SOLAR TODAY Blog

By Seth Masia
SOLAR TODAY deputy editor

The 26-hour flight of the Solar Impulse made front-page news around the world, and was great press for progress in solar technology. In particular it proved that photovoltaic cell and battery efficiencies have reached the stage where a vehicle can store sufficient energy during 15 hours of daylight to sustain flight through a 9-hour night.

Part of that energy storage was in the form of altitude: during the long day, pilot André Borschberg climbed to 8,700 meters (28,500 feet), and during the course of the night descended to 1,500 meters (5,000 feet). That's 7.2 vertical kilometers (4.5 miles) of potential energy converted into flying speed.

At a cruise speed of 23 knots (42 kph), Solar Impulse isn't going anywhere very fast, though a transatlantic flight is the next big goal. The shortest Atlantic crossing, from Senegal to Brazil, has prevailing easterly trade winds. With a 25-knot tailwind, the plane could make that trip westbound in about 27 hours. That close to the equator, the plane would only get 12 hours of daylight for battery charging. The alternative is to ride the westerly winds eastbound along low-arctic route from Labrador to Greenland to Scotland, taking advantage of 20 hours or more of midsummer daylight. With a 25-knot tailwind, that might be a 40-hour flight, with just four or five hours of darkness in the middle.

Solar Impulse is essentially a motor-glider. It climbs using four 10-horsepower motors, so its maximum power draw is 30 kilowatts. A 400-kg lithium-ion battery pack stores about 88 kilowatt-hours, so in theory the motors could run at half power for just under six hours. Half power (15 kilowatts) is marginal to sustain level flight, hence the gradual loss of altitude at night. 200 square meters of monocrystalline silicon PV cells on the wing and horizontal stabilizer, at 22 percent efficiency, ought to generate 44 kw in full sun, so basking on the tarmac for two hours at midday would bring the batteries to full charge. Cruising at altitude during the six or seven hours of high sun, at least 30 watts is available to recharge the batteries after the initial climb.

Solar Impulse is not the first man-carrying solar aircraft capable of long-distance flight. That honor went to Paul MacCready's Solar Challenger, which in 1981 flew from Paris to Manston in England, 163 miles (262 km), piloted by Steve Ptacek. In 1990, Eric Raymond flew his 2.4 hp Sunseeker, powered by 1.8 kw of PV cells, across the United States in 100 hours, spread over 21 days. In 1999 he flew Sunseeker II across the Alps.

Solar Impulse is not the largest solar aircraft, nor the altitude champ: both those records are held by Helios, a MacCready-designed aircraft built under a NASA contract. With a wingspan of 247 feet (75 meters), Helios climbed to 96,863 feet (29,500 meters) in August 2001, remotely piloted by Greg Kendall. Helios was being prepared for a 24-hour flight in 2003 when it was destroyed in wind-shear turbulence.