The main limitation currently facing the Unmanned Aerial Vehicle (UAV) market is the short flight time of these aircraft- that’s where Alta Devices comes in. Alta solar cells offer a way to capture energy while flying without compromising the weight, size, or maneuverability of an aircraft.
Researchers at the U.S. Naval Research Laboratory (NRL) chose our thin flexible GaAs technology to integrate into the surface of a UAV wing in order to extend flight time- and we delivered!
The following summary is based on information gathered from the article “Enhanced Endurance of a Unmanned Aerial Vehicles Using High Efficiency Si and III-V Solar Cells” (Scheiman, David, et al.).
Step One: Solar Integration
NRL retrofitted custom-built solar wings to the RnR Products SBXC sailplane kit. The UAV used has a wingspan of 4.5m and a total surface area of 1.3 square meters. It is hand launched and requires 70W minimum electrical power to fly.
In standard conditions the plane can fly for approximately 4 hours on battery power only. The battery is 6s6p arrangement with approximately 400 W-hrs. Single junction Alta Devices thin film GaAs solar cells measure 2 cm x 5 cm, and each half wing array was populated with 4 laminated matrices of 4 parallel x 18 series cells. The cells are interconnected with a conductive adhesive and a shingle pattern that maximizes the cell packing factor. NRL determined that ‘this (Alta’s) was the easiest of all wing assembly builds’.
Step Two: Flight Test
An endurance flight using the Alta Devices solar wing was conducted to start around sunrise, to maximize the number of hours of daylight and therefore available solar energy. Solar resource was fair with mostly sunny conditions experienced. The solar-covered UAV flew in a fixed rectangular area with a set minimum altitude. The flight duration was 11.2 hours between 7:30 and 18:52. The flight used 907 Whrs, with 587 Whrs supplied by the solar panel and 320 Whrs from the battery. Alta’s solar was able to provide almost two thirds of the energy for this flight!
This flight demonstrated that solar power can easily extend the fight duration by more than a factor of 2, and potentially even longer if soaring and optimized flying algorithms are used. When the UAV landed, the battery still had over 40% capacity, indicating the flight could have been further extended by 1 hour (at least) to over 12 hours of flight time.
Flight time can be further extended by optimizing a few parameters such as weather conditions (level of solar resource) and by using soaring and flying algorithms.
Because this aircraft was just for demonstration, the cells weren’t installed in a way to optimize surface area; fully populating the wing with solar would provide additional solar power and could extend the duration of the flight beyond sunrise and sunset. Longer flight times in commercial applications may reduce the need for additional aircraft, shorten mission timelines, or enable greater payload capacity.
The solar cells used for these flights were Alta Devices single junction Gen 3 (third generation). Recently we released our Gen 4 (fourth generation) single junction solar technology which is 30% lighter and will further extend endurance. Next year (in 2018) we will release our dual junction (2J) Gen4 product which in addition to being very lightweight will have increased efficiency. Performance data for these new products is currently available on our website.
NRL’s flight tests using Alta Devices solar cells proved that solar is an effective way to significantly extend flight duration and in this case by almost a factor of 3.
The Alta Devices solar-powered flight demonstrated a viable solar integration process and long endurance capability. The information for this paper was gathered from the article “Enhanced Endurance of a Unmanned Aerial Vehicles Using High Efficiency Si and III-V Solar Cells” written by David Scheiman, Raymond Hoheisel, Daniel J Edward, Andrew Paulsen, Justin Lorentzen, Steve Carruthers, Sam Carter, Matthew Kelly, Phillip Jenkins, and Robert Walters.