It can never be "free" even the energy evolution of solar energy is not free. Running that place costs money, even if its less than the original ways its still has a operational cost, from salaries of people who run those places to maintenance and stuff we don't know. So it can never be free, it could totally be clean and cheaper but not free. That is one part, on top of that bitcoin miners needs to profit as well, so even if electricity gets really cheap we need to put miners profit on top of that as well and we have a cost at our hands.

Now miners do mine bitcoin in return so that could mean they will be paid in the bitcoins they mine but the transactions could be free but looking at how the halving is getting close I am pretty sure just the bitcoins mined can't be enough, its a possibility but a very low one at that. We already have only 3 million bitcoins left to mine so I doubt it will be feasible but even if it is not going to last too long.