Attila's Thesis
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PEM 100 Watt Fuel Cell System Demo
PEMFC mass and energy model Stephan Strahl
Options to measure Impedance of a Fuel Cell stack
H100 Autonomous 100W Fuel Cell Demo System
IRI PEMFC Organizational Meetings
High Temperature ZBT PEMFC stack Project
Contents
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This is were it all begins
Insert Thesis Title Here (Working title: Identification and Characterization of the Physical Phenomenons in an Open Cathode Simplified PEMFC Systems)
The Test Station: Test Station 3 EnCh
The Fuel Cell: Horizon H100
Water balance equations across the membrane in a Fuel Cell
Energy and mass balance for a fuel cell
The Simulation Model
Experimental methodology:
This section explains the main aspects of the experiments which include the open cathode fuel cell stack configuration, the test station configuration and calibration, the environmental chamber operation and the experimental setups.
Stack Hardware
The open cathode fuel cell used in this study is a commercially available stack, with 21 cells and an active area of 19.4 cm2. The stack operates on dry hydrogen and untreated ambient air for cooling as well as for the oxygen needed for the reaction which is what makes it an open cathode design.
Test station configuration, calibration and verification
- Air velocity to mass flow correlation
- Dew point temperature sensor calibration and verification
- Temperature sensor calibration and verification
Cathode inlet flow rate measurement calibration and verification
To be able to calculate the mass and energy balance of the fuel cell the mass flow rate of the air flow through the cell needs to be determined. Since the air supplied to the cell is with a fan it is difficult to measure the exact flow rate. A air velocity meter has been installed at the inlet of the open cathode fuel cell and all the air is forced through an area were this sensor is located. In theory since the area is constant and the inlet temperatures and dew points are controlled and/or measured the mass flow rate can be calculated, See figure (XXX). However there are several issues associated to measuring velocity instead of mass flow.
- The relationship between mass flow and velocity is dependent on density so therefore has to be corrected for temperature, pressure, gas concentration.
- The velocity meter takes up a significant portion of the flow area and therefore has an affect on the measurement.
Air velocity sensor calibration
Restrictions
- The sensor has to be calibrated inside the fuel cell structure assembly. This is because of the area effect of the actual sensor which has a diameter of 12 mm and is dependent on how far it protrudes into the measuring area.
- The sensor needs to be first verified with a known flow rate at the fuel cell flow conditions.
- The measurement has to be stable
Description of experiments
This first section will describe how the model will be calibrated based on the initial experiments for the purpose of applying them to the second section where the dynamics of the system will be defined based on the three control mechanisms.
Water Transport Experimental Characterization
There are two dominant transport mechanisms for water vapor across the membrane: diffusion which is driven by difference in water vapor partial pressure and electro-osmotic drag which is driven by protons traversing the membrane. As discussed previously in the hypothesis section the relationship between stack temperature, membrane resistance and current will be examined. The results from these to sets of experiments will allow for the completion of the 5 layer MEA water balance sub-model.
Diffusion Coefficient Experimental Design
The effective water vapor diffusion coefficient and its dynamic evolution through the membranes in the stack under open circuit voltage (zero load) conditions will be determined by isolating this water transfer phenomenon across the 5 layer MEA (which include the two gas diffusion layers the two catalysts and the membrane) in relation to stack temperature and membrane resistance.
This coefficient is dependent on the material properties of the components in the fuel cell (gas diffusion layer porosity, membrane thickness, exc.).
The dynamics of temperature and water content will be established experimentally.
The controlled variables in this experiment are the water vapor partial pressures at the inlets and the temperature of the fuel cell stack. Experimental procedure is to independently vary the water vapor partial pressures and fuel cell temperature and measure the water vapor partial pressure at the outlets.
