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International Journal of the Physical Sciences Vol. 6(14), pp. 3539-3550, 18 July, 2011Available online at http://www.academicjournals.org/IJPSISSN 1992 - 1950 2011 Academic Journals

Full Length Research Paper

Thermodynamic performance analysis of gas-turbinepower-plant

M. M. Rahman1,2, Thamir K. Ibrahim1* and Ahmed N. Abdalla3

1Faculty of Mechanical Engineering, Universiti Malaysia Pahang, Tun Razak Highway, 26300 Gambang, Kuantan,

Pahang, Malaysia2Automotive Engineering Centre, Universiti Malaysia Pahang, Tun Razak Highway, 26300 Gambang, Kuantan, Pahang,

Malaysia3Faculty of Electrical and Electronic Engineering, Universiti Malaysia Pahang, Tun Razak Highway, 26300 Gambang,

Kuantan, Pahang, Malaysia.

Accepted 04 April, 2011

This paper was presented the parametric study of thermodynamic performance on gas turbine powerplant. The variation of operating conditions (compression ratio, turbine inlet and exhaust temperature,air to fuel ratio, isentropic compressor and turbine efficiency, and ambient temperature) on theperformance of gas turbine (thermal efficiency, compressor work, power, specific fuel consumption,heat rate) were investigated. The analytical formula for the specific work and efficiency were derivedand analyzed. The programming of performance model for gas turbine was developed utilizing theMATLAB software. The results show that the compression ratio, ambient temperature, air to fuel ratioas well as the isentropic efficiencies are strongly influence on the thermal efficiency. In addition, thethermal efficiency and power output decreases linearly with increase of the ambient temperature and airto fuel ratio. However, the specific fuel consumption and heat rate increases linearly with increase ofboth ambient temperature and air to fuel ratio. Thus the thermodynamic parameters on cycleperformance are economically feasible and beneficial for the gas turbine operations.

Key words: Gas turbine, compression ratio, air to fuel ratio, thermal efficiency, power, turbine inlet temperature.

INTRODUCTION

The gas turbine (GT) performance is affected bycomponent efficiencies and turbine working temperature.The effect of temperature is very predominant for every56C increase in temperature; the work output increasesapproximately 10% and gives about 1.5% increase in

efficiency (Johnke and Mast, 2002). Overall efficiency ofthe gas turbine cycle depends primarily upon thepressure ratio of the compressor. It is important to realizethat in the gas turbine the processes of compression,combustion and expansion do not occur in a singlecomponent as they occurred in a reciprocating engine.

It is well known that the performance can be qualified

*Corresponding author. E-mail: [email protected].

with respect to its efficiency, power output, specific fueconsumption as well as work ratio. There are severaparameters that affect its performance including the compressor compression ratio, combustion inlet temperatureand turbine inlet temperature (TIT) (Mahmood and Mahdi

2009, Rahman et al., 2010). Taniquchi and Miyamae(2000) were carried out the study on the effects ofambient temperature, ambient pressure as well as thetemperature of exhaust gases on performance of gasturbine as shown in Figure 1. There is an obvious drop inthe power output as the ambient air temperatureincreases, if an increase of intake air ambienttemperature from ISO condition 15 to 30C which is 10%decrease in the net power output. This is particularlyrelevant in tropical climates where the temperature varies25 to 35C throughout the year (Boonnasa et al., 2006)


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3540 Int. J. Phys. Sci.

0 10 20 30 40 50

80

90

100

110

Exhaust Temperature (C)

Exhaust Flow (t/h)

Power Output (MW)

Heat Rate (kJ/kWh)

Ambient temperature (C)

Percentageofdesignoutput(%)

(C)Exhaust Temperature

Exhaust Flow (t/h)

Power Output (MW)

Heat Rate (kJ/kWh)

Figure 1. Effect of ambient temperature on gas turbine performance.Source: Taniquchi and Miyamae, 2000.

