Are Electric Cars Actually Better For the Climate Than Gas Ones?

Introduction

Fig. 1: Total local electrical energy generation by type of power plant in indicated year. These fractions are used to calculate the kgCO2 per km of an electric vehicle in each region. [4,5] (Source: T. Dyson)

To mitigate climate change, we must reduce greenhouse gas (GHG) emission. [1] Transport accounted for 14% of GHG emissions worldwide in 2014, most of which is attributed to road transport. [1] Adopting electric vehicles will supposedly reduce this number, since they emit no GHGs directly. However the power for these vehicles comes from power plants, many of which emit GHGs. Electric cars effectively kick the problem upstairs, making gauging their true climate impact difficult.

To make their effect more transparent, a useful quantity to calculate is an electric car's CO₂ emission per kilometer. An electric car's emitted kilograms of CO₂ (kgCO₂) per kilometer can then be compared directly with the kgCO₂ per km of gas cars (whose direct emissions are straightforward to calculate). Unlike for gas cars, your electric car's kgCO₂ per km depends on where in the world you're charging your car, since different regions get their power from different sources.

The two regions analyzed here are the U.S. state of California and the Canadian province of Quebec. These regions were chosen because both governments are encouraging electric car purchases as part of their clean energy projects, but have contrasting electric power sources (see Fig. 1). [2-5] The two regions thus serve as pertinent case studies in determining the impact of electric power sources on the carbon footprint of electric cars.

Calculation

Here I go over my calculation for the CO₂ emission per km of an electric car. First I justify and write down a formula, then find real-world numbers to use in that formula, and provide sources for those numbers.

The Formula

We are interested in getting the kgCO₂ emitted per km travelled in an electric car. We start with the energy per distance for an electric car, which can be given in units of kilowatt-hours (kWh) per 100 km. This number is the electric 'fuel' efficiency of the car, analogous to liters per 100 km for gas cars, and can be found online (see the next subsection).

Next, we need to know how many kgCO₂ are emitted from the power plants for each kWh that goes into the car's battery. I'll ignore electrical energy loss from power transmission, since it's not significant. [6] I'll also neglect indirect sources of emission, such as supply chain and construction. [1] So, all renewables (hydro, wind, solar, and geothermal) as well as nuclear are considered zero-emission. More subtly, since biomass plants burn mostly waste that would decompose (thus emitting its CO₂) relatively soon anyway, its direct emission is also zero. For further reading and caveats (responsible forestry chief among them), see the IEA Bioenergy report. [7] That leaves coal and natural gas as the two CO₂ emitting types of power plant.

Neither California nor Quebec have any appreciable coal power generation, so we only need to consider natural gas. We can calculate the kgCO₂ per kWh for natural gas power plants (see next subsection). Then the overall kgCO₂ per kWh for a region is just the kgCO₂ per kWh for natural gas times the fraction of total electrical energy that's produced by natural gas there. This step is where regional differences are introduced, since California depends much more on natural gas than Quebec, which is almost entirely renewable (see Fig. 1).

All together, the equation is as follows:

Note that (something)^-1 means "per something."

Finding Real-World Numbers

This sub-section will present the numbers used for the electric car emission calculation, and provide sources. These numbers as well as their sources are summarized in Table 1.

The first term, kWh per 100 km, is reported publicly for all new electric cars. For this analysis, the 2018 Kia Soul EV was chosen as a representative electric car. Its kWh per 100 km is from EPRI. [8] In this analysis, the combined fuel efficiency is used, as a balance between city and highway driving.

The fractions of total electric energy generated for each type of power plant in both California and Quebec can be seen in Fig. 1. The data for California were obtained from the EIA, and those for Quebec from the Canada Energy Regulator. [4,5] The fraction of electrical energy supplied by natural gas for each region can be seen in Table 1. This analysis takes into account local power generation only, as the plant type breakdown of imported electric power is opaque. California imports a significant fraction of its electricity, so this approximation is not negligible.

