Yilmazoglu Et Al, 2013

7/18/2019 Yilmazoglu Et Al, 2013

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Comparison of Absorption Cooling and

Fogging Inlet Air Cooling Systems in Gas

Turbine Power PlantsM. Zeki YILMAZOĞLU, Murad RAHİM, Ehsan AMİRABEDİN

Gazi University Faculty of Engineering Department of Mechanical Engineering, Maltepe, Ankara, TURKEY

[email protected] , [email protected]

ABSTRACT - Gas turbine power plants operation performance is a function of ambient conditions. Gas turbines aremass flow devices and the power production capacity is related to the mass flow rate of air passing through the

turbine and ambient conditions. , The density of inlet air increases with decreasing the ambient air temperature thus,mass flow rate of compressor increases and which can be obtain high capacity and performance from gas turbinecycles. Because of high electric consumption in summer season, system power demand typically peaks during thehottest period of the day; this coincides with the periods when gas turbine generators in the system are at their lowestcapacity. It has a vital importance in today’s competitive economy to increase energy conversion efficiency and the productivity minimizing unit cost of products. Decreasing the inlet temperature of gas turbine is the simplest way to

optimize the power output from gas turbines cycles and reducing NOx and CO2 emissions. As the results show thatthe power and efficiency improvements in the gas turbines are depends on reducing inlet air temperature. A simple

gas turbine combined cycle power plant is simulated by Thermoflex software. In this study a fogging system and anabsorption chiller system is commissioned before the air inlet of the compressor. Heat consumption of the absorptionchiller is supplied from bled steam from steam cycle. Three cases; inlet air cooling with fogging, absorption chillersand without inlet air cooling are compared in case of net electricity production, net efficiency (LHV) of the power plant, water consumption, heat consumption and energy economy.

Keywords: Inlet air cooling, Power Enhancement, Power Plant, Gas Turbines.

I. INTRODUCTION

Gas turbines which operate in hot climates, likeMediterranean countries, affected from the ambient airtemperature and a decline takes place on the net powergeneration of system. In summer seasons air temperature

increases and gas turbine power plant’s performancedecreases. Also, in summer season the electricity demandis more than winter season due to the cooling demand.Thermal comfort conditions can be obtained by cooling

the media with a cooling device which consumes largeamounts of electricity. As known, general cooling devices

(vapor compression) work with a compressor whichconsumes electricity due to the compression of vapor. The

net power production of gas turbine decreases and coolingdemand increases, while ambient air temperature rises. Itis very important to run the system efficiently especially inhot climates.

There are different methods for inlet air cooling which

are fogging, evaporative cooling, absorption coolingsystems, and mechanical compression systems [1]. If the power plant uses LNG as fuel, the evaporation heat of

LNG might be used for inlet air cooling. Inlet air coolingmethods have advantages and disadvantages when

compared to each other. Evaporative cooling and fogging

systems have lower specific investment cost. However, inhot climates water is not abundant and there is atemperature limit, the wet bulb temperature, for thesesystems. Absorption and mechanical compression systemsspecific investment costs are higher than evaporative andfogging systems. On the other hand, electricity

consumption is very high for mechanical cooling systems.

Also heat has an economic value when used in absorptionchillers. The wet bulb temperature is not a limit for thesesystems.

Alhazmy and Najjar [2], studied two inlet air cooling

systems which are, cooling coils and water spray.According to their results, spray coolers are able to

increase the power output, much cheaper than the coolingcoils, however, spray coolers operates more efficiently athot and dry climates. Cooling coils give a full control onthe compressor inlet conditions, however, they consumeconsiderable amount of power, causing a large drop in theoverall plant performance and initial cost is higher than

water spray systems. Evaporative cooling and cooling ofthe compressor discharge using water injection is

