Winseer:
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.
Seawater is where we get our drinking water supply in the form of rain.
It also doesn’t burn that well unless it’s electrolyzed into its component parts of hydrogen and oxygen on the basis of a lot more energy in to do it than out.
Burning hydrogen also creates loads of water vapour which is a proper greenhouse gas unlike CO2.
We’ve already got your criteria in the form of clean enough, lesser of all evils, fossil fuel.
Although seawater could provide limitless energy by somehow utilising massive underwater pressure to compress air.