Recent interest in fuel cell-gas turbine hybrid applications for the aerospace industry has led to the need for accurate computer simulation models to aid in system design and performance evaluation. To meet his requirement, solid oxide fuel cell (SOFC) and fuel processor models have been developed and incorporated into the Numerical Propulsion Systems Simulation (NPSS) software package. The SOFC and reformer models solve systems of equations governing steady-state performance using common theoretical and semi-empirical terms. An example hybrid configuration is presented that demonstrates the new capability as well as the interaction with pre-existing gas turbine and heat exchanger models. Finally, a comparison of calculated SOFC performance with experimental data is presented to demonstrate model validity.
We review liquid hydrogen (LH2) as a maritime vessel fuel, from descriptions of its fundamental properties to its practical application and safety aspects, in the context of the San Francisco Bay Renewable Energy Electric Vessel with Zero Emissions (SF-BREEZE) high-speed ferry. Since marine regulations have been formulated to cover liquid natural gas (LNG) as a primary propulsion fuel, we frame our examination of LH2 as a comparison to LNG, for both maritime use in general, and the SF-BREEZE in particular. Due to weaker attractions between molecules, LH2 is colder than LNG, and evaporates more easily. We describe the consequences of these physical differences for the size and duration of spills of the two cryogenic fuels. The classical flammability ranges are reviewed, with a focus on how fuel buoyancy modifies these combustion limits. We examine the conditions for direct fuel explosion (detonation) and contrast them with initiation of normal (laminar) combustion. Direct fuel detonation is not a credible accident scenario for the SF-BREEZE. For both fuels, we review experiments and theory elucidating the deflagration to detonation transition (DDT). LH2 fires have a shorter duration than energyequivalent LNG fires, and produce significantly less thermal radiation. The thermal (infrared) radiation from hydrogen fires is also strongly absorbed by humidity in the air. Hydrogen permeability is not a leak issue for practical hydrogen plumbing. We describe the chemistry of hydrogen and methane at iron surfaces, clarifying their impact on steel-based hydrogen storage and transport materials. These physical, chemical and combustion properties are pulled together in a comparison of how a LH2 or LNG pool fire on the Top Deck of the SF-BREEZE might influence the structural integrity of the aluminum deck. Neither pool fire scenario leads to net heating of the aluminum decking. Overall, LH2 and LNG are very similar in their physical and combustion properties, thereby posing similar safety risks. For ships utilizing LH2 or LNG, precautions are needed to avoid fuel leaks, minimize ignition sources, minimize confined spaces, 1
Deployed on a commercial airplane, proton exchange membrane fuel cells may offer emissions reductions, thermal efficiency gains, and enable locating the power near the point of use. This work seeks to understand whether on-board fuel cell systems are technically feasible, and, if so, if they offer a performance advantage for the airplane as a whole.Through hardware analysis and thermodynamic and electrical simulation, we found that while adding a fuel cell system using today's technology for the PEM fuel cell and hydrogen storage is technically feasible, it will not likely give the airplane a performance benefit. However, when we re-did the analysis using DOE-target technology for the PEM fuel cell and hydrogen storage, we found that the fuel cell system would provide a performance benefit to the airplane (i.e., it can save the airplane some fuel), depending on the way it is configured. 3 AcknowledgementsThe authors of this study had a great deal of enthusiastic support from both within and outside of Sandia National Laboratories.Dr. Joe Breit of The Boeing Company directly or indirectly provided most of the information about airplane electrical systems and issues in current airplane design that might be able to leverage the capabilities of a fuel cell. In the rare cases he could not answer our frequent questions, he referred us to others at Boeing who were just as happy to help: Andy Bayliss for issues concerning airplane performance, Trevor Laib for information on the existing environmental control systems, and Farhad Nozari and Casey Roberts for more electrical system details.Ryan Sookhoo of Hydrogenics was gracious in his support of our project, offering details on their PEM fuel cell technology to assist us in making reasonable estimates for the current state of the art as well as predictions of what may be possible for future aviation-designed fuel cell systems. His insight into the European world of aviation fuel cells was also useful to help put this study into perspective.Dr. Andy Lutz of Sandia (now at the University of Pacific) provided his extensive knowledge on the Simulink thermodynamic models that were used in this study.Of course, this project would not have taken place without the support of the Department of Energy's Fuel Cell Technologies Program, Pete Devlin and Nancy Garland in the Market Transportation group in particular. The encouragement and suggestions we received from them was invaluable. 4 SummaryFuel cells have become increasingly important as alternative sources of power, offering the potential for drastic reduction in emissions in particulate matter (PM), nitrogen oxides (NO x ), and CO 2 . In addition, they offer exceptionally quiet operation, highly efficient use of the fuel energy, and a high energy storage density compared to batteries. For a number of years, the manufacturers of commercial aircraft, most notably Boeing and Airbus, have realized that fuel cells may offer advantages for commercial aircraft operation. Apart from emissions reductions and thermal efficiency...
