Enhanced energy recovery using a cascaded reverse electrodialysis stack for salinity gradient power generation.

Despite significant advances in the field applications of reserve electrodialysis (RED) to produce salinity gradient power, net energy production remains an issue owing to limitations such as high energy requirement for high flow rates of feed solutions, and severe fouling and pressure build up when thin spacers are used. Therefore, to maximize the performance and efficiency of energy harvesting in the RED, a cascaded RED stack, with multiple stages between the anode and cathode electrodes, was investigated. In cascaded stacks, 100-cell paired stacks were divided into several stages, so the feed water flowed into the first stage, and the effluent from the first stage was then reused in the next stages. This cascaded stack could overcome the typical drawbacks of RED (large amount of feed water required, intensive pumping energy, and low net energy production). Although 25% of the feed water volume was used in the 4-stage cascaded stack (100-cell-pairs) compared to the conventional stack (100-cell-pairs with a parallel flow operation), much more energy was produced with the 4-stage cascaded stack. The net power density and net specific energy with the 4-stage cascaded stack were the highest at 0.5 cm/s (0.48 W/m2) and 0.25 cm/s (0.06 kWh/m3), respectively. This is very promising for the practical application of RED since feed water volumes can be greatly reduced, which could reduce the burden on the feed water pretreatment step. Consequently, we can build a compact RED plant with smaller pretreatment processes and fewer RED unit stacks.

Assessing the behavior of the feed-water constituents of a pilot-scale 1000-cell-pair reverse electrodialysis with seawater and municipal wastewater effluent.

Reverse electrodialysis (RED) has vast potential as a clean, nonpolluting, and sustainable renewable energy source; however, pilot-scale RED studies employing real waters remain rare. This study reports the largest RED (1000 cell pairs, 250 m2) with municipal wastewater effluent (1.3-5.7 mS/cm) and seawater (52.9-53.8 mS/cm) as feed solutions. The RED stack was operated at a velocity of 1.5 cm/s and the pilot plant produced 95.8 W of power (0.38 W/m2total membrane or 0.76 W/m2cell pair). During operation of the RED, the inlet design of the stack, comprising thin spacers, and the water dissociation reaction at the cathode were revealed as vulnerabilities of the stack. Specifically, pressure drops at the fluid inlet parts had the most detrimental effects on power output due to clogged spacers around the inlet parts. In addition, precipitates resulting in inorganic fouling were inevitable during the water dissociation reaction due to significant potential generated by the stack in the cathode chamber. Na+ and Cl- accounted for the majority of ions transferred from seawater to wastewater effluent through ion exchange membranes (IEMs). Moreover, some divalent cations in seawater, Mg2+ and Ca2+, were also transferred to the wastewater effluent. Some organics with relatively low molecular weights in the wastewater effluent passed through the IEMs, and their hydrophobic properties elevated the specific UV absorbance (SUVA) level in the seawater.