Performance assessment and multi-objective optimization of a novel transcritical CO2 trigeneration system for a low-grade heat resource

Abstract

This work focuses on designing a self-sufficient trigeneration system for low-grade heat resource applications. For this purpose, a novel transcritical CO2 combined cooling, heating, and power (CCHP) system is proposed, which integrates a Rankine power cycle and an ejector refrigeration cycle (ERC). To evaluate the feasibility of the proposed system, the mathematical model of the combined cycle is built and validated. The effects of seven key parameters on system performance are investigated from the thermoeconomic viewpoint. Furthermore, multi-objective optimization is performed for the system when it generates both cooling and power or simultaneously produces heating and power. The results show that when using low-grade heat resource, the proposed plant not only has desirable net power output under all considered conditions, but also achieves adjustable output for cooling and heating in a broad range. The exergy efficiency of the proposed system and the coefficient of performance (COP) of ERC under base case conditions are respectively improved by 13.3% and 167.7% compared to the reference cycle. Under optimal conditions, the total useful energy and exergy efficiency of the system respectively are 127.3 kW and 22.7% for the combined cooling and power (CCP) mode, while corresponding values are 126.2 kW and 43.6% for the combined heating and power (CHP) mode, respectively. The cost per unit of exergy products for the system in the CCP mode is 3.4 times more than that in the CHP mode.