Energy Conversion from Salinity Gradients by Forward Osmosis-Electrokinetics

A direct energy conversion technology based on electrokinetic (EK) phenomenon has attracted increasing attention during recent years. However, an external driving source (e.g., hydrostatic pressure) is needed to produce an EK flow in microchannels. Since the required driving pressure becomes significant when the size of channels shrinks, the EK energy conversion efficiency is usually low. We recently have developed a novel hybrid energy conversion technique using combined principles of EK and forward osmosis (FO). The proposed FO-EK energy conversion technique has been demonstrated, by using an experimental system comprising two submodules, a FO submodule and an EK submodule. Through the use of a salinity gradient, a suction force is created to induce a hydrodynamic flow in the FO submodule based on the principle of FO. Accordingly, electric energy, in forms of EK streaming potential and streaming current, is generated across a porous glass housed in the EK submodule. This proposed power generation technique converts the salinity gradient energy into the electric energy without need of external pressure input. In this work, thorough experiments are carried out to investigate the FO-EK energy conversion processes. The results show that the generated power density decays with an increase of the pore size of the porous glass and monotonically increases with increasing salinity difference between the draw and the feed solutions. It is also demonstrated that a maximum power density of 0.165 W m(-3) can be achieved by using the porous glass with an average pore size 6 mu m at 4 M salinity difference. Theoretically, two kinds of mathematical models, namely a uniform-capillary model and a heterogeneous-capillary model, are presented. Compared to the uniform-capillary model, the heterogeneous-capillary model proposed in this work has better agreement with the experimental data.