Promises of plug-in hybrids
The true CO2 savings from plug-in hybrid electric vehicles will depend on the fuel economy of the vehicles they replace.
As the automotive industry readies itself to churn out new test models of plug-in hybrid electric vehicles (PHEVs), scientists are trying to figure out just how environmentally friendly these automobiles are. Now, a new study in ES&T (DOI: 10.1021/es062314d) corroborates previous findings that PHEVs can significantly reduce CO2 emissions compared with conventional vehicles. However, the results also show that if PHEVs were to fully penetrate today's fleet, the ultimate energy savings and reductions in emissions would depend on the fuel economy of the vehicles they replace.
Much like the hybrid vehicles on the road today, PHEVs are battery-powered automobiles with a backup gasoline engine. Once the battery's charge is exhausted, the vehicles switch to gasoline for power, allowing the battery to be recharged. However, unlike the hybrids of today, PHEVs rely primarily on electricity to charge their batteries to full capacity.
Because electricity demand is much lower at night, many power plants (about 40%) in the U.S. operate at reduced nighttime loads. According to the new ES&T study, the U.S. currently has sufficient spare nighttime electricity capacity to charge a large fleet of PHEVs.
"Given any sensible realistic penetration of [PHEVs] into the marketplace, assuming they are accepted by consumers, the electricity infrastructure can handle it with ease," says study author John Sullivan, now a scientist at the University of Michigan Transportation Research Institute. PHEVs need about 10 hours of charging every night. If all the spare nighttime electricity were used to charge PHEVs, it would consume 13% of all the fuel used nationally for electricity generation. This could allow PHEVs to replace up to 34% of today's light-duty vehicle fleet.
Michael Kintner-Meyer says that the results reported in the ES&T study are "pretty similar" to those of his group at the Pacific Northwest National Laboratory. Kintner-Meyer found that 43% of the current fleet could be replaced by PHEVs with existing power-generation capacity in the U.S. when charging during the night.
"Having plug-in hybrids charge at night will make our product less expensive," says Hawk Asgeirsson of Detroit, Mich.-based DTE Energy. This is because it would enable some of the most efficient power plants to operate at full capacity (instead of being used only at peak hours), thus driving down maintenance costs.
PHEVs are not commercially available today, primarily because of high battery costs. However, the big automakers, including Ford Motor Co., are busy making test PHEV models. Yet even if the issue of cost is resolved, questions remain about the impacts of these vehicles compared with conventional gasoline and hybrid vehicles. Some of these questions center around energy consumption and emissions of CO2 and other pollutants (plug-ins shift the source of emissions from the vehicles themselves to whatever fuel is used to generate the electricity to charge them).
In the current ES&T study, Sullivan and coauthor Craig Stephan, both at Ford at the time of the study, find that the energy consumed by PHEVs is indeed lower than (by roughly one-fourth) that used by conventional hybrids. They also find that when compared with today's hybrids, PHEVs reduce CO2 emissions by 25% in the short term. Over time, as more plug-ins are introduced into the fleet and utilities develop cleaner and more efficient power plants, these vehicles would slice emissions by 50%. A 2007 report published collaboratively by the Electric Power Research Institute (EPRI) and the Natural Resources Defense Council (NRDC) showed that PHEVs could yield significant reductions in CO2 emissions at various levels of penetration into the existing fleet of vehicles.
What happens if more PHEVs replace the existing fleet? That is where the situation gets trickier, the new study finds. Increasing penetration of the existing fleet by PHEVs would increase the demand for electricity. However, electricity generation also releases CO2 into the atmosphere, and whether the U.S. can achieve greater reductions in emissions by replacing the existing fleet with PHEVs or by replacing existing inefficient coal-fired power plants with cleaner ones will depend on the fuel economy of the cars that the plug-ins replace.
"If you're replacing poor fuel-economy conventional vehicles, then you're better off supplying electricity for plug-in hybrids. If, on the other hand, people are going to be buying high-fuel-economy conventional hybrids instead, then it would tip the other way," says Stephan, Stephan, currently at the Argonne National Laboratory In that case, closing down and replacing the dirtier coal-fired power plants would be a better bargain.