The effects of oxygen concentration and ambient pressure on fuel cell performance are explored both in theory and in experiment. For fuel cells in general the effect due to a change in oxygen concentration is shown to be fundamentally different than the effect due to a change in cathode pressure, even if partial pressure is held constant. For a proton exchange membrane fuel cell, a significant reason for this difference comes from the nature of mass diffusion processes in the fuel cell structure, which infers that there is an optimum fuel cell design (macroscale and microscale) for a given operating pressure and oxygen concentration. In the experimental work a proton exchange membrane fuel cell was subjected to varying atmospheric conditions from sea level to 53,500 ft (16,307 m) with results analyzed up to 35,000 ft (10,668 m). The results showed that at low current density operation a decrease in either cathode pressure or concentration led to an increase in irreversible losses associated with reaction kinetics (activation polarization) and confirmed the differing effects of cathode pressure and oxygen concentration. Consideration of all these effects enables both fuel cell-and system-level optimization of aeronautical fuel cell-based power systems.
Fischer-Tropsch synthesis (FTS) is an attractive option in the process of converting biomass-derived syngas to liquid fuels in a small-scale mobile bio-refinery. Computer simulation can be an efficient method of designing a compact FTS reactor, but no known comprehensive model exists that is able to predict performance of the needed non-traditional designs.This work developed a generalized model framework that can be used to examine a variety of FTS reactor configurations. It is based on the four fundamental physics areas that underlie FTS reactors: momentum transport, mass transport, energy transport, and chemical kinetics, rather than empirical data of traditional reactors.The mathematics developed were applied to an example application and solved numerically using COMSOL. The results compare well to the literature and give insights into the operation of FTS that are then used to propose a new reactor concept that may be suitable for the mobile bio-refinery application. 4 AcknowledgementsI am deeply grateful to the Sandia LDRD office and the ECIS investment area for sponsoring this project.I would also like to thank the following people who participated in this project:• Chris Shaddix for his mentorship, both technically and personally, throughout the project.• Hank Westrich and Sheri Martinez at the LDRD Office for their assistance and encouragement.• Daniel Dedrick for his work in forming the concept of this LDRD topic.• Blake Simmons for providing project management so I could focus on the technical challenges.• Deanna Agosta for financial planning.• Tiffany Vargas and Karen McWilliams at the CA Technical Library for providing and/or helping me find the countless papers and texts used to develop my understanding of everything from the detailed theory to the history and applications of FTS. 5 SummaryLiquid fuels synthesized from renewable CO and H 2 could displace petroleum-based fuels to provide clean, renewable energy for the future. Small scale (<20 bbl/day) reactors for synthetic liquid fuels production are an emerging development area that may enable mobile biomass-to-liquids plants suited for on-demand liquid fuel production from diverse, underutilized, local resources. The design of such a "mobile bio-refinery" is a significant departure from the traditional large-scale industrial designs and requires predictive tools such as computer models to efficiently produce a successful facility. Because existing models inherently incorporate the empirical results of the traditional designs they are not suitable for this application. The need for a model that can examine the drastic design changes and innovations needed for the mobile bio-refinery is clear.The goal of this work was to create a gas-to-liquids synthesis model with the minimum amount of empiricism and assumptions allowed by the current state of the theory, through integrated physics component models across varying length scales, with the ultimate purpose being to enable design of efficient, durable, and flexible small scale and/or novel re...
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