Impacts of Vehicle Weight Reduction via Material ... - ACS Publications

Sep 22, 2015 - This study examines the vehicle-cycle and vehicle total life-cycle impacts of substituting lightweight materials into vehicles. We dete...
0 downloads 8 Views 934KB Size
Download PDF
Article pubs.acs.org/est

Impacts of Vehicle Weight Reduction via Material Substitution on Life-Cycle Greenhouse Gas Emissions Jarod C. Kelly,* John L. Sullivan, Andrew Burnham, and Amgad Elgowainy Systems Assessment Group, Energy Systems Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: This study examines the vehicle-cycle and vehicle total life-cycle impacts of substituting lightweight materials into vehicles. We determine part-based greenhouse gas (GHG) emission ratios by collecting material substitution data and evaluating that alongside known mass-based GHG ratios (using and updating Argonne National Laboratory’s GREET model) associated with material pair substitutions. Several vehicle parts are lightweighted via material substitution, using substitution ratios from a U.S. Department of Energy report, to determine GHG emissions. We then examine fuelcycle GHG reductions from lightweighting. The fuel reduction value methodology is applied using FRV estimates of 0.15− 0.25, and 0.25−0.5 L/(100km·100 kg), with and without powertrain adjustments, respectively. GHG breakeven values are derived for both driving distance and material substitution ratio. While material substitution can reduce vehicle weight, it often increases vehicle-cycle GHGs. It is likely that replacing steel (the dominant vehicle material) with wrought aluminum, carbon fiber reinforced plastic (CRFP), or magnesium will increase vehiclecycle GHGs. However, lifetime fuel economy benefits often outweigh the vehicle-cycle, resulting in a net total life-cycle GHG benefit. This is the case for steel replaced by wrought aluminum in all assumed cases, and for CFRP and magnesium except for high substitution ratio and low FRV. conventional spark-ignition vehicles.14−17 Improvements to conventional vehicle powertrain efficiency, through direct injection, turbocharging, variable camshaft time, and automatic and manual transmissions with six and more gears, have also gained traction within the automotive market. Shares of these technologies in new vehicles are increasing rapidly.13 A recent report estimates that fuel consumption of conventional sparkignition vehicles could decrease by between 10% (low uncertainty) and 42% (high uncertainty) by 2045.18 Beyond the powertrain, automakers have sought to use lightweight materials and reduce aerodynamic drag and vehicle rolling resistance as ways to improve overall vehicle fuel efficiency.13 Lightweighting can be achieved by making a smaller vehicle, but automotive makers could instead reduce weight through material substitution, coupled with vehicle component redesign, while maintaining vehicle size (occupant and cargo capacity). This allows them to deliver comparably sized vehicles to consumers, satisfying consumer demand while achieving fuel economy requirements19 The new lightweight aluminum intensive Ford F-150, and the aluminum intensive Tesla Model S, have performed well in safety tests, and a

1. INTRODUCTION Increased concerns regarding U.S. dependence on foreign oil and global climate change have led to efforts to reduce fossil fuel consumption.1−6 In 2012, transportation accounted for 28% of consumed energy in the U.S., and light-duty vehicles comprised 59% of that, accounting for nearly 9% of the world’s (and 42% of U.S.’s) annual petroleum consumption.7 Increased fuel efficiency efforts could lead to reductions in petroleum consumption. In light of this, new Corporate Average Fuel Economy (CAFE) regulations require automakers to raise the average fuel economy of passenger vehicles to 35.5 miles per gallon gasoline equivalent (mpgge) by 2016 and to 54.5 mpgge by 2025 (6.63 and 4.32 L/100 km, respectively).8,9 Increased fuel economy will be achieved through a combination of reductions in vehicle size, weight, aerodynamic drag, and rolling resistance and through increased powertrain efficiency. The use of alternative fuels, such as biofuels, natural gas, hydrogen, and electricity, can also reduce well-to-wheels (WTW) GHG emissions.10−12 New powertrains and improvements to conventional powertrains have been used to reduce fuel consumption.13 Vehicle hybridization has led to successful deployments as both grid-dependent (Chevrolet Volt, etc.) and grid-independent (Toyota Prius, etc.) variants, while fully electric vehicles have also been released (Nissan Leaf, etc.). Analysis has shown that plug-in electric vehicles have lower WTW fossil fuel consumption and GHG emissions than © 2015 American Chemical Society

Received: Revised: Accepted: Published: 12535

February 18, 2015 September 15, 2015 September 22, 2015 September 22, 2015 DOI: 10.1021/acs.est.5b03192 Environ. Sci. Technol. 2015, 49, 12535−12542

