Comparison of evaporative inlet air cooling systems to enhance the gas turbine generated power

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2007; 31:14831503Published online 7 March 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/er.1315

Comparison of evaporative inlet air cooling systems to enhancethe gas turbine generated power

Mohammad Ameri*,y, H. R. Shahbazian and M. Nabizadeh

Combined Heat & Power Specialized Unit (CHP), Energy Engineering Department, Power & Water Universityof Technology, PO Box 16765-1719 Tehran, Islamic Republic of Iran

SUMMARY

The gas turbine performance is highly sensitive to the compressor inlet temperature. The output of gasturbine falls to a value that is less than the rated output under high temperature conditions. In fact increasein inlet air temperature by 18C will decrease the output power by 0.7% approximately. The solution of thisproblem is very important because the peak demand season also happens in the summer. One of theconvenient methods of inlet air cooling is evaporating cooling which is appropriate for warm and dryweather. As most of the gas turbines in Iran are installed in such ambient conditions regions, therefore thismethod can be used to enhance the performance of the gas turbines.In this paper, an overview of technical and economic comparison of media system and fog system is

given. The performance test results show that the mean output power of Frame-9 gas turbines is increasedby 11MW (14.5%) by the application of media cooling system in Fars power plant and 8.1MW (8.9%)and 9.5MW (11%) by the application of fog cooling system in Ghom and Shahid Rajaie power plants,respectively. The total enhanced power generation in the summer of 2004 was 2970, 1701 and 1340MWhfor the Fars, Ghom and Shahid Rajaie power plants, respectively.The economical studies show that the payback periods are estimated to be around 2 and 3 years for fog

and media systems, respectively. This study has shown that both methods are suitable for the dry and hotareas for gas turbine power augmentation. Copyright # 2007 John Wiley & Sons, Ltd.

KEY WORDS: gas turbine; inlet air cooling; evaporating cooling; media; fog; power augmentation

1. INTRODUCTION

Gas turbines are used widely in power generation, gas transfer stations and petrochemicalindustries (GE Energy, 2006). The site ambient conditions, especially the temperature, havegreat inuence on gas turbines performance (Brook, 1998). Since the air density is decreasedduring warm days, the mass ow rate through the turbine is decreased. Therefore, it causes a

*Correspondence to: Mohammad Ameri, Combined Heat & Power Specialized Unit (CHP), Energy EngineeringDepartment, Power & Water University of Technology, PO Box 16765-1719 Tehran, Islamic Republic of Iran.

yE-mail: ameri [email protected]

Received 29 April 2006Revised 28 December 2006Accepted 31 January 2007Copyright # 2007 John Wiley & Sons, Ltd.

drop in the output power. Moreover, the compressor work increases due to the divergence ofconstant pressure lines in TS diagram (Figure 1). On the other hand, the compressor nalpressure decreases. Given the fact that the turbine inlet temperature is constant, it will reducethe turbine work. As a result the net output of gas turbine falls. Moreover, there is a peakdemand of electricity in summer. Therefore, the gas turbine inlet air cooling is one of the usefulmethods which can be applied for the gas turbine power enhancement.Kraneis et al. (2000) studied the eects of an evaporative cooler on the available power plant

capacity with a detailed outline of the climatic conditions prevailing on the various continents.Nixdorf et al. (2002) investigated the economic benets of some dierent ambient airconditioning methods for reducing the gas turbine intake air temperature in order to enhancethe gas turbine power. Johnson (1998) discussed the theory of evaporative cooling and explainedthe evaporative cooler design, installation, operation, feed water quality, and the causes andprevention of water carry over. Kakaras et al. presented a computer simulation of theintegration of an evaporative cooler and of the air-cooling system and discussed the eect ofambient air temperature variation on the power output and eciency (Kakaras, 2004). Ameriet al. (2004) have studied the installation of fog inlet air cooling system for six Frame-5 (25MW)gas turbines. The results of that study showed that the output power of each gas turbine wasincreased by 3MW. Also, they showed that the fog system was very cheap in comparison withthe installation of new gas turbines. McNeilly (1997) presented a new method for the testcorrection error of evaporative coolers. Ameri et al. (2004) presented a good overview of anintake air-cooling system that used a steam absorption chiller and an air cooler to increase thepower output of Frame-6 gas turbines.The fog and media evaporative inlet cooling are the most economical alternatives for hot and

dry areas. In fact the other techniques such as the absorption and vapour compression chiller

Figure 1. TS diagram for a hot day.

M. AMERI, H. R. SHAHBAZIAN AND M. NABIZADEH1484

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2007; 31:14831503DOI: 10.1002/er

methods are not suitable for that ambient condition as they are very expensive alternatives. Thepurpose of this paper is to present the state-of-the-art application of gas turbine evaporativeinlet air cooling systems for three large gas turbine power plants (Shahid Rajaie, Ghom andFars) in Irans electrical grid and to compare the technical and economical aspects of fog andmedia inlet air cooling systems. Although one may nd various papers in literature regarding thetopic, however, there is neither many reliable actual power plant test data available for the largegas turbines nor there exist many unbiased comparisons between dierent evaporative inlet aircooling systems. In fact, most of the similar results are given by commercial companies whichhave not been justied by an independent research. Each company claims that its system eitherfog or media is the best.Following the discussion on the dierent inlet air cooling methods, the actual performance

test results are presented. Finally, the economic benets of the dierent cooling systems areexplained.

