Faster Time Response by the Use of Wire Electrodes in Capacitive Salinity Gradient Energy Systems

Abstract

Capacitive energy extraction based on Donnan potential (CDP) and capacitive energy extraction based on double layer expansion (CDLE) are novel electroctrochemical processes to convert the potential free energy of mixing sea and river water into electric work. This is done by the use of supercapacitor electrodes with and without ion exchange membranes. Currently, these techniques rely on improved mass transport in order to become more efficient and give higher power output. In this paper we evaluate the transport phenomena by diffusion and the electrode geometry when switching between sea and river water at open circuit potential (OCP). By changing the electrode geometry from a flat plate to a cylindrical one, experiments and analytical models in combination show that mass transport by diffusion is increased. This is demonstrated without any changes in the hydrodynamic conditions. Improving mass transport without changing the hydrodynamic conditions breaks with what has been the convention in the scientific community of salinity gradient power. Moreover, in sea water the transport phenomena appear to be controlled by diffusion, and the response time for building open circuit potential in CDP and CDLE under this condition is reduced by a factor of 2 when using wire electrodes instead of flat plate electrodes. In river water, the trend is similar though the response time is generally larger.