DK202100886A1

DK202100886A1 - A thermal energy storage system with environmental air exchange and a method of its operation

Info

Publication number: DK202100886A1
Application number: DKPA202100886A
Authority: DK
Inventors: Stiesdal Henrik; Vinther Voigt Niels
Original assignee: Stiesdal Storage As
Priority date: 2021-09-20
Filing date: 2021-09-20
Publication date: 2023-04-25
Also published as: DK181199B1; WO2023041132A1; CN117980685A

Abstract

A thermal energy storage, TES, system (100) comprises a thermodynamic gas flow circuit containing air as a working fluid for transport of thermal energy through the TES system. The air is maintained in gas phase throughout the entire gas flow circuit, which is open to the environment. Air is received (7) from the environment and pressurized by a compressor (2), driven by a motor (1A), for use as a working fluid with increased temperature, followed by transfer of thermal energy to a first TES medium (5’) during charging prior to release (7’) of the working fluid to the environment again. In discharging cycles, the air is received from the environment and used as working fluid in reverse flow through the first TES medium (5’) for being heated by the first TES medium prior to driving an expander (3), typically a turbine, which in turn drives an electrical generator (1B) for recovery of electrical energy.

Description

DK 2021 00886 A1 1

FIELD OF THE INVENTION

The present invention relates to a thermal energy storage system and a method of its operation. In particular, it relates to a system and method as described in the preamble of the independent claims.

BACKGROUND OF THE INVENTION

Sustainable electricity production by wind and solar power suffers from the fact that electricity is not necessarily demanded at the time of production and not necessarily available at the time of demand. Accordingly, various energy storage facilities have been proposed, where the electrical energy is transformed into heat and stored until there is a demand for transforming it back into electricity.

US patent No. 8,826,664 discloses a system comprising a first thermal energy storage (TES) container and a second TES container at a lower temperature, which are inter- connected through a compressor/expander arrangement for increasing the temperature in the first TES container during charging and or decreasing the temperature during discharging of the system. When there is surplus electricity, a compressor is driven by an electrical engine, increasing the temperature of gas by compression, which is then used to heat a TES medium in the form of a bed of gravel in the first TES container.

When there is a demand for electricity, the compressed hot gas is released from the first

TES container through an expander which drives an electrical generator for recovering the electrical energy. During charging and discharging processes, a thermal front be- tween hot and cold regions moves through the TES container from one end towards the other due to the gradual temperature changes in the TES container.

During such movements of the thermal front, especially when the charging/discharging process is repeated, the temperature gradient tends to flatten between the two ends of the storage container, which is called thermocline degradation. Thermocline degrada- tion is an effect of the temperature transition zone, also called thermocline zone or

DK 2021 00886 A1 2 thermocline region, becoming wider. Thermocline degradation is not wanted because it decreases the overall efficiency of the system.

The system of US 8,826,604 implies another not well-recognized problem when applied in practice, namely the fact that the specific heat capacity for the gravel decreases with temperature, so that the cold storage container needs more gravel than the hot storage container in order to balance correctly. When having in mind that costs for energy stor- age systems are sensitive to container sizes, this appears not as optimum conditions.

Even further, the larger amount of gravel and its low heat capacity at low temperature increases the length of the thermocline, which is unwanted as it decreases efficiency, as discussed above.

Accordingly, in contrast to idealized theoretical models, system designs for TES are a compromise between different advantages and disadvantages, including costs for estab- lishing and maintaining the system as well as long term efficiency in order to provide long term profitability. There is a general need for improvement in the technical field with respect to practicability and profitability.

With respect to minimizing thermocline degradation, various methods have been pro- posed for steepening the gradient and reducing the width of the thermocline zone.

WO2018/073049 mentions various thermocline control concepts, of which one is to push the thermocline out of the storage container or, in other words, extract the thermo- cline. This terminology is used when heating of the TES medium in the container is continued until the temperature at the end of the TES container is increased above the minimum temperature in the container, potentially up to the maximum temperature where no temperature gradient exists any more inside the container.

Alternative systems use phase change materials (PCM) in TES systems, which control the temperature at one end or both ends of the TES containers. Various examples of multilayer PCM and its influence on the thermocline are discussed in US2017/226900.

