FUEL CELLS-POROUS INSERTS IN 70 CM² PEMFC IN ZIGZAG FLOW PATTERN

RAVI SIVA MANI KANDAN(11A248)

Dissertation submitted in partial fulfillment of the requirements for the degree of

BACHELOR OF ENGINEERING

Branch: AUTOMOBILE ENGINEERING

(08A720 Project Work - Phase I)

of Anna University

OCTOBER2014

DEPARTMENT OF AUTOMOBILE ENGINEERING

PSG COLLEGE OF TECHNOLOGY(Autonomous Institution)

COIMBATORE – 641 004.

FUEL CELLS- POROUS INSERTS IN 70 CM² PEMFC IN ZIGZAG FLOW PATTERN

Bonafide record of work done by

RAVI SIVA MANI KANDAN(Roll No: 11A248)

Dissertation submitted in partial fulfillment of the requirements for the degree of

BACHELOR OF ENGINEERING

Branch: AUTOMOBILE ENGINEERING(08A720 Project Work - Phase I)

of Anna University

OCTOBER 2014

-------------------- --------------------Mr.M.Karthikeyan Dr.S.Neelakrishnan Faculty guide Head of the Department

CONTENTS

CHAPTER Page No.

Acknowledgement………………………………………………………………………..iList of Figures…………………………………………………………………………….ii

1. PREFACE………………………………………………….………………………….1

2. FUEL CELLS......................................................................................................22.1 Introduction to Fuel Cells 2

2.2 Proton Exchange Membrane Fuel Cells (PEMFC) 4

3. LITERATURE REVIEW………………………………………………………...……5

4. PROJECT DESCRIPTION…………………………………………………………..64.1 Problem Definition 6

4.2 Incorporation of Porous Inserts 7

4.3 Modeling of Fuel Cell 8 4.4 Meshing of Fuel Cell Assembly 9

5. CONCLUSION…………………………………………………………………...……14

BIBLIOGRAPHY………………………………………………………………….......……15

ACKNOWLEDGEMENT

I wish to express my sincere gratitude to our respected Principal Dr. R. Rudramoorthy for

having given me the opportunity to undertake my project work.

I also express my sincere thanks to Dr. S. Neelakrishnan, Professor and Head, Department of

Automobile Engineering, for his encouragement and support that he had extended towards the success of

the project work.

I wish to thank Dr. P. Karthikeyan, Associate Professor, Department of Automobile

Engineering, for his evergreen ideas and motivation.

I owe a deep sense of reverence and gratitude to my guide, Mr. M. Karthikeyan, Assistant

Professor, Department of Automobile Engineering, for his efficient and excellent guidance, timely help,

encouragement, and for his sincere inspiration throughout the completion of the project.

I express my thanks and gratitude to my class tutor, Mr. R. Karthikeyan, Assistant Professor,

Department of Automobile Engineering for her help and encouragement in the duration of my course.

Finally, I would like to express my sincere thanks to my parents, all my student colleagues and

staff members in my department without whom this project would not have been completed successfully.

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LIST OF FIGURES

FIGURE NO TITLE PAGE

1 Structure of a fuel cell 2

2 Layout of a fuel cell 3

3 PEM Fuel Cell 4

4 Effect of water generation on performance 7

5 Rib area and flow channel 8

6 2mm zigzag porous inserts 8

7 5mm zigzag porous inserts 9

8 Assembled fuel cell in Ansys Workbench 9

9 Blocking -XY plane 10

10 Blocking - YZ plane 10

11 Blocking - XZ plane 11

11 Node Counts – XY plane 11

12 Node Counts – YZ plane 13

13 Node Counts – XZ plane 14

14 Meshing - XY plane 14

15 Meshing - YZ plane 15

16 Meshing - XZ plane 15

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1. PREFACE

In a world where transportation and power generation is primarily driven by use of fossil fuel, there are two main problems. The first problem is that these fossil fuels are available in a limited amount and eventually there will be a gap between their production and demand. The second problem is that they are harmful to the environment, resulting in global warming, pollution, acid rain, ozone layer depletion, land damage by continuous surface mining for coal and so on.

