There can be no patent on the idea of wind wheels, as they are medieval "prior art". Likewise, many generator patents are no longer applicable, the first were invented back in 1831 by Faraday. Recombining a windwheel with a dynamo is almost as old - so the idea and basic setup of even the worst efficient wind turbines is not patentable: You can not patent anything that is prior art.
Many types of Wind turbines are prior art and had their patents - if they existed - expired for at times decades. As a result, those plans are open to anyone to use.
What is patentable are parts or designs that make them more efficient or cheaper to make, more sturdy or such, and there are tons of parts in that category.
So in the end we got many companies that make either patent-free windmills or have their own patents or licensed the patents of others. But there can't be a monopoly on windmills anymore since there are so many prior-art designs.