The current work highlights development of novel thin-film composite hollow fiber membranes for pressure retarded osmosis applications to harvest salinity gradient energy. The membranes were developed with a specific target of harnessing the salinity gradient energy between wastewater reverse osmosis retentate and seawater reverse osmosis brine via osmotic mixing. The hollow fiber membranes were prepared by coating a polyethersulfone substrate with a thin-film composite polyamide layer via interfacial polymerization, which were assembled into modules of different diameters for lab scale and pilot scale evaluation. As the module size increased from 1-in. to 2-in., and 4-in., the water permeability, tested against a 1000 mg/L sodium chloride solution at 15 bar, decreased from 2.6 L m−2 h−1 bar−1 to 2.0 L m−2 h−1 bar−1, and 1.2 L m−2 h−1 bar−1, respectively. The power density, measured in the lab-scale unit using 1 M sodium chloride draw solution, and DI water feed solution, also decreased from 9.1 W/m2 to 5.3 W/m2 with increasing module size. Pilot scale evaluation of the 4-and 8-in. modules on a 24 m3/day unit resulted in lower power densities of 2.5 W/m2 and 1.5 W/m2 which translated to 0.024 kWh/m3 and 0.05 kWh/m3 of salinity-gradient energy harvested, respectively.