2. GAS TURBINE INLET AIR COOLING METHODS

There are several inlet air cooling methods available for gas turbine power augmentation(Omidvar, 2001). They can be classied into three types:

(1) thermal energy storage systems;(2) refrigerated cooling system (utilizing absorption or mechanical refrigeration);(3) evaporative coolers (media and fog).

3. EVAPORATIVE COOLERS OVERVIEW

Using the evaporative coolers will cause the maximum reduction in inlet air temperature if theinlet air dry bulb temperature approaches the wet bulb temperature (relative humidity of 100%).This can take place for dry and hot weather. There are two types of evaporating cooling:

3.1. Media evaporating cooling

Media surfaces are exuous and consist of beehive-shaped cells which make up an evaporatingcooler. The inlet air can be cooled with surface evaporating by spraying water on these cells andhumidifying them. Increasing the contact area between water and air will cause the surfaceevaporation to be more and faster. The exuous and beehive-shaped cells increase the contactarea between water and air.The following important points should be considered for the selection of a media system:

(a) Pressure drop in this system is more than other evaporating cooling systems. However,this pressure drop has not much inuence on the gas turbine output.

(b) Power consumption of this system is less than other systems.(c) This system does not need demineralized water and can use the raw water. However, it is

better to use distilled water.(d) Since this system needs periodical replacement of media (every 3 or 4 years), its

maintenance cost is more than other systems.

COMPARISON OF EVAPORATIVE INLET AIR COOLING SYSTEMS 1485


(e) The installation cost of this system is more expensive.(f) If the system is installed in lter room, the shutdown period of the unit is considerable.

This system which uses a media surface to evaporate water is widely used for gas turbinesespecially in dry and hot areas.The performance of this system is based on water evaporation, which consumes thermal

energy by the latent heat of vaporization value and reduces the ambient temperature.Figure 2 shows the schematic of media evaporating cooling system. Media system equipment

includes piping, programmable logic controller (PLC) control system and measuring equipment,water distributing headers, media surfaces, mist eliminator, movable wall in the media case,circulation pump, water tank, and blow down system.

3.1.1. Media surfaces. The material of these surfaces is cellulose bre. A media evaporatingcooler, which consists of these surfaces, is like beehive. Figure 3 shows the mediasurface. Water is spread over the media area, so the ratio of vaporization area (m2) to themedia volume is increased and the air cooling is improved. These surfaces are coveredwith special chemical material to prevent corrosion. Since the material of media surfaces iscellulose, they are ammable. However, they can be made from bre glass surfaces which arenot ammable.Each of these surfaces is installed with a certain angle (the angles are 15 and 458 with respect

to the horizontal line) to increase the contact area between water and air (Figure 4). The airowis parallel to the horizontal plane (AAF Power & Industrial, 2002).

3.1.2. Media evaporating cooler function. In this system the water is pumped from the tankbelow the cooler to the distributing header above the cooler (Figure 2). This water is distributedover the surfaces and humidies them. The outlet water is gathered below the cooler anddrained to the water tank. These surfaces which are called media surfaces have a thicknessof about 20 cm or more and cover the entire cross section of inlet air duct or air room. Thecompressor inlet air passes through the media surfaces and evaporates the water to the

Figure 2. Media evaporating cooling system.



saturation limit. The remaining water is used for continuous purging and discharging ofthe other materials from the media surfaces. The amount of circulating water should be at least23 times of evaporated water.To prevent the removing of water droplets from media surfaces and damaging the compressor

blades due to the high velocity of air, the air velocity should be limited. The cooler air is passedthrough a mist eliminator after the evaporating cooler and the water droplets are removed.Figure 5 shows the reduction of saturation eciency of the evaporating cooler at high velocitiesfor Celdek media surfaces with various thicknesses (Munters Co., 2001). On the other hand asthe inlet air velocity increases the pressure drop in the system increases as well. Also Figure 5shows the pressure drop in Celdek media surfaces with various thicknesses versus the inlet airvelocity. Based on these facts, it is obvious that if the inlet air velocity is high, the evaporatingcooler saturation eciency is reduced and the pressure drop is increased. Therefore, a diuser isused at the inlet air duct of evaporative cooler to reduce the inlet air velocity. However, it shouldbe noted that the pressure drop is very small.

Figure 3. Evaporative cooler media surfaces (Munters Co., 2001)

Figure 4. Certain angles of media surface installation (AAF Power & Industrial, 2002).



