When most of us think about the energy we use to drive a car, we focus on the gas mileage. We buy fuel, use it to propel the car, and expect that the exhaust (largely carbon dioxide and water vapor) will leave the tailpipe. Few of us think of the energy used to build the car, including the energy it takes to convert the raw materials into car parts.
What is often called “embodied” or “embedded” energy is the energy that has been used to produce the materials of everyday life: steel, glass, PVC pipe and wood. When you look at a skyscraper being constructed, it is easy to overlook the embodied energy in all the construction materials involved. Carpet and aluminum have high embodied energy content, 150 kWhr and 53 kWhr, respectively.[1] It will be of little surprise that wood (2 kWhr) and stone (1.0 kWhr) have relatively little embodied energy content, and it is largely derived from how they are cut and shipped to the site of use.
Nearly all of the embodied energy in aluminum is derived from the electricity that is used to smelt aluminum ore, which is why recycling aluminum is so important. Jet airliners use a lot of fuel, but they are also made of aluminum, with a lot of embodied energy.
David MacKay of the University of Cambridge estimates that the embodied energy in the average car amounts to about 76000 kWhr. That is equivalent to about five years of electricity used in a 3200-square-foot house in Durham, NC.
Scaled across the range of available sizes, the energy used to manufacture a car is about equivalent to one year’s worth of gasoline to drive it. But there are important differences between brands. When Ford changed the bed liners in its pickup trucks from steel to aluminum, the decision was made to increase the expected gas mileage, since aluminum is a lighter material. The embodied energy in steel is only about 13% of that in the same weight of aluminum. We can hope that the improved mileage will pay back the carbon dioxide emissions associated with smelting aluminum.
Overall, embodied energy represents about a third of the energy used in modern society. This accounts for the energy used to produce the products of everyday life, dominated by chemicals (especially fertilizers), steel, paper and cement. The rest of the energy is used in more obvious ways: transportation, heating and lighting.
References
Dixit, M.K. 2017. Embodied energy analysis of building materials: An improved IO-based hybrid method using sectoral disaggregation. Energy 124: 45-58.
MacKay, D.J.C. 2009. Sustainable Energy-without the hot air. Cambridge University Press.
MacLean, H.L. and L.B. Lave. 2003. Life-cycle assessment of automobile/fuel options. Environmental Science and Technology 37: 5445-5452.
[1] (One kilowatt hour of electricity is equivalent to 3.6 megajoules).
