from electric vehicles depend on ambient conditions and when and where they are
charged. Ambient conditions like temperature and drive cycle influence vehicle
efficiency, while the carbon intensity of the power plants used to charge the
vehicle differs by location and time of charging. In this work, we-
Critical Emissions Factors (CEFs), defined as the electricity emissions
intensity that needs to be achieved during charging to ensure that electric
vehicles achieve lifecycle greenhouse gas emissions parity with some of the
most efficient gasoline hybrids in the United States.
Identify regions that have achieved the required CEFs for vehicle pairs and
regions that will require further emissions reductions to do so.
Update previous results of comparative lifecycle emissions between
electric vehicles and gasoline hybrids as presented in Yuksel
et al. (cite) using new data.
1 shows the dependence of electric vehicle efficiency with temperature (20 F, 72 F, and 95 F)
and drive cycle (UDDS and HWET drive cycles, which approximate urban and
highway driving). Temperature affects both the battery performance, in the case
of EVs, as well as increases the overall energy required for heating, ventilation,
and air conditioning during high and low temperatures for vehicles. Gasoline hybrids
are more sensitive to drive cycles than electric vehicles.
When conducting this study, the latest data available was from the American
Automotive Association for 2018 and 2017 models of Nissan Leaf, Chevrolet Bolt,
and Tesla Model S 75D, a luxury electric vehicle. In recent months, Argonne
National Lab’s D3 laboratory has released more vehicle test data. The models
used in our study have higher energy consumption in highway drive cycles but similar
values for urban drive cycles compared to other models. To reflect the new data,
we add the 2020 Tesla Model 3 results to exhaustively characterize electric
Electric vehicle emissions also depend on which power plants are being
used to charge the vehicle at a given time and location. In this work, we
benchmark our results using marginal emissions factors for 2018. Marginal
emissions factors are the emissions intensity of the last generator(s) kicked
in to serve the incremental load from vehicle charging. Recent research shows that
marginal electricity emissions factors have remained persistently high despite steady,
expected reductions in average electricity emissions. We assume that vehicles
are charged after the last trip of the day based on data from the National
Household Transport Survey, which collects a day’s worth of driving
characteristics from 275,000 people in the United States. To test these
assumptions, the paper's supplementary information shows results using average
emissions factors and vehicle charging during the lowest emissions hours.
Critical Emissions Factors: Break-even emissions intensity of the grid
for EV-hybrid carbon parity
achieve emissions parity with efficient gasoline hybrids, battery electric
vehicles (with US-produced NMC batteries) require power grid emissions between
than 421 to 1101gCO2/kWh, depending on the vehicle pair and
location. For context, emissions intensity of natural gas generated power varies
between 395-461 gCO2/kWh, while that from coal varies between
911-1079 gCO2/kWh. Some regions – shown here in green and yellow –
would require lower carbon electricity to achieve carbon parity between the
pair of vehicles, while others – in black and grey – would do so even with high
carbon electricity. The paper also presents results with varying battery
locations and chemistry. While battery emissions are usually a small portion of
the overall life-cycle emissions of electric vehicles, battery chemistry,
manufacturing locations, and lifetime assumptions can impact the level of
decarbonization needed in the electricity grid as the supply chain of electric
vehicles becomes more diversified. Lithium Iron Phosphate (LFP) and manufacturing
in locations with lower emissions intensity help reduce EV emissions.
Where do the marginal emissions from the grid need to be reduced further?
In Fig. xxx, we compare CEFs to current regional marginal emissions to identify which regions already have reached the required levels of local grid intensity to achieve carbon parity between battery electric and gasoline vehicles and which have yet to. We show the difference between CEFs and current marginal emissions factors for NERC regions weighted by the convenience charging profile (to reflect the charging time). All and or almost all parts of the U.S. have achieved the required CEF for Tesla Model 3, Leaf, and Bolt to break even with gasoline hybrids. All regions of the US, except parts of the Northeast and Florida, would need to reduce marginal emission factors up to 342 gCO2/kWh for the Tesla Model S to have lower lifecycle emissions than these gasoline hybrids.
Comparative life-cycle emissions estimates
We update Yuksel et al.’s comparative life-cycle
emissions estimate for vehicle pairs using marginal emissions factors and convenience
charging profiles. We provide sensitivity to these assumptions in supplementary
We find that Bolt and Leaf electric vehicles have lower emissions than
the Prius and Accord hybrids in almost all counties of the West, Texas,
Florida, and New England. In comparison, they have higher emissions in rural
counties of the Midwest and the South. Tesla Model 3 has lower or comparable emissions
across the entire country. In contrast, the Tesla Model S, a luxury EV, has
higher emissions than the Prius and Accord.
Comparative lifecycle emissions between EVs and hybrids depend on the two
vehicles chosen for comparison, the emissions intensity of the grid, charging
time, and ambient factors like temperature and drive cycle. We introduce Critical
Emissions Factors as the break-even emissions intensity needed for electric
vehicles to achieve carbon parity with gasoline hybrids. CEFs are agnostic to assumptions
baked in emissions accounting of the electricity grid – they present what
should be upper-bound of the electricity emissions intensity.
We find Efficient EVs such as Tesla Model 3 have lower emissions than gasoline
hybrids across the country, but parts of the Midwest and Southwest still need
reductions for certain vehicle pairs to achieve carbon parity. A big motivation
for this project is to peel through different assumptions buried in emissions
accounting to find the best strategies for climate mitigation with EVs.
Decarbonizing the grid further in certain locations (Midwest and parts of
Southwest), charging during low emissions hours, and considering the energy
requirements of battery chemistries and their production location can further
reduce emissions from EVs.