Exergetic analysis of gas turbine plants
Mohamad Javad Ebadi* and Mofid Gorji-Bandpy
Department of Mechanical Engineering,
University of Mazandaran, P.O. Box 484, Babol, Iran
E-mail: ebadi220@gmail.com E-mail: gorji@nit.ac.ir
*Corresponding author
Abstract: An exergetic analysis was performed for a 116-MW gas-turbine
power plant. Mass and energy conservation laws were applied to each
component of the system. Quantitative exergy balance for each component
and for the whole system was considered. In this study, the exergy of a
material stream is decomposed into thermal, mechanical and chemical
exergy and an entropy-production flow. The effect of a change in the inlet
turbine temperature on the exergetic efficiency and exergy destruction in the
plant was evaluated. The crucial dependency of the exergetic efficiency and
the exergy destruction on the change in the turbine inlet temperature was
confirmed.
Keywords: efficiency; exergy; gas-turbine; irreversibility.
Reference to this paper should be made as follows: Ebadi, M.J. and
Gorji-Bandpy, M. (2005) `Exergetic analysis of gas turbine plants', Int. J.
Exergy, Vol. 2, No. 1, pp.31±39.
Biographical notes: Mohamad Javad Ebadi received his BS degree in
mechanical engineering from the Azad University of Ahwaz, Ahwaz, Iran.
At present he is an MSc Student studying mechanical engineering (energy
conversion) at the University of Mazandaran, Babol, Iran.
Mofid Gorji-Bandpy received his MS in mechanical engineering from the
Faculty of Engineering University of Tehran, Iran, in 1978, and in 1990
obtained his PhD degree in hydraulic engineering from the School of
Engineering, University of Wales College of Cardiff (UWCC), UK. At
present he is an A ssociate Professo r of mechanical engineering at
Mazandaran University, Iran, and a Visiting Professor in the department
of Mechanical and Industrial Engineering at the University of Toronto,
Canada. His major interests are advanced methods of energy conversion
systems, turbomachinery, fluid mechanics, water distribution networks and
the solution of both energy and environmental problems.
1 Introduction
In recent years, the use of exergy analysis in thermal design has been discussed and
demonstrated by numerous authors (Cengel and Boles, 1998; Jones and Dugan,
1996; Moran and Shapiro, 2000; Verkhivker and Kosoy, 2001).
An exergy-based performance analysis is the performance analysis of a system based
on the second law of thermodynamics that overcomes the limit of an energy-based
Int. J. Exergy, Vol. 2, No. 1, 2005 31
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#
2005 Inderscience Enterprises Ltd.
analysis. Exergy is generally not conserved as energy but destroyed in the system.
Exergy destruction is the measure of irreversibility that is the source of performance
loss. Therefore, an exergy analysis assessing the magnitude of exergy destruction
identifies the location, the magnitude and the source of thermodynamic inefficiencies
in a thermal system (Flavio et al., 2000; Zhang et al., 2000).
Exergy analysis usually predicts the thermodynamic performance of an energy
system and the efficiency of the system components by accurately quantifying the
entropy-generation of the components.
In our study, an exergetic analysis was performed for a 116-MW gas-turbine
plant, which is an existing plant located in Mahshahr, Iran. Mass and energy
conservation laws were applied to each component. A quantitative exergy balance of
each component was also delivered carefully. The exergy-balance equation developed
by Oh et al. (1996) was used in this analysis.
2 The gas turbine plant
A schematic of a 116-MW gas turbine system is given in Figure 1 and shows the main
work and exergy flows and the state points which we accounted for in this analysis.
The system consists of an air-compressor (AC), a combustion chamber (CC), an
air-preheater (APH), and a gas-turbine (GT). The mass flow rate of air to the
compressor at 26
C is 497 ks s
ÿ1
and the air-fuel ratio at full load is 50 on a mass basis.
Figure 1 Gas turbine system
The incoming air has a temperature of 26
C and a pressure of 1.013 bar. The pressure
increases to 8.611 bar through the compressor, which has an isentropic efficie ncy of
83%. The inlet temperature to the turbine is 1048
C. The turbine has an isentropic
efficiency of 88%. The regenerative heat exchanger has an effecti veness of 75%.
The pressure drop through the air preheater is 4% of the inlet pressure for both
flow streams an d through the combustion chamber is 3% of the inlet pressure. The
fuel (natural gas) is injected at 26
C and 30 bar. The pressure of the hot gas exhaust
from the air preheater is 1.032 bar.
M.J. Ebadi and M. Gorji-B andpy32
3 Formulation of exergy-balance equation
A general exergy-balance equation, applicable to any component of a thermal system
may be formulated by utilising the first and second laws of thermodynamics (Oh
et al., 1996). The thermomechanical exergy stream may be decomposed into its
thermal and mechanical components (Kwon et al., 2001). The balance gives:
_
E
m
i
ÿ
_
E
m
o
_
E
T
i
ÿ
_
E
T
o
ÿ
_
E
P
i
ÿ
_
E
P
o
ÿ
; 1
where the subscripts i and o denote, respectively, exergy flow streams entering or
leaving the plant component.
