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Module 11 Energy Management (continued) Energy management basics Energy audit

Module 11 Energy Management (continued) Energy management basics Energy audit Demand-side management Life-cycle assessment Exergy analysis Carbon and ecological footprints Clean development mechanism. Selected topics in Energy Management:. Energy audit  Demand-side management 

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Module 11 Energy Management (continued) Energy management basics Energy audit

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  1. Module 11 Energy Management (continued) Energy management basics Energy audit Demand-side management Life-cycle assessment Exergy analysis Carbon and ecological footprints Clean development mechanism

  2. Selected topics in Energy Management: Energy audit  Demand-side management  Life-cycle assessment  Exergy analysis (continued) Carbon and ecological footprints Clean development mechanism

  3. Exergy is the maximum theoretical work that can be obtained from an amount of energy. Energy is conserved. Exergy (which is the useful work potential of the energy) is not conserved. Once the exergy is wasted, it can never be recovered.

  4. Exergy (formal definition) and the Dead State The useful work potential of a system is the amount of energy we could have extracted as useful work. The useful work potential of a system at the specified state is called exergy. Exergy is a property and is associated with the state of the system and the environment. A system that is in equilibrium with its surroundings has zero exergy and is said to be at the dead state. YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  5. Exergy Forms There are 4 components: • Kinetic exergy of bulk motion • Potential exergy of gravitational or electro-magnetic field differentials • Physical exergy from temperature and pressure differentials • Chemical exergy arising from differences in chemical composition We can ignore the first two for many industrial and economic applications. YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  6. Exergy of kinetic energy Kinetic energy is a form of mechanical energy and can be converted directly into work. Kinetic energy itself is the work potential or exergy of kinetic energy independent of the temperature and pressure of the environment. Specific exergy of kinetic energy: It is also known as kinetic exergy EKN YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  7. Exergy of potential energy Potential energy is a form of mechanical energy and can be converted directly into work. Potential energy itself is the work potential or exergy of potential energy independent of the temperature and pressure of the environment. Specific exergy of potential energy: It is also known as potential exergy EPT YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  8. Physical exergy Physical exergy from temperature and pressure differentials It is also known as physical exergy EPH YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  9. Exergy of a flow stream The exergy of a flow (stream) on a unit mass basis is written as follows: The exergy change of a fluid stream as it undergoes a process from state 1 to state 2 is YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  10. Exergy transfer by heat transfer By the second law we know that only a portion of heat transfer at a temperature above the environment temperature can be converted into work. The maximum useful work is produced from it by passing this heat transfer through a reversible heat engine. The exergy transfer by heat is YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  11. Derivation is given on the following slides. YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  12. Heat engine converts heat into work Wout ηthermal = Qin Hot reservoir at TH K Wmax ηCarnot = Qin Qin Wout Tenv ηCarnot - 1 = TH Wmax Exergy = Qout = ηCarnot Qin Environment at Tenv K Tenv - = Qin 1 TH

  13. Derivation ends here. YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  14. Exergy transfer by work Exergy is the useful work potential, and the exergy transfer by work can simply be expressed as where , P0 is atmospheric pressure, and V1 and V2 are the initial and final volumes of the system. The exergy transfer for shaft work and electrical work is equal to the work W itself. Note that exergy transfer by work is zero for systems that have no work. YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  15. Exergy transfer by mass Mass flow is a mechanism to transport exergy, entropy, and energy into or out of a system. As mass in the amount m enters or leaves a system the exergy transfer is given by where Note that exergy transfer by mass is zero for systems that involve no flow. YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  16. The Decrease of Exergy Principle The exergy of an isolated system during a process always decreases or, in the limiting case of a reversible process, remains constant. This is known as the decrease of exergy principle and is expressed as YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  17. Exergy Destruction Irreversibilities such as friction, mixing, chemical reactions, heat transfer through finite temperature difference, unrestrained expansion, non-quasi-equilibrium compression, or expansion always generate entropy, and anything that generates entropy always destroys exergy. The exergy destroyed is proportional to the entropy generated as expressed as YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  18. The decrease of exergy principle does not imply that the exergy of a system cannot increase. The exergy change of a system can be positive or negative during a process, but exergy destroyed cannot be negative. The decrease of exergy principle can be summarized as follows: YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  19. Exergy Balances Exergy balance for any system undergoing any process can be expressed as For a reversible process, the exergy destruction term is zero. YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  20. General: General, unit-mass basis: General, rate form: YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  21. General, rate form: where YA Çengel and MA Boles, Thermodynamics: An Engineering Approach, 5th Ed.

