In the future, aircraft pilots may not have to rely solely on their jet fuel to power their planes. They could turn to fuel cells to power at least some onboard systems.

NASA researchers and aerospace engineers are working to do just that, develop fuel cells that could supply aircraft with the power needed to run onboard systems while jet fuel pushes the plane through the sky. More advanced fuel cells could even keep airships aloft for weeks at a time.

"What we're doing is mainly along the same lines as the [U.S.] Department of Energy, which is to focus on reducing the cost of these aircraft systems," said Anita Liang, chief of the aeropropulsion projects at NASA's Glenn Research Center in Cleveland, Ohio.

Fuel cells can convert a variety of fuels, such as hydrogen, natural gas or even the Jet A fuel used in commercial airplanes into energy without combustion, making them a cleaner and quieter alternative for aircraft.

Hydrogen fuel cells like those developed for automobiles produce water vapor as a byproduct of generating power. Since water contains the hydrogen and oxygen necessary for a fuel cell to work, an airship could reuse the wet stuff in a regenerative system by using solar energy to separate it back into gases.

During the day, solar panels plastered across the top of an airship would provide the power necessary to turn propellers and separate water into its component parts via electrolysis. The fuel cell then runs at night on the separated hydrogen and oxygen, combines the gases into water and in the morning the whole system starts again.

"These airships are something very closely tied with our science missions or surveillance," Liang told SPACE.com. "So we're trying to get the [operational] time up to something more like 14 days at an altitude of 30,000 feet."

But the project is still in the technology development stage and a working regenerative fuel cell plane will probably take about five years to complete.

Closer to home than high-flying blimps is a project to adapt fuel cells into commercial jets in order to both save fuel and cut back on noxious emissions.

Aerospace engineers at Boeing, Co. are working with Liang's team to develop an auxiliary power unit (APU) for airplanes capable of using Jet A fuel to generate energy. Instead of hydrogen fuel cells, the plan calls for a solid oxide version that can handle the impurities of jet fuel. While the APU wouldn't power an airplane's engines, it would reduce the amount of jet fuel needed provide the electricity need for most other onboard systems.

During a presentation to the Solid State Energy Conversion Alliance (SECA) earlier this year Boeing's David Daggett, as associate technical fellow, presented the aerospace company's plan for a fuel cell APU.

According to Daggett, a tail-mounted solid oxide fuel cell APU could save up to 40 percent of fuel during a flight's cruise and be put into effect by 2015. The project does depend on the relationship between how heavy a fuel cell is and how much energy it can produce. A Boeing solid oxide fuel cell APU would have to be able to generate at 450 kilowatts total, with a power density of about one-kilowatt per kilogram in weight to be efficient.

"We need to improve fuel cells to be about 10 times better than what's available now," Liang added.

In the meantime, a determined pilot with an environmental state of mind could put together an eco-friendly electric plane using off-the-shelf fuel cell parts.

A small two-seater kit plane like the MCR-01 model could be modified to run on a hydrogen fuel cell largely due to the similarity of its power consumption with that of an automobile.

"Anything bigger and things don't scale well in terms of power requirements," said Jeff Berton, an aerospace engineer at Glenn Research Center who has analyzed the feasibility of small, fuel cell-powered aircraft.

A small electric airplane would be much quieter than a combustion plane, which would be useful in small airports near homes. But there are major hurdles too; not the least of which is infrastructure. A pilot would need a dedicated hydrogen production facility at the airplane's home airport, no small feat for a recreation flyer.

"And that's a tough nut to crack," said Berton.

Putting fuel cells aboard aircraft is not the newest of ideas. Over the last few years, NASA scientists have been working to put such power plants aboard unmanned aircraft such as Helios, a large pilotless vehicle that was recently lost during test flights in Hawaii.

"In a sense, it's an automotive-type fuel cell system," said John Del Frate, NASA's Helios program manager at Dryden Research Center in California, before the crash. "But there are some challenges, particularly the higher you go."

Fuel cells developed today are built for sea level atmospheric pressure, he explained. So any airplane fuel cell flying over 50,000 feet (15,240 meters) has to contend with extremely low pressures and temperatures of about -110 Fahrenheit (-78 degrees Celsius).

With a wingspan of about 247 feet (about 75 meters), the unmanned Helios would have been the largest aircraft to be powered by fuel cells to date. Solar panels would provide the necessary power for liftoff with ground controllers switching to the fuel cell once Helios was in flight. But Helios crashed 29 minutes into a June 26 test flight.

Del Frate is currently regrouping his project, but did say that fuel cell technology is going to be a widely discussed and applied technology in the coming years.

"I think we're all going to be tired of fuel cells by the time we're done," he said.