Why we should count secondary effects when calculating the material footprint in cars
Carbon footprint is gradually becoming a factor in the material selection process of automakers, which is terrific. And I have a suggestion that I think can make the footprint calculations even better, especially when lightweighting with aluminium.
Today, these calculations are often based on simple comparisons. We see comparisons of the footprint of the various materials – kilogram by kilogram – or the footprint of a component made with the different materials or designs. This method is OK as such, but because there are secondary effects that come into play when different materials are used for the components, it does not show the entire picture.
I have a suggestion.
Rather than using that method when evaluating carbon footprint in the context of material selection for automotive components, a better method for OEMs would be to consider the footprint of individual parts while also including the impact of footprint reduction from the secondary effects.
This is not as complex as it sounds. Let me give you an example.
Comparing a steel body with an aluminium-intensive design Fka, the German research facility, recently completed a study on this topic. They did the study together with the aluminium company Hydro, where I work. Their analysis compared a steel body design with an aluminium-intensive body design, and their comparison included the secondary effects that you get as a result of rescaling the drive train, the suspension, the brakes, and more.
As a reference car, they used a battery-electric sport utility vehicle with a 400-kilometer range. The steel-intensive SUV weighed 2,306 kgs, with the body itself weighing 700 kgs. The weight of the aluminium-intensive vehicle, including secondary savings, was 2,059 kgs. Its body weighed 516 kgs. The primary weight savings were obtained by reducing the body’s steel content, while increasing the aluminium content.
All told, the aluminium vehicle was 247 kgs lighter.
The primary savings of 161 kgs in the body led to another 86 kgs of secondary savings in the drive train, body and chassis. In other words, some 35 percent of the weight reduction came from secondary effects.
Estimating the carbon footprint in a better way Fka then estimated the carbon footprint for the car – both the steel and aluminium versions. Actually, we did two calculations for the aluminium variant: One for primary aluminium produced in Europe, and one for primary metal produced with renewable energy. We compared:
- Footprint per kg material
- Footprint of the steel/aluminium content in the body as an indication of component footprint
- Footprint of the entire car, including footprint reduction from secondary effects
This example demonstrated that, as it is with comparing costs, it is also essential for carbon footprint to include secondary effects. For instance, comparing the steel-intensive variant with the aluminium one, assuming the aluminium was produced with renewable energy, the 2x difference in footprint per kg of the material gives the aluminium in the body a footprint that is around 30 percent higher than the steel (+350 kgs CO2).
However, when you include all the secondary effects, considering the footprint of the entire car from cradle to gate, the footprint of the aluminium example is 1,669 kgs CO2 lower than the steel-intensive reference vehicle.
This gives a complete and more accurate picture of the reality.