Icarus unbound

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Aug 29th 2008
From Economist.com

The solar plane is finally becoming a possibility


ICARUS had the right idea about climbing above the clouds where the sun can work its wonders. Unfortunately, solar energy and aeronautical engineering were not two of his stronger points. Today, we know enough about both technologies to make a better job of it.

Yet whenever your correspondent gazes out from a window seat at the shiny surface of a plane’s wings and fuselage, he wonders why aircraft designers don’t take advantage of such an ideal platform for solar cells. Back-of-the-envelope calculations suggest they could help offset fuel costs by enough to make a difference.

Sure, better engine design could perhaps more readily achieve such an improvement, but that would take billions in research and development.

Also, adding thin films of solar arrays to an aircraft adds weight—though it’s less of a burden today than in the past, when airliners were left in their pristine metal state to save the weight of a coat of paint.

Solar ImpulseBut two events this past week have given your correspondent cause for optimism. One was the appeal by two Swiss adventurers, Bertrand Piccard (of the famous family of explorers) and Andre Borschberg, at the Beijing Olympics for sponsors to help them build a solar-powered aircraft capable of circumnavigating the globe.

With the wingspan of a commercial airliner, the one-man Solar Impulse (pictured above) is designed to climb on solar power to almost 30,000 feet during the day, and then gently glide on thermals, uplift and battery power down to 6,000 feet at night—repeating the procedure for several days at a time. While the plane should be able to fly more or less indefinitely, the pilot will need to make numerous landings to sleep, and to replenish his food, water and oxygen supplies.

The other interesting development was the three-and-a-half day flight by an unmanned, solar-powered plane called Zephyr. The 66-pound reconnaissance plane, built by the QinetiQ group in Britain, was guided by autopilot and satellite to an altitude of over 60,000 feet, drawing power from the sun during the day while relying on rechargeable lithium-sulphur batteries at night. That demonstration flight, made from an American army testing ground in Arizona, more than doubled the world record for unmanned flight, set by Northrop Grumman’s Global Hawk in 2001.

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The answer, at least to him, was clearly a rowing eight. Suitably designed, your correspondent felt a man-powered plane based loosely on the architecture of a lightweight racing shell would not only deliver more horsepower per pound, but could also serve as a handy amphibious craft in case of mid-Channel ditching.

Luckier than Icarus, the design never got off the drawing-board. Calculations showed the wing area needed to support eight strapping oarsmen plus a weedy coxswain would have been too unwieldy to fabricate with materials of the day.

Ever since, your correspondent has restricted his enthusiasm to machine-powered flight. But now, thanks to the Chinese, that both lightweight batteries and more efficient solar cells are coming within his grasp, an electric microlite is beginning to seem a surprisingly attractive proposition.

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The energy density of batteries has improved by leaps and bounds over the past decade, thanks to laptop and mobile-phone makers on the one hand, and the motor industry’s growing enthusiasm for hybrid cars on the other. Rechargeable batteries based on once-exotic pairings such as nickel-metal hydride, lithium-ion, lithium-polymer and even silver-zinc are now finding their way into everyday life.

Meanwhile, the price of photovoltaic solar cells is set to plummet from $3.80 per watt today to $1.40 by 2010, as the industry quintuples capacity (mostly in China) and embraces cheaper and more efficient designs based on cadmium telluride instead of polycrystalline silicon. At those prices, solar power could finally achieve “grid parity” with fossil-fuel electricity, costing 10-15 cents per kilowatt-hour.

Solar aircraft would therefore seem poised for take off.

In a sense, the two solar planes in the news this past week are a validation of battery technology more than solar-cell developments. Given enough wing area, the lightweight solar arrays available today can generate enough electricity to drive the plane’s propellers (four on the Solar Impulse; one on the Zephyr) as well as recharge the batteries for flying at night.

The pilot’s only goal, says Mr Piccard, is “to reach the next sunrise before his batteries are empty.” The solar cells can then take care of the rest.

That’s the big difference. No previous solar plane could fly at night. In 1980, the Gossamer Penguin, built by Paul MacCready of AeroVironment in California, became the first manned plane to demonstrate sustained flight under solar power alone. A derivative, called the Solar Challenger, crossed the English Channel in 1981. But neither of these—nor the Sunseeker used in 1990 by Eric Raymond of Solar Flight in California to cross America in 21 hops—could stay aloft for more than a few hundred miles at a time.

Interestingly, the Gossamer Penguin was a scaled-down version of the man-powered Gossamer Albatross, which won the Royal Aeronautical Society’s £100,000 Kremer prize in 1979 for “pedalling” across the English Channel. A chain-driven pedal crank turned a propeller, using motive power supplied by a young Californian cyclist named Bryan Allen. The magnificent Gossamer Albatross is nowadays on display at the National Air and Space Museum in Washington, DC.

As a fledgling aeronautical engineer at the time, with Kremer ambitions of his own, your correspondent was convinced that pedal power alone was not enough. The muscle output per unit of body weight from a just pair of legs—rather than from the pilot’s arms and back as well—seemed sorely lacking.