Lecture 16

Thermodynamics: An Engineering Approach
8th Edition
Yunus A. Çengel, Michael A. Boles
McGraw-Hill, 2015
CHAPTER 6
THE SECOND LAW OF
THERMODYNAMICS
Adapted from the lecture slides by Mehmet Kanoglu Copyright © The McGraw-Hill Education.
Permission required for reproduction or display.
Objectives
• Introduce the second law of thermodynamics.
• Identify valid processes as those that satisfy both the first and second
laws of thermodynamics.
• Discuss thermal energy reservoirs, reversible and irreversible
processes, heat engines, refrigerators, and heat pumps.
• Describe the Kelvin–Planck and Clausius statements of the second law
of thermodynamics.
• Discuss the concepts of perpetual-motion machines.
• Apply the second law of thermodynamics to cycles and cyclic devices.
• Apply the second law to develop the absolute thermodynamic
temperature scale.
• Describe the Carnot cycle.
• Examine the Carnot principles, idealized Carnot heat engines,
refrigerators, and heat pumps.
• Determine the expressions for the thermal efficiencies and coefficients
of performance for reversible heat engines, heat pumps, and
refrigerators.
2
INTRODUCTION TO THE SECOND LAW
These processes
cannot occur even
though they are not in
violation of the first law. 3
MAJOR USES OF THE
SECOND LAW
1. The second law may be used to identify
the direction of processes.
2. The second law also asserts that energy
has quality as well as quantity. The first
law is concerned with the quantity of
energy and the transformations of
energy from one form to another with no
regard to its quality. The second law
provides the necessary means to
determine the quality as well as the
degree of degradation of energy during
a process.
3. 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.
4
THERMAL ENERGY RESERVOIRS
• A hypothetical body with a relatively large thermal energy capacity (mass x

specific heat) that can supply or absorb finite amounts of heat without
undergoing any change in temperature is called a thermal energy reservoir,
or just a reservoir.
• 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. 5
HEAT HEAT ENGINES: The devices that convert heat to work.
ENGINES 1. They receive heat from a high-temperature source

(solar energy, oil furnace, nuclear reactor, etc.).
2. They convert part of this heat to work (usually in the
form of a rotating shaft.)
3. They reject the remaining waste heat to a
low-temperature sink (the atmosphere, rivers, etc.).
4. They operate on a cycle.
Heat engines and other

cyclic devices usually
involve a fluid to and from
which heat is transferred
while undergoing a cycle.
This fluid is called the
working fluid.
6
A steam power plant
7
Thermal
efficiency
8
9
Can we
save Qout?
In a steam power plant,
the condenser is the
device where large
quantities of waste heat is
rejected to rivers, lakes,
or the atmosphere.
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,
Every heat engine must waste some energy the cycle cannot be
completed.
by transferring it to a low-temperature
reservoir in order to complete the cycle, even
under idealized conditions. 10
Net Power Production
of a Heat Engine
11
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.
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.
The impossibility of having a 100%
efficient heat engine is not due to friction
or other dissipative effects. It is a
limitation that applies to both the
idealized and the actual heat engines.
12
REFRIGERATORS AND HEAT PUMPS
• The transfer of heat from a
low-temperature medium to a
high-temperature one requires
special devices called
refrigerators.
• Refrigerators, like heat engines,
are cyclic devices.
• The working fluid used in the
refrigeration cycle is called a
refrigerant.
• The most frequently used
refrigeration cycle is the
vapor-compression
refrigeration cycle.
In a household refrigerator, the freezer
compartment where heat is absorbed by
the refrigerant serves as the evaporator,
and the coils usually behind the
refrigerator where heat is dissipated to the
kitchen air serve as the condenser. 13

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Thermodynamics: An Engineering Approach

• A hypothetical body with a relatively large thermal energy capacity (mass x

ENGINES 1. They receive heat from a high-temperature source

Heat engines and other

No heat engine can have a thermal

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