Article

Environmental Science & Technology

developed ISO 14040, which describes the principles and framework of LCA.43 Those are incorporated within GREET, which includes life-cycle inventory data for numerous materials and fuels and provides tools for determining the life-cycle impacts of fuel/vehicle systems.32,41,44 This study uses GREET for energy and emissions data and modifies vehicle models within GREET to explore material substitution effects. This study focuses on the impact of material substitution on the vehicle cycle, using government reports, lightweighting studies, the automotive design literature, and discussions with automotive design experts to obtain likely material substitution ratios.22,36−40 This study is not an exercise in vehicle design; rather it is an environmental analysis of vehicle lightweighting options. Material substitution scenarios based solely on density and strength considerations, while directionally informative, are insufficient from a vehicle design perspective, which must consider issues such as crash test requirements, manufacturability, joining, and more. This study focuses on material substitutions that have engineering decisions and expert opinion as their basis, but it neither attempts nor purports to be a design exercise. 2.1. Life-Cycle Formulation. The total life-cycle burden (Btot) of any product consists of contributions from all stages during its useful “life,” from raw material extraction through use to end of life treatment. We write Btot = Bmp + Bmfg + Bop + Bmnt + Beol (1)

content analysis of automotive reviews suggests that mass reduction was perceived as positive, but it is possible that lightweight material vehicles may not be accepted by consumers.20,21 Manufacturers are likely to pursue all methods available to them, in the most cost-effective manner, to comply with fuel economy mandates. In this Article, vehicle lightweighting is examined using lifecycle assessment (LCA) to determine potential environmental consequences of changing vehicle materials and the associated impacts on fuel economy. The vehicle total life-cycle consists of the vehicle cycle (all processes related to vehicle manufacturing) and the fuel cycle (all processes related to fuel consumption). This article pays special attention to the vehicle cycle. Several studies have explored the potential for reduction in total life-cycle energy and GHG emissions through vehicle lightweighting.22−31 They show that for material substitution there are trade-offs between the vehicle-cycle and fuel-cycle stages of the life-cycle, and that the benefit of weight reduction via material substitution decreases as powertrain efficiency increases. This study extends that work, deriving and enumerating breakeven GHG distances and substitution ratios for several material pairs. The greatest potential for life-cycle savings arises from vehicle petroleum consumption reductions. Petroleum consumption contributes 80−90% of a conventional vehicle’s lifecycle GHG emissions, when considering the fuel’s well-topump and combustion phases (this may change with new technologies).32,33 However, it is important to consider the entire life-cycle of the vehicle, including the potential shifts in GHG burdens associated with lightweight materials, such as aluminum, magnesium, carbon fiber reinforced plastics (CFRP), and high-strength and advanced high-strength steels (HSS and AHSS). Many lightweight materials have increased GHG burdens compared to conventional vehicle materials.10 Further, a recent study highlights the regional variability of GHG emissions for electricity-intensive material production, such as aluminum.34,35 Of concern, then, is whether or not increases in vehicle-cycle burdens are outweighed by reductions in the fuel cycle. This article examines the total life-cycle GHG emissions implications of vehicle weight reduction through the use of material substitution, and uses the fuel reduction value methodology to account for reduced fuel consumption from reduced weight. It provides an extensive literature collection of substitution ratios for numerous material pairs,36−40,22 and applies both a part- and system-level approach to determine vehicle cycle GHG impacts associated with material substitutions. Ultimately, material substitution ratios obtained from a recent U.S. Department of Energy report are used within the analysis.38 GHG emissions for the vehicle cycle are from Argonne National Laboratory’s GREET (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) model (version 2014).41,42 Finally, this article extends current understanding by presenting a detailed mathematical examination of the underlying substitution equations, providing resolved breakeven equations for driven distance and material pair substitution ratio, along with detailed discussion of each variable’s influence.

where burdens are Bmp for material production, Bmfg for part manufacturing and vehicle assembly, Bop for vehicle operation, Bmnt for vehicle maintenance and repair, and Beol for vehicle end of life. B stands for any life-cycle burden. Here the burden of interest is GHG emissions. The vehicle’s total life-cycle burden is broken into vehiclecycle and fuel-cycle burdens. Vehicle-cycle burdens consists of all burdens associated with manufacturing, maintaining, and retiring the vehicle (= Bmp + Bmfg + Bmnt + Beol), while the fuelcycle burden consists of all burdens associated with the production and use of fuel (= Bop). The fuel-cycle burden for an internal combustion engine vehicle (ICEV) is directly impacted by the vehicle’s fuel economy. GREET provides a platform for analyzing changes to the vehicle-cycle burden through vehicle system material composition. Here, we make the distinction between the terms part, component, and system. Consistent with GREET, a part is a small vehicle element (e.g., a steering knuckle), while a component is a collection of parts (e.g., front suspension). Finally, a system is a collection of many components (e.g., chassis). One means of quantifying the effectiveness of weight reduction for reducing life-cycle burdens is to examine the change in Btot because of a weight change via material substitution, that is, replacing material β with material α. We write this as ΔBtot ≅ ΔBmp + ΔBop

(2)

ΔBtot is written as approximately the sum of material production and vehicle operational burdens because, while there are changes in Bmfg, Bmnt, and Beol, they are much smaller in magnitude than the terms in eq 2, this is especially true for GHG emissions burdens.22,45 This assumption is consistent with that from Keoleian and Sullivan.45 Bop depends on numerous factors including fuel production efficiency, powertrain efficiency, vehicle mass, aerodynamics, accessory loads, and lifetime vehicle driven distance (dlifetime) driven. Bmp

2. METHODOLOGY Life-cycle assessment has been effectively used in many fields to quantify the environmental impacts of products and processes. The International Organization for Standardization (ISO) 12536