3.2. Fog system

High-pressure fogging of gas turbine inlets has been applied for 15 years (Cyrus et al., 2000,2002). In essence, it generates droplets of sizes 520 mm which are injected into the air streamwhere they evaporate and provide air cooling. The test data have shown that this process can be100% eective (i.e. wet bulb temperature can be reached) even in humid regions. Fog isgenerated by the application of high-pressure demineralized water between 70 and 200 bar to anarray of specially designed fog nozzles. A typical fog system consists of a series of high-pressurepumps that are mounted on a skid, PLC-based control system with temperature and humiditysensors, and array of fog nozzles installed in the inlet air duct (Figure 6).The nozzle is usually made of 316 stainless steel (SS) and consists of a small orice from 127

to 178 mm for gas turbine applications. The water emanating from this orice impacts a speciallydesigned impaction pin that breaks up the jet into billions of micro ne fog droplets, whose sizesare between 10 and 40 mm. The rate of evaporation of the droplets essentially depends on thesurface area of water exposed to the air.Figure 7 shows the distribution of droplets diameter in a specic nozzle. Typically, the

plunger type pumps are used to produce 130200 bar pressures for gas turbine inlet air foggingsystems. They are positive displacement ceramic-plunger stainless steel pumps with stainlesssteel heads. The pumps can be turned on sequentially to control the amount of cooling. Forexample, 8.48C drop in temperature may be managed in three 2.88C increments.

3.2.1. Cooling system control. The control system incorporates a PLC, which is typicallymounted on the high-pressure pump skid. Sensors are provided to measure relative humidityand dry bulb temperature. Special programming codes use these measured parameters tocompute the ambient wet bulb temperature and the wet bulb depression (i.e. the dierencebetween the dry bulb and wet bulb temperatures). They quantify and control the amount ofevaporative cooling that is possible with the ambient conditions. The system turns on or o fogcooling stages to match the ability of the ambient conditions to absorb water vapour. Thecontrol system also monitors pump skid operating parameters such as water ow rates and

Figure 5. The media cooler saturation eciency and its pressure drop versus air velocity for dierent mediawidth (width of media surfaces are 75; 100; 150; 200 and 300mm) (Munters Co., 2001).



operating pressure, and provides alarms when these parameters are outside acceptable ranges.Table I indicates the typical cooling control system stages.

3.2.2. Fog nozzles position in inlet duct. There are two main options for installing the inletfogging system, i.e. locating them either upstream or downstream of the lters.

Figure 6. Schematic of fog inlet air cooling system.

Figure 7. Distribution of droplet diameter for a typical nozzle (Omidvar, 2001).



(a) Upstream of the inlet lters: One advantage of positioning the fog nozzle manifoldupstream of the air lters is that the installation can be accomplished without gas turbine outage.In this case, a fog droplet mist eliminator lter must be added downstream of the fog nozzlemanifold to remove any unevaporated droplet. By denition, the droplet lter would not allowany fog intercooling. Typically about half the water droplets by the fog nozzles is captured by thedroplet mist eliminator and drained away. This type of system while used on some earlyinstallations is rarely applied to gas turbine installations today. It requires more fog nozzlesand more water and it is generally more expensive to operate and install. However the turbineoperators, who have experienced excessive loading of inlet air lters, might nd this optiona cost-eective one.(b) Downstream of the inlet lters: The most common location for the high-pressure fog nozzle

manifold is downstream of the air lters and upstream of silencers and trash screens. Installationof the fog system in this location requires an outage of 12 days and calls for only minormodications to the turbine inlet structure. This type of installation allows fog intercooling.While the fog nozzle manifolds can be also installed downstream of the silencers, it is generallyconsidered best to locate them upstream of the silencers, as this would allow more residence timefor the fog droplets to evaporate. Fog nozzle manifolds are usually installed upstream of thetrash screens to avoid any possibility of foreign object damage (FOD).

4. COMPARISON OF MEDIA AND FOG INLET AIR COOLING SYSTEMS

Comparisons of media and fog system for cooling air of gas turbine are shown in Table II(Grance et al., 2001). Care should be taken for using these cooling systems as they may changethe performance curve of the gas turbine towards surge point. Moreover if the humidity of air ishigh, the power increase is limited due to the low eciency of the evaporative system. Therefore,the required cooling is not achieved.

5. THE EFFECT OF EVAPORATING COOLER ON THE COMPRESSOR INLET AIRCOOLING AND THE ENHANCEMENT OF GAS TURBINE OUTPUT POWER

The media and fog cooling systems enhance the output power by reducing the inlet airtemperature as follows:

(1) The density and the ow rate of air passing through the gas turbine are increasedby reducing the compressor inlet air temperature. Therefore, the output power isenhanced.

Table I. Typical fog cooling control stages (four stages of 2.88C each).

Time Dry bulb (8C) Wet bulb (8C) Dierence (8C) Stage ON Cooling (8C)

9:00 21.1 20 1.1 None }10:00 23.9 20.6 3.3 1 2.811:00 26.7 21.1 5.6 2 5.612:00 30 21.1 8.9 3 8.4



(2) The consumption power of compressor is decreased as its inlet air temperature is reduced(The consumption power of compressor is proportional to the inlet air temperaturedirectly). Therefore, the net power output of turbine increases.

(3) If the inlet air temperature is reduced, the exhaust ue gas temperature is reduced. This isdue to the fact that if all other parameters are kept constant, the energy balance causesthe ue gas temperature to reduce. Therefore, the error signal which is the dierencebetween exhaust gas temperature and its set point temperature is reduced. The turbinecontrol system increases the fuel ow rate and therefore the output power of gas turbineis enhanced. The ue gas temperature returns to its set point value.

