Thermoeconomic analysis of a novel cogeneration system for cascade recovery of waste heat from exhaust flue gases

Abstract

In low-grade waste heat recovery scenarios, an absorption refrigeration cycle (ARC) or organic Rankine cycle (ORC) can be used because of its widespread acceptance in combined cooling and power applications. This paper introduces a cogeneration configuration employing a cascade waste-heat recovery architecture (including an ARC, ORC, and intermediate heating loop) to harness waste heat from the flue gas of a gas turbine (GT). The results demonstrated that the first-law efficiency and total power generation were largely influenced by the ARC, whereas the second-law efficiency and total available energy were primarily affected by the ORC. When the ARC generation temperature difference and ORC evaporation temperature were adjusted, the first- and second-law efficiencies exhibited divergent trends, suggesting the absence of a unified strategy to enhance both efficiencies simultaneously in the current system. The system achieved its highest first-law efficiency of 54.84 % and maximum total power generation of 1866.10 kW at an ORC evaporation temperature of 145 °C and a heat-source inlet temperature of 160 °C. Furthermore, at an ORC evaporation temperature of 150 °C and a heat-source inlet temperature of 200 °C, the system reached its highest second-law efficiency of 34.57 % and maximum total available energy of 1233.3 kW. The case study revealed that the GT system (including air compressor, air preheater, combustion chamber, and gas turbine) accounted for the highest level of irreversibility, comprising 74 % of the overall system. Specifically, the combustion chamber in the GT system produced a significant irreversibility due to chemical reactions. Compared with traditional combined cooling and power systems, the proposed system can increase the first-law efficiency and lower the unit energy cost of the output.