The first thing Dax Kepshire shows a visitor to SustainX is foam. When he flips a switch on a 10-foot-tall assemblage of pneumatic tubes and mesh screens, the machine starts spewing a creamy white goo that resembles soft-serve ice cream into a 300-gallon plastic tub. In a few minutes the foam is 3 feet deep, and Kepshire plunges his hand in. “It’s completely nontoxic,” he says, and derived from an industrial foaming agent found in shampoo and carwash soap. The seven-year-old company is betting the substance can solve the biggest challenge for renewable energy: how to store it...

l to r: Ben Bollinger ’04 Th’04, ’08, Troy McBride Th’01, Professor Charles Hutchinson, and Dax Kepshire Th’06, ’09 founded SustainX to develop a fuel-free compressed air system to efficiently capture and store energy.

...Kepshire’s discovery of the foam was an accident. In 2006 he was pursuing his Ph.D. in engineering at Dartmouth when he and fellow student Ben Bollinger sketched out plans for a water-cooled air-compression system for renewable energy, aided by Charles Hutchinson, the dean of their engineering school, and Troy McBride Th’01, at the time a professor of physics and engineering at Elizabethtown College in Pennsylvania. They founded SustainX the following year: “We raised venture capital right out of the gate—all from a napkin idea,” Kepshire says. They hit a wall in 2009, as their prototype proved ineffective beyond small trials. They added anticorrosive and disinfectant chemicals to the water to reduce wear on their machinery and to kill bacteria and algae, and the mixture began to foam up. SustainX tried to figure out how to stop that from happening, until engineers noticed that the foam was preventing the wild temperature swings.

At that point, the SustainX system could store about two days worth of juice for the average U.S. household. The company’s latest prototype, modified from a marine diesel engine typically used on cruise ships, uses foam throughout and feeds compressed air into 84 steel tanks housed in a 6,000-square-foot warehouse. The setup stores enough air to produce as much as 1,500 kilowatt hours of electricity, and power 50 typical households for a day. “Like any development, there are always some tweaks to make,” says Stephen Brown, the company’s vice president for engineering. Upgrades to various seals and valves last month have bumped up the system’s efficiency, meaning the ratio of its useful power output to total power input, to 54 percent, he says.