Extending the endurance of electric unmanned aerial vehicles (Drones, UAV, UAS, RPAS) is an important consideration for many drone, UAV and autonomous vehicle manufacturers. While this can be achieved by adding batteries, the extra weight and space often makes this counterproductive. Developments in solar power technology have made photovoltaic (PV) technology a possible alternative for powering UAVs, drones and other unmanned aircraft.
Rotary UAVs generally do not have enough usable space on the aircraft to place solar panels. They are less efficient at generating lift than fixed-wing aircraft, meaning that scaling up in size presents a significant engineering challenge. An experimental 100% solar powered quadcopter has been developed by a team at the University of Singapore – however, it is extremely thin and currently incapable of carrying a significant payload.
Small fixed-wing UAS may have enough surface area to integrate solar panels that will increase the endurance of the aircraft. For existing UAV platforms, if a sufficiently thin and flexible solar technology is selected, this can even be done without redesigning the structure of the UAV.
HALE UAS, also known as HAPS (High-Altitude Pseudo-Satellites), are intended to maintain continuous stratospheric operation for periods of many months. Applications of HALE UAVs include persistent surveillance and providing communications in areas where network infrastructure is subpar or non-existent.
Currently, no HALE UAS can perform year-round missions. However, due to growth in the solar industry and unmanned systems technology sectors, research and development of HALE platforms is increasing, and performance continues to improve.
Batteries alone cannot sustain continuous operation of these High Altitude Long Endurance UAVs, and so solar cells are a critical part of their power systems.
Due to the altitudes at which they operate, HALE UAS face unique conditions. Higher altitudes mean lower temperatures, which actually increase the power output of solar cells. The factor by which performance is boosted depends on the temperature coefficient of the material, which in turn can vary over the wide temperature ranges found at these altitudes.
HALE UAS typically operate at altitudes of 15 to 24km, providing them with unobstructed solar irradiation as well as a degree of protection from ultraviolet radiation due to the ozone layer. In addition to temperature, the solar performance of these cells also depends on spectral irradiance, which in turn varies with time of day, time of year, and angle of incidence of the solar cell to the sun.
Because of their size, weight is an exponentially growing concern for HALE UAVs. Solar cells for HALE UAS need to be lightweight as well as flexible in order to allow for easy integration into the wing structure. Due to the harsh environments in which these aircraft operate, the UAV solar cells and their encapsulation system need to be highly resistant to extreme temperatures, vibration and flexing.
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