In todays world the focus on environmentally friendly energy sources is ever increasing. Perhaps Penrose’s idea from the seventies must be used… I originally wrote the below post for the Facebook page “UiS Cosmology” (https://www.facebook.com/uiscosmology/?fref=nf):
In 1971 Penrose and Floyd published a paper suggesting a mechanism to extract energy from a black hole. In their paper  they motivate their study from an analogous mechanism in the theory of pulsars (discovered in 1967), which are rotating neutron stars emitting energy in the form of radiation. These rotating stars have an enormous magnetic field (about a billion times as strong as that of the earth!) attached to it. Not only would compasses work outside the star, but one could extract energy from these fields. Penrose and Floyd now wanted to see if it was possible to extract energy from a rotating black hole! A black hole is extremely massive, so one could potentially expect this to be a huge battery of energy. Their hope wasn’t to extract energy from magnetic fields, however, but rotational energy.
You might wonder how this could possibly work, since no information can escape out over the horizon of a black hole. The reason for the speculation is, however, grounded in the so-called Kerr solution to Einstein’s equations, which describes a rotating black hole. For a rotating black hole there is the so-called “stationary limit”, which lies outside the horizon, and a particle travelling with the local speed of light will appear stationary from the outside. More importantly in this case: it is theoretically possible for a particle to have negative energy in this region. Penrose therefore suggested that one could drop a particle from the outside and into this region. In this region the particle should split in two in such a way that one of the particles get negative energy, whereas the other (due to energy conservation) gets more energy. This latter particle would therefore escape back out with more energy than it had when it came in. Voila! Rotational energy extracted from a black hole.
The process nowadays bears the name “The Penrose process”, but has certainly not been physically realised as of yet.
 Penrose and Floyd (1971): http://www.nature.com/…/jour…/v229/n6/pdf/physci229177a0.pdf