ENTC 370: Announcements

1. ENTC 370: Announcements • Homework assignment No.6: • Assigned Problems: • 6.4, 6.16, 6.21, 6.39, 6.44, 6.56, 6.62, 6.71, 6.78, 6.98, 6.129. • Due Tuesday, Nov 4th before 10:50 am • For more information, go to: • http://etidweb.tamu.edu/classes/entc370

2. Thermal Efficiency & Performance Heat Engine Refrigerator Heat Pump

3. Example • Refrigerator is maintained at 4° C by removing heat at a rate of 6 kW. If the required power input is 2 kW, determine COP, and heat rejection.

4. Why systems cannot be a 100% efficient? 100 °C

5. Observation Heat Reservoir Q1 Work During Cycle, WCycle Second Law: A cyclic heat engine cannotoperate with one heat reservoir. (Kevin-Planck) Note that if Q1 = WCycle, then a perpetual machine would be possible! So, no, not all of Q1 can be converted into useful work

6. Central ideas & observations • 2nd Law: There is tendency in all processes and cyclic devices to produce unrecoverable energy in either doing or consuming work.

7. Heat Reservoir Q1 Work During Cycle, WCycle 2nd LawClausius Statement • It is impossible to construct a device (refrigerator or heat pump) 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 Impossible

8. Second Law Revisited Imagine:

9. Kelvin-Planck Statement 1. No heat engine can convert ALL Q to W 2. A cyclic heat engine cannot operate with one heat reservoir only. NOT POSSIBLE

10. Central ideas and observations • Perpetual motion machinesare not possible, specifically machines that utilize all of the energy from a reservoir and convert it all into work.

11. Perpetual-Motion Machines • Perpetual motion machines are not possible, specifically machines that utilize all of the energy from a reservoir and convert it into work. • PMM1: Violate 1st Law • PMM2: Violate 2nd Law

12. What type of Perpetual-Machine is this one?

13. What type of Perpetual-Machine is this one?

14. What makes a system viable from the 2nd Law point of view • Reversible Processes • Can be reversed without leaving any trace on the surroundings (idealized) • Irreversible Processes • Is not reversible, examples include: • Friction: Energy is dissipated and cannot be recovered later in the cycle • Mixing of particles: Once mixed, particles cannot be re-segregated on their own • I2R losses: Energy is dissipated and cannot be recovered later in the cycle • Unrestrained expansion of gases • Heat transfer through a finite temperature difference • Hysteresis effects • Any deviation from a quasi-static process • Minimize irreversibilities

15. Carnot Cycle • Is an ideal cycle for heat engines (and refrigerators/heat pumps) • Reversible cycle (Idealized) • Because can deliver the highest desired output and still satisfies 1st and 2nd Law of Thermodynamics

16. Carnot Cycle • Consist of four reversible processes • Two isothermal processes • Two adiabatic processes • Why reversible processes? • Can deliver the most work • Can consume the least work • Make best use of energy available by not dissipating it • Carnot cycle is the most efficient cycle • Satisfies 1st and 2nd Laws of Thermodynamics

17. Execution of the Carnot cycle in a closed system Isothermal Expansion Even for a reversible engine, certain amount of QH (QL) cannot be used Adiabatic Expansion Isothermal Compression http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/carnot.htm http://brod.sfsb.hr/test/testhome/vtAnimations/animations/chapter06/carnotcycle/index.html Adiabatic Compression

18. The Carnot Cycle Reversible heat addition at TH = Const. p Reversible work transfer, Q = 0 Reversible work transfer, Q = 0 Reversible heat Rejection at TC = Const. V

19. Insulation THgTL source TL=Const Insulation TLgTH sink TH=Const The Reversed Carnot Cycle (ReversibleRefrigerator/Heat Pump) Adiabatic Expansion Reversible work out Isothermal Expansion Reversible Heat Addition Reversible work out Adiabatic Compression Reversible Work In Isothermal Compression Reversible Heat Rejection Reversible Work In

20. What is the maximum efficiency that a heat engine can achieve? hth < 100% (not all Q is converted to W) We know a heat engine cannot achieve a 100% efficiency, but, can it achieve a 90% efficiency? What is the efficiency of the Carnot cycle?

21. Carnot Principles • Efficiency of any irreversible heat engine is always less than a Carnot Engine • The efficiencies of all reversible heat engines (Carnot) operating between the same two reservoirs are the same. • The Carnot engine (or refrigerator/heat pump) is the best device that can be conceived • Represents the upper limit in terms of efficiency

22. Carnot Heat Engine (reversible and irreversible) (Carnot cycle is reversible) It can be proven that only when using reversible devices (i.e. engines, refrigerators and heat pumps) (reversible only) TL and TH are in Kelvin

23. Carnot Heat Engine

24. Maximum Efficiency: Carnot Efficiency Can be proven that: TL and TH are in Kelvin

25. Energy has quality: Higher TH, higher energy quality

26. CarnotRefrigerator (reversible and irreversible) (Carnot cycle is reversible) (reversible only) TL and TH are in Kelvin

27. CarnotHeat Pump (reversible and irreversible) (Carnot cycle is reversible) (reversible only) TL and TH are in Kelvin

29. Carnot Engine Example • A Carnot engine receives 500 kJ from a high-temperature source at 652 °C and rejects heat to a low-temperature sink at 30 °C. Determine the Carnot efficiency and the amount of heat rejected per cycle.

30. Refrigerator and Heat Pump Examples • Claim: A refrigerator is kept at 35 °F while the temperature outside is 75 °F and has a COP of 13.5. Is this claim reasonable?