heat transfer fluid may be a liquid solution or a solid solution.

In one embodiment, the working fluid is organic, e.g. a hydrocarbon (e.g. butane, pentane, propane, etc.).

In one embodiment, the heat transfer fluid is water or water-based (e.g. water with solid solution such as brine).

In one embodiment, the three-phase store stores the heat transfer fluid at a temperature below ambient.

In one embodiment, the compressor/expander apparatus comprises one or more combined compressor/expander devices (e.g. devices configured to compress a fluid flow provided in a first direction and to expand a fluid flow provided in a second (opposed) direction). In another embodiment, the compressor/expander apparatus may comprise one or more dedicated compressors and one or more dedicated expanders.

In one embodiment, in the charging mode the working fluid is returned to the three- phase store after thermal energy is transferred from the working fluid to the thermal energy store.

In one embodiment, in the charging mode after thermal energy is transferred to the thermal energy store the compressor/expander apparatus is operative to expand the liquid phase working fluid before the working fluid is returned to the three-phase store.

In one embodiment, in the discharging mode the compressor/expander apparatus is operative to compress the working fluid (e.g. in a liquid phase) before exposure to the thermal energy store.

In one embodiment, in the discharging mode the working fluid is returned to the three- phase store after expansion of the working fluid by the compressor/expander apparatus.

In one embodiment, the three-phase store comprises a chamber having a lower portion defining a liquid/solid containment region and an upper portion defining a gas containment region.

In one embodiment, the three-phase store is configured to mix (e.g. continually or continuously mix) the liquid phase of the working fluid with the liquid/solid phases of the heat transfer fluid (e.g. to form an agitated mixture).

In one embodiment, the three-phase store comprises a mixing device operative to provide a mixing action in the lower portion of the chamber.

In one embodiment, the three-phase store is configured to pump the liquid phase of the working fluid or a mixture of the liquid phase of the working fluid and heat transfer fluid (e.g. liquid phase of the heat transfer fluid or liquid/solid phases of the heat transfer fluid) from the lower portion of the chamber to a spray device (e.g. spray nozzle) provided at an upper portion of the chamber.

In one embodiment, the three-phase store is configured to separate the liquid phase of the heat transfer fluid from the mixture of liquid/ solid phases of the heat transfer fluid and supply substantially only the liquid phases of the heat transfer fluid and working fluid to the spray device.

In one embodiment, during the charging mode the working fluid is condensed by the processes of transferring thermal energy to the thermal energy store.

In one embodiment, during the discharging mode the working fluid is evaporated by the processes of receiving thermal energy from the thermal energy store.

In one embodiment, thermal energy is transferred from the working fluid to the thermal energy store by means of a heat exchanger located within the thermal energy store (e.g. with the working fluid passing through the thermal energy store to achieve thermal transfer).

In one embodiment, the working fluid exchanges thermal energy with the thermal energy store via an intermediate stage (e.g. via at least one heat exchanger or via secondary thermodynamic cycle).

In one embodiment, the secondary thermodynamic cycle is a Rankine cycle.

In one embodiment, the working fluid exchanges thermal energy with the thermal energy store via first and second heat exchangers wherein the compressor/expander apparatus comprises a first compressor (e.g. first compressor/expander) provided between the three-phase store and the first heat exchanger and a second compressor (e.g. second compressor/expander) provided between first and second heat exchangers.

In one embodiment, the thermal energy store comprises first and second alternately chargeable stores.

In one embodiment, the thermal energy store comprises a liquid store containing a heat storage liquid.

In one embodiment, the heat storage liquid is the liquid phase of the heat transfer fluid.

In one embodiment, during the charging mode the system is operative to allow a flow of the liquid phase of the heat transfer fluid from the three-phase store (or supply store) to the thermal energy store (e.g. to the liquid store).

In one embodiment, the thermal energy store is operative to store the heat storage liquid received in the liquid store such that the temperature of the heat storage liquid is substantially unvarying with store depth.

In one embodiment, during the charging mode the system is operative, following transfer of thermal energy from the working fluid to the thermal energy store, to transfer additional thermal energy from the working fluid to the flow of the liquid phase of the heat transfer fluid as it passes from the three-phase store to the thermal energy store.

In one embodiment, during the charging mode the system is configured to allow any working fluid entering the thermal energy store to be evaporated from the thermal energy store and returned to the three-phase store.

In one embodiment, in the discharging mode the liquid phase of the working fluid flows from the three-phase store to receive thermal energy from the thermal energy store before flowing to the compressor/expander apparatus for expansion to generate power.

In one embodiment, the liquid phase of the working fluid is separated from the heat transfer fluid (e.g. using a settling tank).

In one embodiment, in the discharging mode the working fluid expanded by the compressor/expander is returned to the three-phase tank in gaseous form.

In one embodiment, in the discharging mode the compressor/expander apparatus is operative to pressurise the working fluid received from the three-phase store in the liquid phase prior to exposure to the thermal energy store.

In one embodiment, the system further comprises a secondary heat store operative to receive thermal energy from the working fluid after exposure to the thermal energy store (e.g. direct or via an intermediate stage (e.g. via a heat exchanger)).

In one embodiment, the secondary heat store comprises a liquid store containing a heat storage liquid.

In one embodiment, the heat storage liquid is the liquid phase of the heat transfer fluid.

In one embodiment, the secondary heat store is operative to store the heat storage liquid received in the liquid store such that the temperature of the heat storage liquid is substantially unvarying with store depth (e.g. the secondary heat store is configured to circulate the heat storage liquid through the liquid store).

In one embodiment, the secondary heat store is operative to store the heat storage liquid such that the temperature of the heat storage liquid received in the liquid store varies substantially with store depth. In one embodiment, the secondary heat store is configured to generate a body of heat storage liquid with a temperature profile including at least one thermocline.

In one embodiment, the secondary heat store is further configured to receive thermal energy from the working fluid prior to exposure of the working fluid to the thermal energy store. In this way, the heat storage liquid at the top of the liquid store of the secondary heat store may be hotter than the thermal energy store to provide for superheating of the working fluid prior to expansion during the discharging mode.

In one embodiment, the liquid store of the secondary heat store is connected to the three- phase store or supply store.

In one embodiment, during the charging mode the system is operative to allow a flow of the liquid phase of the heat transfer fluid from the three-phase store (or supply store) to the liquid store of the secondary heat store.

In one embodiment, during the discharging mode the system is operative to allow a flow of the liquid phase of the heat transfer fluid from the liquid store of the secondary heat store to the three-phase store (or supply store).

In one embodiment, the liquid store is fluidly unconnected to the three-phase store or supply store. In one embodiment, during the charging mode the system is operative to allow a flow of the liquid phase of the heat transfer fluid from the three-phase store to the secondary heat store (e.g. to the liquid store).

In one embodiment, during the charging mode the system is operative, following transfer of thermal energy from the working fluid to the thermal energy store, to transfer additional thermal energy from the working fluid to the flow of the liquid phase of the heat transfer fluid as it passes from the three-phase store to the secondary heat store.

In one embodiment, during the charging mode the system is configured to allow any working fluid entering the secondary heat store to be evaporated from the secondary heat store and returned to the three-phase store.

In one embodiment, the compressor/expander apparatus operative to compress the gaseous flow of the working fluid during the charging mode comprises a first compressor/expander device and a second compressor/expander device arranged in series with the first compressor/expander device.

In one embodiment, the first compressor/expander device is a fixed speed device (e.g. comprises a moving compressor/expander part that can only be operated at a single speed) and the second compressor/expander device is a variable speed device (e.g. comprises a moving compressor/expander part that can be operated at different speeds).

In one embodiment, the second compressor/expander is configured to operate over a range of different compression/expansion pressure ratios.

In one embodiment, the first compressor/expander is configured to operate at a substantially fixed compression/expansion pressure ratio.

In one embodiment, at least one (e.g. both) of the first and second compressor/expander devices is configured to provide a volumetric flow proportional to speed of operation of the device.

In one embodiment, at least one (e.g. both) of the first and second compressor/expander devices is a positive displacement device.

In one embodiment, the system further comprises a heat rejection device (e.g. operative to reject heat to an external process or to atmosphere) operative to receive thermal energy from the flow of the working fluid at a point between the first and second compressor/expander devices.

In one embodiment, the heat rejection device is operable during one or both or the charging mode and the discharging mode.

In one embodiment, the system is configured to allow the degree of thermal coupling between the working fluid flowing between the first and second compressor/expander devices and the heat rejection device to be varied.

In one embodiment, the heat rejection stage is provided along a bypass line connected between the first and second compressor/expander devices. In one embodiment, the degree of thermal coupling may be varied by varying the proportion of working fluid flow directed to the heat rejection device relative to the working fluid flow that bypasses the heat rejection device. In one embodiment the proportion of fluid flow directed to the heat rejection device may be varied during one or both of the charging mode and the discharging mode.

In one embodiment, the system further comprises a low pressure evaporator (e.g. direct evaporator) operative to receive fluid from the thermal energy store and generate vapour (e.g. steam in the case that the fluid in the thermal energy store is water). In one embodiment the vapour is generated at subatmospheric pressure. In one embodiment the system further comprises a vapour compressor (e.g. steam compressor) operative to compress vapour received from the low pressure evaporator and deliver the compressed vapour to a further process (e.g. external heating process).

In accordance with a second aspect of the present invention, there is provided a method of operating an energy storage system, comprising: providing an energy storage system including: a three-phase store comprising: a heat transfer fluid; and a working fluid that is immiscible with the heat transfer fluid, the three-phase store being operative during a charging mode to: maintain the heat transfer fluid simultaneously in three phases in a manner that allows each of the three phases of the heat transfer fluid to exchange heat direct with at least one other of the three phases of the heat transfer fluid; and with the working fluid being present in at least two phases, mix the working fluid with the heat transfer fluid to allow direct heat exchange between the working fluid and the heat transfer fluid; compressor/expander apparatus; and a thermal energy store; the method further comprising the steps of: in the charging mode: maintaining a mixture of the heat transfer fluid and a working fluid in the three-phase store; evaporating a liquid phase of the working fluid in the three-phase phase store to form a gaseous flow of the working fluid, whereby a portion of the liquid phase of the heat transfer fluid is converted to solid phase as the liquid phase of the working fluid evaporates to form the gaseous flow; passing the gaseous flow of the working fluid from the three-phase store to the compressor/expander apparatus; compressing the working fluid using the compressor/expander apparatus; and transferring thermal energy from the compressed working fluid to the thermal energy store; in a discharging mode: transferring thermal energy from the thermal energy store to the working fluid or a further working fluid; and expanding the working fluid or the further working fluid to generate power.

