Wolfgang Winkler

The design of stationary and mobile solid oxide fuel cell–gas turbine systems

Abstract A general thermodynamic model has shown that combined fuel cell cycles may reach an electric-efficiency of more than 80%. This value is one of the targets of the Department of Energy (DOE) solid oxide fuel cell–gas turbine (SOFC–GT) program. The combination of a SOFC and GT connects the air flow of the heat engine and the cell cooling. The principle strategy in order to reach high electrical-efficiencies is to avoid a high excess air for the cell cooling and heat losses. Simple combined SOFC–GT cycles show an efficiency between 60 and 72%. The combination of the SOFC and the GT can be done by using an external cooling or by dividing the stack into multiple sub-stacks with a GT behind each sub-stack as the necessary heat sink. The heat exchangers (HEXs) of a system with an external cooling have the benefit of a pressurization on both sides and therefore, have a high heat exchange coefficient. The pressurization on both sides delivers a low stress to the HEX material. The combination of both principles leads to a reheat (RH)-SOFC–GT cycle that can be improved by a steam turbine (ST) cycle. The first results of a study of such a RH-SOFC–GT–ST cycle indicate that a cycle design with an efficiency of more than 80% is possible and confirm the predictions by the theoretical thermodynamic model mentioned above. The extremely short heat-up time of a thin tubular SOFC and the market entrance of the micro-turbines give the option of using these SOFC–GT designs for mobile applications. The possible use of hydrocarbons such as diesel oil is an important benefit of the SOFC. The micro-turbine and the SOFC stack will be matched depending on the start-up requirements of the mobile system. The minimization of the volume needed is a key issue. The efficiency of small GTs is lower than the efficiency of large GTs due to the influence of the leakage within the stages of GTs increasing with a decreasing size of the GT. Thus, the SOFC module pressure must be lower than in larger stationary SOFC–GT systems. This leads to an electrical-efficiency of 45% of a cycle used as a basis for a design study. The result of the design study is that the space available in a mid-class car allows the placement of such a system, including space reserves. A further improvement of the system might allow an electrical-efficiency of about 55%.

Design studies of mobile applications with SOFC-heat engine modules

Abstract The recent development of thin tubular solid oxide fuel cells (SOFCs), microturbines and Stirling engines has inspired design studies of the integration of a SOFC–heat engine (HE) system within a car. The total power system consists of a SOFC–HE power generation unit, a power storage (battery) system, a power management system and electric motors at the wheels. The sizes of the HE and the SOFC stack are to be matched by the start-up requirements. The use of micro tubes allows a very high power density of the stack. The thermodynamic calculation of the cycle gives the actual design values for the study and indicates further steps for system optimisation. The first SOFC–GT layout lead to an electric efficiency of 45% for the cycle used as a base for a design study [The Design of Stationary and Mobile SOFC–GT Systems, UECT, 2001]. The design study shows that the space available in a mid-class car allows the integration of such a system including space reserves. A further improvement of the system might allow an electric efficiency of more than 55%. The integration of a Stirling engine instead of the microturbine is a second possibility and the object of an ongoing study. This was motivated by interesting results from the development of solar powered Stirling engines. Generally, the analyses show that the optimal match of the SOFC and the HE will be a key issue for any engineering solution.

The influence of the mass transfer on the geometric design of SOFC stacks

Abstract The system cost of any SOFC plant and the chances of development of a mobile SOFC application strongly depend on the power density of the stack. Good mass transfer within the stack is one important physical requirement for high power density. The influences on the mass transfer in a tubular, a tubular–annular and a planar stack are analyzed and discussed. It results in a proposal for a tubular–helix SOFC stack to increase the mass transfer by better mixing.

Chapter 3 – Thermodynamics

This chapter discusses the thermodynamic considerations to understand the processes of energy conversion in solid oxide fuel cells (SOFC). Theoretical studies of the behavior of the reversible processes have a high practical value in helping to understand complex systems. The reversible work of a fuel cell is defined by the free or Gibbs enthalpy of the reaction. If one uses the assumption of the ideal gas, one can immediately get the equation of the Nernst voltage from the Gibbs enthalpy of the reaction. The consideration of the electrical effects shows that the molar flow of the spent fuel is proportional to the electric current and the reversible work is proportional to the reversible voltage. A coupling between the thermodynamic data and the electrical data is only possible using the quantities power or heat flow and not by using work and heat. This is caused by the use of a mass or substance transport as the basis for thermodynamic considerations and the use of a charge transport to describe electrical phenomena. The combination of a SOFC with a heat engine allows an extremely high electric efficiency. Any real combination of a SOFC and a heat engine is based on a reversible system but a simplified version can be used to analyze the principles of the design of a combined SOFC-heat engine. It is important that the cell itself and not the flue gas be considered the heat source.

