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INTERNATIONAL JOURNAL OF ENERGY RESEARCH
Int. J. Energy Res. 2012; 36: –
Published online February 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.1814
486 498
7
Exergy, economic and environment (3E) analysis
of absorption chiller inlet air cooler used in gas
turbine power plants
M. A. Ehyaei
1
, S. Hakimzadeh
2
, N. Enadi
3
and P. Ahmadi
4,
,y
1
Islamic Azad University, Pardis Branch, Pardis New City, Tehran, Iran
2
Islamic Azad University, Dezful Branch, Dezful City, Khuzestan, Iran
3
Energy Engineering Department, Power and Water University of Technology (PWUT), Tehran, Iran
4
Clean Energy Research Lab (CERL), Department of Mechanical Engineering, Faculty of Engineering and Applied Science, University
of Ontario, Institute of Technology (UOIT), 23–19 Niagara DR, Oshawa, Ont., Canada L1G 8G2
SUMMARY
Gas turbine (GT) output power is affected by temperature, gas turbine inlet air-cooling systems are used to solve
this. In the present work, the effect of using absorption chiller in GT power plants for two regions in Iran, namely
Tabas with hot–dry and Bushehr with hot–humid climate conditions is conducted. Therefore, output power, first
and second law efficiencies, environmental and electrical costs for GT power plant with inlet air cooler are
calculated for two mentioned regions, respectively. Results show that using this system in hot months of a year is
economical. In addition, using absorption chiller leads to increasing the output power 11.5 and 10.3%, for Tabas
and Bushehr cities, respectively. Moreover, by using this method the second law efficiency is increased to 22.9 and
29.4% for Tabas and Bushehr cities, respectively. In addition, the cost of electricity production for Tabas and
Bushehr cities decreases to about 5.04 and 2.97%, respectively. Copyright
r
2011 John Wiley & Sons, Ltd.
KEY WORDS
exergy; gas turbine; absorption chiller; cost of electricity production
Correspondence
*P. Ahmadi, Clean Energy Research Lab (CERL), Department of Mechanical Engineering, Faculty of Engineering and Applied Science,
University of Ontario Institute of Technology (UOIT), 23-19 Niagara DR, Oshawa, Ont., Canada L1G 8G2.
y
E-mail: Pouryaahmadi81@gmail.com, Pouria.ahmadi@uoit.ca
Received 21 May 2010; Revised 6 December 2010; Accepted 7 December 2010
1. INTRODUCTION
strong function of the ambient air temperature with
power output dropping by 0.5–0.9% for every 11C rise
in the ambient temperature. On several heavy duty
frame GTs, power output drops of around 20% can be
experienced when the ambient temperature reaches
351C, coupled with a heat rate increase of about 5%.
This is due to reduced inlet air density and mass flow
rate [1]. Therefore, the solution of this problem is very
important because the peak demand season also
happens in hot days of summer. One approach to
overcome this problem during periods of high ambient
temperature (high demand period) is to cool the inlet air.
There are several inlet air-cooling technologies available
such as inlet air fogging system, direct refrigeration,
electrically driven chillers or absorption chillers.
One of these options is absorption cooling system.
While the mechanical refrigeration can reduce the
GT inlet temperature below the wet bulb temperature,
the absorption system is relatively simple and has
a lower operation and maintenance cost than the
Gas turbine (GT) power plants are significantly
impacted by the ambient air temperature. Hence,
output power of these cycles decreases with increase
in the ambient temperature.
The GT is known to feature low capital cost to
power ratio, high flexibility, high reliability without
complexity, short delivery time, early commissioning
and commercial operation.
According to a survey [1,2], there are more than
170 GT units used in Iran. The total capacity of these
units is around 9500MW. However, the power output
of the units is about 80% of their rated capacity in the
summer. It means that around 1900MW are lost
during the hot season.
GT output power strongly depends on the inlet air
mass flow rate. Therefore, the available output power
considerably reduces when the air density decreases at
high ambient temperatures. Thus, GT output power is a
486
Copyright
r
2011 John Wiley & Sons, Ltd.
Exergy, economic and environment (3E) analysis
M. A. Ehyaei et al.
2. DESCRIPTION OF LOCATIONS
mechanical refrigeration, and it can cool the inlet air
to 101C [2].
