Physicists at Columbia University and Adolfo Ibanez University in Chile have found a way to extract the energy of a rotating black hole by reconnecting the magnetic field lines near the event horizon. This is reported in a paper published in the journal Physical Review D.
Black holes are surrounded by a cloud of charged particles, creating a magnetic field inside the ergosphere, the region between the event horizon and the static limit in which a body cannot be at rest. When the force lines reconnect in the right way, plasma particles can acquire negative energy and fall to the event horizon. At the same time, other particles accelerated almost to the speed of light leave the vicinity of the black hole, taking away rotational energy.
The energy can be extracted with the suitable magnetization of the plasma and the orientation of the reconnecting magnetic field lines. According to scientists, the energy return from reconnecting plasma can be as high as 150 percent, which is explained by the fact that the black hole gives away free rotational energy to particles that fly away. This process may explain the powerful bursts of radiation in space registered on Earth.
There are other theoretical ways to extract black hole energy. For example, black holes are capable of emitting Hawking's quantum mechanical radiation, and Roger Blandford and Roman Znajek suggested using electromagnetic torque.