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The Solar Impulse 2 in flight (Courtesy of Solar Impulse 2)

Can a Plane Fly Around the World on Solar Power Alone?

With a wingspan greater than a 747, but weighing less than most cars, the Solar Impulse 2 will attempt to circumnavigate the planet.

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Last week, the Obama Administration unveiled new guidelines to control greenhouse gas emissions at U.S. power plants, a move that was quickly characterized as an effort to cement the president’s legacy in the fight against climate change.

A few hours earlier, in the skies above Switzerland, there was another notable event in the evolution of how we create and use power. It didn’t produce nearly the headlines as the White House’s announcement, but in its own way, could help change our view of what’s possible with energy still described as “alternative.”

I’m referring to the maiden flight of the Solar Impulse 2, a bizarre-looking but remarkable aircraft that relies solely on solar energy to get around. It's a plane with a wingspan of almost 240 feet—wider than a Boeing 747—and a weight of just over 5,000 pounds (which is lighter than most cars).

Those long wings are covered with more than 17,000 solar cells, which power four electric motors and spin the propellers. The sun keeps it airborne during the day, and batteries running on stored energy do the work at night.  

Last Monday, the Solar Impulse 2 flew for two hours without a problem, albeit at a speed most of us would consider absurdly slow for a plane. Its top speed at maximum cruising altitude is 88 miles per hour.  Most of the time it will fly much more slowly, closer to 40 miles per hour, and at night, slower still to save battery power.

Here's how the Solar Impulse 2 handled its first takeoff:

So it seems more than a little crazy to hear that that flight and the other trial runs later this year are leading up to what’s seen as the ultimate test: a planned trip around the world beginning next March.

You could make it around the world in fewer than 80 days in this plane, if you flew non-stop. The latest estimate is that it can be done in 25 solid days and nights—or roughly 500 hours—of flying. But the trip is going to be broken into five or six stages over several months, primarily for the benefit of the two pilots who will take turns at the controls. 

Since there’s room for only one person in the plane, this means some very long stretches of time in the cockpit—as long as five whole days when the plane crosses the Pacific Ocean. To accommodate the pilot’s basic needs, the seat both converts into a recliner and serves as a toilet. The pilot will essentially live in a space not much bigger than the inside of a mini-Cooper, though the seat does expand enough for him to do physical exercises. The cockpit is not pressurized, nor does it have heat, but it is lined with high-density thermal insulation. And just in case, there’s a parachute and life raft packed into the back of the seat.

The two men who will fly the Solar Impulse 2, Bertrand Piccard and André Borschberg, know full well what they’re getting themselves into. They've been developing the technology for 12 years, and had a taste of it last year when they flew the plane’s smaller predecessor, Solar Impulse 1, in a series of hops across the U.S.

To prepare for their flight around the Earth, Piccard and Borschberg were taught self-hypnosis and meditation techniques to help them maintain concentration. They’ve also trained themselves how to engage in polyphasic sleep—taking multiple naps throughout the day, though nothing longer than 20 minutes. And because additional weight must be kept to a minimum, they can only take along about 5 pounds of food and 84 ounces of water a day; experts in high-altitude medicine have devised a personalized nutrition plan each will need to closely follow as they mosey across the sky.

Not so impossible dreams

So why are they doing this?

It’s no exaggeration to say it’s in Piccard’s blood. In 1960, his father, Jacques, was one of the two men aboard the bathysphere lowered into the Marianas Trench, the deepest part of the world’s oceans. In 1931, his grandfather, Auguste, was the first balloonist to enter the Earth’s stratosphere.

In 1999, Bertrand himself co-piloted the first gas-powered balloon to travel non-stop around the world. It was during that trip, after he almost ran out of propane while crossing the Atlantic, that he decided to try to find a way to fly without relying on fuel at all. He and Borschberg spent years planning, designing and finding investors–that was no small challenge–but they persevered and, in 2010, the Solar Impulse 1 made the first solar-powered night flight over Switzerland.

As they build up to next year’s planned circumnavigation of the planet, the pair have no illusions about the impact of their mission. They aren't out to reshape human flight—that’s just not realistic in the foreseeable future. What they are doing is pushing the envelope on solar technology. The batteries that power the motors, for instance, have been designed with a particularly high energy density, and the carbon fiber tubes with which the plane is built are lighter than paper.

The point, says Piccard, is to expand the possibilities of what can be done with renewable energy.  

“Our goal,” he told Solar Power World, “is to show that it is now possible to achieve things considered impossible without fossil fuels. In today’s world we have to cultivate the pioneering spirit to liberate oneself from those certainties and habits that hold us prisoner to old ways of doing and thinking.”

Other experimental solar planes are out there, but most are designed to be unmanned.  One of them, an aircraft called Solara, designed by the company Titan Aerospace, is aiming to function more like a satellite. It can fly at higher altitudes and stay airborne for five years, according to Tree Hugger, and could be used for things like disaster response or surveillance.

The Solar Impulse 2 is closer to reality than that effort, but isn't expected to be used more broadly just yet. The plane is still experimental, with no commercial application, for now. The focus at the moment, Piccard said, is not only the round-the-world March flight, but also increasing awareness around the future of the technology.

"This is a symbol of what we can achieve ... it's not any more completely stupid to get rid of fossil energies," Piccard said in a TED talk (below). "Success will not come if we just fly around the world in a solar-powered airplane. The real success will come if enough people are motivated to do exactly the same in their daily life: save energy, go to renewables. This is possible."   

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