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.
• 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.

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 2–1 Introduction 1

If we take the entire room— Figure 2–1

including the air and the
refrigerator (or fan)—as the A refrigerator operating with its door open in a
well-sealed and well-insulated room.
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.

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 2–1 Introduction 2

Figure 2–2
A fan running in a well-sealed and well-insulated room will raise the temperature of air
in the room.

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 2–2 Forms of energy 1

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.

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 2–2 Forms of energy 2

Figure 2–3
At least six different forms of energy are encountered in bringing power from a nuclear
plant to your home: nuclear, thermal, mechanical, kinetic, magnetic, and electrical.

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 2–2 Forms of energy 3

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.

Figure 2–4
The macroscopic energy of an object changes with velocity and elevation.

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 2–2 Forms of energy 4

V2
KE = m ( kJ ) Kinetic energy
2

V2
Ke = ( kJ/kg ) Kinetic energy per unit mass
2
PE = mgz Potential energy

pe = gz ( kJ/kg ) Potential energy per unit mass

V2
E = U + KE + PE = U + m + mgz ( kJ/kg ) Total energy of a system
2
V2
e = u + ke + pe = u + + gz ( kJ/kg ) Energy of a system per unit mass
2

E& = me
& ( kJ/s or kW ) Energy flow rate

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 2–2 Forms of energy 5

Figure 2–5
Mass and energy flow rates associated with the flow of steam in a pipe of inner
diameter D with an average velocity of Vavg.

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Mass flow rate: m& = rV& = r AcVavg ( kg/s )

Energy flow rate: E& = me
& ( kJ/s or kw )

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 2–2 Forms of energy 6

Figure 2–6
Some Physical Insight to
The various forms of microscopic
Internal Energy energies that make up sensible 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
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 2–2 Forms of energy 7

Thermal = Sensible + Latent

Internal = Sensible + Latent + Chemical + Nuclear
Figure 2–7
The internal energy of a system is the sum of all forms of the microscopic energies.

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 2–2 Forms of energy 8

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
– 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.

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 2–2 Forms of energy 9

Figure 2–8
The macroscopic kinetic energy is an organized form of energy and is much more
useful than the disorganized microscopic kinetic energies of the molecules.

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 2–2 Forms of energy 10

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, 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.

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 2–2 Forms of energy 11

Figure 2–9
The fission of uranium and the fusion of hydrogen during nuclear reactions, and the
release of nuclear energy.

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 2–2 Forms of energy 12

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.
P V2 Mechanical energy of a
emech = + + gz
r 2 flowing fluid per unit mass

æ P V2 ö
E& mech = me
& mech = m& ç + + gz ÷ Rate of mechanical
èr 2 ø energy of a flowing fluid

Mechanical energy change of a fluid during incompressible flow per unit mass
P2 - P1 V22 - V12
Demech = + + g ( z2 - z1 ) ( kJ/kg )
r 2
Rate of mechanical energy change of a fluid during incompressible flow
æ P2 - P1 V22 - V12 ö
DE& mech = m& Demech = m& ç + + g ( z2 - z1 ) ÷ ( kW )
è r 2 ø
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 2–2 Forms of energy 13

Figure 2–11
Mechanical energy is a useful concept for flows that do not involve significant heat
transfer or energy conversion, such as the flow of gasoline from an underground tank
into a car.

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 2–2 Forms of energy 14

Figure 2–12
Mechanical energy is illustrated by an ideal hydraulic turbine coupled with an ideal
generator. In the absence of irreversible losses, the maximum produced power is
proportional to (a) the change in water surface elevation from the upstream to the
downstream reservoir or (b) (close-up view) the drop in water pressure from just
upstream to just downstream of the turbine.

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 2–3 Energy Transfer By Heat 1

Heat: The form of energy that is transferred between two systems (or a system
and its surroundings) by virtue of a temperature difference.
Figure 2–14 Figure 2–15
Energy can cross the boundaries of a Temperature difference is the driving
closed system in the form of heat and force for heat transfer. The larger the
work. temperature difference, the higher is the
rate of heat transfer.

