100% efficiancy would be “once you start it, you’ll never have to put anything further into the system to keep it running again”.
That engineer’s dream of “Perpetual Motion” that, is just that - a pipe dream.
A power source for the future, would be a non-renewable but clean and limitless supply of “once it’s gone - it’s gone”.
On this planet, SEAWATER would be the thing there.
Out in space - Solar panels - might actually have a use there, rather than the high degree of uselessness they have down here on earth, what with clouds, night time, and smudgy collectors all getting in the way of any degree of “efficiancy”.
The materials they are made from - are not exactly “renewable” neither, - are they?