SOFC Thermal Transients: Modeling by Application of Experimental System Identification Techniques

Solid oxide fuel cell (SOFC) is one of the most promising technologies for future power generation. In order to make this technology marketable, many issues as cost reduction, durability, and operational management have to be overcome. Therefore, the understanding of thermodynamic and electrochemical mechanisms, that govern the SOFC behavior in steady‐state and in transient operation, becomes fundamental. In this context, the modeling of fuel cell (FC) thermal transient is of great interest because it can predict the temperature time variation, useful to the dimensioning of auxiliary devices and to avoid unwanted operational states affecting cell durability. In the present study, a 0‐D model of SOFC thermal transients was developed by applying system identification techniques, starting from experimental tests carried out on a stack made up of four single cells. Moreover, it was successfully validated in reference to further experimental data. The model allows to evaluate, in term of dynamic response, the effect of the main operating parameters on FC temperature. As further result, some control/regulation considerations useful to limit thermal stresses were proposed.     

History and Aesthetics of the Bronx-Whitestone Bridge

This paper is a study of the Bronx-Whitestone Bridge. Othmar Ammann’s bridge, completed in 1939, has strong similarities to the ill-fated Tacoma Narrows, sharing many structural characteristics. While the New York bridge has served well for two-thirds of a century, the deck has oscillated noticeably for most of its lifetime; in attempts to calm these motions, the Whitestone has undergone an unusual number of retrofits—in 1940, 1946, 1988, and a new retrofit begun in 2003. From a historical perspective, these retrofits have kept pace with the development of suspension bridge technology, each representing the state of the art at the time they were implemented. We can therefore view the Whitestone as a valuable physical record of developments in the profession’s understanding of structural vibrations and aerodynamics. The goal of this paper is to place these retrofits in context, as well as to evaluate them on technical and aesthetic levels.     

A two-phase flow model for geothermal wells in the presence of non-condensable gas

Quantitative information on the phenomena occuring during the upward flow of a geothermal fluid in water-dominated wells is a requisite for designing the wellhead system and optimizing resource exploitation. The geothermal fluid consists, for the most part, of a two-phase mixture of water containing dissolved salts, steam and non-condensable gases. Various, closely interrelated effects must therefore be taken into consideration: pressure drop of the rising fluid; heat and mass transfer between the phases (due to evaporation and desorption); heat exchange with rock formations. Simultaneous application of the mass, energy and momentum equations results in a rather complex model that can be solved by a numerical computer program. The model described here accounts for the effects of: the presence of salts, when computing all the thermodynamic properties of the fluids, especially enthalpy, density, vapour pressure of the brine and superheated steam enthalpy; the presence of non-condensable gases, considering their deviations from ideal behaviour and their contribution to density; the heat exchange with the surrounding rock formations; variation in salt concentration along the flow-path; possible variation in pipe diameter and surface roughness with height. The simplified hypotheses adopted are: fluid flow is stationary; thermodynamic equilibrium conditions exist between the phases in each point along the well; the non-condensable gases are assumed to be CO₂; Henry's law is assumed valid and the quantity dissolved chemically is assumed negligible; the salts are assumed to be NaCl; the activity coefficients are unitary; liquid surface tension and viscosity values are assumed equal to those of pure water. Comparison of the results of the computer program and the experimental pressure and temperature profiles shows that these are in satisfactory agreement within a rather wide range of operative conditions. The noncondensable gases, even in very low concentrations, were shown to be of importance to these calculations. Once the experimental temperature and pressure profiles are known, the model will also permit calculation of the concentration of non-condensable gases. The most efficient of the two correlations used to compute pressure drop in two-phase regimes seems to be that devised by CISE^‡, which is based on global parameters not correlated to the different flow regimes.     