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 2–3 Energy Transfer By Heat 2

Heat transfer per unit mass Figure 2–16

Energy is recognized as heat transfer only
Q as it crosses the system boundary.
q= (kJ/kg )
m

Amount of heat transfer when heat

transfer rate is constant
t2
&
Q = ò Qdt (kJ)
t1

Amount of heat transfer when heat

transfer rate changes with time
Q = Q& Dt

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 2–3 Energy Transfer By Heat 3

Figure 2–17 Figure 2–18

During an adiabatic process, a system The relationships among q, Q, and Q·
exchanges no heat with its surroundings.

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 2–3 Energy Transfer By Heat 4

Figure 2–19
Historical
Background on Heat In the early 19th century, heat was thought
to be an invisible fluid called the caloric
Caloric theory: It asserts that heat that flowed from warmer bodies to cooler
is a fluidlike substance called the ones.
caloric that is a massless, colorless,
odorless, and tasteless substance
that can be poured from one body
into another
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
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 2–3 Energy Transfer By Heat 5

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).

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 2–4 Energy transfer by work 1

Work: The energy transfer associated Figure 2–21

with a force acting through a distance.
Specifying the directions of heat and work.
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.
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 2–4 Energy transfer by work 2

W
w= ( kJ/kg ) Work done per unit mass
m
Figure 2–20
The relationships among w, W, and W& .

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 2–4 Energy transfer by work 3

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).
2
Properties are point functions
have exact differentials (d). ò dV = V
1
2 -V1 = DV

2
Path functions have inexact ( not DW )
differentials (d ) ò dW = W
1
12

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 2–4 Energy transfer by work 4

Figure 2–22
Properties are point functions; but heat and work are path functions (their magnitudes
depend on the path followed).

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 2–4 Energy transfer by work 5

Electrical Work
We = VN Electrical work Figure 2–27
Electrical power in terms of resistance R,
W&e = VI (W) Electrical power current I, and potential difference V.

When potential difference and

current change with time
2
We = ò VI dt ( kJ )
1

When potential difference and

current remain constant

We = VI Dt ( kJ )
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 2–5 Mechanical forms of work 1

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 Figure 2–28

The work done is proportional to the force applied
W = Fs ( kJ ) (F) and the distance traveled (s).

When force is not constant

2
W = ò F ds ( kJ )
1

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 2–5 Mechanical forms of work 2

Shaft Work
T A force F acting through a moment arm r
T = Fr ® F=
r generates a torque T
s = ( 2p r ) n This force acts through a distance s
æT ö
Wsh = Fs = ç ÷ ( 2p rn ) = 2p nT ( kJ ) Shaft work
èrø
Figure 2–30
The power transmitted through Shaft work is proportional to the torque applied
the shaft is the shaft work done and the number of revolutions of the shaft.
per unit time

W&sh = 2p nT
& ( kW )

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 2–5 Mechanical forms of work 3

Figure 2–29
Energy transmission through rotating shafts is commonly encountered in practice.

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 2–5 Mechanical forms of work 4

Spring Work Figure 2–32

When the length of the spring changes by a Elongation of a spring under the
differential amount dx under the influence influence of a force.
of a force F, the work done is
d Wspring = F dx ( kW )
For linear elastic springs, the displacement x
is proportional to the force applied
F = kx (kN)
k: spring constant (kN/m)
Spring work
1
Wspring = k ( x2 2 - x12 ) ( kJ )
2
x1 and x2: the initial and the final
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displacements

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 2–5 Mechanical forms of work 5

Figure 2–33
The displacement of a linear spring doubles when the force is doubled.

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 2–5 Mechanical forms of work 6

Work Done on Elastic Solid Bars

Figure 2–34
Solid bars behave as springs under the influence of a force.
2 2
Welastic = ò F dx = ò s n A dx ( kJ )
1 1

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 2–5 Mechanical forms of work 7

Work Associated Figure 2–35

with the Stretching Stretching a liquid film with a U-shaped wire, and
of a Liquid Film the forces acting on the movable wire of length b.

