Comparative and parametric study of double flash and single flash/ORC combined cycles based on exergoeconomic criteria

Abstract

In this study, double flash geothermal power cycle and single flash/ORC combined cycles with different organic fluids are used for power generation from geothermal fluid reservoir with given temperature. n-heptane, R141b, R113 and steam are selected as organic working fluids in ORC and NH3 is used as a reference working fluid in ORC in this study, as it is a common working fluid in geothermal power plants and Kalina. After energy and exergy analyses of the mentioned cycles, a comprehensive exergoeconomic analysis by developing cost balance and auxiliary equations for all components of the cycles using SPECO approach is done and all exergoeconomic parameters are calculated and these cycles are compared from thermodynamic and exergoeconomic viewpoints. Also a parametric study is performed and the effects on the thermodynamic and exergoeconomic performance of the considered cycles of such operating parameters as separator pressures in double flash geothermal power cycle and separator pressure and evaporation temperature in single flash/ORC combined cycles. The results show that although the highest values of first law efficiency and exergy efficiency among the mentioned cycles are related to single flash/ORC with steam but the minimum unit cost of produced power is related to double flash cycle. Also according to parametric study, with increasing evaporation temperature in single flash/ORC the value of C˙D,tot+Z˙tot parameters for all combined cycles except single flash/ORC with ammonia increases continuously.