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 ${}^{\circ }$ 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 ${}^{\circ }$ 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.

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 $1.5\pm 0.2$ W m ${}_{\mathrm{uc}}^{-2}$ at 25 ${}^{°}$ C and $2.0\pm 0.3$ W m ${}_{\mathrm{uc}}^{-2}$ 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 CO₂ 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.

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 in order to precipitate excess salt. For the precipitation concept to work, a salt where the solubility is highly dependent on temperature must be used. KNO₃ fulfils this requirement. As part of a proof of concept, the conductivity of membranes soaked in KNO₃ 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 RED unit cell is 1.29 W m⁻² with the precipitation system, and 28.1 W m⁻² when evaporation is used.

Clara Venture Labs AS (CVL), previously Prototech AS, led the development of a closed-cycle hydrogen-oxygen Regenerative Fuel Cell System (RFCS) for Lunar Night Survival. A breadboard model was successfully designed, built, and tested, operating for three Lunar day/night charge/discharge cycles. The project was supported by subcontractors Air Liquide Advanced Technologies (ALaT) and Airbus. The breadboard utilized a PEM fuel cell (FC) stack (ALaT) delivering 60–200 watts of electrical power and heat during a total of 415 hours of discharge. The high-pressure PEM water electrolyser stack (CVL) delivered hydrogen and oxygen gases to storage tanks at 50-100 bar, using external electric power that emulated solar panel output. The life test of the breadboard lasted around 800 hours. Summary of results and lessons learned is presented. TRL 4+ was reached by the RFCS for Lunar night survival after the testing was completed.