6. THE CHARACTERISTICS OF GAS TURBINE UNITS AND INSTALLED COOLINGSYSTEMS FOR FARS, SHAHID RAJAIE AND GHOM POWER PLANTS

There are many gas turbines in Iran favoured for coping with the peak electricity demand of theutilities due to their specic properties. However due to the high ambient temperature in the hotseasons, the output power and eciency of gas turbines decrease considerably.The power output of the gas turbine is as low as 70% of its rated output when the

temperature increases in the summer and the electricity is most needed for air conditioning.Each of these facilities serves as a vital equipment for the Iran electricity grid. For existinginstallations the possible output enhancement can be determined by comparing the turbineoutput at the design conditions with the output recorded at the desired inlet air temperature.Essentially the inlet air cooling gives approximately winter performance during hot climaticconditions. Two Frame-9 (PG9171E) gas turbines with the rated output of 99.3MW in Fars,two Frame-9 (PG9171E) gas turbines with the rated power of 98MW in Shahid Rajaie and twogas turbines (701D) with the rated output of 100MW in Ghom power plants are selected forthe inlet air cooling system installation. Table III shows the characteristics of gas turbine unitsand installed cooling systems. The detailed specications of those gas turbines are given in thetechnical documents of Fars, Shahid Rajaie and Ghom combined cycle power plants and arepresented by Shahbazian and Hoseinzadeh (2004) and Nabizadeh and Keshtgar (2004). All gasturbines use the natural gas as their fuel. However, the heating values of their fuels are dierent.

Table II. Comparisons of media and fog system for cooling air of gas turbine (Grance et al., 2004).

Main parameter Media Fog

Eciency of humidifying (%) 8590 90100Capital cost (US$ kW1 installed) 4555 3545Quality of consumption water Raw water Fully demineralizedPressure drop (Pa) 200 Very lowDroplet diameter (mm) Less than 100 Less than 20Time of unit exit forinstallation (days)

57 12

Change of unit structure Need to change of lter room Installation in lter room

Another advantage Increasing the life of lter,reducing the amount of NOx

Ability to produce 100% humidity,lower consumption of water



7. TECHNICAL AND ECONOMIC EVALUATION OF APPLICATION OF MEDIAAND FOG COOLING SYSTEM FOR GAS TURBINE INLET AIR COOLING

7.1. Prediction of gas turbine power enhancement using evaporative cooling system performance atdierent ambient conditions

The evaporative cooling system is designed for a specic ambient condition. The design point isnot usually the maximum ambient temperature and minimum relative humidity temperature atwhich the evaporative cooler has the maximum capacity. In fact as the power enhancement ismost required at the peak load, the design point is selected either based on this point or thecombination of this point and the maximum capacity point. Although it would be useful to testthe system at the design ambient conditions, however, it is impossible due to practical reasons.Therefore, it is necessary to predict the evaporative cooler performance at the dierent ambientconditions. This is usually important to check the guarantee requirements. In this section of thepaper, a method is presented to predict the gas turbine power enhancement.The function of evaporating cooling system is cooling air by humidifying it in an adiabatic

process. During this process the wet bulb of air is constant. Figure 8 shows this process on thepsychometrics chart.The saturation (or humidifying) eciency of the system is dened as follows (Ameri et al.,

2004; AAF, 2002):

Zhumidifying T1db T2dbT1db T1wb

1

The amount of required water in this system is determined from the following equation:

mwater mairo2 o1 2

where mair is the inlet mass ow rate of dry air into the gas turbine.As an example, the maximum power recovery under Fars power plant condition is determined

as follows.At T1db 358C and relative humidity of j1 20%; the specic humidity is o1 0:008464:

Assuming maximum saturation eciency (i.e. 100%, the best conditions for the evaporatingcooling system performance) the minimum achievable temperature is the wet bulb temperature.Therefore, T2db T1wb 17:868C and the relative humidity and specic humidity are j1 100% and o2 0:015568; respectively. Since the altitude of this power plant is 1530m from thesea level, the above values should be corrected. According to the tables of altitude correction(ASHRAE, 1992), one can estimate Do to be 0.007104. The mass ow rate of inlet air to the

Table III. Characteristics of gas turbine units and installed cooling systems.

Power plant

No. ofcase study

units

Power output atsite conditions

(MW)

Min. & Max.Temperatures

(8C)Type of installedcooling system

Design conditionfor installed

cooling system

Fars 2 99.3 14! 43 Media 388C and j=20%Shahid Rajaie 2 98 10! 40 Fog 408C and j=10%Ghom 2 100 12! 45 Fog 458C and j=8%



compressor is 406 kg s1 under design condition. Therefore, the required water mass ow rate ismwater 2:88 kg s1: According to the above values and the diagram of gas turbine outputpower versus the inlet temperature for GE-F9 units of Fars power plant, the augmented power iscalculated (Shahbazian and Hoseinzadeh, 2004). The output power of gas turbine at the designpoint is 79.13MW and after the operation of media system it is estimated to be 90.18MW.Therefore, the maximum recovered power is 11.05MW or 13.96%. For other power plants anddierent ambient conditions the maximum power enhancement is determined by this method aswell (Nabizadeh and Keshtgar, 2004). It should be noted that the evaporative cooler eciency isestimated from the test results. Figure 9 shows the schematic for the prediction of gas turbinepower enhancement using the evaporative cooling system. The equations of evaporative coolersystem box are based on thermodynamical properties and evaporative cooler saturationeciency. The evaporative cooler eciency is calculated using the test point results. Theequations of gas turbine box are based on the characteristics curve of the specic gas turbine.A simple computer code has been prepared to calculate the temperature reduction and powerenhancement for dierent cases (Shahbazian and Hoseinzadeh, 2004; Nabizadeh and Keshtgar,2004).

