Gestão de Sistemas Energéticos 2017/2018

Transcription

1 Gestão de Sistemas Energéticos 2017/2018 Exergy Analysis Prof. Tânia Sousa

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

3 Total Exergy Total exergy is: First evaluate thermomechanical exergy (the system goes from T, P to T 0, P 0 ) and then evaluate the chemical exergy (at constant T 0, P 0 the system goes to the reference chemical composition of the environment)

4 Example What about an ideal mixture of gases present in the environment when T 0 = K, p 0 = 1 atm?

5 Example What about an ideal mixture of gases present in the environment when T 0 = K, p 0 = 1 atm?

6 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.

7 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 0 480K Combustion Products yp 0 25ºC Combustion Products at y e P 0 25ºC

8 Exergetic efficiency of an internal combustion engine Liquid octane enters an internal combustion engine operating at steady state with a mass flow rate of kg/s and is mixed with the theoretical amount of air. Determine the exergetic efficiency. Substance Model for HHV LHV e ch Liquid octane Gasoline 47,900 44,430 47,390 a b

9 Fuel Cells Fuel (H 2 ) and oxidizer (O 2 ) 40 a 60 % efficiency

10 Exergy Balances to Reacting Systems What is the exergy balance to the reversible steadystate combustion of C a H b?

11 Chemical Exergy The exergy value of C a H b is

12 Primary, Final, Useful Energy & Energy Services

13 Forms of Energy - Primary energy

14 Forms of Energy - Final energy

15 Forms of Energy Useful Energy Other Electrical Uses Light Mechanical power Heat Muscle work Heat/Cooling

16 Forms of Energy Primary energy embodied in resources as it is found in nature (coal, oil, natural gas in the ground) Final energy sold to final consumers such as households or firms (electricity, diesel, processed natural gas) Useful energy in the form that is used: light, heat, cooling and mechanical power (stationary or transport) Productive energy the fraction of useful energy that we actually use

17 Energy Services

18 From Primary Energy to Energy Services

19 From Primary Energy to Energy Services Energy Supply energy flows driven by resource availability and conversion technologies IAASA - Global Energy Assessment 2012

20 From Primary Energy to Energy Services Energy Demand Energy system is service driven IAASA - Global Energy Assessment 2012

21 From Primary Energy to Energy Services Quality and cost of energy services IAASA - Global Energy Assessment 2012

22 Sankey Diagrams & First Law Efficiency

23 Sankey Diagrams Sankey Diagrams: a flow diagram used to represent flows of mass (water, CO2, etc) or energy or money or... by arrows where the thickness of arrows are proportional to the amount Miguel Águas (2009)

24 Example? Sankey Diagram

25 Sankey Diagram Schematic representation of the energy flow (natural gas electricity light reading) Natural gas 50% E E final primary W Q out in Electricity 50% E E useful final Light 20% E productive E useful What is the aggregate efficiency?

26 Sankey Diagram Schematic representation of the energy flow (natural gas electricity light reading) Natural gas 50% E E final primary W Q out in Electricity 50% E E useful final Light 20% E productive E useful What is the aggregate efficiency? 5%

27 World Sankey Diagram in 2005 E final? Eprimary E E useful final? US 94 EJ Portugal 1.1 EJ 1st law efficiencies from primary to final energy and primary to useful are 66 % and 34% IAASA Global Energy Assessment 2012

28 Typical values of 1 st law efficiencies 1 st Law efficiencies from primary to final energy 1 st Law efficiencies from final to useful energy

29 First vs. Second Law energy efficiencies IAASA - Global Energy Assessment 2012 Overall 2 nd law efficiency in converting primary to final is 76% and primary to useful energy is 10%

30 First vs. Second Law energy efficiencies Second law efficiencies provide information on how much you can improve your efficiency Rosen and Dincer, 1997

31 World Sankey Diagram in 2005 IAASA Global Energy Assessment 2012

32 World Sankey Diagram in 2005 IAASA Global Energy Assessment 2012

33 World Sankey Diagram in 2005 IAASA Global Energy Assessment 2012

34 World Sankey Diagram in 2005 IAASA Global Energy Assessment 2012

35 World Sankey Diagram in 2005 IAASA Global Energy Assessment 2012

36 World Sankey Diagram in 2005 IAASA Global Energy Assessment 2012

37 World Sankey Diagram in 2005 IAASA Global Energy Assessment 2012

38 From useful energy to energy services Passive systems: where useful energy is delivered into and mostly lost as unwanted heat, in exchange for energy services such as thermal comfort, illumination and transport (Cullen et al., 2011).

39 From useful energy to energy services Human behaviour & quality

40 kahoot