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Applied Thermal Engineering 31 (2011) 2735e2739

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Gas turbine performance at varying ambient temperature

Ashley De Sa*, Sarim Al Zubaidy 1

Heriot Watt University, Dubai Campus, PO Box 294345, Dubai, United Arab Emirates

a r t i c l e i n f o

Article history:Received 4 November 2010Accepted 28 April 2011Available online 12 June 2011

Keywords:Gas turbine powerEfficiencyAmbient temperatureEmpirical relationship

* Corresponding author. Tel.: þ971 55 4145683; faxE-mail addresses: ad162@hw.ac.uk (A. De

(S. Al Zubaidy).1 Tel.: þ971 55 8042653; fax: þ971 4 4464273.

1359-4311/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.applthermaleng.2011.04.045

a b s t r a c t

The difference between the actual power generated by a gas turbine and the design rated power taggedon the gas turbine is observed whenever a gas turbine operates at site ambient conditions that vary fromthe stipulated ISO conditions. A detailed study and extensive logging of data has endorsed the wellknown existence of a direct relationship between the ambient temperature and the de-rating of gasturbine power output. The paper proposes an empirical relationship between the gas turbine’s ability togenerate power when exposed to site ambient conditions, such as the ambient temperature, which differfrom ISO conditions. For every K rise in ambient temperature above ISO conditions the Gas Turbine loses0.1% in terms of thermal efficiency and 1.47 MW of its Gross (useful) Power Output. This establishedrelationship will assist the proper assessment of local power generation for installation planning andforecasting with special reference to Middle-eastern countries which are rapidly developing the appli-cation of Gas Turbine Inlet Air Cooling (GTIAC) technologies. This study was conducted for specificturbines SGT 94.2 and SGT 94.3 installed at the DEWA Power Station located at Al Aweer, H Phase II andIII in Dubai, UAE.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Al Aweer Gas Turbine Power Plant H Phase II comprises of threeSIEMENS Gas Turbines e SGT 94.2 and Phase III comprises of fourSIEMENS Gas turbinese SGT 94.3. Each of these types of gas turbinehas a power generation capacity of 160 MW and 265 MW respec-tively at 50 Hz frequency in dual fuel firing (natural gas and dieselfuel oil) at STP or ISO conditions of 1.01 bar pressure, 15 K and RH60% [1]. The gas turbines, along with the balance of plant and therequired auxiliaries are working in an open cycle mode. Theambient atmospheric condition in which the power plant operatesis harsh desert condition. The site is located in Dubai, where theambient air temperatures vary from the cold of winter e approx11 K, to the extreme hot of summere approx 55 K, with humidity invarying proportions throughout the year going to a maximum of 90percent with occasional scant rain. Windy conditions bring with itsevere sand storms and dust haze which at times last for a numberof days. The plant was commissioned in the early 2008 and isoperational with natural gas as the primary fuel. Fig. 1 showsa simple schematic of the power plant.

The Gas Turbine power plant works on a Joule-Brayton cycle [2].The use of heat sinks in the basic Joule-Brayton cycle in order toexploit its available heat sources leads to a more advanced mixed

: þ971 4 4589768.Sa), S.Al-Zubaidy@hw.ac.uk

All rights reserved.

(auto-combined) cycles [3]. The gas turbine is a complex machine,and its performance and reliability are governed bymany standards.TheAmerican Society ofMechanical Engineers (ASME) performancetest codes [4] have beenwritten to ensure that test are conducted inamanner that guarantees that all turbines are tested under the sameset of rules and conditions to ensure that the test results can becompared in a judicious manner. The reliability of the turbinesdepends on the mechanical codes that govern the design of gasturbines [4]. Themechanical standards and codes have beenwrittenby both ASME and the American Petroleum Institute (API) amongstothers. The major variables that affect the gas turbines are:

1. Type of application2. Plant location and site configuration3. Plant size and efficiency4. Type of fuel5. Enclosures6. Plant operation mode; base or peaking

2. Brief literature review

2.1. Effect of temperature on gas compression and turbineperformance

The performance of the gas turbine is reliant on the efficiencyachieved at the compressor of the turbine. Hot air, being less dense,

Nomenclature

cp Specific Heat Capacity at constant pressure, kJ/kgKPt Gas Turbine Power at ambient temperature, MWPx Gas Turbine Power at ISO condition, MWP1 Pressure at Gas Turbine Compressor Inlet, barP2 Pressure at Gas Turbine Compressor Outlet, barP3 Pressure at Gas Turbine Inlet, barP4 Pressure at Gas Turbine Outlet, barr Pressure ratio, -t Temperature difference from ISO condition of 15 K, KT1 Temperature at Gas Turbine Compressor Inlet, K

