Content uploaded by Dodeye Igbong
Author content
All content in this area was uploaded by Dodeye Igbong on Sep 09, 2015
Content may be subject to copyright.
International Journal of Engineering and Technology Volume 4 No. 8, August, 2014
ISSN: 20493444 © 2014 – IJET Publications UK. All rights reserved.
463
Exergoeconomic Analysis of A 100MW Unit GE Frame 9 Gas Turbine Plant
in Ughelli, Nigeria.
D. I. Igbong1 and D.O. Fakorede2
1Department of Mechanical Engineering, Cross River University of Technology, Calabar Nigeria
2Department of Mechanical Engineering, Cross River University of Technology, Calabar Nigeria
ABSTRACT
A 100 MW gas turbine power plant at Ughell, Nigeria was evaluated using exergoeconomic analysis method. These optimization was
performed on engineering equation solver (EES) software to estimate the cost rate associated with all the exergy streams at cycle state
points and the cost of plant final product which is electricity. Two parameters were chosen as decision variables: turbine inlet
temperature, and compressor pressure ratio, . Studies were carried out on the effect of variation of these parameters on the unit
cost of the final product of the plant. Results establishes a relation between variation in decision variables and unit cost of product.
The unit cost of product decreases to a minimum point as the decision variables increases, beyond which, it increases with further
increase in the value of the parameters.. Thus, the least unit cost of product was achieved at = 1474 K and = 11.4. The plant
thermal efficiency is 31.05%, and overall exergy efficiency of 30.81%, identified the combustion chamber with the lowest exergetic
efficiency of 54.05% accounting for the component with the largest total inlet exergy destruction value of 238.681 MW.
Keywords: Gas turbine, exergoeconomic analysis, exergy analysis, exergy cost rates.
1. INTRODUCTION
The rapid increase in global energy demand, couple with the
challenges of limited energy resources and environmental
concerns have spurred research efforts towards the efficient
utilization of available energy resources as well as the
development of renewable forms of energy. Pursuance of the
former, have led to the combined utilization of energy, exergy
and economic principles in the evaluation of energy
consumption efficiency and exploring cost minimization
potential in thermal process systems. Exergy analysis asserts
the fact that energy cannot be destroyed, but the quality can be
degraded such that it reduces its ability to do useful work [1].
Exergoeconomic analysis is a system optimization tool that
uses both the secondlaw of thermodynamics (Exergy concept)
and economic principles to evaluate thermal energy systems ,
in order to provide designers with useful information for
system improvement and costeffective operations. The
method predicts energy system's thermodynamic performance
and inefficiencies (irriversibilities), estimates cost of product
from the system and associate cost rate to irriversibilies [2].
These concept of system cost optimization became popular
among researchers in the 80'es, with works by Antonio Valero,
Elsayed Yehia, Tadeusz Kotas, Richard Gaggioli, and others
[3]. In recent times, comprehensive work have been done on
the application of exergoeconomic concept in the analysis and
optimization of energy systems for better design and cost
effective operation [2,3,4,5,6,7,8 ]. In this study,
exergoeconomic analysis were applied on 100 MW unit GE
Frame 9 gas turbine plant in Ughelli, Nigeria.
Figure 1: Schematic diagram of Gas turbine plant
1.2 Plant Description
The system under consideration, is a 100MW Unit
singleshaft open cycle active gas turbine plant located at
Ughelli in Nigeria. It uses natural gas of low heating
value (LHV = 43,000KJ/Kg) and its been evaluated from
International Journal of Engineering and Technology (IJET) – Volume 4 No. 8, August, 2014
ISSN: 20493444 © 2014 – IJET Publications UK. All rights reserved.
464
a reference base condition of = 298K and
1.013bar. As shown in Fig. 1, the plant consists of
an aircompressor (AC), combustion chamber (CC), and
gas turbine (GT).
