NL2034612B1

NL2034612B1 - Energy storage system and method having a thermal storage reservoir

Info

Publication number: NL2034612B1
Application number: NL2034612A
Authority: NL
Inventors: Sprakel Lisette; Alfons Bos Ronald; Akerboom Sebastiaan; Antonio De Araujo Passos Luigi
Original assignee: Wilgenhaege Invest B V
Priority date: 2023-04-18
Filing date: 2023-04-18
Publication date: 2024-10-28

Abstract

There is provided an energy storage system and a method for recoverable storage of thermal energy. The system has a hot-side thermal storage medium; a hot-side heat exchanger; a working fluid circuit for circulating a working fluid through the hot-side heat exchanger for heat exchange with the thermal storage medium; wherein the working fluid undergoes a transcritical process during heat transfer in the hot-side heat exchanger; and a working fluid contained within the working fluid circuit. The working fluid has greater than 50 wt.% C02; and an additive influencing one or more of the critical temperature; critical pressure; heat transfer; conductivity and heat capacity of the working fluid; wherein the working fluid has a global warming potential (GWP) of less than 200, preferably less than 175, more preferably less than 150.

Description

ENERGY STORAGE SYSTEM AND METHOD HAVING A THERMAL STORAGE

RESERVOIR

BACKGROUND OF THE INVENTION

1. Field of the Invention {0001} The invention relates generally to the storage of energy. in particular the storage of electrical energy and/or thermal energy, that may be in excess of an immediate demand. For example the storage preferably concerns conversion of the excess energy into electrical/thermal energy, storage of thermal energy in a medium, and reconversion of the stored thermal energy to thermal/electrical energy as required. {0002] Thermal energy to be stored may preferably be generated using (excess, e.g. off- peak) electrical energy. Other sources of excess thermal energy may be geothermal, solar, and waste industrial heat. {0003} The stored thermal energy may be converted to electrical energy, e.g. to meet shortfalls in demand (peak demands). The stored heat may also be used for any general purpose making use of thermal energy. for example heating of a space or energy supply for industrial processes. {0004] In preferred examples the storage preferably concerns conversion of electrical energy to thermal energy, storage of said thermal energy in a medium, and re-conversion of the stored thermal energy to electrical energy as may be required. It relates in particular to a system and method for storing electrical energy in the form of thermal energy in thermal energy storage. jo00s] The invention may also relate to components and materials for use in such energy storage systems, for example advantageous thermal energy storage media, and thermal energy storage media reservoirs. 2. Description of the Related Art {ooo6} Electricity generators having minimal output levels at which the unit can be maintained in service, such as nuclear power plants, and electricity generators having highly variable and low predictability, intermittent output levels, such as wind, tidal, wave and solar power etc., often generate electrical output in excess of needs during off-peak usage time, and/or may provide inadequate electrical output during peak usage times. To address excessive or fluctuating electrical energy production, storage systems have been proposed for (temporary) storage of electrical energy (in various forms) during excess generation or low off-peak usage times, for later release in times of higher demand (i.e., the electrical energy supply can be time-shifted through storage).

[0007] Effective electrical energy storage may in that way assist in reducing overall energy consumption or wastage; and facilitate the adoption of wind, solar, wave or tidal energy systems as a replacement of traditional fossil fuel electricity generation by acting as a buffer to reduce the unpredictability and intermittency of their output. An aim is to achieve flexible and reliable electricity supply despite the unpredictable and intermittent nature of inherently unpredictable/intermittent energy sources. jooos] Unfortunately, current energy storage solutions are not fully suited for grid-scale (electrical energy supply networks, such as national energy grids) application due to use of unsustainable materials (e.g. limited supplies of materials and limited recyclability of materials used in energy storage such as batteries), short lifetimes (e.g. less than 10,000 cycles), poor scalability (e.g., safety hazards or limited economies of scale bevond 1 MWh of storing capacity), and geographical limitations (e.g. pumped hydro stations and compressed air energy storage). j0009] Altematively, or in addition to excess electrical sources. thermal energy sources may also generate energy in excess of immediate demands. Such sources may include geothermal. solar, industrial heat output (e.g. district heating, industrial process heating). It may be beneficial to store that excess thermal energy for later release as heat, and possibly conversion to electricity. {ooto] Several attempts have been made to achieve sustainable energy storage solutions in forms including chemical, gravitational potential, electrical potential, elevated temperature, latent heat, thermochemical heat, and kinetic. Examples have been reported in scientific and commercial literature.

[001] A process of particular interest is thermal electrical energy storage (TEES). Various

TEES systems have been discussed for example in patent publications WO05088122,

WO10020480, WO10006942, WO10118915, WO10145963, and WO14027093. {oo12] TEES systems convert excess electricity to heat in a charging cycle. The heat is stored and when demand for electricity is higher than supply, the stored thermal energy is converted back to electricity in a discharging cycle. j0013f The charging cycle is characterized as a heat pump cycle, which can also be referred to as a refrigeration cycle. This is because a heat pump takes heat from a cold source (i.e., making it colder) and moves it to a hot source (i.e., making it hotter). The heat pump cycle thereby consumes mechanical work (generated from excess electricity) to cool down a first control volume while simultaneously heating up a second control volume. Refrigeration cycles are a well-known principle and have been used in refrigerators, chillers. room heaters and air conditioners.

[0014] The discharging cycle is characterized as a heat engine cycle, operating substantially as the reverse operation of the heat pump cycle. A temperature difference between the two control volumes (or between the control volume and the environment) can be converted into mechanical work and preferably thereafter electricity.

[0015] In the TEES concept. heat is transferred from a hot working fluid (a fluid is a flowable phase, preferably a gas, liquid, or combination thereof) to a thermal storage medium during the charging/heat pump cycle and back from the thermal storage medium to the working fluid during the heat engine cycle. The use of CO: as a working fluid in these applications has been practised in the past, however, it required relatively high pressures compared to other working fluids which made it impractical and may raise safety concerns. {0016} In charging-discharging cycles in which the working fluid undergoes a liquid-gas phase change, the heat engine often follows the principles of a Rankine cycle. Cycles in which the working fluid experiences a supercritical-liquid and supercritical-gas state transitions are also known as a transcritical Rankine cycle. {0017] For storage of the thermal energy generated during the charging cycle, this may be done in the form of sensible heat via a change in temperature, or in the form of latent heat via a change of phase, or in thermochemical heat e.g. in reversible exo/endo-thermic chemical reactions, or a combination of the foregoing examples. The storage medium for the sensible heat can be a solid, liquid, or a gas. The storage medium for the latent heat occurs via a change of phase and can involve any of the phases or a combination of them in series or in parallel.

[0018] TEES systems have the potential to provide sustainable energy storage solutions with roundtrip efficiencies exceeding 50%. They are based on the employment of thermodynamic cycles, where an energy potential is created and maintained by introducing temperature differences between a thermal storage unit and a colder base reference. The base reference can comprise another thermal storage unit, can be an ambient environment, or a combination thereof. The thermal storage unit is a hot-side, and the base reference is a cold- side. The terms hot-side and cold-side are hot and cold respectively relative to one another.

[0019] The round-trip efficiency of an electrical energy storage system can be defined as the percentage of electrical energy that can be discharged from the storage in comparison to the electrical energy used to charge the storage, provided that the state of the energy storage system after discharging returns to its initial condition before charging of the storage. {0020] Storage solutions based on TEES may be attractive and convenient for ready implementation because they make use of components with working principles familiar in industry, including components such as compressors, pumps, turbines, and heat exchangers.

However, there is a challenge in implementing such components into energy storage systems where the components are subjected to non-usual or non-standard operating conditions (e.g., high pressure, high/low temperature, power flexibility). j0021] A further advantage of a TEES system is that the temperature and pressure ranges remain unchanged during operation because the whole system is operated in a closed loop.

The components in a TEES system are therefore more likely to operate in a narrower range of pressure and temperature conditions, easing a faster implementation for a sustainable storage solution based on already available knowledge from industry. {0022} With modern technology and accelerated need for harmless working fluids and low- grade waste heat recovery, the use of CO2-based thermodynamic cycles has gained interest.

More specifically, the ability to withstand the mechanical loads of high-pressure systems and seal its contents, allow systems to operate at pressures well above the critical pressure of CO2 (>71 bar). Driven by the need for more environmentally friendly systems. many harmful working fluids are slowly being phased out and replaced by more sustainable alternatives. of which CO: is among the possibilities. In addition, the liquid-like density combined with gas- like viscosity associated with supercritical CO2 may allow for high mass flow in small channels and therefore high-power density of components. j0023] The use of CO: in thermodynamic cycles has been known previously where operation above the critical point has been advantageous leading to high volumetric efficiency and good heat transfer characteristics. The use of CO21in a TEES transcritical

Rankine cycle is discussed in patent publication WO 10020480. Water is the thermal storage medium due to its high heat capacity and environmental properties. CO: working fluid and water thermal storage were used in combination because operating conditions of CO: (due to low critical temperature of 31°C) and water-based thermal storage system match well. j00247 WO10020480 discusses a system including a charging cycle with a compressor: a transcritical cooler; a work recovering expander; and an evaporator. The discharging cvcle comprises a pump; a transcritical heater; a turbine; and a condenser. Surplus energy, for example excess electrical energy from a main grid or external heat sources, is used to compress and heat the working fluid to a supercritical state in the charging cycle, transferring thermal energy into a thermal storage medium and absorbing heat from ambient or cold storage. Discharging to produce electrical energy from the supercritical working fluid is done using a turbine in a discharging cycle, absorbing heat from the thermal storage medium and releasing heat into ambient or cold storage. 19025} There remains a gap for improved energy storage systems and methods. In particular, there is a need to attain high round-trip efficiencies, stable storage, minimal loss of thermal stored energy, robust operations, an adequate longevity.