The mainly fashionable way to improve the capacity ofthe combined cycle power plant is to lower the intake airtemperature to around 15C (ISO) and relative humidity(RH) of 100% before entering the air compressor of the

gas turbine (Mohanty and Paloso, 1995; Ibrahim et al.,2010). Usually, the operation conditions for the gasturbine module are measured to calculate the outputpower and the efficiency (Horlock et al., 2003). Howeverin many cases, the estimated parameters are not alwaysoptimal inside the gas turbine. It is required to control theinput parameters with the aim of enhancing theperformance of gas turbine. Obviously, these parametershave been actually improved by various gas turbinemanufactures as mentioned above. Meanwhile, theoperating parameters including the compression ratio,ambient temperature, air to fuel ratio, turbine inlettemperature, and both compressor and turbine

efficiencies on the performance of gas turbine powerplant were carried out. Consequently, a parametric studyon the effect of operation conditions requires managingthe operation conditions of the system. Thus, the aim ofthe present work is to develop a strategy to determine theperformance of gas turbine power plant utilizing the effectof operating conditions.

THERMODYNAMIC MODELING OF GAS TURBINE

The first practical success was obtained by the Societe Anonymedes Turbomoteurs French Company, which built a gas turbine in

1905. This engine, the first constant pressure gas turbine to rununder its own power, had an efficiency of 3% which is used into theengine with multistage centrifugal compressor (20 stages or morehaving a pressure ratio of 4 and compressor efficiency not morethan 60% as well as the maximum gas temperature was about

393C. However there was an elapse of many years, until in 1939a Brown Boveri (BBC) unit for emergency electrical-power supplywas put into operation in Neuchatel, Switzerland (Figure 2). Thepower output was 4000 kW and efficiency of 18%. The turbine withinlet temperature 550C was provided 15,400 kW at 3000 rpm(Zurcher et al., 1988).

Basically, the gas turbine power plants consist of foucomponents including the compressor, combustion chamber (CC)turbine and generator. A schematic diagram for a simple gasturbine is shown in Figure 3. The fresh atmospheric air is drawn intothe circuit continuously and energy is added by the combustion othe fuel in the working fluid itself. The products of combustion areexpanded through the turbine which produces the work and finallydischarges to the atmosphere.

It is assumed that the compressor efficiency and the turbineefficiency are represented

cand

trespectively. The ideal and

actual processes on the temperature-entropy diagram arerepresented in full and dashed line respectively as shown in (Figure4) (Al-Sayed, 2008).

The compressor compression ratio (pr ) can be defined as (Al

Sayed, 2008):

1

2

p

prp = (1)

where1

p and2

pare compressor inlet and outlet air pressure


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Figure 2. Worlds first industrial gas turbine set with singlecombustor (Zurcher et al., 1988).

4

CC

Compressor Turbine

32

1 Air

Fuel

Exhaust

G

Wshaft

Figure 3. The schematic diagram for a simple gas turbine.

Figure 4. Temperature-Entropy diagram for gas turbine.

Rahman et al. 3541

respectively.The isentropic efficiency for compressor and turbine in the range

of 85 to 90% is expressed as (Rahman et al., 2011):

12

12

TT

TT sC

= (2)

where T1 and T2 are compressor inlet and outlet air temperaturerespectively, and T2s compressor isentropic out let temperature.

The final temperature of the compressor is calculated fromEquation (3) (Rahman et al., 2010):

+=

c

pa

a

rTT

11

1

12(3)

Therefore it can be simplified these relations by Equation (4):

andr

Rpac

pa

a

1

1

=

=

g

g

pr

Rpg

1

)(

11 (4)

where 4.1=a and 33.1=g .

The work of the compressor ( cW ) when blade cooling is not takeninto account can be calculated as:

m

pa

cm

ppa

c

RpaTCrTc

W

a

a

=

=

1

1

11

(5)

wherepa

C is the specific heat of air which can be fitted by

Equation (6) for the range of 200K


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From the energy balance in the combustion chamber is expressedas:

TITCmmTCmLHVmTCm pgfafpfffpaa +=++ )(2&&&&&

(8)

where, fm& is fuel mass flow rate (kg/s), am& is air mass flow rate

(kg/s), LHV is low heating value, T3 = TIT = turbine inlettemperature

pfC is specific heat of fuel anf

fT is temperature of

fuel.

