News•April 25, 2012
Special Climate Central Report on Best Cars for Climate
Research Report by Climate Central
America’s high-carbon electricity grid is short- circuiting efforts to give consumers climate-friendly, electric-vehicle options. Depending on where you live, generating the electricity to charge an electric car can produce more greenhouse gas pollution than driving a fuel-efficient gasoline-powered car.
The good news is that Americans have lots of choices to reduce the carbon footprint from their daily driving. Anywhere in the country, an electric car is much better for the climate than the average- mileage vehicle. But in many states, popular high- mileage hybrid and conventional gas powered cars are climate-friendlier alternatives to electric cars today, and new fuel economy standards should lead to even more climate-friendly options in the coming few years.
This report provides a state-by-state roadmap to the most climate friendly cars on the market today. The analysis is benchmarked to emissions associated with the Nissan Leaf and Chevy Volt because these are the top-selling all-electric and plug–in electric vehicles on the road today.
In 36 states, the hybrid electric Toyota Prius produces less greenhouse gas pollution than the all-electric Nissan Leaf, because when you plug in a Leaf to recharge, you are tapping into electricity generated largely by burning coal and natural gas in those states. (The Prius, which is the most efficient gasoline car sold in the US today, is called a hybrid electric vehicle, but it can be thought of simply as a high-efficiency gasoline car because it derives all of its power from gasoline: its batteries are recharged by running its engine and recovering braking energy.)
Coal is the largest contributor to the high carbon footprint of our electrical grid today. In states like Wyoming or Indiana, where 90 percent or more of the electricity comes from coal, driving a Leaf is responsible for much more greenhouse gas emissions per mile (about 0.9 pounds) than a Prius (about 0.5 pounds). The Leaf fares better in states that get a significant share of their electricity from natural gas, like Rhode Island or Nevada (about 0.6 pounds per mile), but typically still produces more emissions than a Prius. The Leaf does best in states that rely heavily on nuclear, like Connecticut (0.3 pounds), or on hydropower, like Idaho or Washington (0.1 pounds).
It isn’t only the Prius that out-performs the Leaf. In the 10 states with the most carbon-polluting electricity generation, there are 20 cars that are better for the climate than the Leaf; 13 of them are gas-powered vehicles with conventional engines. The rest are gas-powered hybrids.
The partially-electric Chevy Volt has a similar profile, depending on how often a driver engages its gasoline engine. A Volt, like a Leaf, plugs in to charge its battery, but when the charge is depleted during driving it switches to its onboard gasoline engine to keep going. If a Volt drives half its miles using gasoline and half using electricity from plug-in charging of its battery, it is a bigger carbon polluter than the Prius in 45 states.
But this doesn’t mean that electric cars are not an important option for fighting climate change. They can help address our oil addiction and save consumers thousands of dollars on gasoline over the life of a vehicle. And in the long term, once the grid becomes low-carbon, electric cars, unlike gas-powered automobiles, could be a cornerstone of personal mobility in a world where carbon emissions are next to zero, which will be required to stabilize the climate.
In the meantime, as we work to shift much more of our electricity generation to low-carbon alternatives, there are many high-mileage hybrids, diesels, and other gas-powered cars available today that can offer substantial reductions in climate impacts.
STATE-BY-STATE ROADMAP TO CLIMATE-FRIENDLY CARS
Electric cars critical to zero-emissions future, but not always best choice today
Dependence on coal and natural gas short-circuits electric’s potential
Interactive State-by-State MapContacts:Eric Larson, 609-986-1987
Alyson Kenward, 609-613-1127
Richard Wiles, 609-751-1215
(Princeton, N.J.) - America’s high-carbon electricity grid is short-circuiting efforts to give consumers climate-friendly, electric-vehicle options. Depending on where you live, generating the electricity to charge an electric car can produce more greenhouse-gas pollution than driving a fuel-efficient gasoline-powered car.
