Aerodynamic design and optimization of wind turbines: Investigations on Double Rotor HAWT

Increase of energy demand and detrimental environmental impact of the usage of fossil energy resources encourage the exploitation of renewable energy resources, while keeping the investment and production costs low. Wind energy considered as one of the most promising energy sources for the next decades and wind turbines are playing a significantly increasing role in the generation of electrical power. Fast development of wind-turbines and distribution networks is owing to the steadily growing industry and increasing multi-disciplinary R & D activities of research institutes working on wind energy.

Double rotor HAWT prototype of LSTM-Erlangen

The aerodynamics of the wind turbine is still revealing many challenges, like understanding and predicting the unsteady rotor blades performance, dynamic stress and aero-elastic response of the blades. Moreover, the wind turbines often subjected to complicated environmental conditions, such as atmospheric turbulent flow, ground effects, spatial and directional variation in wind shear. Hence, the understanding of these environmental phenomena and their impact on the wind energy conversion process are essential for efficient exploitation of wind-energy. Recent developments have  shown that the easiest way to increase power coefficient is to increase the rotor size, however, the associated challenges will also increase.

At the Institute of Fluid Mechanics, aerodynamic investigations are being done on the double rotor horizontal axis wind turbines (HAWT). The aim of the activities is to improve the total power extracted from wind in comparison to single rotor.

The impacts of following parameters on the performance are being investigated:

  • Blade section profiles
  • Rotational direction of turbines
  • Distance between the turbines
  • Relative size of the rotors
  • Number of blades
  • Pitching angle
  • Tip speed ratios for both rotors
  • Turbulent flow effects


Design and optimization approach

Design and optimization method

Theoretical design

Using Schmitz theory and blade element method (BEM), initial design of the turbines are made. Governing equations resolving the aerodynamics of single and double rotor HAWT are analyzed to get maximum power coefficient.

Numerical optimization

Numerical optimization is done by integrating BEM and commercially available CFD programs into an optimization platform. In this platform, both gradient based and genetic  algorithms were used to optimize the shape to get maximum power coefficient while matching the torque of the turbine to that required by the generator.

Numerical validation

High fidelity numerical simulations are conducted to validate the performance of the optimized wind turbine and extract information on the interaction between double rotor  HAWT.


Experimental validation

Wind-tunnel tests are conducted to explore the flow physics of double rotor HAWT and experimental validation of the optimization. For that purpose, torque, power and rotational speed measurements are conducted, while varying the wind-speed, pitch angle and distance between the rotors.

High-fidelity numerical simulation of wind turbine Measured quantities for the determination of wind turbine performance Wind tunnel of LSTM-Erlangen

Numerical simulations are used for the validation of optimization

Measured variables for the determination of wind-turbine performance

The rotors are tested in the wind-tunnel of LSTM-Erlangen





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