Investigation on geothermal binary-flashing cycle employing zeotropic mixtures as working fluids
Introduction The accelerated energy demand causes growing consumption of fossil fuels and massive discharge of pollutants, which promotes the development of renewable energy utilization. Among other renewable energy sources, the geothermal energy has the advantages of reliability, sustainability, high capacity factor and less ecological effect (Liu et al. 2017) . Over 70% of the geothermal sources available in the world are of low-enthalpy with the temperature lower than 150 °C (Franco and
... °C (Franco and Vaccaro 2014). As an efficient technology in the exploitation of the low-medium temperature geothermal energy, Organic Rankine Cycle (ORC) systems have drawn great attention (Pollet et al. 2018; Sadeghi et al. 2016) . Nevertheless, the thermal efficiency of the ORC system is generally less than 12% (Basaran and Ozgerer 2013), which is a serious obstacle for further application. It is not well-optimized from the view of thermodynamics. One major disadvantage of the ORC Abstract 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 (η th ), exergy efficiency (η 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 η 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 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. Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.