Publications

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Sandia National Laboratories (SNL) is evaluating the potential of an innovative approach for splitting water into hydrogen and oxygen using two-step thermochemical cycles. Thermochemical cycles are heat engines that utilize high-temperature heat to produce chemical work. Like their mechanical work-producing counterparts, their efficiency depends on operating temperature and on the irreversibility of their internal processes. With this in mind, we have invented innovative design concepts for two-step solar-driven thermochemical heat engines based on iron oxide and iron oxide mixed with other metal oxides (ferrites). The design concepts utilize two sets of moving beds of ferrite reactant material in close proximity and moving in opposite directions to overcome a major impediment to achieving high efficiency--thermal recuperation between solids in efficient counter-current arrangements. They also provide inherent separation of the product hydrogen and oxygen and are an excellent match with high-concentration solar flux. However, they also impose unique requirements on the ferrite reactants and materials of construction as well as an understanding of the chemical and cycle thermodynamics. In this report the Counter-Rotating-Ring Receiver/Reactor/Recuperator (CR5) solar thermochemical heat engine and its basic operating principals are described. Preliminary thermal efficiency estimates are presented and discussed. Our ferrite reactant material development activities, thermodynamic studies, test results, and prototype hardware development are also presented.

A thermodynamic analysis of the two-step water splitting process for the production of hydrogen is reported in this paper. Calculations simulating the preparation of ferrite samples, their thermal reduction to form a mixture of metal oxides, and subsequent reoxidation with steam to produce hydrogen were performed. Mixed-metal spinel ferrites of the general form MFe2O 4, where M = Co, Ni, or Zn, are compared with iron spinel, Fe 3O4. The results indicate that of the four ferrites examined, nickel spinel has the most favorable combination of properties for use in two-step water splitting. Copyright © 2006 by ASME.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The counter-rotating-ring receiver/reactor/recuperator (CR5) solar thermochemical heat engine is a new concept for production of hydrogen that allows for thermal recuperation between solids in an efficient counter-current arrangement. At the heart of the CR5 system are annular rings of a reactive solid ferrite that are thermally and chemically cycled to produce oxygen and hydrogen from water in separate and isolated steps. This design is very demanding from a materials point of view. The ferrite rings must maintain structural integrity and high reactivity after months of thermal cycling and exposure to temperatures in excess of 1100°C. In addition, the design of the rings must have high geometric surface area for gas-solid contact and for adsorption of incident solar radiation. After performing a series of initial screenings, we chose Co0.67Fe2.33O4 as our baseline working material for a planned demonstration of CR5 and have begun additional characterization and development of this material. Our results to date with powders are consistent with the expectation that small particle sizes and the application of a support to inhibit ferrite sintering and enhance the chemistry are critical considerations for a practical operating device. Concurrent with the powder studies, we are using Robocasting, a Sandia-developed technique for free form processing of ceramics, to manufacture monolithic structures with complex three-dimensional geometries for chemical, physical, and mechanical evaluation. We have demonstrated that ferrite/zirconia mixtures can be fabricated into small three-dimensional monolithic lattice structures that give reproducible hydrogen yields over multiple cycles. Copyright © 2006 by ASME.

In this report, we document the accomplishments in our Laboratory Directed Research and Development project in which we employed a technical approach of combining experiments with computational modeling and analyses to elucidate the performance of hydrogen-fed proton exchange membrane fuel cells (PEMFCs). In the first part of this report, we document our focused efforts on understanding water transport in and removal from a hydrogen-fed PEMFC. Using a transparent cell, we directly visualized the evolution and growth of liquid-water droplets at the gas diffusion layer (GDL)/gas flow channel (GFC) interface. We further carried out a detailed experimental study to observe, via direct visualization, the formation, growth, and instability of water droplets at the GDL/GFC interface using a specially-designed apparatus, which simulates the cathode operation of a PEMFC. We developed a simplified model, based on our experimental observation and data, for predicting the onset of water-droplet instability at the GDL/GFC interface. Using a state-of-the-art neutron imaging instrument available at NIST (National Institute of Standard and Technology), we probed liquid-water distribution inside an operating PEMFC under a variety of operating conditions and investigated effects of evaporation due to local heating by waste heat on water removal. Moreover, we developed computational models for analyzing the effects of micro-porous layer on net water transport across the membrane and GDL anisotropy on the temperature and water distributions in the cathode of a PEMFC. We further developed a two-phase model based on the multiphase mixture formulation for predicting the liquid saturation, pressure drop, and flow maldistribution across the PEMFC cathode channels. In the second part of this report, we document our efforts on modeling the electrochemical performance of PEMFCs. We developed a constitutive model for predicting proton conductivity in polymer electrolyte membranes and compared model prediction with experimental data obtained in our laboratory and from literature. Moreover, we developed a one-dimensional analytical model for predicting electrochemical performance of an idealized PEMFC with small surface over-potentials. Furthermore, we developed a multi-dimensional computer model, which is based on the finite-element method and a fully-coupled implicit solution scheme via Newton's technique, for simulating the performance of PEMFCs. We demonstrated utility of our finite-element model by comparing the computed current density distribution and overall polarization with those measured using a segmented cell. In the last part of this report, we document an exploratory experimental study on MEA (membrane electrode assembly) degradation.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Solar power towers can be used to make hydrogen on a large scale. Electrolyzers could be used to convert solar electricity produced by the power tower to hydrogen, but this process is relatively inefficient. Rather, efficiency can be much improved if solar heat is directly converted to hydrogen via a thermochemical process. In the research summarized here, the marriage of a high-temperature ({approx}1000 C) power tower with a sulfuric acid/hybrid thermochemical cycle was studied. The concept combines a solar power tower, a solid-particle receiver, a particle thermal energy storage system, and a hybrid-sulfuric-acid cycle. The cycle is 'hybrid' because it produces hydrogen with a combination of thermal input and an electrolyzer. This solar thermochemical plant is predicted to produce hydrogen at a much lower cost than a solar-electrolyzer plant of similar size. To date, only small lab-scale tests have been conducted to demonstrate the feasibility of a few of the subsystems and a key immediate issue is demonstration of flow stability within the solid-particle receiver. The paper describes the systems analysis that led to the favorable economic conclusions and discusses the future development path.