EP4244470A1

EP4244470A1 - Thermal energy store for storing electrical energy

Info

Publication number: EP4244470A1
Application number: EP21814718.9A
Authority: EP
Inventors: Aiko BERNEHED
Original assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Current assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Priority date: 2020-11-12
Filing date: 2021-11-11
Publication date: 2023-09-20
Also published as: WO2022101348A1; DE102020129902A1

Abstract

The present invention relates to a method for storing electrical energy by means of thermal energy stores and to an apparatus for carrying out said method.

Description

Thermal energy storage for storing electrical energy

The present invention relates to a method for storing electrical energy using thermal energy storage devices and a device for carrying out the method.

In the context of climate protection, energy supply is increasingly becoming the focus of research. Renewable energies, such as wind energy or solar energy, are not constantly available, but this is essential to ensure a constant supply of energy. Energy storage systems thus represent a major development in order to use energy from renewable sources as a standard energy source.

One approach that is currently being researched is so-called "Pumped Thermal Energy Storage" (PTES) systems. A mechanically or electrically operated heat pump is used to extract heat from a cold reservoir and feed it into a hot reservoir. This cold and heat can be converted into corresponding thermal storage and used at a later point in time to generate mechanical/electrical energy again.The special thing about PTES systems is that they can theoretically achieve electrical storage efficiencies of up to 100%, although both the charging and discharging cycle follow the thermal Carnot - Subject to limit.

The working principle of a PTES system works in the Brayton cycle as follows: The first step is loading:

1. Gas (working medium) at ambient pressure and temperature is isentropically compressed in a compressor. It uses external (electrical) energy to push the gas to high pressure and temperature.

2. The thermal energy of the hot gas is stored in a hot storage tank, where the gas cools down isobaric.

3. The cold, pressurized gas is expanded isentropically via a turbine, whereby energy is extracted from the gas. The kinetic energy gained is less than is required in the compressor (under 1.), so that net energy remains in the storage. The gas is now pressureless (ambient pressure) at low temperatures. Theoretically, temperatures of -100°C and below are also possible.

4. The gas is heated isobaric in a cold storage tank so that thermal energy is extracted from it. The memory cools down. The "cold" is clearly stored. The gas is now again at ambient pressure and temperature, so that you can start again at 1.

A loaded, hot storage tank and a loaded, cold storage tank are now available, whereby a heat exchanger can be used to cover the temperatures between steps 2 and 3 or 4 and 1. For discharging, the cycle is reversed:

1. Unpressurized gas at ambient temperature is fed into the cold store and gives off heat to it isobaric, with the store warming up and the gas cooling down.

2. The cold gas is fed to a compressor and is thus pressurized isentropically, which also causes the temperature to rise. For this purpose (electrical) energy is supplied from the outside. 3. The (slightly) warm, pressurized gas is heated isobaric in the hot storage tank, thereby extracting heat from the hot storage tank. This cools down.

4. The hot, pressurized gas is isentropically expanded in a turbine, giving off energy in the form of work. The total work gained is greater than the work expended in the compressor (2.), so that the stores are discharged net in order to make electrical/mechanical work available.

If all heat exchangers and perfect flow machines (turbines, compressors) are precisely matched, up to 100% efficiency (current-to-current) can be achieved. In reality, irreversible processes take place, so that additional waste heat has to be dissipated and further heat exchangers have to be used in order to bring the temperatures in the circuit to the same level. The principle of the PTES systems is, for example, by T. Desrues et al. (A thermal energy storage process for large scale electric applications, Applied Thermal Engineering 30 (2010) 425-423) and A. White et al. {Thermodynamic analysis of pumped thermal electricity storage, Applied Thermal Engineering 53 (2013) 291-298).

Robert B. Laughlin Pumped thermal grid storage with heat exchange, Journal of Renewable and Sustainable Energy 9, 044103 (2017))) has developed a PTES energy storage system using argon, nitrogen or air as the working medium. Molten salt (solar salt: 60% NaNCh - 40% KNO3) and liquid hexane are used as hot and cold storage. The energy transfer to the storage takes place with the help of heat exchangers. The system has a total of four stores, a cold and a hot salt store, and a warm and a cold hexane store. The process corresponds to the Brayton cycle already described. Furthermore, Laughlin uses an internal gas-gas heat exchanger in his system, which is characterized by a small temperature difference between the two sides characterized at the same time by a very high amount of heat transferred. This is technically very difficult to implement and therefore calls into question the economics of the process described.

From DLR. a system called "Compressed Heat Energy Storage" (CHEST) (Steinmann et al: The CHEST (Compressed Heat Energy Storage) concept for facility scale thermo mechanical energy storage; Energy 69 (2014) 543-552; doi : 10.1016/j. energy.2014.03.049) that differs fundamentally from those previously described: a. For loading, energy is extracted from the environment to boil water. The resulting steam is then compressed several times in a multi-step process and thus brought to pressure and temperature. b. The hot accumulator is not a uniform system but consists of sensitive and latent heat accumulators, which are required to bring the temperatures during the cooling of the vapor to the same level. c. A conventional Rankine water-steam cycle is used for discharging. Heat is extracted from the storage tanks and the steam is then condensed using the ambient temperatures.

The most serious difference in the CHEST system lies in the use of a Rankine cycle instead of a Brayton cycle. What is particularly interesting is that the environment can be used as cold storage, so that there are no costs for additional tanks and storage media. The CHEST system requires a six-stage compressor with five-fold intercooling, as well as an industrial-scale latent heat storage at 300-350 °C, according to the previously cited publication. Both components question the economy of the system. In addition, the system requires an ammonia cycle to boil water at 100 °C using the ambient heat. This also poses extreme challenges for machines. There is therefore still a need for a system and method with which heat can be stored effectively. The process should be easy to carry out and contain as few irreversible processes as possible which would reduce the efficiency of the process.

Surprisingly, it has been shown that the use of a Clausius-Rankine cycle makes it possible to store electrical energy. In a first embodiment, the object on which the present invention is based is therefore achieved by a method according to claim 1 and a device for carrying out the method according to claim 8. Preferred embodiments are described in the dependent claims.

