This is the final hurdle for the hydrogen economy to negotiate before it reaches the final straight. Fuel cells depend on a steady supply of hydrogen, which means that storage backup is an essential component of the system where reformation is not involved.
The conventional storage method for hydrogen is pressurized tanks. The pressure varies according to volume: up to 50 litres requires 200-250 bar; larger amounts, 500-600 bar. Some very large containers can be as low as 16 bar.
It can be stored as a liquid but this necessitates cooling to -253°C and requires a heavily insulated tank.
Bonded hydrogen is another option. Granular metal hydrides store hydrogen by bonding it chemically to the surface of the material. The metal particles are charged by being heated and then receiving the hydrogen at high pressure. Some metals can absorb up to one thousand times their volume of hydrogen. On cooling, the hydrogen is locked into the metal and released by heating. The heat may come from a high-temperature fuel cell. For storage in buildings the most appropriate metal hydride is probably iron-titanium. It is too heavy for vehicle application but ideal for buildings having a relatively low operating temperature.
Recently, interest has been aroused by a storage technology which has emerged from Japan and Hong Kong. It consists of nanotubes of carbon, that is, sheets of carbon rolled into minute tubes 0.4 nm (0.4 billionths of a metre) in diameter. This is just the size to accommodate hydrogen atoms. A pack of carbon nanotubes has the potential to store up to 70% of hydrogen by weight compared to 2-4% of metal hydrides. Research is progressing into nanofibre graphite which could be a winner provided the team at Northeastern University in Boston can surmount its predilection for water over hydrogen.
A more exotic possibility was reported in Scientific American in May 2000. It referred to the capacity of solid molecular hydrogen to turn to metal at a pressure of 400-620 gigapascals (4-6 million atmospheres). Solid metallic hydrogen can store huge amounts of energy which is released as it returns to the gas phase.
If, as seems likely, the global warming curve is steeper than the official projections, then the pressure to switch to a hydrogen economy will become irresistible. Fortune will favour those countries who have developed a strong manufacturing base for a range of renew-ables which will be in heavy demand, not least in the developing countries. The problem of the 2 billion of the world's population who do not have access to electricity can only be solved by renewables and a distributed supply system.
The country which is on course to become the first hydrogen economy because it has a head start with its immense resources of geothermal and hydropower is Iceland. It plans to convert all cars, trucks, buses and boats to hydrogen over the next 30 years. It will also export hydrogen to Europe. Even more ambitious is the island of Vanuata in the Pacific Ocean. It is en route to achieve a 100% hydrogen economy by 2010 due to its abundance of renewable resources - geothermal, wind, solar and hydropower. Not to be outdone, Hawaii, which is rich in solar and geothermal resources (and, no doubt, wave), recently established a public-private partnership to promote hydrogen as a major player in the island's economy, even exporting the gas to California.
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