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CHAPTER 2
ENERGY, ENERGY TRANSFER, AND
GENERAL ENERGY ANALYSIS
Lecture slides by
Fawzi Elfghi
Thermodynamics: An Engineering Approach 8th Edition in SI Units
Yunus A. Çengel, Michael A. Boles
2
Objectives
• Introduce the concept of energy and define its various forms.
• Discuss the nature of internal energy.
• Define the concept of heat and the terminology associated with energy transfer by heat.
• Discuss the three mechanisms of heat transfer: conduction, convection, and radiation.
• Define the concept of work, including electrical work and several forms of mechanical work.
• Introduce the first law of thermodynamics, energy balances, and mechanisms of energy transfer to or from a system.
• Determine that a fluid flowing across a control surface of a control volume carries energy across the control surface in addition to any energy transfer across the control surface that may be in the form of heat and/or work.
• Define energy conversion efficiencies.
• Discuss the implications of energy conversion on the environment.
3
INTRODUCTION • If we take the entire room—including the air and the refrigerator (or fan)—as
the system, which is an adiabatic closed system since the room is well-sealed and well-insulated, the only energy interaction involved is the electrical energy crossing the system boundary and entering the room.
• As a result of the conversion of electric energy consumed by the device to heat, the room temperature will rise.
A refrigerator
operating with its
door open in a well-
sealed and well-
insulated room
A fan running in a
well-sealed and
well-insulated room
will raise the
temperature of air in
the room.
4
FORMS OF ENERGY
• Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy, E of a system.
• Thermodynamics deals only with the change of the total energy.
• Macroscopic forms of energy: Those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies.
• Microscopic forms of energy: Those related to the molecular structure of a system and the degree of the molecular activity.
• Internal energy, U: The sum of all the microscopic forms of energy.
• Kinetic energy, KE: The energy that a system possesses as a result of its motion relative to some reference frame.
• Potential energy, PE: The energy that a system possesses as a result of its elevation in a gravitational field.
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6
The total energy E of a system is the sum of all forms of energy that can exist within the system such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear. The total energy of the system is normally thought of as the sum of the internal energy, kinetic energy, and potential energy. The internal energy U is that energy associated with the molecular structure of a system and the degree of the molecular activity. The kinetic energy KE exists as a result of the system's motion relative to an external reference frame. When the system moves with velocity the kinetic energy is expressed as
KE mV
kJ
2
2( )
The energy that a system possesses as a result of its elevation in a gravitational field relative to the external reference frame is called potential energy PE and is expressed as
PE mgZ kJ ( )
where g is the gravitational acceleration and z is the elevation of the center of gravity of a system relative to the reference frame. The total energy of the system is expressed as
E U KE PE kJ ( )
or, on a unit mass basis,
7
eE
m
U
m
KE
m
PE
m
kJ
kg
uV
gZ
( )
2
2
where e = E/m is the specific stored energy, and u = U/m is the specific internal energy. The change in stored energy of a system is given by
E U KE PE kJ ( )
Most closed systems remain stationary during a process and, thus, experience no change in their kinetic and potential energies. The change in the stored energy is identical to the change in internal energy for stationary systems. If KE = PE = 0,
E U kJ ( )
8
Total energy
of a system
Energy of a system
per unit mass
Potential energy
per unit mass
Kinetic energy
per unit mass
Potential energy
Total energy
per unit mass
Kinetic energy
Mass flow rate
Energy flow rate
9
10
Example 1: A car loaded with passengers is pulled at constant speed along an inclined plane to the height of a set-top. If the mass of the ;loaded cart is 3000 kg and the height of the seat-top is 500 meters, what is the potential energy of the loaded cart at the height of the seat-top? (g = 10 m/s2) Solution : PE= mxgxh PE= (300kg) x (10m/s2) x (500 m) PE= 15 MJ 1 MJ = 1000 kJ
11
Example 2: A 55 kg man runs at speed of 5 m/s, find his kinetics energy, KE Solution: M= 55 kg V= 5 m/s KE = ½ m x v2
= ½ x 55 kg x 52 (m/s)2
= 687.5 J ================================================================ Example 3: Determine the kinetic energy of a 1000 kg roller coaster that is moving with a velocity of 86 m/s. Solution: KE = ½ m x v2
= ½ x 1000 kg x 862 (m/s)2
= 3,7 MJ
12
Example 4: A duct has a cross section of 0.2 m x 0.4 m. steam flows through it at a volumetric flow rate of 2.6 m3/s with a presssure of 2 bar. Calculate the kientic energy being transported Solution Cross sectional area, A = 0.2 x 0.4 = 0.08 m2
Volumetric flow rate = 2.6 m3/s Velocity = v = Volumetric flow rate / Area = V/A = 2.6/0.08 = 32.5 m/s Kinetic energy being transported = 1/2 mv2 = ½ x 3 x (32.5)2
= 1584 Watts
13
Some Physical Insight to Internal Energy
Sensible energy: The portion of the internal energy of a system associated with the kinetic energies of the molecules.
