The system will be geared toward producing carbon-free H2 in a process powered entirely by the sun.

MIT engineers have developed a new solar energy reactor system designed somewhat like a train and that can be used for the production of green hydrogen.

The conceptual design for “solar thermochemical hydrogen” was published in the Solar Energy Journal.

In the study published within the Solar Energy Journal, the MIT team described a system that captures heat from the sun to directly split water molecules for green hydrogen production. The system is highly efficient and does not lead to the production of greenhouse gas emissions.

Though there are other methods of green hydrogen production, the majority of today’s H2 is produced using natural gas and other fossil fuels, with unmitigated carbon emissions. The result is considered a form of “gray” fuel, because while it can still be used carbon emission-free, its production involves carbon emissions.

Conversely, the team’s solar thermochemical hydrogen (STCH) relies entirely on the sun and does not produce carbon emissions.

Until now, solar energy has offered an efficiency level of only about 7 percent for hydrogen production.

The new system developed by the MIT team could harness as much as 40 percent of the sun’s heat for powering the hydrogen production process. This helps to overcome prior barriers of low yield and high cost. The efficiency improvement could considerably reduce the cost of the system, making STCH an affordable, scalable decarbonization option for the transportation industry.

“We’re thinking of hydrogen as the fuel of the future, and there’s a need to generate it cheaply and at scale,” explained Ahmed Ghoniem, the Ronald C. Crane Professor of Mechanical Engineering at MIT and the Director of the Center for Energy and Propulsion Research and the Reacting Gas Dynamics Laboratory, who was the study’s lead author. “We’re trying to achieve the Department of Energy’s goal, which is to make green hydrogen by 2030, at $1 per kilogram. To improve the economics, we have to improve the efficiency and make sure most of the solar energy we collect is used in the production of hydrogen.”

Ghoniem’s team and the co-authors of the study consisted of MIT postdoc Aniket Patankar, MIT professor of materials science and engineering Harry Tuller, the University of Waterloo’s Xiao-Yu Wu, and the Ewha Womans University in South Korea’s Wonjae Choi.

While the MIT system is similar to other types of solar energy design in that it would be paired with an existing source of heat from the sun – such as a concentrated solar plant (CSP) – this design would be built with a circular array consisting of hundreds of mirrors that would reflect the sunlight they collect into a central receiving tower. From there, an STCH system would absorb the heat from the receiver, directing it to split water for green hydrogen production.

In this way, it differs from the more conventional way of using the sun to produce electricity which then powers an electrolyzer that splits water into hydrogen and oxygen. The MIT system uses solar heat for splitting water instead.

The essence of a conceptual STCH system is a thermochemical reaction that occurs in two steps. The first step exposes water steam to metal. The metal takes hold of the oxygen in the steam, leaving the hydrogen behind. The process of “oxidation” with the metal can be compared to the way iron rusts when exposed to water. That said, this particular process occurs much more quickly.

When the hydrogen is separated by the heat from the solar energy and the exposure to metal, the oxidized metal is reheated within a vacuum. This way, the rusting process is essentially reversed, regenerating the metal because the oxygen has been removed. Without the oxygen, the metal cools and is then re-exposed to the steam for further hydrogen production. This process can be repeated hundreds of times.