How Things Work: Flying Fuel Cells
Out of gas? Not a problem.
When airlines shop for new aircraft, they follow some of the same principles car buyers do: Green is in; gas hogs are out. So airplane manufacturers are studying ways to reduce emissions and make engines less greedy. That’s why Cecilio Barberán was able to fly a powered glider last year with no avgas.
Instead, his HK 36 Super Dimona carried a 200-pound hydrogen fuel cell that ran an electric motor to turn its propeller. The fuel cell couldn’t quite put out the energy required for takeoff—45 kilowatts—and got help from a lithium ion battery to lift off the runway in Ocaña, Spain. At 3,300 feet Barberán disconnected the battery, and for the next 20 minutes the Super Dimona flew straight and level at about 60 mph on just the fuel cell. It was the first time a piloted airplane had flown powered by a fuel cell alone.
“Demonstrations like this lead the way toward using this technology in small manned and unmanned air vehicles,” says Nieves Lapeña. She heads the demonstrator program, run by Boeing Research & Technology Europe, a far-flung clutch of engineers within the company’s advanced projects arm, Phantom Works. Ordinarily the glider flies with an 80-horsepower Rotax engine. According to Lapeña, the hydrogen fuel cell-electric motor combination used only 28 percent of the energy that the internal combustion engine would have used on a similar flight.
Fuel cells could increase the range of small UAVs now operating on battery power, according to Lapeña. Both batteries and fuel cells deliver electrical power from a chemical reaction, but the battery quits when its stored reactants are used up; a fuel cell continues to work as long as its external fuel supply lasts.
Boeing studies fuel cells because they could also eventually increase the efficiency of airliner engines. Today, a fraction of the power from the engines is diverted to generators that run onboard lighting and other systems. In the future, fuel cells could provide that energy. According to Bill Glover, managing director for environmental strategy at Boeing Commercial Airplanes, relieving the engines of the burden of running generators would save only a small percentage of an airliner’s fuel. But multiply even a tiny fuel savings by the tens of thousands of flights a major airline makes a year: That’s a lot of gas.
For the demonstration flight, the Boeing group used a type of fuel cell called a polymer electrolyte membrane, also known as a proton exchange membrane (PEM). It contains stacks of electrolytes sandwiched between conductive surfaces of opposing charges. Hydrogen protons pass through the membranes, but electrons travel around them. The flow of electrons generates a current to run the engine. The hydrogen protons that have traveled across the membrane recombine with electrons and oxygen from the air to produce water, which can be used aboard an airliner.
PEMs have been around a long time; they were originally developed for NASA’s Gemini program. “The technology has improved tremendously since then,” says Mark Hoberecht, fuel cell manager at NASA’s Glenn Research Center in Ohio. Because of advances in materials used as catalysts in the cells’ chemical reactions, cells produce more electrical power with less waste heat, Hoberecht explains. The efficiency could be improved further if operating temperatures could be increased. Current PEMs are limited to temperatures around 180 degrees Fahrenheit. “A lot of the automobile companies are investigating more advanced membranes that would allow the cells to operate at higher temperatures,” Hoberecht says.
The PEM has drawn the interest of the aerospace industry because it’s portable and can tolerate the expansion and contraction of on-off cycles. It’s also simple—no moving parts—and silent.
Airbus is toying with fuel cell applications too. Last spring, the company ran trials on an A320 airliner proving that a fuel cell could provide power for an electric pump that, through a hydraulic system, moved the airplane’s ailerons, rudder, and other control surfaces while in flight. Before fuel cells will be useful on airliners, however, engineers must find a way to increase their power density. “They have to provide more power at lighter and lighter structure,” Glover says.
As for replacing the raw power of a petroleum-fed jet engine, don’t expect fuel cells to do that any time soon, or ever. They will work quietly behind the scenes to keep the lights on, run the air conditioning, and produce water for the bathrooms, reducing water weight at takeoff. As a key figure bringing fuel cells to aviation, Bill Glover warns that there are still unanswered questions about fuel cell dependability: No one yet knows, for example, how fuel cells will perform in turbulence. So while fuel cells won’t debut on the 787, Boeing has designed the airplane to incorporate them later. Glover says that in the next five years the concepts will be tested in the laboratory, “then we’ll migrate them into flight situations when they mature. We want to make sure this works anywhere around the world.”
It already works out of this world. Three 255-pound fuel cells supply electric power to the space shuttle in orbit and produce more than 26 gallons of fresh water a day.
Michael Klesius is an associate editor at Air & Space/Smithsonian.