performance improvement of a gas turbine cycle by using a desiccant-based evaporative cooling system

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Energy 31 (2006) 2652–2664

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Performance improvement of a gas turbine cycle by using adesiccant-based evaporative cooling system

Amir Abbas Zadpoor�, Ali Hamedani Golshan

Department of Mechanical Engineering, Iran University of Science and Technology, Narmak, Tehran 16844, Iran

Received 6 October 2005

Abstract

This paper focuses on power augmentation of a typical gas turbine cycle by using a desiccant-based evaporative cooling

system. This technique requires a desiccant-based dehumidifying process be used to direct the air through an evaporative

cooler, which could be either media-based or spray type. This could assist the evaporative cooling cycle to make necessary

adjustment for any possible installation defects in a hot and humid climate. We make a comparison between performance

improvement achieved by this technique and those of other evaporative cooling systems in different climatic conditions.

We will show that our proposed technique, at least for hot and humid climates, is more effective than other evaporative

cooling techniques.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Power augmentation; Gas turbine; Desiccant cooling; Evaporative cooling

1. Introduction

Several gas turbines are being widely used for power generation in several countries all over the world.Obviously, many of these countries have a wide range of climatic conditions, which impact the performance ofgas turbines.

Problems rise when a gas turbine is used in a geographic location with hot summers. Hot inlet air results in agas turbine’s generating less power, during summer season, when the demand for electricity is possibly higher.In such conditions, power augmentation techniques are highly desirable. Indeed, a little increment of thermalefficiency could result in a significant amount of fuel being saved and a higher level of power being generated.The simplest remedy to this problem is to reduce the temperature of the inlet air. Several different inlet coolingmethods are currently employed in various systems.

As Boyce [1] discusses, power augmentation methods, which could be applied to existing gas turbines, canbe divided into two main categories. The first category includes inlet air cooling techniques and the secondinvolves techniques based on the injection of compressed air, steam, or water. Since our objective, in thispaper, is to study inlet air cooling techniques, we will only review techniques employed in the first category.

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ess: [email protected] (A.A. Zadpoor).

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Nomenclature

ZThermal thermal efficiency of the gas turbine cycle (%)mf1 ; mf2 fuel flow of the first and the second combustor (kg/s)ma air mass flow rate (kg/s)WTurb:I; WTurb:II produced work of the first and the second turbine (W)ZTurb:I; ZTurb:II efficiency of the first and the second turbine (%)WComp:I; WComp:II consumed work of the first and the second compressor (W)ZComp:I; ZComp:II efficiency of the first and the second compressor (%)Q1, Q2 input heat of the first and the second combustor (W)ZCmbst:I; ZCmbst:II efficiency of the first and the second combustor (%)Wnet net produced work of the gas turbine cycle (W)Qin sum of inputted heat (W)LHVI low heat value of the first combustor’s fuel (kJ/kg)LHVII low heat value of the second combustor’s fuel (kJ/kg)Cp isobaric specific heat of humid air (kJ/kgK)T absolute temperature (K)Zi indirect evaporative cooling effectiveness (%)Zd direct evaporative cooling effectiveness (%)Tdb;i dry bulb temperature of evaporative cooler’s inlet air (K)Twb;i wet bulb temperature of evaporative cooler’s inlet air (K)Tdb;o dry bulb temperature of evaporative cooler’s outlet air (K)Twb;o wet bulb temperature of evaporative cooler’s outlet air (K)hi enthalpy of evaporative cooler’s entering air (kJ/kg)ho enthalpy of evaporative cooler’s exiting air (kJ/kg)wi humidity ratio of evaporative cooler’s entering air (kg/kg)wo humidity ratio of evaporative cooler’s exiting air (kg/kg)Tc combustion temperature (K)P combustion pressure (atm)w humidity ratio (kg/kg)Cair isobaric specific heat of dry air (kJ/kgK)Cvapor isobaric specific heat of water vapor (kJ/kgK)Ti absolute temperature of stage ith of the cycle (see Fig. 1) (K)Pi pressure of stage ith of the cycle (see Fig. 1) (Pa)n1 ratio of pressure of the first turbine’s inlet to the ambient pressuren2 ratio of pressure of the first turbine’s inlet to the second turbine’s inlet pressureZRegenerator regenerator’s effectiveness (%)ZIntercooler intercooler’s effectiveness (%)k ratio of specific heatsh enthalpy (kJ/kg)Tw absolute temperature of the intercooler’s water (K)TCmbst:I outlet temperature of the first combustor (K)TCmbst:II outlet temperature of the second combustor (K)

