Policy Analysis pubs.acs.org/est
Life-Cycle Energy and Greenhouse Gas Emission Benefits of Lightweighting in Automobiles: Review and Harmonization Hyung Chul Kim* and Timothy J. Wallington Systems Analytics and Environmental Sciences Department, Ford Motor Company, Mail Drop RIC-2122, Dearborn, Michigan 48121-2053, United States S Supporting Information *
ABSTRACT: Replacing conventional materials (steel and iron) with lighter alternatives (e.g., aluminum, magnesium, and composites) decreases energy consumption and greenhouse gas (GHG) emissions during vehicle use but may increase energy consumption and GHG emissions during vehicle production. There have been many life cycle assessment (LCA) studies on the benefits of vehicle lightweighting, but the wide variety of assumptions used makes it difficult to compare results from the studies. To clarify the benefits of vehicle lightweighting we have reviewed the available literature (43 studies). The GHG emissions and primary energy results from 33 studies that passed a screening process were harmonized 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). After harmonization, all studies indicate that using aluminum, glass-fiber reinforced plastic, and high strength steel to replace conventional steel decreases the vehicle life cycle energy use and GHG emissions. Given the flexibility in options implied by the variety of materials available and consensus that these materials have substantial energy and emissions benefits, it seems likely that lightweighting will be used increasingly to improve fuel economy and reduce life cycle GHG emissions from vehicles.
1. INTRODUCTION Energy security and climate change are long-term challenges. Approximately 93% of U.S. road transportation energy demand in 2011 was supplied by petroleum, and 45% of this petroleum was imported.1 The nearly complete dependence of a vital economic sector on a limited energy resource is clearly a source of concern. Climate change is caused by increasing levels of greenhouse gases (GHGs) in the Earth’s atmosphere resulting from human activities. CO2 released during fossil fuel combustion and deforestation is the largest contributor to radiative forcing of climate change. On a global basis, in 2009, road transportation was responsible for approximately 5 gigatonnes (Gt) of CO2 emissions2 which represents about 17% of the approximately 30 Gt total global fossil fuel CO2 emissions. The desire for improved energy security and reduced CO2 emissions has led to a substantial research effort to provide more sustainable road transportation. Increased use of lightweight materials together with the development of more energy efficient powertrain technologies (e.g., more efficient internal combustion engines, electric vehicles) and low-CO2 fuels (e.g., biofuels, hydrogen, wind and solar electricity) are critical elements in a transition to more sustainable road transportation. Using lightweight materials (e.g., aluminum, magnesium, or composites) to replace conventional materials (e.g., steel, iron) © 2013 American Chemical Society
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. Received: Revised: Accepted: Published: 6089
October 15, 2012 May 8, 2013 May 13, 2013 May 13, 2013 dx.doi.org/10.1021/es3042115 | Environ. Sci. Technol. 2013, 47, 6089−6097
Environmental Science & Technology
Policy Analysis
We reviewed a total of 43 LCA studies on lightweighting of vehicles in a comparative context. We screened out 10 studies which have a format that cannot be compared with other studies and studies that lack sufficient details to be reviewed in a comparative context. The 33 studies which passed the screening process and were subsequently normalized and harmonized are listed in Tables S1−2; 11 for total vehicle and 22 for vehicle components. The LCA modeling and parameters used in the 43 studies are listed in Table S3. We focus on primary energy benefits of replacing steel with lighter materials in our analysis. The complete analysis for GHG emissions is available in the Supporting Information.
2. OVERVIEW OF LIGHTWEIGHTING LCAS Our review of LCA results encompasses 33 studies and 119 scenarios: 11 studies of total vehicles with 40 lightweighting scenarios and 22 studies of vehicle components with 79 lightweighting scenarios. Of the 79 lightweighting scenarios, 70 scenarios address substitution of steel or iron used in body or engine with lighter materials, 5 scenarios compare aluminum with other materials for powertrain components, and 4 scenarios address mixed materials. 2.1. Component LCA. In the reviewed studies, the usephase 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%. To measure lightweighting effects, we normalized, i.e., divided the component weight, life-cycle energy and GHG emissions of the lightweight scenarios to those of baseline figures, thereby representing the lightweight parameters and results relative to the baseline, which is set to 1. Figure 1 (a) shows the correlation between the normalized primary energy demand and vehicle component weight in 51 of the 70 scenarios that use steel as baseline and report primary energy use; the remaining 19 scenarios report GHG emissions but not energy use. The lightweighting rate in the scenarios was evaluated up to ∼70%, which results in a similar rate of energy saving. As expected, the normalized use-phase primary energy (filled circles) has a strong linear relationship with normalized weight. However, for the entire component life cycle (open circles), the linear correlation is much weaker than for the usephase only (R2 = 0.3937 vs 0.912) with some LCA studies reporting that lightweighting leads to increased energy demand. For example, substitution of steel with 100% primary aluminum in a fender in the studies by Saur et al. (1997; 2000) was reported to increase the life cycle energy by 15% although it reduced the component mass by 50%.3,4 The scenarios using magnesium in a crossbeam by Kiefer et al. (1998) and carbonfiber reinforced plastics (CFRPs) in a floor pan by Das (2011) were reported to increase the life cycle energy by 15−40% and 3%, respectively.5,6 The irregularity in the component LCA results reflects differences in the assumptions across different models and differences in the inherent material properties. The harmonization procedure in the present study removes the effect of the different assumptions and provides a clearer basis on which to compare the results from the studies. 2.2. Vehicle LCA. In the total vehicle LCAs reviewed, 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
