An artist's conception of a commercial hydrogen production plant (Credit: University of Colorado Boulder)

Solar towers could fuel hydrogen-based economy

A hydrogen production plant has been designed that could fuel a sustainable green economy with sunlight and water.

A team of researchers has developed a radically new technique in which sunlight is concentrated by a vast array of mirrors onto a single point on top of a central tower several hundred feet tall.

The tower would heat up to around 1,350°C – enough to liberate hydrogen from steam with the help of a metal oxide compound.

"We have designed something here that is very different from other methods and frankly something that nobody thought was possible before," said lead scientist Professor Alan Weimer, from the University of Colorado at Boulder.

"Splitting water with sunlight is the Holy Grail of a sustainable hydrogen economy."

Writing in the journal Science, the researchers explain how their plant would adopt the same principle as using a magnifying glass to start a fire.

As the metal oxide compound heats up, it releases oxygen atoms, changing its material composition and causing the newly formed compound to seek out new oxygen atoms, said Prof Weimer.

The team showed that the addition of steam to the system which could be produced by boiling water in the reactor with the concentrated sunlight beamed to the tower, would cause oxygen from the water molecules to adhere to the surface of the metal oxide, freeing up hydrogen molecules for collection as hydrogen gas.

"We can concentrate sunlight until it is really hot and use it to drive these chemical reactions," said Dr Christopher Muhich, a member of the University of Colorado team.

"While we can easily heat it up to more than 1,350°C, we want to heat it to the lowest temperature possible for these chemical reactions to still occur. Hotter temperatures can cause rapid thermal expansion and contraction, potentially causing damage to both the chemical materials and to the reactors themselves."

One of the key differences between the researchers’ new method and other methods developed to split water is the ability to conduct two chemical reactions at the same temperature, said co-lead author Professor Charles Musgrave, also of the chemical and biological engineering department.

While there are no working models, conventional theory holds that producing hydrogen through the metal oxide process requires heating the reactor to a high temperature to remove oxygen, then cooling it to a low temperature before injecting steam to re-oxidize the compound in order to release hydrogen gas for collection.

“The more conventional approaches require the control of both the switching of the temperature in the reactor from a hot to a cool state and the introduction of steam into the system,” said Prof Musgrave. “One of the big innovations in our system is that there is no swing in the temperature. The whole process is driven by either turning a steam valve on or off.”

The quantity of hydrogen produced is entirely dependent on the amount of metal oxide, a combination of iron, cobalt, aluminium and oxygen, and how much steam is introduced into the system.

One of the designs proposed by the team is to build reactor tubes roughly a foot in diameter and several feet long, fill them with the metal oxide material and stack them on top of each other, but a working system would require a number of towers each surrounded by several acres of mirrors.

But commercialisation of such a solar-thermal reactor is only likely to happen when the economic conditions are right, said Prof Weimer.

“With the price of natural gas so low, there is no incentive to burn clean energy,” he said. "There would have to be a substantial monetary penalty for putting carbon into the atmosphere, or the price of fossil fuels would have to go way up."

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