A.A. Zadpoor, A.H. Golshan / Energy 31 (2006) 2652–2664 2653

1.1. Evaporative cooling methods

Evaporative methods are among the most widely used power augmentation techniques. This is primarilybecause the machinery is cheaper, and the installation and operating costs are also lower. These methods andrelated technical issues have been subject of several studies [2–10]. Evaporative coolers are divided into twomain subcategories: the first subcategory includes media-based methods in which the inlet air passes through a

ARTICLE IN PRESSA.A. Zadpoor, A.H. Golshan / Energy 31 (2006) 2652–26642654

wet media causing the water to evaporate. The evaporating water needs to absorb its evaporation enthalpy,and the absorbed enthalpy decreases the dry bulb temperature of the air. The humidity ratio is increased whilethe enthalpy remains constant. Fogging is another evaporative cooling method in which demineralized wateris converted to the fog by means of high pressure nozzles. This fog cools the air down in a manner similar tothe previous method. Evaporative cooling techniques are most effective in hot and dry climates but not soeffective in humid climates.

This paper suggests that a desiccant-based dehumidifying system can be used for absorption of air humiditybefore the air is passed to the evaporative cooler. Absorption of water by desiccant causes the dry bulbtemperature of the air to increase. Then, the air is cooled by using an evaporation-based system. Addition ofthe desiccant-based system improves capabilities of the evaporative cooler making it suitable even for hot andhumid climates. Desiccant cooling systems are currently used in air-conditioning systems [11–15] and areproven to be effective and practical. Commercialized versions of desiccant cooling systems are currently mass-produced and studies are being conducted to development of more effective systems.

1.2. Refrigerated inlet cooling systems

Refrigerated inlet air cooling systems are more effective than evaporative cooling systems; because air drybulb temperature is lower in these systems. However, the price of the machinery, and the installation, andoperating costs are much higher. Two main subcategories of refrigerated cooling systems are mechanicalrefrigeration and absorption cooling. In mechanical refrigeration, a centrifugal, screw, or reciprocatingcompressor is utilized for compression of refrigerant vapor. These systems have extremely high powerconsumption and so many auxiliary equipments such as heat exchangers, pumps, compressors, and expansionvalves are also needed. Chlorofluorocarbon refrigerants are normally used in these systems. These systemscause certain environmental problems too. In addition to environmental issues, high power consumption, highcapital and maintenance cost, and poor part load performance are other deficiencies of mechanicalrefrigeration systems. For a literature review of mechanical refrigeration systems see [16].

Absorption chillers use the heat provided by gas, steam, or gas turbine’s exhaust for cooling the water whichacts as refrigerant. Lithium bromide is used as absorber in these systems. Part load performance of thesesystems, in comparison with mechanical refrigeration systems, is fairly good. Some researchers have recentlyconducted studies dealing with the absorption cooling systems [17,18].

Depending on the specifics of the project, a combination of evaporative and refrigerating cooling systemsmight be the best choice. Possibility of such combination should be studied prior to selection of any particulartype of inlet cooling system.

1.3. Thermal energy storage systems

In these systems, extra power of off-peak hours is used for generating ice pieces to be used in peak hours.The inlet air is channeled through a path where it comes into contact with these ice pieces. This causes the inletair to cool down. There are some problems associated with these systems, one of which is the need forauxiliary ice generating equipment and large insulated spaces for stocking of the ice.

There are other procedures for cooling off the inlet air which can be found in the literature on this topic[19–21].

