Reinforced Wind Turbine Blades - An Environmental Life Cycle

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Reinforced Wind Turbine Blades - An Environmental Life Cycle Evaluation Laura Merugula,† Vikas Khanna,‡ and Bhavik R. Bakshi*,† †

William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, Ohio 43210, United States ‡ Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *

ABSTRACT: A fiberglass composite reinforced with carbon nanofibers (CNF) at the resin−fiber interface is being developed for potential use in wind turbine blades. An energy and midpoint impact assessment was performed to gauge impacts of scaling production to blades 40 m and longer. Higher loadings force trade-offs in energy return on investment and midpoint impacts relative to the base case while remaining superior to thermoelectric power generation in these indicators. Energy-intensive production of CNFs forces impacts disproportionate to mass contribution. The polymer nanocomposite increases a 2 MW plant’s global warming potential nearly 100% per kWh electricity generated with 5% CNF by mass in the blades if no increase in electrical output is realized. The relative scale of impact must be compensated by systematic improvements whether by deployment in higher potential zones or by increased life span; the trade-offs are expected to be significantly lessened with CNF manufacturing maturity. Significant challenges are faced in evaluating emerging technologies including uncertainty in future scenarios and process scaling. Inventories available for raw materials and monte carlos analysis have been used to gain insight to impacts of this development.

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INTRODUCTION With wind energy being among the most attractive renewable energy technologies, commercial wind turbines rated 1.5 to 3 MW dominate today’s market.1 Turbines rated 5 to 7 MW are available, one being LM Glasfiber’s LM 61.5 P model with rotor diameter of 126 m and blades 61.5 m from root to tip. Because blade material is one driving factor for growth in wind turbine size and penetration, technologies for weight reduction with preservation of stiffness and dampening properties are essential.1 Carbon fibers are expected to account for up to 50% of fiber in blades by 2025,1 but this may come with higher production and tooling costs. Denser glass fibers, exhibiting lower stiffness, are more economical with better elongation at breakage.2 Blade manufacturers integrate carbon fibers discriminately for local reinforcement to increase edgewise fatigue resistance and in the spar cap for improved bulk properties.3 Vestas and Gamesa use carbon fiber as reinforcement in the spars of all blades above critical lengths (near 40 m), whereas LM Glasfiber is focusing on reducing the need for carbon fiber due to cost, reliability, and supply challenges.4 © 2012 American Chemical Society

Vapor-grown carbon nanofibers (CNF) are attractive since they reinforce polymer resins at low mass percentages yielding plastics that are competitively low in weight and high in strength and stiffness. Adding carbon nanofibers to long fiber composites can prevent generation and propagation of cracks in the resin between the long fibers.5 As little as 1% CNF by weight has been shown to improve mode-I delamination resistance by 100%.6 Nonetheless, manufacture of CNFs is highly energy-intensive, where 13 to 50 times the amount of energy is required to make an equal mass of CNFs as primary aluminum.7 Therefore, it is not clear if a CNF-reinforced blade will indeed be better than other alternatives. Such evaluation requires a life cycle view. This life cycle assessment (LCA) incorporates the life cycle inventory (LCI) of CNF production into that of a multimegawatt wind energy converter (WEC). Very few LCIs for Received: Revised: Accepted: Published: 9785

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Whereas there is considerable diversity in the boundaries and input parameters of each of these LCAs, results are consistent in energy analysis and midpoint impacts. This indicates order of magnitude results that can be expected for environmental performance of modern wind turbine design and placement in the associated categories. Impacts such as noise and visual aesthetics are outside of the scope of this study. The article is organized into four parts. The introduction has laid out the premise of the study. The second section gives background of the material production and LCA approach. Then, results of the energy analysis and midpoint analysis are presented followed by discussion.

