Thermodynamic and economic performance evaluations of double-stage organic flash cycle using hydrofluoroolefins (HFOs)

Organic flash cycle is an important way to achieve geothermal power generation. Double-stage organic flash cycle (DOFC) has two flash processes, remarkably enhancing the heat matching effect of heat absorption process compared to single-stage type. Working fluid is an important factor influencing the performance superiority of DOFC. However, traditional organic fluids are being phased out due to environmental requirements, the new-type hydrofluoroolefins are emerging with good environmental performance. For new-type hydrofluoroolefins, the characteristics of DOFC are unclear and its advantages over single-stage type need to be quantified. This work evaluates the thermodynamic, exergy and economic performance of DOFC using ten eco-friendly hydrofluoroolefins. Influences of key parameters on thermodynamic and economic performance are analyzed. For hydrofluoroolefins, the superiorities of DOFC over single-stage type were quantitatively evaluated. Results show that the advantages of DOFC will become greater at the lower heat source temperature and the lower critical temperature of hydrofluoroolefins. R513A shows the greatest advantages of DOFC over single-stage type, and the maximum relative increment in the net power is 111.2 % and the maximum relative decrement in specific investment cost is 49.5 %. The optimum hydrofluoroolefin is R1366mzz(Z) with a net power of 348.29 kW and a specific investment cost of 7.88 k$·kW⁻¹.