In 1999, Bertrand Piccard of Switzerland and Brian Jones of England became the first people in history to circumnavigate the world nonstop in a hot-air balloon. Their successful 19-day journey in the Breitling Orbiter 3 earned the pair a jubilant welcome home to Geneva and a visit with the Queen of England at a balloon factory in Bristol. Now, 45-year-old psychiatrist Piccard proposes to duplicate the feat in an airplane powered entirely by photovoltaic cells.
Piccard is developing his project, dubbed Solar Impulse, in consultation with scientists and engineers at the Swisscole Polytechnique Fdrale de Lausanne (EPFL). The EPFL previously contributed technical expertise both to Piccard's balloon adventure and to the Swiss yacht Alinghi, the America's Cup winner in 2003.
The first piloted solar aircraft with enough energy to stay up and to climb was the Solar Challenger, made by AeroVironment Inc., in Monrovia, Calif., which in 1981 flew 262 kilometers from Paris to London in 5 hours and 22 minutes. AeroVironment was founded by Paul B. MacCready Jr., whose human-powered Gossamer Albatross was pedaled across the English Channel in 1979 by a bicycle racer.
AeroVironment has moved on to other projects, among them unmanned air vehicles such as Helios, the high-altitude solar-powered communications platform that crashed on 26 June 2003. Now, Piccard is taking the baton in the race to promote manned solar aviation.
The EPFL team, which includes some 30 experts from 10 laboratories, just completed a yearlong feasibility study that involved analyzing existing relevant technology and designs. As a result, says Yves Perriard, director of the EPFL Integrated Actuators Laboratory and one of the lead scientists of the study, "we know that it is possible to create a structure completely powered by the sun."
Though the feasibility of photovoltaic-powered flight has been demonstrated, solar-powered airplanes differ a lot from conventional aircraft, posing special challenges. First of all, getting above the clouds is essential, as all available sunlight must be captured. But flying at high altitudes, where the air is thinner, eats more power than flying low, making for nasty tradeoffs involving wingspan, solar-cell carrying capacity, lift, and weight. Requirements are all the tougher because Solar Impulse will fly around the clock and therefore must capture and store enough solar energy during the day to carry it through the night.
To do that, it will need a very broad wingspan, on the order of 60 meters [see photo, " Wide Wings"]. That adds structural complexity and weight, at some cost to performance. Compounding the difficulty is the need to keep a human comfortable at an altitude of 10 000 to 11 000 meters, where the temperature is around -55 degrees C. When the plane is ready to test on longer flights, the cockpit will have to be pressurized. And the autopilot, besides taking care of altitude and direction, will have to regulate the entire electrical system and govern allocation of incoming energy.
Of course, construction of the plane will depend on the latest in ultralight, multifunctional materials. But the plane's electrical requirements pose the most demanding engineering challenges. "It's really a war against all the losses we will have in the power system from the solar cells to the motors," EPFL's Perriard told IEEE Spectrum.
Most helpful of all would be batteries with higher energy densities. Right now, off-the-shelf lithium-ion batteries provide just under 200 watthours per kilogram, enough to support a plane with a single pilot. Two pilots would be safer and more fun, but that would require getting the capacity up to 250 or, ideally, to 300 Wh/kg.
Piccard, who is currently looking for sponsors for the project, has assembled a separate team of specialists--including meteorologists, aerodynamicists, and pilots--whose job it will be to actually design and build the aircraft. Prototypes will be constructed over the next two years, with short test flights to begin in 2006. Ultra-long-distance flights are planned for 2009. "It will take an elegantly crafted vehicle, flown in meteorological conditions that are hard to find," says AeroVironment's MacCready, "but it's doable."
In the 1930s, Piccard's grandfather Auguste pioneered manned balloon flight into the stratosphere and invented a submersible he called a bathyscaph. In 1960, his father, Jacques, and a fellow diver set a world record by plunging 11 km to the deepest point of Earth in a later-model bathyscaph. Piccard, who by his own account has "piloted many things," has an even grander vision for Solar Impulse.
He wants to channel the public's enthusiasm for a major technological development into support for renewable energy. People usually associate renewable energy with having to give something up, says Piccard. "But the goal [of Solar Impulse] is to pull them into a big adventure."