Comparative analysis of CO2-based transcritical Rankine cycle and HFC245fa-based subcritical organic Rankine cycle using low-temperature geothermal source

A detailed thermodynamic and techno-economic comparison is presented for a CO2-based transcritical Rankine cycle and a subcritical organic Rankine cycle (ORC) using HFC245fa (1,1,1,3,3-pentafluoro-propane) as the working fluid driven by the low-temperature geothermal source, in order to determine the configuration that presents the maximum net power output with a minimum investment. The evaluations of both Rankine cycles have been performed based on equal thermodynamic mean heat rejection temperature by varying certain system operating parameters to achieve each Rankine cycle's optimum design at various geothermal source temperature levels ranging from 80A degrees C to 120A degrees C. The results obtained show that the optimum thermodynamic mean heat injection temperatures of both Rankine cycles are distributed in the scope of 55% to 65% of a given geothermal source temperature level, and that the CO2-based transcritical Rankine cycle presents 3% to 7% higher net power output, 84% reduction of turbine inlet volume flow rate, 47% reduction of expansion ratio and 1.68 times higher total heat transfer capacity compared with the HFC245fa-based subcritical ORC. It is also indicated that using the CO2-based transcritical system can reduce the dimension of turbine design. However, it requires larger heat transfer areas with higher strength heat exchanger materials because of the higher system pressure.