Design of the measurements validation procedure and the expert system architecture for a cogeneration internal combustion engine

A research activity has been initiated to study the development of a diagnostic methodology, for the optimization of energy efficiency and the maximization of the operational time in those conditions, based on artificial intelligence (AI) techniques such as artificial neural network (ANN) and fuzzy logic.The diagnostic procedure, developed specifically for the cogeneration plant located at the Engineering Department of the University of Perugia, must be characterized by a modular architecture to obtain a flexible architecture applicable to different systems. The first part of the study deals with the identifying the principal modules and the corresponding variables necessary to evaluate the module “health state”.Also the consequent upgrade of the monitoring system is described in this paper. Moreover it describes the structure proposed for the diagnostic procedure, consisting of a procedure for measurement validation and a fuzzy logic-based inference system. The first reveals the presence of abnormal conditions and localizes their source distinguishing between system failure and instrumentation malfunctions. The second provides an evaluation of module health state and the classification of the failures which have possibly occurred. The procedure was implemented in C++.     

Gas-water interface rise during early exploitation tests in alfina geothermal field (Northern Latium, Italy)

Alfina geothermal field is made up of a gas-cap (CO₂) overlying an almost closed aquifer. The wells drilled in the highest part of the reservoir produce gas while the others either produce hot water or are sterile.During the first production tests the analysis of pressure and water-levels showed that the gas-water interface had risen. This was later confirmed when a gas-producing well began carrying water.This paper attempts to explain field behaviour by means of simple calculations and assuming an idealized geological structure.This study gives an estimation of the average porosity and initial gas in place.     

Part load operation of SOFC/GT hybrid systems: Stationary analysis

In a global energetic context characterized by the increasing demand of oil and gas, the depletion of fossil resources and the global warming, more efficient energy systems and, consequently, innovative energy conversion processes are urgently required. A possible solution can be found in the fuel cells technology coupled with classical thermodynamic cycle technologies in order to make hybrid systems able to achieve high energy/power efficiency with low environmental impact. Moreover, due to the synergistic effect of using a high temperature fuel cell such as solid oxide fuel cell (SOFC) and a recuperative gas turbine (GT), the integrated system efficiency can be significantly improved. In this paper a steady zero dimensional model of a SOFC/GT hybrid system is presented. The core of the work consists of a performance analysis focused on the influence of the GT part load functioning on the overall system efficiency maintaining the SOFC power set to the nominal one. Also the proper design and management of the heat recovery section is object of the present study, with target a global electric efficiency almost constant in part load functioning respect to nominal operation. The results of this study have been used as basis to the development of a dynamic model, presented in the following part of the study focused on the plant dynamic analysis.     

High temperature electrolysis using Molten Carbonate Electrolyzer

Molten Carbonate Fuel Cells (MCFC) are a well-developed and commercial technology that can operate also as an electrolyzer producing hydrogen from steam. In this study, a system for the production of hydrogen based on Molten Carbonate Electrolyzer (MCE) is presented. The system receives, as an external input, water and recovers internally the additional gas streams required as input to the electrolyzer. The system products are, separately, pure oxygen and hydrogen. A calculation sheet was implemented to analyze the energy equilibrium and gas mix compositions. The system can produce 0.074 Nl h^?1 cm^?2 of hydrogen with an inlet power density of 0.213 W cm^?2 for an energy consumption of 3.40 kWh Nm_H2^?3. Sensitivity studies on current density, utilization factors of both steam and CO₂ were analyzed considering energy equilibrium of the stack unit and the post processing processes. Results show how current density has higher impact on system equilibrium compared to the other parameters.     

