Exergy - I​
Course Instructor
Goutam Deo
Department of Chemical Engineering
Indian Institute of Technology Kanpur
Acknowledgements: Prof. A. Agarwal, ME department, for sharing the course material
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Learning Objectives
Exergy: Definition
Dead State
Exergy associated with KE and PE
Unavailable Energy
Reversible work and Irreversibility
Irreversibility of Heat Engine
Second Law Efficiency
General Definition of Second law efficiency
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Exergy: Work Potential of Energy
• 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.
• But, Availability is a function of initial state and the
environment alone and considers that the process followed is
reversible and the final state is a dead state.
Dead State System
• A system delivers the maximum possible work as it undergoes
a reversible process from the specified initial state to the state
of its environment, that is, the dead state.
• A system is said to be in the dead state when it is in
thermodynamic equilibrium withESO201
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environment it is in.
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Dead State
• At dead state, the system:
• Is at the temperature and pressure of the
surrounding.
• Has no Kinetic and potential energy relative to
the surrounding.
• Is Chemically inert with surrounding.
• Has no unbalanced magnetic, electrical and
surface tension force.
Dead State System
Dead States are denoted by subscript zero.
Unless specified, the dead state has:
𝑇_0=25 ℃, 𝑃_0=1 𝑎𝑡𝑚
The atmosphere around us contains a tremendous amount of energy. However, the
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atmosphere is in the dead state, andESO201
the
energy it contains has no work potential
Note Points
• Exergy is not the actual work that a device can produce at the given state upon
installation
• But no system can generate more work than exergy at a given state without
breaking the thermodynamic laws.
• exergy is a property of the system–environment combination so altering the
environment can alter the exergy.
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Exergy Associated with Kinetic and Potential Energy
• . Kinetic and Potential energies are forms of
mechanical energy and hence can be
completely converted to work irrespective of
the pressure and temperature of the
surroundings.
• The exergies of kinetic and potential energies
are equal to themselves, and they are entirely
available for work.
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Unavailable Energy
• Unavailable energy is the part of total energy that cannot be converted into work
no matter how efficient the engine is (reversible engine).
• Unavailable energy is accessed to get work only when the second law of
thermodynamics is violated.
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Reversible Work and Irreversibility
Importance:
• Final state is always assumed to be the dead state,
which is hardly ever the case for actual engineering
systems.
• Reversible work and irreversibility are related to
the actual initial and final state.
Surrounding work:
• The work done by or against the surroundings
during a process.
• It is applicable to only when the volume of the
system changes
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Reversible Work
• . Useful work: The difference between the actual work
and the surroundings work.
• Reversible work Wrev: 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.
• This is the useful work output (or input) obtained (or
expended) when the process between the initial and final
states is executed in a totally reversible
manner.
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Irreversibility
• The difference between the reversible work and
the useful work is due to irreversibilities and
this difference is called irreversibility.
• Irreversibility can be viewed as the wasted work
potential or the lost opportunity to do work.
• The performance of a system can be improved by
minimizing the irreversibility associated with it.
• Irreversibility is a positive quantity for all actual
(irreversible) processes
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The Rate of Irreversibility of a Heat Engine
Calculate reversible power and irreversibility rate for
this process.
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Irreversibility during the Cooling of an Iron Block
Calculate reversible work and irreversibility in this process.
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Second-law Efficiency
• The thermal efficiency and the coefficient of
performance studied earlier are called first law
efficiencies.
• First law efficiency does not consider the maximum
potential of the engine and hence do not give a
realistic measure.
The two engines have same thermal
efficiency but different maximum
potential
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Second-law Efficiency
 Second-law
efficiency
of
all
reversible devices is 100 percent.
 Second-law
efficiency
of
all
reversible devices is 100 percent.
 the reversible work should be
determined by using the same initial
and final states as in the actual
process.
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General definition of exergy efficiency
The second-law efficiency definition discussed earlier do not apply to devices that
are not intended to produce or consume work. Therefore, we need a more general
definition.
• In a reversible operation, we should be able to recover
entirely the exergy expended during the process, and the
irreversibility in this case should be zero.
•
In a reversible operation, we should be able to recover
entirely the exergy expended during the process, and the
irreversibility in this case should be zero. T
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Second-Law Efficiency of Resistance Heaters
• In electric resistance heating, the exergy expended is the electrical energy the
resistance heater consumes from the resource of electric grid.
• The exergy recovered is the exergy content of the heat supplied to the room,
which is the work that can be produced by a Carnot engine receiving this heat.
• If the heater maintains the heated space at a constant temperature of TH in an
environment at T0, the second-law efficiency for the electric heater becomes
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Learning Objectives
The Thermodynamic Temperature Scale.
The Carnot Heat Engine
The Quality of Energy
The Carnot Refrigerator and Heat Pump
Examples
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