<b>Hydrogen Propulsion</b> Hydrogen reacts both in fuel cells and in direct combustion with oxygen to form pure water, releasing high levels of usable energy without releasing carbon dioxide (CO2), as is the case with fossil fuels. Hydrogen - bound in water - is theoretically available in unlimited quantities and should be produced with the help of regenerative energy (electrolysis). However, the storage of the fuel is a major problem. As a gas, hydrogen is extremely volatile and has only a very low density. In order to be able to use hydrogen sensibly as a fuel on board a vehicle, it is necessary to achieve a high energy density in the smallest possible space (high storage density) in order not only to have sufficient cargo space capacity (volume / load capacity) for commercial use, but also ensure an adequate range of the vehicle. To do this, however, the hydrogen must be liquefied using additional energy and cooled to -253 ° C. This makes it clear that hydrogen tanks require complex insulation in order to minimize unwanted evaporation. For this purpose, vacuum-insulated pressure tanks are used, which are constructed according to the concept of a thermos flask. But even then the hydrogen needs 4 - 5 times the volume compared to mineral oil. Even if there are engine types with the possibility of direct combustion of hydrogen, the development of fuel cells is currently in the foreground. With this technology it is possible to convert the energy generated by a chemical reaction directly into electrical energy. Compared to combustion engines, fuel cells achieve an above-average high degree of efficiency. However, it will be some time before such fuel cells are used on board merchant ships, since the effort required for this technology is very high, especially when it comes to making high outputs of up to 80,000 kW available. But there are already ships that use and test hydrogen propulsion.