DE102013006814A1 - Storage system and method for storing and utilizing temporary electrical energy surpluses - Google Patents

Abstract

Excess energy from wind turbines or other non-continuous amounts is used to increase the enthalpy of a liquid, preferably a solution of inorganic salts, which acts as a heat storage material and is stored in the form of hot liquid in a buffer store. The thermal energy withdrawn is either converted back into electrical energy in a heat engine or used in whole or in part as thermal energy for heating purposes. The enthalpy increase in the storage liquid is brought about either by a heat pump system or a direct electric heating system or a combination of both. Either insulated tanks that work on the principle of stratified storage, or caverns or cavities created in rock salt that work as thermal liquid storage are used for storage.

Description

The invention relates to a storage system and a method for storage and recovery for temporary electrical energy surpluses, in particular from wind turbines or other energy sources supplying energy at irregular intervals.

The stored energy can be given off when energy is needed as electrical energy and / or as thermal energy. The storage of the energy takes place in liquids, mainly concentrated, aqueous solutions of inorganic salts, which are raised by the fed energy to a higher temperature level and then stored in thermally insulated liquid storage for a long time.

When energy is needed, the energy stored as the enthalpy of a liquid is converted back into electrical energy or released as thermal energy for heating purposes.

The storage of irregular amounts of energy is becoming increasingly important as a result of the increased use of renewable energies, in particular from wind power plants, since this energy is generated in varying quantities and not in conformity with the requirements.

The most common way of energy storage of such amounts of energy are pumped storage, which pump liquids, mainly water from a lower level to a higher level of energy, thereby save the energy used as potential energy and recover the energy stored energy as electrical energy, in a short time large amounts of water from the upper basin (upper basin) via a pressure line of a turbine with generator are fed and discharged into a lower lower basin (lower basin).

In the case of pumped storage, the electricity used is used to increase the potential energy of a liquid, which however requires the greatest possible height differences, which are generally only present in mountainous areas.

In addition to the plant of upper and lower basin with natural level differences exists after DE 19720700 A1 also the possibility to use the differences in level of mining facilities for energy storage.

This further possibility of energy storage according to the pumped storage principle has been proposed, in which the height difference between surface and underground mine parts should be used for the creation of liquid reservoirs with sufficient level differences. In particular, in suitable disused mines, the geodetic height difference between the surface of the day and lower parts of the mine should be used and the energy storage in the form of potential energy of liquids between higher and deeper possible reservoirs. This form of energy storage is also tied to the existence of large differences in space.

Another way of storing excess energy is to increase the pressure of compressed air or other gases, to store the compressed gases into an accumulator, and to reconvert the pressure energy into electrical energy through an expansion turbine with generator. This form of energy storage requires the creation or presence of large-scale pressure-resistant storage cavities and the control of the considerable amounts of heat occurring during the compression of the gas and relaxation of the control of the adiabatic cooling of the gas. Also for storage caverns in salt or other mining cavities are used or proposed.

The object of the invention is to provide a storage method for temporary energy surpluses, which requires neither liquid reservoirs at widely different geodetic levels nor a pressure accumulator for gases.

In addition, the stored energy should be able to be delivered either as usable secondary electric energy or as useful for heating heat energy regardless of the fluctuating energy from the power generation.

This object solves the present invention in that the temporarily existing excess energy is used, not the liquid itself, but the heat content of a To raise liquid from a lower to a higher temperature level and to lower this higher temperature level with energy recovery when energy is needed. In the proposed storage principle, not the liquid itself, but the heat content contained in it is raised to a higher level. Although it would be conceivable to use electrical energy according to the immersion lower principle directly for liquid heating, significantly higher efficiencies can be achieved by electrically driven heat pumps.

