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 .
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 fl ows 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 fl ooded mine shafts, other underground cavities and the open sea are also technically