Flow-through CDI cells: a new architecture
Yatian Qu and Juan G. Santiago
Collaborator: Michael Stadermann of LLNL
We have developed a new type of CDI architecture which we call flow-through CDI (ftCDI) (US Patent Application 20120273359).
In ftCDI, we flow feed water through customized porous electrodes (rather than between electrodes as with other CDI methods) to achieve 4-10 times
greater throughput of other CDI systems with similar energy efficiency. This increase in throughput translates to improved scalability and a reduction
in the initial capital cost of CDI. Figure 1 shows a schematic of a flow through CDI cell.
We have developed new methods to analyze and model the traditional flow-between CDI cells (where feed water is flowed in a space between electrode pairs).
Figure 1 below shows a schematic of such a cell and the microstructure of the porous electrodes. We treat the electrodes as bimodal structures where so-called micropores
(order 1 nanometer) are responsible for most of the capacitance and electrostatically adsorb salt ions and so-called macropores (order 1 micron) allow for intra-electrode
transport via electromigration and diffusion.
Figure 1: Flow-through CDI cells, ftCDI. (a) Schematic of ftCDI cell architecture. Flowing water through electrodes removes
the diffusive transport limitation found in traditional CDI cells and results in much faster charging at similar energy cost. (b) Simple transmission line model
We have used models and experiments with ftCDI cells to demonstrate that charging under constant current conditions is more energy efficient than constant voltage charging (for CDI in general). We performed a fair comparison wherein an identical cell was charged for the same time and resulting in the same amount of charge. Figure 2 below summarizes our findings (from Qu et al., 2016).
Figure 2: Energy consumption of a CDI cell as a function of charging cycle time. Shown are raw energy consumption (a)
and energy per salt removed (b) for constant current (CC) and constant voltage (CV) cases (Qu et al., 2016).
Reference
Qu, Y., T.F. Baumann, J.G. Santiago, and M. Stadermann, "Characterization of resistances of a capacitive deionization system," Environmental Science & Technology, 2015.(click here for pdf)
Qu, Y., Campbell, P.G., Gu, L., Knipe, J.M., Santiago, J.G., Stadermann, M., "Energy consumption analysis of constant voltage and constant current operations in capacitive deionization," Desalination,2016.(click here for pdf)
Suss, M.E., Baumann, T.F. Bourcier, W.L., Spadaccini, C.M., Rose, K.A., Santiago, J.G. and Stadermann, M., "Capacitive desalination with flow-through electrodes," Energy and Environmental Science, 2012.(click here for pdf)