Nieves Lapena-Rey, chief engineer on Boeing’s fuel-cell-powered aviation project

Visitors to Madrid’s Museum ‘Del Aire’ could be forgiven for walking straight past the small motor glider that sits in a quiet corner of one of its many hangars.

But although it might not quite be up there with the Wright flyer or Concorde, this diminutive exhibit may one day be regarded as a symbolic landmark in aviation history; the initial stuttering step towards emissions-free flight.

That’s because, during a series of flight tests last spring, the Boeing-developed plane became the world’s first- ever manned fuel-cell-powered aircraft.

Reflecting on this pioneering effort, the project’s chief engineer, Nieves Lapena-Rey, recently spoke to The Engineer about what the plane could mean for the future of hydrogen-powered flight.

Lapena-Rey, who heads up the environmental technologies team at Boeing’s European Research and Technology Centre (BRTE) in Madrid, explained that the aircraft, based on the lightweight Austrian-manufactured Diamond Dimona, was equipped with both a Proton Exchange Membrane (PEM) fuel cell (developed in the UK by Intelligent Energy) and a lithium-ion battery system. These were used to power an electric motor coupled to a propeller.

During the flights, the pilot climbed to an altitude of around 1,000m before disconnecting the batteries and flying solely on fuel-cell power for 20 minutes. And although this was enough to secure its place in the history books, Lapena-Rey said the aircraft could have gone farther. ‘We started with a tank with 350 bar of hydrogen and at the end still had 100 bar of hydrogen and some battery charge left. According to our calculations, this would have given the aircraft endurance of around one hour.’

Clearly, replacing an aircraft’s primary power source with a fuel cell requires some fairly fundamental changes. It was therefore, perhaps, a strong indicator of the technology’s promise that the engineering team was able to achieve this without interfering too much with the aerodynamic profile of the two-seater, 16.3m-wingspan plane. ‘Our idea was to try to modify the aerodynamics of the aircraft as little as possible,’ said Lapena-Rey. ‘We had to modify some of the bulkheads to install the battery, the engine bay in order to house the new electric motor, and we had to introduce a few inlets in order to reduce the risk of explosion should a leak occur, but we consider that the effect on the aerodynamics of the prototype were almost negligible.’

Promising stuff then. But even if the aircraft had exceeded expectations and stayed aloft for an hour, that is still not a very long time. To really capture the imagination, and begin to compete with existing systems, fuel-cell technology must offer a lot more endurance. According to Lapena-Rey, there are two key elements to making this a reality: optimising energy density and reducing overall weight. ‘The most important things are weight reduction and optimisation of the energy density. That takes into account not just the fuel cell but also the fuel,’ she said.

Indeed, for all its high-tech credentials as ‘fuel of the future’, one of the biggest technical stumbling blocks for hydrogen is, on the surface, actually quite mundane: to improve energy density you need to store more hydrogen in a smaller space, and high-pressure storage vessels are typically rather heavy.

Boeing’s plane used a composite tank to store the hydrogen at 350 bar and although some companies already offer systems that store hydrogen at 700 bar, they tend to be made from steel and therefore weigh a lot more.

Developments in hydrogen pressure vessels will, said Lapena-Rey, considerably boost the technology’s potential in aerospace applications.

Nevertheless, for certain missions and power ranges the technology already has plenty to recommend it. ‘This technology has very short-term applications in areas such as sport aviation or small manned or unmanned vehicles,’ said Lapena-Rey.

She added that a number of applications on small unmanned and manned aircraft are already in the pipeline. ‘You will be hearing from us on applications of fuel cells on smaller aircraft fairly soon.’

As for the longer term, Lapena-Rey thinks that it’s unlikely that we will see hydrogen-powered passenger jets anytime soon but she did say that fuel cells could, in the longer term, be used on auxiliary power units for larger aircraft.

To put this into perspective, the ‘longer term’ is not as far off as it might sound. Lapena-Rey’s brief — to look at technology 15 to 20 years away from commercialisation — may amount to little more than crystal-ball gazing in some of today’s fast-moving technology industries. But this is not the case in the risk-averse, traditionally conservative aerospace industry, where advances are measured in decades, rather than months.

And the reason for this conservatism? An emphasis on safety, which is perhaps more onerous than any other industry and presents Lapena-Rey and her team with some of their sternest and most frustrating engineering challenges. ‘The problem that we have is that aircraft safety is paramount,’ she said. ‘Often, engineering solutions are available from other industrial sectors, but they need to be thoroughly tested in aviation before they can be implemented.’

A good example, and the subject of an ongoing research project, is an effort to eliminate the toxic heavy metals chromium and cadmium from the corrosion protection systems of aluminium and stainless steel. ‘There are existing solutions for the automotive sector but they do not comply with the safety requirements of an aerospace application. We base our knowledge on the strides that have been made in automotive but we need to put our requirements up front and test them thoroughly. If you have corrosion in an aircraft it becomes a safety issue,’ said Lapena-Rey.

Against this backdrop, it’s not difficult to see why hydrogen-fuelled passenger planes are seen as a no-go, and no one would want the aviation industry to abandon its essential emphasis on safety.

But despite all this, it is still tempting to imagine some far-off future, where the curator of a dusty old museum in Madrid points at a remote corner of the hangar and tells his visitors: ‘This is where it all began…’