PPT: Availability & Irreversibility Mechanical Engineering Notes | EduRev

 Page 1


Introduction 
• There are many forms in which an energy can exist. But even under ideal conditions all these
forms cannot be converted completely into work. This indicates that energy has two parts :
- Available part
- Unavailable part
• ‘Available energy’ or ‘Exergy’: is the maximum portion of energy which could be converted into
useful work by ideal processes which reduce the system to a dead state (a state in equilibrium
with the earth and its atmosphere).
- There can be only one value for maximum work which the system alone could do while descending
to its dead state, therefore 'Available energy’ is a property
• ‘Unavailable energy’ or Anergy’: is the portion of energy which could not be converted into useful
work and is rejected to the surroundings
Page 2


Introduction 
• There are many forms in which an energy can exist. But even under ideal conditions all these
forms cannot be converted completely into work. This indicates that energy has two parts :
- Available part
- Unavailable part
• ‘Available energy’ or ‘Exergy’: is the maximum portion of energy which could be converted into
useful work by ideal processes which reduce the system to a dead state (a state in equilibrium
with the earth and its atmosphere).
- There can be only one value for maximum work which the system alone could do while descending
to its dead state, therefore 'Available energy’ is a property
• ‘Unavailable energy’ or Anergy’: is the portion of energy which could not be converted into useful
work and is rejected to the surroundings
• A system which has a pressure difference from that of surroundings, work can be obtained from
an expansion process, and if the system has a different temperature, heat can be transferred to
a cycle and work can be obtained. But when the temperature and pressure becomes equal to that
of the earth, transfer of energy ceases, and although the system contains internal energy, this
energy is unavailable
• Summarily available energy denote, the latent capability of energy to do work, and in this sense it
can be applied to energy in the system or in the surroundings.
• The theoretical maximum amount of work which can be obtained from a system at any state p
1
and
T
1
when operating with a reservoir at the constant pressure and temperature p
0
and T
0
is called
‘availability’.
Page 3


Introduction 
• There are many forms in which an energy can exist. But even under ideal conditions all these
forms cannot be converted completely into work. This indicates that energy has two parts :
- Available part
- Unavailable part
• ‘Available energy’ or ‘Exergy’: is the maximum portion of energy which could be converted into
useful work by ideal processes which reduce the system to a dead state (a state in equilibrium
with the earth and its atmosphere).
- There can be only one value for maximum work which the system alone could do while descending
to its dead state, therefore 'Available energy’ is a property
• ‘Unavailable energy’ or Anergy’: is the portion of energy which could not be converted into useful
work and is rejected to the surroundings
• A system which has a pressure difference from that of surroundings, work can be obtained from
an expansion process, and if the system has a different temperature, heat can be transferred to
a cycle and work can be obtained. But when the temperature and pressure becomes equal to that
of the earth, transfer of energy ceases, and although the system contains internal energy, this
energy is unavailable
• Summarily available energy denote, the latent capability of energy to do work, and in this sense it
can be applied to energy in the system or in the surroundings.
• The theoretical maximum amount of work which can be obtained from a system at any state p
1
and
T
1
when operating with a reservoir at the constant pressure and temperature p
0
and T
0
is called
‘availability’.
• First Law of Thermodynamics (law of energy conservation) used for may analyses performed
• Second Law of Thermodynamics simply through its derived property - entropy (S)
• Other ‘Second Law’ properties my be defined to measure the maximum amounts of work
achievable from certain systems
• This section considers how the maximum amount of work available from a system, when
interacting with surroundings, can be estimated
• All the energy in a system cannot be converted to work: the Second Law stated that it is
impossible to construct a heat engine that does not reject energy to the surroundings
Page 4


