ABSTRACT
Vehicle lightweighting strategies must deliver sustainable returns to customers and society. This work evaluates the sustainable return on investment (SROI) of lightweighted advanced high strength steel (AHSS) and carbon fiber reinforced polymer (CFRP)-intensive multimaterial bodies in white (BIWs) for automobiles. The SROI depends on the lightweighted BIW’s manufacturing cost and the difference in sustainable cost between a baseline (mild steel) BIW and the lightweighted alternative. The sustainable cost is the sum of the customer’s lifetime fuel (or electricity) costs and the costs of environmental externalities. A cradle-to-grave life cycle assessment (LCA) was conducted to quantify the environmental impacts of CFRP and AHSS BIWs in gasoline-fueled cars, bioethanol (E85)-fueled cars, and battery electric vehicles (BEVs) driven for a lifetime distance of 200 000 km. For cars fueled with gasoline- or corn-based bioethanol, the CFRP BIW yielded the lowest SROI; the AHSS BIW performed best for BEVs and cars fueled with wood bioethanol. However, the commercial availability of recycled carbon fiber should increase the SROI of the CFRP BIW in the future. Additionally, the SROI of CFRP BIWs is maximized when carbon fiber production is done using energy from a low carbon-intensity electric grid or decentralized sources such as waste-to energy incineration plants.
INTRODUCTION
Because the automotive sector is a major source of greenhouse gas (GHG) emissions, regulatory authorities around the world have introduced stringent emissions standards for new vehicles. For example, the European Union will require new passenger vehicles to have GHG emissions of no more than 95 g/km by 2021.1 Similarly, the United States Environmental Protection Agency (USEPA) and National Highway Traffic Safety Administration (NHTSA) jointly issued a standard that requires car makers to make significant fuel economy improvements in order to reduce fleetwide GHG emissions of US light duty vehicles.2 Although currently being reviewed by US policymakers,3 the standard requires a reduction of fleet-level GHG emissions from 225 g/mile (140 g/km) in the base year of 2016 to 143 g/mile (89 g/km) by 2025.2 Other countries have developed similar emissions targets.4 To improve the fleetwide fuel economy of passenger cars and satisfy new regulatory standards, automotive original equip- ment manufacturers (OEMs) have made significant R&D investments (US $100 billion/year),5,6 focusing particularly on the development of alternative powertrain technologies, fuels from renewable feedstocks (biofuels), and lightweight automotive components.
Unlike fuel-side initiatives (i.e., new powertrains and biofuels), lightweighting approaches can be easily integrated into traditional automotive material supply chains, and can be applied to any vehicle, irrespective of its powertrain and fuel. Steel parts account for ∼56% of the weight of a typical passenger vehicle;7 replacing them with lightweight alternatives can yield part-level weight savings of 10−70%7 and reduce fuel consumption by 6−42%.8 Materials commonly used in the design and construction of lightweight automotive components include advanced high strength steel (AHSS), aluminum alloys, and carbon fiber reinforced polymer (CFRP) composites. Replacing conventional mild steel automotive parts with lightweight alternatives can reduce vehicle weight by 10− 20% if the replacement parts are made from AHSS, 40% if they are made from aluminum, and 60% if they are made from CFRP.9 CFRP composite parts offer the greatest weight reduction potential and thus the greatest weight-related fuel savings in passenger vehicles.10,11
Metal parts are widely used in the automotive industry, but CFRP composites have not achieved comparable acceptance. Their limited use is partly due to their comparatively high manufacturing costs (CRFP composite parts for passenger vehicles cost around 20 times as much as steel equivalents12) and poor end-of-life recyclability. The high cost of manufactur- ing CFRP composite parts is largely due to (i) the high price of polyacrylonitrile (PAN), which is the main raw material used in CF synthesis, and (ii) the need to cure CFRP composite parts, which increases manufacturing cycle times (and thus costs). Considerable efforts are being made to reduce cycle times in CF manufacturing.13
The costliness of CFRP composite parts is also partly due to the poor circularity of carbon fiber in the technosphere. Today, CFRP composite scrap is typically disposed of by incineration with energy recovery. However, EU regulations require producers to adopt hierarchical waste management practices that prioritize reuse and recycling.14 The use of secondary carbon fiber with the strength and functional performance of virgin carbon fiber would reduce manufacturing costs and the need to synthesize virgin CF. Pyrolysis, solvolysis, and fluidized bed processes could all potentially be used to recover carbon fiber from CFRP composite scrap and thereby increase the circularity of CF without compromising its mechanical properties. These technologies could make recycled CF an inexpensive alternative to virgin CF,15,16 enabling large scale automotive utilization of CFRP composites.10
Previous life cycle assessment (LCA) studies have assessed the environmental performance of lightweight materials when used in individual components (i.e., structural parts such as the body in white (BIW), engine hood, chassis, cross-members, and front-end parts)17−23 and entire cars, examining both single-material and multimaterial designs.18,24−26 However, these studies only examined metal-based lightweighting materials. Although the environmental performance of CFRP composite parts has been studied extensively, few LCA studies in the open literature comprehensively assess the cradle-to- grave environmental performance of lightweighted CFRP auto parts. The scarcity of such assessments of CFRP automotive parts is partly due to the heterogeneity of CF production, which introduces considerable uncertainty into LCA models,27 and partly because OEMs prefer to maintain confidentiality regarding the formulation of CFRP composites. The few published LCAs for CFRP composite parts were restricted to cradle-to-gate assessments28 or evaluated a limited subset of impacts (cumulative energy demand and global warming potential) and only addressed the use of CFRP composite parts in internal combustion engine (ICE)-powered ve- hicles.18,29 There are thus no published assessments of the environmental performance of CFRP automotive components in vehicles with ICEs using renewable fuels (e.g., E-85) or alternate powertrains such as battery electric vehicles (BEV). This work is thus the first report describing the environmental performance of CFRP BIWs in bioethanol-driven ICEs and BEVs.
