1. INTRODUCTION TO 2 nd LAW and EXERGY Yunus Çengel Yildiz Technical University, Istanbul University of Nevada, Reno

2. 2 Outline Introduce the 2 nd law of thermodynamics, and Describe the Kelvin–Planck and Clausius statements of the second law of thermodynamics. Define exergy, which is the maximum useful work that could be obtained from the system at a given state in a specified environment. Define the exergy destruction, which is the wasted work potential during a process as a result of irreversibilities. Define the second-law efficiency. Develop the exergy balance relation, and apply it to processes encountered in practice.

3. 1 st and 2 nd Laws The 1 st law of thermodynamics is mundane since it deals with conserved quantities. The 2 nd law is quite exciting and sometimes even bizarre since it baffles the mind and intrigues the imagination by dealing with quantities that are created and destroyed. Many people, including some engineers, have difficulty grasping the 2 nd law concepts such as entropy, exergy, and 2 nd law efficiency, and question the utility of the 2 nd law analysis. As a result, they tend to limit thermodynamics to an energy analysis only. Energy analysis provides a one-sided view of a process or system, and a study is not complete without an accompanying 2 nd -law analysis, which enables one to examine the system or process from a different angle. The 1 st law (or energy) analysis provides a map for energy flow and conversion for a process, whereas the 2 nd law provides a map of inefficiencies and waste occurring throughout the process.

4. Mass balance:Mass change = Mass transfer Energy balance: Energy change = Energy transfer Entropy balance: Entropy change = Entropy transfer + Entropy generation Exergy balance: Exergy change = Exergy transfer - Exergy destruction Thermodynamics in a nutshell

5. 5 Entropy Generation and Exergy destruction associated with heat transfer Energy is conserved, Entropy is generated, Exergy is destroyed.

6. The 2 nd Law of Thermodynamics: The 2 nd Dimension of Energy 1 st Law: Quantity Based Energy is energy. All energies are equal. Quantity is always conserved; it cannot be destroyed. 2 nd law: Quality Based There is difference from energy to energy. All energies are not equal. Quality diminishes in all processes, except the ‘perfect’ ones.

7. Is the theoretical upper limit in energy conversion %100? If not, what is it? How close are we to Perfection? Or, how much room do we have for improvement? Basic Questions

8. ANOTHER QUESTION: From a thermodynamic point of view, which is a better energy conversion/transfer process? 1 st law efficiency: The level of performance achieved compared to the resources provided. 2 nd law efficiency: The level of performance achieved compared to the best possible performance under the circumstances. Process A: 1 st Law efficiency=100%; 2 nd Law efficiency <100%. Process B: 1 st Law efficiency<100%; 2 nd Law efficiency =100%..

9. 2 nd law efficiency – A measure of perfection A process with a 2 nd law efficiency of 100%: - Is perfect (even if its 1 st law efficiency is less than 100%). - Entropy generation = 0 - Exergy destruction = 0 - Waste = 0 Something cannot be more perfect than perfect. The 2 nd law defines the upper limit. Perfection is good and beautiful. Beauty and perfection are liked for what they are. GOAL: Perfection and zero waste.

10. 10 1 st LAW vs. 2 nd LAW: Matter vs. Non-matter 1 st law deals with Matter and Energy - Their existence is certain - Physical quantities - Conserved (subject to conservation laws) - Can be perceived by 5 senses - Conforms to the matter-energy universe that started with big-bang 2 nd law deals with Entropy and Exergy - Their existence is certain - Non-physical quantities (beyond physics) - Non-matter (or meaning) - Non-Conserved (not subject to conservation laws) - Can NOT be perceived by 5 senses - Outside the matter-energy universe that started with big-bang We are blinded by matter, and conditioned with conserved quantities. Some have difficulty grasping Entropy and Exergy since they are invisible non-matter quantities. They are like spirits working behind the scenes and governing physical phenomena. Entropy is like the bad spirit, and exergy the good spirit.

11. Comparison of mass and energy with entropy and exergy Mass, energyEntropy, exergy Associated with1 st law2 nd law ExistenceYes Physical propertyYes Material quantityYesNo ConservedYesNo Matter, energyYesNo Part of big-bang universeYesNo Perceived by 5 sensesYesNo Valuable engineering toolYes

12. Practical utility of the 2 nd law The 2 nd law (or exergy) analysis serves as a mirror to see the effectiveness of each segment of the process, and to identify waste and inefficiencies. The 2 nd law efficiency is a measure of the perfection of a process. The destruction of exergy is a measure of imperfections associated with a process, and it shows the room we have for improvement. Global warming and the associated climate change that pose the greatest risk for the future of planet earth are closely related to the 2 nd -law concepts. Combating climate change and the associated green practices are closely related to avoiding waste and thus minimizing entropy generation or exergy destruction. “Sustainability” is also a 2 nd law concept, as it involves the practice of best resource utilization and waste elimination. Even “reliability” is related to the 2 nd law as wasted energy often causes excessive operating temperatures, and thus a higher rate of failure.