DOI: 10.1021/acs.est.5b03192 Environ. Sci. Technol. 2015, 49, 12535−12542

Article

Environmental Science & Technology

(2) FRV* denotes a reduction value for weight change and powertrain adjustment to maintain the same performance. A discussion of typical values for these two FRVs can be found elsewhere, but they generally range between 0.15 and 0.5 L/ (100km·100 kg).30,45,47,48 In this study, we estimate an FRV range of 0.15−0.25 L/(100km·100 kg), and an FRV* range of 0.25−0.5 L/(100km·100 kg) to be consistent with literature.47,48 Clearly, larger values imply larger fuel savings during vehicle operation. This is utilized to compute the operational term, ((ΔBop)/(ΔP)) = FRVbgasolined, where bgasoline is the burden associated with gasoline (assumed motive fuel) and d is the vehicle’s driven distance. 2.2. Materials and Substitution Ratios. Many potential lightweight materials have been identified in the literature and are examined here along with the material they replace. Table S1 presents material properties, along with the energy use and GHG emissions, bi′, associated with their production, as defined in GREET.39,41,49 GHG emissions ratios can be determined on a mass basis for each material substitution pair (i.e., simple division), as shown in Table S2 (Table S3 presents energy use ratios). The SF6 value for magnesium in GREET was set to zero in this study; magnesium suppliers are moving away from SF6 as a cover gas because of GHG potency, but the uncorrected value is also presented in Table S2. Also, GREET does not differentiate between steel, HSS, and AHSS with respect to energy use or GHG emission intensity because industry suggests that the life-cycle burdens for those materials are comparable;50 the same approach has been used by Geyer.51 It is generally true that mass-based GHG emissions ratios indicate a penalty during the material production stage of a vehicle when using lightweight materials in place of conventional materials.10 However, GHG ratios are not the same as material substitution ratios, fβαj, in vehicle parts. Those ratios are simply the mass ratio of the new material α to the old material β for part j. For example, 0.7 kg of aluminum might replace 1 kg of steel on a fender, yielding a substitution ratio of 0.7 kg/kg. Material properties can be used with fβαj for different materials in vehicle parts, allowing for part-based, not massbased, GHG emissions comparisons. Figure 1 shows the range of fβαj examined in this study covering major vehicle systems (also available in Table S4). Data were gathered from numerous sources including light-

depends on the amounts and types of materials in the vehicle in question. Bmp = ∑b′i mi where b′i is the specific burden for producing a kg of material i (e.g., the burden represented by X kg CO2/kg of the material) and mi is the mass of that material. We rewrite eq 2 as b′βj

ΔBtot =

∑ j

Cβj

bα′ j

− fβαj C

1 − fβαj

αj

Δmj +

ΔBop ΔP

ΔP (3)

where j is the index for a material substitution on part j, fβαj is the mass ratio of substituted material α over replaced material β on part j, cβj and cαj are production efficiencies for transforming materials β and α into part j, P is total vehicle mass, Δmj is the change in mass of part j after substitution, and ΔP = ∑Δmj. Also, Δmj = mβj − mαj = mβj(1 − fβαj). The indices for fβαj denote that for any given pair of replaced and substituted materials, the substitution ratio can vary from part to part. A detailed representation of eq 3 for GHG emissions has been presented by Geyer.9 His expression is general and includes terms for substitutions of multiple materials on the vehicle, for secondary weight changes, and for factors representing the impacts of end-of-life recycling of vehicle materials on future values of b′i . Two common methods to handle recycled materials in lifecycle assessments are the “avoided burden” (end-of-life) approach and the “cut-off” (recycled content) approach. A more complete discussion of them appears elsewhere.46 Briefly, the former gives an end-of-life credit of primary material burdens in proportion to the amount of material recycled at a product’s end of life; the latter accounts for life-cycle burdens in proportion to the amount of recycled materials used during manufacture. The avoided burden approach assumes that a recycled material, at product end of life, will displace primary production in the future. The recycled content approach does not anticipate the impact of material recycled today on future (primary) production, but accounts for use of recycled materials at production. Recycled materials generally have lower environmental burdens at product manufacture. GREET uses the recycled content approach, which is employed in this study. This study focuses on primary weight reductions and does not consider secondary weight reductions. Because secondary weight reductions referred to in life-cycle circles tend to be highly speculative, we have chosen to omit them while acknowledging that they can further reduce life-cycle GHG emissions. Values for the operational term, ΔBop/ΔP, in eq 3, often referred to as the fuel reduction value, depend on the conditions under which the determinations were made. There are two basic conditions under which ΔBop/ΔP is computed: (1) for a change in vehicle weight and (2) for a change in both vehicle weight and the powertrain, with the latter being an adjustment to return the lightweighted vehicle’s performance (e.g., 0−60 mph times, t0−60) to its original value. Because making a vehicle lighter while all other factors (e.g., powertrain) remain constant reduces its t0−60, vehicle manufacturers sometimes adjust the powertrain to return the vehicle’s t0−60 to its original value. This results in additional fuel savings. Hence, fuel reduction value estimates determined at constant performance are larger than estimates only based on weight reduction. We use the following nomenclature: (1) FRV denotes the fuel reduction value for a weight change only and

Figure 1. Substitution ratios for various material pairs classified by vehicle system. Data derived from (1) EPA,8 (2) Singh,27 and (3) calculated from Malen35 (see Tables S5−S11), (4) DOE,33 (5) Sullivan and Hu,15 (6) Geyer,34 and (7) automotive expert opinions. 12537