There is a need for meeting the requirements of human beings in a sustainable and efficient manner. The generation of electricity through electrochemical reactions involving hydrogen forms the basis of fuel cell technology. The operation of fuel cell results in near zero emission as the production of hydrogen is the only source for emissions. Additionally the electrochemical process is not subject to Carnot cycle limitations and has high efficiency. These advantages of fuel cell are similar to those of batteries but fuel cells have higher energy density and also quick recharge capability for sustained usage. Also, Hydrogen is seen as a fuel which is abundant and is very light and clean. With proper storage and utilization, hydrogen as a fuel can replace fossil fuels.

Fuel cells have a wide range of application. Depending on the type of fuel cell used and the stack, fuel cells can power portable electronic equipment, cars, and buses and be the source for power generation. The prospect of high efficiency, zero emission, higher energy density than batteries, fast recharging and quiet operation make fuel cell technology a lucrative and sustainable choice as an energy source, especially in the fields of transportation and power generation.

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2. FUEL CELLS2.1 Introduction to Fuel Cell:

A fuel cell is an electrochemical energy converter which converts the chemical energy of a fuel directly into electricity using an oxidant. Typically the process of electricity generation from fuels consist of various stages of conversion but a fuel cell circumvents all these stages and produces electricity in a single process, similar to a battery .In case of a PEMFC(Proton Exchange Membrane Fuel Cell or Polymer Electrolyte Membrane Fuel Cell), the fuel is pure hydrogen gas and the oxidant is oxygen.

The structure of a fuel cell consists of a membrane electrolyte assembly (MEA). This consists of a membrane which is flanked by the anode and cathode. The function of the membrane is to conduct protons (H+) from anode to the cathode as well as act as a seal between the fuel and oxidant. The MEA also consists of a gas diffusion layer which is coated with a catalyst. Uniform gas distribution and proper conduction is ensured by the gas diffusion layer and the catalyst promotes the reactions. The reactants are fed into graphite plates in which they move along the flow channels which denotes the active area of the fuel cell. The entire assembly is then sandwiched between end plates which hold all the components together.

Fig 1: Structure of a fuel cell

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The conversion of chemical to electrical energy is by the electrochemical oxidation and reduction reactions occurring at the anode and cathode respectively. The voltage produced by these reactions depends upon the reactants (fuel and oxidant) themselves. For pure hydrogen gas oxidized by oxygen (as in the case of a PEMFC), the resulting voltage as per the electrochemical series is 1.23V. The hydrogen oxidation reaction results in two protons and two electrons formed from one hydrogen molecule. This reaction is denoted as follows:

At anode(Hydrogen Oxidation Reaction):

At the cathode side the oxygen reacts with the protons (conducted through the membrane) and the electrons (conducted through an external circuit) to form water molecules. This reaction is denoted as:

At cathode (Oxygen Reduction Reaction):

Overall Reaction:

The reversible reaction is expressed in the equation and shows the reincorporation of the hydrogen protons and electrons together with the oxygen molecule and the formation of one water molecule along with 1.229V.

Fig 2: Layout of a fuel cell

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2.2 Proton Exchange Membrane Fuel Cells (PEMFC):

Among the types of fuel cells, the one most suited for automobile applications is the Polymer Electrolyte Membrane or Proton Exchange Membrane Fuel Cell. In PEMFC, the fuel is pure hydrogen gas and the oxidant is oxygen. The membrane is made of perfluro sulphonic acid (Nafion) and the gas diffusion layer is carbon paper/cloth. Platinum is sprayed onto the gas diffusion layer for catalysis of the electrochemical reactions. The flow plates which act as the electrodes are made of graphite.

The PEM Fuel Cells operate at relatively low temperature range (50-100°C) which makes them easy to contain and reduce thermal losses. They are also smaller in volume and weight which makes them suitable for powering automobiles. They also have quick start up compared to other types of fuel cells and high energy density compared to batteries.In addition to these benefits, PEM Fuel Cells also have high operating efficiency and is under a lot of research into its possible commercialization.

Key factors to consider in maximizing the utility of PEM Fuel Cells include flow channel optimization, water management, alternatives to catalyst and the membrane, scaling up and stacking for the required applications.

Fig 3: PEM Fuel Cell

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3. LITERATURE SURVEY:

FatamehHashemi et al studied the performance characteristics for various flow channel designs. The study concluded that the Reaction gas distribution, current density and temperature distributions are more uniform for serpentine flow channels.

