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March-April 2008
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< previous | 1 | 2 | 3 | 4 The study concluded that the reduction in emissions associated with the HEV 60 option could be as much as 60 percent compared to the conventional gasoline-powered automobile, and 30 percent compared to the HEV 0 vehicle. There would also be significant savings in emissions of CO2 even if the electricity used to charge the batteries of the plug-in hybrid were generated using coal (the “dirtiest” fuel in terms of greenhouse gases). If the electricity taken from the grid to power the plug-in HEV were produced using non-fossil sources of energy (nuclear, solar, hydro, or wind, for example), the savings in emissions could be even greater. It is instructive to compare the cost of driving using gasoline in a conventional internal-combustion engine with the cost of using power taken from the commercial electricity grid. Approximately 80 percent of the energy content of gasoline consumed in a motor vehicle is wasted: it is converted to heat. Storing energy in a battery and then employing it to drive a motor vehicle is much more efficient: the loss to heat amounts to only about 10 percent. Because the energy content of a gallon of gas is equivalent to about 33.6 kilowatt hours (kWh) of electricity, we would need about 7.5 kWh of electricity to obtain the same driving performance as derived from a gallon of gasoline. Electricity delivered to retail customers in Cambridge last December cost 19.7 cents per kWh. At this rate it would take $1.47 to drive the distance covered by a gallon of gas using electricity, a little less than half the then-prevailing price of $3 a gallon for gas. Given that electricity prices in Massachusetts are higher than the national average, savings on a national scale would have been even greater. Further, since one would normally expect to recharge the batteries of the plug-in HEV at night, when demand for electricity is at a minimum, customers could reasonably argue for a reduction in price to go with this off-peak demand. The average length of a typical vehicle trip in the United States is about 10 miles, according to the 2001 National Household Travel Survey. Of such trips, 17 percent were for travel to and from work, 3.2 percent for work-related business, 46.2 percent for family/personal business, 8 percent for travel to and from school and church, 25 percent for recreation, with the balance, 0.6 percent, for unspecified purposes. To replace 90 percent of current gasoline consumption with power derived from electricity using plug-in hybrids with an electrical range of 60 miles (HEV 60) would require that less than 10 percent of travel by the average vehicle should involve distances of more than 60 miles following the last recharge of the vehicle’s batteries. Given that the distance traveled per day by the typical car or light truck in the United States amounts on average to a little more than 30 miles (11,000 miles per year), it seems reasonable to expect that the objective of restricting gasoline-assisted travel to less than 10 percent could be satisfied by a fleet of HEV 60s (which typically would be charged on a daily basis, most likely overnight). Replacing 90 percent of gasoline consumption by electricity would be equivalent to raising the fleet’s average fuel efficiency from the present level of about 17 miles per gallon to close to 150 miles per gallon. Were we to accomplish this objective, total oil use would be reduced by 36 percent, cutting the demand for imported oil by as much as 60 percent (a savings of $270 billion per year at current prices for oil). With such a large reduction in demand, one would expect the price of oil to drop and the bill for imported oil to decrease even more, to the point where its contribution to the U.S. international trade deficit would be of minimal concern. Replacing 90 percent of current U.S. gasoline consumption with electricity would require an increase of 23 percent in demand for electricity. If a significant fraction of this additional energy were supplied off-peak—at night, for example, when the supply of electricity is normally at a minimum—the increase in electric-generating capacity required to supply it would be relatively modest, potentially as little as 10 percent. If the proposed transition to plug-in hybrids were implemented, and if the electricity used to charge the batteries of these vehicles were derived from non-fossil sources, the implied reduction in U.S. emissions of CO2 could amount to as much as 16 percent (assuming a 36 percent reduction in overall consumption of oil). The Climate ConsequencesGiven the proposed increase in electricity consumption (and production) from converting to hybrid vehicles, it is important to examine the climate-change effects of this shift in power demand. Combustion of coal accounted for 49 percent of electricity generated in the United States in 2006, followed by natural gas (20 percent), nuclear (19 percent), hydro (7 percent), and oil (1.6 percent), with the balance from renewables other than hydro (wind, biomass, and solar). Coal was responsible for 82 percent of CO2 emissions from the electric-power sector, and natural gas produced 16 percent of emissions, with oil accounting for only 2 percent. If we are to seriously reduce emissions of CO2 from the power sector, our objective should clearly be to reduce the relative importance of coal, and to increase the contribution from low-carbon sources such as nuclear, wind, and potentially solar (opportunities for increasing the contribution from hydro in the United States are minimal). 1 | 2 | 3 | 4 | continued >  
 
 
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