Even further attempts are using two phase working fluids, for example liquid/ice as disclosed in US 8,931,277 in which water with glycol is used as working fluid.

DK 2021 00886 A1 3

Another example of a two phase working fluid is disclosed in GB2534914A, in which air is exchanged with the environment. In particular, air is compressed by a compressor to a higher temperature and heat transferred to a TES medium. The cooled air is then expanded through a throttle to form liquid air, which is then stored. In discharge mode, the liquid air is pumped through the TES medium and reheated and vaporized and re- leased into the environment. For adjustment of the temperature of the working fluid, a heat exchanger is provided in which heat is exchanged with the air of the environment.

For increase of efficiency, an electrical pre-heater is used for the air upstream of the compressor to increase the temperature of the ambient air. Despite its advantages, the storge of liquid air makes the system complex and expensive in practice.

As this discussion of the prior art reveals, the problem of thermocline degradation in

TES systems is very well known as well as various countermeasures. However, when considering optimization of efficiency in view of costs for establishing, maintaining, and operating the TES systems, no consensus has yet been reached. Various approaches, especially with respect to minimizing thermocline degradation, go in opposite directions and are incompatible alternatives.

Accordingly, there is still a need for improvement with respect to optimization of ther- mal energy storage systems.

DESCRIPTION / SUMMARY OF THE INVENTION

It is therefore an objective of the invention to provide an improvement in the art. In particular, it is an objective to provide a simplified though efficient thermal energy stor- age (TES) system and a method of its operating. It is also an objective to provide an improvement based on a balance between costs for establishing the system and costs for operating the system. This objective and further advantages are achieved with a sys- tem and method as described below and in the claims.

By balancing the various factors for profitability, a system is herein presented that is simple and implies relatively low establishing costs but which still provides a proper long term profitability.

DK 2021 00886 A1 4

The TES system comprises a thermodynamic gas flow circuit containing air as a work- ing fluid for transport of thermal energy through the TES system. The air is not in liquid phase but maintained in gas phase throughout the entire gas flow circuit. In particular, the gas flow circuit is open to the environment. Air is received from the environment and pressurized by a motor-driven compressor for use as a working fluid with increased temperature, followed by transfer of thermal energy to a first TES medium during charging prior to release of the working fluid to the environment again. In discharging cycles, the air is received from the environment and used as working fluid in reverse flow through the first TES medium for being heated by the first TES medium prior to driving an expander, typically a turbine, which in turn drives an electrical generator for recovery of electrical energy.

Accordingly, the temperature at the entrance of the cycle has the temperature of the surrounding environment, which is typically in the range of zero to 25°C, idealized to 15°C, which is commonly used a standard value in modelling.

In advantageous embodiments, thermal energy is transferred between a first and a sec- ond TES medium during charging and discharging by using gaseous air as working fluid for the thermal energy transport between the TES media. In the latter technical solution, the low-temperature end of the second TES medium receives air from the surroundings as a working fluid, which is then released at higher temperature from the high-temper- ature end of the second TES media. As a result, the temperature at the entrance of the cycle at the cold end of the second TES medium has the temperature of the surround- ings.

It is pointed out for better understanding of the invention that minimization of costs for establishment and technical maintenance of the TES system favours simple systems, relatively small containers and minimized use of pipes and valves. Optimization of ef- ficiency implies keeping the gradient steep and the thermocline region narrow for in- creased conversion efficiency. Profitability of a TES system includes a balancing be- tween, on the one hand, minimization of the construction and maintenance costs and, on the other hand, that efficiency that can be reached between charging and discharging, including the conversion between electricity and thermal energy.

DK 2021 00886 A1

In comparison with some prior art, the described setup implies a simplification of the low-temperature, low-pressure section of the TES system, as this is exchanging the working fluid in the form of gaseous air with the environment instead of being con- 5 nected through pipes and potential heat exchangers. Although, the principle of the de- scribed system has certain similarities with heat exchange of the working fluid to achieve a temperature of the surroundings, it implies the cost advantage and ease of operation in that it avoids the need of heat exchangers and the necessary connection pipes and valves, while still having a sufficient efficiency for being profitable.

