Study on the efficiency of a convergent-divergent two-phase nozzle as a motive force for power generation from low temperature geothermal resources

Two-phase nozzles could be used as energy conversion devices in geothermal total flow systems or binary fluid systems followed by a trilateral cycle for power generation. In this paper, the efficiency of such nozzles is investigated. Also, a profound research has been done on similar area in the past where mostly high pressure and high temperature energy resources were considered; so, the possibility of utilizing low temperature energy resources remains limited in the literature. In order to bridge the knowledge gap, the feasibility of utilizing low temperature resources for power generation is studied in this paper. In this regards, experiments are carried out with the following conditions: a convergent-divergent nozzle is supplied with water at atmospheric pressure with various temperatures at / below 100oC. This nozzle is connected to a tank that is evacuated by a vacuum pump. The driving force for water to flow through the nozzle is the pressure difference between atmosphere and vacuum pressure in the flash tank. As water is passed through the nozzle, the thermal energy is converted to kinetic energy as a motive force for power generation. The impulse force caused by the jet exiting the nozzle is measured and compared against the ideal case (i.e. isentropic expansion assumption) to calculate the thrust coefficient of the nozzle and evaluate the efficiency of the process. Also, the pressure and temperature profiles along the nozzle are obtained and compared against saturation pressure corresponding to measured temperatures. The results encourage the utilization of low temperature geothermal energy resources for power generation.