Even using the renewable source of energy, you still need to cover the initial investments of the equipment used. So you still need to charge fee for every transaction, though you have the option to lower such charges. And in that regard, future bitcoin transaction fees might be lower as compared to current rates, even if you want to add more satoshis to accelerate your transaction.
pretty much this. I don't think it would be completey free but it can definitely be much lower if you're talking about using renewable energy. There is still cost associated with equipment that you'll have to purchase and their maintenance and so on but it would be much cheaper than today.