Even if all PoW energy came from renewable sources, it's extremely wasteful and I'm sure we could have used that energy on something more productive.
That's not to say the allocation mechanism in PoW isn't interesting, but that's not really good enough in itself.
I've been experimenting with energy limited PoW, something called NuPoW [1], that reduces the reward if too much energy is collectively used. But this only works if you outsource the security to an underlying chain.