The storage method according to the invention thus consists of a very efficient heat pump, a thermally insulated large-volume liquid storage tank and a heat consumer, which recovers the stored thermal energy. This heat consumer may be a heat engine, such as a so-called Organic Rankine Cycle (ORC) and / or a thermal consumer. In the first case, the energy stored as thermal energy (enthalpy) is recovered as electrical energy, albeit with efficiency losses or when using the stored energy for heating purposes directly as usable heat energy.

The electric energy temporarily available from non-stationary power generation plants such as wind turbines drives a suitable heat pump for large amounts of heat. The heat pump system consumes excess electrical energy and uses it as a driving power to pump heat to a higher level. At the same time heat is removed from a medium and this cooled. The generated hot liquid with a higher temperature level is stored in a large-volume buffer memory, optionally, the cooled liquid, the heat was removed, in a second memory. The electrical energy thus leads to the spread of the temperature of two liquids, which can be undone on the occurrence of an energy demand and release of useful energy. Depending on whether this useful energy is delivered as thermal heat energy or used as electrical power, hot heat transfer fluid is removed from the high-temperature buffer or so that a reversible working as the heat pump heat engine, preferably an ORC system heated and thereby generates electrical energy. The cooling of the ORC system according to the invention takes over the cooled and also cached liquid.

In principle, water could be used as working fluid for the heat / cold storage, but the usable temperature range and the achievable storage volume would be limited. It has been found that concentrated salt solutions are more suitable, since their boiling point is up to 120 ° C and can be cooled well below 0 ° C without ice formation, so allow a significantly greater temperature spread than water in pressureless working. Such salt solutions are binary or multi-component solutions of, for example, NaCl, KCl, MgCl ₂ , CaCl ₂ and water. These are easy to produce, inexpensive and stackable.

Likewise, as in the case of water, there is a pronounced density-temperature gradient, as a result of which specifically lighter hot salt solution can be stored in a stratified storage tank without mixing via a specifically heavier, colder solution. Such salt solutions can be stored analogously to hot water in large tanks made of steel or GRP, whose size can be 10 ³ to 10 ⁴ cubic meters and more, allow ground-level installation and not require special terrain forms and can be built in batteries from several tanks.

Since the discontinuity of the energy supply in wind farms requires storage capacities in the range of several gigawatt hours, the dimension of the buffer store or storage should be as large as possible. For the gigawatt range, liquid-tight rock cavities or excavated caverns in rock salt formations are suitable, stable and without overpressure, but using the hydrostatic pressure, for storing up to 120 ° C hot salt solution or for storing cold solution can be used and their volume up to 10 ⁵ or 10 ⁶ cubic meters.

The only condition is that the liquid used as a heat storage fluid is saturated as much as possible of sodium chloride and no inadmissible dissolution phenomena of the Halitflözes can occur underground. Since a saturated NaCl brine has only a low temperature-solubility coefficient, the solubility of NaCl at higher temperature is similar to the solubility at lower temperature and possible dissolution phenomena of salt rock are limited. Analogously, the NaCl solubility in highly concentrated MgCl ₂ solution differs both absolutely and relatively only slightly with temperature, as a result of which the usability of MgCl ₂ solutions as storage liquid is likewise possible in cavern stores for the secure long-term operation of storage cavities in rock salt.

The simplest form of converting injected energy into storable heat by raising the temperature level of a storage fluid is direct heating. However, higher energy efficiencies are achievable when the injected energy serves to "pump" heat to a higher level with a heat pump system.

Of course, both methods of increasing the temperature of storage fluids can be combined. Preferable from the point of view of the achievable efficiency are multi-stage efficient heat pumps both for the later reactivation of the stored energy as electric energy and as thermal energy.

The application of the inventive concept - use of temporary energy surplus by heat pump systems for temperature spread of suitable storage fluids - primarily salt solutions whose storage in liquid buffers that work as stratified storage without internals and their removal if necessary, allows as already stated on the one hand the provision of hot solution for heating purposes as well Provision of secondary electric energy via a per se known power generation by means of heat engine, which operates between a higher and a lower temperature level.

