Thermodynamic, and Economic Performance of Novel ORC Designs Powered by Low-Grade Waste Heat

Abstract

Waste heat recovery is one of the alternative energy sources that have been investigated by scientists in recent decades to address ongoing environmental problems, such as global warming. The organic Rankine cycle is a promising waste heat recovery technology to exploit industrial waste heat, even at low-temperature levels (<100 o C), for electricity production. The proposed study presents and compares two innovative organic cycle designs feeding with the same available heat source. The first cycle includes a nearly isothermal expander, where a small fraction of the supplied waste heat is continuously provided to the expander, aiming to approach a quasi-isothermal process instead of an adiabatic one, avoiding the temperature decrease due to the expansion process. The result is an increase in the cycle's thermal efficiency and greater power output production compared to the adiabatic expansion process. The second configuration is called the trilateral flash cycle, where the working fluid does not reach the saturated vapor state during heating at the heat recovery system, while it expands into the two-phase region of the fluid. The aforementioned cycles are investigated parametrically in terms of thermodynamics with a low-temperature heat source (80-100 o C) for different organic working fluids, such as the R1234ze(E), R1234yf, R1233zd(E), and R134a. Parametric studies are carried out through Aspen Plus software, while a techno-economic comparison of the organic cycle designs is conducted based on Aspen Process Economic Analyzer and literature data. According to the final results, R1233zd(E) seems to be the most proper working fluid thermodynamically, while the organic Rankine cycle with nearly isothermal expansion achieves higher values of both electrical and exergy efficiencies, reaching the maximum values of 10.51%, and 52.27%, respectively. In terms of finance, both cycles achieve similar payback period values, reaching the value of 1.56 years in the case of the trilateral flash cycle and assuming 8,000 operating hours per year. Finally, for the trilateral flash cycle, lower net present value levels of about 30% compared to the corresponding values for the other cycle, are determined despite its lower installation cost.