Researchers at the U.S. Naval Research Laboratory (NRL) are developing technology for unmanned aerial vehicles (UAV) that has given them the ability to fly for more than 12 hours by harvesting energy from the atmosphere and the sun. Solar-Soaring is a pair of endurance-enhancing technologies. They aid the warfighter by enabling a UAV to fly longer without carrying extra weight in batteries.
“One of the common complaints that we hear across industry and the warfighters is that they want aircraft to fly longer,” said Dr. Dan Edwards, senior aerospace engineer in NRL’s Tactical Electronic Warfare Division. “One great way to do this is to capture atmospheric wind energy or solar energy to extend the endurance.”
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,” said Edwards. “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.”
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 and 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 on-board 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.
The NRL-developed technologies are applicable to platforms that are already in use by the military, such as the Raven, a small hand-launched remote-controlled UAV or the Predator, a larger UAV.
“In the case of Solar-Soaring, we’re demonstrating the techniques to fly aircraft with a higher endurance,” said Edwards. “These techniques are portable to a lot of the programs of record, like the small-size Raven or potentially the larger Predator, so it’s a pretty broad application space.”
Having a UAV with extended endurance capabilities is important for military information, surveillance, and reconnaissance missions, or a communications relay. The technology also has important uses for civilian applications, including monitoring and inspection of railways and oil pipelines, surveying crops, and search and rescue.
“The technology could be very useful for coastal monitoring or pollution monitoring, for example,” said Jenkins. “In these cases, you just want eyes up there for hours and hours, and Solar-Soaring makes that possible.”
Both Edwards and Jenkins identified a hurdle they would eventually have to overcome with Solar Soaring – the ability to fly through the night.
“We still can’t fly through the night because the batteries are just too heavy, but we currently get dawn to dusk-enhanced endurance,” said Jenkins.
For Edwards, the next step in solving this problem is swapping out the battery for a hydrogen fuel cell.
“Fuel cells have much more energy per unit mass than a battery,” said Edwards. “So we’re marrying the fuel cells, which are great for getting through the night, and the Solar-Soaring, which is great in the daytime for getting energy directly from the sun and wind.”
Although the Solar Soaring technology was a success, it did not come without its challenges.
“There’s always an interesting jump from pure math, pure theory, to the application space,” said Edwards. “Some algorithms look great in simulation, but just doesn’t give the desired results in the real world with noisy data. Real thermals are so much more complex than in simulations, so we have had to fly a lot to find out what is robust in the real world.”
Jenkins spends his time at the lab developing solar cells on a small scale, so having the opportunity to take the solar cell technology and apply it was both challenging and rewarding.
“It is fun to see the application of advanced solar cells at work, as opposed to when you’re developing new solar cell technology, where the end product is usually a published report,” said Jenkins. “Here we have something that is very close to the end application.”