7.2. Performance test results and discussion

Since the evaporative systems performance test is an important issue in the feasibility study, thecomplete performance tests have been done on all units. The main parameters in those tests were

Figure 8. Cooling process on the psychometric diagram.

Gas TurbineEvap.CoolerSystem

Ambient Pressure

Ambient Temperature

Relative Humidity

Comp. Inlet Tdb

Comp. Inlet Pressure

Comp. Inlet Twb Power Output

Exhaust Gas Temp.

Figure 9. The schematic for the prediction of gas turbine power enhancement by evaporative coolingsystem at dierent ambient conditions.



the increase in output power and also the temperature and the humidity ratio at the compressorinlet. Regarding these parameters, measuring points were specied. Since this system is used atdierent times both day and night (especially in the summer nights when the ambienttemperatures are high enough that it may make the use of evaporating cooling units necessary)the performance tests at night were also carried out to evaluate the performance of the units atdierent conditions.The correct functioning of sensors installed on PLC should be tested. Therefore, the

temperature and the air humidity data were also measured by another instrument and then theresults were compared to the data of PLC. The test procedure is as follows:

* Starting up gas turbines and turning their automatic control on, letting each unit to reachits full load (base load) (at this time 15-min waiting period was regarded for assurance thatthe system had reached the steady-state operating conditions).

* Recording parameters which were needed for performance test.* Turning on the cooling system and waiting for 20min until the operating conditions

approach steady-state conditions.* Controlling and recording necessary parameters.

Table IV presents the results of performance tests of media evaporative cooling system forunits 1 and 2 of Fars power plant in August 2004.Also, Tables V and VI present the results of performance tests of fogging cooling system for

units 1 and 2 of Shahid Rajaie and Ghom power plants in July and June 2004, respectively. Itshould be noted the maximum calibration error for the output power measurement is 0.1%.

Table IV. The performance test results of media for units 1, 2 of Fars power plant (August 2004)(Shahbazian and Hosseinzadeh, 2004).

GE (Frame-9) gas turbine unit 1 GE (Frame-9) gas turbine unit 2

Parameters

Beforeoperationof mediasystem

Afteroperationof mediasystem Variation

Percentvariation

Beforeoperationof mediasystem

Afteroperationof mediasystem Variation

Percentvariation

Ambienttemp. (8C)

38.17 38.27 0.103 0.033 38.37 38.60 0.23 0.074

Relativehumidity (%)

8.3 8.2 0.08 1 8.03 8.33 0.3 3.73

Ambientpressure (kPa)

83.85 83.82 0.023 0.028 83.95 83.90 0.05 0.06

Comp. inletair temp. (8C)

40.6 22.66 17.93 5.72 38 20 18 5.78

Comp. outputair temp. (8C)

371 347 24 3.73 373.66 348.66 25 3.87

Comp. outputair pressure(bar)

8.23 8.92 0.069 352 8.87 9.42 0.65 7.45

Exhaust gastemp. (8C)

559.6 549 10.66 1.22 551 540 11 1.33

Poweroutput (MW)

76.6 87.71 11.11 14.5 81.48 92.29 10.81 13.27



Figures 1014 show the gas turbine inlet air temperature reduction, power increase andoutput power prediction for the dierent ambient conditions using the media evaporative cooleror fog systems. The test point and the design points are also shown for reference. The maximumerror which is the dierence between the actual (i.e. test) measured power and the predictedpower value is estimated to be around 2%. According to Figure 10 for media evaporatingsystem in Fars power plant, the inlet temperature drop increases due to the air relative humiditydecrease and temperature increase. For example, if the ambient temperature is 388C and therelative humidity is 8%, the inlet air temperature is decreased by at least 178C using mediacooling system. Also according to Figures 11 and 12 the output power is enhanced by11.11MW. On the other hand, according to Figure 13 for fog cooling system in Ghom powerplant (for the ambient temperature of 338C and the relative humidity 16%) the inlet airtemperature is decreased by at least 168C. The output power is enhanced by 8.1MW (Figures 14and 15).It should be noted that the power augmentation are clearly the results at the time of testing. If

an entire summer season or an entire year is considered, the overall power production increasemay be estimated. Figure 16 presents the total power generation for those three power plants inthe summer of 2004. It shows that the maximum power enhancement for the Fars power plant is2970MWh which is much more than the other two power plants i.e. 1701MWh for Ghom and1340MWh for Shahid Rajaie power plants. It is 74.1 and 121.6% larger than those two otherpower plants. In fact although the media evaporative cooler eciency (85%) is generally a little

Table V. The performance test results of fog for units 1 & 2 of Shahid Rajaie power plant (July 2004)(Nabizadeh and Keshtgar, 2004).

