A technology used in ancient Greece to power clocks and fire a cannon is undergoing a revival as a way to store energy from wind turbines and solar panels. Compressed air, already used to power carnival rides, jackhammers and medical equipment, joins the crowded field of innovations chasing what could be a $US21.5 billion ($27 billion) market by 2024. Contenders include Tesla Motors Chairman Elon Musk, who this month unveiled a suite of batteries to store electricity for homes, businesses and utilities. Tesla plans to begin delivering its rechargeable lithium-ion model in late summer. But compressed air storage systems, or CAES, may have an edge: The technology can be used to store large amounts of power for weeks at less than the cost of batteries.