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researched by Bassily [3]. According to his resultsevaporative cooling boosts the efficiency 3.2%. Ameri andHejazi [4], studied on Chabahar thermal power plant andanalysis are performed for an absorption chiller which is

used for inlet air cooling. They showed that, 11.3% powerenhancement is achieved and the payback period is

estimated 4.2 years. Gareta et al [5], presented amethodology for the economic evaluation of inlet aircooling systems. In their study six alternatives;evaporative cooling, absorption cooling (20 MW-8MW),

mechanical compression (20MW-8MW) and ice storageare discussed. As a result cash flow and pay back periods

of the alternatives were given. The most economic systemis evaporative cooling with 0.77 years payback time. 20MW mechanical compression systems have 13.75 years payback time. Erdem and Sevilgen [6] studied two modelson seven different climatic conditions in Turkey andelectricity production variations were studied according to

climatic conditions. As a result 0.27-10.28% electricity

generation increase was found. Dawoud et al [7],compared different inlet air cooling techniques at twodifferent locations in Oman. The considered techniqueswere evaporative cooling, fogging cooling, absorptioncooling using both LiBr–H2O and aqua-ammonia, and

vapor-compression cooling systems. For evaporativecooling, an 88% approach to the wet-bulb temperature has been considered, compared with a 98% approach forfogging cooling. According to their results, foggingcooling is accompanied with 11.4% more electrical energyin comparison with evaporative cooling in both locations.

The LiBr–H2O cooling offers 40% and 55% more energythan fogging cooling at Fahud and Marmul, respectively.

Applying aqua-ammonia–water and vapor-compressioncooling, a further annual energy production enhancementof 39% and 46% is expected in comparison with LiBr– H2O cooling at Fahud and Marmul, respectively. Kakaraset al. [8], showed the effects of the ambient air temperatureon the power output and efficiency for two cases which are

simple cycle gas turbine and combined cycle. Yõlmazoğluand Rahim [9] examined the effects of the ambientconditions on gas turbine power plants. It is showed that,increasing temperature and decreasing pressure badlyaffects the system performance.

In this study, a fogging and an absorption cooling

systems are considered for inlet air cooling of gas turbine.The simulations are performed in THERMOFLEXsoftware. Three cases are performed in the simulation. InCase 1 without inlet air cooling, in Case 2 inlet air coolingwith fogging and in Case 3 inlet air cooling with a single

effect absorption chiller, which uses LiBr-H2O as workingfluid, are examined.

II. SYSTEM STRUCTURE

A flow chart of a combined cycle is given in Fig. 1. Theambient air enters the cycle from the compressor of the gasturbine (C). The compressed air and fuel is mixed to

satisfy the turbulence, which is one of the combustionconditions. After the combustion chamber (CC), hot gases

enter the gas turbine (GT). Then hot gases passes througha dual pressure heat recovery steam generator (HRSG) to produce steam for Rankine cycle. Steam is produced to runthe steam turbines (ST) with two pressure levels and flue

gas leaves the cycle from the stack. In HRSG, anadditional burner can be considered to satisfy the steam

conditions at the inlet of HRSG.

Ambient air enters the cycle from the compressor. Specific power consumption of a compressor is related to the

ambient air temperature as shown in (1).

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡−⎟⎟

⎠

⎞⎜⎜⎝

⎛ =

−

11

1

1

21

k k

air C

C P

P T Cpw

η (1)

where; T 1 is ambient air temperature (before the inlet ofcompressor), Cp is the specific heat of air [kJ/kgK], k is

the ratio of specific heats, p is the pressure and η is theefficiency of the compressor. The specific net power of the

compressor is indicated as w, in [kJ/kg]. The specific power consumption of the compressor is linearly proportional to the ambient air temperature which means,if the temperature of the air at the inlet of compressor isdecreased, the specific power consumption of thecompressor decreases. As a result, more power generationfrom Brayton cycle might be obtained as shown in (2). In

Equation (2), P [kW], is the net electric power obtained

from gas turbine, is the mass flow rate of working

fluid (air), wGT and wC are the specific power of gasturbine and compressor, η M and ηG are the mechanical andgenerator efficiencies.

.

m

( ) M GC GT GT elGT wwm P η η −=.

(2)

Overall efficiency of a combined cycle is given in (3).