This study presents a cradle-to-grave LCA of a CFRP- intensive multimaterial BIW for a passenger vehicle. A BIW was chosen because it is the structural component that offers the greatest potential weight savings. The environmental performance of a CFRP BIW in gasoline-driven ICE vehicles was compared to that of a state-of-the-art mild steel (MS) BIW. In addition, scenario analyses were conducted to assess (a) how the environmental performance of the CFRP BIW would be improved if the circularity of CF increased, (b) how the environmental impact of the CFRP BIW compares to that of a BIW made from AHSS, and (c) the environmental performance of the CFRP BIW in BEVs and vehicles with ICEs using bioethanol (E85). The LCA results were used to estimate the sustainable return on investment (SROI) of AHSS and CFRP BIWs. We expect that the findings presented herein will help OEMs identify optimal material lightweighting strategies for passenger cars operating under different propulsion modes.
LIFE CYCLE ASSESSMENT METHODOLOGY
Goal and Scope Definition. The goal of this LCA is to quantify the cradle-to-grave environmental performance of a CFRP-intensive multimaterial BIW in a midsize passenger car and compare the results to those for a conventional mild steel BIW. It is assumed that the CFRP components of the BIW are manufactured in Sweden and that all other life cycle stages occur within Europe.
Functional Unit. The equivalent functional unit (FU) is a BIW for a compact car with a service life of 200 000 km traveled over 12 years. Some studies suggest that the mass of a CFRP BIW is 171 kg;30,31 others use a value of 139 kgs.32 The latter value is used here. Table 1 shows the CFRP BIW’s assumed material composition. The reference component, that is, the MS BIW, is assumed to have a mass of 280 kg,33 50% greater than that of the CFRP BIW.
Table 1. Weight and Material Composition of the CFRP and MS BIWs
System Boundary. Figure 1 shows the system boundary representing the cradle-to-grave life cycle stages of the CFRP- BIW. The life cycle is divided into five stages: (1) raw material acquisition; (2) manufacturing of the individual BIW components including the stamped steel, extruded aluminum, casted magnesium, and CF-reinforced-epoxy−resin (CFRP) parts; (3) assembly of the BIW and its integration into the passenger car; (4) the car’s use phase, with an assumed service life of 200 000 km; and (5) end-of-life management. In this final stage, the BIW is removed from the car and its metal and CFRP components are separated. The metal parts are shredded and recycled while the CFRP components undergo incineration with energy recovery.
Figure 1. Cradle-to-grave life cycle stages of a CFRP BIW for a lightweight passenger car (life cycle stages are indicated by numbers).
Table 2. Midpoint Impact Categories Chosen for the Study
The CFRP parts of the CFRP BIW are assumed to be sourced from Sweden, which has three major advantages as a supplier of CFRP parts. First, CF production from PAN is energy-intensive, requiring 116 MJ energy per kg CF synthesized.27 This is the single largest contributor to the environmental impact of manufacturing CFRP auto parts. High manufacturing stage impacts may dilute or even outweigh the use-stage environmental benefits of lightweighting. The low carbon intensity of Sweden’s electric grid makes it a good location for manufacturing CFRP components with low greenhouse gas (GHG) emissions.34 The overall impact of using electricity from the Swedish grid for CF production must however be evaluated. Second, Sweden offers good opportunities for symbiotic colocation of CF production sites with facilities such as waste to energy (WtE) plants; Sweden generated 2 TWh of electricity and 14.6 TWh of heat from WtE plants in 2014.35 Third, Sweden has a feedstock advantage because it produces large quantities of low energy- intensity materials (e.g., lignin) that can serve as precursors for PAN synthesis. European boundary conditions were assumed for all other life cycle stages.
Life Cycle Inventory (LCI) Data for CFRP-BIW. Foreground LCI data pertaining to the life cycle stages of the CFRP-BIW were obtained from the academic literature and reports produced by industry associations. The SimaPro LCA software package (version 8.2.0)36 and the Ecoinvent data- base37 were used to obtain background data for the life cycle model. LCI data for cold-rolled steel coils (including end of life credits) were obtained from the World Steel Association. LCI data for aluminum (Al) alloys and CFRP composites were obtained from the Ecoinvent database and the academic literature, respectively. The Swedish (SE) electrical grid’s composition was used to estimate the emissions due to the manufacturing of CFRP composite parts. Details of the LCI modeling process (i.e., the major assumptions made and the key data sources used to develop the LCI data) are provided in section S1 of the Supporting Information together with LCI data for individual BIW components (i.e., stamped steel, cast aluminum, and fabricated CFRP parts) and the MS and AHSS BIWs.
