From Tony Castaldo, San Antonio, Texas, US
Gravitricity’s energy-storage scheme uses a 25-tonne weight dangled in an old mine. Since a great deal of wind energy is generated offshore, we could use excess electricity to pull a large air-filled float down to the sea floor and, in times of need, use its rise to generate electricity (20 July, p 36).
To displace 25 tonnes of water, we would need a 25-cubic-metre float – a sphere with a radius of less than 2 metres. We might want a much larger one, of course.
