Evaluation of flow fields on bubble removal and system performance in an ammonium bicarbonate reverse electrodialysis stack

Ammonium bicarbonate has recently been demonstrated to be an excellent thermolytic solution for energy generation in reverse electrodialysis (RED) stacks. However, operating RED stacks at room temperatures can promote gaseous bubble (CO2, NH3) accumulation within the stack, reducing overall system performance. The management and minimization of bubbles formed in RED flow fields is an important operational issue which has yet to be addressed. Flow fields with and without spacers in RED stacks were analyzed to determine how both fluid flow and the buildup and removal of bubbles affected performance. In the presence of a spacer, the membrane resistance increased by ∼50 Ω, resulting in a decrease in power density by 30% from 0.140 W m−2 to 0.093 W m−2. Shorter channels reduced concentration polarization affects, and resulted in 3−23% higher limiting current density. Gas accumulation was minimized through the use of short vertically aligned channels, and consequently the amount of the membrane area covered by bubbles was reduced from ∼20% to 7% which caused a 12% increase in power density. As ammonium bicarbonate RED systems are scaled up, attention to channel aspect ratio, length, and alignment will enable more stable performance.