Design and Analysis of Wind Turbine Blade Hub using Aluminium Alloy AA 6061-T6

This work presents the design and analysis of horizontal axis wind turbine blade hub using different material. The hub is very crucial part of the wind turbine, which experience the loads from the blades and the loads were transmitted to the main shaft. At present wind turbine is more expensive and weights more than a million pounds, with the nacelle, rotor hub and blades accounting for most of the weight. In this work Spheroidal graphite cast iron GGG 40.3 is replaced by aluminium alloy 6061-T6 to enhance the casting properties and also to improve the strength-weight ratio. This transition of material leads to reduction in weight of the wind turbine. All the loads caused by wind and extreme loads on the blades are transferred to the hub. Considering the IEC 61400-1 standard for defining extreme loads on the hub the stress and deflection were calculated on the hub by using Finite element Analysis. Result obtained from ANSYS is compared and discussed with the existing design.


Introduction
A wind turbine is a device that converts kinetic energy from the wind, also called wind energy, into mechanical energy; a process known as wind power. If the mechanical energy is used to produce electricity, the device may be called a wind turbine or wind power plant. If the mechanical energy is used to drive machinery, such as for grinding grain or pumping water, the device is called a windmill or wind pump. Similarly, it may be referred to as a wind charger when used for charging batteries. The result of over a millennium of windmill development and modern engineering, today's wind turbines are manufactured in a wide range of vertical and horizontal axis types. Fig.1 represents the image of Horizontal Axis Wind Turbine components.
Ali Muhammad et al [1] explained about Modern mega watt class wind turbines are exposed to high and complex loads. The influence of the manufacturing process on material properties is investigated includes the simulation of fatigue life and refining the existing material models lead to a more efficient utilization of the material. Arvind singh rathore et al [2] discussed an optimization model for rotor design of 750 kW horizontal axis wind turbine. In this work a blade of length 21.0 m is taken and airfoil for the blade is S809.The airfoil taken is same from root to tip. All the loads caused by wind and inertia on the blades are transferred to the hub. The stress and deflection were calculated on blades and hub by Finite element analysis method. The maximum stress in the model is less than maximum allowable stress. Tony Burton et al [3] give the information about wind resources, various theories, coordinates, performances related to wind turbine. It also describes about various type of geometries of turbine, different loads acting on the wind turbine and all the fundamentals of wind turbine.

Figure.1 Components of Horizontal Axis Wind Turbine
Vinay V. Kuppast et al [6] study of effect of vibration characteristics of aluminium alloys of different compositions. The modeling and analysis is carried out using ANSYS software. Young's Modulus and the ultimate tensile strength of the 380 alloys increase with the increase in copper and silicon content. Deformation is least in case of 380 alloys and is recommended for low vibration applications.IEC-61400 [7] is an international standard published by the international Electro technical Commission regarding wind turbines. It is a set of design requirements made to ensure that wind turbines are appropriately engineered against damage from hazards within the planned lifetime. Wind classes determine which turbine is suitable for the normal wind conditions of a particular site. Turbine classes are determined by three parameters -the average wind speed, extreme 50-year gust, and turbulence. In this work aluminium alloy 6061-T6 is chosen as an alternativefor the wind turbine hub to reduce the strength-weight ratio and gradual decrease in cost of production.In ANSYS both the materials existing and proposed material are analysed under IEC 61400-1 load cases and the results of total deformation and equivalent stress is taken for comparison.

2.Experimental Set Up
The aim of the project is to reduce strength-weight ratio and decrease in cost by changing material of hub. A 3D model is designed in Pro-E and it analysed in ANSYS for both existing material and an aluminium alloy. After analyzing both the results are compared and fatigue analysis is made for aluminium alloy in order to determine whether it is capable in environmental conditions.

2.2Model of hub
The 3D model of the hub is modelled using the software PRO/E. PRO/E is widely used software to model the three dimensional model due to its user friendly options compared to other similar software's. The 3D model is imported to ANSYS for analysis. In order to import the model is converted into global file format namely .stp format. After converting the file into .stp format the file is imported into ANSYS. Fig. 2 (a) and (b) represents 3D model of wind turbine rotor hub and meshing of wind turbine rotor hub.   Figure.

3.2Results of Aluminium alloy for load case1and 2
Aluminium alloy6061 is a precipitation -hardened aluminium alloy, containing magnesium and silicon as its major alloying elements. T6 temper 6061 has an ultimate tensile strength of at least 3000 MPa and yield strength.Change in the shape of a body caused by the application of a force (stress). Deformation is proportional to the stress applied within the elastic limits of the material. A deformation may be caused by external loads, body forces (such as gravity or electromagnetic forces), or changes in temperature, moisture content, or chemical reactions, etc.   Factor of safety (FOS), also known as (and used interchangeably with) safety factor (SF), is a term describing the capacity of a system beyond the expected loads or actual loads.N f , as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs. For some materials, notably steel and titanium, there is a theoretical value for stress amplitude below which the material will not fail for any number of cycles, called a fatigue limit, endurance limit or fatigue strength. Fig. 7 represents (a) Fatigue life for Aluminium alloy hub (b), Equivalent alternating stresses for Aluminium alloy hub. Table 4 gives the comparison of materials study analysis.

4.Conclusions
 Wind turbine Rotor Hub has been analyzed for two extreme load conditions as per the most important load cases according to IEC 61400-1 (wind turbine loads)  Existing material is used for analysis (GGG40.3). Stress levels are high. Factor of safety of the analysis shows 7.6 and required is 3. This shows that the current design is much higher in safety.  So we can use aluminum alloy for reducing material cost and total weight reduction. The stress (32MPa) and deflection (1.2mm) a value from the analysis. Further Fatigue analysis is carried out and alternating stress for the aluminum alloy is 103.2MPa which is less than the allowable yield limit(170MPa).  The fatigue life of aluminum alloy rotor hub is safe for 20 years time of wind turbine life time.
Hence using aluminum alloy rotor hub will be beneficial to the industries in cost wise.