GE (Frame-9) gas turbine unit 1 GE (Frame-9) gas turbine unit 2

Parameters

Beforeoperationof fogsystem

Afteroperationof fogsystem Variation

Percentvariation



Percentvariation

Ambienttemp. (8C)

35 35.33 0.33 0.107 34.66 35 0.33 0.107


11 11 0.0 0.0 11 11 0.0 0.0

Ambientpressure (mbar)

868.6 868.7 0.1 0.012 868.6 868.7 0.1 0.012


32.66 16 16.66 0.054 33 19 14 0.046


363.33 345 18.33 2.88 361.33 341.33 20 3.15

Comp. outputair pressure (bar)

8.823 9.303 0.48 5.44 8.75 9.26 0.51 5.86


558.33 550.66 7.66 0.92 560 552 8 0.96

Consumptionfuel rate(m3min1)

463.5 498.06 34 7.44 480.1 519.53 39.36 8.19

Poweroutput (MW)

86.55 95.03 8.48 9.79 83.02 92.22 9.2 11.08



bit less than the fog system (90%), however, the climate in the Fars is more favourable forinstallation of the evaporative cooling system. The high temperature, less humidity and morehot hours per each day are the main reasons for higher power augmentation at the Fars site.

Table VI. The performance test results of fog for units 1 & 2 of Ghom power plant (June 2004)(Nabizadeh and Keshtgar, 2004).

MW701D gas turbine unit 1 MW-701D gas turbine unit 2

Parameters



Percentvariation



Percentvariation

Ambienttemp. (8C)

31.96 32.66 0.7 0.23 32 33 1 0.33


16.66 15.66 1 6 16 16 0.0 0.00

Ambientpressure (kPa)

88.87 88.87 0.00 0.00 88.87 88.87 0.0 0.00


31.8 16.58 15.22 4.99 31.83 15.76 16.06 5.27


415.66 404.06 11 1.6 413.66 400.4 13.26 1.93

Comp. outputair pressure(kPa)

1080 1120 40 3.7 1068 1113.33 45.33 4.24


528.93 523.7 5.22 0.65 528.76 523.63 5.13 0.64

Fuel consumptionrate (kNm3 h1)

29.45 30.76 1.31 4.44 28.60 30.14 1.53 5.37

Poweroutput (MW)

94.93 100.53 5.60 5.89 90.1 98.2 8.1 8.99

Gas Turbine 1 in Fars Power Plant

5

8

11

14

17

20

23

26

15 20 25 30 35 40 45 50 55Inlet Air Temperature (C)

Tem

pera

ture

Dec

reas

e (C

)

=5%

=8.3%

=15%

=20%

=25%

=30%

Test Point

Design Point

Figure 10. Temperature decrease prediction using media system at various ambient conditions.



Moreover in order to compare the media and fog system performances, Figure 17 presents thetemperature decrease and percent of power augmentation prediction due to installation ofmedia and fog system at three power plants (i.e. Fars, Rajaie and Ghom) for dierent ambientconditions. It shows that the power augmentation for the media system (Fars) and the fogsystem (Ghom and Rajaie) are almost the same at the same ambient conditions. In fact theevaporative cooler eciencies for the media and fog system are almost the same. These resultsare quite dierent from the MEE Industries (2002) report (i.e. one of the major fog systemmanufacturer) for a Frame 7111EA gas turbine. In fact, MEE has assumed an eciency of8085% for the media evaporative cooling system and up to 100% eciency for the fog system.Therefore, MEE concluded that the percent of power boost attained by the use of fogger overmedia type cooler power was up to 2.2% for dierent ambient temperatures and humidityratios. The reason for this dierence can be due to the fact that for the fog system, the design of


4

6

8

10

12

14

16


Pow

er In

crea

se (M

W)

= 5%

= 8.3%

= 15%

= 20%

= 25%

= 30%

Test Point

Design Point

Figure 11. Power increase prediction using media system at various ambient conditions.

76

80

84

88

92

96

100

Pow

er (M

W)



=5%

=8.3%

=15%

=20%

=25%

=30%

Test Point

Design Point

Figure 12. Gas turbine output power prediction using media system at various ambient conditions.



the nozzle manifold, type of nozzle and inlet duct length from the manifold to the compressorinlet are the critical factors which aects the evaporative cooling system performance. In fact,our fog systems have obtained the eciency of 90% compared with the excellent MEE fogsystem with the eciency of up to 100%. Moreover, our media evaporative cooler system hasa very good design. Therefore, it has been able to achieve an eciency of approximately 90%(i.e. the same as our fog system eciency).

7.3. Economic evaluation and discussion

The cost of an inlet cooling system is often evaluated in terms of US$ kW1. This can bemisleading because the output enhancement as a result of inlet air cooling varies with the

= 5%

= 8%

Gas Turbine 2 in Ghom Power Plant

8

11

14

17

20

23

26

29


Tem

pera

ture

Dec

reas

e (C

)

=10%

=16%

=20%

=25%

Test Point

Design Point

Figure 13. Temperature decrease prediction using fog system at various ambient conditions.

