Multi-objective optimisation of supercritical CO₂combined cycles based on energy-exergy-economy balanced analysis

In this paper, the supercritical CO2 (sCO2) recompression cycle combined cycles are analysed, where the bottom cycles include transcritical CO2 (tCO2), organic Rankine cycle (ORC) or trilateral flash cycle (TFC). The working fluid for the ORC and the TFC can be R123, R245fa, or n-Pentane. We implemented four sub-models in our calculation: 1) thermodynamic model; 2) heat transfer model; 3) economic model; 4) exergoeconomic model. The specific exergy costing (SPECO) method is utilised in the exergoeconomic model, and the multi-objective genetic algorithm is adopted to give the Pareto front of total unit exergy cost of the product (cPtot) and the thermal efficiency. Our model has been validated with the existing literature. The results show that the thermal efficiency standalone CO2 achieved 41.66% while attaching the tCO2, ORC, or TFC bottom cycle could improve the thermal efficiency by 1.1%, 1.68%, or 0.05%, respectively. In the meantime, the cPtot, which is the representation of the cost, decreased by 0.2$/GJ for ORC and increased by 0.06$/GJ and 1.05$/GJ for tCO2 and TFC respectively. The influence of different working fluids for ORC and TFC is not obvious. Therefore, attaching the ORC as the bottom cycle will bring about the best performance and lowest cost.