Cleantech Investor

First published in Cleantech magazine, November 2008. Copyright Cleantech Investor 2008

by Dr Denis Gross

The first ever flight of a manned aircraft powered by hydrogen fuel cells in February 2008 was both a milestone in flight history and an indicator of the advantages and possible limitations of fuel cells in airborne applications.

February’s flight was the outcome of the work done in Spain by Boeing Research and Technology Europe (BR&TE), and its industry partners in Austria, France, Germany, Spain, the United Kingdom and the United States. The partners included Loughborough-based Intelligent Energy, the provider of the fuel cell power system itself. The aircraft used in this programme is a two-seat Diamond Aircraft Dimona motor-glider frame with a 16.3m wingspan. It was modified by BR&TE to incorporate a hybrid low emission 20kW system, containing Intelligent Energy’s power dense proton exchange membrane (PEM) fuel cell power system and lithium-ion batteries, to power an electric motor coupled to a conventional propeller. Three test flights took place in February and March at an airfield south of Madrid. 

During the flights, the pilot of the motor-glider climbed to an altitude of 1,000m (3,300 ft) 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 level at a cruising speed of 100 kph for approximately 20 minutes on power generated solely by the fuel cells.

In October 2008 an Antares motor-glider became the first manned aircraft to take off and land using only a fuel cell to power its electric engine. The German Aerospace Research Centre, DLR, with BASF and Serenergy, a Danish designer of fuel cell stacks and power modules, collaborated in this project. The air-cooled stack in the fuel cell system used PEM technology based on BASF's Celtec membrane electrode assemblies and a stack supplied by Serenergy.

Also in Europe, but still at a relatively early stage, is a EU-funded project, initiated in 2007, to develop the first fuel cell manned intercity aircraft. Representing a significant step-up from powering motor-gliders, the ‘Environmentally Friendly Inter City Aircraft powered by Fuel Cells’ (ENFICA-FC) project is receiving €2.9 million from the EU as part of the aeronautics and space priority of the Sixth Framework Programme (FP6). Led by the Polytechnic of Turin, the goal of the project is to develop an intercity aircraft that uses fuel cell technology for the propulsion system, and hydrogen storage. The Czech company Jihlavan‘s Rapid 200 will be used as a flying test bed.

Underpinning the drive to deploying these technologies are the low emissions and silent running of fuel cells. The attraction of hydrogen-powered aircraft is that the technology would address both the greenhouse gas and nitrous oxide emissions problem, but also offer greater efficiency. Both of these characteristics are advantageous for commuter aircraft, which often operate from airfields in urban areas. The possibility to take off and land without contravening the noise abatement regulations set for small airfields, in urban areas and near population centres, will allow the use of airfields late at night, when noise regulations are the most stringent.

Although future rigorous regulatory pressure might help sway the balance in a fuel cell versus turbine contest in commercial aviation, the capital and installation costs of a fuel cell solution and the amount of further progress required still present serious obstacles.

Cranfield University, UK, which has a well-established fuel cell research activity, recently noted that fuel cells are still too heavy for propulsion: in addition the electric motors are still too large and heavy. Furthermore, hydrogen and oxygen storage pose significant size and weight problems. Overall, the use of fuel cells to propel commercial aircraft remains a distant proposition.

An aircraft’s engines, however, provide more than propulsion – they are used to generate the electrical power needed to supply the systems for aircraft control and cabin comfort, and they power the hydraulic and pneumatic systems that operate the aircraft.  Fuel cells can generate electrical power much more efficiently than conventional engine-driven generators while silently delivering emissions reductions. A further advantage is that fuel cells can generate some of the water needed for galleys and lavatories in the aircraft.

Consequently, the auxiliary power unit (APU) is seen as a suitable candidate for fuel cells. The primary purpose of an aircraft APU is to provide power to start the main engines, which is done by turning the heavy rotors of the engines up to a sufficient rate of rotation for self-sustaining operation. The APU is started by an electric motor, with power supplied by a battery or ground power unit. When it reaches its operational rpm the APU can provide a much larger amount of power to start the aircraft's main engines, either by turning an electrical generator or by providing compressed air to the air turbine of the starter motor. In addition, APUs are used for supplying electrical and pneumatic power before the engines are started, e.g. during passenger boarding.

In February 2008, Airbus successfully tested a fuel cell system in flight. For the first time on a civil aircraft fuel cells were used to power the aircraft's back-up hydraulic and electric power systems. During the test, the emission free PEM fuel cell system, which produces water as a ‘waste’ product, generated up to 20kW of electrical power. The fuel cell system powered the aircraft's electric motor pump and the back-up hydraulic circuit and also operated the aircraft's ailerons. The system's robustness was confirmed at high gravity loads (‘g’ loads) during turns and zero gravity aircraft manoeuvres. In the course of the flight test, the fuel cells produced around 10 litres of pure water.

Boeing is looking at the possibility of using hydrogen-powered fuel cells to provide emergency back-up power in aircraft. The company is collaborating with Sandia National Laboratories to examine their feasibility in commercial and military aircraft, which currently use a variety of techniques for providing back-up electrical power to critical subsystems during emergencies. Back-up systems currently in use include batteries and running the APU during flight.

Perhaps the nearest term commercial opportunity for fuel cell systems in aviation is in small unmanned aircraft (UAVs), which at present use electrical propulsion systems where a performance advantage over batteries already exists. UAVs currently represent the fastest growing segment of the aerospace industry, a market estimated by industry analysts to be worth $54 billion over the next ten years.

Small UAVs are one of the most demanding applications for emerging hydrogen fuel cell power technologies due to the constraints of size, weight and aerodynamics.  Companies like Horizon Fuel Cell Systems in Singapore and the AIM-listed Protonex have demonstrated that extremely high power density PEM fuel cell systems can power these small aircraft for longer distances than today's available battery technologies.

Boeing researchers believe, based on the successful flights of the Dimona motor-glider, that PEM fuel cell technology potentially could power small manned as well as unmanned air vehicles. Over the longer term, solid oxide fuel cells (SOFCs) could be applied to secondary power-generating systems, such as APUs for large commercial airplanes. By 2010 the technology could be sufficiently mature to be offered on future Boeing 787s.

For the time being, however, Boeing and other leading aircraft manufacturers do not envision that fuel cells will ever provide primary power for large passenger airplanes. Nevertheless, we can expect that these companies will continue to investigate their potential, as well as other sustainable alternative fuel and energy sources that improve environmental performance.