Bill Butterworth 23 September 15
Having thought about diesel engines (to help VW out) I began to think about fuel cell driven vehicles and I consulted my brother, a physicist and the following discussion is the result. Some arithmetic, ref hydrogen versus petrol, produces results that might interest the reader, even if they have not yet bought a hydrogen-driven car. If we assume that the relative conversion efficiency of a petrol internal combustion engine versus a fuel cell and electric motor are similar, then the following figures may not be too misleading.
The “Gasoline Gallon Equivalent” of hydrogen at one atmosphere is 357 cubic feet. That is, 1 US Gallon of petrol will potentially release the same amount of energy as 357 cubic feet of Hydrogen gas at 1 atmosphere pressure. There are approximately 7 US gallons to the cubic foot. The density of petrol (gasoline) about 0.75. The density of hydrogen at 1 atmosphere is 0.09 gm/litre. Hydrogen is frequently handled in bottles at 350 bar. At 350 bar, that make 0.9 kilo per cubic foot So 0.9 kilos of hydrogen can be stuffed into 1 cubic foot and have roughly the same energy content as 1 US gallon of petrol. In other words even at 350 bar the equivalent energy storage takes about eight times as much space.
Now add the heavy ironmongery (would aluminium bottles be strong enough in an accident?) needed to keep the gas contained. You can get to higher pressure but that means stronger bottles. Now add energy needed to get the gas into bottles at 350 bars. It won’t be insignificant.
This is all a bit “back of envelope” but I don’t think it is very far out.
So even if you have a very lightweight chassis, the bottles capable of providing a range in the 100’s of miles are going to slow things up a bit. Maybe the answer is to generate the hydrogen very locally and store it in small bottles fitted with a very quick change device. So you go from one hydrogen point to the next. Frankly, it ain’t going to work. QED
Clearly, some much better ideas than that are needed! One might be that methanol, ethanol, biodiesel and biogas are all relatively easily prepared locally from locally available plant material, are much easier to handle and can be used in certain types of fuel cell.