In one embodiment, in the three-phase store the heat transfer fluid has a vapour pressure substantially lower than the vapour pressure of the working fluid (e.g. at least 5 times smaller, e.g. at least 10 times smaller, e.g. at least 20 times smaller).

The heat transfer fluid may be a substantially pure fluid or a mixture (e.g. solution). In the case of a solution, the heat transfer fluid may be a working fluid may be a liquid solution or a solid solution.

Where the heat transfer fluid is a substantially pure fluid, the three phases of the heat transfer fluid will be three phases of the pure fluid (i.e. substantially the same substance in three different phases). Where the heat transfer fluid is a mixture, the solid phase may be a solid phase of only a component of the mixture (e.g. solid phase of the solvent in the case of a solution). Similarly, the gas phase may be a gas phase of only a component of the mixture (e.g. gas phase of the solvent in the case of a solution).

In one embodiment, the solid/liquid component of the heat transfer fluid is maintained within 15°C (e.g. within 10°C, e.g. within 5°C) of the triple point temperature or the freezing point temperature of the heat transfer fluid. In the case that the heat transfer fluid is a substantially pure fluid, the solid/liquid component of the heat transfer fluid may be maintained within 15°C (e.g. within 10°C, e.g. within 5°C) of the triple point temperature or the freezing point temperature of the heat transfer fluid. In the case that the heat transfer fluid is a mixture, the solid/liquid component of the heat transfer fluid may be maintained within 15°C (e.g. within 10°C, e.g. within 5°C) of the freezing point temperature of the heat transfer fluid.

In one embodiment, during the charging mode at least 10% by mass of the liquid phase of the heat transfer fluid is converted to solid phase (e.g. at least 20%, e.g. at least 30%).

In one embodiment, the mass fraction of the solid phase of the heat transfer fluid relative to the combined liquid and solid phase of the heat transfer fluid increases by over 30% during the charging mode (e.g. increases by over 40% during the charging mode, e.g. increases by over 50% during the charging mode).

In one embodiment, the method further comprises maintaining a minimum ratio of solid to liquid phase of the heat transfer fluid in the three-phase store.

In one embodiment, the heat transfer fluid is maintained in the three-phase store at a temperature below ambient.

In one embodiment, in the charging mode the method further comprises returning at least a portion of the working fluid to the heat transfer fluid in three-phase store. In one embodiment, in the charging mode the method further comprises expanding/reducing the pressure of the working fluid prior to returning the at least a portion of the working fluid to the heat transfer fluid in three-phase store.

In one embodiment, in the charging mode the method comprises storing at least a portion of the working fluid in a holding store after transferring thermal energy from the working fluid to the thermal energy store.

In one embodiment, in the discharging mode the method further comprises pressurising the working fluid before transferring the thermal energy from the thermal energy store to the working fluid.

In one embodiment, in the discharging mode the method further comprises returning the working fluid to the heat transfer fluid in three-phase store after expansion.

In one embodiment, the energy storage system is an energy storage system in accordance with any embodiment of the first aspect of the present invention.

In accordance with a third aspect of the present invention, there is provided a combined energy storage and freeze crystallisation water purification system, comprising: an unpurified water supply; a purified water store; and an energy storage system comprising: a three-phase store comprising: a heat transfer fluid comprising unpurified water received from the unpurified water supply; and a working fluid that is immiscible with the heat transfer fluid, the three-phase store being operative during a charging mode to: maintain the heat transfer fluid simultaneously in three phases in a manner that allows each of the three phases of the heat transfer fluid to exchange heat direct with at least one other of the three phases of the heat transfer fluid; and with the working fluid being present in at least two phases, mix the working fluid with the heat transfer fluid to allow direct heat exchange between the working fluid and the heat transfer fluid; compressor/expander apparatus; and a thermal energy store; wherein the system is configured: to store energy in the charging mode by: evaporating a liquid phase of the working fluid in the three-phase phase store to form a gaseous flow of the working fluid, whereby a portion of the liquid phase of the unpurified water of the heat transfer fluid is converted to form (e.g. substantially pure) ice crystals as the liquid phase of the working fluid evaporates to form the gaseous flow; passing the gaseous flow of the working fluid from the three-phase store to the compressor/expander apparatus for compression by the compressor/expander apparatus; transferring thermal energy from the working fluid compressed by the compressor/expander apparatus to the thermal energy store; and transferring the formed ice crystals to the purified water store; and to generate power in a discharging mode by: transferring thermal energy from the thermal energy store to the or a further working fluid; and passing the or the further working fluid to the compressor/expander apparatus for expansion to generate power.

In this way, a combined function system is provided in which during the charging mode of the energy storage system pure water in the form of ice is separated from a water solution 5 (e.g. seawater or contaminated water) in the three-phase store and then passed to the purified water store. Advantageously the system of the present invention allows water purification to be provided in addition to the main energy storage function with minimal additional hardware.

In one embodiment, the heat transfer fluid substantially comprises the unpurified water.

In one embodiment, the solid/liquid component of the heat transfer fluid is maintained 10 in the three-phase store within 15°C (e.g. within 10°C, e.g. within 5°C) of the eutectic point temperature of the heat transfer fluid (e.g. eutectic point temperature of the unpurified water).

Typically the direct heat exchange will be any of (or any combination of): between the gas phase of the working fluid and the gas phase of the heat transfer fluid (typically pure water vapour); between the gas phase of the working fluid and the liquid phase of the heat transfer 15 fluid; between the gas phase of the working fluid and the solid phase of the heat transfer fluid (typically pure water in the form of ice); between the liquid phase of the working fluid and the liquid phase of the heat transfer fluid; and/or between the liquid phase of the working fluid and the solid phase of the unpurified water.

In one embodiment, during the charging mode at least 10% by mass of the liquid phase 20 of the heat transfer fluid in the three-phase store(s) is converted to solid phase (e.g. at least 20%, e.g. at least 30%).

In one embodiment, the mass fraction of the solid phase of the heat transfer fluid in the three-phase store(s) relative to the combined liquid and solid phase of the heat transfer fluid in the three-phase store(s) increases by over 30% during the charging mode (e.g. increases by over 25 40% during the charging mode, e.g. increases by over 50% during the charging mode).

In one embodiment, the system further comprises a pressure changing device operative to change (e.g. raise or lower) the pressure of unpurified water bound for the three-phase store. In this way, the pressure of the unpurified water may be set to achieve the conditions required for the heat transfer fluid to form three phases in the three-phase store.

30 In one embodiment, the system may comprise a further pressure changing device operative to change (lower or raise) the pressure of ice crystal flow from the three-phase store to the purified water store.

In one embodiment, the three-phase store comprises one or more of: a water inlet (e.g. for receiving unpurified water from the unpurified water supply); an ice outlet (e.g. for transferring the formed ice crystals to the purified water store); and a gas outlet (e.g. for transferring the gaseous flow of the working fluid from the three-phase store to the compressor/expander apparatus for compression during the charging mode).

Typically, the purified water store will comprise a mix of substantially pure (liquid phase) water and ice crystals at a temperature substantially below that of the thermal energy store. Accordingly, the thermal energy store and the purified water store act as hot and cold stores respectively during the discharging mode.

In one embodiment, the energy storage system is operative during the discharging mode to condense working fluid expanded by the compressor/expander apparatus by transferring (e.g. indirectly or directly by means of a spray condenser) thermal energy from the expanded working fluid to the purified water store. In this way, the three-phase store operates as an evaporator only with the cooling for condensation of the working fluid being provided by the water/ice crystals contained within the purified water store.

In one embodiment, the energy storage system comprises a discharge heat exchanger operative during the discharging mode to transfer thermal energy from working fluid expanded by the compressor/expander apparatus to the purified water store.

In one embodiment, the system comprises a water outlet for discharging substantially pure water from the purified water store as ice crystals are transferred into the purified water store from the three-phase store (e.g. so that the mass of water/ice crystals in the purified water store remains substantially constant). In one embodiment, the system is operative to discharge water from the purified water store at substantially the same mass flow rate as ice crystals are transferred into the purified water store. In this way, a purified water supply may be provided by the system (e.g. for use in an external process requiring relatively pure water).

In the case of a system comprising a water outlet to form a purified water supply, in one embodiment the system comprises a heat exchanger for transferring thermal energy from the unpurified water supply to the purified water supply.

In one embodiment, the system further comprises a separator for removing working fluid from the purified water supply.

In one embodiment, the three-phase store forms part of a multi-step freeze crystallisation stage further comprising a further three-phase store arranged in series with the first-defined three-phase store, the further three-phase store comprising: heat transfer fluid received from the first-defined three-phase store; and a working fluid that is immiscible with the heat transfer fluid, the further three-phase store being operative during the charging mode to: maintain the heat transfer fluid received from the first-defined three-phase store simultaneously in three phases in a manner that allows each of the three phases of the heat transfer fluid to exchange heat direct with at least one other of the three phases of the heat transfer fluid; and with the working fluid being present in at least two phases, mix the working fluid with the heat transfer fluid to allow direct heat exchange between the working fluid and heat transfer fluid.

In one embodiment, the system is operative in the charging mode to: evaporate a liquid phase of the working fluid in the further three-phase phase store to form a further gaseous flow of the working fluid, whereby a portion of the liquid phase of the unpurified water of the heat transfer fluid is converted to form (e.g. substantially pure) ice crystals as the liquid phase of the working fluid evaporates to form the further gaseous flow; pass the further gaseous flow of the working fluid from the further three-phase store to the compressor/expander apparatus for compression by the compressor/expander apparatus; transfer thermal energy from the working fluid compressed by the compressor/expander apparatus to the thermal energy store; and transfer the formed ice crystals to the purified water store.

In one embodiment, the further three-phase store comprises one or more of: a heat transfer fluid inlet (e.g. receiving heat transfer fluid from the first-defined three-phase store); an ice outlet (e.g. for transferring the formed ice crystals to the purified water store or to a further purified water store); and a gas outlet (e.g. for transferring the gaseous flow of the working fluid from the further three-phase store to the compressor/expander apparatus for compression during the charging mode).