This paper studies power augmentation of a gas turbine by using a desiccant-based evaporative cooling(DBEC) system. A code was developed for thermodynamic simulation of a typical gas turbine cycle. The codewas used for the computation of indicators of thermodynamic performance of the gas turbine cycle and itsNOx emissions. The addition of a DBEC system was also suggested. This system consisted of three stages.These stages were a desiccant wheel accompanied by one direct evaporative cooler (DEC) and one indirectevaporative cooler (IEC). A comparison was made between the performances of the gas turbine when no inletcooling technique was applied and when different versions of evaporative cooler were employed. Differentclimatic conditions, each represented by a single town, were considered and the effect of the inlet coolingtechniques on performance improvement of the gas turbine cycle was simulated. It was shown that the DBECsystem can improve performance of the gas turbine especially in hot and humid climates.

ARTICLE IN PRESSA.A. Zadpoor, A.H. Golshan / Energy 31 (2006) 2652–2664 2655

2. Gas turbine model

A typical gas turbine was used for evaluation of several inlet cooling techniques. Fig. 1 depicts a schematicof the gas turbine model. Two compressors, two turbines, one regenerator, one compressor intercooler, andtwo combustors are included in this model. Our assumption was that the traveling air can be approximated byideal gas relations. Using relations of classical thermodynamic, we could calculate the governing equations ofthe cycle. There are some design variables by which the cycle can be solved. A code was developed forsimulation of the gas turbine cycle. Numerical integration techniques, approximate experimental correlations,iterative computations, and classic thermodynamic relations were all used to simulate this typical gas turbine(see Appendix A for thermodynamic relations used in the code). In addition to design parameters, one shouldintroduce the inlet air conditions. This gas turbine thermal performance simulation program (GTTPSP) willcompute temperatures, pressures, gross generated power, compressor work, net work, cycle’s thermalefficiency, and intermediate values of state variables. This program along with some other codes was used tostudy the effect of different inlet air conditions on thermal performance of the gas turbine cycle. The othercodes produced conditions of the inlet air for GTTPSP.

An empirical correlation was used to estimate NOx emission of the gas turbine. Several other correlationsare available for predicting other pollutants such as CO, UHC, and smoke but they tend to be less reliable;therefore, they were not used. According to Lewis [22], NOx emission of gas turbine could be predicted byfollowing relation:

NOx ¼ 3:32� 10�6 e0:008T cffiffiffiffiPp

ppmv: (1)

This empirical equation was used in the aforementioned code for prediction of the gas turbine’s NOx

emission in terms of g/kWh. Conversion form unit ppmv to unit grams per kilogram of fuel (EI), which shouldbe used in computation of the NOx emission in terms of g/kWh, cannot be undertaken unless the equivalenceratio is known; however, as a rough guide, 1 EI is equivalent to around 12 ppmv [23]. This equivalence ratiowas used for conversion from ppmv to EI.

As previously stated, performance of the gas turbine is dependent on thermodynamics properties of the inletair. It was assumed that specific heat of the traveling air is a function of the specific heat of the dry air, thespecific heat of the water vapor, and the humidity ratio. This function could be stated as follows:

Cp ¼ Cair þ wCvapor. (2)

In order to have a qualitative understanding of the gas turbine’s performance, the code was used forproducing two graphs in which performance indicators of the gas turbine were plotted vs. the inlet airproperties. Fig. 2 depicts these indicators vs. the inlet air temperature. In computing values of this figure, theinlet air was assumed to be dry, i.e. w ¼ 0, and other parameters were fixed as per ISO. Fig. 3 is to depict how

Intercooler

Comp. I Comp. II Turb. I Turb. II

Cmbst. I Cmbst. II

Regenerator9

1

2 3

4

5

6 7

1

8

Fig. 1. Schematic representation of the typical gas turbine cycle.


the gas turbine’s performance indicators change as a result of variations in the inlet air humidity. Otherthermodynamics parameters of inlet air were as per ISO. All values were normalized with respect to ISOconditions in both of Figs. 2 and 3. It could be seen in Fig. 2 that all performance indicators of the gasturbine cycle were worse for higher values of the inlet air temperature. Specifically, output power andthermal efficiency were decreased and specific NOx emission was increased by increasing the temperature ofthe inlet air.

In Fig. 3, it could be seen that mass of the flow-through air decreased as the humidity ratio increased;however, the output power increased. Thermal efficiency increased and specific NOx emission decreased byincreasing the humidity ratio. However, the rates of the change of these two indicators were smaller than therate of the change in output power.

