My previous post comparing the nitrogen oxide emissions of electric and fossil-powered vehicles generated a number of comments (via social media and the blog) to the effect of:
“Power plants may be dirty, but you’re leaving out the upstream impact of refining the fossil fuels to power an ICE vehicle!”
That I did. It was only an analysis of (mostly) local NOx emissions, either from the car’s tailpipe, or the power plant used to generate the electricity (in MInnesota). NOx emissions can have significant local health impacts (inhalation, formation of ozone and smog, etc), but they can also travel long distances. Lifecycle or upstream impacts are also important to consider since we shouldn’t only be concerned about the local air quality impact of our transportation choices.
In an attempt to respond to these comments, I went digging for information on the upstream NOx impacts of the different vehicle fuels discussed in the post (electricity, gasoline and diesel).
The best, most usable, resource I found was the National Energy Technology Laboratory’s Upstream Dashboard Tool (NETL is part of the US Department of Energy). The UDT is “a fast and easy to use tool to determine the environmental profile of various energy feedstocks” according to the documentation. It includes emissions impact information on various “upstream” portions of energy fuels acquisition like raw material acquisition (mining or drilling), raw material transport (the truck, train or pipeline trip fuels must make before they are used) and the “energy conversion facility” (in the case of gas and diesel, this means the refinery process). Basically, you tell it to look at a fuel, and it will give you outputs like air and water emissions, solid waste generation, and water and land use requirements. And it’s all in an easy-to-use excel spreadsheet, just like they advertised!
This tool allowed me to look at the “upstream” impact of the coal and natural gas used in Minnesota’s power plants (to “fuel” an EV), and gasoline and diesel which would be used in ICE vehicles. So now I can answer the question: how do the fuels of ICE and electric vehicles used in Minnesota compare to each other in terms of NOx emissions, accounting for both the “tailpipe” emissions and the upstream emissions?
The upstream emissions
Here are the results from the Dashboard, translated into grams of NOx pollution per mile of vehicle travel:
The Tesla is in fact cleaner in terms of upstream emissions (before the creation of any kWh sent to the electric grid) – responsible for one third as much upstream NOx as either an average gas vehicle or one select diesel vehicle (without a defeat device). This is assuming Minnesota’s electricity mix, which includes 50 percent coal and about 14 percent natural gas. The tesla has no “energy conversion facility” emissions equivalent to a refinery for the liquid fuels, since I’m counting the burning of the coal as part of the downstream emissions.
So, extracting and refining liquid fossil fuels is in fact dirtier (in terms of NOx pollutants) than the extracting and transporting the fuel for an electric vehicle (powered from roughly 75 percent fossil fuels).
The complete picture
Now let’s take a look at the whole picture: from extraction to turning wheels. Here are the results with the downstream emissions included:
Electricity production in Minnesota is responsible for 1.4 pounds of NOx pollution per MWh, meaning an electric vehicle like the Tesla produces 0.21 grams of NOx per mile driven. With Minnesota’s current electricity generation mix, an EV is responsible for emitting about 30 percent more NOx “well to wheels” than typical gasoline vehicle, and 50 percent more than a diesel vehicle.
Electric vehicles can be better for NOx, if we kick coal
NOx rates change significantly based on the amount of coal in the electricity mix. In Oregon, which only gets 6 percent of its electricity from coal (and 32 percent from natural gas), the lifecycle NOx emissions of an EV like the Tesla would be 20 percent lower than an average gasoline vehicle. In Washington, where 75 percent of electricity comes from hyrdopower or other renewables, emissions would be less than half of a gas vehicle. In Wyoming, where 89 percent of electricity generation comes from coal, an EV would emit 80 percent more NOx per mile than a gasoline vehicle on a lifecycle basis. The United States as a whole emits 1.2 lbs of NOx per MWh, translating into a lifecycle per mile emissions rate for an EV that is about 10 percent higher than a gasoline vehicle.
Some Minnesota utilities are on a trajectory to reduce their coal use. However, there is an active political discussion going on right now about the future of coal-fired electricity in Minnesota, and the outcome is uncertain. As I said in the previous post, I think we need to electrify the transport sector to reduce climate risks. However, the choice cannot be between deploying electric vehicles or cleaning up the grid, both must be done simultaneously.
- If you’d like to check my math, here is the spreadsheet.
- This is still not a full supply-chain analysis, which would account for things like the mining of materials used to build EV batteries and car parts. This is just an analysis of fuels.
- Liquid fossil fuels, like gasoline, are getting dirtier over time, measured by carbon impacts, with the addition of new sources like tar sands. The UDT is based on national average compositions for gasoline, so the impact on pollutants like NOx is unclear using this approach. Regardless, electricity still needs to get a lot cleaner.