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The Science of Making Torque from Wind (TORQUE 2018)

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Room: BL.27.0.2
Chaired by: Katherine Dykes | NREL
Topic: DSE. Design, System Engineering and New Concepts
Form of presentation: Oral
Duration: 90 minutes

Authors:
Helena Canet, Pietro Bortolotti, Carlo Luigi Bottasso

Abstract:
This work focuses on the design of rotors of wind tunnel size that match the aerodynamic (for both rotor and wake) and aeroelastic behavior of multi-MW machines, including gravitational effects. The approach implies the definition of length, time and mass scaling ratios to respect nondimensional quantities such as TSR (tip speed ratio), Froude number, Lock number and placement of natural frequencies. The full-scale quantities are enforced in the sub-scale model through a complete aero-structural re-design, considering airfoils with similar polars at sub-scale Reynolds and adequate materials. The approach is applied to the scaling of the rotor of a 10 MW wind turbine to the size of a 2.8 meter diameter machine.

Authors:
Mauro Civati, Luca Sartori, Alessandro Croce

Abstract:
Two-bladed rotors are emerging as a viable alternative to classic three-bladed onesfor driving down the cost of energy of large sized wind turbines. By eliminating a blade onecould easily reduce the cost of the rotor. However, design challenges arise due to the reducedpower output and the resonance typically occurring between the frequency of the tower and thetwo-per-revolution. In this work, we perform a dedicated design of a two-bladed configurationfor a 10 MW wind turbine: the solution is upwind and equipped with a teetering hinge at thehub in order to alleviate the loads on the fixed infrastructure. The rotor and the tower arethen optimized by using a holistic design algorithm, and a complete technical and economicassessment of the optimal design is conducted against a reference three-bladed one in order tocompare the main performance including loads, energy yielding and the cost of energy.

Authors:
D. Todd Griffith, Mayank Chetan

Abstract:
With the progression of novel design, material and manufacturing technologies, the wind energy industry has produced larger and larger wind turbine rotor blades while driving down the Cost of Energy (C.O.E). Though the benefits of larger turbine blades are appealing, larger blades are prone to instabilities due to the long and slender nature of the blades, and one of the concerning aero-elastic instabilities of these blades is classical flutter. In this work we develop and assess classical flutter prediction tools to predict flutter speeds. Flutter predictions are compared with and validated against results of previous studies. Then, a sensitivity study is performed to assess the impacts of blade design choices (e.g. airfoils, materials choice, material placement) on flutter speed for large blades. Using the understanding the effects of flutter on the design parameters, a redesign of the SNL100-03 blade is proposed with an increased flutter speed. Finally, comments are made regarding modifications to the tool to account for novel features in design of large-scale rotors such as blade edge-wise stiffening & aero-elastic tailoring and rotor morphing.

Authors:
Michael Mcwilliam

Abstract:
Passive Bend-Twist Coupling (BTC) can be used in blades to alleviate loads and generate more Annual Energy Production (AEP). However, BTC is inherently aero-elastic, thus difficult to incorporate into the design with sequential design process. Multi-disciplinary Design Optimization (MDO) is an attractive approach for overcoming these challenges. This paper presents the re-design of a 100kW BTC rotor using the MDO rotor design package HAWTOpt2. In the preliminary design phase, MDO was used to assess the differences between elastic BTC (i.e. off-axis fibers) and geometric BTC (i.e. sweep). This work found that aero-elastic design optimization without BTC was able to achieve a 16% improvement, then with sweep a 18% improvement and with material coupling a 17% improvement. Due to the reduced stiffness of off-axis fibers, material coupled designs had more difficulty satisfying the tip deflection constraint. The geometric BTC concept was chosen for the final design. The design optimization was repeated with additional manufacturing constraints. The final design achieved a 12% improvement.

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