Enhancing the thermal and economic performance of supercritical CO2 plant by waste heat recovery using an ejector refrigeration cycle

Abstract

Supercritical CO2 cycle has an optimal performance when the cycle minimum temperature is around the critical temperature (31 °C), which is impossible at hot climatic conditions. To solve this problem, this work hybridizes a supercritical CO2 cycle with an ejector refrigeration cycle (ERC) to cool the minimum temperature of the cycle to be about 31 °C and hence achieving the highest possible performance. Comprehensive energy, exergy, and economic analyses are carried out to explore the mechanisms of performance improvement of the novel combined plant. Sensitivity analysis is performed to recognize the most influencing parameters on the performance of the combined plant. Based on the sensitivity analysis, the effect of different operating and design parameters on the system performance is investigated. Furthermore, a multi-objective optimization study is performed to find the trade-off between exergy efficiency and cost-saving. Among the different the five refrigerants used for ERC, the results illustrate that R717 is the most efficient one for the present hybridization. The exergy destruction in the precooler reduces from 15.5% to 0.7% when ERC is combined with the sCO2 cycle. Thus, the energy efficiency (ηth) and exergy efficiency (ηex) increase by 9.5%, while the levelized cost of energy (LCOE) declines by 10.7%. Compared with the standalone sCO2 cycle, the produced power, ηth, ηex, and LCOE of the optimized plant improve by 94.3%, 36.2%, 28.6%, and 18.3%, respectively.