Elsevier

Energy

Volume 170, 1 March 2019, Pages 139-148
Energy

An internal-integrated RED/ED system for energy-saving seawater desalination: A model study

https://doi.org/10.1016/j.energy.2018.12.111Get rights and content

Highlights

  • Modeling of an internal-integrated RED/ED hybrid system is first established.

  • The effects of design parameters on the performance of RED/ED were investigated.

  • Brine/river is the optional combination to realize fast seawater desalination.

  • RED/ED shows a more economic and better performance than does the RED + ED system.

Abstract

Salinity gradient energy extracting by a reverse electrodialysis (RED) unit using for electrodialysis (ED) desalination process is a potential way to achieve energy-economic and sustainable production of freshwater. However, the parameters in RED and ED unit synergistically influence the desalination process, resulting to the hybrid process controlled by multi-parameters. Modeling of an RED/ED is a simple way to describe the desalination process and reveal the effects of these parameters on the performance of system and then to find the better adaption of RED/ED. In this study, a model of an internal-integrated RED/ED hybrid system is first established. It found that the ratio of desalination in RED/ED is higher than 90%. The brine/river is the alternative combination to realize seawater desalination with a desalination rate of 0.38 h m2/mol. The desalination capacity of RED/ED (0.43–2.6 mol/h·m2) is much higher than that of the external-integrated RED + ED system (0.10–0.15 mol/h·m2), but it is of simpler configuration and has a lower energy requirement. Moreover, the RED/ED system is preferred for using in the pre-desalination process. The outcome of this model is helpful in the design of practical RED/ED systems, and points out the development potential of RED/ED in practical applications.

Introduction

Desalination of seawater has become the most attractive way to solve freshwater scarcity [1]. Generally, desalination technologies include membrane-based (reverse osmosis or electrodialysis) and thermal distillation-based (multieffect distillation, multistage flash, or vapor compression) desalination processes [2,3]. Among them, electrodialysis (ED) has high water recovery and low maintenance costs, being a potential alternative for desalination, especially in water-stressed remote locations [4]. However, the energy consumption of ED, in the range of 0.7–5.5 kwh/m3, is higher than that of other membrane based desalination processes, like reverse osmosis [5,6]. To find an economic ED process, renewable energy (wind or solar energy) is often used to drive the ED process [4,7]. However, expensive capital energy conversion systems and complicated technology designs make hybrid systems difficult to apply [5]. For example, in a solar/ED process, a photovoltaic solar panel, charge regulator, storage batteries, etc., are required to first transform solar energy into electricity, then drive ED desalination [5]. Therefore, a simply hybrid system that utilizes renewable energy in the ED process is of great importance for economic desalination.

Reverse electrodialysis (RED) is the inverse process of ED [[8], [9], [10]]. It can draw electrical energy from salinity gradient energy between two streams at different concentrations [9,11]. The configuration of an RED device is similar to that of an ED device, consisting an electrode system, alternately stacking ion exchange membranes (IEMs) and two streams [12]. Therefore, the RED device and the ED device can be easily coupled in one module without any other special connection. Meanwhile, RED is an excellent method for brine management, in which the brine generated from ED is capable of acting as the high-salinity stream of RED and can then be adequately diluted to assist in its environmental discharge [13,14]. Additionally, energy extracted from RED can be utilized to offset the energy consumption of ED to achieve low-energy desalination [15]. The theoretical energy extracted by RED is in the range of 0.22–14 kwh/m3, which is enough to drive an ED for desalination (0.7–5.5 kwh/m3) [10]. Therefore, coupling RED with ED will realize economic desalination in an easy operation mode meanwhile with good brine management.

