Plug-in Vehicles and Their Dirty Little Secret


John Petersen

Over the last few months I’ve had a running debate with some die-hard EVangelicals who insist that plug-in cars will be cleaner than simple, reliable and relatively inexpensive Prius class HEVs. Since most of my readers have enough to do without slogging through the comments section, it’s high time we lay the cards on the table and show why the myth of zero emissions vehicles is one of the most outrageous lies ever foisted on the American public.

The following graph comparing the life-cycle CO2 emissions of conventional, hybrid and plug-in vehicles comes from a March 15, 2010 presentation by Dr. Constantine Samaras of Rand Corporation. It clearly shows that HEVs and PHEVs are equivalent emitters of CO2 if you take the analysis all the way back to the black earth and base the comparisons on national average CO2 emissions from electric power generation.

1.5.11 GHG PHEV.png

While the graph suggests that there is no meaningful air quality advantage to plug-in vehicles, the reality is much worse because the specific power generation assets that will be used for night-time charging of plug-in vehicles are dirtier than the national average.

The following table is based on data extracted from US Energy Information Administration’s recently released “Electric Power Industry 2009: Year in Review.” It lists high emissions power from fossil fuels in the top section, zero emissions power from conventional sources in the middle section and “clean power” from renewable sources in the bottom section. Since the data was pulled from different parts of the report, estimates of total power generated from specific renewable sources can’t be provided. Since renewables as a class are inconsequential to national power production, I don’t think the missing data is relevant.

1.5.11 US Power.png

The most intriguing facts in the table are the capacity utilization rates for both natural gas and hydro power facilities. Natural gas facilities operated at 25% of capacity in 2009, which works out to a national average of six hours per day. You see the same thing with hydro power facilities which operated at 40% of capacity in 2009, or about ten hours per day. While some natural gas and hydro power plants run 24/7, the nation tends to operate both types of facilities as peak power providers rather than baseload power providers. We turn off the clean hydro power and natural gas at night.

The two baseload elements of US power production are nuclear, which usually runs at a steady state 24 hours a day, and coal, which can be ramped up and down within a limited range to help match supply and demand. During night-time hours, the prime time for electric vehicle recharging, the vast bulk of electric power nationwide comes from nuclear and coal because operators want to conserve their more flexible resources including natural gas and hydro power for high value peak demand periods. As a result, coal accounts for a higher percentage of night-time power than it does day-time power or 24 hour power. There’s just no avoiding the reality that electricity produced at night is significantly dirtier than the national average while electricity produced during the day is cleaner than the national average.

As you shift the US average emissions line in the Rand graph to the right to reflect the differences between day-time and night-time power, plug-ins become seriously sub-optimal. The conclusions are inescapable when you study the data.

I have searched without luck for a scholarly technical analysis that quantifies the emissions differential between relatively clean day-time power, which has a high proportion of variable hydro power and natural gas, and dirtier night-time power, which has a much higher proportion of coal. If you know of a credible study, I’d love to have a reference.

The dirty little secret of plug-in vehicles is that they’ll all charge their batteries with inherently dirty night-time power and be responsible for more CO2 emissions than a fuel efficient Prius-class HEV that costs a third less and doesn’t have any pesky issues with plugs, charging infrastructure or range limitations.

News stories, speeches and press releases can only maintain the zero emissions mythology for so long. Sooner or later the public is going to realize that it’s all hype, blue smoke and mirrors, and that plug-in vehicles have little of substance to offer consumers. When the public comes to the realization that plug-in vehicles:

  • Won’t save their owners significant amounts of money;
  • Won’t be as efficient as HEVs when utility fuel consumption is factored into the equation;
  • Won’t be as CO2 efficient as HEVs when utility emissions are factored into the equation; and
  • Are little more than feel-good, taxpayer subsidized eco-bling for the politically powerful elite,

the backlash against EV developers like Tesla Motors (TSLA), General Motors (GM) and Nissan (NSANY.PK), together with battery suppliers like Ener1 (HEV) and A123 Systems (AONE), could be unpleasant.

Disclosure: None.


  1. I think you’re making an error here, John. If you want to decide how dirty the electricity that charges a PHEV is, you should look at the incremental power source.
    In other words, when I plug in my Volt or Leaf, what poer plant ramps up to supply the additional increment of demand? If it’s a coal plant that has been operating at below full capacity, then I’m charging my vehicle with dirty power, but if it is a gas turbine that supplies the incremental power, then I have relatively clean fuel. And if my additional bit of demand allows the integration of more wind power on a cold and windy night then my fuel is completely clean, even if the nighttime energy mix is 95% coal and only 5% wind.
    I don’t know if your conclusions are right or wrong, but steps you took to get there are not very rigorous.

