Study Onthe Effective Parameter of Gas Turbine Model with Intercooled

Full Length Research Paper

Study on the effective parameter of gas turbine model with intercooled compression process

Thamir K. Ibrahim1*, M. M. Rahman2 and Ahmed N. Abd Alla3

1Faculty of Mechanical Eng., Universiti Malaysia Pahang, Pekan 26600, Malaysia. 2Automotive Excellence Center, Faculty of Mechanical Eng., Universiti Malaysia Pahang, Pekan 26600, Malaysia. 3Faculty of Electrical and Electronic Eng., University Malaysia Pahang, Kuantan 26600, Malaysia.

Accepted 22 November, 2010

In this paper parametric study of a gas turbine cycle model power plant with intercooler compression process was proposed. The power output and the efficiency are simulating with respect to the cycle temperatures and pressure ratio for a typical set of operating conditions. Simple gas turbine cycle calculations with realistic parameters are made and confirm that increasing the turbine inlet temperature no longer means an increase in cycle efficiency, but increases the work done. The analytical study is done to investigate the performance improvement by intercooling. The analytical formula for specific work and efficiency are derived and analyzed. The simulation results shows that increasing turbine inlet temperature and pressure ratio can still improve the performance of the intercooled gas turbine cycle.

Key words: Gas turbine, power plant, performance, intercooler.

INTRODUCTION

Over the past decade, gas turbines have turned out to be real gas effects cause the peaks in cycle performance, one of the most interesting techniques for electric power even without cooling. Higher fuel/air ratios give higher production (Mahmood and Mahdi, 2009). Therefore, water contents, which hint to higher values of specific enhancing the performance of Gas turbine was heat capacitances cv and cp. Wang et al. (2008) arrived successfully through raising the turbine inlet temperature at an overview of their investigations about the effect of (TIT) and the compressor pressure ratio and advances in the cooling on cycle performance. They conclude that cooling technology and material science caused high enhancement allowable blade metal temperature and film turbine inlet temperature conceivable. The challenge of cooling effectiveness are salutary, but not as important as constantly enhancing the gas turbine performance has enhancing the turbomachinery aerodynamic got to a critical moment as Horlock et al. (2003) looked performance. into the limits of raising the combustor outlet temperature. Various means have been employed by many Highest performance is suggested to be achieved at TIT researchers to improve the power product of the turbines, much lower than the stoichiometric combustion particularly the gas turbine. One of the means is to use temperature due to the increase in losses affiliated with intercooler. The intercooler is used to reduced the the cooling flows. Unless new materials and improved temperature at the high-pressure compressor, causing heat transfer mechanisms can confine the increase in the reduce consumption power on compressor and lower requirement cooling air flow rates, it will not be valuable output temperature at high pressure (Canie`re et al., raising the TIT much boost. Saidi et al. (2002) made a 2006). The overall result is a lowering of the net work careful study on the gas properties as a limit on input wanted for a given pressure ratio. According to performance in the absence of cooling. It was shown that Yadav and Jumhare (2004) the intercooling is especially effective when used in a cycle with heat recovery. Even so, intercooling used without reheating causes decrease of the efficiency at least for low pressure ratios. It is *Corresponding author. E-mail: [email protected]. explicated by the drop of temperature after the Ibrahim et al. 3761

required pressure, and then goes to combustion chamber, after additional heating to maximum permissible temperature in the combustion chamber. The network output of the cycle is thus proportional to the temperature drop in the turbine (Bassily, 2004).

PROBLEM FORMULATION

It is assumed that the effectiveness of intercooler (heat exchanger) is , the compressor efficiency in , and the turbine efficiency is . The ideal processes and actual processes are represented in dashed line and full line, respectively, on the T-S diagram (Figure 1). These parameters in terms of temperature are defined as (Al-

