Now think if a solar energy farm with a battery like that were used to create bitcoin transactions. It's an infinite and free source or energy, right? At least after the investment is paid back. My doubt is how miners could return the fees to the transaction senders in this case. Would it be possible? Would you like to add any thoughts?
You are so wrong... A business always has its costs.
Let's say electricity is for free and the devices are paid, those for the electricity and those for mining.
This or that will break from time to time (miners, light cells, network routers, name it) and some of them are not cheap, so some money has to be put aside for those.
The internet is not for free.
The space where everything is running is not for free.
The cooling.. OK, let's add the cooling to the things already paid and which will only break now and then.
Good. We have a better image, only one thing is still missing:
nobody likes to work for free.
So no matter what, the miners will have to be paid. The fees are not big (usually). It's up to them to evolve and switch to free electricity to stay in competition... and that's all.