Life Cycle Inventory (LCI) Data for CFRP-BIW. Foreground LCI data pertaining to the life cycle stages of the CFRP-BIW were obtained from the academic literature and reports produced by industry associations. The SimaPro LCA software package (version 8.2.0)36 and the Ecoinvent data- base37 were used to obtain background data for the life cycle model. LCI data for cold-rolled steel coils (including end of life credits) were obtained from the World Steel Association. LCI data for aluminum (Al) alloys and CFRP composites were obtained from the Ecoinvent database and the academic literature, respectively. The Swedish (SE) electrical grid’s composition was used to estimate the emissions due to the manufacturing of CFRP composite parts. Details of the LCI modeling process (i.e., the major assumptions made and the key data sources used to develop the LCI data) are provided in section S1 of the Supporting Information together with LCI data for individual BIW components (i.e., stamped steel, cast aluminum, and fabricated CFRP parts) and the MS and AHSS BIWs.
Calculation of Mass-Induced Fuel Consumption for a Gasoline-Driven ICE. Use-stage fuel consumption values for the reference component (MS BIW) and the CFRP BIW were estimated using eqs 1 and 2.
Here FCSteelBIW and FCCFRPBIW are the fuel consumption of the specified component in liters; MIF is the mass-induces fuel consumption (0.27 L/(100 km × 100 kg)); FRV is the fuel reduction value with powertrain adaptation (0.32 L/(100 km× 100 kg)); WtSteelBIW is the weight of Steel BIW (280 kg); WtCFRPBIW is the weight of CFRP − Instensive multimaterial BIW (139 kg); and LD is the lifetime distance (200 000 km) Details of the fuel consumption calculations are provided in section S2 of the Supporting Information.
The mass-induced fuel consumption (MIF) (0.27 L/(100 km × 100 kg)) and fuel reduction value (FRV) (0.32 L/(100 km × 100 kg)) used here are based on the averages for six compact car variants.38 The fuel consumption of the reference (MS BIW) and lightweight (CFRP BIW) components was calculated using previously reported methods.39,40
Life Cycle Impact Assessment Methodology. Environmental impacts were quantified in terms of 10 midpoint categories shown in Table 2.
Uncertainty Analysis. The LCA results for the baseline CFRP BIW are sensitive to four key modeling parameters: (a) the difference in weight between the CFRP and MS BIWs, which is a key determinant of the environmental benefit of lightweighting-induced fuel savings; (b) the composition of the materials used to produce the CFRP BIW; (c) the processing conditions during fabrication of individual CFRP BIW components (e.g., material efficiencies); and (d) the FRV for the CFRP BIW. Variation of these key parameters introduces uncertainty in LCA results. Therefore, an uncertainty analysis was performed to determine how variation in four selected parameters (see Table 3) influenced the predicted overall environmental performance of the CFRP and MS BIWs. The analysis was conducted by performing Monte Carlo simulations in SimaPro with 5000 steps and a 95% confidence level.
Table 3. Key Modeling Parameters Varied in the Uncertainty Analysis
a The 280 kg lower limit was chosen because it is the value yielding a weight saving of 50%; the upper limit of 325 kg was chosen based on the literature30,31 and gives a 57% weight saving. CFRP components can potentially yield weight savings of 70%. bThe material efficiency (i.e., the proportion of material not lost as offcuts during cutting) for precured CF fabric is 89%44 (which was used as the baseline value); a conservative estimate of 80%45 was used as a lower bound for the uncertainty analysis. The postcuring material efficiency was taken to be 100% as a baseline, representing the best-case scenario (techniques such as resin infusion molding cause zero material waste46), and a lower bound of 80%45 was used in the uncertainty analysis. c0.21 is the FRV of a compact car without powertrain adaptation (worst case scenario).38,47
The energy consumed during CF production is the dominant contributor to the GWP of CFRP composites. Therefore, it was assumed that the CFRP composite parts would be manufactured in Sweden, the electrical grid of which has the lowest carbon intensity in Europe (46 g CO2/KWh).48 To further evaluate the potential GWP reduction achievable by lightweighting with CFRP, an additional uncertainty analysis was performed to assess the GWP impact of varying the source of electricity used in CF synthesis and CFRP composite production.
The energy consumed during CF production is the dominant contributor to the GWP of CFRP composites. Therefore, it was assumed that the CFRP composite parts would be manufactured in Sweden, the electrical grid of which has the lowest carbon intensity in Europe (46 g CO2/KWh).48 To further evaluate the potential GWP reduction achievable by lightweighting with CFRP, an additional uncertainty analysis was performed to assess the GWP impact of varying the source of electricity used in CF synthesis and CFRP composite production.
