Transcript Chapter 1 INTRODUCTION AND BASIC CONCEPTS

INTRODUCTION TO 2nd LAW
and EXERGY
Yunus Çengel
Adnan Menderes University, Aydin
University of Nevada, Reno (USA)
Üniversite Yönetişiminde 1. ve 2. Kanun
1nci Kanun Uygulaması: Sayıştay (Girdi = Çıktı). Verim her zaman %100
2nci Kanun Uygulaması: Etkin Yönetim (Fayda gözetimi). Verim?
2. Kanunda Hedef: Mevcut kaynaklarla (insan, mekan, maddi kaynak)
amaçlanan en yüksek faydayı sağlamak
Çıktı odaklı yaklaşım:
Öğrenci kazanımları (bilgi, beceri, yetkinlik)
Havanda su dövmenin (kaynak israfı) önlenmesi
Örnek 1: Çok sayıda Bölüm açılması (Elektrik, Elektronik, Elektrik/Elektronik,
Kontrol, Haberleşme, Bilgisayar, …. )
ABD: Electrical and Computer Engineering Department)
Örnek 3: Her bölüm için ayrı bir Termodinamik dersi açılması.
Örnek 23: 5i derslerinin sınıf ortamında yapılması (İnkilap Tarihi, Türk Dili, …)
Yerlerine ‘Türkçe İletişim, İngilizce İletişim, ve (Sosyal Seçmeli) Tarih’
derslerinin açılması
8 Anahtar Yetkinlik (Temel Yetkinlikler)
(Avrupa Parlamentosu AP, 2006)
Her öğrencinin sahip olması gereken temel beceriler (anahtar yetkinlikler):
1- anadilde iletişim,
2- yabancı dillerde iletişim,
3- matematikte yetkinlik ile bilim ve teknolojide temel yetkinlikler
(analiz etme, kritik düşünce, bilimsel yaklaşım),
4- dijital yetkinlik (bilişim/iletişim teknolojilerini etkin kullanma),
5- öğrenmeyi öğrenme,
6- sosyal ve medeni yetkinlikler (demokrasi, kişisel haklar, vs),
7- girişkenlik (insiyatif kullanma) ve girişimcilik (yaratıcılık,
inovasyon, etiklik, liderlik)
8- kültürel farkındalık ve yaratıcı ifade (sanat, müzik, edebiyat,
tiyatro, kültürel faaliyetler).
Outline
• Introduce the 2nd 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.
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1st and 2nd Laws
• The 1st law of thermodynamics is mundane since it deals with
conserved quantities.
• The 2nd 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
2nd law concepts such as entropy, exergy, and 2nd law efficiency,
and question the utility of the 2nd 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 2nd -law
analysis, which enables one to examine the system or process from
a different angle.
• The 1st law (or energy) analysis provides a map for energy flow and
conversion for a process, whereas the 2nd law provides a map of
inefficiencies and waste occurring throughout the process.
Thermodynamics in a nutshell
• Mass balance: Mass change = Mass transfer
min  mout  msystem
• Energy balance: Energy change = Energy transfer
Ein  Eout


Net energy transfer
by heat, work, and mass

Esystem



Change in internal, kinetic,
potential, etc.energies
• Entropy balance: Entropy change = Entropy transfer + Entropy
generation
S S
 S
 S
out
in


Net entropy transfer
by heat and mass
gen

Entropy
generation
system




Change
in entropy
• Exergy balance: Exergy change = Exergy transfer - Exergy
destruction
X in  X out
 X destroyed  X system


 