Figure 1. T-S diagram for gas turbine with Sayed, 2008): intercooler. T2s − T1 T4s − T3 ηc = = ηclp = ηchp = T − T T − T 2 1 4 3 T −T T −T compressor, which is compensated by the increment of η = 5 6 , x = 2 3 the temperature in the combustion chamber. The range t T T T T 5 − 6s 2 − 1 of power output that is concerned when talking about "mid-size" plants is typically 30-200 MW Where represent low and high pressure (Saravanamuttoo et al., 2009). compressor efficiency respectively. This paper focuses on investigating the effective parametric such as ambient temperature, compression The work required to run the compressor is, ratio, turbine inlet-temperature, and the effectiveness of the intercooler to improve the performance of gas turbine ≈ γ a −1 ’» ≈ γ a −1 ’ÿ ∆ ÷ ∆ ÷ power plant with intercooled compression process. In the γ a … γ a Ÿ ∆ r p − 1 ÷ ∆ rp − 1 ÷ process the fluid is compressed in the first compressor to Wc = c T …2 + (1 − x ) Ÿ pa 1 ∆ η ÷… ∆ η ÷Ÿ some average pressure and then it is passed across an ∆ c ÷… ∆ c ÷Ÿ intercooler, where it is chilled to a lower temperature at « ◊ « ◊⁄ basically constant pressure. It is suitable that the lower temperature is as low as possible. The cooled fluid is Where the specific heat of air is given by (Naradasu et directed to second compressor, where it increases in the al., 2007), pressure and then it directs the fluid to the combustion 3 −4 2 chamber and later to the expander. cpa = 1.0189*10 − 0.13784Ta +1.9843*10 Ta −7T 3 −10T 4 + 4.2399*10 a − 3.7632*10 a

MODEL DESCRIPTION The specific heat of flue gas is given by (Naradasu et al., 2007), Intercooling is a way to reduce the power consumption for compression of an air. Thus, the inlet temperature of c =1.8083−2.3127*10−3T +4.045*10−6T 2 −1.7363*10−9T 3 pg the second compressor stage can be kept low. For a given compression ratio, the power consumed in a The work developed by turbine is given by, compressor is directly proportional to the inlet temperature. Consider Figure 1 and assume that the » ≈ ’ÿ … ∆ ÷Ÿ compressor is working between the thermodynamic ∆ 1 ÷ W t = c pg T5 …η t 1 − Ÿ states 1 and 2. If the air is cooled from state 2 to 3 the … ∆ γ g −1 ÷Ÿ ∆ 2 γ ÷ … « (r p ) g ◊Ÿ required compressor power is decreased and the net ⁄ cycle power delivered is increased if the inlet temperature is reduced (Cengel and Boles, 2008 ). Figure 2 shows a Where (turbine inlet temperature) gas turbine power plant with intercooler has a single shaft gas turbine. In this gas turbine cycle with intercooler, air » ≈ ’ÿ … ∆ ÷Ÿ after compression in the first stage compressor enters ∆ 1 ÷ W t = c pg TIT …η t 1 − Ÿ into an intercooler where it is cooled. The cooled air then … ∆ γ g −1 ÷Ÿ ∆ 2 γ ÷ enters into second stage compressor to compress to … « (rp ) g ◊⁄Ÿ 3762 Sci. Res. Essays

Fuel Intercooler 4 5 C.C 2 3 Wshaft

L.P.Comp. H.P.Comp. Turbine G

6 1 Air Exhaust

Figure 2. Block diagram for gas turbine with intercooler.

The network is, RESULT

» ÿ ≈ γa−1 ’» ≈ γa−1 ’ÿ In this paper, the effects of pressure ratio across the ≈ ’ ∆ ÷ ∆ ÷ … ∆ ÷Ÿ γa … γa Ÿ compressor rp, turbine inlet temperature (TIT), ambient 1 ∆rp −1÷ ∆rp −1÷ W =c TIT.…η ∆1− ÷Ÿ−c T …2+ 1−x Ÿ n pg t γ −1 pa 1∆ ÷ ( )∆ ÷ temperature and the effectiveness of intercooler on … ∆ g ÷Ÿ η … η Ÿ ∆ 2 ÷ ∆ c ÷ ∆ c ÷ r γg … Ÿ the first-law efficiency and power are obtained by the … « ( p ) ◊⁄Ÿ « ◊ « ◊⁄ energy-balance approach or the first-law analysis of the cycle programming using matlab software. In the combustion chamber, the heat supplied by the fuel The results of the above analysis are shown in Figures is equal to the heat absorbed by air, hence, 3 to 15. Figure 3 shows the effect of ambient temperature on the efficiency of gas turbine cycle with intercooler. For

» ≈ γ a −1 ’≈ ≈ γ a −1 ’’ÿ ∆ ÷∆ ∆ ÷÷ that figure, TIT=1450 K, rp=12, … γ a γ a Ÿ … ∆ rp −1÷∆ ∆ rp −1÷÷Ÿ . It is clear from the figure that Qadd = cpgm…TIT −T1 +T2∆ ÷∆(2 − x)+ (1− x)∆ ÷÷Ÿ ηc ηc decreasing the ambient temperature increases the gain in … ∆ ÷∆ ∆ ÷÷Ÿ … « ◊« « ◊◊⁄Ÿ efficiency. A direct effect of inlet temperature on the standard air thermal efficiency and the thermal efficiency

of regenerative cycle is shown in Figure 4. As the The power output is (Saravanamuttoo et al., 2009), ambient temperature increases, the specific work of the

compressors increases (Nag, 2008), thus reducing cycle Power = m .W a net efficiency for the intercooler gas turbine cycles as shown

