Thermo-economic optimization and comparative analysis of different organic flash cycles for the supercritical CO₂ recompression Brayton cycle waste heat recovery

It is well known that the energy efficiency of the supercritical carbon dioxide recompression Brayton cycle (sCO₂RBC) can be improved by employing the bottoming cycles to recover its large waste heat. To achieve better performance, three bottoming cycles with a good temperature matching with the sCO₂ stream, namely the basic organic flash cycle (BOFC), the regenerative organic flash cycle (ROFC), and the organic flash Rankine cycle (OFRC), are employed to combine with the sCO₂RBC in this work. The parametric analysis is performed to determine the variation trends of systems’ thermodynamic and exergoeconomic performance with five important parameters. The single-objective optimization is conducted for the sCO₂RBC/BOFC, sCO₂RBC/ROFC and sCO₂RBC/OFRC systems with six organic fluids. The single-objective optimization results show that the highest exergy efficiency of the sCO₂RBC/OFRC is higher than that of the rest combined sCO₂RBC/OFCs by up to 1.95%. And the lowest total product unit cost for the sCO₂RBC/OFRC is up to 0.93% lower than that of the rest combined sCO₂RBC/OFCs. Finally, a comparative analysis is performed for the three combined sCO₂RBC/OFCs, sCO₂RBC/ORC and stand-alone sCO₂RBC systems under the multi-objective optimization conditions. Results show that the integrated systems can improve the η_ex by 4.62–6.06% and reduce the c_p,tot by 2.68–3.14% when taking the sCO₂RBC system as the baseline system. The sCO₂RBC/OFCs and sCO₂RBC/ORC are recommended to use R1336mzz(Z) and R245fa as working fluids, respectively. Moreover, the sCO₂RBC/OFRC system achieves an improvement of 0.41%–1.37% in exergy efficiency with a comparable total product unit cost, compared to other aforementioned integrated systems.