Exergetic Efficiency of Reverse Osmosis Desalination Systems: Definitions and Discussions

Reverse osmosis (RO) desalination is the dominant desalination technology worldwide. The exergetic efficiency—which is recognized as a true measure from a thermodynamic viewpoint—of RO systems has been addressed by many researchers, but the inconsistencies in existing definitions prevent not only objective evaluation of the exergetic performance of an individual RO system, but also a logical comparison of the results among different systems and studies. Strictly following the thermodynamic and exergoeconomic principles, this study presents general and consistent definitions of exergetic efficiency for RO systems, aiming to promote uniformity in this important criterion. Considering the purpose and parametric features of RO systems, total exergy is decomposed into chemical and physical (mechanical plus thermal) exergies. The exergetic efficiencies of over 50 cases are calculated, and the results from different relationships and studies are compared and discussed. For the first time, the exergetic efficiency of thermally-enhanced RO systems is discussed, and the influence of thermal exergy consumption and the thermal exergy increments of permeate and brine is analyzed. Furthermore, the reasons behind the significant deviations in the results of some studies are revealed, and the defects or even mistakes in some of the existing definitions are pointed out. This work clarifies the definition and improves the understanding of the exergetic efficiency of RO desalination systems.