…is an exercise on the efficiency of a wind turbine, check it out first.
How is the power (or energy per unit time) from the wind actually used? Or, putting another way, some energy is wasted, obviously. But how, and where?
1. If the initial speed of the wind is (say) 7m/s, the wind speed won’t fall to zero after the air passes through the blades. It might fall to (say) 2m/s. It’s the difference in speed before and after that is used in a KE calculation, i.e, 5m/s.
2. The turbine will have down times when there is either too little or too much wind.
3. Thermal energy is generated in the form of friction between the moving parts in the turbine. This energy can’t be used.