According to the IEA says that, producing all of today’s hydrogen just using electricity would require 3600 TWh—more than the annual electricity generation of the European Union. A new approach developed by researchers at the Norwegian University of Science and Technology (NTNU) could alleviate that situation a bit by using waste heat from other industrial processes.

Energy experts say that the waste heat from Norway’s businesses and industries is the equivalent of 20 TWh of energy. To put this in perspective, Norway’s entire hydropower system produces 140 TWh of electricity a year. An open-access paper on the process is published in journal Energies.

This work presents an integrated hydrogen production system using reverse electrodialysis (RED) and waste heat, termed Heat to H₂. 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 KNO₃ fulfils. Experimental data of ion conductivity of K⁺ and NO₃^- in ion-exchange membranes is obtained.

The ion conductivity of KNO₃ 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 m³ 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 m³ 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 H₂-process comes from low-grade waste heat.

—Krakhella et al.

Visualization of a RED cell. The hydrogen is produced at the cathode end and oxygen is produced at the anode end. Illustration: NTNU.

In RED, the anion exchange membrane (AEM), allows negatively charged electrons (anions) to move through the membrane, while the cation exchange membrane (CEM) allows positively charged electrons (cations) to flow through the membrane.

The membranes separate a dilute salt solution from a concentrated salt solution. The ions migrate from the concentrated to the dilute solution, and because the two different types of membranes are alternated, they force the anions and cations to migrate in opposite directions.

When these alternating columns are sandwiched between two electrodes the stack can generate enough energy to split water into hydrogen (on the cathode side) and oxygen (on the anode side).

This approach was developed in the 1950s and first used saltwater and river water.

The researchers used a potassium nitrate salt; the use of this kind of salt enabled them to use waste heat as part of the process.

In running the RED stacks, at some point the concentrate and dilute salt solutions become more and more alike, so they have to be refreshed. I.e., there needs to be a way to increase the concentration of the salt in the concentrated solution and remove salt from the dilute solution. That’s where the waste heat comes in.

The researchers tested two systems. The first used waste heat to evaporate water from the concentrated solution to make it more concentrated. The second system used waste heat to cause salt to precipitate out of the diluted solution.

The researchers found that using existing membrane technology and waste heat to evaporate water from their system produced more hydrogen per membrane area than the precipitation approach.

The production of hydrogen was four times higher for the evaporation system operated at 25 ˚C and two times higher for a system operated at 40 ˚C compared to their precipitation system. That made it a better candidate from a cost perspective.

However, the precipitation process was better in terms of energy demand, the researchers found. For example, the energy needed to produce a cubic meter of hydrogen using the precipitation process was just 8.2 kWh, compared to 55 kWh for the evaporation process.

Although the researchers chose potassium nitrate for their salt system, but other salts could also work, said first author Kjersti Wergeland Krakhella. Another issue that continues to limit hydrogen production is that the membranes themselves remain extremely costly.

Krakhella hopes that as societies look to move away from fossil fuels, increased demand will drive the price of membranes down, as well as improving the characteristics of the membranes themselves.

Resources

• Krakhella, K.W.; Bock, R.; Burheim, O.S.; Seland, F.; Einarsrud, K.E. (2019) “Heat to H₂: Using Waste Heat for Hydrogen Production through Reverse Electrodialysis.” Energies 12, 3428 doi: 10.3390/en12183428