JNTU B.Tech III WIND AND SOLAR ENERGY SYSTEMS Previous Question Paper (February/March - 2022 - R18)

Wind turbines are primarily classified into two configurations based on the orientation of the rotor axis: Horizontal Axis Wind Turbines (HAWT) and Vertical Axis Wind Turbines (VAWT).

Horizontal Axis Wind Turbines (HAWT): These feature a rotor shaft mounted horizontally. They are the most common commercial configuration.

Advantages: High efficiency due to the blades moving perpendicular to the wind; ability to adjust blade pitch for optimal performance; suitable for large-scale power generation.

Disadvantages: Require a yaw mechanism to face the wind; tall towers are necessary, increasing structural costs; maintenance is difficult as the gearbox and generator are located at the top of the tower.

Vertical Axis Wind Turbines (VAWT): These feature a rotor shaft mounted vertically. Common types include Savonius (drag-based) and Darrieus (lift-based) designs.

Advantages: Omnidirectional, meaning they do not require a yaw mechanism; the gearbox and generator can be placed at ground level, simplifying maintenance; lower center of gravity improves structural stability.

Disadvantages: Generally lower aerodynamic efficiency compared to HAWTs; they are often not self-starting (requiring a motor to initiate rotation); subject to higher fatigue stress due to cyclic loading as blades pass through the wind stream.

The performance of a wind turbine is defined by its power output relative to wind speed, characterized by the Power Curve and the Power Coefficient curve.

Power Curve: This represents the relationship between the electrical power output and the wind speed. It is defined by four key points:

1. Cut-in speed: The minimum wind speed at which the turbine starts generating power.
2. Rated wind speed: The speed at which the turbine reaches its maximum rated power output.
3. Rated power: The constant power output maintained between the rated wind speed and the cut-out speed.
4. Cut-out speed: The maximum wind speed at which the turbine is shut down to prevent mechanical damage.

Power Coefficient (Cp): This represents the aerodynamic efficiency of the turbine, defined as the ratio of the power extracted by the rotor to the total power available in the wind. According to the Betz Limit, the theoretical maximum efficiency of a wind turbine is 59.3% (16/27).

Where P is the mechanical power, ρ is air density, A is the swept area, and v is the wind velocity. The performance is also dependent on the Tip Speed Ratio (TSR), which is the ratio of the blade tip speed to the wind speed; each turbine design has an optimal TSR where Cp is maximized.