DOI: 10.1021/acs.est.5b03192 Environ. Sci. Technol. 2015, 49, 12535−12542

Article

Environmental Science & Technology

and chassis are lightweighted via material substitution, as described in Table S13, but only replacing a specified percentage of the base material with the lightweight material. This approach is used, as opposed to replacing all of the base material, because it is unlikely that 100% material replacement is feasible across an entire system. Finally, the fuel cycle is considered in depth alongside the vehicle cycle in a life-cycle GHG examination. The five parts identified in Table S12 are obtained from a National Highway Traffic Safety Administration study on lightweighting light-duty vehicles.36 The baseline vehicle described in that report identifies the listed parts and their materials. Likely substitution materials were chosen for each part, and the fβαj of each material pair is based on suggested DOE ranges.38 The baseline vehicle material composition within GREET was modified using part weights and fβαj, and GREET was used to obtain resulting vehicle-cycle GHG emissions. The system-level material substitution process was similar to the part substitution procedure. Baseline materials were identified within the body and chassis along with substitution materials. The substitution ratios for each material pair (Table S13) were from DOE.38 The percent of baseline material replaced by a lightweight material was explored parametrically. The baseline GREET vehicle was modified for each scenario to obtain the vehicle-cycle GHG emissions for material substitution. Finally, while we are most concerned with the impact of lightweighting on the vehicle cycle within this study, it is important to recognize that life-cycle GHG benefits of lightweighting are sensibly the sum of two contributions, namely, ΔBmp and ΔBop. We use the baseline fuel consumption information for the GREET vehicle, and then use the FRV and FRV* ranges described earlier to obtain operational fuel savings. We convert fuel savings into GHG savings, ΔBop, by using GREET fuel-cycle GHG intensities for gasoline (10 881 g/gallon, 27 845 g CO2e/L, inclusive of well-to-pump and operation). We finish by providing an extended mathematical treatment of the total life cycle.

weighting studies, industry experts, and automotive design texts22,36,37,39,40 The range shown for each material pair in Figure 1 as a dashed outline is inclusive of all ratios found in the literature and also of material substitution ratio ranges presented in a recent U.S. Department of Energy (DOE) vehicle lightweighting workshop,38 which are shown in solid outline. Because of the level of industry expertise present at that DOE workshop we consider those ranges as the most representative of actual practice, and we employ them throughout this study. For material pairs without a DOE estimation, the mean and standard deviation for that pair are used. The DOE fβα ranges used in this analysis are in good agreement with the values obtained via other literature sources.22,36,37,39,40 These DOE values were provided and agreed upon by a collection of automotive and materials industry experts, but they did not provide details regarding their assumption for these values, or about the accuracy of the high and low values (the midpoint value here, was derived from the high and low value provided). We use these values to inform broad-based analyses of material substitution within vehicles. Figure 2 shows the range of GHG ratios for material pairs based on Table S1 combined with fβαj in Figure 1 (i.e., GHGβα

Figure 2. GHG emission ratios on an application basis, a product of material GHG emissions with corresponding substitution ratios. Data derived from (1) EPA,8 (2) Singh,27 (3) calculated from Malen35 (see Tables S5−S11, (4) DOE,33 (5) Sullivan and Hu,15 (6) Geyer,34 and (7) automotive expert opinions.

3. RESULTS AND DISCUSSION We examine the impact of vehicle lightweighting via material substitution on vehicle-cycle GHG emissions. The results of lightweighting several vehicle parts are presented, then a system-level analysis is discussed, and, finally, use-phase benefits based on FRV/FRV* are shown. The functional unit within this examination changes as we expand the scope of analysis from the part (kg CO2e/part) to the system (kg CO2e/system) to the full vehicle lifetime (kg CO2e/260,000 km vehicle lifetime). The first two focus only on the vehicle cycle and are specific to the parts and systems being replaced while the last one evaluates the vehicle total life-cycle. Part-based material substitution results are presented in Figure 3. Five parts (engine block, door frame, IP beam, rear kframe, and front steering knuckles) are examined from three different systems (powertrain, body, and chassis). The engine block is constructed of cast aluminum and lightweighted with magnesium. Examining Figure 3, on the far left we see the weight of the original cast iron engine block, and, to its right, the new engine block weight for magnesium. The bar representing the new magnesium engine block weight uses the midpoint fβαj value from the DOE range for the material pair, while the distribution range represents the corresponding

= GHGβ/GHGα × fβα for various parts j). This is reflective of the vehicle cycle only; we have yet to consider the operational stage. Results in the figure show a significant range within each pair; the high end could suggest which lightweight material substitutions might lead to increases in vehicle-cycle GHG burden. For instance, CFRP and magnesium offer the potential for substantial mass savings, as indicated by their small fβαj values. But, their GHG intensity suggests that those materials might increase GHGs associated with the vehicle cycle and possibly the entire vehicle life cycle. To determine the latter, an assessment of vehicle operation is also needed. 2.3. Lightweighting Studies. The material pairs above are used to examine the impact of lightweighting from case studies on vehicle-cycle GHG emissions. The 2014 release of GREET is used to conduct this analysis. The vehicle model denoted “Passenger Vehicle 2ICEV” is used as the baseline vehicle, against which various lightweight vehicle options are presented. This examination first focuses on lightweighting a few parts within the body, chassis, and engine systems based in Table S12. Next, to examine broader system-level benefits, the body 12538

DOI: 10.1021/acs.est.5b03192 Environ. Sci. Technol. 2015, 49, 12535−12542

Article

Environmental Science & Technology

Figure 3. Resulting part weights based on material substitution along with the change in GHG emissions. Original part material and weight are presented as the leftmost bar within each part set (labeled with an *). Distribution bars indicate the different substitution ratios for each material.