emerging nanoproducts exist. This article contributes to existing literature by assessing CNFs as integrated material for renewable wind power generation. The study takes into account that the polymer nanocomposite (PNC) may be an enabling technology leading to increased energy generation and/or deployment of WEC technology, a claim that will need to be supported with further research. Despite the challenges in data precision, life cycle studies at the design stage are imperative, especially in light of the observed pace of nanoproducts’ time-to-market. Among existing life cycle studies of WECs, two independent energy analyses of 1.5 MW turbines reported cumulative energy demand (CED) around 14 000 GJ/turbine for three siting scenarios,8,9 accounting for the life cycle primary energy demands for a single turbine. Despite similar CED, energy return on investment (EROI) and energy payback time (EPBT) varied with geographical site, country of installation and manufacture, and material primary energy sources. In 2002, a meta-analysis of energy and CO2 intensities for over 70 wind turbine systems indicated high variability in reported figures.10 Variability was assessed for influences as turbine size, country of manufacture, technology, maturity, and methodology. Upon normalization, reported mean energy intensity, an aggregate measure of kilowatt hours required for every kilowatt hour delivered (kWhin/kWhel), was reported as 0.063, and energy payback period was 5.2 months. CO2 intensity was reported from 7.9 to 123.7 g CO2/kWhel, where additional variability is exhibited with dependence on regions’ fuel mixes. The meta-analysis was extended in 2009 to a total of 119 turbines from 50 published studies between 1977 and 2007.11 Average operational EROI was 19.8, whereas the average EROI of all studies (both operational, or empirical, and conceptual) was 25.2. The study found average EROI generally increases with power ratings over 1 MW. LCAs of power generation covering a number of midpoint indicators show wind as favorable to thermoelectric generation in emissions as greenhouse gases, SO2, and NOx.12,13 The manufacturing phase is dominant for WEC life cycles with demonstrated sensitivity to recycling and disposal scenarios.12,14−19 Associated transmission grids are demonstrated to contribute less than 10% of impacts; impact per kWh of production is consistently sensitive to projections of life span and capacity factor.12 Offshore multimegawatt WECs generate more electricity in a lifespan, yet have higher resource demands hence higher CED.12,14 The only publicly available LCI for a utility-scale wind turbine, although not free, is via the Ecoinvent database.20 An accompanying report demonstrated global warming potential of 13 g CO2-eq/kWh for the 2 MW WEC.15 EPBT and EROI were not reported in the public document. The Ecoinvent inventory has previously been used as surrogate data for a 2 MW plant with modifications made to the inventory for known variables.16 Very few LCAs are available for systems larger than 3 MW,17−19 where the study in ref 17 is one of four studies of high capacity included in the meta-analysis referenced.11 CED values for a 4.5 MW installation and two 5 MW prototypes have been reported between 70 000 and 85 000 GJ/turbine. The reported ranges for primary energy payback time, CO2 intensity, and energy intensity are 4 to 7 months, 11.5 to 15.8 g CO2/kWh, and 0.054 to 0.083 kWhin/kWhel, respectively. Consistent to other LCAs, reported values show high dependency on capacity factor.

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BACKGROUND Blade Reinforcement Technology. Current megawatt scale blade production tends toward infusion molding and prepreg composites, the latter especially with carbon fiber use.12,14,21,22 Economical use of carbon fibers is achieved with prepreg technology because compressive properties decrease 30 to 45% alternatively with resin infusion.2−4,23 While allowing lay up of less material, carbon fiber has greater requirements than glass for avoiding processing inaccuracies, which impact mechanical properties.3 Premixing is the predominant method for CNF incorporation into polymer resin.2,5,24−29 It can take various forms (high sheer mixing, melt blending, and in situ polymerization 5) but increased resin viscosity inhibits scalable processing.24 A method termed prebinding uses acetone as a dispersing solvent for the nanofibers. Employing vacuum-assisted spraying, developers coat glass fiber mats with the solution; the solvent evaporates leaving a layer of CNFs on a glass mat, which proceeds to resin infusion processing following the same predictive flow model with adjustments in the permeability term.5,30 The result is improved reinforcement at the fiber− resin interface and improved manufacturability of the PNC. The material is a candidate for developments in wind turbine blades. Where premixing can lead to in-plane filtration and higher incidence of microvoids with increasing nanomaterial loading of fiber composites,6 prebinding yields uniform distribution and makes PNC tooling straightforward. PNC mechanical properties show greater improvement from prebinding than premixing24,30 though the material will require more testing for typical blade failure types, which are discussed in ref 31. This PNC is expected to come at a competitive cost to long carbon fiber usage, provided projected increases in CNF production capacity are met. Energy Equations. There is diurnal and seasonal variability in wind speed and duration, captured by statistical distributions of site potential, which determines the most effective match of generator and blade size. A turbine’s lifetime output is determined by its generator rating, capacity factor, and projected lifespan. Capacity factor is a ratio of actual or projected energy production to theoretical production at continuous nameplate operation.32 From these parameters, lifetime power generation in LCA is estimated as shown in the following equation. Boundaries do not extend to transformer losses, and grid integration and load management are outside of the scope. Egen = Pγ 8.76ts 9786

