INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,
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ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue VIII, August 2025
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Comparative Study of Inlet Air Cooling Technologies for Gas
Turbines Under Tropical Conditions
Ojeh-Oziegbe E. O1, Oyedepo SO2
1Department of Mechanical Engineering, Faculty of Engineering, University of Benin, PMB 1154, Benin City, Nigeria.
2 Department of Mechanical Engineering, College of Engineering, Covenant University, Ota, Ogun State.
DOI: https://doi.org/10.51583/IJLTEMAS.2025.1408000162
Received: 18 Aug 2025; Accepted: 22 Aug 2025; Published: 18 September
Abstract: In this paper, a comparative thermodynamic performance analysis is presented for Evaporative Cooling and Mechanical
Chilling, two inlet air cooling techniques for simple cycle gas turbines in tropical environments. While numerous papers in the
literature have examined different cooling methods, this study is particularly focused on the evaluation and comparison of the
performance of these two widely employed techniques under Nigerian Climatic Conditions.
The research employs operating data from a Nigerian gas turbine power station combined with MATLAB-based modeling to
simulate the effect of each cooling method. Performance was assessed under two Relative Humidities, 30% and 70%, typical of the
country's two major climatic zones. The key parameters assessed are net work output, Thermal Efficiency, and heat rate.
The findings indicate that, against common belief, Evaporative Cooling performs more positively in both low- and high-humidity
environments. Although Mechanical Chillers achieve ambient-independent cooling performance regardless of the ambient
conditions, their advantage is limited to extremely high ambient temperature with high Relative Humidity conditions, where
Evaporative Cooling becomes inefficient. In most other situations, Evaporative Cooling performs better in enhancing Gas Turbine
performance.
These results necessitate the need for a climate-sensitive approach in the selection of Inlet Air Cooling technologies. Rather than
adopting a single-fit-all approach, power plant operators are encouraged to align their choice of cooling systems with the prevailing
environmental conditions in specific locations. Practical suggestions that can assist in improving overall efficiency and reliability
of power generation in Nigeria's different climatic regions are provided by the study. By determining the conditions under which
each cooling technology is optimally effective, the paper contributes to more informed decision-making in Gas Gurbine
performance enhancement in tropical climates.
Keywords: gas turbine, inlet air cooling, evaporative cooler, mechanical chiller, tropical climate, thermal efficiency, relative
humidity
I. Introduction
Gas turbine power plants are widely used for electricity generation due to their high power-to-weight ratio, rapid start-up capability,
and relatively low installation costs (Ozgoli et al., 2015). In Nigeria, they form a critical part of the national grid, providing both
base-load and peak-load generation capacity. However, their performance is highly sensitive to ambient conditions, particularly
temperature and humidity—two parameters that vary considerably across Nigeria’s diverse climate zones.
The thermal efficiency and power output of a gas turbine depend strongly on the density of the inlet air, which depend greatly on
ambient temperatures, and seasonal humidity variations. As ambient temperature increases, air density decreases, resulting in a
lower mass flow rate through the compressor. This leads to reduced turbine work output and higher specific fuel consumption. A
study done by Kakaras et al.,(2004) showed a power loss of over 20%, in combination with a substantial increase in specific fuel
consumption, when ambient air temperatures are significantly higher than ISO conditions. In a converse but supporting study,
Zuniga (2005), shows an increase in air density, and consequently, air mass flow rate, power output and efficiency by about 0.7%
for every degree Celsius drop in temperature in heavy duty gas turbines. Da Costa et al (2021) in a study done in an integrated steel
mill in Brazil, to analyse the techno-economic feasibility of installing inlet air cooling on the gas turbines there. The results showed
an estimated inprovement of 4.22% on power output, equivalent to about 3.92 million USD saved per year.
Relative humidity is also a factor, especially in inlet air cooling (IAC) systems involving the use of evaporation. Evaporation
processes work less efficiently in humid locations because there is less water evaporation potential in extremely moist air.
To offset such losses, inlet air cooling technologies have been developed that reduce compressor inlet temperature, thereby
increasing air density and overall cycle performance for simple gas turbines as well as combined cycle power plants. (Ehyaei et al.,
2014; Goldborough et al., 2011; Zhiyan and Obed, 2024). Of these, two are the most widespread methods in use: evaporative
cooling and mechanical chilling.
