MIT’s Innovative System Could Utilize 40% of Solar Heat to Generate Eco-Friendly Hydrogen Fuel

Engineers at the Massachusetts Institute of Technology have conceived a groundbreaking mechanism that employs an array of solar-powered reactors to produce hydrogen fuel devoid of carbon emissions. This design elevates the system’s efficiency dramatically from 7% to 40%, presenting the possibility of making the production of green hydrogen both scalable and economically viable.

The engineering team at MIT is focused on creating an entirely green, carbon-neutral hydrogen fuel through a novel, locomotive-like set of reactors that operate exclusively on solar energy.

Published recently in the Solar Energy Journal, the engineers elucidate the blueprint for a system capable of efficient “solar thermochemical hydrogen” production. The system capitalizes on solar heat to directly decompose water, thereby creating hydrogen— an environmentally clean fuel suitable for powering long-haul trucks, vessels, and aircraft, all while emitting zero greenhouse gases.

Conventional hydrogen production predominantly relies on methods that incorporate natural gas and fossil fuels. This transforms what could be an eco-friendly fuel into more of a “grey” energy source, when examined from the initial point of production to end use. Unlike this, solar thermochemical hydrogen, abbreviated as STCH, stands as a genuinely emissions-free alternative, driven wholly by renewable solar energy. However, prevailing STCH systems suffer from limited efficiency: they convert just around 7% of the incoming solar radiation into hydrogen, resulting in low yield and high operational costs.

The MIT engineers believe their newly-designed system could capture up to 40% of the solar heat to produce an equivalent amount of hydrogen. This surge in efficiency could significantly reduce the overall system costs, making STCH an increasingly viable and cost-effective choice for decarbonizing the transportation sector.

Ahmed Ghoniem, the Ronald C. Crane Professor of Mechanical Engineering at MIT and the lead author of the study, states, “The objective is to meet the Department of Energy’s goal of generating green hydrogen by 2030 at $1 per kilogram. To make this financially sound, it’s crucial to enhance the efficiency so that the majority of the collected solar energy is put to use in hydrogen production.”

The co-authors of Ghoniem’s study include Aniket Patankar, a postdoctoral fellow at MIT; Harry Tuller, a professor of materials science and engineering at MIT; Xiao-Yu Wu from the University of Waterloo; and Wonjae Choi from Ewha Womans University in South Korea.

Solar Infrastructure

Unlike some other proposed models, the MIT scheme would be combined with a pre-existing solar heat source, for example, a concentrated solar plant (CSP)—an arrangement of hundreds of mirrors that gather and focus sunlight onto a central receiver tower. An STCH system would then take in the heat from the receiver to break down water and generate hydrogen. This method is distinct from electrolysis, which employs electricity rather than heat to separate water.

The core mechanism of the hypothetical STCH system comprises a two-phase thermochemical reaction. Initially, steam contacts a specific metal, causing the metal to absorb the oxygen, leaving hydrogen gas behind. This ‘oxidation’ resembles the rusting of iron but occurs at a significantly accelerated rate. Once the hydrogen is isolated, the oxidized metal is reheated in a vacuum to reverse the oxidation, thereby regenerating the metal for future use. This cycle is replicable hundreds of times.

The proposed MIT system is engineered to optimize this cycle. The complete setup mimics a train of cubical reactors operating on a circular track, potentially surrounding a CSP tower. Each reactor would contain the metal subjected to the reversible rusting, also known as redox, process.

The reactors first traverse a hot station, subjected to solar temperatures as high as 1,500 degrees Celsius, which effectively removes the oxygen from the reactor’s metal. Subsequently, the reactor advances to a cooler station at approximately 1,000 degrees Celsius to interact with steam and produce hydrogen.

Overcoming Challenges

Traditional STCH systems encounter two primary challenges: efficient heat recovery from the cooling reactor and the establishment of an energy-efficient vacuum for metal regeneration. MIT’s design tackles these issues by incorporating several energy-saving features. The reactors on either side of the circular track are designed to exchange heat through thermal radiation, thereby retaining heat within the system. Additionally, an outer loop of reactors travels in the opposite direction to facilitate oxygen removal from the hotter inner loop, obviating the need for energy-intensive mechanical pumps.

After running comprehensive simulations, the researchers concluded that their design could potentially elevate the efficiency of solar thermochemical hydrogen production from the current 7% to 40%.

Ghoniem sums up, “Every fragment of energy within the system must be meticulously managed to minimize costs. With our design, we’ve discovered that all required energy can be sourced from the sun, capturing 40% of its heat to generate hydrogen.”

In the forthcoming year, the team intends to construct a prototype to undergo testing at the Department of Energy’s concentrated solar power facilities, which are currently funding this initiative.

The research was financially supported by the Centers for Mechanical Engineering Research and Education at MIT and SUSTech.

Reference: “A Comparative Analysis of Integrating Thermochemical Oxygen Pumping in Water-Splitting Redox Cycles for Hydrogen Production” by Aniket S. Patankar, Xiao-Yu Wu, Wonjae Choi, Harry L. Tuller and Ahmed F. Ghoniem, 16 October 2023, Solar Energy. DOI: 10.1016/j.solener.2023.111960.

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More about Green Hydrogen Production Efficiency

• Solar Energy Journal Publication
• MIT Department of Mechanical Engineering
• Department of Energy Green Hydrogen Goals
• Concentrated Solar Plants (CSP)
• Hydrogen Fuel in Transportation Sector
• Renewable Energy and Decarbonization