  22. Chemical exergy Even when a system is in a state of thermomechanical equilibrium with the environment, it may still be out of equilibrium with that environment owing to the difference in the composition and nature of the components making up the system and the environment, respectively. These differences lead to values for the chemical exergy. For example, pure nitrogen and oxygen have nonzero chemical exergies because their mole fraction in the environment is different from unity (1).

  23. Chemical exergy in Iron Production Production of pure iron (Fe) from iron oxide (Fe2O3). This requires exergy from burning coke (pure carbon). The reaction is as follows: 2Fe2O3 + 3C  4Fe + 3CO2 Carbon dioxide (CO2) is the waste product from the reaction. 3/4 moles of CO2 is produced per mole of Fe manufactured.

  24. 2Fe2O3 + 3C  4Fe + 3CO2 Exergy imbalance = 1565.3 – 1263.9 = 301.4 2Fe2O3 + 3C  4Fe + 3CO2 has the correct mass balance, but incorrect exergy balance

  25. 2 Fe2O3 + 3.76 C + 0.76 O2  4 Fe + 3.76 CO2 On the input side oxygen has been added to fulfill the balance of the extra C required Exergy balance = 1580.4 – 1279.0 ≈ 0

  26. 2 Fe2O3 + 3.76 C + 0.76 O2  4 Fe + 3.76 CO2 3.76/4 moles of CO2 is produced per mole of Fe manufactured Molecular mass of Fe is 56 and that of CO2 is 44. (3.76/4) x 44 kg of CO2 is produced per 56 kg of Fe. 0.74 kg of waste CO2 is produced per kg of Fe manufactured. This is the thermodynamic minimum. In reality, blast furnace has an average efficiency of 33%. So, a mole of C accounts for only 135.4 kJ instead of 410.3 kJ. As a result, we require 12.42 moles of C instead of 3.76 moles.

  27. 2 Fe2O3 + 12.42 C + 9.42 O2  4 Fe + 12.42 CO2 + heat Exergy balance = -3417.6 (12.42/4) x 44 kg of CO2 is produced per 56 kg of Fe. 2.44 kg of waste CO2 is produced per kg of Fe manufactured.

  28. Types of exergy service • Prime movers ( electricity) • Transport • High temperature process heat • Mid and low temperature process heat • Lighting • Non-fuel application

  29. Exergy Analysis: Transactions of the ASME, Vol 119, Sept 1997, pp200-204

  30. Exergy Analysis: Transactions of the ASME, Vol 119, Sept 1997, pp200-204

  31. Exergy Analysis: Transactions of the ASME, Vol 119, Sept 1997, pp200-204

  32. Exergy efficiency of useful work categories: B. Warr et al. / Ecological Economics 69 (2010) 1904–1917

  33. Exergy efficiency of useful work categories: B. Warr et al. / Ecological Economics 69 (2010) 1904–1917

  34. Exergy efficiency of useful work categories: B. Warr et al. / Ecological Economics 69 (2010) 1904–1917

  35. Exergy efficiency of useful work categories: B. Warr et al. / Ecological Economics 69 (2010) 1904–1917

  36. Energy and exergy efficiencies of space heating technologies: B. Warr et al. / Ecological Economics 69 (2010) 1904–1917

  37. Aggregate exergy efficiency: B. Warr et al. / Ecological Economics 69 (2010) 1904–1917

  38. Exergy maps also available at http://gcep.stanford.edu/research/exergycharts.html

  39. Selected topics in Energy Management: Energy audit  Demand-side management  Life-cycle assessment  Exergy analysis  Carbon and ecological footprints  Clean development mechanism

  40. Let’s take a look at how globalization assists in combating global warming. Global warming is said to have caused by greenhouse gases (GHG). GHGs are gases in an atmosphere that absorb and emit radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect.

  41. The Greenhouse effect A T M O S P H E R E S U N G R E E N H O U S E G A S E S

  42. The main GHGs in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Without GHGs, Earth's surface would be on average about 33°C colder than at present.

  43. Rise in the concentration of four GHGs

  44. Global Warming Potential (GWP) of different GHGs

  45. The burning of fossil fuels, land use change and other industrial activities since the Industrial revolution have increased the GHGs in the atmosphere to such a level that the earth’s surface is heating up to temperatures that are very destructive to life on earth.

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