**Contents**

#### **Preface XI**


Preface

This book is concerned with the production of energy that we all need in order to sustain our living standards without, at the same time, harming the environment that makes life possible and worth living. There is enough coal for the next hundreds of years, and the sun pours onto the earth far more energy than we could ever need. The problem is to make it

To tackle this problem, we have to consider all possible ways of obtaining energy and then see how they affect the environment. We have to take into consideration their capacities, their costs, their reliabilities and their safeties, because these considerations determine

Other energy sources such as wind and the sun are also controversial. It is not easy at all to evaluate the conflicting claims made by their optimistic supporters and to decide

It is extremely important in all these discussions to present energy resources and needs, production capacities, costs and safety in a quantitative way, and to give some indication of the reliability of the numbers quoted. The broad conclusion of the discussion on energy supplies is that, providing that the population growth is moderate, we should have enough energy worldwide to satisfy our basic needs. A much more difficult question is whether we can do this without polluting our environment, which might seriously reduce quality of life. This is the real question, and it should strongly influence our choice of energy sources.

Our hope is that this book provides facts about various competing technologies that are necessary for making informed choices and for stimulating a rational debate. We doubt that an immediate and international consensus will emerge out of such a debate but sincerely hope that sufficient number of people will find common ground to work on and engage even larger number of people to join them in addressing this crucial issue. Our book is intended to instigate a quality discussion and decision-making, as communities and nations

This book is a product of several authors and their extensive experience in research and teaching on energy. In composing such a book, the authors assume that the reader has a

I would like to extend my thankfulness to the staff of the publisher, particularly Miss

**Cumhur Aydinalp** Uludag University Bursa, Turkey

Martina Blecicfor their consideration and helpfulness in the preparation of this book.

available in usable form without devastating the Earth.

whether they will be acceptable or not.

prepare themselves to meet the future.

reasonable knowledge of the energy.

whichenergy policy is the best.

## Preface

This book is concerned with the production of energy that we all need in order to sustain our living standards without, at the same time, harming the environment that makes life possible and worth living. There is enough coal for the next hundreds of years, and the sun pours onto the earth far more energy than we could ever need. The problem is to make it available in usable form without devastating the Earth.

To tackle this problem, we have to consider all possible ways of obtaining energy and then see how they affect the environment. We have to take into consideration their capacities, their costs, their reliabilities and their safeties, because these considerations determine whether they will be acceptable or not.

Other energy sources such as wind and the sun are also controversial. It is not easy at all to evaluate the conflicting claims made by their optimistic supporters and to decide whichenergy policy is the best.

It is extremely important in all these discussions to present energy resources and needs, production capacities, costs and safety in a quantitative way, and to give some indication of the reliability of the numbers quoted. The broad conclusion of the discussion on energy supplies is that, providing that the population growth is moderate, we should have enough energy worldwide to satisfy our basic needs. A much more difficult question is whether we can do this without polluting our environment, which might seriously reduce quality of life. This is the real question, and it should strongly influence our choice of energy sources.

Our hope is that this book provides facts about various competing technologies that are necessary for making informed choices and for stimulating a rational debate. We doubt that an immediate and international consensus will emerge out of such a debate but sincerely hope that sufficient number of people will find common ground to work on and engage even larger number of people to join them in addressing this crucial issue. Our book is intended to instigate a quality discussion and decision-making, as communities and nations prepare themselves to meet the future.

This book is a product of several authors and their extensive experience in research and teaching on energy. In composing such a book, the authors assume that the reader has a reasonable knowledge of the energy.

I would like to extend my thankfulness to the staff of the publisher, particularly Miss Martina Blecicfor their consideration and helpfulness in the preparation of this book.

> **Cumhur Aydinalp** Uludag University Bursa, Turkey

**Chapter 1**

**Exergy Analysis of 1.2 kW Nexa™ Fuel Cell Module**

There are two key problems with continued use of fossil fuels, which provide about 80% of the world energy demand today. The first problem is that they are limited in amount and sooner or later depleted. The second problem is that fossil fuels are causing serious environ‐

The fuel cell technology is friendly energy conversion with a high potential for environmental‐ ly. Fuel cells are ideally suited for applications that require electrical energy as the end. Fuel cell systems operate at higher thermodynamic efficiency than heat engines and turbines.

The fuel cell converts chemical energy directly into electricity by combining oxygen from the air with hydrogen gas without combustion. If pure hydrogen is used, the only material out‐ put is water and almost no pollutants are produced. Very low levels of nitrogen oxides are emitted, but usually in the undetectable range. The hydrogen can be produced from water using renewable energy forms like solar, wind, hydro or geothermal energy. Hydrogen also can be extracted from hydrocarbons, including gasoline, natural gas, biomass, landfill gas,

Today, practical fuel cell systems are becoming available and are expected to attract a grow‐ ing share of the markets for automotive power and generation equipment as costs decrease to competitive levels. Depending on the type of fuel cells, stationary applications include small residential, medium-sized cogeneration or large power plant applications. In the mo‐ bile sector particularly low-temperature fuel cells, can be used for passenger vehicles, trains,

*Proton Exchange Membrane (PEM) fuel cell:* Proton Exchange Membrane (PEM) fuel cells are currently the most promising type of fuel cell for automotive use and have been used in the

and reproduction in any medium, provided the original work is properly cited.

© 2012 Sevjidsuren et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

G. Sevjidsuren, E. Uyanga, B. Bumaa, E. Temujin,

mental problems, such as global warming climate changes.

methanol, ethanol, methane and coal-based gas.

boats, and air planes [1-2].

Additional information is available at the end of the chapter

P. Altantsog and D. Sangaa

http://dx.doi.org/10.5772/50602

**1. Introduction**

## **Exergy Analysis of 1.2 kW Nexa™ Fuel Cell Module**

G. Sevjidsuren, E. Uyanga, B. Bumaa, E. Temujin, P. Altantsog and D. Sangaa

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/50602

**1. Introduction**

There are two key problems with continued use of fossil fuels, which provide about 80% of the world energy demand today. The first problem is that they are limited in amount and sooner or later depleted. The second problem is that fossil fuels are causing serious environ‐ mental problems, such as global warming climate changes.

The fuel cell technology is friendly energy conversion with a high potential for environmental‐ ly. Fuel cells are ideally suited for applications that require electrical energy as the end. Fuel cell systems operate at higher thermodynamic efficiency than heat engines and turbines.

The fuel cell converts chemical energy directly into electricity by combining oxygen from the air with hydrogen gas without combustion. If pure hydrogen is used, the only material out‐ put is water and almost no pollutants are produced. Very low levels of nitrogen oxides are emitted, but usually in the undetectable range. The hydrogen can be produced from water using renewable energy forms like solar, wind, hydro or geothermal energy. Hydrogen also can be extracted from hydrocarbons, including gasoline, natural gas, biomass, landfill gas, methanol, ethanol, methane and coal-based gas.

Today, practical fuel cell systems are becoming available and are expected to attract a grow‐ ing share of the markets for automotive power and generation equipment as costs decrease to competitive levels. Depending on the type of fuel cells, stationary applications include small residential, medium-sized cogeneration or large power plant applications. In the mo‐ bile sector particularly low-temperature fuel cells, can be used for passenger vehicles, trains, boats, and air planes [1-2].

*Proton Exchange Membrane (PEM) fuel cell:* Proton Exchange Membrane (PEM) fuel cells are currently the most promising type of fuel cell for automotive use and have been used in the

© 2012 Sevjidsuren et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

majority of prototypes built to date. PEM fuel cells (membrane or solid polymer) typically operated at relatively low temperatures (~50-100o C), have high power densities, can vary their output quickly to meet shifts in power demand, and are suited for many applications.