Pow

er In

crea

se (M

W)

= 5%

= 8%

Gas Turbine 2 in Ghom Power Plant18

16

14

12

10

8

6

415 20 25 30 35 40 45 50 55

Inlet Air Temperature (C)

=10%

=16%

=20%

=25%

Test Point

Design Point

Figure 14. Power increase prediction using fog system at various ambient conditions.



ambient temperature. A better way of evaluating the economic feasibility of a cooling system isthrough cost-benet analysis in which the additional revenues are calculated as a result of costof electricity (COE) and rate of return (ROR) for additional MWh (Jones and Jacobs, 2002;Grance et al., 2001).The economic evaluation criterion may vary from one power producer to another. For some

producers, it may be revenues from enhanced capacity. For others, it may be the bonus formeeting or exceeding capacity or avoiding any penalties for not meeting capacity. All thesefactors contribute to total revenues and should be properly accounted for in economicevaluation.Table VII shows the capital cost for media and fog inlet air cooling system. They include the

initial installation investment cost, annual O&M cost, consumption water cost and consumptionfuel cost.

Pow

er (M

W)

=5%

=8%

Gas Turbine 2 in Ghom Power Plant108

105

102

99

96

93

9015 20 25 30 35 40 45 50 55

Inlet Air Temperature (C)

=10%

=16%

=20%

=25%

Test PointDesign Point

Test Point

Figure 15. Gas turbine output power prediction using fog system at various ambient conditions.

Figure 16. The comparison of gas turbine power augmentation (kWh) using the evaporative coolingsystems for the Fars, Ghom and Rajaie power plants during summer 2004.



For the economic calculations, the following assumptions have been made:

* The investment cost is assumed to be 55US$ kW1 and 45US$ kW1 for media and fogsystems, respectively.

0

5

10

15

20

25

30

Temperature Decrease (Deg C) 14.45 16.92 16.79 19.05 23.68 23.54 20.97 26.94 25.87Percent of Power Augmentation 10.63 11.81 9.98 16.93 18.46 16.39 18.59 20.44 17.93

Fars Rajaei Ghom Fars Rajaei Ghom Fars Rajaei Ghom

=10 %

=10 %T= 45C and

T= 40C and

T= 30C and

=5 %

Figure 17. The comparison of temperature decrease and percent of gas turbine poweraugmentation prediction for the media (Fars) and fog systems (Rajaie and Ghom)

installations at various ambient conditions.

Table VII. Final economical results for media evaporating system and fog system cases.

Case Fars Shahid Rajaie Ghom

Mean power increase (MW) 11 10 8Annual generated power due to using cooling systemfor 8 h per day for 4 months (kWhyear1)

10 912 000 9 920 000 7 936 000

Annual power decrease due to using cooling system(kWhyear1)

2 102 400 } }

Net power increase (kWyear1) 8 809 600 9 920 000 7 936 000Annual consumption of fuel due to using coolingsystem (kNm3 year1)

2281.6 2083.2 1517.7

Annual consumption of water due to using coolingsystem (m3 year1)

10277.1 7285.2 9285.1

Initial investment costs (US$) 605 000 450 000 360 000Annual O&M costs (US$ year1) 24 200 18 000 14 400Consumption fuel costs (US$ year1) 22 816 20 832 15 177Consumption water costs (US$ year1) 51 385 36 426 46 425

Payback period (year)Cost of electricity (COE)3Cents kWh1 6.14 3.80 4.524Cents kWh1 3.30 2.29 2.645Cents kWh1 2.26 1.67 1.88

Rate of return (ROR, %) 24.37 36.90 31.24



* The O&M cost is assumed to be 35% of initial investment cost.* The installation cost of one purication plant for production of demineralized water for

fog system with capacity 100m3 day1 is assumed to be 125 000US$ and cost of waterconsumption is assumed to be 5US$m3.

* The actual fuel consumption cost in Iran is 0.02US$ l1 for gas oil and 0.01US$m3 fornatural gas.

* The current electricity price in Iran is 0.04US$ kWh1.* For economical calculation we have considered 17% for domestic interest rate and 7% for

the foreign interest rate and 10 years for the equipment life.

Since the cooling system is operated in summer and under peak demand condition, its dailyoperation time is assumed to be 8 h a day (although its actual operation time was much lessthan this value due to some technical problems). If it is assumed that this system is operating for4 months per year (MaySeptember), the number of operation hours will be a 992 h year1.Also the power decrease in gas turbines power due to the pressure drop (200 Pa) of media

system is considered in economical calculation.Based on the economic analysis (Table VII), it is clear that both media evaporative and fog

systems are economical as their rates of return (24.3736%) are higher than the domestic andforeign rates of return (RORs) (i.e. 17 and 7%, respectively). However, the fog systems are moreeconomical in comparison with media evaporating system. There are some reasons for thisconclusion:

* The ROR for media and fog systems are more than domestic interest rate but the ROR forfog system is absolutely more than the ROR for media system.

* The capital cost for media system is more than the capital cost for fog system. Therefore,fog systems are more attractive than the media system at the rst view.

* The payback periods for fog system are shorter than media evaporating system for variouselectricity costs.