After manipulating Equation (8), the fuel air ratio ( f ) is expressed

as:

TITCLHV

TCTITC

m

mf

pg

papg

a

f

==2

&

& (9)

The exhaust gases temperature from gas turbine is given by Eq.(10):

( )RpgT

r

T t

g

g

p

t =

=

1

111T 3134

(10)

The shaft work (t

W ) of the turbine is g iven by Equation (11):

mtpgt RpgTITCW /= (11)

The network of the gas turbine (Wnet) is calculated from the

equation:

CtWWW =

net(12)

The output power from the turbine (P) is expressed as:

netaWmP = & (13)

The specific fuel consumption (SFC) is determined by Equation(13):

netW

fSFC

3600=

(14)

The heat supplied is also expressed as:

))1((1add RpaTTITCQ mpg +=

(15)

The gas turbine efficiency (th

) can be determined by Equation

(15) (Ibrahim et al., 2010):

add

net

Q

Wth =

(16)

The heat rate (HR) is the consumed heat to generate unit energy ofelectricity can be expressed as (Saravanamuttoo et al., 2009):

th

HR

3600= (17)

RESULTS AND DISCUSSION

The parameter influence in terms of compression ratioturbine inlet temperature, air to fuel ratio, and ambienttemperature on the performance of gas turbine cyclepower plant are presented in this section. The effects ofoperation conditions on the power output and efficiencyare obtained by the energy-balance utilizing MATLAB10software. The flowchart of simulation of performanceprocess for simple gas turbine power plant is shown inFigure 5.

Effect of compression ratio

Figure 6a presents a relation between the gas turbinecycle thermal efficiency versus compression ratios fodifferent turbine inlet temperature. It can be seen that thethermal efficiency increases with compression ratio ahigher turbine inlet temperature. The deviation of thermaefficiency at lower compression ratio is not significanwhile the variation at higher compression ratio is vital fothermal efficiency. The certain limit depends on theturbine inlet temperature which reveals an ejective

relationship as the efficiency increases as turbine inletemperature increases. Also the thermal efficiency increase with increase the compression ratio and decreasethe ambient temperature as shown in Figure 6b, but theincrease in the air to fuel ratio cause low decrease in thethermal efficiency comparing with effect the compressionratio that caused high increased in the thermal efficiencyas shown in Figure 7. Figure 8 present the effect ocompression ratio on compressor work, this workincrease with increase the compression ratio and theambient temperature.

Figure 9a shows the effect of compression ratio on thethermal efficiency with variation isentropic compressoefficiency. Note that the thermal efficiency is increasedwith compression ratio and isentropic compressorefficiency. Also the increased in compression ratio andisentropic turbine efficiency caused increased in thethermal efficiency as shown in Figure 9b. Figure 10represented the relation between the thermal efficiencyand turbine inlet temperatures for five values of air to fueratio (40 to 56 kg air/fuel) and six values compressionratios (3 to 23). Thermal efficiency has an ejectiverelationship with turbine inlet temperature, the efficiencyincreases when turbine inlet temperature increases, alsothe decreased in air to fuel ratio caused incased therma


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Rahman et al. 3543

Figure 5. Flowchart of simulation of performance process for simple gas turbine powerplant.


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(a) Turbine inlet temperature

(b) Ambient temperature

Figure 6. Variation of compression ratio, turbine inlet temperatureand ambient temperature on thermal efficiency.

efficiency and turbine inlet temperature.

Effect of ambient temperature

Figure 11 show that the gas turbine thermal efficiency isaffected by ambient temperature due to the change of airdensity and compressor work since; a lower ambienttemperature leads to a higher air density and a lowercompressor work that in turn gives a higher gas turbine

0 5 10 15 20 25

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Thermalefficiency

Compression ratio

AFR=40 kg air/fuel

AFR=44 kg air/fuel

AFR=48 kg air/fuel

AFR=52 kg air/fuel

AFR=56 kg air/fuel

Figure 7. Variation of compression ratio and air to fuel ratio onthermal efficiency.

