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Petroleum Engineering Thermodynamics

Chapter 6
The Second Law of Thermodynamics
Dr Muhammad Ayoub
Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh,
Perak, Malaysia | Tel: +605 368 7045 | Fax: +605 365 5670
e-mail : muhammad
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com
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2
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4
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• Identify valid processes as those that satisfy both the first and second
laws of thermodynamics
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• Describe the Kelvin–Planck and Clausius statements of the second law
of thermodynamics
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• Apply the second law of thermodynamics to cycles and cyclic devices
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• Describe the Carnot cycle
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• Determine the expressions for the thermal efficiencies and coefficients of
performance for reversible heat engines, heat pumps, and refrigerators
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A cup of hot coffee does not
get hotter in a cooler room
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• The first law places no restriction
on the direction of a process, but
satisfying the first law does not
Transferring heat to a wire
ensure that the process can
will not generate electricity
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A process must satisfy both
the first and second laws of
thermodynamics to proceed
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The second law may be used to identify the direction of processes
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The second law also asserts that energy has quality as well as quantity
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The second
law provides the necessary means to determine the quality as well as the
degree of degradation of energy during a process
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The second law of thermodynamics is also used in determining the
theoretical limits for the performance of commonly used engineering
systems, such as heat engines and refrigerators, as well as predicting the
degree of completion of chemical reactions
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Statement 2 (Clausius statement)
Heat cannot spontaneously flow from a cold object to a hot
one, whereas the reverse, a spontaneous flow of heat from a
hot object to a cold one, is possible
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Open

The Second Law of Thermodynamics
Kelvin–Planck Statement
It is impossible for any
device that operates on a
cycle to receive heat from a
single reservoir and produce
a net amount of work
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For a power
plant to operate, the working fluid
must exchange heat with the
environment as well as the furnace
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It
is a limitation that applies to both the
idealized and the actual heat engines
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Kelvin–Planck Statement
It is impossible for any system to operate in a thermodynamic cycle &
deliver a net amount of work to its surroundings while receiving energy
by heat transfer from a single thermal reservoir
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It only denies this possibility if the system undergoes a
thermodynamic cycle
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Bodies with relatively large thermal masses
can be modeled as thermal energy reservoirs
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• In practice, large bodies of water such as oceans, lakes, and rivers
as well as the atmospheric air can be modeled accurately as
thermal energy reservoirs because of their large thermal energy
storage capabilities or thermal masses
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o Another familiar example of a thermal energy reservoir is the
industrial furnace
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Therefore,
they can be modeled as reservoirs
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Any physical body whose thermal energy capacity is large
relative to the amount of energy it supplies or absorbs can be modeled
as one
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Open

HEAT ENGINES

1
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Work can always be
converted to heat
directly and completely,
but the reverse is not
true
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4
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Open

The devices that convert
heat to work
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They convert part of this heat
to work (usually in the form of a
rotating shaft
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They operate on a cycle
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This fluid is called the
working fluid
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• Engines that involve internal
combustion such as gas
turbines and car engines fall
into this category
• These devices operate in a
mechanical cycle but not in a
thermodynamic cycle since the
working fluid (the combustion
gases) does not undergo a
complete cycle
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The work-producing device that best fits into the definition of a heat engine is the
steam power plant, which is an external-combustion engine
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The change in internal
energy ∆U is zero, and
therefore the net work
output of the system is
also equal to the net
heat transfer to the
system
Open

THERMAL EFFICIENCY

Some heat engines perform better than
others (convert more of the heat they
receive to work)
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Even the most efficient
heat engines reject
almost one-half of the
energy they receive as
waste heat
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Every heat engine must waste some energy
by transferring it to a low-temperature
reservoir in order to complete the cycle,
even under idealized conditions
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Can we not just take the
condenser out of the plant
and save all that waste
energy?
The answer is, unfortunately,
a firm no for the simple
reason that without a heat
rejection process in a
condenser, the cycle
cannot be completed
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No heat engine can have a thermal efficiency
of 100 percent, or as for a power plant to
operate, the working fluid must exchange
heat with the environment as well as the
furnace
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It is a limitation that
applies to both the idealized and the actual
heat engines
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REFRIGERATORS AND HEAT PUMPS
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•

