Energy Footprint of Car Aluminum Growing

Aluminum use in automobiles has been increasing steadily. Today, the average aluminum content per vehicle is almost 150 kilograms, or 9 percent of the total vehicle weight. Around one fourth of the aluminum consumed in the U.S. is used in automobiles.

The advantage of using aluminum lies in its potential to reduce vehicle weight. Replacing iron and steel with aluminum can help lower fuel consumption. However, the energy required to produce aluminum is nontrivial.

Each kilogram of aluminum requires 160 megajoules of energy to process — from mining the bauxite ore to smelting the alumina and casting an ingot — a value that is about seven times that of steel. When examining energy use in road transportation, one should also consider the production phase of the automobile's life-cycle. A new study by researchers at MIT examined the production-energy impact of aluminum in U.S. passenger vehicles in the present and the future.

The cumulative amount of energy required to produce aluminum for new vehicles is reported to rise to 8 billion gigajoules by 2035. This is a quarter of the amount of energy used by the U.S. transportation sector in 2008 alone.

This production impact can be lowered by using more recycled aluminum and/or improving the energy efficiency of aluminum production at a faster rate. However, this may potentially be countered by efforts to use lighter weight, stamped aluminum in vehicles, which uses less recycled aluminum and therefore requires more energy to process. Use of stamped aluminum, such as in body panels and closures, has been making inroads in vehicles, appearing in vehicle models like the Jaguar XJ (shown above), Audi A8, and Acura NSX.

The MIT study also looked at the recovery of aluminum from scrapped vehicles. The stock of 250 million passenger vehicles on the roads today embodies a significant stock of the material — 30 million metric tons of aluminum. That material stock is expected to continue growing, to reach 54 million metric tons by 2035.

It takes time for aluminum in automobiles to return to the scrap market, due to long vehicle lifetimes. Despite that, the researchers found that scrap aluminum in retired vehicles will almost always exceed the secondary aluminum demand for the domestic manufacture of new vehicles, so this automotive aluminum scrap system will remain in balance.

Details of these findings are available in the fall issue of the Journal of Industrial Ecology as part of a special issue on Applications of Material Flow Analysis, funded in part by the Environmental Protection Agency and the National Science Foundation.

- Lynette Cheah, Massachusetts Institute of Technology