. in Figure 5. Also in Figure 3, if the effectiveness for Where = air mass flow rate, also air to fuel ratio is, intercooler changes from , the efficiency will decrease from 43.8 to 42.5%. This is AFR = LHV because the entry air to the combustion chamber with low Qadd temperature causes increasing fuel consumption as shown in Figure 6, thus reducing the efficiency in spite of The specific fuel consumption is determined by the the increased power output from the gas turbine as formula: shown in Figure 7. Figure 8 shows the effect of compression ratio and 3600 SFC = intercooler effectiveness on the thermal efficiency. Note (AFR.Wnet ) that the thermal efficiency is increased with compression ratio, but the effect of intercooler effectiveness is very low Further the thermal efficiency of the cycle, to increase the thermal efficiency compared with pressure ratio. » ≈ ’ÿ ≈ γ a −1 ’» ≈ γ a −1 ’ÿ ∆ ÷ ∆ ÷… ∆ ÷Ÿ Figure 9 shows the variation of the thermal efficiency … Ÿ r γ a − 1 r γ a − 1 … ∆ 1 ÷Ÿ ∆ p ÷… ∆ p ÷Ÿ c pgTIT ηt 1 − − c paT1∆ ÷ 2 + (1 − x)∆ ÷ with compression ratio. The increase in compression ratio … ∆ γ g −1 ÷Ÿ η … η Ÿ ∆ 2 ÷ ∆ c ÷ ∆ c ÷ … r γ g Ÿ … Ÿ « ( p ) ◊⁄ « ◊ « ◊⁄ means an increase in power output, so the thermal ηth = » ≈ γ a −1 ’ ≈ γ a −1 ’ÿ efficiency must increase too. Also, the effect of ∆ ÷ ∆ ÷ … γ a γ a Ÿ … ∆ rp − 1÷ ∆ rp − 1÷Ÿ decreasing ambient temperature has low effect on c pgm TIT − T1 + T2 ∆ ÷(2 − x) + (1 − x)∆ ÷ … η η Ÿ … ∆ c ÷ ∆ c ÷Ÿ thermal efficiency for gas turbine with intercooler. A direct « ◊ « ◊⁄ effect of compression ratio on the simple gas turbine Ibrahim et al. 3763

0.44

0.435

0.43 y

c 0.425 n e i c i f f E

0.42 l a m r e

h 0.415 T x=0.5 0.41 x=0.6 x=0.7 x=0.8 0.405 x=0.9 x=1 0.4 260 270 280 290 300 310 320 330 Ambient Temperature(K)

Figure 3. Effect of ambient temperature and intercooler effectiveness on thermal efficiency.

0.42

0.40 y c n

e Intercooler Gas Turbine i c i f

f Simple Gas Turbine E

0.38 l a m r e h T

0.36

0.34 260 280 300 320 340 Ambient Temperature

Figure 4. Effect of ambient temperature on thermal efficiency for simple and gas turbine cycle with intercooler. 3764 Sci. Res. Essays

450 x=0.5 x=0.6 x=0.7 x=0.8 x=0.9 ) g

k 400 x=1 / J k ( k r o W

r o s s e r p m

o 350 C

300 260 270 280 290 300 310 320 330 Ambient Temperature(K)

Figure 5. Effect of ambient temperature and intercooler effectiveness on compressor work.

0.18

0.178

) h . 0.176 W k / g k (

n 0.174 o i t p m u

s 0.172 n o C

l e

u 0.17 F x=0.5 c i f i

c x=0.6

e 0.168

p x=0.7 S x=0.8 0.166 x=0.9 x=1 0.164 260 270 280 290 300 310 320 330 Ambient Temperature(K)

Figure 6. Effect of ambient temperature and intercooler effectiveness on specific fuel consumption. Ibrahim et al. 3765

5 x 10 2.6

2.5

2.4

2.3 ) W k ( r 2.2 e w o P 2.1 x=0.5 2 x=0.6 x=0.7 x=0.8 1.9 x=0.9 x=1 1.8 260 270 280 290 300 310 320 330 Ambient Temperature(K)

Figure 7. Effect of ambient temperature and intercooler effectiveness on power output.