Recently, a RED + ED hybrid system in an external-integrated form was proposed, in which high-salt wastewater first flowed across the standalone RED unit for pre-desalting and was then guided into the ED unit for deep desalination, which would cost large amount of energy (17.5–20.1 kWh/m3) [15]. Although a power-free ED for desalination is feasible in RED + ED, a large amount of gradient difference energy from RED is consumed in the conversion of chemical energy to electrical energy and then to chemical energy again, resulting to a low energy efficiency [16]. By contrast, a RED/ED hybrid unit in an internal-integrated form, in which the desalination process is directly driven by the ion current, would reduce energy waste in the process of energy conversion. Moreover, only one electrode (note: Pt/Ti as the electrode) is required in RED/ED, while two couples of electrodes are used in the external-integrated form. It has been proven to be energy self-sufficient to produce drinkable water through RED/ED [17]. However, the performance of internal-integrated RED/ED devices is affected by many factors, such as the concentration of each stream, the numbers of stacks, the thickness of channels, etc. As a multi-parameter control process, modeling of RED/ED is a simple way to explore the effects of these parameters on the performance of system and then to find the better adaption of RED/ED. Although extensive models of RED or ED were constructed previously, the model of RED/ED coupling is still required because the parameters of RED and ED synergistically affect the process of desalination. For example, both of the stacking numbers and concentration of streams of RED or ED contribute to the resistance and electromotive force. Besides, the resistance and electromotive force of system changes with desalination process. Until now, few studies have systematically investigated the effects of these factors on the RED/ED desalination process. Therefore, a systematic investigation of RED/ED is necessary and meaningful to reveal the future of RED/ED.

In this study, a mathematical model of an internal-integrated RED/ED device was established. Through modeling of the RED/ED hybrid system, the basic requirements, the desalination process and the design parameters of the RED/ED system were investigated to identify the better method of integration in a practical RED/ED system. The capacity for desalination and the energy consumption of internal-integrated RED/ED, external-integrated RED + ED, standalone ED, solar/ED and wind/ED systems were compared. It is anticipated that this model might be helpful in the design of RED/ED systems, and might show the development potential of RED/ED for practical application.

Section snippets

Modeling of ED unit operations

A simplified scheme of the ED system is shown in Fig. 1a. A conventional ED device mainly contains four parts: a direct current supply, two electrodes, alternately stacked IEMs and feed/permeate solutions [18]. The feed and permeate solutions are alternately separated by a cation exchange membrane (CEM) and an anion exchange membrane (AEM). When an electric field is applied to the electrodes, cations transport through th CEM to th cathode, while anions transport through the AEM to the anode.

Basic parameters used for model simulation

Based on the modeling of the RED/ED hybrid system in this work, there are several important independent parameters in the design. First are parameters relating to the electromotive force of the system: the ratio of RED and ED stacking pairs (NRED/NED), the ratio between concentration of the high-salinity solution and low-salinity solution (Ch/Cl) and the ratio of the initial concentration of the permeate solution and feed solution (Cp,0/Cf,0); Second are parameters relating to the resistance of

Perspectives

RED is always combined with brine as RED can extract the chemical energy from brine for a higher energy output, meanwhile solving the disposal of brine [34,35]. In the RED/ED system, a higher performance of desalination is obtained from using brine than that from using seawater as the high-salinity water, with a maximum desalination rate of 0.38 h m2/mol. In this model, we assumed that all four streams are independent (Fig. 9a). However, for practical application, the brine generated from ED

Conclusions

In this study, an analytical model of an internal-integrated RED/ED hybrid system is developed to investigate the effects of design parameters on the performance of self-sufficient desalination. The desalination capacity of RED/ED (0.43–2.6 mol/h·m2) is much higher than that of external-integrated RED + ED system (0.1–0.15 mol/h·m2), but the system is of simpler configuration and lower energy requirement. Although the capacity for desalination by RED/ED is lower than that of conventional

Acknowledgements

This work is supported by a grant from the Research Grants Council of the Hong Kong Special Administration Region, China (C7051-17G) and National Natural Science Foundation of China (21607023).

References (36)

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