http://www.designfax.net/cms/dfx/opens/article-view-dfx.php?nid=4&bid=849&et=mechanical&pn=01

Researchers at the U.S. Naval Research Laboratory are developing
technology for unmanned aerial vehicles that has given them the ability to fly
for more than 12 hours by harvesting energy from the atmosphere and the sun.

"Sunlight heats up the surface of the Earth, which in turn heats the lowest
layer of air. That warm air eventually bubbles up as a rising air mass called a
thermal, which the airplane can use to gain altitude. It's indirectly solar
powered," said Edwards.

Solar power is also used directly to power the UAV using solar cells, which are
semiconductor devices that convert light into electricity. While these devices
have been around for some time, it was only recently that photovoltaic
technology advanced to the point where a UAV could be solar powered. For an
aircraft, every gram of weight has to be justified; essentially, it has to pull
its own weight. Until recently, solar cells were not worth the added weight.

"For a long time, even though there has been solar aircraft since the 1990s, the
efficiency of the solar cells wasn't high enough to pay the mass penalty,
meaning you weren't getting enough energy to justify the additional mass," said
Phil Jenkins, head of the Photovoltaics Section in NRL's Electronics Science &
Technology Division. "But over the last 10 years, that has really changed. The
cells have gotten more efficient and lighter."

The aircraft still carries a battery. However, the battery can be smaller
because of the solar and soaring capabilities on board.

"With Solar-Soaring, the UAV doesn't need a huge battery because it is getting
energy from the environment," said Edwards. "It just carries more intelligent
software in the case of the autonomous soaring algorithms, or a lightweight
integrated solar array that captures much more energy from the sun compared to
the amount of mass."

Bringing these two technologies together, NRL found the combination works better
than either individually. While soaring, the motor is turned off and the solar
array can recharge the onboard battery faster. This increases the mission
availability of a UAV for warfighters.

"Between the two, you have the most robust energy-harvesting platform because
sometimes you'll be able to soar and sometimes you won't have the solar, and
vice versa," said Jenkins.

Thanks to Dutcher Sterling.