The Challenge of Hydrogen Storage

Producing hydrogen is only the first step in the envisioned hydrogen economy (Fig. 9.7). The next is the storage of the generated hydrogen in a form that should be economical, practical, safe, and user-friendly. Because hydrogen is a very light gas, it contains much less energy per unit volume than conventional liquid fuels under the same pressure. Under normal conditions, hydrogen requires about 3000 times more space than gasoline for an equivalent amount of energy. Thus, hydrogen must be compressed, liquefied or absorbed on a solid material to be of any practical use for energy storage.

Figure 9.7 Different routes for the production of hydrogen.

Depending on the use for stationary or mobile applications, hydrogen will have very different storage characteristics. In stationary applications, including heating and air-conditioning of homes and buildings, electricity generation and varied industrial uses, hydrogen storage systems can occupy a relatively large space and their weight is not a major factor. In contrast, hydrogen storage in transportation such as in cars is limited by volume and weight which, to provide a driving range of some expected 500 km, must remain minimal. Hydrogen storage is therefore a key factor for the successful introduction of hydrogen as a transportation fuel, the presently considered major application for the hydrogen economy. The challenges faced by hydrogen storage in transportation are great and far from solved. With a driving range requirement of 500 km, a storage capacity of 5 to 10 kg of hydrogen will be needed even for a fuel cell-propelled vehicle. At the same time, refueling should take less than 5 minutes and should be as easy and safe as with hydrocarbon fuels today. The storage system should be also, of course, affordable.

Current technology involves the physical or chemical storage of hydrogen. Physical storage is effected in insulated or high-pressure containers in which hydrogen is stored as a liquefied or compressed gas. Chemical storage includes metals and other materials which absorb hydrogen. Each of these methods has its advantages, but also at the same time has serious drawbacks.

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