Hydrogen propulsion: five questions and answers
1. How does hydrogen propulsion work using the fuel cell principle?
Fuel cell vehicles (FCVs) are driven by an electric motor, just like electric cars. However, according to the definition supplied by the German motoring organisation ADAC, the electricity required for this is not supplied by an on-board battery but is actually generated within the fuel cell in the vehicle. This comes about through reverse electrolysis. The fuel cell converts hydrogen and oxygen (drawn from the atmosphere) into water and, in the process, produces electricity and heat. Fuel cell propulsion in vehicles is efficient and quiet. The high torque of the electric motor permits gear-free acceleration.
A vehicle fuel cell is made up of a large number of individual cells which, when they are connected in series to form a 'stack', generate sufficient power to drive an electric motor. This process causes no emissions; only heat and water vapour are released. This means that a fuel cell car is a Zero Emission Vehicle (ZEV). The fuel cell also replaces the alternator in a car; it provides the electricity for the ever-increasing electronic controls now found in cars and the heating and air-conditioning systems.
2. Can hydrogen also be used a direct source of fuel in a combustion engine?
Hydrogen can also be used as a fuel in suitably modified petrol engines. Such current engines are of a bivalent design, which means that they can run on both petrol and hydrogen (e.g. the BMW Hydrogen 7). Hydrogen combustion engines are based on a normal production engine, but with a modified intake system which permits both direct petrol injection and the injection of hydrogen into the intake manifolds. A further difference lies in the altered combustion process and treatment of the exhaust gases, with the result that practically the only emissions produced when driving using hydrogen are water and minimal quantities of NOx.
3. How is the hydrogen carried in the vehicle?
The tanks in hydrogen vehicles are different from those in a conventional car. They store the hydrogen either as a gas under high pressure (350 bar) or in liquid form at very low temperatures (minus 253°C). Hydrogen has its highest energy density in this physical state. Super insulated, double-walled tanks are required to store it. The vacuum between the two walls contains insulating materials which keep the tank cold, regardless of the external temperature, and minimise loss through evaporation.
4. Are hydrogen cars already on sale?
To date, there are no series production hydrogen vehicles available on the market.
Car manufacturers' current projects, and those planned for the near future, are concept cars to enable them to gain new insights into both the everyday use of this new drive technology and the demands of the infrastructure required. In June 2002, the Clean Energy Partnership (CEP) was founded to explore all the options in this area. In May 2008, the CEP embarked on phase two, in a new configuration with 12 partners: Berliner Verkehrsbetriebe (BVG), BMW, Daimler, Ford, GM/Opel, Hamburger Hochbahn, Linde, Shell, StatoilHydro, TOTAL, Vattenfall Europe and Volkswagen.
The Clean Energy Partnership aims to tap the technological potential of hydrogen as a source of energy and to test the effectiveness of the system and its suitability for routine use. The CEP is supported by the German Federal Government as part of the National Strategy on Sustainability. For more information about this project and the hydrogen models of the participating manufacturers, visit www.cleanenergypartnership.de.
5. Are hydrogen cars more environmentally friendly?
One clear advantage of fuel cell vehicles is that they themselves produce no emissions whilst being driven. However, one must also take into account the emissions that are caused by the production of hydrogen and the level of energy consumption involved. Hydrogen is only found in nature as a compound, and this means that a large amount of energy is required to separate it.
It is therefore only possible to make a significant contribution to the reduction of CO2 emissions in traffic if hydrogen can be produced using renewable sources of energy. Otherwise, we are simply shifting the problem of emissions from the area of cars in movement to that of hydrogen production in a fixed location. Possible sources of energy for the production of hydrogen include photovoltaics, wind and wave power, solar thermal energy, geothermal power and the use of biomass. Scientists see wind power as the most important renewable source of energy in this context. But it is here that the problems really start, as the infrastructure for the mass production of renewable energy does not exist. In addition, the requisite network of filling stations for hydrogen must first be put in place.