Methods for storing and recovering electrical energy

Abstract

Methods for storing and recovering electrical energy, comprising the following steps • Filling a pressure vessel (01) with ethane as a storage medium (02); • Providing water as a coolant (04); • during a storage phase: - Generating an operating pressure in the pressure vessel (01) filled with the storage medium (02), wherein the operating pressure in the pressure vessel (01) is set so that the storage medium (02) is in a wet vapor phase; - Adjusting the operating pressure in the pressure vessel (01) so that the steam temperature of the storage medium (02) is equal to the temperature of the coolant (04), and so that the coolant (04) has a temperature upon entering the pressure vessel (01) that is not higher than the temperature prevailing in the pressure vessel (01); - Providing electrical energy to a charging pump (06); - Pumping the liquid coolant (04) with the charging pump (06) to a spray element (03) which is located in the pressure vessel (01), and spraying the coolant (04) in the pressure vessel (01); • during a recovery phase: - Supplying pressurized coolant (04) from the pressure vessel (01) to a discharge turbine (09); - Absorption of electrical energy at the discharge turbine (09).

Description

[0001] The present invention relates to a method for storing and recovering electrical energy, which—at least in a simplified view—is based on adiabatic, preferably phase-changing energy storage. This method is particularly suitable for the short- and medium-term storage of electrical energy obtained from renewable sources after its conversion into mechanical energy. For this purpose, a storage medium in the vapor or gas phase is used as a compressed gas storage medium and compressed with the aid of a coolant, whereby the coolant simultaneously serves as a heat storage medium for the condensation heat of the medium.

[0002] As part of the desired increased use of renewable energies, there is an increasing need for short- and medium-term storage of electrical energy generated from fluctuating sources, particularly wind and solar power plants. A wide variety of energy storage systems based on different physical principles have been proposed for this purpose.

[0003] DE 10 2014 008 120 A1 describes a method for storing electrical energy using gas compression. For this purpose, a gas, preferably ambient air, is compressed, with compression heat being dissipated. This is followed by at least one further compression of the gas, and subsequent dissipation of compression heat. In the next step, the compressed gas is stored in a gas pressure accumulator. When energy is required, the compressed gas is released and expanded. When the compression heat is dissipated, it is transferred to a storage medium, which is stored in a heat accumulator. When the storage medium is stored, the temperature of the storage medium is at least partially raised to a storage temperature by means of an electric heater. The stored compression heat is used to heat the gas when the compressed gas is expanded.Due to the multiple compression stages required, this process has a comparatively low efficiency and requires a high level of equipment complexity.

[0004] EP 2 494 169 B1 describes an adiabatic compressed air energy storage system (ACAES). A compressor system compresses air and comprises a plurality of compressors, a compressor line, an air storage unit, and a turbine system designed to expand compressed air. Furthermore, a thermal energy storage system is provided, which removes thermal energy from the compressed air and reintroduces it during expansion. The thermal energy storage system comprises a vessel, a plurality of heat exchangers, a medium line system, a liquid medium stored within the vessel, a plurality of pumps, and a thermal separation system positioned within the vessel, which thermally insulates a first portion of the liquid medium at a lower temperature from a second portion of the liquid medium at a higher temperature.The system has a complex structure and also shows poor efficiency.

[0005] DE 10 2015 005 345 A1 discloses a method and device that combine compressed gas storage with a heat transfer medium circuit and hydraulic pressure maintenance in such a way that the gas pressure in the compressed gas container can be regulated or kept constant both during storage and during withdrawal. This is intended to enable thermodynamically advantageous loading and unloading of the storage with gas, as well as its compression and expansion with optimal pressure differences. The heat balance of the heat transfer medium circuit is ensured by a flash evaporator integrated into the circuit and a saturated steam turbine system for power generation. The flow and return flows of the heat transfer medium circuit are stored separately with pneumatic pressure maintenance. This system also has a comparatively complex structure and requires numerous components.

[0006] EP 2 067 942 A2 describes a process for converting and storing renewable energy. The process uses a liquid as a storage medium in a reversible cycle. This liquid is cyclically mechanically pumped into a pressure vessel filled with gas or vapor of a working medium and then released again, generating mechanical energy.

[0007] DE 10 2013 112 196 A1 describes a compressed air storage power plant that is intended to operate almost isothermally. The compressed air storage power plant comprises a pressure vessel filled with a storage medium, a charging pump that delivers liquid coolant to a spray element during the storage process and sprays it in the pressure vessel, and a discharge turbine that absorbs electrical energy during the discharge process.

