|Fig. 1: Solar Impulse 2 aircraft, unveiled in 2014. (Source: Wikimedia Commons)|
The hardest part about solar flight is the past was being able to create an airplane that could handle the payload of a human being. Hence the first few solar-powered aircraft were unmanned. One of the first solar airplanes to take flight was named the Sunrise 1. It first flew in 1974 and reached an altitude of 5000 m in 1975. 
The first manned solar powered flight was performed by Fred Militky (his son actually flew the plane). The airplane was a 75% scale version of the Gossamer Albatross, which was a human powered aircraft. Soon, other aircraft, such as the Solair 1 in 1990 and planes created by Alan Coccini in 2005, were making huge headway on the ultimate goal of emission-less free flight. 
The use of solar power in the United States has rapidly increased in recent years. Between 2000-2009, the millions of kWh generated jumped from 909 to 3588( Just under a 40% increase in a decade).  The demand for electricity is growing at a faster rate than the limits of installable generation capacity. The market for solar energy as an efficient fuel source for not just planes, but for energy countrywide is massive.
A non-powered airplane will fly in the air with velocity v and will sink with velocity vs. The aerodynamic efficiency of a plane is characterized by the ratio vs/v.  For gliding, this value is equal to the Lift/Drag Ratio (L/D). For example, a plane with an L/D of 20 can glide for 20 km from an initial altitude of 1 km. If altitude is to be preserved, the sink rate must be compensated for. The higher the weight of the aircraft, the higher the sink rate, and thus the higher the power required to maintain flight altitude. The power required to compensate in horizontal flight is v × D, or velocity times drag. 
In 2001, Swiss Bertrand Piccard proposed a project to design a manned solar powered aircraft and fly it around the world. Up to this point, no solar aircraft had been able to fly both day and night. The primary goal of Piccard's endeavor is to raise awareness that conventional energy resources are running out, and that renewable energy can, and will, be used to meet future demand. 
The Solar Impulse Foundation is built on the principle of bridging the gap between ecology and economy. Their vision is to advocate the protection of the environment through economically viable options.
The Solar Impulse 2 (see Fig. 1) was 21.85 m long, 6.4 m tall, and featured a wingspan of 72 m. Having a wingspan longer than a 747 jumbo jet minimizes drag and provides ample surface area for the solar cells. The Solar Impulse 2 is composed of mainly carbon fiber and honeycomb sandwich panels. 
The plane has 17,248 monocrystalline solar cells, each which are 135 μm thick. This allows the solar cells to charge the lithium ion batteries with a storage capacity of 260 wh/kg. The plane climbs to around 8500m during the day to collect as much sunlight (and thus energy) as possible and then it glides down to about 1500 m during the night to conserve battery power. 
© Ben Hallock. 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.
 H. Ross, "Fly around the World with a Solar Powered Airplane," Americna Institute of Aeronautics and Astronautics, AIAA 2008-8954, 14 Sep 08.
 V. Devabhaktuni et al., "Solar Energy: Trends and Enabling Technologies," Renew. Sust. Energy Rev. 19, 555 (2013).
 E. Velazquez, "A Soaring Vision," ABB Review 2015, Asea Brown Boveri, 2015, p. 16.