Boeing has announced perhaps the most dramatic technological breakthrough in the race to reduce the impact of air travel on the environment. In April 2008, the company revealed that it had flown a manned airplane powered by hydrogen fuel cells.

According to Boeing, fuel-cell technology could be used to power small manned and unmanned air vehicles.

Fuel cell applications

Over the longer term, solid oxide fuel cells could be applied to secondary power-generating systems, such as auxiliary power units for large commercial airplanes. However, Boeing does not anticipate that fuel cells will ever provide primary power for large passenger airplanes, although it plans to investigate their potential further and to continue research into other sustainable alternative fuels and energy sources that improve environmental performance.

A team from Boeing Research & Technology Europe (BR&TE) in Madrid led the fuel-cell project, with assistance from industry partners in Austria, France, Germany, Spain, the UK and the US. A two-seat Dimona motor-glider with a 16.3m wingspan was used as the airframe. Built by Diamond Aircraft Industries of Austria, it was modified by BR&TE to include a proton exchange membrane (PEM) fuel cell / lithium-ion battery hybrid system to power an electric motor coupled to a conventional

Testing and demonstration

Three test flights took place in Spain, in February and March 2008. During the flights, the pilot of the experimental airplane climbed to an altitude of 1,000m (3,300ft) above sea level using a combination of battery power and power generated by hydrogen fuel cells. Then, after reaching the cruise altitude and disconnecting the batteries, the pilot flew straight and level at a cruising speed of 100km/h (62mph) for approximately 20 minutes on power generated solely by the fuel cells

Boeing demonstrated the fuel cell at the Farnborough Air Show in July 2008 and Jon Moore, director of communications for Intelligent Energy, which supplies the hydrogen fuel-cell power system, told Aerospace Technology that “Boeing’s demonstration and our research have proved that fuel cells have the potential to become a standard feature on airliners to power auxiliary power units, air conditioning and on-board electrics.”

“Boeing demonstrated the fuel cell at the Farnborough Air Show in July 2008.”

“With energy costs rising and airlines increasingly under pressure to address environmental concerns, it’s a neat solution. Ultimately, our technology will have to meet the performance standards demanded by the aerospace industry, which we’re confident it will do once we begin to trial fuel cells systems for these applications.”

Intelligent Energy believes that they have shown that fuel cells could eventually power aircraft engines.

“However, the power required to get a large passenger aircraft off the ground is substantial and fuel cells are better suited to other applications – some within aerospace, others in commercial and passenger road transport or distributed power,” according to Moore.

But it is difficult to put a time frame on when fuel cells will become standard features in aircraft. Moore points out, “One thing we’re acutely aware of is the long development cycles that new technologies go through, so even those manufacturers with the greatest foresight and environmental focus will be working on introducing new systems for years before they come to market.

“Having fuel cell technology on board airliners will take time, but the limiting factor here is the level of commitment that OEMs [original equipment manufacturers] put into this development work. We hope what we’ve achieved on this project will provide some momentum in that respect.”

Airbus also searching for answers

Airbus is also seeking technological solutions to lessen the environmental impact of its aircraft. In February 2008, an Airbus A380 aircraft successfully completed the world’s first ever flight by a commercial aircraft using a liquid fuel processed from gas (gas to tiquids – GTL). The flight from Filton, UK, to Toulouse, France, lasted three hours. Airbus says that its alternative-fuel research programme is the first step in a long-term testing programme to evaluate viable and sustainable alternative fuels in the future.

GTL has attractive characteristics for local air quality, as well as some benefits in terms of aircraft fuel burn relative to existing jet fuel, according to Airbus. It is virtually free of sulphur and can be made from a range of hydrocarbon source material, including natural gas or organic plant matter made by a process called Fischer-Tropsch.

Airbus has set itself the goal of cutting carbon dioxide and noise by half from 2000 levels, and nitrogen oxides by 80%, to lower their impact on the environment and reduce the fuel costs incurred by airlines. “We have put the bar very high,” Airbus’s senior vice president for research and technology, Axel Krein, said at the Farnborough Air Show in July 2008.

“Solid oxide fuel cells could be applied to secondary power – generating systems.”

Engine technology offers best hope

Engine technology holds the key to improving fuel efficiency and lessening the impact of aircraft on the environment. Pratt & Whitney’s new geared turbofan engine appears to be leading the field at present. In July 2008, Pratt & Whitney announced at Farnborough that its geared turbofan engine would be used to power Bombardier’s C Series narrow-body passenger jets. Pratt & Whitney has already invested $1bn in the geared turbofan technology and Airbus plans to fly one of its A340 aircraft using Pratt & Whitney’s new engine later this year.

Innovative use of gearing allows the main front fan and the rear sections of the engine to operate at different, optimum speeds. “The result of that is much better fuel burn, much lower noise, much lower emissions,” says Bob Keady, Senior Vice President of Sales & Marketing at Pratt & Whitney. He believes the engine offers a “complete solution” for the industry.

A number of engine manufacturers are working on even more radical new engines that promise significant advances in efficiency. The most complicated of these is the so-called open-rotor engine, which could cut fuel burn by up to 30%.

These engines have several rows of propellers that jut out at different angles and rotate in different directions, giving the plane greater lift. Open rotors require less fuel than turbofans because they have a much higher bypass ratio – the proportion of air bypassing the engine core. Consequently, the engines run more slowly but are more fuel efficient.

Engineers are working on problems such as noise and vibrations from the open-rotor engine, which would have a massive fan, would not be encased and would not fit under the wings. Rolls-Royce, which is working on an open-rotor design, says it is pursuing different engine strategies, which could begin delivering results to airline customers by about 2015. In order to justify the cost of new planes, airlines generally require a 15% improvement in fuel efficiency and a 25% reduction in maintenance costs.

Radical designs

Airframe design also offers opportunities to save on weight and thus reduce the size of the industry’s carbon footprint. Airbus said at Farnborough that the next generation of aircraft could benefit from increased use of composite materials. The A380 contained 25% composite materials and the A350, which is not yet in service, will contain 53%.

“Fuel-cell technology could be used to power small manned and unmanned air vehicles.”

New aircraft could contain even more. Boeing and Airbus are also seeking to develop more aerodynamic planes. Both are looking at blended wing-body (BWB) aircraft, which have the potential to carry more than 1,000 passengers a flight and hold significant advantages over conventional aircraft in terms of performance and weight. However, the BWB is a revolutionary aircraft concept and will require a large and expensive engineering effort to become a reality. The design is likely to be developed by the military before it is applied to the commercial aerospace sector.

While the open rotor and the blended wing may not be seen on commercial aircraft until well into the next decade, there is little doubt that the planes flying in five years’ time could be radically different from those flying today. Many will feature geared turbofans and much greater use of composite materials. They will also feature the blended winglets that are already being installed on aircraft and which can save up to 6.5% in fuel costs. The use of fuel cells will also arrive at some point, not to power the aircraft’s engines but to provide energy for other uses on the aircraft, and airliners could be flying on alternative fuels.