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Fuel Cell Stack

Implement generic hydrogen fuel cell stack model

Library

Electric Drives/Extra Sources

Description

The Fuel Cell Stack block implements a generic model parameterized to represent most popular types of fuel cell stacks fed with hydrogen and air.

The block represents two versions of the stack model: a simplified model and a detailed model. You can switch between the two models by selecting the level in the mask under Model detail level in the block dialog box.

Simplified Model

This model is based on the equivalent circuit of a fuel cell stack shown below:

The simplified model represents a particular fuel cell stack operating at nominal conditions of temperature and pressure. The parameters of the equivalent circuit can be modified based on the polarization curve obtained from the manufacturer datasheet. You just have to input in the mask the value of the voltage at 0 and 1 A, the nominal and the maximum operating points, for the parameters to be calculated. A diode is used to prevent the flow of negative current into the stack. A typical polarization curve consists of three regions:

The first region represents the activation voltage drop due to the slowness of the chemical reactions taking place at electrode surfaces. Depending on the temperature and operating pressure, type of electrode, and catalyst used, this region is more or less wide. The second region represents the resistive losses due the internal resistance of the fuel cell stack. Finally, the third region represents the mass transport losses resulting from the change in concentration of reactants as the fuel is used.

Detailed Model

The detailed model represents a particular fuel cell stack when the parameters such as pressures, temperature, compositions and flow rates of fuel and air vary. You can select which parameters to vary on the Signal variation pane on the block dialog box. These variations affect the open circuit voltage (Eoc), the exchange current (i0) and the Tafel slope (A). Eoc, i0 and A are modified as follows:

where

R = 8.3145 J/(mol K)

F = 96485 A s/mol

z = Number of moving electrons

En = Nernst voltage, which is the thermodynamics voltage of the cells and depends on the temperatures and partial pressures of reactants and products inside the stack (V)

α = Charge transfer coefficient, which depends on the type of electrodes and catalysts used

PH2 = Partial pressure of hydrogen inside the stack (atm)

PO2 = Partial pressure of oxygen inside the stack (atm)

k = Boltzmann's constant = 1.38 × 10–23 J/K

h = Planck's constant = 6.626 × 10–34 J s

ΔG = Size of the activation barrier which depends on the type of electrode and catalyst used

T = Temperature of operation (K)

Kc = Voltage constant at nominal condition of operation

The equivalent circuit is the same as for the simplified model, except that the parameters Eoc, i0 and Α have to be updated on-line as shown below:

The rates of conversion (utilizations) of hydrogen (UfH2) and oxygen (UfO2) are determined in Block A as follows:

where

Pfuel = Absolute supply pressure of fuel (atm)

Pair = Absolute supply pressure of air (atm)

Vlpm(fuel) = Fuel flow rate (l/min)

Vlpm(air) = Air flow rate (l/min)

x = Percentage of hydrogen in the fuel (%)

y = Percentage of oxygen in the oxidant (%)

N = Number of cells

The 60000 constant comes from the conversion from the liter/min flow rate used in the model to m3/s (1 liter/min = 1/60000 m3/s).

The partial pressures and the Nernst voltage are determined in Block B as follows:

and

where

PH2O = Partial pressure of water vapor inside the stack (atm)

w = Percentage of water vapor in the oxidant (%)

From the partial pressures of gases and the Nernst voltage, the new values of the open circuit voltage (Eoc) and the exchange current (i0) can be calculated.

Block C calculates the new value of the Tafel slope (A).

The parameters α, ΔG and Kc are calculated based on the polarization curve at nominal conditions of operation along with some additional parameters, such as the low heating value (LHV) efficiency of the stack, composition of fuel and air, supply pressures and temperatures. They can be easily obtained from the manufacturer datasheet.

The nominal rates of conversion of gases are calculated as follows:

where

ηnom= Nominal LHV efficiency of the stack (%)

Δh0(H2O(gas))=241.83 × 103 J/mol

Vnom = Nominal voltage (V)

Inom=Nominal current (A)

Vlpm(air)nom= Nominal air flow rate (l/min)

Pairnom= Nominal absolute air supply pressure (Pa)

Tnom= Nominal operating temperature (K)

From these rates of conversion, the nominal partial pressures of gases and the Nernst voltage can be derived. With Eoc, i0 and Α known and assuming that the stack operates at constant rates of conversion or utilizations at nominal condition, α, ΔG and Kc can be determined.

If there is no fuel or air at the stack input, it is assumed that the stack is operating at a fixed rate of conversion of gases (nominal rate of conversion), that is, the supply of gases is adjusted according to the current so that they are always supplied with just a bit more than needed by the stack at any load.

The maximum current the stack can deliver is limited by the maximum flow rates of fuel and air that can be reached. Beyond that maximum current, the voltage output by the stack decreases abruptly as more current is drawn.

The dynamics of the fuel cell are represented if you specify the response time and the parameters for flow dynamics (peak utilization and corresponding voltage undershoot) on the Fuel Cell Dynamics pane on the dialog box.

