Abstract
Renewable energy can be generated when mixing seawater and river water. This energy is captured in reverse electrodialysis (RED) using ion exchange membranes. Although natural sources of seawater and river water are composed of a mixture of monovalent and multivalent ions, laboratory research on RED is generally performed with artificial solutions of sodium chloride. This research demonstrates that the presence of magnesium- and sulphate ions in feed solutions with NaCl has a major effect on the obtained open circuit voltage and power density for three different membrane types. When using a mixture with a molar fraction of 10% MgSO4 and 90% of NaCl in both feed waters, the experimentally obtained power density in steady state decreases from 29% to 50% compared to the case where the feed solutions contain only NaCl as a salt. This effect is among others explained by the transport of Mg2+ and SO42− against their concentration gradient, as is elaborated in a theoretical framework and which is justified by experimental data. Non-stationary cases, where feed water is switched from a NaCl solution to a mixture of NaCl and MgSO4, show that the voltage response time is in the order of tens of minutes up to several hours, due to ion exchange between the membranes and the feed water. The knowledge gained from electrochemical measurements under stationary and non-stationary conditions and a novel technique to monitor the ion transport inside cation exchange membranes can be used to improve the obtained power density in practical applications of RED using natural feed water.
Highlights
This available energy from salinity differences can be captured using ion exchange membranes, which are selective for cations or anions
The results show how reverse electrodialysis (RED) can be operated using feed mixtures of NaCl and MgSO4, as a representative for natural feed waters, while maintaining a high power density
One stack was composed of heterogeneous membranes (Ralex CMH/AMH, MEGA AS, Czech Republic), a second stack was composed of commercial homogeneous membranes (Neosepta CMX/AMX, Tokuyama, Japan) and a third stack was composed of a new type of homogeneous membranes (V1 cation exchange membrane (CEM)/V1 anion exchange membrane (AEM), Fuji lm Europe, The Netherlands)
Summary
The potential to generate power from the mixing of seawater and river water is huge, as approximately 2 TW can be generated theoretically from the global river water runoff.[10,11] This is close to the current worldwide electricity demand.[12]. This would signi cantly improve the obtained voltage, and the power density, the existing monovalent selective membranes do still allow a substantial part of the multivalent ion transport.[19,32]. The electrical resistance is composed of an ohmic contribution, due to the membrane, feed water and spacers, and a non-ohmic contribution, due to the concentration changes when ions are transported under the in uence of an electrical current This theoretical framework helps to understand the effect of MgSO4, the actual open circuit voltage, electrical resistance and power density are more complex to calculate. The generated voltage, electrical resistance and obtained power in RED need to be investigated experimentally, which is described
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