2. Capacitive deionization (CDI)

CDI technology uses an electric field created by a pair of electrodes to induce the mobility and separation of the dissolved salts in the water towards the corresponding electrodes (Figure 1). The salts (their ionic components) are retained at the interface between the electrode and the aqueous medium, in an electrochemical structure called double-layer. Therefore, CDI is a lowpressure desalination process where an energy recovery process can be used to minimize the energy consumption.

## 2.1. Operating principle

As it was mentioned, CDI technology uses the electric field created between the electrodes to retain the ions on the electrode surface by means of electrostatic attraction [4]. This accumulation of ions on the electrodes is a thermodynamically reversible process and can be eliminated in a later step, during which the electrodes are depolarized. The behavior of the structure is similar to a supercapacitor that can be charged or discharged. In order to increase the effective surface of the electrode, nanoporous carbon materials are used to cover the surface of the electrode [5–7].

The first phase of operation is called deionization or desalination phase, and more technically, the ionic adsorption phase. This phase lasts as long as it takes the equivalent capacitor to

Figure 1. Schematic of ion adsorption during the polarization of electrode plates.

overpopulation, global warming, the increase of polluting emissions and the growing energy demand of the world productive model are at the base of numerous studies that indicate an alarming water scarcity in the medium term. Thus, some sources [1] estimate a 40% increase in water demand with respect to its availability within a period of 20 years, which makes it possible to calculate that one-third of the world's population will have access to only 50% of

With this forecast and the potential increase of the world population, it is estimated that by 2050 a global water crisis without precedents could be established that would create very high levels of scarcity in large regions of the planet. This necessarily implies investments in the improvement of water saving and the optimization of current methods of water regeneration/

Over the past decades, processes such as reverse osmosis (RO), electrodialysis and various forms of distillation, such as multi-effect distillation and multi-stage flash distillation, have reached a high level of technological and industrial maturity [2, 3], and are currently the reference processes and mostly used for the regeneration/production of drinking water on a large scale. These methods have the main drawback of high energy consumption (e.g. 4–7kWh/m3 for RO) so that new strategies for purification are being investigated in order to reduce their consumption.

CDI technology is presented as an efficient alternative to the previously mentioned technolo-

CDI technology uses an electric field created by a pair of electrodes to induce the mobility and separation of the dissolved salts in the water towards the corresponding electrodes (Figure 1). The salts (their ionic components) are retained at the interface between the electrode and the aqueous medium, in an electrochemical structure called double-layer. Therefore, CDI is a lowpressure desalination process where an energy recovery process can be used to minimize the

As it was mentioned, CDI technology uses the electric field created between the electrodes to retain the ions on the electrode surface by means of electrostatic attraction [4]. This accumulation of ions on the electrodes is a thermodynamically reversible process and can be eliminated in a later step, during which the electrodes are depolarized. The behavior of the structure is similar to a supercapacitor that can be charged or discharged. In order to increase the effective surface of the electrode, nanoporous carbon materials are used to cover the surface of the

The first phase of operation is called deionization or desalination phase, and more technically, the ionic adsorption phase. This phase lasts as long as it takes the equivalent capacitor to

gies, as it allows an important recovery of the energy involved in the process.

the amount of water necessary to cover their basic needs.

2. Capacitive deionization (CDI)

energy consumption.

electrode [5–7].

2.1. Operating principle

purification.

38 Desalination and Water Treatment

charge to the desired potential, always lower than 1.2 V to avoid water dissociation. The water between the electrodes will be partially deionized during this stage. During the second phase, the electrodes are depolarized or discharged while circulating a flow of water that is used to facilitate the removal of the ions adsorbed by the electrode plates, which will be evacuated in the form of a concentrate or brine. This second phase is called the regeneration or cleaning phase since it makes the electrodes be ready for a new cycle again.

This principle and mode of operation, based on alternating charge-discharge of the equivalent capacitor, suggests the use of the energy accumulated in the electrodes during the regeneration phase (once the electrodes are saturated of salt ions) to supply another deionization module that starts the desalination phase. This principle of energy reuse (transferring energy between deionization modules) implies the possibility of significantly reducing the energy consumption of the system.

The primary energy source will only have to provide the necessary amount of energy to compensate the losses that occur during the energy transfer between the CDI cells.

#### 2.2. Membrane capacitive deionization (MCDI)

In this variant of capacitive deionization, ion-selective membranes are interposed between the electrodes and the solution. In this way, it is possible to use the polarity reversal in the electrodes periodically, drastically improving the efficiency in the cleaning phase [8, 9].

In addition, the use of membranes improves the performance of the process, since it increases the electronic efficiency, which is the ratio between the amount of salt eliminated and the amount of electric current supplied to the CDI cell to achieve that goal.

Figure 2 shows a diagram of the MCDI operation composed of two capacitive units or cells. Each of the cathodes has a cation-selective membrane interposed, and each anode has an anion-selective membrane so that the ions charged with a sign opposite to that of the electrode

Figure 2. Schematic of a membrane capacitive deionization cell.

(or counter-ions) can move freely through the membrane, while the movement of ions with a charge of the same sign is blocked.

During the purification phase, the electrodes invert the polarity to facilitate the desorption of the adsorbed ions in the previous desalination phase. The re-adsorption of ions is avoided thanks to the barrier posed by the membranes.

In both cases, CDI or MCDI, another important parameter, is the effective electrode surface. This parameter is related to the equivalent capacitance that represents the deionization cell and, therefore, to the quantity of salt that can be adsorbed.

CH ¼ A �

"d," and the number of electrodes placed in series, "n."

Figure 3. Classical double-layer model of a flat-parallel capacitor.

during which the parallel resistance, RP, can be neglected:

module.

expressed in Ω.

The desalination cell is built by piling several electrodes to increase the capability of water processing. In Figure 4, several electrodes are piled controlling the distance between them,

Although there are more complex models of the ion distribution around the electrodes [10–12], the whole desalination cell can be modeled with the traditional circuit used to characterize a capacitor C. In this model two additional resistances are included, a series resistance RS to model conduction losses, and a parallel resistance RP that represents the self-discharge of the

The proposed electric model of the CDI cell (Figure 5) will allow the desalinization system to be simulated together with the power topology used for the energy recovery. The electrical parameters defined, RS, RP and C, depending on the geometrical characteristics of the CDI cell and on the salt molar concentration (M). In order to obtain their values, a current source is

The CDI cell is initially completely discharged. At t = 0, a constant current, IDC, is applied to initiate the charging process (Figure 6). Therefore, since the equivalent capacitor C is initially discharged, the value of the voltage VC(t = 0+) measured will determine the value of RS

> RS <sup>¼</sup> <sup>Δ</sup>V<sup>1</sup> IDC

The capacitance of the CDI module, C, can be obtained from the linear charging process,

applied to the cell that generates a linear evolution in the voltage across the terminals.

ε<sup>r</sup> � ε<sup>0</sup> χH

Energy Recovery in Capacitive Deionization Technology http://dx.doi.org/10.5772/intechopen.75537

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