As seen from the (3), overall efficiency of a combinedcycle can be increased by performance enhancement of the

gas turbine. In Equation (3), Pel GT , Pel ST and Q pr are thenet electric power of gas cycle, net electric power of steamcycle and process heat respectively. Hu, is the lower

heating value of the fuel and is the mass flow rate of

fuel.

f m.

f

pr elST elGT CC

m Hu

Q P P

.

++=η (3)

Dual pressure heat recovery steam generator decreasesthe stack losses of gas turbine and produces steam withdifferent pressure levels. The area between the hot gasesand steam in a T-Q diagram decreases by splitting steamside different pressures. Hot gases passes through

superheater, evaporator and economizer of high pressuresteam, then passes through the superheater, evaporator andeconomizer of low pressure steam. For simplicity, these

sections are showed as resistances in Fig. 1.



Three cases are considered in this study; inlet aircooling (IAC) with fogging, IAC with absorption chillerand without IAC. The advantages and disadvantages of

fogging and absorption cooling for inlet air coolingsystems are given in Table 1.

Figure 1 - Connection schema of combined cycle with inlet air cooling system

Table 1 - Comparison of fogging and absorption cooling

Inlet air cooling with fogging Inlet air cooling with absorption cooling

Advantages Disadvantages Advantages Disadvantages

Low first investment cost Temperature limit (Wet bulb

temperature)

More efficient thanevaporative cooling and

fogging

High first investmentcost

Low operation andmaintenance cost

High quality water necessityHigh operation andmaintenance cost

More efficient thanevaporative cooling

Assembling and start-uptime shorter

Assembling and start-uptime shorter

Overspray or wet

compression

Hard to control inlet air

conditions

Easy to control inlet airconditions

Qualified personnel

In order to compare the effects of inlet air coolingmethods the simple system without inlet air cooling is

simulated. The simulation parameters are given in Table 2which are obtained from ISO conditions for base case. Gasturbine exit temperature changes according to the

conditions. The values, shown in Table 2, for gas turbineare catalogue values which indicates these results obtained

from standard ISO conditions. For the simulations wetmechanical draft type cooling tower is chosen.



Table 2 - Input values of the simulated system for base case (without inlet air cooling)

Temperature [ºC] Pressure [bar] Relative Humidity [%] Ambient Conditions

40 1.013 60

Pressure Ratio Turbine Exit Temp. [ºC] Efficiency [%] (LHV)

Gas Turbine 13.5 503 34

Pressure [bar] Isentropic Efficiency [%] Shaft Speed [rpm]High Pressure Steam Turbine

65.5 90.92 3600

Pressure [bar] Isentropic Efficiency [%] Shaft Speed [rpm]Low Pressure Steam Turbine

2.068 88.69 3600

Type LHV [kJ/kg] Pressure [bar]Fuel

Methane 50047 23

Pressure [bar] Temperature [ºC] Gas side Pressure Drop [mbar]Deaerator

1.241 105.8 2.029

Pressure [bar] Sub-cooling [ºC] Total head Loss [m]Condenser

0.1034 2 9.144

Type Temperature Rise [ºC] Pump Efficiency [%]Cooling Tower Wet Mechanical Draft 10 0.72

III. RESULTS

Cycle performance outputs of the simulations are givenin Table 3. In Table 3, net power, net electric efficiency,net heat rate, net fuel input, plant auxiliary, gas turbine net

power, steam turbine net power and mass flow rate of inletair to the compressor are compared to each case.

Table 3 - Simulation results of each case

Cycle Performance Parameters Case 1 Case 2 Case 3

Net Power [MW] 110.89 115.97 119.96

Net electric efficiency [%] 47.95 48.17 45.55Net heat rate [kJ/kWh] 7507 7474 7903