E Birgerrson et al conducted a study on the interdigitated flow distributor. They concluded that the interdigitated flow distributor can sustain highest current density but requires very precise porous backing to avoid loss of power density.

Shawn Lister et al investigated the effect of porous inserts when incorporated into the fuel cell structure. This revealed that Porous inserts in fuel cell acts as absorber for water which increases performance compared to non-porous fuel cells.

EC Kumber studied the saturation for different pore size subjected to a constant liquid pressure. The result was that the Liquid saturation is less for smaller pores at constant liquid pressure.

Jer-Huan Jang observed the impact of water generation on the electrodes on the overall performance of the fuel cell. The conclusion was that Water generation adversely affects performance greatly at low voltage.

MM Mench conducted a study on the impact of a MPL (Micro Porous Layer) on addressing water management issues. The result was that Micro Porous Layer acts as water flow barrier at low liquid pressure.

Eikerling studied the effect of the reactant gas flow through the flow channels over water. He concluded that the Gas flow (in flow channel) over water has induced drag in gas motion.

Jixin Chen investigated the effect of air flow on water removal. This was conducted to study the movement of water into neighboring pores depending on the flow rate of the reactant gases.

Xianguao Li et al studied the importance and the adverse effects of water on the operation of fuel cell. The study revealed that the Water generation adversely affects performance but MEA requires hydration for proton conduction.

Runzhang Yuan et. Alstudied the effect of porosity on the gas diffusion layer. The result showed that the Gas diffusion layer with porosity is more optimal for liquid water discharge from catalyst into the gas channel than a Gas.

HuaMeng studied the effect of Micro Porous Layer through numerical models. He concluded that the micro porous layer serves as a barrier for liquid water transport on cathode side of a PEM Fuel cell.

Wang YL et al.performed a study by comparison on porous and non-porous fuel cells. The performance was better for a porous fuel cell as it addressed water management by absorption.

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4. PROJECT DESCRIPTION

4.1 Problem definition:

Although PEM Fuel Cells are most suitable for automotive applications, they are still not able to completely propel the vehicle on their own without great costs. They are mostly used as auxiliary power sources instead. The major problems in the commercialization of a fuel cell are:

1. Cost – This is mainly due to the usage of platinum as the catalyst. Platinum is the best element for the catalysis of the fuel cell reactions in the given conditions. Alternatives include cerium oxide and some iron based alloys and this is an active research area. Additionally the usage of Nafion membrane increases the cost.

2. Water generation –The basic electrochemical reaction results in water generation on the cathode side. This can diffuse through to the anode side as well during operation. This results in flooding, thus resulting in reduced overall performance (current density, power density and voltage). An additional problem is that water should not be completely removed from the fuel cell as some hydration is required for the protonic conduction through the nafionmembrance.

3. Heat generation – A single fuel cell does not produce enough voltage to supply the required power. Hence multiple fuel cells are stacked up to get the required power. This results in heat generation which adversely affects the performance of the fuel cells, thus reducing stack efficiency which drops the voltage per cell to around 0.6V from a theoretical 1.23V.

In addition to addressing these problems, some of the areas to consider in increasing the performance of fuel cells include increasing catalytic activity and reducing catalytic poisoning.

In scaling up of fuel cells (which increases the current output), the major problem is water generation. Once this is rectified, the performance of the fuel cells increase. This reduces the cost per unit watt of the fuel cell – thus reducing the overall cost of the fuel cell for a particular application. Thus proper water management in the fuel cell is essential towards usage of PEM Fuel Cells in practical applications. There are two ways of addressing this

1. Active methods – This includes usage of an external fan which causes forced convention or using an electro-osmotic pump. Considering the power required for these external devices, the overall increase in performance is not as high as that achieved by passive methods.

2. Passive methods – This includes design optimization such as flow channel design, incorporation of porosity in the gas diffusion layer or using a micro porous layer and also the usage of porous inserts in the flow channel.

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Fig 4: Effect of water generation on performance

4.2 Incorporation of Porous Inserts:

Among the passive methods, the incorporation of porous inserts in the rib area of the flow plates drastically reduces water formation on the surface. This leads to overall performance of the fuel cell. This is especially true for scaled up fuel cell models such as 70cm² (active area). The flow channel type used is serpertine 2x2. The placement of the porous inserts is in the rib area. The water formed in the flow channel is removed by the reactant gas and the water formed in the rib area is now removed by the porous inserts. Also the types of porous inserts used are classified based on length and by porosity.

