Thermodynamic Analyses of Sub- and Supercritical ORCs Using R1234yf, R236ea and Their Mixtures as Working Fluids for Geothermal Power Generation

Abstract

Organic Rankine cycles (ORCs) have been widely used to convert medium-low-temperature geothermal energy to electricity. Proper cycle layout is generally determined by considering both the thermo-physical properties of the working fluid and the geothermal brine temperature. This work investigates saturated, superheated and supercritical ORCs using R1234yf/R236ea for brine temperatures of 383.15 K, 403.15 K and 423.15 K. The evaporation and condensation pressures were optimized to maximize the net power outputs. The thermodynamic characteristics of the cycles at the optimal conditions were analyzed. The saturated ORCs produced slightly more net power than superheated cycles for the R1234yf mole fraction less than 0.2 due to lower exergy losses in the evaporator and condenser; however, the limited evaporation pressure by the turning point at the higher R1234yf mole fraction led to excessive exergy losses in the evaporator. Two R1234yf mole fractions maximized the net power and exergy efficiency in a superheated cycle, with the maximum net power output occurring at the R1234yf mole fraction of 0.8 for brine temperatures of 383.15 K and 403.15 K. The exergy losses for evaporation were reduced by 6–12.7% due to the use of an IHE, while those for condensation were reduced up to 42% in a superheated cycle for a brine temperature of 423.15 K, resulting in a 1–17.8% increase in the exergy efficiency. A supercritical cycle with an IHE using R1234yf/R236ea (0.85/0.15) generated the maximum net power output for a brine temperature of 423.15 K, 8.2–17.5% higher than a superheated cycle with an IHE.