Solar energy is abundant and infinitely renewable.
Airplanes fly thousands of feet above the ground – thus absorbing a lot of solar energy.
Then why don’t commercial airplanes fly on solar?
Most of the planes that are flying nowadays were designed or manufactured many years ago. It wasn’t until recently (late 2008-early 2009) that the costs of solar panels have fallen dramatically. So the answer could simply be that the economics of it didn’t make sense for the planes you see in the air.
But let’s just do some simple back-of-the-envelope calculation. Let’s take the thin-film solar cells from First Solar, an Arizona-based leader in solar panels. They are very cheap to produce (80 cents per watt or so), and as the name implies, very thin and light. They are producing cells with 11.2% efficiency. Combined with an above-air solar irradiance of about 1,200 watt/m^2, that means each square meter of their cells will generate 134.4 watts during the day, so maybe 60 watts on average (half day, half night; above clouds most of the time but sometimes on the ground).
The Boeing 747 has a wing area of 560 square meters so covering just the wings will allow one to generate about 33600 watts on average. At under a dollar per watt, covering the wings should cost only about $50,000 in solar panels, and it wouldn’t weigh much. With an average of about 500 passengers, that works out to be around 67.2 watts per person, which isn’t bad if all you need are the lights, some AC, and the TV.
Implications: The extra 33,600 watts over 12 hours, a typical 747 flight, is the equivalent of 403.2 kwh that doesn’t have to come from other sources like burning kerosene (jet fuel). Kerosene has an energy density of 43.1 MJ/kg,or 11.97 kwh/kg. At a 40% conversion rate, that means those 403.2 kwh will save 84.2 kg of kerosene.
So my conclusion is that it might actually be economically feasible for future planes to cover their wings with thin film solar panels to offset the energy used to power in-cabin services like TVs, lights and such. Just because something hasn’t been done in the past doesn’t mean it is infeasible with new technology.