Abstract: A helical screw expander for deriving energy from geothermally heated water. The expander includes a pair of helical rotors with helical structures fitted in an expansion chamber. The geothermally heated water is introduced into said chamber through a throttle port located at an end face of the expansion chamber, whereby precipitation which occurs as a consequence of reduction of pressure and temperature is formed in the expansion chamber where the helical rotors dispose of it. The throttle is preferably adjustable, and also preferably forms a substantially isentropic converging nozzle which discharges fluid at the helical structures.