Peter Johnstone of Wyndham asks :-

Why is it necessary to compress hydrogen gas to run a car of the future?

John Campbell, a physicist at the University of Canterbury, responded.

In liquid form molecules touch each other, so that is the highest density of chemical fuel we can attain. When a liquid is vapourised into a gas at atmospheric pressure it occupies 700 times more volume, ie the gas is mostly empty space with the molecules about ten diameters apart. Thus the gaseous form is not an efficient way to store chemical energy. Compressed to high pressures is better but liquid is best.

During the second world war in Britain, when petrol wasn't available for civilians, some cars had a burner on the back to convert solid fuel, such as wood or coal, into burnable vapours (gas). This was stored in a very large gas bag on top of the car. They couldn't go far or fast.

To burn the hydrogen efficiently it must be supplied with half as many oxygen molecules. (Air is only one fifth oxygen.) So think how an engine works. The cylinder is filled with hydrogen and air in the right proportions and then exploded so that the hydrogen and oxygen recombine back into water. A car piston goes up and down several hundred times per minute. So you can work out the rate at which hydrogen is used up. It would take a very big plant to generate hydrogen at this rate. Note that for the same reason, cars driven by natural gas have that stored as liquid or compressed gas.

The next generation cars may be run on fuel cells in which hydrogen is chemically combined with the oxygen in air to directly produce electric energy which will run an electric car. For these the hydrogen will be produced at hydro power plants at times of little demand (eg nights and summer) when there is stored water in excess of what is needed at the time (eg when water is being spilled over the dam.) The hydrogen will be stored in compressed or liquid form, or possibly dissolved into special metals which store it at nearly the same density as a liquid.