What energy is stored in steam
In order to be able to compare the flywheel energy storage with other technologies, an overview of further possibilities for storage in the energy supply should be given at this point.
A storage system in the electrical energy supply basically consists of a storage unit and a converter. The various storages can be divided into first and second class storages according to their losses during the storage phase . In the case of first-class storage, the amount of storage material practically does not change over the storage time, so they store without losses. Second-class storage systems store lossy, the amount of storage goods decreases over time. The converter has the task of converting the electrical energy into a storable form of energy during the charging process and converting it back when discharging. He always works lossy according to the second law of thermodynamics.
In the case of these storage facilities, a distinction must be made between storage power plants with and without natural inflow. For better comparability of the accumulators with each other, only the hydraulic accumulators without natural inflow are considered here.
In pumped storage power plants, the electrical energy is stored in the form of the potential energy of the water. The storage medium water is moved between two basins with different height levels. The storage tank is charged with the help of electrically driven pumps. When the water flows in the opposite direction, turbines are driven, which are coupled to generators, and the stored energy is converted back into electrical energy. The pumps and turbines of modern pumped storage power plants are combined to form pump turbines, which saves space and costs.
The ratio of usable energy to energy expended is influenced on the one hand by the efficiency of the pump turbines and the pipe friction losses of the downhill section and on the other hand by the unavoidable resting losses due to evaporation and seepage of the water. The conversion losses are in the order of 15% for the pumping operation and in the order of 10% for the turbine operation. The dormant losses, on the other hand, are negligible at 0-0.05% per day, and the storage basin can be viewed as first-class storage [6,10].
The storable energy is determined by the available height of fall and the volume of the basin. The relationships between the characteristic quantities of power, efficiency, head and water flow and the energy content are strongly non-linear. The large number of systems available show that pumped storage power plants can store large amounts of electrical energy and provide services, so that this technology is very suitable for both long-term and short-term storage. The Vianden pumped storage power station should be mentioned as an example. It is designed for an electrical output of 1100 MW in turbine mode and 836 MW in pump mode.
A major disadvantage of pumped storage power plants is their dependence on suitable topographical conditions. The pumped storage power plants are therefore mostly far from the centers of energy consumption, and the efficiency of the hydraulic storage is thus further reduced.
Pumped storage power plants are the only large-scale storage facilities that are used today on a larger scale in electrical energy supply.
Thermal storage is characterized by the conversion-free storage of thermal energy, which is taken in the intermediate state of energy conversion in thermal power plants.
The steam is temporarily stored in a store consisting of one or more units. A distinction is made between gradient and equal pressure accumulators.
The storage tanks are pressure-tight steel boilers. With the gradient storage, the hot steam flows into this boiler, condensing and releasing its heat of evaporation to the surrounding water. This increases the pressure and temperature of the water. When fully charged, the steam storage tank is filled to around 90% with water.
When it is withdrawn, the remaining 10% water vapor is withdrawn, reducing the pressure in the storage tank and re-evaporation of the hot water. Steam can be withdrawn until the temperature in the memory rises above the saturation temperature at the respective pressure. The extracted steam can then be coupled back into the steam turbine, but at a lower pressure level than the extracted steam.
The equal pressure accumulator is basically a heat exchanger and accumulator in one. During the charging process, more boiler feed water than necessary is heated and stored by excess steam. When discharging, the hot feed water is removed and fed to the boiler. Since less cold feed water has to be preheated during discharge, less steam is required for this. This means that more steam can be fed to the turbine and thus more power can be generated.
The storage and withdrawal of heat takes place in gradient and constant pressure storage tanks with efficiencies of 0.76-0.985% depending on the switching of the storage tanks in the overall system .
During the storage phase, despite the insulation of the container, heat is lost to the environment, which is why thermal storage systems are classified in the second class.
The principle of thermal storage has been used since the twenties. Today, steam accumulators are mainly used for the fail-safe supply of process steam in the chemical industry. The Charlottenburg cogeneration plant in Berlin is an exception. A storage facility with a usable volume of 4480 m was built here from 1929 to at least 19873 initially used to cover power demand peaks and later used as an immediate reserve .
The compressed air storage is tied directly to the location where the electricity is generated by a gas turbine system. The air compression process of the compressor can be temporally decoupled from the expansion process in the turbine and thus from the generation of electricity by the compressed air storage. The compression takes place during off-peak times. The compressor is driven by the generator alone, the turbine is at a standstill.
The compressed air is usually stored in large underground cavities such as salt caverns at pressures of up to 70 bar. During the storage phase there is a pressure loss due to sealing problems and solubility in water, which is stated as 7% per day. The compressed air storage tank is therefore also a second-class storage facility.
During peak load times, the compressed air is then fed to the combustion chamber and finally to the turbine. The turbine power can then be completely delivered to the generator and does not have to additionally drive the compressor.
