The hydrogen economy is a proposal for the distribution of energy by using hydrogen. Hydrogen (H2) gives off energy when it is combined with oxygen, but the hydrogen itself has to first be produced, which requires more energy than is released when it is used as a fuel.

Some futurists promote hydrogen as potential fuel for motive power (including cars, boats and airplanes), the energy needs of buildings and portable electronics. They believe a hydrogen economy could greatly reduce the emission of carbon dioxide and therefore play a major role in tackling global warming. Countries without oil, but with renewable energy resources, could use a combination of renewable energy and hydrogen instead of fuels derived from petroleum, which are becoming scarcer, to achieve energy independence.

Proponents of a world-scale hydrogen economy argue that hydrogen can be an environmentally cleaner source of energy to end-users, particularly in transportation applications, without release of pollutants (such as particulate matter) or greenhouse gases at the point of end use.

A hydrogen vehicle is a vehicle that uses hydrogen as its on-board fuel for motive power. The term may refer to a personal transportation vehicle, such as an automobile, or any other vehicle that uses hydrogen in a similar fashion, such as an aircraft. The power plants of such vehicles convert the chemical energy of hydrogen to mechanical energy (torque) in one of two methods: combustion, or electrochemical conversion in a fuel-cell:

• In hydrogen internal combustion engine vehicles, the hydrogen is combusted in engines in fundamentally the same method as traditional internal combustion engine vehicles.
• In fuel-cell conversion, the hydrogen is reacted with oxygen to produce water and electricity, the latter being used to power an electric traction motor.

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)

For More Information:
Hydrogen Economy
Hydrogen Fuel
Hydrogen Vehicle