Mapping of novel multi-stage expansion supercritical CO2 Rankine cycle for composite waste heat recovery

To further enhance energy utilization efficiency, this paper proposes novel multi-stage expansion S-CO2 Rankine cycles for composite waste heat recovery. Concept of optimal number of expansion stages in the multi-stage configuration is defined, with a selection mapping for the optimal numbers of expansion stages being developed as tool to rapidly identify the multi-stage cycle configuration for target heat source. The research shows that the highest optimal number of expansion stages in multi-stage cycle is quintic-stage, and the maximum net power output is 79.5 kW, which is 16.1 % higher than that of the single-stage cycle. The efficiency gained in multi-stage cycles arises because the decreased value of exergy loss in the heaters is significantly greater than the increased value in the regenerators. Heat source parameters influence the optimal number of expansion stages by affecting thermal matching between single-stage cycle and high temperature heat source. The matching is quantified by a dimensionless parameter, which can serve as metric to determine optimal number of expansion stages. When the value of this dimensionless parameter is in the range of 4.27–5.20, the optimal number of expansion stages is quintic-stage. Furthermore, guidance is also provided for designing multi-stage configuration to recovery waste heat from different engines.