Net fuel input [kW] 231224 240756 263342

Plant auxiliary [kW] 4204 4336 4304

Gas turbine net power [MW] 72.23 77.11 88.13

Steam turbine shaft power [MW] 43.72 44.06 36.89

Mass flow rate of inlet air [kg/s] 283.1 289.5 313.1

The net power production of the combined cycle isincreased an inlet air cooling system is applied. Foggingand absorption cooling systems increased the net power production 4.6% and 8.2% respectively. The main reason

of this difference is the temperature limit of the foggingsystem. Fogging system decreased air temperature from40ºC to 32ºC and absorption cooling system constituted30ºC temperature difference. As a result of temperaturedifference the density of air changes and inlet air massflow rate increases. This means more net power

production of gas turbine is acquired as seen from Table 3.On the other hand, a reduction is occurred in the steamturbine shaft power when absorption cooling is used. Thereason of this situation is explained by taking bled steamfor using in the generator of absorption chiller to producecooling effect. In the simulation a single effect absorption

chiller is used with a COP of 0.67 and 25.8 MW cooling

load. Total heat rejection of absorption chiller is found64.4 MW and this heat is rejected from the cooling towerof power plant. According to bled steam some useful

energy is not converted to electricity in steam turbine and

therefore steam turbine shaft power production isdecreased. This also affects net electricity efficiency of power plant and diminution of efficiency can be explained

by this way when absorption chillers are used for IAC.Besides, net fuel input is increased depending on massflow rate of air. According to Eq. 3, increased net fuel

input decreases the net efficiency.

In the second part of this study, the effects of ambienttemperature for three different cases are analyzed.Ambient air temperature is changed between 5ºC and40ºC. In Fig. 2, air temperature at the inlet of gas turbine

compressor is shown. Without an inlet air cooling systemambient temperature enters at same temperature. When

fogging system is applied a temperature difference can beobtained. As an example, when ambient temperature is 40

ºC, after foggers the temperature is decreased 8ºC. In thethird case absorption cooling system is applied and when

ambient air temperature becomes 15ºC cooling system is



driven by bled steam, taken from low pressure turbine. Asa result, ambient air temperature only affects bled steammass flow rate due to the cooling load necessity.

Figure 2 - Air temperature at the inlet of compressor

In Fig. 3, mass flow rate of inlet air to the compressor is

shown. When ambient air temperature increases, thedensity of air decreases and mass flow rate of inlet air tothe compressor also decreases. As a result, net powergeneration from gas turbine decreases. This result also

affects steam turbine power production. Decreased massflow rate of air results less heat transfer in HRSG and less

steam production means less power generation from steamturbines. In the third case, mass flow rate to thecompressor is not affected from ambient temperature whenabsorption cooling system is started to run.

Figure 3 - Mass flow rate of air to the compressor

In Fig. 4, net power production curves are given. Asseen from Fig. 4, after 20ºC, case 3 is the best solution

when increasing net power production is the main aim. Ifthe ambient air temperature is less than 20ºC inlet aircooling with absorption chillers badly affects the net power production due to steam consumption in order to

satisfy the cooling necessity.

Figure 4 - Net power production

In Fig. 5, the net electric efficiency curves are given

with respect to ambient air temperature. The efficiency ofcase 3 is less than other cases according to the reduction of

steam turbine power generation. Also, increased net fuelinput affects the efficiency of power plant which wasshown in Table 2.

Figure 5 - Net electric efficiency of the system

Net fuel input is related to the mass flow rate of workingfluid (air) and as a predictable result, characteristics ofcurves in Fig. 3 and 6 are same.

Figure 6 - Net fuel input to the system



In Figure 7, the relationship of steam turbine totalelectricity generation and mass flow rate of bled steam isgiven for case 3. At 5ºC, bled steam consumption ofabsorption cooling system is zero and steam turbine total

electricity generation is the highest. When absorptioncooling system is started to run by bled steam total

electricity generation of steam turbine is started todecrease.

Figure 7 - Relationship of steam turbine total electricitygeneration and mass flow rate of bled steam

IV. CONCLUSION

Inlet air cooling systems are useful tools to increase thenet power generation of gas turbine power plantsespecially in hot climates. In this study, three cases aresimulated and system performance datas are analyzed withrespect to ambient air conditions. In Case 1, a power plantmodeled without inlet air cooling in order to make

comparision to other inlet air cooling systems. In Case 2, afogging system was considered at the inlet of compressor.In this case, wet bulb temperature is a limit of coolingcapacity. Relative humidity of ambient air was takenconstant. Water sources near the power plant and thequality of sprayed water are the main obstackles offoggers. In case 3, an absorption cooling system was

commissioned at the inlet of compressor. In this case, wet bulb temperature limit was surpassed. Absorption chillersuse bled steam to satisfy the cooling effect of inlet air.However, the net electric efficiency of the plant decreasessharply when the ambient air temperature reaches 20ºCdue to the increased cooling necessity of inlet air. If themain aim is to increase the net power generation,

absorption chillers are the best solution. According to theresults, a thermoeconomic optimization problem occurs.Before deciding the type of inlet air cooling system to beinstalled, economic and source audits should be performed.

