The First Step in Solar Hydrogen Production: Development of a Solar Thermal Reactor for the Reduction of Metal Oxide Particles

A solar thermal reactor has been designed to experimentally investigate promising paths for reducing metal oxide particles to reduced oxidation states (e.g. Fe2O3 to Fe3O4) utilizing concentrated solar energy. This reactor is windowless and is able to handle internal cavity temperatures in excess of 1700 K. It also has a quasi-continuous feed system that allows the particle residence times to be varied for particles between 0.044 mm and 1 mm in diameter. Furthermore, this reactor utilizes an instrumentation system for the measurement of temperature, particle residence time, particle mass flow rate, and solar flux. In an industrial setting, a large-scale metal oxide reactor would serve as the first step in a metal oxide solar thermal electro-chemical cycle. After the particles are reduced at elevated temperatures using concentrated solar energy, they are used in an electrolysis process to facilitate the production of hydrogen from water. In this process, the reduced metal oxide particles are reoxidized at the anode and hydrogen is liberated at the cathode. The presence of the metal oxide enables hydrogen to be produced with an ideal cell potential of 0.21 V, a potential substantially below the ideal value of 1.2 V for traditional water electrolysis

Solar Thermal Reactor Materials Characterization

Current research into hydrogen production through high temperature metal oxide water splitting cycles has created a need for robust high temperature materials. Such cycles are further enhanced by the use of concentrated solar energy as a power source. However, samples subjected to concentrated solar radiation exhibited lifetimes much shorter than expected. Characterization of the power and flux distributions representative of the High Flux Solar Furnace(HFSF) at the National Renewable Energy Laboratory(NREL) were compared to ray trace modeling of the facility. In addition, samples of candidate reactor materials were thermally cycled at the HFSF and tensile failure testing was performed to quantify material degradation. Thermal cycling tests have been completed on super alloy Haynes 214 samples and results indicate that maximum temperature plays a significant role in reduction of strength. The number of cycles was too small to establish long term failure trends for this material due to the high ductility of the material

The First Step in Solar Hydrogen Production: Development of a Solar Thermal Reactor for the Reduction of Metal Oxide Particles

A solar thermal reactor has been designed to experimentally investigate promising paths for reducing metal oxide particles to reduced oxidation states (e.g. Fe2O3 to Fe3O4) utilizing concentrated solar energy. This reactor is windowless and is able to handle internal cavity temperatures in excess of 1700 K. It also has a quasi-continuous feed system that allows the particle residence times to be varied for particles between 0.044 mm and 1 mm in diameter. Furthermore, this reactor utilizes an instrumentation system for the measurement of temperature, particle residence time, particle mass flow rate, and solar flux. In an industrial setting, a large-scale metal oxide reactor would serve as the first step in a metal oxide solar thermal electro-chemical cycle. After the particles are reduced at elevated temperatures using concentrated solar energy, they are used in an electrolysis process to facilitate the production of hydrogen from water. In this process, the reduced metal oxide particles are reoxidized at the anode and hydrogen is liberated at the cathode. The presence of the metal oxide enables hydrogen to be produced with an ideal cell potential of 0.21 V, a potential substantially below the ideal value of 1.2 V for traditional water electrolysis

Modeling and performance analysis of biomass fast pyrolysis in a solar-thermal reactor

Solar-thermal conversion of biomass through pyrolysis process is an alternative option to store energy in the form of liquid fuel, gas and bio-char. Fast pyrolysis is a highly endothermic process and essentially requires high heating rate and temperature >400 °C. This study presents a theoretical study on biomass fast pyrolysis in a solar-thermal reactor heated by a parabolic trough concentrator. The reactor is part of a novel closed loop pyrolysis-gasification process. A Eulerian-Eulerian flow model, with constitutive closure equation derived from the kinetic theory of granular flow and incorporating heat transfer, drying and pyrolysis reaction equations, was solved using ANSYS Fluent computational fluid dynamics (CFD) software. The highly endothermic pyrolysis was assumed to be satisfied by a constant solar heat flux concentrated on the reactor external wall. At the operating conditions considered, the reactor overall energy efficiency was found equal to 67.8% with the product consisting of 51.5% bio-oil, 43.7% char and 4.8% non-condensable gases. Performance analysis is presented to show the competitiveness of the proposed reactor in terms of thermal conversion efficiency and environmental impact. It is hoped that this study will contribute to the global effort on securing diverse and sustainable energy generation technologies

Model Predictive Control of a Solar-Thermal Reactor

Solar-thermal reactors represent a promising alternative to fossil fuels because they can harvest solar energy and transform it into storable and transportable fuels. The operation of solar-thermal reactors is restricted by the available sunlight and its inherently transient behavior, which affects the performance of the reactors and limits their efficiency. Before solar-thermal reactors can become commercially viable, they need to be able to maintain a continuous high-performance operation, even in the presence of passing clouds. A well-designed control system can preserve product quality and maintain stable product compositions, resulting in a more efficient and cost-effective operation, which can ultimately lead to scale-up and commercialization of solar thermochemical technologies. In this work, we propose a model predictive control (MPC) system for a solar-thermal reactor for the steam-gasification of biomass. The proposed controller aims at rejecting the disturbances in solar irradiation caused by the presence of clouds. A first-principles dynamic model of the process was developed. The model was used to study the dynamic responses of the process variables and to identify a linear time-invariant model used in the MPC algorithm. To provide an estimation of the disturbances for the control algorithm, a one-minute-ahead direct normal irradiance (DNI) predictor was developed. The proposed predictor utilizes information obtained through the analysis of sky images, in combination with current atmospheric measurements, to produce the DNI forecast. In the end, a robust controller was designed capable of rejecting disturbances within the operating region. Extensive simulation experiments showed that the controller outperforms a finely-tuned multi-loop feedback control strategy. The results obtained suggest that our controller is suitable for practical implementation

