Solar Impulse: One Giant Leap for Green Energy

Four days after takeoff, the sun-powered aircraft is still going strong.

(Solar Impulse)
July 3 update: Solar Impulse 2 landed safely in Hawaii around noon (Eastern time) on Friday.

As the U.S. East Coast enjoyed a lazy Sunday afternoon on June 28, an airplane took off from Nagoya, in central Japan, without any gas aboard. Four days later it’s still in the air, proceeding across the Pacific Ocean to Hawaii at the stately pace of around 30 knots. Its pilot and lone occupant, André Borschberg, has already set the world record for longest-duration solo flight, and as of this afternoon he has nearly another day ahead of him at the controls. The weather is not cooperating. It is a tense time.

The airplane, Solar Impulse 2, is only the second of its kind; powered purely by batteries, which are themselves recharged only by solar power, it was built to circle the globe, spreading environmental awareness in its wake. Its predecessor, the original Solar Impulse, flew across the U.S. in 2013. Landing scarcely faster than a running person, it was an impressive and bizarre sight at commercial airports.

Solar Impulse 2 took off from Abu Dhabi, UAE on March 9, with the goal of completing the round-the-world journey east-to-west in 12 legs. Since the first try at a trans-Pacific flight, leaving from Nanjing, China, was aborted due to weather, the aircraft landed in Nagoya to await better conditions, and is now set to finish after 13 legs.

Clearly the team isn’t in a hurry. Commercial airplanes cross the Pacific many times every day, moving much, much faster, and they serve coffee onboard. What’s the point of this odd, one-off airplane?

The aviation world, and global transportation generally, is entirely dependent on fossil fuels, which consist of decayed organic matter from once-living organisms. Those fuels, burned and cast into the atmosphere, represent the major contributor to global warming. Aviation contributes only a small fraction of total carbon emissions, but has an outsize importance: emissions from aircraft are injected directly into the upper atmosphere, and are almost entirely unregulated (though there are steps in that direction), unlike the emissions from automobiles and ships. Large jet airliners like the 747s that cross the Pacific every day can burn in the neighborhood of 36,000 gallons on a single flight. With 500 people aboard, it’s actually a fairly fuel-efficient way to travel, but 36,000 gallons of burning gasoline emits a whole lot of carbon.

Fuel was once dirt-cheap, but that changed for good after the oil crises of the late 1970s, though it is still far cheaper on average than milk ($2.77/gal vs. 3.38/gal as of today). Gas has been among aviation’s biggest costs ever since then—a huge concern to the airline industry, which often operates on a razor’s edge between profitability and bankruptcy. Little tricks help burn less gas. Lighter mechanical components require less energy to push through the air, winglets improve aerodynamic efficiency, and every successive generation of jet engines burns around 10-12 percent less fuel than its predecessor. But that will only get you so far. Jet engines are already among the most efficient machines we have, and we’re not far off from the thermodynamic limits of the Brayton Cycle. Soon there will simply be no way to get more energy out of burning fuel.

B-52s demonstrate how it once was (SSgt. Phil Schmitten/USAF)

There really is no ready replacement for jet engines, much less engines that could compete economically. As a result, those people proposing alternatives were generally ignored as utterly impractical. Gasoline-battery hybrid cars have taken to the streets only recently, and the aerospace industry tends to be much more conservative than their ground-bound car-building colleagues, generally with good reason.

But by flying all the way around the planet without a single drop of gas, the Solar Impulse team hopes to prove the practicality of solar power, and so jolt the world into noticing that batteries really can be useful. Granted that, with the lightest airplane manageable, and with a pilot as its only cargo, it is still moving slower than some legged animals. But it is an achievement nonetheless, when skeptics say that battery-powered aircraft have no use at all.

It will be years, at least, before battery-powered flyers take to the air in number, and absent a radical breakthrough in energy storage there is still no practical way for them to do much more than lift themselves, much less compete with gas-burning jet propulsion. But there are some uses—staying aloft for 72 hours at a time, for example—and small general aviation aircraft and unmanned air vehicles (UAVs) will certainly be among the first adopters. There are a great many challenges yet to overcome, but proponents will at least have a concrete achievement to point to.

Assuming Borschberg makes it. It’s a long way from the middle of the Pacific to Abu Dhabi, and after four days of sleepless flight, the remainder of the flight to Hawaii must seem just as long.

You can follow his flight at this live feed:


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