If only Icarus had waited a couple hundred years, he too may have been able to fly close enough to the sun as to not worry to much about his wings melting. But ever since the Wright Brothers first put a propeller at the end of an engine and slapped it between a couple of wings, the fuel of choice has been fossil based and airplanes use quite a lot of fuel which makes it expensive to fly.
But now, several companies are developing electric trainer planes. Aero Electric Aircraft Corporation will shortly launch the twin-seat Sun Flyer which will cost approximately $5 an hour to fly as opposed to the $73 per hour of a quad-seat Cessna training aircraft. The Sun Flyer uses batteries to power its single propeller but those batteries are charged from the rows of solar panels that line up along the leading edge of each of the wings and two days of sunshine will fully charge the battery pack.
But other companies are also developing planes and Airbus is planning on launching its first electric airplane in 2017. Initially, its plan is to have an aircraft that can fly for two hours on a single charge, which is plenty for a trainer aircraft but never a company to wait for things to happen, this is part of a 30-year plan to develop a 90 passenger seat plane using either electric or hybrid propulsion.
But the problem with electric aircraft is the weight of the batteries. No matter how efficient, they are very heavy and this is one of the issues with developing them for commercial use since it means that the plane has to work harder to take off. But now researchers may have come up with an efficient way of producing one of the Holy Grail of renewable energy: hydrogen power. Hydrogen has long been touted as one of the most efficient methods of powering a vehicle since it's easy to install and doesn't produce any pollutants. It is however, not particularly eco-friendly to produce as to get it either involves mixing steam with natural gas or using electrolysis to separate oxygen and hydrogen in water. Both of these use so much energy that it's cost prohibitive. But now, according to this article in Popular Science, Nate Lewis and his colleagues at Caltech, "a thing coating of nickel oxide that can be applied to semi-conductors made of silicon or other materials-a setup that acts like an artificial leaf, using sunlight to power the system. When introduced to water, one side of the 'leaf' oxidizes the water, releasing oxygen, while the other side gathers the hydrogen".
So, by using sunlight to produce the hydrogen, it's possible that in years to come, hydrogen planes maybe powered by sunlight and latter-day Icarus's can finally go as high as they wish.