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NRL flies solar UAV

Research flight crew (l-r) Dan Edwards and Trent Young (not shown: Chris Bovais, Sam Carter, Matthew Kelly, and Dave Scheiman). (NRL)

Project to fly surveillance UAV using PV array and battery shows progress

Researchers at the US Naval Research Laboratory (NRL), Vehicle Research Section and Photovoltaic Section are building on the proven concept of autonomous cooperative soaring of unmanned aerial vehicles (UAVs). Their research has paired solar photovoltaics (PV) with “autonomous soaring techniques” with a view to enabling endurance flights of unmanned sailplanes that use the power of the Sun.

Together, the “Solar Photovoltaic and Autonomous Soaring Base Program” and the Marine Corps’ Expeditionary Energy Office (E2O) want are looking to develop unmanned platforms to support so-called information, surveillance, and reconnaissance (ISR) missions, and to reduce the fuel and/or batteries needed to be resupplied.

Similar efforts have already been made in the commercial sphere, although projects like Facebook’s solar-powered internet drone have had mixed results.

“NRL has twice flown our solar UAV [based on the SBXC sailplane] over 10 hours using a combination of solar photovoltaics and autonomous soaring as part of the ‘solar-soaring’ research program,” aerospace engineer Dr. Dan Edwards commented.

In the PV-SBXC aircraft itself, the centre wing panel has been fitted with a custom-built PV array as a drop-in replacement to the original wing. A power management and distribution system converts the power from the solar arrays into direct current (DC) voltage, which the electric motor can use for direct propulsion, or to recharge its battery. The power management system was provided by Packet Digital, Inc., as part of a grant from the North Dakota Renewable Energy Council.

In addition to solar power, natural thermal updrafts keep the craft airborne. The autonomous soaring algorithm —typically used to monitor the local vertical winds around the aircraft — commands the aircraft to orbit in any nearby updrafts, very similar to soaring birds (although this capacity was not tested on recent flights). Passive soaring was permitted, where the aircraft could decide to turn the motor off if altitude increased because of an updraft during its flight path.

UAV takes off at sunrise (U.S. Naval Research Laboratory)
UAV takes off at sunrise (NRL)

Test flights

A UAV with solar arrays built at NRL using SunPower solar cells, flew for 10 hours, 50 minutes on October 14, 2016. The craft took off with a 95% battery charge, and landed 10% state of charge, although 40% of the flight time was spent with the motor off thanks to favourable conditions, during which the battery was recharged.

A second UAV equipped with PV arrays from Alta Devices flew for 11 hours, 2 minutes on April 19, 2017. Takeoff occurred with the battery at 90%, and landing at 26%. Here, thermal activity was very weak and almost all of the flight was spent running the motor. Near solar noon, the solar array provided sufficient power to cruise on solar power alone.

“The experiments confirm significant endurance gains are possible by leveraging thermal updrafts and incident solar radiation, rather than ignoring these free sources of energy,” Edwards stated. “Future testing will focus on quantifying the trade space between improvements in solar cell efficiency and combining with autonomous soaring for improved solar-recharging.”

Source: Navy Research Laboratory


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