Maximum Power Tracking simulation for a Low- Temperature fuel cell Doudou N. Luta 1, Atanda K. Raji2

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 7, Issue 12, December 2017)

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Maximum Power Tracking simulation for a

Low-Temperature fuel cell

Doudou N. Luta

1

, Atanda K. Raji

2

Cape Peninsula University of Technology, Symphony Way, Bellville 7535, South Africa

Abstract—Fuel cells are promising alternative power sources to substitute the use of fossil fuels in transportation as well as in a variety of portable and stationary applications to cover the increasing energy demand and promote cleaner environment. A diversity of these technologies is available into the market, they are distinguished based on their working temperature, reactants and electrolytes used. Amongst them, Low-Temperature Polymer Electrolyte Membrane Fuel Cell (PEMFC) is the most attractive. PEMFC display a nonlinear output characteristic due to a number of internal limits, which may cause a low efficient operation. It is therefore important to constrain the system to run at its largest power to avoid low efficiency and excessive fuel use. In this paper, a PEMFC interfaced to a DC/DC boost converter and a MPPT controller based on the incremental conductance algorithm is presented, simulated and analysed. The simulation is performed under Matlab/Simulink environment.

Keywords—Fuel Cell, PEM, Boost converter, MPPT

I. INTRODUCTION

Fuel cells are emerging technologies considered as promising alternative to substitute fossil fuel for power generation. With their gaining interest, Fuel cells show

important opportunities in applications such as

transportation, portable electronics devices and stationary power systems, etc. They offer the possibility of clean and efficient power sources with high energy density and rapid fuel recharge by converting chemical energy from an electrolytic reaction to electricity with heat and water rejection. The conversion process does not involve any

combustion, hence there is no NOx released.

A typical fuel cell comprises a negatively and positively charge electrodes named anode cathode respectively and an

electrolyte membrane [1], it produces electricity

continuously as long as the fuel and the oxidant are supplied. Hydrogen is the principal fuel or reactant while oxygen serves as an oxidant. In a fuel cell stack, cells are connected in series/parallel to obtain the desired voltage and current. Typical cell voltages range from 0.6 to 0.7 in high performance fuel cells, whereas the number of cells and their series/parallel connection dictates the stack voltage.

Several different types of reactants are used depending upon the fuel cell technology, and the most current fuel cells are [1] [2] [3] [4] [5] [6]:

 Proton Exchange or Polymer Electrolytic Membrane

Fuel Cell (PEMFC),

 Phosphoric Acid Fuel Cell (PAFC),

 Alkaline Fuel Cell (AFC),

 Molten Carbonate Fuel Cell (MCFC),

 Solid Oxide Fuel Cell (SOFC) and

 Direct Methanol Fuel Cell (DMFC).

From the technologies above, PEMFC, AFC and DMFC are referred to as Low-Temperature fuel cells due to their lower operating temperature, which is in the order of 60 to

900 Celsius [6] [7] [8] . The performance characteristics

such as high-power density, rapid star-up and zero or low emission make PEMFC the more attractive of fuel cell technologies.

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The remaining of the paper is organised as follows, the next section is dedicated to the system description, modelling, section 3 gives the simulation results, and the last section deals with the conclusion.

II. SYSTEM DESCRIPTION AND MODELLING

The system consists of a PEMFC stack, DC-to-DC converter and MPPT controller and PWM for the duty cycle adjustment (See Figure 1). There are various DC-to-DC converter topologies suitable to interface a fuel cell and a load. In this research, a DC-to-DC boost converter operating at a switching frequency of 50 kHz is used to step-up a 45 V dc supply of a 6-kW fuel cell to 100 V. To ensure a reliable operation of the converter, the inductor is considered 20% greater than the minimum value and the voltage ripple is 1V.

[image:2.612.84.559.170.588.2]

PEM fuel cell

Figure 2. PEM fuel cell stack schematic [17]

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Polymer Electrolyte Membrane fuel cells (PEMFC) also known as Proton Exchange fuel cells or Polymer Electrolyte Fuel cells offer the advantages of low eight and volume as compared to other fuel cell technologies, they require only hydrogen and oxygen which are combined in a cell to generate electricity with water and heat as the by-product. A simple analogy with fuel cells as opposed to batteries is the continuous supply of the reactants while that of batteries are finite.

At the anode of a PEMFC, the hydrogen gas ionises releasing electrons and creating H+ as shown in equation (1), whereas at the cathode, oxygen combines with electrons taken from the electrode, and H+ from the electrolyte to form water (Equation (2)) [1] [18]

(1)

(2)

The overall reaction-taking place in a fuel cell follows the equation (3) below [10]:

⁄ (3)

The output voltage of the fuel-cell stacks can be obtained as [19] [20]:

(4)

In this equation, VFC is the fuel cell output voltage, E is

the open circuit voltage, whereas Vact, Vconc, and Vohmic are

the activation, concentration, and the ohmic fuel cell overvoltage respectively. Relationship between the voltage and the current in the cell can be modelled as [19] [20]:

( ) (

₎

(5)

In which VFC is the output voltage of the fuel cell, E is

the open circuit voltage, IFC is the fuel cell output current,

IO is the exchange current, IL is the limiting current, Rint is

the internal resistance, A is the activation coefficient and B is the concentration coefficient. Furthermore, E, A, and B are dependent on the operating temperature of the fuel cell,

whereas the internal resistance Rint in the fuel cell depends

on the membrane characteristic.