The method according to the invention is a method for storing and releasing electrical energy and comprises the following steps:

(i) in a loading cycle a) a liquid working medium at a temperature TAI and a

Pressure PAI provided, b) then heat is transferred from at least one cold storage material to the working medium by means of at least one first heat exchanger, whereby the temperature of the working medium rises from TAI to a temperature TA2, whereby the working medium becomes gaseous and has the pressure pA2, c) by means mechanical work from electrical energy, the gaseous working medium is heated from the temperature TA2 to a temperature TAS and the pressure is increased from a pressure pA2 to a pressure pA3, d) the heat obtained from the mechanical work and stored in the working medium is then removed from the working medium by at least a heat exchanger on at least one Transfer hot storage material, causing the temperature of the working medium from TAS to TA4 drops, and e) the pressure of the working medium is then reduced from pA4 to PAI, so that one again obtains a liquid working medium with a temperature TAI and a pressure PAI, so that a Cycle process from steps b) to e) can be carried out; and

(ii) in a discharge cycle a) the liquid working medium at a temperature TEI and a

Pressure PEI provided, b) the working medium is compressed to a pressure pE4 and has the temperature TE4, c) heat from at least one hot storage material is transferred to the working medium by means of at least one heat exchanger, whereby the temperature of the working material increases from TE4 to TE3, d ) the then hot, pressurized working medium with the temperature

TE3 and the pressure pE3 is expanded to a temperature TE2 and a pressure pE2, whereby mechanical energy is released, and e) heat of the gaseous working medium with the temperature TE2 and the pressure pE2 is transferred to the at least one cold storage material by means of at least one heat exchanger, whereby the working medium condenses and then has a temperature TEI and a pressure PEI, so that a cyclic process from steps b) to e) can be carried out; wherein the working medium is a zeotropic mixture. In a further embodiment, the object on which the present invention is based is achieved by a device (10) for carrying out the method according to the invention, comprising: a working medium circuit (11) with a working medium; the working medium circuit (11) comprises at least one cold accumulator (12a, 12b) with at least one first heat exchanger (13); at least one hot accumulator (14a, 14b) with at least one second heat exchanger (15); a first fluid energy machine, in particular a turbomachine (16); and a second fluid energy machine, in particular a turbomachine (17), wherein in a loading cycle of the device (10) from the at least one cold store (12a, 12b) via the at least one first heat exchanger (13) thermal energy from the at least one cold store (12a, 12b ) can be supplied to the working medium for converting the working medium from a liquid into a gaseous state of aggregation, with energy being supplied via the first fluid energy machine, in particular a turbomachine (16), preferably designed as a compressor (16a), the pressure and the temperature of the working medium can be increased, wherein heat can be extracted from the working medium via the at least one second heat exchanger (15) and can be supplied to the at least one hot accumulator (14a, 14b), and wherein a second fluid energy machine, in particular a turbomachine, (17), preferably designed as a throttle (17a), for reducing the Pressure of the working medium is formed and / or being in a discharge cycle With the device (10), the working medium can be compressed via the second fluid energy machine, in particular a turbomachine, (17), preferably designed as a pump (17b), with the supply of energy, the temperature of the working medium being increased by the at least one hot accumulator (14a, 14b ) can be increased via the at least one second heat exchanger (15), the pressure of the working medium being able to be relieved via the first fluid energy machine, in particular a turbomachine (16), preferably designed as a turbine (16b), releasing mechanical energy, and heat energy being transferred from the working medium via the at least a first heat exchanger (13) can be discharged to the at least one cold accumulator (12a, 12b) in order to convert the gaseous state of aggregation of the working medium into the liquid state of aggregation.

Preferably, the device (10) for carrying out the method according to the invention can further comprise at least one third heat exchanger, wherein the at least one third heat exchanger is used to transfer heat from the area between the hot accumulator (14a, 14b) and the second turbine (16) to the area between the Cold storage (12a, 12b) and the first turbine (M) is arranged.

The first fluid energy machine, in particular a turbomachine, (16) can preferably be connected to a motor (M) in the loading cycle, via which the fluid energy machine, in particular a turbomachine, (16), which can be designed as a compressor (16a), can be driven. Provision can also be made according to the invention to connect the first fluid energy machine, in particular a turbomachine, (16) to a generator (G) in the discharge cycle, in which case the first fluid energy machine, in particular a turbomachine, (16) can be designed as a turbine (16b) whose mechanical energy is the generator (G) can be converted into electrical energy.

Both the first and the second fluid energy machine can each preferably be designed as a turbomachine or as a reciprocating piston machine.

1 schematically shows a device according to the invention for carrying out the loading cycle. A preferred embodiment is shown in FIG.

2 schematically shows a device according to the invention for carrying out the discharge cycle. A preferred embodiment is shown in FIG. The black lines represent conduits, respectively, with the arrows representing the direction of flow inside the conduits.

The method according to the invention and the device according to the invention are explained in more detail below.

The method according to the invention enables electrical energy to be stored in the charging cycle. This energy is first transferred to the working medium in step (i) c). From there, the energy is stored in a hot store (step (i) d)). In the loading cycle, the working medium is circulated. This can be done as long as hot storage material is available to store the electrical energy. The memory is then full.

If the memory is full, the cycle can be reversed. The working medium now absorbs the heat stored in the hot accumulator (step (ii) c)) and can transport it to a desired location within a device in which the method according to the invention takes place, in which the heat is released and this release of heat is used to obtain of mechanical energy is used (step (ii) d)). This can be carried out until all the heat stored in the storage heater has been used. The loading cycle can then be carried out again.

The method according to the invention thus enables energy to be stored. This can be used, for example, to ensure that regenerative energies are effectively stored and energy supply can be ensured even at times when they are not available (e.g. at night with solar energy, with no wind with wind energy).

