FUEL TECHNOLOGIES

Hydrogen fuel cell technology has potential for aviation use

System’s sole byproduct is clean, harmless water vapor.


Schematic core structure of a PEM fuel cell.

Despite these difficulties, in 2006 researchers at the Georgia Institute of Technology successfully flew an unmanned aircraft with a 22-ft wingspan powered by a 500-watt PEM fuel cell system.

It was one of the first demonstrations of an aircraft powered solely by hydrogen fuel cells following a standard flight procedure from takeoff to landing. Previous aircraft had required either hand launching or auxiliary battery systems for takeoff and lacked full landing gear.

The success of the Georgia Tech design was based on attention to every detail, from reducing drag through improved aerodynamics to the use of lightweight carbon foam as a radiator. The Georgia Tech system is not nearly large enough to power a commercial passenger aircraft.

However, it has potential applications ranging from low-cost satellites to unmanned aerial vehicles (UAVs), which are most commonly used by the military on scouting missions, eliminating the risk of loss of life on basic missions.

UAVs powered by fuel cells would provide enhanced stealth due to lower heat signatures. The technology also has the potential to expand into other risky applications, such as hurricane tracking.

Boeing is pushing the envelope further by adding a lightweight lithium-ion battery system to provide additional power for takeoff and climb. (The Boeing news release can be found at boeing.com/news/ releases/2008/q2/080403a_nr.html.)

This additional power is enough to enable manned flight, while maintaining the advantage of pollution-free operation. In Boeing’s 3 test flights in 2008, the batteries were disconnected after the aircraft reached a cruising altitude of 1000 meters, marking the first time a manned aircraft had been flown powered solely by a fuel cell system.

Its design enabled the 53 ft 6 in wingspan airplane to maintain straight and level flight at a cruising speed of 100 kph for approximately 20 minutes. While the aircraft carried only the pilot, it is conceivable that future advances in fuel cell technology, such as reduced stack weight and more efficient hydrogen storage, could add passenger capacity.

Nevertheless, fuel cell technologies are probably not yet ready to power large commercial passenger airplanes. A more likely immediate application of fuel cell technology for commercial air transport is as a replacement for the auxiliary power unit (APU).

The main purpose of an APU is to provide power to start the main engines. It also serves to power basic functions such as preflight checks and initial heating, and cooling and ventilation of the cabin before the main jet engines have started.

Finally, the APU provides backup in case of system failures during flight. APUs are commonly powered by batteries, accumulators, or ground power units. However, for many of the reasons outlined above, a well designed fuel cell system has the potential to be lighter and more reliable than a battery system.

The opportunities and advantages of fuel cells are exciting, but significant cost reductions and efficiency improvement are necessary for fuel cells to emerge as a practical energy alternative. Large reductions in weight are also required for applications in the transportation industry, particularly aircraft.

A 2004 report entitled The Hyd­rogen Economy from the Committee on Alternatives and Strategies for Future Hydrogen Production and Use, National Research Council, National Academy of Engineering, notes that the primary barriers to fuel cell adoption are both performance and implementation-based.

Fuel cell performance is limited by high cost and durability—and, while their efficiency is higher than most alternatives, it is not yet high enough to overcome the shortcomings. The implementation issue is twofold.

A viable hydrogen economy will require large investments in both hydrogen distribution and production infrastructure. In addition, while the fuel cell itself can be compact, ground and air transportation applications require hydrogen storage systems, which are currently under development.

Fortunately, research and development are providing new answers to many of these questions at a rapid pace. Innovative materials are being developed to store hydrogen more densely and efficiently than compressed gas or liquid storage systems, bringing down total system weight while increasing its overall energy efficiency.

Efficient use of expensive catalysts such as platinum is bringing down the cost. And new designs are improving durability and increasing efficiency. All these advances and new approaches to be discovered would certainly make fuel cells an exciting option to explore for a wide variety of applications, not the least of which is a new type of small aircraft with zero emissions.

Steven Barceló is a researcher at Lawrence Berkeley National Laboratory (LBNL) who specializes in solid‐state hydrogen storage technologies for fuel cell vehicle applications.

Samuel Mao is a career staff scientist at LBNL who leads a team of researchers developing clean energy technologies for applications that include hydrogen storage, fuel cells and solid‐state lighting. He is a judge of R&D 100 technology awards, and has served as a review panel member for government and private funding programs.

 

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