Thermodynamics Assignment Report

7/31/2019 Thermodynamics Assignment Report

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Submitted To: Dr. T.P. Ashok Babu

Wednesday, 05 September 2012

Submitted By:

A. Vinay Bharath 11M101 Avinash Kumar 11M133

Gagandeep K V 11M145

MD. Shahid 11M177


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REPORT

FUEL CELL

Basics

A fuel cell uses the chemical energy of hydrogen to cleanly and efficiently produce

electricity with water and heat as by products. Fuel cells are unique in terms of the variety of their potential applications; they can provide energy for systems as large as a utility power station and

as small as a laptop computer.

Fuel cells have several benefits over conventional combustion-based technologies currently used

in many power plants and passenger vehicles. They emit no emissions at the point of operation,

including greenhouse gases and air pollutants that create smog and cause health problems. On a

life-cycle basis, if pure hydrogen is used as a fuel, fuel cells emit only heat and water as by

products.
http://www.gophoto.it/view.php?i=/wiki/File:Fuel_cell_NASA_p48600ac.jpg


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How Does A Fuel Cell Works?

A fuel cell is a device that uses a fuel and oxygen to create electricity by an electrochemical

process. A single fuel cell consists of an electrolyte and two catalyst-coated electrodes (a porous

anode and cathode). While there are different fuel cell types, all fuel cells work similarly:

charged electrons from positively charged ions (protons).

with species such as

protons or water, resulting in water or hydroxide ions, respectively.

electrolyte to the cathode to combine with oxygen and electrons, producing water and heat.

cells, negative ions travel through the

electrolyte to the anode where they combine with hydrogen to generate water and electrons.

e electrolyte to the positively charged

cathode; they must travel around it via an electrical circuit to reach the other side of the cell. This

movement of electrons is an electrical current.
http://en.wikipedia.org/wiki/File:PEM_fuelcell.svg


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Fuel Cell Systems

The design of fuel cell systems is complex and can vary significantly depending upon fuel

cell type and application. However, most fuel cell systems consist of four basic components:

Fuel cell stack

Fuel processor

Current inverters and conditioners

Heat recovery system

Most fuel cell systems also include other components and subsystems to control fuel cell humidity,

temperature, gas pressure, and wastewater.

Fuel Cell Stack

The fuel cell stack is the heart of a fuel cell power system. It generates electricity in the form

of direct current (DC) from chemical reactions that take place in the fuel cell. A single fuel cellproduces enough electricity for only the smallest applications. Therefore, individual fuel cells are

typically combined in series into a fuel cell stack. A typical fuel cell stack may consist of hundreds

of fuel cells. The amount of power produced by a fuel cell depends upon several factors, such as

fuel cell type, cell size, the temperature at which it operates, and the pressure at which the gases

are supplied to the cell.

Fuel ProcessorThe fuel processor converts fuel into a form useable by the fuel cell. If hydrogen is fed to

the system, a processor may not be required, or it may be needed only to filter impurities out of the

hydrogen gas.

If the system is powered by a hydrogen-rich, conventional fuel, such as methanol, gasoline, diesel,

or gaseous coal, a reformer is typically used to convert hydrocarbons into a gas mixture of

hydrogen and carbon compounds called "reformate." In many cases, the reformate is then sent toanother reactor to remove impurities, such as carbon oxides or sulphur, before it is sent to the fuel


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cell stack. This process prevents impurities in the gas from binding with the fuel cell catalysts. This

binding process is also called "poisoning" because it reduces the efficiency and life expectancy of

the fuel cell.

Some fuel cells, such as molten carbonate and solid oxide fuel cell operate at temperatures high

enough that the fuel can be reformed in the fuel cell itself. This type is called internal reforming.

Fuel cells that use internal reforming still need traps to remove impurities from the unreformed fuel

before it reaches the fuel cell.

Both internal and external reforming release carbon dioxide, but less than the amount emitted by

internal-combustion engines, such as those used in gasoline-powered vehicles.

