No 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

Reverse electrodialysis and electrodialysis can be combined into a closed energy storage system, allowing for storing surplus energy through a salinity difference between two solutions. A closed system benefits from simple temperature control, the ability to use higher salt concentrations and mitigation of membrane fouling. In this work, the permselectivity of two membranes from Fumatech, FAS-50 and FKS-50, is found to be ranging from 0.7 to 0.5 and from 0.8 to 0.7 respectively. The maximum unit cell open-circuit voltage was measured to be 115 ± 9 mV and 118 ± 8 mV at 25 • C and 40 • C, respectively, and the power density was found to be 1.5 ± 0.2 W m -2 uc at 25 • C and 2.0 ± 0.3 W m -2 uc at 40 • C. Given a lifetime of 10 years, three hours of operation per day and 3% downtime, the membrane price can be 2.5 ± 0.3 $ m −2 and 1.4 ± 0.2 $ m −2 to match the energy price in the EU and the USA, respectively. A life-cycle analysis was conducted for a storage capacity of 1 GWh and 2 h of discharging. The global warming impact is 4.53·10 5 kg CO 2 equivalents/MWh and the cumulative energy demand is 1.61·10 3 MWh/MWh, which are 30% and 2 times higher than a lithium-ion battery pack with equivalent capacity, respectively. An electrodialytic energy storage system reaches a comparable global warming impact and a lower cumulative energy demand than a lithium-ion battery for an average life span of 20 and 3 years, respectively.

No abstract

The present work suggests two concepts for producing hydrogen by reverse electrodialysis. Reverse electrodialysis is a technology that uses concentration differences to create electrical energy. In this work, the energy is utilised as direct hydrogen production within a closed-loop system. For both system alternatives, waste heat is used to set up the mentioned concentration differences. The first concept is evaporation, where heat is added to boil off excess water from a concentrated solution and thereby increase its concentration. The second concept removes heat to precipitate excess salt. For the precipitation concept to work, a salt where the solubility is highly dependent on temperature must be used. KNO 3 fulfils this requirement. As part of a proof of concept, the conductivity of membranes soaked in KNO 3 was investigated. The conductivity of the salt in two commercialised membranes, Fumatech FKE-50 and FAS-30, was measured and compared to NaCl in the same membranes. The conductivity of K + in FKE-50 was found to be 4.5 and 6.6 mS cm −1 at 25 • C and 40 • C respectively. The conductivity of NO − 3 in FAS-30 was found to be 4.3 mS cm −1 and 6.5 mS cm −1 at 25 • C and 40 • C respectively. Neither of the membranes change conductivity with soaking concentrations. The conductivity at 40 • C compared to 25 • C is significantly better in the FKE membrane, and seemingly better in the FAS membrane. Potential peak power densities for a This is the accepted version of an article published in ECS Transactions.