Surface tension work

2
Wsurface = ò s s dA ( kJ )
1

dA = 2b dx

F = 2bs s

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 2–5 Mechanical forms of work 8

Work Done to Raise or to Accelerate a Body

1. The work transfer needed to raise Figure 2–36
a body is equal to the change in
the potential energy of the body. The energy transferred to a body while
being raised is equal to the change in its
2. The work transfer needed to potential energy.
accelerate a body is equal to the
change in the kinetic energy of
the body.

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 2–5 Mechanical forms of work 9

Nonmechanical Forms of Work

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.

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 2–6 The first law of thermodynamics 1

The first law of thermodynamics Figure 2–39

(the conservation of energy
principle) provides a sound basis for Energy cannot be created or destroyed; it
can only change forms.
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.
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.

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 2–6 The first law of thermodynamics 2

Figure 2–40
The increase in the energy of a potato in an oven is equal to the amount of heat
transferred to it.

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 2–6 The first law of thermodynamics 3

Figure 2–41 Figure 2–42

In the absence of any work interactions, The work (electrical) done on an adiabatic
the energy change of a system is equal to system is equal to the increase in the
the net heat transfer. energy of the system.

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 2–6 The first law of thermodynamics 4

Figure 2–43 Figure 2–44

The work (shaft) done on an adiabatic The work (boundary) done on an adiabatic
system is equal to the increase in the system is equal to the increase in the
energy of the system energy of the system.

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 2–6 The first law of thermodynamics 5

Energy Balance
æ Total energy ö æ Total energy ö æ Change in the total ö
ç ÷-ç ÷=ç ÷
è entering the system ø è leaving the system ø è energy of the system ø
Figure 2–45
Ein - Eout = DEsystem
The energy change of a system during a
The net change (increase or process is equal to the net work and heat
decrease) in the total energy of transfer between the system and its
the system during a process is surroundings.
equal to the difference between
the total energy entering and the
total energy leaving the system
during that process.

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 2–6 The first law of thermodynamics 6

Energy Change of a System, DEsystem

Energy change = Energy at final state - Energy at initial state
DEsystem = Efinal - Einitial = E2 - E1 Figure 2–46
For stationary systems, ΔKE = ΔPE = 0; thus
DE = DU + DKE + DPE ΔE = ΔU.
Internal, kinetic, and potential
energy changes

DU = m ( u2 - u1 )
1
DKE = m (V2 2 - V12 )
2
DPE = mg ( z2 - z1 )

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 2–6 The first law of thermodynamics 7

Mechanisms of Energy Transfer, Ein and Eout

Energy balance for any system undergoing any kind of process can be expressed
more compactly as
Ein - Eout
14 24 3
= DEsystem
123
( KJ ) (2–35)
Net energy transfer Change in internal, kinetic
by heat, work, and mass potential, etc., energies

or, in the rate form,

as
E& in - E& out = DEsystem ( Kw ) (2–36)
1424 3 123
Rate of net energy transfer Rate of change in internal,
by heat, work, and mass kinetic, potential, etc., energies

For constant rates, the total quantities during a time interval Δt are related to the
quantities per unit time as
æ dE ö
Q = Q& Dt , W = W& Dt , and DE = ç ÷ Dt ( KJ ) (2–37)
è dt ø
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 2–6 The first law of thermodynamics 8

Mechanisms of Energy Transfer, Ein and Eout

The energy balance can be expressed on a per unit mass basis as

ein - eout = Desystem ( KJ/kg ) (2–38)

which is obtained by dividing all the quantities in Eq. 2–35 by the mass m of the
system. Energy balance can also be expressed in the differential form as

d Ein - d Eout = dEsystem or d ein - d eout = desystem (2–39)

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 2–6 The first law of thermodynamics 9

Mechanisms of energy transfer:

• Heat transfer
• Work transfer
• Mass flow

A closed mass involves only heat transfer and work.

( )
Ein - Eout = ( Qin - Qout ) + (Win - Wout ) + Emass,in - Emass,out = DEsystem

Wnet,out = Qnet,in or W&net,out = Q&net,in ( for a cycle )

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 2–6 The first law of thermodynamics 10

Figure 2–47 Figure 2–48

The energy content of a control volume can For a cycle ΔE = 0, thus Q = W.
be changed by mass flow as well as by heat
and work interactions.

