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.
This is a good idea to lower the price of transactions which is hounding everyone of us, just last week one of my transaction took me 4 hours to receive, but the biggest question is do we have good samaritans that will help set up this renewable energy for Bitcoin transactions and not charge us with the set up.
Yes, in some we might see the transaction confirmation are taking little longer but once the price of Bitcoin starts increasing we might see higher transaction rate. Maybe in future with the development of technology the transaction fee can be reduced easily.