Electric cars are much better for the climate than the average-mileage vehicle in the U.S. But in 36 states, the hybrid electric Toyota Prius produces less greenhouse-gas pollution than the all-electric Nissan Leaf, because when you plug in a Leaf to recharge, you are tapping into electricity generated largely by burning coal and natural gas in those states. This is one of the main findings in a new report, A Roadmap to Climate-Friendly Cars, released today by Climate Central, a science and communications organization.
“The good news is that Americans have lots of choices to reduce the carbon footprint from their daily driving,” said Eric Larson, who is on the research faculty at Princeton University and is a senior scientist at Climate Central where he was the lead author of the report. “In many states, popular high-mileage hybrid and conventional gas-powered cars are excellent climate-friendly alternatives to electric cars”.
The report provides a state-by-state roadmap to the most climate-friendly cars on the market today. The analysis compares life-cycle emissions associated with the Nissan Leaf and Chevy Volt, the top-selling all-electric and plug–in electric vehicles, with hybrids like the Prius and other high-mileage conventional gas-powered cars. (Hybrid electric vehicles like the Prius can be thought of simply as high-efficiency gasoline cars because they derive all of their power from gasoline: their batteries are recharged by running the engine and recovering braking energy.)
In the 10 states with the most carbon-polluting electricity generation, there are 20 cars that are better for the climate than the Leaf; 13 of them are gas-powered vehicles with conventional engines. The rest are gas-powered hybrids.
The partially electric Chevy Volt has a similar profile, depending on how often a driver engages its gasoline engine. A Volt, like the Leaf, plugs in to charge its battery, but when the charge is depleted during driving it switches to its onboard gasoline engine to keep going. If a Volt drives half its miles using gasoline and half using electricity from plug-in charging of its battery, it is a bigger carbon polluter than the Prius in 45 states.
Coal is the largest contributor to the high-carbon footprint of our electrical grid today. In states like Wyoming or Indiana, where 90 percent or more of the electricity comes from coal, driving a Leaf is responsible for much more greenhouse-gas emissions per mile (about 0.9 pounds) than a Prius (about 0.5 pounds). The Leaf fares better in states that get a significant share of their electricity from natural gas, like Rhode Island or Nevada (about 0.6 pounds per mile), but typically still produces more emissions than a Prius.
The Leaf does best in states that rely heavily on nuclear, like Connecticut (0.3 pounds), or on hydro power, like Idaho or Washington (0.1 pounds).
“Our findings don’t mean that we won’t need electric cars as an option for fighting climate change,” Larson said. “In the long term, electric cars may be the cornerstone of personal mobility in a world where carbon emissions are next to zero, which will be required to stabilize the climate.”
But the report highlights the importance of fuel-efficient, gasoline-powered vehicles as a practical, immediate, and technologically viable strategy to begin to stabilize the climate.
This is a technology we understand and that consumers want,” said Alyson Kenward, PhD, scientist at Climate Central and co-author of the report. “We expect to see a lot more climate-friendly hybrid, diesel, and conventional gas options in the near future as new fuel economy standards are phased in and raise gas mileage to nearly 55 mpg for the average new car by 2025 – about double today’s level.”
“No matter where you live, the Leaf and the Volt have better mileage than the average car on the road today. But, until we shift much more of our electricity generation to lower-carbon alternatives, in many states, efficient gasoline cars will be the best way to minimize the carbon footprint of daily driving,” Larson said.
Headquartered in Princeton, New Jersey, Climate Central (climatecentral.org) is a non-profit research and journalism organization providing authoritative and up-to-date information to help the public and policymakers make sound decisions about climate and energy.