Biogas use in high temperature fuel cells: Enhancement of KOH-KI activated carbon performance toward H2S removal

Activated carbons (ACs) treated with KOH-KI are very effective sorbents for deep H₂S removal, as required by biogas use in high temperature fuel cell systems. For this application, the performance of a commercial KOH-KI treated AC was investigated through a systematic study based on dynamic adsorption tests. With reference to the composition of a real biogas produced in a wastewater treatment plant located in Barcelona, the present work presents a sensitivity performance analysis on singular and synergetic effects of gas matrix, humidity and oxygen on AC KOH-KI performance.The results revealed a positive role of water (up to 90% of relative humidity (R.H.)) for different gas matrices, enhanced by the simultaneous presence of small percentages of oxygen (2%_v). A relevant influence of gas matrix composition was found (except for the case of oxygen addition to dry inlet streams), specifically in terms of a marked negative effect of CO₂ and a significant sorption capacity increase for high percentage of methane. Sulfur dioxide was not detected in the outlet gas-phase for the investigated operating parameters (O₂ 2%_v, R.H. 0–90%, H₂S 100 ppm_v, temperature 45 °C). Therefore, even in the case of further oxidation of adsorbed elemental sulfur to SO₂, this product could be completely removed by AC KOH-KI.     

Well-testing in travale-radicondoli field. Part V. Concluding statement on pressure transient studies made from well tests in the Travele-Radicondoli reservoir

The theory and applications of pressure transient (well test) analysis have been studied intensively for more than 40 yr by petroleum reservoir engineers and groundwater hydrologists. Only in the past decade, however, have geothermal-fluid wells been tested for the purpose of making pressure transient studies. Results of these studies disclose various well conditions, for example, restrictions to fluid flow into the wellbore. They also disclose reservoir heterogeneities, boundaries and permeability-thickness products of reservoir rocks. Probably most important, they can be used in estimations of energy reserves. This powerful analytical tool is discussed with special reference to the Travale reservoir.This reservoir is complicated geologically and hydrologically. It lies on the margin of a graben near a widespread outcrop of the reservoir rocks, which also form an absorption area for the meteoric waters. The area explored can be divided into three zones: in one of these (the nearest to the absorption area) some noncommercial wells produce two-phase water-steam mixtures; in the second zone the wells produce superheated steam, while a well drilled in the graben itself produces a fluid with an uncondensable gas content of about 80%. The reservoir is described in relation to defining areas for further exploration. The nature of the reservoir has affected the design of programs for collecting pressure-production data and other well performance data. The performance history prior to the advent of pressure transient studies pertains mainly to what is known as the ‘old’ Travale reservoir to the southwest of the ‘new’ Travale-Radicondoli reservoir in which the more recent wells are drilled and in which modern well test analysis methods have been applied. Data on the “old” reservoir are discussed first.Because of its initial performance and relationship to nearby wells the most important well in the “new” reservoir is Travale well 22. It has been subjected to extensive well testing. Nearly all the wells in the “new” reservoir have been involved, however, through well-interference tests. In these tests the wells surrounding Travale well 22 are shut in and their pressure responses to different Travale well 22 production rates are measured. Well interference tests indicate the characteristics of fluid flow in the reservoir between test wells and in a qualitative way the heterogeneous nature of the reservoir itself.Pressure transient theory is developed from ideal system behavior: one vertical, fully-penetrating well producing at a constant rate from a horizontal reservoir of uniform thickness and of infinite extent in any direction from the wellbore. A great deal of research has been done to aid well-test analysts in their interpretation of pressure buildup and pressure drawdown curves constructed from data taken on wells in actual reservoirs. This research generally is accomplished with model studies. Some of the models developed in the present research fit reasonably well with the build-up behavior of Travale well 22.The research done on the Travale reservoir is summarized here with the objective of showing what has been learned, how it can be applied, and what should be done next. Confidence in applications of pressure transient analyses in the Travale reservoir has been gained. New concepts of the reservoir system have emerged as a result of the research. Additional testing and more precise measurements in the field should lead to good engineering estimates of energy reserves.     

An energetic–exergetic analysis of a residential CHP system based on PEM fuel cell

The use of fuel cell systems for distributed residential power generation represents an interesting alternative to traditional thermoelectric plants due to their high efficiency and the potential recovering of the heat generated by the internal electrochemical reactions. In this paper the study of a micro cogenerative (CHP) energy system based on a Proton Exchange Membrane fuel cell (PEMFC) is reported.