In one embodiment, the compressor/expander apparatus comprises: a first compressor/expander stage operative to process working fluid received from the first-defined three-phase store at a first pressure; and a further compressor/expander stage operative to process working fluid received from the further three-phase store at a second pressure (e.g. lower than the first pressure).

In one embodiment, the multi-step freeze crystallisation stage further comprises a yet further three-phase store arranged in series with the further three-phase store, the yet further three-phase store comprising: heat transfer fluid received from the further three-phase store; and a working fluid that is immiscible with the heat transfer fluid, the yet further three-phase store being operative during a charging mode to: maintain the heat transfer fluid received from the further three-phase store simultaneously in three phases in a manner that allows each of the three phases of the heat transfer fluid to exchange heat direct with at least one other of the three phases of the heat transfer fluid; and with the working fluid being present in at least two phases, mix the working fluid with the heat transfer fluid to allow direct heat exchange between the working fluid and heat transfer fluid.

In one embodiment, the system is operative in the charging mode to: evaporate a liquid phase of the working fluid in the yet further three-phase phase store to form a yet further gaseous flow of the working fluid, whereby a portion of the liquid phase of the unpurified water of the heat transfer fluid is converted to form (e.g. substantially pure) ice crystals as the liquid phase of the working fluid evaporates to form the yet further gaseous flow; pass the yet further gaseous flow of the working fluid from the yet further three-phase store to the compressor/expander apparatus for compression by the compressor/expander apparatus; transfer thermal energy from the working fluid compressed by the compressor/expander apparatus to the thermal energy store; and transfer the formed ice crystals to the purified water store.

In one embodiment, the yet further three-phase store comprises one or more of: a heat transfer fluid inlet (e.g. receiving heat transfer fluid from the further three-phase store); an ice outlet (e.g. for transferring the formed ice crystals to the purified water store or to a further purified water store); and a gas outlet (e.g. for transferring the gaseous flow of the working fluid from the yet further three-phase store to the compressor/expander apparatus for compression during the charging mode).

In one embodiment, the compressor/expander apparatus further comprises a yet further compressor/expander stage operative to process working fluid received from the yet further three-phase store at a third pressure (e.g. lower than the second pressure).

In one embodiment, the ice collector of the or each of the three-phase stores is connected to an ice collector (e.g. ice collector pipe) and the ice collector is connectable to the purified water store. In one embodiment, the multi-step freeze crystallisation stage is configured to output a solid contaminant (e.g. collected in a solid contaminant collector).

In one embodiment, the multi-step freeze crystallisation stage is configured to output a concentrated supply of unpurified water.

In one embodiment, the energy storage system is an energy storage system in accordance with any embodiment of the first aspect of the present invention.

In accordance with a fourth aspect of the present invention, there is provided a method of operating a combined energy storage and freeze crystallisation water purification system, comprising: providing a combined energy storage and freeze crystallisation water purification system including: an unpurified water supply; a purified water store; and an energy storage system comprising: a three-phase store comprising: a heat transfer fluid comprising unpurified water received from the unpurified water supply; and a working fluid that is immiscible with the heat transfer fluid, the three-phase store being operative during a charging mode to: maintain the heat transfer fluid simultaneously in three phases in a manner that allows each of the three phases of the heat transfer fluid to exchange heat direct with at least one other of the three phases of the heat transfer fluid; and with the working fluid being present in at least two phases, mix the working fluid with the heat transfer fluid to allow direct heat exchange between the working fluid and the heat transfer fluid; compressor/expander apparatus; and a thermal energy store; the method further comprising the steps of: during the charging mode: maintaining a mixture of the heat transfer fluid and the working fluid in the three-phase store; evaporating a liquid phase of the working fluid in the three-phase phase store to form a gaseous flow of the working fluid, whereby a portion of the liquid phase of the unpurified water of the heat transfer fluid is converted to form (e.g. substantially pure) ice crystals as the liquid phase of the working fluid evaporates to form the gaseous flow; passing the gaseous flow of the working fluid from the three-phase store to the compressor/expander apparatus; compressing the working fluid using the compressor/expander apparatus; transferring thermal energy from the compressed working fluid to the thermal energy store; transferring ice crystals formed in the three-phase store to the purified water store; and during a discharging mode: transferring thermal energy from the thermal energy store to the working fluid or a further working fluid; and expanding the working fluid or the further working fluid to generate power.

In one embodiment, the method further comprises after expanding the working fluid during the discharging mode condensing the working fluid by transferring (e.g. indirectly or directly by means of a spray condenser) thermal energy from the expanded working fluid to the purified water store.

In one embodiment, the method further comprises dispensing purified water from the system. In one embodiment, the dispensing step comprises dispensing purified water from the system at substantially the same mass flow rate as ice crystals are transferred into the purified water store.

In one embodiment, the combined energy storage and water purification system is a combined energy storage and water purification system in accordance with any embodiment of the third aspect of the present invention.

In one embodiment, the method comprises a method in accordance with any embodiment of the second aspect of the present invention. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure la is a schematic view of an energy storage system in accordance with a first embodiment of the present invention;

Figure lb is a schematic view of an energy storage system in accordance with a second embodiment of the present invention;

Figure lc is a Temperature/Entropy diagram showing an illustration of a charge/discharge cycle of the energy storage system of Figures la and lb;

Figure Id is a schematic illustration of the stores in the energy storage system of Figures la and lb when charged and discharged;

Figure 2a is a schematic view of an energy storage system in accordance with a third embodiment of the present invention;

Figure 2b is a schematic view of an energy storage system in accordance with a fourth embodiment of the present invention;

Figure 3a is a schematic view of an energy storage system in accordance with a fifth embodiment of the present invention during a charging cycle;

Figure 3b is a schematic view of the energy storage system of Figure 3a during a discharging cycle;

Figure 4a is a schematic view of an energy storage system in accordance with a sixth embodiment of the present invention;

Figure 4b is a schematic view of an energy storage system in accordance with a seventh embodiment of the present invention;

Figure 4c is a Temperature/Entropy diagram showing an illustration of a charge/discharge cycle of the energy storage system of Figure 4b;

Figure 4d is a schematic illustration of the stores in the energy storage system of

Figures 4b when charged and discharged;

Figure 4e is a schematic view of an energy storage system in accordance with an eighth embodiment of the present invention;

Figure 4f is a schematic illustration of the stores in the energy storage system of Figure 4e when charged and discharged;

Figure 5a is a schematic view of a combined energy storage and freeze crystallisation water purification system in accordance with a further embodiment of the present invention during a charging cycle; Figure 5b is a schematic view of parts of the combined energy storage and freeze crystallisation water purification system of Figure 5a;

Figure 5c is a schematic view of the combined energy storage and freeze crystallisation water purification system of Figure 5a during a discharging cycle;

Figure 5d is a schematic view of a combined energy storage and freeze crystallisation water purification system in accordance with a yet further embodiment of the present invention during a charging cycle;

Figure 5e is a schematic view of parts of the combined energy storage and freeze crystallisation water purification system of Figure 5d;

Figure 5f is a schematic view of the combined energy storage and freeze crystallisation water purification system of Figure 5d during a discharging cycle;

Figure 5g is a Temperature/Entropy diagram showing an illustration of a charge/discharge cycle of the combined energy storage and freeze crystallisation water purification of system of Figure 5a;

Figure 6a is a schematic view of a first alternative compressor/expander arrangement for use in the system of Figure 4e;

Figure 6b is a schematic view of a second alternative compressor/expander arrangement for use in the system of Figure 4e;

Figure 6c is a schematic view of a third alternative compressor/expander arrangement for use in the system of Figure 4e;

Figure 6d is a schematic view of an alternative hot thermal store for use in the system of Figure 4e; and

Figure 6e is a schematic view of an alternative three-phase store/supply store arrangement for use in the system of Figure 4e.

As illustrated in the following Figures, embodiments of the present invention relate to improved energy storage systems in which a heat transfer fluid such as water acts as a cold thermal storage material with vapour, liquid and solid phases present.

In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid and solid) of that substance can coexist in thermodynamic equilibrium. Some examples of triple points of fluids are given below: Substance Temperature Pressure

Ammonia 195.4K 0.060 atm

Carbon Dioxide 216.6K 5.100 atm

Methane 90.7K 0.115 atm

Nitrogen 63.2K 0.124 atm

Water 273.2K 0.006 atm

Advantageously, the use of the three simultaneous phases allows an amount of evaporation to take place at constant temperature and pressure, with the heat to drive the evaporation being provided by some of the liquid converting from liquid to solid phase. In this way, the latent heat of fusion is used to offset the latent heat of evaporation and the evaporation can continue at constant temperature and pressure until all of the liquid has been converted to a solid.

A further advantage of this process is that heat exchange from liquid to solid to vapour is all direct and hence temperature differences can be kept very low. The most interesting fluid from a cost and safety perspective is water. However, the vapour pressure of water at the triple point is 7mbar and 1kg of water occupies about 200m ³ . This means that the design of any equipment to process the water directly will have to be very large and also deal with very large volumetric flows.

In accordance with the present invention, an alternative approach is provided that allows water to be used as the cold storage fluid but uses an immiscible working fluid in combination with the water. This deals with the low vapour pressure issue by using the vapour pressure of the immiscible fluid added to that of the water. In this way, a mixture of immiscible fluids are used having different vapour pressures (e.g. butane mixed with water), whereby the fluid with the higher vapour pressure acts predominantly as the working fluid in the system and the other fluid, with the lower vapour pressure acts predominantly as a heat transfer fluid in the three-phase store.

Two liquids are said to be immiscible if they are completely or substantially insoluble in each other. Such a system actually consists of two phases, though it is usually referred to as a mixture. Examples include benzene and water, kerosene and water, etc. During evaporation the mixture may be stirred or agitated, so that the two liquids are broken up into drops and so that there will be drops of both liquids on the surface. That means that both liquids contribute their respective vapour pressure to the overall vapour pressure of the mixture. The total vapour pressure is then simply the sum of the individual vapour pressures. This is independent of the amount of each type of liquid present. All that is required is sufficient quantity of each liquid so that both can exist in equilibrium with their vapour.