In the following, the TES systems will be described as comprising TES containers that contain TES media. The TES container has a first end and a second end in between which the TES medium is provided and where the air is traversing the TES medium on its travel from one to the other end.

In a more detailed practical approach, the system comprises the following components.

A first TES container is containing a first TES medium for storing thermal energy. The first TES medium is in thermal connection with the gas flow circuit along a first gas flow path for exchange of thermal energy with the working fluid. The first gas flow path is part of the gas flow circuit.

An energy converter is provided for converting between electrical energy and thermal energy of the working fluid in the gas flow circuit. The energy converter comprises an electrical motor, an electrical generator, and a compressor/expander system. In some embodiments, the electrical motor and electrical generator are implemented as a single device with dual mode function. When electrical energy is added to the energy con- verter, the converter converts the electrical energy to thermal energy in the form of added heat in the working fluid. The compressor/expander system comprises a com- pressor and an expander. The compressor is functionally connected to the motor for being driven by the motor during a charging period. In turn, the expander is functionally connected to the generator for driving the generator during a discharging period. As will become apparent below, the compressor/expander system can comprise further com- pressors and expanders.

DK 2021 00886 A1 6

Typically, the functional connection comprises a mechanical coupling, such as a rota- tional axis connecting the motor with the compressor and the generator with the ex- pander, although a hydraulic or pneumatic coupling could also be used in principle.

During a charging period, the compressor receives air from the environment at atmos- pheric pressure as a working fluid and pressurizes it from a first pressure, which is at- mospheric pressure, or close to atmospheric pressure due to some suction effect for moving the air into the compressor, to a second, higher, pressure, which also raises the temperature of the air, for providing the air as pressurized working fluid. The pressur- ized working fluid flows through the first gas flow path for transferring thermal energy from the working fluid to the first TES medium. Once having traversed the first TES container, the working fluid is depressurized again downstream of the first TES con- tainer and released to the environment.

For example, the depressurization of the working fluid into the environment in the charging phase is through the expander or through a further expander, which is mechan- ically coupled by a coupling to the compressor for adding driving force to the compres-

SOT.

During a discharging period, a reverse flow of the working fluid is provided through the first gas flow path for transferring thermal energy from the first TES medium to the working fluid to heat the working fluid. For this, air is received at atmospheric pressure from the environment and compressed by the compressor or by a further compressor prior to traversing the first TES medium in reverse. The heated working fluid is then depressurized to the first atmospheric pressure through the expander for driving the electrical generator. After the expansion, the air is released to the environment.

For example, in the discharging phase, the compressor or further compressor are me- chanically coupled and driven by the expander.

In practical embodiments, the first TES container has a top and a bottom and the first

TES medium is provided therein between, wherein the top is flow-connected to a

DK 2021 00886 A1 7 downstream side of the compressor in the charging period and to an upstream side of the expander in the discharging period.

As mentioned, in some embodiments, the TES system comprises a second TES con- tainer containing a second TES medium for storing thermal energy. The second TES medium is in thermal connection with the gas flow circuit along a second gas flow path for exchange of thermal energy with the working fluid. The second gas flow path is part of the gas flow circuit and flow-connected upstream of the compressor during charging and downstream of the expander during discharging. During a charging period, air is received from the environment through the second gas flow path, for example drawn by suction of the compressor, and thermal energy is transferred from the second TES me- dium to the received air for preheating the air prior to compression by the compressor.

In this connection, it is noted that the second TES medium is heated during the dis- charging period by transfer of thermal energy from the working fluid to the second TES medium after driving the expander. After such traversing of the second TES medium in the discharging period, the working fluid is released to the environment.

In practical embodiments, the second TES container has a top and a bottom and the second TES medium is provided therein between, wherein the top is flow-connected to a downstream side of the expander in the charging period and to an upstream side of the compressor in the discharging period.

For example, during charging, the temperature of the air is raised by the compressor to a temperature in the range of 400°C to 700°C, optionally in the range of 400°C-600°C or 500°C-600°C. This would then become the temperature of the first TES medium at least at the entrance, typically top, of the first TES container.

