Published on May 24th, 2017 | by Tina Casey0
US Navy Lab Eyeballs Solar + Soaring For 24/7 Autonomous Flight
May 24th, 2017 by Tina Casey
You’ve probably heard of solar plus storage, and now the US Naval Research Laboratory has a new twist: solar plus soaring. The lab is looking to amp up the long distance capabilities of unmanned aircraft to provide 24-7 reconnaissance, by combining autonomous soaring algorithms with an onboard photovoltaic system.
Why Solar For US Navy?
For the Air Force, solar provides for energy security at its facilities and shaves a huge chunk out of energy costs. The Army has those concerns in mind plus the advantage of on-the-go solar charging for lightening soldiers’ loads, and transportable solar for forward operating bases.
For both branches, the green war of the future means less risk to warfighters and civilians working fuel drops and fuel convoys.
All of these advantages apply to the US Navy through the Marine Corps.
The Naval Research Lab’s so-named “Solar Photovoltaic and Autonomous Soaring Base” project is in support of the Marine Corps’s Expeditionary Energy Office, aka E2O.
E2O is tasked with this:
To change the way the Marine Corps employs energy and resources to increase combat effectiveness and reduce our need for logistics support.
The office does not foresee solar-powered electric vehicles and aircraft completely replacing liquid fuel in the near future, but it has set a pretty ambitious marker for everything else:
By 2025, we will deploy Marine Expeditionary Forces that can maneuver from the sea and sustain C4I and life support systems in place; the only liquid fuel needed will be for mobility systems which will be more efficient than systems are today.
C4I refers to command, control, computers and intelligence — in other words a whole mess of integrated electronic systems that require a reliable energy supply.
It’s also worth noting that the US military conducts and/or funds a significant amount of foundational research on solar energy.
Solar Plus Soaring For Long Range Aircraft
Where were we? Oh right, the new solar-integrated sailcraft.
Aircraft powered by photovoltaic arrays are practically old hat by now — and NRL is adding another layer of efficiency by combining solar with soaring.
To be clear, soaring and gliding are sometimes used interchangeably, but they are not the same thing. Let’s turn to Stanford University for an explanation — and a tip that modern soaring fans are already beginning to deploy solar power to extend their range:
Soaring is the sport of riding air currents to gain altitude which then is used to glide some distance through still or sinking air, to another source of lift where the process is repeated. In this manner, modern sailplanes (high performance gliders) have soared well over 2,000 km (1,200 miles) in a single day. Except for a brief initial launch using an auxiliary engine or a powered aircraft tug, this is done entirely on solar power, the source of the lift.
PV As A Range Extender
Expert soar-ers like the pilot cited above can keep a sailplane aloft for an impressive length of time. The challenge for the Naval Research Laboratory was to translate that kind of human-directed performance into an autonomous, unmanned aircraft and keep it running day and night.
Using the SB-XC sailplane as a platform, NRL deployed drop-in replacement wings with a photovoltaic array.
So far NRL has flown its autonomous sailplane for more than 10 hours on two test runs, to monitor the performance of two different PV systems (the system recharges a battery as well as providing DC current directly to the electric motor).
One system was built at NRL with SunPower solar cells. Here’s the rundown from NRL on that one:
The UAV with solar arrays built at NRL using SunPower Inc. solar cells, flew for 10 hours, 50 minutes on October 14, 2016. Takeoff occurred at 7:20 a.m. at 95 percent battery state of charge and landing occurred at 6:10 p.m. with the battery at 10 percent state of charge. Thermal activity was very good in the middle of the day and 40 percent of the flight was spent with the motor off, and the solar array partly recharged the battery while the motor was off.
The other test used solar wings developed by Alta Devices. That one flew a hair longer and ended up the flight with more charge in its battery, despite weak thermal activity. It appears that strong sunshine made up the difference, enabling the sailcraft to run entirely on its PV system for a period around noon.
In both tests, the sailplane took advantage of any thermal activity that it happened to come across in its flight path. The autonomous soaring algorithm will provide it with the capability to actively seek out and orbit nearby updrafts, “very similar to soaring birds” as the Navy puts it. That system was tested separately back in October 2015.
The tricky part is putting the two systems together while keeping bulk and weight to a minimum, so stay tuned for next steps.
By the way, if you’d like to DIY this at home the SB-XC is produced by a company called RnR. It comes in a kit and the wings dis-assemble for ease of transportation.
Photo: “Holding the photovoltaic (PV) UAV based on the SBXC sailplane, are members of the ‘Solar-Soaring’ research flight crew (l-r) Dan Edwards and Trent Young (not shown: Chris Bovais, Sam Carter, Matthew Kelly, and Dave Scheiman)” via NRL.