T2 Temperature at Gas Turbine Compressor Outlet, KT3 Temperature at Gas Turbine Inlet, KT4 Temperature at Gas Turbine Outlet, KTA Temperature at Gas Turbine Compressor Inlet, KTB Temperature at Gas Turbine Compressor Outlet, KTes TemperatureeEntropy, KekJ/kgKg Ratio of Specific heats, -h Cycle Efficiency, -hc Compressor Efficiency, -ht Gas Turbine Efficiency at ambient temperature, -hx Gas Turbine Efficiency at ISO condition, -

A. De Sa, S. Al Zubaidy / Applied Thermal Engineering 31 (2011) 2735e27392736

de-rates the gas turbine’s performance [5]. In case studies carriedout previously, the effect of ambient temperature on electricityproduction and fuel consumption of a simple cycle plant has beendocumented at temperatures closer to ISO conditions in Turkey [6].Further, the performance improvement of the gas turbine isdependent on the maximum temperature tolerance of the firststage blades and is also reliant on inter stage cooling at thecompression stage [7]. Several methods and technologies areavailable to augment this power loss but this entails additionalplant and equipment installation as well as additional operationalrequirements [8]. Many of these methods such as use of air cooler[9], regenerative steam injection [10], effusive blade cooling tech-niques [11e13], use of desiccant-based evaporative cooling [14] orabsorption chillers [15] are commonplace. The effect of relativehumidity [16] on the gas turbine power plant addresses issues ofthe air cooling [17e19] and enhances compressor efficiency.However, humidity prior to filtration system imposes a penalty ongas turbine performance. Analytical methods have been researchedfor evaluation of gas turbine performance when subjected to inletair cooling in combined cycle power plant [20]. The effects, whetherpositive or detrimental on the gas turbine compressor and engineperformance, as well as their operability to use of water coolingtechniques for inlet air can be effectively assessed for their merits[21]. Also, analytical studies are performed to confirm thatincreasing the turbine inlet temperature no longer means anincrease in cycle efficiency, but increases the work. When appliedwith intercooled gas turbines, these studies have shown thatincreasing turbine inlet temperature and pressure ratio can stillimprove the performance of the intercooled gas turbine [22].However, such use of additional plant is seldom encountered indesert conditions, primarily due to the high cost of such applicationand its maintenance as water for such application needs to bespecially generated using desalination technologies, which are highcost applications. The effect of ambient temperature on gas turbineperformance is known. However, the arriving at an empiricalrelationship between gas turbine power, efficiency and ambient

Fig. 1. Simple schematic showing

temperature with special reference to middle-eastern desertconditions has not been generally undertaken.

2.2. Re-visiting the reversible simple (Joule-Brayton) cycle

Here the original Joule-Brayton cycle (i.e. an internally revers-ible closed gas turbine cycle 1, 2, 3, 4,with amaximum temperatureT3 ¼ TB and a pressure ratio r) as a standard [2] is used. Theminimum temperature is taken as TA (the ambient temperature) sothat, T1 ¼ TA. Refer Fig. 2.

The concept of compressor polytropic efficiency can be devel-oped from considering small compression processes and by usingthe Gibbs equation for an ideal (or isentropic) process. Using thedefinition of the isentropic (or overall) compressor efficiency withreference to the pressure ratio the following expression could bewritten:

hc ¼

�P2P1

��g�1g

��1

�P2P1

�� g�1ghpc

��1

(1)

The same thermodynamic principles can be applied to the gasturbine expansion process to produce the following:

ht ¼1�

�P3P4

��hPt

�g�1g

��

1��P3P4

�g�1g

� (2)

Fig. 3 shows the relationships given between the isentropicefficiencies of the compressor and the turbine (for a constant

an open cycle power plant.

Fig. 2. Tes Diagram for reversible closed simple cycle.

Fig. 4. Behavior of gas turbine SGT 94.2 thermal efficiency under various operatingloads at varying ambient temperature during the performance tests.

A. De Sa, S. Al Zubaidy / Applied Thermal Engineering 31 (2011) 2735e2739 2737

polytropic efficiency of 0.85) against the pressure ratio. For thecompressor, it can be observed that as the pressure ratio increases(the compressor is bigger), the overall compressor efficiency ispenalized. For the turbine it can be observed that the isentropicefficiency is higher than the polytropic (or small-stage) efficiency.

