Thermodynamics Lecture Series

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1 Thermodynamics Lecture Series Second Law uality of Energy Applied Sciences Education Research Group (ASERG) Faculty of Applied Sciences Universiti Teknologi MARA

2 uotes One who learns by fdg out has a sevenfold the skill of the one who learned by beg told Arthur Gutterman You do not really understand somethg unless you can expla it to your grandfather Este

3 Symbols W ω V ν E ϑ W υ

4 Review - First Law All processes must obey energy conservation Processes that do not obey cannot happen Piston-cylders, rigid tanks Turbes, compressors, Nozzles, heat exchangers

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

6 Review - First Law Energy Enterg a system - Energy Leavg a system Change of system s energy Energy Balance Amount of energy causg change must be eual to amount of energy change of system

7 Review - First Law Energy Enterg a system - Energy Leavg a system Change of system s energy Energy Balance E E out E sys, kj or e e out e sys, kj/kg or E E out E sys,kw

8 Review - First Law Mass Enterg a system - Mass Leavg a system Change of system s mass Mass Balance m m out m sys, kg or m m out m sys,kg / s

9 Review - First Law Energy Balance Control Volume Steady-Flow Steady-flow is a flow where all properties with boundary of the system remas constant with time E sys 0, kj; e sys 0, kj/kg, V sys 0, m 3 ; m sys 0 or m m out, kg m sys 0, kg/s m m out 0 or m m out, kg/s

10 Review - First Law Mass balance m sys 0, So, Mass & Energy Balance Steady- Flow CV m m out or m m out, kg/s Energy balance E sys 0, So, E E out kj/s + W + m ϑ out + W out + m ϑ out, kw + ω + θ out + ω out + θ out, kj/kg

11 Mass balance m sys Review - First Law Mass & Energy Balance Steady- Flow: Sgle Stream 0. So, m m out, kg/s Energy balance out + W E sys W out 0. So, m ϑ out E E out m ϑ out + ω ω out θ out θ, kj/kg, kj/s, kw

12 First Law volves uantity or amount of energy to be conserved processes Second Law 0 - out u u -u 2, kj/kg out T sys,itial 40 C T sys,fal 25 C out T surr 25 C This is a natural process!!! flows from high T to low T medium until thermal euilibrium is reached OK for this cup

13 Second Law u u 2 -u, kj/kg T sys,itial 25 C T sys,fal 40 C T surr 25 C This is NOT a natural process!!! does not flow from high T to low T medium. Never will it return to its itial state.

14 Second Law u u 2 -u, kj/kg T sys,itial 25 C T sys,fal 40 C T surr 25 C This is NOT a natural process!!! does not flow from high T to low T medium. Never will it return to its itial state.

15 First Law volves uantity or amount of energy to be conserved processes But is the process this cup possible?? Second Law u u 2 -u kj/kg T sys,itial 25 C T sys,fal 40 C T surr 25 C This is a NOT a natural process!!! does not flow from low T to high T medium. Never will euilibrium be reached

16 Second Law First Law not sufficient First Law not sufficient to determe if a process can or cannot proceed

17 Second Law First Law not sufficient to determe if a process can or cannot proceed Introduce the second law of thermodynamics processes occur its natural direction. Heat (thermal energy) flows from high temperature medium to low temperature medium. Energy has uality & uality is higher with higher temperature. More work can be done.

18 Considerations: Second Law Work can be converted to heat directly & totally. Heat cannot be converted to work directly & totally. Reuires a special device heat enge.

19 Second Law Heat Enge Characteristics: Receive heat from a high T source. Convert part of the heat to work. Reject excess heat to a low T sk. Operates a cycle.

20 Heat Enges Second Law Thermodynamics heat enges external combustion: steam power plants Combustion outside system Mechanical heat enges ternal combustion: jets, cars, motorcycles Combustion side system Performance Desired output / Reuired put

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

22 Workg fluid: Water Second Law High T Res., T H Furnace H Boiler ω Pump Turbe ω out Condenser - out ω out - ω out L Low T Res., T L Water from river - out ω net,out A Schematic diagram for a Steam Power Plant

23 Second Law Thermal Efficiency for steam power plants η desired output ω η reuired put ω net,out out net,out out out L H

24 Second Law η Thermal Efficiency for steam power plants η W net desired reuired,out output put out W net,out out out L H

25 Second Law Kelv Planck Statement for steam power plants It is impossible for enges operatg a cycle to receive heat from a sgle reservoir and convert all of the heat to work. Heat enges cannot be 00% efficient.

