in the process the working gas cools to 5 u00b0 c after the first stage and to u2212 100 u00b0 c after the second stage the formula specified above serves as a basis for this calculation too in the part of the line 31 that connects the two stages of the fourth fluid energy machine 35 a 35 b in the form of a high pressure turbine and a low pressure turbine there is additionally provided a water separator 29 said water separator makes it possible for the air to be dried after a first expansion such that the air moisture contained in said air does not lead to icing of the turbine blades in the second stage 35 b of the fourth fluid energy machine 35 in the further course of the process the expanded and thus cooled working gas extracts heat from the cold accumulator 16 and is thereby heated to 0 u00b0 c in this way cold energy is stored in the cold store 16 which cold energy can be utilized for subsequent energy production comparing the temperature of the working gas at the outlet of the cold accumulator 16 and at the inlet of the auxiliary heat accumulator 12 it is clear why the heat exchanger 17 b must be provided in the case of a closed charging circuit here the working gas.