For example, phenylamine and water can be treated as if they were completely immiscible and at 98°C the saturated vapour pressures of the two pure liquids are:

Phenylamine 7.07 kPa

Water 94.30 kPa

The total vapour pressure of an agitated mixture would just be the sum of these - 101.37 kPa.

In another example, water and butane at 0°C have saturated vapour pressures as follows:

Butane 102.6 kPa

Water 0.7 kPa

In this scenario the mixture will boil at 0°C when the combined vapour pressure is below 103.3kPa, which is a fraction above atmospheric pressure.

In summary, agitated mixtures of immiscible liquids will boil at a temperature lower than the boiling point of either of the pure liquids. Their combined vapour pressures will typically reach the external pressure before the vapour pressure of either of the individual components get there.

The heat transfer fluid and the working fluid can each be a solution or a pure fluid. For example, the heat transfer fluid could be water (pure fluid) or ammonia and water (solution with liquid) or it could be water and sodium chloride (solution with solid). Accordingly, the mixture of heat transfer and working fluids can be pure (e.g. water and butane) or one or both of them can be solutions (e.g. brine and butane).

Where a solution is used, such as sodium chloride and water, the freezing point of the water will vary depending upon the amount of sodium chloride present. As the amount of saturation the increases the temperature at which ice forms will drop. Fully saturated solution will freeze at around -21°C. Any further freezing of water at this point will create a super saturated solution where sodium chloride will also start to form solid crystals. At the eutectic point both solid water (ice) and solid sodium chloride will be formed separately. In the more normal scenario where the solution is not fully saturated only ice will form and the temperature at which this formation occurs will be related to the level of saturation. Hence, by way of example, the freezing point of the water when mixed with sodium chloride is likely to be between the triple point temperature (0°C) and the eutectic point temperature (-21°C)

The difference between using a solution and two immiscible fluids is explained as follows. If ammonia and water are mixed together, when the water content is near 100% the vapour pressure is near that of water and when the ammonia content is near 100% the vapour pressure is near that of ammonia. If butane and water are agitated together, the vapour pressure is always constant at 103.3kPa regardless of the quantity of butane present. This means that one of the key advantages of using an immiscible fluid is that only a small quantity of one of the fluids is needed to achieve a certain vapour pressure. This can be helpful in terms of cost and also reducing the quantity of flammable or toxic materials needed.

For example, butane at 273K has a vapour pressure around lbar and only about 0.1% will dissolve in the water. If liquid butane and water are mixed together the vapour will be predominantly butane at 1 bar pressure. This means that it is possible to use butane as the working fluid that evaporates while in direct contact with water. The water provides the latent heat of evaporation to the butane while freezing. In this case the three-phase storage system will have both the heat transfer fluid (water) present in three phases and the predominant working fluid (butane) present in two phases (liquid and vapour). Heat exchange within the three-phase store is direct as both fluids are mixed in with each other. While some water vapour will be present in the working fluid that is evaporated from the three-phase store, the properties of the vapour will be predominantly those of butane. While the example of butane and water is given there are numerous combinations of immiscible fluids that could be used. The fact that butane has a vapour pressure similar to atmospheric pressure while at the freezing point of water can be an advantage from a vessel construction perspective.

In all embodiments, compressors can include but are not limited to positive displacement (e.g. screw, reciprocating piston, vane etc .), turbo, centrifugal and axial flow compressors.

In all embodiments, expanders can include but are not limited to positive displacement (e.g. screw, reciprocating piston, vane etc .), turbo, centrifugal and axial flow turbines.

Where the term compressor/expander is used it is understood that this may be a single reversible device or it may two or more separate devices in parallel that are connected with suitable ducting and valves. In the case of a single reversible device during charging or discharging the device will be operating as either a compressor or expander depending upon its function.

Pumps may be referred to as compressors and may be used to raise the pressure of a fluid in liquid state and can include but are not limited to positive displacement and centrifugal pumps.

Where the pressure of liquid is lowered from a high pressure to a low pressure it can be through a pressure reducing device where no work is extracted such as a throttle valve or 5 a liquid expander where some work is extracted. Some liquid expanders can tolerate a proportion of vapour in the flow.

In all embodiments, compressors and pumps may be electrically or mechanically driven. Expanders may drive an electric generator. Combinations of machinery may be attached to the same motor or generator with a net input or output of electrical power 10 depending upon the components.

Hot thermal storage can be provided by any suitable thermal storage material, for example water, crushed rocks, concrete, thermal oils, molten salts, graphite, phase change materials or the like.

Figures la, lb, 2a and 2b show an examples of an energy storage systems in which a

15 three-phase store houses a thermal transfer fluid maintained simultaneously in three-phases and mixed with an immiscible working fluid in a manner that allows thermal transfer between the thermal transfer fluid and the working fluid. Typically the direct heat exchange between the heat transfer fluid and the working fluid will occur: between the gas phase of the working fluid and the gas phase of the heat transfer fluid; between the gas phase of the working fluid and

20 the liquid phase of the heat transfer fluid; between the gas phase of the working fluid and the solid phase of the heat transfer fluid; between the liquid phase of the working fluid and the liquid phase of the heat transfer fluid; and between the liquid phase of the working fluid and the solid phase of the heat transfer fluid.

The heat transfer fluid may be water or a thermal oil. The working fluid may be n-butane,

25 iso-butane, propane or some other immiscible refrigerant with a vapour pressure substantially higher than the heat transfer fluid.

Figure la shows an energy storage system 10 comprising a circuit including a three- phase store 20, a compressor/expander 11, a heat exchanger 13 thermally coupled to a hot thermal store 30, a heat exchanger 14, a liquid expander/pump 12 and reversible pumps 21,

30 22. There is a fluid connection circuit 23 between the three-phase store 20 and the hot thermal store 30 that allows heat transfer fluid to be transferred between the two stores via heat exchanger 14.

The three-phase store 20 comprises a heat transfer fluid with three phases present (solid, liquid and vapour) and a working fluid that is immiscible to some extent and present in two phases (liquid and vapour). An example of this could be water/ice (heat transfer fluid) mixed with butane (working fluid). Only a small quantity of water vapour will be present in the vapour as the vapour pressure of water at the freezing point of water is only 7 mbar. The vapour pressure of the butane around the freezing point of pure water is approximately 1000 mbar. A small amount of butane will dissolve in the water at the water freezing point which is equal to about 0.1% of the water.

The charging cycle of system 10 works as follows: Working fluid (predominantly the two-phase working fluid) is evaporated from the three-phase store 20 with the heat to drive this process comes from the heat transfer liquid phase converting to solid phase. The evaporated working fluid is then compressed in compressor/expander 11 to raise the pressure and temperature and is then subsequently cooled and condensed in heat exchanger 13 transferring heat to hot thermal store 30. The hot thermal store in this example is a single temperature thermal storage system - one where most of the liquid is at the same temperature. For example, it could be a tank of water where the water is circulated repeatedly past the heat exchanger by reversible pump 22, which has the effect of slowly increasing the entire tank temperature. The heat exchange from the working fluid is substantially at constant temperature as this is a condensing process. The working fluid, now as a liquid, is further cooled in heat exchanger 14 in counter-flow with some of the heat transfer fluid pumped by reversible pump 21 from the three-phase store 20. The heated heat transfer fluid can be added to the hot thermal store tank 30 or else stored in a separate tank. The cooled working fluid is now expanded in expander/pump 12 back to liquid (or predominantly liquid) and returned to the three-phase store 20. The effect of the hot thermal store 30 rising in temperature is that the pressure ratio that the working fluid is compressed over in compressor/expander 11 will increase over time. As the transferred heat transfer fluid is heated, some of the working fluid may evaporate from the heat transfer fluid in the hot thermal store 30. This vapour can be returned to the three-phase store 20 by fluid connection circuit 23, where it will be re- condensed. In this way, a cycle has been designed that requires an input of electrical energy to move heat from the three-phase store 20 to the hot thermal store 30. During a charge cycle the temperature of the hot thermal store 30 will steadily rise and the power stored per unit mass of working fluid processed will increase as well.

The discharge cycle of system 10 works as follows: Liquid working fluid is removed from three-phase store 20 and is pumped to a higher pressure in expander/pump 12. In one embodiment the liquid removed is broadly the same composition as the working fluid evaporated. The separation of the liquid phases can be achieved easily in a settling tank where the densities of the two fluids are substantially different. The working fluid is then heated in heat exchanger 14 in counterflow with some heat transfer fluid that has been pumped by reversible pump 21 from hot thermal store 30 to three-phase store 20. The liquid working fluid is then evaporated in heat exchanger 13 with heat from the hot thermal store 30. This has the effect of lowering the temperature of the hot thermal store 30. The liquid in hot thermal store 30 is passed repeatedly through heat exchanger 13 by reversible pump 22. The working fluid is then expanded in compressor/expander 11 driving a generator before the expanded vapour enters three-phase store 20, where it is cooled and condensed. As an example this could occur using a spray of liquid drawn from three-phase store 20 and sprayed over structured packings, i.e. a direct contact condenser. In this way, a cycle has been designed that generates electrical energy when heat is transferred from the hot thermal store 30 to the three-phase store 20. During discharging the pressure ratio across compressor/expander 11 will decrease as will the power generated per unit mass of working fluid processed.

Miscible components may be added to the fluid in the three-phase store 20. For example, if salt (NaCl) is added to a heat transfer fluid of water it will have the effect of lowering the boiling point of the heat transfer fluid to below zero. If it is a fully saturated solution of brine then it will lower the freezing point to around -21°C and raise the boiling point to around 108°C. If it is not a saturated solution then some ice will form below 0°C. This ice that forms will be almost pure water, while the remaining solution will become more concentrated. Pure ice will continue forming as the temperature drops until the remaining liquid will be fully saturated brine. At this point the temperature should remain substantially constant until all the remaining liquid freezes.

Figure lb shows an energy storage system 10' based upon the system 10 of Figure la (corresponding features are labelled accordingly). However, in this embodiment heat exchanger 13' is embedded within the hot thermal store 30. In this way there is direct heat transfer between the working fluid and the hot thermal storage 30.

