When reading about energy and climate science, I often come across very broad statements regarding the benefits of different technologies. While these statements are supported by competent science, the details of their origins are most often left out. Enter the do-it-yourself (DIY) part of science that people rarely realize is within their grasp.
I’ll give an example that I was thinking about last week, based on the statement “electric cars have lower overall emissions than gasoline cars.” A fairly intuitive, even obvious statement, but I found myself wondering “why is this so obvious?” I haven’t heard any quotes on how much less the emissions are. Being a scientist, it is best to not just take someone’s word for it, and to do a little digging.
There are a number or great resources for looking up data and statistics on energy-related topics, some of which I reference below. In this specific case, we are looking to compare how much CO2 is emitted for each mile driven by an average electric versus by a typical gasoline car. To get this quantity, we must take the CO2 emissions per unit of energy and divide it by the amount of energy required to drive one mile. For the latter statistic, this is simply the energy efficiency of the car, which is expressed as the common miles per gallon (mpg) for a gasoline car, and miles per kilowatt-hour (kWh) for an electric car.
Focusing first on the gasoline car, the EPA estimates that 19.4 pounds of CO2 are emitted per gallon of burned gasoline. Additionally, the U.S. Department of Transportation calculates that the average U.S. vehicle efficiency is 19.8 miles per gallon. Dividing the first number by the second yields a value of 0.98 pounds of CO2 per mile.
For an electric car the calculation is just as simple. While electric cars themselves do not emit CO2, the power plants that produce the electricity used by the car do have emissions (the majority of electricity in the United States comes from coal powered plants and other fossil fuels). According to The Energy Information Administration, 1.33 pounds of CO2 on average were emitted per kWh of electricity production in the US in 2006. Additionally, the current standard for electric vehicles is an efficiency of 4 miles per kWh. Dividing again the first number by the second yields a value of 0.33 pounds of CO2 per mile.
Comparing these two figures, the electric vehicle emissions are 66% less than those of gasoline powered vehicles traveling the same distance. So the original statement is supported, but now we know how much cleaner an electric car is to operate. Having this actual figure also allows us to extract further information, such as the fact that a gasoline powered car with increased fuel efficiency of 58 mpg will have the same CO2 emissions as that of an electric car. By digging a little deeper, our understanding got a lot broader.
The above is a very basic analysis of emissions associated with vehicle use. A full life-cycle analysis would involve more detailed calculations that take into account the entire Production-Manufacturing-Use-Disposal pathway of each type of car as well as their fuel sources (for example, how much energy is required to drill, refine, and transport the gallon of gasoline used?). In either case, the key is to break the problem down and to make sure you use accurate sources of data. People like easily quotable facts and statistics, and sometimes you just have to make your own. All the information is there on the internet. So next time you find yourself wondering “why is that the case,” follow The Home Depot method and do it yourself.
2 comments:
Very nice quick calculation quantifying a statement that is often repeated but rarely backed up with numbers.
One aspect of the calculation missing is the energy losses associated with transport of
electricity, and with charging and discharging the battery of the car. These losses maybe 20%, so your electricity consumption is increased by a factor ~1.2, not enough to change your main conclusion.
Finally, CO2 produced in a centralized manner to produce electricity to power a car can be captured and sequestered, which is impossible for CO2 produced at the tailpipe.
Good points, thank you for the comments.
Electricity transport losses should certainly be included, but I am not sure whether the efficiency numbers quoted for the miles/kWh are averages based on actual input into the car (which would include battery inefficiencies). As you said though, either way, this would make a minor change to the overall figure.
Also, carbon sequestration certainly adds more advantages to the centralized production of electricity for electric vehicles, but that was not included. Perhaps a calculation for a future entry...
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