Add it up: Doing the math on electric cars

October 31, 2012

Editor’s note: The opinions expressed by our contributing staff writers are their own and do not necessarily represent the opinions of the Houston Museum of Natural Science.

Electric cars are a popular idea. You see them in movies, hear about them in songs, and especially get to know them via inventive commercials. They claim that they produce no pollution, unlike their dinosaur automobiles with the internal combustion engines. But are they as green as they claim to be? (Note: For this blog I’ll be talking about pure electric cars, not hybrids.)

Doing the math on electric cars

A normal gasoline-using car produces pollutants as a result of converting fuel into movement. An electric car uses stored electricity to propel the vehicle. But how much pollution was created while creating the electricity? To compare the two, we’ll have to find some way to make gasoline and electricity equivalent. Fortunately, we can convert both to one unit: joules. While you might want to wear a jewel, a joule will help you get work done. A joule (abbreviated by “J” ) is a unit of energy. It’s the equivalent of applying 1 ampere through a resistance of 1 ohm for 1 second, or the force of 1 Newton over 1 meter.

A gallon of gasoline contains about 1,300,000,000 joules. One kilowatt of electricity contains 36,000,000 joules. So 1 gallon of gas produces about 36 kilowatt hours.

Burning a gallon of gasoline to move your car produces about 20 pounds of carbon dioxide. One kilowatt hour can produce different amounts of carbon dioxide, depending on what energy source was used to make it. In the United States, much of our electricity (about 42 percent) comes from coal-fired power plants. One kilogram of coal can produce 2 kilowatt hours and 2.93 kilograms of carbon dioxide. That’s about 3.3 pounds of carbon dioxide per kilowatt hour, which means that 1 gallon of gas’ equivalent in electricity produces 118 pounds of carbon dioxide if all the electricity is made from coal-fired power plants. From this information, it seems that the internal combustion engine outperforms the electric, but not all electricity comes from coal.

While the majority of our electrical generation comes from coal-fired power plants, there are other energy sources. Thirteen percent of our electricity comes from renewables such as wind and solar power, which produce no carbon dioxide. Nuclear power gives us 19 percent of our electricity and produces no carbon dioxide, either. Using this division of power sources, the amount of carbon dioxide produced making electricity for an electric car has been reduced from 118 pounds to just 8. But what about natural gas?

Natural Gas is measured by the MMBTu (one million British thermal units), which is about 1,000 cubic feet (1 mcf). One mcf of natural gas produces 122 pounds of carbon dioxide and can produce about 29 kilowatt hours. Are you still with me? This means that natural gas produces about 4 pounds of carbon dioxide per kilowatt hour. So when we add that back into the mix, our electric car is producing about 9 pounds of carbon dioxide per kilowatt hour. A gallon of gas is about 36 kilowatt hours and produces 20 lbs of carbon dioxide, or about half a pound of carbon dioxide per kilowatt hour.

Does that mean that electric cars produce more carbon dioxide than ones that run on gas? Maybe, maybe not. All those numbers are based on the national average of the energy mix. If renewables provide more electricity in your area, the amount of carbon will decrease. If you get your electricity from an all-renewable company, then you’re producing no carbon. Also, this blog has only addressed the amount of carbon dioxide produced directly by energy sources. It has not included all the other pollutants produced. It has not included the entire life cycle of the energy source. For example, a nuclear reactor produces no carbon dioxide, but mining uranium is a very energy intense project. Wind turbines produce no carbon dioxide while creating electricity, but carbon dioxide is produced when they are built.

The amount of carbon dioxide produced by electric cars can be brought down easily, where the amount produced by internal combustion engines can not. yo could switch the source of electricity. You could take stored electricity and use it for you car. Because our grid is a stupid grid and not a smart grid electricity is put on the grid as needed. If there is a moment with high wind generation and a high need for electricity, then the amount of carbon produced decreases. If the wind stops blowing and the need is still there, then the more traditional sources kick in and the amount of carbon produced goes right back up.

So while an electric car, on average, may currently produce more carbon dioxide than a gas-powered car, depending or your location and your electric provider, your electric car may be producing no carbon dioxide. Also, while there is little hope to improve the internal combustion engine to eliminate the production of carbon dioxide, researchers hope to eventually eliminate the carbon produced by an electric car. So “Let’s take a ride in an electric car/To the west side in an electric car/How can you deny an electric car/Won’t you take a ride with me/Come on and take a ride with me!”

Authored By Daniel Burch

An inveterate punster, amateur chef, and fencer, Daniel B has a double degree in History and Museum Science from Baylor. He currently serves as the Assistant Program Coordinator for the Wiess Energy Hall and Adult Education at HMNS.

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