Sunshine to petrol: Thermochemistry for solar fuels

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

Thermokinetic/mass-transfer analysis of carbon capture for reuse/sequestration.

Effective capture of atmospheric carbon is a key bottleneck preventing non bio-based, carbon-neutral production of synthetic liquid hydrocarbon fuels using CO{sub 2} as the carbon feedstock. Here we outline the boundary conditions of atmospheric carbon capture for recycle to liquid hydrocarbon fuels production and re-use options and we also identify the technical advances that must be made for such a process to become technically and commercially viable at scale. While conversion of atmospheric CO{sub 2} into a pure feedstock for hydrocarbon fuels synthesis is presently feasible at the bench-scale - albeit at high cost energetically and economically - the methods and materials needed to concentrate large amounts of CO{sub 2} at low cost and high efficiency remain technically immature. Industrial-scale capture must entail: (1) Processing of large volumes of air through an effective CO{sub 2} capture media and (2) Efficient separation of CO{sub 2} from the processed air flow into a pure stream of CO{sub 2}

Hydrogen from Sunlight and Water: A Side-by-Side Comparison between Photoelectrochemical and Solar Thermochemical Water-Splitting

Photoelectrochemical (PEC) and solar thermochemical (STCH) water-splitting represent two promising pathways for direct solar hydrogen generation. PEC water-splitting integrates multiple functional materials and utilizes energetic electrons and holes generated from sunlight to produce hydrogen and oxygen in two half-reactions, while STCH water-splitting couples a series of consecutive chemical reactions and uses absorbed heat from sunlight to generate hydrogen and oxygen in two full reactions. In this Focus Review, the basic operating principles, sunlight utilization, device architecture, reactor design, instantaneous and annually averaged solar-to-hydrogen (STH) conversion efficiency, and the operating conditions and constraints of both pathways are compared. A side-by-side comparison addresses some common sources of confusion and misinterpretation, especially in the evaluation of STH conversion efficiencies, and reveals distinct features and challenges in both PEC and STCH technologies. This Focus Review also addresses materials and device challenges in PEC and STCH for cost-competitive hydrogen generation

A New Reactor Concept for Efficient Solar-Thermochemical Fuel Production

We describe and analyze the efficiency of a new solar-thermochemical reactor concept, which employs a moving packed bed of reactive particles produce of H2 or CO from solar energy and H2O or CO2. The packed bed reactor incorporates several features essential to achieving high efficiency: spatial separation of pressures, temperature, and reaction products in the reactor; solid–solid sensible heat recovery between reaction steps; continuous on-sun operation; and direct solar illumination of the working material. Our efficiency analysis includes material thermodynamics and a detailed accounting of energy losses, and demonstrates that vacuum pumping, made possible by the innovative pressure separation approach in our reactor, has a decisive efficiency advantage over inert gas sweeping. We show that in a fully developed system, using CeO2 as a reactive material, the conversion efficiency of solar energy into H2 and CO at the design point can exceed 30%. The reactor operational flexibility makes it suitable for a wide range of operating conditions, allowing for high efficiency on an annual average basis. The mixture of H2 and CO, known as synthesis gas, is not only usable as a fuel but is also a universal starting point for the production of synthetic fuels compatible with the existing energy infrastructure. This would make it possible to replace petroleum derivatives used in transportation in the U.S., by using less than 0.7% of the U.S. land area, a roughly two orders of magnitude improvement over mature biofuel approaches. In addition, the packed bed reactor design is flexible and can be adapted to new, better performing reactive materials

5th International Workshop on Desorption Induced by Electronic Transitions

This volume in the Springer Series on Surface Sciences presents a recent account of advances in the ever-broadening field of electron-and photon-stimulated sur­ face processes. As in previous volumes, these advances are presented as the proceedings of the International Workshop on Desorption Induced by Electronic Transitions; the fifth workshop (DIET V) was held in Taos, New Mexico, April 1-4, 1992. It will be abundantly clear to the reader that "DIET" is not restricted to desorption, but has for several years included photochemistry, non-thermal surface modification, exciton self-trapping, and many other phenomena that are induced by electron or photon bombardment. However, most stimulated surface processes do share a common physics: initial electronic excitation, localization of the excitation, and conversion of electronic energy into nuclear kinetic energy. It is the rich variation of this theme which makes the field so interesting and fruitful. We have divided the book into eleven parts in order to emphasize the wide range of materials that are examined and to highlight recent experimental and theoretical advances. Naturally, there is considerable overlap between sections, and many papers would be appropriate in more than one part. Part I focuses on perhaps the most active area in the field today: electron attachment. Here the detection and characterization of negative ions formed by attachment of elec­ trons supplied externally from the vacuum are discussed. In addition, the first observations of negative ions formed by substrate photoelectrons are presented

Theory and modeling in nanoscience: Report of the May 10-11, 2002 Workshop

On May 10 and 11, 2002, a workshop entitled "Theory and Modeling in Nanoscience" was held in San Francisco, California, sponsored by the offices of Basic Energy Science and Advanced Scientific Computing Research of the Department of Energy. The Basic Energy Sciences Advisory Committee and the Advanced Scientific Computing Advisory Committee convened the workshop to identify challenges and opportunities for theory, modeling, and simulation in nanoscience and nanotechnology, and additionally to investigate the growing and promising role of applied mathematics and computer science in meeting those challenges. This report is the result of those contributions and the discussions at the workshop

Theory and modeling in nanoscience: Report of the May 10-11, 2002 Workshop

On May 10 and 11, 2002, a workshop entitled "Theory and Modeling in Nanoscience" was held in San Francisco, California, sponsored by the offices of Basic Energy Science and Advanced Scientific Computing Research of the Department of Energy. The Basic Energy Sciences Advisory Committee and the Advanced Scientific Computing Advisory Committee convened the workshop to identify challenges and opportunities for theory, modeling, and simulation in nanoscience and nanotechnology, and additionally to investigate the growing and promising role of applied mathematics and computer science in meeting those challenges. This report is the result of those contributions and the discussions at the workshop