It was concluded that dry bulb temperature and humidity ratio have different effects on the thermalperformance of the gas turbine cycle. This fact is more important when dealing with desiccant cooling systems.Simulations had to be carried out to see which effect is predominant when a desiccant cooling system is beingused for cooling of the inlet air. These simulations were carried out and are discussed in following sections.

5 10 15 20 25 30 35 40 45 500.85

0.9

0.95

1

1.05

Inlet Air Temperature, C

Output PowerThermal EfficiencyNOx EmissionAir Mass

Fig. 2. Gas turbine cycle dependency on inlet air temperature—dry air.

1 2 3 4 5 6 7 8 9 10 11x10-3

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

Humidity Ratio, kg/kg

Output PowerThermal EfficiencyNOx EmissionAir Mass

Fig. 3. Gas turbine cycle dependency on humidity of the inlet air.


3. Desiccant cooling system

Materials that absorb and hold water vapor are called desiccant materials. Commercial desiccants absorband release large amounts of water vapor depending on moisture available in their environment. The processof absorbing moisture in the desiccant material is classified as either absorption or adsorption depending onwhether the material goes through a chemical or a physical change. Absorbing materials require strictlycareful precautions during storage and operation especially in warm and humid environments and are notcommercially used. Desiccant wheels are normally made of adsorptive materials such as silica gel, activatedalumina, lithium chloride, lithium bromide, etc. A structure, in which the substance is deposited, supportsadsorptive material and a honeycomb-like pattern is formed.

A desiccant cooling system is a system utilizing a desiccant wheel to remove humidity from the ambient air.The resulted dry air is hot due to the latent heat of dehumidification and must be brought back to a lower drybulb temperature by allowing the excess heat to escape. This is done, in our study, by using an IEC. Then, theair can be cooled by a DEC in which air becomes re-humidified by spraying water.

To ensure the continuous operation of the plant, it is necessary to regenerate the desiccant material.Regeneration of the desiccant is by heating in an unsaturated air stream. After drying, it should be cooled sothat it will be able to adsorb the moisture again. The regeneration of the air, in our system, is performed byusing the exhaust air of the gas turbine. In combined cycle systems, steam can be utilized for regenerationpurpose. Furthermore, because of the fact that inlet cooling systems are used only in summers, it is alsopossible to use solar assisted desiccant cooling systems in which regeneration is by means of solar energy [24].

As previously mentioned, a rotary desiccant wheel partitioned into two sections is employed in the soliddesiccant systems. Normally, the processed air sheds off its moisture to the desiccant through one of thesesections. Hot air is passed through the other section to regenerate the desiccant and to maintain the plant’soperation. The processed air is then cooled by using DES and/or IEC. Desiccant wheels are commerciallyavailable from several manufactures all over the world and are currently used in commercial systems alreadyshipping and running here and there. For underlying theories of adsorption see [25–28]. In most systems, awheel that contains a desiccant turns slowly to pick up humidity from inlet air and discharge it to the outdoorsthrough the regeneration air, which warms the desiccant up and removes its moisture. Conditions of the airexiting the desiccant wheel can be determined by solving the governing equations or by use of the performancecurves supplied by wheels manufactures. Novelaire Technologies [29] desiccant wheel simulation software wasused in our study for determining the conditions of the wheel’s exiting air.

Desiccant W

heel

Indirect Evaporative

Cooling

Direct E

vaporative Cooling

Water from and to Cooling Tower

Outdoor Air

Regeneration Air

Gas Turbine Inlet

Fig. 4. Schematics of the desiccant-based evaporative cooling system. This system was consisted of indirect evaporative, direct

evaporative, and desiccant wheel stages. Functionality of desiccant wheel is maintained by using the regeneration air.


