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Solar Technology for Drones Overview
Solar Power for Drones & Unmanned Systems
Recent developments in photovoltaic (PV) technology have made solar power a viable alternative for powering unmanned aircraft (UAV, UAS, RPAS, drones) as well as ground and marine based autonomous platforms USVs, ASVs. There are now many proven autonomous vehicle and aircraft designs that incorporate solar power technology.
Solar Powered Technology
Metrics used to gage photovoltaic technologies & suitability for solar UAV applications
Solar Technology Efficiency
The most commonly used parameter for comparing the performance of solar technologies is efficiency. Efficiency is a measure of how much of the incoming solar energy can be converted into usable electricity, and is affected by wavelength, reflectance, temperature, and a number of effects at the quantum level.
The records for solar efficiency currently stand at 29.1% for a single junction solar cell and 31.6% for a multi-junction solar cell. Multi-junction solar cells utilise multiple layers to capture light at multiple wavelengths, and are thus capable of achieving higher efficiencies than single-junction cells. The downside is that the manufacturing process is more challenging and expensive, making them more suitable for special high-budget solar UAV projects rather than mass-produced unmanned systems.
Photovoltaic Power to Weight Ratio
Power to weight ratio is calculated by dividing the power output of the PV system by its total weight. Integration of solar cells into the wings of a UAV may require structural adjustments, as well as protective encasing that will allow the solar cells to survive the demanding environments that the solar drone operates in. There may also be extra weight due to additional interconnects and cabling. All of these factors should be taken into account when calculating the total weight of the solar subsystem.
Solar Cells Power to Area Ratio
Power to area ratio is an important metric to consider, as the usable surface area for solar cells is limited even on large fixed-wing solar UAVs. Increasing the wing area of an unmanned aircraft design in order to accommodate more solar cells may result in an increase in structural weight and drag that is not compensated for by the gain in solar power.
Relative size of individual solar cells compared to the wing size is also as important characteristic, as smaller cells allow for higher packing densities.
Combined Performance Index
Considering the above metrics in isolation may not be sufficient to gauge the suitability of a solar technology for a particular drone application. High efficiency may come at the expense of an unacceptable increase in weight, and a high power-to-weight ratio may not be enough to overcome a low conversion efficiency, especially if the available wing surface area of the UAV is too small to provide sufficient power.
A combined solar performance index for UAVs has been proposed by photovoltaic technology developer Alta Devices, which takes into account both power-to-area and power-to-mass ratios, since high values for both parameters are extremely desirable for solar UAV applications.
The proposed solar index multiplies the two ratios and takes the square root of the product in order to provide an output with the dimension of power (watts).
Solar Powered Drone Technologies
A wide variety of photovoltaic technologies with different chemistries have been developed by manufacturers and research groups. They can be broadly divided into two groups – wafer-based and thin film-based. Below is a selection of photovoltaic technologies that could be used to produce solar power systems that can be integrated into drones and UAVs.
Alta Devices solar cell powered satellite
Crystalline Silicon Solar Cells
A large portion of the existing solar cell industry is centred around the manufacture of crystalline silicon wafers. This is a highly mature technology, and typically provides good efficiencies. However, the wafers are relatively thick, and are also brittle, requiring extra laminates to provide adequate protection. This can increase the total weight of the system by a significant amount and allow less flexible integration options, both factors that will particularly affect smaller UAVs.
Copper Indium Gallium Selenide (CIGS) Thin-Film Solar Cells
CIGS is a flexible thin-film technology that provides up to 13% efficiency. The cells are sensitive to moisture and a,ir and need to be encased in hermetically-sealing housings which add weight and reduce integration options. CIGS cells also suffer from reduced yield in high temperatures and low light intensity.
Amorphous Silicon (a-Si) Solar Cells
Amorphous silicon cells provide a lot of advantages – they are inexpensive, easy to integrate into structures, insensitive to moisture and air, and the technology is mature. However, they suffer from low efficiencies, meaning that most drones will have insufficient area on which to mount enough cells to meet the system’s power needs.
Alta Devices flexible GaAs Solar Cells
Gallium Arsenide (GaAs) Thin-Film Solar Cells
Thin-film GaAs is a newer technology that provides lightweight, flexible cells with high conversion efficiencies and high energy yields. GaAs thin-film cells are less sensitive to air and moisture and easier to integrate than CIGS cells, and the small cell size leads to high packing densities. These factors make them ideal for providing power to a wide range of solar UAV sizes and classes.
GaAs thin-film cells score higher than any other solar power technology on the combined solar performance index detailed earlier.
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