Scenario Analysis. The purpose of the scenario analysis was to compare the environmental performance of steel and CFRP BIWs under optimal conditions, that is, accounting for likely future developments in the metal and composite industries. The steel industry is strongly advocating the use of advanced high strength steel (AHSS) in structural parts such as BIWs. On the other hand, the composites industry is trying to improve the circularity of CF by exploring the potential to introduce recycled (secondary) CF into the market. CF can be recovered from postconsumer CFRP composite scrap by chemical or thermal treatment (solvolysis and pyrolysis, respectively) without significantly reducing its mechanical strength. The resulting recovered CF can be used to make structurally competent lightweight parts for diverse applications, reducing the need for virgin CF derived from the energyintensive precursor PAN.
Our LCA compared the environmental impacts of the AHSS and CFRP BIWs with postconsumer recycling (PCR) for passenger vehicles with conventional and alternative propulsion systems. The AHSS BIW was treated as the reference component in this scenario. The weight of the AHSS BIW is 235 kg, which is 16% lower than the typical weight of an MS BIW. The CFRP composite scrap was assumed to be recycled chemically by the solvolysis technique. LCI data for the solvolysis of CFRP composite scrap were obtained from the literature49,50 and are shown in section S3 of the Supporting Information. These data account for both the energetic cost of solvolysis and a credit for not using virgin CF synthesized from PAN. The proposed scenarios are summarized in Table 4.
When considering the impact of the lightweighted AHSS and CRFP BIWs in BEVs, the use-stage electricity consumption attributed to the BIWs was calculated as described by Kim and Wallington.47 In this case, the AHSS BIW served as the reference component.
where EAHSSBIW and ECRFPBIW denote the electricity consumption of respective components (KWh); MIFBEV is the mass induced fuel equivalent of the reference component; FRVBEV is the fuel reduction value equivalent of the lightweight component. For BEVs, MIFBEV and FRVBEV is taken as (0.05 Le/(100 km × 100 kg)) from Kim et al., study.47 WtAHSSBIW is the weight of steel BIW (235 kg); WtCRFPBIW is the weight of CFRP instensive multimaterial BIW (139 kg). The factor of 9.1 in these equations is used for unit conversion (1 L equiv petrol = 9.1 KWh). Details of the electricity consumption calculations are presented in section S2 of the Supporting Information.
Table 4. Scenarios Considered When Comparing the Environmental Performance of AHSS and CFRP BIWs with Postconsumer Recycling (PCR)
When considering the impact of the lightweighted AHSS and CRFP BIWs in BEVs, the use-stage electricity consumption attributed to the BIWs was calculated as described by Kim and Wallington.47 In this case, the AHSS BIW served as the reference component.
where EAHSSBIW and ECRFPBIW denote the electricity consumption of respective components (KWh); MIFBEV is the mass induced fuel equivalent of the reference component; FRVBEV is the fuel reduction value equivalent of the lightweight component. For BEVs, MIFBEV and FRVBEV is taken as (0.05 Le/(100 km × 100 kg)) from Kim et al., study.47 WtAHSSBIW is the weight of steel BIW (235 kg); WtCRFPBIW is the weight of CFRP instensive multimaterial BIW (139 kg). The factor of 9.1 in these equations is used for unit conversion (1 L equiv petrol = 9.1 KWh). Details of the electricity consumption calculations are presented in section S2 of the Supporting Information.
Figure 2. Environmental impacts of the MS BIW and the CRFP BIW in a gasoline-burning ICEV based on a cradle-to-grave LCA. Results are shown for 10 midpoint environmental impact categories. The maximum score for each category is shown at the top of the figure.
Sustainable Return on Investment (SROI) of Passenger Cars with Lightweighted BIWs. In financial terms, the return on investment (ROI) is the ratio of the net gains from an investment (i.e., the difference between the revenues due to the investment and the investment’s cost) to the investment’s cost. The SROI is a similar quantity that is used to evaluate an investment based on its sustainable returns, i.e. its benefits in terms of reducing both the cost to the consumer and the environmental externalities imposed on society. A transition from mild steel (MS) to lightweighted parts would be a major strategic move for automotive OEMs. Therefore, before the transition is made, it is essential to properly evaluate the SROI of the available lightweighting strategies.
An earlier assessment of the sustainability of replacing MS with lightweight materials determined the breakeven ratio,39 that is, the ratio at which the environmental impact (assessed in terms of GHG emissions) of a lightweighted solution is identical to that of the market incumbent. The time taken to reach this breakeven point is referred as the payback time.39,52 The payback time is shorter for lightweight material options such as AHSS;52 longer driving distances and/or higher FRVs are required for CFRP composites because of the environ- mental impact of their production. In this work, the SROI of CFRP lightweighting was estimated using a method inspired by the concept of the breakeven ratio. However, instead of calculating breakeven in terms of GHG emissions, the SROI metric uses cost as an indicator. Our decision to use SROI as an indicator metric was motivated by the expectation that it would help OEMs evaluate the sustainable returns of lightweighting solutions, allowing them to be reported in tandem with and compared to investment costs.
