http://www.avweb.com/avwebflash/news/Solar-Powered-Aircraft-Still-Hobbled-by-Batteries-224100-1.html
Solar Powered Aircraft: Still Hobbled By Batteries

By Paul Bertorelli

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
http://www.altadevices.com/ 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.
http://www.avweb.com/podcast/Podcast-Batteries-the-Solar-Airplane-Challenge-224102-1.html

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.
Podcast: Batteries the Solar Airplane Challenge:
http://www.avweb.com/podcast/Podcast-Batteries-the-Solar-Airplane-Challenge-224102-1.html

By Paul Bertorelli

Producing electrical power from the sun has come a long way, and solar cells
are getting remarkably efficient. But where to put all that power remains the
issue for pure electric aircraft. AVweb's Paul Bertorelli spoke with Rich
Kapusta of California-based Alta Devices about the issues and the
opportunities.