2. MATERIALS AND METHODS
2.1 Energy Analysis
The thermodynamic characterization of the plant is
obtained from the operating parameters at various state
point. The decision variables used for the plant
optimization are compression ratio (), and turbine inlet
temperature (
Aircompressor exit pressure and temperature are
(1)
(2)
Where
Work required to drive the compressor per unit mass is
(3)
From combustion chamber energy balance equation [9]
(4)
Fuel air ratio is expressed as
(5)
Heat supplied to the combustion chamber
(6)
Work done by the turbine per unit mass
(7)
From computations with the models above, the state
point characterization of plant with reference base
decision variables; aircompressor ratio ( of 10.336,
turbine inlet temperature of 1324K, isentropic
compressor and turbine efficiencies of 88% and 89%
respectively, are presented in Table 1. A combustion
chamber pressure drop of 4% have been consider during
the analysis.
Table 1: State point for the simple gas turbine
State
Point
Temperature
(K)
Pressure
(bar)
Mass flow
rate
(Kg/s)
1
298
1.013
427
2
619.4
10.47
427
3
1324
10.05
436
4
805.7
1.358
436
5
308
30
8.997
2.2 Exergy Analysis
The limitations of energy analysis to properly account for
energy losses due to irreversibilities in systems, have led
to the application of the secondlaw or exergy
(availability) concept to optimize energy utilization
efficiency in thermal systems. Exergy, defined as the
maximum theoretical useful work obtainable from the an
energy carrier, assesses the quality of an energy carrier to
be converted into work, and account for component
irriversibilities in gas turbine plant. Exergy in a flow
stream consist of physical, chemical, potential and
kinetic exergy components. If we assume the potential
and kinetic component of exergy to be negligible, the
physical and chemical energy in the stream are properly
accounted for by the models presented below.
Exergy flow rate of air stream exiting the compressor.
(8)
Exergy destruction rate or irriversibilities in compressor
exit stream
(9)
The exergy  balance in the combustion chamber is
expressed as
(10)
Exergy rate in fuel is expressed as [10].
(11)
Exergy destruction rate in the combustion chamber exit
stream
(12)
The plant overall exergetic efficiency expresses the ratio
of the useful work output to the maximum obtainable
work input [11].
(13)
International Journal of Engineering and Technology (IJET) – Volume 4 No. 8, August, 2014
ISSN: 20493444 © 2014 – IJET Publications UK. All rights reserved.
465
The rate of exergy destroyed in a component accounts for
the component efficiency defects and can be represent by
the component exergy destruction ratios
(14)
Table 2: Exergy Stream of the Plant
Component
Exergy Flow Rate
(MW)
Exergy Destroyed
(MW)
Exergy Efficiency
(%)
Air Compressor
129.6
9.66
93.07
Combustion Chamber
280.8
238.7
54.05
Gas Turbine
116.7
27.97
65.27
Fuel
389.803


Overall Exergetic Efficiency = 30.81% Thermal Efficiency of plant = 31.05%
2.3 Economics Analysis
The economics of gas turbine assess the nonexergy
related cost; which is the cost of the various components
of the system. This cost comprise of the cost of
ownership and operations of the plant, its value is
dependent on the component life expectancy, capital
requirement, financing structure, etc. The Annual
levelized cost of system kth component is expressed as
follows:
($/year)
(15)
where PEC is the purchase equipment cost for the kth
components, i is the interest rate and n, the time period .
For gas turbine power plant, the purchase equipment cost
for the components are computed from the models bellow
according to Bejan et al., 1996.
Air compressor;
(16)
Combustion Chamber;
(17)
Gas Turbine;
(18)
Therefore, the cost rate for the kth component of the plant
is expressed as
($/hours) (19)
where H is the plant operating hour and is the
maintenance cost factor for the kth component of the
plant.
Table 3: Plant nonexergy associated costs
Component
Annual Levelized Cost
($/year)
Purchased Equipment
Cost (PEC)$
Capital Cost Rate,
($/hour)
Air Compressor (AC)
3.134x106
18.32x106
415.2
Combustion Chamber (CC)
1.035x105
0.605x106
13.72
Gas Turbine (GT)
2.336x106
13.65x106
309.5
International Journal of Engineering and Technology (IJET) – Volume 4 No. 8, August, 2014
ISSN: 20493444 © 2014 – IJET Publications UK. All rights reserved.
