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.
Exactly, mining equipment is very expensive and not only that miners need to pay their employees as well which raises their costs, however miners are not the ones that determine the costs of transactions, that is done by us, since the space in each block is limited this means the supply is low and when the demand gets higher like when the price of bitcoin goes up then it is natural to see a spike in the fees since everyone is trying to outbid one another to get their transactions to be confirmed as fast as possible.