Transitioning from electrodialysis to reverse electrodialysis stack design for energy generation from high concentration salinity gradients

A.M Hulme,C.J Davey,S Tyrrel,M Pidou,E.J Mcadam

doi:10.1016/j.enconman.2021.114493

Abstract

In this study, stack design for high concentration gradient reverse electrodialysis operating in recycle is addressed. High concentration gradients introduce complex transport phenomena, which are exacerbated when recycling feeds; a strategy employed to improve system level energy efficiency. This unique challenge indicates that membrane properties and spacer thickness requirements may differ considerably from reverse electrodialysis for lower concentration gradients (e.g. seawater/river water), drawing closer parallels to electrodialysis stack design. Consequently, commercially available electrodialysis and reverse electrodialysis stack design was first compared for power generation from high concentration gradients. Higher gross power densities were identified for the reverse electrodialysis stack, due to the use of thinner membranes characterised by a higher permselectivity, which improved current. However, energy efficiency of the electrodialysis stack was twice that recorded for the reverse electrodialysis stack at low current densities, which was attributed to: (i) an increased residence time provided by the larger intermembrane distance, and (ii) reduced exergy losses of the electrodialysis membranes, which provided comparatively lower water permeance. Further in-depth investigation into membrane properties and spacer thickness identified that membranes characterised by an intermediate water permeability and ohmic resistance provided the highest power density and energy efficiency (Neosepta ACS/CMS), while wider intermembrane distances up to 0.3 mm improved energy efficiency. This study confirms that reverse electrodialysis stacks for high concentration gradients in recycle therefore demand design more comparable to electrodialysis stacks to drive energy efficiency, but when selecting membrane properties, the trade-off with permselectivity must also be considered to ensure economic viability.

Highlights

Electrodialysis (ED) is a commercially mature technology, with ap plications in multiple industries ranging from the food industry to wastewater treatment [1]
Improved power density of 4.78 W m− 2 was obtained using an reverse electrodialysis (RED) module compared to 0.33 W m− 2 using an ED module, demonstrating its suitability for high salinity gradient applications
Energy efficiency doubled from 4.6% in the RED stack to 9.7% using the ED stack when operated at low current densities (0.1 A) in recycle

Summary

Introduction

Electrodialysis (ED) is a commercially mature technology, with ap plications in multiple industries ranging from the food industry to wastewater treatment [1]. ED has been demonstrated to be economically [2] and energetically [3] competitive to reverse osmosis for the desalination of brackish waters. In an ED stack, anion and cation ion exchange membranes are alternately ar ranged between two electrodes to form concentrate and dilute com partments, with spacers and gaskets separating the membranes. The controlled movement of ions across the ion exchange membranes (IEMs) is driven by an applied electrical current to produce desalinated water [4]. In reverse electrodialysis (RED), the opposite process to ED is employed, where ionic transport across alternately stacked IEMs is driven by a concentration gradient, to liberate the Gibbs free energy of mixing between solutions of different salinities. A redox couple circulating across the electrodes converts the ionic flow to an electric current [5]

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