Latent energy: The internal energy associated with the phase of a system.
Chemical energy: The internal energy associated with the atomic bonds in a molecule.
Nuclear energy: The tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself.
Internal = Sensible + Latent + Chemical + Nuclear
Thermal = Sensible + Latent
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• The total energy of a system, can be contained or stored in a system, and thus can be viewed as the static forms of energy.
• The forms of energy not stored in a system can be viewed as the dynamic forms of energy or as energy interactions.
• The dynamic forms of energy are recognized at the system boundary as they cross it, and they represent the energy gained or lost by a system during a process.
• The only two forms of energy interactions associated with a closed system are heat transfer and work.
• The difference between heat transfer and work: An energy interaction is heat transfer if its driving force is a temperature difference. Otherwise it is work.
15
More on Nuclear Energy
• The best known fission reaction involves the split of the uranium atom (the U-235 isotope) into other elements and is commonly used to generate electricity in nuclear power plants (440 of them in 2004, generating 363,000 MW worldwide), to power nuclear submarines and aircraft carriers, and even to power spacecraft as well as building nuclear bombs.
• Nuclear energy by fusion is released when two small nuclei combine into a larger one.
• The uncontrolled fusion reaction was achieved in the early 1950s, but all the efforts since then to achieve controlled fusion by massive lasers, powerful magnetic fields, and electric currents to generate power have failed.
16
Mechanical Energy Mechanical energy: The form of energy that can be converted to
mechanical work completely and directly by an ideal mechanical device such
as an ideal turbine.
Kinetic and potential energies: The familiar forms of mechanical energy.
Mechanical energy of a
flowing fluid per unit mass
Rate of mechanical
energy of a flowing fluid
Mechanical energy change of a fluid during incompressible flow per unit mass
Rate of mechanical energy change of a fluid during incompressible flow
17
18
19
Heat vs. Work • Both are recognized at the boundaries of a
system as they cross the boundaries. That
is, both heat and work are boundary
phenomena.
• Systems possess energy, but not heat or
work.
• Both are associated with a process, not a
state.
• Unlike properties, heat or work has no
meaning at a state.
• Both are path functions (i.e., their
magnitudes depend on the path followed
during a process as well as the end states).
Properties are point functions
have exact differentials (d ).
Path functions
have inexact
differentials ( )
20
Energy Transport by Heat and Work and the Classical Sign Convention
Energy may cross the boundary of a closed system only by heat or work. Energy transfer across a system boundary due solely to the temperature difference between a system and its surroundings is called heat. Energy transferred across a system boundary that can be thought of as the energy expended to lift a weight is called work. Heat and work are energy transport mechanisms between a system and its surroundings. The similarities between heat and work are as follows: 1.Both are recognized at the boundaries of a system as they cross the boundaries. They are both boundary phenomena. 2.Systems possess energy, but not heat or work. 3.Both are associated with a process, not a state. Unlike properties, heat or work has no meaning at a state. 4.Both are path functions (i.e., their magnitudes depends on the path followed during a process as well as the end states.
21
Since heat and work are path dependent functions, they have inexact differentials designated by the symbol . The differentials of heat and work are expressed as Q and W. The integral of the differentials of heat and work over the process path gives the amount of heat or work transfer that occurred at the system boundary during a process.
2
12
1,
2
12
1,
(not Q)
(not )
along path
along path
Q Q
W W W
That is, the total heat transfer or work is obtained by following the process path and adding the differential amounts of heat (Q) or work (W) along the way. The integrals of Q and W are not Q2 – Q1 and W2 – W1, respectively, which are meaningless since both heat and work are not properties and systems do not possess heat or work at a state.
The following figure illustrates that properties (P, T, v, u, etc.) are point functions, that is, they depend only on the states. However, heat and work are path functions, that is, their magnitudes depend on the path followed.
22
700 kPa
100 kPa
0.01 m3 0.03 m3
A sign convention is required for heat and work energy transfers, and the classical thermodynamic sign convention is selected for
these notes. According to the classical sign convention, heat transfer to a system and work done by a system are positive; heat
transfer from a system and work a system are negative. The system shown below has heat supplied to it and work done by it.
In this study guide we will use the concept of net heat and net work.
