Hydrogen is a well known alternative energy source that has more than three times the energy density by mass than currently used hydrocarbon fuels, such as gasoline. However, hydrogen has the disadvantage of being difficult to store and transport. Using current technology, hydrogen storage has a low energy storage density by volume relative to hydrocarbon fuels. Therefore, with all other factors beings equal, in order to store the same amount of energy, hydrogen storage requires a much larger and heavier storage tank than hydrocarbon fuel storage.
Gravimetric capacity is a measure of the amount of hydrogen that can be stored per unit mass of the storage system. Volumetric capacity is a measure of the amount hydrogen that can be stored per unit volume of the storage system. The United States Department of Energy (DOE) has set targets for hydrogen storage for 2010 and 2015. By 2010, the DOE target is to store hydrogen at a gravimetric capacity of about 6 wt % and a volumetric capacity of about 60 kg/m3. By 2015, the DOE target is to store hydrogen at a gravimetric capacity of about 9 wt % and a volumetric capacity of about 80 kg/m3.
Compression techniques have been used to increase gas pressure and improve the energy storage density by volume for hydrogen. This allows for the storage tanks to be smaller. However, the compressing of hydrogen requires a significant amount of energy, often accounting for as much as 30% of the stored energy. Furthermore, large pressure vessels are required for such compression techniques.
Another technique for storing hydrogen involves converting hydrogen gas to liquid hydrogen. This technique requires cryogenic storage because hydrogen has a very low boiling point of −252.882° C. or −423.188° F. The liquification of hydrogen requires a large amount of energy to maintain these extremely low temperatures. Furthermore, the storage tank for liquid hydrogen requires complex and expensive insulation in order to prevent the liquid hydrogen from boiling off. In addition, liquid hydrogen has a lower energy density by volume than hydrocarbon fuels, such as gasoline, by a factor of about 4.
A further technique of hydrogen storage involves reacting H2 with another compound. Metal hydrides, such as LiH and NaAlH4, are commonly used in this technique. However, hydrides have the disadvantage of being generally combustible upon exposure to moist air, and are toxic to humans.
When hydrogen is reacted with the metal hydride, significant amounts of heat are given off. When this heat is given off, a step of cooling must be carried out to prevent a significant rise in temperature in the system, and this cooling step constitutes an energy loss to the system.
An automobile or truck can have a storage tank containing metal hydrides to be reacted with hydrogen. However, filling this storage tank with hydrogen is a slow process, often requiring more than 3 minutes. Also, storing hydrogen using metal hydrides requires a larger and heavier storage tank relative to the storage tanks used for storing hydrocarbon fuels.
In order to release the hydrogen from the metal hydride, heating of the metal hydrides to temperatures as high as 250° C. is required. This heating step results in a significant loss of energy.