It is easy to see the appeal of a solar-powered aircraft. Next time you are aboard a plane, try to get a window seat. As you taxi out for takeoff, look at the amount of light hitting the plane’s wings. In perfect flying conditions, the polished metal surface will brilliantly reflect the sun.
Then, as you are pushed back into your seat when the aircraft accelerates along the runway, perhaps the sun will catch a wing edge, suddenly illuminating the cabin.
Once the aircraft climbs through the clouds, the seatbelt sign pings off and the light can rapidly become piercing. The wings and fuselage are often bathed in constant sunlight at these higher altitudes during the daytime, prompting passengers to shield their eyes and pull down their window blinds.
In these kinds of conditions, flying by the sun could seem a relatively easy proposition – cover the plane in solar panels and you will have an aircraft that taps this free, limitless and clean energy source that abounds at altitude.
But then think about other flights you have taken. Foul weather or low cloud can cut out almost all sunlight. A winter sun can mean that the light barely glances off the plane as it soars into the sky. A night flight in the wrong direction can end up never seeing sunlight at all.
These kinds of considerations give some insight into the massive hurdles that designers of solar-powered aircraft face. But that has not stopped people trying.
“The promise is almost unlimited duration – as long as the sun is out they can fly,” says Bob van der Linden, the curator of special purpose aircraft at the Smithsonian National Air and Space museum in Washington DC.
For example, his collection contains Pathfinder Plus, an unmanned aircraft which set an altitude record of over 80,000ft back in 1998.
“It’s a bizarre-looking thing with a really, really long wingspan and very short length. If you look at it in flight, the wings are extremely flexible, and under load the wing takes the shape of a U with 8 motors on it,” says van der Linden.
Other high-profile craft include the fragile Nasa Helios, which disintegrated above the Pacific Ocean near Hawaii after hitting unexpected turbulence. Prior to the crash, it had set a new altitude record of 96,836ft (29515.6m).
Although embarrassing, accidents like this have not stopped the development of solar-powered craft, particularly by the military.
One solar-powered high-altitude long-endurance (Hale) craft under development is called Zephyr. The unmanned aerial vehicle (UAV), built by UK defence firm Qinetiq, has shown that it can stay aloft for more than two weeks, a valuable asset for the armed forces.
“You have a machine with solar cells and a battery that can fly, at high or low altitudes, over a troubled area and send back data as a substitute for a satellite,” says van der Linden.
“If the point is to circle over a couple of spots and take pictures, it’s perfect.”
And these have other uses too. They could become slow-flying perpetual aircraft, never having to land, never having to refuel. They could perform long-term science missions, sampling the atmosphere and supplementing the types of data we can get from satellites in much higher orbits. They could also be used to broadcast or relay TV and radio signals, providing a cheap way to extend coverage to areas where it is needed, after natural disasters or during major sporting events like the Olympics.
But it is the idea of a manned aircraft that captures people’s attention.
In 1981, for example, Solar Challenger flew across the channel between England and France. Currently the most high-profile project is Solar Impulse, a team led by Swiss pilot Bertrand Piccard. He hopes to fly into the record books in 2014 by travelling around the world without using any fuel.
The team has already entered the record books after they built an aircraft that managed to stay aloft for 26 straight hours.
HB-SIA, as the one-man plane is known, has the mammoth wingspan of an Airbus A340, but with a tiny body that almost disappears under the wings. It packs 12,000 photovoltaic cells on to its wings, powering four 10 horsepower electric motors that are used to drive 3.5m diameter, twin-bladed propellers. But now the Solar Impulse team is working on a second-generation plane, ready for a 25-day global circumnavigation attempt, flying day and night.
“In the daytime he needs to charge his batteries with sunlight, and then during the night he should have enough energy to power the motors and fly on,” said Joern Juergens of SunPower, the company that supplies Solar Impulse’s solar cells.
“That is a real challenge.”
To increase the amount of energy it can collect, the new plane will have an extra 10,000 cells and a wingspan of 80m (262 ft), slightly wider than the wingspan of an Airbus A380, the largest passenger airliner in the world.
The plane will reportedly take off and land in the United Arab Emirates and will track the equator to maximize the amount of sun falling on its cells.
But the challenges don’t stop there.
The plane needs to be as lightweight as possible, which means a delicate balancing act between the numbers of batteries it needs to power the craft and the number it can carry.
The weight issue carries over to the pilot as well – meaning it can only have one.
As a result, the circumnavigation attempt will require at least five stops to change pilots.
It is this kind of extreme limitation that means nobody – including Piccard – will claim that Solar Impulse provides a practical blueprint for mass solar-powered air travel.
Instead, he says, Solar Impulse is a “demonstration craft”, aimed at highlighting how far renewable energy sources have come.
He has even admitted that the idea of a passenger plane may be impossible.
But, according to van der Linden, the curator at the Smithsonian, that should not stop people trying.
“Right now it’s more a novelty than anything else, but the idea of unlimited power is a wonderful promise and needs to be pursued,” says Bob.
“It’s free energy. You could fly a solar-powered sailplane, or motorised glider almost indefinitely.”
And even if the sun cannot be tapped directly, SunPower’s Juergens sees another way to harness the sun.
"If you use sunlight to produce hydrogen gas, you could run a plane on that, no problem."
His plan is to keep the solar cells on the ground and use them to create a clean, green liquid fuel for aircraft.
"You could use solar energy to produce the fuel for a 747 equivalent.
“You would have a large solar plant producing hydrogen. Then you would pump it into the plane, and the plane would fly."
You can hear Jon discuss the latest advances in science in Science in Action, every week on the BBC World Service.