People here say that energy can't be free, but I think it actually can happen, though I doubt it's very likely. Right now air is free because its supply is much higher than demand and its everywhere. Maybe in some super distant future same will happen to energy - scientists will be able to get energy from anti-matter or some other cool stuff so the supply of energy will just explode.
The thing is, even if that scenario comes true (and it won't be for a really, really long time), states or corporations will still need to profit from it, and it'll still be charged to citizens. I mean, hydropower in my state is practically free and in plentiful abundance to the state (other than maintenance and operations) but it still charges for it. We even have to burn off natural gas (flaring), as there's simply too much, but we're still not giving it away for free;)
There are 2 scenario's for Bitcoin (though I doubt that Bitcoin in current form will survive till then):
1. People still sticking to PoW and throwing gigantic amounts of energy into it
2, People switching to PoS or some other algo. Maybe instead of burning energy we'll be able to burn something more rare in a way that can be linked to the network.
But yeah, even with a theoretically limitless amount of energy, people would simply be throwing more computing power into the network. The reward system for miners will still work. But like everyone says, no real use in matching a distant variable with a current one. Bitcoin will be very different when/if that free energy scenario comes true.