Thermochemical production of ammonia a two-step metal nitride cycle - materials screening and the strontium-based system.

Ammonia synthesis the catalytic Haber-Bosch process is characterized by its high pressures and low single-pass conversions, as well as by the energy-intensive production of the precursors H and N and their concomitant greenhouse gas emissions. Alternatively, thermochemical cycles based on metal nitrides stand as a promising pathway to green ammonia production because they can be conducted at moderate pressures without added catalysts and be further driven by concentrated solar energy as the source of high-temperature process heat. The ideal two-step cycle consists of the nitridation of a metal to form a metal nitride, followed by the hydrogenation of the metal nitride to synthetize NH and reform the metal.

Technical, economic and environmental analysis of solar thermochemical production of drop-in fuels.

This study analyzes the technical performance, costs and life-cycle greenhouse gas (GHG) emissions of the production of various fuels using air-captured water and CO, and concentrated solar energy as the source of high-temperature process heat. The solar thermochemical fuel production pathway utilizes a ceria-based redox cycle for splitting water and CO to syngas - a tailored mixture of H and CO - which in turn is further converted to liquid hydrocarbon fuels. The cycle is driven by concentrated solar heat and supplemented by a high-temperature thermal energy storage for round-the-clock continuous operation.

A solar tower fuel plant for the thermochemical production of kerosene from HO and CO.

Developing solar technologies for producing carbon-neutral aviation fuels has become a global energy challenge, but their readiness level has largely been limited to laboratory-scale studies. Here, we report on the experimental demonstration of a fully integrated thermochemical production chain from HO and CO to kerosene using concentrated solar energy in a solar tower configuration. The co-splitting of HO and CO was performed via a ceria-based thermochemical redox cycle to produce a tailored mixture of H and CO (syngas) with full selectivity, which was further processed to kerosene.

Drop-in fuels from sunlight and air.

Aviation and shipping currently contribute approximately 8% of total anthropogenic CO emissions, with growth in tourism and global trade projected to increase this contribution further. Carbon-neutral transportation is feasible with electric motors powered by rechargeable batteries, but is challenging, if not impossible, for long-haul commercial travel, particularly air travel. A promising solution are drop-in fuels (synthetic alternatives for petroleum-derived liquid hydrocarbon fuels such as kerosene, gasoline or diesel) made from HO and CO by solar-driven processes.

Design principles of perovskites for solar-driven thermochemical splitting of CO.

Perovskites are attractive redox materials for thermo/electrochemical fuel synthesis. To design perovskites with balanced redox energetics for thermochemically splitting CO, the activity of lattice oxygen vacancies and stability against crystal phase changes and detrimental carbonate formation are predicted for a representative range of perovskites by electronic structure computations. Systematic trends in these materials properties when doping with selected metal cations are described in the free energy range defined for isothermal and temperature-swing redox cycles.

Solar-Driven Thermochemical Splitting of CO and Separation of CO and O across a Ceria Redox Membrane Reactor.

Splitting CO with a thermochemical redox cycle utilizes the entire solar spectrum and provides a favorable path to the synthesis of solar fuels at high rates and efficiencies. However, the temperature/pressure swing commonly applied between reduction and oxidation steps incurs irreversible energy losses and severe material stresses. Here, we experimentally demonstrate for the first time the single-step continuous splitting of CO into separate streams of CO and O under steady-state isothermal/isobaric conditions.

Combined Ceria Reduction and Methane Reforming in a Solar-Driven Particle-Transport Reactor.

We report on the experimental performance of a solar aerosol reactor for carrying out the combined thermochemical reduction of CeO and reforming of CH using concentrated radiation as the source of process heat. The 2 kW solar reactor prototype utilizes a cavity receiver enclosing a vertical AlO tube which contains a downward gravity-driven particle flow of ceria particles, either co-current or counter-current to a CH flow. Experimentation under a peak radiative flux of 2264 suns yielded methane conversions up to 89% at 1300 °C for residence times under 1 s.

