BEYOND THE SALINITY GRADIENT ENERGY HARVESTING

Abstract

Recently, salinity gradient energy – known as blue energy – has sparked the interest of researchers in identifying a new source of renewable energy, available at the contact between river water and seawater. In this paper, which is a mini-review of the salinity gradient energy harvesting, the appreciable potential of this energy source is emphasized by calculating the thermodynamic effect of mixing a water of high salinity, which simulates seawater, with water of low salinity, characteristic of rivers. Unfortunately, this potential of salinity gradient energy is difficult to exploit due to technical limitations. In such circumstances, the seawater-river water system was approached as an electrochemical thermodynamic system, at the interface of which an electric potential difference occurs. In the simplest case, this is a diffusion potential, evaluated based on Henderson equation. The value of the diffusion potential is low (about 20 mV) because the interface between these two media is crossed by both cations and anions. If, however, the two media of different salinity are separated by an ion exchange membrane, there is a much larger potential difference between them (about 100 mV), called membrane potential, which can be capitalized using a concentration cell. Moreover, the main methods applied so far for the recovery of the salinity gradient energy are highlighted: pressure retarded osmosis, reverse electrodialysis and capacitive mixing methods.