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Intuitively, radial rotors (like these you would have on a wind speed meter) equipped with cups/cones/sails look like they should be very efficient at converting air flow into energy, given that they should "bite into" and trap the airflow giving it no possibility of "escaping" short of propelling the rotor.

If that was the case, however, sure such designs would be used for such purposes.

Why are such rotors NOT efficient windmills/wind turbines?

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  • $\begingroup$ Google for "vertical axis wind turbine." I don't know what makes them less desirable for large-scale installations, but their inherent simplicity--no need to track the wind direction--does make them useful in some applications. $\endgroup$
    – Solomon Slow
    Commented Dec 12, 2019 at 16:11

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An anemometer is trying to measure the speed of the air: you want the rotor to move at the speed of the air. To do this, you create as little rotational load as possible, and pick a shape that works best at very small relative motion.

A power wind turbine is trying to extract the maximal energy: you want it to turn as fast as possible with a large load on the shaft. It’s ok if the air is moving over the vanes, so you use wing-shaped, flow-maximized shapes to get the most force possible.

Different designs for different purposes.

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  • $\begingroup$ In support of this answer: sailboats and turbine blades can actually move faster than the wind. The cups in an anemometer generally cannot. $\endgroup$
    – S. McGrew
    Commented Dec 12, 2019 at 16:27

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