23
System Boundary Energy Transport by Heat
Recall that heat is energy in transition across the system boundary solely due to the temperature difference between the system and its surroundings. The net heat transferred to a system is defined as
Q Q Qnet in out Here, Qin and Qout are the magnitudes of the heat transfer values. In most thermodynamics texts, the quantity Q is meant to be the net heat transferred to the system, Qnet. Since heat transfer is process dependent, the differential of heat transfer Q is called inexact. We often think about the heat transfer per unit mass of the system, q.
24
m
Heat transfer has the units of energy measured in joules (we will use kilojoules, kJ) or the units of energy per unit mass, kJ/kg. Since heat transfer is energy in transition across the system boundary due to a temperature difference, there are three modes of heat transfer at the boundary that depend on the temperature difference between the boundary surface and the surroundings. These are conduction, convection, and radiation. However, when solving problems in thermodynamics involving heat transfer to a system, the heat transfer is usually given or is calculated by applying the first law, or the conservation of energy, to the system. An adiabatic process is one in which the system is perfectly insulated and the heat transfer is zero.
25
ENERGY TRANSFER BY HEAT Heat: The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.
26
Historical Background on Heat • Kinetic theory: Treats molecules as tiny
balls that are in motion and thus possess kinetic energy.
• Heat: The energy associated with the random motion of atoms and molecules.
Heat transfer mechanisms:
• Conduction: The transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between particles.
• Convection: The transfer of energy between a solid surface and the adjacent fluid that is in motion, and it involves the combined effects of conduction and fluid motion.
• Radiation: The transfer of energy due to the emission of electromagnetic waves (or photons).
27
Heat transfer per unit mass
Amount of heat transfer when heat transfer rate changes with time
Amount of heat transfer when heat transfer rate is constant
28
Fourier's law of heat conduction is
Q A kdT
dxcond t
Qcond
dT
dx
here = heat flow per unit time (W) kt = thermal conductivity (W/mK) A = area normal to heat flow (m2) = temperature gradient in the direction of heat flow (C/m)
Integrating Fourier's law
cond t
TQ k A
x
Since T2>T1, the heat flows from right to left in the above figure.
29
Conduction through Plane Walls
Conduction heat transfer is a progressive exchange of energy between the molecules of a substance.
30
Example 5
A flat wall is composed of 20 cm of brick having a thermal conductivity kt = 0.72 W/mK. The right face temperature of the brick is 900C, and the left face temperature of the brick is 20C. Determine the rate of heat conduction through the wall per unit area of wall.
Tright =
900C Tleft =
20C
20 cm
31
Tright = 900C
Tleft =
20C
20 cm
23168
2.0
)20900(72.0
m
W
m
K
Km
W
x
Tk
A
Q
x
TAkQ
tcond
tcond
32
The rate of heat transfer by convection is determined from Newton's law of cooling.
Qconv
Convection Heat Transfer
Convection heat transfer is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion.
33
( )Q h A T Tconv s f
here Qconv = heat transfer rate (W) A = heat transfer area (m2) h = convective heat transfer coefficient (W/m2K) Ts = surface temperature (K) Tf = bulk fluid temperature away from the surface (K)
The convective heat transfer coefficient is an experimentally determined parameter that depends upon the surface geometry, the nature of the fluid motion, the properties of the fluid, and the bulk fluid velocity. Ranges of the convective heat transfer coefficient are given below.
h W/m2K free convection of gases 2-25 free convection of liquids 50-100 forced convection of gases 25-250 forced convection of liquids 50-20,000 convection in boiling and condensation 2500-100,000
Newton's law of cooling is expressed as
34
Radiative Heat Transfer
Radiative heat transfer is energy in transition from the surface of one body to the
surface of another due to electromagnetic radiation. The Stefan-Boltzmann law
states that the maximum radiative heat transfer per unit surface area that may be
emitted by a surface is given by product of the Stefan-Boltzmann constant and the
fourth power of the absolute temperature of the surface. The radiative energy
transferred is proportional to the difference in the fourth power of the absolute
temperatures of the bodies exchanging energy.
35
44
surrsrad TTAQ
here
= heat transfer per unit time (W)
A = surface area for heat transfer (m2)
σ = Stefan-Boltzmann constant, 5.67x10-8 W/m2K4 and 0.1713x10-8 BTU/h ft2 R4
= emissivity
Ts = absolute temperature of surface (K)
Tsurr = absolute temperature of surroundings (K)
For a small surface exchanging net radiative energy with its larger
surroundings, the rate of radiative heat transfer exchange
between the two surfaces is given by
radQ
36
Example 2-2
A vehicle is to be parked overnight in the open away from large surrounding
objects. It is desired to know if dew or frost may form on the vehicle top. Assume
the following:
•Convection coefficient h from ambient air to vehicle top is 6.0 W/m2C.
