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 method as described in the preamble of the independent claim.

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 interconnected 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 between 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 degradation is an effect of the temperature transition zone, also called thermocline zone or thermocline region, becoming wider. Thermocline degradation is not wanted because it decreases the overall efficiency of the system.

The system of US 8,826,664 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 storage 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 establishing 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 proposed for steepening the gradient and reducing the width of the thermocline zone.

W02018/073049 mentions various thermocline control concepts, of which one is to push the thermocline out of the storage container or, in other words, extract the thermocline. 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.

Thermoclines are also discussed in W02015/011438A1 and WO2017/055447A1. 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.

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 released 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.

Use of air as working fluid and exchange of it with the environment is disclosed in US2018/142577A1, US2019/277196A1, US2014/0352295 and US2015/0059342.

US2014/0352295 and US2015/0059342 disclose a thermal energy storage system with compressors and expanders as well as three reservoirs, which are used as two heat accumulators and one cold accumulator. During charging of the system, air from the environment is preheated in a first of the two heat accumulators and then compressed to increase the temperature for charging the second heat accumulator. After decompression, the air is cooling the cold reservoir to keep it at sub-zero temperatures. In the discharging phase, the air from the environment is used to increase the sub-zero temperature to a higher sub-zero temperature in the cold reservoir and is after compression heated further by the second heat accumulator, then expanded, and then used to re-heat the second heat accumulator. The system in US2014/0352295 and US2015/0059342 has a number of disadvantages. First of all, the use of three thermal accumulators makes the system complex. Furthermore, because a portion of the cold accumulator is kept at sub-zero temperatures at all times, and air is taken in from the environment through the cold accumulator, icing is caused inside the cold accumulator due to the humidity in the air. Icing decreases the efficacy and may even stop the flow through the cold accumulator, which is a great disadvantage. In general, the system in US2014/0352295 and US2015/0059342 has some features which are not clear and therefore cause doubt as to the functionality. For example, FIG.3 of US2014/0352295 and US2015/0059342 discloses a discharging period where the air as working fluid has a temperature of 100°C when entering the second heat accumulator (with reference number 14) prior to the working fluid being warmed up to 500°C inside the second heat accumulator. This limits the temperature in the warm reservoir to a minimum of 100°C and most of the second heat accumulator having a temperature above this minimum temperature of 100°C. As illustrated in FIG. 2 of US2014/0352295 and US2015/0059342, when charging, the working fluid that enters the second heat accumulator has a temperature of 547°C and is then in FIG. 2 illustrated to have been cooled down to 20°C at the exit of the second heat accumulator. However, as just explained, the temperature of the second heat accumulator is above 100°C, this is not possible. It would be desirable to provide a properly functioning system.

Accordingly, there is still a need for improvement with respect to optimization of thermal 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 storage (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 system and method as described below and in the claims.

The system provided herein has a high degree of efficiency, despite being relatively simple in the construction. 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.

The TES system comprises a thermodynamic gas flow circuit containing air as a working 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.

Thermal energy is transferred between a first and a second TES medium during charging and discharging by using gaseous air as working fluid for the thermal energy transport between the TES media. 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-temperature end of the second TES media. As a result, the working fluid at the entrance of the cycle at the cold end of the second TES medium has the temperature of the surroundings.

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 efficiency implies keeping the gradient steep and the thermocline region narrow for increased conversion efficiency. Profitability of a TES system includes a balancing between, 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.

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 connected through pipes and potential heat exchangers. Although, the principle of the described 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 converter, the converter converts the electrical energy to thermal energy in the form of added heat in the working fluid. The compressor/expander system comprises at least one compressor and one expander. A compressor is functionally connected to the motor for being driven by the motor during a charging period. In turn, an 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 compressors and expanders. Herein, the term compressor and expander is used for simplicity, and also covers multistage compressors and multi-stage expanders, respectively.

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

During a charging period, the compressor receives air from the environment at atmospheric pressure as a working fluid and pressurizes it from a first pressure, which is atmospheric 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 pressurized 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 container 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 mechanically coupled by a coupling to the compressor for adding driving force to the compressor.

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 mechanically coupled and driven by the expander.

