Economic optimization of gas turbine air-cooling systems in ... - circe

ECONOMIC OPTIMISATION OF GAS TURBINE INLET
AIR – COOLING SYSTEMS IN COMBINED CYCLE APPLICATIONS
Raquel Gareta, CIRCE
Luis M. Romeo, CIRCE
Antonia Gil, CIRCE
CIRCE (Center for Power Plant Efficiency Research). University of Zaragoza.
Centro Politécnico Superior, Maria de Luna, 3, 50015 Zaragoza (Spain)
ABSTRACT
It is well-known the strong dependence of
climate conditions on gas turbine (GT)
behaviour. A proper solution to minimise this
negative effect is to reduce GT inlet air
temperature by means of an air cooling
system. The criteria to select the most suitable
alternative among cooling technologies and
cooling sizing are complex.
On one hand, bigger cooling sizing means an
increase in GT power output and, as a
consequence, an improvement in money
incomes. This option is generally adopted by
manufacturers. Nevertheless, it should be
considered carefully, because the size of the
cooling equipment increases substantially
without an important power augmentation in
certain situations.
On the other hand, another criteria could be
selected in order to maximise combined cycle
efficiency or cooling system profitability. The
former leads to an optimum behaviour of the
global system, minimising the emissions per
kilowatt and the cost of electricity. Due to the
fact that money incomes directly depends on
the cost of electricity and the power
augmentation, the latter combines the
advantage of taking into account the power
augmentation and the global system
efficiency.
This paper shows a comparison between all of
these criteria, optimising the size of a cooling
system for a combined cycle application in
order to achieve: (i) maximum power
augmentation, (ii) maximum combined cycle
efficiency or (iii) maximum cooling system
profitability, and analysing the key points in
order to properly size the cooling equipment.
INTRODUCTION
Combined cycles (CC) based on gas turbines
are a competitive alternative to produce
electricity power. CC are high efficiency cycles
with low production costs and emissions
which are essential issues in the current
economic and environmental framework.
Despite their high efficiency, GT and also CC
performance strongly depends on ambient air
temperature. Different air cooling systems
have been developed to reduce this influence
and to improve GT performance. A wide range
of cooling technologies can be selected.
Evaporative coolers, mechanical and
absorption chillers are potentially among the
best cooling technologies to install.
Three different criteria can be adopted to
select the most suitable sizing and cooling
technologies. Cooling equipment manufacturers
generally choose the system which
maximise GT power output (or CC power
output) in order to show the maximum benefits
of an air cooling system. In these cases, to

obtain the GT power output augmentation due
to cooling, minimum cooling temperature and
maximum ambient temperature are selected.
However, the latter is scarcely representative
of climate conditions, hence cooling
equipment is generally overestimated. The
result of this criterion is to maximise the power
output regardless the ambient temperature.
In order to solve this limitation, a second
criterion can be selected. Taking the
environment into account, the most suitable
criteria is to achieve CC maximum efficiency.
Given that the point of maximum efficiency is
generally near the design temperature (10 –
15ºC), the cooling equipment selected are of
lower size as compared to the previous
criteria. Although, CC power augmentation is
also reduced, efficiency is maximised and
generation cost are minimised.
Finally, a third criterion could be adopted to
maximise the cooling system profitability. This
criterion includes the effect of the cooling
system in efficiency or in power augmentation
and integrates all these variables with gas and
electricity markets. The size of the cooling
equipment is usually between those resulting
from the first and the second criteria, although
in certain economic scenarios the same
results are obtained.
In the last case, the cooling system not only
depends on climate conditions or GT/CC type,
but also on its coefficient of operation (COP),
fuel and electricity prices and combined cycle
arrangement. These complex variables add
new difficulties in the technical and
economical assessment of cooling systems.
Some examples [1]-[4], [7], [8] and
methodologies [8] have been developed in
order to evaluate these influences in a
complex system as cogeneration or CC
applications.
The aim of this paper is to compare above
criteria in order to size a mechanical chiller
cooling system in a CC application. The size
of a cooling system has been optimised to
obtain: (i) maximum power augmentation, (ii)
maximum combined cycle efficiency or (iii)
maximum cooling system profitability. The key
points to size properly the cooling equipment
have been analysed.
In a particular CC, an economical optimisation
(based on cooling system profitability) is
achieved in different scenarios influenced by
electricity and fuel prices.
COMBINED CYCLE SIMULATION.
To calculate power output improvements
caused by the air cooling and to quantify the
CC efficiency and cooling system profitability,
a simulation of a CC has been carried out.
Simulation inputs are: ambient conditions
(relative humidity and temperature), cooling
system COP, gas and electricity prices.
Variables obtained as results are: CC
efficiency and power output (gas and steam
turbines), air conditions at GT inlet, cost of
electricity, cooling profitability (hourly incomes
including system amortisation) and properties
of different CC points.
Three CC optimisations have been carried
out: (i) CC power augmentation, (ii) CC global
efficiency and (iii) CC hourly incomes (cooling
profitability), with the temperature of GT inlet
(size of the cooling system) as the
independent variable.
A three-level steam pressure combined cycle
has been chosen for the analysis. It is based
on three 52.8 MW industrial GT and a 67.4
MW steam turbine [9]-[10]. Both gas and
steam cycles have been simulated in order to
study the influence of climatic data and GT
inlet air-cooling installation.
GT corrections for power output, heat rate,
mass flow rate and exhaust gases
temperature at different ambient tem-
peratures, as well as inlet and outlet pressure
drop parameters have been calculated by
means of performance curves. The variation
of exhaust gases properties has been
considered, due to its relevant influence on
the temperature profile of the Heat Recovery
Steam Generator (HRSG). Additional GT inlet
and outlet pressure drops have been taken
into account because of the influence of
cooling coils and HRSG. Ambient conditions
are input variables for this model. Output