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 2–7 Energy Conversion Efficiencies 1

Efficiency is one of the most frequently used terms in

thermodynamics, and it indicates how well an energy
conversion or transfer process is accomplished.
Desired output
Efficiency =
Require d input
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.
Figure 2–53
Typical efficiencies of conventional and high-efficiency
electric and natural gas water heaters.
Type Efficiency
Gas, conventional 55%
Gas, high-efficiency 62%
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Electric, conventional 90% Education. Permission required
for reproduction or display.
Electric, high-efficiency 94%
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 2–7 Energy Conversion Efficiencies 2

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 in the combustion gases is 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.

Combustion equipment efficiency

Q useful Useful heat delivered by the combustion equipment

hcomb.equip. = =
HV Heating value of the fuel burned

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 2–7 Energy Conversion Efficiencies 3

Figure 2–54
The definition of the heating value of gasoline.

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 2–7 Energy Conversion Efficiencies 4

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 equipment efficiency
as well as other losses such as heat losses to unheated areas and start-up and
cool down losses.
The AFUE of most new heating systems is about 85 percent, although the AF
UE of some old heating systems is under 60 percent.
The AFUE of some new high-efficiency furnaces exceeds 96 percent, but the
high cost of such furnaces cannot be justified for locations with mild to
moderate winters.
Such high efficiencies are achieved by reclaiming most of the heat in the flue
gases, condensing the water vapor, and discharging the flue gases at
temperatures as low as 38°C (or 100°F) instead of about 200°C (or 400°F) for
the conventional models.

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 2–7 Energy Conversion Efficiencies 5

Overall efficiency of a power plant

W&net, electric
hoverall = hcomb.equip.h thermalhgenerator =
HHV? m& fuel

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 shaft work output
of the turbine to the heat input to the working fluid.

The overall efficiencies are about 25–30 percent for gasoline

automotive engines, 35–40 percent for diesel engines, and up to 60
percent for large power plants.

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 2–7 Energy Conversion Efficiencies 6

Table 2–1 The efficacy of different lighting systems

Lighting efficacy: The amount of
Type of lighting Efficacy, lumens/W
Combustion
light output in lumens per W of
Candle 0.3 electricity consumed.
Kerosene lamp 1-2
Incandescent
Ordinary 6-20
Halogen 15-35
Fluorescent
Compact 40-87
Tube 60-120
High-intensity discharge
Mercury vapor 40-60
Metal halide 65-118
High-pressure sodium 85-140
Low-pressure sodium 70-200 *This value depends on the spectral distribution of the
Solid-State
assumed ideal light source. For white light sources, the
upper limit is about 300 lm/W for metal halide, 350 lm/W
LED 20-160 for fluorescents, and 400 lm/W for LEDs. Spectral
OLED 15-60 maximum occurs at a wavelength of 555 nm (green) with a
Theoretical limit 300* light output of 683 lm/W.
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 2–7 Energy Conversion Efficiencies 7

Figure 2–55

A 15-W compact fluorescent lamp provides as much light as a 60-W incandescent lamp.

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 2–7 Energy Conversion Efficiencies 8

Using energy-efficient appliances Figure 2–56

conserve energy. The efficiency of a cooking appliance
represents the fraction of the energy
It helps the environment by reducing supplied to the appliance that is
the amount of pollutants emitted to the transferred to the food.
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
Energy utilized
Efficiency =
• sulfur dioxide, causes acid rain. Energy supplied to appliance
3 kWh
= = 0.60
5 kWh
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 2–7 Energy Conversion Efficiencies 9

Table 2–2 Energy costs of cooking a casserole with different appliances*

Cooking appliance Cooking Cooking time Energy used Cost of energy
temperature
Electric oven 350°F (177°C) 1h 2.0 kWh $0.19

Convection oven (elect.) 325°F (163°C) 45 min 1.39 kWh $0.13

Gas oven 350°F (177°C) 1h 0.112 therm $0.13

Frying pan 420°F (216°C) 1h 0.9 kWh $0.09

Toaster oven 425°F (218°C) 50 min 0.95 kWh $0.09

Crockpot 200°F (93°C) 7h 0.7 kWh $0.07

Microwave oven “High” 15 min 0.36 kWh $0.03

*Assumes a unit cost of $0.095/kWh for electricity and $1.20/therm for gas.

