Performance assessment and multi-objective optimization of a novel transcritical CO₂ Rankine cycle for engine waste heat recovery

In this study, a novel self-condensing transcritical CO₂ Rankine cycle, which integrates the three-stage expansion process with an ejector cycle, is proposed to recover engine waste heat deeply and overcome the condensation issue caused by the relatively low critical temperature of CO₂. Thermodynamic and economic mathematical models are developed, and the detailed parametric analysis is carried out to investigate the effect of main parameters on both thermodynamic and economic performances of the system. Thereafter, a multi-objective optimization is conducted to trade off the two different performances. Results show that the proposed system could operate under higher temperature cold source conditions with desirable performance. The increases of turbine1 inlet pressure and LT gas heater outlet temperature, the decreases of LT gas heater outlet pressure and turbine3 back pressure are beneficial to achieve better thermodynamic and economic performances. On the basis of multi-objective optimization, the maximum net power output is 70.04 kW, which is a 10.20 % improvement compared to the reference cycle. Meanwhile, the engine power output could be increased by 7.03 % through adopting the novel system. Furthermore, the optimal exergy efficiency and unit net power cost are 37.02 % and 0.1567$/kWh, respectively.