3. EXERGY:
“The useful work potential of the system at the
specified state and is called exergy.”
It is important to realize that exergy does not
represent the amount of work that a work-producing
device will actually deliver upon installation.
Rather, it represents the upper limit on the amount of
work a device can deliver without violating any
thermodynamic laws.
For more understanding of exergy see this fig.
4.
5. Exergy of closed system:
A system at a given state can attain a new state
through work and heat interactions with its
surroundings. Since the exergy value associated with
the new state would generally differ from the value at
the initial state, transfers of exergy across the system
boundary can be inferred to accompany heat and
work interactions.
The change in exergy of a system during a process
would not necessarily equal the net exergy
transferred, for exergy would be destroyed if
irreversibilities were present within the system during
6.
7. Exergy of a closed system, per unit mass j, can be
found be adding all the terms
This gives us the maximum work we could possibly get
out of a system.
Usually we will be more interested in the change in
exergy from the beginning to end of a process.
Note that = 0 at dead state.
8. When properties are not uniform, exergy can be
determined by integration:
Where V is the volume of the system and is density.
9. Note that exergy is a property, and the value of a
property does not change unless the state changes.
If the state of system or the state of the environment
do not change, the exergy does not change.
Therefore, the exergy change of a system is zero if the
state of the system or the environment does not change
during the process. For example, the exergy change of
steady flow devices such as nozzles, compressors,
turbines, pumps, and heat exchangers in a given
environment is zero during steady operation.
The exergy of a closed system is either positive or
zero. It is never negative.
10. Irreversibility:
It represents energy that could have been converted
into work but was instead wasted
Difference between reversible work and useful work
is called irreversibility
Irreversibility is equal to the exergy destroyed
Totally reversible process, I = 0
I, a positive quantity for actual, irreversible processes
To have high system efficiency, we want I to be as
small as possible.
11. (work output device, like a turbine)
OR
(work input device, like a pump)
Wu= useful work; the amount of work done that can actually be used for
something desirable.
where W=actual work done.
12. Gouy-Stodola theorem:
We seen that,
The Gouy stodola theorem states that the rate of loss of
exergy in a process is proportional to the rate of entropy
generation,
This is known as the Gouy-stodola eqn. A thermody-
namically efficient process would involve minimum
exergy loss with minimum rate of entropy generation.
13. Second Law Efficiency II
The first-law efficiency alone is not a realistic
measure of performance of engineering devices. To
overcome this deficiency, we define a second-law
efficiency II as the ratio of the actual thermal
efficiency to the maximum possible (reversible)
thermal efficiency.
For understand this point see this eg.
14. Eg.:
Consider two heat engines, both having a thermal
efficiency of 30 percent as shown in Fig.
One of the engines (engine A) is supplied with heat
from a source at 600 K, and the other one (engine B)
from a source at 1000 K. Both engines reject heat to a
medium at 300 K. At first glance, both engines seem
to convert to work the same fraction of heat that they
receive; thus they are performing equally well.
When we take a second look at these engines in light
of the second law of thermodynamics, however, we
see a totally different picture. These engines, at best,
can perform as reversible engines, in which case their
efficiencies would be
15. Now it is becoming apparent that engine B has a
greater work potential available to it (70 percent of the
heat supplied as compared to 50 percent for engine A),
and thus should do a lot better than engine A.
Therefore, we can say that engine B is performing
poorly relative to engine A even though both have the
same thermal efficiency.
16. Based on definition of the second-law efficiencies,
Now for the two heat engines discussed above are:
That is, engine A is converting 60% of the available
work potential to useful work. This ratio is only 43%
for engine B.
17. The second-law efficiency can also be expressed as
the ratio of the useful work output and the maximum
possible (reversible) work output:
This definition is more general since it can be applied
to processes (in turbines, piston–cylinder devices,
etc.) as well as to cycles.
Note that the second law efficiency cannot exceed
100%.
18. We can also define a second-law efficiency for work-
consuming noncyclic (such as compressors) and
cyclic (such as refrigerators) devices as the ratio of the
minimum (reversible) work input to the useful work
input
For cyclic devices such as refrigerators and heat
pumps, it can also be expressed in terms of the
coefficients of performance as
