Solar Impulse Airplane Prepares for Cross-Country Sun-Powered Flight

By · April 25, 2013

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The Solar Impulse HB-SIA flying over the Golden Gate Bridge this week.

What do you get when you build an airplane with a battery pack the size of the Tesla Model S, and a solar panel array 10 times bigger than ones found on most homes? Answer: The Solar Impulse, a prototype solar-powered airplane designed to to make an around-the-world flight in 2015. This month, the Solar Impulse is at Moffett Field in Silicon Valley, getting prepped for a flight across the U.S. beginning on May 1.

The Solar Impulse is the brainchild of Bertrand Picard, the psychiatrist and adventurer who flew a balloon, the Breitling Orbiter, around the world in 1999. After that flight, he developed the idea of using renewable energy to power an airplane for a flight around the world, and rolled up his sleeves to make it happen. He formed a brave team that when told by aircraft manufacturers that the idea was impossible, went ahead anyway.

Huge Wingspan, Small Body

The HB-SIA Solar Impulse at Moffett Field is the team's first prototype. The team used that model since 2010, setting several flight records. The team is now preparing a second airplane, the HB-SIB, which it hopes to fly around the world in 2015.

The airplane looks like a glider, with a huge wingspan and small body. The 208-foot wingspan, similar in size to a jumbo jet's dimensions. The top skin of the wings has a solar panel array (using flexible cells from Sun Power) of nearly 12,000 cells for a peak power of 45 kilowatts. There are four electric motors distributed along the wings, each rated for 10 horsepower, or 7.5 kilowatts apiece, for 40 horsepower total (30 kilowatts). Each motor module contains a 21 kilowatt-hour battery pack for approximately 85 kilowatt-hours total on-board energy storage capacity. The total weight is a relatively slim 3,500 lbs, thanks to extensive use of carbon fiber and other low weight materials. The pack weighs 882 pounds.

The team reassembled the Solar Impulse at Moffett Field, after air-freighting the plane from Switzerland. The test flight began on Tuesday at dawn and continued until late evening, in part to verify the aircraft's night-flying ability. It was also an opportunity to snap pictures of the airplane flying above the Golden Gate Bridge.

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The Solar Impulse HB-SIA landing at Moffett Field

For comparison, the Solar Impulse's battery pack has the same storage capacity as the Tesla Model S, while the total power for the electric motors is is only slightly stronger than what's used in the 2013 Zero S electric motorcycle.

The Solar Impulse was designed for extreme energy efficiency, and cruises at 40 m.p.h. flight speed, with a takeoff speed of 27 miles an hour. During the day, it flies at an altitude of 28,000 feet, charging the battery pack at the same time. The battery pack is full at the end of daylight.

At night, the pilots shut off the electric motor and glide for approximately four to five hours, losing altitude in the process, before turning the motors back on for another eight hours of night flight. The power is balanced so the solar panels and battery pack provide enough capacity to fly all night. In theory, the Solar Impulse could fly indefinitely—if not for pilot fatigue and weather.

Scheduled Flight

The upcoming flight from the San Francisco area to New York City is expected to take two months to complete. The entire itinerary hasn't been settled, but the first destination is Phoenix, followed by the Dallas-Fort Worth area. The team could take longer flights per leg of the trip, but the lack of a proper on-board toilet system stands in the way. This problem will be fixed in the second version of the Solar Impulse.

In 2012, the team flew the Solar Impulse on the first intercontinental solar powered air flight, when they soared from Switzerland to Madrid and then to Morocco. In the process, they established a record for single flight time at more than 26 hours.

But this project isn't just about setting records and having an adventure. Many of the partners are using the initiative to test and refine technologies that can be used in other applications. Better solar power and battery technology would not only allow the Solar Impulse to fly further or faster, but make for better electric cars and stationary photovoltaic systems.

You can follow the team via their website, which it plans to use for live-streaming video during the flights.

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