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Solar cells on the wings of the Solar Impulse.
Credit: Solar Impulse
To Fly Around the World - Without Fuel
By Stephen Leahy

In the era of climate change, the sun could be the future energy source for the aviation industry.

DÜBENDORF, Switzerland, Sep 14 (Tierramérica).- A solar-powered aircraft will take flight next month from Switzerland with hopes ultimately to circle the Earth in 2012, without any stops or any fuel.

"I'm intrigued by the vision of perpetual flight," mechanical engineer Andre Borschberg, chief executive of the 100-million-dollar Solar Impulse project, told Tierramérica.

Designed to use only energy from the sun during the day and run on sun-charged batteries at night, it could stay aloft perpetually, like the giant, thin-winged Arctic tern that migrates annually from the Antarctic to the Arctic, non-stop.

"The big lesson of the Wright brothers is that if you don't try you never succeed," Borschberg told Tierramérica in the Dübendorf Airfield hanger outside of Zurich where the first prototype was being assembled for a test flight in October or November.

The U.S. inventors Orville and Wilber Wright are credited with the first airplane flight in 1903. "They never dreamed that a plane could cross the Atlantic Ocean, and yet less than 25 years later (in 1927) Charles Lindbergh flew from New York to Paris," Borschberg said.

"It is our hope that the Solar Impulse will be a symbol to the world and create awareness about our own energy use," he added.

The Solar Impulse HB-SIA prototype plane is essentially a 64-meter long, thin wing with four small propellers and narrow pilot pod and tail attached underneath.

Sitting in the airport hangar, even with its inner workings exposed, it doesn't look like much more than a big paper glider - not much for 100 million dollars.

It is in fact part glider. When the sun is shinning the Solar Impulse will slowly climb to an altitude of 8,000 or 9,000 meters. When the sun sets, it begins a powered glide, using energy from the batteries as it slowly falls to 1,500 meters. And when the sun rises again it brings new energy to recharge the aircraft for it to climb back into the sky.

"The key is energy efficiency, extracting maximal power from minimal energy and using as little of it as possible," Borschberg explained.

Weight was the biggest challenge to overcome. The answer was a very basic structure made of extremely light carbon fiber.

While the prototype has a wingspan identical in length to an Airbus A340, a large commercial passenger airplane that weighs in at 260 metric tons without cargo, this one-man plane is just 1,500 kilograms - about the weight of a small car.

There has been no breakthrough in solar or battery technology here. The solar cells are actually a thin film glued to the top of the wing. Rated at 22 percent efficiency, they are not the most effective available - but produce the most energy for their weight.

A major part of that weight is the 400 kg of lithium-ion batteries, which are similar to those in mobile phones but twice as efficient, Borschberg said.

The four electric propellers generate less than 10 horsepower in total, similar to small motor scooter.

The airplane will take off and land under its own power and fly at a relatively leisurely speed of 70 km/hour, far too slow for almost anything else to fly - except birds.

It can only stay aloft because it is so lightweight. The best sail planes have a weight-to-surface area of 40 kg per square meter while the Solar Impulse is eight kg per square meter, he said.

"With the existing technology, this is the best that can be done," he said.

It has taken Borschberg, an accomplished pilot, and partner Bertrand Piccard, a well-known Swiss adventurer, six years to get this far.

The idea came to Piccard while he was completing the first around-the-world balloon flight in 1999. He resolved to try again but without fuel or polluting emissions. The grand exploits of the 21st century will be about preserving or improving "the quality of life on our planet," he said in a statement.

More than 70 people are now working on the project, including 50 engineers, physicists, materials specialists and computer scientists. The next big hurdle is the first test flight in the next couple months, when they will find out how such a large but extremely lightweight aircraft handles.

Next year Borschberg hopes the Solar Impulse will be become the first solar airplane to fly day and night, for 36 hours non-stop, without fuel.

If all goes well then, the final version of the aircraft will built to circumnavigate the planet in 2012, with five giant hops of 5,000 km over five days of non-stop flying.

Borschberg and Piccard will take turns piloting the flight legs. "That is the other big challenge: how to pilot, eat and sleep for five days and nights," said Borschberg.

Sleep deprivation, even for just 24 hours, impairs anyone's abilities and perception. However, the Solar Impulse project has conducted intensive research into the matter and found that 20-minute catnaps, spaced out properly, can forestall those effects.

Eschewing an alarm clock, they have developed a special shirt with sensors and a vibrating system that can be remotely activated to make sure the pilot only sleeps in increments of 20 minutes.

Even if they are successful, Borschberg does not foresee solar powered commercial air travel. "Solar could be a complementary energy source, but without some major breakthrough I don't see it," he said.

The International Air Transport Association, which represents virtually all 230 commercial airlines, has a goal of carbon emission free flight by 2050, he said. "That's a very ambitious target."

* IPS correspondent.

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