Figure lc shows an example of the predicted cycle for systems 10, 10' on a Temperature Entropy diagram going from a start of charge to an end of charge. The working fluid in this diagram is n-butane. Likewise the start and end of discharge are also shown. It can be seen that when n-butane is compressed there is almost no superheat generated. Figure Id shows, using water as the example, how the different thermal stores in systems 10, 10' change between being charged and discharged. For example, the mass of water in the three-phase store drops as it is charged while at the same time the fraction of ice increases. The temperature of the three-phase store remains broadly constant at a temperature of 0°C. The temperature and mass of water in the hot thermal store both increase during charging.

Figure 2a shows an energy storage system 10" based upon the system 10 of Figure la (corresponding features are labelled accordingly) including a refrigeration circuit 35 comprising an additional compressor 15, a further heat exchanger 19, a heat exchanger 17 coupled to an ambient heat rejection system 18, and an expander/throttle 16.

There are certain losses in the systems 10, 10' that mean that for constant mass flow rates during charge and discharge, the discharge cycle cannot operate for as long as the charge cycle. Depending upon the efficiency of machinery and heat exchangers it is likely that the system cannot fully discharge the hot thermal store before all of the solid in the three-phase store 20', 20" has melted. The system 10" in Figure 2a compensates for this by running a refrigeration cycle where some of the working fluid is evaporated from the three-phase store by additional compressor 15. The fluid is condensed in heat exchanger 17 with further heat rejection to ambient heat rejection system 18. The condensed liquid is expanded in expander/throttle 16.

There is a further source of potential loss in the system 10 of Figure la in that compression of some fluids (such as butane) has the result of creating some liquid during the compression process. The process of forming liquid droplets is normally quite an irreversible process. Consequently, as shown in Figure 2a, it is also possible while running compressor 15 to add a small amount of superheat to the working fluid before it enters compressor 11 ". This can be achieved by using some of the heat from the refrigeration cycle via heat exchanger 19.

The charging cycle of system 10" works as follows: A first quantity of working fluid (predominantly the two-phase fluid) is evaporated from the three-phase store 20" and is heated slightly in heat exchanger 19 before being compressed in compressor/expander 11 " to raise the pressure and temperature. The working fluid is then cooled and condensed in heat exchanger 13" transferring heat to a hot thermal store 30" . The hot thermal store in this example is a single temperature thermal storage system - one where most of the liquid is at the same temperature. The working fluid, now as a liquid, is further cooled in heat exchanger 14" in counter-flow with some of the heat transfer fluid pumped by reversible pump 21 " from the three-phase store 20" . The heated heat transfer fluid can be added to the hot thermal store 30" or else stored in a separate tank. The cooled working fluid is now expanded in expander/pump 12" back to liquid (or predominantly liquid) and returned to the three-phase store 20" . A second quantity of working fluid (predominantly the two-phase fluid) is evaporated from the three-phase store 20" and is compressed in compressor 15. The working fluid is cooled and partially condensed in heat exchanger 19 before being fully condensed in heat exchanger 17 with the heat from 17 being rejected via ambient heat rejection system 18. The amount of heat transferred by heat exchanger 19 is preferably controllable (i.e. superheat is only added when required). The second quantity of working fluid is returned to three-phase store means 20" via expander/throttle 16.

The discharge cycle of system 10" works as described above for Figure la. It is possible to run the refrigeration cycle at the same time as the discharge cycle, however when this occurs there is no benefit in adding superheat to the working fluid after it has been expanded in compressor/expander 11 ".

Figure 2b shows an alternative energy storage system 10" ' based on the system 10 of Figure la (corresponding feature are labelled accordingly) where a secondary storage cycle 80 with a secondary working fluid is added between the primary cycle and hot thermal store 30' " with the condensing working fluid being used to evaporate the secondary working fluid. This results in the possibility of a constant pressure operating system as the heat transfer to the secondary cycle is at a constant temperature. Energy storage system 10" ' further comprises a secondary hot thermal store in the form of sensible heat storage 40 for receiving heat transfer fluid from three-phase store 20"'. This secondary store differs from the hot thermal store 30"' in that the tank is not kept at broadly constant temperature, but instead a thermocline is developed in the store as the heat transfer fluid is added. During charge the temperature of the heat transfer fluid being added slowly increases, so it is preferable to add this to the liquid at the top of the tank. During discharge the reverse is true.

Secondary cycle 80 comprises compressor/expanders 82a, 82b, heat exchangers 86a, 86b thermally coupled to hot thermal store 30"', heat exchanger 86c thermally coupled to an ambient heat rejection system 88, expander/pump 84 and heat exchanger 13" ' which is also part of the primary cycle.

The charging cycle of system 10" ' works as follows: Working fluid is evaporated from the three-phase store 20"' and is compressed in compressor/expander 11 "' to raise the pressure and temperature. The working fluid is then cooled and condensed in heat exchanger 13" ' transferring heat to the secondary cycle 80. The working fluid, now as a liquid, is further cooled in heat exchanger 14"' in counter-flow with some of the liquid heat transfer fluid pumped by reversible pump 21 " from the three-phase store 20"' . The heated liquid heat transfer fluid can be stored in sensible heat store 40. The cooled working fluid is now expanded in expander/pump 12" ' back to liquid (or predominantly liquid) and returned to the three-phase store 20"' .

The secondary working fluid is evaporated in heat exchanger 13" ' before being compressed in compressor/expander 82a to a higher pressure and temperature. The secondary working fluid is then cooled in heat exchanger 86a with heat transferred to hot thermal store 30" '. The cooled secondary working fluid is then further compressed in compressor/expander 82b before being cooled in heat exchanger 86b with heat being transferred to hot thermal store 30"'. Secondary working fluid exits heat exchanger 86b and is then further cooled in heat exchanger 86c using ambient heat rejection system 88. Secondary working fluid is then expanded through expander/pump 84 (which may be a single reversible device or two devices of which the expander may be a throttle valve) before reentering heat exchanger 13"' . In this way, a cycle has been designed that requires an input of electrical energy to move heat from the three-phase store 20"' to the sensible heat store 40 and hot thermal store 30" '.

The discharge cycle of system 10" ' works as follows: Working fluid is removed from three-phase store 20"' and is pumped to a higher pressure in expander/pump 12" '. The working fluid is then heated in heat exchanger 14" ' in counterflow with some liquid heat transfer fluid that has been pumped by reversible pump 21 "' from sensible heat store 40' to three-phase store 20" '. The liquid working fluid is then evaporated in heat exchanger 13" ' with heat from condensing the working fluid in the secondary cycle 80. The working fluid is then expanded in compressor/expander 1 Γ " before the expanded vapour enters three-phase store 20"', where it is cooled and condensed.

The secondary working fluid is condensed in heat exchanger 13" ' before being pumped to higher pressure in expander/pump 84. Heat exchange with ambient may be bypassed on this stage. The secondary working fluid is then heated in heat exchanger 86b with heat from hot thermal store 30"'. The heated secondary working fluid is expanded in compressor/expander 82b before being further heated in heat exchanger 86a with heat from hot thermal store 30"'. The secondary working fluid is then expanded in compressor/expander 82a before being condensed in heat exchanger 13" '. In this way, a cycle has been designed that generates electrical energy when heat is transferred from the sensible heat store 40 and hot thermal store 30" ' to the three-phase store 20"'.

If the secondary cycle uses water (and steam) and is designed to be compressed above the critical pressure then it is possible to design a cycle where the energy density of the three- phase store is increased by a factor of 4 or 5 times, that is to say for the same sized three- phase store the system in Figure 2b can store 4 or 5 times as much energy as the system in Figure la. The use of steam is advantageous as steam turbines are highly developed and cost- effective pieces of equipment. They are available and designed to operate at higher temperatures and pressures than most other industrial equipment.

Figures 3a and 3b show an energy storage system 100 comprising a circuit including a three-phase store 120 connected to a supply store 125, a heat exchanger 106 thermally couplable to supply store 125, a compressor/expander 111, a heat exchanger 113 thermally coupled to a hot thermal store 130, a heat exchanger 114, a liquid expander/pump 112, reversible pumps 121, 122, and a fluid connection 126 between the supply store 125 and the hot thermal store 130 that allows heat transfer fluid to be transferred between the two stores.

Three-phase store 120 comprises an evaporator vessel 120a and a mixing system 120b operative to the keep the solid and liquid well mixed as a "slurry". The mixing system 120b may comprise one or more rotatable paddles driven by an electric motor and operative to stir/agitate liquid and solid phases present in the lower part of the evaporator vessel 120a to ensure that there is regular mixing of the heat transfer fluid and the working fluid. The rotatable paddles may be configured to operate at different speeds to vary the amount of mixing.

The supply store 125 comprises a cold vessel 125a containing a liquid/solid mixture of heat transfer fluid at a low pressure above or equal to atmospheric pressure, a pressure raising device 101 (e.g. pump), a pressure reducing device 105 (e.g. expander or throttle valve), a separator 104 and a circulation pump 102.

As with the previously described embodiments, the heat transfer fluid may be water or a thermal oil and the working fluid may be n-butane, iso-butane, propane or some other immiscible refrigerant with a vapour pressure substantially higher than the heat transfer fluid.

Figure 3a shows the charging configuration and Figure 3b the discharging configuration. The charging cycle uses direct heat transfer and the discharging cycle a heat exchanger. This configuration might be used where the vapour pressure of the working fluid is above atmospheric pressure. For example, if the working fluid was propane, at 0°C it has a vapour pressure close to 4.7 bar (abs).

The charging cycle in Figure 3a works as follows: Working fluid (predominantly the two-phase fluid) is evaporated from the three-phase store 120 and is compressed in compressor/expander 111 to raise the pressure and temperature. In the charge cycle the diagram shows the flow passing through heat exchanger 106 before the compressor/expander 111, however heat exchanger 106 is not used in the charge cycle and may be bypassed. The working fluid is then cooled and condensed in heat exchanger 113 transferring heat to the hot thermal store 130. The heat transfer fluid in hot thermal store 130 is pumped through heat exchanger 113 by reversible pump 122. The working fluid, now as a liquid, is further cooled in heat exchanger 114 in counter-flow with some of the liquid heat transfer fluid that is pumped by reversible pump 121 from supply store 125 and along fluid connection 126. The heated liquid heat transfer fluid can be stored in hot thermal store 130. The cooled working fluid is now expanded in expander/pump 112 back to liquid (or predominantly liquid) and returned to three-phase store 120 along with any additional working fluid supplied from the separator vessel 104 via pump 102.