Zeotropic mixtures are binary mixtures, i.e. they consist of two different pure substances. A mixture of chemical substances is called a zeotrope denoted when the composition of liquid and vapor is always different in vapor-liquid equilibrium. This means that the dew curve and the boiling curve do not touch at any point. Mixtures whose dew and boiling curves touch at least one point and therefore have the same composition in vapor and liquid are called azeotropic mixtures.

In the process according to the invention, zeotropic mixtures are particularly well suited as the working medium, since they have one in the once-through evaporation in a heat exchanger in the loading mode (steps (i) b) and c)) and the condensation in the discharge mode (steps (ii) d) and e)). exhibit temperature slip. The saturated vapor phase (working medium at temperature and pressure TE3, PES or TA3, PA3) has a higher temperature than the boiling liquid (working medium at temperature and pressure TE2, PE2 or TA2, PA2), so that the evaporation and condensation temperatures in Loading and unloading cycle of the storage system can be better adapted to the temperature profile of the cold storage.

If one compares the boiling curves (entropy vs. temperature) of pure substances in comparison to zeotropic mixtures, it can be seen that a small temperature difference between the charging and discharging cycle and a high temperature spread of the cold storage system can be achieved with a zeotropic mixture. This achieves a high level of system efficiency and a smaller size for the cold storage tank at the same time.

A zeotropic mixture is a combination of a primary substance and a secondary substance. According to the invention, the primary and secondary substances can be selected from different substances, such as organic or inorganic solvents. In particular, they are selected as shown in Table 1 below:

Table 1 :

The primary substance is preferably ammonia or CO2, in each case in combination with the aforementioned secondary substances. Ammonia and CO2 have high decomposition temperatures, allowing the working fluid to be at high temperature, allowing for high storage temperatures. High storage temperatures are necessary for a large temperature rise and the associated low costs. Furthermore, these substances react little or not at all with other substances and the metals required for the device for carrying out the method.

Methanol, ethanol and water are preferred as secondary materials. These also have high decomposition temperatures with the associated advantages.

A particularly preferred working medium comprises a mixture of ammonia and water.

FIG. 7 shows an example of how a cyclic process for the method according to the invention could work using a mixture of a primary material and a secondary material, in particular an ammonia-water mixture, as the working medium. The loading cycle, here in solid lines (1-2-3-4-5-6-1), is run through to the left, the unloading cycle in dashed lines (AB-4-3-CDA) to the right. Clearly, the overall efficiency of the process (current-to-current) is highest when the charging cycle and the discharging cycle are as close as possible to each other. In addition, the total area within the curves must be maximized in order to minimize the influence of irreversible thermodynamic processes in machines and heat exchangers. This goal can be achieved with a number of measures.

The "spread" of the hot and cold side of the system, i.e. the thermodynamic mean temperatures of the two sides, should diverge as far as possible. This speaks for the highest possible pressure difference, since the boiling and condensation temperatures of a given mixture of substances can only be regulated via the pressure. The specific amount of energy to be stored, i.e. the enthalpy difference between points 2 and 5, should be as high as possible. Therefore, substances are required that have the highest possible enthalpy spread. As already stated, the thermodynamic mean temperature of the hot storage side should be as high as possible, which means , that the working media used must not decompose.This excludes most organic working media, as these have maximum application temperatures between 250 and 300 °C.

Surprisingly, it has been shown that a mixture of ammonia and water in particular has the advantageous properties mentioned, which is why a mixture of ammonia and water is particularly suitable for the present invention. They have the respectively highest and second highest critical point of all industrially applicable pure substances and thus also have the highest achievable enthalpy difference. In addition, they are stable at high temperatures and can be mixed with one another very well. Attention must be paid to any secondary reactions that may occur, in which the nitrogen in the ammonia can attack the machine steel used and can therefore lead to nitriding of the same. In order to avoid this, the temperatures can preferably be raised to 400 to 420.degree be restricted. Furthermore, a second high-temperature storage stage can preferably be introduced in order to be able to achieve the highest possible thermodynamic mean temperature. In this case, a second compression of the working medium is carried out while the heat is being released to a hot accumulator (step (i) d)), with renewed heating and pressure increase taking place and further heat being able to be released, in particular to the same hot accumulator.

Ethanol can particularly preferably be added in an amount of 0 to 10% to the mixture of water and ammonia which is preferred according to the invention. By adding small amounts of ethanol, the mixture is influenced in such a way that the phase transitions are less "curvy".

In a particularly preferred embodiment, the working medium comprises 35 to 45% by weight ammonia, 50 to 60% by weight water and 0 to 10% by weight ethanol.

The charging cycle and the discharging cycle are further described below.

In the loading cycle, the working medium is heated from a temperature TAI and a pressure PAI to a temperature TA2 and a pressure pA2 (step (i) b)). This is followed by further heating and compression of TA2, PA2 to form TAS, PAS (step (i) c)). The compressed hot working fluid gives up energy to the hot storage material so that the working fluid changes temperature and pressure from TA3, PAS TO TA4, PA4 (step (i)d)). Finally, the expansion takes place, so that the working medium changes from state TA4, PA4 back to the initial state TAI, PAI (step (i) e)). Accordingly, PAI and PA2 are roughly the same. pA3 and pA4 also differ only slightly. The pressure difference Ap between pA2 and pA3 is preferably in the range from 5 bar to 100 bar, preferably 10 bar to 90 bar, in particular 15 bar to 80 bar, particularly preferably 20 bar to 60 bar. The exact pressure difference depends on the type of working medium.

The same applies to the temperature. The exact temperatures depend on the nature of the zeotropic mixture. Under the conditions TAI, PAI AT the start of the loading cycle, the zeotropic mixture is in the form of a liquid. Under the conditions TA2, PA2, the working medium is a boiling liquid, which is present as a vapor under TAS, PAS. After the heat has been released to the at least one hot accumulator, the working medium at TA4, PA4 is in the form of a compressed liquid.

In a first step ((i) b)), the working medium is thus increased from TAI to a temperature TA2. This is done by means of at least one so-called cold store. The cold storage comprises at least one cold storage material. This at least one cold storage material gives off heat to the working medium. This can be done, for example, by means of at least one heat exchanger.