•Equivalent sky temperature is -18C.
•Emissivity of vehicle top is 0.84.
•Negligible conduction from inside vehicle to top of vehicle.
Determine the temperature of the vehicle top when the air temperature is 5oF. State
which formation (dew or frost) occurs.
Ttop
Ts ky = -18 CTa i r = 5 C
QconvQrad
37
Ttop
Ts ky = -18 CTa i r = 5 C
QconvQrad
Under steady-state conditions, the energy convected to the vehicle top is equal to the
energy radiated to the sky.
Q Qconv rad
The energy convected from the ambient air to the vehicle top is
( )Q A h T Tconv top air top
The energy radiated from the top to the night sky is
44
skytoptoprad TTAQ
Setting these two heat transfers equal gives
38
44
44
skytoptopair
skytoptoptopairtop
TTTTh
TTATThA
444
42
8
2
273181067.584.0
27350.6
KTKm
Wx
KTKm
W
top
top
Write the equation for Ttop in C (T K = TC + 273)
4
4
55.2100
273
0.6
67.584.05
top
top
TT
Using the EES software package
Ttop 338. C
Since Ttop is below the triple point of water, 0.01C, the water vapor in the air will form
frost on the car top (see Chapter 14).
39
ENERGY TRANSFER BY WORK • Work: The energy transfer associated with a force acting through a distance.
A rising piston, a rotating shaft, and an electric wire crossing the system boundaries are all associated with work interactions
• Formal sign convention: Heat transfer to a system and work done by a system are positive; heat transfer from a system and work done on a system are negative.
• Alternative to sign convention is to use the subscripts in and out to indicate direction. This is the primary approach in this text.
Work done
per unit mass
40
Electrical Work
Electrical work
Electrical power
When potential difference
and current change with time
When potential difference
and current remain constant
The rate of electrical work done by electrons
crossing a system boundary is called electrical power and is given by the product of the
voltage drop in volts and the current in amps.
The amount of electrical work done in a time period is found by integrating the
rate of electrical work over the time period.
41
MECHANICAL FORMS OF WORK
• There are two requirements for a work interaction between a system and its surroundings to exist:
there must be a force acting on the boundary.
the boundary must move.
Work = Force Distance
When force is not constant
42
Mechanical Forms of Work
Work is energy expended by a force acting through a distance. Thermodynamic work
is defined as energy in transition across the system boundary and is done by a
system if the sole effect external to the boundaries could have been the raising of a
weight.
Mathematically, the differential of work is expressed as
W F ds Fds
cos
here is the angle between the force vector and the displacement vector.
As with the heat transfer, the Greek symbol means that work is a path-dependent
function and has an inexact differential. If the angle between the force and the
displacement is zero, the work done between two states is
2
1
2
112 FdsWW
43
Shaft
Work
A force F acting through
a moment arm r
generates a torque T
This force acts through a distance s
The power transmitted through the shaft
is the shaft work done per unit time
Shaft
work
44
Spring Work When the length of the spring changes by
a differential amount dx under the influence
of a force F, the work done is
For linear elastic springs, the displacement
x is proportional to the force applied
k: spring constant (kN/m)
Substituting and integrating yield
x1 and x2: the initial and the final
displacements
45
Work Done on Elastic Solid Bars
Work Associated with
the Stretching of a
Liquid Film
46
Work Done to Raise or to Accelerate a Body
Nonmechanical Forms of Work
1. The work transfer needed to raise a body is equal to
the change in the potential energy of the body.
2. The work transfer needed to accelerate a body is
equal to the change in the kinetic energy of the body.
Electrical work: The generalized force is
the voltage (the electrical potential) and the
generalized displacement is the electrical
charge.
Magnetic work: The generalized force is
the magnetic field strength and the
generalized displacement is the total
magnetic dipole moment.
Electrical polarization work: The
generalized force is the electric field
strength and the generalized displacement
is the polarization of the medium.
47
Work has the units of energy and is defined as force times displacement or newton
times meter or joule (we will use kilojoules). Work per unit mass of a system is
measured in kJ/kg.
Common Types of Mechanical Work Energy (See text for discussion of these
topics)
•Shaft Work
•Spring Work
•Work done of Elastic Solid Bars
•Work Associated with the Stretching of a Liquid Film
•Work Done to Raise or to Accelerate a Body
Net Work Done By A System
The net work done by a system may be in two forms other work and boundary work.