The cycle efficiency could be expressed as:

h ¼ cpðT3 � T4Þ � cpðT2 � T1ÞcpðT3 � T2Þ

(3)

Making use of the isentropic relationship between the pressureand temperature,

T2T1

¼ P2P1

¼ r ¼ P3P4

¼ T3T4

(4)

The simple cycle efficiency is readily shown as:

h ¼ 1��1r

��g�1g

�(5)

Polytropic efficiency for compressor and turbine = 0.85

Pressure ratio1.00 6.00 11.00 16.00

Isen

trop

ic e

ffic

ienc

y

0.75

0.80

0.85

0.90

0.95

Turbine

Compressor

Fig. 3. Turbine and compressor isentropic efficiency variance with pressure ratio atconstant polytropic efficiency.

3. Gas turbine performance test results

Fig. 4 shows the input data associated with the performance teston the Gas Turbine SGT 94.2. The figure portrays the actual varia-tion of the thermal efficiency during the operation and perfor-mance test conducted on the Gas Turbine when the ambient airtemperature undergoes changes at three specific loads (60%, 80%and 100%). It can be observed that at higher loads the deviation isless pronounced while at lower loads it is much sharper. Theprobable reason is that when the Gas Turbine operates at designbase loads with inlet guide vanes in open position, the Gas Turbineinternal polytropic losses are at its minimum. On the other hand theGas Turbine internal polytropic losses increase when the GasTurbine operates at part loads with inlet guide vanes partially open.Therefore, it is interesting to note this difference in variance of GasTurbine efficiency response in varying ambient temperatures whilethe Gas Turbine is made to operate at part loads and base loads.

Fig. 5 highlights the actual variation of the Gas Turbine SGT 94.3thermal efficiency and the useful power output it undergoes versusthe ambient air temperature at base loads during annual moni-toring exercise conducted on the Gas Turbine. A set of sixteenreadings has been selected from the data to give a continuous visualperspective of results as the ambient temperature varies fromapproximately 42 K to approximately 20 K. With Gas Turbine atbase load (inlet guide vanes open) it is demonstrated that the usefulPower Output varies from approximately 226 MW to 257 MWwithAmbient Temperature dips.

Fig. 6 shows the actual variation of the Gas Turbine SGT 94.3thermal efficiency with the ambient air temperature and relativehumidity at base loads and at approximately sixty percent of baseloads, during annual monitoring exercise conducted on the GasTurbine. Once again it is interesting to observe that at base loads,the deviation in thermal efficiency and power output is lesspronounced. The variation in thermal efficiency and useful power

Fig. 5. The behavior of gas turbine SGT 94.3 thermal efficiency and power variancewhen at base load at varying ambient temperature during the annual continuousmonitored period.

Fig. 6. Behavior of gas turbine SGT 94.3 thermal efficiency and power variance at fixed%IGV openings (Base Loads and % of Base load) at varying ambient temperature duringthe annual continuous monitored period.

Table 1Gas turbine power and efficiency at ambient temperatures different from ISOcondition.

Model SGT 94.3

GT Inlet Temp (Ambient) K 15 21.59 26.26 35.44 40.84 46.72GT Power Output MW 265a 257.39a 247.96a 235.8a 227.7a 220.35a

GT Thermal Efficiency % 37 33.96 33.72 32.58 32.38 32.26Decrease in Power Output

with respect to ISO GTInlet temperature (15 K)

% 0 2.87 6.43 11.02 14.08 16.85

Decrease in ThermalEfficiency with respectto ISO GT Inlettemperature (15 K)

% 0 8.21 8.86 11.94 12.48 12.81

a Operation of Gas Turbine with Hydraulic Clearance Optimization (HCO) active.

A. De Sa, S. Al Zubaidy / Applied Thermal Engineering 31 (2011) 2735e27392738

output reacts much more sharply when Gas Turbine is operating atpart loads and is exposed to variance in ambient temperatures. Theprobable reasons for such a variation may be assigned to polytropiclosses.

Fig. 7 illustrates the actual variation of the Gas Turbine SGT 94.3thermal efficiency as it undergoes variation in the ambient airtemperature and relative humidity values at base loads and atapproximately eighty percent of base loads. Examination of thefigures illustrates how at base loads the deviation is smaller whileat eighty percent of base loads it is much sharper. The plausiblereasons for such a variation are as explained above.