Introduction 
• There are many forms in which an energy can exist. But even under ideal conditions all these
forms cannot be converted completely into work. This indicates that energy has two parts :
- Available part
- Unavailable part
• ‘Available energy’ or ‘Exergy’: is the maximum portion of energy which could be converted into
useful work by ideal processes which reduce the system to a dead state (a state in equilibrium
with the earth and its atmosphere).
- There can be only one value for maximum work which the system alone could do while descending
to its dead state, therefore 'Available energy’ is a property
• ‘Unavailable energy’ or Anergy’: is the portion of energy which could not be converted into useful
work and is rejected to the surroundings
• A system which has a pressure difference from that of surroundings, work can be obtained from
an expansion process, and if the system has a different temperature, heat can be transferred to
a cycle and work can be obtained. But when the temperature and pressure becomes equal to that
of the earth, transfer of energy ceases, and although the system contains internal energy, this
energy is unavailable
• Summarily available energy denote, the latent capability of energy to do work, and in this sense it
can be applied to energy in the system or in the surroundings.
• The theoretical maximum amount of work which can be obtained from a system at any state p
1
and
T
1
when operating with a reservoir at the constant pressure and temperature p
0
and T
0
is called
‘availability’.
• First Law of Thermodynamics (law of energy conservation) used for may analyses performed
• Second Law of Thermodynamics simply through its derived property - entropy (S)
• Other ‘Second Law’ properties my be defined to measure the maximum amounts of work
achievable from certain systems
• This section considers how the maximum amount of work available from a system, when
interacting with surroundings, can be estimated
• All the energy in a system cannot be converted to work: the Second Law stated that it is
impossible to construct a heat engine that does not reject energy to the surroundings
• For stability of any system it is necessary and sufficient that, in all possible variations of
the state of the system which do not alter its energy, the variation of entropy shall be
negative
• This can be stated mathematically as ?S < 0
• It can be seen that the statements of equilibrium based on energy and entropy, namely ?E > 0
and ?S < 0
Page 5


Introduction 
• There are many forms in which an energy can exist. But even under ideal conditions all these
forms cannot be converted completely into work. This indicates that energy has two parts :
- Available part
- Unavailable part
• ‘Available energy’ or ‘Exergy’: is the maximum portion of energy which could be converted into
useful work by ideal processes which reduce the system to a dead state (a state in equilibrium
with the earth and its atmosphere).
- There can be only one value for maximum work which the system alone could do while descending
to its dead state, therefore 'Available energy’ is a property
• ‘Unavailable energy’ or Anergy’: is the portion of energy which could not be converted into useful
work and is rejected to the surroundings
• A system which has a pressure difference from that of surroundings, work can be obtained from
an expansion process, and if the system has a different temperature, heat can be transferred to
a cycle and work can be obtained. But when the temperature and pressure becomes equal to that
of the earth, transfer of energy ceases, and although the system contains internal energy, this
energy is unavailable
• Summarily available energy denote, the latent capability of energy to do work, and in this sense it
can be applied to energy in the system or in the surroundings.
• The theoretical maximum amount of work which can be obtained from a system at any state p
1
and
T
1
when operating with a reservoir at the constant pressure and temperature p
0
and T
0
is called
‘availability’.
• First Law of Thermodynamics (law of energy conservation) used for may analyses performed
• Second Law of Thermodynamics simply through its derived property - entropy (S)
• Other ‘Second Law’ properties my be defined to measure the maximum amounts of work
achievable from certain systems
• This section considers how the maximum amount of work available from a system, when
interacting with surroundings, can be estimated
• All the energy in a system cannot be converted to work: the Second Law stated that it is
impossible to construct a heat engine that does not reject energy to the surroundings
• For stability of any system it is necessary and sufficient that, in all possible variations of
the state of the system which do not alter its energy, the variation of entropy shall be
negative
• This can be stated mathematically as ?S < 0
• It can be seen that the statements of equilibrium based on energy and entropy, namely ?E > 0
and ?S < 0
• System A, which is a general system of constant composition in which the work output, ??W, can 
be either shaft or displacement work, or a combination of both
• Figure b, the work output is displacement work, p ??V
Helmholtz Energy (Helmholtz function) 
Thermal Reservoir T
o
??Q
??Q
o
p
o p
System A
????
??
System B
E
R