energy is free is you can grab it for less than the price it cost to grab it from other sources (meaning there is cost for building the infrastructure but no cost to operate it) , and can be considered as perpetual if the source last long enough. (let's say 10 thousands years)
So put like this , solar is free energy since solar panel are dirty cheap and perpetual since sun is expectedly lasting a long time.
Trees are even better, since they ask nothing than a seed and after a while ou can get a full forest for free.
They can survive also long enough to be considered as perpetual.
nuclear power is great too, but it cost so much to produce an exploitable form of energy (usually electricity) that is totally uninteresting on earth.
it could be another story if you make a plant on the moon or a starship and need heat/electricity and don't bother too much with radioactivity.
radioactivity has long life, but at least there is a limit, so it cannot be really considered as perpetual.
The USA has done that in greenland (and also in north pole in the 50's but this one they brought it back to USA, at least that's what they said) were they put a nuclear reactor ( the camp century ) and simply left it behind when the mission was over.
with the global warming this become a issue since the ice is not covering the dangerous littering anymore.
Also you can build a small portable nuclear battery that deliver heat or electricity for a very long time, but again this is usually limited to mission critical use.
US Navy used that to warm divers suits, but that was long time ago.
Sailing branch struggled to keep its divers warm
medium.com