Unmanned Systems Technology 008 | Alti Transition UAS | Ground control systems | Xponential 2016 report | Insitu Orbital N20 | UAVs | Solar power | Oceanology International 2016 report

80 Focus | Solar power idea of reducing general wear and tear on UAVs through longer fights that replace three or even four previously shorter missions. Now that there are real products, real services and real customers, the value proposition starts to make sense. As the manufacturer says, “As everything, if you give them 10 Mbytes of data they will say, ‘This is great, but can you give me 20, or 100 is what I’d really like to see.’ We’re always starved for data, we’re always starved for technology, and power is really the enabler for that. So even in just the past year I think we’ve seen people ready to embrace much higher power at a bit of a higher dollar rate because the systems are no longer hobbies; they’re actually fight systems that are demanding more service, and our customers are willing to pay more.” Solar power as an enabler The large horizontal surfaces on fxed-wing UAVs can accommodate a signifcant quantity of solar cells. Aircraft such as the NASA/AeroVironment Helios demonstrated in the 1980s the potential for solar cells to transform the wing from a passive mechanical component into a primary power source, or to provide payload power with minimal impact on aerodynamics. Recent improvements in electric propulsion technology and wireless connectivity are now driving strong commercial interest in solar power for UAVs of all sizes. The dominant performance metric is effciency, but there are other metrics that are more relevant to UAV applications, such as the power-to-weight ratio. The objective of the UAV is to carry a payload, so any excess weight detracts from this ability. Since the solar panels are usually integrated into cantilevered wings, their weight can lead to an increase in the aircraft’s weight because of increased structural requirements. Therefore, the power-to-weight ratio of the solar technology being evaluated is a primary consideration in a UAV application. The total weight of the solar panel system is the sum of the weight of the cells themselves, plus the weight of the protective packaging needed for them to survive the expected operating environment. Some cell technologies need special thick or multi-layer packaging for acceptable lifetimes. This packaging can be heavy, so the total weight of the solar sub-module needs to be considered when comparing technologies using power-to-weight ratios. Another key metric is the power-to- area ratio. Surface area is limited, even on fxed-wing UAVs, so effciency is vital. Some solar technologies have high power-to-weight ratios but suffer from low conversion effciencies, which can be a handicap in UAV applications since the available area for mounting the panels is limited. Increasing wing area purely to accommodate additional solar power may not pay off, owing to increased structural weight and drag, and the relative size of the individual solar cells compared with the wing size also becomes a consideration, with smaller cells enabling higher packing densities. The ability and willingness of the vendor to provide customised sizes and shapes of solar cells and cell assemblies to ensure maximum use of the available area is therefore important. Choice of cells There are more than 20 photovoltaic technologies being actively pursued by manufacturers and research groups. Broadly, they can be divided into wafer- based and thin-flm types. The relevant wafer-based technologies for our purposes here are crystalline Silicon (c- Si) and Gallium Arsenide (GaAs), while the thin-flm technologies are amorphous Silicon (a-Si), Copper Indium Gallium Selenide (CIGS) and those based on thin Gallium-Arsenide (GaAs). The vast majority of existing solar cells and manufacturing capacity is of c-Si, typically made from wafers 6 in wide. The typical effciency of this technology is about 17% (or up to 20% for some newer versions of the technology) as measured under standard test conditions, but the effciency can drop off under real- world operating conditions of elevated temperatures and reduced illumination. By contrast, thin-flm solar cells use layers of semiconductor material that are 100 times thinner than c-Si wafers, so these cells can potentially be made into very thin and lightweight panels. In terms of solar cell chemistry there are three categories of cell. One is silicon, as used typically on rooftop solar arrays. It is a crystalline material, and you can have single-crystalline or poly-crystalline silicon, although usually the single-crystal variety gives higher performance. June/July 2016 | Unmanned Systems Technology A single solar cell (Courtesy of Alta Devices)