Step 1: Understanding windmill operation.
Windmills require a minimum wind speed (cut-in speed) to commence functioning and generate power.
Step 2: Ideal range for operation.
For most traditional windmills and small turbines, the cut-in speed is approximately 10–15 km/hr, with an efficient operational range of around 20–25 km/hr.
Below this range, power generation is negligible. At very high speeds, safety mechanisms deactivate the blades.
Step 3: Analysis of options.
- (1) 40–45 km/hr: Excessive speed, typically unsafe for continuous operation.
- (2) 30–35 km/hr: High speed, but outside the standard required operational range.
- (3) 20–25 km/hr: Correct; this is the typical operating range.
- (4) 10–15 km/hr: This is the cut-in speed, not the efficient operating speed.
Step 4: Conclusion.
Therefore, the optimal wind speed for windmill operation is 20–25 km/hr.