Fuel Cell Hybrids, Their Thermodynamics and Sustainable Development

The increasing demand on primary energy and the increasing concern on climatic change demand immediately a sustainable development, but still there remain open questions regarding its technical realisation. The second law of thermodynamics is a very simple but efficient way to define the principle design rules of sustainable technologies in minimising the irreversible entropy production. The ideal, but real process chain is defined by a still reversible structure or logic of the process chain—the reversible reference process chain—but consisting of real components with an irreversible entropy production on a certain level. It can easily be shown for energy conversion and for transportation that hybridisation in general can be indeed a measure to meet the reversible process chain and to minimise the entropy flow to the environment. Fuel cells are principal reversible converters of chemical energy and thus a key element within hybridisation. Depending on application, CHP may be a hybridisation step or only a slight improvement. There is a fundamental difference in heating a house or in supplying an endothermic chemical reaction with reaction entropy. The use of heat recovery and isolation is a necessary measure to minimise the entropy flow to the environment and can be described by a reversible reference process as well. The application of reversible reference process chains shows that hybrid systems with fuel cells are a technical feasibility to approach very closely the thermodynamic potential. This development differs from the past where the technical possibilities of materials and manufacturing limited the technology to meet reversibility and thus sustainability.Copyright

A Proposal of a Harmonized Testing Format for Fuel Cell Technology: FCTESTNET

FCTESTNET (Fuel Cell Testing and Standardization Network) is an ongoing European network project within framework program 5. It is a three-year project that commenced 2003, with 55 partners from European research centers, universities, and industry, working in the fuel cell field. The main objective of FCTESTNET is to promote the harmonization of testing procedures and methodologies within the European Union. The lack of standardized test methods for fuel cell technology is a fact. The development of standardized test methods is very important for the commercialization of the fuel cell technology. Standardized test methods are one of the most important instruments in the quality management work of any industrial process. The players that have a common interest to promote and develop harmonized test methods for fuel cell technology are: • Standardization bodies (IEC and ISO); • Fuel cell manufacturing industry (type testing and routine testing); • OEMs (acceptance testing); • Research institutes and universities (R&D). The current work presents one of the core results from the FCTESTNET project, namely a proposal of a harmonized testing format for fuel cell technology. The harmonized testing format has been developed based on a testing model that was proposed in the initial phase of FCTESTNET. The testing model describes the common process steps in testing and has been a valuable tool to communicate testing activities and develop test format within the network. The testing model describes testing in general and fuel cell testing in particular. It is a three-step model. The first step is the planning step and comprises the listing of standardized test methods and other references that are required for the execution of any specific testing activity. The second step of the testing model is the testing execution, which is where the actual testing is carried out. The result of the testing execution is here referred to as test output data or test output. The test output is analysed and compared with input data from the planning step and finally reported in the third step, that is the evaluation step. Some examples of specific test inputs, in the context of fuel cell testing, are temperature, vibration, fuel flow, rain, etc. Examples of specific test outputs are current, voltage, gas emissions, heat, degradation, etc. In professional testing, the internal function of the test object is of secondary importance. The object is to be treated as a “black box”. It is the test output and the test result that are of primary importance. Based on terminology originating from the testing model, such as test object, test inputs, test outputs, etc, a harmonized testing format has been developed and proposed. The key terms in the harmonized testing format are test programme and test module. The test programme is defined as a programme comprising two or more test modules. A test module is a test method defined as the variation of one single test input, for example the testing of power output as function of ambient temperature. Furthermore, a test module comprises the objectives, the scope, the test input varied, the test outputs tested, test object class (fuel cell, fuel cell stack or fuel cell system), test procedure, test report, etc.Copyright

