Correct Answer

Concept & PrincipleElectricalPower Generation

Option B

Wind mills

Quick Summary: Windmills (or wind turbines) are classified based on the orientation of their axis of rotation relative to the ground. The two primary types are Horizontal Axis Wind Turbines (HAWT) and Vertical Axis Wind Turbines (VAWT).

💡 Explanation

Windmills (or wind turbines) are classified based on the orientation of their axis of rotation relative to the ground. The two primary types are Horizontal Axis Wind Turbines (HAWT) and Vertical Axis Wind Turbines (VAWT).

🔢 Key Formulas

$P = \frac{1}{2} \rho A v^3 C_p$ — represents the power available in wind, where $\rho$ is air density, $A$ is swept area, $v$ is wind velocity, and $C_p$ is power coefficient.

⚙️ Working Principle

In Horizontal Axis Wind Turbines (HAWT), the blades rotate around a horizontal shaft parallel to the ground, requiring a yaw mechanism to face the wind. In Vertical Axis Wind Turbines (VAWT), the blades rotate around a vertical axis perpendicular to the ground, allowing them to capture wind from any direction without needing orientation adjustments.

📌 Key Points

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HAWTs are the most common commercial type due to higher efficiency.
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VAWTs are omni-directional, making them suitable for turbulent wind areas.
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The power output is proportional to the cube of wind velocity ( $v^3$ ).
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Betz's Law limits the maximum theoretical efficiency ( $C_p$ ) to 59.3%.

✅ Advantages

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Renewable and clean energy source
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Reduces dependence on fossil fuels
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Scalable from small units to large wind farms

❌ Disadvantages / Limitations

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Intermittent energy supply
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Requires high initial capital expenditure
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Visual and noise impact on surrounding environment

🛠️ Applications / Uses

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Grid-connected power generation
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Off-grid power for remote rural areas
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Battery charging systems for small remote sensors

🔄 Comparison Table

Feature	HAWT	VAWT
Alignment	Parallel to wind	Perpendicular to wind
Yaw System	Required	Not required

📄 Additional Information

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HAWT is standard for large-scale utility power plants (e.g., three-bladed Danish design).
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VAWT designs include Savonius (drag-based) and Darrieus (lift-based) rotors.

📊 Diagram / Illustration

✅

B is correct — Horizontal and vertical axes refer to the orientation of the rotor shaft in wind turbine technology.

Core Concepts Used

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Aerodynamics of wind energy Power extraction from fluid flow Mechanical classification of rotating machinery

💡 EXAM TIP

Always remember that wind power output is highly sensitive to wind speed fluctuations due to the cubic relation $P \propto v^3$ .

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