1. Thermodynamics Lecture Series
11/20/2018
Thermodynamics Lecture Series
Assoc. Prof. Dr. J.J.
Entropy – Quantifying Energy Degradation
Applied Sciences Education Research Group (ASERG)
Faculty of Applied Sciences
Universiti Teknologi MARA
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

2. Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005
Quotes
“The principal goal of education is to create men and women who are capable of doing new things, not simply repeating what other generations have done” Jean Piaget
“What we have to learn to do, we learn by doing” Einstein
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

3. Review - First Law Properties will change indicating change of state
How to relate changes to the cause
System
E1, P1, T1, V1 To
E2, P2, T2, V2
Win
Wout
Mass in
Mass out
Qin
Qout
Dynamic Energies as causes (agents) of change
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

4. Review - First Law Energy Balance Ein – Eout = Esys, kJ or
Energy Entering a system -
Energy Leaving a system =
Change of system’s energy
Energy Balance
Ein – Eout = Esys, kJ or
ein – eout = esys, kJ/kg or
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

5. Review - First Law Mass Balance min – mout = msys, kg or
Mass Entering a system -
Mass Leaving a system =
Change of system’s mass
Mass Balance
min – mout = msys, kg or
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

6. Review - First Law Energy Balance – Control Volume Steady-Flow
Steady-flow is a flow where all properties within boundary of the system remains constant with time
DEsys= 0, kJ; Desys= 0 , kJ/kg, DVsys= 0, m3; Dmsys= 0 or min = mout , kg
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

7. Review - First Law Mass & Energy Balance–Steady-Flow: Single Stream
Mass balance
Energy balance
qin – qout+ win – wout = qout – qin, kJ/kg
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

8. Second Law Steam Power Plant An Energy-Flow diagram for a SPP
Working fluid:
Water
High T Res., TH
Furnace
Purpose:
Produce work,
Wout, out
qin = qH
qin - qout = out - in
qin = net,out + qout
Steam Power Plant
net,out
net,out = qin - qout
qout = qL
Low T Res., TL
Water from river
An Energy-Flow diagram for a SPP
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

9. Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005
Second Law
Thermal Efficiency for steam power plants
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

10. Second Law Refrigerator/ Air Cond Working fluid: Ref-134a net,in
High T Res., TH, Kitchen room / Outside house
qout – qin = in - out
qout = qH
Refrigerator/ Air Cond
net,in
qin = qL
net,in = qout - qin
Purpose:
Maintain space
at low T by
Removing qL
Low Temperature
Res., TL, Inside fridge or house
net,in = qH - qL
An Energy-Flow diagram for a Refrigerator/Air Cond.
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

11. Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005
Second Law
Coefficient of Performance for a Refrigerator
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

12. Second Law Heat Pump An Energy-Flow diagram for a Heat Pump
Working fluid:
Ref-134a
High Temperature
Res., TH, Inside house
Purpose:
Maintain space
at high T by
supplying qH
qout = net,in + qin
qout = qH
Heat Pump
net,in
net,in = qout - qin
qin = qL
net,in = qH - qL
Low Temperature
Res., TL, Outside house
An Energy-Flow diagram for a Heat Pump
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

13. Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005
Second Law
Coefficient of Performance for a Heat Pump
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

14. Second Law – Energy Degrade
What is the maximum performance of real engines if it can never achieve 100%??
Factors of irreversibilities
less heat can be converted to work
Friction between 2 moving surfaces
Processes happen too fast
Non-isothermal heat transfer
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

15. Second Law – Dream Engine
Carnot Cycle
Isothermal expansion
Slow adding of Q resulting in work done by system (system expand)
Qin – Wout = U = 0. So, Qin = Wout . Pressure drops.
Adiabatic expansion
0 – Wout = U. Final U smaller than initial U.
T & P drops.
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

16. Second Law – Dream Engine
Carnot Cycle
Isothermal compression
Work done on the system
Slow rejection of Q
- Qout + Win = U = 0. So, Qout = Win .
Pressure increases.
Adiabatic compression
0 + Win = U. Final U higher than initial U.
T & P increases.
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

17. Second Law – Dream Engine
Carnot Cycle
P -  diagram for a Carnot (ideal) power plant
P, kPa
, m3/kg
qout
qin 2 3 4 1
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Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

18. Second Law – Dream Engine
Reverse Carnot Cycle
P, kPa
qout
qin 3 4 2 1
, m3/kg
P -  diagram for a Carnot (ideal) refrigerator
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

19. Second Law – Dream Engine
Carnot Principles
For heat engines in contact with the same hot and cold reservoir
All reversible engines have the same performance.
Real engines will have lower performance than the ideal engines.
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

20. Second Law Steam Power Plants P1: 1 = 2 = 3 P2: real < rev
Working fluid:
Not a factor
High T Res., TH
Furnace
qin = qH
P1: 1 = 2 = 3
Steam Power Plants
net,out
P2: real < rev
qout = qL
Low T Res., TL
Water from river
An Energy-Flow diagram for a Carnot SPPs
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

21. Second Law Rev. Fridge/ Heat Pump Working fluid: Not a factor net,in
High T Res., TH, Kitchen room / Outside house
qout = qH
Rev. Fridge/ Heat Pump
net,in
qin = qL
Low Temperature
Res., TL, Inside fridge or house
An Energy-Flow diagram for Carnot Fridge/Heat Pump
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

