Energy, exergy, economic, and environment (4E) assessment of trans-critical organic Rankine cycle for combined heating and power in wastewater treatment plant

Abstract

In order to realize energy utilization of the biogas produced by anaerobic digestion of wastewater and sludge, this paper proposes an organic Rankine cycle (ORC) based cogeneration system for wastewater treatment plants (WWTPs). However, due to the seasonal changes in human eating habits, there are significant changes in organic matter content in wastewater and sludge, biogas production and exhaust gas discharged from the internal combustion engine all change consequently. The rated value of the exhaust gas was firstly ascertained. In addition, the thermodynamic model of the trans-critical organic Rankine cycle (TCORC) was constructed based on the first and second laws of thermodynamics. Finally, the economic and environmental benefits of the TCORC system were also analyzed. Results show that the annual weighted average value of the exhaust gas is used as the selection rule to ascertain the rated value of exhaust gas. There exists supercritical temperature and pressure to maximize the power output, thermal and exergetic efficiencies. Moreover, the choice of the working fluid has a greater impact on the performance of the TCORC system, and the higher the critical state point, the easier it is to obtain better performance. For various working fluids in the trans-critical cycle, the optimal operating point are taken at supercritical pressure and temperature slightly above the critical parameters. For n-Decane, the maximum annual fossil fuel savings and the annual reduction of CO2 are 0.36 × 106 L and 1.15 × 106 kg, respectively, when supercritical pressure and temperature are 4.21 MPa and 352 °C, respectively. The TCORC enhances the overall performance of the WWTPs, which should be employed in engineering applications.