First, work may cross a system boundary in the form of a rotating shaft work,
electrical work or other the work forms listed above. We will call these work forms
“other” work, that is, work not associated with a moving boundary. In
thermodynamics electrical energy is normally considered to be work energy rather
than heat energy; however, the placement of the system boundary dictates whether
48
Here, Wout and Win are the magnitudes of the other work forms crossing the
boundary. Wb is the work due to the moving boundary as would occur when a gas
contained in a piston cylinder device expands and does work to move the piston.
The boundary work will be positive or negative depending upon the process.
Boundary work is discussed in detail in Chapter 4.
to include electrical energy as work or heat. Second, the system may do work on its
surroundings because of moving boundaries due to expansion or compression
processes that a fluid may experience in a piston-cylinder device.
The net work done by a closed system is defined by
botherinoutnet WWWW
bothernetnet WWW
Several types of “other” work (shaft work, electrical work, etc.) are discussed in the
text.
49
Example 2-3
A fluid contained in a piston-cylinder device receives 500 kJ of electrical work as the
gas expands against the piston and does 600 kJ of boundary work on the piston.
What is the net work done by the fluid?
Wele =500 kJ Wb=600 kJ
,
0 500 600
100
net net bother
net out in ele bother
net
net
W W W
W W W W
W kJ kJ
W kJ
50
THE FIRST LAW OF THERMODYNAMICS
• The first law of thermodynamics (the conservation of energy
principle) provides a sound basis for studying the relationships
among the various forms of energy and energy interactions.
• The first law states that energy can be neither created nor
destroyed during a process; it can only change forms.
The First Law: For
all adiabatic
processes between
two specified states
of a closed system,
the net work done
is the same
regardless of the
nature of the closed
system and the
details of the
process.
51
The First Law of Thermodynamics
The first law of thermodynamics is known as the conservation of energy principle. It states that
energy can be neither created nor destroyed; it can only change forms. Joule’s experiments
lead to the conclusion: For all adiabatic processes between two specified states of a closed
system, the net work done is the same regardless of the nature of the closed system and the
details of the process. A major consequence of the first law is the existence and definition of the
property total energy E introduced earlier.
The First Law and the Conservation of Energy
The first law of thermodynamics is an expression of the conservation of energy principle.
Energy can cross the boundaries of a closed system in the form of heat or work. Energy may
cross a system boundary (control surface) of an open system by heat, work and mass transfer.
A system moving relative to a reference plane is shown below where z is the elevation of the
center of mass above the reference plane and is the velocity of the center of mass.
V
Energyin Energyout
z
System
Reference Plane, z = 0
CM
V
52
53
54
Energy Balance
The net change (increase
or decrease) in the total
energy of the system
during a process is equal
to the difference between
the total energy entering
and the total energy
leaving the system during
that process.
55
Energy Change of a System, Esystem
Internal, kinetic, and
potential energy changes
56
Normally the stored energy, or total energy, of a system is expressed as the
sum of three separate energies. The total energy of the system, Esystem, is given as
For the system shown above, the conservation of energy principle or the first law
of thermodynamics is expressed as
system theofenergy
in total change The
system theleaving
energy
system theentering
energy TotalTotal
or
E E Ein out system
E Internal energy Kinetic energy Potential energy
E U KE PE
= + +
= + +
Recall that U is the sum of the energy contained within the molecules of the system
other than the kinetic and potential energies of the system as a whole and is called
the internal energy. The internal energy U is dependent on the state of the system
and the mass of the system.
For a system moving relative to a reference plane, the kinetic energy KE and the
potential energy PE are given by
57
2
0
0
2
V
V
z
z
mVKE mV dV
PE mg dz mgz
The change in stored energy for the system is
E U KE PE
Now the conservation of energy principle, or the first law of thermodynamics for
closed systems, is written as
in outE E U KE PE
If the system does not move with a velocity and has no change in elevation, it is
called a stationary system, and the conservation of energy equation reduces to
in outE E U
Mechanisms of Energy Transfer, Ein and Eout
The mechanisms of energy transfer at a system boundary are: Heat, Work, mass
flow. Only heat and work energy transfers occur at the boundary of a closed (fixed
mass) system. Open systems or control volumes have energy transfer across the
control surfaces by mass flow as well as heat and work.
58
• Heat transfer
• Work transfer
• Mass flow
A closed mass
involves only heat
transfer and work.
Mechanisms
of energy
transfer:
59
1. Heat Transfer, Q: Heat is energy transfer caused by a temperature difference
between the system and its surroundings. When added to a system heat transfer
causes the energy of a system to increase and heat transfer from a system
causes the energy to decrease. Q is zero for adiabatic systems.
2. Work, W: Work is energy transfer at a system boundary could have caused a
weight to be raised. When added to a system, the energy of the system
increases; and when done by a system, the energy of the system decreases. W
is zero for systems having no work interactions at its boundaries.
