Abu Dhabi is an appropriate place for launching an attempt to fly around the world on solar power. But after leaving the sun-drenched desert of the United Arab Emirates, Switzerland’s Solar Impulse 2 (Si2) quickly faces the real world of changing weather and night flying. Which is why solar energy is not a practical power source for aviation—or is it?

Solar Impulse is less an everyday aviation endeavor than an environmental rallying call, the round-the-world flight intended to inspire enthusiasm for renewable energy and sustainable technology. But Si2 itself is an aerospace achievement: an all-composite aircraft with the weight of a car, a wingspan greater than a Boeing 747’s and the most efficient propulsion system yet flown.

From Abu Dhabi, Si2 is planned to fly almost 19,000 nm (35,000 km) in 25 flight days over five months, with stops in Oman, India, Myanmar, China, Hawaii, the continental U.S., and Southern Europe or North Africa, before returning to Abu Dhabi. Solar Impulse co-founders Andre Borschberg and Bertrand Piccard will take turns flying, alone in an unheated, unpressurized cockpit for up to five days and nights.

Solar Impulse prototype HB-SIA was the first solar-powered manned aircraft to fly for more than 24 hr., proving the solar cells and batteries could collect and store enough energy to fly through the night. Si2, registered HB-SIB, has been designed to extend that capability to multiple days while enabling the pilot to rest, exercise and stay alert over the long flights.

With a span of 236 ft., the wing has a high aspect ratio to maximize aerodynamic efficiency, but Si2 weighs only 5,070 lb. and slightly more than a quarter of that is for the batteries. The airframe is made of carbon fiber and honeycomb; the single wingspar is 230 ft. long with 140 ribs spaced 20 in. apart to maintain the airfoil shape and rigidity. Carbon-fiber sheets weighing just 0.07 oz./sq. ft. were used in construction.

A total of 17,248 monocrystalline silicon solar cells are encapsulated in the upper-surface skins of the wing, tail and fuselage. Operating at 23% efficiency, these generate electricity to be stored in 1,395 lb. of lithium-polymer batteries housed in the nacelles for the four 17.4-hp brushless electric motors. These drive 13-ft.-dia. propellers at 525 rpm via reduction gears. Overall efficiency is a record 94%, says Solar Impulse.

The round-the-world attempt is as much about the pilot’s endurance as the aircraft’s. Compared with the prototype, Si2 has a much larger, 134-cu.-ft. cockpit to allow the pilot to move around, and the seat, which also functions as a toilet, allows him to exercise when fully reclined. The aircraft flies up to 28,000 ft. during the day, requiring oxygen, and descends to 5,000 ft. at night to save energy.

The pilot is allowed to sleep. A monitoring and alerting system continuously checks the autopilot and will alert the pilot via a vibrating sleeve if bank angle exceeds a limit of 5 deg. Another system controls the charging thresholds and temperatures in the batteries to prevent a thermal runaway. Aircraft data are telemetered continuously to the Solar Impulse mission control center (MCC) in Monaco.

The MCC is responsible for all decisions on departures and routes, and for monitoring aircraft status and position, and the pilot. Si2 has a limited flight envelope, its low wing-loading making it sensitive to turbulence. Takeoffs and landings are at night to minimize bumpiness, and wind speeds must be less than 10 kt. The average cruise speed is expected to be only 30-55 kt.

None of that sounds like a practical aircraft. But Solar Impulse is making a statement about sustainable energy and climate change. And solar-powered aircraft are coming, although much smaller than Si2. The obvious application is to unmanned aircraft, with Google to begin tests this year of high-altitude, long-endurance UAVs for Internet delivery under Project Titan.

But there are manned aircraft, too. Colorado-based Aero Electric Aircraft is developing the Sun Flyer solar-electric training aircraft, flying a single-seat demonstrator while a two-seat prototype is built. The first two-seat solar-powered aircraft to fly is Solar Flight’s Sunseeker Duo. Low operating cost and noise are benefits. Performance is low, if perhaps adequate for a trainer or recreational aircraft.

But solar power could impact commercial aviation if hybrid turbine/electric propulsion becomes a reality. A Boeing study for NASA suggests the environmental benefits of hybridization are only substantial if the grid power used to recharge the batteries comes from renewable sources. So Solar Impulse’s message about solar power and sustainability may yet prove significant for air transport.

This article was first published on March 6.