"Research by Harvard professor..." Who cares where he does his armchair science from? The real research is being done by building windmills around the world. Has there been a documented lessening of windflow anywhere as a result?
"as wind farms grow larger, they start to interact"
So don't build them too close together?
"If we were to cover the entire Earth with wind farms, he notes, "the system could potentially generate enormous amounts of power, well in excess of 100 terawatts, but at that point my guess, based on our climate modeling, is..."
Guess we'll just have to get to 100 terawatts, then, and then we can test his 'guess' in the real world.