Energy consumption and charging dynamics of flow-through capacitive deionization systems [electronic resource]
Keywords: Mechanical Engineering.

Water scarcity is an emerging global issue. For the last few decades, fresh water shortage is becoming a threat to sustainable development of human society. Currently, four billion people, two-thirds of the world population, experience severe water scarcity during at least 1 month of the year. Desalination of seawater or brackish water can potentially help address this water shortage crisis, by increasing fresh water supplies. Particularly, the desalination of brackish water is more appealing and feasible to inland regions where seawater resources are not accessible. Capacitive deionization (CDI) is a promising desalination technology, which operates at low pressure, ambient temperature, requires little infrastructure, and has the potential to consume less energy than the state-of-the-art technique reverse osmosis (RO) for brackish water desalination. Recently developed flow-through CDI (ftCDI) devices demonstrate effective salt removal with a desalination rate 4 to 10 times faster than traditional CDI cells. However, ftCDI faces operational challenges such as low water recovery and non-uniform effluent concentration. In addition, CDI devices consume significantly more energy than the theoretical thermodynamic minimum. To tackle challenges of high energy consumption, we present our efforts to characterize electric resistances in a CDI system, with a focus on the resistance associated with the contact between current collectors and porous electrodes. We present an equivalent circuit model to describe resistive components in a CDI cell. We propose measurable figures of merit to characterize cell resistance. We also show that contact pressure between porous electrodes and current collectors can significantly reduce contact resistance. In addition, we propose and test an alternative electrical contact configuration which uses a pore-filling conductive adhesive (silver epoxy) and achieves significant reductions in contact resistance. To further optimize energy consumption, we also present 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. Finally, to address water recovery and non-uniform effluent concentration problems, we present a study of the interplay among electric charging rate, capacitance, salt removal, and fluid flow in ftCDI systems. We develop two models describing coupled transport and electro- adsorption/desorption which capture salt removal dynamics. The first model is a simplified, zero-dimensional lumped- parameter model which identifies dimensionless parameters and figures of merits associated with cell performance in advection-limited transport regime. The second model is a high-fidelity numerical model which captures spatial and temporal responses of charging for both advection-limited and dispersion-limited transport regimes. We further conducted an experimental study of these charging dynamics. We use these experimental data to validate models. The study shows that, in the advection-limited regime, differential charge efficiency determines the salt adsorption at the early stage of deionization process. Charging subsequently transitions to a quasi-steady state where salt removal rate is proportional to applied current scaled by the inlet flow rate. In the diffusion- dominated regime, differential efficiency, cell volume, and diffusion rates govern adsorption dynamics while flow rate has little effect. In both regimes, the interplay among mass transport rate, differential charge efficiency, cell capacitance, and (electric) charging current governs salt removal in ftCDI.