Blade Design for Modern Wind Turbines P M V Subbarao Professor Mechanical Engineering Department A Procedure to Design Lift Machines……

The Lift Machine A significant difference in the performance between lift and drag machines is that much higher relative wind velocities can be achieved with lift machines. Relative velocities are always greater than the free stream wind speed, sometimes by an order of magnitude. The relative wind velocity at the airfoil of a lift machine is: Speed ratios of up to 10, and forces proportional to the square of the relative speed, make the a lift machine to generate significantly greater force than a drag machine with the same surface area. The larger forces allow for much greater power coefficients.

Current Rotor Design Practice The complex operating environment that wind turbine blades experience and the interaction between (1) the boundary layers around the airfoils, (2) the production of power, and (3) the flow field around the wind turbine necessitate the use of computer codes for blade design. These computer codes may calculate some or all of the following: Overall steady state rotor performance (energy yield), fluctuating aerodynamic loads along the blade, the flow field around the wind turbine, and noise emissions generated by aerodynamic effects.

Theory for Description of Steady State WT Behaviour Blade Element Momentum (BEM) Theory is a popular theory. Also known as Strip Theory. Blade element theory refers to an analysis of forces at a section of the blade. The required local forces is estimated as a function of blade geometry. This theory can be used to relate blade shape to the rotor’s ability to extract power from the wind.

Blade Element Theory The forces on the blades of a wind turbine are expressed as a function of lift and drag coefficients and the angle of attack. For this analysis, the blade is assumed to be divided into N sections (or elements).

Major Assumptions The following assumptions are made: There is no aerodynamic interaction between elements (thus, no radial flow). This known as Radial Equilibrium of blade. The forces on the blades are determined solely by the lift and drag characteristics of the airfoil shape of the blades. The lift and drag forces are perpendicular and parallel, respectively, to relative, wind. The relative wind is the vector sum of the wind velocity at the rotor, U(1-a), and the wind velocity due to rotation of the blade.

An Overall Flow Past an WT The wind velocity due to blade rotation is identified of wind velocity. This rotational component is the vector sum of the blade section velocity and the induced angular velocity at the blades.

Comprehensive Geometrical Detailing of Blade (HWT) Wind velocity at just upstream of Blades

Local Forces on A Blade Element dFL is the incremental lift force; dFD is the incremental drag force; dFN is the incremental force normal to the plane of rotation dFT is the incremental force tangential to the circle swept by the rotor. Thrust is the force creating useful torque.

The relation Between Kinetic & Kinematics

Definitions of Force Increments The incremental lift force The incremental Drag force The incremental force normal to the plane of rotation The incremental force tangential to the circle swept by the rotor If the rotor has B blades, the total normal force on the section at a distance, r, from the center is:

The Diffrrential Torque Contributed by a Blade Element The differential torque due to the tangential force operating at a distance, r, from the center is given by: Note that the effect of drag is to decrease torque and hence power, but to increase the Normal loading.