The most difficult number to find is kgCO₂ emitted per kWh. Here I calculate that quantity for natural gas for use in the main calculation Eq. (1). I also calculate that quantity for coal plants for comparison in the results section.

The end goal of these calculations is the kg of CO₂ produced per kWh of electrical energy generated, for coal and natural gas plants. To find this, we find the number of atoms of carbon converted into CO₂ per energy released by the reaction (in number of atoms per electron volt (eV)). We then convert that number into moles per kWh, then multiply by the molar mass of CO₂ to find kgCO₂ per kWh. Finally, since power plants are not 100% efficient at converting the reaction's heat into electricity, we divide by the efficiency factor for that kind of power plant (effectively multiplying the energy, on the denominator, by that factor). This procedure is represented by

I'll use this equation to calculate kgCO₂ emitted per kWh for natural gas and coal below.

Natural Gas

Natural gas consists of a chain of carbon with two hydrogens bonded to each carbon atom. Combusting one link of this chain thus involves breaking two H-C bonds and one C-C bond (only one, because the other one is considered by the next link - to break one loop off a chain, you only need to cut one loop, not both!). The combustion also involves 3/2 of an O₂ molecule, and forms one H₂O and one CO₂. Summing all the bond energies gives 6.22 eV per CO₂ emitted (or 1/6.22 C atoms per eV). Plugging that into Eq. (2), with an efficiency of 0.4 (the average efficiency for California natural gas generators, see Fig. 4 in Nyberg), gives a result of 0.65 kgCO₂ kWh^-1. [9] This is the number given in Table 1. Note since we only looked at the part that actually burns, i.e. there is no reference to kg of natural gas, there is no need to worry about the purity of the fuel here (or in the coal calculation below).

Coal

The calculations for coal are a bit less straightforward, since coal isn't a simple hydrocarbon chain. I assume the part of coal that burns is purely carbon. This is not entirely true, since coal contains a mix of things including hydrocarbon chains of various lengths, but for this analysis only the pure carbon is considered. I also assume the carbon is arranged in 2-dimensional sheets. With this assumption, the carbon atoms form a repeating hexagonal pattern, with bonds forming the edges and carbon atoms at the points. Since carbon atoms form four bonds, two of the three bonds each atom makes are single C-C bonds, and one is a double C=C bond.

Now let's examine what happens when we remove one carbon atom from the hexagonal pattern. Looking at the carbon we just removed, it's now free of its C=C bond and both its C-C bonds. Looking at its neighbors, one of them is free of its C=C bond, and two of them are free of one of their C-C bonds. So in total, two carbons are free of their C=C bonds, one is free of two C-C's, and two are free of one C-C each. We just broke two carbon's worth of bonds (future reactions won't have to spend the energy to break them). Thus the smallest unit of this reaction we can look at actually frees two carbon atoms energy-wise, so involves two O₂ and produces two CO₂. Counting up the energy spent breaking one C=C, two C-C, and two O=O, subtracting that from the energy released from forming four C=O bonds, gives 8.93 eV per two carbon atoms, or 2/8.93 C atoms per eV. Plugging that number into Eq. (2) with an efficiency of 0.35 (that of a typical U.S. coal plant), gives 1.11 kgCO₂ kWh^-1. [10]

Both kgCO₂ emitted per kWh values calculated agree well with direct emissions reported in the IPCC report. [1] This gives confidence in these calculations, especially in the unintuitive coal calculation.

Results and Discussion

Fig. 2: kgCO2e per km for the 2018 Kia Soul EV charged in California, Quebec, and entirely using coal generation. Compare to the kgCO2 per km for several gas cars: the 2018 Toyota Prius (a typical no-plug hybrid), 2018 Volkswagen Beetle (a typical car), and 2018 Honda Odyssey (a typical minivan). [8] (Source: T. Dyson)

This section reports the kgCO₂ per km for a Kia Soul EV 2018 in California and in Quebec, and compares the result to the kgCO₂ per km of a few gas vehicles. For comparison's sake, the kgCO₂ per km for that model of electric car in a world where all electricity is obtained from coal plants, the most GHG-emitting type, is also reported. That calculation was also done using Eq. (1), but with the kgCO₂ kWh^-1 of coal plants rather than natural gas. These results can be seen in Fig. 2.