466
2.4 Exergoeconomic Analysis
This analysis assess the costs of all the flows involve in
the plant and associates cost to each exergy stream in
individual plant component. This is achieved by
formulating a cost balance equation for each component
of the plant. The general equation according to Bejan,
1996 [12], is expressed as
(20)
Auxiliary equations, which are formulations of cost
balance equations for individual components of the plant
are based on the following principles
FPrinciple: The cost of exergy removal from a stream
must be equal to the cost of supplying the exergy to the
same stream in a component located upstream.
PPrinciple: Associates the same average cost to any
exergy unit supplied to any stream that is related to the
product.
Air compressor:
(21)
Combustion chamber:
(22)
Gas Turbine:
(23)
Frule:
or
(24)
But
Prule:
or
(25)
where are average cost per unit
exergy in dollar gigajoule ($/GJ).
Another cost associated with components is the hidden
cost or cost associated with the rate of exergy destruction
. This, alongside with other exergoeconomic
variables like average cost of fuel per unit exergy
, the average unit cost of product
, and the exergoeconomic factor, are very
vital in the analysis of the system.
(26)
3. RESULTS AND DISCUSSION
Table 4: Average unit exergy cost and levelized cost rates associated with various state point
State
points
, $/h
c, $/KWh
c, $/GJ
1
0
0
0
2
1,769
0.01363
3.786
3
6,092
0.02170
6.026
4
2,532
0.02170
6.026
5
4,311
0.01106
3.072
6
1,352
0.009705
2.695
7
2,518
0.009705
2.695
Table 5: Gas Turbine Components Exergoeconomic Parameters
Component
, $/GJ
, $/GJ
,
MJ
,
$/h
,
$/h
+,
$/h
,
%
Air
Compressor
3.786
2.695
9.66
26.0337
415.2
441.234
94.099
Combustion
Chamber
6.026
3.072
238.7
733.286
13.72
747.006
1.836
Gas Turbine
2.695
6.026
27.97
168.547
309.5
478.047
64.74
International Journal of Engineering and Technology (IJET) – Volume 4 No. 8, August, 2014
ISSN: 20493444 © 2014 – IJET Publications UK. All rights reserved.
467
This study looks at the exergoeconomic analysis of one
of the Gas turbine power plants operating in Nigeria.
Table 1 shows the state point characteristics of the plant
calculated based on the design point ISO data, such as
ambient temperature, mass flow rate entering the
compressor, turbine inlet temperature, compressor ratio,
compressor isentropic efficiency, and turbine isentropic
efficiency. In Table 2, the exergy flow rate, exergy
destroyed or irriversibility and the exergetic efficiency of
the various components of the plant are given. The
combustion chamber recorded the lowest efficiency of
54.05% , accounting for the component with the largest
total inlet exergy destroyed value of 238.681MW. This
results in a low plant overall exergy efficiency of
30.81%.
Table 3 presents the nonexergy associated costs of the
plant; initial investment (purchased equipment) costs,
capital cost rates, and the annual levelized costs for each
component of the plant. The average unit cost of exergy
and the levelized cost rates associated with every state
point in the plant is shown in Table 4. The cost of exergy
associated with the product of the plant; which is
electricity is given as 2.695$/GJ. And Table 5 presents
the exergoeconomic parameters for each component of
the plant. The combustion chamber component which
has the highest value of
+ and lowest
exergoeconomic factor is the component of interest.
This values implies that the component accounts for the
highest cost rate of exergy destroyed in the system, thus
the isentropic efficiency should be improved by
increasing capital investment costs. For other
components (compressor and turbine) with high values
of and lower value of , figure 1 shows that a
low exergy cost rate is achieved through reduction in
component isentropic efficiencies, which implies a
reduction in capital investment.
Further analysis, examines the effect of the decision
variables on the unit cost of product. Figure 2 and 3,
shows that increasing pressure ratio and turbine inlet
temperature decreases the unit cost of product to a
minimum point beyond which further increase in
decision variables increases the unit cost of product.
