Solar-powered hydrogen production scales beyond the lab

The Sun is a tremendous source of energy in places with long, consistent exposure. But what about at night or on a cloudy day? One solution is to bottle the Sun’s energy into hydrogen or other chemical fuel made from carbon dioxide, water, and other common substances. (For more on clean hydrogen fuel, see Physics Today, August 2022, page 22 , and the article by Joan Ogden, Physics Today, April 2002, page 69 .)

Researchers have already developed multiple proof-of-concept devices that use photoelectrochemistry to store solar energy as hydrogen for later use, but those efforts have been limited mostly to laboratory demonstrations. In an effort to generate energy approaching commercial scales, Sophia Haussener (EPFL, the Federal Institute of Technology Lausanne) and her colleagues have now developed an energy plant that’s capable of generating about 2 kW of hydrogen fuel, or about half the electricity consumption of a four-person Swiss household.

The system collects and concentrates solar light with a 7-m-diameter parabolic dish (shown above and in a simplified schematic below). Installed on the EPFL’s main campus, it tracks the Sun’s movement across the sky. A steady stream of tap water is then sent to the reactor’s electrolyzer subsystem, where it’s split with solar energy to generate hydrogen. Before reaching the electrolyzer, the water is heated as it passes through the solar reactor—specifically, a light homogenizer, photovoltaic module, and light shield. The researchers intentionally designed the system to generate heat, which not only accelerates the water-splitting reaction but also can be used for warming water and other elements.

Although the physics of the power plant is fairly straightforward, Haussener and her colleagues had to solve many engineering problems. For example, the new system needed a water-circulation design with two pumps: A global pump pushes water from the storage tank to the electrochemical reactor, and the second recirculates the water—at a different flow rate found only through trial and error—through the photovoltaic module to improve the reaction’s efficiency and to protect the module from burning. In addition, the parabolic dish demanded a water-cooled shield to absorb excess concentrated light and protect the reactor module.

The reactor operated at about 20% solar-to-hydrogen efficiency in summer and winter conditions, producing an average 2.1 kW of hydrogen fuel and about 15 kW worth of heat. Previous lab-scale work achieved only about 15% efficiency and produced just 32 W of hydrogen fuel. Haussener and coauthor Saurabh Tembhurne have founded an energy company called SoHHytec and are now developing a prototype system based on their design that simultaneously produces hydrogen, oxygen, electricity, and heat. They’re also designing it so that multiple systems can be linked to scale to the energy demands of the application. (I. Holmes-Gentle et al., Nat. Energy, 2023, doi:10.1038/s41560-023-01247-2 .)