From a societal perspective, the benefits of lightweighted BIWs (or indeed any lightweighted car components) are 2- fold. First, customers benefit from lower driving costs due to fuel savings. Second, lightweighting can reduce environmental externalities by reducing greenhouse gas emissions and air pollution. The SROI accounts for these benefits and is calculated using eq 5.
CCC and CCCLWX represent the climate change cost of baseline and lightweight BIWs, respectively, and are given by the expression lifetime GHG emissions in tons × costs of climate change in € /ton GHG) for the relevant fuel.
APC and APCLWX represent the air pollution costs of baseline and lightweight BIWs, respectively, and are given by the expression (exhaust emissions in tons of PM2.5 during the vehicle’s use-stage × cost of PM2.5 in €/ton) + (nonexhaust emissions in tons of PM10 during the manufacturing and EOL stages of the BIW’s life cycle × cost of PM10 in €/ton) + (lifetime NMVOC emissions in tons × costs of NMVOC in €/ton) + (lifetime SO2 emissions in tons × costs of SO2 in €/ton) for the appropriate fuel.
For BIWs in BEVs, only nonexhaust emissions (in ton of PM10) were considered over the component’s life cycle (including the use stage).
Cost of BIW to OEM is the manufacturing cost of the BIW incurred by the OEM.
Figure 3. Results of a contribution analysis showing the environmental impacts of manufacturing individual CFRP BIW components as well as the impacts due to the use of the CRFP BIW in a gasoline-powered ICE and its end-of life processing by incineration with energy recovery.
RESULTS AND DISCUSSION
Baseline Scenario: MS-BIW vs CFRP-BIW in ICEVs. The CFRP-BIW outperformed the MS BIW with respect to 5 of the 10 chosen midpoint environmental impact categories (Figure 2). The life cycle impacts of the CFRP-BIW were 12−84% lower than those of the MS BIW with respect to HH-CP, HH- NCP, GWP, ODP, and CED. On the other hand, the life cycle FETP, AP, FEP, and PMFP impacts of the MS BIW were 30− 48% lower than those of the CFRP BIW.
Contribution Analysis of the CFRP-BIW. A contribution analysis of the CRFP-BIW was performed to identify hotspots for potential improvement at each stage of its life cycle (Figure 3). The manufacturing stage impacts were assessed by considering the contributions of five materials (mild steel, aluminum, thermoplastics, structural adhesives, and CFRP composites) used in the construction of a CFRP BIW.
During the manufacturing stage, mild steel, thermoplastics, and structural adhesive collectively contribute only 1−13% of the total impact. The environmental burden of the manufacturing stage is thus largely due to the fabrication of CFRP composites and the production of aluminum components.
The contributions of CFRP composite parts ranged from 28% to 58% in 7 out of 10 impact categories. These impact scores were mainly due to the synthesis of PAN-derived CF, which is a raw material for CFRP composite fabrication. CF synthesis is environmentally impactful for three reasons. Its AP impact is primarily due to air emissions from CF manufacturing facilities, particularly releases of ammonia.
The PMFP impact is also partly due to these releases of ammonia. Atmospheric ammonia concentrations correlate strongly with particulate matter concentrations53 and acid deposition.54 The use of advanced emissions control systems such as regenerative thermal oxidizers (RTO)55 as an abatement control strategy could significantly reduce the burden in these categories. The HH-NCP, ODP, and FEP impacts of CRFP composite parts were largely due to the composition of Sweden’s electrical grid. The CF and CFRP composites were assumed to be manufactured in Sweden to exploit the country’s low carbon-intensity grid. The Swedish electricity generation mix is dominated by hydro (42%) and nuclear (41%) power.56 The ODP impact is due to nuclear power plants specifically, the coolants used in uranium enrichment. The Ecoinvent data set37 for nuclear power production in Sweden uses global average values for enriched uranium as key raw material inputs, which is important because some coolants used for uranium enrichment in certain geographical locations are ozone-depleting (e.g., CFCs).57 Conversely, hydroelectric power is a major source of FEP and HH-NCP impacts. Hydroelectric power plants alter the nutrient budgets of surface water systems and may increase eutrophication.58 Hydropower may also exacerbate non- carcinogenic human health effects due to assimilation of neurotoxic pollutants such as methylmercury.59,60 CF manufacturing also accounted for 30% and 50% of the total GWP and CED impacts of the CRFP-BIW, respectively. The CED impact is due to the fossil energy used in PAN synthesis and the nuclear energy used in CF synthesis. The GWP of CF manufacturing was primarily due to fossil energy consumption during PAN synthesis.
Figure 4. (a) Uncertainty analysis for the MS and CFRP BIWs; (b) Results of a differential uncertainty analysis comparing the GWP impacts of steel and CRFP BIWs assuming CF and CRFP production using energy sourced from the energy grids of different countries (SE, Sweden; FR, France; FI, Finland; BE, Belgium; AT, Austria; EU, EU-27 Average; and UK).
