two black holes in highly elliptical orbit, so they they nearly collide when they come together. at the center of mass is a cloud of iron atoms. When the black holes nearly collide tidal forces across iron nuclei rip them apart. These ripped apart atoms can then be re-harvested and used as fuel in a fusion engine. The energy loss from ripping the atoms apart is then recovered by the black holes by taking advantage of dark energy when they go apart, which increases the distance between them, increasing the gravitational potential energy.
I could definitely be wrong, but I don't see how dark energy would enter this scenario. DE doesn't seem to play a role in gravitationally bound systems like two black holes orbiting each other.
dark energy increases the distance between objects, adding to the system's energy by increasing the gravitational potential energy. Without the contribution from dark energy, after several cycles the pull from the iron atoms would cause the black hole orbit to decay.
