# INTRODUCTION TO 2 LAW and EXERGY

Mechanics

Oct 27, 2013 (5 years and 2 months ago)

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INTRODUCTION TO 2
nd

LAW
and EXERGY

Yunus
Çengel

Y
i
ld
i
z Te
chnical University
,
I
stanbul

University of
, Reno

2

Outline

Introduce the
2
nd

law
of thermodynamics, and Describe
the
Kelvin

Planck

and
Clausius
statements of the
second law of thermodynamics.

Define
exergy
, which is the maximum useful work that
could be obtained from the system at a given state in a
specified environment.

Define the
exergy destruction
, which is the wasted work
potential during a process as a result of irreversibilities.

Define the
second
-
law efficiency
.

Develop the
exergy balance
relation, and apply it to
processes

encountered in practice.

1
st

and 2
nd

Laws

The
1
st

law
of thermodynamics is
mundane
since it deals with
conserved quantities
.

The
2
nd

law
is quite exciting and sometimes even bizarre since
it
baffles the mind and intrigues the imagination

by dealing with
quantities that are created and destroyed.

Many people, including some engineers, have difficulty grasping the
2
nd

law concepts such as entropy, exergy, and 2
nd

law efficiency
,
and question the utility of the 2
nd

law analysis. As a result, they tend
to limit thermodynamics to an energy analysis only.

Energy analysis provides a
one
-
sided view
of a process or system,
and a study is not complete without an
accompanying 2
nd

-
law
analysis,

which enables one to examine the system or process from
a different angle.

The 1
st

law (or energy) analysis provides a
map for energy flow and
conversion
for a process, whereas the 2
nd

law provides a map of
inefficiencies

and
waste

occurring throughout the process.

Mass balance
:

Mass change = Mass transfer

Energy balance
:
Energy change = Energy transfer

Entropy balance
:
Entropy change = Entropy transfer + Entropy
generation

Exergy balance
:
Exergy change = Exergy transfer
-

Exergy
destruction

Thermodynamics in a nutshell

5

Entropy Generation and
Exergy destruction

associated with heat transfer

Energy is conserved,

Entropy is generated,

Exergy is destroyed.

The 2
nd

Law of Thermodynamics
:

The 2
nd

Dimension of Energy

1
st

Law: Quantity Based

Energy is energy.

All energies are equal.

Quantity is always conserved; it cannot be destroyed.

2
nd

law: Quality Based

There is difference from energy to energy.

All energies are not equal.

Quality diminishes in all processes, except the ‘perfect’ ones.

Is the theoretical upper limit in energy
conversion
%100
? If not, what is it?

How close are we to
Perfection
? Or, how
much room do we have for improvement?

Basic Questions

ANOTHER QUESTION:

From a thermodynamic point of view, which is a better
energy conversion/transfer process?

1
st

law efficiency
: The level of performance achieved compared
to the resources provided.

2
nd

law efficiency
: The level of performance achieved compared
to the best possible performance under the circumstances.

Process
A
: 1
st

Law efficiency
=100%; 2
nd

Law efficiency

<100%.

Process
B
: 1
st

Law efficiency<
100%; 2
nd

Law efficiency

=
100%.

.

2
nd

law efficiency

A measure of perfection

A process with a 2
nd

law efficiency of 100%:

-

Is perfect (even if its 1
st

law efficiency is less than 100%).

-

Entropy generation = 0

-

Exergy destruction = 0

-

Waste = 0

Something cannot be
more perfect than perfect
. The 2
nd

law
defines the upper limit.

Perfection is
good and beautiful
.

Beauty and perfection are liked for what they are.

GOAL
:
Perfection and zero waste
.

10

1
st

LAW vs. 2
nd

LAW: Matter vs. Non
-
matter

1
st

law deals with
Matter and Energy

-

Their existence is
certain

-

Physical quantities

-

Conserved (subject to conservation laws)

-

Can be perceived by 5 senses

-

Conforms to the matter
-
energy universe that started with big
-
bang

2
nd

law deals with
Entropy and Exergy

-

Their existence is
certain

-

Non
-
physical quantities (beyond physics)

-

Non
-
matter (or meaning)

-

Non
-
Conserved (not subject to conservation laws)

-

Can NOT be perceived by 5 senses

-

Outside the matter
-
energy universe that started with big
-
bang

We are blinded by
matter
, and conditioned with
conserved quantities
.

Some have difficulty grasping
Entropy and Exergy
since they are
invisible non
-
matter quantities
.

They are like
spirits

working behind the scenes and governing physical
phenomena. Entropy is like the bad spirit, and exergy the good spirit.

Comparison of mass and energy
with entropy and exergy

Mass, energy

Entropy, exergy

Associated with

1
st

law

2
nd

law

Existence

Yes

Yes

Physical property

Yes

Yes

Material quantity

Yes

No

Conserved

Yes

No

Matter, energy

Yes

No

Part of big
-
bang universe

Yes

No

Perceived by 5 senses

Yes

No

Valuable engineering tool

Yes

Yes

Practical utility of the 2
nd

law

The
2
nd

law (or exergy) analysis
serves as a mirror to see the
effectiveness

of each segment of the process, and to
identify waste and
inefficiencies.

