Fuel Cell Lecture 02

8/11/2019 Fuel Cell Lecture 02

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Chapter 02

Fuel Cell Thermodynamics

Lecture Notes

Dr. Sammia Shahid


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What is Thermodynamics?

Thermo

means heat" and Dynamicsrelates to "movement"; in essence

thermodynamics studies the movement of

heat energy and how that energy makesmechanical movement (i.e. does work).


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Thermodynamics is a science about theeffects of changes in temperature,

pressure, and volume

and how these

changes effect a physical system.

(e.g. a car engine, an air conditioner)


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When Energy flows from a hot object to acold object, the energy is called Heat

Heat is the energy that flows from one object to

another due to a temperature difference.


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Fast moving atoms with a lot of random motion collide with slower moving

atoms.

As kinetic energy is transferred from the fast moving atoms to the slowermoving atoms, we say that the warmer side gave up heat to the colder side and

that heat was transferred.

Two objects in contact on a microscopic level:

Fast

movingSlow

moving


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What is Temperature?

Temperature is a measurement of the average thermalenergy

of the particles in a substance.

Heat flows due to temperature differences.

No heat is transferred between two objects that are at thesame temperature (i.e. in thermal equilibrium). If two bodiesare in thermal equilibrium with a third body, they are also inthermal equilibrium with each other. This simple fact is known

as the zeroth law of thermodynamics.

A cup of boiling water is at the same temperature as a gallonof boiling water, but the gallon of boiling water has morethermal energy than the cup.


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How does Heat flow?

Conduction

the transfer

of heat energy by makingdirect contact with theatoms/molecules of thehotter object

Convection

the transferof heat due to a bulkmovement of matter fromhotter to colder areas

Radiation

energy

transferred byelectromagnetic waves


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Conduction

When two objects are in direct contact,particles in the hotter object are moving

faster and will collide with slower

moving objects in the colder object.

When this happens, heat flows.

Energy is transferred from the hot objectto the cold object.


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Convection

Buoyancy forces

cause bulk

movement of the

water.

www.physics.arizona.ed


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More examples of Convection

Rising hot air and falling

cool air sets up convectioncells.


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Convection Ovens

A fan circulates the air so hot

air is not trapped at the top of

the oven. More cookies can

be baked at one time and all

will cook at the same rate.


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Heat Transfer from Radiation

All matter that has thermal energy will emit infraredelectromagnetic radiation.

We can feel this when we put our hands close to a fire.

This type of heat transfer requires no medium.Electromagnetic radiation travels at the speed of light

through a vacuum.

http://www.newt.com

http://www.charlesandhudson.com


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Energy

Definition: the capacity to do WORK

Units are Joules (J) = kg.m2/s2

(from KE=1/2mv2)

Work done on a system -

system gains energy (w +ve)

Work done by the system -

system loses energy (w -ve)

Heat absorbed by the system (endothermic) -

system gains energy (q +ve)Heat released by the system (exothermic) -

system loses energy (q +ve)

SYSTEM TOTAL ENERGY (kinetic plus potential) is the

INTERNAL ENERGY (U sometimes E)

Usually measure CHANGE in internal energy ( U )

U=Ufinal Uinitial

U is a STATE FUNCTION (independent of path)


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Energy

There are three broad concepts of energy:

Kinetic Energy

is the energy associated with anobject by virtue of its motion.

Potential Energy

is the energy an object has by

virtue of its position in a field of force.

Internal Energy

is the sum of the kinetic and

potential energies of the particles making up a

substance.


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A system is said to be in thermodynamic equilibrium if it maintains

thermal (uniform temperature), mechanical (uniform pressure),

phase (the mass of two phases, e.g., ice and liquid water, inequilibrium) and chemical equilibrium.

Equilibrium

Process

Any change from one state to another is called a process. In most ofthe processes that we will study, one thermodynamic property is held

constant. Some of these processes are:

Process Property held

constant

isobaric pressure

isothermal temperature

isochoric volume

isentropic entropy


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Energy

The Law of Conservation of Energy:Energy may be converted from one

form to another, but the total quantities

of energy remain constant.


