Electric vehicles charged in coal-heavy regions can create more human health and environmental damages from life cycle air emissions than gasoline vehicles, according to a new consequential life cycle analysis by researchers from Carnegie Mellon University. However, the anticipated—albeit now possibly delayed, per the recent Supreme Court decision—retirement of coal-fired power plants will make electric vehicles more competitive on an air emissions basis, the researchers found.
Among the findings of the study, published as an open-access paper in the journal Environmental Research Letters, was that battery electric vehicles with large battery capacity can produce two to three times as much air emissions damage as gasoline hybrid electric vehicles, depending on charge timing.
(Lifecycle emissions for greenhouse gases, PM, NOx and SO2 are higher under controlled charging scenarios than in uncontrolled charging due to the generation mix; the researchers found that charging late at night reduces power generation costs by a quarter to a third—largely by shifting to cheaper coal-fired power plants. But the extra emissions released as a result can cause 50% higher costs to human health and the environment.)
In the study’s future 2018 grid scenarios that account for predicted coal plant retirements, PEVs would produce air emissions damages comparable to or slightly lower than HEVs.
The researchers also found a significant difference between coal and natural gas generation. Even in one of the power systems in the country with the highest coal generation, PEVs could reduce transportation health and environmental damages in the near future, long before a zero-carbon electricity mix is achieved, due primarily to substitution of natural gas for coal on the margin.
|Life cycle emissions by pollutant and life cycle stage for each vehicle type in the recent (a) and future (b) PJM grid. UC stands for uncontrolled charging and CC stands for controlled charging. Source: Weis et al. Click to enlarge.|
Carnegie Mellon Associate Professor of Engineering and Public Policy Paulina Jaramillo; Professor of Engineering and Public Policy and Mechanical Engineering Jeremy Michalek; and former Engineering and Public Policy PhD student Allison Weis studied the electricity grid in the PJM region, which includes Washington DC, Philadelphia, Pittsburgh, and Chicago.
The team modeled the power plants in the PJM region and looked at how plant operation would change in response to electric vehicle charging load, said Jaramillo. They then modeled the emissions from those power plants, the effects of emissions on air pollution in downwind counties, and the resulting implications for human health and the environment.
The study modeled a conventional gasoline vehicle, conventional hybrid, two plug-in hybrids (one modeled on the 10-mile AER Prius PHV and one on the 35-mile AER Chevy Volt) and a battery-electric vehicles (modeled on the Tesla Model S).
The most recent year for which all of the necessary data are available to make this assessment is 2010, noted Michalek.
The largest source of damage stems from sulfur dioxide emissions from coal-fired power plants, which produce airborne particles that people breathe, according to the study.
The study, which was funded by the Doris Duke Charitable Foundation, the Richard King Mellon Foundation, the Electric Power Research Institute, the Heinz Endowment, the National Science Foundation, and Toyota Motor Corporation, also examined a hypothetical case with increased wind power.
However, the air emissions damages resulting from electric vehicle charging hinge primarily on the amount of coal in the system, not the amount of wind or solar power, said Jaramillo.
When EV charging load is added to a power system, wind and solar plant output can’t be turned up to respond because they are typically already fully utilized. Fossil fuel plants are the ones dispatched in response to new charging load. That’s why the shift away from coal is so important for EVs.Jeremy Michalek
While PEVs can double or triple air emission damages in the recent grid relative to HEVs, they could reduce damages in a future grid. However, we estimate that near future (~2018) potential air emissions benefits from PEV adoption in PJM are small relative to HEVs (or even negative when considering the net effect on the automaker’s fleet under federal fuel economy policy). Nevertheless, electrification may offer a promising long term option to significantly reduce air emissions from the transportation sector compared to some other alternative transportation fuels, including biofuels and natural gas, that have been shown to offer small-to-no reductions in GHG emissions and could have unintended consequences like higher global food prices. Indeed, the logistics of regulating emissions from individual vehicles over their functional lives are more difficult than regulation of power plant emissions.
Continued regulation of the electricity system can increase the benefits of vehicle electrification, and consequential air emissions implications of PEV charging are already lower in many regions than in PJM. While near-term benefits of PEV adoption in PJM are estimated to be small or negative, a transition of the transportation system could lead to long-term benefits outside the scope of this analysis, including greater benefits in other regions and future emissions savings enabled by a transition to electric vehicles as the electricity grid becomes cleaner and as public policy adjusts.—Weis et al.
Allison Weis, Paulina Jaramillo and Jeremy Michalek (2016) “Consequential life cycle air emissions externalities for plug-in electric vehicles in the PJM interconnection” Environmental Research Letters, Volume 11, Number 2