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a 2 kWh cell arrangement. The battery system is shown in Fig. 6. The Phoenix prototype was used as a test bed for evaluating various (COTS) PV technologies that could potentially be available for an operational vehicle; their development and selection was managed by the Centre for Process Innovation and IQE. Given the emphasis on minimising weight, and their application to curved surfaces, the PV cells need to be thin, lightweight and flexible. Consequently, three types were identified and selected for assessment on the flight test vehicle, based on the following criteria – low cost, availability in large areas, low mass and high performance (conversion efficiency). However, after evaluating the COTS products available, it was determined that none of them met all those criteria, such that any choice inevitably involved some sort of compromise. Subsequently the following products were selected – III-V semiconductor, three-junction inverted metamorphic cells from Microlink Devices, low-cost silicon crystalline PV cells from Sunpower, and flexible CIGS cells from Global Solar. The first is the best-performing technology available (and the most expensive), the second is a relatively high-efficiency low- cost semi-flexible silicon solar cell, and the third are low-cost flexible solar cells with reasonable efficiency. The Microlink cells were located on the tail, while the Sunpower and Global Solar cells covered most of the upper surfaces of the wings. The latter, being the most flexible, were located at the wing leading edge where the surface curvature is greatest. The silicon and CIGS options are the least costly but have far lower performance than the III-V solution, so they yield much lower energy per unit area. On the other hand, the Microlink technology is far more expensive, and would therefore significantly add to the aircraft’s costs, should it be the chosen technology eventually. Although the flight trials were conducted at night, the performance of the cells as part of the complete power supply storage and generation system during separate tests were carried out during daylight, as shown in Fig. 7 (page 30). Those tests concluded that this PV configuration was sufficient to recharge the battery during daylight hours, and that the battery could power the aircraft. The only element of the power solution initially envisaged that did not make it onto the aircraft was a reversible hydrogen fuel cell, developed by the University of Newcastle. This was prohibited for the indoor flight trials owing to Health & Safety requirements, but it also remains at a lower Technology Readiness level than the rest of the constituent components, although enough data to model its eventual inclusion and effects have been acquired. Phoenix UAS | Dossier