Any materials that are able to store heat can be used as the cold storage material. The heat capacity required is to be regarded as low. The cold storage material can be a sensible heat storage, a latent heat storage or another storage material. The storage material is preferably present as a fluid, so that it can easily transfer the heat to the working medium or absorb heat from the working medium. In addition, it can preferably be pumped from a first cold storage container into a second cold storage container.

For example, water or air, for example ambient air, can be used as cold storage material. If air, i.e. the environment, is used as cold storage, no separate cold storage container is required. instead, heat is released and absorbed with the ambient air, i.e. in an open system.

However, a closed system with a cold accumulator that is present in a closed system, ie at least two cold accumulator containers, is preferred. In such a system, a fluid can be used as a cold storage material. Water is preferably used as the cold storage material. The storage temperature is not limited to a temperature range of 0 °C to 100 °C, even for water. Due to the use of zeotropic mixtures and the associated use of very low and high temperatures, it is better to use a closed system. For example, individual mixtures can be cooled down to temperatures as low as -80°C and reach upper evaporation temperatures of up to 150°C. In order to be able to cover these temperatures with a two-tank system with water as the storage medium, the freezing point can be lowered using antifreeze (salt (such as NaCl or CaCh), glycol, methanol, ethanol, or a combination of these substances), and/or an increase in the boiling point through use pressurized tanks (max. 10 bar) can be used.

Before the storage system is loaded, it is preferable to remove entropy from the system that previously occurred during the discharge operation. This entropy is in the form of a warming of the hot, i.e. the discharged, side of the cold storage system (in the second cold storage container). This entropy can be dissipated with the aid of external cooling. Under certain operating conditions, it may be necessary to cool the hot side of the cold storage system below the ambient temperature with additional cooling before charging. For this purpose, in addition to the ambient cooling, an additional cold pump, for example with ammonia as a refrigerant, can be used. This is activated in loading mode to pre-cool the cold storage material before it absorbs cold from the working cycle. Electrical energy is then supplied to the system in the form of mechanical work. This is done, for example, with a motor or compressor. As a result, the working medium is heated to the temperature TAS. The pressure rises to a value PAS. The electrical power supplied to the system can come from any power source. The method according to the invention can thus be used as a buffer in a conventional power plant. However, the method according to the invention is particularly suitable for storing regenerative energies. According to the invention, regenerative energies refer to energy sources that are available in practically inexhaustible terms for a sustainable energy supply in the human time horizon or that renew themselves relatively quickly. This distinguishes them from fossil energy sources, which are finite or only regenerate over a period of millions of years. According to the invention, regenerative energies include bioenergy (biomass potential), geothermal energy, hydropower, ocean energy, solar energy (solar energy) and wind energy. The method according to the invention now makes it possible to store electrical energy, which is obtained from regenerative energies, and to receive and use it again at any point in time. The method according to the invention thus enables the effective use of regenerative energies and in particular such energies that are not constantly available, such as wind energy and solar energy. For the purposes of the present invention, the electrical energy therefore preferably comes from wind energy and/or solar energy. The electrical energy is particularly preferably obtained from solar energy, such as concentrated solar radiation or from photovoltaic cells.

According to the invention, the compression and heating step (step (i) c)) is carried out in one working step, whereas in the prior art (CHEST method) intermediate cooling is still necessary. The heated, compressed working medium then gives off heat to a hot accumulator (step (i) d) of the method according to the invention). As a result, the working medium cools down from temperature TAS to temperature TA4. The pressure pA3 remains essentially unchanged. The pressure can drop by a few mbar due to friction losses. The extent of the friction and thus the pressure loss, i.e. Ap between pA3 and pA4, depends on the precise technical implementation. The heat of the working medium is transferred, for example, by means of at least one heat exchanger to a hot storage material, as a result of which its temperature rises from temperature THI to temperature TH2.

In particular, a second compression step can take place while the heat is being released from the working medium to a hot accumulator. Such is particularly advantageous for the use of a mixture of ammonia and water as the working medium, as described above. In this case, the system works with two compressor stages in loading mode. First, after the first compressor stage, heat is given off to a hot accumulator and then the temperature and pressure are increased again in a second compressor stage and more heat is given off to the same or another hot accumulator. In order to be able to achieve even higher pressures during loading, in a further preferred embodiment almost 10% cold, liquid working medium can be extracted from the main cycle and injected behind the first hot storage stage and before the second compressor stage. As a result, the pressure in the subsequent compressor stage can be increased further than without this intermediate cooling, while at the same time maintaining the previously specified temperature limit. In particular, the previously mentioned thermodynamic mean temperature of the hot storage side can also rise above that of the process without a second compression step, so that storage efficiency is surprisingly increased. Compared to the prior art, the use of zeotropic mixtures as the working material allows the process to be carried out entirely in the subcritical state. It is currently assumed that when the working medium cools down from TAS to TA4, the working medium first undergoes cooling down to the boiling point. It then condenses to form a liquid and is then finally cooled to TA4. The phase change of the zeotropic mixture is associated with a temperature glide. Therefore, according to the invention, a sensible heat accumulator can be used as the heat accumulator material. A latent heat storage device, such as that required in the CHEST system, is not necessary in this case.

Sensitive heat accumulators work according to the principle of changing the temperature of the storage medium. When selecting the material for the heat accumulator, care must be taken to ensure that it has a sufficiently high specific heat capacity and a sufficiently high storage density. In addition, it must cover a temperature range in which the working medium is used. Accordingly, the sensible heat accumulator can be selected differently for different work materials. Salt systems, which are also used in concentrated solar systems, have proven to be suitable heat storage materials. The following are examples, whereby the melting temperature must be less than Tm:

In addition, ternary salt systems of nitrate salts (for example in DE 102014212051 A) or halogen salts (for example in WO 2017/093030 A) are also known.

