Globally, direct combustion of fuels for transportation and heating accounts for more than half of greenhouse gas emissions, a significant fraction of air pollutant emissions, and about two-thirds of primary energy use. Even with continuing incremental progress in energy technologies, most energy forecasts project that primary energy use and emissions of greenhouse gases and air pollutants from the use of fuels will grow over the next century because of increasing demand, especially in developing countries. Energy-supply security is a serious concern, particularly for transportation fuels.

A variety of alternative fuels have been proposed that could help address future environmental and energy-supply challenges. These fuels include reformulated gasoline or diesel, methanol, ethanol, synthetic liquids such as dimethyl ether made from natural gas or coal, compressed natural gas, and hydrogen.

Of these, hydrogen offers the greatest potential environmental and energy-supply benefits. Like electricity, hydrogen is a versatile energy carrier that can be...

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