The world's first compressed air storage gas turbine system has been in operation in Huntdorf near Bremen since 1977. It has a generator output of 290 MW and the storage capacity is sufficient for a four-hour discharge at full load. The system works with an efficiency of 41.8%, which, however, cannot be compared with a storage efficiency, since chemical energy is also supplied in the combustion chamber [26,27].
In battery storage systems, electrical energy is stored in the form of chemically bound energy. At the time of the direct current supply networks, battery storage systems were used to a considerable extent to cover peak loads and for immediate reserve power. In 1930 batteries with a total output of 186 MW with a 20-minute discharge were installed in Berlin alone .
The lead-acid battery is the most advanced technology. It is used today in electrical island networks for frequency control. For example, the Berlin energy supply company BEWAG operated a battery storage system with a nominal energy content of 14.4 MWh from 1987 to 1994. This system was planned when West Berlin was still an island network. With the connection to the Western European network, the system was no longer required for frequency control and was taken out of service. It was able to provide immediate reserve power of 17 MW. Other types with aqueous electrolyte are the nickel-cadmium battery and the nickel-iron battery. In contrast, more recent developments are based on high-temperature processes with solid electrolytes. The most popular battery type is the sodium-sulfur battery (NaS). However, commercial use of the NaS battery is questionable because of its very high cost. Another development is the zinc-bromine battery. However, these types of batteries are currently still in the prototype phase.
Batteries achieve efficiencies of around 65-75%. Added to this are the idle losses due to self-discharge and the auxiliary energy consumption. The resting losses with conventional lead-acid batteries are around 0.1-0.5% per day, NaS batteries achieve more favorable values. Batteries can therefore be classified as first or second class, depending on their design and type. The big disadvantage of batteries is their short lifespan, which is limited to a few thousand full cycles.
What is special about batteries is that the battery is both a storage device and a converter. Batteries only work with direct voltage, and a rectifier circuit must also be available when used in the electrical power supply. A general problem with batteries is the endangerment of the environment due to the sometimes toxic ingredients.
The use of hydrogen technology for energy purposes will be of great importance for the future. Because of its physical and chemical properties, hydrogen can be used in a variety of ways for indirect storage of electrical energy.
In electrolysis, water is broken down into its components hydrogen and oxygen with the help of electrical energy. As things stand today, electrolysis is the most effective process for producing hydrogen. Here, alkaline electrolysis is the most advanced technology. However, high-temperature electrolysis promises better development opportunities with more favorable degrees of efficiency. The efficiency will be of the order of up to 65% with alkaline and up to 95% with high-temperature electrolysis.
The storage of hydrogen gas requires similar components as the storage of natural gas. In addition to conventional storage methods such as underground storage, surface pressurized gas storage and liquid hydrogen storage, there are other concepts such as metal hydride and cryoadsorber storage that achieve significantly higher energy densities. From today's perspective, the underground or surface pressurized gas storage for the short term and the underground or liquid hydrogen storage for the long term are the most economical options.
Due to the low resting losses of less than 0.01% per day and the high storage volumes that can be achieved, both long-term and short-term storage are possible. Hydrogen storage systems are therefore among the first-class storage systems.
The conversion of the hydrogen back into electrical energy can take place with the help of conventional thermodynamic processes. The use of fuel cells allows the chemically stored energy to be converted directly into electrical energy. The conversion back takes place at efficiencies that are greater than 55% even in partial load operation. The only reaction product is water. The compactness of the fuel cells also makes mobile use possible.
Since the electrolysis can only be operated with direct voltage, and the fuel cell only supplies direct voltage, a converter must also be present when it is used in the electrical energy supply.
In the magnetic field of superconducting coils, large amounts of energy can be stored in a very small space, since superconductivity has high current densities of up to 300 A / mm2 and thus enables high inductions. In addition to capacitors, which, however, hardly play a role as energy stores, superconducting magnetic energy stores are the only stores with the property that they store electrical energy directly and do not have to convert it into another form of energy.
Today, NbTi or NbSn is usually used to manufacture superconductors. The problem of superconductivity is the provision of the very low temperatures and the associated energy expenditure in the cooling systems. The information on volume-related energy density given in the literature usually relates to the volume of the magnetic field enclosed by the coil. With an induction of 10 Tesla, the energy density is approx. 11 kWh / m3.
As with the flywheel storage system, the overall efficiency of this type of storage is very much dependent on the storage times. Long-term storage can be ruled out from the outset in the event of dormant losses of around 12% per day, so the superconducting magnetic energy storage device is classified as a second-class storage device. Here, too, the only area of application is short-term storage, whereby these storage systems can achieve very good levels of efficiency. In the case of superconducting magnetic storage devices, too, a current rectifier must also be available when used in the electrical power supply.
Superconducting memories are still in the development phase today. Installed systems are mainly used to provide short-term power. Larger systems with outputs in the GW range are planned, but not yet implemented .
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