Direct osmosis process for power generation using salinity gradient: FO/PRO pilot plant investigation using hollow fiber modules

The phenomenon of osmosis is known for more than 150 years and involves the contact of a semi-permeable membrane with solutions of different salinity. The osmotic flux that permeates through the membrane can be used for electric power generation as a renewable source of energy. This process can be accomplished by means of the pressure retarded osmosis (PRO). This work consisted of the construction of a demonstrative FO/PRO power pilot plant. Assymetric polymeric cellulose acetate hollow fiber membranes were produced and conditioned in permeation modules. The pilot plant consists of three countercurrent flow hollow fiber modules connected in parallel. According to the modules and fibers dimensions, the membrane area per module can reach 1.5 m2 that means a packing density of 1500 m2 m−3. The pilot scale enables the activation of a hydroturbine/generator device coupled to a 9-LEDs panel which can establish the technological and scientific bases for further industrial upscaling. A forward osmosis (FO) test using a MgSO4 solution (0.8 M, π: 22 bar) and an intermittent PRO test using a NaCl solution (0.8 M, π: 37 bar) were performed, where the maximum osmotic flux values were 33 and 8 L m−2 h−1, corresponding to an energy performance of 5 and 2 W m−2, respectively. These values are comparable with most of the latest bench-scale FO membrane investigations.