variables are GT efficiency and power output
and exhaust gases properties. The HRSG and
the steam turbine have been simulated with
usual equations and strategies [7].
Although there are different cooling
techniques in order to improve the GT and CC
performance, only mechanical chillers have
been considered for the optimisation. A
mechanical chiller refrigerates the air stream
by means of a cold refrigerant circuit. This
refrigerant is cooled with a mechanical
compressor system. In the simulation, the
refrigerant cycle is not relevant, since
electricity consumption in electrically driven
chillers mainly depends on the coefficient of
performance (COP), and variations in this
parameter at different operation loads have
not been considered. The COP is a function of
the refrigerant type and technology selected
([1]-[4]). The model pays special attention to
analyse changes in air-water mixture
properties. Cooling of the humid air causes
saturation at the dew temperature, fixed by
the initial humid air properties. If the final air
temperature is below the dew temperature,
water content in the mixture condenses. This
process has an important chilling energy cost,
since the energy to produce the phasechange
is necessary to be transferred. In
consequence, these systems allow the
temperature to be reduced at the desired
level, but subject to high cooling power
consumption. It is assumed that this cooling
system causes a pressure drop of 0.3kPa at
GT inlet duct. The minimum air temperature
achieved is around 5ºC, because lower air
temperatures may damage the compressor
blades.
RESULTS OF THE OPTIMISATION
As it is aforementioned, it is possible to select
three different criteria to select or to operate
the inlet air-cooling systems integrated in a
combined cycle. The first criterion is to design
the cooling system in order to obtain the
highest power output improvement. To reach
this operation point, the minimum inlet
temperature must be reached. This minimum
temperature is fixed by technological issues to
5ºC.
Results obtained with the first assumption are
shown in Table 1. The power output increase
is high, but the energy cost of this
improvement is also high, due to the efficiency
loss. High temperatures and high relative
humidity implies the use of powerful chillers.
The efficiency loss is caused by the high
electricity consumption of the chillers. The
high investment of a 38 000 kW cooling power
system probably makes unfeasible its
installation. The cost of the electricity
production and hourly incomes are blank
because the power output maximisation is
independent of the energy market framework,
and it is not possible to evaluate this cost
without an energy market framework.
Amb. Temp. Rel. Hum. Inlet Temp. M. C. P. a ∆Power Efficiency Prod. Cost Incomes
(ºC) (%) (ºC) (kW) (kW) (%) (cent/kW) (€/hour)
5,0 60 5,0 0 0 44,69 - -
15,0 60 5,0 6 917 10 294 44,24 - -
25,0 60 5,0 19 957 19 596 43,14 - -
35,0 60 5,0 38 005 29 392 41,69 - -
15,0 40 5,0 5 620 10 618 44,30 - -
15,0 60 5,0 6 917 10 294 44,24 - -
15,0 80 5,0 25 232 9 575 44,10 - -
Table 1. Cooling results when the performance criteria is maximise the power improvement
( a M. C. P. means Mechanical Chiller Power)

