Depths of imagination

WIND farms and solar-energy plants have the advantage that their fuel is free, but the disadvantage that the availability of that fuel may change from minute to minute. If they are to become the large-scale contributors to power generation that their boosters suggest, then cheap and reliable means of smoothing their output, by storing surpluses for use during times of scarcity, need to be developed. At the moment, there is only one good way of saving surplus grid electricity, regardless of how i...