Should We Wait To Electrify Ground Transportation?

Introduction

Fig. 1: Share of U.S. electricity grid by fuel source. [1]

The transportation sector accounts for approximately a quarter of all U.S. CO₂ emissions annually, and personal transport with light-duty vehicles accounts for almost 60% of all transportation emissions. [1] When including distillate fuel oil, which is commonly used in heavy transportation machinery, and other oils commonly used in personal transportation, ground transportation accounts for around 88% of all transportation emissions. [1] Given that ground transportation is a substantial contributor to annual U.S. CO₂ emissions, electrification is necessary to create a net zero society. Since electrification is necessary: how quickly must electrification be achieved? Also, because the U.S. electricity sector is still dominated by fossil fuels, would electrification actually decrease emissions?

This study estimates the CO₂ emissions for two scenarios, the first is assuming every car on the road is electric (EV), and the second is assuming that every car on the road is internal combustion (ICV). It then compares those values to each other and to the current emissions from ground transportation to determine what would happen to carbon emissions if, assuming all the infrastructure was in place, all U.S. ground transport vehicles became electric.

Analysis

To start the analysis, we need to know how many kg of CO₂ are released per kWh of electricity production given the current grid. As can be seen in Fig. 1, the electricity grid is mostly powered by fossil fuels, however, the low-carbon segment of the grid is nontrivial. This analysis assumes that any electricity produced by renewables or nuclear is carbon neutral, which means that almost 40% of electricity produced in the U.S. is essentially carbon-free. [1] Given this breakdown, we can estimate the kg of CO₂ emitted per kWh by multiplying the kg of CO₂ released per kWh of each fuel source with their share and then summing them up. The kg of CO₂ emitted per kWh of each fuel source can be seen in Table 1.

This results in an average emissions per kWh produced of 0.378 kg/kWh. Using the fact that the average EV travels 3 miles per kWh and the total estimated number of miles that will be driven in 2022 is 3.268 trillion, we can estimate the carbon emissions from an electric ground transportation fleet in one year. [2,3]

Now using the same analysis method, in which we determine emissions per energy unit and multiply that by total energy units needed, we can find the amount of emissions assuming all cars on the road were gas. This analysis assumes that all miles driven by cars on the road were driven by the average light-duty vehicle. These are like the common cars and trucks driven during personal commuting. This underestimates the emissions by gas-powered cars because it assumes that heavy-duty vehicle miles (like those driven by semi-trucks) were instead driven by light-duty vehicles with much better fuel efficiency. According to the EIA, the most common fuel used by light-duty vehicles is E10 which emits 8.596 kg of CO₂ per gallon burned, and according to the EPA, the average fuel efficiency of a 2019 light-duty vehicle was 24.9 mpg. [4,5]

Using 24.9 mpg as the average mpg in the analysis introduced another element of underestimation. This is because not all cars on the road are as new as 2019, and there has generally been an upward trend in efficiency. The results of this analysis are graphed in Fig. 2 where the black bar represents the total amount of emissions from ground transport in 2021 based on data from the EIA, the red bar is the estimated emissions value given this analysis for only ICVs on the road, the blue bar is the estimated emissions value given this analysis for only EVs on the road, and the purple bar is the difference between only ICVs and only EVs given this analysis.

Conclusion

Fig. 2: Estimated annual CO2 emissions compared to real emissions. (Source: A. Tidd)

As can be seen in Fig. 2, the estimated emissions assuming only gas-powered vehicles is an underestimate of the EIA's emissions value. In addition to the mpg estimate creating an underestimation, this analysis also assumes all ICVs are light-duty vehicles, causing more underestimation. The emissions value for only EVs is also an underestimate. This is because the average efficiency of EVs in miles per kWh was given based on light-duty EVs. Furthermore, this analysis ignores electricity transmission and distribution losses because they tend to stay under 10% and average out to be only 6-7% of production. [6] Although this analysis underestimates the emissions by both EVs and ICVs, it is unlikely that the amount of underestimation of each differs significantly given the values were determined under the same assumptions and with the same methodology.

Given this data, it is probable that fully electrifying ground transportation will have a relative emissions decrease of about 64%. Ground transportation accounts for 88% of transportation emissions and transportation emissions account for about 27% of total U.S. emissions, therefore, an overnight switch to EVs could decrease total U.S. emissions by about 15% based on this analysis. [1] This analysis shows that the switch to EVs needs to happen as fast as possible. Because the U.S. grid derives almost 40% of its energy through low-carbon sources, electrifying transportation will have a sizeable impact on yearly U.S. emissions. Therefore, there needs to be much more investment in electrifying transportation because it is not the case that we need to wait until the grid is cleaner to start the transition. The transition must start now.

© Andrew Tidd. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] "Monthly Energy Review - November 2022," U.S. Energy Information Administration, DOE/EIA-0035(2022/11), November 2022.

[2] "Consumer Guide to Electric Vehicles," Electric Power and Research Institute, June 2021.

[3] "Traffic Volume Trends," U.S. Federal Highway Administration, March 2022.

[4] "Frequently Asked Questions: How Much Carbon Dioxide Is Produced By Burning Gasoline and Diesel Euel?," U.S. Energy Information Administration, May 2014.

[5] "2020 EPA Automotive Trends Report," U.S. Environmental Protection Agency, EPA-420-R-21-003, January 2021.

[6] "Energy Efficiency," ABB, February 2007.