|Schematic of the solar reactor for the two-step, solar-driven thermochemical production of fuels. Click to enlarge.|
A team from Caltech, ETH Zürich and the Paul Scherrer Institute have devised a solar reactor for the two-step, solar-driven thermochemical production of fuels. In a paper published in the journal Science, they report stable and rapid generation of fuel over 500 cycles. They achieved solar-to-fuel efficiencies of 0.7 to 0.8%, and showed that the efficiency was largely limited by system scale and design, rather than by its chemistry.
The basis for the system is a solar-driven thermochemical cycle for dissociating H2O and CO2 using nonstoichiometric ceria (CeO2). The design of the reactor exposes porous ceria directly to concentrated solar radiation, heating it to between 1,420 and 1,640 °C, thereby liberating oxygen from its lattice. The material then readily strips oxygen atoms from carbon dioxide and water, forming CO and hydrogen, respectively, which are combined to create fuels.
Solar-driven thermochemical approaches to CO2 and H2O dissociation inherently operate at high temperatures and use the entire solar spectrum; as such, they provide an attractive path to solar fuel production at high rates and efficiencies in the absence of precious metal catalysts...Cerium oxide (ceria) has emerged as a highly attractive redox active material choice for two-step thermochemical cycling because it displays rapid fuel production kinetics and high selectivity, where such features result, in part, from the absence of distinct oxidized and reduced phases. However, ceria-based thermochemical studies to date have largely been limited to bench-top demonstrations of components or individual steps of the solar fuel production cycle; assessment of cyclability has been limited, and the energy conversion efficiency has remained uncertain because of the relatively low gravimetric fuel productivity inherent to the nonstoichiometric process.
Here, we demonstrate high-rate solar fuel production from both CO2 and H2O using a solar reactor subjected directly to concentrated radiation under realistic operating conditions relevant to large-scale industrial implementation, without the need for complex material microstructures and/or system design (e.g., additional quench or separation steps). The results provide compelling evidence for the viability of thermochemical approaches to solar fuel generation while clarifying the efforts required to transform the concept into a central technology in a sustainable energy future.—Chueh et al.
The solar-to-fuel energy conversion efficiency obtained in the work for CO2 dissociation was about two orders of magnitude greater than that observed with state-of-the-art photocatalytic approaches. The gravimetric hydrogen production rate exceeds that of other solar-driven thermochemical processes by more than an order of magnitude.
The researchers found that both the efficiency and the cycling rates in the reactor were limited largely by thermal losses, resulting from conductive and radiative heat transfer. A thermodynamic analysis of efficiency based solely on the material properties of CeO2 suggests that values in the range of 16 to 19% are attainable, even in the absence of sensible heat recovery.
Given that, the team anticipates that reactor optimization and system integration will result in substantial increases in both efficiency and fuel production rates. Furthermore, they note, the abundance of cerium, which is comparable to that of copper, is such that the approach is applicable at scales relevant to global energy consumption.
William C. Chueh, Christoph Falter, Mandy Abbott, Danien Scipio, Philipp Furler, Sossina M. Haile, and Aldo Steinfeld (2010)High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria. Science Vol. 330 no. 6012 pp. 1797-1801 doi: 10.1126/science.1197834