As an offset, the invention is explained in the following wherein the compressor/ex- pander system comprises a compressor, a further compressor, an expander, and a further expander. These will be used in the following when explaining the system, where the compressor and expander are used in the charging period and the further expander and further compressor in the discharging period. However, as mentioned already, this does not necessarily imply that there are four separate machines. For example, at least two of them are provided as an apparatus with multi -function, for example bi-directional compressor-expander function. Alternatively, a compressor is used both for charging and discharging and an expander is used both for charging and discharging, and the flow connections between the compressor/expander system and the TES containers are changed when changing between charging and discharging periods.

For example, the method comprises at least one of the following:

- providing the compressor and the further expander as a single bi-directional functioning device and changing from compressor function during charging to expander function during discharging; in this case, the pipe connections to the TES containers need not be changed;

- providing the expander and the further compressor as a single bi-directional functioning device and changing from expander function during charging to compressor function during discharging; in this case, the pipe connections to the TES containers need not be changed;

- providing the compressor and the further compressor as a single compressor device and changing the upstream connection of the single compressor device between an upstream connection to the second flow path during charging and an upstream connection to the environment during discharging; in this case, the downstream connection is correspondingly changed between opposite ends of the first flow path;

- providing the expander and the further expander as a single expander device and changing the downstream connection of the single expander device between a downstream connection to the environment during charging and a downstream connection to the second flow path when discharging; in this case, the upstream connection is correspondingly changed between opposite ends of the first flow path.

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 downstream side of the compressor in the charging period and to an upstream side of the further expander in the discharging period.

As mentioned, the TES system comprises a second TES container 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 further expander during discharging.

During a charging period, air is received from the environment through the second gas flow path by suction of the compressor, and thermal energy is transferred from the second TES medium to the received air for preheating the air prior to compression by the compressor. It is noted that the second TES medium is heated during the discharging period by transfer of thermal energy from the working fluid to the second TES medium after driving the further 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 further expander in the discharging period and to an upstream side of the compressor in the charging period.

Releasing the working fluid from the downstream side of the expander to the environment during the charging period at a temperature achieved by the depressurization is an advantage.

In particular, in comparison to the aforementioned system US2014/0352295 and US2015/0059342, it is pointed out that the air is released directly from the expander, without flowing through a cold accumulator prior to release to the environment. This simplifies the system.

Advantageously, the air from the environment is received by the further compressor during the discharging period at the temperature of the environment.

In particular, in comparison to the aforementioned system US2014/0352295 and US2015/0059342, it is pointed out that the air enters the second TES container directly during the charging phase, without pre-cooling, which has an advantage in that icing in the second TES container is prevented. Despite higher simplicity, the system has a high efficiency.

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 450°C, for example in the range of 200°C to 400°C or 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 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, and correspondingly further compressor and further 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-adi- abatic 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 properties.

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 discharging and before start of charging. The temperature gradient is contained in a thermocline zone of the TES medium. During charging, the thermocline zone with the gradient moves towards the lower end. Depending on the charging time, the temperature Tend at the lower end may 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 drying is achieved with a corresponding quick start of normal operation of the system.

ASPECTS

In the following, a number of interrelated aspects are summarised, which can be combined with the features above.

ASPECT 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 maintained 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 (5’) 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 (IB), and a compressor/expander system (2, 3, 2’, 3’), the compressor/expander system comprising a compressor (2) and an expander (3), wherein the compressor (2) is functionally connected to the motor (1 A) for being driven by the motor (1A) during a charging period, and the expander (3) is functionally connected to the generator (IB) for driving the generator (IB) 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 (1 A) 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 (5’);

- 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 transferring thermal energy from the first TES medium (5’) 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 (IB) by the expander (3); wherein the gas flow circuit is an open circuit connected to the environment 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 pressurized 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 environment 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 (5’) and releasing the working fluid to the environment after expansion through the expander (3).

ASPECT 2. Method according to aspect 1, 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 (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’). ASPECT 3. Method according to any aspect 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 of the compressor (2) in the charging period and to a downstream side of the expander (3) in the discharging period.

ASPECT 4. Method according to any preceding aspect, wherein the first TES container (5) has a top and a bottom and the first TES medium (5’) 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.

ASPECT 5. Method according to any preceding aspect, wherein the method comprises depressurizing the working fluid into the environment in the charging phase through the expander (3) or through a further expander (3’), which is functionally coupled, for example mechanically coupled by a coupling (8), to the compressor (2) for adding driving force to the compressor (2).