4. Comparison of actual gas turbine efficiency and usefulpower at varying ambient temperature and relative humidity

The effect of ambient temperature on gas turbine efficiency andpower is well noticed, however the effect of humidity on the gasturbine efficiency and power proved to be difficult to quantifywithin the existing set-up and instrumentation capabilities.Therefore, the trend encompassing this needs further investigationin order for a relationship to be established. An attempt has beenmade to project the established trends to gauge the upward limitsat which the useful power and actual efficiency will reach, as theambient temperature dips to ISO conditions of 15 K. It is clear thatwhen the Gas Turbine operates at an ambient temperature ofapproximately 30 K higher than the ISO condition, it loses around44 MW of useful power generation to the grid and its thermalefficiency decreases by approximately 2.1 percent. The comparisonof the deviation of power and efficiency as observed at ambienttemperatures in Dubai is tabulated at Table 1. This considerable lossin power generation occurs due to the intricate role the Gascompressor plays in the balance of internal power consumptionfrom the gas in order to compress the hot ambient air. Also, Fig. 8shows the Gas Turbine Efficiency and Power Output when theambient temperature varies between the maximum and minimumvalues as prevalent at Dubai, in the United Arab Emirates.

Fig. 7. Behavior of gas turbine SGT 94.3 thermal efficiency under various operatingloads at varying ambient temperature during the Performance tests.

4.1. Empirical relationship between gas turbine efficiency, gasturbine power output and ambient temperature

Using available data a direct relationship between the variationof ambient temperature and the effect it has on both (a) The gasturbine efficiency and (b) Its power output is established. Ignoringall other effects of polytropic nature including the ambienthumidity, for Gas Turbine SGT 94.3, it is found that for every degreerise in ambient temperature there will occur a fall in gas turbineefficiency and power output to the equivalent of 0.07% and1.47 MW.

The empirical relationship is therefore established and practi-cally confirmed over repeated tabulation of data and is as statedbelow:

“For every K rise in ambient temperature above ISO conditionthe Gas Turbine looses 0.1% in terms of thermal efficiency and1.47 MW of its Gross (useful) Power Output”.

1. For Gas Turbine efficiency ¼ hx at ISO condition then,

For predicted Gas Turbine efficiency at ambient temperaturediffering from ISO condition by t (or�twhen less than ISO) shall be:

Fig. 8. SGT 94.3 gas turbine thermal efficiency and power output variance at base loadat varying ambient temperature and varying humidity during annual monitoring.

A. De Sa, S. Al Zubaidy / Applied Thermal Engineering 31 (2011) 2735e2739 2739

ht ¼ hx � ð0:1ÞðtÞ (6)

2. For Gas Turbine Useful Power ¼ Px at ISO condition then,

For predicted Gas Turbine power at ambient temperaturediffering from ISO condition by t (or�twhen less than ISO) shall be:

Pt ¼ Px � ð1:47ÞðtÞ (7)

The empirical constants thus obtained are a direct result of thedifference in calculated values of Gas Turbine power output of44 MW and thermal efficiency of 2.1% when measured overa change of ambient temperature of 30 K.

5. Conclusion

The graphical representation of data as seen in Figs. 4e8,demonstrate that the gas turbine thermal efficiency and its usefulpower output varies with the ambient temperature. At higherambient temperatures (than ISO conditions) the thermal efficiencyand useful power output tend to be lower. The gas turbine inlettemperature being a limiting factor as dictated by the turbineblade metallurgy and mass flow of air being reduced at highertemperatures, hence we observe a turbine thermal efficiency de-rating upon rise in ambient temperature. As a direct conse-quence of this, the power generated by the gas turbine andsupplied to the grid has a significant drop and therefore is a matterof concern.

For gas turbine power plants operating in theMiddle-East, thereis a tremendous de-rating factor due to higher ambient tempera-tures. Coupled with this, these gas turbines are made to operatewithout the application of gas turbine inlet air cooling equipmentand technology applications [23] primarily due to the costs as wellas logistics associated with the generation and sourcing of thisadditional requirement of coolingwater. Themain source of coolingwater is from desalination processes which can only be carried outprimarily at coastal regions.

A direct comparison between the Gas Turbine useful poweroutput, its thermal efficiency and the operational ambienttemperature has been established with data readings in excess of8000 for Gas Turbine operations over 280 days covering summerand winter periods [24].

The next stage of the work is to develop an empirical relation-ship between ambient humidity and the system performancewhenthe turbines are tested for their performance during the end ofwarranty inspection; currently scheduled to take place mid of year2011.

Acknowledgements

The authors acknowledge their gratitude to Dubai Electricityand Water Authority, SIEMENS and colleagues within these orga-nizations for the necessary access to Gas Turbine data [24].

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