Which one of the following rotors is commonly used for electricity generation?

Concept: A wind rotor is classified by its axis of rotation, horizontal or vertical, and by the aerodynamic force that drives it, drag or lift. Its solidity, the fraction of the swept area covered by blades, together with its blade aerodynamics sets the tip-speed-ratio range in which it works best: low-solidity lift-driven rotors work best running several times faster than the wind and reach the highest power coefficients, while high-solidity drag-driven rotors run at about wind speed and develop large torque from rest.

Application: A wind-electric machine is judged on how much of the wind's power its rotor captures and on how practically that rotor can be controlled, scaled and coupled to a generator. The propeller rotor is the horizontal-axis machine carrying two or three slender aerofoil blades on a hub held facing the wind. Its solidity is low, it is lift-driven, it works best at a tip-speed ratio of about 5 to 8, and it reaches a power coefficient near 0.45 against the Betz limit of 0.593. That high aerodynamic efficiency, together with proven pitch control and scaling to multi-megawatt sizes, is why the propeller rotor drives practically all wind-electric capacity installed worldwide.

• Savonius rotor: vertical-axis drag-driven scoops, tip-speed ratio near 1 and power coefficient about 0.15 to 0.20, so its output is small and its natural duty is high-torque low-power work such as pumping and ventilation.

• Darrieus rotor: vertical-axis lift-driven troposkein blades, efficient once running and capable of generating electricity, but a classical fixed-blade Darrieus develops very little torque at rest and generally needs a starting aid, and it has not been scaled to utility sizes the way the horizontal-axis machine has.

• Multiblade rotor: horizontal-axis wheel of 12 to 24 sheet-metal blades whose very high solidity gives large starting torque at a tip-speed ratio near 1, the classic farm windmill used to lift water.

Cross-check: the Darrieus rotor can generate electricity too, so the distinction the stem draws is prevalence rather than possibility, and it is the propeller-type horizontal-axis rotor that is commonly deployed, on the strength of its efficiency, control and scaling record. The National Testing Agency final answer key for this UGC NET June 2024 paper, held on 27 August 2024 in Shift 1, also marks the propeller rotor as correct.

Result: the propeller rotor is the rotor commonly used for electricity generation.