Project Se(L)ect(i)vity (PTDC/EQU-EQU/6193/2020)

Power generation by reverse electrodialysis (RED) depends on ionic mass transfer through alternately arranged cation- and anion-exchange membranes. Chronopotentiometric measurements were carried out in an EDR-Z Mini stack (MEGA a.s.), equipped with Ralex heterogeneous membranes, separated by either sheet flow spacers or mesh-free gaskets. Various concentrations of model NaCl solutions were used to study the ohmic and non-ohmic resistances in the stack under different hydrodynamic conditions. In order to eliminate the uncertainties associated with a non-uniform distribution of the applied current over the membrane surface, the relaxation zone of the obtained chronopotentiograms was used to estimate the diffusion boundary layer thickness. It was found that this approach provide more accurate data under RED operating conditions, which are strongly influenced by entrance effects on mass transfer, especially for a spacer-free channel configuration. For shorter flow channels, since the salt concentration profile across the diffusion boundary layer is not yet fully developed, more power can be obtained than in the case of longer channels. The presence of spacers was found to reduce the boundary layer thickness, but also increased the ohmic resistance, due to their shadow effect over the membrane surface. The description of the impact of flow entrance effects on mass transfer, and therefore on diffusion boundary layer thickness, can be useful for characterization and further design and/or optimization of RED stacks performance.

Reverse electrodialysis (RED) is a sustainable technology for salinity gradient energy harvesting. In order to make the process economically competitive, it is desirable to operate it at the highest possible net power density, which depends on the RED stack geometry and on the pressure drop along its pathways and, thus, on the energy spent for solutions pumping. The fluid flow in RED stacks generally occurs in rectangular compartment channels, equipped with spacers. The effects of spacers design and properties have been studied extensively in recent years. However, the other possible causes for a RED stack and their relative impact on the process performance have not yet been systematically studied. In this study the partial pressure drops in (1) distribution ducts, (2) branches, (3) beams, (4) due to sudden section expansion between the beam and the compartment channel and (5) in the compartment channel were taken into consideration. A model for the total pressure drop inside a RED stack, with a parallel fluid flow distribution through the compartments, is proposed and experimentally validated for lab-scale RED stacks with sheet flow spacers and compared with an open channel (spacer-free) design. The importance of each partial pressure drop was then evaluated quantitatively through model simulations for industrial-scale stacks with an increasing number of cell pairs. It was found that the net power density decreases when the cell-pair number increases, since the partial pressure drop in the branches becomes dominant. Moreover, the possible reasons for a non-uniform fluid flow distribution are discussed, thus making the proposed model useful for planning and/or optimization of RED stacks design.