|Fig. 1: Estimated Daily Irradiance from the Sun (Annual Average)  (Source: V. Troutman, after the DOE)|
Unmanned aerial vehicles (UAVs) are growing in popularity for military, commercial and personal applications. Small UAVs are generally battery powered, and larger UAVs utilize engines, both flight times are limited by either battery or fuel storage. One way to extend flight time is by utilizing the energy radiated by the sun. Two current solar-powered drones of note are Helios (NASA) and Aquila (Facebook).
The energy from the sun can be converted to photovoltaic (PV) electricity by converting the energy in sunlight into free charged particles. The amount of local sunlight that arrives to the earth depends on a large number of variables including atmospheric absorption, cloud cover, and the tilt of the earth with respect to the sun.  Irradiance is the measurement of energy per unit area (W / m²). Utilizing satellites images, the SUNY model produces estimates of irradiance at hourly intervals. Those values are used to understand throughout the United States an annual average of the irradiance within a day. The Southwest area of the United States sees the largest amount of irradiance each day throughout the year, averaging about 270 W/m² each day (6.5 kW hr / m² / day ) (Fig. 1). 
|Fig. 2: A) History of Solar Cell Efficiency  B) History of Battery Density  (Source: V. Troutman)|
Over decades, commercial PV modules that are used to construct solar panels have continued to increase in efficiency (Figure 2A). The efficiency of the solar module is the amount of sunlight that can be converted to electricity. Many factors play into the efficiency of solar panels including reflections, light with too much or too little energy, electrical resistance, and operating temperature. Currently the most efficient solar panels operate around 20% efficiency. 
A battery is a key component of any solar panel system. This allows for components that rely on electricity to function while sunlight is temporarily covered by clouds and during night hours. In the context of a solar drone, the limiting factor of the battery is the weight. Electricity storage is necessary, but there is a tradeoff, due to the additional energy required for the aircraft to carry the battery. The storage capacity per unit weight continues to increase (Figure 2B), with Lithium-ion batteries at the upper limit. Currently, the energy storage capacity of Lithium-ion batteries is around 8e5 J/kg. 
Under NASA's Environmental Research Aircraft and Sensor Technology project, a solar-powered UAV, Helios, has been under development to carry scientific instruments or telecommunications relay equipment. The approximately 730 kg aircraft is able to reach cruising speeds of 8.5-12 m/s with the use of fourteen 1.5 kW motors that drive two-blade propellers. It's wingspan of 75.3 m, allows for the wing area of 183.6 m² (wing thickness of .3 m) to be covered with solar cells that are approximately 19% efficient. The maximum wing loading of this aircraft is 40 Pa (N/m²), with the ability to carry a payload of about 90 kg . Helios is able to perform during daylight hours plus an additional five hours at night, and is designed to fly at an altitude of 15,000-21,000 m. 
As solar cells continue to increase in efficiency and batteries continue to increase in storage density, the capabilities of solar drones will continue to improve with respect to weight of payload and duration of flight. This will allow for a wider range of applications to be pursued, including heavier scientific instrumentation (air pollution, temperature), mail delivery or communication equipment. Utilizing solar-power not only increases potential flight times, but also eliminates pollutants that are emitted by traditional jet engines.
© Valerie Troutman. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Basic Photovoltaic Principles and Methods," U.S. Solar Energy Research Institute, SERI/SP-290-1448, February 1982.
 D. Renné et al., "Solar Resource Assessment", U.S. National Renewable Energy Laboratory, NREL/TP-581-42301, February 2008.
 C.X. Zu and H. Li, "Thermodynamic Analysis on Energy Densities of Natteries," Energy Environ. Sci. 4, 2614 (2011).
 "Helios Prototype", U.S. National Aeronautics and Space Administration, FS-2002-08-068 DFRC, August 2002.
 M.J. de Wild-Scholten, "Energy Payback Time and Carbon Footprint of Commercial Photovoltaic Systems," Sol. Energy Mat. Sol. C. 119, 296 (2013).