DOE fβαj range (see Table S14). Above that is the change in GHG emissions, ΔBmp, associated with each substitution. A value greater than zero is an increase in GHG emissions, while a less-than-zero value is a reduction. The central value represents ΔBmp for the midpoint fβαj while the range corresponds to the substitution range for that material pair. The remaining parts and materials examined in this substitution study are presented and grouped accordingly within the figure. For the parts shown, the door frames offer the greatest potential for lightweighting, because they have the highest initial weight. GHG emissions are increased most by replacing steel with CFRP, magnesium, and wrought aluminum. Using CFRP as the substitution material for the door frame results in the largest GHG increase. But, if we consider magnesium replacement, and assume that SF6 is used as a cover gas during its production,52 then the change in GHGs increases from 145 kg CO2e to 1300 kg CO2e for the midpoint substitution ratio, and overtake CFRP in GHG impact. Figure 4 presents the results of body system lightweighting, while Figure S1 presents results for chassis lightweighting (see also Table S15). These figures are similar to the part-basis examination of Figure 3. The leftmost bar in each major group in Figure 4 represents the system’s original weight for the material shown. The associated lightweight material bars show the amount of remaining base material in that component (bottom bar) along with the weight of the replacing material (top bar, distribution bars indicate substitution ratios, as before). Above each replacing material set is the ΔBmp associated with that material pair replacement. The line connecting those sets shows the impacts of increasing the percentage of replaced material. Due to steel’s dominance as a base material in the chassis, we present only those results in

Figure 4. Body System. Resulting component weights based on material substitution along with the change in GHG emissions. Original system material and weight are presented as the leftmost bar (labeled with an *). Distribution bars indicate the different substitution ratios for each material.

Figure 4. Results for other material pairs are available in Figures S1. Several trends are observed in Figures 3, 4, and S1, which are all based on data from GREET.41 First, steel affords a large potential for weight reduction within the vehicle. Second, replacing steel with HSS and AHSS yields both mass and vehicle-cycle GHG emissions reductions. This is due to essentially a source reduction, since HSS and AHSS GHG emissions intensities are equal to steel’s. Cast aluminum enables reductions in vehicle weight and vehicle-cycle GHG emissions, while wrought aluminum reduces weight but increases vehicle12539

DOI: 10.1021/acs.est.5b03192 Environ. Sci. Technol. 2015, 49, 12535−12542

Article

Environmental Science & Technology

causes a 66% reduction in breakeven driving distance. This differs depending upon material pair. Figure 5 presents the breakeven distance for different material pairs and substitution ratios, assuming the range of

cycle GHG emissions. The difference in vehicle-cycle GHG emission changes between cast and wrought aluminum is due to the high amount of recycled content used within cast aluminum vs the lower recycled content of wrought aluminum.41 The GHG burden for recycled cast and wrought aluminum is only 8−10% of their virgin counterparts. But, cast and wrought aluminum are often not suitable for the same applications. As with wrought aluminum, replacing steel with CFRP and magnesium allows appreciable weight reductions, but at the cost of increased vehicle-cycle GHG emissions. For context, the typical GHG emissions for the vehicle-cycle of a conventional gasoline powered vehicle is about 7500 kg CO2e. These results provide insight for decisions makers seeking to lightweight vehicles. They cannot be used for engineering design, but do provide guidance toward promising material pairs, both for weight and vehicle-cycle GHG reductions. They do not, however, provide a complete picture of the life-cycle benefits. eq 3 presents the change in life-cycle GHG emissions, showing that fuel cycle changes must also be accounted for over the vehicle lifetime (260 000 km, 161 557 miles). Setting ((ΔBop)/(ΔP)) = FRV*bgasolined, where FRV is an assumed fuel reduction value, bgasoline is the GHG burden of gasoline (10 881 g CO2e/gallon, 27 845 g CO2e/L), and d is a driven distance, and assuming that Cβj = Cαj = 1 and that j = 1 for convenience, eq 4 allows a deeper examination of how substitution ratios, mass, lifetime distance driven, and material GHG burdens influence the vehicle total life-cycle change in GHGs because of material substitution. ⎛ b′β − f bα′ ⎞ βα ΔBtot = ⎜⎜ + FRV ·bgasoline ·d⎟⎟Δm ⎝ 1 − fβα ⎠

Figure 5. Breakeven driving distance for different material substitution pairs and substitution ratios, assuming different FRV/FRV* values.

FRV and FRV* values from 0.15−0.25 and 0.25−0.5 L/(100 km·100 kg), respectively. Some material pair/substitution ratio combinations are inherently beneficial because they result in a lower total production burden; those are represented by a value of zero (e.g., steel to GFRP with substitution ratios of 0.65 and 0.7). Figure S2 (SI) shows additional material pairs. As substitution ratio decreases breakeven driving distance decreases, and as FRV/FRV* increases breakeven driving distance decreases. Finally, substitution ratio is examined by further rearranging eq 4 to determine a breakeven ratio (i.e., a substitution ratio at which there is no emissions difference between the baseline and lightweight material) as shown in eq 6.

