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

83 Solar power | Focus value indicates the amount of current that the cell will provide when irradiated by photons of a particular wavelength. Solar cells can also be manufactured with multiple junctions, however, where layers of cells are grown on top of one another and with each layer fne-tuned for a particular spectrum of sunlight. So, one junction would be manufactured with chemistry optimised for blue, others for yellow, red and infrared. In this case it is possible to harvest more energy out of each captured photon. If these four junctions are connected in series, each produces its own voltage. Multi-junction cells have multiple p-n junctions made of different semiconductor materials. A p-n junction is a boundary or interface between two types of semiconductor material, p-type and n-type, inside a single crystal of semiconductor. The p (positive) side contains an excess of electron holes – the lack of an electron at a position where one could exist in an atom – while the n (negative) side contains an excess of electrons. Each material’s p-n junction will produce electric current in response to different wavelengths of light. The use of multiple semiconducting materials allows a broader range of wavelengths to be absorbed, improving the cell’s effciency at converting sunlight to electrical energy. However, this effciency is gained at the cost of increased complexity and manufacturing price. Despite their higher price-to- performance ratio though, they are well suited to aerospace, where their high power-to-weight ratio is desirable. There are cells on the market that are single, double, triple and quad junctions. Triple junction is very typical for, and is the state of the art in, space applications right now. Several cell manufacturers are developing effcient and cost-effective four-junction cells, which will deliver about another 10% of power over triple- junction versions. The best triple-junction cells in a terrestrial environment at the moment would achieve about 32% effciency, and moving to four junctions would increase that to about 35-36% in terrestrial sunlight. One supplier points out however that while simply adding more and more junctions is all well and good, increasing the effciency of each junction matters more. Series, parallel, voltage and current To formulate the most practical solar panel for a particular UAV involves connecting numerous cells in various confgurations. Nominally each cell produces 2.2 V, so to achieve an 18 V system for example, nine of them (perhaps ten to allow for a little headroom) would be connected in series. For a 50 V system more cells are stacked to get a voltage closer to the operating voltage. The advantage of this ability to connect cells in series to deliver the required voltage is that there is no need for step-up or -down electronics. In fact, power electronics technology, although sophisticated, is so well advanced that a UAV solar system can use off-the-shelf electronic converters and charge controllers that offer effciencies as high as 90%. Cells are connected in series to get more voltage. In a series connection, each cell makes its own contribution to the total, but it’s the current that does all the work, and because a solar cell is converting photons of light into electrons, the electrons are the current: the more electrons, the more current. Each solar cell has a particular inherent voltage, and that doesn’t change with its area but its current increases with area, so you choose a size for an individual solar cell that puts out a certain current. As an example, a typical triple-junction cell that covers 25 cm 2 will produce something like 0.5 A at 2.2 V. According to basic electrical theory, the power in Watts of a system is voltage multiplied by current, so with 2.2 multiplied by 0.5, each cell will deliver 1.1 W. If you need 22 V out of the system, there will be ten cells connected in series. Each will provide the 2.2 V and the current will be the same, 0.5 A, in each of the ten. With ten cells of course, 22 V at 0.5 A gives a total of 11 W. If you need more current, then more cells will be connected in parallel. If two were connected in parallel they would deliver 0.5 A each, so 1 A. With the same Unmanned Systems Technology | June/July 2016 Thin-film solar cells use layers of semiconductor material that are 100 times thinner than c-Si wafers (Courtesy of SolAero Technologies)