**Energy Recovery in Capacitive Deionization Technology** Energy Recovery in Capacitive Deionization Technology

DOI: 10.5772/intechopen.75537

Alberto M. Pernía, Miguel J. Prieto, Juan A. Martín-Ramos, Pedro J. Villegas and Francisco J. Álvarez-González Juan A. Martín-Ramos, Pedro J. Villegas and Francisco J. Álvarez-González Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.75537

Alberto M. Pernía, Miguel J. Prieto,

#### Abstract

[43] Suss ME, Biesheuvel PM, Baumann TF, Stadermann M, Santiago JG. In situ spatially and temporally resolved measurements of salt concentration between charging porous electrodes for desalination by capacitive deionization. Environmental Science and Technology.

[44] Lee JB, Park KK, Eum HM, Lee CW. Desalination of a thermal power plant wastewater

[45] Zhao R, Biesheuvel PM, van der Wal A. Energy consumption and constant current operation in membrane capacitive deionization. Energy and Environmental Science.

[46] Gao X, Omosebi A, Landon J, Liu K. Surface charge enhanced carbon electrodes for stable and efficient capacitive deionization using inverted adsorption–desorption behavior.

[47] Pasta M, Wessells CD, Cui Y, La Mantia F. A desalination battery. Nano Letters. 2012;

[48] Porada S, Biesheuvel PM, Presser V. Comment on sponge-templated preparation of high surface area graphene with ultrahigh capacitive deionization performance. Advanced

[49] Lee J, Kim S, Kim C, Yoon J. Hybrid capacitive deionization to enhance the desalination performance of capacitive techniques. Energy and Environmental Science. 2014;

[50] Jeon S et al. Desalination via a new membrane capacitive deionization process utilizing

[51] Mossad M, Zou L. A study of the capacitive deionisation performance under various operational conditions. Journal of Hazardous Materials. 2012;**213-214**:491-497

[52] Huang ZH, Wang M, Wang L, Kang F. Relation between the charge efficiency of activated carbon fiber and its desalination performance. Langmuir. 2012;**28**(11):5079-5084

[53] Ahn HJ, Lee JH, Jeong Y, Lee JH, Chi CS, Oh HJ. Nanostructured carbon cloth electrode for desalination from aqueous solutions. Materials Science and Engineering A. 2007;

[54] Ryoo MW, Seo G. Improvement in capacitive deionization function of activated carbon

[55] Ryoo MW, Kim JH, Seo G. Role of titania incorporated on activated carbon cloth for capacitive deionization of NaCl solution. Journal of Colloid and Interface Science. 2003;

[56] Chang LM, Duan XY, Liu W. Preparation and electrosorption desalination performance of activated carbon electrode with titania. Desalination. 2011;**270**(1-3):285-290

flow-electrodes. Energy and Environmental Science. 2013;**6**(5):1471

cloth by titania modification. Water Research. 2003;**37**(7):1527-1534

by membrane capacitive deionization. Desalination. 2006;**196**(1-3):125-134

Energy and Environmental Science. 2015;**8**(3):897-909

Functional Materials. 2015;**25**(2):179-181

2014;**48**(3):2008-2015

36 Desalination and Water Treatment

2012;**5**(11):9520

**12**(2):839-843

**7**(11):3683-3689

**448-451**:841-845

**264**(2):414-419

Capacitive deionization technique (CDI) represents an interesting alternative to compete with reverse osmosis by reducing energy consumption. It is based on creating an electric field between two electrodes to retain the salt ions on the electrode surface by electrostatic attraction; thus the CDI cell operates as a supercapacitor storing energy during the desalination process. Most of the CDI research is oriented to improving the electrode materials in order to increase the effective surface and ionic retention. However, if the CDI overall efficiency is to be improved, it is necessary to optimize the CDI cell geometry and the charge/discharge current used during the deionization process. A DC/DC converter is required to transfer the stored energy from one cell to another with the maximum possible efficiency during energy recovery, thus allowing the desalination process to continue. A detailed description of energy losses and the DC/DC converter used to recover part of the energy involved in the CDI process will provide the hints to optimize the efficiency of the CDI technique for water desalination. The proposed chapter presents an electric model to characterize the power losses in CDI cells and the power converter required for the energy recovery process.

Keywords: capacitive deionization, energy recovery, up-down DC/DC converter, desalination, carbon electrodes, supercapacitors

#### 1. Introduction

Nowadays it is evident that fresh water, suitable for different types of consumption, is a resource of paramount importance and growing scarcity, although throughout modern history the fact that it is a limited resource has been overlooked. Several factors, such as

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 the amount of water necessary to cover their basic needs.

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/ purification.

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.

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

Energy Recovery in Capacitive Deionization Technology http://dx.doi.org/10.5772/intechopen.75537 39

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

The primary energy source will only have to provide the necessary amount of energy to

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

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

compensate the losses that occur during the energy transfer between the CDI cells.

amount of electric current supplied to the CDI cell to achieve that goal.

phase since it makes the electrodes be ready for a new cycle again.

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

2.2. Membrane capacitive deionization (MCDI)

of the system.

CDI technology is presented as an efficient alternative to the previously mentioned technologies, as it allows an important recovery of the energy involved in the process.