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Lifetime energy production, Egen (GWh), is a function of generator rating P (MW), capacity factor γ (percent), and lifespan, ts (years). The conversion factor is 8.76 GWh/MWy. Energy return on investment, EROI, for WEC systems is the dimensionless ratio of lifetime electricity generated, Egen, to CED, which is typically reported as MJ-eq/kWh or GJ-eq/ turbine. EROI is sometimes referred to as energy yield ratio and defined as, Egen E EROI = ts 0 = E iot CED

Figure 1. Life cycle inventory modifications. Ecoinvent 2 MW offshore wind turbine inventory is augmented and parametrized with cradle-togate inventories for CNF and acetone with respective changes in waste scenarios. Blade mass reductions are tested, and lifetime power generation is varied.

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where E0 is the annual production of electricity (GWh/yr) and Eiot is the total of one-time energy input (GWh) required for activities as manufacture, installation, and disposal of capital goods and supplies. Energy payback time, EPBT, is the time for the system to generate as much electricity as needed for its manufacture, installation, operation and disposal. Taking the convention of including O&M cumulative energy costs in Eiot, the energy payback time is the inverse of EROI multiplied by the life span, shown in the equation: EPBT =

E iot ts = E0 EROI

is no test data available for the PNC performance in large structures. Thus, long carbon fiber is neglected, as in reviewed literature, whereas coarse substitutions of the new material are made for neat glass fiber composites with carbon nanofiber loadings up to 5% by weight of the blade. Amounts greater than 5 wt % are not expected because agglomeration of nanofibers begins to reduce performance.30 Integration of CNF is assumed to occur by the laboratory method of prebinding, but solvent feed reductions of up to 99% are evaluated. This level of solvent reduction is assumed reasonable by a combination of recovery and recycling of acetone in a closed unit operation and expected increases in the nanofiber-to-solvent ratio with development. The substitution of carbon fibers for glass fibers leads to weight savings from lower density and an associated overall reduction in material. With carbon and glass fibers of similar size, thinning of the blade is not induced by the substitution. To the contrary, a CNF-prebinded glass fiber mat infused with resin could experience a mass reduction only with thinning of the material. Mass reductions by way of reduced partial safety factors are limited, but composition of the spar cap can lead to meaningful mass reductions that are less subtle. If the new material could lead to enhancement of the spar cap or reduction in partial safety factors from increased confidence by material testing, then a mass reduction may result.35 To account for this possibility, the range of 0−20% reduction in the amount of glass fiber and epoxy is factored for sensitivity. Determining the upper limit requires further materials research. As evidenced by developments in increased carbon fiber loading in Vestas V90 3.0 MW turbine, an analogy was drawn that the new material could potentially enable increased power generation by extended life span or increased capacity factor. The V90 effects were not in isolation from other modifications, such as improving tower fatigue strength and making significant changes in the nacelle.36 However, for the purpose of this analysis, it can be extrapolated that the material under development could make a comparable contribution to such redesign and improvement as Vestas achieved with carbon fiber. An increase in production up to 67% is evaluated; this limit is analogous to an increase in capacity factor from 30% to 50%. It is not claimed that inclusion of the PNC will cause increased power generation, but that the new material may enable a more resilient blade. Accompanied by other design developments, it is of interest to see if inclusion of the nanofibers would cause disproportionate impacts in life cycle EROI and emissions for future systems. The benchmark and its modifications were evaluated for energy intensity and midpoint impacts. Preliminary results of CED for the 2MW and 5MW systems with the PNC were

