Solar Impulse is a long-range solar plane project currently under study at the EPFL. The project is promoted by Bertrand Piccard and aims at completely solar-powered circumnavigation. The aircraft is intended to be a one-seater, capable of taking off autonomously, and remain airborne for days. Once the efficiency of the batteries makes it possible to reduce the weight, a two-seater is planned to make circumnavigation possible. Take-off is proposed for May 2011, for a flight around the world near the equator, but essentially in the northern hemisphere. Five stops are planned to change pilots. Each leg will last 3-4 days, limited by the physiology of the human pilot.
The wingspan of Solar Impulse will be 80 metres, slightly wider than the wingspan of an Airbus A380, in order to minimise drag and offer a maximum surface for solar cells. Such light wing loading (8 kg/m�) creates greater sensitivity to turbulence. The ultra-light structure must use customised carbon fibres. While traditional sandwich composites have an area density of 10 kg/m�, those developed for Solar Impulse should weigh 0.5 kg/m�. These materials could also have functionality integrated, such as integrity sensors, active control of the form, etc. A layer of ultra-thin solar cells will be integrated to the wings. These cells are designed to be flexible enough to withstand deformations and vibrations. Photovoltaic cells will generate electricity during the day, which will serve both to propel the plane and to recharge the batteries to allow flight at night. Energy accumulated during the day will be stored in lithium batteries in the wings, the density of which must be close to 200 Wh/kg, in spite of temperatures ranging from +80 degree C to �60 degree C. The average power provided to the engines will be around 12 hp. The cockpit will provide pressurisation, oxygen and various environmental support to the pilot to allow a cruise altitude of 12,000 metres.
There are eight thermoplastic parts already developed for the Solar Impulse. A prototype with a 200-ft wingspan is under construction in Switzerland and initial test flights are expected in one year. The purpose of the prototype is to validate mechanical engineering concepts being developed for the plane. These include new lightweight plastic parts from Solvay, one of the principal sponsors of the project. The newly developed parts include:
    •  Battery Binder: Solef polyvinylidene fluoride
    •  Hinges (Inserts): Ixef polyarylamide
    •  Throttle Housing: Radel polyphenylsulfone
    •  Bushings: Torlon polyamide-imide and KetaSpire polyetheretherketone
    •  Foam Cockpit Eggshell: Solkane foam
    •  Circuit Board Spacers: PrimoSpire self-reinforced polyphenylene
    •  Bolts & Screws: PrimoSpire SRP
    •  Lubricant: Fomblin perfluoropolyether