Fig. 4 gives a schematic of DBEC system proposed and studied in this paper. Two path lines are depicted inthis figure. While solid lines demonstrate paths of the air, dashed lines are for water paths. Desiccant wheelrotates by a low rotational speed. Entering air is first passed through the wheel. Consequently, its dry bulbtemperature increases to a significantly higher value while its humidity diminishes drastically. The air is, then,passed through a cooling coil. Circulating water of this cooling coil comes from cooling tower and runs backto it. The dry bulb temperature of the cooling tower’s exiting water is close to wet bulb temperature of theinstallation site. IEC performs a sensible cooling on the traveling air, and conditions of the air after passingthrough it could be computed as follows:

Tdb;o ¼ Tdb;i � ZiðTdb;i � Twb;iÞ,

wo ¼ wi. ð3Þ

After indirect cooling, dry bulb temperature of the air is decreased but its humidity does not change.The third stage is the DEC in which water is sprayed into the traveling air. Spraying of water causes dry

bulb temperature of the air to decrease. The humidity ratio, however, increases. DEC is completely similar toadiabatic saturation process and preserves the wet bulb and the enthalpy of the processing air. But, the drybulb temperature of the air is reduced because sensible enthalpy is replaced by latent enthalpy. Thus,conditions of air after passing through DEC process could be computed as follows:

Tdb;o ¼ Tdb;i � ZdðTdb;i � Twb;iÞ,

Twb;o ¼ Twb;i,

ho ¼ hi. ð4Þ

In this paper, the effectiveness of the IEC was assumed 70% while the effectiveness of the DEC wasassumed 90%.

The air is, then, directed to the gas turbine inlet. In order to maintain functionality of the desiccant wheel,some amount of the regeneration air should be passed through upper half of the desiccant wheel. Theregeneration air must be hot. Exhausting air of the gas turbine can be used for heating of the regeneration air.

4. Simulation results

Simulations were carried out for three different geographic locations of Iran: Siri, a hot and humid island;Chabahar, a hot and moderately humid town; and Qom, a hot and dry town. Table 1 gives climatic conditionsof these locations. Four different cases were studied for each location: simple gas turbine cycle (default), DECapplied gas turbine, indirect and direct evaporative cooling (IDEC) applied gas turbine, and DBEC appliedgas turbine. Table 2 summarizes design variables of the gas turbine cycle for which simulations were carriedout. Table 3 summarizes specifications of desiccant wheel used for simulation of the desiccant-based system.Conditions of the inlet air for all cases were entered to GTTPSP and results of simulation were presented inTables 4–6. Table 4 summarizes simulation results for Qom. Tables 5 and 6 do the same for Chabahr and Siri.These tables give thermal conditions of simulated gas turbine cycle as well as predicted NOx emission. Three

Table 1

Climatic conditions of geographic locations

ISO standard Qom Chabahar Siri

Dry bulb (1C) 15 42.2 40 37.22

Wet bulb (1C) 10.8 22.7 32.2 32.22

Altitude (m) 0 918 6.1 17.06

Relative humidity (%) 60 19.70 58.41 70.86

Humidity ratio (kg/kg) 0.00637 0.01145 0.02777 0.02908

Dew point (1C) 7.27 14.24 30.32 31.07

Specific volume (m3/kg) 0.82 1.02 0.93 0.92

Enthalpy (kJ/kg) 31.11 71.71 111.46 111.91

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Table 2

Gas turbine cycle design data

Inlet air volume (m3/s) 10 Intercooler’s effectiveness (%) 70

LHV (kJ/kg) 43,000 First compressor’s efficiency (%) 85

Ratio of pressure of the high-pressure turbine to

ambient pressure

20 Second compressor’s efficiency (%) 85

Ratio of pressure of the high-pressure turbine to the

low-pressure turbine

2 First turbine’s efficiency (%) 83

First combustor’s outlet temperature (1C) 1600 Second turbine’s efficiency (%) 83

Second combustor’s outlet temperature (1C) 1500 First combustor’s efficiency (%) 87

Temperature of the intercooling fluid (1C) 22 Second combustor’s efficiency (%) 87

Regenerator’s effectiveness (%) 95

Table 3

Desiccant wheel design data

Desiccant media Wound silica gel Hub diameter (m) 0.254

Regeneration/process air volume ratio 0.333 Cassette height (m) 3.3528

Wheel diameter (m) 3.0505 Cassette width (m) 3.3528

Wheel depth (m) 0.20 Cassette depth (m) 0.45212

Regeneration portion (%) 25 Heater outlet temperature (1C) 100

Wheel speed (rph) 24 Process side face velocity (m/s) 1.823

Regeneration side face velocity (m/s) 1.823 Air flow (m3/s) 10


first rows of each table specify conditions of the inlet air and, thus, demonstrate how effective each coolingtechnique was for that location. Table 6 gives performance indicators of the gas turbine cycle for ISOconditions. These results were used for comparison purposes.