0 5 10 15 20 25100

150

200

250

300

350

400

450

500

550

600

Compression Ratio

Compres

sorWork(kJ/kg)

T1=268

T1=278

T1=288

T1=298

T1=308

T1=318

T1=328

Figure 8. Effect of compression ratio and ambient temperatureon compressor work.

output power as shown in Figure 12. Figure 11 showsthat when the ambient temperature increases the thermaefficiency decreases. This is because, the air mass flowrate inlet to compressor increases with decrease of theambient temperature. So, the fuel mass flow rate wilincrease, since air to fuel ratio is kept constant. Thepower increase is less than that of the inlet compressoair mass flow rate; therefore, the specific fuel consumption increases with the increase of ambient temperatureThis occurs because of increased losses due to theincreased amount of flue gases. The output power from


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(a)

(b)

Figure 9. Variation of compression ratio, isentropic compressor

efficiency and isentropic turbine efficiency on thermal efficiency. (a)Isentropic compressor efficiency (b) Isentropic turbineefficiency.

simulation model is higher than the practical data fromBaiji gas turbine power plant as shown in Figure 13.However, increased the ambient temperature andincreased the air to fuel ratio caused increased of specificfuel consumption and heat rate as shown in Figures 14and 15.

Figure 16 shown effect of air to fuel ratio on thermalefficiency with variation ambient temperatures. Thethermal efficiency decreases with increase the air to fuel

Rahman et al. 3545

AFR=56 kg air/fuel

AFR=40 kg air/fuel

Figure 10. Variation of turbine inlet temperatures with thermaefficiency for several compression ratio and air to fuel ratio.

260 270 280 290 300 310 320 3300.32

0.325

0.33

0.335

0.34

0.345

0.35

0.355

0.36

0.365

Therma

leffic

iency

Ambient temperature (K)

AFR=40 kg air/fuel

AFR=44 kg air/fuel

AFR=48 kg air/fuel

AFR=52 kg air/fuel

AFR=56 kg air/fuel

Figure 11. Effect of ambient temperature and air to fuel ratio onthermal efficiency.

fuel ratio because increases the flue gases losses, buthe specific fuel consumption increased with increase aito fuel ratio as shown in Figure 17. Figure 18 representthe variation of exhaust temperatures with thermaefficiency for several ambient temperature and air to fueratio. Noted the thermal efficiency is increased withdecrease the ambient temperature and air to fuel ratiobut the thermal efficiency decreases with increase theexhaust temperature.

The thermal efficiency decreases with increases the


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260 270 280 290 300 310 320 3301.22

1.24

1.26

1.28

1.3

1.32

1.34

1.36

1.38x 10

5


Power

(kW)

AFR=40 kg air/fuel

AFR=44 kg air/fuel

AFR=48 kg air/fuel

AFR=52 kg air/fuel

AFR=56 kg air/fuel

Figure 12. Effect of ambient temperature and air to fuel ratio onpower.

260 280 300 320 340

Ambient Temperature (K)

100000

110000

120000

130000

140000

Power(kW)

Practical Simple Gas Turbine

Simple Gas Turbine

Figure 13. Effect of ambient temperature on power output.

exhaust temperature.

Effect of compressor and turbine efficiencies

Figures 19 and 20 highlight the effect of the compressorand turbine isentropic efficiencies on the thermalefficiency for different air fuel ratio. The thermal efficiencyincrease with increase the compressor and turbine isen-tropic efficiencies, this is mean the thermal losses have

260 270 280 290 300 310 320 3300.245

0.25

0.255

0.26

0.265

0.27

0.275

0.28

Spec

ificFue

lConsump

tion

(kg

/kW

.h)


AFR=40 kg air/fuel

AFR=44 kg air/fuel

AFR=48 kg air/fuel

AFR=52 kg air/fuel

AFR=56 kg air/fuel

Figure 14. Effect of ambient temperature and air to fuel ratio onspecific fuel consumption.

260 270 280 290 300 310 320 3300.98

1

1.02

1.04

1.06

1.08

1.1

1.12x 10

4


Heatr

ate

AFR=40 kg air/fuel

AFR=44 kg air/fuel

AFR=48 kg air/fuel

AFR=52 kg air/fuel

AFR=56 kg air/fuel

Figure 15. Effect of ambient temperature and air to fuel ratioon heat rate.

been reduced in both compressor and turbinerespectively, this lead to increased power output.

DISCUSSION

The efficiency and power output of gas turbine dependson the operation conditions were presented in earliesection. The variation of thermal efficiency at higher


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Rahman et al. 3547

40 42 44 46 48 50 52 54 560.32

0.325

0.33

0.335

0.34

0.345

0.35

0.355

0.36

0.365

Therma

lEfficiency

Air Fuel Ratio (kg air/fuel)

T1=268T1=278

T1=288T1=298

T1=308

T1=318T1=328

Figure 16. Effect of air to fuel ratio and ambient temperature on thermal efficiency.