Although many researches have been carried out
about GT power plant, the numbers of researches
performed on absorption chiller inlet air cooler for GT
power plant are restricted. Mohanty et al. [3] analyti-
cally studied the GT power plant enhancement in
Bangkok, Thailand, and found that 11% increase
in output power was observed by using the absorption
chiller as inlet air cooler. Bies et al. [4] used the
absorption chiller units without a cooling water sto-
rage system to enhance the power production by GT
unit. Ameri et al. [5] proposed the improvement of the
new power plant (16.6MW) and focused on the GT
unit. The study showed that the power output could be
increased by about 11.3% by using the absorption
chiller inlet air cooler. Namprakai et al. [6] studied the
combined cycle power plant by intake air cooling using
an absorption chiller. This study showed that the
power output of a GT was increased by about 10.6%
and the CC power plant by around 6.24% annually [6].
According to the literature, although there are several
papers including GT inlet air cooling with absorption
chiller, these researches do not pay much attention to
economical aspects as well as environmental impacts.
Therefore, the objective of this paper is to study the
effect of using absorption chiller inlet air-cooling system
for the GT power plant by exergy and economic ap-
proaches. For this analysis, the GT model used is V94.2
of Siemens and it is located in Bushehr and Tabas cities.
Moreover, the social cost of air pollution has been
considered in the economic analysis. The social cost of
air pollution is based on the negative effects of air
pollution on the health of society and environment. The
economic aspect of these effects is called external social
cost of air pollution. Other pollution sources such as
water, soil, etc. produced by an operational power
generation system are ignored in this research.
In summary, the following are the specific contri-
bution of this study in the subject matter area:
2.1. Bushehr city
Bushehr is a city in southwest Iran, located near the
Persian Gulf in a vast plain running along the coastal
region, the capital of Bushehr Province. Bushehr is a
major fishing and commercial port (so-called Bandar-e
Bushehr). It is one of the chief ports of Iran and its
location is 281 59
0
N, 501 49
0
E, about 1.281 km
(796mile) south of Tehran, and it has a hot, humid
climate. The maximum average temperature and
relative humidity of Bushehr city during the months
of a year are shown in Figure 1 [7].
2.2. Description of location, Tabas
Tabas city located in the centralIran is a city of 30 000
people, 950 km southeast of Tehran, in the province
of Yazd. Formerly it was part of Khorasan province.
The maximum average temperature and relative
humidity of Tabas city during the months of a year
are shown in Figure 1 [7].
3. SYSTEM DESCRIPTION AND
MATHEMATICAL MODELING
Figures 2 and 3 show the schematic diagram of a GT
power plant with and without absorption chiller.
In a simple GT power plant, a radial flow (centrifugal)
compressor (‘C’) compresses the inlet air. The com-
pressed air mixes and reacts with compressed fuel
(from the booster compressor) in the combustion
chamber (CC) and then the hot combustion gases
expand in the turbine. The turbine runs the compressor,
the booster compressor and the generator. A booster
compressor is needed to increase the fuel pressure to
that of the combustion chamber. In GT power plants
with absorption chiller, the hot flue gas from the GT
Exergy and economic analysis of a GT power
plant with inlet air-cooling method has been
conducted.
A computer program code is developed in the
FORTRAN language which can be applied for
simulation of any types of GT with and without
absorption chiller inlet air-cooling method.
The comparison between our developed code and
an actual power plant is conducted and good
agreement was obtained.
Comparison of costs of electricity production in two
cases with and without absorption chiller for two
conditions (i.e. hot–dry and hot–humid) is shown.
Average entropy production rate for both the
cases is calculated and explained in detail.
The cost of environmental impacts has been
considered for the economical consideration.
90
110
85
Maximum average dry bulb temperature (Bushehr city)
Maximum average wet bulb temperature (Bushehr city)
Maximum average dry bulb temperature (Tabas city)
Maximum average wet bulb temperature (Tabas city)
Relative humidity (Bushehr city)
Relative humidity (Tabas city)
100
80
75
90
70
80
65
60
70
55
50
60
45
50
40
35
40
30
25
30
20
20
15
10
10
5
0
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Months
Figure 1.
Maximum average dry bulb and wet bulb tempera-
tures and relative humidity of Bushehr and Tabas cities during
the months of a year [7].