Increase in decision variable implies an increase in the
investment cost. Therefore, increasing capital investment
cost on the combustion chamber (which has the highest
exergy destruction cost value) by increasing turbine inlet
temperature, leads to an increase in the exergeoeconomic
factor, decreases exergy destruction cost rate and a
corresponding decrease in unit cost of product. The
turbine inlet temperature that gives the least unit cost of
product is In the other hand, increase in
pressure ratio (in compressor and turbine) decreases the
exergy destruction cost and a decrease in unit cost of
product. The pressure ratio that gives the least unit cost
of product is 11.4.
4. CONCLUSION
The unit cost of product for the 100MW gas turbine
power plant was estimated through the exergoeconomic
Figure 2: Pressure ratio Vs Unit Cost of
Product
Figure 3: Gas Turbine Inlet Temperature Vs Unit Cost of Product
International Journal of Engineering and Technology (IJET) – Volume 4 No. 8, August, 2014
ISSN: 20493444 © 2014 – IJET Publications UK. All rights reserved.
468
analysis of the plant. The effect of the turbine inlet
temperature and the pressure ratio on the unit cost of
product was investigated, and result shows that the unit
cost of product decreases to a minimum point as the
decision variables increases, beyond which, it increases
with further increase in the value of the parameters. Thus,
the value of the turbine inlet temperature and
compression ratio where the least cost of product is
achieved are and , respectively.
REFERENCES
[1] Dincer, I. A.(2001)"Thermodynamic analysis of
reheats cycle steam power plants":Int. J. Energy
Research, Vol. 25, pp 727739.
[2] GorjiBandpy, M., Goodarzian, H., and Biglar,
M.(2010)"The Costeffective Analysis of a Gas
Turbine Power Plant":Energy Sources, Part B,
Vol. 5:pp 348358.
[3] Mesin, J. K., and Kemerdekaan, J.
P.(2009)"Thermoeconomic Analysis of Gas
Turbine Power Plant (GE MS 6001B PLTG
PLNSektor Tello Makassar)":Jurnal Penelitian
Enjiniring, Vol. 12, No. 2,Tahun.
[4] Petrakopoulou, F., Tsatsaronis, G., Morosuk, T.,
and Carassai, A.(2012)"Advanced
Exergoeconomic Analysis Applied to a complex
Energy Conversion System":Journal of
Engineering for Gas Turbines and Power, Vol.
134/0318011.
[5] Fellah, G. M., Mgherbi, F. A., and Aboghre, S.
M.(2010)"Exergoeconomic Analysis for Unit
Gt14 of south Tripoli Gas Turbine Power
Plant":Jordan Journal of Mechanical and
Industrial Engineering, Vol.4 No.4, pp 507516.
[6] Bagdanavicius, A., Sansom, R, Jenkins, N., and
Strbac, G.(2012)"Economic and
exergoeconomic analysis of microGTand ORC
cogeneration systems", Proceedings of Ecos
2012The 25th International Conference on
Efficiency, cost, optimization, simulation and
environmental Impact of energy systems, June
2629, Perugia, Italy.
[7] Almasi, A., Avval, H. B., Ahmadi, P., and Najafi,
A. F.(2011)"Thermodynamic modelling, energy
and exergoeconomic analysis and optimization
of Mahashar gas turbine power plant":
Proceedings of the Global coference on Global
warming.
[8] Ozkan, D. B., Kiziler, O., and Bilge, D.(2012)
"Exergy Analysis of a Cogeneration Plant":
World Academy of Science, Engineering and
Technology, Vol 61;pp 774778.
[9] Thamir, K. I., and Rahman, M. M.(2002)"Effect
of compression ratio on performance of
combined cycle gas turbine":International
Journal of Engineering, Vol.2, pp. 914.
[10] Abam, F. I., Ugot, I. U., and Igbong, D.
I.(2012)"Effect of Operating variables on
exergetic efficiency of an active gas turbine
power plant":Journal of Emerging Trends in
Engineering and Applied sciences (JETEAS),
Vol. 3(1),pp. 131136.
[11] Kotas, T. J.(1985)"The Exergy Method of
Thermal Plant Analysis": Butter Worths,
London.
[12] Bejan, A., Tsatsaronis, G., and Moran,
M.(1996)"Thermal Design and Optimization":
New York, Wiley.