For example, during discharging the temperature of the working fluid is decreased by the expander to a temperature in the range of 200°C to 380°C. This would then become the temperature of the second TES medium at least at the entrance, typically top, of the second TES container.

As it appears from the above, in contrast to many systems in the prior art, the cold ends, typically the bottoms, of the first and second TES containers are not interconnected by

DK 2021 00886 A1 8 pipes. Instead, the cold ends, typically the bottoms, of the first TES container and the second TES container are connected to the environment for exchanging air with it. In this sense, the gas flow circuit is open to the environment.

Advantageously, the compression in the compressor and the expansion in the expander are adiabatic and the thermal transfer between the gas and the thermal storage media is isobaric. This is the ideal case, though, in practice, there is a deviation from this ideal model. For a practical approach, the theoretical term adiabatic should in this case be understood as quasi-adiabatic or near-adiabatic to the extent that practical embodiments allow such compression and expansion. Similarly for the term isobaric. Typically, the deviation from the theoretical adiabatic and isobaric case to the more realistic quasi- adiabatic and quasi-isobaric situation in real practice is in the region of less than 10% deviation from the ideal theoretical model.

During a period of charging, air as the working fluid is provided to the top of the first

TES container at a maximum temperature level Tmax for the storage of thermal energy.

The thermal energy in the working fluid is transferred from the working fluid to the first

TES medium by flow of the working fluid from the upper end to the first TES medium.

For example, the first TES medium is gas permeable, for example a bed of gravel, and the heated gas is traversing the medium from the upper to the lower end and leaving the

TES container at the bottom.

The TES medium is advantageously a sensible TES medium, for example gravel, but can alternatively or in addition comprise a latent heat medium with phase change prop- erties.

The transfer of thermal energy in the first TES medium provides a temperature gradient from Tmax tO Tmin where Tmin < Tmax . Here, Tmax is the temperature of the working fluid added at the top of the first TES container and the upper end of the first TES medium, and Tmin is the minimum temperature of the TES medium at the lower end after dis- charging and before start of charging. The temperature gradient is contained in a ther- mocline zone of the TES medium. During charging, the gradient moves towards the lower end. Depending on the charging time, the temperature Tenq at the lower end may

DK 2021 00886 A1 9 rise to a level above Tmin. This situation is equivalent to pushing the thermocline out of the TES container, which in some cases is used for controlling steepness of the gradient, which in common terminology is also termed thermocline control.

Apart from the above mentioned advantages, an even further advantage is achieved with the system as described herein. When establishing a TES system with gravel as TES medium, the gravel is typically delivered in wet conditions. The drying of the relatively large amount of gravel in large-scale systems requires substantial energy and time. Due to the fact that the air is constantly exchanged in the described system, the air transports the water vapor promptly out of the system, so that a quick and implicitly efficient dry- ing is achieved with a corresponding quick start of normal operation of the system.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawing, where

FIG. 1 illustrates a principle sketch of an energy storage system in A) charging cycle and B) discharging cycle;

FIG. 2 illustrates an example of a volume-pressure diagram for a charging period,

FIG. 3 illustrates an example of a volume-pressure diagram for a discharging period.

DETAILED DESCRIPTION / PREFERRED EMBODIMENT

The system 100 comprises an electrical motor/generator system with an electrical motor 1A and an electrical generator 1B, shaft-connected to a compressor/expander system with a compressor 2 and an expander 3, connected by a common rotational shaft 8, for example a co-functional compressor/expander unit. The electrical motor and electrical generator are optionally implemented as a single device working in dual modes period- ically as generator or motor.

Notice, however, that other functional connections, for example hydraulic or pneumatic, can be used between the motor/generator 1A/1B and the compressor/expander system 2/3.

DK 2021 00886 A1 10

The system 100 also comprises a first thermal energy storage (TES) container 5 con- taining a first gas-permeable TES medium 5’, and a second TES container 4 containing a second gas-permeable TES medium 4’. For example, the medium is gravel.

The working fluid is gaseous throughout the circuit and does not attain a liquid state.