THE INVENTION joo26] According to aspects of the invention there is provided a method in accordance with the claims and/or clauses. {0027} According to an aspect of the invention there is provided a system in accordance with the claims and/or clauses. {0028} According to an aspect of the invention there is provided a thermal storage medium in accordance with the claims and/or clauses. j0029] According to an aspect of the invention there is provided a thermal storage medium reservoir in accordance with the claims and/or clauses. {0030] Subject matter in embodiments of this invention may concern variations of TEES in which advantageous levels of performance are achieved in combination with operating conditions allowing for longevity of components and operations and/or cost-efficient production and accessibility of components. {0031} More specifically, preferably embodiments employ a pumped thermal energy storage (PTES). PTES is a subcategory of TEES in which heat pumps are emploved to generate heat. In other forms of TEES, resistive heating may be used. In a PTES a heat pump (comprising of a compressor, expander and at least two heat exchangers) is employed in a charging cycle of the storage system to increase the thermal energy in thermal storage medium. In addition, a heat engine (comprising of a pump, expander and at least two heat exchangers) is used in the discharging cycle to discharge the thermal energy in the thermal storage medium, converting the thermal energy potential into mechanical work and finally electricity. As opposed to resistive heating, a heat pump can reach much higher energetic efficiency. In addition, or alternatively, local (waste) heat, industrial or solar/geothermal heat can be used to provide thermal energy potential in parallel or alternatively to the heat pump.

0032] In further embodiments, in addition to electricity generation by a heat engine it is possible to directly utilize the stored thermal energy to serve as a local heat source for a local heat demand without first conversion to electricity. {0033] PTES systems may be efficient and practical when based on the working principles of the transcritical Rankine cycle, in particular when the working fluid is based on CO:. {0034} In PTES systems the work ratio is an important measure of efficiency. The work ratio is the ratio of the compressor work over expander work in the heat pump cycle.

Academic studies have indicated that PTES systems using supercritical CO: (sCOz) as their working fluid can reach more advantageous (e.g. higher, or optimized levels) work ratios than ideal gas cycles operating in the same temperature limits, and thereby reach higher roundtrip efficiency and lower sensitivity to losses in compression and expansion devices. Without wishing to be bound bv theory, it is believed that a contributor to this is the real gas behaviour of CO: near its critical point. That critical point is achievable already at relatively mild operating conditions (e.g. approx. 74 bar and 31°C). Operation in the zone around the critical point can give large density differences between the compressor and expander fluid flow and may result in an ability to reach more optimal work ratios.

[0035] Best utilization of the density difference to give optimal work ratio in the charging cycle is done by compressing in the gaseous/supercritical state and by expanding in the supercritical/liquid state.

[0036] In embodiments of the present invention, a PTES system and method is provided that is suitable for construction and operation with sustainable design choices; i.e. the system and method can be constructed and operated using materials and processes that are readily available resources, possibly naturally regenerating resources, and without a need for highly engineered components. Moreover, long cyclic lifetimes may be facilitated by way of the ability to employ relatively mild operating conditions while achieving good charge and discharge efficiencies.

[0037] A transcritical Rankine cycle based on CO: is useful in achieving one or more of the above.

[0038] In preferred embodiments the following features are employed. Features of one embodiment may be combined with features of other embodiments.

[0039] In an aspect of the invention there is provided an energy storage system for recoverable storage of thermal energy. The system comprises a hot-side thermal storage medium; a hot-side heat exchanger; a working fluid circuit for circulating a working fluid through the hot-side heat exchanger for heat exchange with the thermal storage medium,

wherein the working fluid undergoes a transcritical process during heat transfer; and a working fluid contained within the working fluid circuit.

The working fluid in this aspect comprises greater than 50 wt.% CO: preferably greater than 75wt.% COz2, and more preferably greater than 90 wt.% CO, and most preferably 95 wt.% CO2. In addition the working fluid comprises an additive for influencing one or more of the critical point, heat transfer characteristics, conductivity and heat capacity of the working fluid.

Preferably the working fluid comprises from 0.1 wt.% to 25 wt.% of the additive.

Preferably the additive alters the critical point as compared to the critical point of pure CO:. jooao] There is also provided a method for recoverable storage of thermal energy, the method comprising; circulating a working fluid through a hot-side heat exchanger for heat transfer with a hot-side thermal storage medium and transferring heat with the thermal storage medium in a transcritical process.

The working fluid in this aspect comprises greater than 50 wt.% CO: preferably greater than 75wt.% CO:, and more preferably greater than 90 wt.% CO», and most preferably 95 wt.% CO». In addition the working fluid comprises an additive for influencing one or more of the heat transfer characteristics, conductivity and heat capacity of the working fluid.

{0041} Preferably the working fluid comprising CO: and an additive has a global warming potential (GWP) of less than 200, preferably less than 175. more preferably less than 150;

over 100 years.

{0042} Global warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (CO2).

{0043] While it has been known to make use of pure CO: as working fluid in prior systems and that this may provide useful results, difficulties have been identified.

For example, it has been identified that mechanical components, and heat storage media may present difficulties in matching the fluid thermodynamic behaviour of the working fluid.

For example this may concem aimed power production aimed, efficiencies, heat transfer area, and heat sources and heat sinks.

[00447 While alternative working fluids to CO: are known, they may demonstrate disadvantages from an environmental point of view, for example in terms of GWP and ozone depletion potential (ODP). or less advantageous operating characteristics.

For example, in literature, synergistic effects of mixing multiple working fluids have been described, such as provision of an increased heat transfer of two-phase flow by blending of CO: with a non-

condensable gas like nitrogen. Other attempts have been associated with components having characteristics that resemble chlorofluorocarbons (CFC), without adding the toxic and environmentally harmful CFCs themselves. Further attempts have been made using halogens, saturated alkanes (C1-5), monocyclic aromatics like benzene, toluene, and noble gases. 10045] Itis desirable to provide a more versatile, system, method or equipment working fluid, while simultaneously maintaining suppressed environmental impact. {0046} In line with the above, without wishing to be limited to theory, the present inventors have identified relevant additives for combination with a CO: based working fluid. joo47| The following compounds are useful additives for inclusion in a CO: working fluid:

Pentafluoropropane; 1.1,1,2-Tetrafluoroethane: Difluoromethane; Pentafluoroethane; 1,1.1-

Trifluoroethane; 1,1-Dichloro-1-fluoroethane; 1,1.1,3.3,3-Hexafluoropropane; 1,1,1,3,3-

Pentafluorobutane; Trifluoromethane; Sulfur hexafluoride; 1-Chloro-1,2,2,2- tetrafluoroethane; 1-Chloro-1,1-difluoroethane; Chlorodifluoromethane; ethane; and 1.1,1,3,3,3-Hexafluoropropane. The additives may be used in combination or individually as additives in the working fluid. joo48] Itis preferable that the GWP of a CO2-based working fluid is maintained under 150, and in that respect the additives may be restricted to maximum inclusion levels in CO2 as follows: up to 14.48 wt.% of Pentafluoropropane; up to 10.42 wt.% of 1,1,1.2-

Tetrafluoroethane: up to 22.1 wt.% of Difluoromethane; up to 4.25 wt.% of

Pentafluoroethane; up to 3.33 wt.% of 1,1, 1-Tnfluoroethane; up to 0.21 wt.% of 1,1-

Dichloro-1-fluoroethane; up to 12.42 wt.% of 1,1,1,3,3,3-Hexafluoropropane;

[0049] up to 18.78 wt.% of 1,1,1.3,3-Pentafluorobutane; up to 1.00 wt.% of

Trifluoromethane: up to 0.65 wt.% of Sulfur hexafluoride; up to 24.5 wt.% of 1-Chloro- 1.2.2. 2-tetrafluoroethane; up to 6.45 wt.% of 1-Chloro-1,1-difluoroethane; up to 8.23 wt.% of

Chlorodifluoromethane; up to 50wt.% ethane; and up to 1.52 wt.% of 1,1,1,3,3,3-

Hexafluoropropane. [0pos0} Of the preferred additives, Trifluoromethane; Sulfur hexafluoride; and ethane are preferred, with ethane being most preferred. In some embodiments the working fluid may comprise up to 100 wt.% ethane, up to 90 wt.% ethane, up to 80 wt.% ethane, up to 70 wt.% ethane, up to 60 wt.% ethane, up to 50 wt.% ethane, up to 40 wt.% ethane, up to 30 wt.% ethane, up to 20 wt.% ethane, or up to 10 wt.% ethane.

[0051] The preferred additives assist in provision of a working fluid critical point and boiling point that aligns with a phase-change temperature and pressure in which the working fluid will operate in the charge/discharge cycle. Particularly, critical points under 250°C may

<9. be preferred because these allow for relatively mild operating conditions. For example, a low critical temperature allows for a low-grade hot storage temperature. A preferred boiling point for the working fluid is from 10 to 30°C, preferably from 10 to 25°C, more preferably from 10 to 20°C, more preferably about 15°C at the system operating pressure. For example, a low boiling temperature allows for a low sink temperature in the charging cycle that is close to ambient conditions. The reference pressure for determination of the boiling point is from 3.5 - 7.2 MPa absolute. {0052} The preferred additives discussed herein may provide low GWP and low ODP working fluid, with low/medium-boiling temperatures as discussed. 0053] The system and method are preferably configured such that the working fluid undergoes a transcritical cooling in the hot-side heat exchanger during a charging cycle of the energy storage svstem.