One could see that the gas turbine cycle’s performance was improved for all the inlet air cooling techniques.However, magnitude of the improvement was not the same. It depends not only on the cooling technique butalso on climatic conditions of the installation site. Several conclusions were made from simulation results ofTables 4–6. First, regardless of the evaporative cooling technique being used, the performance improvementwas better for dry climate. Second, it became clear that the smaller the site altitude, the greater the producedwork. It confirms what we mentioned previously, namely that smaller altitude means greater ambient pressureand greater ambient pressure results in the greater air density. Since produced work of the gas turbine isdependent on mass flow rate of the air, increment of inlet mass flow rate causes the produced work to beimproved.

Tables 4–6 showed that the net produced work is increased by using DEC. It improves by employing IDEC.The results are even better for desiccant-based inlet air cooling. By application of inlet cooling techniques,NOx emission was also improved. Thermal efficiency behaved like output power except from IDEC to DBECfor Qom. For power, simulation results showed that desiccant-based cooling is better than other coolingtechniques. But, it does not necessarily hold true for emissions. For example, NOx emission for IDEC is insome cases better than desiccant-based cooling technique (see Table 4 and 5).

One can see that in Qom, a dry location, addition of a desiccant wheel to the IDEC improves theperformance slightly. Besides, we observed that thermal efficiency decreases. But, Table 5 shows a largerimprovement of the produced power from IDEC to DBEC. Thermal efficiency and specific NOx emission werealso improved. These simulations show that desiccant-based cooling has a little advantage over IDEC for dryclimates. Thus, it is not feasible to use desiccant wheel for such climates. Instead, the desiccant wheel has animproving effect on the gas turbine cycle in humid climates. Therefore, it can be feasible to implement such asystem in the inlet of the gas turbines installed in humid climates.

In hot and dry climate, i.e. Qom, it was seen that the produced work of the gas turbine cycle was increasedby 8.45% for DEC, by 10.54% for DEC and IEC and by 10.57% for DBEC. Drop in dry bulb temperature

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Table 4

Qom conditions gas turbine cycle performance

Default standard DEC IDEC Desiccant

Dry bulb (1C) 42.2 24.65 19.91 20.05

Wet bulb (1C) 22.7 22.70 18.95 18.66

Altitude (m) 918 918 918 918

Relative humidity (%) 19.70 85.29 91.81 88.27


Dew point (1C) 14.24 21.96 18.45 18.04


Specific NOx (g/kWh) 0.8221 0.8063 0.8025 0.8032

Thermal efficiency (%) 65.8879 67.1730 67.4933 67.4812

Air mass flow rate (kg/s) 9.8390 10.3000 10.5277 10.5443

Net produced work (W) 8.6751e6 9.4087e6 9.5896e6 9.5927e+006

Power output improvement (%) NA 8.45 10.54 10.57

Table 5

Chabahar conditions gas turbine cycle performance


Dry bulb (1C) 40 32.98 31.48 31.13

Wet bulb (1C) 32.2 32.2 31.14 30.17

Altitude (m) 6.1 6.1 6.1 6.1



Dew point (1C) 30.32 32.09 31.11 29.95





Net produced work (W) 9.8709e6 1.0197e7 1.0252e7 1.0261e7


Table 6

Siri conditions gas turbine cycle performance


Dry bulb (1C) 37.22 32.72 31.76 31.30

Wet bulb (1C) 32.22 32.22 31.55 30.41

Altitude (m) 17.06 17.06 17.06 17.06



Dew point (1C) 31.07 32.18 31.56 30.21





Net produced work (W) 9.9895e6 1.0199e7 1.0235e7 1.0242e7


A.A. Zadpoor, A.H. Golshan / Energy 31 (2006) 2652–26642660

was 17.55 1C for DEC technique. This is in agreement with Alhazmy and Najjar’s [16] findings. In hot andmoderately humid conditions, i.e. Chabahar, output power was increased by 3.30% for DEC, by 3.86% forDEC and IEC, and by 3.95% for DBEC. In hot humid climate, i.e. Siri, it was seen that output power wasincreased by 2.1% and dry bulb temperature of inlet air was dropped by 4.5 1C for DEC. These results, too,

