Aldo Steinfeld

High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria.

Fuel from Heat Plants grow by using energy from the Sun to convert carbon dioxide into sugar-based polymers and aromatics. These compounds in turn can be stripped of their oxygen, either through millennia of underground degradation to yield fossil fuels, or through a rather more rapid process of dissolution, fermentation, and hydrogenation to yield biofuels. Can we use sunlight to turn CO2 into hydrocarbon fuel without relying on the intervening steps of plant growth and breakdown? Chueh et al. (p. 1797) demonstrate one possible approach, in which concentrated sunlight heats cerium oxide to a sufficiently high temperature (∼1500°C) to liberate some oxygen from its lattice. The material then readily strips O atoms from either water or CO2, yielding hydrogen or CO, which can then be combined to form fuels. Solar heating of ceric oxide enables a cycle for conversion of carbon dioxide to carbon monoxide or water to hydrogen. Because solar energy is available in large excess relative to current rates of energy consumption, effective conversion of this renewable yet intermittent resource into a transportable and dispatchable chemical fuel may ensure the goal of a sustainable energy future. However, low conversion efficiencies, particularly with CO2 reduction, as well as utilization of precious materials have limited the practical generation of solar fuels. By using a solar cavity-receiver reactor, we combined the oxygen uptake and release capacity of cerium oxide and facile catalysis at elevated temperatures to thermochemically dissociate CO2 and H2O, yielding CO and H2, respectively. Stable and rapid generation of fuel was demonstrated over 500 cycles. Solar-to-fuel efficiencies of 0.7 to 0.8% were achieved and shown to be largely limited by the system scale and design rather than by chemistry.

Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn=ZnO redox reactions

Abstract The production of hydrogen from water using solar energy via a two-step thermochemical cycle is considered. The first, endothermic step is the thermal dissociation of ZnO(s) into Zn(g) and O 2 at 2300 K using concentrated solar energy as the source of process heat. The second, non-solar, exothermic step is the hydrolysis of Zn(l) at 700 K to form H 2 and ZnO(s); the latter separates naturally and is recycled to the first step. Hydrogen and oxygen are derived in different steps, thereby eliminating the need for high-temperature gas separation. A 2nd-law analysis performed on the closed cyclic process indicates a maximum exergy conversion efficiency of 29% (ratio of Δ G 298 K °| H 2 +0.5 O 2 → H 2 O for the H 2 produced to the solar power input), when using a solar cavity-receiver operated at 2300 K and subjected to a solar flux concentration ratio of 5000. The major sources of irreversibility are associated with the re-radiation losses from the solar reactor and the quenching of Zn(g) and O 2 to avoid their recombination. An economic assessment for a large-scale chemical plant, having a solar thermal power input into the solar reactor of 90 MW and a hydrogen production output from the hydrolyser of 61 million-kWh/yr, indicates that the cost of solar hydrogen ranges between 0.13 and 0.15

Syngas production by simultaneous splitting of H2O and CO2via ceria redox reactions in a high-temperature solar reactor

/kWh (based on its low heating value and a heliostat field cost at 100–150

Concentrating solar thermal power and thermochemical fuels

/m 2 ) and, thus, might be competitive vis-a-vis other renewables-based routes such as electrolysis of water using solar-generated electricity. The economic feasibility of the proposed solar process is strongly dependent on the development of an effective Zn/O 2 separation technique (either by quench or by in situ electrolytic separation) that eliminates the need for an inert gas. The chemical aspects of the reactions involved and the present status of the pertinent chemical reactor technology are summarized.