[0054] In the system and method, the working fluid preferably undergoes a transcritical heating in the hot-side heat exchanger during a discharging cycle of the energy storage system. j0055] In the system and method, the working fluid is in a supercritical state on entering the hot-side heat exchanger during a charging cycle of the energy storage system.

[0056] In the system and method. the working fluid is in a supercritical state on exiting the hot-side heat exchanger during a discharging cycle of the energy storage svstem. 10057} The system and method preferably further comprise an expander positioned in the working fluid circuit for recovering energy from the working fluid during the charging cycle, wherein the recovered energy is used to reduce the net required power input to operate the thermodynamic cycle, preferably it may be supplied to a compressor in the working fluid circuit for compressing the working fluid to a supercritical state. 10058] In the system and method the energy storage system preferably comprises a compressor for conversion of electrical energy to thermal energy in the working fluid, which thermal energy is exchanged with the thermal storage medium.

[0059] In some aspects, a different working fluid may be employed in the charging and discharging cycles, either as two separate systems in parallel, or by adjusting the composition of the working fluid between charging and discharging. Alternatively, or in addition, varying the composition during one cycle may be achieved by inclusion of components or additives that can be separated from the working fluid elsewhere in the cycle, and therefore are active in only one of the cycles.

{0060} The hot and cold storage media may be any media discussed herein, including water, liquid polyols, hydrated salt solutions, or mixtures thereof. joost] The system and method are preferably such that the energy storage system is a thermoelectric energy storage system for provision of thermal energy to a thermodynamic machine for generating electricity.

[0062] The system and method are preferably such that the energy storage system is a thermoelectric energy storage system for conversion of electricity to thermal energy, preferably configured for regeneration of electricity from the stored thermal energy. joo63] In an aspect of the invention there is provided an energy storage system for recoverable storage of thermal energy. The system comprises a hot-side thermal storage medium; a hot-side heat exchanger; and a working fluid circuit for circulating a working fluid through the hot-side heat exchanger for heat transfer with the thermal storage medium, wherein the working fluid undergoes a transcritical process in the hot-side heat exchanger during heat transfer; and a working fluid contained within the working fluid circuit, the working fluid comprising at least 50 wt.% CQO2; wherein the hot-side thermal storage medium comprises at least one polyol of the formula C2-4He-1002-1, preferably wherein the thermal storage medium comprises glycerol.

[0064] There is also provided a method for storing thermal energy, the method comprising; circulating a working fluid through a hot-side heat exchanger for heat transfer with a hot-side thermal storage medium and transferring heat with the working fluid in a transcritical process; and wherein the hot-side thermal storage medium comprises at least one liquid polyol of the formula C2-1He-1002-4, preferably comprising glycerol. j0065] The hot-side (higher temperature storage) is preferably realized by use of a liquid thermal storage medium, preferably water or a polyol-based liquid of C2-4Hs-1502s, optionally comprising solid or liquid additives. It is preferred to operate the hot-side storage process in temperature ranges from 20 to 300°C, more preferably from 30 to 260°C, and most preferably from 40 to 220°C. The hot storage medium is preferably configured to operate in that temperature range and simultaneously minimize occupied volume. More preferably the thermal storage medium has a low environmental impact. joo66; Additives may be included in the thermal storage medium, which solid particles having a higher heat capacity per volume than a fluid thermal storage medium in which they are distributed. This may assist in minimizing a required volume while achieving similar or greater total heat capacity.

0067] In the system and method the hot-side thermal storage medium preferably has a melting point below 40°C, preferably below 30°C. {0068} The hot storage medium may also comprise or (substantially) consist of other known heat storage media, for example water. 10069] The system and method are preferably such that the energy storage system is a thermoelectric energy storage system for provision of thermal energy to a thermodynamic machine for generating electricity.

[0070] The system and method are preferably such that the energy storage system is a thermoelectric energy storage system for conversion of electricity to thermal energy. preferably configured for regeneration of electricity from the stored thermal energy.

[0071] In an aspect there is provided a thermal energy storage svstem comprising; a hot-side thermal storage medium; a hot-side heat exchanger; a cold-side thermal storage medium; and a cold-side heat exchanger; a working fluid circuit for circulating a working fluid through the hot-side and cold-side heat exchangers for heat transfer with the hot-side and cold-side thermal storage mediums respectively, wherein the working fluid undergoes a transcritical process during heat transfer in the hot-side heat exchanger; and a working fluid contained within the working fluid circuit. The cold-side thermal storage medium comprises a hydrate of at least one inorganic salt, preferably comprising 2-10 moles of crystal water per mole of the at least one inorganic salt.

[0072] There is also provided a method for recoverable storage of thermal energy, the method comprising: circulating a working fluid through a hot-side heat exchanger for heat transfer with a hot-side thermal storage medium, and transferring heat with the thermal storage medium in a transcritical process in the hot-side heat exchanger, and circulating the working fluid through a cold-side heat exchanger for heat transfer with a cold-side thermal storage medium, and transferring heat with the cold-side thermal storage medium; wherein the cold-side thermal storage medium comprises a hydrate of at least one inorganic salt. preferably comprising 2-10 moles of crystal water per mole of the at least one inorganic salt.

The mechanism of energy storage is based on the crystal lattice of the salt hydrate, which preferably has a low melting point. That is, the energy storage is based on the latent heat of crystallization associated with the structure of the salt hydrate in the solid state. When the salt hydrate melts, energy is absorbed and an aqueous solution with a specific salt concentration is formed. Conversely, energy is released upon formation of the crystal structure of the solid salt hydrate.

{0073} Prior attempts have been made in thermal energy storage systems in which a cold storage reservoir has been provided. Such attempts have included the use of water and ice transitions for cold storage. Reliance of ice for cold storage may be disadvantageous.

[0074] This aspect of the invention makes use of aqueous salt hvdrates and the latent heat of crystallization associated with melting and freezing of these low-melting point compounds.

It has been determined that by configuration of the crystallization point of the solution, optionally by inclusion of additives, may assist in improving the lower temperature range of both the charging and discharging cycle. This may assist in facilitating more robust operating conditions. j0075] Identification of suitable salt hydrates is complex. Salt hydrates discussed herein have been identified as advantageous and may assist in providing low wear and/or corrosion of both thermally insulated storage vessels, and highly thermally conductive heat exchanger surfaces. {0076} Suitable salt hydrates include one or more of CaCl::6H20, Na2S0:: 10H:0,

Zn(NO3)::6H20, CaBr2:6H20. These salt hydrates may be suitable for operation in temperature ranges of 30 to 40 °C with melting and freezing in this range. A preferred salt hydrate is CaCl2-6H20. {0077f The system and process may be improved by inclusion of additives to the cold-side thermal storage medium. The additives may include any one or more of: a melting point reducing additive, which may comprise a polar component, preferably those with a protic functionality, preferably selected from the group consisting of glycerol, urea or ethanol: a further salt hydrate, for example the combination of CaCl2:6H2O0 and MgClz-6H:0, preferably up to 33 wt.% by weight of the thermal storage medium, which may provide congruent behaviour of the salt hydrate during charging/discharging operations: a viscosity modifier which may suppress phase separation during charging/discharging operations or form a gel when in water, preferred viscosity modifiers comprise compounds with hydroxyl functional groups, preferably selected from the group of silica, bentonite, C2-C4 polyols, modified cellulose, and polvacrylic acid; and a nucleating agent which may reduce supercooling of the salt hydrate during the discharge cycle, preferably a homogeneous nucleating agent, preferably a compound selected from the group of alkaline earth metal halides, -hydroxides, or carbonates.

[0078] In preferred embodiments the cold-side thermal storage medium may comprise a eutectic mixture of CaCl2:6H20 and up to 33 wt.% of MgCl2-6H20, preferably from 16 to 20 wt.%, more preferably about 18 wt. %. {0079] In preferred embodiments the cold-side thermal storage medium may comprise one or more melting point reducing additives selected from Ethanol, NH4C1, NH4NO:. Urea, and

Ethylene glycol. {0080} In preferred embodiments the cold-side thermal storage medium comprises one or more nucleating agents selected from Ba(OH)2-8H:0, Bal2-6H20, BaCl2-6H:0, BaF:, SrF2,

SrC12:6H20. SrBr2-6H20, Str(OH)2:8H20, SrCO3, Mg(OH)2, Mg(NO3)26H20,

Na4B407:5H20, LiNO3-3H20, NaCl, KCI. and KNO:3, y-Al:0:3 nanoparticles. [oosi] In preferred embodiments the cold-side thermal storage medium comprises a viscosity increasing modifier selected from fumed silica. polyethylene glvcol, ethylene glycol, hydroxyethylcellulose, carboxymethylcellulose. methylcellulose. {0082} In preferred embodiments the salt hvdrate comprises Na2S04-10H20. {0083} In embodiments where the salt hydrate comprises Na:SO4 10H:0, the cold-side thermal storage medium may comprise a eutectic mixture of Na2SOs-10H20 and up to 77 wt.% Na2HPO: 12H20, preferably from 73 to 77 wt.%. more preferably about 75 wt.%; or

NaxCOs5-10H:20; or up to 7 wt.% KCL.

