2. 5 Exergy Exergy and Anergy Evaluation of heat Quantity Quality 5.1 Introduction Full convertible energy: mechanical Partial convertible energy: heat Unconvertible energy: environmental

3. （ 1 ） based on environment; exergy of environmental energy is zero （ 2 ） reversible process （ 3 ） there is no other heat resource in the process. Exergy: useful work potential; available energy; The maximum useful work a system can delivered from a specified state to the state of its environment in theory. Anergy: unavailable energy, unconvertible energy Conditions for definition of exergy

4. 5.2 Calculation of exergy Kinetic Distribution Chemical potential Temperature and pressure Velocity Position Concentration unbalance Position Physical Chemical Type

5. Source Electricity Hydraulic Pneumatic Wave voltage Water level Wind pressure Difference between interior and surface Unbalance

6. 5.2.1 Work resource Electricity, mechanical energy, pneumatic energy, hydraulic energy, can be converted to work entirely. Exergy of work resource = its total energy

7. T T0T0 5.2.2 Heat exergy Potential work of heat W0W0

8. For constant temperature heat resource

9. For finite heat resource T0T0

10. Influential factors: Heat quantity Heat resource temperature Environmental temperature T 0 If the system absorbs heat, it absorbs exergy; If the system discharges heat, it discharges exergy;

11. Example 1kg air with temperature of 200 ℃ was cooled to 40 ℃. Please calculate the heat exergy. The specific heat of air is c p =1.004kJ/(kg.K). the environmental temperature is T 0 =25 ℃。

12. Solution ：

13. 5.2.3 Low temperature heat exergy Potential work of heat at a temperature below the environmental temperature T0T0 T W0W0

14. For constant temperature

15. For finite heat resource

16. Influential factors: Heat quantity Heat resource temperature Environmental temperature T 0 If the system absorbs heat, it discharges exergy; If the system discharges heat, it absorbs exergy;

17. For closed system From initial state to final state 5.2.4 Inner energy exergy

18. For A→B→O The dead work to resist environment The maximum work

19. For m kg substance The maximum available work

21. Please calculate the inner energy exergy of air in the state of 1MPa and 50 ℃. The environmental pressure is p 0 =0.1MPa ， the temperature is T 0 =25 ℃ and the specific heat is c v =0.716kJ/(kg·K) 。 Example

24. 5.2.5 Enthalpy exergy For steady flow system From initial state to final state

25. For A→B→O For m kg substance The maximum available work

26. 开系工质物理

27. Please compare the enthalpy exergy value of saturated steam of 0.5MPa with 5MPa. The environmental state is p 0 =0.1MPa and T 0 =20 ℃。 Example

28. Look up properties in the table of water or steam Solution : h 0 =84 kJ/kg s 0 =0.2963 kJ/(kg·K) h 1 =2747.5 kJ/kg s 1 =6.8192 kJ/(kg·K) h 2 =2794.2 kJ/kg s 2 =5.9735 kJ/(kg·K)

29. e x1 =(h 1 － h 0 ) － T 0 (s 1 － s 0 ) =(2747.5 － 84) － 293×(6.8192 － 0.2963) =752.3kJ/kg e x2 =(h 2 － h 0 ) － T 0 (s 2 － s 0 ) =(2791.2 － 84) － 293×(5.9735 － 0.2963) =1046.8kJ/kg

30. 5.3 Exergy loss Exergy loss caused by temperature difference TATA H.E T0T0 Q

31. TATA T0T0 TBTB Q Q

32. Exergy loss Entropy production

33. T0T0 H.E T

34. 温差传热引起的损失

35. Exergy loss caused by temperature difference

36. If discharging temperature is The whole exergy loss

37. 5.4 exergy equation

38. For many streams Heat exergy Enthalpy exergy Technical work Exergy loss

39. 2 1 5.5 Exergy efficiency

41. Example The high temperature is T H =1800K and the low temperature is the environmental temperature that is T 0 =290K. A heat engine absorbs heat at T 1 =900K and discharges heat at T 2 =320K. The engine efficiency is 70% of that of corresponding carnot cycle 。 If each kilogram substance absorbs heat 100kJ ， please calculate: (1)the practical work of heat engine; (2)the heat exergy at given temperature; (3)the entropy production and exergy loss of each process; (4)the entropy increase of isolated system and the whole exergy loss.

42. T H = 1800 K T 1 = 900 K T 0 = 290 K T 2 = 320 K R W

43. 1 2 4 3 THTH T1T1 T2T2 T0T0 s

44. Solution: (1) The carnot cycle works between T 1 and T 2 Practical work of engine

45. Discharged heat (2) Heat exergy Heat exergy at 1800K Heat exergy at 900K

46. Heat exergy at 320K (3) Entropy production and exergy loss Entropy production caused by temperature difference in absorbing heat process Exergy loss or

47. Friction work loss Exergy loss 0r Entropy production

48. Entropy production caused by temperature difference in discharging heat process Exergy loss 0r

49. (4)Entropy increase of isolated system 0r

50. The whole exergy loss 0r