one-third of which is projected to be from petroleum use (EIA, 2003). The projections show that the entire transportation sector, not simply the light-duty vehicles, will account for 37 percent of these emissions. Thus, gasoline use in light-duty vehicles is an important component of the release of CO2 into the atmosphere, comprising roughly two-thirds of the carbon emissions from the transportation sector (EIA, 2002), but it is not the dominant component.
In Chapter 5, the committee presented estimates of the amount of CO2 that would be released into the atmosphere per kilogram of hydrogen produced for each of the technological pathways considered; it also gave estimates of the amount of CO2 that would be released into the atmosphere per gallon of gasoline used. These estimates can be applied to the committee’s estimates of gasoline consumption and hydrogen consumption over time in order to estimate the impacts of a transition to hydrogen on the carbon releases into the atmosphere. These estimates appear in Figures 6-7 and 6-9 for current hydrogen production technologies and in Figures 6-8 and 6-10 for possible future technologies.
Figures 6-7 through 6-10 show that a transition from conventional fueled vehicles to hybrids alone, without the introduction of hydrogen-fueled vehicles, would reduce carbon emissions by 200 million metric tons annually by 2050. A further transition from GHEVs to hydrogen vehicles would have sharply different impacts, depending on which technology was utilized. At one extreme, the use of coal without sequestration or of distributed electrolysis using grid-supplied electricity would lead to little or no further reductions in CO2 releases than would occur through a transition to GHEVs.
Distributed generation of hydrogen by electrolysis using photovoltaics or wind turbines when they were available, and using grid-supplied electricity when the wind turbines or photovoltaics were not supplying electricity, could further reduce CO2 emissions by a moderate amount (on the order of 100 million to 150 million metric tons per year by 2045). The reductions in CO2 emissions from the possible future technologies could be somewhat greater than those obtainable using the current technologies, but the differences between the two are not great. However, distributed electrolysis using electricity exclusively from wind turbines could bring CO2 emissions down to zero by 2050 if it were possible to generate all of the hydrogen by this means. The committee shows this particular technology for the possible future state of technology development and shows wind turbines combined with grid-supplied electricity for the current state of development.7
The committee shows the particular technologies in this way because for the current state of technology development it will be less costly to have the grid-based electricity used with wind-based electricity, and for the possible future technologies it would be less costly to have an entirely wind-based system without the use of electricity from the grid.