There is a great deal of movement toward electric cars. There are also efforts to create cars that burn natural gas (or use it to power fuel cells). Is it better from a GHG or energy efficiency standpoint to use gas to make electricity to power a car? Or is it best to just burn the gas in the car directly?
Asked by Roy Johnson, MBA ’86, Portola Valley, Calif.
When comparing vehicles, it is easy to get lost in the details. I discovered this firsthand when I set out to calculate the emissions from electricity production versus compressed natural gas combustion. Certain factors, such as total mileage considered, were constants for both vehicle types. Other variables, such as associated emissions factors, were different.
According to my calculations and the information provided by the EPA, outlined below, I found the following associated greenhouse gas emissions for each scenario over one year: Chevy Volt with electricity generated by natural gas, 1.2 tons; Chevy Volt with California electricity, 1.5 tons; Chevy Volt with Louisiana electricity, 3.2 tons; Honda Civic CNG with compressed natural gas, 5.6 tons; and Chevy Volt with Wyoming electricity, 6.1 tons. Also, the Honda Civic gasoline car emits 6.5 tons, the most of all the scenarios explored. As is evident, the Volt has fewer associated driving emissions than does the Civic CNG unless the electricity used to power the Volt is relatively “dirty,” and both vehicles have fewer emissions than the gasoline automobile.
The first consideration for my calculation was what resources are used to make the electricity. In the United States we use many resources, including coal, natural gas, hydropower and nuclear power. When the electricity is delivered to the user, it is usually from a combination of sources. The specific combination depends on the location, and can thus have a wide range of associated emissions. I compared a natural gas-powered car to an electric car in several scenarios using electricity mixes from across the country: one run on electricity solely from natural gas, one on electricity from California, one on electricity from Louisiana, and one on electricity from Wyoming. For these comparisons, I used the 2011 Chevy Volt, a plug-in hybrid that can run on electricity, and the 2011 Honda Civic CNG that runs on compressed natural gas.
The figure below illustrates the where electricity comes from in each state of the United States in Gigawatt-Hours for 2009. Note that coal comprises a large percentage of electricity in many eastern states, whereas the southern and western states have a higher percentage of electricity generated from natural gas and hydropower.
Source: EPA: Fuel Economy. 2009. http://www.fueleconomy.gov/feg/phevghg.shtml
The EPA’s fuel economy site helped me track down how much electricity the Volt requires to operate. Then, using the Stationary Combustion Tool provided by the Greenhouse Gas Protocol, I established the emissions associated with combustion of natural gas to make the appropriate amount of electricity to drive the Volt 15,000 miles, the annual mileage convention used by the EPA. Then, I used the information provided by the EPA about regional electricity generation emissions in conjunction with information from a report by the Intergovernmental Panel on Climate Change to calculate the emissions for that same amount of electricity in California, Louisiana and Wyoming.
The next step was to calculate the emissions associated with direct combustion of compressed natural gas in the Honda Civic CNG. To obtain this figure, I used the information provided by the EPA fuel economy site for the Honda Civic CNG for 15,000 miles. Refer to the beginning of this section for my results.
The graph below shows the carbon emissions associated with different gas mileage values. The graph also demonstrates the emissions associated with electric vehicles powered by electricity from different sources. The figure can be utilized to find the equivalent mileage of a gasoline vehicle for an electric vehicle powered by a particular electricity source. For example, an electric vehicle getting 4 miles/kilowatt-hour from the average U.S. grid is equivalent to a gasoline vehicle getting 67 miles/gallon; while one getting 3 miles/kilowatt-hour from an old coal steam plant is equivalent to a gasoline vehicle with 27 miles/gallon.
Figure created by: Dr. Gilbert M. Masters. 5/9/2011 Typically, there are fewer greenhouse gases emitted from the electricity used to power an electric vehicle than from a vehicle fueled by compressed natural gas, especially when that electricity is generated from natural gas or other relatively clean sources. This is because of the specific processes involved in the way each type of energy is used and processed. For example, large power plants that combust natural gas use large-scale technology to monitor and clean emissions, whereas individual vehicles that combust natural gas are limited by the structure of the car itself.
However, the distinction between which vehicle is better is much more convoluted than it appears. Dr. Sven Beiker, executive director of the Center for Automotive Research (CARS) at Stanford, reinforces the industry-wide understanding that manufacturing batteries for electric vehicles is very energy intensive. Additionally, electric vehicle batteries are very heavy. Thus, when the CNG car has less fuel, it will be lighter and possibly have higher gas mileage and lower emissions per mile than the weighed-down electric vehicle. Battery weight has been an area of much focus in the electric vehicle industry and Department of Energy forecasts show that battery weight should decrease by 33 percent by 2015, and by 75 percent by 2030.
Researchers also predict that battery life will increase to more than three times what it is today, decreasing the overall energy and material needed for an electric vehicle. Check out this report by researchers at Carnegie Mellon University that outlines the economic and environmental impacts of battery weight and charging patterns. Also, please note that the emissions figures I reported above are not based on the production of the vehicles themselves, but from driving only.
The issue of automobile emissions is an important one. Though the overall impact of a car is an incredibly complex matter, extensive research, such as that being conducted by Dr. Beiker and his team, is dedicated to just this topic. How do we pick the best way to drive if we don’t know the full impact of each type of vehicle? Well, for that answer, stay tuned, and in the meantime, steer clear of gas-guzzlers, no matter what fuel that “gas” may be. Electric vehicles with “clean” sources are better and more widely available than CNG vehicles. Hybrids are a good option for those who don’t have access to clean electricity or require features that electric vehicles can’t offer. Regardless, the trend is away from gasoline vehicles and toward those that offer reduced emissions in a variety of ways.
Hillary Hull is a graduate student in civil and environmental engineering.