In particular, exergy analysis yields efficiencies which provide a true measure of how nearly actual performance approaches the ideal, and identifies more clearly than energy analysis the causes and locations of thermodynamic losses. Consequently, exergy analysis can assist in improving and optimizing designs. A main aim of exergy analysis is to identify exergy efficiencies and the causes of exergy losses. The exergy of a system is defined as the maxi‐ mum shaft work that can be done by the composite of the system and a specified reference environment. Typically, the environment is specified by stating its temperature, pressure

Exergy Analysis of 1.2 kW Nexa™ Fuel Cell Module

http://dx.doi.org/10.5772/50602

5

*Exergy efficiency:* Exergetic efficiency, which is defined as the second law efficiency, gives the true value of the performance of an energy system from the thermodynamic view‐ point. The exergy efficiency of a fuel cell system is the ratio of the electrical output pow‐

Actual exergy defined as difference between the exergy of the reactants (hydrogen + air) and the exergy of the products (air + hydrogen). In the PEMFC module, a basic reaction

The exergy efficiency of a fuel cell system is the ratio of the power output, over the exergy of

*Actual exergy*

where: *W*˙ *elect – electrical output power [kW]; <sup>E</sup>*˙ *air*,*R*, *<sup>E</sup>*˙ *<sup>H</sup>*2,*R*, *<sup>E</sup>*˙ *air*,*P*, *<sup>E</sup>*˙ *<sup>H</sup>*2*O*,*<sup>P</sup> – total exergies of the*

*Electrical output power:* The electrical power (gross power) production is the sum of the para‐ sitic load (i.e. the NEXA™ blower, compressor, and control system load) and the external

The external load (*W*˙ *net*-net power) is calculated directly from the voltage and current meas‐

*<sup>W</sup>*˙ *elect* <sup>=</sup>*W*˙ *para* <sup>+</sup> *<sup>W</sup>*˙ *net* (3)

(*Exergy*)*<sup>R</sup>* <sup>−</sup> (*Exergy*)*<sup>P</sup>* =

(1)

(2)

*H*2*O* + *UnusedAir*(*Oxygen depletedair*)+

the reactants (hydrogen + air), which can be determined [9-11] by following formula:

*<sup>η</sup>exergy system* <sup>=</sup> *Electrical output power*

*<sup>η</sup>exergy system* <sup>=</sup> *Electrical output power*

*W*˙ *elect* (*E*˙ *air*,*<sup>R</sup>* <sup>+</sup> *<sup>E</sup>*˙ *<sup>H</sup>* 2,*R*) <sup>−</sup> (*E*˙ *air*,*<sup>P</sup>* <sup>+</sup> *<sup>E</sup>*˙ *<sup>H</sup>* <sup>2</sup>*<sup>O</sup>* ,*<sup>P</sup>* )

*reactants [kW]; air and hydrogen, and the products air and water, respectively.*

*H*<sup>2</sup> + *Air*→

*ElectricalPower* + *Heat*

and chemical composition.

er and actual exergy.

load (e.g. residential load).

ured at the load.

occurs as below.

PEM fuel cells used an electrolyte such as conducted hydrogen ions from the anode to cathode. The electrolyte is composed of a solid polymer film that consists of a form acidified Nafion membrane. The membrane is coated on both sides with highly dispersed metal alloy particles (mostly platinum or platinum alloys) that are active catalysts. Hydrogen is fed to the anode side of the fuel cell where, due to the effect of the catalyst, hydrogen atoms release electrons and become hydrogen ions (protons). The electrons travel in the form of an electric current that can be utilized before it returns to the cathode side of the fuel cell where oxygen is fed. The pro‐ tons diffuse through the membrane to the cathode, where the hydrogen atom is recombined and reacted with oxygen to produce water, thus completing the overall process [3].

In this work, we will focus on the efficiency of a PEM fuel cell system at variable operating con‐ ditions such as working temperature, pressure and air stoichiometry. Determination of an ef‐ fective utilization of a PEM fuel cell and measuring its true performance based on thermodynamic laws are considered to be extremely essential. Thus, it would be very desira‐ ble to have a property to enable us to determine the work potential of a given amount of energy at power plant. This property is exergy, which is also called the availability or available energy.

In an energy analysis, based on the first law of thermodynamics, all forms of energy are con‐ sidered to be equivalent. The loss of quality of energy is not taken into account.

An exergy analysis, based on the first and second law of thermodynamics, shows the ther‐ modynamic imperfection of a process, including all quality losses of materials and energy, including the one just described. An energy balance is always closed as stated in the first law of thermodynamics. There can never be an energy loss, only energy transfer to the envi‐ ronment in which case it is useless. From the second law of thermodynamics, the exergy analysis of the irreversibility of a process due to increase in entropy. Exergy is always de‐ stroyed when a process involves a temperature change. This destruction is proportional to the entropy increase of the system together with its surroundings. Therefore, exergy is a property of the system–environment combination and not of the system alone.

Theoretically, the efficiency of a PEM fuel cell based on the first law of thermodynamics makes no reference to the best possible performance of the fuel cell, and thus, it could be misleading. On the other hand, the second law efficiency or exergetic efficiency of a PEM fuel cell, which is the ratio of the electrical output over the maximum possible work output, could give a true measure of the PEM fuel cell performance. Energy analysis performed on a system based on the second law of thermodynamics is known as exergy analysis [4-8].

## **2. Exergy analysis of 1.2kW Nexa™ PEM fuel cell**

Exergy analysis is a thermodynamic analysis technique based on the second law of thermo‐ dynamics, considering of all components and parametric in the system.

In particular, exergy analysis yields efficiencies which provide a true measure of how nearly actual performance approaches the ideal, and identifies more clearly than energy analysis the causes and locations of thermodynamic losses. Consequently, exergy analysis can assist in improving and optimizing designs. A main aim of exergy analysis is to identify exergy efficiencies and the causes of exergy losses. The exergy of a system is defined as the maxi‐ mum shaft work that can be done by the composite of the system and a specified reference environment. Typically, the environment is specified by stating its temperature, pressure and chemical composition.

for Better Environment

4 Clean Energy

operated at relatively low temperatures (~50-100o

majority of prototypes built to date. PEM fuel cells (membrane or solid polymer) typically

their output quickly to meet shifts in power demand, and are suited for many applications. PEM fuel cells used an electrolyte such as conducted hydrogen ions from the anode to cathode. The electrolyte is composed of a solid polymer film that consists of a form acidified Nafion membrane. The membrane is coated on both sides with highly dispersed metal alloy particles (mostly platinum or platinum alloys) that are active catalysts. Hydrogen is fed to the anode side of the fuel cell where, due to the effect of the catalyst, hydrogen atoms release electrons and become hydrogen ions (protons). The electrons travel in the form of an electric current that can be utilized before it returns to the cathode side of the fuel cell where oxygen is fed. The pro‐ tons diffuse through the membrane to the cathode, where the hydrogen atom is recombined

and reacted with oxygen to produce water, thus completing the overall process [3].

sidered to be equivalent. The loss of quality of energy is not taken into account.

property of the system–environment combination and not of the system alone.

**2. Exergy analysis of 1.2kW Nexa™ PEM fuel cell**

dynamics, considering of all components and parametric in the system.

In this work, we will focus on the efficiency of a PEM fuel cell system at variable operating con‐ ditions such as working temperature, pressure and air stoichiometry. Determination of an ef‐ fective utilization of a PEM fuel cell and measuring its true performance based on thermodynamic laws are considered to be extremely essential. Thus, it would be very desira‐ ble to have a property to enable us to determine the work potential of a given amount of energy at power plant. This property is exergy, which is also called the availability or available energy. In an energy analysis, based on the first law of thermodynamics, all forms of energy are con‐

An exergy analysis, based on the first and second law of thermodynamics, shows the ther‐ modynamic imperfection of a process, including all quality losses of materials and energy, including the one just described. An energy balance is always closed as stated in the first law of thermodynamics. There can never be an energy loss, only energy transfer to the envi‐ ronment in which case it is useless. From the second law of thermodynamics, the exergy analysis of the irreversibility of a process due to increase in entropy. Exergy is always de‐ stroyed when a process involves a temperature change. This destruction is proportional to the entropy increase of the system together with its surroundings. Therefore, exergy is a

Theoretically, the efficiency of a PEM fuel cell based on the first law of thermodynamics makes no reference to the best possible performance of the fuel cell, and thus, it could be misleading. On the other hand, the second law efficiency or exergetic efficiency of a PEM fuel cell, which is the ratio of the electrical output over the maximum possible work output, could give a true measure of the PEM fuel cell performance. Energy analysis performed on a system based on the second law of thermodynamics is known as exergy analysis [4-8].

Exergy analysis is a thermodynamic analysis technique based on the second law of thermo‐

C), have high power densities, can vary

*Exergy efficiency:* Exergetic efficiency, which is defined as the second law efficiency, gives the true value of the performance of an energy system from the thermodynamic view‐ point. The exergy efficiency of a fuel cell system is the ratio of the electrical output pow‐ er and actual exergy.

