Design of Wind Turbine Rotor (With Diagram) | Wind Energy | Energy Management

In this article we will discuss about the design of wind turbine rotor with the help of suitable diagrams.

In a propeller type wind turbine, the following two forces operate on the blades:

1. Axial Thrust:

It acts in the same direction as that of the flowing wind stream.

2. Circumferential Force:

It acts in the direction of wheel rotation that provides the torque.

Thrust on Turbine Rotor:

A turbine extracts wind energy, causing the difference in momentum of air streams between the upstream and down-steam sides as shown in Fig. 5.18.

Eqn. (5.20) indicates that for designing a wind energy generator (WEG) a large axial force can be obtained by using large diameter turbines. The upper limit of the diameter needs to be optimized by matching structural design with economy.

Torque on Turbine Rotor:

The torque on a turbine rotor would be maximum when maximum thrust can be applied at the blade tip farthest from the axis, Maximum torque (T_max) in a propeller turbine of radius ‘R’ is given as:

Solidity:

Solidity (S) is defined as the ratio of the blade area to the rotor circumference. It determines the quantity of blade material required to intercept a certain wind area.

Mathematically, solidity,

where, N = No. of blades,

b = Blade width and

D = Diameter of the circle described by a blade.

“Solidity” represents the fraction of the swept area of the rotor which is covered with metal.

Variation of Solidity (S) with Tip Speed Ratio (TSR):

The variation of ‘S’ with ‘TSR’ is shown in Fig. 5.19.

i. The faster a rotor runs with respect to the wind speed less ‘solidity’ is required to intercept the entire stream-tube of wind passing through the rotor disc. In other words, a rotor with high TSR needs less material and can have relatively slender blades. However, because it rotates at higher speeds and its twist angles require only slight angles of attack at the tip, a high LDR (Lift/drog ratio) is essential to produce a driving torque.

Therefore, high speed low-solidity wind rotors need good quality airfoils (similar to those used for aircraft). They must also be made with good surface finish and the structural integrity necessary to withstand rotational speeds.

a. Conversely, low TSRs require rotor discs with relatively solid blades or soils, so that wind energy will not be lost through the gaps between them. These blades need courser pitch setting. Because lift is stronger in the plane of rotation and drag is weaker, the LDR is less critical. For this reason, mindmills with low TSRs tend to have many blades, and the lifting surfaces can be much cruder without causing any serious loss of efficiency.

b. As TSR increases, the number of blades decreases.

Effect of Tip Speed Ratio (TSR) on Torque and Solidity:

i. Fig. 5.20 shows how the relative torque of various Wind energy conversion systems (WECS) decreases with increasing TSR. High torque requires a high solidity, and that type of WECS works best at low tip speed ratios.

ii. Fig. 5.21 shows how the best operating tip speed ratio changes with solidity.