Current Inverters and Conditioners

Current inverters and conditioners adapt the electrical current from the fuel cell to suit the

electrical needs of the application, whether it is a simple electrical motor or a complex utility power

grid.

Fuel cells produce electricity in the form of direct current (DC). In a direct current circuit, electricity

flows in only one direction. The electricity in your home and workplace is in the form of alternating

current (AC), which flows in both directions on alternating cycles. If the fuel cell is used to power

equipment using AC, the direct current will have to be converted to alternating current.

Both AC and DC power must be conditioned. Power conditioning includes controlling current flow

(amperes), voltage, frequency, and other characteristics of the electrical current to meet the needs

of the application. Conversion and conditioning reduce system efficiency only slightly, around 2%

6%.

Heat Recovery System

Fuel cell systems are not primarily used to generate heat. However, because significant

amounts of heat are generated by some fuel cell systems especially those that operate at high

temperatures, such as solid oxide and molten carbonate systems this excess energy can be

used to produce steam or hot water or to be converted to electricity via a gas turbine or other

technology. These methods increase the overall energy efficiency of the systems.


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Types of Fuel Cells

Fuel cells are classified primarily by the kind of electrolyte they employ. This classification

determines the kind of chemical reactions that take place in the cell, the kind of catalysts required,

the temperature range in which the cell operates, the fuel required, and other factors. These

characteristics, in turn, affect the applications for which these cells are most suitable. There are

several types of fuel cells currently under development, each with its own advantages, limitations,

and potential applications.

-Polymer electrolyte fuel cell

-Direct methanol fuel cells

- Alkaline fuel cells

-Phosphoric acid fuel cells

-Molten carbonate fuel cells

-Solid oxide fuel cells

-Regenerative fuel cells

Polymer Electrolyte Membrane (PEM) Fuel Cells


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Polymer electrolyte membrane (PEM) fuel cells also called proton exchange membrane fuel

cells deliver high-power density and offer the advantages of low weight and volume,

compared with other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous

carbon electrodes containing a platinum catalyst. They need only hydrogen, oxygen from the air,

and water to operate and do not require corrosive fluids like some fuel cells. They are typicallyfuelled with pure hydrogen supplied from storage tanks or on-board reformers.

Polymer electrolyte membrane fuel cells operate at relatively low temperatures, around 80C

(176F). Low-temperature operation allows them to start quickly (less warm-up time) and results in

less wear on system components, resulting in better durability. However, it requires that a noble-

metal catalyst (typically platinum) be used to separate the hydrogen's electrons and protons,

adding to system cost. The platinum catalyst is also extremely sensitive to CO poisoning, making it

necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen is derived

from an alcohol or hydrocarbon fuel. This also adds cost. Developers are currently exploring

platinum/ruthenium catalysts that are more resistant to CO.

PEM fuel cells are used primarily for transportation applications and some stationary applications.

Due to their fast start up time, low sensitivity to orientation, and favourable power-to-weight ratio,

PEM fuel cells are particularly suitable for use in passenger vehicles, such as cars and buses.

A significant barrier to using these fuel cells in vehicles is hydrogen storage. Most fuel cell vehicles

(FCVs) powered by pure hydrogen must store the hydrogen on-board as a compressed gas in

pressurized tanks. Due to the low-energy density of hydrogen, it is difficult to store enough

hydrogen on-board to allow vehicles to travel the same distance as gasoline-powered vehicles

before refuelling, typically 300 400 miles. Higher-density liquid fuels, such as methanol, ethanol,

natural gas, liquefied petroleum gas, and gasoline, can be used for fuel, but the vehicles must

have an on-board fuel processor to reform the methanol to hydrogen. This requirement increases

costs and maintenance. The reformer also releases carbon dioxide (a greenhouse gas), though

less than that emitted from current gasoline-powered engines.

Direct Methanol Fuel Cells

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or

can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol,

ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered bypure methanol, which is mixed with steam and fed directly to the fuel cell anode.


