UAVs fly for more than 12 hours by harvesting solar energy directly and indirectly. Source: NRLUAVs fly for more than 12 hours by harvesting solar energy directly and indirectly. Source: NRLCombining autonomous soaring and onboard solar power, new technology developed at the U.S. Naval Research Laboratory (NRL) allows unmanned aerial vehicles (UAVs) to fly for more than 12 hours without increasing battery weight.

With solar-soaring technology, the UAV autopilot system is directed by sensing and guidance algorithms until a thermal updraft is encountered. The aircraft is then commanded to circle in the rising air, similar to the way a bird flies. Lightweight solar cells co-molded into the wing composite structure also provide power for UAV propulsion.

The onboard battery lasts for approximately four hours, with a 30-minute reserve. From about two hours after sunrise, the built-in solar arrays provide enough electrical power to stay aloft until about two hours before sunset. During those hours of adequate sunlight, any excess power is used to recharge the battery. While soaring, the motor is turned off and the solar array can recharge the onboard battery faster, increasing the mission availability of a UAV for warfighters.

A UAV with extended endurance capabilities is important for military information, surveillance and reconnaissance missions, or a communications relay. The technology can also benefit civilian applications, including monitoring and inspection of railways and oil pipelines, surveying crops, and search and rescue.

The researchers next want to develop the capacity for night flight. Use of hydrogen fuel cells instead of batteries in the solar-soaring system might enable the realization of nocturnal missions.

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