Since 2005, Edwards has been exploring how to teach an autopilot how to soar using thermals in the atmosphere, much like how a bird flies. Using special sensing and guidance algorithms, the UAV flies a waypoint route until it senses a thermal updraft, then commands the aircraft to circle in the rising air.
“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.