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Direct methanol fuel cells do not have many of the fuel storage problems typical of some fuel

cells because methanol has a higher energy density than hydrogen though less than gasoline or

diesel fuel. Methanol is also easier to transport and supply to the public using our current

infrastructure because it is a liquid, like gasoline.

Direct methanol fuel cell technology is relatively new compared with that of fuel cells powered by

pure hydrogen, and DMFC research and development is roughly 3 4 years behind that for other

fuel cell types.

Alkaline Fuel Cells

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the

first type widely used in the U.S. space program to produce electrical energy and water on-board

space crafts. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and

can use a variety of non-precious metals as a catalyst at the anode and cathode. High-

temperature AFCs operate at temperatures between 100C and 250C (212F and 482F).However, newer AFC designs operate at lower temperatures of roughly 23C to 70C (74F to

158F)

AFCs' high performance is due to the rate at which chemical reactions take place in the cell. They

have also demonstrated efficiencies near 60% in space applications.

The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide (CO 2). In fact,

even the small amount of CO 2 in the air can affect this cell's operation, making it necessary to

purify both the hydrogen and oxygen used in the cell. This purification process is costly.


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Susceptibility to poisoning also affects the cell's lifetime (the amount of time before it must be

replaced), further adding to cost.

Cost is less of a factor for remote locations, such as space or under the sea. However, to

effectively compete in most mainstream commercial markets, these fuel cells will have to become

more cost-effective. AFC stacks have been shown to maintain sufficiently stable operation for

more than 8,000 operating hours. To be economically viable in large-scale utility applications,

these fuel cells need to reach operating times exceeding 40,000 hours, something that has not yet

been achieved due to material durability issues. This obstacle is possibly the most significant in

commercializing this fuel cell technology.

Phosphoric Acid Fuel Cells

Phosphoric acid fuel cells use liquid phosphoric acid as an

electrolyte the acid is contained in a Teflon-bonded silicon carbide

matrix and porous carbon electrodes containing a platinum

catalyst. The chemical reactions that take place in the cell are

shown in the diagram to the right.

The phosphoric acid fuel cell (PAFC) is considered the "firstgeneration" of modern fuel cells. It is one of the most mature cell

types and the first to be used commercially. This type of fuel cell is

typically used for stationary power generation, but some PAFCs

have been used to power large vehicles such as city buses.

PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than

PEM cells, which are easily "poisoned" by carbon monoxide because carbon monoxide binds to

the platinum catalyst at the anode, decreasing the fuel cell's efficiency. They are 85% efficient

when used for the co-generation of electricity and heat but less efficient at generating electricity

alone (37% 42%). This is only slightly more efficient than combustion-based power plants, which

typically operate at 33% 35% efficiency. PAFCs are also less powerful than other fuel cells, given

the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are

also expensive. Like PEM fuel cells, PAFCs require an expensive platinum catalyst, which raises

the cost of the fuel cell.

Molten Carbonate Fuel Cells


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Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based

power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature

fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a

porous, chemically inert ceramic lithium aluminium oxide (LiAlO 2) matrix. Because they operate at

extremely high temperatures of 650C (roughly 1,200F) and above, non-precious metals can be

used as catalysts at the anode and cathode, reducing costs.

Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric

acid fuel cells (PAFCs). Molten carbonate fuel cells, when coupled with a turbine, can reach

efficiencies approaching 65%, considerably higher than the 37% 42% efficiencies of a phosphoric

acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be as

high as 85%.

Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, MCFCs do

not require an external reformer to convert more energy-dense fuels to hydrogen. Due to the high

temperatures at which MCFCs operate, these fuels are converted to hydrogen within the fuel cell

itself by a process called internal reforming, which also reduces cost.

