The limits of solar power in aviation are amply demonstrated by the Solar Impulse project.
With the ultimate objective of circumnavigating non-stop in a second-iteration aircraft, in 2010 the programme succeeded in flying non-stop for 26h - including, obviously, through the night - and, now, crossing continents on a multi-leg trip from its home base in Switzerland to Morocco, back across the Mediterranean to Madrid last week, with a final return to Payerne in Switzerland imminent.
But for all its success, Solar Impulse may not be paving the way for more capable sun-powered aircraft. The aircraft has the wingspan of an Airbus A340 but flies with, on average, the power output of a scooter. Generating even that much energy takes nearly 12,000 silicon solar cells covering the wing and horizontal stabiliser. At 22% these very thin (150 microns) cells provide good efficiency, but even better performance was foregone to avoid adding weight to the aircraft, which has the mass of an average-sized car, a quarter of which - 400kg (880lb)- is in batteries.
The huge wingspan houses 12,000 silicon solar cells
According to Bernd Rothe of Bayer MaterialScience, one of the project's key partners, it is best not to think about Solar Impulse as an aircraft project. Rather, he says, this is a renewable energy project and, at least from Bayer's perspective: "It's mainly about weight."
However, when it comes to weight, aircraft designers may ultimately owe much to Solar Impulse. Rothe is keen to point out that the project's pioneering use of a carbon nanotube-reinforced epoxy resin is dramatically improving the mechanical properties of the carbonfibre-reinforced plastic that makes up much of the aircraft's structure.
So far, he stresses, it is still midway through the project and there is a second-generation aircraft to build for the circumnavigation attempt. But now, he says, much of the aircraft is built of foam sandwiched between CFRP sheets - which, thanks to Bayer's nanotube-reinforced resin, are just "paper thin".