487
Int. J. Energy Res. 2012;
36
:486–498
r
2011 John Wiley & Sons, Ltd.
DOI: 10.1002/er
M. A. Ehyaei et al.
Exergy, economic and environment (3E) analysis
Table I.
V94.2 gas turbine characteristics [8].
159
Output power (MW)
50
Frequency (Hz)
11.1
Compression ratio
Turbine exhaust gas mass flow rate (kg s
1
)
519
539
Exhaust temperature (
1
C)
34.5
Efficiency (%)
85
Compressor efficiency (%)
76
Booster compressor efficiency (%)
85
Turbine efficiency (%)
250
Pipeline pressure (kPa)
Low heat value fuel (kJ kg
1
)
48916.9
225
Percent excess air (%)
Input fuel mass flow (kg s
1
)
9.316
Figure 2.
Schematic diagram of simple gas turbine power plant.
Table II.
Specifications of natural gas components [13,15].
Component, i
CH
4
C
2
H
6
C
3
H
8
C
4
H
10
CO
2
N
2
y
i
, % by mole
81
7.9
4.2
4.7
1.2
1
x
i
, % by mass
62.58
11.44
8.92
13.16
2.55
1.35
M
i
, kg kmole
1
16
30
44
58
44
28
C
pi,
kJ kg
1
K
1
2.25
1.76
1.67
1.64
0.84
1.04
M
f
¼
X
y
i
M
i
¼
20:712
ð
kg=kmoles
Þ
ð
c
p
Þ
f
¼
X
x
i
c
p
i
¼
2:013
ð
kJ=kg K
Þ
R
f
¼
R
u
M
f
¼
0:401
ð
kJ=kg K
Þ
ð
C
v
Þ
f
¼ð
C
p
Þ
f
R
f
¼
1:612
ð
kJ=kg K
Þ
k
f
¼ð
C
p
Þ
f
=
ð
C
v
Þ
f
¼
1:249
Figure 3.
Schematic diagram of simple gas turbine power plant
with absorption chiller inlet air cooling.
exhaust is used to generate the hot water in a heat
exchanger. The hot water is usually used in a double-
effect lithium-bromide absorption chiller to produce
the chilled water. A compact heat exchanger should be
designed for installation at the compressor inlet duct.
The chilled water from the absorption chiller flows
through the heat exchanger and cools the inlet air.
It is worth mentioning that GT model used is Siemens
GT V94.2 with 159 (MW) output power, and a frequency
of 50Hz is a heavy industrial turbine that has been de-
signed for reliable, effective and flexible application [8].
The V94.2 GT’s characteristics are also shown in Table I.
Compressor outlet temperature, compressor outlet
pressure and input work of air compressor per mass
flow rate are calculated as follows [9,10]:
Compressor:
T
2
¼
T
1
11
1
Z
c
compressor inlet pressure and R is the gas constant,
respectively. Information about gas components is
given in Table II.
Combustion chamber:
m
a
h
2
1 m
6
LHV
¼
m
g
h
3
1
ð
1
Z
cc
Þ
m
6
LHV
ð
4
Þ
In which Z
cc
is a combustion efficiency, LHV is the
fuel lower heating value, m
a
is the inlet air mass flow
rate to combustion chamber, m
6
is the fuel mass flow
rate, m
g
is the outlet gas mass flow rate, h
3
is outlet gas
enthalpy.
Combustion equation:
lC
x1
H
y1
1
ð
x
O
2
O
2
1x
N
2
N
2
1x
H
2
O
H
2
O1x
CO
2
CO
2
1x
Ar
Ar
Þ
!
y
CO
2
CO
2
1y
N
2
N
2
1y
O
2
O
2
1y
H
2
O
H
2
O
1y
NO
NO1y
CO
CO1y
Ar
Ar
y
CO
2
¼ð
l
x
1
1x
CO
2
y
CO
Þ
y
N
2
¼
x
N
2
y
NO
y
H
2
O
¼
x
H
2
O
1
l
y
1
2
y
O
2
¼
x
O
2
l
x
1
l
y
1
4
h
i
r
k
1
k
c
1
ð
1
Þ
P
2
¼
r
c
P
1
ð
2
Þ
T
1
h
i
kR
k
1
1
Z
c
r
k
1
W
c
¼
k
c
1
ð
3
Þ
y
CO
2
y
NO
2
where T
1
is the compressor inlet temperature, Z
c
is the
compressor isentropic efficiency, r
c
is the compressor
pressure ratio, k is the ratio of constant pressure spe-
cific heat to constant volume specific heat, P
1
is the air
y
Ar
¼
x
Ar
l
¼
n
fuel
n
air
ð
5
Þ
488
Int. J. Energy Res. 2012;
36
:486–498
r
2011 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Exergy, economic and environment (3E) analysis
