Energy question of the week: How can electricity be generated from hydrogen?
Fuel cells are surely the most elegant way of converting energy-rich fuels into electricity without emitting any carbon dioxide or other exhaust gases if hydrogen is the type of fuel used. Back in 1838, the German-Swiss chemist Christian Friedrich Schönbein successfully generated an electrical voltage from hydrogen and oxygen for the first time, using two platinum wires to accomplish this feat. Since then, many different types of fuel cells have been developed to a commercially marketable standard. What underlying principle do they all have in common?
Hydrogen and oxygen react with each other in a fuel cell in much the same way as they do in an explosion of oxyhydrogen or 'electrolytic' gas. The only difference is that, in a fuel cell, this process runs slowly and in a controlled fashion. To achieve this, it is important to have two sub-chambers divided by a membrane or diaphragm. Hydrogen flows into one chamber and oxygen flows into the other. In order to combine and form water, hydrogen molecules break up, lose their electrons and migrate through the membrane and into the oxygen in the other half of the fuel cell. The surplus of electrons on the hydrogen side causes the electrode to develop a negative charge. On the oxygen side, ions are captured by a second electrode, causing a charge gradient to form and direct current to start flowing.
Flying with a fuel cell
Fuel cells can now be operated with methanol instead of hydrogen or using a 'reformer' that uses kerosene, liquefied or natural gas. While it is true that the high cost of components has prevented the fuel cell from achieving broad mass appeal until now, it is winning hearts and minds in a growing number of niche markets. Aviation is certainly one sector where it makes sense to use fuel cells. "Using fuel cells, we can generate electricity for the electrical system of an aircraft. That means that we can replace the auxiliary power unit or 'APU' used to generate that power on board," says Professor Andreas Friedrich, Department Manager of Electrochemical Energy Technology at the DLR Institute of Technical Thermodynamics in Stuttgart. Advanced polymer fuel cells in jet aircraft now generate sufficient levels of electricity from pure hydrogen to supply all on-board systems, such as the air conditioning system, or to start the main engines.
Antares DLR-H2 is the first aircraft in the world capable of taking off and landing with a fuel cell as its power source. Credit: DLR.
On board an aircraft, the fuel cell not only generates the electricity required, but it also does so efficiently. It also has additional benefits: in course of a flight within Europe, some 200 to 400 litres of water are produced, and this can be used on board the aircraft. This means that a smaller quantity of water can be loaded at the airport, reducing the take-off weight of the aircraft. Further, the fuel cell can increase on-board safety – oxygen-depleted air, following its passage through the fuel cells, can be directed into the kerosene tanks. The reduced oxygen level lowers the fuel's flashpoint, making flying safer.
Along with many DLR researchers, experts around the globe are hard at work on developing the fuel cell concept. The aim is to extend the lifetime and to cut the costs involved. This process, while admittedly a slow one, is steadily laying the groundwork for the ongoing success of the fuel cell.
Elektrochemische Energietechnik beim DLR-Institut für Technische Thermodynamik (Electrochemical Energy Technology at the DLR Institute of Technical Thermodynamics)
The DLR Energy question of the week in 'The future of energy' Year of Science
The Federal Ministry of Education and Research (BMBF) has given the Year of Science 2010 the motto 'The future of energy'. For this reason the science journalist Jan Oliver Löfken will this year answer a question on the subject of energy in his blog each week. Do you have a question about how our energy supply might look in the future? Or do you want to know, for example, how a wave power plant works and how it can efficiently generate electricity? Then send us your question by email. Science journalist Jan Oliver Löfken will investigate the answers and publish them each week in this blog.