The 2
nd

law efficiency is a
measure of the
perfection

of a process.

The
destruction of exergy is a measure of imperfections
associated with a
process, and it shows the room we have for improvement.

Global warming
and the associated climate change that pose the greatest
risk for the future of planet earth are closely
related to the 2
nd
-
law concepts
.

Combating climate change
and the associated
green practices
are closely
related to avoiding waste and thus minimizing entropy generation or exergy
destruction.

Sustainability
” is also a 2
nd

law concept, as it involves the practice of
best
resource utilization
and waste elimination.

Even “
reliability
” is related to the 2
nd

law as wasted energy often causes
excessive operating temperatures, and thus a higher rate of failure.

13

1.
T
he direction of processes can be identified.

2.
The second law 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.

3.
The second law provides the necessary means to determine
the quality as well as the degree of degradation of energy
during a process.

4.
The second law is used in determining the
theoretical limits
for
the performance of commonly used engineering systems.

5.
E
xergy efficiencies provide a measure of how nearly actual
performance approaches the ideal, and identifies the causes
and locations of thermodynamic losses.

6.
Exergy analysis can assist in improving and optimizing
designs.

PRIMARY USES OF THE 2
nd

LAW AND EXERGY

14

INTRODUCTION TO THE 2
nd

LAW

A cup of hot coffee
does not get hotter in
a cooler room.

Transferring
heat to a wire
will not
generate
electricity.

Transferring
heat to a
will not cause
it to rotate.

These processes
cannot occur
even though they
are not in violation
of the first law.

The 2
nd

Law and Exergy

The Second Law of Thermodynamics:

Kelvin

Planck Statement

A heat engine that violates the
Kelvin

Planck statement of the
second law.

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.

17

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 lower
-
temperature body to a higher
-
temperature body.

It states that a refrigerator cannot operate
unless its compressor is driven by an
external power source, such as an electric
motor.

A refrigerator that
violates the Clausius
statement of the
second law.

THE CARNOT HEAT ENGINE

No heat engine can have a higher
efficiency than a reversible heat
engine operating between the same
high
-

and low
-
temperature
reservoirs.

Any heat
engine

Carnot heat
engine

The Quality of Energy

The fraction of heat that can be
converted to work as a function
of source temperature.

The higher the
temperature of the
thermal energy, the
higher its quality.

20

THE CARNOT REFRIGERATOR

AND HEAT PUMP

No refrigerator can have a higher COP

than a reversible refrigerator operating

between the same temperature limits.

Any refrigerator or heat pump

Carnot refrigerator or heat pump

How do you increase the
COP of a Carnot
refrigerator or heat pump?

EXERGY: WORK POTENTIAL OF ENERGY

A system that is in equilibrium
with its environment is said to be

The useful work potential of a given amount of energy at some
specified state is called
exergy
,
which is also called the
availability

or
available energy
.

A system is said to be in the

when it is in thermodynamic
equilibrium with the environment it is in.

The atmosphere contains a
tremendous amount of energy, but
no exergy.

22

REVERSIBLE WORK and EXERGY DESTRUCTION

As a closed
system expands,
some work needs
to be done to push
the atmospheric
air out of the way
(
W
surr
).

For constant
-
volume
systems, the total
actual and useful
works are identical
(
W
u

=

W
).

Reversible work
W
rev
:

The maximum amount of
useful work that can be produced (or the
minimum work that needs to be supplied) as a
system undergoes a process between the
specified initial and final states.

The difference between
reversible work and
actual useful work is the
irreversibility.

Exergy (Work Potential) Associated with
Kinetic and Potential Energy

The
work potential
or
exergy
of potential
energy is equal to the
potential energy itself.

Exergy of kinetic energy:

Exergy of potential energy:

The exergies of kinetic
and potential energies
are equal to
themselves, and they
are entirely available for
work.

24

Exergy of a Fixed Mass:

Nonflow (or Closed System) Exergy

The
exergy
of a specified
mass at a specified state is
the useful work that can be
produced as the mass
undergoes a reversible
process to the state of the
environment.

25

Exergy of a Flow Stream: Flow (or Stream) Exergy

Exegy of flow
energy

Flow
exergy

The
exergy
associated with
flow energy
is the
useful work that
would be
delivered by an
imaginary piston
in the flow section.

26

EXERGY TRANSFER BY HEAT,
WORK, AND MASS

Exergy by Heat Transfer,
Q

Exergy transfer
by heat

The Carnot efficiency

c
=
1

T
0

/
T
represents the
fraction of the energy transferred from a heat
source at temperature
T
that can be converted to
work in an environment at temperature
T
0
.

The transfer and
destruction of exergy
during a heat transfer
process through a
finite temperature
difference.

27

EXERGY TRANSFER BY WORK,
W

There is no useful
work transfer
associated with
boundary work when
the pressure of the
system is maintained
constant at
atmospheric
pressure.

