Comparative exergetic sustainability analysis of subcritical and supercritical organic Rankine cycle plants for a waste heat recovery application

This study was aimed at comparing the exergetic sustainability and exergoenvironmental performance of subcritical (SUBORC) and supercritical (SUPERORC) organic Rankine cycle (ORC) plants for waste heat recovery from a marine vessel engine. A liquid-gas shell-and-tube heat exchanger was employed to recover the engine exhaust gases’ thermal energy. The hot exhaust gases flowing through the gas side heat the Therminol 66 heat transfer fluid employed on the liquid side, which in turn heats the ORC working fluid (R600a). Zero-dimensional mass, energy, and exergy balance equations defined by the Laws of Thermodynamics were implemented in MATLAB for the design of the two ORC configurations. Results showed that the evaporator and the preheater are the most sustainable components with exergetic sustainability index (ESI) values of 7.7 and 39.4 for the SUBORC and the SUPERORC, respectively. Additionally, the condenser was obtained in both cases with the worst environmental performance with exergoenvironmental impact of electricity (EIE) values of 12 and 13 eco-indicator points per 1 MW of electricity produced by the SUBORC and the SUPERORC, respectively. Moreover, results showed that a switch from the SUBORC to the SUPERORC would have an insignificant effect on the ESI at the system level; it would reduce the recoverability ratio by about 4%, from about 0.47 in the SUBORC to around 0.45 in the SUPERORC; and it would reduce the EIE by 1.4 Pt/MW. In sum, a supercritical ORC system would exhibit better exergetic environmental sustainability than a subcritical one for waste heat recovery from a marine vessel engine.