Solar Funnels that Convert Sunlight into Fuel

An ongoing criticism of solar energy is that it only works during theday. This problem is being tackled in a plethora of different ways, from oversizing solar systems and connecting them to battery backups andthermal storage to creating syngas (synthetic gas) via algae. Newresearch from CalTech and the Swiss Federal Institute of Technology arelooking at a new way of producing hydrogen and syngasses in another way, by using solar funnels.

CalTech Professor and researcher Sossina Haile and her colleaguespublished their research in the Dec. 24, 2010, edition of Science. Thedevice that they developed is able to concentrate solar radiation andheat it up to 1,600 degrees Celsius. The resulting heat is used to split water or carbon dioxide into their constituent elements.

The device consists of a quartz lens that focusses the solar radiationon a reaction chamber. The reaction chamber is internally reflective,capturing most of the photons that enter the chamber and converting them to heat. The device heats up at a rate of 140 degrees Celsius a minuteuntil it reaches about 1,250 degrees Celsius, and stabilizing at morethan 1,400 degrees Celsius.

Through a two-step process, the device’s catalyst ceria (cerium dioxide) converts carbon dioxide or water into its constituent elements. “Ceriais a metal oxide, what that material will do when heated is it willrelease oxygen.…It happens at high temperatures, when we cool it backdown it wants to absorb oxygen,” Haile said. The ceria replaces theoxygen by stripping it from the supplied material, carbon dioxide orwater, thereby creating carbon monoxide—used for syngas, orhydrogen—which can be used directly. Either resulting fuel could be used to store the sun’s energy for use in power generation.

The funnels can be small, but they’re not nano-sized. “It’s like asponge it’s porous and the gasses flow through it,” Haile said. But“it’s not nano because these temperatures are too high fornano-structures.”

At this point, the material isn’t efficient enough for commercial use.The prototype is inefficient, converting between 0.7 percent and 0.8percent of the solar energy in the funnel into fuel. With advances thatcould change. “We calculated efficiency should be between 15 percent and 19 percent,” according to Haile. “We’re working with University ofMinnesota on that. Right now it’s limited by the thermal design of thereactor. We need a better thermal design,” she said. 

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