(4)

It is first obvious that increasing d, driving distance, causes increased reductions in life-cycle GHG emissions over its original counterpart. And, increasing FRV directly correlates to increased GHG reductions. Additionally, increasing the magnitude of Δm, which is negative for a weight reduction, causes an increase in scale of both the material production phase of GHG emissions (which could be either an increase or decrease) and the operational phase emissions. The breakeven distance for any material pair is mass independent, as can be shown by setting eq 4 to zero. The result is shown in eq 5, and represents a generalized breakeven distance, dbreakeven, for lightweighting using a single material pair.

dbreakeven =

fβα

⎛ b′β − f bα′ ⎞ −⎜ 1 − fβα ⎟ ⎝ βα ⎠ FRV ·bgasoline

breakeven

=

b′β + FRV ·bgasolined bα′ + FRV ·bgasolined

(6)

If d = 0 in the above formulation, then the breakeven substitution ratio, fβα_breakeven, is the ratio of the emissions burden of the baseline material to the lightweighting material. The breakeven substitution ratio increases as driven distance increases. This basically means that the further a vehicle is expected to be driven, the higher the substitution ratio can be in order to achieve a GHG payback. Also, as FRV increases the breakeven substitution ratio increases. Figure 6 presents the breakeven substitution ratio for different material pairs with FRV = 0.15, FRV/FRV* = 0.25, FRV* = 0.5 L/(100 km·100 kg), and for driven distances of d = 0 km or d = 260k km. The results are overlaid on the literature substitution ranges presented in Figure 1. The breakeven substitution ratio for d = 0 km is the same for all FRV values, as can be seen from eq 6, and is presented by the red “X’ marker. This indicates that any substitution ratio less than that marker

(5)

According to eq 5 if bβ′ > fβαbα′ then material production GHG emissions are reduced via substitution, and dbreakeven is zero. If b′β < fβαb′α, then material production GHG emissions are increased, so dbreakeven is positive and that distance increases as fβα increases, meaning that higher substitution ratios (i.e., less benefit of weight reduction) implies longer breakeven distances. As FRV increases dbreakeven decreases. For the range of FRV and FRV* examined in this study, a change from the low FRV of 0.15 to the high FRV* of 0.5 L/(100 km·100 kg) shows a 70% reduction in breakeven driving distance for any given material pair. To put this in perspective, if we consider a change from steel to wrought aluminum, then changing from a substitution ratio of 0.7 to 0.55 (the high and midpoint ratios in this study) 12540

DOI: 10.1021/acs.est.5b03192 Environ. Sci. Technol. 2015, 49, 12535−12542

Article

Environmental Science & Technology Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This study was supported by the Vehicle Technologies Office of the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy under Contract Number DE-AC0206CH11357. We are grateful to Connie Bezanson and Jake Ward of that office for their guidance and support.

(1) Steffen, W.; Noble, I.; Canadell, J.; Apps, M.; Schulze, E.-D.; Jarvis, P. G.; et al. The terrestrial carbon cycle: implications for the Kyoto Protocol. Science 1998, 280 (5368), 1393−1394. (2) O’Neill, B. C.; Oppenheimer, M. Climate change: dangerous climate impacts and the Kyoto Protocol. Science 2002, 296 (5575), 1971−1972. (3) Susan, S. Climate Change 2007The Physical Science Basis, Working group I Contribution to the Fourth Assessment Report of the IPCC; Cambridge University Press: Cambridge, U.K., 2007; Vol. 4. (4) Ford, J. D.; Berrang-Ford, L.; Paterson, J. A systematic review of observed climate change adaptation in developed nations. Clim. Change 2011, 106 (2), 327−336. (5) IPCC. Climate Change 2013: The Physical Science Basis, Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change; Cambridge University Press: Cambridge, U.K., 2013. (6) Mashayekh, Y.; Jaramillo, P.; Samaras, C.; Hendrickson, C. T.; Blackhurst, M.; MacLean, H. L.; Matthews, H. S. Potentials for sustainable transportation in cities to alleviate climate change impacts. Environ. Sci. Technol. 2012, 46 (5), 2529−2537. (7) Davis, S. C.; Diegel S. W.; Boundy, R. G. Transportation Energy Data Book, edtion 33,ORNL-6990; Oak Ridge National Laboratory: Knoxville, TN, 2014. (8) U.S. Environmental Protection Agency. EPA and NHTSA Finalize Historic National Program to Reduce Greenhouse Gases and Improve Fuel Economy for Cars and Trucks, EPA-420-F-10-014; U.S. EPA: Washington, DC, 2010. (9) U.S. Environmental Protection Agency; U.S. National Highway Traffic Safety Administration. 2017 and Later Model Year Light-Duty Vehicle Greenhouse Gas Emissions and Corporate Average Fuel Economy Standards; U.S. EPA; U.S. NHTSA: Washington, DC, 2012. (10) Kim, H. C.; Wallington, T. J. Life-cycle energy and greenhouse gas emission benefits of lightweighting in automobiles: Review and harmonization. Environ. Sci. Technol. 2013, 47 (12), 6089−6097. (11) Cheah, L.; Heywood, J. Meeting US passenger vehicle fuel economy standards in 2016 and beyond. Energy Policy 2011, 39 (1), 454−466. (12) Kromer, M. A.; Heywood, J. B. A Comparative Assessment of Electric Propulsion Systems in the 2030 US Light-Duty Vehicle Fleet. SAE Int. J. Engines 2009, 1, 372−391. (13) U.S. Environmental Protection Agency. Light Duty Automotive Technology, Carbon Dioxide Emissions, and Fuel Economy Trends: 1975−2014, PA-420-R-14-023; U.S. EPA: Washington, DC, 2014. (14) Elgowainy, A.; Burnham, A.; Wang, M.; Molburg, J.; Rousseau, A. Well-to-Wheels Energy Use and Greenhouse Gas Emissions Analysis of Plug-in Hybrid Electric Vehicles, Report #ANL/ESD/09-2; Argonne National Laboratory: Lemont, IL, 2009. (15) Elgowainy, A.; Han, J.; Poch, L.; Wang, M.; Vyas, A.; Mahalik, M.; Rousseau, A. Well-to-Wheels Analysis of Energy Use and Greenhouse Gas Emissions of Plug-In Hybrid Electric Vehicles, ANL/ESD/10-1; Argonne National Laboratory: Lemont, IL, 2010. (16) Elgowainy, A.; Rousseau, A.; Wang, M.; Ruth, M.; Andress, D.; Ward, J.; Joseck, F.; Nguyen, T.; Das, S. Cost of ownership and wellto-wheels carbon emissions/oil use of alternative fuels and advanced light-duty vehicle technologies. Energy Sustainable Dev. 2013, 17 (6), 626−641.

