The electric car, seemingly on its death bed throughout the 90s and much of this decade, appears over the past two years to be rising from the grave. Every car company worth its road salt is rushing to put a plug-in hybrid or an all-electric car on the market.
All-electric cars directly release no air emissions, and they seem for many a perfect green alternative to “clunkers,” SUVs, and other road hogs dependent for their get-up-n’go on internal combustion engines.
Cars of course need energy to move, so when someone plugs in an electric car, the battery is charged with electricity from the electric grid. In the U.S., electricity generation is responsible for about 40 percent of total carbon emissions. So from a full life-cycle standpoint, electric cars are hardly zero-emissions when it comes to carbon; and depending on where one lives, a new electric car may not be the only – or the optimum – choice, at least until we get more renewable energy on our grid.
|How the Calculations Were Made In Reporting this Article|
Electric devices are often more greenhouse-gas intensive than equivalent natural gas or even oil devices. In most of the U.S., an electric stove, furnace, or water heater will be much more carbon-intensive and more expensive than a comparable gas device. The reason? Primarily because of the conversion efficiencies and losses involved in electricity generation, transmission, and use: Converting one form of energy to another comes at the expense of a loss of some of the energy.
An average coal power plant (pdf) in the United States has a conversion efficiency of slightly more than 33 percent. So only about 33 percent of the energy stored in coal is converted to electricity, with the rest lost in the form of waste heat. An additional 6 to 8 percent of electricity is lost during the transmission from the power plant to the residence. The electricity reaching that house must then be stored in the electric car’s lithium ion battery (where another 14 percent (pdf) of the electricity is lost).
Finally, the electricity in the car’s battery must be converted into mechanical energy by the motor to turn the wheels. That process leads to an additional 10 to 20 percent loss of energy as waste heat.
Electric car fuel efficiency is best measured in miles per kilowatt-hour (kWh) rather than miles per gallon. However, because scientists know the carbon associated with both a kWh of electricity and with a gallon of gas, comparing the net carbon emissions from driving an electric car and driving a car with an internal combustion engine leads to an informed decision on which has a lower carbon footprint.
The most efficient widely available hybrid internal combustion engine (ICE) car currently being produced is the 2009 Toyota Prius, which is rated around 46 miles per gallon for combined city/highway driving. The most common all-electric car currently commercially available is the Tesla Roadster, which gets about 4.9 miles per kWh. The map below compares the difference in annual carbon emissions between driving a 2009 Prius and a Tesla Roadster 12,000 miles a year for different parts of the country, based on the electricity generation mix and transmission losses for states and regions.
The Tesla Roadster comes out the clear winner in most places, with the exception of some parts of Montana, Kansas, Oklahoma, and Missouri, where the amount of coal in the grid and transmission losses associated with each state hold sway. In the places with the cleanest grid mix – like the Pacific Northwest, parts of California, New England, and New York City – the Tesla produces only about one-third the emissions of a Prius. The Tesla also has the benefit of changing carbon-efficiency over its life, as it reflects the fuel mix of the grid. If the U.S. were to start using more renewable energy, or if Tesla owners were to install solar panels on their houses, the Tesla would emit even less carbon.
However, the comparison between the Prius and the Tesla is not entirely fair: Not only is the Tesla far more expensive than a Prius to buy, but the Prius is a four-door sedan, the Tesla a two-door sports car. A better comparison might look at the now-discontinued two-door Honda Insight, which had a more impressive 65 miles per gallon combined city/highway fuel economy. In this case, the benefit of the electric car versus the hybrid is less clear-cut, as shown in the map below.
Electric car size matters for vehicle efficiency, just as it does with vehicles fueled by traditional internal combustion engines.
Comparing the all-electric version of the Toyota RAV4 SUV, for example, to the 2009 Prius, the hybrid car beats the electric SUV hands-down in most places (though the electric SUV would probably beat a hybrid SUV).
Electric cars are a promising and important part of future efforts to reduce emissions from transportation, and they likely will become greener with time as more renewable energy is added to the grid. Nonetheless, it is important, in making comparisons between fuel types, to keep in mind that electricity, like any other fuel, is not zero-emission, even if the all-electric vehicle is while headed down the road. Depending on where the vehicle is to be used, a hybrid car for now may well be a “greener” choice than an electric car when it comes to considering the total carbon footprint.
How the Calculations Were Made
In Reporting this Article
Electrical-outlet-to-wheel efficiency for Tesla Roadsters is given as 2.18 megajoules (MJ) per kilometer (km). We convert this to kilowatt hours (kWh) per mile using the following equation:
EVEff = 1 MJ / 2.18 km * 1.609 km / mile * 1 kWh / 3.6 MJ = 0.205 kWh per mile
Efficiency for the electric Toyota RAV4 is calculated based on Department of Energy tests that found that electric RAV4s get 1.9 miles per kWh on average. This converts to:
EVEff = 0.526 kWh per mile
Annual emissions from electric vehicles are calculated based on the following equation:
EVCO2-eqAnnual = milesAnnual * EVEff * CO2-eqElectric
- EVCO2-eqAnnual is annual greenhouse gas emissions from electric vehicle driving in pounds of carbon dioxide-equivilent
- milesAnnual is the number of miles driven per year
- VEff is the efficiency of the electric vehicle in question (in kWh per mile)
- CO2-eqElectric is the emission factor for the user’s zip code based on their NERC (North American Electricity Reliability Corporation) subregion, their state’s transmission losses, and indirect emissions associated with fuel extraction, refinement, and transportation as well as power plant construction and decommissioning
Emissions from hybrid-electric vehicles are calculated using the following equation:
HEVCO2-eqAnnual = milesAnnual / HEVmpg * DirectCO2Gasoline / (1 – IndirectCO2-eqGasoline)
- HEVCO2-eqAnnual is annual greenhouse gas emissions from electric vehicle driving in pounds of carbon dioxide equivalent
- HEVmpg is the efficiency of the hybrid (in miles per gallon)
- DirectCO2-eqGasoline is the emission factor for gasoline combustion based on numbers from the EPA
- IndirectCO2-eqGasoline is the percent of total gasoline emissions that are a result of indirect emissions caused by the extraction, refinement, and transportation of gasoline
Specifically, we assume that:
- milesAnnual is 12,000 miles per year, based on EPA estimates of average annual vehicle mileage
- DirectCO2-eqGasoline is 19.675 lbs carbon dioxide equivalent per gallon of gasoline
- IndirectCO2-eqGasoline is 4.860 percent of total gasoline carbon emissions based on work by Meier (2003).Combined city/highway miles per gallon are 46 for the 2009 Toyota Prius and 65 for the 2006 Honda Insight.The net carbon difference of using an all-electric vehicle instead of a hybrid-electric vehicle for each zip code can be calculated by:NetCO2-eq = EVCO2-eqAnnual – HEVCO2-eqAnnual
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