Exergy Analysis of various systems

1. Exergy Analysis of Thermal
Systems
Prof. Debajyoti Bose
Alternate Energy, UPES

2. Exergy analysis is based on three pillars:
a. Conservation of mass
b. Conservation of energy
c. Second law of thermodynamics
Uses of exergy analysis:
a. Design of thermal systems
b. Reduce source of inefficiency in existing systems
c. Evaluate system economics

4. • Total energy associated with the system is unchanged
• But initial air-fuel mixture has greater economic value and more
useful than the final product gases
• For example the fuel can be used to generate electricity or
generate superheated steam
• Whereas the use of combustion product is limited
“System has greater potential in initial use.”
But once the match is lit the process is irreversible
This shows unlike energy, EXERGY is not conserved
“Exergy can be destroyed by irreversibilities”

5. Defining Exergy:
Exergy is the maximum theoretical work obtained as a system tends
towards equilibrium.
The reference environment is very important

6. • Suppose a steam based TPP operates around a surrounding
• It is important to distinguish the environment when calculating
exergy
• For exergy analysis we introduce “immediate environment.”
Where intensive property may vary for the system
• However, it is unaffected by the environment
Without the Exergy Line = Energy analysis of TPP

7. Exergy of a system:
where E= (U+ KE + PE)
U0, V0, and S0 are the values of the same properties
if the system were at the dead state
“The units of exergy are seen to be the
same as those of energy.”

8. Dead State:
• If the state of a fixed quantity of matter, a closed system, departs from
that of the environment, an opportunity exists for developing work.
However, as the system changes state toward that of the environment,
the opportunity diminishes, ceasing to exist when the two are in
equilibrium with one another. This state of the system is called the dead
state
• At the dead state, the fixed quantity of matter under consideration is
imagined to be sealed in an envelope impervious to mass flow, at rest
relative to the environment, and internally in equilibrium at the
temperature T0 and pressure p0 of the environment
• At the dead state, both the system and environment possess energy, but
the value of exergy is zero because there is no possibility of a
spontaneous change within the system or the environment, nor can
there be an interaction between them

9. EXERGY Aspects:
• Exergy is a measure of the departure of the
state of a system from that of the environment.
It is therefore an attribute of the system and
environment together
• The value of exergy cannot be negative. If a system were
at any state other than the dead state, the system would be
able to change its condition spontaneously toward the dead
State
• Exergy is not conserved but is destroyed by irreversibilities.

10. The specific exergy on a unit mass basis is given by:
we can determine the change in exergy between two states of a closed system as the
difference:
By inspection, the units of specific exergy are the same as those of specific energy.

11. Numericals
1. Determine the specific exergy of saturated water vapor at 120°C,
having a velocity of 30 m/s and an elevation of 6 m, each relative to
an exergy reference environment where T0 = 298 K (25°C), p0= 1 atm,
and g = 9.8 m/s2. For water as saturated vapor at 120°C, Given v =
0.8919 m3/kg, u= 2529.3 kJ/kg, s= 7.1296 kJ/kg K. At the dead state,
where T0 = 298°K (25C) and p0 = 1 atm, water is a liquid.
• Additional Data given:

12. 2. A cylinder of an internal combustion engine contains 2450 cm3
of gaseous combustion products at a pressure of 7 bar and a
temperature of 867°C just before the exhaust valve opens.
Determine the specific exergy of the gas, in kJ/kg.
Ignore the effects of motion and gravity, and model the combustion
products as air as an ideal gas. Take T0= 300 K (27°C) and p0=1.013
bar.