Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)33

Power Output Augmentation of Gas Turbine Combined
Cycle by Inlet-Air Cooling System of Chiller Typeunder High Ambient Air Temperature
TAKANOBU KOMURO*1

EISAKU ITO*2TAKASHI SONODA*2

YASUOKI TOMITA*3

KOHEI HIDAKA*4

SEIJI SHIBUTANI*1

At Mitsubishi Heavy Industries, Ltd. (MHI), large-scale gas turbines for electric power
plants are designed and manufactured at the Takasago Machinery Works. Because of the highly
advanced design and manufacturing technologies used for turbo machines and heat exchangers,
large-capacity, high-efficient centrifugal-chillers are also designed and manufactured at theAir-Conditioning & Refrigeration Systems Headquarters Takasago Plant that is part of theTakasago Machinery Works. By combining these two product technologies, an inlet-air coolingsystem with a gas-turbine inlet section has been developed for gas turbine combined cycle power
plants (GTCC). For refrigeration equipment, a centrifugal-chiller, cooling tower, water supply
pump and power supply unit are packaged in a single unit. An inlet-air cooling coil unit placedin an air-inlet duct is devised so as to be installed during the major inspection period of gasturbines. Our Power Systems Headquarter and Air-Conditioning & Refrigeration Headquarterare integrated to provide one-stop services for our customers.

|1. Introduction
At Mitsubishi Heavy Industries, Ltd. (MHI), large-scale gas turbines for electric power plants
are designed and manufactured at the Takasago Machinery Works. Because of the highly advanced
design and manufacturing technologies used for turbo machines and heat exchangers,
large-capacity, high-efficient centrifugal-chillers are also designed and manufactured at theAir-Conditioning & Refrigeration Systems Headquarters Takasago Plant that is part of theTakasago Machinery Works. By combining these two product technologies, an inlet-air coolingsystem with a gas-turbine inlet section has been developed for gas turbine combined cycle power
plants (GTCC). The relationship of the output power of a gas turbine – the core of a combined
cycle power plant – to ambient air temperature is shown in Figure 1.

Figure 1 Power-output recovery mechanism by inlet-air cooling system in gas turbine
combined cycle power plant (GTCC)
Air density increases by lowering inlet-air temperature, contributing to power output recovery.
*1

Manager, Centrifugal & Absorption Chiller Department, Air-Conditioning & Refrigeration Systems Headquarters*2

Manager, Takasago Research & Development Center, Technical Headquarters*3

Manager, Service Department, Takasago Machinery Works*4Service Department, Takasago Machinery Works

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)34

In a gas turbine, atmospheric air is suctioned, compressed, mixed with fuel and burned in acombustor to generate electricity. The air density falls as the ambient air temperature rises, suchthat the power output decreases depending on the decrease in the mass of air that is sent to thecombustor. When air at a temperature of 35°C is cooled to 15°C, the air density increases by about
10 percent, increasing the power output by about 10 percent.

|2. Inlet-air cooling system
2.1

Cooling mechanism of centrifugal-chiller

Figure 2 shows the cooling mechanism of the centrifugal-chiller.
The cooling mechanisms are as follows:(1) A refrigerant gas contained in the top section of an evaporator under conditions below
10°C and around 0.3 MPa is compressed by a turbo (centrifugal) compressor and sent to acondenser, with the conditions of the gas over 40°C and around 1 MPa. (2) In a wet cooling tower(one example here, but many other methods of heat exchange are known), cooling water at around
30°C that has been heat-exchanged with atmospheric air is sent to the above condenser and again
heat-exchanged with the refrigerant, liquefying the refrigerant. (3) The high pressure liquefiedrefrigerant in the condenser is adiabatically expanded to the evaporator via an expansion valve,
resulting in a low pressure liquefied refrigerant at a single-digit temperature level. (4) In anevaporator, the low pressure liquefied refrigerant is heat-exchanged with the returned cooling waterat 10-some °C from the cooling coil, returning the refrigerant to a gaseous condition and where thecooling water is cooled to a single-digit temperature (for instance, 5°C). MHI’s centrifugal-chiller
has a feature that can provide cold energy corresponding to 6.5 times the electric power for thedrive motor of a compressor to a cooling coil, which is referred to as a coefficient of performance(COP) of 6.5.

