Date of final exam: 18/05/2005

E-mail: canevese@elet.polimi.it

Tutor: Prof. S. Bittanti, Politecnico di Milano

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Modelling and control of fuel cells   ___________________________________________________________________________________________________________

Advisor:

Prof. S. Bittanti, Politecnico di Milano; Eng. Antonio De Marco, Politecnico di Milano

Summary of the thesis

 

A fuel cell is a device which converts chemical energy, stored inside a fuel, directly to electrical energy, i.e. without the need to burn the fuel and to generate intermediate thermal energy. This way, as compared with conventional combustion-based power stations or engines, efficiency is improved: 50-60% can be reached. Besides, the basic electrochemical reaction involved, which gives water from hydrogen and oxygen, is exothermic: the heat it produces can then be usefully employed in other steps or parts of the whole process, thus further enhancing efficiency issues. Moreover, the amount of toxic emissions, like those of nitrates and sulphates, is very low, so that risks of pollution are greatly reduced, in comparison with fossil fuel-based generation. These observations contribute to justify the growing interest that research has been devoting and is devoting to the study of how to design and operate fuel cells in more and more effective ways.

 

Here, attention is focused on a Molten Carbonate Fuel Cell (MCFC) power plant for distributed production of electrical energy and heat; the cell activity is complemented by other devices, such as a reformer and a catalytic combustor for anode fuelling (that is an indirect cell, i.e. the conversion of the fuel, methane in this case, to hydrogen does not occur inside the cell itself); a microturbine exploits the spare heat of the gas outlets of the rest of the plant. A prototype of such a kind of plant is under construction at CESI, a research institute belonging to ENEL. They are the promoters of this research (together with the Politecnico) and one of the sources of technical and practical data.

 

The first objective is to build up a model of the electrochemical and thermodynamic phenomena involved in cell operation; such phenomena are complex, because not only the structure of each single MCFC is composed of many layers, i.e. separating (from the adjacent cell) metal plate, anodic gas channels, anodic electrode with diffusor, electrolyte, cathodic electrode with diffusor, cathodic gas channels and separating plate, but also the gas mixtures are in cross-flow to each other in othogonal directions with respect to the cell stack axis. To achieve better performance and reliability, a two-three-dimensional model is worked out, based on mass, momentum, energy and charge balance and implemented in a Matlab-Simulink environment. Incidentally, with the same kind of approach as that followed for the cell, a thermodynamical and chemical model is built and implemented also for the other devices in the cell part of the hybrid plant taken as a reference. As for electrochemical phenomena, the study is based on reaction kinetics and on analysing how potential differences, contributing to build up the whole potential difference between the cell ends, arise across cell sublayers, from the anode to the cathode electrode, and how they are linked to charged species concentrations; this last aspect is important to understand the presence of capacitive behaviours affecting fast dynamics.

 

Many control purposes can be conceived, according to different strategies, such as how to operate the plant in the most efficient way and without asking its materials and components for excessive performance, or how to face sudden changes in power request; therefore, one has to study not only the features of stationary conditions and slow transient paths, but also how to deal with emergency situations, like load loss. Here, steady states for some different electrical loads are simulated, so as to be able to plan suitable feed-forward actions to meet operating constraints, like those on temperatures at cell anode and cathode or at the combustor; some industrial controllers (PIDs) are also designed, for the several control loops inside the plant, for various situations, with special care, as just underlined, for emergencies.
 

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