Hydrogen in the air: electric aircraft

Aerospace companies are investigating several options for reducing the dependency of aircraft on fossil fuels, one of which is the use of hydrogen to power on-board fuel cells. These would generate electricity that could be used for a variety of purposes, from powering the aircraft itself to powering the electrical systems on board a larger airliner.

There are several electrical systems that are powered by electricity generated by the aircraft’s engines. These include the hydraulic and pneumatic systems that control the aircraft; the air-conditioning systems, lighting and heating; the anti-icing systems for wings and control surfaces; and the on-board avionics and electronic systems such as in-flight entertainment.

Another important use for electricity on a large aircraft is to start the engines, with a system known as the auxiliary power unit (APU) running motors that turn the engine rotors until they are running fast enough to compress air into the engine and start burning fuel. The APU is started up from batteries or a ground-based power unit.

Hydrogen is flammable and highly explosive – keeping it in one place, and keeping it safe, is vital to its use

The generators driven by the engines are not as efficient at generating electricity as a fuel cell, and removing the need to run generators would considerably reduce the amount of fuel an aircraft needs to carry. Therefore, replacing the APU with a fuel-cell system would be a major step towards a low-emission, fuel-efficient aircraft. It would also mean that a ground-based generator wouldn’t be necessary, making the operation of the aircraft on the ground emissions free at point of use (the actual emissions would depend on the processes and materials used to make the hydrogen electrolysing water using renewable electricity would be emissions free, but other methods would generate CO2. Both Airbus and Boeing are looking at this technology.

One of the major stumbling blocks, however, is familiar from the problems in developing fuel-cell-powered cars: how do you store the hydrogen? The simplest substance in the universe, it has smaller molecules than any other, and a tendency to leak out of containers. Moreover, it’s highly flammable and explosive; keeping it in one place, and keeping it safe, is vital to its use.

One company looking at this problem is Magna, an Austro-Canadian outfit whose roots are in the automotive components business. Presenting its developments in hydrogen storage at the recent Aerodays conference in Madrid, Karl Langensteiner,from the space technology department of the company’s Magna Steyr division, explained that its involvement in aerospace hydrogen came from two sources. ’We’re a supplier of cryogenic equipment for the Ariane rocket launcher,’ he said. ’We’ve produced the main-stage feed line that carries liquid oxygen and hydrogen to the Vulcaine engine on the Ariane 5 since 1990, and the seamless and welded tubing system for the upper-stage motors since 1996.’

“The main problem was weight reduction – this ruled out a stainless steel construction”

Karl Lengensteiner, Magna Steyr

As a consequence of this, Magna Steyr worked with BMW on a liquid hydrogen storage system for the BMW Hydrogen 7 series, which uses the gas directly as a combustion fuel in place of petrol. The car carries two fuel tanks and can switch between the two fuels automatically or under the driver’s control. For this project, Magna Steyr developed a 110-litre tank, holding 8.8kg of liquefied hydrogen at a temperature that cannot rise above -253°C. The tank was constructed of two layers of stainless steel, with a vacuum gap between the layers, giving an insulation claimed to be equivalent to a 17m-thick wall of expanded polystyrene. As a safety system, the tank incorporated a valve to vent excess pressure once the hydrogen started to vapourise, as it inevitably would; if left full and out of use, the tank would empty completely in 10-12 days.

Development of this system, and subsequent work on hydrogen tanks for fuel-cell-powered buses, led to a collaboration with Airbus to develop highly insulated hydrogen tanks for aircraft. ’The main problem was weight reduction,’ Langensteiner said. This ruled out a stainless steel construction; instead, it looked at composites, with filament-wound carbon fibre forming the reinforcing structure of the tank outer and multi-layer insulation systems as the lining, and reducing the use of stainless steel to the end cap of the tank. ’We minimised the amount of equipment inside the tank to avoid unwanted heat transfer,’ he added.

The tank was designed to supply gaseous hydrogen to a fuel cell at a pressure of 5 bar and at temperatures above 5°C, so the system had to incorporate a heat exchanger and valves. These were grouped together into a single assembly mounted on the tank for easy maintainability.

The tank, which holds 25kg of liquid hydrogen, is designed to be mounted inside the tail cone of an A320 aircraft, which Airbus plans to use to test the system. The fuel cell would not only provide electricity. Langensteiner said: ’The exhaust air would be fed through a condenser to provide both water and nitrogen-enriched air. The water would be used to humidify the cabin air, and for the toilets on board; with minerals added, it would also be suitable as drinking water or for on-board showers.’ The nitrogen-enriched air would be pumped into the fuel tanks as a safety feature to reduce the risk of fire, he added.