Investigation on geothermal binary-flashing cycle employing zeotropic mixtures as working fluids

The binary-flashing cycle (BFC) is supposed to be a promising technology for geothermal recovery due to the full use of geofluid. For further performance improvement, the potential of using mixtures of a hydrocarbon and a retardant in the BFC system is investigated. R245fa is selected as a retardant and blended with R600 to form zeotropic mixtures. With the thermal efficiency (eta(th)), exergy efficiency (eta(ex)), net power output per ton geofluid (PRW), and exergy destruction rate (E-d) as evaluation indexes, the flowsheet modeling and optimization are conducted to explore the optimal compositions and operating parameters. It is revealed the optimal mass fraction of R600/R245fa is 0.44/0.56, at which the flammability of R600 is suppressed, the global warming potential (GWP) of R245fa is reduced. The maximum eta(ex) and PRW are higher than those of component fluids. The maximum E-d occurs in the heat exchanger, which should be optimized. The recommended generation pressures are 1200-1430, 1240-1480, 1220-1460, and 1170-1420 kPa, respectively, for R600 mass fraction of 0.2, 0.4, 0.6, and 0.8. In addition, the flashing temperature is also optimized. Although the mixtures do not always yield superior performance, it is still beneficial to apply the mixtures to the BFC system through systematic consideration of safety and environmental friendliness