[0008] An object of the present invention, based on the prior art, is to provide an improved method for storing and recovering electrical energy, which shows improved efficiency, can be used regardless of location and requires simplified plant engineering expenditure.

[0009] This object is achieved by a method according to the appended claim 1.

[0010] The inventive method for storing and recovering electrical energy is based primarily on the realization that numerous disadvantages of prior art adiabatic compressed air energy storage systems (ACAES) can be avoided by maintaining the adiabatic principle and dispensing with a technical separation between the compression / decompression of a gas serving as the storage medium and the heat transfer from / to this gas. This is achieved by simultaneously using the coolant, which is used to extract heat from the storage medium during compression, as the compression medium.

[0011] In a storage phase of the process according to the invention, electrical energy, which is preferably supplied by a renewable source, is converted and stored. For this purpose, an operating pressure is first generated in a pressure vessel filled with gas. The gas acts as a storage medium. The operating pressure in the pressure vessel is not undercut during the further process, so that energy only needs to be expended once to produce it, in particular by pressurizing the pressure vessel with the gas. In the following step, the electrical energy to be stored is supplied to a charging pump. The charging pump then pumps a coolant, preferably water from a coolant reservoir, to a spray element located in the pressure vessel. The coolant reservoir can be provided as a standalone unit or formed by a natural reservoir.The charge pump is dimensioned to pump the coolant against the operating pressure into the pressure vessel, where it is sprayed via the spray element. The coolant cools the gas in the pressure vessel, and at the same time, the gas pressure increases, as a growing portion of the volume in the pressure vessel is occupied by the coolant. The pressure thus increases, but when steam is used, this is counteracted by the fact that the steam condenses during cooling, so the pressure increase is much smaller than if the steam were compressed without cooling.

[0012] The spray element is arranged in the pressure vessel above a maximum filling level so that the sprayed coolant travels as far as possible before collecting at the bottom of the pressure vessel.

[0013] In a recovery phase of the process according to the invention, the pressurized coolant, in particular water, is supplied from the pressure vessel to a discharge turbine, which is driven thereby and generates electrical power. To recover the stored energy, the electrical energy generated by the discharge turbine is extracted so that it can be supplied to an electrical consumer.

[0014] Water is preferably used as a coolant or heat storage medium for the heat generated during gas compression, as it has a high heat capacity, low cost, and high availability. By using the compression medium simultaneously as a coolant, heat dissipation occurs continuously as the coolant absorbs the compression heat.

[0015] The initial operating pressure in the pressure vessel is predetermined depending on the selected gas and is selected so that, on the one hand, as much coolant as possible can be pumped in to store as much energy as possible, and on the other hand, that the coolant is essentially completely forced out of the pressure vessel due to the gas pressure if stored energy is to be recovered. Since neither the coolant reservoir nor the pressure vessel are preferably insulated, the ambient temperature, particularly that of the coolant, must be taken into account during operation. The pressure in the pressure vessel is set so that the steam temperature is approximately the same as the coolant (water) temperature. This can be achieved by pumping the wet steam into or feeding it from a storage tank.

[0016] Preferably, a minimum amount of coolant always remains in the pressure vessel during the recovery phase so that the liquid level prevents the escape of gas from the pressure vessel into the discharge turbine.

[0017] Ethane is used as the gas for filling the pressure vessel. Higher efficiency can be achieved with ethane. According to a preferred embodiment, when using steam, the initial operating pressure in the pressure vessel is adjusted so that it is close to the dew line of the wet steam region.

[0018] It is advantageous if the coolant is pumped from a coolant reservoir by the charging pump and returned to the coolant reservoir via the discharge turbine. Alternatively, however, a "lost" coolant can also be used, for example, if sufficient water is available as a coolant near a flowing body of water.

[0019] An energy storage arrangement not claimed as an invention, with which in particular the method described above can be carried out, comprises a pressure vessel for holding a pressurized gas. Furthermore, a coolant reservoir is provided in which a coolant can be stored. An electrically driven charging pump serves to convey coolant from the coolant reservoir into the pressure vessel when electrical energy is to be stored, which energy is used to drive the charging pump. A discharge turbine is connected to the pressure vessel and generates electrical energy when coolant flows through it from the pressure vessel back to the coolant reservoir. Furthermore, the energy storage arrangement comprises a control unit and preferably a plurality of control valves and / or converters controlled by the control unit in order to regulate the flow at the charging pump and the discharge turbine.