Amb. Temp. Rel. Hum. Inlet Temp. M. C. P. ∆ Power Efficiency Prod. Cost Incomes
(ºC) (%) (ºC) (kW) (kW) (%) (cent/kW) (€/hour)
5,0 60 5,0 0 0 44,69 - -
15,0 60 11,7 1 813 3 550 44,35 - -
25,0 60 16,7 4 550 9 341 43,55 - -
35,0 60 26,1 4 808 10 520 42,13 - -
15,0 40 11,7 1 822 3 583 44,35 - -
15,0 60 11,7 1 813 3 550 44,35 - -
15,0 80 11,8 1 766 3 446 44,35 - -
Table 2. Cooling results when the performance criteria is to maximise efficiency
The second possibility is optimising CC
efficiency. With this option, the power output
improvement is reduced in order to reach the
point of lowest fuel consumption per kilowatt.
This efficiency is calculated considering the
chiller consumption. As shown in Table 1,
since high power improvement is obtained,
efficiency loss appears as a result of the high
chiller consumption. Consequently, with this
criterion the optimum inlet air-cooling system
is obtained by reducing slightly inlet air
temperature without modify the efficiency of
the global system (working both combined
cycle and chiller).
The results of the second criteria are
presented in Table 2. The most remarkable
aspect is the reduction of the chiller size. As a
result of that, the power improvement is also
extremely reduced. That means that a
reduced cost of the chilling equipment allows
the efficiency to be optimised but prevents the
power from the highest improvements.
The last criterion is to optimise the hourly
incomes. In this study, the hourly incomes are
considered in a simplified way, as the
difference between the electricity sale and the
fuel payment. Obviously, to select an
economic parameter for the optimisation
involves the knowledge of the market
framework.
In consequence, three energy market
situations are considered: (i) a favourable
energy market, where electricity price is
clearly higher than the electricity production
cost; (ii) a non-favourable energy market,
where the electricity production cost is clearly
higher than the electricity price; and finally, (iii)
an intermediate situation, where the electricity
production cost is slightly lower than the
electricity price.
Amb. Temp. Rel. Hum. Inlet Temp. M. C. P. ∆ Power Efficiency Prod. Cost Incomes
(ºC) (%) (ºC) (kW) (kW) (%) (cent/kW) (€/hour)
5,0 60 5,0 0 0 44,69 2,90 4 568,56
15,0 60 5,0 6 916 10 293 44,24 2,93 4 451,45
25,0 60 5,0 19 957 19 596 43,14 3,00 4 167,83
35,0 60 5,0 38 005 29 392 41,69 3,11 3 785,01
15,0 40 5,0 5 620 10 617 44,30 2,92 4 467,16
15,0 60 5,0 6 916 10 293 44,24 2,93 4 451,45
15,0 80 5,0 9 789 9 575 44,10 2,93 4 416,54
Table 3. Cooling results when the performance criteria is to optimise hourly incomes in a favourable
energy market framework
(Electricity price: 4,81 cent €/kW; natural gas price: 1,50 cent €/therm)

Amb. Temp. Rel. Hum. Inlet Temp. M. C. P. ∆ Power Efficiency Prod. Cost Incomes
(ºC) (%) (ºC) (kW) (kW) (%) (cent/kW) (€/hour)
5,0 60 5,0 - - 44,69 2,90 -1 174,26
15,0 60 15,0 - - 44,34 2,92 -1 163,41
25,0 60 25,0 - - 43,36 2,99 -1 224,05
35,0 60 35,0 - - 41,79 3,10 -1 340,31
15,0 40 15,0 - - 44,34 2,92 -1 163,41
15,0 60 15,0 - - 44,34 2,92 -1 163,41
15,0 80 15,0 - - 44,34 2,92 -1 163,41
Table 4. Cooling results when the performance criteria is optimising the hourly incomes in a nonfavourable
energy market framework
(Electricity price: 2,40 cent/kW; natural gas price: 1,50 cent/therm)
Amb. Temp. Rel. Hum. Inlet Temp. M. C. P. ∆ Power Efficiency Prod. Cost Incomes
(ºC) (%) (ºC) (kW) (kW) (%) (cent/kW) (€/hour)
5,0 60 5,0 0 0 44,69 2,90 1 697,15
15,0 60 6,8 4 885 8 665 44,31 2,92 1 609,58
25,0 60 11,2 12 351 14 127 43,36 2,99 1 398,82
35,0 60 20,0 16 301 16 642 42,11 3,07 1 125,28
15,0 40 5,0 5 620 10 618 44,30 2,92 1 621,04
15,0 60 6,8 4 885 8 665 44,31 2,92 1 609,58
15,0 80 7,5 6 931 7 330 44,20 2,93 1 583,62
Table 5. Cooling results when the performance criteria is optimising the hourly incomes in an
intermediate energy market framework
(Electricity price: 6,61 cent/kW; natural gas price: 1,50 cent/therm)
The results of the third criterion are shown in
Table 3. As the sold electricity implies an
important benefit, the main result obtained
from optimisation is to reduce the inlet
temperature as possible in order to maximise
the power improvement (first criterion).
Therefore, despite the initial investment or the
operation costs, in a good energy market the
installation of a big chiller is justified.
The results in a non-favourable energy market
framework are presented in Table 4. When
costs are higher than benefits, the advice is to
produce only the obligatory electricity quantity
(set by technological or strategic purposes).
From the optimisation results, it is
recommended not to cool the inlet air since
any improvement is penalised.
The situation could be less clear as in the
former assumptions, as shown in Table 5. In
this case, the optimisation results are
intermediate between power improvement and
efficiency maximisation. The chiller sizes are
also intermediate. It is remarkable that high
temperatures and relative humidity involve a
result near to an efficiency optimisation, while
an intermediate ambient temperature allows
the power output to be maximised.
Finally, Tables 6 and 7 show the influence of
the above operation criteria in the combined
cycle and chiller system. As expected, the
third criteria allows average results to be
obtained
In Figure 1, data obtained for the different
operation criteria are presented. Since an
intermediate market situation has been
assumed, the results for the incomes
optimisation are located between the other
two cases, namely, near to the maximum
power at low temperatures and close to the
maximum efficiency at high temperatures.

