Thermodynamics Lecture Series Applied Sciences Education Research Group (ASERG) Faculty of Applied Sciences Universiti Teknologi MARA Entropy – Quantifying Energy Degradation

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

How to relate changes to the cause Dynamic Energies as causes (agents) of change System E 1, P 1, T 1, V 1 To E 2, P 2, T 2, V 2 Properties will change indicating change of state Mass out Mass in W in W out Q in Q out Review - First Law

Energy Balance E in – E out =  E sys, kJ or e in – e out =  e sys, kJ/kg or Energy Entering a system - Energy Leaving a system = Change of system’s energy Review - First Law

Mass Balance m in – m out =  m sys, kg or Mass Entering a system - Mass Leaving a system = Change of system’s mass Review - First Law

 E sys = 0, kJ;  e sys = 0, kJ/kg,  V sys = 0, m 3 ;  m sys = 0 or m in = m out, kg Energy Balance – Control Volume Steady-Flow Steady-flowall properties remains constant with time Steady-flow is a flow where all properties within boundary of the system remains constant with time Review - First Law

Mass & Energy Balance–Steady-Flow: Single Stream q in – q out +  in –  out =  out –  in, kJ/kg Energy balance Mass balance Review - First Law

Second Law Steam Power Plant High T Res., T H Furnace q in = q H  net,out Low T Res., T L Water from river An Energy-Flow diagram for a SPP q out = q L Working fluid: Water q in - q out =  out -  in  net,out = q in - q out Purpose: Produce work, W out,  out q in =  net,out + q out

Second Law Thermal Efficiency for steam power plants

Second Law Refrigerator/ Air Cond High T Res., T H, Kitchen room / Outside house q out = q H  net,in Low Temperature Res., T L, Inside fridge or house An Energy-Flow diagram for a Refrigerator/Air Cond. q in = q L Working fluid: Ref-134a q out – q in =  in -  out  net,in = q out - q in Purpose: Maintain space at low T by Removing qLqL  net,in = q H - q L

Second Law Coefficient of Performance for a Refrigerator

Second Law Heat Pump High Temperature Res., T H, Inside house q out = q H  net,in Low Temperature Res., T L, Outside house An Energy-Flow diagram for a Heat Pump q in = q L Working fluid: Ref-134a q out =  net,in + q in  net,in = q out - q in Purpose: Maintain space at high T by supplying qHqH  net,in = q H - q L

Second Law Coefficient of Performance for a Heat Pump

Second Law – Energy Degrade Factors of irreversibilities less heat can be converted to work –Friction between 2 moving surfaces –Processes happen too fast –Non-isothermal heat transfer What is the maximum performance of real engines if it can never achieve 100%??

Second Law – Dream Engine Carnot Cycle Isothermal expansionIsothermal expansion  Slow adding of Q resulting in work done by system (system expand)  Q in – W out =  U = 0. So, Q in = W out. Pressure drops. Adiabatic expansionAdiabatic expansion  0 – W out =  U. Final U smaller than initial U.  T & P drops.

Second Law – Dream Engine Carnot Cycle Isothermal compression  Work done on the system  Slow rejection of Q  - Q out + W in =  U = 0. So, Q out = W in.  Pressure increases. Adiabatic compression  0 + W in =  U. Final U higher than initial U.  T & P increases.

Second Law – Dream Engine Carnot Cycle P - diagram for a Carnot (ideal) power plant P, kPa, m 3 /kg q out q in

Second Law – Dream Engine Reverse Carnot Cycle P - diagram for a Carnot (ideal) refrigerator P, kPa q out qinqin , m 3 /kg

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.

