In the history of powered flight there's an unbroken line of development from the hot-air balloon to the jet engine. And now the next stage in that evolution looks to be the fuel cell.
A fuel cell is a device that converts hydrogen or some other source fuel such as hydrocarbons and alcohols directly into electricity, with water and heat as the by-products. There are currently more than 20 types of fuel cell, and although the underlying technology is not that new – it's already been applied to a wide range of transport applications from submarines, boats, buses and cars to the Space Shuttle – in recent years the aviation industry has started seriously considering it as a way to help it cut fuel costs and emissions.
As technical fellow at Boeing Dr Jim Kinder says: "Although from our point of view the technology is still very much at the preliminary stage, the principle is that any time we can create onboard power rather than pull it from the engines means saving fuel – the number one fixed cost in aviation."
The technology's commercial viability is still some way off. "No particular fuel cell technology appears to be the best way forward at the moment," Kinder says. "We’re looking at all of them, as well as ways to store hydrogen on board the aircraft, how to use the water and heat generated by the cells, and ways of converting liquid fuels to hydrogen."
Balaji Srimoolanathan, programme manager for aerospace and defence at consultancy Frost & Sullivan says: "The main obstacles in adopting the technology in commercial aviation are cost and size. Both are too big, and I don’t see fuel cells being adopted for anything other than auxiliary use until at least 2020, probably later. The technology is still very much at the development and testing stage."
There's also the issue of power density, given the ratio of power output to weight. It's estimated by Boeing and Airbus that this needs to be raised from current levels of about 0.3kW/kg to about 1kW/kg for the technology to be acceptable for onboard use. But with rising power density comes more heat, which also needs to be managed.
The fuel cell challenge
Because of this, fuel cells have been evaluated only experimentally. Boeing, for example, successfully tested a motor-glider in 2008 powered by a hydrogen fuel cell, but only to demonstrate that a manned airplane can maintain a straight level flight with fuel cells as the only power source. And in the same year, Airbus got good results with an in-flight test of fuel cells to power the back-up and hydraulic systems on an A320.
Real applications are emerging though. On board, and reflecting the A320 test, auxiliary power units (APUs) are a prime early candidate for fuel cell power, while environmental controls and power for galleys and entertainment systems also show potential.
And on the ground, fuel cells look set to debut even sooner, in ground power units and transit buses for example. "A key way forward here is to adopt the technology on ground vehicles," says Srimoolanathan. "The technology is already in place in cars and so on, and testing and safety is less of an issue here, so it should be an easier transition than for in-flight use."
In time, fuel cells could even provide primary power. As Dale Carlson, executive for advanced engine systems at GE Aviation, says he feels we could perhaps see fuel cells developing from applications for APUs, say, to providing the main propulsive power in aircraft.
"That's because, with jet engines, airlines are looking for something like a 20% improvement in fuel economy with each new generation of power plant. But there's only so much efficiency you can get out of the Brayton thermodynamic cycle, on which the workings of a jet engine are based, so eventually you have to look at new sources of power."
Waiting on legislation
There's also the issue of legislation to reduce the carbon footprint of air travel as much as possible, as Carlson's colleague John Kinney, director of advanced technology business development, points out.
"These reductions are set to be pretty draconian, especially in the longer term, and there's no way the industry can meet them with current jet technology."
But as Kinder explains, it's important to look at the technology from a system-wide point of view. "Advances in the technology do not take place in isolation; they are complementary with advances in other areas," Kinder says. "For example, the new 787, with its extensive use of lightweight composite materials and a distributed power network on board, represents a dramatic change in the way we build airframes."
Boeing, along with GE Aviation and others, recently completed a study for Nasa, called N+3, into possible commercial aircraft designs for 2030 and beyond, and Carlson and Kinney echo Dr Kinder’s point. You need to look at a wide range of issues, they say, including the cells' power density, where to site them, their reliability, how to get power from them and so on, as well as the materials used in the cell itself. So integrating fuel cells into an airframe is really an issue of system of systems integration.
"It's likely that airframes will be designed around fuel cells as a prime component from the outset rather than there being provision for them to be retrofitted according to future developments in the technology," Carlson says. "I can’t see them being used in a 'plug and play' manner, as you have now with some types of military equipment."
Kinney, for one, sees a bright future for fuel cells. "There's no reason at the moment not to see potential for all the different technologies. Fuel cells definitely have a future in aviation – there are many applications for the technology," Kinning says.
"Commercially viable fuel cells look to represent the new paradigm, and power plant manufacturers have to be in the frame here – and the winners with this technology will be those who get there first."