1. delayed charging (i.e., starting at midnight) leads to higher emissions in most cases due largely to increased coal in the marginal generation mix at night;

2. the Chevrolet Volt has higher expected life cycle emissions than the Toyota Prius hybrid electric vehicle (the most efficient US gasoline vehicle) across the US in nearly all scenarios (a breakout of the findings re: the Volt is available in the Supplemental Information for the paper);

3. the Nissan LEAF BEV has lower life cycle emissions than the Prius in the western US and in Texas, but the Prius has lower emissions in the northern Midwest regardless of assumed charging scheme and marginal emissions estimation method; and

4. in other regions the lowest emitting vehicle depends on charge timing and emission factor estimation assumptions.

Probability that the Nissan LEAF is lower CO2 emitting than the Toyota Prius Hybrid by region and charging scheme. Green indicates that the Nissan Leaf is lower emitting than the gasoline vehicle (Toyota Prius Hybrid or sales-weighted ICEV), while red means that the opposite holds. Credit: ACS, Tamayao et al. Click to enlarge.

Most studies indicate that the key factor when comparing PEVs and gasoline vehicles is the magnitude of emissions associated with electricity production. However, many of these studies rely on a single electricity production emission factor estimate or conduct sensitivity analyses on grid emission factors over a range of power plant types. A more detailed assessment is needed to estimate regionally specific emissions from those power plants that respond to PEV charging. … In particular, variation in grid emission factors and regional boundaries are key drivers of differences in the estimates of regional PEV benefits.

… to properly assess the CO₂ emissions implications of adding new PEV charging demand in a particular region, one should estimate and use marginal consumption emission factors. … In this work, we assess regional variation in electric and conventional vehicle CO₂ emissions under a range of assumptions for regional boundaries of analysis, electricity emission factors, and charging patterns.

—Tamayao et al.

For the study, the researchers used the Nissan LEAF and Chevrolet Volt as representative of battery-electric and plug-in hybrid electric vehicles, respectively. They compared these vehicles with the Toyota Prius HEV. They also assumed lifetime vehicle miles traveled ranging from 100k to 150k miles with a best estimate of 125k miles used for the base case analysis.

Given substantial regional differences in PEV GHG emissions implications, differential regional policy may be warranted, but current differences in state subsidies do not align particularly well with regions where PEVs provide the largest GHG emissions benefits. For example, the state with the largest state subsidies ($7500) for BEVs is West Virginia, which is under the RFC region, where the Nissan LEAF and the Chevy Volt are likely higher emitting than the Toyota Prius. Under the Clean Power Plan Proposal, West Virginia is expected to bring down its carbon rate to about 730 kg/MWh, but that level is not yet low enough for PEVs to be lower emitting than the Prius, and the effect of average emissions reductions on marginal emissions has not yet been characterized. The second highest state subsidies ($6000) are in Colorado, part of the WECC region where the LEAF is likely lower emitting than the Prius and the Volt may be higher or lower. The third highest state subsidies ($5000) are in Georgia, where the comparison of the Leaf and Prius is inconclusive and the Volt is higher emitting. Of course, GHG benefits must be balanced against other goals, including reduction of air pollution and oil dependency as well as economic factors.

—Tamayao et al.

The researchers offer policymakers a number of recommendations based on the study’s findings:

1. Be wary of regional claims about electric vehicle air emissions implications based only on regional electricity generation mix, since the emissions associated with new PEV charging in a region can differ substantially from the average generation mix in that region.

2. Consider federal and regional strategies for promoting electric vehicle adoption most strongly in the regions where they can do the most good. (Earlier post.) When considering GHG reductions alone, this would mean the western US and Texas (where there is high confidence that GHG emissions of the Nissan LEAF are lower than the best gasoline vehicles) and in Florida and New England (where the LEAF also likely has lower GHG emissions). However, other factors beyond the scope of this analysis, such as air quality implications, should be considered as well.

3. Continue to reduce the emissions intensity of the electricity grid. When electricity generation is sufficiently clean, electric vehicles have lower GHG emissions than the most efficient gasoline vehicles.

4. Avoid treating PEVs as though they are all the same. While the Nissan LEAF has lower GHG emissions than the gasoline Toyota Prius in several regions, the Chevy Volt has higher GHG emissions than the Toyota Prius across much of the US. Policies that target outcomes (e.g., GHG emissions reduction) rather than specific technologies are generally preferred.

5. Incentivizing nighttime charging should be avoided: while night charging can be preferred by grid operators and can lower costs, in most regions nighttime charging increases GHG emissions, and nighttime charging can also increase health costs in some regions due primarily to increased air pollution from coal-fired power plants.

Resources

• Mili-Ann M. Tamayao, Jeremy J. Michalek, Chris Hendrickson, and Inês M. L. Azevedo (2015) “Regional Variability and Uncertainty of Electric Vehicle Life Cycle CO2 Emissions across the United States” Environmental Science & Technology doi: 10.1021/acs.est.5b00815