You keep using this word "used", I do not think it means what you think it means...
Energy is one of those strange things, where consumption is not necessarily destruction. You 'used' a joule of energy when you heated up your coffee this morning, then dumped the waste heat to the rest of your kitchen, where it became an input for another part of the cycle. The energy did not go away when you consumed it, and in this case most of the energy 'used' in the production of a cup of coffee was never lost.
[I kid, I kid, I know you meant negentropy, but I just couldn't help myself.]
Energy is the only resource I know about where there is a physical law stating that this must be true. (To be fair, you could have less energy offset by more mass. But the point stands.)
... That's exactly what I was referring to? You seem to be confused. Your responded to a post stating that energy is never destroyed, with the sentence:
"Energy is about the only resource where this doesn't hold true."
First, heat is energy. Second, you can use heat as heat. Third, you can use heat to boil water to turn a turbine to create electricity, which you can use to do very many things, including create heat.
Now, whether we can effectively use the energy is an entirely other question. There's plenty of technical limitations. But in a closed system there's no reason an object cannot have perpetual motion with perfect energy recovery. Of course, closed systems don't help us, because they cannot exert useful work outside of the system.
You can't. You can increase the efficiency that way, but you can't use it to produce limitless energy. We wouldn't be burning fossil fuels if that was an option.
You're not "producing" limitless energy. You're reusing the existing energy, because the energy cannot be destroyed. Again, there's no reason why this can't be done within a closed system, except for our technical limitations. Indeed, tracking and recovering energy is the entire idea behind reversible computing.
In theory at least, all non-radioactive elemental resources are recoverable from whatever combinations in which they are found, given sufficient energy and information.
The marginal energy is probably not very much. The average energy might be significant, but it's probably dwarfed by a single can of soda if you include the can.
