Case study LCA on automotive light-weighting, using different datasets

Övrigt konferensbidrag, 2019

Goal(s)

Practitioners need quick but accurate methods for decision making when choosing materials in e.g. car chassis components. Simplified LCA is commonly used for this but choice of database, impact assessment method, material source and system boundaries can distort results and decision-making.

This case study investigating steel versus aluminium in a cars roof evaluates effects from making lighter vehicles and demonstrates how the calculation results differ depending on choice of datasets.  The impact of sourcing recycled or virgin material and depending on carbon intensity of energy source is assessed. Implications regarding future development and LCA modelling of e.g. light-weighting of electric cars is discussed.

Method(s)

The model considers manufacturing of a car roof from two optional materials, aluminium and steel, and the difference in fuel/electrical consumption for 10 years.  The LCA system model compares the environmental impact of the material production with the weight-induced incremental fuel consumption impact of carrying the roof.  The system boundary includes all life cycle phases, but in line with EPD system rules, end of life recycling is considered at the input material level. Cars are regulated to 85% material recycling, with near 100% for metal chassis.[MZ1]  Also, from overview analysis of component production plants (stamping and joining) the chassis production difference was omitted.

The functional unit is use of a car roof during car life cycle. A 17 kg steel roof is compared to a 7 kg aluminium roof, where the requirement of functionality stays the same for the roof regardless of the material chosen.  

In this study the following impact categories are used:
-Climate impact
-Acidification
-Ground-level ozone (Photochemical oxidation)
-Overfertilization (Eutrophication)
Cost impact is also calculated to compare with literature.

The evaluation only considers the effect of light-weighting a car depending on its energy source. It cannot be used to compare cars using different energy sources. The system boundaries and functional unit does not include the production of engines nor anything else of the car except the roof. Thus, the study is limited to comparing differences in the material production with the weight-induced energy consumption changes.

Results.

The study validates earlier studies on climate, eutrophication and economic gains in changing car body. For photochemical oxidation and acidification, the result differs depending on database, no robust conclusion can be. One dataset always gives reduced environmental impact on the chosen categories. The reason is that these data incorporates the effects of future recycling. This could be interpreted as in a system with effective end-of-life recycling and a high use of recycled materials, all investigated impacts would be reduced by light-weighting. For electric cars, light-weighting of chassis can minimise battery size and/or increase drive range.

Adaptions for other materials must consider that the assumption of negligible impact changes in the car manufacturing plant is only true for metal plate in a mixed model line, thus the component plant has to be accounted for. Also, many reinforced plastic compounds are not recycled but incinerated, thus EoL combustion need to be accounted for.