1. Gestão de Sistemas Energéticos 2016/2017
Exergy Analysis
Prof. Tânia Sousa

2. Evaluating Enthalpy for Reactive Systems
The specific enthalpy of a compound (in this datum) at a state where temperature is T and pressure is p is determined from
What is the value of in the datum used to assign the enthalpies of formation?
h is associated with the change in state from the standard state to the state where temperature is T and the pressure is p (obtained from any table).

3. Absolute Entropy
The specific absolute entropy of a compound at a state where temperature is T and pressure is P is determined from
For the ideal gases in Table A-23, the absolute entropy at a state where temperature is T and pressure is p is given by
For the component i of an ideal gas mixture
ref
ref
Partial pressure

4. Evaluating Gibbs Function for Reacting Systems
The specific Gibbs function of a compound (in this datum) at a state where temperature is T and pressure is p is determined from
In an ideal gas mixture, the specific Gibbs function is evaluated at Pi

5. Exergy
Exergy is …
Thermomechanical exergy: if temperature and/or pressure of a system differ from that of the environment;
Chemical exergy – if there is a composition difference between the system and environment;

6. Conceptualizing Chemical Exergy

7. Chemical Exergy

8. Exergy Balances to Reacting Systems
What is the exergy balance to the reversible steady-state combustion of CaHb?

9. Standard Chemical Exergy
The standard exergy value of CaHb is

10. Example
Compute the exergy of methane, CH4, when T0 = K (25oC), p0 = 1 atm.
gfo (CH4)= -50,79kJ/kmol

11. Example
The exergy of methane, CH4, when T0 = K (25oC), p0 = 1 atm.
=831,680kJ/kmol=51980kJ/kg

12. Example
What is the HHV of methane at T0 = K (25oC), p0 = 1 atm?
=831,680kJ/kmol=51980kJ/kg

13. Example
What is the HHV of methane at T0 = K (25oC), p0 = 1 atm?
=831,680kJ/kmol=51980kJ/kg

14. Heating Values of Hydrocarbon Fuels
The heating value of a fuel is the difference between the enthalpy of the reactants and the enthalpy of the products when the fuel burns completely with air, reactants and products being at the same temperature T and pressure p.

15. Standard Chemical Exergy
The chemical exergy of hydrocarbon fuels are approximated by …

16. Standard Chemical Exergy
The chemical exergy of hydrocarbon fuels are approximated by their fuel heating values.

17. Standard Chemical Exergy
“The chemical exergy of a substance is the maximum work that can be obtained from it by taking it to chemical equilibrium with the reference environment at constant temperature and pressure.” (Rivero & Garfias, 2006)
The standard chemical exergy is chemical exergy at T= K and P=1 atm
For any substance the chemical exergy can be estimated using:

18. Total Exergy Total exergy is:
First evaluate thermomechanical exergy (the system goes from T, P to T0, P0) and then evaluate the chemical exergy (at constant T0, P0 the system goes to the reference chemical composition of the environment)

19. Example
Steam at 5 bar, 240ºC leaks from a line in a vapor power plant. Evaluate the flow exergy of the steam, in kJ/kg, relative to an environment at T=25ºC, P=1 atm in which the mole fraction of water vapor is yeH2O=0.0303

20. Example
Steam at 5 bar, 240ºC leaks from a line in a vapor power plant. Evaluate the flow exergy of the steam, in kJ/kg, relative to an environment at T=25ºC, P=1 atm in which the mole fraction of water vapor is yeH2O=0.0303
Steam 5 bar
240ºC
Liquid Water 1 bar
25ºC
Vapour bar
25ºC

21. Example
Steam at 5 bar, 240ºC leaks from a line in a vapor power plant. Evaluate the flow exergy of the steam, in kJ/kg, relative to an environment at T=25ºC, P=1 atm in which the mole fraction of water vapor is yeH2O=0.0303

22. Example
Methane gas enters a reactor and burns completely with 140% theoretical air. Combustion products exit as a mixture at temperature T and a pressure of 1 atm. For T 480 and 1560 K, evaluate the flow exergy of the combustion products, in kJ per kmol of fuel.

23. Example
Methane gas enters a reactor and burns completely with 140% theoretical air. Combustion products exit as a mixture at temperature T and a pressure of 1 atm. For T 480 and 1560 K, evaluate the flow exergy of the combustion products, in kJ per kmol of fuel.
Combustion Products yP
480K
Combustion Products yP
25ºC
Combustion Products at yeP
25ºC

24. Example
What about an ideal mixture of gases present in the environment when T0 = K (25oC), p0 = 1 atm?

25. Exergetic efficiency of an internal combustion engine
Liquid octane enters an internal combustion engine operating at steady state with a mass flow rate of 1.810-3 kg/s and is mixed with the theoretical amount of air. Determine the exergetic efficiency.

26. Fuel Cells
Fuel (H2) and oxidizer (O2)
40 a 60 % efficiency

27. Example
Compute the standard chemical exergy of hydrogen, H2, when T0 = K (25oC), p0 = 1 atm.

28. Example
Compute the standard chemical exergy of hydrogen, H2, when T0 = K (25oC), p0 = 1 atm.
H O2 → H2O(l)

29. Standard Chemical Exergy
TABLE A-25

30. Standard Chemical Exergy
TABLE A-26

31. Standard Chemical Exergy
TABLE A-26

32. Forms of Energy - Primary energy

33. Forms of Energy - Final energy

34. Forms of Energy – Useful Energy

35. World Sankey Diagram in 2005
Overall 1st law efficiency in converting primary to final energy? 66% ? ?
US – 94 EJ
Portugal – 1.1 EJ
IAASA – Global Energy Assessment 2012

36. World Sankey Diagram in 2005
Overall 1st law efficiency in converting primary to useful energy? 34% ? ?
US – 94 EJ
Portugal – 1.1 EJ
IAASA – Global Energy Assessment 2012

37. Typical values of 2nd law efficiencies
IAASA - Global Energy Assessment 2012
Overall 2nd law efficiency in converting primary to final is 76% and primary to useful energy is 10%

38. Second law efficiencies
Second law efficiencies by providing information on how much you can improve your efficiency show where efforts should be made
Rosen and Dincer, 1997

39. Energy Balance
What is the overall efficiency in thermoelectricity production with and without cogeneration? Compare the values and comment
What is the fraction of thermoelectricity that is renewable?

40. Energy Analysis: Specific Consumption of Operation & Inputs
Specific Consumption of Operation A: CEA
Specific Consumption of Material 1: CE1

41. Benefits of Cogeneration
Substitution of fuel driven cogeneration with fuel driven electrical generation and fuel heating
Comparison using first law efficiencies?
What are the exergy efficiencies assuming that the energy and exergy of the fuel are identical?
423K
423K
T0= 288K

42. Benefits of Cogeneration
Substitution of fuel driven cogeneration with fuel driven electrical generation and electrical heating
Comparison using first law efficiencies?
What are the exergy efficiencies assuming that the energy and exergy of the fuel are identical?
423K
423K
T0= 288K

43. Benefits of Cogeneration
Cogeneration increases the energy and exergy efficiencies compared to separate processes
The exergy efficiency is much lower than the energy efficiency for cogeneration and non-cogeneration. Why?
Separate System
Cogeneration
Caso a)
=66.2%
=30.9%
=92%
=43%
Caso c)
=35.5%
=16.6%