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Electricity from WECs can be considered avoided production from conventional sources. Thus, wind EPBT is often reported as primary energy payback time. It is calculated by dividing EPBT by the reference region’s primary energy ratio, PRreg. U.S. primary energy ratio for the period 2001−2005 is 3.34, that is, 3.34 MJ of primary energy required for every MJ of enduse.33 By the same calculation method in ref 33 an average of 3.182 was found for years 2006−2010.34 Approach. This study began with a 2 MW system representing current technology. Methods for producing wind turbines and the foundations for offshore installation are not expected to change much before the year 2025.1 Informed assumptions in the analysis should help determine areas of focus for development of the material. From this assessment, extensions to a larger system have been considered with expectation that generator size will continue to increase well beyond 5 MW. Benchmark LCA data from Ecoinvent for a 2 MW offshore horizontal axis wind turbine was selected with capacity factor of 30% and lifespan of 20 years.20 The boundary of analysis is before losses in the transforming station. Goodness of the surrogate data from Ecoinvent is assumed as the most comprehensive and tenable inventory available for this system. Both CED and midpoint indicators of the plant were evaluated for consistency with published literature. Modifications to the benchmark LCI include upstream CNF and acetone production for the prebinding phase. Nanofiber surface modification is neglected due to insufficient information. Vacuum-assisted resin transfer molding is assumed for composite production, where it is understood that the manufacturer of the benchmark turbines is known to use this type of processing.15,22 Processing energy difference are assumed negligible at this stage due to insufficient scaling information with a permeability reduction. Similarly, energy requirements for cross-flow assisted spray application are neglected for lack of data. Figure 1 indicates the areas where the life cycle inventory has been modified to model the effects of the new material. There 9787

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benzene feedstock or assuming conceptual figures reported in ref 7. Further results (midpoint) will be given for current CNF manufacturing with methane feedstock, whereas the discussion and Supporting Information cover extensions to the benzene case, theoretical maturity of CNF production, and 5 MW systems. A major update to solvent use from preliminary figures37 has been made with solvent data since reported in literature.38 Without a plan to reduce the solvent ratio to the neighborhood of 90%, the PNC is arguably infeasible in this system for technical, economic, and environmental reasons. It is assumed that maturation and scaling of the dispersion process will address technical and economic feasibility. The energy analysis results are highly dependent on these assumptions and corrected solvent figures. Solvent Loading. Eckelman et al.39 found that material use in many processes for nanomaterial production is dominated by purification stages, exhibiting E-factors of 100−100 000 for material production; the E-factor is a ratio of material input to the material incorporated in the final product. According to ref , the E-factor for the production phase of CNFs from methane feedstock is about 35, mostly resulting from the use of solvents and a mild acid wash. Where this assessment of an emerging PNC extends to the use phase, nontrivial impacts arise from solvent used for dispersion and binding of the nanofibers. At laboratory-scale, the mass ratio of solvent to nanofibers is 499:1 to accomplish 0.2 wt % dispersion of nanofibers in acetone.38 At this scale of solvent use, integration of CNF at 3 wt % loading would contribute an E-factor at the fiber integration phase of about 16; this assumes 499 g acetone waste plus 35 g other waste per gram CNF applied and is in addition to other production material wastes. As a transfer medium in a single unit operation, solvent contamination is minimized making recovery and recycling feasible, pending operations research on acetone use with nanoparticles in a recovery system. Details of projected acetone needs for production of 3 blades of 9.9 t each are available in the Supporting Information. Note that Figure 2 is developed with an assumption of 90% reduction in solvent use from the laboratory scale. Midpoint Assessment. A midpoint assessment was performed for the 2MW system to determine impacts forced by the new material. Uncertainty values are included in much of the inventory for the 2 MW Ecoinvent report, and a pedigree matrix was used to estimate uncertainties for the new material and parameters. This technique is outlined in ref 40 and specifics are in the Supporting Information. Trends that emerged, which were highly coupled to the new material, can be qualitatively extended to the 5 MW case. If no technical improvements are assumed from addition of the new material (e.g., increase in energy production, decrease in other material requirements), all categories show an increase in impact potential. This is expected as it conveys the impact of adding material and processing to an established system. For an upper limit of CNFs, 5% of the blades’ mass, the percentage of the nanofibers in a 2 MW plant is on the order of 10−6 of the total mass, yet the impacts are nontrivial. As in the energy analysis results, there is a high sensitivity in midpoint indicators of these systems based on energy production, which is considered in this work as a function of capacity factor and/or life span of the materials. With the CML method employed,41 5 of the 10 impact categories do not

reported in ref 37. Available LCAs for larger systems indicate only glass fiber and epoxy, no carbon fiber, were used in the inventory accounting,12,14,17−19 which makes substitution of the neat composite by the nanocomposite straightforward and consistent with these reports as with the 2MW system.