The response time (Td) @ 95% is used to model the "charge double layer" phenomenon due to the build up of charges at electrode/electrolyte interface. This affects only the activation voltage (NAln(ifc/i0)) as shown on the equivalent circuits.

The peak utilization (UfO2(peak)) and the corresponding voltage undershoot (Vu) are used to model the effect of oxygen depletion (due to the air compressor delay) on the cell output voltage. The Nernst voltage is modified due to this effect as follows:

where

K = voltage undershoot constant

UfO2(nom) = nominal oxygen utilization

K is determined as follows:

Current step and interrupt tests must be made on a real stack to represent with accuracy its dynamics. The figure below shows the stack response from these tests and the required parameters (Td, UfO2(peak) and Vu).

The response time (Td) depends on the fuel cell stack itself and is usually given on the datasheet. The parameters for flow dynamics (UfO2(peak) and Vu) depend on the dynamics of external equipments (compressor, regulator and loads) and they are not provided by manufacturers as their values vary with the user application. For simulation, the user may assume values of UfO2(peak) between 60% to 70% and Vu between 2-5% of the stack nominal voltage.

Dialog Box and Parameters

Parameters Tab

Preset model

Provides a set of predetermined polarization curves and parameters for particular fuel cell stacks found on the market:

• No (User-Defined)

• PEMFC - 1.26 kW - 24 Vdc

• PEMFC - 6 kW - 45 Vdc

• PEMFC - 50 kW - 625 Vdc

• AFC - 2.4 kW - 48 Vdc

Select one of these preset models to load the corresponding parameters in the entries of the dialog box. Select No (User-Defined) if you do not want to use a preset model.

Model detail level

Provide access to the two versions of the model:

• Simplified

• Detailed

When a simplified model is used, there is no variable under the signal variation pane

Voltage at 0 A and 1 A

The voltage at 0 A and 1 A of the stack (Volts). Assuming nominal and constant gases utilizations.

Nominal operating point

The rated current (Ampere) and rated voltage (Volts) of the stack. Assuming nominal and constant gases utilizations.

Maximum operating point

The current (Ampere) and voltage (Volts) of the stack at maximum power. Assuming nominal and constant gases utilizations.

Number of cells

The number of cells in series in the stack. This parameter is available only for a detailed model.

Nominal stack efficiency

The rated efficiency of the stack relative to the low heating value (LHV) of water. This parameter is available only for a detailed model.

Operating temperature

The nominal temperature of operation in degrees Celsius. This parameter is available only for a detailed model.

Nominal air flow rate

The rated air flow rate (l/min). This parameter is available only for a detailed model.

Nominal supply pressure

Rated supply pressure (absolute) of fuel and air in bars. This parameter is available only for a detailed model.

Nominal composition

The rated percentage of hydrogen (x) in the fuel, oxygen (y) and water (w) in the oxidant. This parameter is available only for a detailed model.

Plot V-I characteristic

Plots a figure containing two graphs. The first graph represents the stack voltage (Volts) vs current (A) and the second graph represents the stack power (kW) vs current (A). To plot the graphs, select the checkbox. This checkbox is available only for a detailed model.

View cell parameters

Presents the overall parameters of the stack. This checkbox is available only for a detailed model. Select the checkbox to view the parameters. The dialog box is shown below.

Signal Variation Tab (Detailed Model Only)

The Signal variation pane provides a list of parameters that can be varied. Select a checkbox for a variable to input a corresponding signal to the block. The following signals can be input to the block:

Fuel composition

Percentage of hydrogen in the fuel.

Oxidant composition

Percentage of oxygen in the oxidant.

Fuel flow rate

Fuel flow rate in liter per minutes.

Air flow rate

Air flow rate in liter per minutes.

System Temperature

Temperature of operation in Kelvin.

Fuel supply pressure

Fuel supply pressure in bars.

Air supply pressure

Air supply pressure in bars.

Fuel Cell Dynamics Tab

Specify Fuel Cell Dynamics?

Asks whether you want to specify the fuel cell dynamics. Select the checkbox to enter the fuel cell response time in seconds.

Fuel Cell response time (sec)

Enter the response time of the cell (at 95% of the final value). This parameter becomes visible only when the Specify Fuel Cell Dynamics? checkbox is selected.

Peak O2 utilization (%)

Enter the peak oxygen utilization at nominal condition of operation. This parameter becomes visible only when the Specify Fuel Cell Dynamics? checkbox is selected and the Air flow rate checkbox is selected under the Signal variation pane.

Voltage undershoot (V) @ peak O2 utilization

Enter the voltage undershoot (Volts) at peak oxygen utilization at nominal condition of operation. This parameter becomes visible only when the Specify Fuel Cell Dynamics? checkbox is selected and the Air flow rate checkbox is selected under the Signal variation pane.

How to Extract Parameters from Data Sheet

Here is the procedure to extract parameters from fuel cell stack manufacturer's data sheet. For this example, the NetStack PS6 data sheet from NetStack is used:

The rated power of the stack is 6 kW and the nominal voltage is 45 V. The following detailed parameters are deduced from the datasheet.