26 Workg fluid: Ref-34a Second Law High T Res., T H, Kitchen room / Outside house out ω - ω out out H Refrigerator/ Air Cond ω net, ω net, out - ω net, H - L Low Temperature Res., T L, Inside fridge or house L Purpose: Mata space at low T by Removg L An Energy-Flow diagram for a Refrigerator/Air Cond.

27 Workg fluid: Refrigerant-34a Second Law High T Res., T H Kitchen/Outside house out H Condenser Throttle Valve Evaporator Com pressor ω L Low T Res., T L Ref. Space/Room A Schematic diagram for a Refrigerator/Air Cond.

28 Second Law Coefficient of Performance for a Refrigerator COP R desired output reuired put ω net, COP R ω net, out out out H L

29 Second Law Second Law Coefficient of Performance for a Refrigerator, net R W put reuired output desired COP, net R W COP out out out L H

30 Workg fluid: Ref-34a Second Law High Temperature Res., T H, Inside house Purpose: Mata space at high T by out ω net, + out H supplyg H Heat Pump ω net, ω net, out - ω net, H - L L Low Temperature Res., T L, Outside house An Energy-Flow diagram for a Heat Pump

31 Workg fluid: Refrigerant-34a Second Law High T Res., T H Inside house out H Condenser Throttle Valve Evaporator Com pressor ω L Low T Res., T L Outside the house A Schematic diagram for a Heat Pump

32 Second Law Coefficient of Performance for a Heat Pump COP COP HP HP desired output out reuired put ω ω out net, out out net, out out out out L H

33 Second Law Clausius Statement on Refrigerators/Heat Pump It is impossible to construct a device operatg a cycle and produces no effect other than the transfer of heat from a low T to a high T medium. Must do external work to the device to make it function. Hence more energy removed to the surroundg.

34 Second Law Energy Degrade What is the maximum performance of real enges if it can never achieve 00%?? Factors of irreversibilities less heat can be converted to work Friction between 2 movg surfaces Processes happen too fast Non-isothermal heat transfer

35 Second Law Dream Enge Carnot Cycle Isothermal expansion Slow addg of resultg work done by system (system expand) W out U 0. So, W out. Pressure drops. Adiabatic expansion 0 W out U. Fal U smaller than itial U. T & P drops.

36 Second Law Dream Enge Carnot Cycle Isothermal compression Work done on the system Slow rejection of - out + W U 0. So, out W. Pressure creases. Adiabatic compression 0 + W U. Fal U higher than itial U. T & P creases.

37 Second Law Dream Enge Carnot Cycle P, kpa P - ν diagram for a Carnot (ideal) power plant out ν, m 3 /kg

38 Second Law Dream Enge Reverse Carnot Cycle P, kpa P - ν diagram for a Carnot (ideal) refrigerator out ν, m 3 /kg

39 Second Law Dream Enge Carnot Prciples For heat enges contact with the same hot and cold reservoir All reversible enges have the same performance. Real enges will have lower performance than the ideal enges. H L rev T T H L (K) (K)

40 Second Law Workg fluid: Not a factor P: η η 2 η 3 High T Res., T H Furnace H η real L H Steam Power Plants ω net,out P2: η real < η rev out L Low T Res., T L Water from river η rev T T L H (K) (K) An Energy-Flow diagram for a Carnot SPPs

41 Workg fluid: COP Not a factor HP L H Second Law High T Res., T H, Kitchen room / Outside house out H Rev. Fridge/ Heat Pump COP R H L ω net, COPHP rev L COPHP rev T T H L H rev L Low Temperature Res., T L, Inside fridge or house COP COP An Energy-Flow diagram for Carnot Fridge/Heat Pump R rev R rev H L T T rev H L

Quotes. Review - First Law. Review - First Law. Review - First Law. Review - First Law. Thermodynamics Lecture Series

Quotes. Review - First Law. Review - First Law. Review - First Law. Review - First Law. Thermodynamics Lecture Series 8//005 herodynaics ecture Series Entropy uantifyg Energy Degradation Applied Sciences Education Research Group (ASERG) Faculty of Applied Sciences Universiti eknologi MARA eail: drjjlanita@hotail.co http://www3.uit.edu.y/staff/drjj/