[image:3.612.333.573.152.261.2]

DC/DC Boost Converter

Figure 3. Boost converter circuit [21]

Boost converters (See Figure 3) convert unregulated voltages into desired regulated voltages by varying the duty

cycle at high switching frequency. The choice

of components like inductors and capacitors is very important to reduce the ripple generation for a given switching frequency. The minimum value of the inductor to ensure continuous conduction mode is given by (6) as [22] [23]:

( ) (6)

With (7)

V0 the output voltage, V the input voltage and T the

switching period

The minimum capacitance is:

(8)

A boost converter operating at a switching frequency of 50 kHz is used to step-up a 45 V dc supply of the fuel cell to 100 V. To ensure a reliable operation, the inductor is considered 20% greater than the minimum value and the voltage ripple is chosen as 1V. The converter simulation parameters are in Table 1.

Table 1. Boost converter parameters.

Parameter Value

Switching frequency 50 kHz

Inductor 1.02.10-6 _H

Capacitor 26.10-4 _F

Voltage ripple 1V

[image:3.612.325.564.621.708.2]

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Incremental Conductance Algorithm

Initialize INC

Measure V, I

Evaluate dI, dV, G, dG

G + dG > 0

V_new=V_old + V

V_new=V_old - V

I_old=I_new V_old=V_new Wait

T

[image:4.612.62.238.158.414.2]

Yes No

Figure 4. Incremental conductance algorithm flowchart [22]

The algorithm in Figure 4 compares the incremental conductance to the instantaneous conductance of the fuel cell. Depending on the result, the voltage is increased or decreased until the achievement of maximum power point. This method stands on the assumption that at the maximum power point the following condition occurs [24] [25] [26]:

( ( )) (9)

( ) (10)

Where P and V are the power and voltage respectively.

At the maximum power point, and

(11)

then V<VMPP (12)

then V=VMPP (13)

then V>VMPP

The equivalent Simulink model of the incremental conductance algorithm is shown is Figure 5.

[image:4.612.68.525.488.623.2]

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III. SIMULATION RESULTS

A 6-kW PEM fuel cell block modelled using pre-defined fuel cell Simulink block, Incremental Conductance MPPT controller block combining Simulink and Matlab Simulink function block and DC-DC converter. The fuel cell considered operates in the range of voltage from 30V to 60V by controlling some parameters such as the hydrogen and oxygen compositions and flow rates, the hydrogen and oxygen pressures and the fuel cell temperature. In this simulation, the fuel cell hydrogen flow rate, hydrogen pressure and temperature are constants. Thus, the operating voltage is maintained at a constant value. Figure 6 (a) and (b) shows that the nominal input voltage and current of the fuel cell are 45 V and 130 A respectively. The voltage and current from Figure 7 (a) and (b) represent the outputs

obtained across the converter using the

Incremental Conductance algorithm model in Figure 5. This algorithm computes the duty cycle required to produce the maximum power. This duty cycle is then used as an input for the PWM pulse generation, which is shown in the figure 9. This PWM pulse is as the IJBT gate signal of the boost converters. The voltage and current across the boost converter (See Figure 7 (a) and (b)) are 100 V and 60 A respectively. Figure 8 shows the converter’s power, which is 6 kW.

Figure 7. (a) Converter voltage; (b) Converter current (b) DC bus current (A)

(a) Fuel cell voltage (V)

Figure 6. (a) Fuel cell voltage; (b) Fuel cell current (b) Fuel cell current (A)

[image:5.612.44.287.450.620.2]

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Figure 9. PWM signal

IV. CONCLUSIONS

Fuel cells display nonlinear output characteristics due to a number of internal parameters, which can cause a low efficient system operation and impact on the output voltage. Similarly to photovoltaic systems, the variations of temperature and of internal parameters in a fuel cell throughout its lifetime makes the position of the maximum power point to change, It is therefore important to constrain the system to operate at its maximum power to avoid low efficiency and excessive fuel use. To ensure that, the power converter interface must be able to self-adjust its parameters at run time, consequently modifying its input voltage and current based upon the fuel cell maximum power point position. This study was targeted to analyze

MPPT implementation on a 6 kW Low Temperature fuel cell stack (PEMFC) interfaced to a boost converter using Incremental Conductance method. The fuel cell hydrogen

flow rate, hydrogen pressure and temperature were

assumed constants. The simulation result shown the performance of the Incremental Conductance algorithm on the converter output voltage and current.

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