In the last decade

In the last decade, while fossil energy resources are rapidly declining, the demand for energy is continuously increasing due to population explosion and technological advancements. Besides this an increase in emission of greenhouse gases originate from the use of fossil energy resources. The energy related greenhouse gas emissions could more than double by 2050, and increased oil demand will increase concerns over the security of supplies incase no strict action is taken. For this reason, an energy optimisation is needed to achieve a 60% reduction of global CO2 emissions by 2050. In this optimisation, renewable energy plays a crucial role in addressing global energy and environmental concerns. For global CO2 emissions reduction to come into play, power generation should strongly depend on renewable energy systems, especially in developing nations.
Among renewable energy sources, wind energy is the best of the new renewable energy techniques. Wind energy is the latest alternative renewable energy source in the recent times and offers potential for CO2 emissions reduction in power production. In few countries, wind energy already provides upto 30% of electricity. This technology keeps rapidly improvising and costs of generation from wind installations greatly reduced . There is now 497 GW of installed wind power capacity in the world and a total of 163.7 GW is now installed in the European Nations
Many efforts are being conducted by the scientists to determine aerodynamic characteristics of HAWT. Kishinami et al. studied theoretical and experimental aerodynamic characteristics of a HAWT. They concludes that optimized design parameters play a crucial role in the performance of the wind turbine. Wind characteristics can be significantly affected by surrounding areas. Numerical and Empirical approaches are being employed to evaluate aerodynamic performance of HAWT by Wekesa et al. Computational fluid dynamics has been successfully introduced with optimized cost and time compared to the experimental method. The performance of optimum HAWT is evaluated and also compared with the optimum theoretical rotor of Glauret and some numerical solutions for constant speed turbines. The optimal blade geometry was obtained by Sedaghat et. al. for which maximum power coefficient is calculated at drag to lift ratios and different design tip speed ratios by assuming variable operational speed. Messina and Lanzafame tested the performance of a wind turbine which continuously operates at the maximum power.

M. Abid et al. evaluated the design, development and testing of a savonius and darrieus horizontal axis wind turbine. This paper shows that horizontal axis wind mill is more efficient when compared to vertical axis wind mill. The darrieus turbine consists of 3 blades which can start at low wind speed. When savonius turbine is attached on the top of existing wind mill the whole assembly can start at low wind speed. This indicates that the darrieus HWAT acts as a self-starter during testing. The function required the starting mechanism which can be provided by the combination of NACA 5050 aerofoil and savonius turbine. The high blade thickness of the NACA 5050 aerofoil will improve the self-starting of the turbine.
The increased efficiency is achieved based on the characteristics such as tip speed ratio, velocity, aspect ratio and other geometry parameter. The experiment is conducted to increase the efficiency and power production of a wind turbine. The design is optimized by combined blade structure and the flow performance. The results indicated that the efficiency of turbine is always based on the wind speed and design of blade. Low aspect ratios improve the power coefficient of the turbine.
The blades are designed using various types of airfoils with unique angle of attack. The design of blade is responsible for the efficiency of the wind turbine. The design of the blade is done using javafoil software. The results indicate that the power output of wind turbine is determined using blade elemental theory.
Abmjit N Roy et al. experimented the design and fabrication of horizontal axis wind mill. This paper indicates that horizontal axis wind mill is a crucial type of wind mill. Performance characteristics such as wind speed versus power output or versus angular velocity must be optimized in order to compete with other energy sources. The experimental result shows that wind turbine on top areas is an ideal position to produce electricity. The power generation is easy and it is used for various applications such as domestic purpose, street lights , agriculture etc.

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The aerodynamic noise sources can be mainly classified as tonal noise which is discrete frequency noise and broadband noise. The characteristics of the tonal noise is that it has a low frequency, due to the disturbance produced by the blade motion (thickness noise), and related pressure field (loading noise). However, the broadband noise is higher frequency and it originates from the mutual interaction between atmospheric turbulence and the rotating blades. For the generation of this noise type, the interaction of turbulence with both the trailing and leading edges is important.
Brooks and Schlinker gave the previous scientific evaluations for the noise sources and their mechanisms. Self-noise of the blade is produced as a result of the interaction between the turbine blade and turbulence which generated on its own boundary layer. The total self-noise is generated when an airfoil encounters smooth non-turbulent inflow.
Self-noise is broadband in character, generated due to several mechanisms, such as turbulent boundary layer which introduces the trailing edge noise. However, relative importance and significant of the various mechanisms is not yet clear, and may depend on the characteristics of the wind turbine. Present models do not hold precise evaluation of the wind turbine blade geometry and its relations to emitted noise.
It is therefore required to comeup with models which consider the blade geometry into account in order to design new blades with reduced noise without losing power. There are two aspects in the design of the computational aero-acoustics analysis; the generation of noise and the propagation of noise. The CFD prediction and simulation can introduce valuable data by studying animations of important flow quantities. It is possible to get a very good idea where the regions of the most serious noise generation are located.
Tadamasa introduced a modelling to wind turbine by using Unsteady Reynolds Averaged Navier Stokes (URANS) method, at low frequencies in particular. However, the URANS techniques are insufficient to predict and simulate the noise emission at high frequencies, due to the simulations based on URANS model has an evident problem to smooth out small turbulence structures and it is impossible to detecting the high frequency with this model. Direct Numerical Simulation method is an optimum method to collect all turbulence structures important in the acoustic problem. DNS has the ability to resolve all turbulence structures without any modelling. However, DNS is costly in terms of computation; it needs a very refined meshes resolution to resolve all turbulent length scale.
The usage of DNS method in wind turbine is impossible due to massive computational cost. The other possibility is to usage a Large Eddy Simulation model, which has less cost than DNS and moreover, LES has a capability to simulate and predict turbulent structures to a specific level. Arakawa et al. developed numerical simulation to the wind turbine blade by using LES model in order to reduce tip blade noise. Due to the fine mesh employed in LES modelling, the acoustic near field of the whole blade has a total number up to 500 million nodes are carried out in parallel with 117 processors. The simulation performed for a total time of 70 milliseconds the turbine blade rotation of 25.4 degrees and the blade tip movement of 2.8 m and this indicated the difficulty and the high computation cost of the LES simulation. It is noted that the time period is too short to obtain a good sound spectrum, the short fall is overcome by employing Large Eddy Simulation (LES) turbulence modelling.
Numerical investigation on spacing between airfoils using Unsteady Reynolds-averaged Naviere Stokes (URANS) in order to reduce noise emission from horizontal axis wind turbines (HAWTs) has shown that the 55% spacing is the best design configuration of the double-airfoil from the noise reduction point of view.
Several parameters were used to reduce and control the noise of the wind turbine such as pitch angles, controlling rotation speed of the wind turbine and torque. The control of these parameters is used to settle the tip speed and blade angle of attack in order to reduce the noise emission.
Leloudas conducted a parametric investigation to obtain optimum performance and minimum noise emissions from a 2 MW wind turbine by variable pitch setting and tip speed. Blade-Element Momentum (BEM) techniques as well as semi-empirical acoustic relations were the base of the acoustic model in the studies of Leloudas. The results have shown that it is possible to reduce the noise level up to 12 dB without losing extravagant the power yield.