Improvement of vibration behaviour of small-scale wind turbine blade

Abstract

Externally applied loads from high winds or impacts may cause structural damage to the wind-turbine blade, and this may further affect the aerodynamic performance of the blade. Wind-turbine blades experience high vibration levels or amplitudes under high winds. Vibrations negatively affect the wind flow on the blade. This project considers the structural dynamic analysis of a small-scale wind turbine with a particular focus on the blade; it involves the finite element model development, model validation and structural analysis of the validated model. The analysis involves a small-scale wind-turbine structural response when subjected to different loading inputs. The analysis is specifically focused on on-shore systems. The use of small-scale wind-turbine systems is common however, apart from initial structural analysis during design stages, these systems have not been studied sufficiently to establish their behaviour under a variation of real-life loading conditions. On-shore wind turbines are often designed for low-wind speeds and their structural strength may be compromised. In addition, these systems experience widely-varying wind speeds from one location to another to an extent that it is extremely difficult to achieve a uniform structural performance. The main reason for solving this problem is to evaluate the structural response of the blade, with special emphasis on an 800 W Kestrel e230i. This involves the calculation of the distribution of blade deflections and stresses over the wind-turbine blade under different loading conditions. To solve the problem, a three-dimensional model of a Kestrel e230i blade was firstly developed in Autodesk Inventor Professional using geometrical measurements that were taken in the mechanical engineering laboratory. A 3D finite element model was developed in ANSYS using approximate material properties for fiberglass obtained from the literature. The model was then validated by comparing its responses with those from a number of static tests, plus a simple impact test for comparison of the first natural frequency. Finally, a number of numerical tests were conducted on the validated finite element model to determine its structural responses. The purpose of the numerical analysis was to obtain the equivalent von Mises stress and deformation produced in the blade. It was determined that under the examined different loading conditions, a higher stress contour was found to occur around the mid-span of the blade. The calculated maximum flexural stress on the blade was observed to be less than the allowable flexural stress for fiberglass which is 1,770 MPa. As expected, the highest deformation occurred at blade tip. The first critical speed of the assembled three-bladed wind turbine was found to be at 4.3 rpm. The first mode shape was observed to be in the flap-wise bending direction and for a range of rotor speeds between zero and 608 rpm, three out of a total of five mode shapes were in the flap-wise bending direction. Future studies should address issues relating blade vibrations with generated power, validation of dynamic tests, fluid-structural interaction and introduction of bio-inspired blade system. Although the performance of the bioinspired blade has not been studied in great detail, preliminary studies indicate that this system has a superior performance.