Figure 2 Cooling mechanism of centrifugal-chiller

2.2

Comparison of inlet-air cooling system

Table 1 shows a comparison of general inlet -air cooling systems.
Inlet-air cooling systems include: (a) An eva-cooler system in which air is cooled with thelatent heat of water vaporization by dripping water from evaporative elements placed in thedownstream of an air-inlet filter housing in an air-inlet duct inlet portion. (b) A fog system in which
air is cooled with the latent heat of water vaporization by atomizing water mist in the downstreamof an air-inlet filter housing. (c) A chiller system in which air is cooled by introducing chilled waterfrom a chiller such as a centrifugal-chiller into a cooling coil placed in the downstream of an
air-inlet filter housing.
In this development of the cooling system, a chiller system that can lower the temperature of
the inlet air below a wet-bulb temperature, which results in a greater increase rate, has been
selected.

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)35

Table 1 Comparison of inlet air cooling systems
Eva-cooler system
Systemoutline

Fog system

Chiller system

Inlet-air cooling with the latent Inlet-air cooling with the latent heat Inlet-air cooling by introducing
heat of water by dripping water of water by atomizing water mist
chilled water from a chiller such as
from evaporative elements of ana centrifugal-chiller into a coolingeva-cooler
coil placed in the downstream of an
inlet-air filter housing

Systemconfiguration

approx. 5°C
approx. 5°C
Inlet-airapprox. 15°C
temperature (atmospheric conditions: 30°C, RH: (atmospheric conditions: 30°C, RH:(atmospheric conditions: 30°C)
60%)
60%)
drops
Outputapprox. 3%
approx. 3%
approx. 9%
recovery rate(1) Air temperature is not attained (1) Drainage system is required in (1) Output recovery rate is high.
Featuresbelow a wet-bulb temperature,(2) Inlet air temperature is attained
an inlet air duct.
so that the effect of system is (2) Air temperature is not attained
independently of humidity.
small in high humidity
below a wet-bulb temperature, (3) A cooling coil and a mist
so that the effect of system isconditions.
eliminator must be installed.
smallinhighhumidity (4) The initial investment cost is(2) A mist eliminator is required.
conditions.
higherfor
chiller-systeminstallation.

2.3

Component of inlet-air cooling system

Figure 3 shows the main components of the newly developed inlet-air cooling system. The
newly developed cooling system includes an air cooling coil (1) (that is installed in the downstreamof a filter housing in the duct-inlet portion), a centrifugal-chiller (2), a cooling tower (3), a chilledwater pump (4) for circulating chilled water from/to an air cooling coil (1) and acentrifugal-chiller (2) a cooling water pump (5) for circulating cooling water from/to acentrifugal-chiller (2) and a cooling tower (3) and electrical facilities (6) for driving acentrifugal-chiller compressor motor, a cooling-tower fan motor and pump motors, wherein thecomponents (2), (3), (4), (5) and (6) are combined in a packaged chiller unit with a cooling towershown in this figure.

Figure 3

Components of inlet air cooling system

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)36

2.4

Selection of refrigeration capacity

Figure 4 shows temperature data and relative humidity data per hour throughout the year inChon Buri City in Thailand.
In the figure, using the same centrifugal-chiller where the inlet air can be cooled to 10°C at
7,800 Rt, with the cooling-water temperature raised slightly to boost a refrigeration capacity to
8,600 Rt, the cooling range that can cool the inlet air to 13.5°C is shown. In this manner, the proper
refrigeration capacity of the chiller can be selected depending on the weather conditions of thetargeted location.

Figure 4 Temperature data and relative humidity data per hour throughout the year in thearea around the power-station and examples of selecting the chiller capacity of theinlet-air cooling-system

Figure 5 shows the output increases of GTCC under each weather condition, along withhumidity parameters.
This shows that greater output increases are obtained, compared with an eva-cooler system.
(Note: 1USRT refers to a refrigeration capacity that cools 1US ton (2,000 lb) of water at 0°C into
ice within 24 hours.)

Figure 5

Example of power output recovery, with inlet air chilled using a centrifugal-chiller

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)37

2.5

Layout of equipment

Figure 6 shows the layout of an inlet-air cooling system as an example. The inlet-air coolingsystem includes two sets of chiller modules and a cooling-coil housing that is placed between thefilter housing and the inlet-air duct; wherein cooling water is sent by water supply pumps placed inchiller modules through piping connected from chiller modules to the cooling-coil housing andthen distributed to cooling coils through headers on both sides. Cooling water is finally returned tothe chiller modules after cooling the inlet air.