Solar-Processed Metals as Clean Energy Carriers and Water-Splitters

Solar syngas production from H2O and CO2 is experimentally investigated using a two-step thermochemical cycle based on cerium oxide redox reactions. A solar cavity-receiver containing porous ceria felt is directly exposed to concentrated thermal radiation at a mean solar concentration ratio of 2865 suns. In the first endothermic step at 1800 K, ceria is thermally reduced to an oxygen deficient state. In the second exothermic step at 1100 K, syngas is produced by re-oxidizing ceria with a gas mixture of H2O and CO2. The syngas composition is experimentally determined as a function of the molar co-feeding ratio H2O:CO2 in the range of 0.8 to 7.7, yielding syngas with H2:CO molar ratios from 0.25 to 2.34. Ten consecutive H2O/CO2-splitting cycles performed over an 8 hour solar experimental run are presented.

Production of solar hydrogen by a novel, 2-step, water-splitting thermochemical cycle

Concentrated solar energy provides a virtually unlimited source of clean, non-polluting, high-temperature heat. This article reviews the underlying principles of concentrating solar radiation and describes the latest technological advances and future prospects of solar thermal power and thermochemical fuel production.

Optimum aperture size and operating temperature of a solar cavity-receiver

Abstract Two-step solar thermochemical cycles and processes for the production of hydrogen, hydrocarbons, and synthesis gas are considered. The first step is based on the thermal, electrothermal, or carbothermal reduction of metal oxides, producing metals, metal nitrides, metal carbides, or lower-valence metal oxides. These are hightemperature highly endothermic reactions that can be driven by concentrated solar energy, reducing the consumption of fossil fuels and their concomitant emissions. The second step involves hydrolysis reactions. The thermodynamics of both reaction steps are examined and relevant experimental studies conducted using solar energy are reviewed.

Amine-Based Nanofibrillated Cellulose As Adsorbent for CO2 Capture from Air

A novel, two-step, water-splitting cycle is presented which, in contrast to previously proposed cycles that require upper operating temperatures above 2300 K, can be conducted at a moderate temperature. In the first endothermic step, Ni0.5Mn0.5Fe2O4 is thermally activated above 1073 K to form an oxygen-deficient ferrite. In the second step, activated ferrite is reacted with water below 1073 K to form hydrogen; the ferrite product is recycled to the first step. H2 and O2 are derived in different steps, eliminating the need for high-temperature gas separation. Both reactions have been demonstrated experimentally using concentrated solar radiation as energy source for the activation.

DESIGN ASPECTS OF SOLAR THERMOCHEMICAL ENGINEERING—A CASE STUDY: TWO-STEP WATER-SPLITTING CYCLE USING THE Fe3O4/FeO REDOX SYSTEM

For solar cavity-receivers operating at high temperatures, the optimum aperture size results from a compromise between maximizing radiation capture and minimizing radiation losses. When the absorbed solar energy is utilized as high temperature process heat, the energy conversion efficiency can be represented as the product of the energy absorption efficiency and the Carnot efficiency. We describe a simple, semiempirical method to determine the optimum aperture size and optimum operating temperature of a solar cavity-receiver for which its energy conversion efficiency is maximum. Such optimization strongly depends on the incident solar flux distribution at the aperture plane of the receiver. We analytically examine the case of a Gaussian distribution of the incident power flux, and we compare theoretical results with the results obtained when using an optically measured flux distribution. Using Monte-Carlo ray tracing, we further investigate the influence of sunshape on the optimal parameters of a cavity-receiver in a paraboloidal concentrator.

A solar chemical reactor for co-production of zinc and synthesis gas

A novel amine-based adsorbent for CO₂ capture from air was developed, which uses biogenic raw materials and an environmentally benign synthesis route without organic solvents. The adsorbent was synthesized through freeze-drying an aqueous suspension of nanofibrillated cellulose (NFC) and N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (AEAPDMS). At a CO₂ concentration of 506 ppm in air and a relative humidity of 40% at 25 °C, 1.39 mmol CO₂/g was absorbed after 12 h. Stability was examined for over 20 consecutive 2-h-adsorption/1-h-desorption cycles, yielding a cyclic capacity of 0.695 mmol CO₂/g.