Study Finds EVs With Smaller Batteries Are Greener

By Scott Doggett October 11, 2011

Toyota Prius.jpg

Electric vehicles (EVs) fitted with small battery packs are more environmentally friendly than EVs fitted with large battery packs, according to a recent study conducted by researchers at Pittsburgh-based Carnegie Mellon University. Plug-in vehicles with small battery packs and hybrid-electric vehicles (HEVs) that don't plug in can reduce life cycle impacts from air emissions and enhance oil security at low or no additional cost over a lifetime, researchers found. But plug-in vehicles with large battery packs are more costly and may have higher or lower emissions than HEVs, depending on where and when they are plugged in.

"Current government policy provides larger subsidies for vehicles with larger battery packs, assuming that larger is better," said Jeremy J. Michalek, an associate professor of engineering and public policy and mechanical engineering at CMU. "While larger battery packs allow plug-in vehicles to drive longer distances on electric power instead of gasoline, they are also expensive and heavy, they are underutilized when the battery capacity is larger than needed for a typical trip, they require more charging infrastructure and they produce more emissions during manufacturing." Vehicles with smaller battery packs such as the 2012 Toyota Pruis Plug-in HEV qualify for a $2,500 federal tax credit, while vehicles with larger battery packs such as the 2012 Fisker Karma and the 2012 Nissan Leaf – both battery-electric vehicles (BEVs) – are eligible for a $7,500 federal tax credit.
 
In the study, Michalek and co-authors concluded that electrified vehicles with smaller battery packs are more efficient in reducing societal costs for health care, environmental damages and oil consumption. The American Recovery and Reinvestment Act of 2009 provides up to $7,500 in tax credits for up to 200,000 plug-in vehicles with larger battery packs. "Because vehicles with larger battery packs are more expensive, fewer of them can be subsidized, and that can result in lower total benefits," said Michalek, who recently received a $400,000 grant from the National Science Foundation to analyze how public policy could help determine the types of vehicles built in coming years and how consumers might respond to these vehicles.
 
"It's possible that in the future plug-in vehicles with large battery packs might offer the largest benefits at competitive costs if the right factors fall into place, including sufficiently low cost batteries, high gasoline prices, low emission electricity and long battery life," said study co-author Mikhail Chester, assistant professor of sustainable engineering at Arizona State University. "But such a future is not certain, and in the near term, HEVs and plug-in vehicles with small battery packs provide more emissions benefits and oil displacement benefits per dollar spent."
 
The research by Michalek, Chester and others was aimed at understanding tradeoffs in the capabilities of new technologies and to predict what near- and long-term strategies should be. "Given the major spending cuts under debate in Washington, it is important that we get the most benefits out of spending designed to improve the environment and energy security," Michalek said. "In the near term, HEVs and plug-in vehicles with small battery packs offer more cost-effective benefits. More research on batteries – especially lowering cost – and a transition to a cleaner electricity grid are needed to pursue a future where large battery packs may also be able to help address climate change, air pollution and oil dependency at competitive costs."
 
Emissions Considerations
The researchers estimated life-cycle emissions damages for comparable new midsize vehicles, including a conventional vehicle (CV), a hybrid-electric vehicle (HEV), plug-in hybrid-electric vehicles (PHEV) with battery packs sized for storing 20 miles worth of range on electricity only (PHEV20) or 60 miles worth of range on electricity only (PHEV60) with the remainder powered by gasoline. They also considered a battery electric vehicle (BEV) with a 240-mile pack (and no gasoline engine). They estimated location-specific externality damages for releases of carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (PM), sulfur dioxide (SO2), and volatile organic compounds (VOCs) using data from a 2010 National Research Council (NRC) study with their $6 million estimate for value of statistical life, and they examined a range of estimates for damages from greenhouse-gas emissions.
 
