Study on supercritical CO2 power cycles for natural gas engine energy cascade utilization

An energy cascade utilization system (ECUS) composed of a natural gas engine (in the first stage) and supercritical CO2 power cycle (in the second stage) in series is established to improve thermal efficiency. From the perspective of energy and exergy, seven classical cycles and three engine coolant pre-heating cycles in the ECUS are investigated. Subsequently, the design criteria for supercritical CO2 power cycles are summarized. By introducing the concept of "Fuel-Product-Loss," the thermodynamics and exergoeconomics of the ECUS are comprehensively investigated. Based on the foregoing, the multi-objective optimization of the ECUS is investigated using the Pareto front. Moreover, the effect of engine load on the ECUS is considered. Results show that the thermal efficiency of the ECUS increases from 42.4% to 48.94%, the exergy efficiency increases from 50.17% to 52.01%, and the unit product cost decreases from 14.98 to 13.19 cent/kW center dot h compared with the original engine (100% load).