How much do electric cars really pollute, even without tailpipes?

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2016 Nissan Leaf

2016 Nissan Leaf

Personal automobiles have given hundreds of millions of people the great luxury of a convenient and comfortable mode of transportation.

But there's no escaping that manufacturing these vehicles, driving them for 10 years or more, and then disposing of them creates pollution that has a wide range of negative impacts.

As advocates and some governments push for more plug-in electric cars, that poses a valid question: what's the total emission impact of driving a vehicle that plugs into the grid for its energy?

Over the last five years, electric vehicles have become a target for some who are not convinced that they will lead to less pollution.

Two different types of emission need to be considered. First, there is carbon dioxide (CO2), a climate-change gas.

Second are what the EPA calls criteria pollutants: other toxic substances that come out of the tailpipes of vehicles with combustion engines—and out of the smokestacks of any electric powerplant that burns any hydrocarbon fuel.

The CO2 question is more easily answered. Studies by numerous groups indicate that electric vehicles recharged on most North American grids are, at minimum, cleaner than almost any gasoline or diesel vehicle.

Electric-car wells-to-wheels emission equivalencies in MPG, Sep 2015 [Union of Concerned Scientists]

Electric-car wells-to-wheels emission equivalencies in MPG, Sep 2015 [Union of Concerned Scientists]

The Union of Concerned Scientists has produced a nice map that depicts the full "wells-to-wheels" carbon footprint of electric vehicles charged on a variety of North American regional grids.

The map makes it very clear that electric vehicles emit significantly less CO2 than gasoline-powered vehicles.

The one exception comes when the electric car is charged on a grid with a heavy mix of coal—an increasingly rare circumstance, as coal plants are retired or retrofitted for natural gas—and is compared to the most fuel-efficient cars sold today, like a 52-mpg Toyota Prius.

In other words, a Nissan Leaf electric car charged in Kansas or Oklahoma today emits slightly more carbon than a Prius driven on the roads of Kansas City.

In the places where most electric cars are sold, however—think California and the Northeast—it's not even close.

The electric car wins no matter what you compare it to.

2014 BMW i3 REx fast-charging at Chargepoint site, June 2016 [photo: Tom Moloughney]

2014 BMW i3 REx fast-charging at Chargepoint site, June 2016 [photo: Tom Moloughney]

Other studies show different numbers, but come to roughly the same conclusion.

But CO2 emissions are not the only type of emissions to be concerned about, so we return to the criteria pollutants.

Nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO), volatile organic components (VOC), and particulates (PM2.5) all represent an alphabet soup of airborne emissions that harm human health, hurt the environment, and damage property.

So are electric cars better or worse than traditional gasoline or diesel cars in their emissions of criteria pollutants?

To answer that question, we used three main tools and data sources.

These three pieces were combined to analyze the relative environmental damage of plug-in electric cars charged from the grid versus those from fossil-fuel-powered vehicles.

Cost of environmental damage from criteria emissions from electric-car use by region [John Briggs]

Cost of environmental damage from criteria emissions from electric-car use by region [John Briggs]

The results are shown in the figure above.

Note that all vehicles are modeled in GREET with a "fuel cycle" that includes its total consumption of resources: (energy in renewable and nonrenewable form), petroleum, coal, natural gas and water.

EDITOR'S NOTE: To clarify the details of the model after a number of readers questioned whether it is comparing apples to apples, we've added the following explanation:

The model includes emissions of CO2-equivalent greenhouse gases, primarily carbon dioxide (CO2), methane (CH4), and nitrogen oxides (NOx). It also consider seven different criteria pollutant associated with the fuel pathway.

GREET models 100 different fuel pathways under which the fuel can be converted to energy to power the vehicle. These techniques provide a truly well-to-wheel comparison of internal-combustion-engine vehicles with electric vehicles.

It specifically includes the emissions associated with extracting, refining, and transporting fossil fuels, and grid loss and battery-charging efficiency in the case of electric vehicles.

Our analysis concludes that charging an electric on grid electricity, on average, creates about 2.6¢/mile of environmental damage.

This is slightly more than the damage caused by diesel, gasoline, or hybrid cars.

But the details indicate a more interesting result than that single, overall, nationwide conclusion.

2016 Nissan Leaf

2016 Nissan Leaf

In regions that operate a very clean grid and purchase lots of electric cars, like California, their environmental damage is predicted to be lower than that produced by cars powered by fossil-fuel gasoline or diesel.

