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92 Focus | Solar power unmanned vehicle could experience a net loss in efficiency after the cells have been integrated. Some metrics can be difficult to quantify but are no less important when looking to develop an efficient vehicle. For example, endurance and payload capacity can be reduced by any notable increases in drag. The flexibility, smoothness and aerodynamics of a given solar module should therefore be closely examined to ensure the material will conform closely to the shape of a UAV’s wings. To maximise uniformity across the integrated solar cells (so that the desired airflow occurs across the length of the wing), the part count should be minimised, especially if the end-user has to undertake a lot of wiring, fastening or gluing. The overall ruggedness of a PV module is also worth considering, since for medium- or low-altitude UAVs, or USVs operating in sea states above 2, waves or hard landings can easily damage the cells. The number (and degree of redundancy) of contact points can also be important in providing multiple circuit paths in the event of such impacts. The nature of the unmanned system in question can be a useful indicator of which metrics matter. For example, for a tactical UAV designed for defence and security operations, ruggedness, power-to-weight ratio and power per square metre tend to be considered the most important. By contrast, for commercial UAVs at competitive prices, the cost per kilogramme of the cells is more important. High production volumes of the cells can also be useful in terms of costs and time spent in manufacture for commercial UAVs intended for manufacture in batches, or for military vehicles designed to be produced in bulk for swarm operations. For HALE applications, weight becomes an exponentially sensitive issue, so cells that are as light as possible will be desirable. The temperature extremes experienced by HALE and orbital craft can also mean needing to know the thermal expansion coefficients of different PV materials and how they differ from those of the vehicles’ hull materials. As noted, resistance to weather, dust and hard landings do not pose any major concerns with pseudo-satellite type aircraft or orbital vehicles. However, the length of service of such systems can place greater emphasis on the importance of a solar cell’s lifespan, and its end-of-life power generation. From low Earth orbit to geosynchronous orbit, mass remains an important consideration because of its close relation to the cost of launching systems into space – tens of thousands of dollars can be saved for each unit of weight eliminated. In addition to the aforementioned UV radiation concerns, thermal management can be critical as the solar cells cannot be cooled by air convection. Temperatures of more than 80 C can be experienced if a PV module cannot radiate heat back into space. At the other end of the scale, their survivability in temperatures as low as -100 C when crossing the Earth’s shadow is also vital. The integration of solar cells into an unmanned vehicle can pose a delicate balancing act. Many of the design qualities and anticipated operating environment of the system, its missions and its lifespan, need to be known before an accurate picture of the optimal solar configuration can be drawn up. However, as the quantity of unmanned systems manufacturers and the range of platforms they offer expands, every advantage counts. The number of vehicles making use of solar cells remains in the minority, but that number is rising. Endurance records are being achieved through the judicious application of PV modules and dedicated architecture of unmanned vehicles centred on the accommodation of solar-augmented powertrains. The list of prior knowledge and understanding needed for going solar is long, but the results can make it extremely worthwhile. Acknowledgements The author would like to thank Rich Kapusta of Alta Devices, Ray Chan of MicroLink Devices, Luca Bonci of Solbian, Kenneth Steele and Brad Clevenger of SolAero Technologies, and Victor Lee and Joe Kigin of Ascent Solar for their help with researching this article. June/July 2018 | Unmanned Systems Technology ITALY Solbian +39 011 966 35 12 www.solbian.eu USA Alta Devices +1 408 988 8600 www.altadevices.com Ascent Solar Technologies +1 720 872 5000 www.ascentsolar.com MicroLink Devices +1 847 588 3001 www.mldevices.com SolAero Technologies +1 505 332 5000 www.solaerotech.com Some examples of solar power-related suppliers