With reference to FIG. 1A, during charging, the motor 1A drives the compressor 2 for compressing the gaseous working fluid by the compressor 2, where the gaseous working fluid is taken from the top of the second TES container 4. The temperature of the gase- ous working fluid from the second TES container increases adiabatically by the com- pression in the compressor 2, and the hot gaseous working fluid from the exit of the compressor 2 is added to the top of the inner volume of the first TES container 5 for heating the first TES medium 5°, for example gravel, inside the first TES container 5.

While the compressed gaseous working fluid flows through the first TES medium 5’ from top to bottom in the first TES container 5, it heats up the contained first TES medium 5’, first in the top and subsequently further down. During the charging, the size of the hot-temperature volume 5A of the first TES medium 5’ that has already attained the temperature of the compressed gaseous working fluid increases gradually with time, so that the heated hot-temperature volume SA expands downwards in the first TES con- tainer 5 by which the low-temperature volume 5B of the first TES medium 5’ corre- spondingly decreases.

For example, the temperature of the compressed gaseous working fluid is in the range of 400°C to 700°C, optionally 500°C to 600°C, which will be the temperature at the top of the first TES container 5 at the start of the charging. As an example, the pressure downstream of the compressor is in the range of 3-8 bar.

While the gaseous working fluid traverses the first TES container 5 it is cooled by ther- mal transfer to the first TES medium 5’ inside the first TES container 5 and leaves the bottom of the first TES container at a lower temperature, for example in the range of 50-150°C during the start of the charging period. It expands in the expander 3 to atmos- pheric pressure, which cools the gas further down before released to the environment,

DK 2021 00886 A1 11 the latter being indicated by arrow 7°. Optionally, a heat recovery system is added for extracting further thermal energy prior to releasing the air to the environment.

Air from the environment, for example at 15°C enters the bottom of the second TES container 4 and passes the second TES medium 4’ in the second TES container 4 on its way from the bottom to the top, so that it gets heated, for example to a temperature in the range of 200°C to 380°C, for example 300°C-380°C. The low-temperature volume 4B of the second TES medium 4’ increases during this process, while the high-temper- ature volume 4A in the second TES container 4 decreases correspondingly during the charging process.

The lowest pressure in the open gas flow circuit is approximately 1 atmosphere due to the exchange of air as working fluid with the environment. In practice, there are slight deviations from atmospheric pressure due to a pressure drop in the second TES con- tainer 4.

An increase of the temperature by the compressor 2 to a level in the interval of 400- 600°C has been found useful. For higher temperatures, the requirements of the materials for the containers and pipes become challenging and the equipment expensive.

Between the high-temperature volume SA and the low-temperature volume 5B in the first TES container 5, the temperature transition region SC with the temperature gradient from the high to the low temperature is called the thermocline zone. Similarly, the tran- sition region with the thermocline zone 4C between the high-temperature volume 4A and the low-temperature volume 4B of the second TES medium 4’ in the second TES container 4 is called a thermocline zone. These transition regions or thermocline zones 4C, 5C are desired narrow with a steep gradient.

In the prior art, typically, as a measure for improving the efficiency, a heat exchanger is provided at the position 6 between the first TES container 5 and the further compres- sor/expander 2/3. Such heat exchanger is typically used in the prior art for avoiding thermocline flattening and compensate for efficiency losses when using the system in practice. However, due to the fact that the system is open to air, which to a large extent is equivalent to an infinitely large buffer container at fixed temperature, such heat

DK 2021 00886 A1 12 exchanger at the position 6 can be avoided. This fact is mentioned here as a further example of simplification relatively to similar prior art systems.

The charging process is done when surplus electricity is available in the electricity sys- tem, for example from a solar power plant or wind turbines or from a more conventional electricity production plant using fossil fuel. The electricity drives the motor 1 for the charging process.

The pressure in the first TES container 5 and in the pipe system above the compressor 2 and the expander 3 is higher than the atmospheric pressure in the second TES con- tainer 4. Accordingly, the region of the thermodynamic cycle above the compressor/ex- pander is a high pressure region, and the region of the thermodynamic cycle below the compressor/expander is a low pressure region, especially at atmospheric pressure. The section between the tops of the TES containers has a temperature higher than the section between the bottoms of the TES containers, why the section between the tops of the

TES containers is called a high temperature section of the thermodynamic cycle, and the section between the bottoms of the TES containers is called a low temperature sec- tion of the thermodynamic cycle.

