PTT 201/4 THERMODYNAMICS SEM 1 ( 2013/2014). CHAPTER 6: Second Law of Thermodynamics. Objectives. • Introduce the second law of thermodynamics. • Identify valid processes as those that satisfy both the first and second laws of thermodynamics.
PTT 201/4 THERMODYNAMICS SEM 1 ( 2013/2014)
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PTT 201/4THERMODYNAMICS SEM 1 (2013/2014) CHAPTER 6: Second Law of Thermodynamics
Objectives • 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. • Apply the second law to develop the absolute thermodynamic temperature scale. 2
INTRODUCTION TO THE SECOND LAW The 1st law of thermodynamics places no restriction on the direction of aprocess, but satisfying the 1st law does not ensure that the process can occur.Due to the lack the 1st law, the 2nd law of thermodynamics is introduced. Transferring heat to a paddle wheel will not cause it to rotate. A cup of hot coffee does notget hotter in a cooler room. These processescannot occur eventhough they are not Transferringheat to a wirewill not in violation of the first generateelectricity. law (energy is conserved). 3
INTRODUCTION TO THE SECOND LAW Processes occur in acertain direction, and notin the reverse direction. A process must satisfy boththe first and second laws ofthermodynamics to proceed. Work can always beconverted to heat directly and completely,but the reverse is nottrue (transferring heatto the water does not cause the shaft torotate).
SECOND LAW OF THERMODYNAMICS MAJOR USES OF THE SECOND LAW 1. The second law may be used to identify the directionof 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 itsquality. The second law provides the necessary means to determine thequality 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. 4
THERMAL ENERGY RESERVOIRS A source supplies energy in the form of heat, Bodies with relatively large thermal masses can be modeled as thermal and a sink energy reservoirs. absorbs it. • A hypothetical body with a relatively large thermal energy capacity(mass xspecific heat) that can supplyor absorb finite amounts of heatwithout undergoing any change in temperatureis called a thermal energyreservoir, or just a reservoir. • In practice, large bodies of water such as oceans, lakes, and rivers as wellas the atmospheric air can be modeled accurately as thermal energyreservoirs because of their large thermal energy storage capabilities or thermal masses. 5
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.). They operate on a cycle. Heat engines and other cyclicdevices usually involve a fluid to and from which heat is transferred Part of the heatreceived by a heat engine is converted to work, while the rest isrejected to a sink. while undergoing acycle. This fluid is called theworking fluid. 6
A steam power plant A portion of the work output ofa heat engine is consumedinternally to maintaincontinuous operation. 7
Thermal efficiency Schematic of a heat engine. Some heat engines perform betterthan others (convert more of theheat they receive to work). Even the most efficient heat engines rejectalmost one-half of the energy they receive as waste heat . 8
Example 6-1: Heat is transferred to heat engine from a furnace at a rate of 80 MW. If the rate of waste heat rejection to a nearby riveris 50 MW, determine the net power output and thermal efficiency for this heat engine? Solution (refer to textbook p.280): Assumption: Heat losses through the pipes and otherscomponents are negligible. Answer: i. Net power output = 30 MW ii. Thermal efficiency = 0.375 or 37.5 % 10
Example 6-2: A car engine with a power output of 65 hp has a thermal Efficiency 24 percent. Determine the fuel consumption rateof this car if the fuel has a heating value of 44,000 kJ/kg (that is, 44,000 kJ of energy is released for each kg of fuel burned). Solution (refer to textbook p.280): Assumption: The power output of the car is constant. Answer: The fuel consumption rate = 16.5 kg/h 10
The Second Law of Thermodynamics: Kelvin-Planck Statement It is impossible for any device that operates on a cycle to receive heat from asingle reservoir and producea net amount of work. No heat engine can have a thermalefficiency of 100 percent, or as for a A heat engine that violates theKelvin-Planck statement of thesecond law. power plant to operate, the working fluidmust 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 alimitation that applies to both the idealized and the actual heat engines. 11
REFRIGERATORS AND HEAT PUMPS The transfer of heat from a low- temperature medium to a high-temperature one requires specialdevices called refrigerators. Refrigerators, like heat engines,are cyclic devices. The working fluid used in therefrigeration cycle is called arefrigerant. The most frequently used refrigeration cycle is the vapor-compression refrigerationcycle. In a home refrigerator, the freezer compartment where heat is absorbed by the refrigerant serves as the evaporator, and the coils usually behind the Basic components of arefrigeration system and refrigerator where heat is dissipated to thekitchen air serve as the condenser. typical operating conditions. 12
Coefficient of Performance The efficiency of a refrigerator is expressedin terms of the coefficient of performance(COP). The objective of a refrigerator is to removeheat (QL) from the refrigerated space. Can the value of COPR begreater than unity? Yes, theamount of QL can be greater The objective of a refrigerator is toremove QLfrom the cooled space. than Wnet,in. 13