Net exergy transfer
by heat, work, and mass
Exergy
destruction
Change
in exergy
Entropy Generation and
Exergy destruction
associated with heat transfer
Entropy transfer
by heat
Exergy transfer
by heat
Energy is conserved,
Entropy is generated,
Exergy is destroyed.
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Energy is conserved, Entropy generated, Exergy
destroyed
2 + 2 = 4? Belki!
• Soru 1: 2 + 2 = ?
1. Kanun: 4 (Enerji gibi korunan her şey için doğru)
2. Kanun: 0, 1, 4, 7, … (Entropi ve ekserji gibi şeyler için bütünün
değeri parçalarının toplamanından büyük veya küçük olabilir)
• Soru 2: Birlikten kuvvet doğar mı?
1. Kanun: Evet, her zaman.
2. Kanun: Hayır, birlikten zaaf da doğabilir.
• Soru 3: 1. ve 2. Kanun verimlerinin farkı nedir?
1. Kanun verimi: Ulaşılan performansın sağlanan kaynaklara
oranı.
2. Kanun verimi: Ulaşılan performansın mevcut şartlar altında
ulaşılabilecek en iyi performansa oranı.
Termodinamiğin 2nci Kanunu:
Enerjinin Görünmeyen İkinci Boyutu
1. Kanun: Miktar tabanlı
• Enerji enerjidir;
• Tüm enerjiler eşittir.
• Miktar her zaman korunur, yok edilemez.
2. Kanun: Kalite tabanlı
• Enerjiden enerjiye fark vardır.
• Enerjiler eşit değildir.
• Kalite varken yok olur.
An overview of the 1st and 2nd Laws
Implications of the 2nd law
• The 2nd 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 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 are closely
related to the 2nd-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 2nd law concept, as it involves the practice
of best resource utilization and waste elimination.
• Even “reliability” is related to the 2nd law since wasted energy often
causes excessive operating temperatures, and thus a higher rate of
failure.
Basic Questions
• 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?
ANOTHER QUESTION:
From a thermodynamic point of view, which is a better
energy conversion/transfer process?
1st law efficiency: The level of performance achieved compared
to the resources provided.
2nd law efficiency: The level of performance achieved compared
to the best possible performance under the circumstances.
Process A: 1st Law efficiency=100%; 2nd Law efficiency <100%.
Process B: 1st Law efficiency<100%; 2nd Law efficiency =100%.
.
2nd law efficiency – A measure of Perfection
• A process with a 2nd law efficiency of 100%:
- Is perfect (even if its 1st law efficiency is less than 100%).
- Entropy generation = 0
- Exergy destruction = 0
- Waste = 0
• Something cannot be more perfect than perfect. The 2nd law
defines the upper limit.
• Perfection is good and beautiful, which are liked for what they
are.
• Aim with 2nd Law: ‘Perfection’ and ‘Zero waste’.
1st LAW vs. 2nd LAW: Matter vs. Non-matter
• 1st 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
• 2nd 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.
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PRIMARY USES OF THE 2nd LAW AND EXERGY
1. The 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. Exergy 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.
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INTRODUCTION TO THE 2nd LAW
A cup of hot coffee
does not get hotter in
a cooler room.
Transferring
heat to a
paddle wheel
will not cause
it to rotate.
Transferring
heat to a wire
will not
generate
electricity.
These processes
cannot occur
even though they
are not in violation
of the first law.
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The 2nd Law and Exergy
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.
A heat engine that violates the
Kelvin–Planck statement of the
second law.
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 highertemperature 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.
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THE CARNOT HEAT ENGINE
Any heat
engine
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.
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.
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.
How do you increase the
COP of a Carnot
refrigerator or heat pump?
How about for actual ones?
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EXERGY: WORK POTENTIAL OF ENERGY
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 dead state when it is in thermodynamic
equilibrium with the environment it is in.
A system that is in equilibrium
with its environment is said to be
at the dead state.
The atmosphere contains a
tremendous amount of energy, but
no exergy.
REVERSIBLE WORK and EXERGY DESTRUCTION
Reversible work Wrev: 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.
As a closed
system expands,
some work needs
to be done to push
the atmospheric
air out of the way
(Wsurr).
The difference between
reversible work and
actual useful work is the
irreversibility.
For constant-volume
systems, the total
actual and useful
works are identical
(Wu = W).
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Exergy (Work Potential) Associated with
Kinetic and Potential Energy
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.
The work potential or
exergy of potential
energy is equal to the
potential energy itself.
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.
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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.
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EXERGY TRANSFER BY HEAT,
WORK, AND MASS
Exergy by Heat Transfer, Q
Exergy transfer
by heat
The transfer and
destruction of exergy
during a heat transfer
process through a
finite temperature
difference.
The Carnot efficiency c=1T0 /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 T0.
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EXERGY TRANSFER BY WORK, W
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.
There is no useful
work transfer
associated with
boundary work when
the pressure of the
system is maintained
constant at
atmospheric
pressure.
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EXERGY DESTRUCTION
Exergy destroyed is a positive
quantity for any actual process and
becomes zero for a reversible
process.
The exergy change of a system
can be negative, but the exergy
destruction cannot.
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.
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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.
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2nd Law (Exergetic) Efficiency, II
SECOND-LAW EFFICIENCY, II
Second-Law Efficiency of Resistance Heaters
A dealer advertises that he has just received a shipment
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.
2nd Law efficiency of Reversible devices
Second-law efficiency of all reversible devices and
processes is100%.
Q: Can the 2nd-law efficiency be greater than 1st-law efficiency?
SECOND-LAW EFFICIENCY, II
2nd-Law Efficiency of Steady-Flow Devices
Turbines:
Compressors:
Heat Exchangers:
Mixing Chambers:
EXAMPLE: Heating with a hot iron block
500 kg
Actual process
Reversible process
EXAMPLE: Minimum work input to a R-134a compressor
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EXAMPLE : Heating of a gas by stirring vs. a heat pump
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1st and 2nd law views of energy flows
HEAT ENERGY FLOW,
1 kJ/s
500 K
• Energy flow = 1 kW
• Entropy flow = 0.002 kW/K
• Exergy flow = 0.4 kW
T0 = 300 K
“IMPURE ENERGY”
ELECTRİCAL ENERGY FLOW,
1 kW = 1 kJ/s
• Energy flow = 1 kW
• Entropy flow = 0
• Exergy flow = 1 kW
“PURE ENERGY”
2nd 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 (2nd law)
• A process with a higher second-law efficiency is a greener
thermodynamic process.
A 2nd 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 GroundSource Heat Pumps.
• Covers green design
techniques applicable to related
technical disciplines, such as
plumbing and lighting.
Sustainability and Ethics: Relation to 2nd Law
• Sustainable development: “Development 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/sustainable practices allow humans to exploit nature,
but to do so without inflicting irreversible damage to the
environment and without disturbing the ecological balance.
• 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.
• A process with a higher second-law efficiency is a greener
process.
Energy Conservation:
A 2nd Law Concept
• 1st law: Energy is always
conserved, even when heat is
lost from a building (conservation
of energy principle).
• 2nd law: Degraded energy is
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 2nd law point
of view:
“ZERO WASTE”
Thank you!
Q: Is 2 + 2 = 4?
A: Maybe!
1st Law: 2 + 2 = 4 is true for conserved quantities like energy.
2nd Law: There are quantities for which the value of the whole is less or
more than the arithmetic sum of the parts, like entropy and exergy.
Question #1: What is 2+2 = ?
1st law: 4
2nd law: 0, 1, 4, 7, …. .
Question #2: Is unity a source of strength?
1st law: Yes, always.
2nd law: Not always – it may be a source of weakness as well.
Question #3: How do the 1st- and 2nd-law efficiencies compare?
1st law efficiency: The level of performance achieved compared to the
resources provided.
2nd law efficiency: The level of performance achieved compared to the best
possible performance under the circumstances.