40 42 44 46 48 50 52 54 560.245

0.25

0.255

0.26

0.265

0.27

0.275

0.28

Spec

ificF

ue

lConsump

tion

(kg

/kW

.h)

Air Fuel Ratio (k air/fuel)

T1=268T1=278T1=288T1=298T1=308T1=318T1=328

Figure 17. Effect of air to fuel ratio ambient temperature on specific fuel consumption.


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700 750 800 850 900 950Exhaust Temperature (K)

0.32

0.33

0.34

0.35

0.36

0.37

AFR=40 Kg air/fuel

AFR=56 Kg air/fuel

T1= 268 K

T1= 328 K

Therma

le

fficiency

Figure 18. Variation of exhaust temperatures with thermal efficiency for several

ambient temperature and air to fuel ratio.

0.75 0.8 0.85 0.9 0.95

0.29

0.3

0.31

0.32

0.33

0.34

0.35

0.36

0.37

ThermalEfficiency

Isontropic Compressor Efficiency

AFR=40 kg air/fuel

AFR=44 kg air/fuel

AFR=48 kg air/fuel

AFR=52 kg air/fuel

AFR=56 kg air/fuel

Figure 19. Effect of isentropic compressor efficiency air to fuel ratio on

thermal efficiency.

compression ratio and ambient temperature are verycrucial. The thermal efficiency is affected by ambienttemperature due to the change of air density and

compressor work since a lower ambient temperatureleads to a higher air density and a lower compressorwork that in turn gives a higher gas turbine output powe


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Rahman et al. 3549

0.75 0.8 0.85 0.9 0.950.2

0.22

0.24

0.26

0.28

0.3

0.32

0.34

0.36

0.38

0.4

ThermalEfficiency

Isontropic Turbine Efficiency

AFR=40 kg air/fuel

AFR=44 kg air/fuel

AFR=48 kg air/fuel

AFR=52 kg air/fuel

AFR=56 kg air/fuel

Figure 20. Effect of isentropic turbine efficiency air to fuel ratio on thermal efficiency.

as shown in Figure 11. The increases of ambienttemperature lead to decrease the thermal efficiency. Itcan also be seen that the specific fuel consumption

increases with increase of air to fuel ratio and ambienttemperature because of the air mass flow rate inlet tocompressor increases with decrease of the ambienttemperature. Thus, the fuel mass flow rate increasessince the air to fuel ratio is kept constant. The variation ofpower out is less than that of the inlet compressor airmass flow rate. Therefore, the specific fuel consumptionincreases with the increase of ambient temperature dueto the flue gases losses. The increase in compressionratio for gas turbine power plant lead to a continuousincrease of thermal efficiency and this result move thegas turbine power plant to reaches the highest efficiencyand then begins to decrease. Therefore, the overallenhancement of the effect of operation conditions on

efficiency of gas turbine can be highly positive, especiallywhen considering the possibility to take advantage fromincrease turbine inlet temperature and then increase theoutput power and the thermal efficiency.

Conclusion

The simulation result from the modeling of the influenceof parameter showed that compression ratio, ambienttemperature, air to fuel ratio and turbine inlet temperature

effect on performance of gas turbine power plant. Theresults were summarized as follows:

1. The compression ratios, ambient temperature, air tofuel ratio as well as the isentropic efficiencies are stronglyinfluence on the thermal efficiency of the gas turbinepower plant.2. The variation of thermal efficiency at highercompression ratio, turbine inlet temperature and ambientemperature are very important.3. The thermal efficiency and power output decreaseslinearly with increase of ambient temperature as well asthe air to fuel ratio.4. The specific fuel consumption and heat rate increaselinearly with increase of both ambient temperature and airto fuel ratio.5. The peak efficiency, power and specific fue

consumption occur at higher compression ratio with lowambient temperature.

ACKNOWLEDGEMENTS

The authors would like to thank University MalaysiaPahang for providing laboratory facilities and financiasupport under Doctoral Scholarship scheme (Project NoGRS100332).


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