David Fridley. August 25, 2009. Embodied and operational energy of vehicles.
I read an article today by Xiaoyu Yan titled Energy demand and greenhouse gas emissions during the production of a passenger car in China in journal Energy Conversion and Management, August 2009. It is a life-cycle assessment study of vehicles that summarized 9 additional studies, all published in peer-reviewed journals.
The findings are quite interesting, and show the impact both of a country’s fuel mix and efficiency level on the amount of energy used to build a vehicle and the proportion it accounts for life-cycle energy use.
For a typical American car of 1324 kg (2,919 lbs), the figures were (in MegaJoules):
Material Production 93,730 MJ
Car Assembly 25,240 MJ
Total 119,150 MJ
For a typical Chinese car of 765 kg (1,767 lbs) , the figures were:
Material Production 149,720 MJ
Car Assembly 17,430 MJ
Total 167,150 MJ
The higher material production figure for China is almost entirely due to the fact that its electricity system is 80% powered by coal, so in primary energy terms, each kWh of electricity used translates into more total primary energy than would be the case in the US.
As for operational energy, each gallon of gasoline contains 121 MJ, so the production energy in the US translates into 985 gallons of gasoline equivalent. If you assume the vehicle gets 28 mpg and is driven 10,000 miles a year, the car would consume more energy in operations than production in just 2.8 years. If the car is used for 10 years and driven the same amount each year, total consumption would reach 3570 gallons of gasoline. In this situation, the embodied energy of a car accounts for just 22% of the energy consumed by the car over its lifetime.
In the case of China, production energy translates into 1237 gallons of gasoline equivalent. The average mpg is 35, and the typical vehicle miles traveled per year is 9000 km, or 5590 miles. In this situation, it would take 7.7 years for the operational energy to exceed the energy used in manufacturing. Similarly, over a 10-year lifetime, the vehicle would consume nearly 1600 gallons of gasoline. As a result, the embodied energy of a Chinese car would account for 47% of the total energy consumed by the car over its lifetime.
This shows that it’s difficult to making blanket statements about the relationship between embodied energy and operational energy-it’s highly dependent on factors that can vary widely, but in all the studies reviewed, operational energy was the largest proportion of total energy use, which is pretty much a logical conclusion.
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