For the first time, one novel closed-loop pressure retarded osmosis (PRO) process promoted by an effective hydroacid complex draw solution has been demonstrated for harvesting the renewable salinity-gradient energy. The complex draw solute was molecularly constructed to possess unique characteristics of high osmotic pressure, large molecular size and relative low viscosity, and easy regeneration. Compared to conventional PRO processes, the newly developed closed-loop PRO process exhibits promising advantages of sustainable high power output, negligible internal concentration polarization and low membrane fouling, as well as no problems of feed water pretreatment and brackish water discharge. Employing a highly permeable (A = 4.30 LMH/bar) and selective (B = 0.47 LMH) thin film composite PRO hollow fiber membrane, a power density of 16.2 W/m(2) can be achieved with an ultralow reverse solute flux (J(s)/J(w) <0.062 g L-1) at 12 bar when using 1 M complex draw solution and deionized water as feeds. The diluted complex draw solution can be regenerated via a solvent precipitation process, and the outstanding PRO performance could be almost fully recovered. We believe the newly developed closed-loop PRO process shows great potential for salinity-gradient energy capture, although the specific benefits have to be fully defined through energy or cost analysis.