In the last decade, wind energy has witnessed faster growth than all other renewable energy sectors, with over 25% annual growth. While this is encouraging news, there are also three technical challenges facing wind energy. First, models used in the design process for simulating turbulence do not cover the range of atmospheric conditions likely to introduce critical events (e.g., high shear across a turbine rotor), especially in the Great Plains region. Second, wind turbines in service are aging, and failures, especially due to fatigue of composite materials used for blades, demand attention. Finally, refinements in design philosophy and practice are needed that are informed by state-of-the-art capabilities in atmospheric boundary layer modeling and stochastic wind turbine aeroelastic response simulation.
Intellectual Merit
This collaborative research seeks to extend the design paradigm for wind turbines to the reliability-based assessment of performance against fatigue and extreme limit states. Specifically, turbine loads will be evaluated under various suites of inflow turbulence flow fields generated with spatial structure that reflects a range of atmospheric stability conditions. The proposed work plan will focus on development of four dimensional synthetic turbulence time series based on tuning-free large-eddy simulations along with mesoscale model forecasts. These simulated flows will feed into aeroelastic models of utility-scale wind turbines whose response statistics will be developed using extensive time-domain simulations representing varying inflow conditions over the long term. Load extreme values will be analyzed in evaluating ultimate limit states, whereas cumulative damage and life assessment will be analyzed in evaluating the fatigue limit states. Refinements to current design specifications (e.g. by the International Electrotechnical Commission) will be proposed based on findings from this study.
This research is innovative and potentially transformative because it will use realistic atmospheric flows as the inflow conditions to the wind turbine simulation. Consequently, the proposed research will uniquely enable the simulation of wind turbine performance under realistic field conditions.
Broader Impacts
The education and outreach activities are centered on wind energy. The education plan will provide opportunities for students to develop skills in state-of-the-art computational techniques at the interface of atmospheric science and wind energy technology. Summer internship opportunities with Sandia National Laboratories in wind energy technology will be provided for undergraduate and graduate students. At the K-12 level, the PIs have established relationships with teachers at local school districts in Central and West Texas to create the Run on the Wind/Engineering a Clean Tomorrow summer camp.
CBET-0967816 Lance Manuel (The University of Texas at Austin) and Sukanta Basu (North Carolina State University) In recent years, wind energy has witnessed faster growth than all other renewables. Aggressive renewable portfolio standards have sought to increase the percentage of renewable sources in the energy portfolio of many states; this, along with production tax credits, has led to a boom in wind-generated electricity production. While this is encouraging news, there are also challenges that are becoming evident. First, models used in the design process (namely, the International Electrotechnical Commission (IEC) design standards) for simulating turbulence do not cover the range of atmospheric conditions that may be associated with critical events (e.g., strong shear and veer across a turbine rotor). Second, turbines in service are aging and failures especially due to fatigue (composite materials used for blades are especially vulnerable) demand attention. Finally, an experience base on turbine usage has accumulated and gaps have been identified: refinements in design philosophy and practice are needed that are informed by improved capabilities in atmospheric boundary layer (ABL) modeling (e.g., using large-eddy simulation (LES)) and wind turbine aeroelastic response simulation. At present, the IEC guidelines explicitly address conditions associated only with the neutral boundary layer (NBL)—i.e., they neglect buoyancy effects in definitions of all design load cases. In this project, the investigators attempted to extend the design paradigm for the reliability-based assessment of wind turbines against fatigue and extreme limit states so as to include the evaluation of turbine loads for suites of inflow turbulence flow fields generated with spatial structure representative of a range of atmospheric stability conditions. Specifically, they developed a coupled atmospheric modeling framework that can reproduce certain characteristics of observed ABL flows (e.g., low-level jets, various scaling regimes of energy spectra, including the so-called spectral gap, etc.). They also showed that turbine rotor-scale variables (e.g., hub height wind speed, standard deviation of the longitudinal wind speed, wind speed shear, wind directional shear and Richardson number) are strongly inter-related. Thus, these variables should not be prescribed as independent input degrees of freedom in any synthetic turbulent inflow generator but rather that any turbulence generation procedure should have the ability to bring about realistic sets of such physically realizable sets of turbine-scale flow variables. Last, specific turbine-scale flow variables responsible for large turbine blade and tower loads were identified—e.g., wind speed shear was found to have a greater influence on out-of-plane blade bending moments for the turbine studied compared with its influence on other loads such as the tower-top yaw moment and fore-aft tower base moment. All these findings and others have been disseminated via peer-reviewed publications, Master’s theses, and numerous conference presentations and posters. As indicated in the figure accompanying this report, this collaborative project has made it possible to bring together two research partners with different areas of expertise to address the challenging problem described above. The team at North Carolina State University (NCSU) developed an extensive database of stable atmospheric boundary layer flow fields using large-eddy simulation. Illustrative fields are shown in the figure for weakly and moderately stable boundary layers. The team at the University of Texas at Austin (UT) attempted to relate the simulated flow fields to rotor-scale variables describing the inflow wind fields to a utility-scale 5-MW wind turbine. The influence of these variables defining shear, turbulence, etc. was then systematically and jointly studied for their effects on turbine loads (the figure shows illustrative blade and tower load maxima for one case and corresponding correlated values of wind speed, wind shear, and turbulence that were obtained using LES) using aeroelastic response simulations. This collaborative project primarily facilitated the educational and research training of graduate students—in Civil Engineering at the University of Texas at Austin and in Atmospheric Sciences at North Carolina State University. These students presented their wind energy-related research at various meetings, received internship opportunities and, in turn, became a part of the next-generation science and engineering workforce to tackle the wind energy industry's future challenges. Throughout the course of this project, the investigators were also engaged in various outreach activities. They presented their research and wind energy-related knowledge to K-12 students, teachers, and the general public via several in-person and remote (via skype) presentations.
