Thermodynamic and economic evaluation of the organic Rankine cycle (ORC) and two-stage series organic Rankine cycle (TSORC) for flue gas heat recovery

Abstract

The flue gas heat can be recovered by organic Rankine cycle (ORC) or two-stage series organic Rankine cycle (TSORC), but their comprehensive performance should be compared in detail. In this paper, ORC and TSORC are compared from thermodynamic and economic points of view. The technical indicators include the net output power, efficiency, thermal conductance, volumetric flow ratio, and the economic indicators the electricity production cost (EPC), payback period (PBP) and rate of return on investment (ROROI). The results show that the TSORC can recover more heat in the low-pressure stage from the heat source so as to decrease the irreversible loss between the heat source and the working fluid in contrast with ORC, thereby enhancing the net output power and exergetic efficiency but decreasing the thermal efficiency. However, from the economic point of view, the TSORC declines the EPC, PBP and ROROI due to that the increased investment is higher than the earnings as a result of the installation of the second-stage evaporation. A higher flue gas temperature from 200 °C to 300 °C tends to enhance the tech-economic performance of the TSORC. As the manufacturing cost goes down over time, the TSORC will further improve its economic performance in the future.