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USF EGN 3343 - Thermo_8e_Chap_6_lecture

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Slide 1Slide 2INTRODUCTION TO THE SECOND LAWSlide 4THERMAL ENERGY RESERVOIRSSlide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37SummaryCHAPTER 6 THE SECOND LAW OF THERMODYNAMICSLecture slides byMehmet KanogluCopyright © The McGraw-Hill Education. Permission required for reproduction or display.Thermodynamics: An Engineering Approach 8th EditionYunus A. Çengel, Michael A. BolesMcGraw-Hill, 20152Objectives•Introduce the second law of thermodynamics.•Identify valid processes as those that satisfy both the first and second laws of thermodynamics.•Discuss thermal energy reservoirs, reversible and irreversible processes, heat engines, refrigerators, and heat pumps.•Describe the Kelvin–Planck and Clausius statements of the second law of thermodynamics.•Discuss the concepts of perpetual-motion machines.•Apply the second law of thermodynamics to cycles and cyclic devices.•Apply the second law to develop the absolute thermodynamic temperature scale.•Describe the Carnot cycle.•Examine the Carnot principles, idealized Carnot heat engines, refrigerators, and heat pumps.•Determine the expressions for the thermal efficiencies and coefficients of performance for reversible heat engines, heat pumps, and refrigerators.3INTRODUCTION TO THE SECOND LAWThese processes cannot occur even though they are not in violation of the first law.4 MAJOR USES OF THE SECOND LAW1. The second law may be used to identify the direction of processes. 2. The second law also 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. The second law provides the necessary means to determine the quality as well as the degree of degradation of energy during a process.3. The second law of thermodynamics is also used in determining the theoretical limits for the performance of commonly used engineering systems, such as heat engines and refrigerators, as well as predicting the degree of completion of chemical reactions.5THERMAL ENERGY RESERVOIRS•A hypothetical body with a relatively large thermal energy capacity (mass x specific heat) that can supply or absorb finite amounts of heat without undergoing any change in temperature is called a thermal energy reservoir, or just a reservoir. •In practice, large bodies of water such as oceans, lakes, and rivers as well as the atmospheric air can be modeled accurately as thermal energy reservoirs because of their large thermal energy storage capabilities or thermal masses.6HEAT ENGINESHeat engines and other cyclic devices usually involve a fluid to and from which heat is transferred while undergoing a cycle. This fluid is called the working fluid.HEAT ENGINES: The devices that convert heat to work.1. They receive heat from a high-temperature source (solar energy, oil furnace, nuclear reactor, etc.).2. They convert part of this heat to work (usually in the form of a rotating shaft.)3. They reject the remaining waste heat to a low-temperature sink (the atmosphere, rivers, etc.).4. They operate on a cycle.7A steam power plant8Thermal efficiency910Can we save Qout?In a steam power plant, the condenser is the device where large quantities of waste heat is rejected to rivers, lakes, or the atmosphere. Can we not just take the condenser out of the plant and save all that waste energy? The answer is, unfortunately, a firm no for the simple reason that without a heat rejection process in a condenser, the cycle cannot be completed. Every heat engine must waste some energy by transferring it to a low-temperature reservoir in order to complete the cycle, even under idealized conditions.11Net Power Production of a Heat Engine12The Second Law of Thermodynamics:Kelvin–Planck StatementIt 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.The impossibility of having a 100% efficient heat engine is not due to friction or other dissipative effects. It is a limitation that applies to both the idealized and the actual heat engines.13REFRIGERATORS AND HEAT PUMPS•The transfer of heat from a low-temperature medium to a high-temperature one requires special devices called refrigerators.•Refrigerators, like heat engines, are cyclic devices. •The working fluid used in the refrigeration cycle is called a refrigerant. •The most frequently used refrigeration cycle is the vapor-compression refrigeration cycle.In a household refrigerator, the freezer compartment where heat is absorbed by the refrigerant serves as the evaporator, and the coils usually behind the refrigerator where heat is dissipated to the kitchen air serve as the condenser.14Coefficient of PerformanceThe efficiency of a refrigerator is expressed in terms of the coefficient of performance (COP). The objective of a refrigerator is to remove heat (QL) from the refrigerated space.Can the value of COPR be greater than unity?15Heat Pumpsfor fixed values of QL and QHCan the value of COPHP be lower than unity?What does COPHP=1 represent?16•Most heat pumps in operation today have a seasonally averaged COP of 2 to 3.•Most existing heat pumps use the cold outside air as the heat source in winter (air-source HP).•In cold climates their efficiency drops considerably when temperatures are below the freezing point. •In such cases, geothermal (ground-source) HP that use the ground as the heat source can be used.•Such heat pumps are more expensive to install, but they are also more efficient.•Air conditioners are basically refrigerators whose refrigerated space is a room or a building instead of the food compartment.•The COP of a refrigerator decreases with decreasing refrigeration temperature. •Therefore, it is not economical to refrigerate to a lower temperature than needed.Energy efficiency rating (EER): The amount of heat removed from the cooled space in Btu’s for 1 Wh (watthour) of electricity consumed.17The Second Law of Thermodynamics: Clasius StatementIt is impossible to construct a device that operates in a cycle and


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