Introduction: The dual-pressure evaporation Organic Rankine Cycle (ORC) consists of two evaporation and compression processes, which yield a better temperature match during evaporation and lower exergy loss than a single-pressure evaporation ORC, resulting in the highest exergy efficiency and maximum net power output.
Methodology: In this study, an assessment process is designed and simulated by AspenPlus (V12) on the dual-pressure evaporation ORC to determine the maximum and assessed net power output, overall exergy efficiency, and thermo-economic parameters, including the Specific Purchased Equipment Cost (SPEC), Net Earning (NE), and Payback Period (PBP). Therefore, we evaluate to make a comparison between R245fa as an old but high-performance refrigerant working fluid with R1336mzz(Z) as a substitute and environmentally friendly refrigerant working fluid with low Global Warming Potential (GWP), Ozon Depletion Potential (ODP), and very low lifetime in the atmosphere in pure and different mole fraction of zeotropic mixtures based on effect of different geothermal water temperatures as a heat source (100, 125, 150, 175, and 200 °C).
Results: The results reveal that, based on the design and simulation of a new assessment process for the dual-pressure evaporation ORC under subcritical conditions at 0.05 bar below the critical pressure of the working fluids, geothermal water temperature is considered a fundamental key parameter in this assessment process. We increased the geothermal water temperatures and discuss the results from two perspectives: thermodynamic and thermo-economic. Regarding the thermodynamic results, the maximum net power output and overall exergy efficiency increase significantly due to the increased heat duty in the evaporators and a better temperature match between pure and zeotropic working fluid mixtures and geothermal water during both high- and low-pressure evaporation. In this case, among pure working fluids, the maximum net power output for R1336mzz(Z) showed 12.05%, 15.55%, 21.09%, 8.84%, and 5.12% improvements compared with R245fa at 100, 125, 150 °C, 175 °C, and 200 °C, respectively, because of higher evaporator heat duty and higher thermal conductivity. Furthermore, for all mole fractions of binary zeotropic mixtures of working fluids, based on this newly designed assessment process, the R1336mzz(Z)/R245fa (0.8/0.2) reveals a maximum improvement of 23.86%, corresponding to 2049 kW, which is the highest maximum net power output among all pure and zeotropic mixtures at a geothermal water temperature of 150 °C. Also, these results are reflected in the overall exergy efficiency of pure R1336mzz(Z) at all five geothermal water temperatures, with 5.01%, 6.22%, 10.11%, 4.03%, and 3.74%, improvements. R1336mzz(Z)/R245fa (0.8/0.2) illustrates a 17.99% improvement at 150 °C and reaches 88.95% as the highest exergy efficiency compared with all pure and zeotropic mixtures. These results are due to the lower irreversibility arising from lower viscosity and a higher reduced temperature of the geothermal water output. According to the thermo-economic results, the SPEC, as an indication of economic efficiency for pure working fluids, shows a reduction of 18.05-24.08% for R1336mzz(Z) across all geothermal water temperatures compared with R245fa due to its higher maximum net power output. For all mole fractions of binary zeotropic mixtures of working fluids, R1336mzz(Z)/R245fa (0.8/0.2) shows a 26.43% reduction in SPEC at 150 °C compared with all pure and zeotropic mixtures. To support this, the higher heat duty during evaporation processes indicates a greater required heat exchanger area. On the other hand, the lower high and low pressures in the compression processes of R1336mzz(Z) have a positive effect on SPEC. However, among the other thermo-economic results, the NE for R1336mzz(Z) is slightly higher than that for R245fa, owing to the higher maximum net power output, and this increase in NE has a positive effect on PBP. Based on the designed assessment process, PBP was reduced to its minimum value, reaching 13.21 years, for R1336mzz(Z)/R245fa (0.8/0.2). Finally, in terms of environmental analysis results, R1336mzz(Z)/R245fa (0.8/0.2) was selected at a geothermal water temperature of 150 °C, with a GWP of 207.6, compared with R245fa with 1030 GWP.
Conclusion: Overall, according to the designed and simulated assessment process for the dual-pressure evaporation ORC, the substantial improvement in overall exergy efficiency for R1336mzz(Z)/R245fa (0.8/0.2) indicates lower irreversibility during evaporation and a smaller temperature reduction of the geothermal water output. Likewise, the lowest SPEC and PBP of R1336mzz(Z)/R245fa (0.8/0.2), as the optimal composition compared with all pure and zeotropic mixtures, are attributed to its higher maximum net power output resulting from higher thermal conductivity and lower viscosity. Therefore, the newly designed and simulated assessment process identified R1336mzz(Z)/R245fa (0.8/0.2) as the best binary zeotropic mixture to substitute R245fa.
