Thermodynamic Modeling and Exergoenvironmental Analysis of a Methane Gas-Powered Combined Heat and Power System

Abstract

A combined heat and power (CHP) system powered by methane gas is modelled and analyzed in this study. The Thermolib MATLAB extension is used to model the system by graphically connecting the Thermolib standard components through fluid flows. An exergoenvironmental analysis is also performed using EES. The results show that, for an input thermal energy rate of 29.9 MW, the Brayton and Rankine cycles generated 9.8 MW and 7.5 MW of net power, respectively. The heat pump was also able to supply 1.4 MW as its output. The total energy efficiency of the cogeneration system was 62% with the Brayton cycle working at 33%, the Rankine cycle at 36%, and the ammonia heat pump at a coefficient of performance (COP) of 9.1. The system also achieved an overall exergy efficiency of 78%. Furthermore, the system was examined at different levels by varying input parameters such as the pressure ratio of both the Brayton cycle and the heat pump, the pressure of the steam in the Rankine cycle, and the inlet energy from the combustion chamber of the system. The exergoenvironmental modeling of the system showed that the exergy stability factor and exergetic sustainability index increased from 0.41 to 0.47 and from 0.6 to 0.64 with increasing inlet combustion energy; this can be seen as a good indicator of its stability and sustainability.