As the working fluid evaporates a certain quantity of heat transfer fluid is converted to solid form. This is likely to create a slurry that can be pumped if the solid fraction remains quite low. With water, it is still pumpable as a slurry with ice fractions of less than 50% with special pumps. At concentrations of less than 20% ice it pumps in a manner that is very like water.

Liquid heat transfer fluid is added to the working fluid in three-phase store 120 from supply store 125 via pressure raising device 101 which increases the pressure of the heat transfer fluid. Ideally this should be pre-dominantly liquid although it is likely that there will be small solid crystals of the heat transfer fluid also entrained in the flow.

Heat transfer fluid consisting of both liquid and solid particles mixed with some working fluid will exit three-phase store 120 via separator 104. The purpose of separator 104 is to separate the working fluid from the heat transfer slurry. The working fluid is separated out and returned to three-phase store 120 via pump 102. The heat transfer slurry is separated our and expanded via pressure reducing device 105 to the correct pressure to return to supply store 125.

If the supply store 125 stores a mixture of water and ice, then gravity will allow the ice crystals to rise. This means that if water is drawn from the bottom of the vessel then it will be predominantly liquid phase.

The discharge cycle in Figure 3b works as follows: Liquid working fluid is withdrawn from heat exchanger 106 and is pumped to a higher pressure in expander/pump 112. The working fluid is then heated in heat exchanger 114 in counterflow with some heat transfer fluid that has been pumped by reversible pump 121 from hot thermal store 130 to supply store 125. The liquid working fluid is then evaporated in heat exchanger 113 with heat from the hot thermal store 130. This has the effect of lowering the temperature of the hot thermal store 130. The liquid in hot thermal store 130 is pumped through heat exchanger 113 by reversible pump 122. The working fluid is then expanded in compressor/expander 111 before the expanded vapour enters heat exchanger 106, where it is cooled and condensed. The heat transfer liquid in supply store 125 is passed repeatedly through heat exchanger 106 by pump 124, which has the effect of heating the heat transfer liquid and melting any solid phase in the flow. The larger the proportion of solid phase the less quantity of liquid needs to be pumped. Hence, it will be beneficial to design the supply store 125 so that a controlled mixture of solid and liquid phase can be removed. This can be achieved by having a combination of stirrers/mixers located within the supply store 125 as well as extraction ports located at different heights and locations within the supply store 125. The warmer heat transfer liquid is cooled in the supply store 125 by contact with and melting of solid phase. In this way, a cycle has been designed that generates electrical energy when heat is transferred from the hot thermal store 130 to the supply store 125.

It can be seen that on the discharge cycle three-phase store 120, separator 104, pressure raising device 101, and circulation pump 102 as well as pressure reducing device 105 are not used. As shown in later embodiments it is possible to design the three-phase store 120 in Figure 3a so that it can act as a condenser vessel on discharge that acts as a direct contact condenser. In this case it is not necessary to have a separate heat exchanger 106 and pump 124.

Figures 4a, 4b and 4e show alternative embodiments based on the system of Figures 3a and 3b in which an additional sensible thermal store is provided. As with the system of Figures 3a and 3b, the heat transfer fluid may be water or a thermal oil and the working fluid may be n-butane, iso-butane, propane or some other immiscible refrigerant with a vapour pressure substantially higher than the heat transfer fluid.

Figure 4a shows an energy storage system 200 comprising a circuit including a three- phase store 220 connected to a supply store 225, a compressor/expander 211, a heat exchanger 213 thermally coupled to a hot thermal store 230, a heat exchanger 214 thermally coupled to an additional sensible heat store 240, a liquid expander/pump 212, reversible pumps 221, 222, and a fluid connection 226 between supply store 225 and sensible heat store 240 that allows heat transfer fluid to be transferred between the two stores.

Three-phase store 220 comprises an evaporator vessel 220a and a mixing system 220b operative to keep the solid and liquid well mixed as a "slurry".

The supply store 225 comprises a cold vessel 225a containing a liquid/solid mixture of heat transfer fluid at a low pressure above or equal to atmospheric pressure, a pressure raising device 201 (e.g. pump), a pressure reducing device 205 (e.g. expander or throttle valve), a separator 204 and a circulation pump 202.

In this way, a system is provided with three thermal stores:- a supply store 225 (cold store); a sensible heat store 240 (intermediate/stratified store); and a hot thermal store 230 (hot store). Like the system in Figure 3a, three-phase store 220 and separator vessel 204 are separate from supply store 225. However, in this case three-phase store also acts as a condenser on the discharge cycle by spraying heat transfer fluid into the flow of expanded working fluid thereby removing the need for a separate heat exchanger equivalent to heat exchanger 106.

Advantageously, the provision of sensible heat store 240 means that the fluid in the hot thermal store does not change level (apart from a small amount of thermal expansion) and there is no dissolved working fluid in the hot store.

During charging working fluid (predominantly the two-phase fluid) is evaporated from three-phase store 220 and is compressed in compressor/expander 211 to raise the pressure and temperature. The working fluid is then cooled and condensed in heat exchanger 213 transferring heat to the hot thermal store 230. The heat transfer fluid in hot thermal store 230 is pumped through heat exchanger 213 by reversible pump 222. The working fluid, now as a liquid, is further cooled in heat exchanger 214 in counter-flow with some of the liquid heat transfer fluid that is pumped by reversible pump 221 from supply store 225 to sensible heat store 240. The heated liquid heat transfer fluid is stored in sensible heat store 240 with it being preferable to add the liquid to the top of the store as the temperature increases during the charge cycle. This means that it is possible to establish and use a thermocline in the sensible heat store 240. The cooled working fluid is now expanded in expander/pump 212 back to liquid (or predominantly liquid) and returned to three-phase store 220 along with any additional working fluid supplied from the separator vessel 204 via pump 202. Liquid heat transfer fluid is added to three-phase store 220 from supply store 225 via pump 201. Heat transfer fluid consisting of both liquid and solid particles mixed with some working fluid will exit three-phase store 220 via separator 204. The working fluid is separated out and returned to three-phase store via pump 202. The heat transfer slurry is separated out 5 and expanded (optionally) via expander 205 to the correct pressure to return to supply store 225.

The discharge cycle works as follows: Liquid working fluid is removed from separator 204 and pumped to a higher pressure via pumps 202 and expander/pump 212. The working fluid is then heated in heat exchanger 214 in counterflow with some heat transfer fluid that

10 has been pumped by reversible pump 221 from sensible heat store 240 to supply store 225.

Again it is preferable if the fluid is withdrawn from the top (hottest) of the fluid in the store. The liquid working fluid is then evaporated in heat exchanger 213 with heat from the hot thermal store 230. The heat transfer fluid in hot thermal store 230 is pumped through heat exchanger 213 by reversible pump 222. This has the effect of lowering the temperature of

15 the hot thermal store 230. The liquid in hot thermal store 230 is passed repeatedly through heat exchanger 213. The working fluid is then expanded in compressor/expander 211 before the expanded vapour enters three-phase store 220, where it is cooled and condensed by a direct contact method such as a spray. The mixture of heat transfer liquid and working fluid is separated in separator 204. The now warmer heat transfer fluid is returned to supply store

20 220 via expander 205. The working fluid is returned to the circuit via pump 202 and expander/pump 212. The warmer heat transfer liquid is cooled in supply store 225 by contact with and melting of solid phase. In this way, a cycle has been designed that generates electrical energy when heat is transferred from the hot thermal store 230 to supply store 225.

Figure 4b shows an energy storage system 200' that is similar to that in Figure 4a

25 (corresponding feature are labelled accordingly), but with an additional heat exchanger 216' that is added between compressor/expander 211 ' and heat exchanger 213' . This additional heat exchanger 216' allows some of the superheat to be stored within the sensible heat store 240' . The result of this is that the sensible heat store 240' (assuming it is stratified) will be hotter on top than the hot thermal store 230' . The effect of storing the superheat is an increase

30 in efficiency and can be seen in the Temperature Entropy diagrams in Figure 4c, where the working fluid (propane in this example) is further heated on discharge before it is expanded in compressor/expander 211 '.

Figure 4d shows the variation in temperature and levels of the different tanks between charge and discharge. The level of supply store 225' drops during charge and the fraction of solid increases. If the fluid is water then the ice is likely to float nearer the top. The hot thermal store 230' increases in temperature during charging, while there is no change in level apart from that which results from thermal expansion of the fluid. The sensible heat store 5 240' starts at a temperature around that of the hot thermal store 230 at discharge. Hotter fluid is added to sensible heat store 240' as the charging continues so that there is thermocline in this tank.

Figure 4e shows a further energy storage system 200" based on system 200' in which supply store 225", hot thermal store 230" and sensible heat store 240" are totally separated0 with sensible heat store 240" being thermally connected to heat exchangers 214" and 216" via a circulation circuit 241 driven by a reversible pump 242. There is a larger thermocline range in the sensible heat store 240", however the levels in all of the tanks remains broadly constant. This can make the construction of the tanks much simpler. There is a change in level of supply store 225 of about 5% due to the lower density of the ice. It is possible to add5 one or more expansion tanks to the system to allow water to be pumped in or out of the different thermal stores to keep the level constant. The size of these tanks is very low relative to the stored volume of fluid. There is a further advantage in that only the supply store has working fluid dissolved in the heat transfer fluid.

During charging working fluid (predominantly the two-phase fluid) is evaporated from0 three-phase store 220" and is compressed in compressor/expander 211 " to raise the pressure and temperature. The working fluid is then cooled in heat exchanger 216" adding some superheat to the fluid in the sensible heat store 240' ' . The working fluid is then further cooled and condensed in heat exchanger 213" transferring heat to the hot thermal store 230". The heat transfer fluid in hot thermal store 230" is pumped through heat exchanger 213" by5 reversible pump 242. The working fluid, now as a liquid, is further cooled in heat exchanger 214" in counter-flow with some of the liquid heat transfer fluid that is pumped by reversible pump 242 from the bottom of the sensible heat store 240" to the top of the sensible heat store 240" via heat exchangers 214" and 216". This means that it is possible to establish and use a thermocline in the sensible heat store 240". The cooled working fluid is now expanded in0 expander/pump 212" back to liquid (or predominantly liquid) and returned to three-phase store 220" along with any additional working fluid supplied from the separator vessel 204" via pump 202".

