Aerodynamic characteristics and optimal pitch control of high-solidity straight-bladed vertical axis wind turbines

Abstract

Gradual depletion of fossil energy sources arouses worldwide attention to renewable energy sources, among which wind energy is one of the most promising alternatives. In the past few decades, as challenging problems on enlarging the scale of horizontal axis wind turbines (HAWTs) become more prominent, vertical axis wind turbines (VAWTs) attract attention in both fields of academic and industry. However, while the VAWTs have the primary advantages of being omnidirectional, insensitive to turbulent wind, having a simple structure, less aerodynamic noises and easy maintenance, they also suffer from the problems of comparatively low power coefficient and unsatisfactory self-start ability. Straight-bladed VAWTs (SBVAWTs) are of potential to gain a better power efficiency by variable-pitch techniques, and among different types of SBVAWTs, the high-solidity SBVAWTs could be of value in application for their comparatively good self-start performance and low operational speed. This thesis therefore mainly investigates aerodynamic characteristics and self-start performance of high-solidity SBVAWT and establishes a practical pitch control technology for enhancing power generation of high-solidity SBVAWTs through a series of wind tunnel tests, computational fluid dynamics (CFD) simulations and analytical analyses. To this end, a wind tunnel test system, which can directly measure the aerodynamic force on the blades of a SBVAWT, is developed. Systematic wind tunnel tests on the aerodynamic forces on the blades of both a moderate-solidity and a high-solidity SBVAWTs are carried out. Both tangential and normal aerodynamic forces are measured for different tip speed ratios and incoming wind speeds. Characteristics of these aerodynamic forces are analyzed and discussed. The aerodynamic force data collected from wind tunnel tests will be used as a database for establishing appropriate analytical tools for analyzing high-solidity SBVAWTs and for validating the reliability of analytical tools and computational fluids dynamics (CFD) simulations in the subsequent studies. A CFD-based model, using 2.5D large eddy simulation (LES) method, for the high-solidity SBVAWT with fixed-pitch is established. The reliability of the 2.5D LES simulation is by the comparison between the simulation result and the test result. The flow field characteristics around the high-solidity and moderate-solidity SBVAWTs are investigated. Based on the details of the flow field around the rotor, the variations of the aerodynamic forces are explained and the mechanisms of aerodynamic forces on the blades are explored. After assessing the availability of the current analytical model to the high-solidity SBVAWT, a test-based double disk multiple stream-tube model (DMST), which is able to predict and evaluate the aerodynamic forces on blades of the high-solidity SBVAWT, is derived and verified by the experimental data. Novel method of establishing the attack angle-dynamic aerodynamic force coefficient relationship based on test data is proposed, which is then applied to the DMST model to develop it to be a test-based DMST model. The aerodynamic forces estimation from the proposed test-based DMST model are verified by the experimental data and the results manifest that the testbased DMST model could predict the aerodynamic forces on blades of the high-solidity SBVAWT with higher accuracy than the existing DMST models. Although the attack angle-dynamic anglenamic force coefficient relationship is obtained from the tested high-solidity SBVAWT, the method to establish this relationship proposed in this thesis is applicable to any other SBVAWTs. Therefore, it is of value in design and optimization of the SBVAWTs. An algorithm to search the optimal pitch angles for the SBVAWTs is also proposed. It takes maximizing the tangential force as the objective function, rather than limiting the forms of pitch curves as most of the current relevant researches. Based on the test-based DMST, the optimal pitch angles are searched at each stream-tube and finally composed as an optimal pitch curve after completing searching at the rotor plane. Optimal pitch curves with tip-speed ratios (TSRs) from 0.7 to 2.2 are computed and obtained, based on which an optimal pitch function is then established for the implementation to the control devices. Several mathematic constraints for practical implementations are proposed in the establishment of optimal pitch function. It is determined to be the function of azimuth angle and TSR, whose parameters are fitted based on the calculated optimal pitch curves. Comparison results of power coefficients indicate that the power coefficients of the high-solidity SBVAWT could be increased considerably with either the calculated optimal pitch curves or the optimal pitch function. CFD simulation with 2.5D LES method is conducted to reveal the flow field features of the variable-pitch high-solidity SBVAWT and to analyse the mechanism of power coefficient enhancement. Details of flow field characteristics are depicted and demonstrated that compared to the high-solidity SBVAWT with fixed-pitch, the high-solidity SBVAWT with the proposed optimal pitch function could diminish the scale of vortex forming in the upwind area and postpone the dynamic stall occurrence to increase the contributed tangential force coefficients and eventually enhance the power efficiency. A novel design of the variable-pitch control scheme with the proposed optimal pitch to the high-solidity SBVAWT is proposed and realized for a prototype of the variable-pitch high-solidity SBVAWT. The pitch angles of three blades are controlled by a control disc with a designed rail and three connecting rods. The designed rail could be changed to make it adaptive to any other pitch control scheme or any other types of SBVAWT. Wind tunnel tests that focus on the power output of the fixed and variable-pitch high-solidity SBVAWT are carried out accordingly. A sinusoidal pitch scheme is also used for comparison. Test results indicate that the power coefficient could be enhanced prominently with the proposed optimal pitch function and demonstrate that the proposed design of variable-pitch control scheme is feasible, functional and effective.