Energy consumption in capacitive deionization – Constant current versus constant voltage operation

Abstract

In the field of Capacitive Deionization (CDI), it has become a common notion that constant current (CC) operation consumes significantly less energy than constant voltage operation (CV). Arguments in support of this claim are that in CC operation the endpoint voltage is reached only at the end of the charging step, and thus the average cell voltage during charging is lower than the endpoint voltage, and that in CC operation we can recover part of the invested energy during discharge. Though these arguments are correct, in the present work based on experiments and theory, we conclude that in operation of a well-defined CDI cycle, this does not lead, for the case we analyze, to the general conclusion that CC operation is more energy efficient. Instead, we find that without energy recovery there is no difference in energy consumption between CC and CV operation. Including 50% energy recovery, we find that indeed CC is more energy efficient, but also in CV much energy can be recovered. Important in the analysis is to precisely define the desalination objective function, such as that per unit total operational time -including both the charge and discharge steps- a certain desalination quantity and water recovery must be achieved. Another point is that also in CV operation energy recovery is possible by discharge at a non-zero cell voltage. To aid the analysis we present a new method of data representation where energy consumption is plotted against desalination. In addition, we propose that one must analyze the full range of combinations of cycle times, voltages and currents, and only compare the best cycles, to be able to conclude which operational mode is optimal for a given desalination objective. We discuss three methods to make this analysis in a rigorous way, two experimental and one combining experiments and theory. We use the last method and present results of this analysis.

Exploring the Function of Ion-Exchange Membrane in Membrane Capacitive Deionization via a Fully Coupled Two-Dimensional Process Model

“…Huang and Tang [75] revealed that high temperature fastened desalination rate but affected the adsorption capacity of CDI. Energy consumption of MCDI is determined by water recovery and salt removal efficiency and can be improved with energy recovery [76]. Energy losses in MCDI largely depend on systematic resistances, including external circuit electronic resistance, electrode electronic resistance, and ionic transfer resistance [34].…”

Object-Oriented Modeling of a Capacitive Deionization Process

“…A characteristic extension of the EDL is the Debye length in which the potential has decreased by a factor of e (Porada et al, 2013;Biesheuvel et al, 2015). In the activated carbon particles which are typically used in electrodes of a CDI cell, the average pore size is around 2 nm (Hemmatifar et al, 2017;Dykstra et al, 2018). As this is generally smaller than the Debye length, it is assumed that the diffuse layers of the EDLs are overlapping.…”

“…In recent years Capacitive Deionization (CDI) has become a commonly known technique for deionization of brackish water, as it is an energy efficient and cost effective technology (Oren, 2008;Zou et al, 2008;Anderson et al, 2010;Cohen et al, 2013;Porada et al, 2013;Han et al, 2014;Biesheuvel et al, 2015;Suss et al, 2015;Gude, 2016;Dykstra et al, 2018;Ma et al, 2018). In general, a CDI cell mainly consists of two porous electrodes oppositely placed to each other and separated by a permeable, non-conducting spacer through which the feed water is pumped.…”

“…By using a dynamic transport model, Wang and Lin (2018) quantified recently the intrinsic trade-off between the energetic and kinetic efficiencies which is caused by the different charging methods CC or CV. The effect of different charging methods in terms of energy recovery was also described by Dykstra et al (2018), who presented a methodology to assess the energy consumption under different operation modes. All the abovementioned models are based on self-programmed algorithms in programming languages such as C++ or MATLAB or commercially available software like COMSOL Multiphysics.…”

New Approach for Investigating Diffusion Kinetics Within Capacitive Deionization Electrodes Using Electrochemical Impedance Spectroscopy

“…Thereby metal-organic frameworks (MOF) in different morphologies play a significant role (Xu et al, 2016b(Xu et al, , 2019b(Xu et al, , 2020bWang et al, 2019) or aerogel-like Ti 3 C 2 Tx MXene structures (Bao et al, 2018). When an electrical potential of around 1V is applied, ions are captured on the large inner surface of the electrodes within the electric double layer (Biesheuvel et al, 2012;Dykstra et al, 2018). As a consequence the fluid passing through the gap between the electrodes is depleted of ions, resulting in a desalted or purified solution (Biesheuvel et al, 2011;Porada et al, 2013;Suss et al, 2015;Kim et al, 2017).…”