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© 2010 AAAS Concentrated solar radiation enters the reactor, is intensified by a compound parabolic concentrator, and is focused on a cerium oxide cylinder. H 2 O and CO 2 enter side inlets, and O 2 , H 2 , and CO exit a bottom outlet.

Researchers have developed a novel thermochemical reactor that uses sunlight to convert carbon dioxide and water into hydrocarbon-fuel precursors at a relatively high efficiency.

The feat is a key step toward using solar energy to produce much-needed liquid fuels more efficiently than may be possible with alternative methods, such as photocatalysis or microbial fermentation-based hydrocarbon-fuel production.

The new thermochemical reactor is believed to be more efficient than previously developed ones, whose efficiencies could not be comparably measured. And it is amenable to continuous operation, suggesting that an industrial-scale version of the process could be developed for solar towers.

The reactor was designed by solar technology specialist Aldo Steinfeld of ETH, the Swiss Federal Institute of Technology, Zurich; materials scientist Sossina M. Haile of California Institute of Technology; and coworkers (Science, DOI: 10.1126/science.1197834). It uses concentrated solar energy to thermochemically dissociate CO 2 and H 2 O via cerium oxide redox reactions to produce CO and H 2 , respectively, with O 2 as a by-product. CO and H 2 form syngas, which can be processed to generate methanol, gasoline, and other liquid fuels.

The reactor’s solar-to-syngas energy conversion efficiency, experimentally measured with a 2-kW prototype, is 0.7 to 0.8%, which Steinfeld says is significantly higher than those of current photocatalytic methods for CO 2 dissociation. A thermodynamic analysis indicates that efficiencies of 16% or more are achievable with the new reactor.

The study’s “solar conversion efficiencies are less than 1%, but these efficiencies set an important benchmark for further improvements in the use of pure solar thermal energy to split CO 2 ,” notes renewable energy researcher Stuart Licht of George Washington University.

The novelty is the experiment’s relatively large scale, “the number of cycles demonstrated, and performing the demonstration long enough and in such a reproducible and controlled way that the efficiency can be carefully determined,” says thermochemistry specialist James E. Miller of Sandia National Laboratories. “It’s a step toward demonstrating what’s possible for a technology that has been underappreciated and deserves more attention.”