[0084] In embodiments where the salt hydrate comprises Na250:+: 10H20 the cold-side thermal storage medium may comprise a nucleating agent selected from one or more of

Na:B407:5H:O and Na:Si03-9H:0. {0085} In embodiments where the salt hydrate comprises Na2804: 10H2O the cold-side thermal storage medium may comprise a viscosity increasing modifier selected from one or more of polyacrylamide, fumed silica, polyacrylic acid polymer, preferably 2-5 wt.% of

Polyacrylic acid polymer.

[0086] In preferred embodiments the salt hydrate comprises Zn(NO3)2:6H:0.

[0087] In embodiments where the salt hydrate comprises Zn(NO3)2:6H20, the cold-side thermal storage medium may comprise a nucleating agent selected from one or more of

Zn3(OH)4(NOs)2; ZnO; Zn(OH):; and Zinc acetate. 10088) In embodiments where the salt hydrate comprises Zn(NO:)2:6H20 the cold-side thermal storage medium may comprise a viscosity increasing modifier comprising carboxymethyl cellulose.

[0089] In embodiments where the cold-side thermal storage medium comprises a salt hydrate, the working fluid preferably undergoes transcritical heating in the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage system.

[0090] In embodiments where the cold-side thermal storage medium comprises a salt hydrate, the working fluid is preferably in a supercritical state on entering the hot-side heat exchanger during a charging cycle of the thermoelectric energy storage system. {0091} In embodiments where the cold-side thermal storage medium comprises a salt hydrate, the working fluid is in a supercritical state on exiting the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage system. §0092] In embodiments where the cold-side thermal storage medium comprises a salt hydrate, an expander is preferably positioned in the working fluid circuit for recovering energy from the working fluid during the charging cycle, wherein the recovered energy is used to reduce the net required power input to operate the thermodynamic cycle, preferably it may be supplied to a compressor in the working fluid circuit for compressing the working fluid to a supercritical state. j0093] The system and method are preferably such that the energy storage system is a thermoelectric energy storage system for provision of thermal energy to a thermodynamic machine for generating electricity.

[0094] The system and method are preferably such that the energy storage system is a thermoelectric energy storage system for conversion of electricity to thermal energy, preferably configured for regeneration of electricity from the stored thermal energy. {0095} The system and method are preferably such that the energy storage system is a thermoelectric energy storage svstem for provision of thermal energy to a thermodynamic machine for generating electricity. 10096] The system and method are preferably such that the energy storage system is a thermoelectric energy storage system for conversion of electricity to thermal energy, preferably configured for regeneration of electricity from the stored thermal energy. {0097} Preferably the cold storage reservoir does not contain ice or an ice slurry, but aqueous salt hydrates. The crystallization point of this solution can be tuned with additives, which in turn can be used to optimize the lower temperature range of both the charging and discharging cycle.

[0098] The materials from which the cold storage reservoir is constructed are preferably corrosion resistant to the aqueous salt, while maximizing the thermal insulation in order to retain the stored thermal energy in the system. The internal heat exchange system in this reservoir should also minimize the corrosion when in contact with the salt hydrate(s) . but instead of insulation a high heat conductance is desired to achieve efficient heat transfer. {00991 Tuning the crystallization temperature of the cold storage reservoir can facilitate more robust operating conditions.

[00109] In an aspect there is provided a thermal energy storage system, preferably a thermoelectric energy storage system for providing thermal energy to a thermodynamic machine for generating electricity, comprising: a hot-side thermal storage medium: a hot-side heat exchanger; a working fluid circuit for circulating a working fluid through the hot-side heat exchanger for heat transfer with the thermal storage medium, wherein the working fluid undergoes a transcritical process during heat transfer in the hot-side heat exchanger; and a hot-side thermal storage medium reservoir, wherein the reservoir comprises an integral, intemal volume divided into a high-temperature region and a low-temperature region, wherein a ratio of volume size of the regions can be altered during operation. jooioi] There is also provided a method for recoverable storage of thermal energy, preferably a method for storing thermoelectric energy in a thermoelectric energy storage svstem, comprising; circulating a working fluid through a hot-side heat exchanger for heat transfer with a hot-side thermal storage medium, and transferring heat with the thermal storage medium in a transcritical process in the hot-side heat exchanger; {001021 wherein the hot-side thermal storage medium is provided a hot-side thermal storage medium reservoir, wherein the reservoir comprises an integral, internal volume divided into a high-temperature region and a low-temperature region, the method further comprising varying a ratio of volume size of regions, increasing the low-temperature region volume during a discharging cycle, and/or increasing the high-temperature region volume.

[00103] In the system and method the reservoir is preferably provided with a conduit fluidly communicating the high-temperature region, a low-temperature region, and the hot-side heat exchanger.

[00104] In the system and method the reservoir is preferably provided with a movable insulating barrier dividing the high-temperature region from the low-temperature region, wherein movement of the dividing barrier increases the volume of one region while decreasing the volume of the other region. Configuration of the hot-side thermal storage medium in this manner may allow for an increased efficiency, reduced footprint or volume requirements. This may be because a single reservoir volume may be utilized as opposed to separate reservoirs for a high-temperature region and a low-temperature region.

[00105] In the system and method the high-temperature region and the low-temperature region are preferably in fluid communication and the reservoir is provided with a tortuous path between the high-temperature region and the low-temperature region whereby one region may increase in volume and encroach upon a reducing volume of the other region.

Configuration of the hot-side thermal storage medium in this manner may allow for an increased efficiency, reduced footprint or volume requirements. This may be because a single reservoir volume may be utilized as opposed to separate reservoirs for a high-temperature region and a low-temperature region. Preferably the tortuous path (e.g. a labyrinth or baffle structure) directs the fluid flow through the reservoir in a plug flow manner, resulting in a gradual transition between the high-temperature region and the low-temperature region of the hot storage reservoir. {oot06] In the system and method the tortuous path is preferably formed by insulating baffles, piping, or a combination thereof. joo107] In the system and method the working fluid preferably undergoes a transcritical cooling in the hot-side heat exchanger during a charging cycle of the thermoelectric energy storage system.

[00108] In the system and method the working fluid preferably undergoes transcritical heating in the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage system.

[00109] In the system and method the working fluid is preferably in a supercritical state on entering the hot-side heat exchanger during a charging cycle of the thermoelectric energy storage system.

[00119] In the system and method the working fluid is preferably in a supercritical state on exiting the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage system. {ootit] In the system and method there is preferably provided an expander positioned in the working fluid circuit for recovering energy from the working fluid during the charging cycle. wherein the recovered energy is wherein the recovered energy is used to reduce the net required power input to operate the thermodynamic cycle, preferably it may be supplied to a compressor in the working fluid circuit for compressing the working fluid to a supercritical state.

00112] In preferred embodiments of the invention as applicable to any of the aspects discussed above, additional heating or cooling of the working fluid may be provided at relevant positions in order to ensure adequate, preferably full, phase change of the working fluid. 100113] In preferred embodiments, a cooler can pre-cool the working fluid where necessary in order to decrease the system’s sensitivity to (minor) unforeseen temperature changes. {oot14] All components are equipped to handle a varying power consumption or generation.

In terms of energy storage services, the ability to regulate the speed at which energy is stored or generated (i.e., charging and discharging power) extends the range of applications. jooi15} All components in the charging cycle (compressor, expander, heat exchanger circulation pumps) are able to handle a variable mass flow. Primarily, the compressor is the leading component, after which the operating conditions of the expander and circulation pumps may be adjusted for optimal efficiency and correct operation of the heat pump cycle. footie} All components in the discharging cycle (pump, turbine, heat exchanger circulation pumps) are able to handle a variable mass flow. Primarily, the turbine is the leading component, after which the operating conditions of the pump and circulation pumps may be adjusted for optimal efficiency and correct operation of the heat engine cycle. {00117} For the charging cycle of the PTES system, i.e. transcritical heat engine, the expander experiences high pressure drops and the fluid goes from a supercritical state with liquid-like density to a subcritical state. In order to maximize the efficiency with a minimum number of expander stages, the expander may follow the principle of an impulse turbine, for example a Pelton turbine.

[00118] The svstem is preferably a modular svstem. A modular nature of the system allows the energy storage capacity to be adjusted to the desired amount. Each thermal energy storage reservoir has a specific energy storage capability (MWh). By increasing the volume or number of reservoirs that are installed on site the desired maximum storage can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

[oo119] The features and advantages of the invention will be further appreciated upon reference to the following drawings, presented by way of example only, in which:

Figure 1 illustrates a PTES system;

Figure 2A illustrates a PTES charging cycle;

Figure 2B illustrates a PTES discharging cycle; and

Figure 3 is a thermodynamic diagram (temperature vs entropy) for showing charging and discharging cycles.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

100120] Figure 1 illustrates a PTES system provided with three thermal circuits. These include a cold-side circuit 1 for a cold-side thermal storage cycle; a hot-side circuit 3 for a hot-side thermal storage cycle; and a working fluid circuit 5 for a transcritical thermodynamic cycle of a working fluid, e.g. CO2, with condensation. jooizij The cold-side circuit is provided with a cold-side thermal storage reservoir 2. It shares a cold-side heat exchanger 10 with a cold-side of the working fluid circuit 5. The cold- side thermal storage reservoir 2 and cold-side heat exchanger 10 are in fluid communication.

A cold-side circuit pump 101 is provided to drive a cold-side heat exchanger fluid via the cold-side heat exchanger 10 and the cold-side thermal storage reservoir 2. {001221 The hot-side circuit 3 is provided with a hot-side thermal storage reservoir 4. It shares a hot-side heat exchanger 12 with a hot-side of the working fluid circuit 5. The hot- side thermal storage reservoir 4 and hot-side heat exchanger 12 are in fluid communication.

