Design and testing of a composite material for modelling wind turbine blade structures in tropical region.
Tefera, Getahun Akulu.
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Currently large wind turbine blades have been installed in several offshore and onshore wind farms around the world, particularly in the desert areas of North East Africa where wind turbine blades and nacelles are affected by elevated temperatures. The aim of this study is to investigate the effect of temperature variation on the mechanical behaviour of composite wind turbine blades installed in tropical wind farms. The blades are constructed from unidirectional carbon fibre/epoxy, glass fibre/epoxy and hybrids of these two composite materials. ASTM standards were taken into account when the composite specimens were manufactured for testing purposes. Short Beam Shear (SBS), Dynamic Mechanical Analysis (DMA) and tensile tests were conducted under increasing temperatures to investigate the mechanical behaviour of composite materials when used for structural modelling of wind turbine blades. Experimental findings revealed that the strength and stiffness properties of composite specimens were reduced when temperatures increased. Betz’s element momentum theory and Glauert’s modelling methods were used to investigate the characteristics of composite wind turbine blades measuring 54m and generating 2MW power. Flap-wise loading was taken into account along the length of the wind turbine blades when they were analysed using the Blade Element Momentum (BEM) theory. The wind turbine blades were developed using carbon fibre/epoxy, glass fibre/epoxy, glass-carbon fibre/epoxy and carbon-glass fibre/epoxy composite materials. The tip deflection of the blades was analysed allowing for different flap-wise and thermal loadings. Simulation results indicated that a glass/epoxy blade has the highest and a carbon/epoxy blade the lowest tip deflection. The values for the tip deflections of the blades show minimal change under thermal loading. To study the mechanical behaviour of the blades under thermal loading, an element-wise approach was developed and the failure index for different composite materials was computed. Tsai-Wu failure criterion was employed to determine the failure index of each composite material under thermal and mechanical loadings. Blades failed when the thermal loading was above 40ºC irrespective of the flap-wise loading. This finding was similar to the experimental results mentioned above. Carbon/epoxy showed non-linear behaviour when the test temperature approached 40ºC. Generally, experimental and numerical results are comparable and can be considered valid. To conclude carbon-glass fibre/epoxy composite wind turbine blades are observed to be a better option for tropical wind farms based on experimental and simulation results.