2.6

Chiller module

Figure 7 shows the chiller module. The chiller module includes two sets ofcentrifugal-chillers, four sets of water supply pumps (two sets for chilled water, two sets forcooling water), their motor panels, transformer panels (e.g., 6kV to 400V, etc.), incoming panels
and control panels, where those panels are placed on the first floor and a cooling tower is installedon the second floor, wherein piping and cable wiring between each piece of equipment areconnected in the module.
Chillers are installed with AART-200s in series and have a capacity of 4,400 USRt, whereinchilled water is circulated at 1,200 m3/hr, under the conditions of incoming cooling water at 16°C
and outgoing water at 5°C.

Figure 6
Layout example of inlet-air
cooling-system equipment

2.7

Figure 7
Configuration of 4,400 USRT
chiller module

Cooling-Coil Housing and Assembly MethodFigure 8 shows an example of the inlet-air cooling-coil housing. The cooling-coil housing isinstalled in a space of 3m in width between the filter housing and the inlet air duct. In this example,cooling coils each having a cooling capacity of 2.3 MW are arranged in two rows and stacked in six
tiers, amounting to a cooling capacity of 28 MW.

Figure 8Configuration of inlet-air cooling coils in turbine inlet portion

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
38

Figure 9 shows an example of the coil-assembly method.
In the case of existing facilities, the coil-assembly method is as follows: (1) The filter
housing is separated from the duct to move to the upstream side by 3 m. (2) Each side of the coilhousing (i.e., the filter housing side and the duct side) is assembled to be connected with the filter
housing and the duct, respectively. (3) The cooling coils are lifted up to be connected with the steel
frame of the floor. (4) The cooling coils are stacked in order and connected with each other. (5) The
nozzles of the cooling coils (i.e., the nozzles of the inlet and outlet of the chilled water) are
extruded to exit out the side portion of the coil housing. The space between the cooling coils andthe upstream side and downstream side of the coil housing are covered by seal plates. (6) The
headers and nozzles are connected. (Each nozzle and each header nozzle are connected by valves
and flexible joints.)
In addition, not shown in step (2) above, a stage structure which has a height corresponding
to the height of the second floor coils, is fabricated in the filter housing side of the coil housing. In
the side face of the housing, a stairway (shown in Figure 8) is assembled, allowing access to eachstage and the top of the coil housing.
This assembly manner is devised so as to make it possible to install the coils during themajor inspection period of the gas turbine.

Figure 9

Installation of coils in air cooling coil housing|3. Engineering and service
As shown in Figure 10, centrifugal-chillers are developed, designed and manufactured attheir own plant in the same area of the Takasago Machinery Works where gas turbines are alsodeveloped, designed and manufactured. This inlet-air cooling system is also developed by a joint
team from the Power Systems Headquarter and Air-Conditioning & Refrigeration Headquarter,
together with members of the Takasago Research & Development Center, which are adjacent toeach other.
In the application of an inlet-air cooling system to the gas turbine combined cycle power
plants of each customer, plant information as the OEM from the Power Systems Headquarter iscommunicated to the Air-Conditioning & Refrigeration Headquarter, so that an optimum systemaligned with weather conditions can be proposed immediately and presented to our customers.
In maintenance services as well, when you contact the Power Systems Services Headquarter,
the information will be communicated to the Air-Conditioning & Refrigeration Headquarter, so thatmaintenance services can be smoothly provided.

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)
39

Figure 10 One-stop services|4. Conclusion
We have developed a chiller type inlet-air cooling system in which a highly efficient
centrifugal-chiller is applied for gas turbine combined cycle power plants. The system includes acooling coil module and a chiller module. The optimum cooling temperature of the inlet air and thecapacity of the chiller for each plant are selected based on plant information as the OEM and theweather conditions of the targeted area. The capacity can be met by combining several chiller
modules. For chiller modules, a centrifugal-chiller, a cooling tower, a water supply pump and anelectric power unit, a transformer and a control unit are packaged in a single unit. Inlet -air coolingmodules, in this example, are placed in a space of 3m in width between the filter housing and theinlet-air duct, with coils arranged in two rows and stacked in six tiers and can be installed during
the major inspection period of a gas turbine. Our Research & Development Center, Design andEngineering Section, Manufacturing Section and Service Section for gas turbines and chillers aregathered in the same area of Takasago, Hyogo Prefecture, allowing one-stop services to beprovided for customers.