ASPECT 6. Method according to any preceding aspect, wherein the method comprises,

- during charging, raising the temperature of the air by the compressor (2) to a temperature 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.

ASPECT 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 maintained 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 (5’) 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 (IB), and a compressor/expander system (2, 3, 2’, 3’), the compressor/expander system comprising a compressor (2) and an expander (3), wherein the compressor (2) is functionally connected to the motor (1 A) for being driven by the motor (1A) during a charging period, and the expander (3) is functionally connected to the generator (IB) for driving the generator (IB) 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 (1 A) 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 (5’);

- 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 transferring thermal energy from the first TES medium (5’) 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 (IB) by the expander (3); wherein the gas flow circuit is an open circuit connected to the environment 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 pressurized 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 environment 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 (5’) and releasing the working fluid to the environment after expansion through the expander (3).

ASPECT 8. A system according to aspect 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 (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 first and second gas flow paths are on opposite sides of the com- pressor/expander 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’).

ASPECT 9. A system according to aspect 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 second 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.

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

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 IB, 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 1 A and electrical generator IB are optionally implemented as a single device working in dual modes periodically 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.

The system 100 also comprises a first thermal energy storage (TES) container 5 containing 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. 1 A, during charging, the motor 1 A 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 gaseous working fluid from the second TES container 4 increases adiabatically by the compression 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 5 A 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 5 A expands downwards in the first TES container 5 by which the low-temperature volume 5B of the first TES medium 5’ correspondingly 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 become 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 thermal transfer to the first TES medium 5’ inside the first TES container 5 and leaves the bottom of the first TES container 5 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 atmospheric pressure, which expansion cools the gas further down before released to the environment, 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, although this is not necessary.

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 during charging, so that it gets pre-heated, for example to a temperature in the range of 200°C to 380°C, for example 300°C-380°C, prior to further becoming hotter by the compression in the compressor 2. The low-temperature volume 4B of the second TES medium 4’ increases during this process, while the high- temperature 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 container 4. However, as these pressure drops are relatively low and are negligible in relation to the pressure increase by the compressor 3, these are dealt with herein as negligible. For case of simplicity, and it is assumed that the pressure at the exit of the second TES container 4 is atmospheric pressure, although a slightly lower pressure that atmospheric pressure may be measured due to the suction by the compressor 2.

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 5A and the low-temperature volume 5B in the first TES container 5, the temperature transition region 5C with the temperature gradient from the high to the low temperature is called the thermocline zone. Similarly, the transition 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 compressor/ex- pander 273. 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 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 system, 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 A 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 container 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 section 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, as illustrated in FIG. IB, 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 5 is leaving the container 5 at the top and expanding in a further expander 3’ towards the low-pressure in the second TES container 4. The further expander 3’ drives the generator IB to produce electricity, for example for giving it back to the electricity grid for general consumption. The expansion of the hot air in the further 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'.

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’. Flow connections in the circuit would have to be swapped in dependence of the desired flow. Similarly, it is possible that compressor 2 and the further expander 3’ are combined in a single device, for example working bidirectional. Similarly for the expander 3 and the further compressor 2’. Functional combinations were explained above.

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 comer, 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. This is indicated by arrow 7. 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 temperature, 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 expansion of the air back to 1 bar through the expander 3 reduces the temperature of the air to 56°C. The driving of the expander 3 by the expanding air is assisting the motor 1 A in driving the compressor 2 for the charging. The air is then released from the downstream side of the expander 3 to the environment, indicated by arrow 7’.

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 further expander 3’ for driving the generator IB. The further expander 3’ is also driving the further compressor 2’. Accordingly, the recovered energy by the further expander 3’ is split between driving the further compressor 2’ and the generator IB. The expansion in the further expander 3’ reduces the temperature from 540°C to 340°C and reduces the pressure largely to environmental pressure of 1 bar, or slightly above determined by some flow resistance downstream of the further expander 3’. 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 environment. The efficiency of such system is lower as compared to a system having elevated pressure in the low temperature section of the cycle including the second TES container. However, the system 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.

In particular in comparison to the aforementioned system US2014/0352295 and US2015/0059342, it is pointed out that the system as just described has advantageously for simplicity only two TES containers and not three. Furthermore, the air that enters the second TES container directly during the charging phase, without pre-cooling, has an advantage in that icing in the second TES container is prevented. Despite higher simplicity, the system has a high efficiency.