Restuccia, Giulio
(2008)
Theoretical and experimental analysis of high temperature solid oxide fuel cells (SOFC).
[Tesi di dottorato]
(Inedito)
Abstract
The technology of solid oxide fuel cells (SOFC) at high temperature represents a particularly promising solution in the realization of innovative systems for energetic conversion at high efficiency, in particular for systems destined to the distributed energetic production on small-medium scale.
In such ambit has been carried out a theoretical analysis for the development of a one-dimensional simulation model of a solid oxide fuel cell stack, capable to predict the course and the variation of the main operational parameters of such systems and characterizing the main sources of irreversibility.
At the theoretical analysis has placed side by side an experimentation program on solid oxide fuel cells based on two main topic of search. First of all has been analyzed the global behavior of a cogenerative module of 5 kWe, manufactured by Acumentrics Corporation (Boston, MA - U.S.A.), with the aim to determine the total energetic performances in cogenerative arrangement for domestic applications. Successively the attention has been concentrated on the energetic performances of single tubular SOFC, through a campaign of tests realized on fuel cell of different dimension and manufactured with diverse materials; the activity has been carried out at the Colorado School of Mines in collaboration with the Prof. Nigel Sammes (Editor in Chief of Journal of Fuel Cells Science and Technology).
For the simulation activities, has been built a detailed axial symmetric model at finite volumes applied to a single tubular Solid Oxide Fuel Cell (SOFC) with internal reforming. The model, with relative 1-D discretization, was later extended to other components present within a fuel cell stack: pre-reformer and catalytic combustor placed on the top of the fuel cells bundle.
In developing the simulation model were applied detailed models related to the kinetics of shift, steam methane reforming and post-combustion reactions, the pressure drop, the heat transfer mechanisms, and overvoltage occurring within the fuel cells. The model is capable of analyzing the profiles of temperature, pressure, chemical composition and power within the single fuel cell as inside other components of the stack. This model was developed through the finite volume technique: the control volume was divided into elementary slice, and for each domain were applied mass, energy and moles balances. By implementing the exergetic analysis techniques were unable to identify the source and location of the irreversibility of the various components of the stack.
Unlike the existing paper in literature the model for the heat transfer was greatly improved by introducing the radiative heat exchange mechanism. The results show the great influence that the radiation has for these types of cells (high temperature).
Temperature, pressure, chemical composition, electrical parameters and exergy destroyed were assessed for each discretization domain of the single tubular SOFC and for pre-reformer and post-combustor components. A sensitivity analysis was developed in order to analyze the influence of the main design and operational parameters to the fuel cell performance.
The experimental activity is part of a research program for the establishment of a Fuel Cell Research Centre at the Ges.En. (Gestioni Energetiche) S.p.A. power plant, for conducting experimental campaigns aimed at determining the energetic and exergetic performance of different fuel cell based on different technologies, of different size and fed with different types of fuel (hydrogen, natural gas, biogas, etc.). The program is conducted in collaboration between the DETEC of University of Naples Federico II and the Ges.En. SpA, a company responsible for post-mortem managing of a landfill located in Schiavi Masseria del Pozzo, in the town of Giugliano in Campania (NA).
The Ges.En. S.p.A. owns with the D.I.M.S.A.T. of University of Cassino (Italy) a cogenerative module CP-SOFC-5000, manufactured by Acumentrics Corporation (Boston, MA - USA) and based on solid oxide fuel cell technology. The cogeneration unit is capable of delivering a maximum power of 5 kWe with an electric gross efficiency of around 30-35%. In addition, the module is capable of producing hot water at temperatures ranging 40-50 °C that could be used for cogeneration for domestic application.
In order to conduct experiments on that cogeneration unit, were first acquired fundamental knowledge for the operation of the same module, through a period of training at the Acumentrics Inc. (Boston, MA): in particular have deepened the issues concerning the design of the cell, the manufacturing and assembly processes of the various components, the solutions adopted for reforming and post-combustion processes and the solutions adopted for the module heat balance.
