Energy consumption analysis of constant voltage and constant current operations in capacitive deionization

Abstract

We report our studies to compare energy consumption of a CDI cell in constant voltage (CV) and constant current (CC) operations, with a focus on understanding the underlying physics of consumption patterns. The comparison is conducted under conditions that the CV and CC operations result in the same amounts of input charge and within identical charging phase durations. We present two electrical circuit models to simulate energy consumption in charging phase: one is a simple RC circuit model, and the other a transmission line circuit model. We built and tested a CDI cell to validate the transmission line model, and performed a series of experiments to compare CV versus CC operation under the condition of equal applied charge and charging duration. The experiments show that CC mode consumes energy at 33.8 kJ per mole of ions removed, which is only 28% of CV mode energy consumption (120.6 kJ/mole), but achieves similar level of salt removals. Together, the models and experiment support our major conclusion that CC is more energy efficient than CV for equal charge and charging duration. The models also suggest that the lower energy consumption of CC in charging is due to its lower resistive dissipation.

Desalination with porous electrodes : Mechanisms of ion transport and adsorption

“…Recent scientific developments are on electrode materials [11,33,51,[88][89][90][91][92][93], novel cell designs, ranging from desalination with wires [80] to flow electrodes [5,72,73], or on new operational modes or ion-selective removal [28,47,[94][95][96]. Also, studies are conducted to quantify resistances and energy losses in the CDI cell [4,97,98], or to identify parasitic reactions that may result in pH changes [8,[99][100][101].…”

“…[155] where on theoretical grounds CC was considered to have a twice lower energy use than CV operation. In another study, Qu et al [97] show that in CC operation with complete energy recovery, the energy consumption can be as low as 28% of CV operation, i.e., about a factor of three less, and Ref. [156] concludes that energy recovery in CC operation is much more favorable than in CV operation.…”

“…Note that this choice does not need to be made for steady state operation of CDI with flowing electrodes, but is only necessary for cyclic processes. For carbon electrodes (which we consider from this point onward), in literature it is reported that CC operation leads to a lower energy consumption than CV operation [36,97,[153][154][155][156]. Kang et al [153] show that CC-CDI consumes approximately 30% less energy than CV-CDI for identical electronic charge storage or identical ion removal, without considering energy recovery.…”

“…4 in Ref. [97]) that in their case the amount of charge is close to proportional to salt removal. Furthermore, data were obtained at the same duration of the charging (either CV or CC) and discharge step (CC in all cases).…”

“…[153] the performance of a single cycle is compared, but not the performance per time period of system operation, and therefore the cycle-averaged salt adsorption rates are not equal. Whereas these papers consider flow-by CDI, Qu et al [97] perform experiments with flow-through CDI and compare data as function of charge transferred. They show (inset picture Fig.…”