Multi-objective optimization and exergoeconomic analysis of supercritical-transcritical CO2 cogeneration cycles with integrated ejector and vapor injection cooling

Cogeneration systems built with conventional supercritical CO 2 topping power cycles cannot achieve sufficiently low exhaust cooling temperatures. Therefore, in the present study, four novel cogeneration systems are developed using regenerative Supercritical-Transcritical (sCO 2 – tCO 2 ) as the toping power cycle. These supercritical transcritical cogeneration (STC) cycles have excellent heat recovery characteristics. Four distinct STCs with different bottoming cycles are Vapor Compression (STC-VCRS), flash tank (STC-FT), ejector enhanced (STC-EE) and ejector injection (STC-EI) refrigeration system for cooling. The STCs are compared against regenerative sCO 2 – tCO 2 cycle (RSTC) which outperforms recompression sCO 2 cycle in internal combustion engine (ICE) exhaust waste heat recovery (WHR) for reference. Parametric analysis is performed to evaluate the impact of pressure ratios, turbine inlet temperatures, evaporator inlet temperature and evaporator mass flow rate on cycle performance. Exergoeconomic performance of the STCs for a particular operational condition is also assessed. Single objective optimization using genetic algorithm (GA) disclosed STC-EE has the highest utilization factor (UF) of 34.91% followed closely by STC-EI. STC-EI also incurs the lowest unit cost, 7.41 $/GJ. Multi-objective optimization is performed using Non-Dominated Sorting Algorithm (NSGA-II) to find tradeoff between maximum UF and minimum unit cost. A lower minimum cost is found in all four of the STCs than the reference RSTC. STC-EE has 16.22% higher UF and STC-FT has 4.13% lower cost which is 7.128 $/GJ than the reference system at their optimum. STC-EI provides 15.49% improved UF at 2.3% reduced unit cost, 7.264 $/GJ, a high COP of 4.013 and 62.9% exergy. These findings reveal ejector vapor injection despite its complexity can provide greater output at lower cost in integrated systems. The study also proves STCs can be the economical choice for IC engine WHR.