Get ready for the world's first solar-powered circumnavigation

After flying a plane across the United States last summer using nothing but the sun for power, Swiss pilots Andre Borschberg and Bertrand Piccard have their eye on the whole enchilada. On Wednesday, April 9, they unveiled a solar-powered aircraft they believe will help them circumnavigate the globe in 2015.

Solar cells generate electricity by allowing photons — the little particles that make up light — to knock an electron off an atom inside a semiconducting material. Think of the solar cell as a pool table, with the photon as a cue ball and the electrons like the colored pool balls. But our pool table is designed to channel the struck balls into just one pocket: The composition of solar cell material creates an electrical imbalance that forces freed electrons to flow in a certain direction. This generates an electrical current, a process known as the photovoltaic effect.

The promise of solar power has long been equally matched by its challenges, particularly when it comes to efficiency and storage. Old solar cells were made of large silicon crystals, which are efficient at converting light to electricity, but expensive to make. Newer materials use films of less efficient, but cheaper materials, like copper-indium-gallium-selenide. Another challenge: Batteries can only capture so much surplus energy from solar panels.

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The first Solar Impulse plane was really a prototype built to prove that it was possible to fly day and — horrors — night, on solar power alone. On Wednesday, Borschberg and Piccard unveiled the Solar Impulse 2, a redesign of their original plane they hope will carry them all the way around the world in 2015.

Though the new model is bigger and heavier than the first Solar Impulse plane, it's still a lightweight compared to conventional aircraft. It has to be: With all aircraft, every pound is precious, and a solar plane's extra weight would have to be counteracted with ever more sunlight (or efficiency) to lift and push it forward.

The Solar Impulse 2's 236-foot wingspan is wider than that of a Boeing 777. But while the Boeing plane can weigh as much as 370,000 pounds empty, the Solar Impulse 2 maintains a relatively trim physique, weighing in at 5,300 pounds. The bulk of the difference is fuel weight: While the Boeing 777 relies on tens of thousands of gallons of jet fuel (tank capacity varies from 31,000 to nearly 48,000 gallons), the Solar Impulse 2 is covered with 17,000 solar cells that power four 20-kilowatt engines.

To fly by night, Solar Impulse 2 relies on batteries that can release surplus energy collected during the day. Together, the plane's batteries can store enough energy for 2,000 flight-hours, a vast improvement over the 500 flight hours stored in the batteries of the first Solar Impulse craft.

It's a long flight, around the world — around 500 flight hours in the cockpit — and so pilot comfort had to be considered too. "The cockpit has been redesigned so the pilots have more room to bend, take short rests," says Solar Impulse spokesman Vincent Colegrave. "They have to be able to get enough energy to fly day and night."

(In case your inner six year old is wondering how the pilots go to the bathroom: The toilets are built into the seats, which can also recline all the way back to allow the pilots to stretch and exercise.)

Could solar-powered planes be feasible on a more massive scale — will we ever see solar-powered 747s? At this point, it's too soon to tell. With present technology, it doesn't seem likely: Wired has an in-depth post explaining just how the figures don't add up.

"So far, our energies are concentrated on working hard to prepare for the first round the world flight [on solar power]," Colegrave says. "We don't know what will happen in the future. The thing we like to say is that between the Wright Brothers' first flight and the first commercial flight, that was 30 years. This technology is evolving pretty fast."

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