22. Second Law – Will a Process Happen
Carnot Principles
For heat engines in contact with the same hot and cold reservoir
P1: 1 = 2 = 3 (Equality)
P2: real < rev (Inequality)
Processes satisfying Carnot Principles obeys the Second Law of Thermodynamics
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

23. Second Law – Will a Process Happen
Clausius Inequality :
Sum of Q/T in a cyclic process must be zero for reversible processes and negative for real processes
reversible
real
impossible
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

24. Second Law – Will a Process Happen
Source
Carnot SPP
qin = qH
Steam Power Plant
net,out
qout = qL
Sink
Processes satisfying Clausius Inequality obeys the Second Law of Thermodynamics
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

25. Entropy – Quantifying Disorder
Source
Entropy
qin = qH
Entropy Change in a process
Steam Power Plant
net,out
qout = qL
Sink
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

26. Entropy – Quantifying Disorder
Quantitative measure of disorder or chaos
Is a system’s property, just like the others
Does not depend on process path
Has values at every state
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

27. Entropy – Quantifying Disorder
Quantifies lost of energy quality
Can be transferred by heat and mass or
generated due to irreversibilty factors:
Frictional forces between moving surfaces.
Fast expansion & compression.
Heat transfer at finite temperature difference.
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

28. Entropy – Quantifying Disorder
Increase of Entropy Principle
The entropy of an isolated (closed and adiabatic) system undergoing any process, will always increase.
Surrounding
System
For pure substance:
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

29. Entropy – Quantifying Disorder
Increase of Entropy Principle Proven
Consider the following cyclic process containing an irreversible forward path and a reversible return path
Clausius Inequality
irreversible
reversible 2 1
Entropy
Change
So:
Then:
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

30. Entropy – Quantifying Disorder
Increase of Entropy Principle Proven
Consider the following cyclic process containing an irreversible forward path and a reversible return path
Then entropy change for the closed system:
irreversible
reversible 2 1
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

31. Entropy – Quantifying Disorder
Increase of Entropy Principle Proven
Consider the following cyclic process containing an irreversible forward path and a reversible return path
Then entropy change for the closed system:
irreversible
reversible 2 1
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

32. Entropy – Quantifying Disorder
Increase of Entropy Principle Proven
Consider the following cyclic process containing an irreversible forward path and a reversible return path
Then entropy change for the closed system:
irreversible
reversible 2 1
For adiabatic process:
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

33. Entropy – Quantifying Disorder
Increase of Entropy Principle Proven
Consider the following cyclic process containing an irreversible forward path and a reversible return path
Then entropy change for the closed system:
irreversible
reversible 2 1
For adiabatic reversible system:
Isentropic or constant entropy process
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

34. Entropy – Quantifying Disorder
Increase of Entropy Principle Proven
Consider the following cyclic process containing an irreversible forward path and a reversible return path
Then entropy change for the closed system:
irreversible
reversible 2 1
For isolated (adiabatic & closed) system:
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

35. Entropy – Quantifying Disorder
T – S diagram
Area of curve under P – V diagram represents total work done
Area of curve under T – S diagram represents total heat transfer
Hence total heat transfer is
Recall
Then
Area under T- S diagram is amount of heat in a process
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

36. Entropy – Quantifying Disorder
T – s diagram
The infinite area dA = area of strip = Tds
s, kJ/kgK 2 1
qin=qH
T, C TH T TL ds
Adding all the area of the strips from state 1 to state 2 will give the total area under process curve. It represents specific heat received for this process
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

37. Entropy – Quantifying Disorder
Factors affecting Entropy (disorder)
Entropy will change when there is
Heat transfer (receiving heat increases entropy)
Mass transfer (moving mass changes entropy)
Irreversibilities (entropy will always be generated)
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

38. Entropy – Quantifying Disorder
Entropy Balance
For any system undergoing any process,
Energy must be conserved (Ein – Eout = Esys)
Mass must be conserved (min – mout = msys)
Entropy will always be generated except for reversible processes
Entropy balance is (Sin – Sout + Sgen = Ssys)
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

39. Entropy – Quantifying Disorder
Entropy Balance –Closed system
Energy Balance:
Entropy Balance:
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Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

40. Entropy – Quantifying Disorder
Entropy Balance –Steady-flow device
Then:
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

41. Entropy – Quantifying Disorder
Entropy Balance –Steady-flow device
Nozzle:
Assume adiabatic,
no work done,
pemass = 0
where
Entropy Balance In
State 1
Out
State 2
A2 << A1
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

42. Entropy – Quantifying Disorder
Entropy Balance –Steady-flow device
Turbine:
Assume adiabatic,
kemass = 0,
pemass = 0
where In
Out
Entropy
Balance
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

43. Entropy – Quantifying Disorder
Entropy Balance –Steady-flow device
Qin
Heat exchanger: energy balance; 2 cases
where
Assume kemass = 0,pemass = 0
Case 1
Case 2
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

44. Entropy – Quantifying Disorder
Entropy Balance –Steady-flow device
Qin
Heat exchanger:
Entropy Balance
where
Case 1
Case 2
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005

45. Entropy – Quantifying Disorder
11/20/2018
Entropy – Quantifying Disorder
Entropy Balance –Steady-flow device
Mixing Chamber: 1 3 2
where
11/20/2018
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005
Copyrights DR JJ, ASERG, FSG, UiTM Shah Alam, 2005