Design and Analysis of a Wind Turbine

ABSTRACTThe objective of this project is to design a small wind turbine that is optimized for the constraints that come with residential use. The design process includes the selection of the wind turbine type and the determination of the blade airfoil, pitch angle distribution along the radius, and chord length distribution along the radius. The pitch angle and chord length distributions are optimized based on conservation of angular momentum and theory of aerodynamic forces on an airfoil. Blade Element Momentum (BEM) theory is first derived then used to conduct a parametric study that will determine if the optimized values of blade pitch and chord length create the most efficient blade geometry. Finally, two different airfoils are analyzed to determine which one creates the most efficient wind turbine blade. The project includes a discussion of the most important parameters in wind turbine blade design to maximize efficiency.INTRODUCTIONAs renewable energies become a growing part of the energy portfolio, focus is being put on increased performance and efficiency of proven sources such as horizontal axis wind turbines. Modern commercial power wind turbines are predominantly horizontal-axis, three-bladed behemoths, with a fixed blade design that is adapted to varying wind conditions by a blade pitch control mechanism. They are complex systems whose design requires the integration of many engineering disciplines including aerodynamics, structures, controls, and electrical engineering. Previous wind turbine design optimization techniques have focused on specific regions of interest, including optimal control,1 optimal blade shape, and site-specific performance increases.2 Our goal was to combine all of these with a simplified analysis model to make computation tractable, but to design a blade from scratch with optimal performance over a site-specific wind profile probability distribution .

PROBLEM IDENTIFICATIONWind as a Resource By the end of 2011, it was reported by the World Wind Energy Association, that there are over 238,351 MW of wind power capacity in the world, as illustrated in Figure 1. The same wind power advocacy group stated that wind power now has the capacity to generate 500 TWh annually, which equates to about 3% of worldwide electricity usage. According to BTM Consult, a company that specializes in independent wind-industry research, the level of annual installed capacity has grown at an average rate of 27.8% per year for the past five years. These statistics demonstrate that wind energy is already a vital source of energy production around the globe and that the demand for wind energy solutions is increasing.

With such increasing demand, it is evident that the benefits of wind energy are real. While wind turbine power capacity is increasing, not many are found in backyards and on top of houses. However, depending on exactly where you live, there is usually an appreciable amount of wind above the tree and houseline. The majority of power 2 generation from wind turbines is currently produced in wind farms, or large fields that have several large commercial wind turbines.

PROBLEM STATEMENT

From an environmental standpoint, a wind farm is much preferred to a coal burning plant because of carbon emissions and other factors, but both methods of power generation require the consumer buy this power from a utility company. What is stopping the average land owner from erecting his own wind turbine? This project is aimed at determining how efficient the small wind turbine can be given the space constraints of a residential area.

DESIGN PARAMETERS

Wind turbines are machines that remove energy from the wind by leveraging the aerodynamic principals of lift and drag. Lift and drag forces move the turbine blades which convert kinetic wind energy to rotational energy. The rotational energy can then be transformed into electrical energy. The rate of energy extracted from the wind is governed by Equation

where P is the power,T is the torque, and is the angular velocity of the turbine blades.Lift and drag forces are measured experimentally in a wind tunnel for airfoils as a function of the angle of attack, . The angle of attack is defined as the angle between the chord line c of the airfoil and the direction of the wind, as shown in Figure 2. For aircraft wing design, it is generally ideal to choose the airfoil that has the greatest lift-to-drag ratio, since there will be the least amount of thrust required to maintain altitude. The objective of turbine blade design is also to maximize the lift force on the blade and reduce drag so that the force on the blade that acts in the tangential direction is maximized. Lift acts in the direction normal to the fluid flow, which is not necessarily acting in the tangential direction once the turbine blades begin to spin. In most wind turbine designs, only the lift force on a blade creates a tangential force in the correct direction, while the drag force creates a small tangential force in the opposite direction. Other than the tangential force, another force, called thrust, is also comprised of lift and drag and acts normal to the plane of rotation. In air turbine design, it is crucial to reduce the thrust on the turbine blades because it wastes energy and it requires a stronger blade to withstand its loading.

Design Methodology

Efficiency of Wind Turbine Wind turbine efficiency is quantified by a non-dimensional value called the coefficient of power CP, which is the ratio of power extracted from the wind, P, to the total power in wind crossing the turbine area. Equation (10) shows that the coefficient of power is a function of the air density , the area inscribed by the turbine blade A, and the wind speed v1.

