TERMODİNAMİĞİN TEMEL LER İ – 2 2nci KANUN ve EKS ERJİ

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TERMODİNAMİĞİN TEMEL LER İ – 2 2nci KANUN ve EKS ERJİ. Prof. Dr. Yunus Çengel Nevada Üniversitesi Reno, Nevada, ABD. Termodinamiğin 2nci Kanunu: Enerjinin görünmüyen İkinci Boyutu. 1. Kanun: Miktar tabanlı Enerji enerjidir; Tüm enerjiler eşittir.

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### TERMODİNAMİĞİN TEMELLERİ – 2 2nci KANUN ve EKSERJİ

Prof. Dr. Yunus Çengel

Termodinamiğin 2nci Kanunu: Enerjinin görünmüyen 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.
Kısım 2: Exerji ve İsrafın Sıfırlanması

Temel Sorular:

• Verimde teorik üst sınır %100 müdür?

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.

A heat engine that violates the Kelvin–Planck statement of the second law.

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

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

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 availabilityor available energy.

A system is said to be in the dead statewhen 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.

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.

SECOND-LAW EFFICIENCY, II

Second-law efficiency is a measure of the performance of a device relative to its performance under reversible conditions.

Two heat engines that have the same thermal efficiency, but different maximum thermal efficiencies.

2nci Kanun verimi:

Mükemmellik Ölçüsü

• Birolayın2nci Kanunverimi%100 ise:
• O olayMÜKEMMELDİR (1inci kanunverimi %100 olmasa bile)
• Entropiüretimi = 0
• Ekserjiimhası = 0
• İsraf = 0
• Mükemmeldendahamükemmelbirşeyyapılamaz.
• MükemmelşeylerGÜZELDİR.
• Güzellikvemükemmellikzatındasevilir, veinsanahazvehuzurverir.
• HEDEF: Kusursuzluk, mükemmellik, veSIFIR İSRAF.

General definition of exergy efficiency

The second-law efficiency of naturally occurring processes is zero if none of the work potential is recovered.

Second-law efficiency of all reversible devices is 100%.

EXERGY OF A SYSTEM

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.

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.

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.

EXERGY TRANSFER BY WORK, W

Exergy Transfer by Mass, m

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

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

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 thedecrease of exergy principle.

EXERGY BALANCE

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.

Mechanisms of exergy transfer.

EXAMPLE: Heating with a hot iron block

500 kg

Actual process

Reversible process

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.

ISI ve İŞ ENERJİLERİ

• ISI ENERJİSİ AKIMI, 1 kJ/s
• Enerjiakımı= 1 kW
• Entropy akımı= 0.002 kW/K
• Ekserjiakımı= 0.4 kW
• “POSALI ENERJİ”

500 K

300 K

• Enerjiakımı= 1 kW
• Entropy akımı= 0
• Ekserjiakımı= 1 kW
• “SAF ENERJİ”
• ELEKTRİK ENERJİSİ AKIMI, 1 kW
Tasarruf: Bir 2. Kanun Kavramı
• 1. Kanun: Enerji zaten her zaman – binadan ısı kaybıolurken dahi – korunur (Enerjinin korunumu kanunu)
• 2. Kanun: Kalitesi düşen enerji, israf edilmiş olan enerjidir. Enerji tasarrufu, enerjiyi en faydalı halinde muhafaza etmektir. Faydasız bir hale dönüşen enerji tamamen kaybolmuştur.

Ekserjiimhasının minimize edilmesi, en yeşil (çevredostu) termodinamikpratiğidir.

2nci Kanun Verimi: Sosyal Hayat
• Siyaset: Şu kadar senede şu kadar fabrika açmak, bu kadar yol yapmak, milli geliri şu kadar dolara yükseltmek , vs pek bir şey ifade etmez. Başarı ölçüsü: Kullanılan para, emek, vs kaynaklarla aynı sürede ne kadar yapılabilirdi? Aynı sürede baska ülkeler ne yapmışlar? Zaman ve kaynakların ne kadarı değerlendirildi, ne kadarı israf edildi?
• Firma yöneticisi: Firma o dönemde ne tür performans gösterebilirdi, nasıl performans gösterdi? Benzer şartlardaki benzer firmaların performansları nedir?
• Yolculuk: İşe veya okula gidip gelirken otobüs veya trende çok iş yapılabilir; hatta bir dil öğrenilebilir.