[0020] A spray element is arranged in the pressure vessel, which is located in particular above a maximum filling level and is connected to the charging pump in order to spray the coolant pumped into the pressure vessel in the form of droplets or mist in the pressure vessel.

[0021] Preferably, the pressure vessel of the energy storage arrangement has no thermal insulation from the environment. This simplifies the design and reduces costs. The additional energy input at high outside temperatures largely compensates for the heat loss at low outside temperatures in the long term. Thermal insulation can also preferably be omitted from the coolant storage.

[0022] Advantageously, the discharge turbine is connected to the coolant storage tank in order to return the coolant there, thus creating a closed coolant circuit.

[0023] Further advantages, details, and modifications will become apparent from the following description of a preferred embodiment with reference to the drawings. They show: Fig. 1 a highly simplified block diagram of an energy storage arrangement; Fig. 2 a Ts diagram for a storage phase and a recovery phase; Fig. 3 a schematic diagram of the energy storage arrangement in connection with a wind turbine.

[0024] Fig. Figure 1 shows a simplified block diagram of an energy storage device, with only the most important components shown to illustrate the operating principle. The device comprises a pressure vessel 01 with a closed volume, which is adapted to the amount of energy to be stored and, for example, in the range of 10,000 to 100,0000 m 3can be. The size of the pressure vessel ultimately depends on the general conditions of energy generation and desired storage. The pressure accumulator 01 contains a gas 02 under operating pressure as the energy storage medium, in particular carbon dioxide or ethane. The operating pressure is, for example, in the range 20 to 70 bar. In the pressure vessel 01, above a predetermined maximum fill level, if possible on the upper wall of the pressure vessel, a spray element 03 is arranged, from which a coolant 04 can be sprayed under pressure. The coolant 04 is preferably water, which is pumped to the spray element 03 by a charging pump 06.

[0025] During a storage phase, the electrically driven charging pump 06 is activated when electrical energy needs to be stored. It then pumps coolant 04 into the pressure vessel 01, where the coolant is sprayed or atomized as finely as possible. By introducing the coolant 04 into the pressure vessel, the volume available for the gas 02 is reduced, so that the pressure rises and the mechanical energy in the system increases. At the same time, heat is dissipated from the compressed gas because it is absorbed by the coolant. This makes it possible to carry out isothermal compression of the gas in the pressure vessel. When the coolant 04 enters the pressure vessel, its temperature should not be higher than the temperature currently prevailing in the pressure vessel. The coolant 04 collects in the lower area of ​​the pressure vessel 01 and covers an outlet there.

[0026] In other words, when coolant is added to the pressure vessel, there is initially an increase in pressure and temperature until the temperature difference between the pumped-in water (coolant) is large enough for the water to absorb enough heat to equal the volume of steam condensed. Thermodynamically, the process can therefore be described as approximately isothermal and approximately isobaric, although an ideal isothermal and isobaric change of state does not occur. If, in a modified embodiment, air is used as the storage medium, an approximately isothermal change of state occurs, with the pressure increasing.

[0027] When steam (for example CO2 or ethane) is used as the storage medium, it is kept in the wet steam range. If carbon dioxide is used as the gas and water as the coolant, the operating pressure in the pressure vessel is approximately 45 bar, which corresponds to the steam pressure at a temperature of the supplied water of approximately 10°C. In this case, compression takes place as an approximately isobaric and isothermal change of state, i.e. pressure and temperature remain approximately constant. The cooling medium 04 pumped into the pressure vessel by the charging pump 06 reduces the volume remaining for the gas, so that the pressure increases and the temperature in the gas rises. The heat of compression is transferred to the water droplets.Since the pressure vessel contains CO2 wet steam, the CO2 condenses on the water droplets, which promotes heat transfer and at the same time reduces the space required for the condensed storage medium, resulting in only a slight pressure increase in the pressure vessel 01.

[0028] In the illustrated embodiment, the coolant 04 is stored in a coolant reservoir 07 and supplied from there to the charging pump 06 via a line system 08. The coolant reservoir can be artificially created or a natural body of water. Typical average temperatures of 10°C in natural bodies of water are suitable for the operation of the energy storage arrangement.

[0029] Pressure vessel 01 is considered adiabatic. During the summer months, additional heat input from the environment benefits the stored energy, thus compensating for losses during the winter months.

