Membrane-based indirect power generation technologies for harvesting salinity gradient energy - A review

The giant and sustainable salinity gradient energy broadly occurs when mixing solution sources with different concentrations can be potentially harvested through the intensively studied membrane-based indirect power generation technologies. This kind of technology commonly has two functional submodules, namely the osmosis submodule to induce driven forces and the electric submodule to produce electricity. However, almost all relevant reviews only concentrate on the traditional pressure retarded osmosis (PRO) technology without involving newly emerged ones such as the forward osmosis-electrokinetic (FO-EK) technology, leading to outdated and incomplete knowledge in this field. Therefore, this study is going to provide a comprehensive and up-to-date review of the membrane-based indirect power generation technologies through adequately outlining most related research. The authors not only provide a systematic overview of the theoretical background and the development of the state-of-the-art membrane-based indirect power generation technologies but also highlight their essential characterizations. Meanwhile, the challenges and the optimizing strategies in aspects of efficient semipermeable membranes, available fluid materials, and operation conditions, as well as future promising applications in different scenarios are also elaborated in detail.