Fuel Cell Lecture 02
Transcript of 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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