Materials for Solar Thermal Chemical Reactor Applications

An extremely important component of concentrated solar energy development is designing robust reactors. ... Many different reactor designs have been proposed in recent literature. Transferring the solar radiation to process reactants can often be difficult. For this study a reactor design was created that uses multiple absorbing tubes. The absorber tubes are housed within a cavity that reflects spilled or emitted radiation to increase solar utilization efficiency. The reactor has been designed to fit the High Flux Solar Furnace at the National Renewable Energy Laboratory. Solar concentrating capabilities of the HFSF facility have been modeled using the ray-trace program SOLTRACE. Absorber tube positions were optimized to intercept a large fraction of the incident radiation. An attempt was also made to more evenly distribute the flux across multiple tubes. The outer cavity was fabricated out of polished, reflective, aluminum to reduce the systems thermal mass and shorten heating and cooling times during testing. The HFSF facility was qualified through black body calorimetery and flux versus inlet attenuation was mapped. Reactor performance was validated by installing absorber tubes and measuring temperature distributions. Maximum temperature differences between the central and surrounding alumina tubes were less than 350 K, at ~1673 K central tube maximum. This temperature difference decreased with higher absorptivity tube materials. Radial distance from the central tube seemed to have the largest effect on the maximum tube temperature as the tubes were farther from the focal point of the incident radiation. Cavity wall temperatures were kept below 50 K, at maximum absorber temperatures, which indicate excellent heat reflection and dissipation. ... Once thermal stresses were quantified an effort was made to find high temperature materials that are better suited to withstand thermal shock. Graphite is a thermal shock resistant high temperature material that would be well suited for solar thermal applications. However, graphite oxidizes above 773 K. By coating graphite with an oxygen barrier material, thermal shock resistant composites could help in a wide range of applications. Graphite powder was coated with alumina via atomic layer deposition (ALD). The powders had a marginal increase in oxidation resistance but coated powders showed a marked improvement in dispersability. Sedimentation and isoelectric tests showed a change in particle-particle interactions which was also validated by decreased particle size distributions of coated particles. Alumina-graphite composites showed enhanced thermal properties such as thermal diffusivity, thermal conductivity, and thermal expansion when compared to uncoated composites. This research provides a method to enhance bulk material properties of composites specifically using hard to disperse additives such as graphite and potentially carbon nanotubes. The work in this thesis represents a broad investigation into the feasibility of using concentrated solar thermal technology for renewable fuels production. Research pertaining to materials robustness, reactor design, and fuel production cycles has advanced the state of the art. Rudimentary experimentation with this technology was conducted decades ago. New research is needed to deal with the engineering challenges that are hindering large scale adaptation of concentrated solar energy today

CFD and experimental studies on a circulating fluidised bed reactor for biomass gasification

Biomass gasification has been extensively studied in different thermochemical systems, as has the potential to produce fuel gas for chemicals, fuel and electricity applications. Circulating fluidised bed systems (CFB) are of particular interest due to the high reaction rates and thermal efficiency. The study of varying particle properties and gas velocities during the solids recirculation in a CFB system has been proved to greatly influence the overall biomass gasification process. A comparison between experimental and modelling gas-solid interactions can represent a comprehensive and analytical approach for further understanding and scaling up this reaction system. However, running several experiments is expensive and time-consuming. In this work, a reliable and accurate computational fluid dynamics (CFD) framework has been developed to evaluate the hydrodynamics performance of a CFB gasifier. The multiphase CFD model was validated using a pilot-scale CFB gasifier and silica sand. The CFD and experimental data showed good agreement for the solid recirculation tests, for example when comparing predicted and measured the spatial distribution of pressure up the gasifier’s riser. It is the first time that the spatial distribution of solids around a CFB system has been numerically predicted, which can provide guidance to evaluate the hydrodynamics performance of CFB

A hydrodynamic model for biomass gasification in a circulating fluidized bed riser

This study presents a three-dimensional Computational Fluid Dynamic (CFD) model and experimental measurements of the hydrodynamics in the riser section of a Circulating Fluidized Bed (CFB) biomass gasifier consisting of a binary mixture of polydisperse particles. The model is based on multi-fluid (Eulerian-Eulerian) approach with constitutive equations adopted from the Kinetic Theory of Granular Flow (KTGF). The study first presents an assessment of the various options of the constitutive and closure equations for a binary mixture followed by sensitivity analysis of the model to the solution time step, cell size, turbulence and the alternative formulations of the granular energy equation. Accordingly, a robust and reliable hydrodynamic model is recommended and validated using conventional pressure measurements and Positron Emission Particle Tracking (PEPT) technique. Furthermore, the model predictions and experiments revealed evidence of the particle re-circulation within the lower part of the riser, which is an important feature contributing to rapid mass and heat transfer in a CFB gasifier. The present hydrodynamic model can be further developed; by incorporating appropriate reactions and heat transfer equations, in order to fully predict the performance and products of a CFB biomass gasifier