ACKNOWLEDGEMENTS

This work is supported by Gazi University ScientificResearch Projects 06/2006-04.

NOMENCLATURE

w Specific work [kJ/kg]Cp Specific heat capacity at constant pressure [kJ/kgK]T Temperature [K] p Pressure [bar]P Power [MW]k Specific heat ratiom Mass flow rate [kg/s]Q Heat [MW]

Hu Lower heating value [kJ/kgfuel]

η Efficiency

SubscriptsC Compressor

GT Gas turbineG Generator

m Mechanicalg Gas pr Processf FuelCC Combined Cycle

REFERENCES

[1] Giampaolo, A. 2006. Gas Turbine HandbookPrinciples and Practices, Fairmont Press Inc., ThirdEdition.

[2] Alhazmy, M.M. and Najjar Y.S.H. 2004.Augmentation of gas turbine performance using air

coolers, Applied Thermal Engineering 24, 415–429.[3] Bassily A. 2001. Effects of evaporative inlet and

aftercooling on the recuperated gas turbine cycleApplied Thermal Engineering 21, 1875-1890.

[4] Ameri M. and Hejazi S.H. 2004. The study ofcapacity enhancement of the Chabahar gas turbine

installation using an absorption chiller, AppliedThermal Engineering 24, 59–68.

[5] Gareta R., Romeo L.M. and Gil A. 2004.Methodology for the economic evaluation of gasturbine air cooling systems in combined cycle

applications, Energy 29, 1805–1818. [6] Erdem H.H. and Sevilgen S.H. 2006. Case study:

Effect of ambient temperature on the electricity production and fuel consumption of a simple cyclegas turbine in Turkey, Applied Thermal Engineering26, 320–326.

[7] Dawoud B., Zurigat Y.H. and Bortmany J. 2005.Thermodynamic assessment of power requirementsand impact of different gas-turbine inlet air cooling

techniques at two different locations in Oman,Applied Thermal Engineering 25, 1579–1598.

[8] Kakaras E., Doukelis A. and Karellas S. 2004.Compressor intake-air cooling in gas turbine plants,

Energy 29, 2347-2358.



[9] Yõlmazoğlu M.Z. and Rahim M. 2010. The effects ofambient temperature and pressure on gas turbine power plants performance, Journal of the Faculty ofEng. and Arch. of Gazi Universty, Article in Press.

BIOGRAPHIES

Mustafa Zeki YILMAZOĞLU- was born inKahramanmaraş in 1977. He graduated as mechanicalengineer from Karadeniz Technical University Department

of Mechanical Engineering in 2002. He has M.S degreefrom Gazi University Department of Mechanical

Engineering in 2006. He is working as a research assistantat Gazi University and studying his PhD on IntegratedGasification Combined Cycles.His major interests are combined cycles, absorptioncooling systems, thermodynamic analysis of energyconversion systems.

Mr. Yõlmazoğlu, is the member of The Chamber of

Mechanical Engineers and The Turkish Society of HVACand Sanitary Engineers.

Murad A. RAHIM- was born in Kirkuk city/Iraq, 1981. Hegraduated as Refrigeration and Air-Conditioning Engineer

from Kirkuk Tecnology Faculty on 2002. He has M.S.Degree from Gazi University Department of MechanicalEngineering on 2008.He is working as a researcher at Gazi University Energy-Environmental and Industrial Rehabilitation ResearchCenter and studying his PhD on Coal Gasification in

Circulating Fluidized Beds.His major interests are combined cycles, Clean Coal

Technologies, thermodynamic analysis of energyconversion systems.

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Yilmazoglu Et Al, 2013