The module is also equipped with a DSP (Digital Signal Process) that acquire the signals from instruments placed on board of the unit and automatically adjust the operation during changes in electric load drawings.
The real testing program started at the Acumentrics factory, analyzing the various stages that compose the start-up procedure of the cogenerative module, and continued when the fuel cell unit was transferred to the Ges.En. Research Center in the town of Giugliano in Campania (NA), where it was connected to the electrical load line (composed of halogen lamps powered in AC ranging between 200W and 500W), and to the thermal loads (2 Fancoil fueled by a circuit of water at low temperature). An Ethernet line allows the remote control of the module.
The campaign has allowed testing the energetic performance of the cogenerative module in different conditions of electrical load, and verifying the response of cogeneration systems in transient operation. The gross electrical efficiency varies between 30% and 35% for variations in electrical load of between 2.1 kW and 3.8 kW, while the coefficient of fuel utilization varies between 60% and 85% respectively.
In the dynamic behavior the cell has shown good ability to adapt at electric load changes, due to the presence, on board of the module, of a series of batteries that act as buffer between the demands of the electrical load and the electricity provided by the tubular cells, interposing in sudden variations of the electrical load.
In parallel to the above mentioned experimental activities, has been conducted a research program involving the analysis of the energetic performance of single tubular solid oxide fuel cells manufactured and tested in the laboratory. The research program was conducted at the Colorado Fuel Cell Center (CFCC) and the Colorado Center for Advanced Ceramics (CCAC) of Colorado School of Mines (Golden, CO - USA) in collaboration with Prof.. Nigel Sammes (Editor in Chief of Journal of Fuel Cells Science and Technology).
This experience has allowed acquiring essential information on manufacturing processes of different types of solid oxide fuel cells using conventional materials and new materials capable of ensuring superior performance and advantages in manufacturing processes. The tests conducted have allowed to analyze the energetic performance of different SOFC operating at different temperatures and with different fuel volume flow of gas entering the cell. Also has been analyzed the behavior of fuel cells when fed with different types of gas (hydrogen, methane, synthesis gas, etc.).
In particular, we were able to analyze the energetic performance of tubular SOFC at anodic support, with an electrolyte consisting of YSZ (yttria Stabilized Zirconia), an anode made with Cermet formed by NiO (Nickel Oxide) and YSZ, and a cathode made of LSM (Strontium - doped Lanthanum Manganite). These cells were tested at operating temperatures ranging from 750 °C and 850 °C, fed with variables volume flow and different types of gas (hydrogen, methane, synthesis gas, etc.).
Subsequently, have been analyzed the energetic performance of anode supported micro-tubular SOFC, with an electrolyte realized in GDC (Gadolinum Doped Ceria), an cermet anode made by mixing NiO and GDC and a cathode in LSCF (La0.6Sr0.4Co0.2Fe0.8O3-y). These cells were tested at operating temperatures ranging from 450 °C and 550 °C, fed by variables volume flow, using different types of gas (hydrogen, methane, synthesis gas, etc.).
The results show performance definitely much higher for these last types of SOFC with lower operating temperatures that allow lower start-up times (start-up) and greater simplicity in the "sealing" of the various stacks components. Tests conducted with different gas confirm the ability of the "Ceria" material to inhibit the phenomenon of carbon deposition in the anode. It should be remembered however, as the presence of nickel in the anode is still the main obstacle to the possibility of direct feed such types of cells with fuels that present levels of carbon more or less variables. Tests conducted in fact confirm the ability of the "ceria" to improve the performance of the cells when fed directly with hydrocarbon, but this behavior is guaranteed for a number of hours still insufficient.
The research program conducted has allowed also learning important information about the main techniques used in laboratories to test the electrochemical performance of single fuel cells and methods used to measure key parameters that influence the behavior of these cells.
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