(Invited) Development of Reverse Electrodialysis System

Abstract

Salinity gradient power (SGP), also called blue energy, is a newly emerging form of marine renewable energy. SGP is the energy available from the salinity difference between seawater and river water, which respectively have high and low salt concentrations. The SGP energy potential between typical seawater and river water is 231 m (~23 atm) of hydraulic water head. The global energy potential is estimated to be 2.4–2.6 TW, which is ~80% of the global electricity demand. Among membrane-based blue energy technologies, reverse electrodialysis (RED), the opposite process of electrodialysis (ED), has been applied practically. Recently, RED has been considered the attractive technology because this new process has huge potential and possibility to generate energy from abundant but largely unused resources. RED uses a stack of alternating cation (CEM) and anion (AEM) exchange membranes. For RED to find greater use, a high power density (power generated per membrane area) is essential. By increasing the power density, the membrane demand and RED stack size can be decreased. To make RED an economically attractive technology, the large scale RED stack should be developed. Recently, we have developed the kW class RED stack and evaluated the performances for the first time in Korea. And the effect of operation conditions on power density was confirmed. Furthermore, a numerical simulation has been conducted for investigating the flow distribution and pressure loss in the stack. Since the performance of RED significantly depends on the inertial fluid characteristics and the required pumping power, it is a primary importance to lower pressure drop and uniform flow distribution in the stack. With respect to the size and the height of cell, and the inlet shape of cell, the flow distribution and the pressure loss have been evaluated. The ion exchange membranes (IEMs) also strongly influences the performance, specifically, the power density, of the RED stack. Although studies have, over the last decade, improved the IEM characteristics, they do not yet match the expected characteristics for RED. Generally, IEMs for ion separation processes require high strength and long lifetime regardless of the electrical resistance and thickness; however, IEMs for RED require low electrical resistance and high permselectivity. Nevertheless, most present IEMs for RED still do not possess both the thinness and the mechanical strength required for higher power density of the RED stack. To solve this problem, we have developed pore-filling-type IEMs for RED. A pore-filling membrane consists of a very thin porous substrate to provide mechanical strength and an electrolyte polymer in the substrate pores to allow ion conductivity. Meanwhile, electrolyte-polymer-filled pores should have high ion exchange capacity (IEC) to increase the ion conductivity. The thickness of the prepared pore filling membranes was controlled between 20 and 25 micrometers to extremely lower membrane resistance and to increase ion transfer. The prepared ion exchange pore filling membranes were compared with commercial membranes. We obtained the highest power density comparing to the result reported in elsewhere.