The solar impulse is an electrical aircraft that use only the power of the sun to flight. It has the wingpan of a A340 and the weiht of a car. The average flying speed is 70 km/h, and just one person can be in the aircraft. The question of energy defines the entire project. At midday, each square metre of land surface, in the form of light energy, receives the equivalent of 1000 watts, or 1.3 horsepower of light power. Over 24 hours, this averages out at just 250W/m². With 200m² of photovoltaic cells and a 12 % total efficiency of the propulsion chain, the plane’s motors achieve an average power of 8 HP or 6kW – roughly the amount of power the Wright brothers had a available to them in 1903 when they made their first powered flight. And it is with that energy, optimized from the solar panel to the propeller, that Solar Impulse achieved to fly day and night without fuel!
The 12 000 or so photovoltaic cells in 145 micron monocrystalline silicon combine lightness and efficiency. Their efficiency could have been even higher, like the panels used in space, but their weight would then have penalized the plane during night flight. This phase being the most critical, the main constraint is the one imposed by the batteries. Still heavy, they require a drastic reduction of the weight of the rest of the plane, so as to optimize the whole energy chain and to maximize the aerodynamic performance provided by a large wing span and a wing profile designed for low speeds.