Interesting - I'm not aware of every YC startup, so cool to hear that that's in the mix (though I feel like fusion is still little more than fantasy, sadly).

I will say that one of the more interesting energy startups that I'm aware of (don't think it's YC though) is Terraform Industries which plans to make hydrocarbon fuels out of captured carbon powered via massive solar installations. I hope that that works out because they really do have a good mission and a plausible economic model to achieve it.