Round-the-world solar flight taxiing towards take off


From a vast hangar in the lush Swiss town of Payerne to a dusty airstrip in Abu Dhabi – the first leg of Solar Impulse 2's (Si2) journey is a little unedifying. Cooped inside a cargo plane, the boundary-breaking Si2 solar-powered plane will only get to metaphorically and literally spread its wings once it has been safely flown to the Middle East in March.

The Solar Impulse team is taking no chances. The Si2 has demonstrated its prowess on a series of recent sorties above the mountains of Switzerland, but the sheer scale of both its wingspan and its ambition means every minute detail of the record-breaking journey has been carefully calculated.

With weight at a premium, the Si2 has no de-icing capabilities, making a European start-point impossible. March in the Middle East is far more clement, and the launch date has been scheduled to avoid the Indian Monsoon as the plane trundles east. Landings and take-offs will be at night to avoid turbulence of more than 10 knots (18km/h), while a multidisciplinary team on the ground charts the best possible route.

This is just a snapshot of the meticulous attention to detail that has become Si2’s stock-in-trade. The brainchild of explorer Bertrand Piccard and engineer and pilot Andre Borschberg, Solar Impulse's round-the-world journey has been 12 years in the making, and last week pv magazine was invited along to the company’s Swiss hangar to see the final touches being put into place.

The tech specs

Si2's 72-meter wingspan is wider than a Boeing 747's, but the plane itself weighs just 2.3 tonnes – around the same as a large passenger car. If the plane was made of polystyrene, it would be heavier than it is currently (the airframe is comprised of a composite material of carbon fiber and honeycomb sandwich). Its wings, fuselage and horizontal tailplane are mounted with 17,248 monocrystalline silicon cells, provided by SunPower. Each cell is a mere 135 microns thick, providing a compromise between lightness, flexibility and – at 22.7% efficiency – performance.

"The solar cells cover a total area of 250 square meters, delivering a combined power output of 45 kWp," said CEO and co-founder of Solar Impulse, Andre Borschberg, who will also pilot the flight in rotation with Piccard. "The solar system uses maximum power point tracking (MPPT) to create a direct current (DC) of 300 volts, which is fed to the four DC motors and the lithium-polymer batteries."

The batteries, developed with the assistance of Solvay, Bayer Material Science and Kokam, have been optimized to 260 watt hours per kg. At 633 kg combined, the battery system is one-quarter of the Si2's total weight, and able to store 164,580 watt hours of power.

"Storage represents the main inefficiency at this point," said Piccard. "The batteries we have used have been specially developed for optimal maintenance but are still 10 x heavier than kerosene. I would imagine that we are still 20 years away from attaining parity on this point, but the hope is that Si2 can inspire better efficiencies and lighter batteries."

Pioneering for PV

Piccard was keen to stress that the Si2 project is not to push the boundaries of the possibility of solar-powered commercial flight, but rather to challenge perceptions of what is possible with solar PV technology and energy storage.

"Solar Impulse was not built to carry passengers, but to carry messages," he said. "The world needs to find new ways of improving the quality of human life. Clean technologies and renewable forms of energy are part of the solution."

Solar-powered flight is nothing new, with a history spanning 40 years. However, stressed Piccard, energy storage – the ability to stay in the air once the sun goes down – has long been the perceived insurmountable hurdle. “To really push the envelope on solar-powered flight, the challenge was always to fly at night,” Piccard said. “We have only recently been able to fit these planes with battery storage ample enough to achieve this."

The first Solar Impulse flights took Piccard and Borschberg across the U.S. in 2013, a year after the first intercontinental flight between Morocco and Spain. This latest project is grander in scope and ambition, involving a new set of challenges that have pushed boundaries in energy storage, efficiency, pilot training and aircraft construction.

Piccard – who in 1999 became the first man to circumnavigate the globe non-stop in a hot air balloon – spoke of how the limitations of fossil fuel dependence were laid bare during that historic flight.

"The idea for Solar Impulse was predicated on that round-the-world balloon flight," he said. "That was when the concept of finite fuel became a reality. During the last leg of that trip we were down to two more hours of possible flight. We made it, just, but that moment fuelled my fear of fuel dependence, and that was when I made a promise to myself that the next time I flew around the world it would be with no fuel."

An engaging speaker, Piccard recalled how the Egyptians could have flown if they had wanted to. The technology was there, but the mindset wasn’t, he adds. "In the 20th century, we finally tried to fly. And then in the space of just 60 years we had the first flight, had reached both the North and South poles, plunged to the bottom of the Marianas Trench, and went to the moon. Innovation is about shaking off old beliefs, challenging those who say it can’t be done. Once you have done that, then you are free to find new solutions.

"Our doubt is our ballast – we have to throw it overboard in order to rise."

The flight

Scheduled to take off in March 2015 from Abu Dhabi, the round-the-world solar flight will take a total of 4-5 months, with 20-25 flying days in total. The Si2 will fly up to five consecutive days and nights, with no fuel and only one pilot. Both Borschberg and Piccard have trained themselves to sleep for 20-minute spurts using a series of self-hypnosis techniques, meditation and yoga.

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"As pilots, we have to be as sustainable as the plane itself," said Borschberg (pictured above, on the right). "This means no caffeine or stimulants." The pilots will only sleep over oceans or seas, remaining awake over land for safety purposes. The 3.8 meter cockpit has no heating and is unpressurized. Instead, the pilot's life is dependent upon high-density thermal insulation, oxygen supplies and a monitoring system that detects anomalies in the plane's angle and autopilot functions. The two pilots will each fly for 4-5 day stints, before swapping in or out. The development and construction of the plane was achieved with meticulous 3D modeling, with Solar Impulse utilizing Dassault Systemes Catia software to calculate every inch and gram of the aircraft.

Si2’s maximum cruising altitude is 8,500 meters (27,000ft) and can reach a maximum speed of 140 km/h – far slower than all kerosene-powered jets. The absence of an expendable fuel means that there is no change to the plane’s center of gravity, which also presents the pilots with additional challenges.

The route will head eastwards from Abu Dhabi, across the Indian subcontinent, calling in on Japan before heading across the Pacific to the U.S. west coast. From there Si2 will head towards New York, then on across the Atlantic, over Europe, before touching down again in Abu Dhabi some time around July.

"Solar Impulse 2 is like a flying laboratory," concluded Piccard. "Not only does it demonstrate the efficiencies of solar technology and captures the imagination, but it will also teach us so much when we take flight next year.

"It is hard to say at this stage what the future possibilities will be, or what Solar Impulse 3 will look like until we have completed the round the world trip, but the possibilities are: to go higher, to go faster, to make the trip non-stop. How we do this will only be determined once we have succeeded in this latest mission – taking Si2 around the world with no fuel."

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