IEC_Elctrical Energy Storage.pdf

Figure 2 2 pumped hydro storage vattenfall iec msbees

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Figure 2-2 – Pumped Hydro Storage (Vattenfall, IEC MSB/EES Workshop, 2011) 4 The largest PHS plant in the world, with 2 100 MW peak power, is the Bath County hydroelectric pumped storage plant located in Virginia, USA [bat85]. 5 Adjustable-speed machines are now being used to improve efficiency.
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22 S E C T I O N 2 Types and features of energy storage systems 2.2.2 Compressed air energy storage (CAES) Compressed air (compressed gas) energy storage (Figure 2-3) is a technology known and used since the 19th century for different industrial applications including mobile ones. Air is used as storage medium due to its availability. Electricity is used to compress air and store it in either an underground structure or an above-ground system of vessels or pipes. When needed the compressed air is mixed with natural gas, burned and expanded in a modified gas turbine. Typical underground storage options are caverns, aquifers or abandoned mines. If the heat released during compression is dissipated by cooling and not stored, the air must be reheated prior to expansion in the turbine. This process is called diabatic CAES and results in low round-trip efficiencies of less than 50 %. Diabatic technology is well- proven; the plants have a high reliability and are capable of starting without extraneous power 6 . The advantage of CAES is its large capacity; disadvantages are low round-trip efficiency and geographic limitation of locations [nak07]. Figure 2-3 – Underground CAES [rid11] 6 In an adiabatic CAES process, currently under development, the released heat is retained in thermal storage (e.g. porous stones) and used again during expansion in a turbine.
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23 Figure 2-4 – Flywheel energy storage [act11] 2.2.3 Flywheel energy storage (FES) In flywheel energy storage (Figure 2-4) rotational energy is stored in an accelerated rotor, a massive rotating cylinder. The main components of a flywheel are the rotating body/cylinder (comprised of a rim attached to a shaft) in a compartment, the bearings and the transmission device (motor/generator mounted onto the stator 7 ). The energy is maintained in the flywheel by keeping the rotating body at a constant speed. An increase in the speed results in a higher amount of energy stored. To accelerate the flywheel electricity is supplied by a transmission device. If the flywheel’s rotational speed is reduced electricity may be extracted from the system by the same transmission device. Flywheels of the first generation, which have been available since about 1970, use a large steel rotating body on mechanical bearings. Advanced FES systems have rotors made of high-strength carbon filaments, suspended by magnetic bearings, and spinning at speeds from 20 000 to over 50 000 rpm in a vacuum enclosure. The main features of flywheels are the excellent cycle stability and a long life, little maintenance, high power density and the use of environmentally inert material. However, flywheels have a high level of self-discharge due to air resistance and bearing losses and suffer from low current efficiency.
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