Once, the charging process has been finished, the energy is stored until a demand for electricity is present, and discharging starts.

During discharging, air from the environment is compressed in a further compressor 2’ and is added to the bottom of the first TES container 5 where it is further heated up by the first TES medium 5' during its flow from the bottom to the top of the first TES container 5. The hot air from the first TES container 5A is leaving the container 5 at the top and expanding in the expander 3 towards the low-pressure in the second TES con- tainer 4. The expander 3 drives the generator 1B to produce electricity, for example for giving it back to the electricity grid for general consumption. The expansion of the hot air in the expander 3 leads to a decrease of temperature of the air. The cooled air is then supplied to the top of the second TES container 4 in which it is further cooled by thermal transfer to the second TES medium 4’ on its way to the bottom. The cold air leaves the second TES container 4 at the bottom into the environment, which is indicated by arrow 7.

DK 2021 00886 A1 13

The compressor 2 and the further compressor 2’ are optionally identical devices or even the same device, optionally working bi-directional. Similarly for the expander 3 and the further expander 3’.

An example of a charging cycle, which is to be read counter-clockwise, is illustrated in a PV diagram in FIG. 2. Starting in the left lowermost corner, the air taken into the bottom of the second TES container 4 at the temperature of the environment, which for generality is taken to be 15°C. The air is flowing through the second TES medium 4 from bottom to top and heated by the second, colder TES medium 4’ to 340°C, which is the temperature at the upper end of the second TES medium 4’ in the second TES container 4. At this temperature of the upper end of the second TES container 4, the air is entering the compressor 2 and compressed from a pressure of the environment at 1 bar (1 bar = 100 kPa) to 2.7 bar, which raises the temperature to 540°C. At this temper- ature, the air as a working gas enters the top of the first TES container 5 and transfers thermal energy to the first TES medium 5’, which is cooling the air to 150°C. The ex- pansion of the air back to 1 bar through the further expander 3” reduces the temperature of the air to 56°C. The driving of the further expander 3’ by the expanding air is assisting the motor 1A in driving the compressor 2 for the charging. The air is then released from the downstream side of the expander to the environment.

An example of a discharging cycle, which is to be read clockwise, is illustrated in FIG. 3. air at the temperature of the environment, which for generality is taken to be 15°C is taken in by the further compressor 2° and compressed to 3.2 bar, which is higher than the pressure of 2.7 bar during charging, which is due to optimization of efficiency in that exemplified embodiment. Due to the compression, the air attains the value of 150°C, which is the temperature at the bottom entrance of the first TES medium 5' in the first TES container 5. By traversing the first TES medium 5' from the bottom to the top, the air is heated to 540°C, which is the temperature at the top of the first TES medium 5°. The air at this temperature of the warmest part of the first TES medium 5’ is entering the expander 3 for driving the generator 1B. The expander 3 is also driving the further compressor 2°. Accordingly, the recovered energy by the expander 2 is split between driving the further compressor 2° and the generator 1B. The expansion in the expander 2 reduces the temperature from 540°C to 340°C and reduces the pressure

DK 2021 00886 A1 14 largely to environmental pressure of 1 bar, or slightly above determined by some flow resistance downstream of the expander 2. The gas at that temperature is fed into the second TES medium 4’ in the second TES container 4 for transferring heat to the second

TES medium 4°. After the transfer of the thermal energy, the air is released to the envi- ronment.

The efficiency of such system is lower as compared to a system having elevated pres- sure in the low temperature section of the cycle between the expander/compressor and the further expander/compressor including the second TES container. However, the sys- tem is less complex and can be established at lower costs. Balancing cost and efficiency, the open, environment-connected system has been found as a useful alternative in those situations where costs for establishing and maintenance and compact size of the system are prioritized over maximum efficiency.

