Sage Geosystems has pioneered a new form of cheap energy storage that uses the Earth as a giant bellows, pumping water into underground fractures, then letting it squirt back up at 70% efficiency – or 200% efficiency if you also harvest heat energy.

The "huff & puff" method, as it's known, is adapted here from a similar technique that's used in oil production, where a fluid – often steam – is injected into a shale oil deposit and left there for several hours to heat the oil, reducing its viscosity and making it easier to pump out.

Sage, however, uses dense drilling mud, forced at high pressure into rock deep underground at disused oil wells, to push slim fractures apart, then pumps water in, again at high pressure, to keep the fractures "inflated." This is done using excess renewable energy collected during daylight hours, and then a valve is closed to lock the water in.