The environmental chamber indirectly controls the fuel cell temperature as well as the inlet dew point temperature and air temperature. The anode reactant inlet water vapor partial pressure is maintained by a membrane based humidifier which is controlled with a dew point sensor. The measured variables will be the anode and cathode outlet dew point temperatures and the membrane resistance. The dew point temperatures will be measured using a humidity sensors and the membrane resistance will be measured using a continuous high frequency EIS.
The experiment will provide insights in the variability of the effective diffusion coefficient under a variety of conditions, temperatures from 10ºC to 60ºC and relative humidities from 10% to 100%.
This experimental setup will be used to evaluate the diffusion coefficient variability with respect to membrane water content which is directly related to membrane resistance and stack temperature.
This data then will be analyzed and incorporated into the model.
Electro-osmotic drag coefficient experimental design
In general the electro-osmotic drag is considered dependent on temperature and water content in the membrane. The proposed experiment will examine the effects of temperature, current density, and water content on the coefficient while isolating it from diffusion.
Electro-osmotic drag will be affected by the catalyst layers since a large volume fraction of the catalyst layer is ionomer. This means that electro-osmotic drag will also occur in the ionomer portion of the catalyst layer. To accurately calculate water diffusion transfer through the membrane, one must use the water contents of the membrane at the anode and cathode sides as boundary conditions. Yet covered with the catalyst layer and the GDL, it is almost impossible to know the water contents at the membrane boundaries.
A supersaturated hydrogen will be supplied to the stack to ensure saturation throughout the anode flow field. The cathode reactant will be saturated as well. This will minimize the transfer of water due to diffusion. Thus, the water that is transferred from the anode to the cathode should depict the water transfer due to electro-osmotic drag. The stack temperature will be maintained to be equal to the temperature in the environmental chamber with the fan so there will be a limitation to the amount of current the can be drawn from the stack do to the temperature gradient that will be produced. Another restraint for the test will be to ensure that both reactant streams leave the stack fully saturated relative to the stack temperature. This will indicate that there is minimal water transfer due to diffusion.
The intended range of conditions that will be tested are temperatures from 10ºC to 60ºC and currents from 1 to 8 amps all with full saturated flows.
Measuring the humidity at the exit streams will then allow for the calculation of water transfer across the membrane due only to electro-osmotic drag. The measurement of the membrane resistance will give an indication of the water content. The goal will be to main the water constant fully saturated at all the temperature and current density to get a map out of the changes in the electro-osmotic drag coefficient due to the previously stated variables.
An equation will be fitted to this data that will be a function of temperature, current density, and water content which will be incorporated into the system model.
Heat Transfer Constants
The coefficient for heat transfer through the fuel cell will be split into two coefficients: heat transfer coefficient, and the effective thermal conductivity coefficient. Both will be experimentally examined and a introduced into the model.
The convective heat transfer coefficient will be experimentally determined between the air, hydrogen and the stack.
Effect of the 3 Control Mechanisms
The 3 control mechanisms of the simple PEMFC system that will be examined are the fan velocity, hydrogen purge valve frequency and duration and stack short circuit frequency and duration. The purpose is to understand how these mechanisms effect the transport of water across the membrane and subsequently its effect on stack voltage at various operating conditions to determine the appropriate control strategy.
Fan velocity effect
The fan velocity will be varied at different current densities, ambient relative humidities, ambient temperatures in order to achieve different stack temperatures. Varying the above conditions will change the partial pressure of water on the cathode side and thus will effect the transport of water. To study these effects the inlet and outlet humidities will be measured. The anode side flow rate and relative humidity will be kept constant. The membrane resistance will be measured to relate it to the membrane water content.
Secondary effects on the cathode outlet due to the change in fan velocity should be: change the air flow rate through the stack for both cooling and oxygen for the reaction. So with a change in the air flow rate on the cathode side there will be a change in the average oxygen concentration, average water vapor partial pressure. A change in the fan voltage also will change the fan power consumption and thus the system efficiency.
