Sunday, August 26, 2012

Making clean energy pay off by storing it as squeezed air

A stumbling block to increasing our reliance on electricity from cleaner energy sources such as solar panels and wind farms has always been figuring out how to efficiently store the energy for use when the wind isn’t blowing and the sun isn’t shining. Danielle Fong could make clean energy significantly more practical on a large scale by introducing a novel way to use tanks of compressed air for energy storage. “It could radically reorient the economics of renewable energy,” she says.

Danielle Fong
The idea of using compressed air to store energy is not new. Electricity from solar panels or wind turbines can turn a motor that’s used to compress the air in a large tank, and the air pressure can then be converted into power to drive a generator when the power is needed. The problem is that during compression the air reaches temperatures of almost 1,000 °C. That means energy is lost in the form of heat, and storage in conventional steel vessels becomes impractical.

Fong stumbled on a possible solution while skimming through a nearly century-old book: water spray is great at cooling air. She asked, why not spray water into the air while compressing it, so that the air stays cool? To make the process practical, she developed a technique for separating the heated water from the compressed air and diverting the water into a tank, so the heat can be recaptured to minimize energy loss. The process is about as efficient as the best batteries: for every 10 kilowatt-hours of electricity that goes into the system, seven kilowatt-hours can be used when needed.

Fong founded a company called LightSail Energy in Berkeley, California, to develop the technology. Initially, she planned to produce compressed-air-powered scooters. But backer Vinod Khosla of the venture capital firm Khosla Ventures convinced her to go after the much bigger market of electricity for the power grid.

Batteries are the current state of the art in storing excess wind and solar energy, but Fong says the LightSail system will cost less to purchase and will last for a decade or more. Over the long term, she says, the system could cost as little as one-tenth as much to own and operate as batteries do. A single system, which is about the size of a shipping container plus a car-size unit, will store the energy generated by a one-megawatt wind turbine running for three hours. More


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