Second Law Steam Power Plants High T Res., T H Furnace q in = q H  net,out Low T Res., T L Water from river An Energy-Flow diagram for a Carnot SPPs q out = q L Working fluid: Not a factor P1: 1 1 = 2 2 = 33 P2:  real <  rev

Second Law Rev. Fridge/ Heat Pump High T Res., T H, Kitchen room / Outside house  net,in Low Temperature Res., T L, Inside fridge or house An Energy-Flow diagram for Carnot Fridge/Heat Pump Working fluid: Not a factor q in = q L q out = q H

Carnot Principles For heat engines in contact with the same hot and cold reservoir  P1:  1 =  2 =  3 (Equality)  P2:  real <  rev (Inequality) Second Law – Will a Process Happen Processes satisfying Carnot Principles obeys the Second Law of Thermodynamics

Clausius Inequality : Sum of Q/T in a cyclic process must be zero for reversible processes and negative for real processes Second Law – Will a Process Happen reversible impossible real

Steam Power Plant Source q in = q H  net,out Sink q out = q L Second Law – Will a Process Happen Processes satisfying Clausius Inequality obeys the Second Law of Thermodynamics Carnot SPP

Steam Power Plant Source  net,out Sink Entropy – Quantifying Disorder Entropy Entropy Change in a process q out = q L q in = q H

Entropy 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 Entropy – Quantifying Disorder

Entropy 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. Entropy – Quantifying Disorder

Increase of Entropy Principle The entropy of an isolated (closed and adiabatic) system undergoing any process, will always increase. Entropy – Quantifying Disorder Surrounding System For pure substance :

Increase of Entropy Principle Proven –Consider the following cyclic process containing an irreversible forward path and a reversible return path Entropy – Quantifying Disorder irreversible reversible 2 1 Clausius Inequality So: Then: Entropy Change

Increase of Entropy Principle Proven –Consider the following cyclic process containing an irreversible forward path and a reversible return path Entropy – Quantifying Disorder irreversible reversible 2 1 Then entropy change for the closed system:

Increase of Entropy Principle Proven –Consider the following cyclic process containing an irreversible forward path and a reversible return path Entropy – Quantifying Disorder irreversible reversible 2 1 Then entropy change for the closed system:

Increase of Entropy Principle Proven –Consider the following cyclic process containing an irreversible forward path and a reversible return path Entropy – Quantifying Disorder irreversible reversible 2 1 Then entropy change for the closed system: For adiabatic process:

Increase of Entropy Principle Proven –Consider the following cyclic process containing an irreversible forward path and a reversible return path Entropy – Quantifying Disorder irreversible reversible 2 1 Then entropy change for the closed system: For adiabatic reversible system: Isentropic or constant entropy process

Increase of Entropy Principle Proven –Consider the following cyclic process containing an irreversible forward path and a reversible return path Entropy – Quantifying Disorder irreversible reversible 2 1 Then entropy change for the closed system: For isolated (adiabatic & closed) system:

T – S diagram Entropy – Quantifying Disorder Area of curve under P – V diagram represents total work done Area of curve under T – S diagram represents total heat transfer Recall Then Hence total heat transfer is Area under T- S diagram is amount of heat in a process

T – s diagram Entropy – Quantifying Disorder 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 The infinite area dA = area of strip = Tds s, kJ/kg  K 2 1 q in =q H T,  C THTH T TLTL ds

Factors affecting Entropy (disorder) Entropy – Quantifying 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)

Entropy Balance Entropy – Quantifying Disorder  For any system undergoing any process,  Energy must be conserved (E in – E out =  E sys )  Mass must be conserved (m in – m out =  m sys )  Entropy will always be generated except for reversible processes  Entropy balance is (S in – S out + S gen =  S sys )

Entropy Balance –Closed system Entropy – Quantifying Disorder Energy Balance: Entropy Balance:

Entropy Balance –Steady-flow device Entropy – Quantifying Disorder Then:

Entropy Balance –Steady-flow device Entropy – Quantifying Disorder Nozzle: Assume adiabatic, no work done,  pe mass = 0 where Entropy BalanceIn State 1 Out State 2 A 2 << A 1

Entropy Balance –Steady-flow device Entropy – Quantifying Disorder Turbine: Assume adiabatic,  ke mass = 0,  pe mass = 0 where Entropy Balance In Out

Entropy Balance –Steady-flow device Entropy – Quantifying Disorder Heat exchanger: energy balance; 2 cases Assume  ke mass = 0,  pe mass = 0 where Q in Case 1 Case 2

Entropy Balance –Steady-flow device Entropy – Quantifying Disorder Heat exchanger: Entropy Balance where Q in Case 1 Case 2

Entropy Balance –Steady-flow device Entropy – Quantifying Disorder Mixing Chamber: where 1 3 2