Claims

DK 2021 00886 A1 15 CLAIMS

1. A method of operating a thermal energy storage, TES, system, wherein the TES system (100) comprises - a thermodynamic gas flow circuit containing air as a working fluid for transport of thermal energy through the TES system, wherein the air is not in liquid phase but main- tained in gas phase throughout the gas flow circuit; - a first TES container (5) containing a first TES medium (57) for storing thermal energy, wherein the first TES medium (57) is in thermal connection with the gas flow circuit along a first gas flow path for exchange of thermal energy with the working fluid, wherein the first gas flow path is part of the gas flow circuit; - an energy converter for converting between electrical energy and thermal energy of the working fluid in the gas flow circuit; the energy converter comprising an electrical motor (1A), an electrical generator (1B), and a compressor/expander system (2, 3, 2’, 37), the compressor/expander system comprising a compressor (2) and an expander (3), wherein the compressor (2) is functionally connected to the motor (1A) for being driven by the motor (1A) during a charging period, and the expander (3) is functionally con- nected to the generator (1B) for driving the generator (1B) during a discharging period, wherein the method comprises during a charging period, - receiving the working fluid at a first pressure by the compressor (2) and driving the compressor (2) by the motor (1A) and compressing the working fluid by the compressor (2) from the first pressure to a second, higher, pressure and raising the temperature of the working fluid by the compression, - providing a flow of the compressed working fluid at the second pressure through the first gas flow path and transferring thermal energy from the working fluid to the first TES medium (57); - downstream of the first TES container (5) depressurizing the working fluid from the second to the first pressure: wherein the method comprises during a discharging period, - providing a reverse flow of working fluid through the first gas flow path and transfer- ring thermal energy from the first TES medium (57) to the working fluid to heat the working fluid;

DK 2021 00886 A1 16 - downstream of the first TES container (5) depressurizing the working fluid to the first pressure through the expander (3) and by the expansion driving the electrical generator (1B) by the expander (3); characterized in that the gas flow circuit is an open circuit connected to the environ- ment for exchange of air with the environment and that the first pressure is atmospheric pressure and the method comprises: - during the charging period, receiving (7) air from the environment at atmospheric pressure by the compressor (2) and by the compressor (2) providing the air as pressur- ized working fluid and releasing (7°) the working fluid to the environment downstream of the first TES container (5) after the depressurization; - during the discharging period, receiving air at atmospheric pressure from the environ- ment and compressing the air by the compressor (2) or by a further compressor (2°) and providing the reverse flow with the thereby compressed working fluid in reverse through the first TES medium (57) and releasing the working fluid to the environment after expansion through the expander (3).

2. Method according to claim 1, wherein the TES system comprises a second TES con- tainer (4) containing a second TES medium (4’) for storing thermal energy and, wherein the second TES medium (4°) is in thermal connection with the gas flow circuit along a second gas flow path for exchange of thermal energy with the working fluid, wherein the second gas flow path is part of the gas flow circuit, wherein the first and second gas flow paths are on opposite sides of the compressor/expander system; wherein the method comprises - during a charging period, receiving the air from the environment through the second gas flow path and transferring thermal energy from the second TES medium (4’) to the received air for preheating the air prior to compression by the compressor (2); - during a discharging period, releasing the working fluid from the expander (3) to the environment only after reverse flow through the second TES container (4) and after transfer of thermal energy from the working fluid to the second TES medium (4°).

3. Method according to any claim 2, wherein the second TES container (4) has a top and a bottom and the second TES medium (4’) therein between, wherein the method comprises flow-connecting the top of the second TES container (4) to an upstream side

DK 2021 00886 A1 17 of the compressor (2) in the charging period and to a downstream side of the expander (3) in the discharging period.

4. Method according to any preceding claim, wherein the first TES container (5) has a top and a bottom and the first TES medium (57) therein between, wherein the method comprises flow-connecting the top of the first TES container (5) to a downstream side of the compressor (2) in the charging period and to an upstream side of the expander (3) in the discharging period.

5. Method according to any preceding claim, wherein the method comprises depressur- izing the working fluid into the environment in the charging phase through the expander (3) or through a further expander (37), which is functionally coupled, for example me- chanically coupled by a coupling (8), to the compressor (2) for adding driving force to the compressor (2).

