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.



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.



Fong and the LightSail team had to come up with a filtering system capable of separating the water from the highly compressed air. Another challenge was to design a system that could handle both compressing the air and expanding it to drive a generator; previous efforts have required two separate systems.



Not only did LightSail meet those challenges, but it managed to find a compound—the company won’t provide details—that can be used more efficiently than steel to make compressed-air storage tanks. Tanks made from this material also don’t need the costly underground installation that’s normally required. And unlike standard systems, LightSail’s doesn’t need the turbine to run at a fairly constant speed to get efficient compression, meaning it is better able to cope with intermittent wind conditions.



Fong says there are no technical barriers to building units large enough to power entire cities. The company plans to manufacture the systems, and she says several renewable-energy developers have already signed on as customers. The first pilot unit is scheduled to ship in late 2013 or 2014—but she is still hoping to see those compressed-­air scooters.



—Rachel Metz