The power extracted from the wind is derived using the Bernoulli equation on both sides of a wind turbine as depicted in Figure

Blade Design

In order to successfully design an efficient wind turbine, the blade contour must take advantage of aerodynamic considerations while the materials it is made from provides the necessary strength and stiffness. By investigating the aerodynamic characteristics of a wind turbine blade, the parameters that make up the blade contour are optimized, and the loads that test its structural adequacy are calculated. Only aerodynamic principles are being analyzed in this study.

Defining the Chord Length and Blade Twist

As shown in Equation (1), the power extracted from the air is the result of a torque and angular velocity in the wind turbine. According to the conservation of angular momentum, the torque in the wind turbine shaft can only be created if there is a rotation in the downstream wake opposite the direction of the rotors rotation. By taking account of the torque producing the wake in the opposing direction, the following equation expresses the relative tangential speed of the rotating blade.

Upstream of the rotor plane, the rotational velocity of the wake is zero. Down stream of the wind turbine plane, the wake has a rotational velocity of w acting in the opposite direction of the turbine motion. The average rotational velocity over the blade due to wake rotation is therefore w/2.

Using conservation of momentum, the following equation relates the lift force for a section of the blade to the change in relative wind velocity w and mass flow rate dq of air through a ring element of width dr at radius r from the hub.

In order to calculate the power created from the lift force for a segment of the foil, the torque is first calculated by taking the tangential component of the lift force and multiplying it by the differential blade segments radius. The assumption is made that the drag of the airfoil is negligible which, if included, would create a torque in the opposite direction and reduce the power generated.

Airfoil Selection

In order to use the relationships derived in the previous section to arrive at the most efficient blade design, the cross sectional properties of the wing must also be defined. The decision of which airfoil to use over the turbine blade defines the coefficients of lift and drag, which directly affect the forces produced on the blade. Most airfoils used in airplane wing design have documented data from a wind tunnel of the coefficients of lift and drag for a range of angles of attack. For aircraft wing design, data is only required for angles of attack up to the first occurrence of a phenomena known as stall, or the angle of attack where the lift coefficient is drastically reduced due to flow separation. Generally, stall occurs in most airfoils between 15 and 20 degrees, depending on the Reynolds number of the fluid. This data is easily found in many handbooks, but since wind turbine blades operate at angles of attack up to 90 degrees, lift and drag coefficient data is required for the angles of attack past 20 degrees. The National Renewable Energy Laboratory (NREL) has developed several families of special-purpose airfoils for HAWTs. The NREL S-Series airfoils come in both thin and thick families and within each family is a set of two of three different airfoils that are designated root, primary, and tip. Each set of three airfoils is defining a single blade with a variable cross section, such that the root airfoil is the cross section shape at the location of largest chord length, the primary airfoil is the shape at 75% of the radius, and the tip airfoil which occurs at 95% of the radius. The cross section of the blade is interpolated between the three main airfoils. The S-Series airfoils are classified according to their blade length. One family of airfoils is made specifically for wind turbine blades ranging from 1 to 3 meters long. This airfoil family, from root to tip, includes S835, S833, and S834. While this airfoil family fits the intent of the small wind turbine design, sufficient experimental lift and drag data does not yet exist, so it will not be used in this blade design study. The data shown in Figure 10 demonstrates how wind turbine performance is drastically improved by using an airfoil that is specifically tailored for use in a HAWT. Even though the NACA airfoil has a greater maximum coefficient of power, the NREL airfoil in Figure 10 is designed to operate at a higher coefficient of power over a larger range of tip speed ratios. While the NREL airfoils are superior to NACA airfoils for use in wind turbines, wind tunnel lift and drag data is very scarce for NREL airfoils, especially those used in small wind turbines. Since there is sufficient wind tunnel data for NACA airfoils, only these will be considered in this analysis.

The air foil sections finally arrived at the following dimensions, based on the previously mentioned formulas.NACA Airfoil Data is plotted for various sections along the guide lines and hence deduced.

Varying the Airfoil The airfoil is another parameter that can be varied to optimize a blade design. Associated with the variation in airfoil is the change in optimal coefficient of lift and optimal angle of attack. While the airfoil changes the blade cross section, it also alters the optimal coefficient of lift and optimal angle of attack, which affects the pitch and chord length distributions. The original airfoil used was NACA 23012, which is a standard cambered airfoil. The second airfoil that will be used for comparison is the NACA 4415. The NACA 4415 airfoil is different than the NACA 23012 in that the maximum glide ratio occurs at an angle of attack of 51.999 (COMUMN C) degrees, not 7 degrees like the NACA 23012. Another difference between the two that will reshape the blade is the coefficient of lift at 34 the maximum glide ratio. The corresponding coefficient of lift for the NACA 4412 is about 1.05 instead of 0.88.