The working fluid used in the
refrigeration cycle is called a
refrigerant
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Basic components of a
refrigeration system and
typical operating conditions
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The most frequently used
refrigeration cycle is the vaporcompression refrigeration cycle
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COEFFICIENT OF PERFORMANCE
The efficiency of a refrigerator is expressed
in terms of the coefficient of performance
(COP)
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Can the value of COPR be greater than unity?
The objective of a refrigerator
is to remove QL from the
cooled space
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The work supplied to a
heat pump is used to
extract energy from the
cold outdoors and carry it
into the warm indoors
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•

Most heat pumps in operation today have a seasonally
averaged COP of 2 to 3
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In cold climates their efficiency drops considerably when
temperatures are below the freezing point
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Such heat pumps are more expensive to install, but they
are also more efficient
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The COP of a refrigerator decreases with decreasing
refrigeration temperature
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Energy efficiency rating (EER): The amount of heat removed
from the cooled space in Btu’s for 1 Wh (watthour) of electricity EER=3
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Open

THE SECOND LAW OF THERMODYNAMICS:
CLAUSIUS STATEMENT

It is impossible to construct a device that
operates in a cycle and produces no effect
other than the transfer of heat from a lowertemperature body to a higher-temperature
body
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This way, the net effect on the surroundings
involves the consumption of some energy in
the form of work, in addition to the transfer
of heat from a colder body to a warmer one
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Open

A refrigerator that violates
the Clausius statement of
the second law
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The Kelvin–Planck and the Clausius statements are equivalent in their
consequences, and either statement can be used as the expression of the
second law of thermodynamics
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Open

PERPETUAL-MOTION MACHINES

A perpetual-motion machine that violates the
first law (PMM1)
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Perpetual-motion machine: Any device that violates the first or the second
law
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• A device that violates the second law is called a PMM2
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If something sounds too good to be true, it probably is
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Irreversible process: A process that is not reversible
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Open

All the processes occurring in nature are irreversible
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Some processes are more irreversible than others
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Why?

Reversible processes deliver the most and
consume the least work
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•

•

The factors that cause a process to be irreversible
are called irreversibilities
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The presence of any of these effects renders a
process irreversible
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Open

Irreversible
compression and
expansion
processes
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Externally reversible: If no irreversibilities occur outside the system boundaries
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A totally reversible process involves no heat transfer through a finite temperature
difference, no nonquasi-equilibrium changes, and no friction or other dissipative
effects
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Open

Totally and internally reversible heat transfer
processes
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Reversible Isothermal Expansion (process 1-2, TH = constant)
Reversible Adiabatic Expansion (process 2-3, temperature drops from TH to TL)
Reversible Isothermal Compression (process 3-4, TL = constant)
Reversible Adiabatic Compression (process 4-1, temperature rises from TL to TH)
Open

THE CARNOT CYCLE

P-V diagram of the Carnot cycle
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The Reversed Carnot Cycle
The Carnot heat-engine cycle is a totally reversible cycle
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Open

THE CARNOT PRINCIPLES

The Carnot principles
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1
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2
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Open

THE CARNOT PRINCIPLES

Open

THE THERMODYNAMIC TEMPERATURE SCALE
• A temperature scale that is independent
of the properties of the substances that
are used to measure temperature is
called a thermodynamic temperature
scale
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The arrangement of
heat engines used to
develop the
thermodynamic
temperature scale
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For reversible cycles, the heat
transfer ratio QH /QL can be
replaced by the absolute
temperature ratio TH /TL
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THE CARNOT HEAT ENGINE

The Carnot heat engine is the most efficient
of all heat engines operating between the
same high- and low-temperature
reservoirs
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THE QUALITY OF ENERGY

Can we use C unit
for temperature
here?

The fraction of heat that can be
converted to work as a function
of source temperature
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• How do you increase the thermal efficiency
of a Carnot heat engine?

• How about for actual heat engines?

THE CARNOT REFRIGERATOR AND HEAT PUMP
Any refrigerator or heat pump

Carnot refrigerator or heat pump

No refrigerator can have a higher COP than a
reversible refrigerator operating between the
same temperature limits
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Actual refrigerators or heat pumps may approach these values as their designs
are improved, but they can never reach them
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That is, it requires more work to absorb heat from lower-temperature media
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4
An inventor claims to have developed a power cycle capable of delivering
a net work output of 410 kJ for an energy input by heat transfer of 1000
kJ
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Evaluate this claim
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It has a thermal efficiency of 75%
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Determine the power output of the engine and the temperature
of the source
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The house is losing heat to the outside
air through the walls and the windows at the rate of 60,000 kJ/h while
the energy generated within the house from people, lights, and
appliances amount of 4000 kJ/h
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