Figure 6. Breakeven substitution ratios for different material substitution pairs, assuming different driving distances (d = 0 km, and 260k km) and FRV/FRV* values, compared against substitution values from literature.

yields a GHG emissions benefit over the life of the vehicle even if it is never driven. The yellow markers indicate d = 260k km driven for different FRVs. Increasing FRV increases the breakeven substitution ratio, below which a GHG benefit will be achieved over the course of a 260 000 km vehicle lifetime. Compared to the literature, many material pairs, such as cast iron to cast aluminum, indicate that the breakeven substitution ratios for d = 260k km is higher than, or within, the literature ratio ranges. However, we see that changing from cast aluminum to cast magnesium, for an FRV of 0.15 L/(100 km·100 kg), would require a substitution ratio below that which has thus been reported in the literature, indicating that the vehicle would need to be driven beyond the proposed 260 000 km to achieve breakeven GHGs for that FRV. This article examined the total life-cycle GHG emissions implications of vehicle weight reduction via material substitution and used the FRV methodology to account for fuel savings. It summarized substitution ratios for numerous material pairs, and used a ranged analysis to examine substitution at the part and system level to determine vehicle cycle GHG impacts. It also provided resolved breakeven equations for driven distance and material pair substitution ratio, which can quickly assist decision makers in identifying material substitutions consistent with their GHG goals.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b03192. Additional tables, figures, detailed derivations, and extended analysis (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 (630) 252-6579. Fax: +1 (630) 252-1342. 12541

DOI: 10.1021/acs.est.5b03192 Environ. Sci. Technol. 2015, 49, 12535−12542

Article

Environmental Science & Technology

(38) U.S. Department of Energy. Light-Duty Vehicles Technical Requirements and Gaps for Lightweight and Propulsion Materials; Workshop Report; U.S. Department of Energy, Vehicle Technologies Office: Washington, DC, 2013. (39) Malen, D. E. Fundamentals of Automobile Body Structure Design; SAE International: Warrendale, PA, 2011. (40) Geyer, R. Parametric assessment of climate change impacts of automotive material substitution. Environ. Sci. Technol. 2008, 42 (18), 6973−6979. (41) Argonne National Laboratory. GREET 2 (Vehicle-Cycle Model) https://greet.es.anl.gov/greet_2_series (accessed Feb 16, 2015). (42) Kelly, J.; Burnham, A.; Elgowainy, A.; Wang, M. Addition of New Conventional and Lightweight Vehicle Models in the GREET Model; Argonne National Laboratory: Lemont, IL, 2014. (43) Finkbeiner, M.; Inaba, A.; Tan, R.; Christiansen, K.; Klüppel, H.J. The new international standards for life cycle assessment: ISO 14040 and ISO 14044. Int. J. Life Cycle Assess. 2006, 11 (2), 80−85. (44) Wang, M. Q. GREET 1.5Transportation Fuel-Cycle Model Vol. 1: Methodology, Development, Use, and Results; ANL/ESD-39 Vol. 1; Argonne National Laboratory: Lemont, IL, 1999. (45) Keoleian, G. A.; Sullivan, J. L. Materials challenges and opportunities for enhancing the sustainability of automobiles. MRS Bull. 2012, 37 (04), 365−373. (46) Frischknecht, R. LCI modelling approaches applied on recycling of materials in view of environmental sustainability, risk perception and eco-efficiency. Int. J. Life Cycle Assess. 2010, 15 (7), 666−671. (47) Kim, H. C.; Wallington, T. J. Life Cycle Assessment of Vehicle Lightweighting: A Physics-Based Model of Mass-Induced Fuel Consumption. Environ. Sci. Technol. 2013, 47 (24), 14358−14366. (48) Kim, H. C.; Wallington, T. J.; Sullivan, J. L.; Keoleian, G. A. Life Cycle Assessment of Vehicle Lightweighting: Novel Mathematical Methods to Estimate Use-Phase Fuel Consumption. Environ. Sci. Technol. 2015, 49 (16), 10209−10216. (49) Norton, R. L. Material Properties. In Machine Design: An Integrated Approach; Prentice-Hall, Inc.: Upper Saddle River, NJ, 2000; p Appendix C. (50) WorldAutoSteel representative. Personal discussion, 2014. (51) Geyer, R. UCSB Automotive Materials GHG Comparison Model; WorldAutoSteel: Brussels, Belgium, 2013. (52) Sivertsen, L. K.; Haagensen, J. Ö .; Albright, D. A review of the life cycle environmental performance of automotive magnesium. SAE Tech. Pap. Ser. 2003, No. 2003-01-0641.

