Ford LCA harmonization study clarifies benefits of lightweighting for vehicle life cycle energy use and GHG emissions
|Correlation between weight reduction and life cycle primary energy demand for (a) component and (b) total vehicle scenarios. Credit: ACS; Kim and Wallington. Click to enlarge.|
While reducing vehicle weight (lightweighting) by replacing steel and iron with alternatives such as aluminum, magnesium, and composites decreases fuel consumption and greenhouse gas emissions during vehicle use, it can increase energy consumption and GHG emissions during vehicle production.
Hyung Chul Kim and Timothy J. Wallington at Ford Motor’s Systems Analytics and Environmental Sciences Department set out to clarify the lifecycle benefits of vehicle lightweighting in a meta-analysis of previously published life cycle assessments (LCAs). While numerous studies assay the benefits of lightweighting, the wide variety of assumptions used makes it difficult to compare results from the studies, the pair noted in their paper, published in the ACS journal Environmental Science & Technology.
Using lightweight materials (e.g., aluminum, magnesium, or composites) to replace conventional materials (e.g., steel, iron) decreases the energy consumption and hence GHG emissions during vehicle use. However, the production of lightweight materials generally requires more energy and generates more GHG emissions than the production of conventional materials. Life cycle assessments (LCAs) must be performed to determine the net energy and GHG benefits of using lightweight materials. Vehicle lightweighting is an area of current interest, and this has led to a large number of LCA studies.
Unfortunately, a wide range of initial assumptions such as recycling rates, vehicle lifetime, and material substitution factors have been assumed, and it is difficult to compare the results from the different studies. To provide clarity in discussions of the energy and GHG benefits of vehicle lightweighting we present a comprehensive review of the results from the published studies. To facilitate a direct comparison of results from the various studies we adjusted the LCA results in two steps. First, the results were normalized by dividing the lightweighted results by the baseline results. Second, the results were harmonized to reflect a common set of input assumptions for key parameters.—Kim and Wallington
They reviewed 43 available studies on the benefits of lightweighting, and selected the GHG emissions and primary energy results from 33 studies (and 119 scenarios) that passed a screening process. Eleven studies and 40 scenarios were of total vehicles; 22 studies and 79 scenarios were of vehicle components.
Of the 79 component scenarios, 70 addressed substitution of steel or iron used in body or engine with lighter materials; 5 scenarios compared aluminum with other materials for powertrain components; and 4 scenarios addressed mixed materials.
They then harmonized the results from these 33 studies using a common set of assumptions (lifetime distance traveled, fuel-mass coefficient, secondary weight reduction factor, fuel consumption allocation, recycling rate, and energy intensity of materials).
Vehicle LCAs. Across the range of the total vehicle LCAs, the use phase accounts for 63−92% of the life cycle energy consumption; materials production 8−32%; manufacturing and assembly 1−4% and the rest <4%. The upper bound of lightweighting and associated life cycle energy savings for the total vehicle LCA is ∼40% and ∼25%, respectively, much smaller than those for component LCA.
All the lightweighting scenarios of total vehicle that describe intensive use of aluminum, magnesium, or composites resulted in reduced life cycle energy; the energy reduction per weight saving during the use-phase is much smaller than for the component level scenarios. This stems from the fact that vehicle weight accounts for only part of the use-phase fuel consumption in total vehicle LCAs with the balance being related to aerodynamic factors, the authors noted. Weight reduction thus results in a less than proportional energy consumption reduction during the use-phase. In component LCAs, on the other hand, the fuel consumptions related to aerodynamic factors are not accounted for in the use-phase.
Component LCAs. In the reviewed studies, the use phase dominates the life cycle energy demand for the baseline steel components accounting for 66−97% of the total, followed by the materials production phase contributing 3−20%. In the lightweighting scenarios, material production accounts for 3− 55% of total life cycle energy demand, while manufacturing and assembly account for 1−16%.
Among their findings and conclusions from the study:
LCA results of lightweight vehicles or vehicle components should be carefully interpreted as the results are sensitive to the methodology used, and values assumed for parameters and a wide range of values and choices have been used. In particular, they noted, component-level LCAs present often inconsistent conclusions across studies regarding the life cycle benefits of lightweighting materials.
Harmonization of each parameter highlights significant uncertainties in the life cycle benefit of lightweight materials. Depending on the materials production method (the Pidgeon or electrolytic process), recycling rate, and to a lesser extent on data source, the available studies indicate that the primary energy demand from magnesium production and recycling is 39−360 MJ/kg, while energy demand for the same phases of aluminum ranges from 26−249 MJ/kg.
The primary energy demand from steel for these phases ranges less widely: 12−54 MJ/kg. Magnesium and aluminum can substitute for steel at a ratio of 0.3−0.6 and 0.4−0.8, respectively. Hence, the authors point out, the energy demand for combined materials production and recycling phase for these metals would be essentially the same as for steel when the lower bound of energy demand is used.
Therefore, choices of material source can eliminate the disadvantage of these metals associated with high energy intensity during materials production, although such choices are often limited by quality or process constraints, they conclude.
We note that the substantial database of LCA studies shows that all the lightweight materials discussed here have a significant potential of reducing life cycle energy when avoiding energy-intensive material production processes. In particular...using aluminum, glass-fiber reinforced plastic, and high strength steel to replace conventional steel was found to significantly decrease the vehicle life cycle energy use in all studies after harmonization to a common set of assumptions. The same conclusions are drawn for GHG emissions after normalization and harmonization...as fossil fuels are the dominant energy source for vehicle operation in the reviewed studies. Further investigations would be needed to reduce uncertainties around the life cycle environmental benefits of using magnesium alloys and carbon-fiber reinforced plastics in vehicles.
The current harmonization shows that whether lightweighting reduces life cycle energy demand and GHG emissions depends on material source and process type or inventory data source for these materials. Given the flexibility in options implied by the variety of materials available and the broad consensus that they have energy and emissions benefits...we conclude that lightweight materials (particularly Al, GFRP, and HSS) are likely to see increased use in automobiles.—Kim and Wallington
Hyung Chul Kim and Timothy J. Wallington (2013) Life-Cycle Energy and Greenhouse Gas Emission Benefits of Lightweighting in Automobiles: Review and Harmonization. Environmental Science & Technology doi: 10.1021/es3042115