Actual exergy defined as difference between the exergy of the reactants (hydrogen + air) and the exergy of the products (air + hydrogen). In the PEMFC module, a basic reaction occurs as below.

$$H\_2 + Air \rightarrow \\ H\_2O + \text{Liused}A\text{ir} \text{(Oxygen depletedair)} + \\ \text{ElectricalPower} + Heat$$

The exergy efficiency of a fuel cell system is the ratio of the power output, over the exergy of the reactants (hydrogen + air), which can be determined [9-11] by following formula:

$$\begin{aligned} \eta\_{\text{exergy system}} &= \frac{\text{Electrical output power}}{\text{Actual energy}}\\ \eta\_{\text{exergy system}} &= \frac{\text{Electrical output power}}{(\text{Exergy})\_R - (\text{Exergy})\_P} = \\ \eta\_{\text{exber}} &= \frac{\dot{W}\_{\text{ckrt}}}{(E\_{air,R} + E\_{H\_2,R}) - (E\_{air,P} + E\_{H\_2O,P})} \end{aligned} \tag{2}$$

where: *W*˙ *elect – electrical output power [kW]; <sup>E</sup>*˙ *air*,*R*, *<sup>E</sup>*˙ *<sup>H</sup>*2,*R*, *<sup>E</sup>*˙ *air*,*P*, *<sup>E</sup>*˙ *<sup>H</sup>*2*O*,*<sup>P</sup> – total exergies of the reactants [kW]; air and hydrogen, and the products air and water, respectively.*

*Electrical output power:* The electrical power (gross power) production is the sum of the para‐ sitic load (i.e. the NEXA™ blower, compressor, and control system load) and the external load (e.g. residential load).

$$
\dot{\mathbf{W}}\_{\text{elect}} = \dot{\mathbf{W}}\_{\text{ para}} + \dot{\mathbf{W}}\_{\text{net}} \tag{3}
$$

The external load (*W*˙ *net*-net power) is calculated directly from the voltage and current meas‐ ured at the load.

$$
\dot{W}\_{net} = I\_{net} \cdot V\_{net} \tag{4}
$$

*exph* =(*H* −*Ho*) −*To*(*S* −*So*) (11)

Exergy Analysis of 1.2 kW Nexa™ Fuel Cell Module

http://dx.doi.org/10.5772/50602

1

) by processes involving heat transfer and exchange of

*<sup>n</sup>* <sup>+</sup> *RTo*∑ *xn*ln*xn* (13)

*<sup>n</sup> – standard chemical exergy kJ* / *kg* ; *R - universal*

(12)

7

*k k*

*where:H -enthalpy kJ* / *kg* ;*H*o*-specific enthalpy at standard condition kJ* / *kg* ;*S- entropy kJ* / *kgK* ;*S*o*- specific entropy at standard condition kJ* / *kgK* ;*T*o*- ambient standard temperature*

The physical exergy of an ideal gas with constant specific heat *cp* and specific heat ratio *k* can

1 ln ln *<sup>o</sup> o oo*

*T TP* - é ù æö æö ê ú = -- + ç÷ ç÷ èø èø ë û

*Chemical exergy:* The chemical exergy is associated with the released of chemical composition of a system from that of the environment. Chemical exergy is equal to the maximum amount of work obtainable when the substance under consideration is brought from the environmental

substances only with the environment. The specific chemical exergy at *Po* can be calculated by

The chemical exergies of gaseous fuels are computed from the stoichiometric combustion chemical reactions. The standard chemical exergies of various fuels can found in the literature.

*Mass flow rates of the products and the reactants:* Depending on the power output (*m*˙ *elect*), and a fuel cell voltage (*Vcell*), and the stoichiometry of air(), the mass flow rates of the reactants and the products in the fuel cell can be easily evaluated from the equations used by Larminie

To calculate the mass flow rate of reactant air, we must to know the oxygen usage firstly. From the basic operation of the fuel cell, we know that four electrons are transferred for each mole of oxygen. So oxygen usage can be evaluated through the following equation:

bringing the pure component in chemical equilibrium with the environment.

*exch* <sup>=</sup>∑ *xnech*

*T TP ex c T*

*ph p*

where: *P -pressure (atm), Po – standard pressure (atm).*

The chemical exergy can be calculated from [11, 12] as:

state (*To*, *Po*) to the dead state (*To*, *Po*, *Ϛoi*

where: *xn - molar fraction of component n, ech*

*gas constant kJ* / *kmolK .*

and Dicks.

*[K];*

be written as:

where: *Inet -current measured at the external load* (*A*) *, Vnet -voltage measured at the external load* (*V* ) *.*

Assuming all hydrogen is reacted, for every mole of hydrogen consumed, two moles of elec‐ trons become available. Using Faraday's constant (F), the mass flow rate of hydrogen *m*˙ *<sup>H</sup>*<sup>2</sup> and the number of cells in the stack, a theoretical current for the NEXA™ power module can be found using following equation:

$$I\_{Nexa} = \frac{2 \cdot F \cdot \dot{m}\_{H\_2}}{47} \tag{5}$$

The total theoretical electrical power output of the Nexa can be computed using the voltage of the stack (*V Nexa*).

$$
\dot{W}\_{\text{elect}} = I\_{\text{Nexa}} \cdot V\_{\text{Nexa}} \tag{6}
$$

Parasitic loads are estimated as the difference between the primary load and the theoretical electrical power calculated from fuel consumption because power consumption by the indi‐ vidual NEXA™ Sub systems was not measured.

*Actual exergy:* We already know it before, that the actual exergy is a difference between the exergy of the reactants and the exergy of the products. So intend to calculate the actual exer‐ gy we must to know the sub exergies (total exergy). The total exergy of the reactants and the products can be determined through the following equations:

$$
\dot{E}\_{a\dot{r},R} = \dot{m}\_{a\dot{r},R} \left( e\mathbf{x}\_{ch} + e\mathbf{x}\_{ph} \right)\_{a\dot{r}r,R} \tag{7}
$$

$$
\dot{E}\_{H\_{2},R} = \dot{m}\_{H\_{2},R} (e\chi\_{ch} + e\chi\_{ph})\_{H\_{2},R} \tag{8}
$$

$$\dot{E}\_{air,P} = \dot{m}\_{air,P} (e\chi\_{ch} + e\chi\_{ph})\_{air,P} \tag{9}$$

$$
\dot{E}\_{H\_2O,P} = \dot{m}\_{H\_2O,P} \left( e\mathbf{x}\_{ch} + e\mathbf{x}\_{ph} \right)\_{H\_2O,P} \tag{10}
$$

where: *exph - physical exergy kJ* / *kg ; exch - chemical exergy kJ* / *kg ; m*˙ *– mass flow rates of the reactants and products kg* /*s .*

*Physical exergy:* Physical exergy, known also as thermo mechanical exergy, is the work ob‐ tainable by taking the substance through reversible process from its initial state (*T* , *P*) to the state of the environment (*To*, *Po*). The general expression of the physical exergy can be de‐ scribed as:

$$e\boldsymbol{x}\_{ph} = \left(\boldsymbol{H} - \boldsymbol{H}\_o\right) - \boldsymbol{T}\_o \left(\boldsymbol{S} - \boldsymbol{S}\_o\right) \tag{11}$$

*where:H -enthalpy kJ* / *kg* ;*H*o*-specific enthalpy at standard condition kJ* / *kg* ;*S- entropy kJ* / *kgK* ;*S*o*- specific entropy at standard condition kJ* / *kgK* ;*T*o*- ambient standard temperature [K];*

The physical exergy of an ideal gas with constant specific heat *cp* and specific heat ratio *k* can be written as:

$$\text{acc}\_{ph} = \mathcal{C}\_p T\_o \left[ \frac{T}{T\_o} - 1 - \ln \left( \frac{T}{T\_o} \right) + \ln \left( \frac{P}{P\_o} \right)^{\frac{k-1}{k}} \right] \tag{12}$$

where: *P -pressure (atm), Po – standard pressure (atm).*

for Better Environment

be found using following equation:

vidual NEXA™ Sub systems was not measured.

products can be determined through the following equations:

*load* (*V* ) *.*

6 Clean Energy

of the stack (*V Nexa*).

