Transforming concentrated solar light, heat and water into hydrogen

Hydrogen is a promising fuel and feedstock for a renewable energy economy and chemical process industry. It is typically produced by steam reforming of fossil fuels, an unsustainable approach. Hydrogen, as carbon-free fuel and feedstock, can be stored and transported, and directly used in fuel cells for mobility applications or electricity generation, or be further processed to synthetic methane for long term storage, or to liquid fuels (e.g. methanol or gasoline) to be used in conventional engines. The state-of-the-art production of hydrogen by sunlight via photovoltaic-driven low-temperature electrolysis demands rare and expensive materials (e.g. platinum group catalysts for electrolyzers). Thus, it is challenging to supply the market with green hydrogen at competitive prices and on a large scale.

Researchers at EPFL’s Laboratory of Renewable Energy Science and Engineering (LRESE) and the Group of Energy Materials (GEM) presented an innovative method to overcome the challenging economic competitiveness. The idea is to use concentrated solar radiation for powering the high-temperature electrolyzer. The benefit is twofold: (1) high-temperature electrolysis allows for the use of earth-abundant materials (i.e. catalysts for electrolyzers), and (2) the solar-to-fuel efficiency is significantly increased given that the electricity demand for the electrolyzers is reduced compared to low-temperature electrolysis. A demonstrator reactor was designed, fabricated and successfully tested in LRESE’s unique high-flux solar simulator. The innovative design followed an integrated approach, i.e., electrolyzer stack and solar absorber were in spatial proximity, which significantly reduces heat losses in the reactor. The novel design approach showed a potential of 20% solar-to-hydrogen efficiency.

The work has just been published in Joule. “Our approach utilizes concentrated solar radiation for both, heat for reactant heating and electricity from PV cells for powering the electrolyzer. Given that heat can be produced more efficiently than electricity from solar light, the endothermic operation of the electrolyzer is preferential as some part of the electricity demand is substituted by heat,” says Clemens Suter, a researcher at the LRESE. “However, the thermal management in the solar absorber is crucial for a safe operation of the electrolyzer. We have found a sophisticated design of the solar absorber using a cavity-receiver shape made of two interlaced double-helical pipes for the reactant heating. This approach enabled the reduction of thermal stress in the electrolyzer made of ceramics“, adds Meng Lin, a co-author of the study and now Assistant Professor at Southern University of Science and Technology (SUSTech) in China.

Upscaling strategies:

The researchers predict that the system can be scaled up on an industrial level. Sophia Haussener, head of the LRESE and the project lead, explains: “The 7-meter parabolic mirror (solar dish) on EPFL’s Lausanne campus is suited for testing such a reactor under real sun conditions. Solar concentrations of around 1’000 are produced, which match perfectly to the concentrated solar radiation input required by this reactor design. An array of such solar concentrators could be used for scaling up of the approach.” A scale-up also allows for higher thermal efficiencies, and thus a further improvement of the overall performance. It is planned to extend this technology to co-electrolysis for the co-production of hydrogen and CO, called synthesis gas, which is perfectly suited as feedstock for the Fischer-Tropsch or methanol synthesis for direct production of liquid solar fuels.