[0030] The energy storage arrangement further comprises a discharge turbine 09, which is fed by the coolant 04 during a recovery phase, releases electrical energy, and feeds it, for example, into a power grid. The coolant is then preferably returned to the coolant storage unit 07 so that it is available again for a later storage phase.

[0031] Finally, the energy storage arrangement has a control unit (not shown) to control the various processes during the storage phase and the recovery phase. This is preferably done by controlling the charging pump 06 and the discharging pump 09, as well as any additional control valves 11, 12.

[0032] Fig. 2 shows, as an example, the Ts diagram of the temperature curve T versus the specific entropy s of the storage medium CO2 in the storage phase (loading) and in the recovery phase (discharging the pressure vessel serving as energy storage).

[0033] An upper curve 20 illustrates loading, i.e., water being pumped into pressure vessel 01. It initially exhibits a sharp temperature rise, which then gradually levels off until a section of nearly isothermal change of state is reached. The total temperature change remains comparatively small at approximately 8.5 K. A temperature difference between CO2 and water is necessary to enable heat transfer. The temperature in the CO2 rises until an equilibrium is established between the heat loss from the gas resulting from compression and the heat absorption of the pumped-in and sprayed water.

[0034] The lower curve 21 illustrates the reverse behavior during unloading, i.e. in the recovery phase. In order to ensure the smallest possible temperature drop in the CO2 vapor during unloading, as soon as the temperature of the vapor has fallen below that of the water O4 in the pressure vessel O1, the water is passed through the boiling CO2 phase before unloading, so that the gas O2 is heated up again. This counteracts the cooling of the gas that occurs during decompression, so that the temperature is ultimately kept largely constant. For example, liquid CO2 can be used for this purpose in the form of a heat pump. To do this, the liquid CO2 is mixed with water and the pressure of this mixture is reduced, for example by pumping the mixture upwards through a vertical pipe. As the pressure drops, the CO2 begins to evaporate at low temperatures.In this way, the water can be cooled below the CO2 temperature before leaving the pressure vessel. The evaporated CO2 must then be pumped back into the pressure vessel via a fan, whereby the pressure differences to be overcome are small.

[0035] Fig. Figure 3 shows a schematic diagram of a combination of the energy storage arrangement with a wind turbine for generating and temporarily storing electrical energy. The wind turbine comprises several wind turbines 30. The pressure vessels 01 and the coolant reservoirs 07 can be housed in the foundations 31 of the wind turbines 30. A shared pumping system can be housed in a central powerhouse 32. For example, the energy storage arrangement can be dimensioned to achieve a specific electrical energy of approximately 1 kWh / m 3 can be stored when excess electrical energy is generated by the wind turbines.

[0036] Since upper and lower reservoirs are not required, the energy storage arrangement according to the invention can be deployed at any location, without any special requirements regarding the terrain. Depending on the storage medium used, the storage efficiency is expected to be between 70 and 80 percent. Reference symbol 01 Pressure accumulator 02 Storage medium / Gas 03 Spray element 04 Coolant / Water 05 -- 06 Charging pump 07 Coolant reservoir 08 Piping system 09 Unloading turbine 10 -- 11 Control valve 12 Control valve 20 Loading curve 21 Discharge curve 30 wind turbines 31 Foundation 32 Powerhouse

Claims

[1] A method for storing and recovering electrical energy, comprising the following steps • Filling a pressure vessel (01) with ethane as a storage medium (02); • Providing water as a coolant (04); • in a storage phase: - generating an operating pressure in the pressure vessel (01) filled with the storage medium (02), wherein the operating pressure in the pressure vessel (01) is adjusted such that the storage medium (02) is in a wet steam phase; - Adjusting the operating pressure in the pressure vessel (01) so that the vapor temperature of the storage medium (02) is equal to the temperature of the coolant (04), and so that the coolant (04) has a temperature upon entering the pressure vessel (01) which is not higher than the temperature prevailing in the pressure vessel (01); - Providing electrical energy to a charging pump (06); - conveying the liquid coolant (04) with the charging pump (06) to a spray element (03) which is located in the pressure vessel (01), and spraying the coolant (04) in the pressure vessel (01); • in a recovery phase: - Providing pressurized coolant (04) from the pressure vessel (01) to a discharge turbine (09); - Electrical energy consumption at the discharge turbine (09). [2] Method according to claim 1, wherein the coolant (04) is conveyed from a coolant reservoir (07) and is returned to the coolant reservoir (07) through the discharge turbine (09).

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