Gaseous storage (compressed hydrogen gas)

Using a compressed gas storage system is probably the most straightforward option at this time. The National Renewable Energy Laboratory (NREL) found that compressed hydrogen gas offers the simplest and least expensive method for onboard storage of hydrogen. Compressed hydrogen gas storage uses technology similar to that used for compressed natural gas, with stainless steel, aluminum or composite cylinders. The refilling time of compressed hydrogen tanks is also similar to that of gasoline tanks.

Hydrogen, however, requires more volume for the same energy equivalent amount of natural gas. One way to increase the fuel stored in the container is to increase pressure, but this requires more expensive storage containers, increasing compression costs and entails investigation into safety issues. Lower pressures, while lessening these concerns, would mean taking up more vehicle space. In addition, hydrogen has a tendency to leak because of its small size. Seals and valves on the containers need to be designed to prevent leaks. If a fuel cell vehicle is stored in a closed garage, hydrogen that has leaked out could accumulate and increase the risk of fire or explosion.

Offboard the vehicle, hydrogen can also be stored in high-pressure gas tanks or even in pipelines. Depleted oil and gas fields and mined salt caverns can serve as underground storage areas for gaseous hydrogen.

Currently, most fuel cell buses that use direct hydrogen for fueling use compressed hydrogen storage.

Liquid storage

Liquefied hydrogen (LH2) does not have the high weight penalty seen with compressed hydrogen, but it is still bulkier than gasoline storage. As with compressed hydrogen, liquid hydrogen storage takes advantage of similar technologies used in liquid natural gas storage. A drawback to this method of hydrogen storage is that the process to liquefy hydrogen is energy intensive.

Hydrogen's low boiling point requires excellent insulation of storage containers; otherwise, left for a period of time, the storage tanks could become depleted. Maintaining the extreme cold temperatures of LH2 during refueling and onboard storage currently poses a significant technical challenge, with 25 percent of LH2 boiled off during refueling and 1 percent lost per day for onboard storage.

Liquid hydrogen is actually the most common method currently used for offboard hydrogen storage because of the high energy density of liquid hydrogen. For example, this was the method used to store hydrogen at the Chicago Transit refueling station for their fuel cell bus demonstration. The LH2 was then pressurized into compressed hydrogen for storage on the bus' roof.

Solid storage (hydrides)

Solid storage of hydrogen is possible with metal hydrides. Basically, hydrogen bonds easily with more than 80 metallic compounds, forming a weak attraction that stores hydrogen until heated. These so-called metal hydride systems can either be low temperature (< 150¡C) or high (300¡C). Since heat is required to release the hydrogen, this method avoids safety concerns surrounding leakage that can be a problem with compressed hydrogen and LH2. In fact, metal hydrides are one of the safest methods for storing hydrogen.

One major obstacle to this method is that the metal compounds used to attract the hydrogen tend to be very heavy resulting in only 1.0 to 1.5 percent hydrogen by weight. In addition, some of the metals used for hydrides are very expensive. There are less expensive options but they are impractical for use in fuel cell vehicles as these cheaper metals require extremely high temperatures to release the hydrogen.

Energy Conversion Devices (ECD) is one technology company working in the field of metal hydride hydrogen storage. ECD's Ovonic proprietary magnesium alloy is a lightweight metal hydride, with a 7.0 percent by weight hydrogen content, which avoids some of the weight penalty encountered with other metal hydrides. Millennium Cell's sodium borohydride is another hydride potential storage option that is currently being used by DaimlerChrysler in their prototype fuel cell vehicle, the Natrium, at the California Fuel Cell Partnership.

Advanced storage

There are other advanced methods of onboard hydrogen storage being researched, but most are in very early research stages. For example, research is being conducted with carbon nanotubes -- microscopic carbon tubes synthesized in the laboratory -- that absorb hydrogen. Initial research reports that these carbon-based methods have extremely high hydrogen storage densities, higher than solid hydrogen. Considering the low costs of carbon materials, this could also be a very cost-effective technique. However, there currently are no commercial applications of this type of technology.

Another advanced method uses glass microspheres. The basic principle behind the glass microsphere method of hydrogen storage is that tiny beads of glass, microscopic in size, are heated up. Heating the spheres allows hydrogen to permeate the glass. Upon cooling, the hydrogen becomes trapped inside. The hydrogen is released when the spheres are reheated. This technology could store as much hydrogen as hydrides but with less weight and also could eliminate the need for high-pressure or low-temperature storage vessels. This technology is still in a very early stage of development.

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