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I'll give it a try.
You, indeed, will have a lower pressure downwind, as there is a resistance effect due to the turbine. However, in order to keep a constant mass flow, would mean that you have different densities between both sides of the turbine. Gases are compressible, hence you could, theoretically, keep the same mass, different velocities and different densities.
It is, actually, this density difference that is directly related to the pressure, the one that causes the flow. And given the question you asked I am quite sure this last paragraph is now more than obvious to you.
Now, I am not an expert in fluid dynamics, specially with gases, but this made sense to me.
Ultimately wind energy comes from nuclear fusion in the sun.
Warm air becomes less dense and lower pressure, it rises, cooler air flows in to take its place, then the explanation you provided takes over from there.
Well, you did ask where the wind energy comes from, not how a wind turbine works. :)
Nuclear decay in the Earth's core, and nuclear fusion + fission in the sun are the only sources of energy that Earth has, everything else is a derivative. This is also why the widespread adoption of solar power is inevitable, because given enough time (300 years or less) all other sources of power for our current population levels require depleting a finite material fuel resource hundreds of thousands of times faster than it can be regenerated. (Wood is a form of solar power less than 1% efficient.) Ultimately no process on Earth is sustainable indefinitely. When you hear this word "sustainability" being thrown around willy-nilly, one has to question what exact definition is being used. Typically there isn't one, it's just eco-feelgoodery.
Industrial civilisation will live fast and die young, we are just quibbling over exactly how fast and exactly how young.
I figure civilisation is going to die anyway, so we may as well shoot for the stars while we can instead of crippling industry and hugging trees. We don't know if we can leave here, nor how long we will have to live on a polluted planet until we figure it out, so none of the above is any excuse for screwing up our own ecological support systems. We may even figure out how to actually escape the planet in the attempt. Any other course of action is irrational, given physics and earth sciences as we know it, and a will to survive, explore, and prosper.
If it weren't for megalomaniacs and banksters screwing us at every opportunity we might be further along towards peace and (dare I say it) utopia. Anyone who says technology can't solve social problems has underestimated the impact of agriculture and washing machines.
For me, a simple question about wind energy leads to a profound central problem of being human.
The turbine operation is basically governed by Bernoulli’s equation. If you do a web search for, say, “wind turbine efficiency” you will find many relevant articles. An interesting outcome is that turbine efficiency cannot exceed a certain limit (the Betz efficiency) which is 0.59.