Zeotropic mixture as a working fluid for cascade Rankine cycle-based reverse osmosis: Energy, exergy, and economic analysis

Abstract

This study investigates the cascade Rankine cycle coupled with a reverse osmosis system for brackish groundwater treatment. The proposed system integrates a steam Rankine cycle (SRC) and an organic Rankine cycle (ORC) in a looped configuration, utilizing solar energy as a heat source. Each Rankine cycle is coupled with reverse osmosis (RO) to produce approximately 1 m³/h of permeate from each RO system. The system is investigated with working fluid combinations from R1233zd(E), R1234ze(Z), and R1336mzz(Z). Through comprehensive energy, exergy, and economic analyses, the system's performance is evaluated with zeotropic mixtures compared to pure R1233zd(E). The results demonstrate reliable performance with zeotropic mixtures, particularly R1233zd(E)/R1234ze(Z) with a mass composition of 0.6/0.4, demonstrating the maximum ORC expander work output of 1.15 kW. Parametric analysis reveals remarkable performance under different ORC system parameters. Variations in SRC condensation pressure show a trade-off performance between SRC and ORC turbine work output. Exergy analysis reveals an increase in exergy destruction by evaporation-based ORC components and a reduction in exergy destruction by condensation-based components, emphasizing improved irreversibility during the condensation process. Economic analysis indicates a marginal impact on the overall system cost, with the treated water cost ranging from 0.891 to 0.919 $/m³.