  2. Thanks Tom. While a detailed analysis of what power will flow to a specific electric outlet might yield a different result, and would certainly yield a different result in cases where an owner had his very own wind turbine to do the charging work, basing advertising claims on plug to wheels emission values is fundamentally fraudulent.
    I would love to see a detailed analysis that packs some muscle on the bones, and one of my greatest frustrations is that the work has never been done, but it’s not so much a lack of rigor in this case as a lack of data.

  3. Its hardly a secret that existing electricity generation is not all clean, but EV adoption creates a viable path for zero emission transportation that other solutions simply cannot. So 1) less local pollution in cities has a positive health impact and 2) replacement of elec generation with cleaner sources.
    as an aside, isnt the idea behind technologies like ZBB that we can have greater control over utilization of power sources through more efficient storage?

  4. Electricity generation isn’t even close to clean and won’t be for decades.
    I’m happy to accept the proposition that there is a virtue in generating green electricity. That virtue, however is not transferable to the device that uses the green electricity. The electricity has virtue of its own and does not care if it powers a light bulb, a refrigerator or an EV.

  5. ” I have searched without luck for a scholarly technical analysis that quantifies the emissions differential between relatively clean day-time power, which has a high proportion of variable hydro power and natural gas, and dirtier night-time power, which has a much higher proportion of coal. If you know of a credible study, I’d love to have a reference. ”
    Here some links for a comparison between France (80% nuclear electricity) and Denmark (now 25% wind power) in electric carbon emissions :
    France 83 gr/kWh
    Denmark 580gr/kWh
    As an aside : I have 100 RE sourced electricity supplied from the grid by my utility Ecopower cvba. They represent 1% of the Belgian electric GWh generated each year, so they are a marginal player, 55% is done by Nuke plants and 6% comes from RE overall in Belgium, which is far better than the US average. So the US should start to clean up it’s electric generation if we want PHEV to have a shot at reducing carbon emissions in cities.
    Why wind power works for Denmark. Denmark has done the most in this respect, with 20 per cent of its average energy coming from wind power. Denmark is exporting most of its fluctuating wind power to larger neighbors while finding other solutions for supply and demand at home. Despite this investment in non-hydro renewables, the carbon intensity for electricity production in Denmark is about 580 grams of carbon dioxide per kilowatt hour. By contrast, the figure for France is 83 grams of carbon dioxide per kilowatt hour, which draws almost 80 per cent of its electricity from nuclear power. This is more than 7 times lower than Denmark, per unit of electricity. This is the stark reality, not the spin.
    Right, I think I have the Denmark electricity EI sorted as best as I can. The most thorough data come from this IEA report from 2006:
    Although this does not report electricity EI directly, there is enough data to reconstruct it. For 2010 electricity generation, the estimate is 42% coal, 2.4% oil, 21.7% gas, 12.7% biomass and 21.3% wind. At EIs (t/MWh), based on Weisser 2008 review, I assign 1, 0.8, 0.6, 0.05, 0.02 respectively. This yields and electricity EI of 580 g/kWh. This is an assumption laden figure, but I believe is more reliable that either the Mackay figure (880) or the CARMA figure (370).

  6. Thanks for pointing out that the Rand graph is based on US data and comparisons in other countries, and for that matter in different regions of the US will result in different numbers.
    It’s also important to recognize that the economic analysis will be different in high fuel price countries like Denmark than they are in low fuel price countries like the US

  7. Here’s a study that confirms the link between power generation fuel and electric car carbon emissions, and which confirms John’s statement that you need to clean up your electric power generation mix during the night to enjoy carbon emissions reductions derived from electrification of car transportation.
    A study by EMPA, the Swiss Federal Laboratories for Materials Science and Technology, casts light on the question : when is an electric car better than a gas-powered vehicle measured with a life cycle analysis ?
    Their findings prove that electric vehicles offer a better lifetime impact than currently available combustion engine technology, but by a smaller margin than one would like to think :
    An electric car charged with power from a coal-fired plant is roughly equivalent to a gas powered vehicle that gets 5.2L/100 km (45.2 mpg).
    If charged on the typical mix of European power, derived from renewables and nuclear power as well as combustion plants, the gas-powered vehicle needs to achieve a fuel consumption of 3-4 L/100 km (78.4 to 58.8 mpg) to compete with electric.
    An electric car charged solely by renewable energy competes with a fuel efficiency of 2L/100 km (117.6 mpg).


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