The researchers combined these externality values with data on U.S. driving patterns from the 2009 National Household Travel Survey and data on manufacturing, fuel cycle, and operation emissions from Argonne National Laboratory to estimate U.S. life-cycle damages for each vehicle. Fig. 1 summarizes the results. In their base case, the researchers assumed average U.S. values for emissions and damage valuation of electricity generation, oil refining, vehicle and battery production, driving location, and upstream supply chain emissions. They used a medium global valuation for greenhouse-gas emissions, and they assumed the battery would last the life of the vehicle. Although gasoline production and combustion produce significant emissions, battery and electricity production emissions are also substantial.
 
"We find that, in the base case, plug-in vehicles (PHEVs and BEVs) may produce more damage on average than today’s HEVs," the authors wrote in their study. "This fact is due in large part to sulfur-dioxide, or SO2, and greenhouse-gas emissions from coal-fired power plants." That said, they noted that power plant emissions associated with charging a plug-in vehicle could be higher or lower than today's U.S. average grid mix, depending on region, time of day and regulations, such as caps on sulfur-dioxide emissions or renewable portfolio standards, and anticipated plant retirement and new plant construction. Additionally, considerable variation in electricity emissions factors exist for each charging location depending on the geographic boundary chosen for analysis. To put bounds on these factors, researchers presented a hypothetical optimistic case, where zero-emission electricity is used to charge the vehicle, and a pessimistic case, where coal-fired power plants are used to charge the vehicle.

Michalek-chart-1.jpgIn the pessimistic case, the study concluded, the BEV could be responsible for more than a $5,000 increase in lifetime damages over the HEV (all costs are in year 2010 U.S. dollars). In the optimistic case, the BEV could reduce lifetime air emissions damages by about $100. Although the costs of damages from vehicle-associated emissions are significant, the damage reductions that can be gained through electrification are small compared to the total cost of owning and operating a vehicle, the researchers reported.
 
Lifetime Ownership Costs And Damages
In another chart, the researchers summarized the lifetime private cost paid to own and operate each vehicle type, plus the cost of the oil premium and damages caused by lifetime emissions charged to the owner at the time of purchase, assuming no change in driving patterns. In the base case, they used U.S. average grid emissions, average gasoline prices in the period 2008–2010, a long battery life, scheduled maintenance costs estimated by Oak Ridge National Laboratories, charging infrastructure cost estimates based on installation data, and Argonne National Laboratory estimates of vehicle costs in the year 2015. "Although the lifetime costs of conventional, HEV, and PHEV20 are comparable, it is clear that the high costs of vehicles with larger battery packs are not balanced by fuel cost savings or emissions damage and oil premium reduction," the report said.

Michalek-chart-2.jpgThe researchers also compared an optimistic scenario, using the highest historical weekly U.S. gasoline price, U.S. Department of Energy targets for vehicle costs in 2030 (which Argonne National Laboratory called "very optimistic"), long battery life, and zero-emission charging. In the optimistic scenario, plug-in vehicles with large battery packs could offer lower damage at lower lifetime cost. But competitiveness in this case was driven by direct costs, not externality damages, and market forces would presumably drive adoption in such a case. Conversely, in a pessimistic scenario using low gasoline prices, shorter battery life, coal-powered charging, and Argonne National Laboratory 2015 cost estimates, plug-in vehicles could produce more damages at substantially higher cost, according to the report.
 
The researchers also examined a range of other factors that affect costs and damages, such as urban versus rural driving, oil supply source, refinery location, emissions valuation and oil premium estimates. "Although large battery packs offer the largest emissions and oil consumption reductions at lowest cost in the most optimistic scenarios, they result in high costs and increased damages if not all of the right factors fall into place, including high gasoline prices and achievement of low battery costs, long battery life, and low electricity production emissions," according to the report. "In contrast, HEVs and PHEVs with small packs are robust, providing emissions reductions and oil displacement benefits at low cost with less infrastructure investment and lower uncertainty."

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