Our calculations begin with the EPA’s combined highway/city driving fuel economy of cars: miles per gallon for gasoline cars and kilowatt-hours per mile for electric cars. (See Appendix Table A1 of Full Report)
For a gasoline car, the bulk of the lifecycle greenhouse gas (GHG) emissions associated with driving are due to the CO2 emitted by combustion of the fuel in the car’s engine. A gallon of gasoline releases about 19 lbs of CO2 when burned, or about three times the weight of the gallon before it burns. To these CO2 emissions we must add the GHG emissions associated with extracting, transporting, and refining the crude oil used to make that gallon of gasoline. When these are included, the total lifecycle GHG emissions for using gasoline in a car come to 25.9 lbs of CO2-equivalent per gallon. (a)
In this report, the term CO2-equivalent (or CO2e) is used to refer to GHG emissions. This measure expresses the combined global warming impact of several different gases in terms of the amount of CO2 alone that would give the same warming. (GHGs in addition to CO2, such as methane (CH4) and nitrous oxide (N2O) are emitted over the lifecycles considered here.) Since different gases have different lifetimes in the atmosphere, the relative warming impact of the non-CO2 molecules depends on the time frame under consideration. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (b) gives global warming potentials (GWPs) relative to CO2 for a large number of gases considering 20-year, 100-year, and 500-year time frames. For the results reported in the main body of this report, we have used the 20-year GWP values since 20 years is close to the typical lifetime of a car — certainly much closer than either 100 years or 500 years. Our results recalculated using 100-year GWP values are given for comparison in the Appendix (Tables A2 and Table A3 of Full Report). (For a gasoline car, the lifecycle emissions assuming a 100-year GWP are 24.3 lbs CO2e/gallon instead of 25.9 for a 20-year GWP. (a))
Estimating GHG emissions associated with electricity use by an electric car is more difficult than estimating emissions for a gasoline car, because it is essentially impossible to say with certainty that an electron generated at a particular power plant is the same electron that ends up in the battery of a particular vehicle. The uncertainties arise because of the nature of electricity flow and the geographical extent and interconnectedness of electricity grids. (c) Additional uncertainty is introduced by the time-varying nature of electricity demand and supply. For example, if an electric vehicle plugs in to charge during a period of peak electricity demand, the mix of power plants generating electricity (and hence the GHG emission profile of the electricity) will be different from the mix of plants during periods of lower electricity use. In general, the greater the temporal or geographic specificity with which we wish to determine the emissions associated with electricity use, the greater will be the uncertainty around whether the emissions accurately represent actual use.
To make our analysis tractable, we have chosen not to consider time-of-use variations in electricity emissions, choosing instead to use annual emissions per megawatt-hour generated from power plants. We also assume electricity generated in a state is consumed in that state. A recent similar study by the Union of Concerned Scientists (d) also uses annual emissions per megawatt hour, but chooses to divide the U.S. into 26 electricity-generating/consuming sub-regions defined by the EPA. (e) Another study in 2007 by the Electric Power Research Institute and the Natural Resources Defense Council divided the U.S. into 13 sub-regions. (f)
The larger the geographic region selected, the more certain one can be of the average emissions associated with each kilowatt-hour used in that region — for example, the average emissions per kilowatt-hour consumed for the entire U.S. can be known with considerable certainty. The drawback of averaging over larger and larger areas is that less and less insight can be gained into the impact of geographic distribution of different electricity generating sources. In an effort to balance these competing considerations, we have chosen to average emissions at the state level. For large states, or for states of any size that have similar electricity generating fuel mixes as neighboring states, the uncertainty introduced by this assumption is small. The uncertainties are larger for smaller states.
To estimate state-level GHG emissions associated with electricity, the following methodology was adopted. The starting point were data published by the Energy Information Administration (EIA) on how much CO2 was emitted on average per kilowatt-2 hour (kWh) of electricity generated in each state in 2010 (TableA4 and Figure A1 of Full Report). (g) This average is most influenced by the types of fuels used in the power plants in the state. For example, a state that relies more on nuclear or hydro power will have lower average CO2 emissions per kWh generated than a natural gas-reliant state or, especially, a coal-reliant state. But CO2 emissions at a power plant alone are not the full emissions story because there are also emissions associated with supplying fuel to the plant (e.g., emissions that occur during coal mining or natural gas extraction). Accurately estimating on a state-by-state basis the emissions other than those at the power plant itself requires detailed lifecycle calculations.
