Numerical Assesment of a Small-Scale and Very Low Tip Speed Ratio Wind Turbine

Martin Bourhis,Ivan Dobrev,Florent Ravelet,Michaël Pereira

doi:10.1051/e3sconf/202132101019

Abstract

The aim of this paper is to study by CFD the performance and to characterize the velocity fields in the wake of an horizontal axis wind turbine. The design of this wind turbine is far from classical as it has been designed to work at very low angular velocity to promote torque. The 8 blades are not isolated but form a high solidity blade cascade. The numerical simulation compares well to experimental data regarding the power coefficients. The analysis of the wake does show that high tangential velocities, close to the order of magnitude that was used as a design requirement, are generated and form a stable rotating wake. This rotating kinetic energy in the wake may be used to rotate a second rotor in a counter-rotating arrangment.

Highlights

Wind energy is one of the most growing source of renewable energy
The best performances are obtained with three-blades horizontal-axis wind turbines, operating at tip-speed ratios Λ of the order of 6 to 7 and that are designed with the help of the “Blade Element / Momentum theory ” [1, 2]
When lowering the mechanical power while keeping a relatively high angular velocity ω, the mechanical torque generated by the wind turbine T may eventually become far too small to cope with mechanical friction in the bearings

Summary

Introduction

Wind energy is one of the most growing source of renewable energy. This energy resource is worldwide distributed and has attracted in the past two decades a lot of research. The best performances are obtained with three-blades horizontal-axis wind turbines, operating at tip-speed ratios Λ of the order of 6 to 7 and that are designed with the help of the “Blade Element / Momentum theory ” [1, 2]. This kind of wind turbines can reach up to 85% of the maximum power that could be extracted from a stream tube of cross section equal to the are that is swept by the rotor —the Betz limit. One way of facing this problem is to lower the angular velocity, that is to design wind turbines with much smaller tip-speed ratios Λ

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Results