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.

The Helion ideas for fusion are pretty interesting. They put a bunch of energy in using electromagnetics to fuse deuterium, and then (if they can do the next bit) harvest the extra energy out of the magnetic fields on the way out using the same equipment.

It's a different approach than most fusion reactors at least.

Both of those are highly experimental and neither seems to have a working/energy producing reactor yet though. So more of a long term solution if anything.