3. Mass flow, m: As mass flows into a system, the energy of the system increases
by the amount of energy carried with the mass into the system. Mass leaving the
system carries energy with it, and the energy of the system decreases. Since no
mass transfer occurs at the boundary of a closed system, energy transfer by mass
is zero for closed systems.
The energy balance for a general system is
, ,
in out in out in out
mass in mass out system
E E Q Q W W
E E E
60
Mechanisms of Energy Transfer, Ein and Eout
61
( )
( / )
in out system
in out system
E E E kJ
e e e kJ kg
First Law for a Cycle
A thermodynamic cycle is composed of processes that cause the working fluid to
undergo a series of state changes through a process or a series of processes. These
processes occur such that the final and initial states are identical and the change in
internal energy of the working fluid is zero for whole numbers of cycles. Since
thermodynamic cycles can be viewed as having heat and work (but not mass)
crossing the cycle system boundary, the first law for a closed system operating in a
thermodynamic cycle becomes
net net cycle
net net
Q W E
Q W
62
Example 2-4
A system receives 5 kJ of heat transfer and experiences a decrease in energy in the
amount of 5 kJ. Determine the amount of work done by the system.
E= -5 kJ Qin =5 kJ Wout=?
System
Boundary
We apply the first law as
5
5
5 5
10
in out system
in in
out out
system
out in system
out
out
E E E
E Q kJ
E W
E kJ
E E E
W kJ
W kJ
63
The work done by the system equals the energy input by heat plus the decrease
in the energy of the working fluid.
Example 2-5
A steam power plant operates on a thermodynamic cycle in which water
circulates through a boiler, turbine, condenser, pump, and back to the boiler.
For each kilogram of steam (water) flowing through the cycle, the cycle receives
2000 kJ of heat in the boiler, rejects 1500 kJ of heat to the environment in the
condenser, and receives 5 kJ of work in the cycle pump. Determine the work
done by the steam in the turbine, in kJ/kg.
For a thermodynamic cycle, the first law becomes
64
Let and
2000 1500 5
505
net net cycle
net net
in out out in
out in out in
out in out in
out
out
Q W E
Q W
Q Q W W
W Q Q W
W Qw q
m m
w q q w
kJw
kg
kJw
kg
65
Example 2-6
Air flows into an open system and carries energy at the rate of 300 kW. As the air
flows through the system it receives 600 kW of work and loses 100 kW of energy by
heat transfer to the surroundings. If the system experiences no energy change as
the air flows through it, how much energy does the air carry as it leaves the system,
in kW?
System sketch:
Open
System
,mass inE
inW
outQ
,mass outE
Conservation of Energy:
, ,
, ,
,
0
300 600 100 800
in out system
mass in in mass out out system
mass out mass in in out
mass out
E E E
E W E Q E
E E W Q
E kW kW
66
ENERGY CONVERSION EFFICIENCIES
Efficiency is one of the most frequently used terms in thermodynamics, and it indicates how well an energy conversion or transfer process is accomplished.
Efficiency of a water
heater: The ratio of
the energy delivered
to the house by hot
water to the energy
supplied to the water
heater.
67
Energy Conversion Efficiencies
A measure of performance for a device is its efficiency and is often given the symbol
. Efficiencies are expressed as follows:
Desired Result
Required Input
How will you measure your efficiency in this thermodynamics course?
Efficiency as the Measure of Performance of a Thermodynamic cycle
A system has completed a thermodynamic cycle when the working fluid undergoes a
series of processes and then returns to its original state, so that the properties of the
system at the end of the cycle are the same as at its beginning.
Thus, for whole numbers of cycles
P P T T u u v v etcf i f i f i f i , , , , .
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.
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The system, or working fluid, undergoes a series of processes that constitute the heat
engine cycle.
The following figure illustrates a steam power plant as a heat engine operating in a
thermodynamic cycle.
69 Photo courtesy of Progress Energy Carolinas, Inc.
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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 (the cost or required input to
obtain the desired result).
th Desired Result
Required Input
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For a heat engine the desired result is the net work done (Wout – Win) and the input is
the heat supplied to make the cycle operate Qin. The thermal efficiency is always
less than 1 or less than 100 percent.
th
net out
in
W
Q
,
where
W W W
Q Q
net out out in
in net
,
Here, the use of the in and out subscripts means to use the magnitude (take the
positive value) of either the work or heat transfer and let the minus sign in the
net expression take care of the direction.
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Example 2-7
A steam power plant received 2000 kJ/kg of heat, 5 kJ/kg of pump work, and
produced 505 kJ/kg of turbine work. Determine the thermal efficiency for this cycle.
