Abstract
Reverse electrodialysis (RED) is an electro-membrane process to harvest renewable energy from salinity gradients. RED process models have been developed in the past, but they mostly assume that only NaCl is present in the feedwaters, which results in unrealistically high predictions. In the present work, an existing simple model is extended to accommodate the presence of magnesium ions and sulfate in the feedwaters, and potentially even more complex mixtures. All power loss mechanisms deriving from the presence of multivalent ions are included in the new model: increased membrane electrical resistance, uphill transport of multivalent ions from the river to the seawater compartment, and membrane permselectivity loss. This new model is validated with experimental and literature data of membrane electrical resistance (at 10 mol. % MgCl2 for the CEMs and 25 mol. % Na2SO4 for the AEMs), RED stack performance (up to 50 mol. % MgCl2 or Na2SO4 in the feedwaters), and ion transport (at 10 mol. % MgCl2 or Na2SO4 in the feedwaters) showing very good agreement between model predictions and experimental data. Finally, we showed that the developed model not only describes experimental data but can also predict RED performances under a variety of conditions and cross-flow configurations (single-stage with and without electrode segmentation, multi-stage in co-current and counter-current mode) and feedwater compositions (only NaCl, with Na2SO4, with MgCl2, and with MgSO4). It thus provides a very valuable tool to design and evaluate RED process systems.
Highlights
In the effort to limit global warming and reduce climate change, renewable energy plays a key role [1,2,3]
Reverse electrodialysis (RED) process models have been developed in the past, but they mostly assume that only NaCl is present in the feedwaters, which results in unrealistically high predictions
To correctly predict RED performance with mixtures of mono- and multivalent ions, the following aspects are taken into account in the model: 1) uphill transport of multivalent ions against their concentration gradient; 2) higher membrane electrical resistance due to the larger size of the multivalent ions and the partitioning of current between different ionic species; 3) membrane permselectivity loss; and 4) change of electrical conductivity of the water compartments when more charges are introduced with the salts containing multivalent ions
Summary
In the effort to limit global warming and reduce climate change, renewable energy plays a key role [1,2,3]. A promising candidate is salinity gradient energy (SGE), known as blue energy, which is the energy derived from the controlled mixing of solutions with different salinities, e.g., river and seawater [4,5,6]. The basic principle of RED consists of a stack of cation exchange membranes (CEMs, selective for cations) and anion exchange membranes (AEMs, selective for anions), piled alternately and separated by feedwater compartments. Kept open by non-conductive spacers or by patterns on the surface of profiled membranes [9,10], river and seawater flow alternately, and the salt gradient across each membrane generates a voltage difference [11]. An electrode pair placed at both ends of the stack and a redox couple recirculating in the electrode compartments allow the conversion of the ionic current flowing through the mem branes into an electronic current when an external load is connected to the electrode and the circuit is closed [12]
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