(17) Cai, H.; Wang, M.; Elgowainy, A.; Han, J. Life-Cycle Greenhouse Gas and Criteria Air Pollutant Emissions of Electric Vehicles in the United States. SAE Int. J. Alt Power 2013, 2 (2), 325− 336. (18) Moawad, A.; Sharer, P.; Rousseau, A. Light-Duty Vehicle Fuel Consumption Displacement Potential up to 2045; ANL/ESD/11-4; Argonne National Laboratory: Lemont, IL, 2013. (19) Whitefoot, K. S.; Skerlos, S. J. Design incentives to increase vehicle size created from the US footprint-based fuel economy standards. Energy Policy 2012, 41, 402−411. (20) 5-Star Safety Ratings|SafercarNational Highway Traffic Safety Administration (NHTSA). http://www.safercar.gov/Safety+Ratings (accessed Sep 15, 2015). (21) Helfand, G.; Revelt, J.-M.; Reichle, L.; Bolon, K.; McWilliams, M.; Sha, M.; Smith, A.; Beach, R. Searching for Hidden Costs: A Technology-Based Approach to the Energy Efficiency Gap in LightDuty Vehicles, Presented at the Association of Environmental and Resource Economists 4th Annual Summer Conference, San Diego, California, 2015. (22) Sullivan, J. L.; Hu, J. Life cycle energy analysis for automobiles. SAE Tech. Pap. Ser. 1995, No. 951829. (23) Kobayashi, O. Car life cycle inventory assessment. SAE Tech. Pap. Ser. 1997, No. 971199. (24) Schuckert, M.; Beddies, H.; Gediga, J.; Florin, H.; Eyerer, P.; Schweimer, G. W. Life cycle inventoriesNew experiences to save environmental loads and costs. SAE Tech. Pap. Ser. 1997, No. 971171. (25) Keoleian, G. A.; Spatari, S.; Beal, R. T.; Stephens, R. D.; Williams, R. L. Application of life cycle inventory analysis to fuel tank system design. Int. J. Life Cycle Assess. 1998, 3 (1), 18−28. (26) Das, S. The life-cycle impacts of aluminum body-in-white automotive material. JOM 2000, 52 (8), 41−44. (27) Schmidt, W.-P.; Dahlqvist, E.; Finkbeiner, M.; Krinke, S.; Lazzari, S.; Oschmann, D.; Pichon, S.; Thiel, C. Life cycle assessment of lightweight and end-of-life scenarios for generic compact class passenger vehicles. Int. J. Life Cycle Assess. 2004, 9 (6), 405−416. (28) Cheah, L. W. Cars on a Diet: The Material and Energy Impacts of Passenger Vehicle Weight Reduction in the US; Massachusetts Institute of Technology: Cambridge, MA, 2010. (29) Kim, H.-J.; McMillan, C.; Keoleian, G. A.; Skerlos, S. J. Greenhouse Gas Emissions Payback for Lightweighted Vehicles Using Aluminum and High-Strength Steel. J. Ind. Ecol. 2010, 14 (6), 929− 946. (30) Koffler, C.; Rohde-Brandenburger, K. On the calculation of fuel savings through lightweight design in automotive life cycle assessments. Int. J. Life Cycle Assess. 2010, 15 (1), 128−135. (31) Modaresi, R.; Pauliuk, S.; Løvik, A. N.; Müller, D. B. Global Carbon Benefits of Material Substitution in Passenger Cars until 2050 and the Impact on the Steel and Aluminum Industries. Environ. Sci. Technol. 2014, 48 (18), 10776−10784. (32) Burnham, A.; Wang, M. Q.; Wu, Y. Development and Applications of GREET 2.7−The Transportation Vehicle-Cycle Model; ANL/ESD/065; ANL (US); US Department of Energy: Washington, DC, 2006. (33) Chester, M. V.; Horvath, A. Environmental assessment of passenger transportation should include infrastructure and supply chains. Environ. Res. Lett. 2009, 4 (2), 024008. (34) Colett, J. S. Impacts of Geographic Variation on Aluminum Lightweighted Plug-in Hybrid Electric Vehicle Greenhouse Gas Emissions. Master’s thesis, University of Michigan, Ann Arbor, MI, 2013. (35) Colett, J. S.; Kelly, J. C.; Keoleian, G. A. Using Nested Average Electricity Allocation Protocols to Characterize Electrical Grids in LCA: A Case Study of U.S. Primary Aluminum Production. J. Ind. Ecol. 2015.10.1111/jiec.12268 (36) Singh, H. Mass Reduction for Light-Duty Vehicles for Model Years 2017−2025, Report No. DOT HS 811 666; U.S. Department of Transportation: Washington, DC, 2012. (37) U.S. Environmental Protection Agency. Light-Duty Vehicle Mass Reduction and Cost Analysis Midsize Crossover Utility Vehicle, EPA420-R-12-026; U.S. EPA: Washington, DC, 2012. 12542

DOI: 10.1021/acs.est.5b03192 Environ. Sci. Technol. 2015, 49, 12535−12542