Molten carbonate fuel cells are not prone to carbon monoxide or carbon dioxide "poisoning" they

can even use carbon oxides as fuel making them more attractive for fuelling with gases made

from coal. Because they are more resistant to impurities than other fuel cell types, scientists

believe that they could even be capable of internal reforming of coal, assuming they can be made


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resistant to impurities such as sulphur and particulates that result from converting coal, a dirtier

fossil fuel source than many others, into hydrogen.

The primary disadvantage of current MCFC technology is durability. The high temperatures at

which these cells operate and the corrosive electrolyte used accelerate component breakdown

and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials

for components as well as fuel cell designs that increase cell life without decreasing performance.

Solid Oxide Fuel Cells


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Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte.

Because the electrolyte is a solid, the cells do not have to be constructed in the plate-like

configuration typical of other fuel cell types. SOFCs are expected to be around 50% 60% efficient

at converting fuel to electricity. In applications designed to capture and utilize the system's waste

heat (co-generation), overall fuel use efficiencies could top 80% 85%.

Solid oxide fuel cells operate at very high temperatures around 1,000C (1,830F). High-

temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also

allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces

the cost associated with adding a reformer to the system.

SOFCs are also the most sulphur-resistant fuel cell type; they can tolerate several orders of

magnitude more of sulphur than other cell types. In addition, they are not poisoned by carbon

monoxide (CO), which can even be used as fuel. This property allows SOFCs to use gases made

from coal.

High-temperature operation has disadvantages. It results in a slow start up and requires significant

thermal shielding to retain heat and protect personnel, which may be acceptable for utility

applications but not for transportation and small portable applications. The high operating

temperatures also place stringent durability requirements on materials. The development of low-

cost materials with high durability at cell operating temperatures is the key technical challenge

facing this technology.

Scientists are currently exploring the potential for developing lower-temperature SOFCs operating

at or below 800C that have fewer durability problems and cost less. Lower-temperature SOFCs

produce less electrical power, however, and stack materials that will function in this lower

temperature range have not been identified.

Regenerative Fuel Cells

Regenerative fuel cells produce electricity from hydrogen and oxygen and generate heat and

water as by products, just like other fuel cells. However, regenerative fuel cell systems can also

use electricity from solar power or some other source to divide the excess water into oxygen and

hydrogen fuel this process is called "electrolysis." This is a comparatively young fuel cell

technology being developed by NASA and others.


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Parts of a Fuel Cell

Polymer electrolyte membrane (PEM) fuel cells are the current focus of research for fuel cell

vehicle applications. PEM fuel cells are made from several layers of different materials, as shown

in the diagram. The three key layers in a PEM fuel cell include:

1. Membrane electrode assembly

2. Catalyst

3. Hardware

Other layers of materials are designed to help draw fuel and air into the cell and to conduct

electrical current through the cell.

Membrane Electrode Assembly

The electrodes (anode and cathode), catalyst, and polymer electrolyte membrane together

form the membrane electrode assembly (MEA) of a PEM fuel cell.


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Anode. The anode, the negative side of the fuel cell, has several jobs. It conducts the

electrons that are freed from the hydrogen molecules so they can be used in an external

circuit. Channels etched into the anode disperse the hydrogen gas equally over the surface

of the catalyst.

Cathode. The cathode, the positive side of the fuel cell, also contains channels thatdistribute the oxygen to the surface of the catalyst. It conducts the electrons back from the

external circuit to the catalyst, where they can recombine with the hydrogen ions and

oxygen to form water.

Polymer electrolyte membrane. The polymer electrolyte membrane (PEM) a specially

treated material that looks something like ordinary kitchen plastic wrap conducts only

positively charged ions and blocks the electrons. The PEM is the key to the fuel cell

technology; it must permit only the necessary ions to pass between the anode and cathode.Other substances passing through the electrolyte would disrupt the chemical reaction.

