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Title: Structural analysis of large scale darrieus type vertical axis wind turbines with monitoring and control
Authors: Lin, Jinghua
Degree: Ph.D.
Issue Date: 2016
Abstract: Compared with horizontal axis wind turbines (HAWT), vertical axis wind turbines (VAWT) have the primary advantages of insensitivity to wind direction and turbulent wind, a simpler structure, less fatigue loading, and easy maintenance. As a result, a renewed interest in VAWTs has been seen in recent years and there is a trend worldwide in building large-scale VAWTs. Nevertheless, VAWTs were not pursued after brief development and failing in the 1980s, which resulted in a lack of design experience of large-scale VAWTs. Furthermore, VAWTs suffer the disadvantages of low power coefficient and difficulties in self-start and shut-down. Hence, great research efforts are urgently required to make VAWTs workable. In this regard, this thesis is devoted to a systematic and novel study of large-scale VAWTs, which includes the determination of wind loads on all the components of the VAWT using computational fluid dynamics (CFD) simulation, the finite element modelling and model updating of laminated composite blades, the fatigue and ultimate strength analyses of blades and other components of the VAWT, the pitch control systems for four states of the VAWT, the structural health monitoring (SHM) of the VAWT, and the concept of the smart VAWT. To conduct fatigue and ultimate strength analyses, dynamic loads on the whole VAWT must be determined. A practical method of wind load simulation for VAWTs is proposed in this study based on the strip analysis method and the 2D shear stress transport (SST) k-w model. The validity of 2D SST k-w model for VAWTs is assessed by comparing simulation results with those obtained by 2.5D large eddy simulation (LES). The influences of the tower, arms and turbulent inflow on the aerodynamic forces of the blades are further studied. The results show that the wind pressure and aerodynamic forces simulated by the 2D SST k-w model match well with those obtained by 2.5D LES. The influences of the mean wind speed profile, turbulence, and interaction of all the components can be included in the proposed method at an acceptable computation cost. The influence of the tower is unapparent while the influence of the arms is obvious. The tangential force, and thus the power coefficient, is reduced due to the existence of the arms. The turbulent inflow wind speed causes fluctuation in the wind loads. In addition to wind loads, a precise finite element (FE) model is also needed for the structural analysis. The blades of modern wind turbines are made of laminated composite materials. In this thesis, an FE model of blades is established using laminated shell elements and a micromechanics-based model updating method is proposed to update the laminar elastic constants of the FE model. Analyses of sensitivity and uncertainty are conducted to determine the parameters of micromechanics models to be updated. Static bending tests are conducted and the measured data are used to update the models. The results show that by applying micromechanics models to the process of updating laminar elastic constants, direct identification of these constants can be avoided. In addition, the number of updating parameters can be reduced. It is found that the fiber volume fraction is the most influential parameter with the largest uncertainty for both unidirectional fiber reinforced plastic (UD FRP) and plain weave fiber reinforced plastic (PW FRP). After updating the fiber volume fractions for UP FRP and PW FRP based on the measured strains and displacements, both the calculated local strains and the global displacement match well with the measured data.
A framework for the fatigue and ultimate strength analyses of composite blades is proposed. First, a refined FE model of a laminated composite straight blade is established. Based on the FE model, fatigue analyses are conducted and the influences of the ultimate tensile and compressive strains, damping ratio and fundamental frequency on fatigue damage are studied. Ultimate strength analysis at the extreme wind speed is also conducted and the influence of wind direction on the response of blade is considered. The results show that for the specific composite straight blade considered, the locations at the supports and the mid-span of the blade have larger fatigue damage than other positions of the blade. The positions subjected to compressive cyclic loads have the larger fatigue damage than those subjected to tensile cyclic loads. The fatigue damage is sensitive to the damping ratio and fundamental frequency. The critical locations of strength failure are near the supports. The largest interlaminar shear stresses occur near the supports while the largest interlaminar normal stresses occur at the leading edge, not in the support section. A framework for fatigue and ultimate strength analyses of other components of VAWTs is also proposed. A FE model of the VAWT is established by beam elements. The rotating frame method is used to eliminate the rigid motion of the VAWT. Based on the FE model, fatigue and ultimate strength analyses are then conducted. The results show that the largest fatigue damage occurs at the root of the main arms. The fatigue critical location of the tower is at the bottom. It is found that larger fatigue damage occurs in the leeward side of the tower. For the rotor, the strength failure critical locations are the roots of main arm and the shaft. Assuming that the direction of the extreme wind speed is at the azimuth angle of 0o, the dangerous azimuth angles of the main arm are 60o and 240o; the dangerous azimuth angles of the shaft are 30o, 150o and 270o. For the tower, the fatigue critical location is at the bottom and in the leeward side and the dangerous azimuth angles are 30o, 150o and 270o. Field tests of a straight-bladed VAWT are conducted to validate the proposed frameworks for the fatigue and ultimate strength analyses of VAWTs. The power spectrum densities (PSDs) of the measured responses are calculated under different conditions. Natural frequencies are determined from the peaks of the normalized PSDs of measured responses. The FE model of the VAWT is updated by the identified natural frequencies. By comparing the simulated responses with the corresponding measured data in the frequency domain, it is found that these two results match well with each other. Therefore, the proposed frameworks are validated to some extent. The pitch control system for large-scale VAWTs is proposed. The operation of VAWTs can be divided into four states: start-up above the cut-in wind speed, operation under the rated wind speed, operation above the rated wind speed, and shut-down over the cut-out wind speed. To improve the power generation, self-starting and shut-down performance of straight-bladed VAWTs, two pitch control algorithms ,the fixed pitch (in one revolution) and the variable pitch (in one revolution), are studied for the four states using the double multiple streamtube theory (DMST). It is found that the sinusoidal pitch algorithm produces better control results than the fixed pitch algorithm. Based on these studies, a pitch control system is defined. Two sets of data acquisition and processing devices are used, one for the rotating parts and the other for the stationary parts. Furthermore, based on the results of fatigue and ultimate strength analyses, a SHM system is proposed for the VAWT. Anemometers are installed to monitor the wind condition; a tachometer is installed to monitor the rotational speed and the azimuth angle; strains gauges are installed at the critical locations of fatigue and ultimate strength failure to monitor the local deformations; accelerometers are installed to monitor global deformations; and load cells are installed to monitor the service loads. Similar to the control system, two sets of data acquisition and processing devices are used. Synthesizing the SHM system, control system and power supply, a smart VAWT concept is defined. Such a smart VAWT has self-sensing, self-inspecting, self-control and self-power capabilities.
Subjects: Vertical axis wind turbines.
Wind turbines -- Design and construction.
Wind turbines -- Aerodynamics.
Hong Kong Polytechnic University -- Dissertations
Pages: 1 online resource (XL, 408 pages) : color illustrations
XL, 408 pages : color illustrations
Appears in Collections:Thesis

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