The Betz Limit is the maximum fraction of kinetic energy available in the wind that any device/turbine/generator can extract. It's 16/27 or about 59% and it is so low because air needs to move on the other side of the turbine to make room for more air, so a minimum of ~41% of the kinetic energy that comes into the turbine must exit to keep the air moving away from the back end so air can keep flowing through and the turbine can keep spinning.
sorry it's poorly worded
Essentially correct. It's also worth noting that once the KE of flowing air is extracted, it stops moving and is stuck behind the blades, thus impeding the rotation of the turbine. THAT is what regulates the Betz limit.
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