Combining 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.
The solar-soaring technology uses both thermal energy in the atmosphere and solar energy. “NRL is pioneering two new aspects,” says Dr Dan Edwards, senior aerospace engineer in the NRL’s Tactical Electronic Warfare Division. “First, the solar arrays are co-molded into the wing composite structure, meaning there is no ‘bump’ in the airfoil surface to disrupt the aerodynamics. Second, NRL is experimenting with combination autonomous soaring and onboard solar power, working to understand how these work together.”
Putting the new solar-soaring technology to the test
Part of the NRL program are flight endurance demonstrations to show how combination solar and soaring technologies can provide additional capability for defense and civilian uses.
The battery onboard the PV-SBXC aircraft 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. “With the PV-SBXC, we can take off roughly two hours before sunrise, fly all daylight hours and fully recharge the battery, then land roughly two hours after sunset,” Edwards says. “The two 11-hour endurance flights NRL has done have been takeoffs exactly at sunrise and landings just prior to sunset, with battery capacity to spare.”
The advantage of combining thermal and solar energy
The solar-soaring technology uses both thermal energy in the atmosphere and solar energy to power the aircraft. Edwards explains the advantage of using both sources of energy together: “Typically, thermal updrafts form beneath cumulous clouds, potentially shading a solar array as the aircraft flies beneath the cloud. Between thermals, however, it is typically sunny. Marrying the ability of soaring to gain energy beneath clouds with the ability of the solar arrays to gain energy in full sunlight provides dual environmental energy extraction. Alternately, the ability to soar can offset the lower efficiency of lower cost solar arrays to get the same performance.”
Teaching a computer how to soar like the birds
Gliding aircrafts basically fly freestyle — without a button to turn on the power. When air close to the Earth is warmed by the sun and starts to rise, it lifts the aircraft up by its wings. The NRL team now had to figure out how UAVs can use sensing and guidance algorithms to detect a thermal updraft to soar. “Soaring is essentially playing chess with the atmosphere, except the pieces are invisible and they move,” Edwards says. “Developing a robust sensing and guidance algorithm has been a challenge that NRL has made great strides toward solving. It has taken a lot of head-scratching, some mathematics, developing behavior-based algorithms and lots of flight testing to teach a computer how to soar like the birds.”
Although solar-powered aircrafts have been around for decades, the solar cells had not been efficient enough to justify the weight. This changed over the past ten years, when solar cells have become remarkably more efficient, lighter and more cost efficient. “However, the integration of solar into an airplane wing is a different challenge than a solar panel on the ground,” Edwards points out. “NRL went through several iterations making test coupons and small wing sections before being comfortable with our co-molding process for trying a full wing. The first time we watched the onboard battery state of charge increasing in charge while flying was the moment we realized the potential.”
Not throwing away free energy
The aerospace engineer notes that nearly every UAV mission — using small or large aircrafts —could use improved endurance, including industrial applications, such as traffic monitoring, farm crop surveying and emergency communications services after a natural disaster. “The core premise is that current aircrafts are throwing away free energy,” Edwards says. “Solar and thermal updraft energy is free for the capture, and NRL is developing the techniques and demonstrating the performance benefits.”
The NRL research team has already completed endurance flights with two of the different solar arrays, but Edwards says they have three more endurance flights to accomplish later this summer. “These remaining three arrays are all higher installed efficiency than the ones already flown, so we are considering adding a camera payload in order to demonstrate mission capability,” he adds.
Furthermore, the team is planning to put all the solar integration and soaring algorithms into the Hybrid Tiger aircraft, a follow-up UAV project with multiple days of endurance. Hybrid Tiger also includes a hydrogen fuel cell, which is able to provide power during nighttime operations, when the battery is otherwise depleted.
Written by Sandra Henderson, research editor Solar Novus Today