This is largely the result of California having less than 1 percent of coal in its generation mix. The cleaner grids in such areas as New England, New York, and the East South Central region also make plug-in vehicles quite clean.

The flip side is that grid electricity in the Midwest, with its high mix of coal today, could mean more pollution if large numbers of people in that area rapidly switched over to electric cars.

So even if they're still equal or better on CO2, addressing the climate-change concern, they may be slightly worse on other emissions that affect public health.

As the grid gets cleaner—and that is exactly what it's doing, due to a transition from coal to natural gas and renewable sources—electric cars get cleaner.

Wells-to-wheels carbon dioxide emissions per mile from electric-car use by region [John Briggs]

Wells-to-wheels carbon dioxide emissions per mile from electric-car use by region [John Briggs]

Tailpipe criteria emissions from fossil-fuel-powered vehicles have been radically reduced since the 1970s, by a factor of 100 or more. But the opportunities for further improvement on that front are both limited and likely to be very costly.

As the analysis makes clear, electric cars represent a huge opportunity for cleaner transportation.

Digging into the data further reveals that SOx emissions from electricity generation, presumably from coal-based power facilities, is by far the biggest factor in the environmental damage numbers. 

In the West North Central area, SOx emissions represent fully 60 percent of all estimated damage.

Another intriguing result from the analysis: of an estimated 1.8 cents per mile of environmental damage caused by an electric in cleaner-grid states like California, more than half the total—1.1 cents per mile—is attributed to the manufacturing of the vehicle itself.

Rendering of Tesla battery gigafactory outside Reno, Nevada, Sep 2014

Rendering of Tesla battery gigafactory outside Reno, Nevada, Sep 2014

Powering parts manufacturing and assembly plants on renewable energy, as electric-car maker Tesla Motors says it plans to do for its battery gigafactory, offers a potentially significant reduction in the overall environmental damage of making, driving, and recycling a car.

While the same general principle applies both to CO2 emissions and criteria pollutants—as the grid gets cleaner, emissions fall—our analysis shows one big difference between the two.

Electric cars already have a lower wells-to-wheels carbon footprint than diesel, gasoline, or hybrid cars today, with a few exceptions.

And homeowners can lower that footprint significantly more by installing photovoltaic solar panels to charge their cars.

There's simply no equivalent to that for fossil-fuel cars. You have no way to generate gasoline or diesel fuel at home.

So what should an eco-friendly motorist concerned about overall emissions drive?

2016 Chevrolet Volt, Catskill Mountains, NY, Dec 2015

2016 Chevrolet Volt, Catskill Mountains, NY, Dec 2015

From a CO2 perspective, electric cars are already significantly better—with a few exceptions, like that Prius in Kansas City, that may evaporate as all grids get cleaner. 

As for criteria pollutants, cars that plug in are already less polluting in many locations. If you live in one of those, electric cars are a win there too as well.

But if you live in an area where electric cars emit higher criteria pollutants due to the fuel mix in your electric grid, the decision of what to drive may be more complicated.

Even if you're good on CO2, the criteria pollutants, particularly SOx, will be likely be higher.

But change needs to start somewhere, so perhaps buying an electric car and installing solar panels as well—or working with advocacy groups and government agencies to clean the grid—is the best choice.

It is no coincidence that the grid is cleaner in California, where in June, more electric cars were sold than in the rest of the country combined.

1970s Los Angeles smog depicted in the Honda short film

1970s Los Angeles smog depicted in the Honda short film

The state has worked for more than 50 years to reduce criteria pollutants from both cars and power plants. That effort now pays additional dividends as residents of the state start to adopt electric cars.

California can be a model for pressuring states with dirty grids to make the same effort , rather than assuming the grid must forever remain as dirty as it is today. Progress comes only gradually and with consistent effort.

2016 Nissan Leaf

2016 Nissan Leaf

Below are the details of our study, which we've grouped here for those interested in digging more deeply into our methodology.

The Life Cycle Analysis presented here necessarily involves a lot of assumptions that leave it open to debate, which is entirely fair.

This particular analysis attempted to leverage the expertise of three different bodies and their scientists and analysts.

They are the Argonne National Lab, which created the GREET model, as well as the folks at the U.S. Energy Information Agency who report the grid mix information, and the professionals at the U.S. Environmental Protection Agency who have estimated the environmental damage caused by various criteria pollutants.

