Solar Powered Aircraft: Still Hobbled By Batteries

With Solar Impulse plying its way around the globe, solar-powered flight is getting plenty of press, but there are dozens of other quieter projects already in service or in development. A California company called Alta Devices is building super-efficient solar panels for these, but the company recently told us that as with pure electric aircraft, battery technology remains a barrier.

"The challenge is, every day the batteries get discharged and recharged and you go through the night and the day cycle again. To find a battery technology with enough energy density that's lightweight and can be recharged with a five-year endurance, that's a tough one. I don't know if that exists today. There are a lot of people working on that problem," says Alta's Rich Kapusta, whom AVweb interviewed at the recent AUVSI convention in Atlanta.

Alta has been in business since 2008 and set a world record for solar cell efficiency of 28.8 percent, in 2011, according to Kapusta. He says there are marginal gains in efficiency still to be had, but single-sun cells—that is, those that don't rely on concentrated sunlight—are approaching their theoretical maximum efficiency.

Nonetheless, Alta's products are already finding wide use in active military programs, including the small man-launched UAVs such as the Raven and Puma. The wings of those aircraft are covered with Alta's lightweight solar cells, which extends their loiter time from two to three hours to as much as nine hours, Kapusta said. Although Alta wouldn't confirm it, the company's products may be in use on Google's planned "atmospheric satellite" long-duration UAV currently being constructed in New Mexico.

Alta is leveraging and improving expensive gallium arsenide technology traditionally limited to satellite power systems. "What we do is make this super thin layer; it's a crystal structure. We grow it on a substrate and etch it off that substrate. We end up with an almost aluminum foil-like solar cell. It keeps all of the attributes of gallium arsenide, but we're able to get the costs out of it because we use less raw material to make that solar cell," Kapusta said. You can hear a podcast of AVweb's interview here.