CMU county-level study shows plug-ins have larger or smaller lifecycle GHG than gasoline ICE depending on regional factors
A US-wide county-level study comparing lifecycle greenhouse gas (GHG) emissions from several light-duty passenger gasoline and plug-in electric vehicles (PEVs) has found that PEVs can have larger or smaller carbon footprints than gasoline vehicles depending on regional factors and the specific vehicle models being compared.
The team from Carnegie Mellon University led by Dr. Jeremy Michalek accounted for regional differences in emissions due to marginal grid mix; ambient temperature; patterns of vehicle miles traveled (VMT); and driving conditions (city versus highway). Their open-access paper is published in the journal Environmental Research Letters.
Past studies have shown that life cycle plug-in electric vehicle (PEV) emissions depend heavily on the assumed electricity grid mix, driving patterns (including drive cycle and distance) and climate (including ambient temperature). These factors vary regionally, so PEV emissions implications also vary regionally. Several studies have assessed regional differences in PEV emissions incorporating subsets of these factors—with most focused on regional grid mix, but no study has accounted for the combined influence of consequential grid emissions, driving patterns, and temperature heterogeneity in assessing regionally-specific life cycle implications of PEVs in the US.—Yuksel et al.
The CMU team compared life cycle CO2 emissions of five existing vehicle models:
- Nissan LEAF BEV
- Chevy Volt PHEV (EREV)
- Toyota Prius PHV PHEV (blended)
- Toyota Prius HEV
- Mazda 3 (with i-ELOOP)
They selected the vehicles based on availability of Argonne National Laboratory vehicle test efficiency data at high, low, and moderate test chamber temperatures.
They assigned driving conditions to each US county based on urbanization level; assigned vehicle miles traveled (VMT) patterns to counties based on data from the National Household Travel Survey (NHTS) for the corresponding state; and assigned marginal grid emission factors for each North American Electric Reliability Corporation (NERC) region to the counties that lie in that region.
They then estimated the energy consumption rate for each vehicle based on Argonne’s Downloadable Dynamometer Database (D3) temperature-controlled chamber vehicle test data together with information on temperature, drive cycle, and VMT patterns for each county. They used energy consumption and VMT patterns to compute timing and duration of vehicle charging.
They then derived estimated life cycle CO2 emissions for each vehicle type and location by adding vehicle and battery manufacturing emissions, gasoline combustion and upstream emissions (based on computed gasoline consumption), and electricity production and upstream emissions (based on computed electricity consumption, timing, and location).
|Framework for the analysis. Yuksel et al. Click to enlarge.|
Among their findings:
The Nissan LEAF produces lower life cycle GHG emissions than the Prius HEV in urban counties of Texas, Florida, and much of the southwestern US. In most of the rest of the country, the LEAF increases GHG emissions relative to the Prius HEV, with those increases being most notable in the Midwest and in the South.
The researchers said this is due to the combined effect of grid carbon intensity, highway driving, and regional temperature. The Northern Midwest has a combination of a coal-heavy electricity grid, rural counties (with an assumed highway driving cycle), and cold weather that all contribute to higher relative emissions for the BEV.
The Chevrolet Volt PHEV has higher life cycle emissions than the Prius HEV in all counties. This is because the Volt consumes more gasoline per mile in charge-sustaining mode (after the battery is depleted) than the Prius HEV, and it consumes more electricity per mile than the LEAF in charge-depleting (CD) mode (when the battery is charged) at high temperatures.
In cold weather the Volt consumes both gasoline and electricity in CD mode.
The PHEV Prius produces lower life cycle GHG emissions than the HEV Prius in Texas, Florida, and the southwestern US as well as in most urban areas, but it produces higher emissions in many rural areas across the country—especially in the Northern Midwest. This is because the PHEV Prius consumes less gasoline than the HEV Prius in city driving conditions and more gasoline than the HEV Prius in highway driving conditions.
Differences between the HEV Prius and the PHEV Prius are generally less pronounced than those comparing the HEV Prius to the Volt or the LEAF, the team found.
Relative to the Mazda 3 conventional vehicle, (1) the LEAF reduces GHG emissions in urban counties across the US as well as suburban and rural counties in Texas, Florida, the Western US, and New England while increasing GHG emissions in the rural Midwest; (2) the Volt reduces GHG emissions in urban counties across the US while increasing GHG emissions in rural counties of the Midwest and the South; and (3) the Prius PHEV reduces emissions in all counties. In all three cases the GHG emission reductions in urban counties can be substantial.
Assessing their findings for life cycle CO2 emissions for each vehicle in various selected counties, the researchers concluded broadly that:
The effects of regional climate and grid mix on emissions become more important for vehicles with higher degrees of electrification. All vehicles have higher emissions in Minnesota, a colder state, compared to California. However, the increase in emissions is largest for the Leaf BEV, whereas only a slight increase is observed with Mazda 3 CV.
(Both batteries and engines are less efficient when cold, but gasoline vehicles are able to use waste heat from the engine to heat the cabin, while BEVs and EREV PHEVs need to draw energy from the battery to heat the cabin. PEVs thus tend to have larger energy penalties in cold weather regions than conventional gasoline vehicles.)
The effect of driving cycle on emissions becomes more prominent for vehicles with lower degrees of electrification. In counties with similar climate conditions and grid mix, they observed that the biggest change in emissions with highway driving compared to city driving occurs with Mazda 3.
|Radar chart showing Nissan Leaf life cycle emissions in gCO2eq mi−1 from different cases in selected counties. Yuksel et al. Click to enlarge.|
Our results suggest that the GHG-reduction benefits of PEVs have significant regional variability due to grid mix, temperature, and driving conditions as well as differences among vehicle alternatives within each technology class. This suggests that a regionally-targeted vehicle-specific strategy to encourage adoption primarily in areas where specific PEVs provide the largest benefits could increase the GHG reductions achievable under a given budget.
… Broadly, regional policies that are more aligned with the GHG benefits we estimate could be more efficient at achieving GHG reductions, though other factors such as regional consumer preferences, political climate, and other externalities also affect regional policy choices. In general, policies that target GHG reductions directly, such as carbon tax or cap-and-trade policies, rather than favoring specific technologies, are likely to be more efficient at achieving GHG reductions, though support for the development and deployment of new technologies can also have dynamic benefits and potentially lead to large long-term benefits if they enable a fleet transition that would not have happened otherwise.—Yuksel et al.
Tugce Yuksel, Mili-Ann M Tamayao, Chris Hendrickson, Inês M L Azevedo and Jeremy J Michalek (2016) “Effect of regional grid mix, driving patterns and climate on the comparative carbon footprint of gasoline and plug-in electric vehicles in the United States” Environmental Research Letters 11 doi: 10.1088/1748-9326/11/4/044007