insufficient to address the need to develop alternative liquid transportation fuels, particularly because the development and demonstration of technology, the construction of plants, and the implementation of infrastructure require 10–20 years per cycle. In addition, investments in alternative fuels have to be protected against crude oil price fluctuations.

Because geologic CO2 storage is key to producing liquid fuels from coal with life-cycle greenhouse gas emissions comparable to those of gasoline, commercial demonstrations of coal-to-liquid and coal-and-biomass-to-liquid fuel technologies integrated with geologic CO2 storage would have to proceed immediately if the goal is to deploy commercial plants by 2020. Moreover, detailed scenarios for market-penetration rates of biofuels and coal-to-liquid fuels would have to be developed to clarify the hurdles preventing full feedstock utilization and to establish the enduring policies required to overcome them. Further, current government and industry programs would have to be evaluated to determine whether emerging biomass- and coal-conversion technologies could further reduce U.S. oil consumption and CO2 emissions over the next decade.

Other Transportation Fuels

Technologies for producing transportation fuels from natural gas—such as gas-to-liquid diesel, dimethyl ether, and methanol—have been deployed or will be ready for deployment by 2020. But only if large supplies of natural gas were available at acceptable costs—for example, from natural gas hydrates—would the United States be likely to use natural gas as the feedstock for transportation fuel production.

Hydrogen has considerable potential, as discussed in Transitions to Alternative Transportation Technologies—A Focus on Hydrogen (NRC, 2008) and The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs (NRC, 2004). Hydrogen fuel-cell vehicles could yield large and sustained reductions in U.S. oil consumption and greenhouse gas emissions, but it will take several decades to realize these potential long-term benefits.

RENEWABLE ENERGY

The level of electricity generation from renewable resources has risen significantly over the past 20 years. Nonhydroelectric renewable sources, however,



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