13. 13 1.T he direction of processes can be identified. 2.The second law asserts that energy has quality as well as quantity. The first law is concerned with the quantity of energy and the transformations of energy from one form to another with no regard to its quality. 3.The second law provides the necessary means to determine the quality as well as the degree of degradation of energy during a process. 4.The second law is used in determining the theoretical limits for the performance of commonly used engineering systems. 5.Exergy efficiencies provide a measure of how nearly actual performance approaches the ideal, and identifies the causes and locations of thermodynamic losses. 6.Exergy analysis can assist in improving and optimizing designs. PRIMARY USES OF THE 2 nd LAW AND EXERGY

14. 14 INTRODUCTION TO THE 2 nd LAW A cup of hot coffee does not get hotter in a cooler room. Transferring heat to a wire will not generate electricity. Transferring heat to a paddle wheel will not cause it to rotate. These processes cannot occur even though they are not in violation of the first law.

15. The 2 nd Law and Exergy

16. The Second Law of Thermodynamics: Kelvin–Planck Statement A heat engine that violates the Kelvin–Planck statement of the second law. It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. No heat engine can have a thermal efficiency of 100 percent, or as for a power plant to operate, the working fluid must exchange heat with the environment as well as the furnace.

17. 17 The Second Law of Thermodynamics: Clausius Statement It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher- temperature body. It states that a refrigerator cannot operate unless its compressor is driven by an external power source, such as an electric motor. A refrigerator that violates the Clausius statement of the second law.

18. THE CARNOT HEAT ENGINE No heat engine can have a higher efficiency than a reversible heat engine operating between the same high- and low-temperature reservoirs. Any heat engine Carnot heat engine

19. The Quality of Energy The fraction of heat that can be converted to work as a function of source temperature. The higher the temperature of the thermal energy, the higher its quality.

20. 20 THE CARNOT REFRIGERATOR AND HEAT PUMP No refrigerator can have a higher COP than a reversible refrigerator operating between the same temperature limits. Any refrigerator or heat pump Carnot refrigerator or heat pump How do you increase the COP of a Carnot refrigerator or heat pump? How about for actual ones?

21. EXERGY: WORK POTENTIAL OF ENERGY A system that is in equilibrium with its environment is said to be at the dead state. The useful work potential of a given amount of energy at some specified state is called exergy, which is also called the availability or available energy. A system is said to be in the dead state when it is in thermodynamic equilibrium with the environment it is in. The atmosphere contains a tremendous amount of energy, but no exergy.

22. 22 REVERSIBLE WORK and EXERGY DESTRUCTION As a closed system expands, some work needs to be done to push the atmospheric air out of the way (W surr ). For constant-volume systems, the total actual and useful works are identical (W u = W). Reversible work W rev : The maximum amount of useful work that can be produced (or the minimum work that needs to be supplied) as a system undergoes a process between the specified initial and final states. The difference between reversible work and actual useful work is the irreversibility.

23. Exergy (Work Potential) Associated with Kinetic and Potential Energy The work potential or exergy of potential energy is equal to the potential energy itself. Exergy of kinetic energy: Exergy of potential energy: The exergies of kinetic and potential energies are equal to themselves, and they are entirely available for work.

24. 24 Exergy of a Fixed Mass: Nonflow (or Closed System) Exergy The exergy of a specified mass at a specified state is the useful work that can be produced as the mass undergoes a reversible process to the state of the environment.

25. 25 Exergy of a Flow Stream: Flow (or Stream) Exergy Exegy of flow energy Flow exergy The exergy associated with flow energy is the useful work that would be delivered by an imaginary piston in the flow section.

26. 26 EXERGY TRANSFER BY HEAT, WORK, AND MASS Exergy by Heat Transfer, Q Exergy transfer by heat The Carnot efficiency  c =1  T 0 /T represents the fraction of the energy transferred from a heat source at temperature T that can be converted to work in an environment at temperature T 0. The transfer and destruction of exergy during a heat transfer process through a finite temperature difference.

