Research Papers: Energy Systems Analysis

Performance Improvement of Capacitive Deionization for Water Desalination Using a Multistep Buffered Approach

[+] Author and Article Information
Yasamin Salamat

Department of Mechanical
and Industrial Engineering,
Northeastern University,
334 Snell Engineering Center,
360 Huntington Avenue,
Boston, MA 02115
e-mail: s.salamat@neu.edu

Carlos A. Rios Perez

Department of Mechanical
and Industrial Engineering,
Northeastern University,
334 Snell Engineering Center,
360 Huntington Ave,
Boston, MA 02115
e-mail: carlos.a.riosperez@gmail.com

Carlos Hidrovo

Department of Mechanical
and Industrial Engineering,
Northeastern University,
207 Snell Engineering Center,
360 Huntington Avenue,
Boston, MA 02115
e-mail: hidrovo@neu.edu

1Corresponding author.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received August 20, 2016; final manuscript received October 5, 2016; published online November 10, 2016. Editor: Hameed Metghalchi.

J. Energy Resour. Technol 139(3), 032003 (Nov 10, 2016) (6 pages) Paper No: JERT-16-1342; doi: 10.1115/1.4035067 History: Received August 20, 2016; Revised October 05, 2016

Due to the increasing demand for clean and potable water stemming from population growth and exacerbated by the scarcity of fresh water resources, more attention has been drawn to innovative methods for water desalination. Capacitive deionization (CDI) is a low maintenance and energy efficient technique for desalinating brackish water, which employs an electrical field to adsorb ions into a high-porous media. After the saturation of the porous electrodes, their adsorption capacity can be restored through a regeneration process. Herein, based on a physical model previously developed, we conjecture that for a given amount of time and volume of water, multiple desalination cycles in a high flow rate regime will outperform desalinating in a single cycle at a low flow rate. Moreover, splitting a CDI unit into two subunits, with the same total length, will lead to higher desalination. Based on these premises, we introduce a new approach aimed at enhancing the overall performance of CDI. An array of CDI cells are sequentially connected to each other with intermediate solutions placed in between them. Desalination tests were conducted to compare the performance of the proposed system, consisting of two CDI units and one intermediate solution buffer, with a two-cascaded-CDI unit system with no intermediate solution. Experimental data demonstrated the improved performance of the buffered system over the nonbuffered system, in terms of desalination percentage and energy consumption. The new proposed method can lead to lower amount of energy consumed per unit volume of the desalinated water.

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Grahic Jump Location
Fig. 1

Schematic of convective–diffusive layers in CDI, and illustration of ionic concentration in two regimes. Fully developed convective–diffusive layer: (a) at low flow rates, diffusion of ions toward the porous walls is more substantial than their advection within the channel. Developing convective–diffusive layer: (b) at high flow rates, due to the dominance of ions advection, the diffusion of the ions toward the porous media is limited to the convective–diffusive layer.

Grahic Jump Location
Fig. 2

Normalized outlet concentration over time for two desalination systems, with the same initial concentration. (a) One cycle of desalination in the fully developed convective–diffusive regime with low flow rate of 1 ml/min. (b) Four consecutive cycles of desalination in the developing convective–diffusive regime at high flow rate of 4 ml/min. Each cycle has the inlet concentration equal to the minimum average concentration of the previous cycle (except for the first cycle).

Grahic Jump Location
Fig. 3

Normalized outlet concentration over time for two architectures, with the same flow rate and initial concentration. These numerical results demonstrate effective impact of splitting the CDI cell on the ultimate desalination performance of the system.

Grahic Jump Location
Fig. 4

Normalized concentration over time for the two-cascaded-CDI unit system (a) and the proposed buffered system (b). In the cascaded system, two CDI units are consecutively connected to each other, with no intermediate solution reservoirs. In the buffered system, first a CDI unit performs a desalination test, and then, the obtained minimum average concentration is assigned as the initial concentration of the second desalination test.

Grahic Jump Location
Fig. 5

Exploded view of the CDI unit. Titanium sheets serve as current collectors. Activated carbon pairs are separated by polyester mesh, and the rubber gasket is placed in the rectangular groove to seal the unit. The inlet and outlet fittings and power supply contact screws are not shown in this figure.

Grahic Jump Location
Fig. 6

Schematic of the experimental setup for two CDI systems. The buffered system: (a) a steady desalination test was conducted with a single CDI unit. The minimum average concentration obtained was used as the inlet concentration for the second steady desalination test, with the same CDI unit. The cascaded system: (b) one steady desalination test was performed with a CDI cell consisting of two pairs of current collectors and activated carbon electrodes, located at a distance from each other.

Grahic Jump Location
Fig. 7

Experimental data for normalized outlet concentration over time for the cascaded (a) and the buffered (b) CDI systems




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