0.5 0.5

0.45 0.45

0.4 y

0.4 c y n c e i n c i e f i f c i E f

0.35 f l a E

0.35 l m r a e m h T1=268(K) r T e 0.3 h T1=278(K) T 0.3 T1=288(K) x=0.5 T1=298(K) x=0.6 0.25 T1=308(K) 0.25 x=0.7 T1=318(K) x=0.8 T1=328(K) 0.2 x=0.9 0 5 10 15 20 25 30 0.2 'Compression Ratio' 0 5 10 15 20 25 30

Compression Ratio Figure 9. Effect of compression ratio and ambient temperature on thermal efficiency. Figure 8. Effect of pressure ratio and intercooler effectiveness on the thermal efficiency.

entering the combustion chamber which decreases the cycle thermal efficiency and the thermal efficiency of gas heat added, that is, increases the thermal efficiency. In turbine cycle with intercooler is shown in Figure 10. The gas turbine cycle with intercooler the thermal efficiency thermal efficiency increases with increase of compression increases with compression ratio. ratio for the same inlet temperatures since the Figure 11 shows the relation between turbine inlet compression ratio will raise the temperature of the air temperature and thermal efficiency for different values of 3766 Sci. Res. Essays

0.6

0.4 y c n e i c i f f E

l a m r e h T 0.2

Intercooler Gas Turbine Simple Gas Turbine

0.0 0 10 20 30 40 50 Compression Ratio

Figure 10. Effect of compression ratio on thermal efficiency for simple and intercooler gas turbine.

0.42

0.4

0.38 y c n e i c i f f E

0.36 l a m r e

h T1=268 T 0.34 T1=278 T1=288 T1=298 0.32 T1=308 T1=318 T1=328 0.3 1000 1200 1400 1600 1800 2000 2200 Turbine Inlet Temperature(K)

Figure 11. Effect of turbine inlet temperature and ambient temperature on thermal efficiency. Ibrahim et al. 3767

0.34 rp=3 0.32 rp=6 rp=9

) 0.3 h

. rp=12 W

k rp=15 / 0.28 g

k rp=18 ( n

o rp=21 i

t 0.26 p rp=24 m u

s 0.24 n o C

e 0.22 u F

c i

f 0.2 i c e p

S 0.18

0.16

1000 1200 1400 1600 1800 2000 2200 Turbine Inlet Temperature (K)

Figure 12. Effect of turbine inlet temperature and compression ratio on specific fuel consumption.

ambient temperature. As the turbine inlet temperature is in the isentropic compressor efficiency and intercooler increased for the same exit temperature, the temperature effectiveness but the effect of isentropic compressor drop will increase giving higher power potential. This efficiency is more than the effect of intercooler increase in power leads to an increase in the thermal effectiveness. Also the thermal efficiency increased with efficiency, then the increase in the thermal efficiency increase in the isentropic turbine efficiency and about (6 to 8%) increases the turbine inlet temperature intercooler effectiveness as shown in Figure 16. from 1000 to 2050K. The relation between specific fuel consumption versus turbine inlet temperature for gas turbine cycle with DISCUSSION intercooler at different compression ratios values is shown in Figure 12. The specific fuel consumption Efficiencies of the simple-cycle early gas turbines were decreases with increase in the turbine inlet temperature practically increased by incorporating intercooling. The and increase in compression ratio. Also the power output output power of a gas-turbine cycle improves as a result increases with increase in the turbine inlet temperature of intercooling. The efficiency of gas turbine with and compression ratio as shown in Figure 13. intercooler depends on the operation conditions; with full Figure 14 represented the relation between the thermal load operation giving the highest efficiency. The efficiency and power output for eight turbine inlet necessary derivation of output power and efficiency for temperatures (1000-2050K) and twelve pressure ratios gas turbine with intercooler were made. Results show the (2-24). The compression ratio for maximum power for the effect of intercooler on performance of the gas turbine turbine inlet temperatures is selecting a compression power plant. The effect of increase of ambient ratio of 9.3 for a turbine inlet temperature of 1000K which temperature leads to decreased efficiency and that is will result in a higher thermal efficiency, but for the turbine identical to all previous studies. Also the increase in inlet temperature (2050K), the maximum power output compression ratio for gas turbine with intercooler leads to was at compression ratio of 24 and that yielded the a continuous increase in the thermal efficiency and this highest thermal efficiency. result runs counter to simple cycle for gas turbine that The effect of isentropic compressor efficiency and reaches the highest efficiency and then begins to intercooler effectiveness on thermal efficiency is shown in decrease. Figure 15. The thermal efficiency increased with increase Therefore, the overall impact of intercooling on 3768 Sci. Res. Essays

5 x 10 4.5 rp=3 4 rp=6 rp=9 rp=12 3.5 rp=15 rp=18 3 rp=21 )

W rp=24 k ( r 2.5 e w o P 2

1.5

0.5 1000 1200 1400 1600 1800 2000 2200 Turbine Inlet Temperature (K)

Figure 13. Effect of turbine inlet temperature and compression ratio on power output.