A hot-side circuit pump 301 is provided to extract and return a fluid thermal storage medium to and from the hot-side thermal storage reservoir 2, passing the fluid thermal storage medium via the hot-side heat exchanger 12. [001231 The working fluid circuit 5 passes a working fluid (e.g. CO: optionally including additives) in a continuous loop via the hot-side heat exchanger 12 and the cold-side heat exchanger 10. The working fluid circuit is provided with an liquid turbine/expander 8, a gas turbine/expander 6, a compressor 14, and a working fluid pump 16. j00124] The system of Fig.1 is capable of operation in a charging cycle (Fig.2A) and a discharging cycle (Fig.2B). {00125} Fig 2A illustrates the PTES in the charging cycle configuration. Compressor 14 does work on the working fluid to compress it and thereby heating the working fluid.

Compressor 14 is driven, for example, by electricity which may be in excess of a current need in an electricity generating system. That is, surplus electrical energy may be used to compress the working fluid to a supercritical state in the charging cycle of Figure 2A. {00126} The thermal energy generated due to compression in the compressor 14 is transferred to the hot-side thermal storage reservoir 4 via the hot-side heat exchanger 12. The working fluid passes through the hot-side heat exchanger 12 transcritically. It heats fluid thermal storage medium that is being pumped in counter current flow via the hot-side heat exchanger 12 under influence of hot-side circuit pump 301. In this manner the fluid thermal storage medium that is extracted and retuned to the hot-side thermal storage reservoir 4 receives and stores the thermal energy. {00127} Following passage through the hot-side heat exchanger 12, the still supercritical working fluid passes to the liquid turbine/expander 8 and expands to a subcritical liquid phase at lower pressure, and cools.

[00128] The liquid working fluid thereafter passes through the cold-side heat exchanger 10 and evaporates by absorbing heat from cold-side heat exchanger fluid pumped in counter flow through the cold-side heat exchanger 10 under influence of cold-side circuit pump 101.

The cold-side heat exchanger fluid is in that manner cooled and returned to the cold-side thermal storage reservoir 2 where it absorbs thermal energy from a cold-side thermal storage medium, in that manner cooling (removing thermal energy) the cold-side thermal storage medium in the cold-side thermal storage reservoir 2. {001291 Following passage through the cold-side heat exchanger 12, the working fluid returns to the compressor 14 for continuation of the charging process.

[00130] Fig.2B illustrates the PTES in a discharging cycle configuration. In the discharging cycle the working fluid is pumped by working fluid pump 16 over the hot-side heat exchanger 12. Hot thermal storage medium is concurrently extracted from the hot-side thermal storage reservoir 4 being pumped in counter flow via the hot-side heat exchanger 12 under influence of hot-side circuit pump 301. The hot thermal storage medium heats the working fluid and is itself cooled before returning to the hot-side thermal storage reservoir 4 where it is stored in a cooler state.

[00131] Following passage through the hot-side heat exchanger 12, the heated working fluid reaches a supercritical state, passes to the gas turbine/expander 6 and is allowed to expand to asubcritical gas driving the turbine to preferably generate electricity.

[00132] The expanded working fluid is thereafter passed to the cold-side heat exchanger 10 where cooled cold-side heat exchanger fluid is pumped in counter flow through the cold-side heat exchanger 10 under influence of cold-side circuit pump 101. The working fluid is in this manner further cooled and condensed by release of its thermal energy to the cold-side heat exchanger fluid. This condensation ensures the working fluid has reached a liquid state in the pump 16 during the discharging process. {00133} The cold-side heat exchanger fluid is passed through the cold-side thermal storage reservoir 2 and is cooled by exchange of its thermal energy to the cooled cold-side thermal storage medium.

[00134] Following passage through the cold-side heat exchanger 12, the working fluid returns to the pump 16 for continuation of the discharging process.

[00135] Switching the system between charging and discharging processes may be done by operation of various valves 102, 103, 302, 303, 501. 502.

[00136] In each of the above-mentioned three thermal circuits distinct fluids may be employed. A heat transfer fluid for the cold-side thermal storage medium circuit: a working fluid for the transcritical thermodynamic cycle: and a heat transfer fluid for the hot-side thermal storage medium circuit. joo137]} In the illustrated system, the heat transfer fluid for the hot-side thermal circuit is concurrently the thermal storage medium, however, an intermediate heat transfer fluid may be used instead of moving the thermal storage medium. {001381 In the illustrated system, heat transfer for the cold-side circuit is done via a separate thermal transfer fluid. carrying thermal energy between cold-side storage 2 and the cold-side heat exchanger 10. However, by combining the transcritical heat exchanger 10 with the heat exchanger in reservoir 2, it may be possible to have no additional heat transfer fluid and to transfer heat directly to the cold-side storage 2. {00139} The working fluid is configured for efficiency and may be predominantly CO: with inclusion of additives to improve the operational conditions, critical point, heat transfer match, heat capacity, viscosity and other specifications as discussed herein. The fluids of the thermal storage medium cycles may be configured to function in useful temperature ranges, and at the same time minimize required volume, while maintaining low environmental impact. joo140] The fluid for the hot-side thermal storage medium may be water or may preferably be a polyol-based liquid as discussed herein, e.g. Cz-1Hs-10024, preferably glycerol, with solid or liquid additions. {00141} The hot-side storage may be operated between 40°C to 250°C. Polvol liquids as discussed may be optimal for the temperature range and at the same time require limited volumes of occupation.

[00142] Additive solid particles may be dispersed in the fluid, said solid particles having a high heat capacity per volume (higher than the main phase of the fluid (e.g. liquid polyol), to decrease the overall required volume while achieving the same total heat capacity.

[00143] The storage-medium of the cold-side thermal storage medium cycle may be operated between 30°C to 40 °C. Suitable fluids may be based upon salt hvdrates as discussed herein.

The cold-side storage medium of the cold-side thermal storage medium cycle may be configured to function in the temperature range 30°C to 40 °C, and at the same time minimize the volume required, while maintaining environmental friendliness and minimizing the environmental footprint. The use of salt hydrates with a latent heat of crystallization is considered useful in this respect, in particular for relatively small temperature ranges of operation.

[00144] Undercooling of salt hydrate solutions and solidification at lower temperatures may be advantageously reduced, minimized or prevented by adding nucleation sites or nucleating agents.

[00145] A lowering of the melting temperature may be achieved by inclusion of one or more polar components, such as glycerol, urea or ethanol in the salt hydrate composition.

[00146] In the illustrated svstem of Figs. 1, 2A and 2B, a hot-side thermal storage reservoir 4 is provided having a single reservoir body. This may be contrasted with separate double reservoirs as may be found in the prior art. Implementation with a single hot-side thermal storage reservoir body that is able to contain both heated and cooled hot-side thermal storage medium in distinct hot and cold regions, may assist in reducing a required volume or footprint of the thermal storage unit. {00147} In such a single reservoir system, as fluid is pumped through the hot-side heat exchanger either the cold region of the reservoir or the hot region of the reservoir is emptied, as the fluid is returned to the single reservoir after heat exchange, the returned fluid flows into the volume made available by the initial removal of the fluid for heat exchange. To maintain a temperature differential, mixing of the returned fluid with fluid present in the reservoir may be limited or prevented. j00148} Limiting mixing can be achieved. for example by inclusion of a labyrinth or tortuous path type construction in the reservoir, which direct the fluid flow gradually through an extended path in the reservoir in a plug flow manner, resulting in a gradual transition of the cold fluid side to the hot fluid side. [001491 With reference to Figs. 1. 2A and 2B, the hot-side thermal storage reservoir is partially divided by baffles 304. The baffles 304 allow fluid flow through the inner volume of the reservoir 4 yet impede that free flow to reduce or prevent excessive mixing of heated and cooled thermal storage medium. {00150} In the charging configuration of Fig.2A, thermal storage medium heated in the hot- side heat exchanger 12 is returned the hot-side thermal storage reservoir in a heated state via valve 302. while cooler thermal storage medium is withdrawn via valve 303. Extraction via valve 303 frees up internal volume in the hot-side thermal storage reservoir for receipt of the heated thermal storage medium entering via valve 302. Preferably the heated thermal storage medium proceeds in a plug flow. The volume containing the heated thermal storage medium is the high-temperature region of the hot-side thermal storage reservoir, and the volume containing cooler thermal storage medium is the low-temperature region of the thermal storage reservoir.

[00151] The volumes of the high-temperature region and a low-temperature region vary according to the state of charge or discharge of the system. In a fully charged state, all or almost all of the internal volume containing the heated thermal storage medium is the high- temperature region. In a fully discharged state, all or almost all of the internal volume containing the heated thermal storage medium is the low-temperature region. {00152} Prevention of mixing between a hot and cold fluid region may alternatively or additional be achieved by way of a movable or stretchable barrier or membrane between the hot region fluid and the cold region fluid. The barrier or membrane may accommodate the changes in volume while blocking mixing of the fluids. {00153} Figure 3 illustrates suitable operating conditions in pressures and temperatures for the charging and discharging cycles. {00154} In the charging cycle compression is preferably done at pressures from 50 to 500 bar, with heat increases from 15°C to 215°C, followed by cooling (to store heat). This is carried out with the working fluid in the supercritical phase. There is supercritical to liquid expansion to retrieve a portion of the pressure energy, followed by evaporation to complete the cycle.

[00155] In the discharging cycle, heat from the hot-side thermal storage is employed to heat and cause a temperature increase at high pressure (400 bar), after which expansion can take place to lower pressure (50 bar) in a turbine which can be used to produce electricity.