The salt composition is selected in such a way that the hot storage material is continuously liquid, i.e. only changes its temperature but not its state of aggregation. The liquid salt or the liquid salt mixture can easily exchange heat with the working medium via a heat exchanger. The hot storage material is particularly preferably selected from nitrate and/or nitrite salts of alkali and/or alkaline earth metals and mixtures of these. In addition to salt compositions, thermal oil such as Therminol VP1, waxes, unpressurized or pressurized water, ceramics or fills of sand, gravel or rock can also be used.

Azeotropic mixtures have a variable heat capacity. If you only use a hot storage, this step cannot be used effectively. On a temperature-enthalpy (TH) diagram, variable heat capacity shows up as a curved line, while thermal storage materials show up as a straight line. However, at high temperature, the working medium and the heat storage material should have similar enthalpy to enable effective heat transfer. This problem can be solved by using several heat accumulators, i.e. transferring the heat from the working medium to the heat storage material via several heat exchangers. The heat storage material can be located in several different heat storage tanks, with buffer tanks being located between the two heat storage tanks. So several storage systems are connected in series. According to the invention, it is preferably provided to use not just one hot accumulator, but two or more hot accumulators that are different from one another. In this case, the first hot storage can be provided with a first hot storage material, whereas a second hot storage material is used in the second hot storage. The first hot storage material and the second hot storage material may be the same or different. Preferably, the first hot storage material and the second hot storage material are different from each other. This enables an effective temperature transfer from the working medium to the storage material, with the hot storage material being able to be adapted to the properties of the working medium under the prevailing temperature conditions. For example, the first thermal storage material may use a salt system as previously described. The second thermal storage material may also contain a salt system. However, a thermal oil, wax, unpressurized or pressurized water, ceramics or fills of sand, gravel or rock can also be used as the second heat storage material. Preferably, the second thermal storage material is a thermal oil, wax, or unpressurized or pressurized water, more preferably pressurized water.

If the method according to the invention has more than two separate hot accumulators, the first hot accumulator preferably uses a salt system as the hot accumulator material. The second and subsequent hot storage tanks are operated using either a salt system or pressurized water as the hot storage material.

After the heat has been transferred from the working medium to the hot storage material, the energy is stored in this material. The working medium is now returned to its initial state in order to be available for further loading cycles. For this purpose, the working medium is brought from the temperature TA4 and the pressure pA3 to the temperature TAI and the pressure PAI. The pressure PAI is well below the pressure PAS, SO that this process of Relaxation of the working medium can be done by means of a throttle or a liquid turbine. As a result, the state of aggregation of the working medium also changes isentropically. It condenses and becomes liquid. At the same time, the temperature drops from TA4 to TAI due to the flash evaporation of the working medium.

The working medium now has the temperature TAI and the pressure PAI again, so that the loading cycle can be run through again. The duration of the loading time depends only on the volume of the storage for the hot storage material or the cold storage material.

To enable an effective method, TA4 > TAI . The temperature difference between TA4 and TAI is preferably 0.5K or more and 25K or less. The temperature is reduced as a result of the isentropic expansion (step (i)e) of the process according to the invention. Depending on the working medium and the pressure difference between pA4 and PAI, the temperature difference AT between TA4 and TAI is in the range from 0.6 K to 20 K, in particular from 1 K to 15 K.

The pressure PAI is about atmospheric pressure. It is preferably in the range from 0.2 bar to 3 bar, in particular from 0.5 bar to 2 bar.

The temperature TAI is chosen so that the zeotropic mixture is liquid at the pressure PAI. In a preferred embodiment, TAI is in the range from -45°C to 60°C, in particular from -40°C to 40°C, preferably from -35°C to 25°C, particularly preferably from -30°C to 10° C, most preferably from -25°C to 0°C.

The temperature TA2 is greater than TAI because in step (i) b) heat is transferred from the cold storage material to the working medium. For example, TA2 can be in the range from 20 °C to 200 °C, in particular from 30 °C to 150 °C, preferably from 35°C to 100°C. In step (i) b), such an amount of heat is usually absorbed that ΔT between TAI and TA2 is from 20 K to 120 K, preferably from 25 K to 100 K, in particular from 30 K to 80 K.

The pressure pA2 essentially corresponds to the values specified for PAI. The pressures PAI and pA2 are approximately equal.

In step (i) c) of the method according to the invention, energy is transferred into the working medium. As a result, the temperature TA2 rises to the temperature TAS. The temperature TAS is below the decomposition temperature of the working medium, which is usually 500°C. Thus, TA3 is preferably 500°C or less. However, a temperature of 200 °C should be exceeded to enable effective heat transfer into the heat storage material. TA3 is thus preferably 400° C. or less, in particular 300° C. or less.

In addition to the supply of heat, the working medium is also compressed in step (i) c) of the method according to the invention, so that the pressure of the working medium increases from pA2 to pA3. The pressure pA3 is preferably in the range from 10 bar to 100 bar, in particular in the range from 15 bar to 90 bar, preferably from 20 bar to 80 bar, particularly preferably from 30 bar to 70 bar, particularly preferably from 40 bar to 60 bar.

The working medium then transfers the heat to the hot storage material in step (i) d), causing the temperature to drop from TA3 to TA4. The pressure remains essentially unchanged, so that pA4 has the same values as PA3.

The temperature TA4 is in the order of magnitude of TAI where, as previously stated, TA4 > TAI . TA4 is preferably in the range of -40°C to 65°C, in particular from -35°C to 50°C, preferably from -30°C to 30°C, particularly preferably from -25°C to 20°C, particularly preferably from -20°C to 10°C.

The energy stored by the charging cycle can be released again in the discharging cycle. In this case, the loading cycle is run through in reverse order, so to speak, with the working medium again going through a change in the state of aggregation.

In detail, the working medium is first provided at a temperature TEI and a pressure PEI. The working medium is liquid. The temperature TEI essentially corresponds to the temperature TAI, with temperature differences of up to 100 K or less, in particular 80 K or less, preferably 60 K or less, being able to occur. The temperature differences depend on the design of the device for carrying out the method according to the invention and should be as small as possible. Temperature differences of 50 K or less, in particular 20 K or less, preferably 15 K or less, preferably 10 K or less are therefore particularly preferred.

