Though electricity is considered as the prototype as far as renewable energy is concerned, it is however, not arbitrarily storable for longer periods, or transportable. Even if the problem of current loss could be solved when transporting it from the hot areas of the earth by super-conduction or direct current, the difficulty of long-term storage remains. In the automobile, the storage of electric energy is even more difficult. The energy density of a full fuel tank is simply without competition. This aspect of hydrogen still is a problem.
Nowadays, using hydrogen fuel cells, an almost emission free operation is possible. This is not valid, by the way, for the combustion of hydrogen in engines. As an 'exhaust gas' only water in vapour form is developed during the operation of the fuel cell with pure hydrogen, no carbon dioxide, and in fact, no noxious exhaust gasses at all. This is only possible, apart from direct solar power, if the vehicle is running on pure hydrogen or electric current. Under identical test conditions, only the fuel cell proves itself more efficient than good diesel engines. Having said that, a true efficiency comparison is difficult. It would have to incorporate, besides the exploitation of the fuels, also the production process. In the noise emission area the hydrogen car could also bring about progress.
With the fuel cell, electric energy is obtained directly from chemically fixed energy without further changes. There are several construction principles, but here only the PEM (Proton Exchange Membrane) fuel cell will be described. Hydrogen (H2) and oxygen (O2) would react with each other immediately when in direct contact and ignited, producing explosive gas.
Therefore, the hydrogen is separated from the oxygenated air by a wafer-thin synthetic membrane (PEM) which is permeable for the protons. This leads to a surplus of protons which is compensated by electricity flowing through a circuit. Higher voltage is necessary for driving motors. Therefore, like in a battery, about 200 elements of about 1 volt each must be closely packed into so-called stacks and switched one after the other. Because the developing energy is not completely used straight away, a battery is necessary for the intermediate storage. This can however, be smaller than for an electric drive motor.
All in all, the development of the fuel cell into a product ready for mass production, is running somewhat slower than expected. Although there are already prototypes which are in regular operation, as far as cost reduction is concerned the crucial breakthrough is still to come. All the technical problems have, by far, not yet been solved. The protection of the sensitive (and expensive to be replaced) synthetic membrane against freezing water, and the protection of the whole system against excessively high temperatures, are a part of the problem. There are the special material requirements, e.g., to the diaphragm, like conductivity, elasticity, shock resistance and a certain swell-behaviour which still stand in the way of a cost-saving serial production.
For the long-term planning, two serious disadvantages are noted here. The operation with pure hydrogen requires a new filling station network at considerable cost. They are being developed currently, with the production of fuel cells. For both of these reasons one estimates that a wide spread availability will be, at the earliest, realized in 2025. Methanol operation would perhaps be earlier realizable, however, with a lower effect and CO2 emission. The combustion of hydrogen in the piston-engine is considerably simpler but e.g., the exhaust gasses are not NOx-free, and pose the same supply problems. Finally, the question must be asked, whether the production of hydrogen is always emission free. 06/07