The thermal and mechanical components of the exergy stream for an ideal gas
with constant specific heat may be written as (Kotas, 1995)
_
E
T
_
mc
p
T ÿ T
ref
ÿ
ÿ T
ref
ln
T
T
ref
2
_
E
P
_
m RT
ref
ln
P
P
ref
3
With the decomposition defined by Equation (1), the general exergy-balance
equation is written as follows (Oh et al., 1996):
_
E
CHE
X
inlet
_
E
T
i
ÿ
X
outlet
_
E
T
o
!
X
inlet
_
E
P
i
ÿ
X
outlet
_
E
P
o
!
T
ref
X
inlet
_
S
i
ÿ
X
outlet
_
S
o
_
Q
CV
T
ref
!
_
E
W
4
The term
_
E
CHE
in Equation (4) denotes the rate of exergy flow of fuel in the plant
and
_
Q
CV
in the fourth term denotes the heat transfer interaction between the
component and the environment.
4 Exergy-balance equation for a gas turbine plant
The exergy-balance equations for each component in the gas turbine plant, can
be derived from the general exergy balance equation given in Equation (4). The
exergy-balance equations for these components are as follows.
Air compressor
_
E
T
1
ÿ
_
E
T
2
ÿ
_
E
P
1
ÿ
_
E
P
2
ÿ
T
0
_
S
1
ÿ
_
S
2
ÿ
_
W
AC
: 5
Air preheat er
_
E
T
2
ÿ
_
E
T
3
_
E
T
6
ÿ
_
E
T
7
ÿ
_
E
P
2
ÿ
_
E
P
3
_
E
P
6
ÿ
_
E
P
7
ÿ
T
0
_
S
2
ÿ
_
S
3
_
S
6
ÿ
_
S
7
_
Q
APH
=T
0
ÿ
0
: 6
Exergetic analysis of gas turbine plants 33
Combustion chamber
_
E
CHE
_
E
T
3
_
E
T
f
ÿ
_
E
T
5
_
E
P
3
_
E
P
f
ÿ
_
E
P
5
T
0
_
S
3
_
S
f
ÿ
_
S
5
_
Q
CC
=T
0
ÿ
0: 7
Gas turbine
_
E
T
5
ÿ
_
E
T
6
ÿ
_
E
P
5
ÿ
_
E
P
6
ÿ
T
0
_
S
5
ÿ
_
S
6
ÿ
_
W
GT
: 8
5 Results and discussions
Table 1 shows chemical, thermal and mechanical exergy flow rates and entropy flow
rates at various state points in the cycle. These flow rates were calculated based on
the values of measured properties such as pressure, temperature, and mass flow rate
at various points. Evaluations of various exergies at the inlet, and outlet of each
system component are obtained by fitting appropriate polynomials (Gordon and
Mcbride, 1971) to the thermophysical data in the JANAF Tables (1971).
Table 1 Property values and chemical, thermal and mechanical exergy flows and entropy
production rates at various state points in the gas turbine plant at rated conditions
State
_
m (kg/s) t (K) p (bar)
_
E
CHE
(MW)
_
E
T
(MW)
_
E
P
(MW)
_
S (MW/K)
1 497.00 299.15 1.013 0.000 0.000 0.000 0.000
2 497.00 603.02 8.611 0.000 47.034 91.318 0.045
3 497.00 796.91 8.267 0.000 102.221 89.580 0.190
4 10.09 299.15 30.000 508.566 0.000 5.298 ÿ0.018
5 507.09 1320.00 8.019 0.000 335.766 91.015 0.563
6 507.09 861.54 1.075 0.000 143.181 2.613 0.607
7 507.09 695.18 1.032 0.000 83.699 0.817 0.488
The net flow rates of the various exergies crossing the boundary of each component
in the gas-turbine plant at rated conditions are shown in Table 2, together with the
exergy destruction in each component. Positive values indicate the exergy flow rate of
products while negative values represent the exergy flow rate of resources or fuel.
Here, the product of a component corresponds to the added exergy whereas the
resource to the consumed exergy (Kwak et al., 2003). The sum of the exergy flow
rates of products, resources and destruction equals zero for each component and for
the total plant; this zero sum indicates that exergy balances are exactly satisfied.
Figure 2 shows the exergetic efficiency
b
of components of the gas-turbine plant.
The exergetic efficiency of the total plant is also shown: it amounts to 39%. It is
shown that the exergetic efficiency of the combustion chamber is much lower than
that of other plant components, due to the high irreversibility in the former.
M.J. Ebadi and M. Gorji-B andpy34
Table 2
Net exergy flow rates and exergy destruction in the gas turbine plant at rated condition
Component
_
E
W
(MW)
_
E
CHE
(MW)
_
E
T
(MW)
_
E
P
(MW)
_
E
D
(MW)
Compressor ÿ151.814 0.000 47.034 91.318 13.462
Air preheater 0.000 0.000 ÿ4.295 ÿ3.534 7.829
Combustion chamber 0.000 ÿ508.566 233.545 ÿ3.863 278.884
Gas turbine 267.824 0.000 ÿ192.585 ÿ88.402 13.163
Total plant 116.010 ÿ508.566 88.699 ÿ4.481 313.338
Figure 2 Exergetic efficiency of components and of total plant in the system
In comparison with other plant components, the combustion chamber destructs the
largest amount of total inlet exergy into the plant, as shown in Figure 3. This figure
shows also that 60.97% of the total inlet exergy is annihilated in the plant.
Figure 3 Exergy destruction in components and in total plant in the system
Exergetic analysis of gas turbine plants 35