Reversible process structures as a base of sustainable engineering

The analysis of exergy losses of a system is a well-known way to determine the influence of the second law on existing systems. Thermo-economics combines this methodology with economic calculations. Using this methodology engineering becomes an evolutionary process. Since system structures are virtual, reversible system structures are possible and inevitable irreversibility is only caused by its real components. They can be described by their exergetic efficiency. Thus reversible system structures can be used as general valid benchmarks for system engineering. It allows easy comparison or a trade-off between possible solutions. The use of a few basic reversible processes allows the building of larger reversible structures including an effective management of released and demanded entropy within the system, as can be shown in different applications and missions. The effective use of renewable sources can be considered as well however bioprocesses are not investigated yet.Copyright

Systemintegration und Anlagenkonzepte

Die thermodynamische Struktur der Vergleichsprozesse, wie sie in den vorangehenden Abschnitten beschrieben wurde, liefert zwar wesentliche Aussagen uber den grundsatzlichen Prozessaufbau, lasst aber definitionsgemas die anlagentechnische Umsetzung offen. Es ist daher fur die technische Umsetzung notwendig, die „beliebige“ Warmekraftmaschine des Vergleichsprozesses zu konkretisieren und dann fur die Varianten von konkreten Anlagenschaltungen Prozessberechnungen als Basis fur die weitere Auslegung durchzufuhren. Der theoretische thermodynamische Vergleichsprozess erweist dabei seinen praktischen Nutzen zur Evaluation und Verifizierung der Modellierung des realen Prozesses. Wenngleich die Ergebnisse solcher Prozessberechnungen eine Vorauswahl erlauben und so eine notwendige Voraussetzung fur die praktische Umsetzung darstellen, zeigt erst eine darauf aufbauende Konstruktionsstudie die tatsachlichen Realisierungsmoglichkeiten moglicher Varianten auf. Insbesondere bei kostenintensiven Entwicklungen, wie sie die Stackentwicklung darstellt, ist der fruhzeitige konstruktive Entwurf des Gesamtsystems eine notwendige Voraussetzung fur eine effiziente Projektsteuerung, da nur so fruhzeitig und mit vergleichsweise geringem Aufwand kritische Bauteile und Konstruktionen auch an den Schnittstellen erkannt werden konnen.

Die Brennstoffzelle in der zukünftigen Energieversorgung

Die einfache Nutzungsmoglichkeit vorhandener Infrastrukturen erleichtert die Markteinfuhrung neuer Energiewandlungssysteme, wie der Brennstoffzelle erheblich. Wahrend die Notwendigkeit, eine vollig neue Infrastruktur zusatzlich errichten zu mussen, eine Innovation erheblich behindert, wenn nicht sogar verhindern konnte [7.1]. Als essentielle Aspekte zur Nutzung der vorhandenen Infrastruktur sind hier insbesondere die Moglichkeiten: zur Einbindung in den Betrieb (elektrisches Netz, Warte, Instandhaltung etc.) und zur Nutzung kommerziell verfugbarer Brennstoffe zu sehen. Die sonst — zumindest in Europa — ebenfalls essentiellen Fragestellungen zur Umweltvertraglichkeit sind hinsichtlich des Einsatzes von Brennstoffzellen zunachst wohl von untergeordneter Bedeutung, da im Regelfall unterstellt werden kann, dass das Brennstoffzellensystem an dieser Stelle anderen Systemen deutlich uberlegen ist [7.2]. Demgegenuber sind aber alle Fragen zur Wirtschaftlichkeit von Brennstoffeellen wieder von erheblicher Relevanz, da gerade hier — angesichts hoch entwickelter Konkurrenztechnologien — mogliche Hemmnisse fur eine Innovation sehr konkret erscheinen.

Brennstoffzellensysteme und kombinierte Anlagen

Bereits im vorangehenden Abschnitt 2. wurde erwahnt, dass die effektive Nutzung der Zellabwarme eine wesentliche Voraussetzung fur die optimale Auslegung von Brennstoffzellenanlagen ist. Dazu sollen im folgenden die thermodynamischen Prinzipien zur Verknupfung einer Brennstoffzelle mit einem Warmekraftprozess und zur Integration der Brenngaserzeugung im Gesamtsystem hergeleitet und diskutiert werden.