The pressure PEI is greater than PAI. This is necessary so that the heat transfer to the cold storage material can take place, the pressure difference Ap between PAI and PEI is in particular in the range from 0.05 to 1 bar, preferably in the range from 0.1 to 0.9 bar, preferably in the range of 0 .15 to 0.5 bar.

The cold working medium (TEI, PEI) is now compressed so that the pressure on PE4 increases (step (ii) b) of the method according to the invention). The temperature TE4, which the working medium has after compression, is usually above the temperature TEI. Thus TE4 > TEI . Due to the isentropic heat input in the form of mechanical work, the temperature can be increased by a few K in the range from 0.5 K to 15 K, in particular 0.6 K to 10 K. Usually the heat difference is less than 10 K. The The temperature of the working medium is not actively changed, but the change takes place solely as a side effect of the work step. After compression, the cold working medium can be fed through a low-temperature recuperator in order to preheat the cold material flow in the liquid state a little.

The pressurized cold working medium is now heated, the heat originating from hot storage material (step (ii) c) of the method according to the invention). This lowers the temperature of the hot storage material from TH2 to Tm and raises the temperature of the working material from TE4 to TES. The energy stored in the hot storage material is thus transferred back to the working medium and a hot, pressurized working medium with the temperature TES and a pressure pE3 is obtained. In this step, too, the pressure can drop by a few mbar due to friction losses.

The energy stored in the hot, pressurized working medium can be released (step (ii) d) of the method according to the invention). In this work step, mechanical work is given off in an expansion machine, which can be used to operate a generator, for example. After the energy has been released, the working medium has a temperature TE2 and a pressure PEI. The temperature TE2 is higher than the temperature TEI. This excess temperature is finally transferred to the cold storage material (step (ii) e) of the method according to the invention). As a result, the working medium becomes liquid and the temperature drops to TEI. The pressure remains more or less constant with PEI. The temperature of the cold storage material increases from TKI to TK2.

In a preferred embodiment, the energy stored in the working medium can be released in two stages. In this case, after passing through a first expansion machine and the delivery of mechanical work, the working medium can once again store heat with one or more hot accumulators exchange, whereby the working medium is heated again. The working medium can then be passed through a second expansion machine, with mechanical work being released again.

The discharge cycle can run again until all energy has been transferred from the hot storage material to the working medium and from there to the generator or a comparable device.

In analogy to the CHEST system, the phase change from liquid to gas is also used in the method according to the invention in order to store the energy on the cold side. According to the invention, the cold accumulator is a stand-alone accumulator with a cold accumulator material.

According to the invention, the excess energy that is supplied to the cold storage material in the discharge cycle (step (ii) e) of the method according to the invention) is used to evaporate the working medium in the loading cycle and thus cool the cold storage material.

In the unloading cycle, the loading cycle runs more or less in reverse order. Accordingly, TAI and TEI are of the same order of magnitude. The same applies to TA2 and TE2 as well as to TAS and TE3 and to TA4 and TE4. The same applies to the pressures. It has been found that preferably pE4 is 90% to 120%, in particular 95% to 110% of pA4.

In a further embodiment, the object on which the present invention is based is achieved by a device (10) for carrying out the method according to the invention, comprising: a working medium circuit (11) with a working medium; the working medium circuit (11) comprises at least one cold accumulator (12a, 12b) with at least one first heat exchanger (13); at least one hot accumulator (14a, 14b) with at least one second heat exchanger (15); a first Fluid energy machine, in particular turbomachine, (16); and a second fluid energy machine, in particular a turbomachine (17), wherein in a loading cycle of the device (10) from the at least one cold store (12a, 12b) via the at least one first heat exchanger (13) thermal energy from the at least one cold store (12a, 12b ) can be supplied to the working medium to convert the working medium from a liquid to a gaseous state of aggregation, with energy being supplied via the first fluid energy machine, in particular a turbomachine, (16) the pressure and the temperature of the working medium can be increased, with the at least one second heat exchanger (15 ) heat can be extracted from the working medium and fed to the at least one hot accumulator (14a, 14b), and wherein a second fluid energy machine, in particular a turbomachine, (17) is designed to reduce the pressure of the working medium and/or wherein in a discharge cycle of the device (10) the Working medium on the second fluid energy machine, esp other turbomachine, (17) can be compressed by supplying energy, the temperature of the working medium being able to be increased by releasing heat to the at least one hot accumulator (14a, 14b) via the at least one second heat exchanger (15), the pressure of the working medium being increased via the first Fluid energy machine, in particular turbomachine, (16) can be expanded with the release of mechanical energy and heat energy can be dissipated from the working medium via the at least one first heat exchanger (13) to the at least one cold accumulator (12a, 12b) in order to convert the gaseous state of aggregation of the working medium into the liquid state of aggregation.

The device (10) for carrying out the method according to the invention can preferably also include at least one third heat exchanger, the at least one third heat exchanger for heat transfer from the area between the hot accumulator (14a, 14b) and the second fluid energy machine, in particular turbomachine, (17). the area between the cold accumulator (12a, 12b) and the first fluid energy machine, in particular a turbomachine (16).

According to the invention, only one compressor 16a and one expander 16b (piston or turbomachine, as the first fluid energy machine 16) are required on the gas side. On the liquid side, liquid pumps 17b and nozzles 17a or liquid turbines (as the second fluid energy machine 17a) are sufficient. Complex devices, which are sometimes required in the prior art, are not required in the present case.

The first and/or second fluid energy machine can each be designed as a turbomachine or a reciprocating piston machine. In particular, a turbomachine or a reciprocating piston machine can be used as the compressor 16a. In particular, an expander 16b can be designed as a turbomachine or a reciprocating piston machine. In a preferred embodiment, a single reciprocating piston machine can also be used as compressor 16a and expander 16b.

Flow machines can basically be used as fluid energy machines within the meaning of the present invention. Turbo machines with isentropic efficiency of over 90% and over a wide power range between 20 MW and 1000 MW have been used successfully for decades in industrial and power plant technology. In full-load operation, turbomachines are therefore outstandingly suitable for being used in the present invention.

