battery recharge requires about 15 kWh, which is 3 kW for 5 hours, and at 110 volts would require close to 30 amps for this time period.

There are many longer-term system issues with PHEVs or electric vehicles. These include the plausible fraction of total vehicles that would satisfy the recharging location, time of day, and charging-power-level constraints; and how the consumer vehicle purchase and use patterns would be affected by the range and recharging limitations of vehicles having different operating characteristics. It is too early in the development of electric-vehicle technology to be able to project how large a fraction of the vehicle market might eventually be met by such vehicles. Note that the impact on greenhouse gas emissions of using electricity as an energy source in transportation will depend on how much electricity is produced, distributed, and used for that purpose and how that energy is generated (i.e., what the primary energy source is, and—if it is fossil fuels—whether carbon capture and storage technology is effectively deployed).

3.5.6
Hydrogen

Hydrogen fuel presents an especially challenging set of issues, since there is currently no hydrogen distribution system. A recent NRC study, Transition to Alternative Transportation Technologies: A Focus on Hydrogen (NRC, 2008c), examines what would be needed to implement such a transition and the timescales involved. It concludes that reductions in petroleum use and greenhouse gas emissions could grow steadily over the 2020–2050 timeframe but that substantial government actions and assistance would be needed for this to happen. Establishing a hydrogen production, distribution, and refueling system that provides a sufficiently widespread availability of the fuel so as not to impede the growth in fuelcell vehicle deployment is a challenging (but doable) task.

3.6
SYSTEM-LEVEL ISSUES

The history of transportation is one of continuous innovation. Most innovations are small and incremental. Some innovations accumulate and lead to a restructuring and reorganization of activities. Energy costs and supply often play a role in motivating innovation—for instance, the transition from sailing ships to steamships—but usually system innovations and changes are motivated by other factors. Major changes in transportation systems are costly and are complicated to



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