* Evaporative Cooling utilizes the latent heat of vaporization of water to cool the inlet air through adiabatic saturation. It is energy-
efficient, relatively low-cost, and easy to install. Its efficiency, though, is limited by the wet-bulb temperature of the ambient air,
and hence it is less effective in warm climatic conditions. Ibrahim et al. (2011) conducted a technical analysis of several air inlet
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,
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ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue VIII, August 2025
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cooling systems and determined that evaporative systems are better suited for hot, dry climates than for hot, humid climates. Under
hot, dry climates, they reported that power output is most likely to be increased by as much as nearly 12%, whereas in hot, humid
climates, the power output is typically improved by no more than 5–7%.
*Mechanical Chilling employs a refrigerant cycle to cool supply air below the wet-bulb temperature, achieving greater cooling
reductions and consistent performance with changing humidities. It is more expensive to purchase initially, more complicated to
operate, and experiences parasitic losses of power through chiller operation. Alhazmy et al. (2004) carried out an experiment
proving that, for an 8 pressure ratio gas turbine, lowering the inlet air temperature from about 50 °C to 40 °C results in a power
boost of 3.85%, but at the expense of 1.037% in thermal efficiency.
Nigeria experiences extremely variable climate conditions between the dry northern areas (low RH, often below 35%) and humid
southern coastal areas (high RH, often above 70%). Such a disparity requires site-specific choice of inlet air cooling technology.
Initial assumptions would assume that while evaporative cooling may be perfect for northern environments, mechanical chilling
may suit humid southern climates better. However studies by Carmona, 2015, show that evaporative cooling may be better in both
conditions.
Previous studies (e.g., Johnson & Cambron, 2006; Al-Ibrahim & Varnham, 2010) have shown that application of inlet cooling in
general can contribute to increased net output by 5–15% depending on climate and type of cooling. However, few studies examining
Nigerian climatic conditions exist comparing evaporative and mechanical cooling systems with similar operating conditions. This
research bridges that gap by conducting a comparative thermodynamic analysis of the two technologies under two common
humidity levels—30% and 70%—using operating data from a Nigerian gas turbine power station.
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The findings are meant to provide practical recommendations for climate-appropriate cooling strategies for power plant operators
in Nigeria and other tropical nations, such that they can make informed decisions for maximum cost-effectiveness and efficiency.
Limitations to Study
This study is limited only to the improvement of the efficiency of existing simple gas turbines in Nigeria. Analysis will be based
on the principles of thermodynamics and mass conservation. The climate of the country brings about certain unique parameters that
would affect the study, but may not be applicable to gas turbines functioning in areas with a different climate. Results may be used
for studies of areas with similar climate. The inlet air cooling technologies being considered are the evaporative cooler and the
mechanical chiller.
II. Materials and Methods
Data Source and Thermodynamic Analysis
For the analysis of this system, the first law of thermodynamics was used, as well as laws of mass and enthalpy conservation. The
reference model was developed from operational records(including average operating temperatures and pressures) of a Nigerian
gas turbine power station collected over an average period of 12 months. Other variables (Wc, Wt, Wnet,ηth, SFC) were derived
from the appropriate equations using MATLAB programming language.
The behavior of the simple gas turbine cycle was evaluated under two modes of inlet cooling: evaporative cooling and mechanical
refrigeration, at relative humidities of 30% and 70%. The performance of each component of the power plant, and consequently the
entire power plant is examined with each air cooler. In order to simplify the analysis, the combustion chamber is assumed to be
insulated. The working fluid passing through the turbine is assumed to be an ideal mixture of flue gases and water vapour, and air
and water vapour and flue gases are assumed to behave as ideal gases.
Ambient temperatures were shifted from 280 K to 325 K in steps of 5 K. Compressor and turbine work equations, cooling load
calculations, and energy balance equations are regulatory equations employed.
Cooling Methods
* Evaporative Cooling: Direct-type media with effectiveness, εevap ≈ 0.85.
*Mechanical Chilling: Vapor compression cycle chiller with Coefficient of Performance (COP) = 4.5.
Assumptions
1. Steady-state, steady-flow operation.
2. Ideal gas behavior for inlet air.
3. There is complete combustion of natural gas fuel.
4. Cooling system pressure drops are negligible.
5. The chiller power consumption is deducted from net turbine output.
Governing Equations
Evaporative Cooling Humidifier and Compressor Process.
Applying the mass balance equation across the humidifier control volume boundary gives
wa,e = wa,i + mw 2.0
where w is the specific humidity, at evaporative cooler exit and inlet respectively, and is calculated for a certain temperature as
w=
0.622Pv
P-Pv
2.1
Where Pv = φ.Psat is the partial pressure of vapour, φ = the relative humidity and Psat is the saturation pressure of air corresponding
to the desired temperature.