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RESULTS Energy Analysis. Figure 2 shows the EROI for the 2MW system assuming a 90% reduction in solvent fresh feed and

Figure 2. Energy return on investment for 2 MW wind turbine with 90% acetone ratio reduction. Possible changes in system performance are shown as range from no improvement to best-case scenario assuming CNF integration can facilitate resin load reduction, fiberglass material reduction, an increase in capacity factor or life span or a combination of these improvements.

carbon nanofibers made from a methane feedstock using current manufacturing practices. Details on life cycle energy analysis for CNF manufacture for various feedstocks are available in ref 7. EROI results indicate a range of scenarios from best- to worst-case. Best-case is considered achieving a capacity factor of 50 while decreasing the resin percentage for higher fiber content and reducing overall fiberglass mass by 20%. Worst-case basis (columns) assumes, all things being equal, CNFs are added at the respective weight percentage with no resulting improvement in resin content, fiberglass mass, or power generation, and the mass of the blade is increased by the respective mass of the nanofibers. A 90% reduction in acetone feed is assumed readily achievable with standard unit operations for solvent recovery and recycle, though the added capital and operation for this has not been included at this stage. What is apparent from the results is the disporportionate impact for the new material, which is a fraction of a percentage of turbine mass. Nonetheless, in the worst-case scenario with 5 wt % loading and no other improvements, EROI remains competitive to coal-fired power plants.13 Furthermore, WECs are not water-dependent as thermoelectric and hydroelectric power generation. Thus, a rough calculation was done to estimate water use given recent focus on what is termed the energy−water nexus. A first-order estimate of life cycle water use yields nearly 90% reduction in consumptive water use for power generation; the calculation is discussed further in Supporting Information along with additional background on the EROI calculations. The span of effect between the designated best and worst case scenarios warrant continued investigation into the material and economic effects of the PNC for this demanding system, where blades experience over 10 million flexural cycles in a span of 20 years and where increasing length increases the difference of wind shear experienced. The EROI results of Figure 2 are solely for current CNF manufacturing. These figures improve substantially when evaluated with CNF production from 9788

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Figure 3. Ozone depletion potential for 90−99% solvent reduction with CNF loadings of 1, 3, and 5 wt %; blade mass reductions of 0 and 20%; and increased lifetime power generation of 33.3% and 66.7% (analogous to capacity factor increase from 30% to 40 and 50, respectively).

Figure 4. Global warming potential for 90−99% solvent reduction with CNF loadings of 1, 3, and 5 wt %; blade mass reductions of 0 and 20%; and increased lifetime power generation of 33.3% and 66.7% (analogous to capacity factor increase from 30% to 40 and 50, respectively).

Global warming potential, acidification potential, abiotic depletion potential, and eutrophication potential exhibit a more coupled correlation to acetone and CNF production phases, whereas photochemical oxidation potential in this analysis is strictly dependent on acetone emissions in the solvent’s use phase and associated waste scenarios for PNC production. Figure 4 gives the trend for global warming potential from the addition of the PNC; the remaining midpoint results are provided in the Supporting Information. Processing energy requirements of temperature ramping and pyrolysis for CNF manufacture, as discussed in ref 7, is driving the global warming potential impacts for PNC integration, whereas solvent production increases impacts slightly moreso. As CNF integration approaches 5 wt %, even in the best case scenario, an increase in the system’s global warming potential is expected. Error bars showing the 95% confidence interval for the life cycle inventory indicate a relatively low coefficient of variance, though the range of uncertainty increases with increasing CNF loading. For comparison, coal-fired power generation has global warming potential on the order of 103 g CO2-eq/kWh,42 whereas worst-case CNF scenario is on the order of 102.

