For clarity, though. Federation don't rely too much on vacuum energy generation. It's kind of free energy but only until you add the various technical issues with it. At least for the Federation. 
They rather prefer their Conversion Reactors which are basically the holy grail of nuclear power to harness energy from nearly any material in a clean, safe and efficient way.
These are safe enough you can put them in a electric toothbrush and never worry about the fact you put a nuclear reactor right next to your open mouth.