1. Gestão de Sistemas Energéticos 2015/2016
Exergy Analysis
Prof. Tânia Sousa

2. 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;

3. Evaluating Chemical Exergy
The exergy reference environment (the Earth and its atmosphere)
Environmental temperature T0 and pressure p0.
Set of reference substances with concentrations closely corresponding to the chemical makeup of the natural environment.
Reference substances may include
Gaseous components of the atmosphere: N2, O2, CO2, H2O(g), and other gases.
Solid substances from the Earth’s crust.
Substances from the oceans.

4. Conceptualizing Chemical Exergy
CaHbOc enters the control volume at T0, p0.
O2 and CO2, H2O(g) enter and exit the control volume at T0 and their respective partial pressures.
The ideal gas model applies to O2, CO2, and H2O(g).

5. Conceptualizing Chemical Exergy
Negligible kinetic and potential energy effects
Heat transfer between the control volume and environment occurs only at temperature T0
Steady state
The chemical exergy per mole of CaHbOc, ech, is the maximum theoretical value of Wcv/nF

6. Conceptualizing Chemical Exergy
The logarithmic term typically contributes only a few percent to the chemical exergy magnitude

7. Example
Compute the exergy of methane, CH4, when T0 = K (25oC), p0 = 1 atm.

8. Example
Compute the exergy of methane, CH4, when T0 = K (25oC), p0 = 1 atm.

9. Example
Compute the exergy of methane, CH4, when T0 = K (25oC), p0 = 1 atm.

10. Example
Compute the exergy of methane, CH4, when T0 = K (25oC), p0 = 1 atm.

11. Example
What is the pure CO2 exergy, when T0 = K (25oC), p0 = 1 atm?

12. Example
Compute the exergy of pure CO2, when T0 = K (25oC), p0 = 1 atm.

13. Example
Compute the exergy of CO2, when T0 = K (25oC), p0 = 1 atm.
Applies also to other gases in the environment

14. Example
What about the exergy of H2O, when T0 = K (25oC), p0 = 1 atm?

15. Example
What about the exergy of H2O, when T0 = K (25oC), p0 = 1 atm?

16. Example
What about an ideal mixture of gases with gases that are present in the environment and with molar fractions of yi when T0 = K (25oC), p0 = 1 atm?

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

18. Conceptualizing Chemical Exergy

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

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

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

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

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

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

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

26. 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:

27. 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)

28. 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

29. 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

30. 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

31. 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.

32. 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

33. 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.

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

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

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

37. Standard Chemical Exergy
TABLE A-25

38. Standard Chemical Exergy
TABLE A-26

39. Standard Chemical Exergy
TABLE A-26