Elsevier

Renewable Energy

Volume 74, February 2015, Pages 749-760
Renewable Energy

Multiple surrogate based optimization of a bidirectional impulse turbine for wave energy conversion

https://doi.org/10.1016/j.renene.2014.09.001Get rights and content

Highlights

  • Relative efficiency enhancement of bidirectional impulse turbine up to 16% by changing number of rotor blade and guide vane.

  • Finding optimal operating zone of turbine operation so that maximum energy can be harvested.

  • Focus on selection of surrogate strategy and the surrogate effectiveness is also reported.

Abstract

Oscillating water column based wave energy extracting system has a low efficiency due to the poor performance of its principal power extracting component, the bidirectional turbine. In the present work, flow over a bidirectional impulse turbine was simulated using CFD technique and optimized using multiple surrogates approach. The surrogates being problem dependent may produce unreliable results, if a wrong surrogate is selected. Hence, multiple surrogates such as response surface approximation, radial basis function, Kriging and weighted average surrogates were incorporated in this problem. Same design points were used to generate multiple optima via multiple surrogates to enhance the robustness of the optimization process. Numbers of guide vanes and rotor blades were chosen as the design variables, and the objective was to maximize the blade efficiency. Reynolds-averaged Navier–Stokes equations were solved for analyzing the flow physics. The computed results were used to train the surrogates and find the optimal points via hybrid genetic algorithm. The surrogates were further applied to find the optimal flow parameters by changing flow velocity and turbine speed. The relative efficiency enhancement through our present approach was about 16%. Detailed methodologies, analysis of the results and surrogate applicability have been presented in this paper.

Section snippets

Introduction:

The depletion of fossil fuels and growing environmental issues has motivated to develop technology for the renewable energy systems [1]. One source of the renewable energy is the ocean wave, which is periodic in nature and having immense potential of producing energy [2]. Many methods like an oscillating water column (OWC), Archimedes wave swing, point absorber, overtopping device etc., have been deployed in the ocean, but the factors such as uncertainty of waves, extreme weather conditions,

Problem description and numerical procedure

A bidirectional flow impulse turbine was chosen as the reference geometry [8] for this problem. The turbine and meshing the flow domain is shown in Fig. 1. The turbine has 30 rotor blade (Nrb = 30) mounted on the hub and 26 guide vanes (Ngv = 26) located on the either side of the rotor and mounted on the outer casing. The GVs guide the inlet flow to the rotor. The GVs may be fixed or self-pitching [17] and the fixed GVs were used in this work. The RB profile has a part of ellipse section on the

Numerical optimization

The optimization procedure is illustrated in Fig. 2. The optimization steps involve a methodology of selecting the best available alternatives in the context of the given designs. The design variables (DVs) or factors from any geometry, which is to be evaluated for fluid flow or structural strength or any other properties, are selected and approximate ranges of the designs are decided by some initial literature searches or some random calculation at the extreme points of design space [15] or

Validation and grid dependency

Initially, the flow domain of an existing geometry [8] was imported and solved using Ansys-CFX solver. The numerical results were validated (Fig. 3) with the previous experimental and the numerical results [8], [22]. The torque and input coefficients and efficiency at different flow rates were compared. The current validation results matched well as compared to the existing CFD results. However, there was a discrepancy observed in accuracy of results over wide range of flow coefficients. CFD is

Conclusion

In this paper, design optimization and flow parameter optimization of a bi-directional impulse turbine are reported. The efficiency was maximized using CFD analysis and multiple surrogates modeling based optimization approach.

The design optimization alone produced an improvement of 12.66% relative efficiency. The optimized design has 38 rotors and 24 guide vanes which are higher and lower than the reference design, respectively. After the flow parameter optimization, total 16% relative

Acknowledgment

The authors gratefully acknowledge the financial support by the Earth System Science Organization, Project no: OEC/1213/115/NIOT/ABDU, Ministry of Earth Sciences, Government of India to conduct the research.

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