However, under certain circumstances, turbomachines have the disadvantage that they are often used in sliding pressure operation at part load. Correspondingly, with a reduction in the injection and extraction lines, there is an expected reduction in the mass flow and an undesirable reduction in the upper pressure levels. The system can thus become unbalanced under certain circumstances.

In the loading mode, the compressor 16a may no longer be able to bring the working medium exactly to the desired pressure and therefore no longer to the desired temperature. The temperature of the working medium at the compressor outlet can therefore be too low to store the thermal energy in the hot accumulator. This can result in insufficient storage. This applies not only to the hottest storage tank, but cascades through to the following storage tanks, since the working medium at their heat exchangers is also at too low a pressure and therefore at a too low condensation temperature.

In the unloading mode, the sliding pressure operation in the partial load range of the Rankine cycle according to the invention leads to a reduction in the boiling temperature, which in turn can lead to insufficient storage and to a low inlet temperature of the working medium in the turboexpander. It follows that the turbine work is greatly reduced in the discharge mode and a large part of the thermal energy transferred has to be dissipated to the environment.

In both cases, the efficiency of the device can be reduced. An alternative is the use of fixed pressure operation, in which the corresponding pressure levels are kept constant. In the loading mode, however, this operation can only be used to a limited extent, since a turbo compressor with the same number of revolutions but a reduced mass flow quickly reaches its surge limit, which can lead to the destruction of the unit. This may also limit partial load loading. In the unloading mode, partial load operation works better, but also leads to possible losses, since the turbine has strong ventilation losses with partial loading. These disadvantages can possibly be avoided if the compressor 16a and/or the expander 16b are configured as reciprocating piston machines. Reciprocating engines work in two-stroke operation using electronically controlled valves and can thus maintain the required pressure or temperature level at all times. The partial load control takes place via the speed of the machine, for example by means of a frequency converter. In particular, a single reciprocating machine can be used for the compressor 16a and expander 16b.

The different method steps can then be designed as follows. In the loading mode, the reciprocating piston machine can be operated at a lower frequency for partial load operation. The valves open as soon as the desired outlet temperature is reached. The discharge throttle can be throttled to maintain the appropriate pressure level in the system. In the unloading mode, the reciprocating piston machine is loaded more slowly by the feed pump than in full-load operation, i.e. with the same pressure but with a lower mass flow. This results in a lower rotational speed, which can be converted back to the mains frequency using a frequency converter.

The part-load efficiencies of reciprocating machines are not quite as high as in full-load mode, but in principle show a lower drop than turbomachines. In addition, the same reciprocating engine can be used as a compressor for the loading mode and as an expander for the unloading mode. This is not readily possible with turbomachines. However, due to the moving masses, reciprocating machines are generally smaller than turbomachines.

The system shown allows the location-independent and economical storage of electrical energy from different energies. It is designed for a storage capacity of 1-200 MW and a storage duration of 5-50 hours. It is therefore suitable for large-scale electricity storage for small businesses, industrial areas and entire cities and allows the integration of wind and solar power into global energy systems. The investment and operating costs are estimated to be significantly lower than is the case with lithium-ion batteries or comparable battery systems, so that this technology also offers many economic advantages. Since no special geological or geographical conditions, energy sources or sinks, or other site-specific features are required for the operation of the system, the technology can be used worldwide.

Execution example:

An exemplary representation of the disclosed invention is presented hereinbelow.

A zeotropic mixture of 80% by weight ammonia and 20% by weight water was used as the working medium. A water-salt (25% by weight CaCl2) mixture represented the cold storage material. Three hot storage tanks with three different hot storage materials were used. A salt mixture (8% NaNOs-48% KNO3- 2% UNO3- 42% CaNCh) was used as the first heat storage material. The second hot storage material was pressurized water and the third hot storage material was non-pressurized water.

The loading cycle is shown schematically in FIG. 4 shows the associated T-s diagram of the working medium during passage through a device preferred according to the invention.

The loading cycle included the following steps: 1. The working medium (TAI: -21 °C, PAI: 1.38 bar) (position 6 in Fig. 3 and 4) was heated by means of a first heat transfer using the cold storage material (TAZ: 69 °C, PAZ: 1, 35 bar) (position 1 in Fig. 3 and 4). The temperature of the cold storage material fell from 70.5 °C (TK2) to - 19.5 °C (TKI).

2. The working medium with a temperature of 69 °C and a pressure of 1.35 bar (TA2, PA2, corresponding to position 1 in Fig. 3 and 4) was pressurized to a high pressure of 57 bar and a temperature of 500 °C (pA3, TAS, corresponding to position 2 in Fig. 3 and 4). As a result, the mechanical work done by the electricity was stored in the working medium.

3. The hot working medium (TAS: 500 °C, PAS. 57 bar) is then cooled by means of a first hot storage material, as a result of which part of the heat is transferred from the working medium to the first hot storage material, causing the working medium to have the temperature T : 190 °C and the pressure p: 55.4 bar (position 3 in Fig. 3 and 4). The working medium is first brought to a temperature T: 110° C. and a pressure p: 54.3 bar (position 4 in FIGS. 3 and 4) and finally to

T: -14 °C, p: 53.2 bar (TA4, PA position 5 in Fig. 3 and 4).

4. The now cold but pressurized working medium (TA4, PA4) was expanded using a nozzle, as a result of which the temperature and pressure of the working medium returned to the initial values TAI, PAI.

