That's the irony of it all. Power plants capable of powering an EmDrive that's practically useful for manned flight can be summed up with one word: nuclear. With the exception of the (lovely but insane) Project Orion, all of the nuclear propulsion designs like NERVA and the nuclear lightbulb generate a ridiculous amount of heat that we have trouble handling here on Earth, let alone in vacuum without convection - most nuclear plants are built with access to large natural or artificial bodies of water as a heat sink.

The future of manned space flight is almost certainly nuclear and it looks like tiny cabins attached to giant passive heatsinks or spacecraft that constantly have to accelerate just to dump all the waste heat and avoid cooking its inhabitants.