Life Cycle Energy Analysis and Midpoint Assessment of Multimegawatt Wind Turbines with Polymer Nanocomposite Blade Material

Life cycle analysis of vapor-grown carbon nanofiber (VGCNF or CNF) manufacturing shows production is highly energy-intensive with a cumulative energy demand that is influenced by feedstock selection and dominated by processing energy needed to maintain reactor temperature for the catalytic pyrolysis of the hydrocarbon feed.  However, at relatively low mass percentages, CNFs can reinforce polymer composites yielding significant improvements in properties such as mode-1 delamination resistance, vibrational damping, tensile strength, fatigue resistance and Young's modulus.  A new method termed “pre-binding” has been developed to reinforce polymer composites with CNFs at the interface of long fibers and resin.  This method improves the manufacturability of large pieces with CNF-reinforced polymer nanocomposites (PNCs) by overcoming processing issues inherent to previous methods of PNC fabrication.  Materials scientists are considering use of this emerging material for wind turbine blade production to meet the demand for larger, more resilient utility-scale wind power plants.

Wind energy converters (WECs) on-line today are a relatively new technology with installations of multimegawatt ratings having been introduced to the modern market only in the last 15-20 years.  The largest 3-blade horizontal axis wind turbine (HAWT) rotor size in operation is 126 meters in diameter with generator ratings from 5 to 7 MW while the growing market is dominated by new installations from 1 to 3 MW.  Apparent limits of fiberglass technology have been reached as rotor blades beyond about 40 meters generally require some inclusion of carbon fiber to address stiffness and edgewise fatigue, which become critical at this length.  The less dense carbon fiber also leads to a positive deviation in trends of weight to increasing size.  Recent press releases communicate progress in R&D for meeting industrial-governmental collaborative goals for 20 MW plants possibly by 2020 using available technologies with rotors expected to have diameters near 200 meters.

In an even shorter time span, numerous nanotechnology-enabled products have been introduced to the consumer market with over 1,300 manufacturer-identified products populating The Project on Emerging Nanotechnologies consumer product inventory today.  These products include automobile body parts with carbon nanotubes (CNTs) for enabling online electrostatic painting and a personal watercraft using nanomaterials for reinforcement of the polymer composite for a “lightweight, stronger material.”

While life cycle energy consumption of CNF-reinforced PNCs for use in automobile body panels has been evaluated, similar trends were not expected for life cycle energy in wind turbines due to differences in operational phase energy consumption.  Also, previous assessments of PNCs have not considered the method of pre-binding, which uses acetone as a vessel for dispersion and distribution of the CNFs onto glass or carbon fiber matting. 

Preliminary studies that assumed replacement of polymer composite blade materials on a 2 MW and 5 MW plant with CNF pre-bound PNCs indicated a range of parameterization values under which the energy return on investment (EROI) would not decrease.  These studies have been updated in a process-based life cycle assessment (LCA) to include more relevant process information and have been extended to a midpoint analysis using the CML and Traci methods to determine areas of focus for optimization.  An energy analysis was extended to a 5 MW WEC using cumulative energy demand and power generation figures from published LCA literature.

Impacts are reported per kilowatt-hour generation of electricity by the wind power plant and compared to the base case, which is a 2 MW WEC in Middelgrund that is inventoried in the proprietary Ecoinvent database.  In every feasible scenario, the EROI of wind power plants with the PNC blade material remained competitive to thermoelectric power generation, and life cycle water consumption remained favorable.  Where CNF production was assumed to be based on benzene feedstock, energy analysis results were much more favorable than methane.  Midpoint analysis, however, was assessed with methane as the preferred hydrocarbon source for pyrolysis in nanofiber manufacture.  Similar trends emerged under extension to the 5 MW system energy analysis.

Assuming a mass contribution of CNFs that ranges from 1 to 5 % by weight of each blade, the overall mass of the nanomaterials is approximately 0.01-0.06% of the overall mass of the WEC system, yet there is direct correlation between the mass fraction of the nanomaterials to nontrivial impacts per kWh compared to the base case.  Where studies have shown manufacturing of virgin nanomaterials to be solvent-intensive, this study finds that the use phase of the nanomaterial, where CNFs are integrated into a product system, is highly solvent-intensive as well.  Current laboratory scale ratios of solvent spent to nanofibers dispersed are on the order of 5x10² g solvent per g CNF.  Maturation of this material development must include a reduction of solvent use by optimization of the dispersion process and evaluation of solvent recovery systems for reuse that considers effects of nanomaterials in the operation.  Photochemical oxidation is a concern in terms of acetone emissions, the degree to which will be dependent on recovery, reuse, and emissions controls.  Also, the degree of solvent fresh feed required has an impact on the overall system's global warming potential and eutrophication potential due to the manufacturing phase of the acetone.

Evaluating impacts of emerging technologies is complicated by the fast-pace of development and deployment, significant gaps in data and uncertainties about the future.  Compilation of life cycle data is unwieldy at best due to complex supply chains and regional variations in categories as energy supply and transportation.  This is further complicated by incomplete understanding of ecosystem service impacts, though work is underway to quantify and compile ecosystem services for inclusion in LCA.  System performance improvements will not occur with PNC integration in isolation of other technological changes within WEC design and operation, yet analyses as this are meant to give insight to areas of concern by identifying impacts and order of magnitude effects associated with this emerging material for the assumed use.