Figure 4. (a) Uncertainty analysis for the MS and CFRP BIWs; (b) Results of a differential uncertainty analysis comparing the GWP impacts of steel and CRFP BIWs assuming CF and CRFP production using energy sourced from the energy grids of different countries (SE, Sweden; FR, France; FI, Finland; BE, Belgium; AT, Austria; EU, EU-27 Average; and UK).
Aluminum components also contributed significantly to the environmental burden of the manufacturing stage, accounting for 31−67% of the overall impact in the FEP, FETP, HH-NCP, and HH-CP impact categories. This was primarily due to the consumption of primary aluminum to produce wrought aluminum components (sheets and extrusions), which comprise 16% of the mass of a CFRP BIW. The HH-CP and HH-NCP impacts are attributed to waste streams from primary aluminum production plants. Red mud is a byproduct of bauxite ore processing that has been linked to both carcinogenic and noncarcinogenic (genotoxicity) risks in humans.61,62 Interestingly, however, the HH-CP and HH-NCP impacts of the CFRP BIW are 82% and 59% lower, respectively, than those of the market incumbent, that is, the MS-BIW (Figure 1). This was due to the impact of recycling of scrap steel at the end of life and particularly the disposal of slag from electric arc furnaces (EAF). The FEP impact was linked to the geography of material supply chains: much of the primary aluminum used in wrought components is sourced from China (China accounts for 56.3% of the world’s primary aluminum production according to the Ecoinvent, Rest of the World data set), which has a coal-intensive electrical grid. This is the main reason why aluminum components contribute 46% of the BIW’s FEP impact. The contribution of aluminum components is reduced to 41% if the primary aluminum is assumed to be sourced from the EU-27 and countries in the European Free Trade Association (EFTA). If the primary aluminum for wrought component production is sourced exclusively from Canada, the contribution of aluminum components to the total FEP impact of the BIW is reduced further still, to just 21%. Sourcing primary aluminum from countries other than China thus reduces the FEP impact of the CFRP BIW by between 10% and 40%.
Aluminum components are also responsible for 31% of the FETP impact. The cast aluminum parts of the BIW are made from AlMg3 alloy, whose aluminum consists of 20% primary and 80% secondary metal ingots. The FETP impact is due to the production of secondary aluminum, specifically the alloying additive (copper) used to prepare postconsumer aluminum scrap for melting.63 Studies on the production of secondary aluminum have demonstrated the ecotoxicity and ecotoxico- logical potency of copper.64−66 The end of life stage also accounts for 48% of the FETP burden, which was mainly attributed to the incineration of CFRP composite scrap and the recycling of steel parts in EAF.
The use stage impacts of the CRFP-BIW are significant (37−83%) for GWP, ODP, PMFP, POFP, and CED, and moderate for other impact categories (9−28%). The use stage accounts for 83% of the POFP impact. Despite the low manufacturing stage impact on POFP, the CRFP-BIW’s lifecycle impact marginally exceeds (by 4%) that of the MS BIW because POFP impact is engine-dependent (it relates to NOx formation due to incomplete combustion) rather than fuel-dependent. Lastly, as expected, the end of life credits for the CFRP-BIW in eight impact categories are small (<10%), highlighting the need for better methods of recycling CF from CFRP composite scrap.
Figure 5. Environmental impact values based on a cradle-to-grave LCA of AHSS-BIW and CFRP-BIW-PCR (using the Swedish electrical grid for CF manufacturing) and CFRP-BIW-PCR (using electricity obtained from a WtE plant).
Uncertainty Analysis of the Mild Steel and CFRP BIWs. Figure 4A shows the results of the uncertainty analysis for the MS and CFRP BIWs.
This analysis revealed some interesting trends in the environmental performance of MS and CFRP BIW. For the MS BIW, the HH-NCP, HH-CP, ODP, and FEP impacts varied significantly (by 66−79%). A contribution analysis for the MS-BIW (shown in section S4 of the Supporting Information) showed that scrap steel processing in EAF during the end of life stage was primarily responsible for its HH-NCP and HH-CP impacts. This implies that these impacts are sensitive to variation in the weight of the MS-BIW. The high uncertainty associated with the ODP impact was also attributed to variation in the weight of the MS BIW because heavier components increase gasoline consumption during the use stage. The variation in FEP was due to both the manufacturing and end of life stages. The FEP impact of manufacturing (which stems from the energetic cost of stamping) increases with the weight of the BIW but results in a correspondingly large end of life recycling credit. The CED varied by 45% upon varying the weight of the MS BIW, which was attributed to differences in weight-induced gasoline consumption during the use stage. The CED, FETP, FEP, and ODP impacts of the CFRP-BIW varied only modestly (by 23− 36%) compared to the variation in these impacts for the MS BIW. The predicted variation was attributed to variation in the material efficiency of CFRP composite parts and changes in fuel consumption based on the absence of powertrain adaptations (a worst-case scenario without such adaptations was considered in the uncertainty analysis). Overall, the results suggest that CFRP BIWs exhibit superior environmental performance to a degree that exceeds the uncertainty in the estimates with respect to GWP, HH-CP, and HH-NCP, whereas MS BIWs perform better with respect to PMFP, AP, and FETP. For the other studied impact categories, there was significant overlap between the results for the two BIWs, suggesting that the results obtained are highly sensitive to variations in the modeling parameters.