strongly correlate to the solvent use. These impacts have their strongest dependencies on energy production and thus exhibit nearly a constant improvement in impact with increased production; reduction of these gains coincides with increased CNF mass. This is not to say other categories are not sensitive to energy production, which is the single largest factor for interpreting the energetic and environmental performance of this material in WEC systems. In Figure 3, mean values for ozone depletion potential are given with 95% confidence intervals for scenarios of 1, 3, and 5 wt % CNF loading coupled with a 33.3% or 66.7% increase in energy production and 0 to 20% reduction in fiberglass mass. The graph shows independence of the impact category on the solvent use and relative dependence on CNF loading. Where ozone depletion potential impacts are increased from the base case at the 33% or less increase in production with higher CNF loadings, most scenarios show an overall decrease in impact with increased generation, offsetting the impact of CNF manufacturing. Ozone depletion potential for coal-fired generation is on the order of 10−5 kg CFC11-eq/kWh,42 where the worst-case CNF scenario (not shown) is on the order of 10−9. This trend is also seen in marine aquatic ecotoxicity potential, whereas human toxicity potential, fresh water ecotoxicity potential, and terrestrial ecotoxicity potential show improvements in the mean impact potential for all production increase scenarios even at high CNF loadings; these graphs are available in the Supporting Information.

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DISCUSSION The manufacture of CNFs is energy-intensive with main impacts from electricity required for pyroloysis of methane in the reactor. The addition of CNFs to any material will force trade-offs that should be evaluated for improvements in overall 9789

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inclusion of the PNC is questionable if a larger plant can be achieved by current technology, going beyond this size will necessarily require additional reinforcement, making assessment of candidate materials necessary. An inventory of the base case with higher resolution, including carbon fiber figures, and better understanding of PNC materials at this scale will help in making decisions. Notably, carbon fibers are only 2−8% as energy intensive than their nanofiber competitor,7,44 yet the two would not substitute on an equal mass basis. Trends suggest final choices will be based on processability and economics, and it is likely PNC integration will be a combination of glass fiber and carbon fiber reinforced by CNFs at the resin interface for maximum strength in both the in-plane and transverse directions. According to the U.S. Energy Information Administration, the nation’s carbon intensity of power generation in 2008 was lower overall primarily because of a 50% increase in wind power generation.45 This change in emissions was a 2.1% drop from 2007 levels, indicating the emerging impact of wind energy generation at current deployment levels. Beyond the impacts of continuous material extraction and combustion for thermoelectric power generation, there is also a need for continuous material and energy input for mitigation of combustion products, including adsorbents, solvents, filters, and requisite pumping energies. Because wind energy requires no associated energy for excavating, transporting, and preparing materials for daily generation, each megajoule of electricity from wind is equivalent to a much higher savings in combustible materials and associated needs. Land use is a contentious issue with respect to comparisons between thermoelectric and wind power generation. Land use information on wind energy is available in ref 46. Siting farms in and around agricultural fields can allow for multiple uses of the land as well as provide income for landowners. As wind energy moves offshore, land use becomes less of an issue, though effects of sea bottoms and surrounding aquatic ecosystems must not be neglected. Furthermore, there are far fewer longterm secondary land use change implications with respect to wind turbines because few disruptive activities are required for continued operation and maintenance. Results have shown that increasing the manufacturing material and energy requirements in conjunction with material advancements in wind turbines will have implications on relative EROI, yet the technology will remain competitive or superior to thermoelectric life cycle impacts, warranting continued investigation if it will lead to more efficient operation and higher deployment rates to meet expected energy demand increases. Interpretation of results is cautioned where fate and toxicity data for carbon nanofibers is lacking. Thus, end-of-life effects regarding the nanomaterials have not been included in this analysis. It may be assumed that the nanofibers will be unavailable to the environment while caged in the resin matrix of the blades and that the incineration and landfilling of the blade materials will proceed the same as with neat fiberglass blades. This does not address exposure, transport, and fate from the dispersion process. Because thermoset polymers are not recyclable, CNF inclusion is not expected to change the end-oflife activities. However, any moves toward thermoplastics would require investigation into the effect of the nanomaterial on recyclability. Emerging technologies do not exhibit historical trends of demand curves, well-known processing technologies, and user