Nonimaging polygonal mirrors achieving uniform irradiance distributions on concentrating photovoltaic cells.

We introduce a design methodology for nonimaging, single-reflection mirrors with polygonal inlet apertures that generate a uniform irradiance distribution on a polygonal outlet aperture, enabling a multitude of applications within the domain of concentrated photovoltaics. Notably, we present single-mirror concentrators of square and hexagonal perimeter that achieve very high irradiance uniformity on a square receiver at concentrations ranging from 100 to 1000 suns. These optical designs can be assembled in compound concentrators with maximized active area fraction by leveraging tessellation.

Experimental Demonstration of the Thermochemical Reduction of Ceria in a Solar Aerosol Reactor.

We report on the experimental demonstration of an aerosol solar reactor for the thermal reduction of ceria, as part of a thermochemical redox cycle for splitting HO and CO. The concept utilizes a cavity-receiver enclosing an array of alumina tubes, each containing a downward gravity-driven aerosol flow of ceria particles countercurrent to an inert sweep gas flow for intrinsic separation of reduced ceria and oxygen. A 2 kW lab-scale prototype with a single tube was tested under radiative fluxes approaching 4000 suns, yielding reaction extents of up to 53% of the thermodynamic equilibrium at 1919 K within residence times below 1 s.

Spectral data of specular reflectance, narrow-angle transmittance and angle-resolved surface scattering of materials for solar concentrators.

The spectral specular reflectance of conventional and novel reflective materials for solar concentrators is measured with an acceptance angle of 17.5 mrad over the wavelength range 300-2500 nm at incidence angles 15-60° using a spectroscopic goniometry system. The same experimental setup is used to determine the spectral narrow-angle transmittance of semi-transparent materials for solar collector covers at incidence angles 0-60°.

Design Principles for Metal Oxide Redox Materials for Solar-Driven Isothermal Fuel Production.

The performance of metal oxides as redox materials is limited by their oxygen conductivity and thermochemical stability. Predicting these properties from the electronic structure can support the screening of advanced metal oxides and accelerate their development for clean energy applications. Specifically, reducible metal oxide catalysts and potential redox materials for the solar-thermochemical splitting of CO and HO via an isothermal redox cycle are examined.

Experimental and Numerical Investigation of Combined Sensible/Latent Thermal Energy Storage for High-Temperature Applications.

Combined sensible/latent heat storage allows the heat-transfer fluid outflow temperature during discharging to be stabilized. A lab-scale combined storage consisting of a packed bed of rocks and steel-encapsulated AlSi(12) was investigated experimentally and numerically. Due to the small tank-to-particle diameter ratio of the lab-scale storage, void-fraction variations were not negligible, leading to channeling effects that cannot be resolved in 1D heat-transfer models.

Two-stage solar concentrators based on parabolic troughs: asymmetric versus symmetric designs.

While nonimaging concentrators can approach the thermodynamic limit of concentration, they generally suffer from poor compactness when designed for small acceptance angles, e.g., to capture direct solar irradiation.

Kinetics of CO Reduction over Nonstoichiometric Ceria.

The kinetics of CO reduction over nonstoichimetric ceria, CeO, a material of high potential for thermochemical conversion of sunlight to fuel, has been investigated for a wide range of nonstoichiometries (0.02 ≤ δ ≤ 0.25), temperatures (693 ≤ ≤ 1273 K), and CO concentrations (0.

Mechanism of Zn Particle Oxidation by HO and CO in the Presence of ZnO.

In this work we investigate the mechanism of Zn oxidation with CO and/or HO to produce solar derived fuels (CO and/or H) as part of the Zn/ZnO thermochemical redox cycle. It has been observed that the ZnO contamination of Zn produced by solar thermal reduction of ZnO (solar Zn) facilitates oxidation of the metallic Zn by CO and HO, allowing for nearly complete conversion at temperatures as low as 350 °C. Reaching the same reaction extent starting with pure Zn requires considerably higher temperatures which imposes use of unconventional hard-to-operate reaction configurations utilizing Zn as vapor.

