[freebeard]

Not the same as the wheel then.

hardware.slashdot.org/.../siemens-gamesa-unveils-world-first-electrothermal-energy-storage-system

The heat storage facility consists of around 1,000 tonnes of volcanic rock which is used as the storage medium. The rock is fed with electrical energy which is then converted into hot air by means of a resistance heater and a blower that, in turn, heats the rock to 750C/1382F. When demand requires the stored energy, ETES uses a steam turbine to re-electrify the stored energy and feeds it back into the grid. The new ETES facility in Hamburg-Altenwerder can store up to 130 MWh of thermal energy for a week, and storage capacity remains constant throughout the charging cycles.