For the biofuel plug-in hybrid electric vehicle scenario, we assume that PHEVs will use biofuels such as cellulosic ethanol (modeled here) instead of gasoline.  We also assume an upper bound to the biomass potential in the US. Various studies have estimated that agricultural and forest land in the US could produce biofuels such as cellulosic ethanol.  These estimates have ranged between 60 billion gallons per year (compared to current corn ethanol production capacity of 8 billion gallons per year) up to 120 billion gallons per year. We have used this upper estimate in this model to provide the best possible outcome for biofuels.

It turns out (coincidentally) that this 120 billion gallon/year biofuel production level produces approximately the same limit on biofuel PHEVs as the 75% charging outlet constraint. In the model, those PHEVs that cannot be supplied with biofuels are run on gasoline up to the 75% charging ceiling, but these gasoline PHEVs are barely visible on the chart above by the end of the century)

Other biofuels such as biodiesel, bio-butanol or bio-methanol among others are also possible in the future.  We are assuming that these biofuels would have approximately the same conversion efficiency from biomass, so the upper bound of 120 billion gallons/year of cellulosic ethanol would still apply, limited by land and biomass growing availability

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