*reactants and products kg* /*s .*

scribed as:

*<sup>W</sup>*˙ *net* <sup>=</sup> *Inet* <sup>⋅</sup>*Vnet* (4)

<sup>47</sup> (5)

*<sup>W</sup>*˙ *elect* <sup>=</sup> *INexa* <sup>⋅</sup>*<sup>V</sup> Nexa* (6)

*<sup>E</sup>*˙ *air*,*<sup>R</sup>* <sup>=</sup>*m*˙ *air*,*R*(*exch* <sup>+</sup> *exph* )*air*,*<sup>R</sup>* (7)

*<sup>E</sup>*˙ *<sup>H</sup>*2,*<sup>R</sup>* <sup>=</sup>*m*˙ *<sup>H</sup>*2,*R*(*exch* <sup>+</sup> *exph* )*H*2,*<sup>R</sup>* (8)

*<sup>E</sup>*˙ *air*,*<sup>P</sup>* <sup>=</sup>*m*˙ *air*,*P*(*exch* <sup>+</sup> *exph* )*air*,*<sup>P</sup>* (9)

*<sup>E</sup>*˙ *<sup>H</sup>*2*O*,*<sup>P</sup>* <sup>=</sup>*m*˙ *<sup>H</sup>*2*O*,*P*(*exch* <sup>+</sup> *exph* )*H*2*O*,*<sup>P</sup>* (10)

where: *Inet -current measured at the external load* (*A*) *, Vnet -voltage measured at the external*

Assuming all hydrogen is reacted, for every mole of hydrogen consumed, two moles of elec‐ trons become available. Using Faraday's constant (F), the mass flow rate of hydrogen *m*˙ *<sup>H</sup>*<sup>2</sup> and the number of cells in the stack, a theoretical current for the NEXA™ power module can

<sup>2</sup> <sup>⋅</sup> *<sup>F</sup>* <sup>⋅</sup> *<sup>m</sup>*˙ *<sup>H</sup>* <sup>2</sup>

The total theoretical electrical power output of the Nexa can be computed using the voltage

Parasitic loads are estimated as the difference between the primary load and the theoretical electrical power calculated from fuel consumption because power consumption by the indi‐

*Actual exergy:* We already know it before, that the actual exergy is a difference between the exergy of the reactants and the exergy of the products. So intend to calculate the actual exer‐ gy we must to know the sub exergies (total exergy). The total exergy of the reactants and the

where: *exph - physical exergy kJ* / *kg ; exch - chemical exergy kJ* / *kg ; m*˙ *– mass flow rates of the*

*Physical exergy:* Physical exergy, known also as thermo mechanical exergy, is the work ob‐ tainable by taking the substance through reversible process from its initial state (*T* , *P*) to the state of the environment (*To*, *Po*). The general expression of the physical exergy can be de‐

*INexa* =

*Chemical exergy:* The chemical exergy is associated with the released of chemical composition of a system from that of the environment. Chemical exergy is equal to the maximum amount of work obtainable when the substance under consideration is brought from the environmental state (*To*, *Po*) to the dead state (*To*, *Po*, *Ϛoi* ) by processes involving heat transfer and exchange of substances only with the environment. The specific chemical exergy at *Po* can be calculated by bringing the pure component in chemical equilibrium with the environment. The chemical exergy can be calculated from [11, 12] as:

$$e\,\mathbf{x}\_{ch} = \sum \mathbf{x}\_{n} e\_{ch}^{n} + RT\_{o} \sum \mathbf{x}\_{n} \text{ln}\mathbf{x}\_{n} \tag{13}$$

where: *xn - molar fraction of component n, ech <sup>n</sup> – standard chemical exergy kJ* / *kg* ; *R - universal gas constant kJ* / *kmolK .*

The chemical exergies of gaseous fuels are computed from the stoichiometric combustion chemical reactions. The standard chemical exergies of various fuels can found in the literature.

*Mass flow rates of the products and the reactants:* Depending on the power output (*m*˙ *elect*), and a fuel cell voltage (*Vcell*), and the stoichiometry of air(), the mass flow rates of the reactants and the products in the fuel cell can be easily evaluated from the equations used by Larminie and Dicks.

To calculate the mass flow rate of reactant air, we must to know the oxygen usage firstly. From the basic operation of the fuel cell, we know that four electrons are transferred for each mole of oxygen. So oxygen usage can be evaluated through the following equation:

$$\text{Oxygen } \iota \text{ asage} = \dot{m}\_{O\_2} - \\\\ \dot{m}\_{O\_2} = \frac{32 \cdot 10^{-3} \cdot W\_{\text{det}}}{4 \cdot F \cdot V\_{\text{cell}}} = 8.29 \times 10^{-8} \left(\frac{W\_{\text{det}}}{V\_{\text{cell}}}\right) \tag{14}$$

*ex* =*exph* + *exch* (19)

Exergy Analysis of 1.2 kW Nexa™ Fuel Cell Module

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9

The exergy analysis of a PEM fuel cell system is defined on the 1.2kW NexaTM PEM power module taken from Ballard Power Systems Inc, the NexaTM module installed at Fuel Cell Laboratory, Institute of Physics and Technology, Mongolian Academy of Sciences in 2010.

*Nexa™ power module description:* This module is capable of providing 1.2kW of unregulat‐ ed DC output. The output module voltage level can vary from 43V at no load to about 26V at the full load. By the way an increasing of load, we were taken the increasing da‐ ta of the current density at real time. The designed operating temperature in the stack is

individual fuel cell element consists of two electrodes, the anode and the cathode, sepa‐ rated by a polymer membrane electrolyte. Each of the electrodes is coated on one side with a thin platinum catalyst layer. The electrodes, catalyst and membrane together form

**Figure 1.** Main components and principle for operation of the NexaTM PEM power module (Adapted from Ballard,

A single fuel cell element produces about 1V at open-circuit and about 0.6V at full current

midification, and the hydrogen pressure to the stack is normally maintained at 0.3 bar. Oxy‐ gen comes from the ambient air. The pressure of the oxidant air is 0.1 bar, and the air is humidified through a built in humidity exchanger to maintain membrane saturation and prolong the life of the membrane. Any drying of the PEM will greatly reduce the life of the fuel cell system. A humidity exchanger transfers both fuel cell product water and heat from the wet cathode outlet to the dry incoming air. The excess product water is discharged from

. The fuel is 99.99% hydrogen with no hu‐

C at the full load. There are totally 47 cells connected in series in the stack. A

**3. Discussion and results**

the membrane electrode assembly.

around 65o

Power systems inc., 2004).

output. The geometric area of the cells is 120 cm2

At standard atmospheric conditions, the air molar analysis (%) would be: 77.48 N2, 20.59 O2, 0.03 CO2 and 1.9 H2O. Therefore molar proportion of air that is oxygen is approximately 0.21, and the molar mass of air is 28.97⋅10-3 kg mol-1. So the air inlet flow rate or mass flow rate of reactant air can be evaluated through the following equation:

$$\begin{aligned} \text{Air inlet flow rate} &= \dot{m}\_{air,R} - \\ \dot{m}\_{air,R} &= \frac{28.97 \cdot 10^{-3} \cdot \lambda \cdot W\_{\text{elect}}}{0.21 \cdot 4 \cdot F \cdot V\_{\text{cell}}} = 3.57 \times 10^{-7} \left(\frac{\lambda \, W\_{\text{act}}}{V}\right) \end{aligned} \tag{15}$$

The exit air flow rate or mass flow rate of the product air can be defined as the difference between the amount of air inlet flow rate and amount of oxygen usage:

Using equations (14), (15) this becomes:

$$\begin{aligned} \text{Exit air flow rate} &= m\_{air, P} - \\ m\_{air, P} &= 3.57 \times 10^{-7} \left( \frac{\lambda \dot{W}\_{elect}}{V} \right) - 8.29 \times 10^{-8} \left( \frac{\dot{W}\_{elect}}{V} \right) \end{aligned} \tag{16}$$

The hydrogen usage or mass flow rate of reactant hydrogen is derived in a way similar to oxygen, except that there are two electrons from each mole of hydrogen. So hydrogen usage can be evaluated through the following equation:

$$\text{Hydrogen usage} = \dot{m}\_{\text{H}\_2\text{R}} \rightarrow \dot{m}\_{\text{H}\_2\text{R}} \rightarrow \dot{m}\_{\text{H}\_2\text{R}} \rightarrow \frac{2.02 \cdot 10^{-3} \cdot \dot{W}\_{\text{slect}}}{2 \cdot \text{F} \cdot \text{V}\_{\text{cell}}} = 1.05 \cdot 10^{-8} \cdot \left(\frac{\dot{W}\_{\text{slect}}}{\text{V}}\right) \tag{17}$$

In a hydrogen-fed fuel cell, water is produced at the rate of one mole for every two elec‐ trons. The molecular mass of water is 18.02⋅10-3 kg mole-1. The amount of water produced by the fuel cell can be calculated by the following equation:

