Hydrogen Fuel From Sunshine And Splitting Water

An artist's conception of a commercial hydrogen production plant that uses sunlight to split water in order to produce clean hydrogen fuel

A University of Colorado Boulder team have devised a new, more efficient technique for converting sunshine and water directly into usable fuel. The technique involves concentrating sunlight in a solar tower to achieve temperatures high enough to drive chemical reactions that split water into its constituent oxygen and hydrogen molecules. In this way, the team says it should be able to cheaply produce massive amounts of hydrogen fuel.

The method would use a vast array of mirrors that would concentrate sunlight onto a single point at the top of a tall tower to produce very high temperatures. The temperatures there could rise to as high as 1,350 Celsius. That energy would then be sent to a reactor vessel containing metal oxides, which, when heated, release oxygen atoms. The reduced metal oxide now gains a chemical composition that makes it ready to bind with oxygen atoms. Introducing steam into the reactor, which can also be produced by heating water with sunlight, causes the compound to draw oxygen atoms out of the water molecules, leaving behind hydrogen molecules that can be collected as hydrogen gas.

"You need this high temperature both to give you the driving force to drive the chemical reactions and also the kinetics to make the reactions go fast enough to make the process practical," says Charles Musgrave, Professor of Chemical and Biological engineering at CU-Boulder.

Diagram explaining the process for producing hydrogen fuel from sunlight and water

“We have designed something here that is very different from other methods and frankly something that nobody thought was possible before,” said University of Colorado Boulder professor Alan Weimer. “Splitting water with sunlight is the Holy Grail of a sustainable hydrogen economy”, he added.

The big breakthrough came about when the team discovered certain active materials that allowed both these chemical reactions (reducing the metal oxide and re-oxidizing it with steam) to occur at the same temperature.

While there are other methods to split water into hydrogen and oxygen, Colorado Boulder scientists say their method is unique because the two chemical reactions can be done at the same temperature.

“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 associate professor Charles 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.”

With the new method, the amount of hydrogen produced for fuel cells or for storage is entirely dependent on the amount of metal oxide -- which is made up of a combination of iron, cobalt, aluminum and oxygen -- and how much steam is introduced into the system.

According to the team, huge solar plants spread across many acres could produce much more fuel per acre than biofuels for the same amount of acreage. Another advantage that this process has over other renewable technologies, such as wind and photovoltaics, is that it directs sunlight to directly drive chemical reactions to produce fuel for use in combustion engines or fuel cells. In contrast, photovoltaic processes first convert sunlight into electricity, reducing overall efficiency.

"Our objective is to produce hydrogen (H2) at $2/kg H2," Weimer told. "This is equivalent to about US$2/gallon (3.7 L) of gasoline based on mileage in a fuel cell car versus a combustion engine today." With the aid of a solar thermal plant, the team believes that on a land area of about 48,500 ha (120,000 acres) they can generate 100,000 kg (222,460 lb) of hydrogen per day, which is enough to run over 5,000 hydrogen-fuel cell buses daily.

Though the technology has the potential to be a game-changer in pushing the hydrogen economy forward, commercialization might still be several years away thanks to continuing stiff competition from fossil fuels.

The National Science Foundation and the U.S. Department of Energy supported the research and a paper on the system was published in the Aug. 2 issue of Science.
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Write by: RC - Tuesday, August 6, 2013

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