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42 Focus | Solar cells microgroove sub-modules, connected in series or parallel, it is far less sensitive to damage or defects. The company has tested demonstrator sheets after puncturing them in various ways to prove that most of the sheet can continue generating energy as normal. Non-reflective microstructures Other architectural solutions designed around surface microstructures have been developed to enhance the quantities of light that modules can absorb. Panels designed with plain, smooth surfaces are prone to reflecting sunlight, and even when specially formulated with anti-reflective properties these glass or plastic outermost layers will often lose around 4% of the sunlight shining on them owing to reflection. One of the most prominent ways to reduce that loss involves designing and constructing the module surfaces with rows of micro-structural pyramids, each one about 5 µm tall. They are positioned next to one another with their faces angled such that most of the light reflected from one face hits another on a neighbour for ‘another pass’, rather than being reflected back into the sky. As a result, panels made in this way (and from materials with the appropriate optical qualities) reduce reflective losses from around 4% down to about 0.3%. This reduction is maintained up to angles of incidence of 60 º , which corresponds to flights occurring with the sun sitting 30 º above the horizon. It also enables greater absorption of sunlight at angles of incidence beyond the critical angle compared with standard modules. Again, this structural innovation can be performed with a variety of materials. Its developer supplies both GaAs and Si modules constructed in this way, albeit with the standard engineering differences between them still included, such as the inclusion of bypass diodes in GaAs modules for when parts of their PV panels become damaged or shaded. This structural approach has been tested in various road and maritime applications, where it has also proved its benefits for structural strength. The brittle nature of solar panels can make them susceptible to damage from gravel or debris, but including a micro-pyramidal outer surface has been found to toughen and even ruggedise the cells as well as improve their efficiency. In the UAV sector, aerospace solar panels designed in line with this technology are thinner and less rugged than their automotive and maritime counterparts, owing to weight concerns. Iterating such products over time though has improved the technology to the point that there is no major difference in weight compared with standard encapsulation. Also, given the size of these micro- structures, aerodynamics and drag remain unaffected by their presence. That is a critical improvement over past attempts at this kind of technology, which historically were prone to inducing micro-turbulences and aerodynamic losses at the structural level. Modern versions incur only an increase in manufacturing costs as the trade-off for their higher solar absorption. Multiple busbars The design of the electrical connectivity between cells is another essential consideration for the viability of solar as a serious power source or range extension medium. To derive power from the sun, electrons have to be carried effectively from sunlight to electrical contacts. However, solar cells are made from semiconductors, not conductors, so peripheral components for electrical wiring must be chosen and modelled carefully to ensure high carrier mobility and minimal resistive losses. Busbars are a critical component in this aspect of solar modules. Often visible as thin metallic strips at the ends of the panels (except in some architectures that install electrical contacts on the backs of panels), they act as conductive highways for electrons to flow from the PV material to the batteries in the form of direct current. Modules can usually continue working after damage to them that causes the loss of one or two cells, or just part of one cell. Adding more wiring or other connections ensures that electrons can still make their way around a non- operational cell to the busbar. However, by forcing increased current into some wires, the damage can eventually lead to bottlenecks of current in those connections. These accelerate electrical resistance losses, causing hotspots. October/November 2020 | Unmanned Systems Technology A comparison of conventional smooth-faced solar modules (top) with modules constructed using pyramidal microstructures (Courtesy of Gochermann Solar Technology)