Full battery electric vehicles are claimed to reduce greenhouse gas emissions by 50% to 75% compared with conventional gasoline-powered cars. As discussed below, a realistic percentage reduction is probably considerably less. And there’s a major problem with electric cars. Their large-scale adoption could ensure that the current generation of carbon-intensive coal-burning power plants continues to operate far into the future. An excellent case can be made that coal combustion must be phased out to avoid dangerous climate change. (See, for example, the book Storms of My Grandchildren, by James Hansen.) This phase-out is unlikely to happen if the industrial world tries to power its transportation system with electricity.
Electric motors seem ideal for vehicles because they are highly efficient – they turn about 90% of supplied energy into motion. But powering them from batteries offsets some of this efficiency. Batteries aren’t so efficient. Energy is lost when they supply power, and it is lost when they are charged. Assuming an overall charge/discharge efficiency for batteries of about 65% seems reasonable. Production of electric power is less efficient still; the national average is about 35%. The net efficiency of the power system of a fully-electric car is thus about 0.9 x 0.65 x 0.35, or 20%. This is no different than a gasoline engine. Can these efficiencies be improved? Certainly. But it's hard to see how electric car power system overall efficiency could get much better than 30% as long as the power to charge the batteries comes from the relatively inefficient electricity production system.
Electric cars' greenhouse gas emission reduction potential depends to a large degree on the carbon intensity of this system. Overall, U.S. electricity production releases about 1.3 pounds of carbon dioxide per kWh. This is about 25% less carbon dioxide than if the electric power was produced by a fuel with the carbon content of gasoline. In some parts of the country such as New Jersey, which has a lot of almost carbon dioxide-free nuclear power, this rate is lower, and electric cars reduction potential is somewhat larger.
Electric motors seem ideal for vehicles because they are highly efficient – they turn about 90% of supplied energy into motion. But powering them from batteries offsets some of this efficiency. Batteries aren’t so efficient. Energy is lost when they supply power, and it is lost when they are charged. Assuming an overall charge/discharge efficiency for batteries of about 65% seems reasonable. Production of electric power is less efficient still; the national average is about 35%. The net efficiency of the power system of a fully-electric car is thus about 0.9 x 0.65 x 0.35, or 20%. This is no different than a gasoline engine. Can these efficiencies be improved? Certainly. But it's hard to see how electric car power system overall efficiency could get much better than 30% as long as the power to charge the batteries comes from the relatively inefficient electricity production system.
Electric cars' greenhouse gas emission reduction potential depends to a large degree on the carbon intensity of this system. Overall, U.S. electricity production releases about 1.3 pounds of carbon dioxide per kWh. This is about 25% less carbon dioxide than if the electric power was produced by a fuel with the carbon content of gasoline. In some parts of the country such as New Jersey, which has a lot of almost carbon dioxide-free nuclear power, this rate is lower, and electric cars reduction potential is somewhat larger.
If the U.S. could produce more electricity from renewable and nuclear sources, and from efficient combined heat and power plants, electric cars would look better from a greenhouse gas perspective. But how likely is a major shift in the generation mix any time soon? It seems more likely that increasing numbers of electric vehicles will provide the excuse for electricity producers to keep old coal-burning plants going.
What to do? We could take a fresh look at diesel engines, which are more efficient than gasoline and comparable to even optimistic estimates for full battery electric when the entire power cycle is considered. Diesels can run on low-carbon fuels like natural gas. We could significantly downsize our cars. This would produce major gains in miles per gallon. And we could get serious about making telecommuting work. It's worth looking seriously at the greenhouse gas reduction potential of such changes before we spend huge amounts of money converting our transportation system to full battery electric.
Mike, how, or where, does a hybrid car fit into this picture? This is really interesting stuff, and I enjoy reading your blog. Even though some of the science can be a stretch for me, it's good to stretch the brain!
ReplyDeleteThanks Louise! In my view, hybrids make more sense than full-battery electric vehicles. They combine the best features of electric motors and internal combustion engines without the drawback of being dependent on the present carbon-intensive electricity production grid. Their already good efficiency will be improved if they become smaller and lighter and equipped with diesel instead of gasoline engines. Where the grid is relatively low-carbon, a plug-in charge feature would improve their carbon efficiency also. But in my view, we still need to seriously work on ways to eliminate the need to drive so much in the first place.
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