The thickness of the membrane in a membrane electrode assembly can vary with the type of

membrane. The thickness of the catalyst layers depends upon how much platinum (Pt.) is used in

each electrode. For catalyst layers containing about 0.15 milligrams (mg) Pt./cm 2, the thickness of

the catalyst layer is close to 10 micrometres (m) less than half the thickness of a sheet of

paper. This membrane/electrode assembly, with a total thickness of about 200 m (or 0.2 mm),

can generate more than half an ampere of current for every square centimetre of assembly area at

a voltage of 0.7 volts, but only when encased in well-engineered components backing layers,

flow fields, and current collectors.

Catalyst

All electrochemical reactions in a fuel cell consist of two separate reactions: an oxidation half-

reaction at the anode and a reduction half-reaction at the cathode. Normally, the two half-reactionswould occur very slowly at the low operating temperature of the PEM fuel cell. Each of the

electrodes is coated on one side with a catalyst layer that speeds up the reaction of oxygen and

hydrogen. It is usually made of platinum powder very thinly coated onto carbon paper or cloth. The

catalyst is rough and porous so the maximum surface area of the platinum can be exposed to the

hydrogen or oxygen. The platinum-coated side of the catalyst faces the PEM. Platinum-group

metals are critical to catalysing reactions in the fuel cell, but they are very expensive. DOE's goal

is to reduce the use of platinum in fuel cell cathodes by at least a factor of 20 or eliminate italtogether to decrease the cost of fuel cells to consumers.


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1. Hardware

The backing layers, flow fields, and current collectors are designed to maximize the current from a

membrane/electrode assembly. The backing layers one next to the anode, the other next to the

cathode are usually made of a porous carbon paper or carbon cloth, about as thick as 4 to 12

sheets of paper. The backing layers have to be made of a material (like carbon) that can conduct

the electrons that leave the anode and enter the cathode. The porous nature of the backing

material ensures effective diffusion (flow of gas molecules from a region of high concentration to a

region of low concentration) of each reactant gas to the catalyst on the membrane/electrode

assembly. The gas spreads out as it diffuses so that when it penetrates the backing, it will be in

contact with the entire surface area of the catalysed membrane.

The backing layers also help in managing water in the fuel cell; too little or too much water can

cause the cell to stop operating. Water can build up in the flow channels of the plates or can clog

the pores in the carbon cloth (or carbon paper), preventing reactive gases from reaching the

electrodes.


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The correct backing material allows the right amount of water vapour to reach the

membrane/electrode assembly and keep the membrane humidified. The backing layers are often

coated with Teflon to ensure that at least some, and preferably most, o f the pores in the carbon

cloth (or carbon paper) does not become clogged with water, which would prevent the rapid gas

diffusion necessary for a good rate of reaction at the electrodes.

Pressed against the outer surface of each backing layer is a piece of hardware called a bipolar

plate that typically serves as both flow field and current collector. In a single fuel cell, these two

plates are the last of the components making up the cell. The plates are made of a lightweight,

strong, gas-impermeable, electron-conducting material graphite or metals are commonly

used even though composite plates are now being developed.

The first task served by each plate is to provide a gas "flow field." Channels are etched into the

side of the plate next to the backing layer. The channels carry the reactant gas from the place

where it enters the fuel cell to the place where it exits. The pattern of the flow field in the plate (as

well as the width and depth of the channels) has a large impact on how evenly the reactant gases

are spread across the active area of the membrane/electrode assembly. Flow field design also

affects water supply to the membrane and water removal from the cathode.

Each plate also acts as a current collector. Electrons produced by the oxidation of hydrogen must(1) be conducted through the anode, through the backing layer, along the length of the stack, and

through the plate before they can exit the cell; (2) travel through an external circuit, and (3) re-

enter the cell at the cathode plate. With the addition of the flow fields and current collectors, the

PEM fuel cell is complete; only a load-containing external circuit, such as an electric motor, is

required for electric current to flow.

