The physics of rotating masses can no longer define the electric grid

Tucked away behind a cash-and-carry in a Liverpool suburb is a 40-tonne cylinder of steel spinning 1,500 times a minute. Its 500 megajoules of kinetic energy are the equivalent of the chemical energy stored in 100kg (220lb) of TNT. To keep the cash and carry and other neighbours safe, the whole contraption is cocooned in steel-reinforced concrete; on one side of the cylinder a rubberised steel pit waits to catch it should the numerous redundant safety systems fail.