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Heat and Work

Heat: transfer of energy that

changes motions of atoms in

the surroundings in a chaoticmanner

Work: transfer of energy that changes

motions of atoms in the surroundings

in a uniform manner= F x d


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Work (W)

Work involves the movement of matter from one

place to another. Examples of Work include:

Pressure Volume Work (Expansion work)

Electrical work

Mechanical work

In Thermodynamics work always involves the

exchange of energy between system and its

surroundings.


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Expansion Work

w = F x dWork

= Force x Distance


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Energy and Enthalpy

Constant Pressure:

E = q + w

w = work = -PVq = heat transferred

q = E + PV

qP

=

E + P

V

Constant Volume (V = 0): qV

= E


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Energy and Enthalpy

= Hproducts

-

Hreactants

H

Enthalpy change

or

Heat of reaction (at constant pressure)

qP

= E + PV =

H = Hfinal

-

HinitialEnthalpy is a state function whose

value depends only on the current

state of the system, not on the

path taken to arrive at that state.


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Fossil Fuels, Fuel Efficiency, and

Heats of Combustion

CO2

(g) + 2H2

O(l)CH4

(g) + 2O2

(g)


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Laws of Thermodynamics

Zeroth Law:

If two objects are in thermal equilibrium

with a third object, then they are also inthermal equilibrium with each other.

Thermal equilibrium means an objects temperature,

pressure, and volume are not changing.


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If two cups of coffee are at

thermal equilibrium with the room,

then the two cups are in thermalequilibrium with each other.

The two cups of coffee have the

same temperature.

If the two cups are put in contact

with each other no heat will flow.

Zeroth Law


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First Law of Thermodynamics: The good news!

Energy is Conserved. Energy can not be destroyed.

In an isolated system, the total energy stays the same.

Energy can be converted from one form to another.

Thermal Energy can be converted into another form ofenergy!


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What is Entropy?

Entropy = total disorder of an object/system

Disorder is the sum of the thermal energy plus

the physical disorder.

Entropy always increases with time!


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Examples of increasing entropy


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Energy flows in one direction

towards amore disordered state

Entropy


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+

Heat, light, ash,particulates, gases

Direction

Is possible

+

Heat, light, ash,

particulates, gases

Direction

Is impossible

Examples of increasing entropy


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An Introduction to Free Energy

Gibbs Free Energy Change (

G)

Entropy

change

G =

Enthalpy of

reaction

Temperature

(Kelvin)

H S- T


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An Introduction to Free Energy

Gibbs Free Energy Change (G)

G = H S- T

G < 0

Process is spontaneous

G = 0

Process is at equilibrium

(neither spontaneous nor nonspontaneous)

G > 0

Process is nonspontaneous


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The Second Law of Thermodynamics:

(The bad news!)

An isolated system gets more disordered with

time.

Entropy always increases with time.


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What does this mean to us?

It is impossible to construct an engine that

converts all its thermal energy into useful work.The exhaust must be hotter than the incomingair.

100% efficiency is impossible there must besome unusable energy because entropy mustincrease.

Were going to get old and die

The house is going to need cleaning again!

Wh i 100% ffi i th ti ll


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Why is 100% efficiency theoretically

impossible?

If machine operates ina cycle, some energy

must be used to resetthe machine.

Parts of machine will

absorb some of theheat.

Exhaust must behotter than incomingair, due to 2nd

law.This hot exhaustrepresents wastedenergy.


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Second Law of Thermodynamics

Maintenance of complex, dynamic

system requires energy input.

Then, energy conversions lose energy

to universe.

Without more energy input, disorder

(entropy) increases.


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Apply to human body

Input of energy

Losses

Energy used to maintain

organization

disorder death.


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Apply to environmental system

Fuel cells to power cars

2H2

(fuel) + O2

(from air) H2O + electricity

Electricity powers car

Technology exists!

Apollo program, Space shuttles

Working cars

Source of H2

??


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The Earths Thermodynamic System


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Energy Conversion of Fuels


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The combustion process is a chemical reaction whereby fuel is

oxidized and energy is released.

Fuels are usually composed of some compound or mixture containingcarbon, C, and hydrogen, H2

.

Examples of hydrocarbon fuels are

CH4

MethaneC8

H18

Octane

Coal

Mixture of C, H2

, S, O2

, N2

and non-combustibles

Initially, we shall consider only those reactions that go to completion.The components prior to the reaction are called reactants and the

components after the reaction are called products.


