Engineers at the Massachusetts Institute of Technology (MIT) have developed a conceptual design to produce totally green, carbon-free hydrogen fuel with a train-like system of reactors that is driven solely by solar energy.

In a study published in Solar Energy Journal, the engineers said the new system efficiently harnesses the sun’s heat to split water and generate hydrogen.

The MIT system would be paired with an existing source of solar heat, such as a concentrated solar plant (CSP) — a circular array of hundreds of mirrors that collect and reflect sunlight to a central receiving tower. A solar thermochemical hydrogen (STCH) system then absorbs the receiver’s heat and directs it to split water and produce hydrogen. This process is very different from electrolysis, which uses electricity instead of heat to split water.

At the heart of the STCH system is a two-step thermochemical reaction. In the first step, water in the form of steam is exposed to a metal. This causes the metal to grab oxygen from steam, leaving hydrogen behind. This metal “oxidation” is similar to the rusting of iron in the presence of water, but it occurs much faster. Once hydrogen is separated, the oxidized metal is reheated in a vacuum, which acts to reverse the rusting process and regenerate the metal. With the oxygen removed, the metal can be cooled and exposed to steam again to produce more hydrogen. This process can be repeated hundreds of times.

The system resembles a train of box-shaped reactors running on a circular track. In practice, this track would be set around a solar thermal source, such as a CSP tower. Each reactor in the train would house the metal that undergoes the redox, or reversible rusting, process.

Each reactor would first pass through a hot station, exposing it to the sun’s heat at temperatures of up to 1,500 degrees Celsius. This extreme heat would effectively pull oxygen out of a reactor’s metal. That metal would then be in a “reduced” state — ready to grab oxygen from steam. For this to happen, the reactor would move to a cooler station at temperatures around 1,000 C, exposing it to steam to produce hydrogen.

This development could pave the way for the cleaner production of green hydrogen and reduce reliance on conventional systems that rely on fossil fuels and natural gas.

STCH offers an emissions-free alternative, where only 7% of the incoming sunlight is used to make hydrogen. As this limits efficiency, the results currently are low yield with high cost. However, with the new design, the MIT team can harness up to 40% of the sun’s heat to generate that much more hydrogen. This increased efficiency can decrease the system’s total cost and make it scalable.

In the coming year, the team will develop a prototype of the system that they will test in concentrated solar power facilities at laboratories of the United States Department of Energy (DOE). “We’re trying to achieve the Department of Energy’s goal, which is to make green hydrogen by 2030, at $1 per kilogram,” said the study’s lead author, Ahmed Ghoniem, the Ronald C. Crane Professor of Mechanical Engineering at MIT.

In July this year, the DOE announced plans to invest $1 billion in the Regional Clean Hydrogen Hubs initiative to reduce emissions from energy-intensive sectors. This aligns with the government’s plan to establish a national clean hydrogen network. The MIT study is also funded by the DOE.

The demand for clean hydrogen is set to grow by 10 million metric tons (MMT) annually by the end of the decade in the U.S., while it will rise to 20 MMT and 50 MMT by 2040 and 2050, respectively, according to the DOE.