|Fig. 1: Model of Solar Impulse (Source: Wikimedia Commons)|
Ever since the Wright brothers created the world's first successful airplane, there has been a question of when will an aircraft be able to fly on its own without the need to stop for fuel. Dr. Bertrand Piccard, who has an ultimate goal of perpetual flight, aims to complete this through Solar Impulse, an solar powered plane. Solar Impulse is a slender plane with a 208-foot wingspan and 12,000 solar panels to power its flight.  Because the plane is entirely powered by solar power, it is extremely lightweight in order to conserve energy. In addition to that, it's cabin only has room for one pilot and no baggage. The aircraft weighs just over the weight of a small car but has the wingspan of a jetliner making it very sensitive to air conditions. 
Dr. Bertrand Piccard since created a second version of the plane called the Solar Impulse 2 which has 17,000 solar panels and is made of carbon fiber.  This second plane had room for two pilots as well as opposed to one previously.
The validity of these aircraft was first determined in 2010 as the Solar Impulse flew for 26 hours consecutively and reached a height of 28,543 feet which set a record for the longest and highest flight ever for a solar powered plane.  This flight had an average speed of 26 miles per hour with a top speed of 78 miles per hour, making it much slower than a standard commercial plane.  The founders of the Solar Impulse project had stated that their goal was to be able to create a solar powered plane that could travel across the world in a single trip. This goal would be realized several years later.
The Solar Impulse 2 would take off from Abu Dhabi to begin its round-the-world trip in March of 2015 under the pilot of both Andr Borschberg and Bertrand Piccard.  This cross country expedition consisted of 13 segments including stops of Nanjing, Hawaii and Southern Europe/ Northern Africa. The longest of these segments was from Japan to Hawaii which took over 117 hours, making it the longest solo flight in history.  Of course, Piccard's vision for this plane is to provide a focus on alternative energy. This plane is cleaner and more environmentally friendly than any diesel or petrol plane; flying the plane in the sun's rays can bring the plane's batteries to a full charge. 
The Solar Impulse 2's flight across the world back to Abu Dhabi can be viewed as a testament to solar and alternative energy sources. While Piccard was not designing the Solar Impulse to replace commercial aircraft, it cannot be ignored that solar powered aviation may be the environmentally friendly way to travel across the skies in the future.  However, this future may be further away than hopeful. A large energy consumption is fundamental to an airplane's ability to carry payloads. For example, Liu has calculated that a jet airliner with the wing area of a Boeing 737 could carry a payload of only 1315 kg - which is less than the mass of the plane itself, much less a plane loaded with cargo and passengers.  The Solar Impulse 2 would still not come close to achieving even this small payload due to its size and weight of lithium batteries. While the Solar Impulse 2 provides hope for future solar powered air transporation, it is likely very far way in the future that these types of aircraft will be flown commercially.
© Bryan McLellan. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A. Cowell, "Solar-Powered Plane Flies for 26 Hours," New York Times, 17 Jul 10.
 H. Smith, "Solar-Powered Plane Resumes Round-the-World Trip," Washington Post, 12 Apr 16.
 "Solar Impulse 2: Test Flight Carried Out in Switzerland," BBC News, 2 Jun 14.
 M. Liu, "Solar Powered Flight," Physics 240, Stanford University, Fall 2010.