In particular, the possible by the invention storage of heat generated by heat pump hot storage fluid in underground cavities in salt formations in the form of caverns, chambers or other mined cavities in rock salt allows an economic use of temporary energy surpluses z. B. from wind farms for the heat supply to district heating networks nearby settlements and cities look promising. The possibility of converting back the stored by heat pumps storage energy stored in secondary electric current, in principle, allows a year-round operation.

Which type of storage liquid and which type and size of the liquid storage tank is appropriate depends on the desired performance range and the desired use of the stored energy.

The following cases are typical: 1) Power range 1 to 10 megawatts - use as electrical energy - Storage fluid: Saline or water - Storage type: pressureless tank storage for hot media pressureless storage tank for cold media (coolant) - Type of heat generation: Heat pump and / or electric direct heating - heat source: heated coolant - Type of energy use: Generation of secondary electric energy 2) Power range 1 to 10 megawatts - Use as heating energy - Storage fluid: Saline or water - Storage type: non-pressurized tank storage for hot media (coolant reservoir is not required) - Type of heat generation: heat pump - heat source: Running waters or stagnant waters - Type of energy use: thermal energy for heating purposes 3) Power range from 100 megawatts to> 1 gigawatt - use as electrical energy - Storage fluid: saline solution - Storage type: Cavern storage in rock salt formations for hot and cold media - Type of heat generation: heat pump - heat source: heated coolant - Type of energy use: Generation of secondary electric energy 4) Power range 100 megawatts to> 1 gigawatt - use as heating energy - Storage fluid: saline solution - Storage type: Cavern storage in rock salt formations for hot (coolant reservoir omitted) - Type of heat generation: heat pump - heat source: Running waters or stagnant waters - Type of energy use: thermal energy for heating purposes (seasonal energy supply of district heating systems)

The invention is illustrated by 3 examples.

Example 1:

This in 1 The overall system shown consists of a heat pump (WP) and a heat engine (WKM). When feeding energy, it is used via the motor (M) to drive the heat pump (WP) and transfer thermal energy via a heat exchanger to the storage liquid salt solution. At the same time salt solution is cooled. Hot and cold liquids are stored in saline stratified storage tanks. The energy is taken in reverse. The heat engine (WKM) is supplied with hot storage liquid and gives its energy to the generator (G). By the in 1 illustrated arrangement, the operation of which is monitored by control modules and temperature sensors, a reversible operation is ensured.

The provided excess energy is fed as electric power in the expedient of several arranged in cascade heat pumps with multiple compressor stages and consumed as drive power of the compressor. The heat pumps pump in the in 2 As shown, heat from the lower temperature level of the storage for cooling liquid to the temperature level of the hot-solution buffer and thereby cool the cooling liquid of the memory and fill the hot-storage tank to the maximum storage content with hot saline solution.

The heat pump system is expedient because of the large temperature span to be bridged from several, for example, 2-3 individual stages, each consisting of compressor, condenser or evaporator and an expansion valve. In the middle stages, the adjacent heat pump cycles are as in 3 shown connected in series and executed by means of a condenser working as the first and at the same time the second stage evaporator heat exchanger.

This known system of heat pump cascade circuit bridges temperatures of 100 and more Kelvin by the heat absorbed by the first stage is delivered to the following higher thermal level and discharged from the last stage to the hot-storage tank. If only secondary electric power is to be generated, hot fluid is removed from the reservoir if necessary and used to heat the ERM system, which works the other way around 2 illustrated heat pump system used.

The cooling of the heat engine, for example, an ORC system, takes over the removed from the memory coolant, which is ready to re-heat as a heat source for the heat pump operation. Both the ERM system and the hot stages of the heat pump system work with higher temperature refrigerants such as pentafluoropropane or similar work equipment (R134A, R407C). Water and steam are also considered as working fluids for the hot stages.

