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Renewable energy is the energy that comes from the nature which should not harm the environment. One of the main renewable energy recourses is the wind. The wind turbines are the devices used to convert the wind kinetic energy to mechanical energy. This converting process to produce electricity is known as the wind power. The wind turbines are manufactured in vertical and horizontal axes types. Three main components of any wind turbine should be presented in it construction, these components are:
This research project will focus on the rotor component because it has the most related problems to the fluid mechanic field. The aerodynamics is an important aspect of the wind turbines, because the method used to extract the power is related to the lift and drag based. The materials used in manufacturing and the inclination angels of the blades are the main concerns in the extraction of the wind power. The efficiency of the wind turbines is determined from the magnitude and distribution of the forces on the blades. The equation below represents the power gained from the wind turbines.
where F is the force vector ( drag and lift forces) and U is the speed of the moving part in wind turbines.
The main challenges for the designing engineers are the designing of the wind turbine that insures the maximum power efficiency and less maintenance. Moreover, having a design that has minimum noise caused by the rotational motion of the blades and that has a fixed speed operation which avoids the mismatch with the wind speed are an important concerns in this field.
2. Literature Review
This paper studied the wind turbine efficiency of the heat pump. The wind turbine can convert the wind power to useful power by a percentage of 59% approximately, this is according to Betz in an idealized laminar-flow model . The power is introduced by the wind kinetic energy at the inlet of the turbine, but completely converting the wind energy to useful power is not easy since The wind is leaving the wind turbine at a smaller velocity, containing residual kinetic energy . The efficiency at the maximum power output of the wind turbines can be expressed by Betz number. The value of this number is 36.2%. Based on this finding, the actual operating efficiency of wind turbines is in the range of 36%ââ‚¬"59% . Figure 1 shows a simple schematic of wind turbine, the wind speed at the upstream is V1 and at the downstream is V2. The cross section area at the upstream side is smaller than the downstream side; this causes the kinetic energy and pressure drop. Turbine blades is responsible to extract Power.
the fluid friction and turbulence is considered, but the energy leaks due to loss mechanism is not considered in this research study. The main findings was that in the wind turbine model the driving forces depend on the square of the wind speed .
The main goal of any wind turbine is to has design that operates at an optimum efficiency. In order to achieve this, many manufacturers are using hydraulics technology to meet some requirements of modern wind turbine manufacture. hydraulic cylinders are being used to allow a quick and precise varying of the angle of the rotor blades through a cam action . The blades are turned according to the wind speed. if the wind speed falls the blades turned into the wind to increase the rotational speed and if the wind speed increase it turns out of the wind. This is achieved by installing one pitch control system in the hub of the turbine for each blade . Controlling the angle of the blades helps to achieve the required net forces from the flowing air on the blades' surfaces that need to rotate the rotor and thus having the mechanical energy that will be converted to useful energy. The problem in this technology is that the flow of the air on each blade surface is effecting the air flow on the other blades of the same wind turbine. This article didn't search for a solutions of this problem.
The researchers introduced the aeroacoustic analysis of the wind turbines to predict the noise generated by the rotation of these turbines. This article considered three main noise sources to study. These sources are the airfoil self-noise, the blade rotation and the propagation of sound over large distances . Regarding the noise from the blades rotation, the incoming gusts and atmospheric turbulence and the wind shear are the reasons of this noise on the blades because of the unsteady flow . The problem that is facing the design engineers is that the blades of the wind turbines can face a large changes in angle of attack because of the changes in the wind direction or the interaction with the unsteady wake shed. The National Wind Technology Center (NWTC) has done a researches that involves experiments, modeling and simulation to analyze the wind turbine blade noise and to find the key variables of this problem. In order to do a simulation for this complex wind turbine flows, The range should be from the largest scale in vortex shedding to the finest scales of turbulence in the boundary layer . According to , the main reasons of the airfoil noise are the trailing edge noise of the turbulent boundary layer and the vortex shedding noise of the laminar boundary layer. This article conclude that Each of the component aerodynamic problems under investigation is very challenging. However, the select of perfect techniques and codes under development will provide a tool for the prediction of the sources of wind turbine noise.
A field test was preformed for two different wind turbines in order to study the noise caused by the blades. As shown in figure 2, Turbine 1 was with a rotor diameter of 94m and height of 100m, and was located in Netherlands. Turbine 2 was with a rotor diameter of 58m and a height of 55m, and was located in northern Spain . The two test turbines were rotated in clockwise direction . The three blades for each turbine were identical. However, for turbine 2 one blade was untreated , one was cleaned and one was tripped but for turbine 1 all blades were untreated. This provide the researchers with information of the effect of the blade roughness. The main result found was that the frequency of rotation for turbine 1 was higher than turbine 2. Moreover, for both turbines all noise was found to be produced during the downward movement of the blades. this aerodynamic blade noise is known as the trailing edge noise (swish) .
Figure , Test set-up for turbine 1 (left) and turbine 2 (right)
The articles in this project research explained the efficiency of the wind turbines. It introduced some technologies that required to increase the extraction of the wind power by controlling the inclination of the wind turbine blades. The aerodynamics and aeroacoustic of the wind turbine blades were discussed in order to investigate the noise generated from the rotation of the rotor. Two wind turbine were tested and the main finding was that for the both turbines the noise was produced from the downward movement of the blades. For future research, it is important to conduct investigations on the optimal design of new turbines while considering the weight of turbine, aerodynamic shape and wind speed. Moreover, it is important to consider the changing in temperature while studying the motion of the wind turbine blades because at different temperatures the air flow will change and the blade material also will be affected.