Secondary effects on the anode outlet: The change in temperature will change the maximum water vapor partial pressure.
Experimental Constants
- Through flow on anode flow rate: ?
- Humidified anode ? not sure about this one
- Anode inlet temperature: ?
- No purge or short circuit
- Constant Anode pressure
Experimental Variables
- Chamber temperature: Range: ??
- Chamber water vapor content: Range: ??.
- Fan flow rate
- Current
Measured Outcome
- Stack resistance
- Voltage
Hypothesis Determine the effect of fan flow rate on stack voltage and it relation to water transport.
The duty cycle and period of the hydrogen purge valve
Opening the hydrogen purge valve flushes the anode flow field. Which removes both liquid water and vapor from the anode side. This reduces the partial pressure of water for a time being and increases the hydrogen concentration. This control action should temporally drive water from the cathode to the anode. A secondary effect is the removal any other impurities such as nitrogen that has diffused from the cathode. The frequency of the purges affects the system efficiency due to hydrogen lost to the environment.
Experimental Constants
- Fan flow rate
- Anode dead ended
- Anode inlet temperature: ?
- No short circuit
Experimental Variables
- Chamber temperature: Range: ??
- Chamber water vapor content: Range: ??.
- Purge frequency and duration
- Current
Measured Outcome
- Stack resistance
- Voltage
Hypothesis Determine the effect of hydrogen purge rate and duration on stack voltage and it relation to water transport.
Stack short circuit duty cycle and period
Short circuiting the stack for a very short period of time (in the order of 40 milliseconds) consumes all the hydrogen and oxygen in and around the catalyst layers and produce heat and water on the cathode catalyst. This heat and water will be distributed to both anode and cathode flow fields depending on there temperature and water vapor partial pressure states. The frequency of the short circuit effects the system efficiency due to the hydrogen that is consumed.
A dynamic full system model and a fuel cell test station will be constructed to accomplish this experiments.
Experimental Constants
- Fan flow rate
- Constant anode humidification and flow
- Anode inlet temperature: ?
- No Hydrogen purge
Experimental Variables
- Chamber temperature: Range: ??
- Chamber water vapor content: Range: ??.
- Short circuit frequency
- Current
Measured Outcome
- Stack resistance
- Voltage
Hypothesis Determine the effect of the short circuit on stack voltage and it relation to water transport.
The Data
Test Station 5 Demo with H100 & MHS225 Metal Hydride
Presentations
- PEMFC Class Presentation:
\\Haydn\control\presentaciones ppt\2009-11-26 Class pres of PEMFC work Attila
- PowerPoint PPT is in Haydn Control Presentations:
\\Haydn\control\presentaciones ppt\Hyceltec 2009 Vila Real FC Test Station Attila
Papers in progress:
Article sent to Journal of Power Sources, related to CONNAPICE 2010: Media:JPS2010_07_30.pdf
Abstract sent to ConAppice May 2010 Sevilla
Media:SStrahl_Comunication_Conappice2010_30-04-2010b.doc
Abstract sent to HyCelTec 2010
Media:Husar_FC_tech_Zaragoza_2010_rev2.doc
Communication (4 page article) Sent to Conappice 2010 Sevia June 15-18 Media:SStrahl_Comunication_Conappice2010_30-04-2010.doc
Presented: HyCelTec in Vila Real Portugal
PPT is in Hyden control Presentations
\\Haydn\control\presentaciones ppt\Hyceltec 2009 Vila Real FC Test Station Attila
Sent: Abstract Jun 30 2009 To HYCELTEC
File:Abstract hyceltec2009 IRIv6.doc
Media:Abstract_hyceltec2009_IRIv6.doc
Sent Abstract Feb 8 2008 To FUCE
File:Abstract for Fuel Cell Science and Tech.doc
Dynamic model of water transport and water content through diffusion in a fuel cell stack
Thesis Resources, Comsol, Work Opportunities in the basque country