To compare the electric car's CO₂ emission efficiency to common gas vehicles, we simply need the kg of CO₂ emitted from burning 1 liter of gasoline (kgCO₂ L^-1). This number is surprisingly difficult to pin down, as it depends on several factors including crude oil source, age, and refinement technique. [11] This analysis uses the approximate number reported by the NRC. [11] Then, the liters of gasoline per 100 km (reported fuel efficiency of the gas vehicle) can be turned into kgCO₂ per km via the formula

The CO₂ emission per km of a typical hybrid without charging cable (2018 Toyota Prius), a typical car (2018 Volkswagen Beetle), and a typical minivan (2018 Honda Odyssey), using fuel efficiency from the NRC, can also be seen in Fig. 2. [8] These numbers can be compared directly to the kgCO₂ numbers from the electric car analysis.

Evidently, electric cars emit far less GHG per distance than gas ones, even hybrids. The source of the electricity has a huge effect, as you might expect. A California Kia emits about 56 grams of CO₂ per km, whereas in Quebec it emits only 0.1 gram per km. In Quebec, the emission jump from electric to no-plug hybrid is larger than from hybrid to gas car. The Kia powered solely by coal electricity, surprisingly, emits more than a typical gas car (though less than a minivan). This tells us that electric cars are not unilaterally better than gas ones.

Conclusion

It was seen that the carbon footprint of electric vehicles varies wildly by the source of their power. Even still, electric cars are much more carbon-efficient than their gas counterparts, and this will only improve with the implementation of more carbon-neutral power plants. In the case of Quebec, electric cars are nearly zero-emission. However, it's important to recognize that electric cars are only as good as their power source. Adopting electric cars alone is not enough to beat down the carbon contribution of road transport. Carbon-neutral electric power must be adopted as well, and enough of it to account for the increased load on the electrical grid by the new electric cars. As such, calling electric cars zero-emission across the board is deceptive. In the ongoing struggle against climate change, it is critical to pierce the veil of greenwashing and elucidate real impact. All that said, in the case of California and Quebec, at least, we may happily conclude that electric cars are actually better for the climate than gas ones.

If you would like to calculate the kgCO₂ per km of an electric car in the region you live, see the python source code used in this analysis is available here.

© Taj Dyson. 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] Intergovernmental Panel on Climate Change, Climate Change 2014: Mitigation of Climate Change, Working Group III Contribution to the IPCC Fifth Assessment Report (Cambridge University Press, 2015).

[2] "Implementation Manual For the Clean Vehicle Rebate Project," California Air Resources Board, July 2021.

[3] "Propulser le Québec par L'Eléctricité," Ministère des Transports du Québec, 2015.

[4] "State of California Energy Sector Risk Profile," U.S. Department of Energy, March 2021.

[5] "Canada's Energy Future 2019," Canada Energy Regulator, 2019.

[6] "Transmission Efficiency Initiative," Electric Power Research Institute, 1017894, October 2009.

[7] "Is Energy From Woody Biomass Positive for the Climate?" International Energy Agency Bioenergy, January 2018.

[8] "2018 Fuel Consumption Guide," Natural Resources Canada, May 2018.

[9] M. Nyberg, "Thermal Efficiency of Gas-Fired Generation in California: 2014 Update," California Energy Commission, CEC-200-2014-005, September 2014.

[10] E. Grol, "Options for Improving the Efficiency of Existing Coal-Fired Power Plants," U.S. National Energy Technology Laboratory, DOE/NETL-2013/1611, April 2014.

[11] "Learn the Facts: Fuel Consumption and CO₂," Natural Resources Canada, 2014.