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Table 7

ISO conditions gas turbine cycle performance

Default standard

Dry bulb (1C) 15

Wet bulb (1C) 10.8

Altitude (m) 0

Relative humidity (%) 60

Humidity ratio (kg/kg) 0.00637

Dew point (1C) 7.27

Specific volume (m3/kg) 0.82

Specific NOx (g/kWh) 0.8445

Thermal efficiency (%) 67.8050

Air mass flow rate (kg/s) 12.1316

Net produced work (W) 1.0902e7


agree with Alhazmy and Najjar’s [16] findings in which a 1.95% improvement in output power and 3.95 1Cdrop in dry bulb temperature is reported for DEC applied in hot and humid climate. Improvement of theoutput power was 2.45% for IDEC. Application of the DBEC increased the output power by 2.52%. For dryclimates, there was only a little difference between simulation results of IDEC and those of the desiccant-basedcooling system. Furthermore, thermal efficiency of the gas turbine was decreased slightly from 67.4933% to67.4812% by application of DBEC. In humid climates, instead, desiccant cooling showed a largerimprovement in the output power comparing with the two other techniques. Thermal efficiency of the gasturbine was increased from 66.7337% to 66.7495% for hot and moderately humid climate and from 66.7174%to 66.7395% for hot and humid climate.

Pressure drop due to existence of the desiccant wheel in process and the regeneration paths was computedbetween 154 and 214Pa. Simulations showed that there existed only a little dependency on parameters of thedesiccant wheel. Although conditions of the exiting air for different wheels with different design parameterswere different, after indirect and direct cooling, the condition indicators became very close. Thus, designparameters of desiccant wheel did not have a significant affect on the performance of the DBEC system.Several design parameters were examined in this dependency analysis including regeneration portion, heateroutlet temperature, entering air conditions, etc. The same adsorptive material was used in all simulations ofthe parameter dependency analysis.

Energy consumed in desiccant-based evaporative cooler is comparable with other evaporative coolers andremains significantly below what is consumed in refrigeration systems such as vapor compression andabsorption chiller systems. However, initial investment is much higher comparing with other evaporativecooling systems but comparable with initial payload of refrigeration systems. Maintenance problems are lessfor desiccant-based systems as they normally have fewer moving parts. Comparing two last columns of Table4 with Table 7, we may conclude that thermal efficiency of the inlet air cooling applied gas turbine in Qom iscompletely close to ISO conditions. This confirms the fact that evaporative cooling systems work much betterin dry climates. However, it should be noted that utilization of evaporation-based cooling techniques isdifficult by the fact that in some dry climates it is difficult to find enough water resources for operation ofcooling apparatus.

5. Conclusions

Application of DBEC systems for the inlet air cooling of gas turbines was studied in this paper.A simulation code was developed and used for simulation of different evaporation-based inlet air coolingtechniques including DEC, IDEC, and DBEC. Improvement of performance of the gas turbine cycle for eachof these methods was studied for different climatic conditions.

In some cases of study, we observed that improvement of output power caused by desiccant-based coolingtechnique was better than other techniques. It should be noted that in comparison with IDEC (see Tables 5and 6), additional performance improvement caused by desiccant-based cooling technique is limited. From


economy viewpoint, initial payload of desiccant-based cooling systems is much higher comparing with otherevaporative cooling systems. So, it seems that advantage of the desiccant-based cooling technique over IDECis not obvious. Accurate feasibility studies are required for individual projects to determine that if it is feasibleto introduce such a system or not. Specific NOx emission was decreased by application of the inlet aircooling techniques. It was concluded that implementation of the desiccant-based cooling system can befeasible only in humid climates. Simulation results showed that performance of the desiccant-based coolingsystem slightly depends on design parameters of the desiccant wheel provided that the same adsorptivematerial is being used.