Heat Pumps The objectiveof a heat pump is to supply heatQHinto thewarmer space. The work suppliedto a heat pump is used to extract energy from thecold outdoors and carry it into the warm indoors. for fixed values of QLand QH 14
Example 6-3: The food compartment of a refrigerator, as shown in figure, is maintained at 4° C by removing heat from it at arate of360 kJ/min. If the required power input to the refrigerator is 2 kW, determine (a) the coefficient of performance of the refrigerator and (b) the rate of heatrejection to the room that houses the refrigerator? Solution (refer to textbook p.285): Assumption: Steady operating conditions exist.Answer: COPR= 3, 3 kJ of heat removed for each kJ of worksupplied. b) QH = QL + Wnet, in = 480 kJ/min 16
Example 6-4: A heat pump is used to meet the heating requirements of ahouse and maintain it at 20° C. On a day when the outdoorair temperature drops to -2 ° C, the house estimated to lose heat at a rate of 80,000 kJ/h. If the heat pump under these conditions has COP of 2.5, determine (a) the power consumed by the heat pump and (b) the rate at which heatis absorbed from the cold outdoor air? Solution (refer to textbook p.286): Assumption: Steady operating conditions exist.Answer: a) Wnet,in = 32,000 kJ/h. b) QL = 48, 000 kJ/h 17
The Second Law of Thermodynamics: Clausius Statement It is impossible to construct a device thatoperates in a cycle and produces no effectother than the transfer of heat from a lower-temperature body to a higher-temperaturebody. It states that a refrigerator cannot operate unlessits compressor is driven by an external powersource, such as an electric motor. This way, the net effect on the surroundings involves the consumption of some energy in theform of work, in addition to the transfer of heatfrom a colder body to a warmer one. A refrigerator that violatesthe Clausius statement ofthe second law. 18
Equivalence of the Two Statements Proof that the violation of theKelvin-Planckstatement leads to the violation of the Clausiusstatement. The Kelvin-Planck and the Clausius statements are equivalent in their consequences, and either statement can be used asthe expression of the second law of thermodynamics. Any device that violates the Kelvin-Planck statement also violates the Clausius statement, and vice versa. 19
REVERSIBLE AND IRREVERSIBLE PROCESSES Reversible process: A process that can be reversed without leaving any traceon the surroundings (actually do not occur in nature). Irreversible process: A process that is not reversible. • All the processes occurring in nature are irreversible. • Why are we interested in reversible processes? • (1)they are easy to analyze and (2)they serve as idealized models (theoretical limits) to whichactual processes can be compared. • We try to approximate reversible processes. Why? Two familiar reversible processes. Reversible processes deliver the most and consume the least work. 20
• The factors that cause a process to be irreversible are called irreversibilities . • They include friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite Frictionrenders aprocess temperature difference, electric resistance, inelastic deformation of solids, and chemicalreactions. irreversible. • The presence of any of these effects renders a process irreversible. Irreversibilities (a) Heat transfer througha temperaturedifference is Irreversible irreversible, and compression (b) the reverseprocess is and expansionprocesses. impossible. 21
Internally and Externally Reversible Processes • Internally reversible process: If no irreversibilities occur within the boundaries of the system during the process. • Externally reversible: If no irreversibilities occur outside the system boundaries. • Totally reversible process: It involves no irreversibilities within the system or its surroundings. • A totally reversible process involves no heat transfer through a finite temperature difference, no nonquasi-equilibrium changes, and no friction or other dissipative effects. A reversible process involves no internal and Totally and internally reversible heat external irreversibilities. transfer processes. 22
THE CARNOT CYCLE Execution of the Carnot cycle in a closed system. Reversible Isothermal Expansion (process 1-2, TH = constant) Reversible Adiabatic Expansion (process 2-3, temperature drops from THto TL) Reversible Isothermal Compression (process 3-4, TL = constant) Reversible Adiabatic Compression (process 4-1, temperature rises from TLto TH)
P-V diagram of the Carnot cycle. P-V diagram of the reversed Carnot cycle. The Reversed Carnot Cycle The Carnot heat-engine cycle is a totally reversible cycle. Therefore, all the processes that comprise it can be reversed, in which case it becomes the Carnot refrigeration cycle.
THE CARNOT PRINCIPLES The Carnot principles. Proof of the first Carnot principle. The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs. The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.
THE CARNOT PRINCIPLES
THE THERMODYNAMIC TEMPERATURESCALE A temperature scale that is independent of the properties ofthe substances that are used to measure temperature is called a thermodynamic temperaturescale. Such a temperature scale offersgreat conveniences in thermodynamic calculations. The arrangement of heat engines used to develop the thermodynamictemperature scale. 23
This temperature scale iscalled the Kelvin scale,and the temperatures onthis scale are calledabsolute temperatures. For reversible cycles, theheat transfer ratio QH /QLcan be replaced by the A conceptual experimental setupto determine thermodynamic temperatures on the Kelvin absolute temperature ratio scale by measuring heattransfers QHand QL. TH /TL. 24