Therefore, based on the economic evaluation, the best alternatives for the gas turbine poweraugmentation can be both the fog and media inlet air cooling systems. However, one shouldnote that the media evaporating system is safer than the fog system due to the possibility of largefog droplets entering the compressor if the fog nozzles types and their orientations are notdesigned very well.

8. CONCLUSION

According to the feasibility study test results, using the media evaporating cooler in Farscombined cycle power plant increased the gas turbine output power by 11MW (or 14.5%).Using the fogging system in Shahid Rajaie and Ghom combined cycle power plants enhancedthe gas turbine output power by 9.2MW (or 11%) and 8.1MW (8.9%). However, the actualaverage power enhancement in the summer of 2004 was 11, 8.5 and 6.6MW for Fars, Ghom andShahid Rajaie power plants, respectively. The results reveal that the media evaporative coolereciency is the same as the fog evaporative system eciency. However due to the fact that theFars climate conditions are more favourable, it has generated more enhanced power.



The results of technical and economic evaluation show that using the evaporative systems ismore cost eective than using new gas turbines for generating more power. Therefore, thesesystems are very suitable for the dry central regions of Iran such as Fars, Ghom, Yazd, etc.where the air temperature is high and the humidity ratio is low.Also the capital costs are very cheap in comparison with the installation of the new gas

turbines (300US$ kW1). The payback period for the application of the evaporative systems isfrom 2 to 3 years.

NOMENCLATURE

mair =air mass ow rate (kg s1)

mwater =water consumption (kg s1)

Tdb =dry bulb temperature (8C)Twb =wet bulb temperature ( 8C)Zhumidifying =evaporative cooler saturation eciencyo =specic humidity (kgwater/kgdryair)

REFERENCES

AAF Power & Industrial. 2002. Gas Turbine Inlet Cooling. http://www.aantl.comAmeri M, Hejazi SH. 2004. The study of capacity enhancement of the Chabahar gas turbine installation using anabsorption chiller. Applied Thermal Engineering Journal 24(1):5968.

Ameri M, Keshtgar A, Nabati H. 2004. Gas turbine power augmentation using fog inlet air-cooling system. Proceedingsof the ASME 7th Biennial Conference on Engineering Systems Design and Analysis, vol. 1, Manchester, U.K., 2004;7378.

ASHRAE Handbook (IP edn). 1992. HVAC Systems & Equipment.Brook FJ. 1998. GE gas turbine performance characteristic. Report NO. GER-356H, GE power system.Cyrus B, Meher-Homji CB, Mee III TR. 2000. Inlet fogging of gas turbine engines, part A: theory, psychometrics andfog generation. Proceedings of the ASME Turbo Expo. 2000, Paper 2000GT-308, Munich.

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GE Energy. 2006. Library of Standard Gas Turbine. http://www.gepower.comGrance D, Boncimino G, Stenzel W. 2001. Combined cycle plant optimization studies. Proceedings of the JPGC01,

International Joint Power Generation Conference, The Netherlands, Paper No. JPGC2001/PWR-1930, New Orleans,Louisiana.

Johnson RS. 1998. Theory and operation of evaporative coolers for industrial gas turbine installations. Presented at TheGas Turbine and Aero Engine Congress, Amsterdam, Netherlands, ASME, 88-GT-41.

Jones C, Jacobs III JA. 2000. Economic and Technical Considerations for Combined-Cycle PerformanceEnhancementOptions, GE Power Systems, GER-4200, Schenectady.

Kakaras E, Doukelis S, Karellas S. 2004. Compressor intake-air cooling in gas turbine plants. Energy Journal (SpecialIssue) 29(1215):23472358.

Kraneis W. 2000. Increased importance of evaporative coolers for gas turbine and combined-cycle power plants. VGBPower Tech 80(9):2225.

McNeilly JD. 1997. Test correction error for evaporative cooler equipped gas turbine power plants. Proceedings of the1997 International Joint Power Generation Conference, Denver, U.S.A., Part 2 (of 2), vol. 32, 399406.

MEE Industries. 2002. Economic benets of replacing gas turbine media based evaporative cooling with inlet foggingsystems. AN-GT-205, MeeFog Technical Application Note, Mee Industries Inc.

Munters Co. 2001. CELdek 7060-15 Evaporative Cooling Pad. http://www.munters.comNabizadeh M, Keshtgar AR. 2004. The study of fog inlet air cooling system for the output power enhancementof gas turbines. B.Sc. Thesis, Energy Engineering Department, Power & Water University of Technology,Tehran, Iran.



Nixdorf M, Prelipceanu A, Hein D. 2002. Thermo-economic analysis of inlet air conditioning methods of a cogenerationgas turbine plant. Proceedings of the ASME TURBO EXPO 2002: Ceramics, Industrial and Cogeneration Structuresand Dynamics, Amsterdam, Netherlands, IGTI, vol. 4 A, 403412.

Omidvar B. 2001. Gas turbine inlet air cooling system. The 3rd Annual Australian Gas Turbine Conference, Melbourne,Australia.

Shahbazian HR, Hoseinzadeh H. 2004. The study of media evaporating cooler for the output power enhancementof fars gas turbines. B.Sc. Thesis, Energy Engineering Department, Power & Water University of Technology,Tehran, Iran.



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Comparison of evaporative inlet air cooling systems to enhance the gas turbine generated power

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