Cooling/condensing is then followed by pumping to higher pressure again to complete the cycle.

[00156] The present invention is able to achieve a relatively efficient thermal energy storage while making use of relatively mild conditions, which assists in real-world achievement of energy storage. Preferably the pressure ratio is about 12 but can be lowered to roughly 4 or 3, which may provide more readily acceptable pressure conditions (with, for example, a minimum pressure of 40 bar) associated with an ability to condense at about 5°C . {00157} Further modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

[00158] Aspects of the invention may be defined by any one or more of the following clauses.

Clause 1. An energy storage system for recoverable storage of thermal energy, comprising; a hot-side thermal storage medium; a hot-side heat exchanger: a working fluid circuit for circulating a working fluid through the hot-side heat exchanger for heat exchange with the thermal storage medium. wherein the working fluid undergoes a transcritical process during heat transfer in the hot- side heat exchanger; and a working fluid contained within the working fluid circuit, the working fluid comprising; greater than 50 wt.% COz; and an additive influencing one or more of the, critical temperature, critical pressure, heat transfer, conductivity, and heat capacity of the working fluid; wherein the working fluid has a global warming potential (GWP) of less than 200, preferably less than 175, more preferably less than 150; preferably the working fluid comprises at least 90 wt.% CO2; and from 0.1 wt.% of the additive influencing one or more of the, critical temperature, critical pressure, heat transfer, conductivity and heat capacity of the working fluid.

Clause 2. The system according to clause 1 wherein the additive is selected from the group consisting of Pentafluoropropane: 1,1,1.2-Tetrafluoroethane;

Difluoromethane; Pentafluoroethane: 1,1,1-Trifluoroethane; 1,1-Dichloro-1- fluoroethane; 1,1,1,3,3,3-Hexafluoropropane; 1,1,1,3,3-Pentafluorobutane;

Trifluoromethane; Sulfur hexafluoride; 1-Chloro-1.2.2,2-tetrafluoroethane; 1-

Chloro-1,1-difluoroethane; Chlorodifluoromethane; ethane: and 1.1,1,3,3,3-

Hexafluoropropane.

Clause 3. The system according to clause 1 wherein the additive is selected from the group consisting of: up to 14.48 wt.% of Pentafluoropropane:

up to 10.42 wt.% of 1,1.1,2-Tetrafluoroethane: up to 22.1 wt.% of Difluoromethane; up to 4.25 wt.% of Pentafluoroethane: up to 3.33 wt.% of 1,1, 1-Trifluoroethane; up to 0.21 wt.% of 1,1-Dichloro-1-fluoroethane: up to 12.42 wt.% of 1,1,1,3,3,3-Hexafluoropropane; up to 18.78 wt.% of 1,1,1,3,3-Pentatluorobutane; up to 1.00 wt.% of Trifluoromethane; up to 0.65 wt.% of Sulfur hexafluoride; up to 24.5 wt. % of 1-Chloro-1,2,2.2-tetrafluoroethane; up to 6.45 wt.% of 1-Chloro-1,1-difluoroethane; up to 8.23 wt.% of Chlorodifluoromethane; up to 49.99 wt.% ethane; and up to 1.52 wt.% of 1,1,1,3,3,3-Hexafluoropropane.

Clause 4. The system according to clause 1 wherein the additive is selected from the group consisting of Trifluoromethane; Sulfur hexafluoride; and ethane: preferably the additive is ethane.

Clause 5. The system according to any of clauses 1 to 4, wherein the working fluid undergoes a transcritical cooling in the hot-side heat exchanger during a charging cycle of the energy storage system.

Clause 6. The system according to any of clauses 1 to 5, wherein the working fluid undergoes a transcritical heating in the hot-side heat exchanger during a discharging cycle of the energy storage svstem.

Clause 7. The system according to any of clauses 1 to 6, wherein the working fluid is in a supercritical state on entering the hot-side heat exchanger during a charging cycle of the energy storage system.

Clause 8. The system according to any of clauses 1 to 7, wherein the working fluid is in a supercritical state on exiting the hot-side heat exchanger during a discharging cycle of the energy storage system.

Clause 9. The system according to any of clause 1 to 8, further comprising; an expander positioned in the working fluid circuit for recovering energy from the working fluid during the charging cycle, wherein the recovered energy is supplied to a compressor in the working fluid circuit for compressing the working fluid to a supercritical state.

Clause 10. The system according to any preceding clause wherein the energy storage system is a thermoelectric energy storage system for provision of thermal energy to a thermodynamic machine for generating electricity.

Clause 11. The system according to any preceding clause wherein the energy storage system comprises a compressor for conversion of electrical energy to thermal energy in the working fluid, which thermal energy is exchanged with the thermal storage medium.

Clause 12. A method for storing thermal energy. preferably thermoelectric energy in a thermoelectric energy storage system. comprising; circulating a working fluid through a hot-side heat exchanger for heat transfer with a hot-side thermal storage medium, and transferring heat with the thermal storage medium in a transcritical process in the hot-side heat exchanger; wherein the working fluid comprises greater than 50 wt.% CO:: and an additive influencing one or more of the, critical temperature, critical pressure, heat transfer, conductivity, and heat capacity of the working fluid; wherein the working fluid has a global warming potential (GWP) of less than 200, preferably less than 175, more preferably less than 150; preferably the working fluid comprises at least 90 wt.% CO2; and from 0.1 wt.% of an additive influencing one or more of the critical temperature, critical pressure, heat transfer, conductivity, and heat capacity of the working fluid. preferably up to 10 wt.%.

Clause 13. The method of clause 12 wherein the additive is selected from the group consisting of Pentafluoropropane; 1,1,1,2-Tetrafluoroethane; Difluoromethane;

Pentafluoroethane; 1,1.1-Trifluoroethane; 1,1-Dichloro-1-fluoroethane: 1,1.1,3.3,3-Hexafluoropropane; 1,1,1,3,3-Pentafluorobutane; Trifluoromethane;

Sulfur hexafluoride; 1-Chloro-1.2,2 2-tetrafluoroethane; 1-Chloro-1,1- difluoroethane; Chlorodifluoromethane; ethane; and 1.1,1.3,3.3-

Hexafluoropropane.

Clause 14. The method of clause 12 wherein the additive is selected from the group consisting of: up to 14.48 wt.% of Pentafluoropropane; up to 10.42 wt.% of 1,1,1,2-Tetrafluoroethane:

up to 22.1 wt.% of Difluoromethane: up to 4.25 wt.% of Pentafluoroethane: up to 3.33 wt.% of 1,1,1-Trifluoroethane: up to 0.21 wt.% of 1,1-Dichloro-1-fluoroethane: up to 12.42 wt.% of 1,1,1,3,3.3-Hexafluoropropane: up to 18.78 wt.% of 1.1,1,3,3-Pentafluorobutane: up to 1.00 wt.% of Trifluoromethane: up to 0.65 wt.% of Sulfur hexafluoride; up to 24.5 wt. % of 1-Chloro-1.2,2,2-tetrafluoroethane: up to 6.45 wt.% of 1-Chloro-1.1-difluoroethane: up to 8.23 wt.% of Chlorodifluoromethane; up to 49.99 wt.% ethane; and up to 1.52 wt.% of 1,1,1,3,3.3-Hexafluoropropane.

Clause 15. The method according to clause 12 wherein the additive is selected from the group consisting of Trifluoromethane; Sulfur hexafluoride; and ethane: preferably the additive is ethane.

Clause 16. The method according to any of clauses 12 to 15, wherein the step of transferring heat comprises transcritical cooling of the working fluid in the hot- side heat exchanger during a charging cycle of the thermoelectric energy storage system.

Clause 17. The method according to any of clauses 12 to 16, wherein the step of transferring heat comprises transcritical heating of the working fluid in the hot- side heat exchanger during a discharging cycle of the thermoelectric energy storage system.

Clause 18. An energy storage system for recoverable storage of thermal energy, preferably a thermoelectric energy storage system for providing thermal energy to a thermodynamic machine for generating electricity, comprising; a hot-side thermal storage medium; a hot-side heat exchanger; and a working fluid circuit for circulating a working fluid through the hot-side heat exchanger for heat transfer with the thermal storage medium, wherein the working fluid undergoes a transcritical process during heat transfer in the hot- side heat exchanger: a working fluid contained within the working fluid circuit,

wherein the hot-side thermal storage medium comprises at least one polyol of the formula — Cz-4He-10024, preferably glycerol.

Clause 19. The system of clause 18. wherein the working fluid comprising at least 50 wt.% CO.

Clause 20. The system according to anv of clauses 18 to 19, wherein the working fluid has a melting point below 40°C, preferably below 30°C.

Clause 21. The system according to any of clauses 18 to 20, wherein the hot-side thermal storage medium further comprises solid particles having a higher heat capacity per volume than the storage medium, the solid particles being dispersed within the storage medium.

Clause 22. The system according to any of clauses 18 to 21, wherein the working fluid undergoes a transcritical heating in the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage system.

Clause 23. The system according to any of clauses 18 to 22, wherein the working fluid is in a supercritical state on entering the hot-side heat exchanger during a charging cycle of the thermoelectric energy storage system.

Clause 24. The system according to any of clauses 18 to 23, wherein the working fluid is in a supercritical state on exiting the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage svstem.

Clause 25. The system according to any of clause 18 to clause 24, further comprising; an expander positioned in the working fluid circuit for recovering energy from the working fluid during the charging cycle, wherein the recovered energy is supplied to a compressor in the working fluid circuit for compressing the working fluid to a supercritical state.

