Solar Impulse 2 landed back in Abu Dhabi yesterday after a 15 month round trip of 17 legs and 40,000 km. The next stage is to develop an unmanned version of the craft to operate at high altitudes as an aerial basestation similar to Facebook’s Aquila UAV which saw its first test flight this week. The prototype of the Solar Impulse version, supported by ABB and Solvay, is planned for 2019.
The single seat Swiss craft uses 17,248 photovoltaic cells that produce a peak power of 66kW from the 270m2 coverage. These supply four 41kWh lithium ion batteries that weigh a total of 633kg, a quarter of the total weight of the craft at 2.3 tonnes. Four 41kWh motors drive the aircraft at speeds up to 140 km/h (87mph) and up to 12,000 m (39,000 ft).
The problem with the batteries occurred during the longest leg of the trip between Japan and Hawaii in 2015. Too much insulation around the cells meant the batteries overheated and had to be replaced. This led to a ten month delay over the winter. There were also problems with the 'virtual co-pilot' electronic system that allowed a single pilot to travel for up to five days. The lessons from the battery management system and the autopilot will be applied to the autonomous version.