What are the alternative H2-powered drives for vehicles?
Although the fuel cell offers the most familiar way of exploiting the chemical energy stored in hydrogen, it also has some fairly serious competitors. One of these is the combustion engine, which offers great flexibility in terms of the fuel on which it can run.
For example, a hydrogen-powered combustion engine burns an ignitable mixture of hydrogen and air in the combustion chamber. What makes this type of engine interesting is not least the fact that it can be modified – by adapting the relevant components – to operate as a bivalent engine that can be run on either hydrogen or a carbon-neutral gasoline fuel. The direct combustion of hydrogen is one of the areas of interest at Fraunhofer ICT. Here, researchers are developing and refining combustion processes and related technology in a program that combines computer simulation and concrete experimentation with single-cylinder research engines. Hydrogen has superb combustion properties, meaning that an internal combustion engine can be run on a very lean mixture of hydrogen and air. As a result, only low levels of nitrogen oxides are produced within the engine, and these can be reduced to almost zero by means of simplified exhaust gas treatment. Furthermore, unlike conventional combustion engines, hydrogen engines do not produce any carbon-based emissions, which otherwise necessitate the use of increasingly sophisticated methods of exhaust gas treatment.
The key question regarding the direct combustion of hydrogen is what if something happens to the vehicle. After all, hydrogen is not only a powerful energy carrier but also highly explosive. A research team at Fraunhofer ICT is currently investigating a range of scenarios, including the worst case. Under what conditions might a critical event occur? Are there cavities in the vehicle where escaped hydrogen might accumulate? In the worst instance, how much pressure would build up there? The researchers calculate potential failures on the basis of key system data, dentify and model likely scenarios and then verify the results on the basis of real tests. This means calculating the theoretical impact of possible failures and then, on this basis, pumping hydrogen into vehicle cavities and generating an explosion under controlled conditions. Fraunhofer ICT has its own test site for this purpose. It is designed to withstand a detonation force equivalent to that of 3 kilograms of TNT.
Hydrogen-based liquid fuels are also in competition with the fuel cell. The basic concept is simple: green hydrogen generated via electrolysis is combined with carbon dioxide or nitrogen – rather than being turned back into electricity via a fuel cell. The result is methanol (CH4O), which in turn can be used to produce highly synthesized fuels – more precisely, oxymethylene ethers, which, similar to E10 in gasoline, can be used directly as a diesel substitute. This type of power-to-liquid (PtL) process makes sense primarily in areas where a rapid renewal of vehicle fleets is impracticable or where the conversion of existing infrastructure is prohibitively expensive. Indeed, one of the key advantages of this type of drive is that, in ideal circumstances, no changes must be made to the engine technology – and yet, compared to fossil fuels, greenhouse gas emissions are reduced by as much as 90 percent over the entire functional chain. Moreover, due to their specific chemical structure, which does not include any C-C bonds, OMEs also produce very low emissions when combusted, which greatly reduces the local emission of pollutants. Similarly, in combination with CatVap®, a process developed at Fraunhofer ISE, power-to-liquid fuels could help to significantly reduce emissions of combustion engines. Fraunhofer ISE is currently investigating this potential in a number of projects, including Sylink and C3-Mobility. “In terms of efficiency, there’s not much to choose between fuel cells and oxymethylene ethers,” Schattauer says. “As far as cars are concerned, it’s really more a question of ideology.”