*Water production* =*m*˙ *<sup>H</sup>*2*O*,*P*→ *<sup>m</sup>*˙ H2O,P <sup>=</sup> 18.02 <sup>⋅</sup> <sup>10</sup>−<sup>3</sup> <sup>⋅</sup> *<sup>W</sup>*˙ elect <sup>2</sup> <sup>⋅</sup> <sup>F</sup> <sup>⋅</sup> Vcell =9.34⋅10−<sup>8</sup> <sup>⋅</sup> ( *<sup>W</sup>*˙ elect <sup>V</sup> ) (18)

Negligible to potential and kinetic energy effects on the fuel cell electrochemical process, the total exergy transfer per unit mass of each reactant and product consists of the combination of both physical and chemical exergies:

$$e\mathbf{x} = e\boldsymbol{\chi}\_{\rm plt} + e\boldsymbol{\chi}\_{\rm ch} \tag{19}$$

## **3. Discussion and results**

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8 Clean Energy

Using equations (14), (15) this becomes:

can be evaluated through the following equation:

*Oxygen usage* =*m*˙ *<sup>O</sup>*<sup>2</sup>

rate of reactant air can be evaluated through the following equation:

*Air inlet flow rate* =*m*˙ *air*,*R*→

between the amount of air inlet flow rate and amount of oxygen usage:

*Exit air flow rate* =*mair*,*P*→ *mair*,*<sup>P</sup>* =3.57×10<sup>−</sup>7( *<sup>λ</sup>W*˙ *elect*

*Hydrogen usage* =*m*˙ H2,R→

*Water production* =*m*˙ *<sup>H</sup>*2*O*,*P*→

*<sup>m</sup>*˙ H2O,P <sup>=</sup> 18.02 <sup>⋅</sup> <sup>10</sup>−<sup>3</sup> <sup>⋅</sup> *<sup>W</sup>*˙ elect

*<sup>m</sup>*˙ H2,R <sup>=</sup> 2.02 <sup>⋅</sup> <sup>10</sup>−<sup>3</sup> <sup>⋅</sup> *<sup>W</sup>*˙ elect

the fuel cell can be calculated by the following equation:

of both physical and chemical exergies:

*<sup>m</sup>*˙ *air*,*<sup>R</sup>* <sup>=</sup> 28.97 <sup>⋅</sup> <sup>10</sup>−<sup>3</sup> <sup>⋅</sup> *<sup>λ</sup>* <sup>⋅</sup> *<sup>W</sup>*˙ *elect*

<sup>=</sup> <sup>32</sup> <sup>⋅</sup> <sup>10</sup>−<sup>3</sup> <sup>⋅</sup> *<sup>W</sup>*˙ *elect*

*m*˙ *<sup>O</sup>*<sup>2</sup>

→

<sup>4</sup> <sup>⋅</sup> *<sup>F</sup>* <sup>⋅</sup> *Vcell* =8.29×10<sup>−</sup>8( *<sup>W</sup>*˙ *elect*

At standard atmospheric conditions, the air molar analysis (%) would be: 77.48 N2, 20.59 O2, 0.03 CO2 and 1.9 H2O. Therefore molar proportion of air that is oxygen is approximately 0.21, and the molar mass of air is 28.97⋅10-3 kg mol-1. So the air inlet flow rate or mass flow

0.21 <sup>⋅</sup> <sup>4</sup> <sup>⋅</sup> *<sup>F</sup>* <sup>⋅</sup> *Vcell* =3.57×10<sup>−</sup>7( *<sup>λ</sup>W*˙ *elect*

The exit air flow rate or mass flow rate of the product air can be defined as the difference

The hydrogen usage or mass flow rate of reactant hydrogen is derived in a way similar to oxygen, except that there are two electrons from each mole of hydrogen. So hydrogen usage

<sup>2</sup> <sup>⋅</sup> <sup>F</sup> <sup>⋅</sup> Vcell =1.05⋅10−<sup>8</sup> <sup>⋅</sup> ( *<sup>W</sup>*˙ elect

In a hydrogen-fed fuel cell, water is produced at the rate of one mole for every two elec‐ trons. The molecular mass of water is 18.02⋅10-3 kg mole-1. The amount of water produced by

<sup>2</sup> <sup>⋅</sup> <sup>F</sup> <sup>⋅</sup> Vcell =9.34⋅10−<sup>8</sup> <sup>⋅</sup> ( *<sup>W</sup>*˙ elect

Negligible to potential and kinetic energy effects on the fuel cell electrochemical process, the total exergy transfer per unit mass of each reactant and product consists of the combination

*<sup>V</sup>* ) <sup>−</sup>8.29×10<sup>−</sup>8( *<sup>W</sup>*˙ *elect*

*Vcell*

) (14)

*<sup>V</sup>* ) (15)

*<sup>V</sup>* ) (16)

<sup>V</sup> ) (17)

<sup>V</sup> ) (18)

The exergy analysis of a PEM fuel cell system is defined on the 1.2kW NexaTM PEM power module taken from Ballard Power Systems Inc, the NexaTM module installed at Fuel Cell Laboratory, Institute of Physics and Technology, Mongolian Academy of Sciences in 2010.

*Nexa™ power module description:* This module is capable of providing 1.2kW of unregulat‐ ed DC output. The output module voltage level can vary from 43V at no load to about 26V at the full load. By the way an increasing of load, we were taken the increasing da‐ ta of the current density at real time. The designed operating temperature in the stack is around 65o C at the full load. There are totally 47 cells connected in series in the stack. A individual fuel cell element consists of two electrodes, the anode and the cathode, sepa‐ rated by a polymer membrane electrolyte. Each of the electrodes is coated on one side with a thin platinum catalyst layer. The electrodes, catalyst and membrane together form the membrane electrode assembly.

**Figure 1.** Main components and principle for operation of the NexaTM PEM power module (Adapted from Ballard, Power systems inc., 2004).

A single fuel cell element produces about 1V at open-circuit and about 0.6V at full current output. The geometric area of the cells is 120 cm2 . The fuel is 99.99% hydrogen with no hu‐ midification, and the hydrogen pressure to the stack is normally maintained at 0.3 bar. Oxy‐ gen comes from the ambient air. The pressure of the oxidant air is 0.1 bar, and the air is humidified through a built in humidity exchanger to maintain membrane saturation and prolong the life of the membrane. Any drying of the PEM will greatly reduce the life of the fuel cell system. A humidity exchanger transfers both fuel cell product water and heat from the wet cathode outlet to the dry incoming air. The excess product water is discharged from the system, as both liquid and vapor in the exhaust. There is a small compressor supplying excess oxidant air to the fuel cell, and the speed of the compressor can be adjusted to match the power demand from the fuel cell stack.

formance of certain key fuel cells, which are termed "purge cells". In response to the purge cell voltage, a hydrogen purge valve at the stack outlet is periodically opened to flush out inert constituents in the anode and restore performance. Only a small amount of hydrogen purges from the system, less than one percent of the overall fuel consump‐ tion rate. Purged hydrogen is discharged into the cooling air stream before it leaves the Nexa™ system, as shown in Figure 2. Hydrogen quickly diffuses into the cooling air stream and is diluted to levels many times less than the lower flammability limit. The hydrogen leak detector, situated in the cooling air exhaust, ensures that flammable lim‐

Exergy Analysis of 1.2 kW Nexa™ Fuel Cell Module

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11

**Figure 3.** Polarization, power density curves at different temperatures and with different gas flows.

Nexa™ module during normal operation.

At high current levels, more heat is generated. It is important to keep the fuel cell stack tem‐ perature at a constant operating temperature; therefore, the fuel cell stack temperature has to be controlled. Fuel cell systems are either liquid-cooled or air-cooled. The Nexa™ fuel cell stack is air-cooled. A cooling fan located at the base of the unit blows air through vertical cooling channels in the fuel cell stack. The fuel cell operating temperature is maintained at 65°C by varying the speed of the cooling fan. The fuel cell stack temperature is measured at the cathode air exhaust, as shown in Figure 2. Hot air from the cooling system may be used for thermal integration purposes. Heat rejected in the air can be used for integration with metal hydrides, for evolving hydrogen. Hot air may also be used for space heating in some cases. The cooling system is also used to dilute hydrogen that is purposely purged from the

Nexa™ system operation is automated by an electronic control system. The control board receives various input signals from onboard sensors. Input signals to the control board in‐

its are not reached.

The NexaTM fuel cell module stack is air-cooled; the cooling fan draws air from the ambient sur‐ roundings in order to cool the fuel cell stack and regulate the operating temperature. Onboard sensors monitor system performance and the control board and microprocessor fully auto‐ mate operation. The Nexa™ system also incorporates operational safety systems for indoor operation [13]. In figure 1 illustrates all components and subsystems of Nexa™ power module.

