Performance analysis and binary working fluid selection of combined flash-binary geothermal cycle

• Performance of the combined flash-binary geothermal cycle is investigated.
• Thirty different fluids are screened to find the most suitable ORC working fluid.
• Optimum cycle operation conditions presented for geofluids between 150 °C and 250 °C.
• Refrigerants are more suitable for the ORC at geothermal sources temperature ≤200 °C.
• Hydrocarbons are more suitable for the ORC at geothermal sources temperature >200 °C.

Performance of the combined flash-binary geothermal power cycle for geofluid temperatures between 150 and 250 °C is studied. A thermodynamic model is developed, and the suitable binary working fluids for different geofluid temperatures are identified from a list of thirty working fluid candidates, consisting environmental friendly refrigerants and hydrocarbons. The overall system exergy destruction and Vapor Expansion Ratio across the binary cycle turbine are selected as key performance indicators. The results show that for low-temperature heat sources using refrigerants as binary working fluids result in higher overall cycle efficiency and for medium and high-temperature resources, hydrocarbons are more suitable. For combined flash-binary cycle, secondary working fluids; R-152a, Butane and Cis-butane show the best performances at geofluid temperatures 150, 200 and 250 °C respectively. The overall second law efficiency is calculated as high as 0.48, 0.55 and 0.58 for geofluid temperatures equal 150, 200 and 250 °C respectively. The flash separator pressure found to has important effects on cycle operation and performance. Separator pressure dictates the work production share of steam and binary parts of the system. And there is an optimal separator pressure at which overall exergy destruction of the cycle achieves its minimum value.