Figure 14. Variation of power with thermal efficiency for several compression ratio and turbine inlet temperatures. Ibrahim et al. 3769

0.45 x=0.5 0.44 x=0.6 x=0.7 0.43 x=0.8 x=0.9 0.42 x=1 y c n e i c

i 0.41 f f E

l a 0.4 m r e h T 0.39

0.38

0.37

0.36 0.75 0.8 0.85 0.9 0.95 1 Isontropic Compressor Efficiency

Figure 15. Effect of isentropic compressor efficiency and intercooler effectiveness on thermal efficiency.

0.5 x=0.5 x=0.6 x=0.7 0.45 x=0.8 x=0.9 x=1 y c

n 0.4 e i c i f f E

l a m r

e 0.35 h T

0.3

0.25 0.75 0.8 0.85 0.9 0.95 1 Isontropic Turbine Efficiency

Figure 16. Effect of isentropic turbine efficiency and intercooler effectiveness on thermal efficiency. 3770 Sci. Res. Essays

efficiency of gas turbine can be highly positive, especially REFERENCES when considering the possibility to take advantage of Al-Sayed AF ( 2008). Aircraft Propulsion and Gas Turbine Engines. lower intercooler effectiveness to increase TIT, and then Taylor and Francis Croup. CRC Press, ISBN 978-0-8493-9196-5. increase the output power and the thermal efficiency. Mahmood FG, Mahdi DD (2009). A New Approach for Enhancing Performance Of A Gas Turbine (Case Study: Khangiran Refinery). Appl. Energy, 86: 2750-2759. Nag PK (2008). Power Plant Engineering. Tata McGraw-Hill Publishing Conclusion Company Limited. NEW DELHI. ISBN-13: 978-0-07-064815-9. Naradasu RK, Konijeti RK, Alluru VR (2007). Thermodynamic Analysis The simulation result from the analysis of the influence of of Heat Recovery Steam Generator in Combined Cycle Power Plant. parameter showed that compression ratio, turbine inlet Therm. Sci., 11(4): 143-156. Saravanamuttoo H, Rogers G, Cohen H and Straznicky P (2009). Gas temperature and ambient temperature effect on Turbine Theory. Prentice Hall. ISBN 978-0-13-222437-6. performance of gas turbine cycle with intercooler can be Cengel AY, Boles AM (2008). Thermodynamics An Engineering summarized as follows: Approach. McGraw-Hill Higher Education. New York. ISBN 978-0-07- 352921-9. Bassily AM (2004). Performance improvements of the intercooled 1. The thermal efficiency decreases and specific fuel reheat recuperated gas-turbine cycle using absorption inlet-cooling consumption increases with increase in the intercooler and evaporative after-cooling. Appl. Energy, 77: 249-272. effectiveness. Canie`re H, Warlock A, Dick E, De Paepe M (2006). Raising cycle 2. The thermal efficiency of the simple gas-turbine cycle efficiency by intercooling in air-cooled gas turbines. Appl. Therm. Eng., 26: 1780-1787. experiences small improvements at large compression Wang X, Hwang Y, Radermacher R (2008). Investigation of potential ratios as compared to gas turbine cycle with intercooler. benefits of compressor cooling. Appl. Therm. Eng., 28: 1791-1797. 3. The peak efficiency, power and specific fuel Horlock JH, Eng FR, FSR (2003). Advance Gas Turbine Cycles. consumption occur when compression ratio increases in ELSEVIER SCIENCE Ltd. British. ISBN: 0-08-044273-0. Yadav R, Jumhare SK (2004). Thermodynamic analysis of intercooled the gas turbine cycle with intercooler. gas-steam combined and steam injected gas turbine power plants. 4. Maximum power for the turbine inlet temperatures is Proceedings of ASME TURBO EXPO:Power for Land. Sea and Air. selecting a compression ratio of 9.3 for a turbine inlet Vienna. Austria, pp. GT2004-54097. temperature of 1000K which will result in a higher thermal Saidi A, Eriksson D, Sunden B (2002). Analysis of some heat exchanger concepts for use as gas turbine intercoolers. Int. J. Heat efficiency. Exchangers, 3: 241-260.

ACKNOWLEDGEMENTS

The authors would like to thank Universiti Malaysia Pahang for providing laboratory facilities and financial support under Doctoral Scholarship scheme.