27. 27 EXERGY TRANSFER BY WORK, W There is no useful work transfer associated with boundary work when the pressure of the system is maintained constant at atmospheric pressure. Exergy Transfer by Mass, m Mass contains energy, entropy, and exergy, and thus mass flow into or out of a system is accompanied by energy, entropy, and exergy transfer.

28. 28 EXERGY DESTRUCTION The exergy change of a system can be negative, but the exergy destruction cannot. Exergy destroyed is a positive quantity for any actual process and becomes zero for a reversible process. The exergy of an isolated system during a process always decreases or, in the limiting case of a reversible process, remains constant. In other words, it never increases and exergy is destroyed during an actual process. This is known as the decrease of exergy principle.

29. 29 EXERGY BALANCE Mechanisms of exergy transfer. The exergy change of a system during a process is equal to the difference between the net exergy transfer through the system boundary and the exergy destroyed within the system boundaries as a result of irreversibilities.

30. 2 nd Law (Exergetic) Efficiency,  II

31. SECOND-LAW EFFICIENCY,  II

32. Second-Law Efficiency of Resistance Heaters A dealer advertises that he has just received a shipment of electric resistance heaters for residential buildings that have an efficiency of 100%. Assuming an indoor temperature of 21°C and outdoor temperature of 10°C, determine the second-law efficiency of these heaters.

33. Second-law efficiency of all reversible devices and processes is100%. 2 nd Law efficiency of Reversible devices Q: Can the 2nd-law efficiency be greater than 1 st -law efficiency?

34. SECOND-LAW EFFICIENCY,  II

35. 2 nd -Law Efficiency of Steady-Flow Devices Heat Exchangers: Mixing Chambers: Compressors: Turbines:

36. EXAMPLE: Heating with a hot iron block Actual process Reversible process 500 kg

37. 37 EXAMPLE: Minimum work input to a R-134a compressor

38. 38 EXAMPLE : Heating of a gas by stirring vs. a heat pump

39. 1 st and 2 nd law views of energy flows Energy flow = 1 kW Entropy flow = 0.002 kW/K Exergy flow = 0.4 kW “IMPURE ENERGY” Energy flow = 1 kW Entropy flow = 0 Exergy flow = 1 kW “PURE ENERGY” HEAT ENERGY FLOW, 1 kJ/s ELECTRİCAL ENERGY FLOW, 1 kW T 0 = 300 K 500 K

40. 2 nd Law as a Guide for Green Practices and Sustainability

41. Green Engineering and Thermodynamics Green engineering is the design, commercialization, and use of processes and products that are feasible and economical while reducing the generation of pollution at the source and minimizing the risk to human health and the environment. Green thermodynamics is a subcategory of green engineering related to energy. In thermodynamics, the concept of green can be associated with an energy source, an energy interaction or transfer, and energy conversion or use. Green thermodynamic practices are closely related to minimizing waste or: - Minimizing entropy generation, - Minimizing exergy destruction (2 nd law) A process with a higher second-law efficiency is a greener thermodynamic process.

42. Sustainability and Ethics Sustainability is usually concerned with long-term impact on resources, environment, and processes on macro scale whereas Green is also concerned with short-term effects, such as the indoor air quality, on micro scale. Green/sustainable practices allow humans to exploit nature, but to do so without inflicting irreversible damage to the environment and without disturbing the ecological balance. Sustainable development: “Development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. (World Commission on Environment and Development, 1987). This definition is closely tied to engineering ethics. Ethical practices require being considerate of the needs of future generations since the world’s resources belong to them as much as they belong to us. Green practices involve doing the right thing and doing it right. The terms “Green” and “Sustainability” are loosely defined. They are used to promote ‘green thinking’, and they should be viewed as goals to strive to reach. “Green” and “Sustainable” are often used interchangeably. A process with a higher second-law efficiency is a greener process.

43. A 2 nd Law Application: ASHRAE GreenGuide Guidance to HVAC&R system designers involved in green or sustainable building design. A step-by-step manual for the entire building lifecycle. Includes 29 Green Tips, specific measures for improving sustainability, such as Ground- Source Heat Pumps. Covers green design techniques applicable to related technical disciplines, such as plumbing and lighting.

44. Conservation: A 2 nd Law Concept Minimization of exergy destruction is the greenest thermodynamic practice! Conservation implies conserving the “exergy content” or “usefulness” of energy. 1 st law: Energy is always conserved, even when heat is lost from a building (conservation of energy principle). 2 nd law: Degraded energy is wasted energy. Conserving energy is preserving it at the most useful form. Energy converted to a useless form is lost forever.

45. Last word from the 2 nd law point of view: “ZERO WASTE”

46. Thank you!