Clause 26. A method for storing thermal energy, preferably thermoelectric energy in a thermoelectric energy storage system, comprising; circulating a working fluid through a hot-side heat exchanger for heat transfer with a hot-side thermal storage medium, and transferring heat with the thermal storage medium in a transcritical process in the hot-side heat exchanger; and wherein the hot-side thermal storage medium comprises at least one liquid polyol of the formula — Cs. 4He-1002-4, preferably glycerol.

Clause 27. The method of clause 26, wherein, the working fluid comprising at least 50 wt.% CO.

Clause 28. The method according to clause 26 or 27, wherein the polyol has a melting point below 40°C, preferably below 30°C.

Clause 29. The method according to any of clauses 26 to 28, wherein the step of transferring heat comprises transcritical cooling of the working fluid in the hot- side heat exchanger during a charging cycle of the thermoelectric energy storage system.

Clause 30. The method according to any of clauses 26 to 29, wherein the step of transferring heat comprises transcritical heating of the working fluid in the hot- side heat exchanger during a discharging cycle of the thermoelectric energy storage system.

Clause 31. An energy storage system for recoverable storage of thermal energy, preferably a thermoelectric energy storage system for providing thermal energy to a thermodynamic machine for generating electricity, comprising; a hot-side thermal storage medium; a hot-side heat exchanger: a cold-side thermal storage medium; and a cold-side heat exchanger: a working fluid circuit for circulating a working fluid through the hot-side and cold-side heat exchangers for heat transfer with the hot-side and cold-side thermal storage mediums respectively, wherein the working fluid undergoes a transcritical process during heat transfer in the hot-side heat exchanger; and a working fluid contained within the working fluid circuit wherein the cold-side thermal storage medium comprises a hvdrate of at least one inorganic salt of at least one inorganic salt, preferably comprising 2-10 moles of crystal water per mole of the at least one inorganic salt.

Clause 32. The system according to clause 31, wherein the salt hydrate comprises one or more of CaCl:6H:0, Na2S0:+ 10H:0, Zn(NO3)2:6H:0, CaBr2-6H:0.

Clause 33. The system according to clause 31, wherein the salt hydrate comprises

CaCl: 6H20.

Clause 34. The system according to any of clause 31 to 33. wherein the cold-side thermal storage medium further comprises one or more of: a melting point reducing additive preferably selected from the group consisting of glycerol, urea or ethanol:

MgCl2-6H20, preferably up to 33 wt.% by weight of the thermal storage medium; a viscosity modifier, preferably a viscosity modifier comprising hydroxyl functional groups, preferably selected from the group of silica, bentonite, C2-C4 polyols, modified cellulose, and polvacrvlic acid; and a nucleating agent, preferably a compound selected from the group of alkaline earth metal halides, hydroxides, or carbonates.

Clause 35. The system according to any of clauses 31 to 34 wherein the cold-side thermal storage medium comprises a eutectic mixture of CaCl2:6H20 and up to 33 wt.% of MgCl2-6H20, preferably from 16 to 20 wt.%, more preferably about 18 wt.%.

Clause 36. The system according to any of clauses 31 to 35 wherein the cold-side thermal storage medium comprises one or more melting point reducing additives selected from Ethanol, NH4CI, NH4NOs3, Urea, and Ethylene glycol.

Clause 37. The system according to any of clauses 31 to 36 wherein the cold-side thermal storage medium comprises one or more nucleating agents selected from

Ba(OH)2-8H20. Bal::6H:0, BaC1::6H:0, BaF:, SrFz. SrCl2-6Hz20, SrBr2-6H20,

Sr(OH)::8H20, SrCO3, Mg(OH)2, Mg(NOs)2-6H20, Na4B407:5H:20,

LiNO:3:3H20, NaCl, KCI, and KNO;3, y-A1:0:3 nanoparticles.

Clause 38. The system according to any of clauses 31 to 37 wherein the cold-side thermal storage medium comprises a viscosity increasing modifier selected from fumed silica, polyethylene glvcol, ethylene glycol, hydroxyethylcellulose, carboxymethylcellulose, methylcellulose.

Clause 39. The system according to clause 31 wherein the salt hydrate comprises

Na250410H20.

Clause 40. The system according to clause 39 wherein the cold-side thermal storage medium comprises a eutectic mixture of Na2S0:4:10H20 and up to 77 wt.%

Na:2HPO4:12H:0, preferably from 73 to 77 wt.%, more preferably about 75 wt.%; or Na2C03:10H20; or up to 7 wt.% KCl.

Clause 41. The system according to any of clauses 39 to 40 wherein the cold-side thermal storage medium comprises a nucleating agent selected from one or more of

Na4B+07:5H20 and Na2S103:9H20.

Clause 42. The system according to any of clauses 39 to 41 wherein the cold-side thermal storage medium comprises a viscosity increasing modifier selected from one or more of polvacrylamide, fumed silica, polyacrylic acid polymer.

Clause 43. The system according to clause 31 wherein the salt hydrate comprises

Zn(NOz)2-6H20.

Clause 44. The system according to clause 43 wherein the cold-side thermal storage medium comprises a nucleating agent selected from one or more of

Zn:(OH)4(NO3):: ZnO: Zn(OB):: and Zinc acetate.

Clause 45. The system according to any of clauses 43 to 44 wherein the cold-side thermal storage medium comprises a viscosity increasing modifier comprising carboxymethyl cellulose.

Clause 46. The system of any of clauses 31 to 45. wherein the working fluid comprises at least 50 wt.% COa.

Clause 47. The system according to any of clauses 31 to 46, wherein the working fluid undergoes transcritical heating in the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage system.

Clause 48. The system according to any of clauses 31 to 47, wherein the working fluid is in a supercritical state on entering the hot-side heat exchanger during a charging cycle of the thermoelectric energy storage system.

Clause 49. The system according to any of clauses 31 to 48, wherein the working fluid is in a supercritical state on exiting the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage svstem.

Clause 50. The system according to any of clauses 31 to 49, further comprising; an expander positioned in the working fluid circuit for recovering energy from the working fluid during the charging cycle, wherein the recovered energy is supplied to a compressor in the working fluid circuit for compressing the working fluid to a supercritical state.

Clause 51. A method for storing thermal energy, preferably thermoelectric energy in a thermoelectric energy storage system, comprising; circulating a working fluid through a hot-side heat exchanger for heat transfer with a hot-side thermal storage medium, and transferring heat with the thermal storage medium in a transcritical process in the hot-side heat exchanger, and circulating the working fluid through a cold-side heat exchanger for heat transfer with a cold-side thermal storage medium, and transferring heat with the cold-side thermal storage medium:

wherein the cold-side thermal storage medium comprises a hydrate of at least one morganic salt. preferably comprising 2-10 moles of crystal water per mole of the at least one inorganic salt.

Clause 52. The method according to clause 51. wherein the salt hydrate comprises one or more of CaCl2::6H:0, Na2$0: 10H:0, Zn{NO3)::6H20, CaBr::6H:0.

Clause 53. The method according to clause 51, wherein the salt hydrate comprises

CaClz-6H20.

Clause 54. The method according to any of clause 51 to 53, wherein the cold-side thermal storage medium further comprises one or more of: a melting point reducing additive preferably selected from the group consisting of glycerol, urea or ethanol;

MgCl2-6H:0, preferably up to up to 33 wt.% by weight of the thermal storage medium; a viscosity modifier, preferably a viscosity modifier comprising hydroxyl functional groups. preferably selected from the group of silica, bentonite, C2-C4 polyols, modified cellulose, and polyacrylic acid; and a nucleating agent, preferably a compound selected from the group of alkaline earth metal halides, -hydroxides, or carbonates.

Clause 55. The method according to any of clauses 51 to 54 wherein the cold-side thermal storage medium comprises a eutectic mixture of CaCl2-6H20 and up to 35 wt.% of MgCl::6H20, preferably from 16 to 20 wt.%, more preferably about 18 wt.%.

Clause 56. The method according to any of clauses 51 to 55 wherein the cold-side thermal storage medium comprises one or more melting point reducing additives selected from Ethanol, NH4Cl, NH:NO3, Urea, and Ethvlene glycol.

Clause 57. The method according to any of clauses 51 to 56 wherein the cold-side thermal storage medium comprises one or more nucleating agents selected from

Ba(OH)2-8H:0, Bal::6H:0, BaCl»6H20, BaF:, StF, SrCla-6H20, SrBrz-6H20,

Sr(OH)2-8H:0, SrCO3, Mg(OH)2, Mg(NOs3)2-6H20, Na:B:075H:0,

LiNO3:3H:0, NaCl, KCI, and KNO: y-Al203 nanoparticles.

Clause 58. The method according to any of clauses 51 to 57 wherein the cold-side thermal storage medium comprises a viscosity increasing modifier selected from fumed silica, polyethylene glycol, ethylene glycol, hydroxyethylcellulose, carboxymethylcellulose. methylcellulose.

Clause 59. The method according to clause 51 wherein the salt hydrate comprises

Na250+10H20.

Clause 60. The method according to clause 59 wherein the cold-side thermal storage medium comprises a eutectic mixture of Na250:4: 10H20 and up to 77 wt.%

Na:HPO:4:12H20, preferably from 73 to 77 wt.%, more preferably about 75 wt.%; or Na2C03:10H20; or up to 7 wt.% KCl.

Clause 61. The method according to any of clauses 59 to 60 wherein the cold-side thermal storage medium comprises a nucleating agent selected from one or more of

Na4B407:5H20 and Na2S103:9H:0.