An additional differential uncertainty analysis was performed to assess the extent of variation in GWP when the electricity used to produce CF and CFRP composites originated from the grids of European countries other than Sweden. The results are shown in Figure 4B.
The carbon intensity of the Swedish electrical grid (46 g CO2/KWh) is lower than that of France (107 g CO2/KWh), Finland (240 g CO2/KWh), Belgium (259 g CO2/KWh), Austria (359 g CO2/KWh), the EU-27 average (482 g CO2/ KWh), and the UK (612 g/KWh). The results clearly show that the likelihood of CFRP BIW having a lower GWP impact than MS BIW decreases as the carbon intensity of the electricity used to manufacture CFRP composite parts increases. The results (Figure 4B) indicate that for a given set of LCA modeling conditions, the CFRP BIW should have a lower GWP impact than the MS BIW if the carbon intensity of the electricity used to produce CFRP parts is below 360 g CO2/KWh. Above this threshold, uncertainty increases and the MS BIW may have the lower GWP impact.
Figure 6. Environmental impact values based on a cradle-to-grave LCA of the AHSS-BIW and CFRP-BIW-PCR for an ICE fueled with E85 containing bioethanol produced from (A) Swedish woody biomass; and (B) corn from the USA. The maximum score in each impact category is listed at the top of the figure.
Scenario Analysis. The results obtained under the three scenarios summarized in Table 4 are discussed below.
Scenario 1: ICE Optimization. The environmental impacts of the AHSS BIW, the CFRP BIW-PCR(SE), and the CFRP BIW-PCR (WtE) are compared in Figure 5.
The absolute scores for the AHH BIW, CFRP BIW-PCR (SE) and CFRP-BIW-PCR(WtE) are lower than those for the corresponding baseline variants (i.e., the MS-BIW and CFRP- BIW with incineration). However, the trends in the impacts resemble those for the baseline cases: both CFRP BIW variants exhibit superior environmental performance with respect to GWP, CED, ODP, HH-CP, and HH-NCP, while the AHSS BIW performs better with respect to PMFP, POFP (by a small margin), AP, FEP, and FETP. A notable difference is that the FEP impact burden of CFRP-BIW-PCR is higher than that for incineration of CFRP composite scrap for energy recovery. This increase is due to (a) the use of acetic acid in solvolysis, because acetic acid is known to increase eutrophication,67 and (b) the energy burden of solvolysis. The use of a decentralized energy source (a WtE plant) reduced environmental impacts by 1−28% compared to using the Swedish grid, and had particularly beneficial effects on the CED, ODP, and HH-NCP impacts. The collocation of a CF production site with a WtE incineration plant would allow the former to use electricity generated by the latter for CF synthesis. This could be a good sustainable business model for CF manufacturing, especially in countries with highly carbon-intensive electrical grids.
Scenario 2: Alternative Fuel (ICE Burning E-85). As shown in Figures 6A,B, the life cycle impacts of the AHSS BIW and CFRP BIW-PCR were also evaluated under the assumption that they would be used in ICE-powered vehicles fueled with bioethanol from woody biomass and corn-based feedstocks.
The AHSS BIW exhibited superior overall environmental performance to the CFRP-BIW-PCR for E85-fueled cars when the bioethanol portion of E-85 was sourced from a woody biomass feedstock. Using the CFRP BIW in woody bioethanol-fueled automobiles would be environmentally unwise because it negates the climate change benefits of the CRFP and introduces significant trade-offs in other impact categories. The CFRP BIW-PCR had a lower impact on PMFP, FETP, AP, and FEP, suggesting a pattern resembling that seen with gasoline-fueled ICEs. As discussed in the contribution analysis, these impacts are primarily due to the manufacturing stage, so changing the fuel employed in the use stage is unlikely to affect them greatly. However, the CFRP BIW-PCR performed poorly with respect to ODP and POFP, which was not seen in the gasoline-fueled case. This is because the well-to-wheel environmental impacts of wood bioethanol are lower than those of gasoline. Consequently, the weight-induced fuel benefits (including GWP) of the lightweight CFRP BIW are lower for wood bioethanol than for gasoline.
Figure 7. Environmental impact values based on a cradle-to-grave LCA of the AHSS BIW and CFRP BIW-PCR for a BEV charged with electricity generated using a grid mix corresponding to the EU27 average. The maximum score in each impact category is listed at the top of the figure.