**Figure 2.** Schematic of the Nexa™ power module module (Adapted from Ballard, Power systems inc., 2004).

Figure 2 illustrates the schematic diagram of Nexa™ system. Hydrogen, oxidant air, and cooling air must be supplied to the unit, as shown in Figure 2. Exhaust air, product water and coolant exhaust is emitted.

The fuel-supply system, as shown in Figure 2, monitors and regulates the supply of hy‐ drogen to the fuel cell stack. The fuel cell stack is pressurized with hydrogen during op‐ eration. The regulator assembly continually replenishes hydrogen, which is consumed in the fuel cell reaction. Nitrogen and product water in the air stream slowly migrates across the fuel cell membranes and gradually accumulates in the hydrogen stream. The accumulation of nitrogen and water in the anode results in the steady decrease in per‐ formance of certain key fuel cells, which are termed "purge cells". In response to the purge cell voltage, a hydrogen purge valve at the stack outlet is periodically opened to flush out inert constituents in the anode and restore performance. Only a small amount of hydrogen purges from the system, less than one percent of the overall fuel consump‐ tion rate. Purged hydrogen is discharged into the cooling air stream before it leaves the Nexa™ system, as shown in Figure 2. Hydrogen quickly diffuses into the cooling air stream and is diluted to levels many times less than the lower flammability limit. The hydrogen leak detector, situated in the cooling air exhaust, ensures that flammable lim‐ its are not reached.

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the power demand from the fuel cell stack.

and coolant exhaust is emitted.

the system, as both liquid and vapor in the exhaust. There is a small compressor supplying excess oxidant air to the fuel cell, and the speed of the compressor can be adjusted to match

The NexaTM fuel cell module stack is air-cooled; the cooling fan draws air from the ambient sur‐ roundings in order to cool the fuel cell stack and regulate the operating temperature. Onboard sensors monitor system performance and the control board and microprocessor fully auto‐ mate operation. The Nexa™ system also incorporates operational safety systems for indoor operation [13]. In figure 1 illustrates all components and subsystems of Nexa™ power module.

**Figure 2.** Schematic of the Nexa™ power module module (Adapted from Ballard, Power systems inc., 2004).

Figure 2 illustrates the schematic diagram of Nexa™ system. Hydrogen, oxidant air, and cooling air must be supplied to the unit, as shown in Figure 2. Exhaust air, product water

The fuel-supply system, as shown in Figure 2, monitors and regulates the supply of hy‐ drogen to the fuel cell stack. The fuel cell stack is pressurized with hydrogen during op‐ eration. The regulator assembly continually replenishes hydrogen, which is consumed in the fuel cell reaction. Nitrogen and product water in the air stream slowly migrates across the fuel cell membranes and gradually accumulates in the hydrogen stream. The accumulation of nitrogen and water in the anode results in the steady decrease in per‐

**Figure 3.** Polarization, power density curves at different temperatures and with different gas flows.

At high current levels, more heat is generated. It is important to keep the fuel cell stack tem‐ perature at a constant operating temperature; therefore, the fuel cell stack temperature has to be controlled. Fuel cell systems are either liquid-cooled or air-cooled. The Nexa™ fuel cell stack is air-cooled. A cooling fan located at the base of the unit blows air through vertical cooling channels in the fuel cell stack. The fuel cell operating temperature is maintained at 65°C by varying the speed of the cooling fan. The fuel cell stack temperature is measured at the cathode air exhaust, as shown in Figure 2. Hot air from the cooling system may be used for thermal integration purposes. Heat rejected in the air can be used for integration with metal hydrides, for evolving hydrogen. Hot air may also be used for space heating in some cases. The cooling system is also used to dilute hydrogen that is purposely purged from the Nexa™ module during normal operation.

Nexa™ system operation is automated by an electronic control system. The control board receives various input signals from onboard sensors. Input signals to the control board in‐ clude: fuel cell stack temperature, hydrogen pressure, hydrogen leak concentrations, fuel cell stack current, air mass flow, fuel cell stack voltage and purge cell voltage.

tively. Hydrogen pressure data and air stoichiometric ratio values we taken from measured data. With increasing current density the hydrogen pressure decreases and its inlet air mass

Variation of stoichiometric air ratio (λ) illustrated in Figure 5a. The mass flow rate of the product air, can be defined as the difference between the amount of oxygen in the elec‐ trochemical reaction and the amount of oxygen consumed by reacting with hydrogen to produce water. The products, water flow rate and unused air calculated through Eq. (16,

**Figure 5.** (a) Hydrogen mass flow rate and stoichiometric air ratio (b) Products flow rates of NexaTM fuel cell module.

In Figure 5b illustrated, water production rate has small amount and production air rate in‐ creases depending on current density increase. Values of the chemical exergies for both the reactants and products are taken from published literature [10] and presented in Table 1.

**Chemical exergy,** *exch* **(kJ/kg)**

The physical exergies of product water and product air calculated from Eq. (11, 19) was used to determine the physical exergies of inlet air and hydrogen. The values of fuel pressure and

NexaTM module 0 159138 2.5 8.58 - MEA by Pt/MWCNT catalyst - 159138 2.5 0 246

**Table 1.** Chemical exergy of the reactants and products of NexaTM module and MEA by Pt/MWCNT catalyst.

The total exergy of the reactants and the products can be determined Eq. (15-18).

**Reactant H2**

**Product H2O**

**Product Air**

Exergy Analysis of 1.2 kW Nexa™ Fuel Cell Module

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**Reactant O2**

flow rate increases as shown in Figure 4.

**Reactant/Product Reactant Air**

operation temperature taken from measured data.

18), respectively.

*Exergy analysis of NEXATMfuel cell module:* The analysis is conducted on cell operating voltag‐ es from 0.001 to 0.84 V at air stoichiometrics from 13.0 to 2.1 in order to determine their ef‐ fects on the efficiency of the fuel cell. The calculations of the physical and chemical exergies, mass flow rates and exergetic efficiency are performed at *temperature ratios* (*T/To*) (*Exit air flow rate* = *Air inlet flow rate* - *oxygen usage*) and *pressure ratios* (*P/Po*) ranging from 0.93 to 1.13 and 7.44 to 4.91, respectively.

In Figure 3 shown calculated load characteristics represent cell voltage depending on cur‐ rent density (I-V curve).

**Figure 4.** Reactants flow rates and hydrogen pressure of Nexa™ fuel cell module.

From the measured data, we calculated the cell voltage and current density by Eq. (19), Eq. (20).

$$V\_{cell} = \frac{V}{47} \tag{20}$$

$$J = \frac{I}{120} \tag{21}$$

where: *V – output voltage* [V] ; *47 – number of stack, J – current density* [A/cm2 ]; *I – output cur‐ rent* [A]; *120 – geometric area of the cell* [cm2 ].

In Figure 4 illustrated the dependence of hydrogen pressure and mass flow rates of the inlet air. Mass flow rates of the inlet air and hydrogen were calculated from Eq. (15, 17), respec‐ tively. Hydrogen pressure data and air stoichiometric ratio values we taken from measured data. With increasing current density the hydrogen pressure decreases and its inlet air mass flow rate increases as shown in Figure 4.

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12 Clean Energy

to 1.13 and 7.44 to 4.91, respectively.

rent density (I-V curve).

Eq. (20).

clude: fuel cell stack temperature, hydrogen pressure, hydrogen leak concentrations, fuel

*Exergy analysis of NEXATMfuel cell module:* The analysis is conducted on cell operating voltag‐ es from 0.001 to 0.84 V at air stoichiometrics from 13.0 to 2.1 in order to determine their ef‐ fects on the efficiency of the fuel cell. The calculations of the physical and chemical exergies, mass flow rates and exergetic efficiency are performed at *temperature ratios* (*T/To*) (*Exit air flow rate* = *Air inlet flow rate* - *oxygen usage*) and *pressure ratios* (*P/Po*) ranging from 0.93

In Figure 3 shown calculated load characteristics represent cell voltage depending on cur‐

From the measured data, we calculated the cell voltage and current density by Eq. (19),

<sup>47</sup> (20)

<sup>120</sup> (21)

]; *I – output cur‐*

*Vcell* <sup>=</sup> *<sup>V</sup>*

*<sup>J</sup>* <sup>=</sup> *<sup>I</sup>*

].