M. A. Ehyaei et al.
Gas Turbine (GT ):
3.1. Absorption chiller capacity calculation
Absorption chiller capacity selection has important
effects on the refrigeration cycle components and
economical benefits. Therefore, one should define the
optimum chiller capacity. To calculate the cooling
load, one should estimate the amount of energy per
hour that is used to change the temperature and
humidity from a given condition to the ISO tempera-
ture of 151C and the relative humidity of 100%.
Therefore, a chiller capacity is a function of tempera-
ture and the relative humidity of the ambient air. Some
of the cooling load is used to decrease the ambient
temperature till the relative humidity reaches 100%.
The remaining of the cooling load is used to reduce the
air temperature below the dew point. Figure 4 shows
the cooling process on a Psychometric diagram.
In this diagram point (a) shows the ambient condi-
tion that should be cooled to the final condition of
point (c). As the sensible heat of air transfers to the
chilled water passing through the cooling coil, the
relative humidity rises. The air temperature decreases
to the dew point (point b). In this study, in order to
choose the size of the absorption chiller, the following
method is used. According to the climate condition
(temperature and relative humidity) in the men-
tioned regions and with respect to the 151C created by
the absorption chiller, the cooling load was achieved
for the whole months during the year and with
respect to the maximum achieved cold load, the
required number of the absorption chillers has been
recognized.
Necessary cooling load is calculated from [14]:
Q
L
¼
m
a
ð
h
a
h
c
Þ ð
17
Þ
where h
a
is the enthalpy in ambient condition and h
c
is
the enthalpy in final condition, respectively.
For instance, for a temperature of 301C and 70%
relative humidity and with respect to inlet air mass flow
rate equals to 506 kg s
1
, the following values have
been achieved:
1
g
g
g
g
T
4
¼
T
3
1
Z
GT
1
ð
r
GT
Þ
ð
6
Þ
W
GT
¼
m
g
C
p;g
ð
T
3
T
4
Þ
ð
7
Þ
W
net
¼
W
GT
W
C
ð
8
Þ
m
g
¼
m
f
1 m
a
ð
9
Þ
where T
3
is the gas turbine inlet temperature (TIT), Z
GT
is the GT isentropic efficiency, r
GT
is the GT pressure
ratio, k is the ratio of constant pressure specific heat to
constant volume specific heat, m
a
is the air mass flow
rate, m
f
is the fuel mass flow rate and W
bc
is the booster
compressor input work per fuel mass flow rate, respec-
tively. The amount of CO and NO
x
produced in the
combustion chamber is mainly by the adiabatic flame
temperature. Accordingly, based on Reference [11], to
determine the pollutant emission in grams per kilogram
of fuel, the proper equations are proposed as follows:
m
NO
x
¼
0:15E16t
0:5
exp
ð
71 100=T
Pz
Þ
P
0:05
3
ð
10
Þ
0:5
DP
3
P
3
m
CO
¼
0:179E9 exp
ð
7800=T
Pz
Þ
P
3
t
ð
11
Þ
0:5
DP
3
P
3
In which T
Pz
is the flame temperature that can be
calculated by [11,12]:
T
Pz
¼
As
a
exp
ð
b
ð
s1l
Þ
2
Þ
p
x
y
y
c
z
ð
12
Þ
Here p is dimensionless pressure (P/P
0
), y is dimension-
less temperature (T/T
0
), c is the H/C atomic ratio, s5f
for f
p
1(f is mass or molar ratio) and s5f
0.7 for
fX1. Moreover, x, y and z are quadric functions of s
based on the following equations [11,12]:
x
¼
a
1
1b
1
s1c
1
s
2
ð
13
Þ
y
¼
a
2
1b
2
s1c
2
s
2
ð
14
Þ
h
a
¼
78 kJ kg
1
z
¼
a
3
1b
3
s1c
3
s
2
ð
15
Þ
h
c
¼
42 kJ kg
1
In Equations (12)–(15), parameters A, a, b, l, a
i
, b
i
and c
i
are constant parameters [11,12].
