Fuel cell vehicles have the potential to significantly reduce greenhouse gas emissions (depending on how hydrogen is produced) as well as U.S. dependence on imported oil over the long term. However, fuel cell vehicle technologies have technical challenges that are severe enough to convince the committee that it is unlikely such vehicles will be deployed in significant numbers within the time horizon of this study.

A recent report states that under the following set of very optimistic assumptions, 2 million fuel cell vehicles could be part of the U.S. fleet in 2020:
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Hybrid vehicles achieve reduced fuel consumption by incorporating in the drive train, in addition to an internal combustion (IC) engine, both an energy storage device and a means of converting the stored energy into mechanical motion. Some hybrids are also able to convert mechanical motion into stored energy. In its most general sense, the storage device can be a battery, flywheel, compressible fluid, elastomer, or ultra capacitor.

The means of converting energy between storage and mechanical motion is through the use of one or more motors/ generators (e.g., electric, pneumatic, hydraulic). In motor mode, these devices convert stored energy into mechanical
motion to propel the vehicle, and in generator mode, these devices convert vehicle motion into stored energy by providing part of the vehicle braking function (regeneration).
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