Currently, nearly all the hydrogen produced in the world is, like electricity, derived from fossil fuels. In the case of hydrogen, steam reforming of natural gas is the most common production method. In this reaction, natural gas and water are mixed at moderate temperatures and pressures in the presence of catalysts. Both molecules dissociate under these conditions to form a mixture of hydrogen, carbon monoxide, carbon dioxide, and other gases. This mixture of "synthesis" gases is fed into a second reaction chamber where a "shift reaction" with additional steam takes place. The result is a nearly complete conversion of the gases into hydrogen and carbon dioxide. Steam reforming energy efficiencies above 70% are common. Partial oxidation and pyrolysis are other, less typical, methods of chemically reformulating hydrocarbon fuels and water into hydrogen and carbon dioxide. As with steam reforming, natural gas is normally the energy feedstock in these reactions, although coal and oil can be used as well.

Like electricity produced from fossil fuels, hydrogen produced from natural gas or other fossil fuels is unsustainable because supplies are limited. Also, the processes, though clean compared to oil refining or electricity generation from coal, are not pollution free. The reactions produce carbon dioxide and trace concentrations of carbon monoxide, hydrocarbons and nitrogen oxide.

The most commonly used method of hydrogen production, after steam reforming of natural gas, is electrolysis of water. In this process, an electrical current passes through electrodes immersed in an electrolytic solution divided into two chambers by a semi-permeable membrane. The electrical energy splits the water molecule into hydrogen and oxygen gases, which are released and collected at the electrodes. Energy efficiencies in electrolysis generally exceed 80%; the process produces no air pollution.

Electricity produced from any source is indistinguishable in its ability to provide the energy to produce hydrogen through electrolysis. Therefore, any renewable resource that can be used to generate electricity is capable of producing hydrogen through electrolysis. In California today, there are two hydrogen vehicle demonstration facilities, one at the University of California at Riverside and one at a Xerox Corporation facility in El Segundo. Both employ photovoltaic cells to generate electricity to power electrolysis units that manufacture hydrogen. Two other photovoltaic fields are used to generate hydrogen in Germany. A handful of other plants exist elsewhere. A project underway in Palm Desert, California will use windpower to provide the electricity to manufacture hydrogen for use in a fleet of small personal utility vehicles.

There are various other methods to produce hydrogen directly from renewable resources without the intervening step of electrical generation. Although none of these technologies is in commercial application, bench-scale or demonstration-scale plants show them to be capable of producing hydrogen sustainably and with little or no environmental impact. Research is underway at multiple sites worldwide to develop and commercialize the following direct hydrogen production technologies:

• Biomass or municipal waste gasification: One direct method of producing hydrogen involves the gasification of municipal waste or biomass, such as fast growing grasses or trees, to extract hydrogen. Biomass or municipal waste gasifiers are similar to the systems that have been developed to gasify coal as part of integrated gasification, combined cycle power generators. The technology can be completely renewable resource-based if biomass or renewable waste is used to provide the heat needed to power the gasification process and the feedstock that is gasified. Because biomass gasification relies on technology that has been developed for other applications, it is the most advanced of the direct approaches to hydrogen production. Although more costly compared to the cost of conventional steam reforming of natural gas, biomass gasification is the least expensive and one of the most promising methods of direct hydrogen production from renewable resources.

• Direct photoelectrochemical conversion: In photochemical systems, sunlight strikes an electrolytic solution in which photosensitive semiconductors and catalysts are suspended. The solar energy is absorbed by the catalysts, creating localized electrical fields that trigger the electrolytic splitting of water into hydrogen and oxygen.

• Direct photobiological conversion: Various bacteria and algae have been isolated that can produce hydrogen either from organic materials through digestion or directly from water and sunlight through photosynthesis. Both methods rely entirely on renewable resources, such as sunlight, biomass, and biological organisms, to produce hydrogen.

• Thermal decomposition: Concentrated solar thermal energy can achieve temperatures exceeding 5,000 degrees Fahrenheit. Under these conditions, water thermally decomposes into hydrogen and oxygen. Separation of the gases before they spontaneously recombine is one problem facing this technology.

Pipeline is the easiest form of hydrogen transmission, although the gas can be compressed or liquefied and transported in tanks as well. Unlike centrally generated electricity, hydrogen can be easily, quickly, and compactly stored and retrieved when needed. Like natural gas, hydrogen can be compressed and held in pressurized storage tanks. Advanced high pressure tanks capable of containing pressures up to 10,000 pounds per square inch have been produced and used in space applications, although most hydrogen is now stored at pressures around the standard of 3,600 pounds per square inch now common in natural gas and other compressed industrial gas storage systems. Alternatively, hydrogen can be liquefied, although the liquefaction temperature - more than 400 degrees Fahrenheit below zero and less than 40 degrees above absolute zero - makes this process difficult. Another set of technologies for storing hydrogen involves reversible adsorption onto the surface of activated carbons. Hydrogen also can be held within the internal structure of metal alloys, called hydrides.

Hydrogen can be stored until needed more easily than electricity. For example, a compressed hydrogen storage system can provide as much energy as a battery meeting the Advanced Battery Consortium's mid-term commercialization goals, while weighing one-tenth as much and consuming one-third the space.¹² Moreover, when hydrogen storage tanks are depleted, they can be replenished in a small fraction of the time that it takes to recharge batteries.

Thus, a sustainable transportation system can be envisioned that uses renewable resources to produce hydrogen. In this scenario, hydrogen is distributed through pipelines and stored on board electric vehicles. When energy is needed, the hydrogen is fed into an on-board fuel cell generating electricity to propel the vehicle. This electric vehicle energy system can operate independent of centrally generated electricity and batteries, relying totally on the fuel cell for electricity. Alternatively, the two systems can be intertwined in "hybrid" electric vehicle configurations that include fuel cells and batteries. Batteries can be recharged from the grid while the vehicle is parked or they can rely on the on-board fuel cells for the electricity they store. In the latter case, batteries can be used to provide energy during fuel cell warm-up upon starting, assist the fuel cell in meeting peak acceleration energy demands, and capture energy through regenerative braking.¹³

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