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          Institute: MPI für Dynamik komplexer technischer Systeme     Collection: Physical and Chemical Process Engineering     Display Documents

ID: 208006.0, MPI für Dynamik komplexer technischer Systeme / Physical and Chemical Process Engineering
Direct Methanol Polymer Electrolyte Fuel Cell : Transfer Function Analysis of Current-Voltage Dynamics
Authors:Krewer, U.; Schultz, T.; Sundmacher, K.
Date of Publication (YYYY-MM-DD):2004
Title of Proceedings:Fuell Cell Research Symposium : Modelling and Experimental Validation : [Abstracts]
Physical Description:CD-ROM
Name of Conference/Meeting:Fuel Cell Research Symposium : Modelling and Experimental Validation
Place of Conference/Meeting:Zuerich, Switzerland
(Start) Date of Conference/Meeting
End Date of Conference/Meeting 
Review Status:not specified
Audience:Experts Only
Intended Educational Use:No
Abstract / Description:The Direct Methanol Fuel Cell (DMFC) is a promising technology as mobile power supply. Typical for mobile systems are dynamically changing load demands, i.e. a DMFC system has to operate reliably even at strongly varying load conditions and dynamic changes of system parameters. In the presented work, DMFCs operated with liquid methanol-water solutions are investigated experimentally under dynamic load conditions. In a series of experiments, the electric cell current was changed stepwise between typical operating regimes, while all other system parameters (temperatures, pressures, methanol feed concentration, air supply etc.) were kept constant. The cell voltage response to such current steps shows significant overshooting and relaxation phenomena on two different time scales: very fast responses within milliseconds after the current step inputs, followed by much slower responses in the range of some seconds up to minutes. Possible explanations for the observed short-time behaviour can be found in the reaction kinetics, double layer charging / discharging and Ohmic drops, while for the long-term response mass transport and methanol accumulation phenomena (transport towards the anode and crossover through the polymer electrolyte membrane to the cathode) as well as slow reaction steps and adsorption / desorption of long-living reaction intermediates such as COx are responsible. For the operation of mobile DMFC power supply systems, it is necessary to design and implement control strategies that ensure that the system meets certain power output requirements. Fundamental for this task is to formulate dynamic process models which are as simple as possible, but which nonetheless reflect the qualitative dynamic behaviour of the real system. Therefore, it is necessary to identify those physico-chemical processes that are mainly responsible for the dynamic system behaviour. For this purpose, the DMFC is decomposed hierarchically into a network of process elements, which are connected to each other via input and output signals. For each of these elements a transfer function connecting in- and output signals is formulated. The transfer function can be are derived from physico-chemical models. Selectively simplifying or neglecting certain transfer elements within the network allows to investigate their relevance for the observed overshooting and relaxation phenomena. As a result, the key phenomena can be identified which are responsible for the characteristic response behaviour and a simple but adequate model can be developed which is suitable for controller design.
External Publication Status:published
Document Type:Conference-Paper
Communicated by:Kai Sundmacher
Affiliations:MPI für Dynamik komplexer technischer Systeme/Physical and Chemical Process Engineering
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