Unmanned Systems Technology 020 | Alpha 800 I Additive Manufacturing focus I USVs insight I Pegasus GE70 I GuardBot I AUVSI Xponential 2018 show report I Solar Power focus I CUAV Expo Europe 2018 show report

85 S olar panels have traditionally been most closely associated with spacecraft such as satellites and unmanned probes. In the pursuit of ever-greater capabilities for electrically powered UAVs, however, the use of photovoltaic (PV) cells has become well-established as a means of enhancing mission flight times, payload weight limits or both. There are now many proven craft that have solar technology integrated into their designs, typically on the wings to maximise the scope for receiving sunlight. In the case of a UAV, integrating solar cells effectively adds a small amount of weight (from the PV module and potentially from larger batteries) to it, in exchange for the improved operating specifications. The technology also enables persistent operations for the high-altitude long endurance (HALE) ‘pseudo-satellite’ UAVs being trialled by companies such as Facebook and formerly Google, for continued flight during night time. In addition to air systems, the past several years have witnessed the emergence of an increasing number of USVs that incorporate solar cells, often used alongside complementary power systems such as diesel generators or varying forms of wave propulsion. In a similar way to HALE-class UAVs, these ocean-going platforms integrate PV modules to better enable long-endurance missions such as coastal surveillance or environmental monitoring. Material choices The global market for solar cells remains dominated by crystalline silicon (c-Si), often referred to as the ‘first generation’ PV cell material. Commercial c-Si modules tend to have efficiencies (rates at which sunlight is converted into electricity) of 16-17%, although in the case of monocrystalline silicon, efficiencies of about 23% can be achieved. Although c-Si has been the most widely used and best-known material for converting light into energy, there are other materials, configurations and production approaches for solar systems. This is critical, as the optimal PV module architecture for a given unmanned vehicle can depend greatly on factors such as the desired voltage, altitude, operating temperature and the vehicle’s angle relative to the sun. And because it is an indirect band-gap material (which allows photons to penetrate much further than in direct band-gap materials before being absorbed), silicon must be manufactured with a certain thickness and concurrent weight in order to capture solar energy. If it is too thin, it will not function, as light will simply pass right through it without being captured. That may not be an issue for USVs and spacecraft that can withstand the weight and operate in environments such as water or in space, but for UAV manufacturers who need to minimise weight or optimise the aerodynamics, c-Si is unsuitable. The purpose of integrating PV cells into an unmanned system could be to replace the weight of a battery with a solar module to extend mission life, but if the PV panel weighs more than a battery of equivalent power then the manufacturer could merely install a larger battery as a quicker and simpler solution. Fortunately, there are a range of alternatives for UAVs as well as some low-Earth orbit spacecraft (or those launched as spaceplanes). For example, thin-film solar cells produced from direct band-gap semiconductors can be made much lighter and more flexible than silicon owing to their faster light Solar power | Focus Unmanned Systems Technology | June/July 2018 Thin-film solar cells can conform precisely to airfoils in order to maintain aerodynamics (Courtesy of Ascent Solar) The optimal PV architecture for a given unmanned vehicle can depend greatly on factors such as its angle relative to the sun