Flight path for fuel cells

NASA’s Revolutionary Aeropropulsion Concepts programme is aiming to produce a fuel cell-powered aircraft the size of a Boeing 737 with zero CO2 emissions.

While cars and power stations are the world’s major greenhouse gas contributors, NASA estimates that around four per cent of all CO2 emissions are produced by aircraft. the Royal Commission on Environmental Pollution recently concluded that this could reach 10 per cent by 2050.

Although question marks remain over the likelihood of hydrogen-powered cars becoming a reality, fuel cells could be the answer to reducing pollution from aircraft, said Peter McCallum, deputy head of NASA’s propulsion and power projects.

‘We think that fuel cells offer the greater long-term benefit if they can be made to work because they have a higher inherent thermal efficiency than conventional aircraft engines,’ he said.

After three years and $21m (£13m) of research at NASA’s Glenn Research Centre, Ohio, several candidate propulsion systems have been taken forward for further development this year, following a call for research proposals in 2002.

But the technology will need to be adapted as aircraft require lighter and more powerful fuel cells, so the researchers are working with General Motors and Canadian fuel cell manufacturer Ballad to develop the devices.

The researchers are investigating solid oxide fuel cells, which use solid electrolytes rather than liquids and are generally used in power plants as they are more efficient in stationary applications. Proton exchange membrane fuel cells, which are being developed for cars and use liquid hydrogen, are under serious consideration. But the fuel is typically stored in spherical tanks to ensure it is kept at a sufficiently high pressure to remain liquid, causing problems for aircraft designers as airliner fuel is normally stored in the wings.

Producing the electric motors needed to drive the propellers and turbine fans in a fuel cell-powered aircraft is another major hurdle. Existing copper coils, used within electric motors to generate electro-magnetic fields, are far too heavy for use on an aircraft as are electric ducted fans and propeller motors.

One possible solution, said McCallum, is to use cryogenics to produce superconducting aluminium. The lighter material would be used in place of copper, although research is still at an early stage, he said.

The programme, costing $7m (£4.3m) a year, is linked with NASA’s high-altitude, long-endurance solar electric Helios aircraft project, and the US military’s fuel cell-powered UAV research. But despite this government-wide effort, McCallum admits it will be some time before zero emission aircraft become a reality.

‘To say this technology will be ready in 20 years is more of an aspiration. There are just too many technological advances that would be required.’

The Revolutionary Aeropropulsion Concepts programme also includes research into more efficient jet turbines and pulse detonation engines, where fuel is exploded within air rather than burnt at lower pressure as in conventional engines. The researchers are also investigating direct hydrogen combustion, where the fuel is burned as a gas in an internal combustion engine. The technology is simpler than that required for fuel cells, but only half as efficient.

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