The energy balance equation for the humidifier is given as
ha,e = ha,i+ (wa,e - wa,i ) hw 2.2
Where ha,e and ha,i are the enthalpy of the moist air at outlet and inlet of the air humidifier respectively and are calculated as follows:
ha,e = Cp,a,eta,e + (2500 + 1.88ta,e) wa,e 2.3a
ha,i =Cp,a,inta,I + (2500 + 1.88ta,i) wa,I 2.3b
Ta,e = ta,e + 273 2.3c
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The equations (2.0 – 2.3) can be solved to determine the value of Ta,e, wa,e, and mw. (Oyedepo and Kilanko, 2014)
The inlet air temperature after the cooling process can be calculated as:
T03 = Tb02 – ε. (Tb02 - Tw02) 2.4
Where Tb02 is the dry-bulb temperature, Tw02 is the corresponding wet-bulb temperature at the specified relative humidity, and ε
is the cooling effectiveness of the cooler.
The cooling load associated with the evaporative cooling system results as:
ǬCL = ṁa .cpa (T02 – T 03) 2.5
Where ṁa is the air mass flow rate and cpa is the specific heat of the dry air at constant pressure, determined as a function of the
average temperature across the evaporative system T as (Oyedepo and Kilanko, 2014):
Cpa = 1.04841 –
3.8371T
104 +
9.4537 T2
107 -
5.49031T3
1010 +
7.9298T4
1014 2.6
The working fluid passing through the compressor is assumed to be an ideal mixture of air and water vapour. The total enthalpy of
the atmospheric air is given as ( Abam et al., 2012):
h = ha + w × hv≅ CpaTa + whv 2.7
where ha is the enthalpy of dry air, and hv is the enthalpy of water vapour.
The enthalpy of water vapour can be evaluated by (Jonsson and Yan, 2005):
hv= 2501.3 + 1.8723Tj 2.8
where j refers to state 04 or 05.
The total mass flow rate of the humid air is given by:
ṁha = ṁda + wṁda = (1+w)ṁda 2.9
where, ṁha and ṁda are the mass flow rates of humid air and dry air respectively. The compressor work for humid air between states
03 and 04 is calculated from the mass flow rate and enthalpy change across the compressor:
ẆC = ṁa(1+w) × Cpa(T4s – T03) + w(h4s – h03) 2.10
Mechanical Chiller Process
The specific humidity of the ambient air, T0 = T2, can be calculated as:
ѡ = 0.622
Pv
P0-Pv
(2.11)
Where P0 is the ambient pressure, and Pv = Psat * is the saturated vapour pressure of the air at T0.
The cooling load carried removed from the air at ambient temperature is given as(using the first law of thermodynamics):
Qmc = ṁa(h2 – h3) – (w2 – w3)hw3) (2.12)
where h2 and h3 are the enthalpy of the air at the chiller inlet and outlet systems respectively.
The power needed to drive the mechanical chiller is given as :
Wmc = Qmc / COP (2.13)
Where COP is the coefficient of performance of the mechanical chiller, and its value is fixed (Santos and Andrade, 2012).
ṁa = ṁdryair (1 + w2) (2.14)
h2 = (1.0029 + T2. 5.4 × 10-5) + w2.2500.9 + (1.856 + (T2.2 × 10-4)T2) (2.15)
h3 = Cp.T3.T0 + w3(2500.9 + 1.82T3) (2.16)
hw3 is the enthalpy of the water present in the air at T3 (cooler outlet temperature), and w3 is the specific humidity of the water
present in the air at chiller outlet temperature T3(Oyedepo and Kilanko, 2014).
The specific heat capacity of the air at T3 is gotten from the equation:
Cp,T3 = 1.048 –
3.837T03
104 +
9.4537 T03
2
107 -
5.4903T03
3
1010 +
7.9298T03
4
1014 (2,17)
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Mechanical Chiller Compressor Process:
Compressor
Compressor outlet temperature T04 is given as:
T04 = T03 +
T03
nc
( rp
γ-1
γ -1) (2.18)
Where compressor efficiency, nc and specific heat ratio γ are fixed values.
