Life Cycle Assessment of Multi-Megawatt Wind Turbines with Carbon Nanofiber-Modified Rotors

It is generally accepted that wind turbine farms are zero-carbon emitters in the use phase, and most emissions are attributed to manufacturing and construction of the parts, periodic maintenance, and decommissioning. Life cycle assessments in open literature have supported this systematically and have allowed conclusions to be drawn from effects of material and site selections. Such assessments are based on a choice of fundamental unit by which systems serving equal purposes can be compared. Wind farms are consistently assessed by impact per kilowatt-hour of electricity produced. To effectively normalize an evaluation with regard to the functional unit, a life span of the materials and a maintenance schedule must be assumed.

For wind turbines, fatigue failure remains as a great uncertainty in materials selection for the rotor, which are composite jointed materials undergoing millions of flexural cycles in the operational phase. While the trend is towards glass fibers in a resin matrix as the main load-bearing materials, carbon fiber-epoxy systems are being considered more for their strength-to-weight ratio as turbine size increases. However, this electrically conductive system makes blades more susceptible to lightning strikes, which can cause generator shorts or turbine fires if not effectively grounded. The incidence of lightning is, coincidentally, higher in the offshore environment for which such larger turbines are slated. Stated research community goals are to develop models of turbines having individual ratings of 8-10 MW and rotor diameters over 120 m. With increase in rotor size, there is concomitant weight increase making resilience to fatigue even more necessary. Maintenance and repair from premature failure can drive down cost-effectiveness, if not make a project cost-prohibitive, so expected life span and presumed downtime must be modeled with such issues in mind.

This current project aims to contribute to modeling by using systems analysis techniques to evaluate emerging methods and materials meant to modify the glass fiber-resin interface of turbine blades by addition of carbon nanofibers. The modifications are meant to improve characteristic fatigue response by incorporating an ultra-high strength nanoscale reinforcing material at the interface so that it better reinforces the resin at the molecular level without creating an additional phase in the composite, which is one way to design damage tolerance into the structure. Also, the modifications are meant to be amenable to current production techniques so that the maturing system of manufacturing is hardly affected, except for the modification of the glass fibers. Thus, with the consistency of functional unit and candidates in the 2-5 MW range, a life cycle assessment is performed to compare the material and energy uses and impacts assuming a replacement of a percentage of glass fibers by an equal volume of carbon nanofibers. The resulting impacts will be compared to those of a conventional wind turbine farm.

Increased energy payback periods are due to energy-intensiveness of carbon nanofiber production. There is also an introduction of solvents and related emissions during the incorporation of the nanofibers, the effects of which may improve as laboratory methods mature. Methods of electrical grounding will be considered in the event of a change in related materials resulting from the addition of the conductive nanofibers. The major method of analysis is process-scale life cycle assessment. Exergy and ecological life cycle analyses are also undertaken to explore the implications of wind energy and the use of fossil fuel-based materials in achieving zero-emission electricity production versus unmodified wind turbines, i.e., without nanomaterials, and traditional electricity generation from non-renewable resources.