Liquid heat transfer fluid is added to three-phase store 220" from supply store 225" via pump 201 ". Heat transfer fluid including both liquid and solid particles mixed with some working fluid will exit three-phase store 220" via separator 204". The working fluid is separated out and returned to three-phase store 220" via pump 202". The heat transfer slurry is separated out and expanded via expander 205" to the correct pressure to return to supply store 225".

The discharge cycle works as follows: Liquid working fluid is removed from separator 204" and pumped to a higher pressure via pumps 202"and expander/pump 212". The working fluid is then heated in heat exchanger 214" in counter-flow with some heat transfer fluid that has been pumped by reversible pump 242 from the top of sensible heat store 240" to the bottom of sensible heat store 240" via heat exchangers 216" and 214". The liquid working fluid is then evaporated in heat exchanger 213" with heat from the hot thermal store 230". The heat transfer fluid in hot thermal store 230" is pumped through heat exchanger 213" by reversible pump 242. This has the effect of lowering the temperature of the hot thermal store 230". The working fluid is then further heated in heat exchanger 216" before being expanded in compressor/expander 211 ". The expanded vapour enters three-phase store 220", where it is cooled and condensed by a direct contact method (such as a spray). The mixture of heat transfer liquid and working fluid is separated in separator 204". The now warmer heat transfer fluid is returned to supply store 225" via expander 205". The working fluid is returned to the circuit via pump 202" and expander/pump 212". The warmer heat transfer liquid is cooled in supply store 225" by contact with and melting of the solid phase. In this way, a cycle has been designed that generates electrical energy when heat is transferred from the hot thermal store 230" to the supply store 225".

Figure 4f shows an example of the almost constant levels in the different tanks as well as the larger thermocline range in the sensible heat store 240". It is assumed that the heat transfer medium in this example is water. As the three tanks 225", 230", 240' ' are no longer connected it is possible that different fluids can be used for each of the tanks. For example one or more of them could use seawater, a brine or even non-water based fluid.

Figure 5a-c and 5d-f show systems where a freeze crystallisation process is used during the charge cycle to separate pure water from a solution. While this is most likely to be seawater it could in principle be any form of contaminated fluid or brine. This might be the result of a mining, chemical or industrial process. For example fracking produces significant contaminants, power stations have an issue with 'blow down' water and landfills need to deal with water ingress into their sites. Seawater desalination by freeze crystallisation of a directly mixed refrigerant was investigated in the United States by the Office for Saline Water in the 1960's. A number of different refrigerants were tried in various prototype systems, although the process was eventually abandoned in favour of different desalination technologies such as reverse osmosis 5 or multi-stage flash distillation.

The process works because the ice crystals formed do not allow salt to enter and are almost pure water. The crystals can be washed to remove any brine residue before being melted back to fresh water.

The proposed energy storage process of the present invention can be adapted to become 10 a combined desalination plant and energy storage system. The combined system may be particularly advantageous in countries with significant annual sunshine since photovoltaic (PV) cells may be used to both charge the energy storage system and create fresh water at the same time from broadly the same equipment. By combining the two systems the additional power consumption resulting from the extra compression work during charging is very modest. This 15 means that the process is potentially both efficient and cost-effective (i.e. with low capital cost requirements).

Figures 5a-c show a combined energy storage and seawater desalination system 300 in accordance with a first embodiment of the invention and configured to provide zero liquid discharge back into the sea. The advantage of zero liquid discharge is that there is no 20 concentrated brine returned to the ocean with potential environmental issues and the solid salts extracted can also be sold for commercial purposes. Combined energy storage and seawater desalination system 300 comprises a seawater desalination stage 305 and an energy storage system 310.

As shown in Figure 5b, seawater desalination stage 305 comprises: a heat exchanger 25 361; a multi-step freeze crystallisation stage 362 operative to receive heat transfer fluid in the form of seawater from a seawater supply and a working fluid immiscible with the heat transfer fluid; a purified water store 325; a working fluid extractor 360, and a solid evaporator 367 with waste heat supply 368. Multi-step freeze crystallisation stage 362 comprises: pressure changing devices 380, 382a, 382b, 382c, 382d and 382e, pump 381, a series of three-phase stores 362a, 30 362b, 362c, 362d and 362e operative to store the heat transfer fluid (seawater) simultaneously in three-phases; and an ice collector pipe 364 connected to purified water store 325.

Energy storage system 310 is based on system 100 of Figures 3a and 3b and comprises for each three-phase store 362a, 362b, 362c, 362d and 362e a separate energy storage circuit 301a-e in thermal communication with a hot thermal store 330 via reversible pump 322 (for simplicity only circuit 301a associated with three-phase store 362a is shown). As with the system of Figures 3a and 3b, the working fluid may be n-butane, iso-butane, propane or some other immiscible refrigerant with a vapour pressure substantially higher than the heat transfer 5 fluid.

Each energy storage circuit 301a-e includes a heat exchanger 306 thermally couplable to purified water store 325, a compressor/expander 311, a heat exchanger 313 thermally coupled to hot thermal store 330, a heat exchanger 314, and a liquid expander/pump 312.

A fluid connection 326 between the purified water store 325 and the hot thermal store

10 330 allows water to be transferred between the two stores using reversible pump 321.

In use, seawater enters the system via pressure changing device 380 (note this will be a pump if the pressure in three phase store is at or above ambient pressure, however it is possible that the pressure is below atmospheric pressure in which case this will be a pressure reducing device) and is cooled in heat exchanger 361 in counterflow with produced fresh water that is

15 leaving the plant before entering three-phase store 362a of freeze crystallisation stage 362. In three-phase store 362a low temperature liquid working fluid is added to chilled seawater present in three-phases in the three-phase store 362a. As in previous embodiments, the gas phase of the three-phase store will be primarily working fluid since the vapour pressure of the sea water at the freezing point of the sea water is significantly lower than that of the working fluid. The 0 working fluid is evaporated off in three-phase store 362a resulting in the formation of ice crystals (with the working fluid being present in the three-phase store in both liquid and gas phases during the charging mode by virtue of this evaporation process). Some ice crystals are removed and added via an output to pressure changing device 382a (again this will depend on the working fluid, if propane this is likely to be a pressure reducing device and if n-butane a pressure 5 increasing device. For small drops in pressure it may be possible to use fluid friction in the pipe as the method of pressure reduction) and ice collector pipe 364 to purified water store 325. The remaining seawater and working fluid is then passed to three-phase store 362b where more low temperature liquid working fluid is added if required. As the brine has become more concentrated the temperature and pressure at which the working fluid evaporates and ice forms

30 will be lower than three-phase store 362a (hence pressure PI > P2 > P3 > P4 > P5 and flow will naturally pass from one stage to the next). Again ice crystals are removed and added to the purified water store 325 via an output to pressure changing device 382b and ice collector pipe 364. This process continues through three-phase stores 362b, 362c and 362d until the seawater entering three-phase store 362e is now a saturated brine. In three-phase store 362e as the ice forms the salt will also start to drop out of solution at the same time. Solid salt and ice can be removed from the vessel during this process. While the concentration of sodium chloride in the saturated solution stays broadly constant the proportion of other minerals will increase. If there is commercial value it is possible to pass the heavily concentrated brine through a solid evaporator that will separate out important elements. This process could be driven by a supply of waste heat 368, for example from the hot storage tank. If waste heat is used then the water separated from this stage is likely to be very pure as it will have been removed by evaporation. As has previously been explained the ice crystals are likely to have some salt carry over on the surface and hence the water is not totally pure. Depending upon the concentrations it may be suitable for drinking or may be better suited for agricultural or industrial uses.

The compressor/expanders 311 of the energy storage circuit 301a-e driving the evaporation from the three-phase stores 362a-e will have to work over increasingly higher pressure ratios as the seawater becomes more concentrated. This additional work of compression is one of the ways in which additional energy is required to power the desalination process.

As ice is added to the store 325 an equal mass of water can be removed and the clean fresh water (still at close to 0°C) is then pumped by pump 381 and heated up in counterflow with incoming seawater via heat exchanger 361. It then passes through the working fluid extractor 360. The working fluid extraction process could be achieved by lowering the pressure above the water or else heating in a manner similar to a deaerator. While the quantity of working fluid in the fresh water is low it is economically better to recover it.

During discharging system 300 operates in a similar manner to system 100 of Figure 3b with purified water store operating to condense the working fluid after the working fluid in each energy storage circuit 301a-e is heating via its respective heat exchanger 313 and expanded by its respective compressor/expander 311. The effect of using pure water/ice on discharge is that while the evaporation of the working fluid on charge will have occurred at temperatures below 0°C, on discharge the condensation of the working fluid will occur at or above 0°C allowing for heat transfer losses. This requirement to compress from a lower temperature and pressure than the discharge circuit is able to expand to is one of the reasons that there is an energy penalty associated with the desalination process.

If all of the compressor/expanders use the same hot and cold stores, then during discharge they will all operate over the same pressure and temperature ranges, i.e. they will appear to be identical. This is different to the charge circuit where they each operate over a different pressure and temperature range. However, if each circuit has a separate hot store then there will be differences between the different circuits, although they will still all expand to similar temperatures and pressures as they all use pure water/ice to drive the condensation process.

5 Figures 5d-f show a combined energy storage and seawater desalination system 300' based on the system of Figures 5a-c (features in common are labelled accordingly; the working fluid is as before) but with a smaller two-step freeze crystallisation stage 362' operating whereby concentrated seawater is returned to the sea. In this case heat transfer fluid in the form of incoming seawater enters via pressure changing device 380' and is cooled in heat exchanger 0 369 in counterflow with both outgoing concentrated brine and fresh water. The heat transfer fluid (i.e. chilled seawater) then enters three-phase store 326a and low temperature liquid working fluid is added to the three-phase store. As in the system of Figures 5a-c, the gas phase of the three-phase store will be primarily working fluid since the vapour pressure of the sea water at the freezing point of the sea water is significantly lower than that of the working fluid. 5 The working fluid is evaporated off in three-phase store 362a' and ice crystals are removed and added to water purification store 325' via an output to pressure changing device 382a' and ice collection pipe 364'. The remaining seawater and liquid working fluid is then passed to three- phase store 362b' where more low temperature liquid working fluid is added if required. As the brine has become more concentrated the temperature and pressure at which the working fluid evaporates and ice forms will be lower than three-phase store 362a'. Again ice crystals are removed and added to water purification store 325' via an output to pressure changing device 382a' and ice collection pipe 364'. The concentrated seawater or brine is passed through a separator 304 before passing through pressure changing device 383 (note this could be either increased or reduced depending upon the working fluid) and heated up in heat exchanger 369 and then passing through working fluid separator 360'.

