We assume in this analysis that all technical and economic challenges are eventually met by each alternative vehicle.  Thus the cost and durability of fuel cells are proven; affordable hybrid batteries that can deliver both deep discharge energy for all-electric range and micro-cycling power for acceleration are developed; and successful cellulosic ethanol plants are built to convert corn stover, energy crops and other agricultural wastes to fuel grade ethanol.

 Given these technical achievements, we compare alternative transportation options with regard to the nation’s energy security, local air pollution, and greenhouse gas emissions over the century.

 One key finding: the composition of the nation’s electrical power grid will be a major factor in determining the carbon footprint for both plug-in hybrids and fuel cell vehicles. The DOE’s Energy Information Administration is projecting a net increase in coal-based power, rising from 51.9% of all electricity generated in 2007 to 60.8% by 2030.  Without a major and relatively rapid shift in the generation mix toward lower-carbon electricity from some combination of renewables, nuclear and coal with carbon sequestration, the hydrogen for fuel cells would have to be produced from natural gas in a carbon-constrained world, which would cut emissions by 45% to 50% compared to conventional cars. 

 Plug-in hybrids would fare even worse, with little or no greenhouse gas reductions compared to running the hybrid on gasoline all the time.  Thus plug-in hybrids using internal combustion engines would make a major near-term contribution to reducing imported oil, but would not significantly reduce greenhouse gas emissions.  Fuel cell vehicles running on hydrogen made from natural gas would take the next step, cutting greenhouse gases in half.  Further reductions will require a major make-over in the nation’s electrical grid to low-carbon alternatives, when electrolyzing water would cut greenhouse gases more than making hydrogen from natural gas.

 Similarly, biofuels will contribute to reduced oil imports in the short term.  Ethanol and particularly cellulosic ethanol will also reduce greenhouse gas emissions, but will do relatively little to reduce local air pollution.  In addition, biofuels according to most experts could only supply 25% to 30% of our nation’s growing consumption of transportation fuels over the next few decades.

 We conclude that hydrogen will be required over the long-term to supplement and greatly expand the contributions of plug-in hybrids and biofuels to reducing environmental and energy security threats to our nation.