The classification on length is:

1. 2mm 2. 5 mm

And classification based on porosity is:

1. 60-70%2. 70-80%3. 80-90%

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Fig 5: Rib area and flow channel

4.3 Modeling of Fuel Cell:

The modeling of 2mm and 5mm zigzag porous inserts are done using Solidworks 2014 software. Once the flow plate is extruded, the flow channel is drawn using linear sketch pattern and extrude-cut along with the inlet and outlet. Then the porous inserts are drawn using linear sketch pattern and extrude cut option.

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Fig6: 2mm zigzag porous inserts

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Fig 7: 5mm zigzag porous inserts

4.4 Meshing of Fuel Cell Assembly:The assembled fuel cell geometry is imported in Ansys Workbench V14.5software and

exported as a parasolid file (.x_b)

Fig 8: Assembled fuel cell in Ansys Workbench

The fully assembled fuel cell contains 7 parts- two graphite plates, two gas diffusion layers, two catalysts and one membrane. From the assembled model, the flow channel is made as a single part by using the ‘Line from points’ and ‘Surface from Edges’ options found from Tools menu.

For the meshing of the assembled fuel cell, the parasolid geometry is imported in ICEM-CFD software. The inlets and outlets are created using ‘create part’ option found in ‘part’ list.

The next step is blocking. Using the Blocking tab, a 3d block is first created around the entire geometry and then split-block is used to block the fuel cell further into sections. The mesh is created by first blocking the points along the edges. The edges and vertices are then. Thus blocks are created for each junction on the fuel cell.

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Fig 8: Blocking – XY plane

Fig 9: Blocking – YZ plane

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Fig 10: Blocking – XZ plane

Once the blocks have been created, the number of nodes on each edge can be set. The accuracy of the results is higher when the number of node points is increased. The number of nodes can be set using the ‘Premesh’ tool. The required edge is selected and the number of nodes is set. For parts with short length, lesser node values can be given.

The number of nodes for individual part can be found out by clicking on blocking menu. Right click on the ‘edge’ menu followed by ‘Counts’

Fig 11 : Node Counts – XY plane

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Fig 12: Node Counts - YZ plane

Fig 13: Node Counts - XZ plane.

After setting up the number of nodes, the meshing parameters are fed. The ’Global mesh’ menu is clicked. Cartesian mesh type is selected. The projection factor is changed to 1. The ‘Compute Mesh’ icon is selected and the Cartesian file is loaded. The mesh is generated when the ‘Compute’ button is clicked.

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Fig 15: Meshing - XZ plane

Fig 16: Meshing - XY plane

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Fig 17: Meshing about YZ plane

The resulting mesh is saved as a mesh file (.uns). Now that the meshing is done for the whole assembly, the mesh can be imported in FLUENT 14.5 software and analysed

5.CONCLUSION:Fuel cells are seen as a sustainable energy source for stationary applications as well as

automotive propulsion by scaling and stacking. However, the major commercialization problems are to be addressed. It is evident that water generation on the electrolyte surface adversely affects overall performance. The incorporation of porous inserts is aimed at managing this issue. From the analysis of the fuel cell assembly, the effect of various porous inserts can be studied. This can address the water management issue and the performance parameters can be increased. From the analysis, the most efficient assembly can be fabricated and tested for performance measurement. A comparison with the performance for a conventional fuel cell will show the effect of the porous inserts for water management in the 70 cm² PEM Fuel Cell.

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BIBLIOGRAPHY:

1. Fuel Cell Engines by Matthew M. Mench2. PEM Fuel Cells – Theory and Practice by Frano Barbir3. Wei Dai, HaijiangWanga, Xiao-Zi Yuan, Jonathan J. Martin, Daijun Yang, JinliQiao, Jianxin

Ma. “A review on water balance in the membrane electrodeassembly of proton exchange membrane fuel cells.”

4. Fuel Cell Science and Engineering: Materials, Processes, Systems and technology by DetlefStolten and Bernd Emonts.

5. Yong Hun Park, Jerald A. Caton- “Development of a PEM stack and performance analysis including the effects of water content in the membrane”

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