Reference: www.eere1.gov.in


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In thermodynamics, the exergy of a system is the maximum useful work possible during a

process that brings the system into equilibirium with a heat reservoir. When the surroundings are

the reservoirs, exergy is the potential of a system to cause a change as it achieves equilibiriumwith its environment. Exergy is the energy that is available to be used. After the system and

surroundings reach equilibirium, the exergy is zero. Determining exergy was also the the first goal

of thermodynamics.

Energy is never destroyed during a process; it changes from one form to another (first lawof thermodynamics). In contrast, exergy accounts for the irreversibility of a process due toincrease in entropy (second law of thermodynamics). In thermodynamics, a change in thethermodynamic state of a system and all of its surroundings cannot be precisely restored to itsinitial state by infinitesimal changes in some property of the system without expenditure of energy.

A system that undergoes an irreversible process may still be capable of returning to its initial state;however, because entropy is a state function, the change in entropy of a system is the samewhether the process is reversible or irreversible. The second law of thermodynamics can be usedto determine whether a process is reversible or not.

Exergy is always destroyed when a process involves a temperature change. Thisdestruction is proportional to the entropy increase of the system together with its surroundings.The destroyed exergy has been called anergy. For an isothermal process exergy and energy areinterchangeable terms, and there is no anergy.

Exergy is a combination property of a system and its environment because unlike energy itdepends on the state of both the system and environment. The exergy of a system in equilibriumwith the environment is zero. Exergy is neither a thermodynamic property of matter nor athermodynamic potential of a system.

Energy is the concept to be conserved so that the energy flowing in must be equal to the sum of the energy stored within the system and the energy flowing out from the system.

This energy balance can be expressed as follows.

(Energy input) = (Energy stored) + (Energy output) ... (1.1)

Since the steady-state condition is being assumed here, there is no energy storage and


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hence the above equation turns out to be the following simpler form.

(Energy input) = (Energy output) ... (1.2)

Secondly, let us set up the entropy equation consistent with the above two equations.

Energy flowing into the system as heat is more or less dispersed energy. Heat is aenergy transfer due to dispersion, thus entropy necessarily flows into the system as heat

flows in and some amount of entropy is generated inevitably within the system in the

course of heat transmission. The sum of the entropy input and the entropy generated

must be in part stored or in part flows out of the system. Therefore the entropy balance

equation can be expressed in the following form.

(Entropy input) + (Entropy generated) = (Entropy stored) + (Entropy output) (1.3)

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Since the steady-state condition is being assumed, there is no entropy storage as well as

no energy storage. Therefore, the above entropy balance equation turns out to be

(Entropy input) + (Entropy generated) = (Entropy output) . (1.4)

The fact that the outgoing entropy from the system includes the entropy generated

within the system suggests that the system disposes of the generated entropy with the

entropy output.

Combining the energy and entropy balance equations brings about the exergy balance

equation. Entropy (or entropy rate) has a dimension of J/K (or W/K) and energy (or

energy rate) has a dimension of J (or W). Therefore we need a kind of trick to combine

the two equations.

Generally speaking, energy contained by a body, which has an ability to disperse, is

called an energy resource. Such an energy resource exists within the environmental

space, which is filled with dispersed energy. The dispersed energy level of the resource

surrounded by the environmental space can be expressed as the product of the entropy

contained by the resource and its environmental temperature in the Kelvin scale. The

same expression applies to the waste discarded by the system. Therefore the entropybalance equation can be rewritten as follows.


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(Entropy input) Te+ (Entropy gener ated) Te= (Entropy output) Te (1.5)

Where Te is the environmental temperature. The product of entropy and environmental

temperature is called anergy, which implies dispersed ene rgy. Using the term

anergy, the above equation can be expressed in the following form, anergy balance equation.