In Figure 4 illustrated the dependence of hydrogen pressure and mass flow rates of the inlet air. Mass flow rates of the inlet air and hydrogen were calculated from Eq. (15, 17), respec‐

where: *V – output voltage* [V] ; *47 – number of stack, J – current density* [A/cm2

cell stack current, air mass flow, fuel cell stack voltage and purge cell voltage.

**Figure 4.** Reactants flow rates and hydrogen pressure of Nexa™ fuel cell module.

*rent* [A]; *120 – geometric area of the cell* [cm2

Variation of stoichiometric air ratio (λ) illustrated in Figure 5a. The mass flow rate of the product air, can be defined as the difference between the amount of oxygen in the elec‐ trochemical reaction and the amount of oxygen consumed by reacting with hydrogen to produce water. The products, water flow rate and unused air calculated through Eq. (16, 18), respectively.

**Figure 5.** (a) Hydrogen mass flow rate and stoichiometric air ratio (b) Products flow rates of NexaTM fuel cell module.

In Figure 5b illustrated, water production rate has small amount and production air rate in‐ creases depending on current density increase. Values of the chemical exergies for both the reactants and products are taken from published literature [10] and presented in Table 1.


**Table 1.** Chemical exergy of the reactants and products of NexaTM module and MEA by Pt/MWCNT catalyst.

The physical exergies of product water and product air calculated from Eq. (11, 19) was used to determine the physical exergies of inlet air and hydrogen. The values of fuel pressure and operation temperature taken from measured data.

The total exergy of the reactants and the products can be determined Eq. (15-18).

Finally, exergy efficiency was calculated by Eq. (2). The energy efficiency of the system can be calculated from Eq. (17) using the experimental *W*˙ *net* and *m*˙ *<sup>H</sup>*2,*R* data [7]:

$$\eta\_{energy, system} = \frac{\dot{W}\_{\text{act}}}{\{HH \, V\_{H\_2} \cdot \dot{m}\_{H\_2}\}\_{R}} \tag{22}$$

and 1 ml Nafion® 5 % solution under sonication. The dispersion was stirred with a high shear mixer at 7000 rpm. For the anode, 200 mg of Pt on carbon (20 wt% Pt, Alfa Aesar HISPEC® 3000) were dispersed in 4 ml of H2O and 2 ml isopropanol. 1.2 ml of Nafion® 5 % solution was added, and the solution was dispersed by sonication and stirred with a high shear mixer. The inks were filled into an airbrush pistol (Evolution by Harder & Steenbeck) and sprayed successive‐ ly onto the heated membrane surface, allowing each layer to dry for 10 seconds. Hydrogen and oxygen or air reactants are fed to the anode and cathode compartments, respectively, with or without pre - humidifying. Usually, the cell is conditioned by operating at low loading to acti‐ vate the MEA. After that, the polarization curve is recorded galvanostatically by stepping the current from zero to the maximum test current density (Figure 7). The polarization curve is ef‐ fective and intuitional to characterize the performance. However, separation of the electro‐ chemical and ohmic contributions to polarization requires additional experimental techniques. This can be done by measuring the electrochemical impedance spectroscopy. The flow rates of both gases were adjusted to H2/O2 55/25 sccm and 83/38 sccm, respectively, and the cell temperatures varied between 25°C, 50°C, and 65°C. Hydrogen was loaded with water in a humidifier (25 °C) and fed into the anode. The voltage at each current density is allowed to stabilize before measurement. MEAs were conditioned overnight until a steady state current achieved at a potential of 0.6V. The operation of the fuel cell test station was controlled and

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**Figure 7.** Polarization and power density curves at different temperatures and with different gas flows.

*Exergy analysis on the Membrane-electrode assembly by the synthesized Pt/MWCNT catalyst:* Based on this exergy analysis of NexaTM power module, we calculated the exergy efficiency on membrane-electrode assembly by the synthesized Pt/MWCNT catalyst. The MEAs fabri‐

monitored by LabView programs [15].

where: *HHV – higher heating value* [MJ/kg]; *m*˙ *<sup>H</sup>*2,*<sup>R</sup> – reactant hydrogen mass flow rate which cal‐ culated by Equation (17) .*

Figure 6 illustrated the power productions and calculated energy, exergy efficiencies at dif‐ ferent current density values. It can be seen that energy and exergy efficiencies decreases while power production increases.

**Figure 6.** Energy, exergy efficiency and power of NexaTM fuel cell module depending on current density.

Energy and Exergy efficiency of 1.2 kW NexaTM power module decreases depending on cur‐ rent density increases. Energy efficiencies vary between 41 – 55%, exergy efficiency from 33% to 42% at the current density of 0.02 - 0.36A/cm2 respectively. From the Figure 6, in‐ creasing of flow rates and decreasing of hydrogen pressure caused to the decreasing of the energy and exergy efficiencies of the module.

*Fabrication of Membrane-electrode assembly by the synthesized Pt/MWCNT catalyst:* The prepara‐ tion of the MEA was carried out using a Nafion® 117 membrane from Dupont. For both electro‐ des an ink solution was prepared using a method slightly modified from the one reported by Gottesfeld et al., [14]. MEAs with an active electrode area of 25 cm2 were fabricated by air‐ brushing the catalyst ink onto one side of the Nafion membrane, heated to and kept at 120 °C. For the cathode, 100 mg of the catalyst were dispersed in 0.5 ml ultrapure water, isopropanol and 1 ml Nafion® 5 % solution under sonication. The dispersion was stirred with a high shear mixer at 7000 rpm. For the anode, 200 mg of Pt on carbon (20 wt% Pt, Alfa Aesar HISPEC® 3000) were dispersed in 4 ml of H2O and 2 ml isopropanol. 1.2 ml of Nafion® 5 % solution was added, and the solution was dispersed by sonication and stirred with a high shear mixer. The inks were filled into an airbrush pistol (Evolution by Harder & Steenbeck) and sprayed successive‐ ly onto the heated membrane surface, allowing each layer to dry for 10 seconds. Hydrogen and oxygen or air reactants are fed to the anode and cathode compartments, respectively, with or without pre - humidifying. Usually, the cell is conditioned by operating at low loading to acti‐ vate the MEA. After that, the polarization curve is recorded galvanostatically by stepping the current from zero to the maximum test current density (Figure 7). The polarization curve is ef‐ fective and intuitional to characterize the performance. However, separation of the electro‐ chemical and ohmic contributions to polarization requires additional experimental techniques. This can be done by measuring the electrochemical impedance spectroscopy. The flow rates of both gases were adjusted to H2/O2 55/25 sccm and 83/38 sccm, respectively, and the cell temperatures varied between 25°C, 50°C, and 65°C. Hydrogen was loaded with water in a humidifier (25 °C) and fed into the anode. The voltage at each current density is allowed to stabilize before measurement. MEAs were conditioned overnight until a steady state current achieved at a potential of 0.6V. The operation of the fuel cell test station was controlled and monitored by LabView programs [15].

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14 Clean Energy

while power production increases.

Finally, exergy efficiency was calculated by Eq. (2). The energy efficiency of the system can

(*HH <sup>V</sup> <sup>H</sup>* <sup>2</sup>

where: *HHV – higher heating value* [MJ/kg]; *m*˙ *<sup>H</sup>*2,*<sup>R</sup> – reactant hydrogen mass flow rate which cal‐*

Figure 6 illustrated the power productions and calculated energy, exergy efficiencies at dif‐ ferent current density values. It can be seen that energy and exergy efficiencies decreases

<sup>⋅</sup> *<sup>m</sup>*˙ *<sup>H</sup>* <sup>2</sup> ) *R* (22)

be calculated from Eq. (17) using the experimental *W*˙ *net* and *m*˙ *<sup>H</sup>*2,*R* data [7]:

*<sup>η</sup>energy*,*system* <sup>=</sup> *<sup>W</sup>*˙ *elect*

**Figure 6.** Energy, exergy efficiency and power of NexaTM fuel cell module depending on current density.

33% to 42% at the current density of 0.02 - 0.36A/cm2

energy and exergy efficiencies of the module.

