The hybrid Chevy Volt is touted by General Motors as producing less carbon dioxide than purely gasoline-powered cars. But that may not be true according to an analysis by Junkscience.com:
According to the EPA the 4-seat Volt is capable of driving 35 miles on its 16 kilowatt hours (kWh) of stored electric charge. The Volt’s gas-only fuel economy rating is 37 mpg.
Since two oxygen atoms from the atmosphere combine with each carbon atom when gasoline is burned, a gallon of gas produces about 19.6 lbs. of carbon dioxide (CO2) when burned. So when operating on gasoline, the Volt produces 0.53 lbs. of CO2 per mile (19.6 lbs. of CO2 per gallon divided by 37 miles per gallon).
Since we can’t quantify accurately just how much transmission loss there is between electricity generation and charging points, we’ll assume an impossible 100 percent efficiency at the charger to work out the CO2 emissions for the Volt’s 16 kWh stored charge.
In 2007, national “average” CO2 emissions were 2.16 lbs per kWh from coal-fired generation and 1.01 lbs per kW for gas-fired generation. according to Power Systems Analysis. Given that 44.46 percent of electricity in the U.S. is coal-fired and 23.31 percent is gas-fired, on a national basis, then, the mean emission of CO2 per kWh is 1.2 lbs/kWh. (2.16 lbs/kWh x 0.4446 = 0.96 lbs/kWh from coal, plus 1.01 lbs/kWh x 0.2331 = 0.24 lbs/kWh from gas).
The Volt’s “emissions mileage” from its stored charge is then 16 kWh x 1.2 lbs/kWh divided by 35 MPG = 0.55 lb CO2/mile.
So on an “average” basis, the Volt emits more CO2 from battery use than from gasoline use (0.55 lbs/mile vs. 0.53 lbs/mile).
Maybe you don’t think that’s a big difference, but the difference becomes more pronounced when the Volt is charged in states that rely more on coal-fired electricity.
When I first read this analysis I wondered how one gallon of gasoline, which weighs about 6 pounds could produce almost 20 pounds of carbon dioxide. Well according to a Department of Energy website, it works like this:
It seems impossible that a gallon of gasoline, which weighs about 6.3 pounds, could produce 20 pounds of carbon dioxide (CO2) when burned. However, most of the weight of the CO2 doesn’t come from the gasoline itself, but the oxygen in the air.
When gasoline burns, the carbon and hydrogen separate. The hydrogen combines with oxygen to form water (H2O), and carbon combines with oxygen to form carbon dioxide (CO2).
A carbon atom has a weight of 12, and each oxygen atom has a weight of 16, giving each single molecule of CO2 an atomic weight of 44 (12 from carbon and 32 from oxygen).
Therefore, to calculate the amount of CO2 produced from a gallon of gasoline, the weight of the carbon in the gasoline is multiplied by 44/12 or 3.7.
Since gasoline is about 87% carbon and 13% hydrogen by weight, the carbon in a gallon of gasoline weighs 5.5 pounds (6.3 lbs. x .87).
We can then multiply the weight of the carbon (5.5 pounds) by 3.7, which equals 20 pounds of CO2!