In three-stage cascade connection and working temperatures between 20 and 55 ° C, 55 and 85 ° C and 85 and 110 ° C from 1 kilowatt hour of electrical energy about a thermal energy of 3 kilowatt hours generated, which is delivered to the heating circuit of the WKM plant after caching , The power of the thermal power plant depends on the number of stages. With only one-stage design, the efficiency is only below 15 percent based on the thermal energy used, but at least three times higher based on the electrical energy used.

With a tank volume of 40,000 m ³ each, power of 5,000 kilowatts of electrical energy can be achieved over approximately 8 to 10 hours.

Example 2:

The plant consists of a salt solution buffer tank operating in the form of a tank battery consisting of 10 thermally insulated 4,000 m ³ stacking tanks, a heat pump operating analogously to Example 1, which is at a temperature level of up to 125 ° C., working with about 30% magnesium chloride brine or 40% calcium chloride brine as heat storage liquid C provides heat and a heat exchanger system which delivers the energy stored in the hot liquid of the brine reservoir to a district heating system.

With a realizable temperature difference of 50 Kelvin and a usable storage volume of 40,000 m ³ , about 40,000 kilowatt hours of heat energy can be stored, which is about one third each from electrical energy and two thirds from environmental energy.

The energy storage can be done in the summer half-year, the energy tax in the winter half-year.

Example 3:

The plant consists of a heat pump cascade and a buffer storage tank in the form of a storage tavern in a rock salt storage or salt dome with a usable volume of 800,000 m ³ .

The storage brine is taken from the lower part of the storage cavern at 40 ° C and heated by the heat pump to 110 ° C. As a result, the sum of the applied electrical energy and enthalpy of the storage liquid is generated as usable heating energy and introduced in the form of hot brine, preferably NaCl brine with> 300 g NaCl / l in the upper part of the storage cavern, where they are on the colder, specific heavier NaCl solution overcoated. The hot brine removed again can serve directly or partially as a heat source for the heat supply of a residential area.

With a usable cavern volume of 800,000 m ³ and a usable temperature difference of 65 Kelvin, stack 50 million kilowatt hours of thermal energy and use the surplus electrical energy of a wind park stored in this form for the district heating system of a middle city.

The heat energy, which must be supplied to the heat pump system, either fresh water from an inland lake, a larger running water or sea water can be withdrawn in the warm season and released after storage as hot NaCl brine in the underground cavern storage in the winter months to heat a district heating system.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 19720700 A1 [0007]

Claims (6)

Storage system and method for storing and recycling temporary electrical energy surplus consisting of operating as a liquid heat pump heat pump system, a liquid storage tank and a consumer thermal energy, characterized in that the fed electrical energy serves as drive energy for one or more heat pumps, thereby di a storage liquid contained heat a higher temperature level is brought and stored in this way to just below the boiling point liquid stored in a working according to the stratified storage principle liquid storage and used as thermal energy. A method according to claim 1, characterized in that the hot storage liquid is stored in energy demand and serves as a heat source for electrical energy production by means of working between a higher and lower temperature level heat engine, preferably an Organic Rankine cycle plant (ORC). A method according to claim 1, characterized in that the hot storage liquid stored in energy demand and serves as a heat source for thermal utilization, preferably district heating systems. A method according to claim 1 to 3, characterized in that as the storage liquid for the heat energy to be stored, a concentrated salt solution of one or more components of the substance group NaCl, KCl, MgCl ₂ and water is used. A method according to claim 1 to 4, characterized in that are used as heat storage for storing the hot storage liquid thermally insulated container in the upper part hot storage liquid with lower specific gravity and in the lower part colder storage liquid with higher specific gravity are. A method according to claim 1 to 5, characterized in that as a heat storage for storing the hot storage liquid in rock salt formations large volume caverns or mined cavities created and as a storage liquid to a sodium chloride indifferent storage brine, preferably NaCl solution with concentrations> 300 g / l NaCl used becomes.

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