Efficiency of Carnot and Conventional Capacitive Deionization Cycles

Brackish water treatment characteristically suffers from low efficiency. Overcoming this challenge requires a fundamental understanding regarding the energetics of both the ion-removal process and the trajectory of the complete deionization cycle. Evaluating the processes and cycles in tandem can elucidate potential sources of inefficiency (entropy production) and may promote new operational schemes leading to higher efficiency low-saline separations. Here, using a Gouy–Chapman–Stern model we elucidate the efficiency of two capacitive deionization cycles. In the first cycle, electroadsorption occurs as the cell voltage increases, which resembles processes employed experimentally. In the second cycle, a theoretical Carnot-like electroadsorption process occurs while maintaining a constant number of ions (N = constant). We show that the projected thermodynamic efficiency of both cycles exhibited similar efficiency, which range from 2–4.5%, while operating with a low-saline feed (Cfeed = 20 mM) stream. Howeve...