Acknowledgements

We would like to thank the anonymous reviewers for their professional review and helpful comments.Furthermore, we wish to express our gratitude and sincere thanks to Dr. S.M. Soleiman-Panah for hisproofreading.

Appendix A

This appendix is devoted to mathematical basis and thermodynamics relations used in the codeGTTPSP. Parameters of the cycle are defined in the nomenclature section of the paper. Index s is appeared asa part of subscripts of some parameters. This index stands for isentropic value of those parameters. Realthermodynamic processes deviate from isentropic value and this deviation is accounted for by utilization ofefficiency.

1.
Calculation of the turbines’ produced work
WTurb:I ¼ ZTurb:Iðma þmf1 ÞðhðT6Þ � hðT7ÞÞ,

WTurb:II ¼ ZTurb:IIðma þmf1 þmf2 ÞðhðT8Þ � hðT9ÞÞ.

2.
Calculation of compressors’ consumed work
WCompI ¼1

ZComp:I

maðhðT2Þ � hðT1ÞÞ,

WCompII ¼1

ZComp:II

maðhðT4Þ � hðT3ÞÞ.

3.
Calculation of the cycle’s net produced work
Wnet ¼WTurb:I þWTurb:II � ðWComp:I þWComp:IIÞ.In order to estimate real produced work of the gas turbine power plant, losses caused by mechanical and

electrical apparatuses as well as losses caused by other sources of power loss should be subtracted fromthis value.

4.
Calculation of thermal energy consumed in the combustors
Q1 ¼ mf1 LHV I,

Q2 ¼ mf2 LHV II,

Qin ¼ Q1 þQ2.


5.
Calculation of thermal properties of the traveling air when it is passed through the combustors
mf1 LHV I ZCmbst:I ¼ ðma þmf1Þ

Z T6

T5

Cp dT ,

mf2 LHV II ZCmbst:II ¼ ðma þmf1 þmf2Þ

Z T8

T7

Cp dT .

6.
Calculation of thermal efficiency of the cycle
ZThermal ¼Wnet

Qin

.

7.
Pressures and temperatures of different stages of the cycle (numbers of the stages are shown in Fig. 1)
P2 ¼ffiffiffiffiffiffiffiffiffiffiffiP1P4

p; P2 ¼ P3; P6 ¼ n1P1; P6 ¼ P5 ¼ P4; P6=P8 ¼ n2,

P9 ¼ P1,

T2s ¼ T1P2

P1

� �ðk�1Þ=k

; T2 ¼ T1 þT2s � T1

ZComp:I

,

T3 ¼ ZIntercoolerðTw � T2Þ þ T2,

T4s ¼ T3P4

P3

� �ðk�1Þ=k

; T4 ¼ T3 þT4s � T3

ZComp:II

,

T5 ¼ ZRegeneratorðT9 � T4Þ þ T4,

T6 ¼ TCmbst:I; T7s ¼ T6=P6

P7

� �ðk�1Þ=k

; T7 ¼ T6 � ZTurb:IðT6 � T7sÞ,

T8 ¼ TCmbst:II,

T9s ¼ T8=P8

P9

� �ðk�1Þ=k

; T9 ¼ T8 � ZTurb:IIðT8 � T9sÞ.

8.
Ambient pressure of the cycle’s site, which is also pressure of the first stage of the cycle, was calculated byrelations given in the ASHRAE handbook [30]. Density of inlet air was computed by using relations givenin the ASHRAE handbook for specific volume of humid air [30].
9.
Specific heat of dry air and water vapor were calculated by using relations given in Ref. [31]. 10. An iterative procedure was used for determination of temperature of the traveling air after it is passed
through the regenerator.
11. Enthalpy of the air was calculated by numerically integrating the specific heat of the humid air, i.e.
hðT2Þ � hðT1Þ ¼R T2

T1CpðTÞdT .

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http://www.novelaire.com

performance improvement of a gas turbine cycle by using a desiccant-based evaporative cooling system

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