Second Law of Thermodynamics

S.Gunabalan Associate Professor Mechanical Engineering Department Bharathiyar College of Engineering & Technology Karaikal - 609 609. e-Mail : gunabalans@yahoo.com

Second law of thermodynamics

• The second law of thermodynamics about the direction of heat transfer

• Second law can be visualized in terms of the waterfall – You can not send water up with out energy

Second law of thermodynamics

• Second Law for Heat Engines It is impossible to extract heat QHot from a hot reservoir and use it all to do work W . Some amount of heat QCold must be exhausted to a cold reservoir.

Called Kelvin-Planck statement

Second law of thermodynamics

• Second Law for Refrigerator • It is not possible for heat to flow from a colder body

to a warmer body without any work having been done to accomplish this flow.

Called Clausius statement

Second law of thermodynamics

Second Law: Entropy A measure of the amount of energy which is unavailable to do work. A state variable whose change is defined for a reversible process at T where Q is the heat absorbed A measure of the disorder of a system.

First Law efficiency

• Efficiency = output energy of device / input energy of device – Irrespective of form of energy – Availability of energy at different temperature

Second Law efficiency

• Efficiency = 풂풗풂풊풍풂풃풍풆풆풏풆풓품풚 푬풙풆풓품풚

• 푬풙풆풓품풚 Defined as the available energy for the work

Exergy Balance For a Closed System

• Exergy balance for a closed system can be developed by combining the energy and entropy balances for a closed system.

• Energy Balance

∫ 푑푄 = ∆퐸 + 푊 ----------(1)

Exergy Balance For a Closed System

• ∫ 푑푄 = ∆퐸 + 푊−−−− −(1)

• Entropy Balance

∫ 푑푄/푇 = ∆푆 --------------(2)

• Multiplying the second equation by T0 and subtracting it from the first one yields

• ∫ 푑푄 = 푇표.∆푆

Exergy Balance For a Closed System

• ∫ 푑푄 = ∆퐸 + 푊−−−− −(1)

• ∫ 푑푄 = 푇표.∆푆

• Subtract ------------------------------------

• ∫ 1 − 푑푄 = ∆퐸 + 푊 − 푇표.∆푆

• E = u + v2/2+gZ

Exergy Balance For a Closed System

• ∫ 1 − 푑푄 = ∆퐸 + 푊 − 푇표.∆푆

• E = u + v2/2+gZ

1 −

푇표푇 푑푄 = 푢2 − 푢1 + (

푉22 − 푣1

2

2 ) + 푝0(푉2 − 푉1) − 푇표(푆2 − 푆1)

Reference • http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html • Rajput, R. K. 2010. Engineering thermodynamics. Jones and Bartlett

Publishers, Sudbury, Mass. • Nag, P. K. 2002. Basic and applied thermodynamics. Tata McGraw-Hill, New

Delhi.