Clause 62. The method according to any of clauses 59 to 61 wherein the cold-side thermal storage medium comprises a viscosity increasing modifier selected from one or more of polvacrylamide, fumed silica, polyacrylic acid polymer.

Clause 63. The method according to clause 51 wherein the salt hydrate comprises

Zn(NO3):6H20.

Clause 64. The method according to clause 63 wherein the cold-side thermal storage medium comprises a nucleating agent selected from one or more of

Zn:(OH)4(NO3)2; ZnO; Zn(OH)2; and Zinc acetate.

Clause 65. The method according to any of clauses 63 to 64 wherein the cold-side thermal storage medium comprises a viscosity increasing modifier comprising carboxymethyl cellulose.

Clause 66. The method of any of clauses 51 to 65, wherein the working fluid comprises at least 50 wt.% COa.

Clause 67. The method according to and of clauses 51 to 66, wherein the step of transferring heat with the hot-side thermal storage medium comprises transcritical cooling of the working fluid during a charging cycle of the thermoelectric energy storage system.

Clause 68. The method according to any of clauses 51 to 67, wherein the step of transferring heat with the hot-side thermal storage medium comprises transcritical heating of the working fluid during a discharging cycle of the thermoelectric energy storage system.

Clause 69. An energy storage system for recoverable storage of thermal energy, preferably a thermoelectric energy storage system for providing thermal energy to a thermodynamic machine for generating electricity, comprising;

a hot-side thermal storage medium; a hot-side heat exchanger; a working fluid circuit for circulating a working fluid through the hot-side heat exchanger for heat transfer with the thermal storage medium, wherein the working fluid undergoes a transcritical process in the hot-side heat exchanger during heat transfer ; and a hot-side thermal storage medium reservoir, wherein the reservoir comprises an integral, internal volume divided into a high-temperature region and a low- temperature region, wherein a ratio of volume size of the regions can vary during operation.

Clause 70. The system according to clause 69 wherein the reservoir is provided with a conduit fluidly communicating the high-temperature region, the low-temperature region, and the hot-side heat exchanger.

Clause 71. The system according to clause 69 or 70 wherein the reservoir is provided with a movable insulating barrier dividing the high-temperature region from the low- temperature region, wherein movement of the dividing barrier increases the volume of one region while decreasing the volume of the other region.

Clause 72. The system according to any of clauses 69 to 71, wherein the high-temperature region from the low-temperature region is in fluid communication and the reservoir is provided with a tortuous path between the high-temperature region and the low-temperature region whereby one region may increase in volume and encroach upon a reducing volume of the other region.

Clause 73. The system according to clause 72, wherein the tortuous path is formed by insulating baffles. piping. or a combination thereof.

Clause 74. The system according to any of clauses 72 to 73, wherein the working fluid undergoes a transcritical cooling in the hot-side heat exchanger during a charging cycle of the thermoelectric energy storage system.

Clause 75. The system according to any of clauses 72 to 74, wherein the working fluid undergoes a transcritical heating in the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage system.

Clause 76. The system according to any of clauses 72 to 75, wherein the working fluid is in a supercritical state on entering the hot-side heat exchanger during a charging cvcle of the thermoelectric energy storage system.

Clause 77. The system according to any of clauses 72 to 76, wherein the working fluid is in a supercritical state on exiting the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage system.

Clause 78. The system according to any of clauses 72 to 77, further comprising; an expander positioned in the working fluid circuit for recovering energy from the working fluid during the charging cycle, wherein the recovered energy is supplied to a compressor in the working fluid circuit for compressing the working fluid to a supercritical state.

Clause 79. A method for storing thermal energy, preferably thermoelectric energy in a thermoelectric energy storage system, comprising; circulating a working fluid through a hot-side heat exchanger for heat transfer with a hot-side thermal storage medium, and transferring heat with the hot-side thermal storage medium in a transcritical process in the hot-side heat exchanger: wherein the hot-side thermal storage medium is provided a hot-side thermal storage medium reservoir, wherein the reservoir comprises an integral, internal volume divided into a high-temperature region and a low-temperature region. the method further comprising varving a ratio of volume size of the regions, increasing the low-temperature region volume during a discharging cycle, and/or increasing the high-temperature region volume during a charging cvcle.

Clause 80. The method according to clause 79 wherein the reservoir is provided with a conduit fluidly communicating the high-temperature region volume, the low- temperature region volume, and the hot-side heat exchanger, and thermal storage medium is passed from one region to the other region via the hot-side heat exchanger.

Clause 81. The method according to clause 79 or 80 wherein the reservoir is provided with a movable barrier dividing the high-temperature region from the low- temperature region volume, wherein the dividing barrier is moved to increase the volume of one region while decreasing the volume of the other region.

Clause 82. The method according to clause 79 or 81 wherein the high-temperature region and the low-temperature region are in fluid communication and the reservoir is provided with a tortuous path between the high-temperature region and the low- temperature region, wherein one region is increased in volume to encroach upon a reducing volume of the other region, preferably in plug-flow.

Clause 83. The method according to clause 82, wherein the tortuous path is formed by baffles, piping, or a combination thereof in the reservoir.

Clause 84. The method according to any of clauses 79 to 83 wherein the step of transferring heat comprises transcritical cooling of the working fluid in the hot- side heat exchanger during a charging cycle of the thermoelectric energy storage system.

Clause 85. The method according to clauses 79 to 84. wherein the step of transferring heat comprises transcritical heating of the working fluid in the hot-side heat exchanger during a discharging cycle of the thermoelectric energy storage system.

Claims

Conclusions

1. An energy storage system for recoverable storage of thermal energy, preferably a thermoelectric energy storage system for supplying thermal energy to a thermodynamic machine for generating electricity. comprising: a hot side thermal storage medium; a hot side heat exchanger; a working fluid circuit for circulating a working fluid through a hot side heat exchanger for heat transfer with the thermal storage medium, the working fluid undergoing a transcritical process in the hot side heat exchanger during heat transfer; and a hot side thermal storage medium reservoir, the reservoir comprising an integral internal volume divided into a high temperature region and a low temperature region. wherein a volume size ratio of the different regions may vary during operation.

2. The system of claim 1, wherein the reservoir is provided with a tube in fluid communication with the high temperature region, the low temperature region and the hot side heat exchanger.

3. A system as claimed in claim 1 or 2, wherein the reservoir is provided with a movable insulating barrier separating the high temperature region from the low temperature region, wherein movement of the separating barrier increases the volume of one region and simultaneously decreases the volume of the other region.

4. A system as claimed in any one of claims 1 to 3, wherein the high temperature region is in fluid communication with the low temperature region and the reservoir is provided with a tortuous path between the high temperature region and the low temperature region, whereby one region can increase in volume and expand after a decrease in volume of the other region.

5. The system of claim 4, wherein the tortuous path is formed by insulating bulkheads, piping or a combination thereof.

6. System according to any of claims 4 to 5, wherein the working fluid undergoes transcritical cooling in the hot side heat exchanger during a charging cycle of the thermoelectric energy storage system.

7. A system according to any one of claims 4 to 6, wherein the working fluid undergoes transcritical heating in the hot side heat exchanger during a discharge cycle of the thermoelectric energy storage system.

8. A system according to any one of claims 4 to 7, wherein the working fluid is in a supercritical state when it enters the hot side heat exchanger during a charging cycle of the thermoelectric energy storage system.

9. A system as claimed in any one of claims 4 to 8, wherein the working fluid is in a supercritical state when it exits the hot side heat exchanger during a discharge cycle of the thermoelectric energy storage system.

10. The system of any of claims 4 to 9, further comprising: an expander disposed in the working fluid circuit to recover energy from the working fluid during the loading cycle, the recovered energy being supplied to a compressor in the working fluid circuit to compress the working fluid to a supercritical state.

11. A method for storing thermal energy, preferably thermoelectric energy in a thermoelectric energy storage system, comprising: circulating a working fluid through a hot side heat exchanger for heat transfer with a hot side thermal storage medium, and transferring heat with the hot side thermal storage medium in a transcritical process in the hot side heat exchanger; wherein the hot side thermal storage medium is provided with a hot side thermal storage medium reservoir. wherein the reservoir comprises an integral internal volume divided into a high temperature region and a low temperature region, the method further comprising varying a volume size ratio of the regions, increasing the volume of the low temperature region during a discharge cycle and/or increasing the volume of the high temperature region during a charge cycle.

12. A method according to claim 11, wherein the reservoir is provided with a tube in fluid communication with the high temperature region volume, the low temperature region volume and the hot side heat exchanger, and thermal storage medium passes from one region to the other region via the hot side heat exchanger.

13. A method according to claim 11 or 12, wherein the reservoir is provided with a movable barrier separating the high temperature region from the volume of the low temperature region, the separating barrier being moved to increase the volume of one region while simultaneously decreasing the volume of the other region.

A method according to claim 11 or 13, wherein the high temperature region and the low temperature region are in fluid communication and the reservoir is provided with a tortuous path between the high temperature region and the low temperature region, whereby one region can increase in volume and expand after a decrease in volume of the other region, preferably in plug flow.

15. The method of claim 14, wherein the tortuous path is formed by bulkheads, pipelines or a combination thereof in the reservoir.

16. A method according to any one of claims 11 to 15, wherein the step of transferring heat comprises transcritically cooling the working fluid in the hot side heat exchanger during a charging cycle of the thermoelectric energy storage system.

17. A method according to any one of claims 11 to 16, wherein the step of transferring heat comprises transcritically heating the working fluid in the hot side heat exchanger during a discharge cycle of the thermoelectric energy storage system.