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90 which this is accomplished will largely depend on the end-user’s preferences. For example, an unmanned vehicle in need of significant voltage would need an increasing number of cells connected in a series circuit, as each cell added to the circuit will generally tack on a fixed voltage increase. Indeed, the required voltage for a UAV power system might often be what primarily determines how many cells are arranged in a string. On the other hand, if increasing the current while minimising the number of modules is desirable, parallel circuit construction may be needed. Beyond the choice of series or parallel connections, several other configuration choices must be made. For example, the shape of a UAV’s solar module ought to be matched to the area and physical dimensions of the wing, between the leading and trailing edges (or any flaps or ailerons, if the UAV uses any). Integration can be far easier if the optimally sized module has been designed as a single piece, so it is essential to consider PV integration in the early design stages. For HALE systems with solar architectures supplying more than 1 kW of power, physically separating PV leads can be a critical safety concern, to prevent arcing and minimise the chances of the leads accidentally touching each other. Otherwise, there could be large voltage differences in close proximity, threatening the operation of the vehicle and the safety of the ground crew when the vehicle is brought down to land. For thin polycrystalline silicon cells on a USV or ground vehicle, say, the electrical connections might be in the form of ultra-thin copper wires that form a fine mesh on the cell surface, potentially resulting in thousands of usable contact points. Metal fasteners or structural adhesives are then typically used to attach the material to the unmanned vehicle. Protective layers After the cells have been configured for arrangement on a vehicle’s surface, it is critical that some form of protective coating or laminate is applied; this is the final layer on top of the junctions and connections of the solar module. There are various qualities and requirements that can determine the kind of coating layer used. As a primary example, with each material interface that the sunlight comes into contact with, there is a difference in its refractive index (the ratio of light’s velocity in a vacuum to its velocity in the material medium concerned). This difference causes reflections in the light, which can increasingly reflect it away from each semiconductor structure. The greater the differences, the more solar energy is lost. As a result, a special anti-reflective coating (ARC) is commonly deposited onto the cells to mitigate these disparities and to enhance the amount of light received by the PV structure. In addition to ARCs, which may need to take into account the vehicle’s altitude and the relative angle to the sun among other factors, the solar material must be protected from its operating environment. If it is not, its lifespan – and potentially that of its host vehicle as well – can be significantly reduced. For low-altitude unmanned systems, or ground or sea vehicles, that means the laminate selected might need to shield against dust, moisture, salt spray and similar issues. In the case of USVs, the coating would need to be highly water resistant and protect against the corrosive effects of constant exposure to saltwater. USVs subjected to frequent moderate waves, sea states or submergence would also benefit from a design aimed at collecting the limited and shorter- wavelength spectrum beneath the water. As the use case moves into the territories of HALE UAVs and unmanned spacecraft in low Earth orbit, which operate in conditions free from dust, moisture and adverse weather, UV radiation becomes a mounting concern, June/July 2018 | Unmanned Systems Technology After manufacture, the cells must be soldered and electrically connected to the exact parameters of their intended environment (Courtesy of Solbian)