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Heat Engine

A heat engine is a thermodynamic system operating in a thermodynamic

cycle to which net heat is transferred and from which net work is delivered.

The system, or working fluid, undergoes a series of processes that constitutethe heat engine cycle.

The following figure illustrates a steam power plant as a heat engine

operating in a thermodynamic cycle.


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Steam Cars

Heavy

Slow to heat up and

start

Required carrying

both fuel and water

http://www.steamcar.net/my-85.html

S


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Steam Engine

Steam Engine

Heat generates steam increasing pressure

Pressurized steam does work driving a piston or shaft

Exhausted steam gets rid of waste heat

Being the cycle over

Fi l i hi l (EV)


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First electric vehicles (EV)

Edison worked on battery

storage believing that

electricity would power

future cars

In 1900, roughly a third ofall vehicles sold are EVs

EVs were marketed to

women and for urbanareas

Thomas Edison circa 1900

Morrisons 4-horse

power EV with arange of 50 miles.


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Early Gas powered cars

Karl Benz was thefirst to commercialize

a gas powered

motorwagon in 1885


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Why did EVs and Steamers fade away?

Gasoline and Diesel have high energydensities

Greatest need for cars and trucks was inrural areas, therefore long range was

needed.

Steamers too heavy on unpaved roads

Gas powered cars started quickly

Henry Ford perfected the assembly-line,making his cars the most affordable

Why was gasoline the chosen fuel


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Why was gasoline the chosen fuel

source for the automobile?

Gasoline has 1000X the energy as an equal

weight of batteries.

Gasoline has 4.5X more energy per gallon than

liquid hydrogen.

Gasoline has 2X the energy of coal for the sameweight

Gas has slightly less energy per volume as

veggie oil

Gasoline combines with Oxygen when it burns.

The Oxygen is free and does not have to becarried.


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Diesel Engine

Diesel Cycle

Isobaric combustionFuel injected into hot

air after compression

permits higher

compression ratio


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Diesel vs. IC

Diesel achieves higher

compression ratios

gives higher efficiency

Direct fuel injection after

compression

Diesel fuel (~C10

paraffins) is cheaper to

recover from petroleum

More particulates in the

emissions

Better acceleration

(power)

Easier to start up,

particularly in cold

weather

Better emissions control

technology


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Th l Effi i


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Thermal Efficiency,th

The thermal efficiency is the index of performance of a work-

producing device or a heat engine and is defined by the ratio of

the net work output (the desired result) to the heat input (thecosts to obtain the desired result).

For a heat engine the desired result is the net work done and

the input is the heat supplied to make the cycle operate. The

thermal efficiency is always less than 1 or less than 100

percent.

th = Desired Result

Required Input


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Why 100% efficiency is impossible?

At least some of the energy must be passed on

to heat a low-temperature energy sink

This is due to the 2nd

Law of Thermodynamics

Entropy must increase!

Engine needs to be reset.

Engine parts will absorb some of the heat

energy.


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Fuel Cell versus CarnotEfficiency


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Result from Thermodynamics

According to thermodynamics, evenunder ideal conditions, a heat engine is

incapable of converting all heat energy

supplied to it into mechanical energy.

Some of the heat is rejected.


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Efficiency of an internal combustion engine

maximum efficiency of an internal combustion engine is

given by

carnot

= (Th

Tc

) / Th , where

Thand T

c

are temperatures of the heat source and heat sink in

degrees Kelvin.

As seen by the above equation, this efficiency cannot exceedthe

Carnot limit!!!


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Efficiency of a Fuel Cell

The theoretical efficiency of a fuel cell is given by

fc

= G0/H0

where G0

is the chemical energy (or Gibbs energy)

and H0

is the total heat energy or Enthalpy of thefuel.

NOTE: This efficiency can exceed the Carnot limitbecause the electrochemical process of the fuel celldoes not involve conversion of thermal to mechanicalenergy!!

F l ll d l t i


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Fuel cell powered electric car

With pure hydrogen it has the potential of having 80%

efficiency.

Since hydrogen is difficult to store in a car, a reformer is

needed to convert methanol to hydrogen.

This drops the efficiency to 30-40%

Conversion of electric energy into mechanical work

requires an electric motor and inverter.

Accounting for all these processes ultimately gives an

efficiency of about 24-32%.


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Fuel Cell Powered Electric Car


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Fuel Cell Lecture 02