For unloading, the cycle was reversed and run through with the aid of a liquid pump. The discharge cycle is shown schematically in FIG. 6 shows the associated Ts diagram. In the discharge cycle, the working medium was at the beginning (position 1 in FIGS. 5 and 6) at TEI: -18° C., PEI: 1.6 bar. After compression, the pressure pE2 was 52.6 bar (TEZ: -17° C.; position 2 in FIGS. 5 and 6). Heat was successively transferred from several heat accumulators to the working medium, resulting in the following temperature rise and pressure development: Position 3 in Fig. 5/6: T: 107 °C, p: 51.6 bar Position 4 in Fig. 5/6 T : 182 °C, p: 49.5 bar position 5 in Fig. 5/6 TES : 496.5 °C, PES : 49.5 bar

The energy stored in the working medium was transferred to a generator G, so that the working medium had the temperature TE2: 152° C. and the pressure PE2: 1.6 bar (position 6 in FIGS. 5 and 6). The last step was the dissipation of the remaining residual heat to the cold storage material, whereby the working medium again exhibited the initial temperature TEI and initial pressure PEI.

Claims

34 patent claims

1. A method for storing and releasing electrical energy comprising the steps:

(i) in a loading cycle a) a liquid working medium at a temperature TAI and a

Pressure PAI provided, b) then heat is transferred from at least one cold storage material to the working medium by means of at least one first heat exchanger, whereby the temperature of the working medium rises from TAI to a temperature TA2, whereby the working medium becomes gaseous and has the pressure pA2, c) by means mechanical work from electrical energy, the gaseous working medium is heated from the temperature TA2 to a temperature TAS and the pressure is increased from a pressure pA2 to a pressure pA3, d) the heat obtained from the mechanical work and stored in the working medium is then transferred from the working medium to at least a heat exchanger is transferred to at least one hot storage material, causing the temperature of the working medium to drop from TAS to TA4, and e) the pressure of the working medium is then reduced from pA4 to PAI, so that a liquid working medium with a temperature TAI and a pressure PAI is again obtained receives, so that a cyclic process from steps b) to e) can be carried out; and

(ii) in a discharge cycle a) the liquid working medium is provided at a temperature TEI and a pressure PEI, 35 b) the working medium is compressed to a pressure pE4 and has the temperature TE4, c) heat from at least one hot storage material is transferred to the working medium by means of at least one heat exchanger, whereby the temperature of the working material rises from TE4 to TE3, d) the then hot, pressurized working medium with temperature

TE3 and the pressure pE3 is relaxed to a temperature TE2 and a pressure pE2, whereby mechanical energy is released, and e) heat of the gaseous working medium with the temperature TE2 and the pressure pE2 is transferred to the at least one cold storage material by means of at least one heat exchanger, whereby the working medium condenses and then has a temperature TEI and a pressure PEI, so that a cyclic process from steps b) to e) can be carried out; wherein the working medium is a zeotropic mixture.

2. The method according to claim 1, characterized in that the zeotropic mixture is a binary system of a primary substance and a secondary substance, the primary substance being selected from ammonia or CO2 and the secondary substance being selected from water, ethanol or methanol.

3. Process according to claim 1, characterized in that the zeotropic mixture comprises 35 to 45% by weight ammonia, 0 to 10% by weight ethanol and 50 to 60% by weight water.

4. The method according to any one of claims 1 to 3, characterized in that the cold storage material comprises water or air.

5. The method according to claim 4, characterized in that the cold storage material has water, wherein the water has additives for freezing point reduction, selected from NaCl, CaCh, glycol, methanol, ethanol or a mixture of these, or the water is pressurized.

6. The method according to any one of claims 1 to 5, characterized in that the hot storage material is a sensitive heat storage medium, in particular a salt mixture, a thermal oil, a wax, unpressurized or pressurized water, ceramics or beds of sand, gravel or rock.

7. The method according to claim 6, characterized in that the salt mixture is selected from nitrate and / or nitrite salts of alkali and / or alkaline earth metals and mixtures of these.

8. The method according to any one of claims 1 to 7, characterized in that the electrical energy in (i) c) comes from renewable energies, in particular from solar energy and / or wind energy.

9. Device for carrying out a method according to one of claims 1 to 8, comprising: a working medium circuit (11) with a working medium; the working medium circuit (11) comprising at least one cold accumulator (12a, 12b) with at least one first heat exchanger (13); at least one hot accumulator (14a, 14b) with at least one second heat exchanger (15); a first fluid energy machine, in particular a turbomachine (16); and a second fluid energy machine, in particular a turbomachine (17), wherein in a loading cycle of the device (10) from the at least one cold store (12a, 12b) via the at least one first heat exchanger (13) thermal energy from the at least one cold store (12a, 12b ) The working medium can be fed to convert the working medium from a liquid to a gaseous state of aggregation, with the supply of energy The pressure and temperature of the working medium can be increased via the first fluid energy machine, in particular a turbomachine (16), with heat being able to be extracted from the working medium via the at least one second heat exchanger (15) and being fed to the at least one hot accumulator (14a, 14b) and with one second turbine (16) is designed to reduce the pressure of the working medium and/or wherein, in a discharge cycle of the device (10), the working medium can be compressed via the second fluid energy machine, in particular a turbomachine (17), with the supply of energy, the temperature of the working medium being reduced by release heat from the at least one hot accumulator (14a, 14b) can be increased via the at least one second heat exchanger (15), the pressure of the working medium being able to be relieved via the first fluid energy machine, in particular a turbomachine, (16) with the release of mechanical energy, and heat energy being transferred from the Working medium on the at least one first n heat exchanger (13) to the at least one cold accumulator (12a, 12b) can be discharged to convert the gaseous state of aggregation of the working medium into the liquid state of aggregation.

10. The device according to claim 9 further comprising at least one third heat exchanger, wherein the at least one third heat exchanger for heat transfer from the area between the hot accumulator (14a, 14b) and the second fluid energy machine, in particular turbomachine, (17) to the area between the cold accumulator ( 12a, 12b) and the first fluid energy machine, in particular a turbomachine (16).

11. The device according to claim 9 or 10, wherein the first fluid energy machine (16) and / or the second fluid energy machine (17) is designed as a reciprocating piston machine. - 38 -

12. The device according to claim 11, wherein the first fluid energy machine (16) is configured as a single reciprocating piston machine in the loading cycle as a compressor (16a) and in the unloading cycle as an expander (16b).

13. Use of a device, preferably according to one of claims 9 to 12, for carrying out the method according to one of claims 1 to 7.