Moreover, t is the residence time in the combustion
zone (t is assumed constant and is equal to 0.002 s), T
3
is the combustion temperature, P
3
is the combustor
inlet pressure, DP
3
/P
3
is the non-dimensional pressure
drop in the combustion chamber [11,12].
The first law efficiency of GT for a simple cycle is
calculated from [9]:
Q
L
¼
18 216 kW
Now by determining the amount of necessary cooling
load, 6 units of selected absorption chiller have been
calculated. The selected absorption chiller is shown in
Table III [16].
3.2. Exergy analysis
Exergy analysis is a method that uses the conservation
of mass and conservation of energy principles together
with the second law of thermodynamics for the
analysis, design and improvement of energy and other
systems [17]. The exergy method is a useful tool for
W
net
m
f
LHV
Z
I
¼
ð
16
Þ
where the value of LHV in this study is 50 143 kJ kg
1
[10,13].
489
Int. J. Energy Res. 2012;
36
:486–498
r
2011 John Wiley & Sons, Ltd.
DOI: 10.1002/er
M. A. Ehyaei et al.
Exergy, economic and environment (3E) analysis
Figure 4.
Air-cooling process in psychometric diagram.
composition of a system from its chemical equilibrium.
The chemical exergy is an important part of exergy in
combustion process. Applying the first and the second
law of thermodynamics, the following exergy balance
is obtained [20]:
Ex
Q
1
X
i
Table III.
The characteristics of absorption chiller [15].
LiBr–Water single effect
Chiller kind
4900 (kW)
Cooling load
120 (
1
C)
Inlet temperature
200 (kPa)
Inlet pressure
m
i
ex
i
¼
X
e
0.7
Performance coefficient
m
e
ex
e
1 Ex
W
1 Ex
D
ð
18
Þ
7(
1
C)
Outlet cool water temperature
285 000(US$)
Price
It should be noted that in Equation (18) subscripts e
and i are the specific exergy of control volume inlet and
outlet flow and Ex
D
, is the exergy destruction. Other
terms in this equation are as follows [21,22]:
Ex
Q
¼
1
T
0
T
i
furthering the goal of more efficient energy-resource
use, it enables the locations, types and magnitudes of
wastes and losses to be identified and meaningful
efficiencies to be determined [15]. Today there is a
much stronger emphasis on exergy aspects of systems
and processes. The emphasis is now on system analysis
and thermodynamic optimization, not only in the
mainstream of engineering but also in physics, biology,
economics and management. As a result of these recent
changes and advances, exergy has gone beyond
thermodynamics and become a new distinct discipline
because of its interdisciplinary character as the
confluence of energy, environment and sustainable
development. According to the literature, exergy can
be divided into four distinct components. The two
important ones are the physical exergy and chemical
exergy. In this study, the two other components,
kinetic exergy and potential exergy, are assumed to be
negligible as the elevation and speed have negligible
changes [18,19]. The physical exergy is defined as the
maximum theoretical useful work obtained as a system
interacts with an equilibrium state. The chemical
exergy is associated with the departure of the chemical
Q
i
ð
19
Þ
Ex
W
¼
W
ð
20
Þ
ex
ph
¼ð
h
h
0
Þ
T
0
ð
S
S
0
Þ
ð
21
Þ
where Ex
Q
and Ex
W
are the corresponding exergy of
heat transfer and work which cross the boundaries of
the control volume, T is the absolute temperature (K)
and 0 refers to the ambient conditions, respectively.
In Equation (18), term Ex is defined as follows:
Ex
¼
Ex
ph
1 Ex
ch
ð
22
Þ
where Ex
¼
m ex.
The mixture chemical exergy is defined as follows
[18–22]:
"
#
ex
c
mix
¼
X
x
i
ex
ch
i
1RT
0
X
n
n
ð
23
Þ
x
i
Lnx
i
i
¼
1
i
¼
1
490
Int. J. Energy Res. 2012;
36
:486–498
r
2011 John Wiley & Sons, Ltd.
DOI: 10.1002/er
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