No attempt has been made to skew the results in a particular direction; this was an honest effort to get good answers to important questions. We acknowledge, however, that it is difficult to rule out poor assumptions either in the analysis or in the data fed into it.

Getting these answers requires understanding of not only the pollutants emitted by a vehicle, but the upstream pollutants associated with generating and refining vehicle fuel or the electricity to charge a car, as well as the pollutants involved in manufacturing a car as well. 

Argonne national Laboratories

Argonne national Laboratories

Gaining that understanding is a complex process.

The analysis made one significant deviation from the default GREET model: it uses data from the December 2015 grid mix, which is significantly cleaner than the older data used in other analyses. This was deemed necessary due to the rapid shift in U.S. grid mix from coal to natural gas.

A second deviation was to modify the GREET assumption that an electric car weighs 4270 pounds. That was changed to the 3256-pound weight of a Nissan Leaf, the world's highest-selling electric vehicle, which reduces the pollutants associated its manufacturing.

The default GREET assumptions for diesel, gasoline, and hybrid cars were maintained, which resulted in average miles-per-gallon figures of 35.9, 25.6, and 36.1 respectively. 

It was tempting to increase those numbers based on crowd-sourced real-world fuel economy results, with diesels in the low 40s and Prius hybrids in the high 40s, but they were left untouched. The electric vehicle efficiency was also a little low at 95.8 MPGe, but that was accepted as a reasonable approximation.

The most difficult assumptions to judge were the EPA-reported environmental damages from different criteria pollutants.  From the agency's analysis, volatile organic compounds create only $1,300 per ton of damage but PM2.5 particulates cause a whopping $300,000 per ton of damage—over 230 times more.

Electric power plant outside Ithaca, New York

Electric power plant outside Ithaca, New York

These numerical estimates vary significantly from one source to the next, so the decision was made to use the EPA values.  But it is clear that the price of SOx emissions, at $32,000 per ton, is a critical factor in the environmental damage calculations for electric cars, so if that value changes, the analysis will change commensurately.

While the details can and should be debated, we conclude that the general trends outlined in the article are on target.

SOURCES

  1. This article was motivated by a post from Loren Mars, a Certified Consulting Meteorologist who kindly explained his methodology via email.
  2. Generation mix data from December 2015 was taken from The Energy Information Agency (EIA)  for nine different regions of the USA, although state-by-state data is also available..
  3. Argonne National Labs GREET model (GREET1_2015.xlsm) was used.
  4. Electricity mix was set to “User Defined Mix” (Section 10.2 of Inputs tab). Data for mix entered on the Fuel_Prod_TS tab under “Electricity Generation Mixes” and “Shares of Technologies for Other Power Plants” sections.
  5. The pollutants (CO, CO2, NOX, etc) created from operating an EV on a given grid mix are shown on the GREET “Results” tab under “Electric Vehicle” (cell 1993).
  6. The GREET model estimates the pollution outputs associated with making an EV in terms of grams/mile over the expected 160,000 mile life of the vehicle.
  7. The 4270 lbs GREET standard weight for an EV was scaled to the weight of a 3256 lbs Nissan LEAF (the most popular EV sold in the USA) for the purposes of calculating the emissions from manufacturing the EV.
  8. The default values were used in GREET, except  "2016" option in section 1.1 drop-down menu was used, and also "well infrastructure" was included in section 3.7 and 4.6, and selected "yes" for section 8.6 and 10.6 to include all associated emissions in the construction of related equipment.
  9. The reference diesel, gasoline, and hybrid cars were selected from pre-existing GREET models called
      CIDI Vehicle: Conventional and LS Diesel
      SIDI Vehicle: Low-Level EtOH Blend with Gasoline (E10, Corn)
      Grid-Independent SI HEV: Low-Level EtOH Blend with Gasoline (E10, Corn)
    Respectively.
  10. The diesel, gasoline, and hybrid vehicles had mpg values of 35.9, 25.6, and 36.1 which are the GREET defaults. The electric vehicle had a 95.8 mpge efficiency which was also the GREET default.
  11. The diesel and gasoline cars had weights of 2980 lbs and the hybrid had a weight of 3220 lbs, all GREET defaults
  12. Criteria pollutants were calculated from EPA “Draft Regulatory Impact Analysis” Table 7.1.

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