Carbon Dioxide Reforming of Methane using an Isothermal Redox Membrane Reactor.

The continuous production of carbon monoxide (CO) and hydrogen (H) by dry reforming of methane (CH) is demonstrated isothermally using a ceramic redox membrane in absence of additional catalysts. The reactor technology realizes the continuous splitting of CO to CO on the inner side of a tubular membrane and the partial oxidation of CH with the lattice oxygen to form syngas on the outer side. LaSrCoFeO (LSCF) membranes evaluated at 840-1030 °C yielded up to 1.

Rational design of metal nitride redox materials for solar-driven ammonia synthesis.

Fixed nitrogen is an essential chemical building block for plant and animal protein, which makes ammonia (NH3) a central component of synthetic fertilizer for the global production of food and biofuels. A global project on artificial photosynthesis may foster the development of production technologies for renewable NH3 fertilizer, hydrogen carrier and combustion fuel. This article presents an alternative path for the production of NH3 from nitrogen, water and solar energy.

Design Principles of Perovskites for Thermochemical Oxygen Separation.

Separation and concentration of O2 from gas mixtures is central to several sustainable energy technologies, such as solar-driven synthesis of liquid hydrocarbon fuels from CO2 , H2 O, and concentrated sunlight. We introduce a rationale for designing metal oxide redox materials for oxygen separation through "thermochemical pumping" of O2 against a pO2 gradient with low-grade process heat. Electronic structure calculations show that the activity of O vacancies in metal oxides pinpoints the ideal oxygen exchange capacity of perovskites.

Fast and reversible direct CO2 capture from air onto all-polymer nanofibrillated cellulose-polyethylenimine foams.

Fully polymeric and biobased CO2 sorbents composed of oxidized nanofibrillated cellulose (NFC) and a high molar mass polyethylenimine (PEI) have been prepared via a freeze-drying process. This resulted in NFC/PEI foams displaying a sheet structure with porosity above 97% and specific surface area in the range 2.7-8.

Morphological Characterization and Effective Thermal Conductivity of Dual-Scale Reticulated Porous Structures.

Reticulated porous ceramic (RPC) made of ceria are promising structures used in solar thermochemical redox cycles for splitting CO₂ and H₂O. They feature dual-scale porosity with mm-size pores for effective radiative heat transfer during reduction and µm-size pores within its struts for enhanced kinetics during oxidation. In this work, the detailed 3D digital representation of the complex dual-scale RPC is obtained using synchrotron submicrometer tomography and X-ray microtomography.

Nonparabolic solar concentrators matching the parabola.

We consider the limit of geometric concentration for a focusing concave mirror, e.g., a parabolic trough or dish, designed to collect all radiation within a finite acceptance angle and direct it to a receiver with a flat or circular cross-section.

Thermochemical CO2 splitting via redox cycling of ceria reticulated foam structures with dual-scale porosities.

Efficient heat transfer of concentrated solar energy and rapid chemical kinetics are desired characteristics of solar thermochemical redox cycles for splitting CO2. We have fabricated reticulated porous ceramic (foam-type) structures made of ceria with dual-scale porosity in the millimeter and micrometer ranges. The larger void size range, with dmean = 2.

Theory and design of line-to-point focus solar concentrators with tracking secondary optics.

The two-stage line-to-point focus solar concentrator with tracking secondary optics is introduced. Its design aims to reduce the cost per m(2) of collecting aperture by maintaining a one-axis tracking trough as the primary concentrator, while allowing the thermodynamic limit of concentration in 2D of 215× to be significantly surpassed by the implementation of a tracking secondary stage. The limits of overall geometric concentration are found to exceed 4000× when hollow secondary concentrators are used, and 6000× when the receiver is immersed in a dielectric material of refractive index n=1.