A University of Colorado Boulder team has developed a radically new technique that uses the power of sunlight to efficiently split water into its components of hydrogen and oxygen. If the team can take the concept to a working model they will have a real new idea for making hydrogen fuel.
CU-Boulder Professor Alan Weimer of the chemical and biological engineering department and the research group leader explains the concept is a solar-thermal system where sunlight would be concentrated by an array of mirrors onto a single point atop a central tower up to several hundred feet tall. The tower would gather heat generated by the mirror system to roughly 2,500º F (1,350º C), then deliver it into a reactor containing chemical compounds known as metal oxides.
Weimer describes that it works based on a metal oxide compound heating up, releasing oxygen atoms, changing its material composition and causing the newly formed compound to seek out new oxygen. 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.
Weimer with many others realize that sunlight as power source for hydrogen production would go along way to achieving the energy sustainability goal. The team’s paper has been published in Science. The team includes co-lead authors Weimer and Associate Professor Charles Musgrave, first author and doctoral student Christopher Muhich, postdoctoral researcher Janna Martinek, undergraduate Kayla Weston, former CU graduate student Paul Lichty, former CU postdoctoral researcher Xinhua Liang and former CU researcher Brian Evanko.
What makes the new concept stand apart and stand out is a key difference between the CU method and other methods developed to split water. Professor Musgrave explains the CU concept has the ability to conduct two chemical reactions at the same temperature.
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. It’s a very energy intense process that loses a lot of the energy.
“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 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.”
Postdoc Muhnich explains, “Just like you would use a magnifying glass to start a fire, we can concentrate sunlight until it is really hot and use it to drive these chemical reactions. While we can easily heat it up to more than 1,350º 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.”
Weimer notes the conventional process waste time as well as the energy and adds, “There are only so many hours of sunlight in a day.”
To grasp the concept note with the new CU-Boulder method, the amount of hydrogen produced for fuel cells or for storage is entirely dependent on the amount of metal oxide and how much steam is introduced into the system. When all the oxygen collection is done, the metal oxide would need to be replaced and recycled. The metal oxide the team suggests to start with is made up of a combination of iron, cobalt, aluminum and oxygen.
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. A working system to produce a significant amount of hydrogen gas would require a number of the tall towers to gather concentrated sunlight from several acres of mirrors surrounding each tower.
Weimer said the new design began percolating within the team about two years ago. “When we saw that we could use this simpler, more effective method, it required a change in our thinking. We had to develop a theory to explain it and make it believable and understandable to other scientists and engineers,” he said.
Weimer suggests that despite the discovery, the commercialization of such a solar-thermal reactor is likely years away. “With the price of natural gas so low, there is no incentive to burn clean energy,” said Weimer, also the executive director of the Colorado Center for Biorefining and Biofuels, or C2B2. “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.”
The good professor may be right, but free energy to drive a process, using only water as a raw material supply and the product used in a highly efficient fuel cell, could be so cost compelling that the concept may well make headway anyway. There is also the reality that natural gas is cheap only in North America, and at the rate the economy is adopting natural gas alternative uses the glut won’t last long.
The CU splitting idea looks good and may be looking for the next great engineering idea to keep it going.