Screw expander with variable volume controlled by the pressure ratio, and plant for recovering electrical energy from heat with a screw expander

A screw expander comprising

• an expander housing with a screw rotor chamber arranged therein,
• two screw rotors which are arranged in the screw rotor chamber and are each mounted in the expander housing so as to be rotatable about a screw rotor axis, which screw rotors intermesh with their screw contours and each cooperate with wall surfaces adjacent to the screw contours and partially surrounding the screw contours, in order to receive working medium supplied by means of a high-pressure chamber arranged in the expander housing adjacent to the screw rotors and to discharge it in the region of a low-pressure chamber arranged in the expander housing, the gaseous working medium being enclosed with an initial volume at high pressure in expansion chambers formed between the screw contours and wall surfaces adjacent thereto and being expanded to a final volume at low pressure, and a generator driven by the screw rotors,
• the screw expander has at least one control slide arranged in a slide channel of the expander housing and adjacent to both screw rotors with slide wall surfaces, which the at least one control slide is movable in a displacement direction parallel to the screw rotor axes and is thereby configured to influence the final volume and/or the initial volume, and
• in that a slide drive is provided for moving the at least one control slide into different slide positions,
• said at least one control slide facing the high pressure chamber which is arranged on a high pressure side of the at least one control slide,
• said at least one control slide forming an inlet edge facing the high pressure chamber and defining an inlet volume of the expansion chambers wherein the inlet edge can be displaced relative to a high-pressure side termination face of the screw rotor chamber for determining the initial volume of the expansion chambers formed and thus a volume ratio, and wherein the inlet edge can be at a minimum spacing from the high-pressure side termination face and can be displaced beyond the high-pressure side termination face into a run-in space for the control slide that is enclosed in the high pressure chamber in order to vary the final volume by a position of an end face of said control slide opposite to said inlet edge without changing the initial volume which then always remains minimal, and
• wherein a control system is provided which controls the slide drive for the at least one control slide to set a volume ratio for the expansion of the working medium, said volume ratio being formed from final volume divided by initial volume, wherein the control system detects the high pressure and the low pressure by means of respective sensors arranged one on the high-pressure side and one on the low-pressure side and determines a pressure ratio of high pressure to low pressure from this, and wherein the control system positions the at least one control slide by actuating the slide drive according to whether the pressure ratio of high pressure to low pressure exceeds and/or undershoots at least one specified limit value.