Amb. Temp. Rel. Hum. Inlet Temp. M. C. P. ∆ Power Efficiency Prod. Cost Incomes
(ºC) (%) (ºC) (kW) (kW) (%) (cent/kW) (€/hour)
A 15,0 60 5,0 6 917 10 294 44,24 2,93 1 609,22
B 15,0 60 11,7 1 813 3 550 44,35 2,91 1 581,31
C 15,0 60 6,8 4 885 8 665 44,31 2,92 1 609,58
Table 6. Comparison of the all criteria results given a climate situation (temperature: 15ºC and
relative humidity: 60%)
(A: highest power improvement; B: best efficiency; C: highest hourly income)
Amb. Temp. Rel. Hum. Inlet Temp. M. C. P. ∆ Power Efficiency Prod. Cost Incomes
(ºC) (%) (ºC) (kW) (Kw) (%) (cent/kW) (€/hour)
A 25,0 60 5,0 19 957 19 596 43,14 3,00 1 395,79
B 25,0 60 16,7 4 550 9 341 43,55 2,97 1 397,86
C 25,0 60 11,2 12 351 14 127 43,36 2,99 1 398,82
Table 7. Comparison of the three criteria result in a giving climate situation (temperature: 25ºC and
relative humidity: 60%)
(A: highest power improvement; B: best efficiency; C: highest hourly income)
Inlet temperature (ºC)
Power output (kW))
30
25
20
15
10
5
0
0 10 20 30 40
245000
240000
235000
230000
225000
220000
215000
210000
205000
Ambient temperature (ºC)
Max. efficiency Max. Pow er output Max. Incomes
(a) Inlet temperatures
200000
0 10 20 30 40
Ambient temperature (ºC)
Max. efficiency Max. Pow er output Max. Incomes
(b) Power output increase
Incomes (€/hour)
1700
1600
1500
1400
1300
1200
1100
0 10 20 30 40
Ambient temperature (ºC)
Max. efficiency Max. Pow er output Max. Incomes
(c) Incomes
Figure 1. Results of the three criteria (maximum
efficiency, maximum power output and
maximum hourly incomes) for different
temperatures and 60% Relative Humidity
(Electricity price: 6,61 cent/kW; natural gas
price: 1,50 cent/therm)
CONCLUSIONS
The selection and sizing of a cooling system
to be integrated in a CC or other power
production system based in GT is a critical
aspect. Due to this, it must not be considered
as a secondary question. The criterion to set
both of the previous characteristics must be
seriously balanced.

In this paper, three optimising criteria have
been compared: (a) The power output
improvement; (b) The efficiency of the global
system; (c) The economical results depending
on the electricity and the fuel prices.
The (a) and (b) criteria lead to extreme
results. The former implies the use of
extremely big cooling systems, if the
maximum improvement are wanted to be held
in adverse climatic situations. The second
reduce the size of the cooling systems, but the
power improvements are slight, and the
problem is only partially mitigated. However,
this operation or sizing criterion is
environmentally friendly, because the fuel
consumption per electricity kilowatt is
reduced.
The third criterion is proposed in order to
include economical aspects in the electricity
generators operation. This option allows better
and more sensible results to be obtained. That
means medium cooling system size, efficiency
near to optimum efficiency and the highest
hourly incomes. Obviously, the results are
adapted to the market situation, allowing the
best investment pay back to be reached.
Therefore, an economic optimisation criteria is
adviced.
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