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

Abstract

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 ηex by 4.62–6.06% and reduce the cp,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.