Air is pumped into large receptacles (e.g. underground caverns or old mines) at night and used to drive gas turbines for peak, day loads. The energy stored is equal to the product of the air pressure and volume. The compressed air allows fuel to be burnt in the gas turbines at twice the normal efficiency. The general scheme is illustrated in Figure 1.13. A German utility has installed a 290 MW scheme. In one discharge/charge cycle it generated 580 MWh of on-peak electricity and consumed 930 MWh of fuel plus 480 MWh of off-peak electricity. A similar plant has been installed in the USA. One disadvantage of these schemes is that much of the input energy to the compressed air manifests itself as heat and is wasted. Heat could be retained after compression, but there would be possible complications with the store walls rising to a temperature of 450 °C at 20 bar pressure. A solution would be to have a separate heat store that could comprise stacks of stones or pebbles which store heat cheaply and effectively. This would enable more air to be stored because it would now be cool. At 100 bar pressure, approximately 30 m3 of air is stored per MWh output.