Which way ahead for hydrogen cars?
Box 2 | Alternative hydrogen storage systems
Hydrogen has many advantages as a transportation fuel but there is one major drawback. At normal temperatures hydrogen is a gas, which makes it difficult to store. Current solutions to the storage problem involve either strongly compressing the hydrogen or liquifying it. Some interesting alternative hydrogen storage systems are being researched, but none has yet emerged as an obvious answer to the problem.
In one approach, hydrogen combines with a pure or alloyed metal to produce a metal hydride. Heating the hydride releases the hydrogen. Studies have shown that this system can store hydrogen at higher densities than simple compression. The challenge remains, though, to identify a metal that will store and release sufficient hydrogen at temperatures suited to a practical system.
Very small glass spheres are the basis of another approach. During filling, hydrogen passes through the glass at high temperatures. Cooling stops this movement, trapping the gas at high pressures inside the ‘microspheres’.
Another technology uses carbon nanotubes – tiny fibres of graphite. Research has shown that these can absorb very large amounts of hydrogen, storing much more fuel per litre than is possible with either liquefied or compressed hydrogen. As with the other approaches though, substantial problems need to be solved before a practical storage system emerges.
Posted May 2001.