By ‘green’ we mean more efficient and less polluting. What is true of energy derived from sun rays is true of wind energy as well. Wind is clearly at a much higher entropy* level than coal is. Therefore we can extract much lower level of energy contained in wind than that in coal. The part of wind energy that can’t be harnessed for doing useful work will get dissipated in the form of vibrations caused by forces (other than those help in rotating the turbine) on wind turbine’s blades, bearings and support structure.
Since wind doesn’t flow at constant speed additional energy losses will take place in acceleration and deceleration of the wind turbine. It is easier to burn coal, for example, at reasonably constant rate.
The much higher entropy in wind (changing speeds, cross directions, turbulence) and sun rays (scattered rays, wavelengths outside electron flow generating quantum bands, changing directions) make them decidedly less efficient than fossil fuels. Energy which can’t be extracted as useful work ends as waste heat.
Heat recovery may seem like a solution, but here too heat is in the form of radiation and convection of air. It is at higher entropy level than heat in the form of say flame(highly concentrated). So we are back at the game of diminishing energy utility.
Wind and solar energy may seem to be free and green but clearly they aren’t.
I will return to the pollution aspect the above energy sources in another post.
*entropy is a measure of disorderliness. For example, Gas has higher entropy than liquid which in turn has higher entropy than solids.