One of the main problems with the use of hydrogen as a transport fuel and energy carrier is how to store it as it is a light gas (Coontz and Hanson, 2004; Zhou, 2005). Compression and subsequent storage at high pressure in cylinders is a common method of storing gases. However, the density of hydrogen is low (0.089 kg/m3) com­pared with methane (0.717 kg/m3) and therefore a pressure some four times higher than normal is required (345-690 bar; 10,000 psi) to contain sufficient hydrogen.

Even at these pressures it still requires the fitting of a tank eight times the size of the equivalent petrol tank, and tanks of this size are not available commercially. Therefore, compressed gas is not likely to be used at present.

Another common method of storing gas is as a liquid. Hydrogen has a boiling point of -253°C and the critical temperature for liquid hydrogen is -240°C. There­fore, the liquid hydrogen will need to be stored in well-insulated tanks in an open system to avoid pressure build-up and thus some gas will be lost on storage. Whether compression or liquefaction is used both require the input of energy, between 214 and 354 MJ/m3 for compression and 15.2 kWh/kg for liquefaction which is some 30% of the energy contained in the fuel (Midilli et al., 2005; de Wit and Faaij, 2007).

Despite these problems the USA has invested US$1.7 billion and the €2.8 billion in hydrogen power vehicles. Ford, Mazda and BMW have produced hydrogen-powered internal combustion engines and Honda, Ford, Toyota, General Motors, Daimler — Chrysler, Renault-Nissan and Volkswagen have produced fuel cell vehicles. Liquid hydrogen-filling stations have been installed in Munich and London. In 2006 there were 159 hydrogen vehicles in the USA not including electric hybrid vehicles (Fig. 5.10).

Because of the need to store hydrogen inexpensively, alternatives to compression and liquefaction have been investigated including complex hydrides, metallic hydrides, physisorption and nanostructures (Zhou, 2005).

Complex hydrides form between hydrogen and group I, II and III elements such as lithium, magnesium, boron and aluminium. The complex hydrides have a high hydrogen density (150 kg/m3) and can release hydrogen at moderate temperatures. This method is still under development as conditions for hydrogen release changes the particle and the effects of repeated adsorption/desorption need to tested.

Some metals absorb hydrogen, forming hydrides. These are usually a rare-earth metal such as lanthium combined with a transition metal such as nickel. Hydrogen density has reached 115 kg/m3 for the metal hydride LaNi5H6. In simple hydrides the metal can absorb and release the hydrogen at room temperature but their hydrogen density is low. The formation of hydrides is an exothermic reaction and the more stable the hydride the more heat is required to release the hydrogen.

Gases can be adsorbed on to a number of adsorbents in a variety of ways pro­vided the gas is below its critical point. The gas can be adsorbed as a single layer on the surface which depends on the surface area of the material and the temperature. Adsorption decreases with a rise in temperature.

Hydrogen can be stored in carbon nanotubes but the capacity appears much lower than was first estimated. As adsorption is dependent on surface area, it is dif­ficult for nanotubes to compete with super-activated carbon.