• Voltage at 0 A and 1 A [Eoc,V1] = [65, 63]

• Nominal operating point [Inom, Vnom]=[133.3, 45]

• Maximum operating point [Iend,Vend]=[225, 37]

• Nominal stack efficiency (ηnom)= 55%

• Operating temperature = 65 0C

• Nominal supply pressure [H2, Air]=[1.5 1]

If the pressure given is relative to the atmospheric pressure, add 1 bar to get the absolute pressure.

• Nominal composition (%)[H2, O2, H2O(Air)]=[99.999, 21, 1]

If air is used as oxidant, assume 21% of O2 and 1% of H2O in case their percentages are not specified.

• Number of cells

If not specified, estimate it from the formulae below:

In this case,

• Nominal air flow rate

If the maximum air flow rate is given, the nominal flow rate can be calculated assuming a constant oxygen utilization at any load. The current drawn by the cell is linearly dependent on air flow rate and the nominal flow rate is given by:

In this case,

In case no information is given, assume the rate of conversion of oxygen to be 50% (as it is usually the case for most fuel cell stacks) and use the formulae below to determine the nominal air flow rate.

• Fuel cell response time = 10s

 Note   The parameters [Eoc, V1], [Inom, Vnom] and [Iend,Vend] are approximate and depend on the precision of the points obtained from the polarization curve. The higher the accuracy of theses parameters, the more closed will be the simulated stack voltage to the data sheet curve. A tool, called ScanIt (from amsterchem) can be used to extract precise values from data sheet curves.

With the above parameters, the polarization curve of the stack operating at fixed nominal rate of conversion of gases is closed to the datasheet curves as shown below: The blue dotted line shows the simulated stack voltage and green dotted line shows the simulated stack power.

Above the maximum current, the flow rate of gases entering the stack is maximum and the stack voltage decreases abruptly as more current is drawn.

Block Inputs and Outputs

m

The Simulink output of the block is a vector containing 11 signals. You can demultiplex these signals by using the Bus Selector block provided in the Simulink library.

 Model detail level(Signal availability) Signal Definition Units Symbol Detailed Simplified 1 Voltage V Vfc Yes Yes 2 Current I Ifc Yes Yes 3 Stack Efficiency % η Yes No(Set to 0) 4 Stack consumption [Air, Fuel] slpm Vslpm Yes No(Set to 0) 5 Flow Rate [Air, Fuel] lpm Frlpm Yes No(Set to 0) 6 Stack consumption [Air, Fuel] lpm Vlpm Yes No(Set to 0) 7 Utilization [Oxygen, Hydrogen] % Uf Yes No(Set to 0) 8 Slope of Tafel curve A Yes No(Set to 0) 9 Exchange current A i0 Yes No(Set to 0) 10 Nernst voltage V En Yes No(Set to 0) 11 Open circuit voltage V Eoc Yes No(Set to 0)

Model Assumptions

• The gases are ideal

• The stack is fed with hydrogen and air

• The stack is equipped with a cooling system which maintains the temperature at the cathode and anode exits stable and equal to the stack temperature

• The stack is equipped with a water management system to maintain the humidity inside the cell at appropriate level at any load

• The cell voltage drops are due to reaction kinetics and charge transport as most fuel cells do not operate in the mass transport region

• Pressure drops across flow channels are negligible

• The cell resistance is constant at any condition of operation

Model Limitations

• Chemical reaction dynamics caused by partial pressure changes of chemical species inside the cell are not considered

• The stack output power is limited by the fuel and air flow rates supplied

• The effect of temperature and humidity of the membrane on the internal resistance is not considered

• The flow of gases or water through the membrane is not considered

Example

The power_fuel_cellpower_fuel_cell example illustrates a 6 kW, 45 volts Proton Exchange Membrane (PEM) Fuel Cell Stack model feeding a 100Vdc DC/DC converter.

The converter is loaded by an RL element of 6kW with a time constant of 1 sec. During the first 10 secs, the utilization of the hydrogen is constant to the nominal value (Uf_H2 = 99.56%) using a fuel flow rate regulator. After 10 secs, the flow rate regulator is bypassed and the rate of fuel is increased to the maximum value of 85 lpm in order to observe the variation in the stack voltage. That will affect the stack efficiency, the fuel consumption and the air consumption.

 Note   As the Air flow rate checkbox under Signal variation pane is not selected, the stack will operate at fixed, nominal oxygen utilization (59.3%).

The simulation produces the followings results:

Results for the Scope1

Results for the Scope2

At t = 0 s, the DC/DC converter applies 100Vdc to the RL load (the initial current of the load is 0A). The fuel utilization is set to the nominal value of 99.56%. The current increases until the value of 133A. The flow rate is automatically set in order to maintain the nominal fuel utilization. Observe the DC bus voltage (Scope2) which is very well regulated by the converter. The peak voltage of 122Vdc at the beginning of the simulation is caused by the transient state of the voltage regulator.

At t = 10 s, the fuel flow rate is increased from 50 liters per minute (lpm) to 85 lpm during 3.5 s reducing by doing so the hydrogen utilization. This causes an increasing of the Nernst voltage so the fuel cell current will decrease. Therefore the stack consumption and the efficiency will decrease (Scope1).