environmental performance, supply chain stability, material performance, and capital expense. As an emerging material, these trade-offs are not yet well-understood, but it has been shown that incorporation of CNFs by prebinding is not expected reduce the EROI or environmental performance, according to the CML midpoints, to a degree that discounts it as a poor trade-off at this stage of development. The view taken is that this emerging material has potential as an enabler for increased deployment and operation intensities. If research and development of the PNC confirms this assumption, then the trade-offs are not expected to be prohibitive. Modern turbines are rated on average for 20 years operation, a time period that has not passed for referenced installations. Improvements in material and aerodynamic design are leading to capacity factors approaching Betz’s limit. Without historical information and an economic trade-off analysis between life span and increased rate of power production, a high capacity factor at 20 years of operation with increased fiber content and reduction in overall fiberglass mass describes a best-case for this system. It has been shown that wind energy outperforms traditional electricity generation in terms of emissions even with back-up generation for intermittence.43 From this, a decrease in EROI for a wind energy system is not a game-changer if it is a compromise in other areas, such as increased penetration of the technology, improved performance of the system over time, and reduced dependency on politically unstable fuel supplies. However, it becomes a critical factor in determining where a technology goes beyond feasibility and becomes an improvement to alternatives of similar mechanical performance. As primary and secondary effects of reinforcing blades with this material become more well-defined, these losses and conditions for improvement can be better resolved. With higher loadings and current manufacturing methods, EROI is expected to be less than current technologies. However, where Khanna et al. generated an LCI for current CNF manufacturing, the work included an extrapolation of the energy analysis for manufacturing process maturity of the nanofibers.7 Taking these figures into account, even higher loadings of CNFs approaching 5 wt %, if accompanied even with modest increased generation, would lead to improvements in EROI and midpoint indicator performance. While acetone has been exempted from the US EPA’s national ambient air quality standards (NAAQS) for ozone, acetone emissions have significant photochemical oxidation potential. Geographical considerations are important because the impact of photochemical smog will be relative to the region in which the manufacture of the PNC will occur. Furthermore, there is a correlation between acetone use, global warming potential and CED due to the acetone production phase, impacts of which are more transboundary in nature. Decisions about solvent recycling will impact the competitive potential for this material in terms of energy and emissions analysis. Investigation into recycling processes for nanoparticle-laden acetone is recommended. A number of modifications will be required to meet the structural and economic constraints for emerging systems of 5 MW and larger. The use of carbon nanofibers as one contributing factor to meeting these constraints is considered likely. Energy analysis results for a 5 MW turbine depend on the point of comparison with respect to the base case capacity factor. Available LCAs assume capacity factors ranging from 30 to 53%, making the point of comparison debatable. Whereas 9790

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trends. Environmental impacts of such technologies as nanotechnology and renewable energy can be overwhelming if not foreseen and must be addressed in the design stage and revisited periodically so as to prevent overinvestment in capital and overreliance on ecosystem services. However, there seems to be much promise in moving toward lower life cycle environmental impact by selectively exploiting nanomaterials for their unique properties and reducing consumption of nonrenewable sources for energy. This study has contributed an assessment of potential impacts for the PNC being developed as it has been proposed as a potential wind turbine blade material. The study has taken the best available base case inventory and made reasonable generalizations based on R&D information about the PNC. The assessment has considered realistic technologies that represent current and future markets for carbon nanofibers and wind energy and determined key operating parameters and areas for further assessment. Parameters include an expressed need for acetone reduction, recovery and reuse in the dispersion phase, and existence of an optimal range of overall CNF mass fraction of the blades for life cycle energy and impact benefits. As the material is further developed, concomitant with the maturity of CNF manufacture for higher commercialization, a more detailed inventory is imperative. This must include better resolution of end-of-life effects.

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ASSOCIATED CONTENT

* Supporting Information S

Additional information on solvent use, energy analysis, water use, and midpoint assessment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge Drs. Jose M. Castro and L. James Lee for experienced and thoughtful insight. This work was supported in part by the Environmental Protection Agency under Grant R832532.

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REFERENCES

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