We can write the thermal efficiency on a per unit mass basis as:
,
505 5
2000
0.25 or 25%
net out
th
in
out in
in
w
q
kJ
w w kg
kJq
kg
Combustion Efficiency
Consider the combustion of a fuel-air mixture as shown below.
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Air
Combustion
Chamber
Fuel CnHm
CO2
H2O
N2
Qout = HV Reactants
TR, PR
Products
PP, TP
Fuels are usually composed of a compound or mixture containing carbon,
C, and hydrogen, H2. During a complete combustion process all of the
carbon is converted to carbon dioxide and all of the hydrogen is converted
to water. For stoichiometric combustion (theoretically correct amount of air
is supplied for complete combustion) where both the reactants (fuel plus
air) and the products (compounds formed during the combustion process)
have the same temperatures, the heat transfer from the combustion
process is called the heating value of the fuel.
Combustion Efficiency
Consider the combustion of a fuel-air mixture as shown below.
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Heating value of the fuel: The amount of heat released when a unit amount of
fuel at room temperature is completely burned and the combustion products are
cooled to the room temperature.
Lower heating value (LHV): When the water leaves as a vapor.
Higher heating value (HHV): When the water in the combustion gases is
completely condensed and thus the heat of vaporization is also recovered.
The efficiency of space heating
systems of residential and
commercial buildings is usually
expressed in terms of the annual
fuel utilization efficiency
(AFUE), which accounts for the
combustion efficiency as well as
other losses such as heat losses
to unheated areas and start-up
and cooldown losses.
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The lower heating value, LHV, is the heating value when water appears as
a gas in the products.
2out vaporLHV Q with H O in products
The lower heating value is often used as the measure of energy per kg of
fuel supplied to the gas turbine engine because the exhaust gases have
such a high temperature that the water formed is a vapor as it leaves the
engine with other products of combustion.
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The higher heating value, HHV, is the heating value when water appears as a liquid
in the products.
2out liquidHHV Q with H O in products
The higher heating value is often used as the measure of energy per
kg of fuel supplied to the steam power cycle because there are heat
transfer processes within the cycle that absorb enough energy from the
products of combustion that some of the water vapor formed during
combustion will condense.
Combustion efficiency is the ratio of the actual heat transfer from the
combustion process to the heating value of the fuel.
outcombustion
Q
HV
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Example 2-8
A steam power plant receives 2000 kJ of heat per unit mass of
steam flowing through the steam generator when the steam flow
rate is 100 kg/s. If the fuel supplied to the combustion chamber
of the steam generator has a higher heating value of 40,000
kJ/kg of fuel and the combustion efficiency is 85%, determine the
required fuel flow rate, in kg/s.
outcombustion
Q
HV
Combustion Efficiency
Combustion efficiency is the ratio of the actual heat transfer from
the combustion process to the heating value of the fuel.
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100 2000
0.85 40000
5.88
steam out to steamoutcombustion
fuel
steam out to steam
fuel
combustion
steam
steam
fuel
fuel
fuel
fuel
m qQ
HV m HHV
m qm
HHV
kg kJ
s kgm
kJ
kg
kgm
s
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• Generator: A device that
converts mechanical energy to
electrical energy.
• Generator efficiency: The ratio
of the electrical power output to
the mechanical power input.
• Thermal efficiency of a power
plant: The ratio of the net
electrical power output to the
rate of fuel energy input.
Overall efficiency of a power plant
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Power Plant Overall Efficiency:
, , ,
, ,
,
in cycle net cycle net electrical output
overall
fuel fuel in cycle net cycle
overall combustion thermal generator
net electrical output
overall
fuel fuel
Q W W
m HHV Q W
W
m HHV
Motor Efficiency:
mechanical output
motor
electrical input
W
W
Generator Efficiency:
electrical output
generator
mechanical input
W
W
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Lighting Efficacy:
Amount of Light in Lumens
Watts of Electricity ConsumedLighting Efficacy
Type of lighting Efficacy, lumens/W
Ordinary Incandescent 6 - 20
Ordinary Fluorescent 40 - 60
Effectiveness of Conversion of Electrical or chemical Energy to
Heat for Cooking, Called Efficacy of a Cooking Appliance:
Useful Energy Transferred to Food
Energy Consumed by ApplianceCooking Efficacy
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Lighting efficacy: The
amount of light output in
lumens per W of
electricity consumed.
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• Using energy-efficient appliances
conserve energy.
• It helps the environment by
reducing the amount of pollutants
emitted to the atmosphere during
the combustion of fuel.
• The combustion of fuel produces
• carbon dioxide, causes global
warming
• nitrogen oxides and
hydrocarbons, cause smog
• carbon monoxide, toxic
• sulfur dioxide, causes acid
rain.
