Isentropic: Rock battery could store electricity

Cambridge firm finds way to save excess power in gravel
Danny Fortson

POWER from the wind and the sun is the future, we are told. There’s just one snag – these power sources aren’t dependable. Unless there is a way to store electricity in times of plenty we will need a lot of back-up power, probably fossil-fuelled, to tide us over when the clouds move in or the wind stops blowing. And that’s not very green.
Isentropic, a five-man outfit in Cambridge, thinks it has the answer: a gravel battery. It may sound distinctly low-tech, but the company has come up with some clever engineering to make tanks of rocks into electricity stores. It comes from an idea pioneered in the 1830s by John Ericsson, the prolific Swedish inventor who designed a hot-air engine and the USS Monitor, the ironclad warship of the American civil war.
Mark Wagner, a former hedge-fund manager who is now the company’s chairman, said Isentropic came about because of another boat. A sailing enthusiast, he was on the lookout for new hydrofoil designs in his attempt to set a world speed record.
When he came across a novel model on the internet, he called its designer, John Howes, an engineer at the Civil Aviation Authority. “We worked on the boat for a while, but then he told me about this heat pump he was working on.” That was eight years ago.
Isentropic’s “battery” works like this. An electrical current feeds a heat pump, a bigger version of the device that keeps your fridge cold. The fridge in your kitchen uses electricity to drive a tiny heat pump to create a temperature split, chilling the inside of the fridge and warming the back.
Isentropic’s system uses Howes’s pump to heat argon gas to 500C on one side while cooling it to -150C on the other. The hot and cold gas is then passed through two giant gravel tanks, heating one and cooling the other. The power is stored in the gravel as a temperature difference.
When power is needed, the process can be instantly reversed and the heat pump now works like an engine — the extreme temperatures are brought together and turn the engine, which powers a generator to create electricity.
Today most of the world’s electricity storage systems are based on hydro-electric schemes with two reservoirs. When power is needed, water is released from the top reservoir to drive a turbine and then pass into the lower reservoir. When there is surplus power, the turbine pumps water back up to the higher reservoir. The disadvantage is that it takes a huge amount of power to pump the water back up. Dinorwig hydro station in Wales is run only at times of peak demand, when prices are highest.
Wagner said his team has proved the technology with a pair of prototypes and is now looking for funding to build a demonstration plant. Between him, the other founders and a handful of private investors, they have invested about £1.5m so far. Other companies are working on a similar technology including Saipem, the French industrial giant.