Hydrogen hop

The next generation of Airbus aircraft could use a fuel cell for emergency back-up power if trials of new membrane technology developed by BASF are successful.

BASF is part of a consortium including the German Aerospace Centre (DLR) that is testing a hydrogen fuel cell system based on a novel, high-temperature polymer electrolyte membrane (PEM).

The system, which will be trialled by Airbus Germany on an A320, was used recently on what was claimed to be the first manned aircraft able to take off and fly powered solely by a fuel cell. A single-seat Antares DLR H2 aircraft, developed by the DLR and Lange Aviation, took off from Stuttgart Airport equipped with the technology.

The electricity produced by the fuel cell was fed to the aircraft's battery, which powered the electric motor of the propeller. A 350 bar pressure hydrogen tank and the fuel cell system were housed in two external pods beneath the aircraft's wings.

The flight was claimed to be a milestone in alternative power technology for use in aerospace. Boeing has demonstrated manned aircraft powered solely by a fuel cell during flight, but is not thought to have used the technology to achieve take-off.

The demonstration was not intended to prove that fuel cells could be used for powering full commercial airline flights.

Instead Josef Kallo, head of electrochemical systems at the DLR, said his group hopes its system could be sold in two years time as an emergency power unit replacement for supplies on board aircraft.

The DLR has already provided a fuel cell auxiliary power supply unit for the hydraulic pumps of Airbus Germany's A320 ATRA research aircraft.

The system for the Antares test flight relied on BASF's Celtec membrane electrode assemblies (MEA) technology. The membrane assembly includes a synthetic fibre that is commonly used in applications such as racing driver suits as it has an extremely high melting point and does not ignite.

The heat-resistant polymer, called polybenzimidazole, is combined with phosphoric acid for the fuel cell's electrolyte. The combination of materials allows the fuel cell to operate at temperatures of 120ºC-180ºC. Normal PEM fuel cells operate at temperatures of 60ºC-80ºC.

Carsten Henschel of BASF said these high operating temperatures allow the membrane to tolerate impurities in hydrogen fuel gas. Theoretically this could mean that one day an aircraft's fuel cell could rely on hydrogen gas generated on board through reforming kerosene jet fuel.

Impurities can stick to a fuel cell's catalyst, which is usually platinum. The surface effectively becomes poisoned and no longer reacts with fuel. 'Even small traces of impurities such as sulphur and carbon monoxide poison the catalyst in low temperature PEM fuel cells,' said Henschel. 'Therefore fuel cell system builders have to include costly and complex technology to clean up the hydrogen gas in order to avoid the destruction of the fuel cell stack.

'At higher temperature of 140ºC-80ºC, the reaction kinetics change dramatically, allowing for a significant tolerance of the membrane electrode assembly and thus the catalyst for those impurities,' said Henschel.

The Celtec membrane electrode assembly can accept carbon monoxide concentrations of 30,000 to 40,000 parts per million, he said, while a low-temperature membrane electrode assembly can tolerate only 10 parts per million of carbon monoxide.

Another benefit is that the Celtec membrane electrode assembly does not need humidification. This versatility makes finding applications for the fuel cells a much easier task, according to the developers.

The DLR's Kallo said they could generate heat on board aircraft which could be used to de-ice wings or warm up the cabin area. The next step will be to prove their concept on the Airbus Germany A320 research aircraft.

'Our goal is to have a fuel cell emergency power unit product for the next Airbus generation in two years,' he said.

Siobhan Wagner