A potentially renewable and sustainable source of energy is the chemical energy associated with solvation of salts. Mixing of two aqueous streams with different saline concentrations is spontaneous and releases energy (Gibbs free energy). The global theoretically obtainable power from salinity gradient energy (SGE) due to World’s rivers discharge into the oceans has been estimated to be within the range of 1.4–2.6 TW.
Reverse electrodialysis (RED) is one of the emerging, membrane-based, technologies for harvesting SGE. A common RED stack is composed by alternately arranged cation- and anion-exchange membranes, stacked between two electrodes. The compartments between the membranes are alternately fed with concentrated (e.g., sea water) and dilute (e.g., river water) saline solutions. Migration of the respective counter-ions through the membranes leads to ionic current between the electrodes, where an appropriate redox pair converts the chemical SGE into electrical energy.
In this chapter, based on authors’ own as well as literature data, the influence of ion-exchange membranes type and solutions compositions on ionic transport, concentration polarization, and power generation in RED is presented and critically compared. The ways of assessment of the relevant electrical resistances, as well as the determination of diffusion boundary layer thickness through chronopotentiometry, under RED-operating conditions, are also discussed.
Currently, the maximal obtainable net power density through RED is close to 2.2 W/m2at 35 % of energy efficiency. The main challenge for improving the process efficiency is most frequently associated with the high electrical resistance offered by the dilute solution compartment and/or the presence of non-conductive spacers. The ionic composition of the two streams (dilute and concentrated saline solutions) and the nature of the used redox pair also influence to a certain extent the process efficiency. From an economical viewpoint, the relatively high current ion-exchange membrane price still remains a crucial factor for RED implementation in practice.