IEC_Elctrical Energy Storage.pdf

This combined electrolysis fuel cell process is an

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This combined electrolysis-fuel cell process is an electrochemical EES. However, both gases are multi-purpose energy carriers. For example, electricity can be generated in a gas or steam turbine. Consequently, they are classified as chemical energy storage systems. In Figure 2-1 thermal energy storage systems are included as well, although in most cases electricity is not the direct input to such storage systems. But with the help of thermal energy storage the energy from renewable energy sources can be buffered and thus electricity can be produced on demand. Examples are hot molten salts in concentrated solar power plants and the storage of heat in compressed air plants using an adiabatic process to gain efficiency. 2.2 Mechanical storage systems The most common mechanical storage systems are pumped hydroelectric power plants (pumped hydro storage, PHS), compressed air energy storage (CAES) and flywheel energy storage (FES). Figure 2-1 – Classification of electrical energy storage systems according to energy form (Fraunhofer ISE) Electrical energy storage systems Mechanical Pumped hydro - PHS Compressed air - CAES Flywheel - FES Secondary batteries Lead acid / NiCd / NiMh / Li / NaS Double-layer Capacitor - DLC Superconducting magnetic coil - SMES Flow batteries Redox flow / Hybrid flow Hydrogen Electrolyser / Fuel cell / SNG Sensible heat storage Molten salt / A-CAES Electrochemical Chemical Electrical Thermal
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21 2.2.1 Pumped hydro storage (PHS) With over 120 GW, pumped hydro storage power plants (Figure 2-2) represent nearly 99 % of world-wide installed electrical storage capacity [doe07], which is about 3 % of global generation capacity 4 . Conventional pumped hydro storage systems use two water reservoirs at different elevations to pump water during off- peak hours from the lower to the upper reservoir (charging). When required, the water flows back from the upper to the lower reservoir, powering a turbine with a generator to produce electricity (discharging). There are different options for the upper and lower reservoirs, e.g. high dams can be used as pumped hydro storage plants. For the lower reservoir flooded mine shafts, other underground cavities and the open sea are also technically possible. A seawater pumped hydro plant was first built in Japan in 1999 (Yanbaru, 30 MW) [fuj98]. PHS has existed for a long time – the first pumped hydro storage plants were used in Italy and Switzerland in the 1890s. By 1933 reversible pump-turbines with motor- generators were available 5 . Typical discharge times range from several hours to a few days. The efficiency of PHS plants is in the range of 70 % to 85 %. Advantages are the very long lifetime and practically unlimited cycle stability of the installation. Main drawbacks are the dependence on topographical conditions and large land use. The main applications are for energy management via time shift, namely non- spinning reserve and supply reserve.
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