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

82 Monocrystalline solar cells are made out of silicon ingots, which are cylindrical in shape. To optimise performance, four sides are cut out of the cylindrical ingots to make silicon wafers. Unlike monocrystalline-based solar panels, for poly-crystalline solar panels raw silicon is melted and poured into a square mould, which is cooled and cut into perfectly square wafers. Some crystalline materials have been used on UAVs but it tends to be relatively heavy, as the cells have to be thicker, and the effciency is typically only in the 15-18% range. The second category is CIGS technology, part of the class of what would be called amorphous solar cells, and there is also amorphous silicon. They are not crystalline, as amorphous implies, and because of that they tend to have much lower effciency. So where CIGS cells are typically in the 10-12% effciency range, amorphous silicon would be even lower than that, perhaps 6-8%. Despite that limited effciency CIGS cells are relatively light and inexpensive, so they are suitable where effciency is not the key factor, but as soon as something more effcient is needed a third category of cells, like a GaAs type or a III-V type cell which can give effciencies of 25-35%, is required. Silicon is in column 4 in the periodic table which means that silicon atoms can bond to silicon atoms and make a perfect crystal structure. If you move one column left, and one column right, you have a 3 and a 5 which can also bond together to make a perfect crystal. This is how Gallium and Arsine form GaAs. There are two benefts of using a 3-5 (III-V) instead of a 4 (or silicon). First – they work better for converting light to electricity. This has to do with something called the band gap of the material. GaAs is a direct band gap material and silicon is an indirect band gap material. The second beneft has to do with junctions. You can mix and match various atoms to create new 3-5 junctions while keeping the crystal intact. This allows for better overall performance. Multi-junction cells There are two ways to make solar cells work more effciently. One is to make each junction better. The other is to add more and more junctions. A junction is a layer in the solar cell that captures a certain portion of light better. A typical solar cell has only a single junction, and captures effciently only in the wavelength that’s closest to the properties of the materials used; for silicon that’s infrared. Any wavelength in the solar spectrum that has higher energy – be it visible red, orange, yellow, green or blue – will have progressively higher energies per photon. It is possible to convert all those photons into electrical power, but with blue photons about half the energy is wasted in conversion in a silicon cell because the cell is converting it at the infrared photon energy. It’s a quantum conversion: a cell’s quantum effciency June/July 2016 | Unmanned Systems Technology The layers of an inverted metamorphic multi-junction cell (Courtesy of ML Devices) Solar panels integrated into the wings of a UAV (Courtesy of ML Devices)