Thermally enhanced osmotic power generation from salinity difference

Abstract

Recently, membrane-based power generation from salinity difference has been in the spotlight as a blue energy harvesting, but achieving a high power density and conversion efficiency still remains as a major challenge in this approach. Instead of developing new membranes, regulating the thermal condition within the membrane has been proposed as a way to enhance the power generation by several numerical studies, but this concept has rarely been explored through the systematic experimental studies due to the difficulty of imposing a controlled temperature gradient within the membrane. In this work, we experimentally and systematically study how the temperature difference can influence osmotic power generation using a commercial polycarbonate membrane and demonstrate that even when a thermal gradient is negligibly small within the nanoporous membrane, it is still possible to achieve a significant enhancement of the power generation. We propose that the effective ion concentration at the interfacial region between the reservoir and the membrane varies with the direction of the imposed temperature difference, such that the opposite direction of salinity and temperature differences can lead up to 5.3 times power enhancement as a result of the increase of the effective ion concentration ratio across the membrane. As an example of practical applications, we apply our findings to a floating type nanogenerator by incorporating a solar absorber to generate the temperature difference spontaneously under solar radiation conditions, and the results with the nanogenerator show that the power generation is indeed enhanced under both simulated and actual solar radiation conditions. We believe that our approach can be applied to any nanoporous membrane regardless of its thermal property, and therefore would provide a practical path to the power enhancement of reverse electrodialysis systems.