Abstract
Abstract. The wind energy industry relies heavily on computational fluid dynamics (CFD) to analyze new turbine designs. To utilize CFD earlier in the design process, where lower-fidelity methods such as blade element momentum (BEM) are more common, requires the development of new tools. Tools that utilize numerical optimization are particularly valuable because they reduce the reliance on design by trial and error. We present the first comprehensive 3-D CFD adjoint-based shape optimization of a modern 10 MW offshore wind turbine. The optimization problem is aligned with a case study from International Energy Agency (IEA) Wind Task 37, making it possible to compare our findings with the BEM results from this case study and therefore allowing us to determine the value of design optimization based on high-fidelity models. The comparison shows that the overall design trends suggested by the two models do agree, and that it is particularly valuable to consult the high-fidelity model in areas such as root and tip where BEM is inaccurate. In addition, we compare two different CFD solvers to quantify the effect of modeling compressibility and to estimate the accuracy of the chosen grid resolution and order of convergence of the solver. Meshes up to 14×106 cells are used in the optimization whereby flow details are resolved. The present work shows that it is now possible to successfully optimize modern wind turbines aerodynamically under normal operating conditions using Reynolds-averaged Navier–Stokes (RANS) models. The key benefit of a 3-D RANS approach is that it is possible to optimize the blade planform and cross-sectional shape simultaneously, thus tailoring the shape to the actual 3-D flow over the rotor. This work does not address evaluation of extreme loads used for structural sizing, where BEM-based methods have proven very accurate, and therefore will likely remain the method of choice.
Highlights
Wind turbine rotor optimization aims to maximize wind energy extraction and has been an important area of research for decades
The optimization problem is aligned with a case study from International Energy Agency (IEA) Wind Task 37, making it possible to compare our findings with the blade element momentum (BEM) results from this case study and allowing us to determine the value of design optimization based on high-fidelity models
As mentioned in Vorspel et al (2017), this number depends on the optimization problem and optimizer settings, meaning that cross-setup comparison is difficult. c Tsiakas et al (2018) only gives the number of mesh nodes. d Reduced geometry where the root section was removed. e Applied symmetric boundary conditions (BCs) double the mesh size compared to others. f In cases where a range of Reynolds numbers were used, we report the maximum values. g We only found high-fidelity shape optimization for three turbine configurations in the literature: two smaller turbines – NREL Phase VI and MEXICO (Schepers et al, 2012) – and the large, commercial-scale IEA 10 MW wind turbine
Summary
Wind turbine rotor optimization aims to maximize wind energy extraction and has been an important area of research for decades. Upscaling the turbine leads to an increase in swept area, which in turn extracts more energy. A major drawback of naive upscaling is that mass increases with the cube of the rotor radius. The industry avoids the prohibitive mass increase by improving the blade design, which has resulted in blades that are more slender for a given power rating, where the increase in loads (and mass) can be kept low. This further results in blades with increased capacity factors
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