Wind Energy Practice Questions

Correct answer: Lift

Modern wind turbine blades use an airfoil shape to create a pressure difference, resulting in lift. This lift force is more efficient than drag for high-speed rotation and electricity generation.

Correct answer: Gearbox

The gearbox sits between the main shaft and the generator. It increases the rotational speed from approximately 15-20 rotations per minute (rpm) to the 1,500-1,800 rpm required by most electrical generators.

Correct answer: 59.3%

Betz's Law states that no turbine can capture more than 59.3% of the kinetic energy in wind. This is because the wind must retain some kinetic energy to move away from the turbine blades after passing through them.

Correct answer: Cube ($v^3$)

The wind power formula is P = 0.5 × ρ × A × v³, where ρ is the air density and A is the rotor swept area. Because power varies with the cube of the wind speed, doubling the wind speed produces an eight-fold increase in the available power.

Correct answer: Anemometer

An anemometer measures the velocity of the wind. This data allows the turbine's control system to decide when to start, stop, or adjust the blade pitch for safety and efficiency.

Correct answer: To turn the turbine into the wind

The yaw drive ensures the rotor is always facing directly into the wind to maximize power capture. It rotates the entire nacelle atop the tower in response to changes in wind direction.

Correct answer: The wind speed at which the turbine begins to produce net power

Cut-in speed is typically between 7 and 10 miles per hour. Below this speed, there is insufficient torque to rotate the blades and overcome the friction in the generator system.

Correct answer: They have lower efficiency and more pulsating torque

VAWTs, like the Darrieus model, are omnidirectional and don't need yaw systems. However, because their blades are often moving against the wind for part of the rotation, they are less efficient than horizontal models.

Correct answer: Winds are stronger and steadier over open water

Water surfaces have lower 'roughness' than land, which has trees and buildings. This lack of friction leads to higher, more stable wind speeds, allowing offshore turbines to generate more electricity.

Correct answer: The enclosure holding the gearbox and generator

The nacelle is the large casing sitting at the top of the tower. It protects the critical mechanical and electrical components from the environment while allowing them to rotate via the yaw system.

Correct answer: Loss of lift due to an excessive angle of attack

Stall occurs when the angle of the blade relative to the wind becomes too steep, disrupting the smooth airflow and reducing lift. This acts as a passive brake to prevent the turbine from spinning at dangerously high speeds.

Correct answer: The ratio of blade area to rotor swept area

Solidity describes how much of the rotor disk is 'filled' by blades. Low-solidity turbines (with fewer, thinner blades) are typically more efficient for high-speed electricity generation.

Correct answer: Wind speed and electricity output

A power curve is a graph that indicates how much electrical power a specific turbine will produce at different wind speeds. It includes the cut-in, rated, and cut-out speeds.

Correct answer: To reach faster, less turbulent air

Due to 'wind shear,' wind speed increases with height above the ground. Higher towers allow turbines to access stronger, more laminar (smooth) wind, significantly increasing energy production.

Correct answer: Acoustic noise and harm to local bird and bat populations

While wind energy is clean and renewable, turbines can pose a risk to flying animals and produce low-frequency noise. Modern siting practices aim to minimize these impacts by avoiding migration paths.

Correct answer: The area of the circle traced by the blades

The swept area determines how much wind the turbine can intercept. Doubling the length of the blades quadruples the swept area and the potential power output.

Correct answer: The variation in wind availability over time

Intermittency refers to the fact that wind does not blow at a constant speed all the time. This requires grid operators to use energy storage or backup power sources to maintain a steady electricity supply.

Correct answer: Doubly Fed Induction Generator (DFIG)

DFIGs are popular because they allow the turbine to operate efficiently over a wide range of wind speeds. They can adjust the electrical frequency to match the grid even when the rotor speed changes.

Correct answer: The moving shadow cast by rotating blades

Shadow flicker occurs when the sun is low in the sky and the rotating blades cast a moving shadow through the windows of nearby buildings. It is an aesthetic and comfort consideration in wind farm planning.

Correct answer: Three

Three-bladed turbines are the industry standard because they offer the best balance of aerodynamic efficiency, structural stability, and aesthetic appeal. They are more stable than two-bladed designs which suffer from 'teetering' issues.