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91 Solar power | Focus and protective layers need to be established against this instead. Furthermore, before the operating environment becomes a concern, the coating must initially protect against rough handling on the ground, otherwise cracks in the semiconductor material can occur. Any damage to the coating itself can also expose the cell to UV radiation or other environmental threats. For these reasons, and to maintain transmissibility of light through ARC layers, a laminate should also be reasonably easy to clean before and after missions. Also, while using as many ingredients as possible in the surface coating might sound appealing (as if to protect against as many operational threats as feasible), minimising weight remains crucial for aerial systems, and to a lesser extent other vehicle types. The protective layer must therefore not add too much weight, even if there are a lot of factors that the UAV designer wishes to protect their solar material from. Given the precise and distinct differences between each unmanned vehicle’s operating environment and envelope, it is to be expected that each PV module’s coating system will be highly customised. Partially as a result of this customisability, and the advantages that a closely tailored coating can bring, the exact chemical nature and constituents of the protective layers offered by industry suppliers tend to remain as confidential intellectual property. Key metrics Given the range of options that an unmanned vehicle developer can choose from when looking to integrate solar cells into their design, it is vital that they understand which key specifications are the most important to consider. Some of these may be the same across different markets, missions and environments. Efficiency for example is a common aim among manufacturers to enhance their vehicles’ mission length and carrying capability. The percentage of solar energy captured by a PV cell is therefore important, and a frequently cited figure for many cell types. Given the limited amount of surface area on a typical unmanned vehicle, the amount of power supplied per square metre is an additional important metric for efficiency that designers should consider. On the other hand, surface area might not be a particular worry for a large, broad, fixed-wing UAV. When there is enough area, the specific power (or power-to-weight ratio) can demonstrate how much power is generated at the cost of mass – hence, as noted above, the reasoning behind choosing solar cells over a larger or additional battery. This metric also helps define why thin-film single-junction or multi- junction cells tend to be the material of choice for UAVs over silicon in its various forms. Adding any significant weight relative to increased power will reduce the overall flight endurance and maximum payload capacity. It should be noted however that the specific power claimed by a solar product should be checked to ensure that the weight used in forming the figure takes all the other relevant factors into account. If the power-to-weight ratio only accounts for the weight of the bare cell, and not the anticipated extra weight of wiring, laminates or other ancillary items required for a final product, the Unmanned Systems Technology | June/July 2018 Solar panels on USVs must be laminated to protect against saltwater and corrosion (Courtesy of Solbian) The wings tend to present the most sizeable surface area for integrating solar cells onto a UAV’s hull (Courtesy of SolAero Technologies)