Specific heat capacity of air at T04 is given as :
Cpa = 1.04841 –
3.8371T4
104 +
9.4537 T4
2
107 -
5.49031T4
3
1010 +
7.9298T4
4
1014 (2.19)
Compressor work Wc, is given as:
Wc = ṁa (1+w3)((Cp,T4.T4) – (Cp,T3.T3)) + w3(h4 – h3) (2.20)
Combustion chamber equation
The mass of fuel flowing through the combustion chamber is
ṁf = Qin/ ncombst.LHV (2.21)
Turbine equation
The energy at the turbine input is given as :
Qin,T5 = Cpg (T05-T04) (2.22)
Turbine work, Wt is given as:
Wt = ṁa + ṁf(1+w2)(Cpg(T05 – T06)) + w2(h05-h06) (2.23)
The net work is given by:
Wnet = Wt – Wc- Wmc (2.24)
And the power output is given as :
Pout =ṁa.ẆN (2.25)
For this cooling method, the gas turbine power output is decreased by the power consumption by mechanical chiller.
III. Results and Discussion
The result data used in this study is presented in this section. Table 1 presents the calculated data comparing parameters for a simple
gas turbine, and ones fitted with evaporative cooling and mechanical chillers under same ambient temperature conditions, at a
relative humidity of 30%. Figures 3.1-3.4 presented show a graphical comparison of calculated parameters comparing operational
results between the simple gas turbine and select retrofitted versions (evaporative cooling, or evaporatoe, and mechanical chilling)
Performance at RH = 30% (Dry Conditions)
Simulation results show that evaporative cooling reduces increasing air density and mass flow rate at a better rate than mechanical
chillers, without the extra energy drain from the system to power the chiller (Figures 3.1 and 3.3). Net work output improves by
34% at an ambient temperature of 300K, and thermal efficiency rises from 40.95% to 53.48%. Heat rate reduces from 87.9 to 67.3.
Mechanical chilling achieves greater cooling, but efficiency gains are reduced once chiller power consumption is deducted. Net
work output increases only by 0.03% after deductions, and thermal efficiency goes up from 40.95% to 42.62%.
Table 1: Performance Gains at RH = 30% , Ambient temperature 300K
Parameter Baseline Evap. Cooling Mech. Chiller
Net Power Output (MW) 31.19 42.03 32.24
Thermal Efficiency(%) 40.95 53.48 42.62
Heat Rate (kJ/kWh) 87.91 67.31 84.45
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Performance at RH = 70% (Humid Conditions)
Evaporative cooling works less effectively in humid conditions. However, there is still a surprising higher net work gain, as well
as thermal efficiency, comparative to that of mechanical chillers only as ambient temperatures increase(Figures 3.2 and 3.4). This
concurs with results gotten from Carmona’s study (2015) which showed that there was high potential for evaporative cooling in
high temperature and relative humidity, areas, using Nigeria as a case study.
Figure 3.1- Graph of Net Work versus Ambient Temperature, RH 30%
Figure 3.2- Graph of Net Work versus Ambient Temperature, RH 70%
Figure 3.3- Graph of Thermal Efficiency versus Ambient Temperature, RH = 30%
25000
30000
35000
40000
45000
50000
270 280 290 300 310 320 330
N
e
t
W
o
rk
, K
w
Ambient Temperature, K
Net Work vs Ambient Temperature, 30RH
SGT without Cooler
Evaporator
Mechanical Chiller
25000
30000
35000
40000
45000
270 280 290 300 310 320 330
N
et
W
o
rk
, K
W
Ambient Temperature T0, K
Net Work Vs Ambient Temperature, 70 RH
SGT without Cooler
Evaporator
Mechanical Chiller
30
35
40
45
50
55
60
270 280 290 300 310 320 330
Th
er
m
al
E
ff
ic
ie
n
cy
, %
Ambient Temperature, K
Thermal Efficiency vs Ambient Temperature, 30 RH
SGT without Cooler
Evaporator
Mechanical Chiller
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Figure 3.4- Graph of Thermal Efficiency versus Ambient Temperature, RH = 70%
Operational Considerations
Evaporative cooling has the advantages of lower capital cost, minimal power consumption, and simple maintenance. However, it
is highly dependent on local humidity, with performance diminishing in coastal or rainy seasons. It still shows better cooling
potential than mechanical chillers, contrary to popular opinion.
Mechanical chilling provides consistent performance regardless of humidity but carries higher capital and operational costs, plus
parasitic power losses from the chiller system.
IV. Conclusion
The comparative analysis shows that evaporative cooling shows good performance in both areas with low and high relative humidity
and ambient temperature. The Nigerian energy sector could take advantage of this information to retrofit our existing gas turbines,
and improve power production with the equipment currently on ground. Other literature show that the cost of retrofit for evaporative
cooling is significantly lesser than that for mechanical chillers, and that the power sector of the country should take advantage of
these cooling technologies, and the associated prospective benefits.
Conflict of Interest
There is no conflict of interest associated with this work.
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