Is there even another energy source to discover? I don't want to sound like someone ho believes we are at the end of technological progress, but i can't think of anything. We know about most of naturally occurring physical phenomenas from which we can harvest energy and we even use most of them. Of course there are physical processes that we don't use - muh annihilation - but those won't occur naturally and make them happen would require more energy than we can get from them.