Materials and Processes for Renewable Energy Technologies

ISSN:1047-4838ISSN:0148-6608ISSN:1543-185

Performance indicators for benchmarking solar thermochemical fuel processes and reactors

ABSTRACT: Concentrated solar energy offers a source for renewable high-temperature process heat that can be used to efficiently drive endothermic chemical processes, converting the entire spectrum of solar radiation into chemical energy. In particular, solar-driven thermochemical processes for the production of fuels include reforming of methane and other hydrocarbons, gasification of biomass, coal, and other carbonaceous feedstock, and metal oxide redox cycles for splitting H2O and CO2. A notable issue in the development of these processes and their associated solar reactors is the lack of consistent reporting methods for experimental demonstrations and modelling studies, which complicates the benchmarking of the corresponding technologies. In this work we formulate dimensionless performance indicators based on mass and energy balances of such reacting systems, namely: energy efficiency, conversion extent, selectivity, and yield. Examples are outlined for the generic processes mention above. We then provide guidelines for reporting on such processes and reactors and suggest performance benchmarking on four key criteria: energy efficiency, conversion extent, product selectivity, and performance stability.info:eu-repo/semantics/publishedVersio

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. The process is based on a thermochemical redox cycle driven by concentrated solar process heat at 700-1200°C that yields NH3 via the oxidation of a metal nitride with water. The metal nitride is recycled via solar-driven reduction of the oxidized redox material with nitrogen at atmospheric pressure. We employ electronic structure theory for the rational high-throughput design of novel metal nitride redox materials and to show how transition-metal doping controls the formation and consumption of nitrogen vacancies in metal nitrides. We confirm experimentally that iron doping of manganese nitride increases the concentration of nitrogen vacancies compared to no doping. The experiments are rationalized through the average energy of the dopant d-states, a descriptor for the theory-based design of advanced metal nitride redox materials to produce sustainable solar thermochemical ammonia

ISEC2004-65035 MONTE CARLO RADIATIVE TRANSFER MODELING OF A SOLAR CHEMICAL REACTOR FOR THE CO-PRODUCTION OF ZINC AND SYNGAS

ABSTRACT Radiation heat transfer within a solar chemical reactor for the co-production of zinc and syngas is analyzed by the Monte Carlo ray-tracing method. The reactor is treated as a 3D nonisothermal cavity-receiver lined with ZnO particles that are directly exposed to concentrated solar irradiation and undergo endothermic reduction by CH 4 at above 1300 K. The analysis includes coupling to conduction/convection heat transfer and chemical kinetics. Calculation of the apparent absorptivity indicates the cavity's approach to a blackbody absorber, for either diffuse or specular reflecting inner walls. Numerically calculated temperature distributions, zinc production rates, and thermal efficiencies are validated with experimental measurements in a solar furnace with a 5-kW prototype reactor. At 1600 K, the zinc production rate reached 0.12 mol/min and the reactor's thermal efficiency exceeded 16 %. Scaling up the reactor to power levels of up to 1 MW while keeping constant the relative geometrical dimensions and the solar power flux at 2000 suns results in thermal efficiencies of up to 54 %. NOMENCLATUR

Concentrated solar energy for thermochemically producing liquid fuels from CO2 and H2O

A two-step solar thermochemical cycle for producing syngas from H2O and CO2 via Zn/ZnO redox reactions is considered. The first, endothermic step is the thermolysis of ZnO to Zn and O2 using concentrated solar radiation as the source of process heat. The second, non-solar, exothermic step is the reaction of Zn with mixtures of H2O and CO2 yielding high-quality syngas (mainly H2 and CO) and ZnO; the latter is recycled to the first solar step, resulting in net reactions CO2 = CO+0.5O2 and H2O= H2 +0.5O2. Syngas is further processed to liquid fuels via Fischer-Tropsch or other catalytic reforming processes. State-of-the-art reactor technologies and experimental results are provided for both steps of the cycl

Thermoelectric oxide modules tested in a solar cavity-receiver

Four-leg thermoelectric oxide modules (TOMs) consisting of two p-type (La1.98Sr0.02CuO4) and two n-type (CaMn0.98Nb0.02O3) thermoelectric (TE) legs were produced with a manufacturing quality factor between 30 and 60%. The pressed sintered TE legs revealed 90% of the theoretical density to ensure a sufficient mechanical stability of the TE modules. The legs were connected electrically in series and sandwiched thermally in parallel between two Al2O3 plates serving as absorber and cooler, respectively. A solar cavity-receiver packed with an array of TOMs was subjected to concentrated thermal radiation with peak solar radiative flux intensities exceeding 600 kW/m2. Temperature distributions in the cavity, open-circuit voltage (VOC), and maximum output power (Pmax) were measured for different external loads and solar radiative fluxes (qin). Finally, the solar-to-electricity conversion efficiency (η) was calculate

Pore-level engineering of macroporous media for increased performance of solar-driven thermochemical fuel processing

The performance of high-temperature solar reactors incorporating porous ceramic materials that serve as radiative absorbers and chemical reaction sites can be improved significantly by tailoring their pore structure. We investigated the changes in their effective heat and mass transport properties with increasing mass loading of porous ceramics fabricated by the replica method. We applied a methodology consisting of the experimental characterization of the structure via 3D tomographic techniques coupled to pore-level direct numerical simulations for the determination of the effective transport properties. This approach was extended by using digital image processing on the structure data to allow for artificial changes in the morphological characteristics – corresponding to actual variations in the fabrication process. We derived transport correlations of porous ceria foam with varying mass loading, i.e. reticulate to dense foams with porosity from 0.85 to 0.45. We observed that the correlations proposed in literature do not accurately describe the behavior of low-porosity foams. The numerical findings of this study provide guidance for pore-level engineering of materials used in solar reactors and other high-temperature heat and mass transfer applications