6. Method according to any preceding claim, wherein the method comprises, - during charging, raising the temperature of the air by the compressor (2) to a temper- ature in the range of 400°C to 700°C, optionally in the range of 500°C to 600°C, - during discharging, decreasing the temperature of the working fluid by the expander (3) to a temperature in the range of 200°C to 400°C.

7. A thermal energy storage, TES, system, wherein the TES system (100) comprises - a thermodynamic gas flow circuit containing air as a working fluid for transport of thermal energy through the TES system, wherein the air is not in liquid phase but main- tained in gas phase throughout the gas flow circuit; - a first TES container (5) containing a first TES medium (5) for storing thermal energy, wherein the first TES medium (57) is in thermal connection with the gas flow circuit along a first gas flow path for exchange of thermal energy with the working fluid, wherein the first gas flow path is part of the gas flow circuit; - an energy converter for converting between electrical energy and thermal energy of the working fluid in the gas flow circuit; the energy converter comprising an electrical motor (1A), an electrical generator (1B), and a compressor/expander system (2, 3, 2’, 37), the compressor/expander system comprising a compressor (2) and an expander (3), wherein the compressor (2) is functionally connected to the motor (1A) for being driven

DK 2021 00886 A1 18 by the motor (1A) during a charging period, and the expander (3) is functionally con- nected to the generator (1B) for driving the generator (1B) during a discharging period, wherein the system during a charging period is configured for:

- receiving the working fluid at a first pressure by the compressor (2) and driving the compressor (2) by the motor (1A) and compressing the working fluid by the compressor

(2) from the first pressure to a second, higher, pressure and raising the temperature of the working fluid by the compression,

- providing a flow of the compressed working fluid at the second pressure through the first gas flow path and transferring thermal energy from the working fluid to the first

TES medium (57);

- downstream of the first TES container (5) depressurizing the working fluid from the second to the first pressure:

wherein the system during a discharging period is configured for: - providing a reverse flow of working fluid through the first gas flow path and transfer-

ring thermal energy from the first TES medium (57) to the working fluid to heat the working fluid;

- downstream of the first TES container (5) depressurizing the working fluid to the first pressure through the expander (3) and by the expansion driving the electrical generator (1B) by the expander (3);

characterized in that the gas flow circuit is an open circuit connected to the environ- ment for exchange of air with the environment and that the first pressure is atmospheric pressure and wherein the system is configured for:

- during the charging period, receiving (7) air from the environment at atmospheric pressure by the compressor (2) and by the compressor (2) providing the air as pressur-

ized working fluid and releasing (7') the working fluid to the environment downstream of the first TES container (5) after the depressurization;

- during the discharging period, receiving air at atmospheric pressure from the environ- ment and compressing the air by the compressor (2) or by a further compressor (2°) and providing the reverse flow with the thereby compressed working fluid in reverse through the first TES medium (57) and releasing the working fluid to the environment after expansion through the expander (3).

DK 2021 00886 A1 19

8. A system according to claim 7, wherein the TES system comprises a second TES container (4) containing a second TES medium (4) for storing thermal energy and, wherein the second TES medium (47) is in thermal connection with the gas flow circuit along a second gas flow path for exchange of thermal energy with the working fluid, , wherein the first and second gas flow paths are on opposite sides of the compressor/ex- pander system; wherein the system is configured for: - during a charging period, receiving the air from the environment through the second gas flow path and transferring thermal energy from the second TES medium (4’) to the received air for preheating the air prior to compression by the compressor (2); - during a discharging period, releasing the working fluid from the expander (3) to the environment only after reverse flow through the second TES container (4) and after transfer of thermal energy from the working fluid to the second TES medium (4°).

9. A system according to claim 8, wherein the second TES container (4) has a top and a bottom and the second TES medium (4”) therein between, wherein the top of the sec- ond TES container (4) is flow-connected to an upstream side of the compressor (2) in the charging period and to a downstream side of the expander (3) in the discharging period; and wherein the first TES container (5) has a top and a bottom and the first TES medium (5°) therein between, wherein the top of the first TES container (5) is flow- connected to a downstream side of the compressor (2) in the charging period and to an upstream side of the expander (3) in the discharging period.

10. A system according to anyone of the claims 7-9, wherein the TES medium is gravel.