April 10-14, 2016
Sandia National Laboratories has for many years been engaged in investigating and developing the science and technology of solar thermochemistry for application to production of solar fuels (“Sunshine to Petrol”), and thermochemical energy storage. The vision of Sunshine to Petrol is captured in one deceptively simple chemical equation:
Solar Energy + xCO2 + (x+1) H2O → CxH2x+2 (liquid fuel) + (1.5x+0.5) O2
Practical implementation of this equation may seem far-fetched, since it effectively describes the use of solar energy to reverse combustion. However, it is also representative of the photosynthetic processes responsible for much of life on earth and, as such, summarizes the biomass approach to fuels production. Our analysis indicates that any such solar-driven conversion process must operate at a relatively high efficiency, at least 10% solar-to-fuel, to meet the dictates of economics and scale. Thus, it is our contention that an alternative that is not limited by the efficiency of photosynthesis and that more directly leads to a liquid fuel is required. The approach we have pursued is the direct application of solar thermal energy to split carbon dioxide and water to obtain carbon monoxide and hydrogen, the basic precursors to synthetic fuels. These conversions are accomplished via two-step metal-oxide based thermochemical cycles (Figure 1.) In one step of the thermochemical cycle, a metal oxide (MOx) is thermally reduced (oxygen is evolved) at high temperatures driven by concentrating solar power; in the other step the oxygen-deficient (MOx-d) material is reoxidized with carbon dioxide (or water) at a lower temperature to restore the material to its original state and to yield carbon monoxide (or hydrogen). As shown in the figure, heat may be recuperated between the high and low temperature steps.
Figure 1 – Schematic depiction of a two-step metal-oxide thermochemical cycle with internal recuperation for carbon dioxide and water splitting.
Thermochemistry promises to provide the high efficiencies that we believe are required for solar fuels. However, the continuous chemical and thermal cycling occurring in these cyclic processes poses numerous chemistry, materials, and engineering challenges. Improvements in both the metal oxides that facilitate the conversion, and the reactors and systems in which they are implemented, are needed to realize high efficiency and reliable operation. The properties that define an ideal material for an efficient process, e.g. the thermodynamics of the redox reaction, and key materials traits for implementation will be discussed. Advances in characterizing and understanding the remarkably dynamic behavior of some of the known active materials will also be presented. Requirements and constraints for efficient design and operation of solar thermochemical reactors will likewise be introduced. Results for an established material, i.e. ceria, in a first-of-kind continuous reactor for on-sun conversion of carbon dioxide to carbon monoxide over a period of days will be presented. Next-generation approaches to materials and reactors will be briefly discussed.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
James Miller, Eric Coker, Andrea Ambrosini, Anthony McDaniel, Ivan Ermanoski, and Ellen Stechel, "Sunshine to petrol: Thermochemistry for solar fuels" in "CO2 Summit II: Technologies and Opportunities", Holly Krutka, Tri-State Generation & Transmission Association Inc. Frank Zhu, UOP/Honeywell Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/co2_summit2/18