(Anergy input) + (Anergy generated) = (Anergy output) (1.6)

Provided that anergy is a portion of energy that is already disperse d, then the other

portion is not yet dispersed. Stating in another way, energy consists of two parts: the

dispersed part and the part, which can disperse. The latter is exergy. Now let us take

the difference of the two equations, energy balance equation (1.2) and anergy balance

equation (1.6). This operation brings about

[(Energy input) (Anergy input)] (Anergy generated) = [(Energy output)

(Anergy output)] (1.7)

Anergy generated is such energy that originally had an abil ity to disperse and that has

just dispersed. We can state this in the other way; that is, exergy is consumed. Anergy

generation is equivalent to exergy consumption. Using the term exergy, the above

equation can be reduced to the following equation.

(Exergy input) (Exergy consumed) = (Exergy output) .(1.8)

This is the exergy balance equation for a system under steady-state condition such as the

building envelope system. Exergy consumed, which is equivalent to

anergy generated, is the product of entropy generated and the environmental

temperature.

(Exergy consumed) = (Environmental temperature) x (Entropy generated) (1.9)

Exergy consumed is exactly proportional to the entropy generated with the proportional constant of environmental temperature.


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Here in this section we briefly present an exergy analysis of the PEM fuel cell system. Note that

determination of an effective utilization of a PEM fuel cell and measuring its true performancebased on

thermodynamic laws are considered to be extremely essential. 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 exergeticefficiency of a PEM

fuel cell, which is the ratio of the electrical output over the maximum possible work output, couldgive a true

measure of the PEM fuel cells performance. Energy analysis performed on a system based on thesecond law of

thermodynamics is known as exergy analysis [4]. In the fuel cell module, a basic reaction occursas below.

H2 + Air H2O + Unused Air (Oxygen-depleted Air) + Electrical Power + Heat

The exergy efficiency of a fuel cell system is the ratio of the power output, over the exergy of thereactants (air + hydrogen), which can be determined by following formula [5, 6]:

Here, if the potential and kinetic exergies are neglected, the total specific exergy transfer consistsof the combination of both physical and chemical exergies as

The physical exergy is calculated from:

ex ph =( h h o ) - T o( s s o )

If reactant gases are assumed asideal gases, it results in


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The chemical exergy is associated with the departure of the chemical composition of a systemfrom that of the environment. The chemical exergy can be calculated from [e.g., 4, 6, 7]:

For the sake of simplicity, the chemical exergy considered in the analysisis rather a standard chemical

exergy that is based on the standard values of the environmentaltemperature of 298 K and pressure of 1 atm.

Generally, these values are in good agreement with the calculatedchemical exergyrelative to alternative

specifications of the environment. The values of the chemical exergies for the reactants are taken from published

literature [e.g., 4] as listed Table 2.Hydrogen and air are assumed tosupply to the fuel cell module at room

temperature, 300 K. Air enters to the module dry and it is h eated at thecell operating temperature.

In this study, exergy analysis of the fuel cell system was carried out toevaluate the fuel cell efficiency and following assumptions and values areused for an exergy analysis:

Flow of reactants is steady, incompressible and laminar.

All gases are ideal gases. Kinetic and potential exergies are neglected.


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Dead state pressure is 1 bar and dead state temperature is 298 K.

Finally, calculated exergy and energy efficiencies are presented in Fig.11. Energy efficiencies of the

module for this experimental setup are between 44% and 30% whenexergy efficiencies vary from 38% to 24.5%

at the current density of 0.047 to 0.348 respectively. It can be said thatenergy and exergy efficiency decreases

because of reactants flow rates and hydrogen pressure. Reactants flowrates increases depending on load

increasing. However, hydrogen pressure decreases when output currentincreases.

Conclusions

In this paper, we have conducted a brief thermodynamic analysis, interms of energy and exergy, of a 1.2 kWp

Nexa PEM fuel cell system in order to investigate its performance interms of energy and exergy efficiencies at

different operating conditions. The results show that the PEM fuel cellsystem has some high irreversibilities,


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exergy destructions, resulting in lesser exergy efficiencies compared tothe corresponding energy efficiencies.

The future work will concentrate of full-scale exergoeconomic analysis of

the system independently and as a partof the solar-hydrogen energy system.


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Thermodynamics Assignment Report

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