Exergy Transfer by Mass,
m

Mass contains
energy, entropy, and
exergy, and thus
mass flow into or out
of a system is
accompanied by
energy, entropy, and
exergy transfer.

28

EXERGY DESTRUCTION

The exergy change of a system
can be negative, but the exergy
destruction cannot.

Exergy destroyed is a
positive
quantity
for any actual process and
becomes
zero
for a reversible
process.

The exergy of an isolated system during a process always decreases
or, in the limiting case of a reversible process, remains constant.
In
other words, it
never
increases and
exergy is destroyed
during an
actual process. This is known as the

decrease of exergy principle
.

29

EXERGY BALANCE

Mechanisms
of exergy
transfer.

The exergy
change of a
system during a
process is equal
to the difference
between the net
exergy transfer
through the
system
boundary and
the exergy
destroyed within
the system
boundaries as a
result of
irreversibilities.

2
nd

Law (
Exergetic
) Efficiency,

II

SECOND
-
LAW EFFICIENCY,

II

Second
-
Law Efficiency of Resistance Heaters

of electric resistance heaters for residential
buildings that have an efficiency of 100%. Assuming
an indoor temperature of 21
°
C and outdoor
temperature of 10
°
C, determine the second
-
law
efficiency of these heaters.

Second
-
law efficiency of all reversible devices and
processes is100%.

2
nd

Law efficiency of Reversible devices

Q: Can the 2nd
-
law efficiency be greater than 1
st
-
law efficiency?

SECOND
-
LAW EFFICIENCY,

II

2
nd
-
-
Flow Devices

Heat Exchangers:

Mixing Chambers
:

Compressors:

Turbines:

EXAMPLE: Heating with a hot iron block

Actual process

Reversible process

500 kg

37

EXAMPLE: Minimum work input to a R
-
134a compressor

38

EXAMPLE : Heating of a gas by stirring vs. a heat pump

1
st

and 2
nd

law views of energy flows

Energy flow = 1 kW

Entropy flow = 0.002 kW/K

Exergy flow = 0.4 kW

“IMPURE ENERGY

Energy flow = 1 kW

Entropy flow = 0

Exergy flow = 1 kW

“PURE ENERGY

HEAT ENERGY FLOW,
1 kJ/s

ELECTRİCAL ENERGY FLOW,
1 kW

T
0

= 300 K

500 K

2
nd

Law as a Guide for

Green Practices and Sustainability

Green Engineering and Thermodynamics

Green engineering
is the design, commercialization, and use
of processes and products that are feasible and economical
while reducing the generation of pollution at the source and
minimizing the risk to human health and the environment.

Green thermodynamics
is a subcategory of green
engineering related to energy.

In thermodynamics, the concept of green can be associated
with an energy source, an energy interaction or transfer, and
energy conversion or use.

Green thermodynamic practices
are closely related to
minimizing waste or:

-

Minimizing
entropy generation
,

-

Minimizing
exergy destruction
(2
nd

law)

A process with a
higher second
-
law efficiency
is a greener
thermodynamic process.

Sustainability and Ethics

Sustainability
is usually concerned with
long
-
term impact
on resources,
environment, and processes on macro scale whereas
Green

is also
concerned with short
-
term effects, such as the indoor air quality, on micro
scale.

Green/sustainable practices
allow humans to exploit nature, but to do so
without inflicting irreversible damage
to the environment
and without
disturbing the ecological balance.

Sustainable development
: “
D
e
velopment that meets the needs of the
present without compromising the ability of future generations to meet their
own needs
”.
(World Commission on Environment and Development, 1987).

This definition is closely tied to
engineering ethics
. Ethical practices
require being considerate of the needs of future generations since the
world’s resources belong to them as much as they belong to us.

Green practices involve
doing the right thing
and
doing it right
.

The terms
“Green”
and

“Sustainability”
are loosely defined. They are
used to promote
‘green thinking’,
and they should be viewed as goals to
strive to reach.

“Green”
and
“Sustainable”
are often used interchangeably.

A process with a
higher second
-
law efficiency
is a
greener process.

A 2
nd

Law Application:

ASHRAE GreenGuide

Guidance to HVAC&R system
designers involved in
green
or
sustainable
building design.

A step
-
by
-
step manual for the
entire building lifecycle.

Includes 29
Green Tips
, specific
measures for improving
sustainability, such as Ground
-
Source Heat Pumps.

Covers
green design
techniques
applicable to related
technical disciplines, such as
plumbing and lighting.

Conservation: A 2
nd

Law Concept

Minimization of
exergy
destruction is
the greenest
thermodynamic
practice!

Conservation

implies
conserving the “exergy
content”
or “usefulness” of
energy.

1
st

law
: Energy is always
conserved
, even when heat is
lost from a building
(conservation of energy
principle).

2
nd

law
:
wasted energy
. Conserving
energy is preserving it at the
most useful form. Energy
converted to a useless form
is lost forever.

Last word from the 2
nd

law point
of view:

“ZERO WASTE”

Thank you!