Abstract
This work presents an integrated hydrogen production system using reverse electrodialysis (RED) and waste heat, termed Heat to H 2 . The driving potential in RED is a concentration difference over alternating anion and cation exchange membranes, where the electrode potential can be used directly for water splitting at the RED electrodes. Low-grade waste heat is used to restore the concentration difference in RED. In this study we investigate two approaches: one water removal process by evaporation and one salt removal process. Salt is precipitated in the thermally driven salt removal, thus introducing the need for a substantial change in solubility with temperature, which KNO 3 fulfils. Experimental data of ion conductivity of K + and NO 3 − in ion-exchange membranes is obtained. The ion conductivity of KNO 3 in the membranes was compared to NaCl and found to be equal in cation exchange membranes, but significantly lower in anion exchange membranes. The membrane resistance constitutes 98% of the total ohmic resistance using concentrations relevant for the precipitation process, while for the evaporation process, the membrane resistance constitutes over 70% of the total ohmic resistance at 40 ∘ C. The modelled hydrogen production per cross-section area from RED using concentrations relevant for the precipitation process is 0.014 ± 0.009 m 3 h − 1 (1.1 ± 0.7 g h − 1 ) at 40 ∘ C, while with concentrations relevant for evaporation, the hydrogen production per cross-section area was 0.034 ± 0.016 m 3 h − 1 (2.6 ± 1.3 g h − 1 ). The modelled energy needed per cubic meter of hydrogen produced is 55 ± 22 kWh (700 ± 300 kWh kg − 1 ) for the evaporation process and 8.22 ± 0.05 kWh (104.8 ± 0.6 kWh kg − 1 ) for the precipitation process. Using RED together with the precipitation process has similar energy consumption per volume hydrogen produced compared to proton exchange membrane water electrolysis and alkaline water electrolysis, where the energy input to the Heat to H 2 -process comes from low-grade waste heat.
Highlights
Renewable energy sources, like wind, solar, and wave energy, are often intermittent and not available when and where they are needed
As a proof of concept for using KNO3 as salt in reverse electrodialysis (RED), conductivity measurements of AEM and CEM soaked in KNO3 at 23 ◦ C and 40 ◦ C were performed for concentrations close to saturation point, and compared with NaCl at the same concentrations and temperatures
The ion conductivity measurements revealed significant differences in ion conductivity for KNO3 and NaCl. The relevance of these results is demonstrated by implementing them into the described model of the hydrogen production from RED using concentrations relevant for precipitation and evaporation
Summary
Like wind, solar, and wave energy, are often intermittent and not available when and where they are needed. The technique of evaporating the salt is presented by other authors [10,11,12], where Raka et al [10] found the energy needed per kg hydrogen produced to be 105–480 kWh. Membrane distillation as a separation technique with RED is mentioned by Long et al, using sodium chloride [13]. Most research on RED and ion exchange membranes (IEM) is reported for low NaCl concentrations or for AmB when waste heat is used for separation. As a proof of concept for using KNO3 as salt in RED, conductivity measurements of AEM and CEM soaked in KNO3 at 23 ◦ C and 40 ◦ C were performed for concentrations close to saturation point, and compared with NaCl at the same concentrations and temperatures. An energy evaluation of the two processes allows for a broader comparison with other technologies, which was lacking in [17]
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