These calculations were undertaken using the Argonne National Laboratory’s Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) Model, version 1_2011, the same model used to estimate the lifecycle GHG emissions for gasoline mentioned above. GREET was run for each state’s electricity system by specifying in GREET the mix of fuels used for electricity generation in the state. (g) The main sources of electricity in any given state in the U.S. are coal, natural gas, nuclear, and/or hydro. (Renewables other than hydro play small roles in most states today.) In the case of natural gas, the power plant technologies used vary significantly from state to state, and the average efficiency of generation from natural gas varies accordingly from state to state. (This is not the case for coal, nuclear, or hydro plants.) Efficiency directly impacts the GHG emissions per unit of electricity produced, so we provided the mix of natural gas power plant technologies in each state as an input to GREET. The mix of natural gas powerplant technologies (combined cycle, simple cycle, or steam cycle) in each state was obtained from EIA data. (h) GREET’s default values for electricity generating efficiencies were then kept for all power plant technologies.
The outputs from running the GREET model for each state include A, the average CO2 emissions at power plants per kWh generated and B, the average total lifecycle GHG emissions in CO2- equivalents per kWh delivered to the end user. (The transmission and distribution losses assumed by the GREET model are 8% of generated electricity.) For each state, the ratio B/A was calculated and multiplied by the average CO2 emissions per kWh of electricity generated (derived from EIA data, as described above) to arrive at the lifecycle GHG emissions associated with electricity used in each state in 2010. For each state, separate calculations were done using 20-year and 100-yr GWP values for non-CO2 gases (Table A4 of Full Report).
(a) This estimate is for gasoline from conventional crude oil, as calculated by the Argonne National Laboratory’s Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) Model, version 1_2011. (See Figure 2 in A. Burnham, J. Han, C.E. Clark, M. Wang, J.B. Dunn, and I. Palou-Rivera, “Life-Cycle Greenhouse Gas Emissions of Shale Gas, Natural Gas, Coal, and Petroleum,” Environmental Science & Technology, 46: 619-627, 2012.)
(b) IPCC, 2007: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. Available at www.ipcc.ch.
(c) For a thoughtful discussion of this issue, see C.L. Weber, P. Jaramillo, J. Marriott, and C. Samaras, “Life Cycle Assessment and Grid Electricity: What Do We Know and What Can We Know,” Environmental Science & Technology, 44: 1895-1901, 2010.
(d) Anair, D. and Mahmassani, A., “State of Charge: Electric Vehicles’ Global Warming Emissions and Fuel-Cost Savings Across the United States,” Union of Concerned Scientists, (prepublication version), April 2012.
(e) The UCS report uses an overall methodology quite similar to the one we have used. Two notable differences are in some key input assumptions, including the use of 2007 emissions data by the UCS (rather than the more recent 2010 data we have used) and 100-yr GWP values for estimating the global warming impact of non-CO2 greenhouse gases. We have used a 20-yr GWP, but also show results for 100-yr GWP in the Appendix.
(f) EPRI and NRDC, “Environmental Assessment of Plug-In Hybrid Electric Vehicles, Vol. 1: Nationwide Greenhouse Gas Emissions,” July 2007.
(g) Energy Information Agency, State Electricity Profiles 2010, US Department of Energy, January 2012. (In this reference, Table 5 for each state gives annual electricity generation by fuel type, and Table 7 gives CO2 emissions from electricity generators.)
(h) Theamountsofelectricitygeneratedineachstatein2010brokendownbytypeofpowerplanttechnologyarecollectedbythe Energy Information Administration on form EIA-923 and published in spreadsheet form. For input to GREET, the natural gas generating technologies were categorized as combined cycle, simple cycle, or steam cycle.