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.
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SEM 1 (2013/2014)
• 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.
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.
heat to a
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
law (energy is
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).
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.
energy in the
form of heat,
Bodies with relatively large thermal
masses can be modeled as thermal
and a sink
• 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
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
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.
A portion of the work output ofa heat engine is consumedinternally to maintaincontinuous operation.
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 .
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.
Net power output = 30 MW
Thermal efficiency = 0.375 or 37.5 %
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.
The fuel consumption rate = 16.5 kg/h
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.
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.
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.
The objectiveof a heat
pump is to
supply heatQHinto thewarmer
The work suppliedto a heat pump is
used to extract
energy from thecold outdoors and carry it into the
for fixed values of QLand QH
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
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
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.
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.
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?
Reversible processes deliver the most
and consume the least work.
• 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
temperature difference, electric resistance,
inelastic deformation of solids, and chemicalreactions.
• The presence of any of these effects renders a process irreversible.
transfer througha temperaturedifference is
(b) the reverseprocess is
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
A totally reversible process involves no heat transfer through a finite temperature
difference, no nonquasi-equilibrium changes, and no friction or other dissipative
A reversible process
involves no internal and
Totally and internally reversible heat
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 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.
Proof of the first Carnot principle.
A temperature scale that is
independent of the properties ofthe substances that are used to
measure temperature is called a
Such a temperature scale offersgreat conveniences in
The arrangement of
heat engines used to
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.