Heat to H2: Using Waste Heat for Hydrogen Production through Reverse Electrodialysis

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 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 ∘ . 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 n a m e d - c o n t e n t c o l o r t y p e r g b c o n t e n t - t y p e b l a c k - 1 (1.1 ± 0.7 g h n a m e d - c o n t e n t c o l o r t y p e r g b c o n t e n t - t y p e b l a c k - 1 ) at 40 ∘ , while with concentrations relevant for evaporation, the hydrogen production per cross-section area was 0.034 ± 0.016 m 3 h n a m e d - c o n t e n t c o l o r t y p e r g b c o n t e n t - t y p e b l a c k - 1 (2.6 ± 1.3 g h n a m e d - c o n t e n t c o l o r t y p e r g b c o n t e n t - t y p e b l a c k - 1 ). The modelled energy needed per cubic meter of hydrogen produced is 55 ± 22 kWh (700 ± 300 kWh kg n a m e d - c o n t e n t c o l o r t y p e r g b c o n t e n t - t y p e b l a c k - 1 ) for the evaporation process and 8.22 ± 0.05 kWh (104.8 ± 0.6 kWh kg n a m e d - c o n t e n t c o l o r t y p e r g b c o n t e n t - t y p e b l a c k - 1 ) for the precipitation process. Using RED together with the precipitation process has similar energy consumption per volume hydrogen produced compared to

Comparison of Combustion Models for Lifted Hydrogen Flames within RANS Framework

“…Hydrogen allows a useful way of storing the surplus power generated by photovoltaics and wind [7]. Moreover, in place of combustion [8,9], the gasification of waste, biomass, and coal [10,11] provides a useful procedure for efficient and clean conversion, whereby the product synthesis gas (syngas) contains rather considerable portions of hydrogen. Furthermore, there is a developing tendency in hydrogen production using nuclear energy [12].…”

Electrodialytic Energy Storage System: Permselectivity, Stack Measurements and Life-Cycle Analysis

“…However, due to the large difference between the concentrations in the present work and the concentrations used by Zlotorowicz et al, the water transport number in AEM and CEM for the model is varied between 0 and 10 [18]. The permselectivity, given salt transport number equal to Equation (5), average water transport number: t w = 5 and activity coefficients following Equation (6), is plotted in Figure 3. Fumatech reports a permselectivity of 0.97-0.99 and 0.92-0.96 for FKS-50 and FAS-50 respectively [13,14] at 25 • C and 0.5 M NaCl.…”

“…Conductivity measurements carried out on FAS-50 and FKS-50 soaked in up to 4.3 mol kg −1 NaCl, indicating a decrease in conductivity with increasing mean concentration. For FAS-50, the conductivity decreased from 4 to 2 mS cm −1 from 0.9 to 4.3 M in concentration at 23 • C and from 8 to 3 mS cm −1 at 40 • C. The conductivity in FKS-50 decreased from 2 to 1 mS cm −1 from 0.9 to 1.8 M and from 4 to 1 mS cm −1 from 0.9 to 1.8 M at 40 • C [5]. These measurements indicate that the calculated resistance in Figure 10b is too low.…”

“…The ohmic losses can be reduced by decreasing the compartment thickness, where 100 µm or lower is recommended by Vermaas et al [47]. Increasing the temperature would also decrease the resistance of the solutions [5]. Jalili et al showed a significant decrease in stack resistance, increasing the temperature from 10 • C to 80 • C. However, for the temperature range used in the present study, Jalili et al [7] did not show a prominent change in stack resistance, where the stack resistance was measured to be between 1 and 2 mΩ m 2 between 25 • C and 40 • C. It is also worth mentioning that the model in Reference [7] uses concentrations 0.01 M to 1.0 M. However, both the study by Jalili et al [7] and our work show that membranes with lower resistance should be prioritised, especially at high concentration difference and temperature.…”

“…The system is discharged by mixing the two solutions, converting the chemical potential to electricity and decreasing the concentration difference. The very same system can also be charged using waste-heat and has hydrogen as output [5,6]. An illustration of a general energy storage system with solutions of different concentrations is given in Figure 1.…”