As ice is added to the purified water store 325' an equal mass of water can be removed and the clean fresh water (still at close to 0°C) is then pumped by pump 38 and heated up in counterflow with incoming seawater via heat exchanger 369. It can then be passed through a working fluid extractor 360'.

In addition there are a number of losses in the energy storage process that appear as low grade heat. The system can be designed to use some of the electrical losses (stored as additional heat in the hot store) to provide some heat for desalination in the form of Multi-Stage Distillation (MSD). This could mean that the MSD system operates almost continually at low level from waste heat in the thermal store, while the storage system only desalinates when charging. The losses would effectively be rejected through the MSD system.

Fresh water storage is potentially much cheaper to build than thermally insulated water stores. Consequently, it is also possible when there is excess power stored in the storage system to generate additional fresh water by running one energy storage system in a discharge mode (to generate power) and a second energy storage system in a charge mode to provide additional desalination. In this way the generation of fresh water is a further method of energy storage by increasing the amount of desalination when power demand is low. Likewise, if the amount of energy stored is low or power demand is high it is possible to replace the seawater with fresh water in the system and eliminate the additional power demand from the desalination process. This means that the desalination process can improve the responsiveness and flexibility of the energy storage system in addition to the added value of fresh water and even solid salt.

Figure 5g shows a Temperature Entropy Diagram of a charging cycle using propane and a saturated brine solution and a discharge cycle where the discharge is to fresh water illustrative of the operation of system 300. It can be seen that the work of the charging cycle is much greater than that of the discharge cycle and this difference in energy is partially attributable to the energy cost of the additional desalination process.

Figure 6a shows part of an energy storage system that is similar to that shown in Figure 4e. The part of the energy storage system 400, has the compressor/expander split into a low pressure (LP) compressor/expander 41 la and a high pressure (HP) compressor/expander 41 lb, evaporator/condenser 403 and turbine/pump 412. The compressor/expanders in this example are positive displacement machines where the volumetric flow is proportional to speed.

It is a feature of this energy storage cycle that the power varies per unit mass flow of working fluid as the temperature in the hot storage tank changes. This means that for constant power output it may be desirable for both the HP compressor/expander 411a and the LP compressor/expander 411b to be variable speed machines. Unfortunately variable speed inverters add an additional electrical loss into the system that lowers the overall round trip efficiency of the storage process.

An alternative approach is that the HP compressor/expander 411b may be a variable speed machine and the LP compressor/expander 411a may be a fixed speed machine. This allows the HP compressor/expander 411b, when operating as an expander, to operate at both higher and lower generator speeds, during discharge, relative to the fixed speed of the LP compressor/expander 411a. As the mass flow through the HP compressor/expander 411b increases it will tend to increase the pressure ratio across the LP unit while reducing the pressure ratio of the HP unit. If the mass flow through the HP unit is reduced it will have the reverse effect. The advantage of this approach is that the HP unit only needs to be varied to maintain constant power output across the combination of both HP and LP compressor/expanders.

Figure 6b shows part of an energy storage system 400', similar to that described in

Figure 6a, but where the heat rejection circuit is located between the HP and LP units. This heat rejection system may be operated during either charge or discharge and consists of heat exchanger 417 connected to ambient heat rejection system 418 and expander/valve 415. Expander/valve 415 may be a liquid/vapour expander or it may be a throttle valve. On charge some or all of a flow of working fluid may be diverted via this heat rejection system rather than passing through HP compressor/expander 41 lb. On discharge some or all of a flow of working fluid may be diverted via this heat rejection system rather than passing through LP compressor/expander 411a.

The location of the heat rejection system at this point adds significant flexibility to the system. For example if the majority of the flow on charge is diverted via the heat rejection system then the system is operating primarily as an 'ice making' circuit. The power required on charge operating in this method is likely to be lower than in full storage mode. Likewise the discharge power can be reduced in a similar manner that has an identical effect, i.e. the amount of ice being melted is much less than would normally occur if the flow was not diverted.

An example of the benefit of this approach is as follows. If the energy storage system is located in a country with high solar insolation then it is likely to be charged by PV. If total decarbonisation is to be achieved then all of the power for the day must be absorbed over 8 hours. This means that the machinery for charging is likely to be much larger (2x or 3x) than the peak demand. On discharge therefore (over 16 hours) it is likely that the same machinery will have significant over capacity. Hence it is possible to run the same machinery at high mass flow, but low power (which is still sufficient to meet demand) as the flow is diverted past the heat rejection system after the HP expander. This mode of operation is directly analogous to 'additional ice making' as it melts significantly less ice on the discharge cycle. Furthermore, heat rej ection at night is likely to occur when ambient is lower with a resulting efficiency benefit.

This ability to vary the amount of ice that is generated (on charge) or melted (on discharge) can have a significant benefit if the whole energy storage system is combined with a district cooling system. The machinery in the energy storage system is likely to be both cheaper and more efficient (partly due to the direct heat exchange) than standalone chiller systems. Hence, using the energy storage machinery to act as large scale chillers combined with large thermal storage systems means that air conditioning and refrigeration loads can be moved to times of either low demand of high generation. When energy is shifted in this way it is possible for the system to have a storage efficiency of over 100%, if the storage machinery is more efficient than the refrigeration machinery that it is replacing.

Further benefits of large cold thermal stores is that it provides redundancy and backup as well as reducing the electrical loads involved. For example a large element of a data centre load is from the cooling required. This means that if there is a power failure the cooling circuit can be provided with no requirement for electricity consumption apart from pumping losses. The energy storage system then only has to provide electricity to cover the load of the computers not the combined computer and cooling load. This ability to provide cooling can be used for a wide variety of applications and has significant benefit as cooling loads, such as air conditioning, can increase electricity demand by more than 40% in certain countries.

The placement of the heat rejection system between the LP and HP compressor/expanders allows the system in hot months to operate in a mode that is more akin to part energy storage and part air conditioning and in cold months to operate as just an energy storage system.

This approach can be further used with desalination where in hot months it may be better to provide additional fresh water and cooling plus storage and in cold months just to provide fresh water and storage.

One problem with solar desalination is that if brine is rejected it will have a higher density than the seawater that it is being added to. It is possible to reject the heat from the heat rejection process into the brine that is being rejected. By raising the temperature it may be possible to lower the density of the brine such that it has a similar density to the surrounding seawater and hence may be rej ected into the sea with good mixing.

Economically many cooling systems are installed where they are sized for peak load with spare capacity and only operate for a few months a year. The use of this combined energy storage and cooling system avoids the capital expenditure of the additional stand-alone cooling machinery.

The use of a variable speed HP compressor/expander with a fixed speed LP compressor/expander, as described in Figure 6a, also means that it is easier to balance the flows through the heat rejection circuit and the different compressor/expanders.

Figure 6c shows part of an energy storage system 400", similar to that described in Figure 6a, but where the heat rejection circuit is located between the FIP and LP units. This heat rejection system may be operated during either charge or discharge and consists of heat exchanger 417" connected to ambient heat rejection system 418". The difference from Figure 6b is that all of the flow of working fluid passes via this heat rejection system either on charge or discharge or both. The amount of heat rejected can be varied as required although this rejection applies to the whole flow.

The location of the heat rejection system at this point adds significant flexibility to the system by controlling the amount of heat rejected in either direction. For example rejecting heat on charge reduces the power requirement of the charge cycle or rejecting heat on discharge reduces the power generated.

While the system in Figure 6b or 6c can provide district or industrial cooling there are also benefits of providing heat to a district or industrial heating system. This could be via a heat exchanger with heat being extracted from the hot tank as has been demonstrated in countries such as Denmark. It could also be via a heat pump where the heating temperature required is above that of the hot tank.

If the heat transfer fluid is water then this can be directly evaporated from the hot store by using a low pressure evaporator and while avoiding the need for an additional heat exchanger. Low pressure steam has significant use in many industries such as food, beverage, petrochemical, textiles, mills and agriculture.

A further embodiment is shown in Figure 6d that can provide low pressure steam. Again this shows part of an energy storage system that is similar to that shown in Figure 4e. The part of the energy storage system 400"', has heat exchanger 413"', reversible pump 422"' and hot thermal store 430"' as well as steam compressor 471, low pressure evaporator 472, circulation pump 473 and water return path 474. Water is drawn from hot thermal store 430"' into low pressure evaporator 472 where steam is evaporated from water at below atmospheric pressure. The water in the evaporator is continually circulated by circulation pump 473 so that the temperature of the water in the evaporator does not drop very much. The evaporated steam is compressed in steam compressor 471 to a higher pressure and temperature. When the steam has been used it is returned as condensed warm water via the water return path 474 and can be added back to the hot thermal store 430" ' .

The advantage of this approach is that energy cost to evaporate the steam is very low compared to the thermal energy required. The energy storage system needs to reject losses, which appear as heat, from the system and this use means that the losses can be converted into something with financial value. The direct evaporation is a very low cost and efficient approach.

Figure 6e shows part of an energy storage system that is similar to that shown in Figure 4e. The part of the energy storage system 400"", has pump/expander 401"" and 405"", separator 404"", three-phase store 420"" and evaporator/condenser 403"". If the working fluid has a vapour pressure above atmospheric pressure then it is advantageous to have the evaporator and separator at least 10 m below the surface of the three-phase store. In this way the static head of water may be used to reduce the pumping and expanding requirements of the pump/expanders 401 "" and 405"". This will lead to a more efficient system. The height difference may be such that the pumping requirements for changing pressure are almost eliminated. For example, if the height difference is 45m and the working fluid is propane then this will almost perfectly match the vapour pressure of propane at 0°C.