MIT engineers want to produce completely green hydrogen with no carbon emissions using a new reactor system in the form of a train powered solely by the sun.
In a study published today in the Solar Energy Journal, engineers present a conceptual design of a system capable of efficiently producing “solar thermochemical hydrogen.” The system uses heat from the sun to directly split water and produce hydrogen, a clean fuel that can power trucks, ships and long-haul aircraft without emitting greenhouse gases.
Today, hydrogen is largely produced through processes using natural gas and other fossil fuels, making this otherwise environmentally friendly fuel a “grey” energy source globally. From the beginning of its production to its final use. Instead, solar thermochemical hydrogen (STCH) offers a completely zero-emissions alternative that relies entirely on renewable solar energy to power hydrogen production. However, so far existing STCH designs have limited efficiency: only about 7% are used of sunlight to produce hydrogen. The results so far have been low performance and high cost.
The MIT team believes so Its new design could harness up to 40% of the sun’s heat to produce much more hydrogen, which would represent a major advance in the production of solar fuels. Greater efficiency could reduce overall system costs and make STCH a potentially scalable and affordable option to help decarbonize the transportation industry.
“We think hydrogen is the fuel of the future, and it needs to be produced cheaply and on a large scale,” says study lead author Ahmed Ghoniem, the Ronald C. Crane Professor of Mechanical Engineering at MIT. “We are trying to meet the Department of Energy’s goal of producing green hydrogen for $1 per kilogram by 2030. To improve the economy, we need to improve efficiency and ensure that the majority of the solar energy we collect is used for hydrogen production.
Solar stations
Like other proposed designs, MIT’s system would be combined with an existing solar heat source, such as a concentrating solar power plant, a circular array of hundreds of mirrors that collect and reflect sunlight onto a central receiving tower. An STCH system then absorbs heat from the receiver and conducts it to split water and produce hydrogen. This process is very different from electrolysis, which uses electricity instead of heat to split water.
The core of a conceptual STCH system is a two-step thermochemical reaction. In the first step, water in vapor form is exposed to a metal. This causes the metal to absorb oxygen from the steam and leaves the hydrogen behind. This “oxidation” of the metal is similar to the oxidation of iron in the presence of water, but occurs much faster. Once the hydrogen is separated, the oxidized (or oxidized) metal is reheated in vacuum, reversing the oxidation process and regenerating the metal. Once the oxygen is removed, the metal can be cooled and exposed to steam again to produce more hydrogen. This process can be repeated hundreds of times.
The MIT system is intended to optimize this process. The system as a whole resembles a train of box-shaped jets traveling in a circular path. In practice, this path would be routed around a solar thermal source, such as a CSP tower. Each reactor on the train would house the metal undergoing the redox, or reversible oxidation, process.
Each reactor would first pass through a hot station, where it would be exposed to the heat of the sun at temperatures of up to 1,500 degrees Celsius. This extreme heat would deprive the reactor metal of oxygen. This metal would then remain in a “reduced” state and would be ready to absorb oxygen from the vapor. To do this, the reactor would be moved to a colder station with temperatures around 1,000 °C, where it would be exposed to water vapor to produce hydrogen.
Rust and rails
Other similar STCH concepts face a common obstacle: what to do with the heat released by the reduced reactor as it cools? If this heat is not recovered and reused, the efficiency of the system is too low to be practical.
A second challenge is to create an energy-efficient vacuum in which the metal can deoxidize. Some prototypes create vacuum using mechanical pumps, but these use too much energy and are too expensive for large-scale hydrogen production.
To solve these problems, MIT’s design includes several energy-saving solutions. To recover most of the heat that would otherwise escape the system, reactors on opposite sides of the circular orbit exchange heat through thermal radiation: hot reactors are cooled and cold reactors are heated. In this way the heat is retained in the system. The researchers also added a second group of reactors that would surround the first train and move in the opposite direction. This outer reactor train would operate at generally colder temperatures and would serve to evacuate oxygen from the hotter inner reactor train without the need for energy-intensive mechanical pumps.
These outdoor reactors would contain a second type of metal that also rusts easily. When turned over, the outer reactors would absorb oxygen from the inner reactors, effectively deoxidizing the original metal without the need to use energy-intensive vacuum pumps. Both reactor trains would run continuously, producing separate streams of pure hydrogen and oxygen.
The researchers conducted detailed simulations of the conceptual design and found that it would significantly increase the efficiency of solar thermochemical hydrogen production, from 7%, as previous designs have shown, to 40%.
“We have to think about every bit of energy in the system and how we can use it to minimize costs,” says Ghoniem. “And with this design we discovered that everything can be powered by solar heat. “It is capable of using 40% of the sun’s heat to produce hydrogen.”
Next year, the team will build a prototype of the system that they plan to test in concentrated solar power plants in laboratories run by the Department of Energy, which is currently funding the project.
“If fully implemented, this system would be housed in a small building in the middle of a solar field,” explains Patankar. “Inside the building there could be one or more trains, each with about 50 reactors. “And we think it could be a modular system where reactors could be added to a conveyor belt to expand hydrogen production.”
REFERENCE
A comparative analysis of the integration of thermochemical oxygen pumps into water-splitting redox cycles for hydrogen production