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Efficiencies of Mechanical and Electrical Devices
The effectiveness of the conversion process between
the mechanical work supplied or extracted and the
mechanical energy of the fluid is expressed by the
pump efficiency and turbine efficiency,
Mechanical efficiency
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Generator
efficiency
Pump-Motor
overall efficiency
Turbine-Generator overall efficiency
Pump
efficiency
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ENERGY AND ENVIRONMENT
• The conversion of energy from one form to another
often affects the environment and the air we breathe in
many ways, and thus the study of energy is not
complete without considering its impact on the
environment.
• Pollutants emitted during the combustion of fossil fuels
are responsible for smog, acid rain, and global
warming.
• The environmental pollution has reached such high
levels that it became a serious threat to vegetation,
wild life, and human health.
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Ozone and Smog
• Smog: Made up mostly of ground-level ozone (O3), but it also contains numerous
other chemicals, including carbon monoxide (CO), particulate matter such as
soot and dust, volatile organic compounds (VOCs) such as benzene, butane, and
other hydrocarbons.
• Hydrocarbons and nitrogen oxides react in the presence of sunlight on hot
calm days to form ground-level ozone.
• Ozone irritates eyes and damages the air sacs in the lungs where oxygen and
carbon dioxide are exchanged, causing eventual hardening of this soft and
spongy tissue.
• It also causes shortness of breath, wheezing, fatigue, headaches, and nausea,
and aggravates respiratory problems such as asthma.
• The other serious pollutant in smog is carbon monoxide, which is a colorless,
odorless, poisonous gas.
• It is mostly emitted by motor vehicles.
• It deprives the body’s organs from getting enough oxygen by binding with the red
blood cells that would otherwise carry oxygen. It is fatal at high levels.
• Suspended particulate matter such as dust and soot are emitted by vehicles and
industrial facilities. Such particles irritate the eyes and the lungs.
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Acid Rain
• The sulfur in the fuel reacts with oxygen to form sulfur dioxide (SO2),
which is an air pollutant.
• The main source of SO2 is the electric power plants that burn high-
sulfur coal.
• Motor vehicles also contribute to SO2 emissions since gasoline and
diesel fuel also contain small amounts of sulfur.
• The sulfur oxides and nitric oxides react with water vapor and other
chemicals high in the atmosphere in the presence of sunlight to form
sulfuric and nitric acids.
• The acids formed usually dissolve in the suspended water droplets in
clouds or fog.
• These acid-laden droplets, which can be as acidic as lemon juice, are
washed from the air on to the soil by rain or snow. This is known as
acid rain.
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The Greenhouse
Effect: Global Warming
and Climate Change
• Greenhouse effect: Glass allows the
solar radiation to enter freely but blocks
the infrared radiation emitted by the
interior surfaces. This causes a rise in
the interior temperature as a result of the
thermal energy buildup in a space (i.e.,
car).
• The surface of the earth, which warms up
during the day as a result of the
absorption of solar energy, cools down at
night by radiating part of its energy into
deep space as infrared radiation.
• Carbon dioxide (CO2), water vapor, and
trace amounts of some other gases such
as methane and nitrogen oxides act like
a blanket and keep the earth warm at
night by blocking the heat radiated from
the earth. The result is global warming.
• These gases are called “greenhouse
gases,” with CO2 being the primary
component.
• CO2 is produced by the burning of
fossil fuels such as coal, oil, and
natural gas.
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• A 1995 report: The earth has already warmed about
0.5°C during the last century, and they estimate that the
earth’s temperature will rise another 2°C by the year 2100.
• A rise of this magnitude can cause severe changes in
weather patterns with storms and heavy rains and
flooding at some parts and drought in others, major floods
due to the melting of ice at the poles, loss of wetlands and
coastal areas due to rising sea levels, and other negative
results.
• Improved energy efficiency,
• energy conservation,
• using renewable energy sources
• help minimize global warming.
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The average car produces several
times its weight in CO2 every year (it is
driven 20,000 km a year, consumes
2300 liters of gasoline, and produces
2.5 kg of CO2 per liter).
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Summary • Forms of energy
Macroscopic = kinetic + potential
Microscopic = Internal energy (sensible + latent + chemical + nuclear)
• Energy transfer by heat
• Energy transfer by work
• Mechanical forms of work
• The first law of thermodynamics
Energy balance
Energy change of a system
Mechanisms of energy transfer (heat, work, mass flow)
• Energy conversion efficiencies
Efficiencies of mechanical and electrical devices (turbines, pumps)
• Energy and environment
Ozone and smog
Acid rain
The Greenhouse effect: Global warming and climate change