My thoughts on the singularity and how they produce energy.
Once you have the singularity in place it creates an event horizon where virtual particles that pop into existence don't annihilate themselves immediately because they get separated on this ultimate razor's edge.
The Romulans then harness those new particles to power their ships.
It is in a way the exact opposite of an antimatter/matter warpcore.
If the singularity is self sustaining it would indeed be free energy and grow slightly more powerful over time, since it's mass would increase (allthough probably not in a measurable way).
Though I don't see how it could be selfsustaining without the mass of a star or an energy input that puts a dyson sphere to shame which Kind of defeats the purpose.