Energy and Exergy efficiency of 1.2 kW NexaTM power module decreases depending on cur‐ rent density increases. Energy efficiencies vary between 41 – 55%, exergy efficiency from

creasing of flow rates and decreasing of hydrogen pressure caused to the decreasing of the

*Fabrication of Membrane-electrode assembly by the synthesized Pt/MWCNT catalyst:* The prepara‐ tion of the MEA was carried out using a Nafion® 117 membrane from Dupont. For both electro‐ des an ink solution was prepared using a method slightly modified from the one reported by Gottesfeld et al., [14]. MEAs with an active electrode area of 25 cm2 were fabricated by air‐ brushing the catalyst ink onto one side of the Nafion membrane, heated to and kept at 120 °C. For the cathode, 100 mg of the catalyst were dispersed in 0.5 ml ultrapure water, isopropanol

respectively. From the Figure 6, in‐

**Figure 7.** Polarization and power density curves at different temperatures and with different gas flows.

*Exergy analysis on the Membrane-electrode assembly by the synthesized Pt/MWCNT catalyst:* Based on this exergy analysis of NexaTM power module, we calculated the exergy efficiency on membrane-electrode assembly by the synthesized Pt/MWCNT catalyst. The MEAs fabri‐ cated from the Pt/MWCNT performed well, and the polarization curves and power densities in different operation conditions can be found in Figure 7.

erating temperature (in our case up to 720

**Author details**

**References**

C), and improved by a higher operating pressure.

Exergy Analysis of 1.2 kW Nexa™ Fuel Cell Module

http://dx.doi.org/10.5772/50602

17

From the result of the exergy efficiency of membrane - electrode assembly of the Pt/ MWCNT higher than NexaTM power module, it might be explained fed pure oxygen (O2), in

Department of Material Science and Nanotechnology, Institute of Physics and Technology,

[1] Barbir, F. (2005). *PEM Fuel Cells Theory and Practice*, Elsevier Academic Press.

[2] Basu, S. (2007). *Recent Trends in Fuel Cell Science and Technology*, Anamaya Publishers,

[3] Gencoglu, M. T., & Ural, Z. (2009). Design of PEM fuel system for residential applica‐

[4] Zhang, Jiujun. (2008). *PEM fuel cell electrocatalysts and catalyst layers*, Springer, Verlag

[5] Smith, J. M., Van Ness, H. C., & Abbott, M. M. (2001). *Introduction to chemical engineer‐*

[6] Dincer, I., & Rosen, M. A. (2007). *Exergy: Energy, environment and sustainable develop‐*

[7] Zang, Z., Huang, X., & Jiang, J. (2006). An improved dynamic model considering ef‐ fects of temperature and equivalent internal resistance for PEM fuel cell power mod‐

[8] Kazim, Ayoub. (2004). Exergy analysis of a PEM fuel cell at variable operating condi‐

[9] Kirubakaran, A., Sh, Jain, & Nema, R. K. (2009). The PEM Fuel Cell System with DC/DC Boost Converter: Design, Modeling and Simulation. *J. Recent Trends in Engi‐*

[10] Larminie, J., & Dicks, A. (2003). *Fuel cell systems explained*, John Wiley & Sons Ltd.

other hand a high air stoichiometry could be improve a fuel cell exergitic efficiency.

G. Sevjidsuren, E. Uyanga, B. Bumaa, E. Temujin, P. Altantsog and D. Sangaa

\*Address all correspondence to: sevjidsuren@ipt.ac.mn

tion. *J. Hydrogen Energy*, 34, 5242-5248.

*ing thermodynamics*, McGraw-Hill.

ules. *J. Power Sources*, 161, 1062-1068.

tions. *Energy Conversion and Management*, 45, 1949-1961.

Mongolian Academy of Sciences, Mongolia

New Delhi, India.

London Ltd.

*neering*, 1, 157-161.

*ment*.

At the current density up to 0.2 A/cm2 , the exergy efficiency decreases from 72% to 35% as shown in Figure 8. We can explain that the exergy efficiency of 72% at the low current densi‐ ty is caused by the only one MEA and to feed pure oxygen (O2). Therefore, the activation loss is main effect to the rapid decrease of the exergy efficiency.

**Figure 8.** Exergy efficiency and power output of PEM fuel cell.

## **4. Conclusion**

We performed the exergy analysis of 1.2 kW NexaTM power module at variable operating conditions such as a different temperatures, pressures, cell voltages and stoichiometry.

The total exergy of the reactants and the products consist of both physical and chemical ex‐ ergies, which are calculated for each element in the electrochemical process. Through the ex‐ perimental data, we were calculated flow rates, energy efficiency, physical, chemical exergy and exergy efficiency.

The results provided that exergy efficiencies of the PEM fuel cell module less than energy efficiencies. From our calculation, it is recommended that the PEM fuel cell should operate at stoichiometric ratios less than 4 in order to optimize the relative humidity level in the product air and to avoid the membrane drying out at high operating temperatures. Exergy efficiency of the NEXA™ PEM fuel cell can be improved through increasing the fuel cell op‐ erating temperature (in our case up to 720 C), and improved by a higher operating pressure. From the result of the exergy efficiency of membrane - electrode assembly of the Pt/ MWCNT higher than NexaTM power module, it might be explained fed pure oxygen (O2), in other hand a high air stoichiometry could be improve a fuel cell exergitic efficiency.

## **Author details**

for Better Environment

16 Clean Energy

At the current density up to 0.2 A/cm2

**Figure 8.** Exergy efficiency and power output of PEM fuel cell.

**4. Conclusion**

and exergy efficiency.

in different operation conditions can be found in Figure 7.

loss is main effect to the rapid decrease of the exergy efficiency.

cated from the Pt/MWCNT performed well, and the polarization curves and power densities

shown in Figure 8. We can explain that the exergy efficiency of 72% at the low current densi‐ ty is caused by the only one MEA and to feed pure oxygen (O2). Therefore, the activation

We performed the exergy analysis of 1.2 kW NexaTM power module at variable operating conditions such as a different temperatures, pressures, cell voltages and stoichiometry.

The total exergy of the reactants and the products consist of both physical and chemical ex‐ ergies, which are calculated for each element in the electrochemical process. Through the ex‐ perimental data, we were calculated flow rates, energy efficiency, physical, chemical exergy

The results provided that exergy efficiencies of the PEM fuel cell module less than energy efficiencies. From our calculation, it is recommended that the PEM fuel cell should operate at stoichiometric ratios less than 4 in order to optimize the relative humidity level in the product air and to avoid the membrane drying out at high operating temperatures. Exergy efficiency of the NEXA™ PEM fuel cell can be improved through increasing the fuel cell op‐

, the exergy efficiency decreases from 72% to 35% as

G. Sevjidsuren, E. Uyanga, B. Bumaa, E. Temujin, P. Altantsog and D. Sangaa

\*Address all correspondence to: sevjidsuren@ipt.ac.mn

Department of Material Science and Nanotechnology, Institute of Physics and Technology, Mongolian Academy of Sciences, Mongolia

## **References**


[11] Cengel, Y., & Boles, M. (1994). *Thermodynamics-an engineering approach* (2nd ed.), Mc Graw-Hill, Inc.

**Chapter 2**

**Grid-Connected Wind Park with Combined Use of**

Over the last decade, driven by limited fossil fuel supply, global warming, and the provision of tax credit or financial support for renewable power production, sustainable power gener‐ ations such as wind power and photovoltaic power etc. have been increasingly integrated into the existing power grids. Among these renewable power generations, wind power has various advantages such as large per unit capacity and easily construction of a large-scale generating station, which lead to a relatively lower generation cost over the other ones, and hence be considered as one of the mature renewable energy alternatives to the conventional fuel-based resources [1]. Fig. 1 and Fig. 2 gives the change tendency of the total introduction capacity and annual introduction capacity of wind power generation in the world [ 2]. It can be known from these figures that the wind power generation has been significantly in‐

However, the main challenge of wind power utilization is associated with the fluctuation and unpredictability in its power generation. Fig. 3 gives a typical wind power output curve based on the measurement data [3]. This data is a good example to obviously indicate the facts that the wind power consists of both considerable fast/short-term fluctuation and slow/ long-term variation. Generally, wind parks are located in the remote area and interconnect‐ ed into the distribution system via a relatively long tie transmission line. Along with the in‐ creasing integration of wind power into a power grid, due to the fluctuation of wind power, considerable fluctuation of power flow in the tie transmission line (the line that connects the wind park to the power grid) may occurs and lead to some problems such as power quality degradation, voltage instability and insufficient available power transferability, etc., and hence imposes difficulties both in terms of operation and planning. These issues need to be

> © 2012 Wu et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

creased in the recent years and is expected to increase further in the later future.

**Battery and EDLC Energy Storages**

Additional information is available at the end of the chapter

Guohong Wu, Yutaka Yoshida and

Tamotsu Minakawa

**1. Introduction**

http://dx.doi.org/10.5772/51521

