Energy extraction from a Reissner-Nordström–de Sitter black hole

The energy extraction from an electrostatic black hole by the decay or splitting of electrically charged particles is analyzed. We determine the energetic conditions that make the extraction process viable and present a general expression for the efficiency in terms of the parameters of the electrostatic black hole and the decaying particles. We also examine the conditions that optimize the efficiency of the extraction process. We analyze two particular cases: the first one is the extraction process from a Reissner-Nordstr"om black hole, for charged test particles with nonvanishing angular momentum; the second one and more interesting corresponds to the energy extraction from a Reissner-Nordstr"om de Sitter black hole. For the latter there are two regions where the energy extraction is possible, the generalized ergosphere and a cosmological ergosphere induced by the cosmological horizon. Under certain conditions the two ergospheres get connected and cover the whole region between the event horizon and the cosmological horizon, and therefore, the energy extraction is possible at any point in the vicinity of the black hole. Moreover, the efficiency of the energy extraction can be the same for different breakup points and also there is the possibility of a different efficiency for the same breakup point. The conditions that maximize the efficiency are determined as well.