The use of CFRP composite lightweight parts in automobiles fueled with corn-based bioethanol presents an interesting value proposition. The CFRP BIW-PCR achieved better environmental performance than the AHSS BIW in all impact categories other than ODP and FETP in a vehicle running on E85 containing corn-derived bioethanol. The end of life stage of the AHSS BIW is responsible for its high HH-CP and HH-NCP impacts (which are due to slag disposal from EAF, as mentioned previously). However, its higher impacts in other categories are due to the environmental impact of corn bioethanol. The FEP impact of corn ethanol is particularly high because of fertilizer-laden agricultural runoff from cornfields. Unlike biofuels derived from woody biomass, biofuels derived from agricultural crops are often criticized because of their effects on eutrophication and terrestrial and freshwater ecotoxicity.68−70 Energy consumed during crop harvesting and drying also increases the GWP, AP, POFP, and PMFP impacts of the AHSS BIW in the corn bioethanol case; the AHSS BIW increases fuel consumption relative to the CFRP BIW-PCR and so exacerbates these impacts. Overall, the LCA results indicate that lightweighting with CFRP composites may be environmentally beneficial in regions (e.g., the USA) where corn bioethanol is a prominent ICE fuel.
Scenario 3: Alternate Powertrain BEV. The life cycle environmental impacts of the AHSS BIW and CFRP BIW- PCR for a BEV are shown in Figure 7.
For a BEV, the absolute impact scores of both the AHSS and CFRP BIW variants are lower than those for the ICE variants. However, the AHSS BIW has a clear environmental advantage over the CFRP BIW PCR in the BEV case. Because of the BEV’s low FRV, weight-induced energy savings do not provide significant impact reductions for BEVs as they do in gasoline- fueled ICEs. The high manufacturing impact of the CFRP BIW and the minimal benefits of lightweighting during the use stage explain the poor environmental performance of the CFRP BIW PCR in seven impact categories in the BEV case; it only outperforms the AHSS BIW with respect to HH-CP, HH- NCP, and (marginally) FEP.
The SROI of Lightweighted BIWs for Various Fuel Options. SROI values for replacing an MS BIW in a gasoline-fueled car with one of the lightweight BIWs considered in this work (and potentially also replacing the gasoline powertrain with a greener alternative) were determined using eq 5, yielding the results shown in Table 5. Manufacturing costs71 and social costs72,73 were calculated using literature data; details of the calculations are presented in section S5 of the Supporting Information
Table 5. SROI of Lightweighted BIWs (per Functional Unit)
The SROI values represent the sustainable returns (€) per euro spent by an OEM on manufacturing lightweight BIWs, and range from 0.54 to 3.13. These values indicate that sustainable returns are maximized by replacing an MS BIW with an AHSS BIW in ICEs fueled with woody biomass- derived bioethanol and BEVs. Conversely, a CFRP BIW is a superior replacement for passenger cars fueled with gasoline or corn bioethanol. Although the social costs of the CFRP BIW are lower than those of the AHSS BIW for both E85 variants, its SROI is lower because of its high manufacturing costs. However, commercialization of CFRP recycling technologies is expected to increase the SROI of the CFRP BIW when used with alternative fuels by increasing the usage of secondary CF and thus reducing manufacturing costs.
In summary, we have conducted a detailed cradle-to-grave LCA of AHSS and CFRP BIWs for three different vehicle propulsion modes. Four key insights were obtained. First, the CFRP BIW exhibits worse environmental performance than the MS BIW with respect to the PMFP, AP, FEP, and FETP impact categories. Its higher impact scores are predominantly due to the release of atmospheric pollutants such as ammonia during CF production, the electricity source (assumed to be the Swedish electric grid in this work) used in CF synthesis, and the alloying additives used to recycle postconsumer aluminum scrap.
Second, the GWP of the CFRP BIW was lower than that of the MS BIW for gasoline-fueled ICEs, which is encouraging from a climate change perspective. This was attributed to the source of electricity used for CF production. Sweden, with its low carbon-intensity electrical grid, should be a favored location for CF production. The likelihood of a CFRP BIW having a lower GWP than an MS BIW is highest for electrical grids with carbon intensities below 400 g CO2/KWh. Moreover, our scenario analysis showed that operating a CF production facility in symbiosis with a decentralized energy source such as a WtE plant would be more beneficial from a climate change perspective in countries where the carbon intensity of electricity is high.
Third, the AHSS BIW exhibited superior overall performance in cars fueled with woody biomass-derived bioethanol and BEVs, but the CFRP BIW performed better for vehicles fueled with corn bioethanol. This is mainly due to the high well-to- tank impact of corn bioethanol, which reduces the impact scores of the CFRP BIW because of its high fuel savings potential.
Finally, the SROI values calculated for the AHSS and CFRP BIWs under different propulsion modes ranged from 0.54 to 3.13. On the basis of these values, the AHSS BIW should be preferred for BEVs and the CFRP BIW for gasoline-fueled ICEs. For ICEs fueled with E85, the CFRP BIW should be generally preferred to the AHSS BIW if its manufacturing costs can be reduced sufficiently. This would require the availability of recycled CF with properties suitable for producing lightweight structural components or the development of a method for preparing CF from an inexpensive lignin-based precursor.
Source: Kavitha Shanmugam - Umeå University, Gadhamshetty Venkataramana - South Dakota School of Mines and Technology, Pooja Yadav - Swedish University of Agricultural Sciences, Dimitris Athanassiadis - Swedish University of Agricultural Sciences
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