Abstract
The ohmic resistances of the anion and cation ion-exchange membranes (IEMs) that constitute a reverse electrodialysis system (RED) are of crucial importance for its performance. In this work, we study the influence of concentration (0.1 M, 0.5 M, 1 M and 2 M) of ammonium bicarbonate solutions on the ohmic resistances of ten commercial IEMs. We also studied the ohmic resistance at elevated temperature 313 K. Measurements have been performed with a direct two-electrode electrochemical impedance spectroscopy (EIS) method. As the ohmic resistance of the IEMs depends linearly on the membrane thickness, we measured the impedance for three different layered thicknesses, and the results were normalised. To gauge the role of the membrane resistances in the use of RED for production of hydrogen by use of waste heat, we used a thermodynamic and an economic model to study the impact of the ohmic resistance of the IEMs on hydrogen production rate, waste heat required, thermochemical conversion efficiency and the levelised cost of hydrogen. The highest performance was achieved with a stack made of FAS30 and CSO Type IEMs, producing hydrogen at 8.48× 10 kg ms with a waste heat requirement of 344 kWh kg hydrogen. This yielded an operating efficiency of 9.7% and a levelised cost of 7.80 € kg.
Highlights
In the imminent transition needed in our energy economy, shifting from fossils to sustainable, a demand for vast amounts of chemically storable energy for transportation and the chemical process industry is emerging
Based on the measured values, we find that a lower ohmic resistance for the membranes gives higher hydrogen production rate and lower waste heat required per kg of hydrogen
The membrane resistance/conductivity were estimated for membranes soaked in a ammonium bicarbonate salt solution at various concentrations and different temperatures
Summary
In the imminent transition needed in our energy economy, shifting from fossils to sustainable, a demand for vast amounts of chemically storable energy for transportation and the chemical process industry is emerging. Our energy economy is transcending from burning chemical energy to generate electricity into an energy economy where electricity is used for producing chemicals and chemical energy In this emerging energy economy, most of the electrical energy is generated from solar and wind power, likely to an extent where utilising waste heat for electricity production will become much less interesting than what has been suggested so far. In a reverse electrodialysis (RED) system, the concentration gradient of salt solutions across an ion-exchange membrane (IEM) acts as a driving force for ions to diffuse through the membrane to create an ionic flux This flux can be converted into either electrical current or a gas with an appropriate combination of electrolyte rinse solutions and electrodes
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