An array of hollow fiber membranes is grouped into bundles is submerged in an enclosed pure-water bath. The bundles are potted into top and bottom membrane headers such that the fiber lumen (the inside of the hollow fiber) is open to the inside of the membrane header and the pure feedwater is prevented from leaking into either the header or into the inside of a hollow fiber. Pure water penetrates the hollow fiber membrane. In accordance with pressure retarded osmosis (PRO) principles, the permeate and draw solution within the lumen are transported to at least one hydroturbine where electricity is generated. The draw solution contains superparamagnetic nanoparticles that are prevented from proceeding beyond a magnetic field. The system has a heater to improve its efficiency.