The use of unmanned aerial vehicles (UAVs) has been increasing worldwide as demands for agile, rapid, targeted response forces have grown. UAVs provide surveillance and detection capabilities to these smaller forces that are unmatched by other technologies. As the reliance on UAVs increases, the short endurance times of existing UAV solutions become a significant factor. Existing commercial solutions have endurance ranging from 45 minutes to 4 hours, limiting both operational range and loiter capabilities. To increase endurance, UAV manufacturers have begun to target technologies ranging from increased performance batteries, improved control algorithms, fuel cells and other alternative power sources. While solar powered UAVs have long been a desired goal, existing solar technologies have characteristics that have made solar integration difficult and limited their integration.
The most basic challenge is the limited surface area with which to generate solar energy. While most small UAVs fly at altitudes of a few thousand feet or less, the available solar resource is typically between 900 and 1000W/m2 on bright, clear days. As such, using 20% efficient solar cells, a UAV with 0.5m2 of wing area and 80% packing density (ratio of active area to inactive area) would generate 80W in optimal conditions (20% * 1000W/m2* 0.5m2 * 80% packing density). The solar resource will significantly reduce in overcast weather or during the hours close to sunrise or sunset. These factors demonstrate how important conversion efficiency as well as packing density is for the selected solar technology.
Another major challenge is weight. As the lift force required for flight has to be equal to the weight of all the elements constituting the UAV, every component used has been engineered to provide all necessary functionality and the lowest possible weight. Solar solutions are no different and can be evaluated on a simplistic “grams per W” basis for first order comparisons. While evaluating weight, encapsulation requirements (materials required to protect the cells for reliability or durability purposes) must not be ignored. Often these materials can add multiples of the weight of the solar cells themselves.
Beyond these basic challenges lie more complex questions. How will the solar integrate into the existing electrical system? How will the control surfaces on the UAV or its flight pattern shade the solar cells and how tolerant are they to this? What electronics are required to maximize the production of the solar system under all conditions? How robust is this material to rough field handling?
The most optimal solar solution is a material that optimizes and responds to all demands: highly efficient, lightweight, easily integrated and robust to varying illumination and handling. This has been the major shortcoming of previous solar solutions. They are capable of answering one or two of the demands but fall short in other areas, leaving the overall solution lacking.
Standard monocrystalline silicon cells have reached efficiencies as high as 24% and are produced by a wide number of vendors. However, the physical form of the cells (130 to 200um thick, 150 x 150mm rigid wafers) makes them difficult to integrate into complex aerodynamic surfaces, fairly weighty and difficult to pack densely into small UAV wings. Typical research airplanes using crystalline silicon cells have focused on designing a plane around the cells rather than integrating the cells into a known, proven UAV platform.
aSi (amorphous silicon) cells are lightweight and flexible, allowing for easier integration. However, their low efficiencies (ranging from 6-8%) make them unsuitable for use. Some of the CIGS (copper indium gallium selenide) cell technologies represent a middle-of-the-road solution. With efficiencies up to 16% and flexible substrates, they seem to be decent options but are still limited by lower efficiencies and added weight to protect them from moisture.
Mobile Power Solution
Recently, Alta Devices commercialized a new type of mobile power technology that is uniquely qualified for use in UAV applications. This new technology is based on GaAs (gallium arsenide) rather than silicon or CIGS and optimized to provide record breaking single junction efficiencies (28.8%) significantly higher than those capable in any other material. Additionally, due to the manufacturing techniques used, these cells are light and flexible, allowing for easy integration directly into existing UAV designs with little to no modification necessary.