[From J. T. Amann, A. Wilson, and K. Ackerly, Consumer Guide to Home Energy Savings, 9th ed.,
American Council for an Energy-Efficient Economy, Washington, D.C., 2007, p. 163.]

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 2–7 Energy Conversion Efficiencies 10

Efficiencies of Mechanical and Electrical Devices

Mechanical efficiency
Mechanical energy output Emech,out Emech,loss
hmetch = = = 1-
Mechanical energy input Emech,in Emech,in

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 energy increase of the fluid DE& mech,fluid W&pump,u

hpump = =
&
=
Mechanical energy input Wshaft,in W&pump

Mechanical energy output W&shaft,out W&turbine

h turbine = = =
Mechanical energy decrease of the fluid DEmech,fluid W&turbine,e
&

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 2–7 Energy Conversion Efficiencies 11

Figure 2–58
The mechanical efficiency of a fan is the ratio of
the rate of increase of the mechanical energy of air
to the mechanical power input.

V1 » 0, V2 =12.1m/s
z1 = z 2
p1 » patm and p2 » patm
DE& mech , fluid mV
& 22 / 2
hmech , fan = & =
Wshaft ,in W&shaft ,in
2
(0.506 kg/s) (12.1 m/s ) / 2
=
50.0 W
= 0.741
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 2–7 Energy Conversion Efficiencies 12

Mechanical power output W&shaft,out

Motor : hmotor = =
Electric power input W&elect,in

Electric power output W&elect,out

Generator : hgenerator = =
Mechanical power input W&shaft,in

W&pump,u DE& mech,fluid

hpump - motor = hpumphmotor = = Pump-Motor
W&elect,in W&elect,in overall efficiency

W&elect,out W&elect,out Turbine-

hturbine - gen = hturbinehgenerator = = Generator
W&turbine,e DE& mech,fluid overall
efficiency

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 2–7 Energy Conversion Efficiencies 13

Figure 2–59
The overall efficiency of a turbine– generator is the product of the efficiency of the
turbine and the efficiency of the generator, and it represents the fraction of the
mechanical power of the fluid converted to electrical power.

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 2–8 Energy and Environment 1

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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 2–8 Energy and Environment 2

Figure 2–62 Figure 2–63

Energy conversion processes are often Motor vehicles are the largest
accompanied by environmental pollution. source of air pollution.

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 2–8 Energy and Environment 3

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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 2–8 Energy and Environment 4

Figure 2–64
Ground-level ozone, which is the primary component of smog, forms when HC and
NOx react in the presence of sunlight on hot, calm days.

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 2–8 Energy and Environment 5

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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 2–8 Energy and Environment 6

Figure 2–65
Sulfuric acid and nitric acid are formed when sulfur oxides and nitric oxides react
with water vapor and other chemicals high in the atmosphere in the presence of
sunlight.

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 2–8 Energy and Environment 7

The Greenhouse Effect: Global Warming and Climate Change

Figure 2–66
Greenhouse effect: Glass allows the
The greenhouse effect on earth.
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.
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 2–8 Energy and Environment 8

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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 2–8 Energy and Environment 9

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.
How to minimize global warming?
• Improved energy efficiency.
• energy conservation.
• using renewable energy sources.

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 2–8 Energy and Environment 10

FIGURE 2–67 Figure 2–68

The average car produces several times its Renewable energies such as wind are
weight in CO2 every year (it is driven called “green energy” since they emit
13,500 miles a year, consumes 600 gallons no pollutants or greenhouse gases.
of gasoline, and produces 20 lbm of CO2 per
gallon).

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 Summary
• Forms of energy
• Energy transfer by heat
• Energy transfer by work
• Mechanical forms of work
• The first law of thermodynamics
• Energy conversion efficiencies
• Energy and environment

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 End of Chapter 2

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