**Energy, Exergy and Thermoeconomics Analysis of Water Chiller Cooler for Gas Turbines Intake Air Cooling**

Rahim K. Jassim1, Majed M. Alhazmy2 and Galal M. Zaki2 *1Department of Mechanical Engineering Technology, Yanbu Industrial College, Yanbu Industrial City, 2Department of Thermal Engineering and Desalination Technology, King Abdulaziz University, Jeddah Saudi Arabia* 

### **1. Introduction**

70 Efficiency, Performance and Robustness of Gas Turbines

[15] Gurrappa, Final Report on "Design and Development of Smart Coatings for Aerospace

[17] H.L.Logan et al., Spec. Tech. Publ. No. 397 (ASTM Materials Park, OH, 1966) p. 215

[9] I.Gurrappa, Oxid. Met., 50 (1999) 353

[16] Gurrappa, Oxid. Met., 59 (2003) 321

[12] I.Gurrappa, Mater.Sci.Technol., 19 (2003) 178 [13] I.Gurrappa, Surf. Coat. Tech., 139 (2001) 272 [14] Gurrappa, J.Coat. Technol. Res., 5 (2008) 385

[10] C.J. Wang and J.H. Lin, Mater.Chem.Phys., 76 (2002) 123

[11] I.Gurrappa and A. Sambasiva Rao, Surf. Coat. Technol., 201 (2006) 3016

Applications" submitted to European Commission, July 2008

During hot summer months, the demand for electricity increases and utilities may experience difficulty meeting the peak loads, unless they have sufficient reserves. In all Gulf States, where the weather is fairly hot year around, air conditioning (A/C) is a driving factor for electricity demand and operation schedules. The utilities employ gas turbine (GT) power plants to meet the A/C peak load. Unfortunately, the power output and thermal efficiency of GT plants decrease in the summer because of the increase in the compressor power. The lighter hot air at the GT intake decreases the mass flow rate and in turn the net output power. For an ideal GT open cycle, the decrease in the net output power is ~ 0.4 % for every 1 K increase in the ambient air temperature. To overcome this problem, air intake cooling methods, such as evaporative (direct method) and/or refrigeration (indirect method) has been widely considered [Cortes and Williams 2003].

In the direct method of evaporative cooling, the air intake cools off by contacts with a cooling fluid, such as atomized water sprays, fog or a combination of both, [Wang 2009]. Evaporative cooling has been extensively studied and successfully implemented for cooling the air intake in GT power plants in dry hot regions [Ameri *et al.* 2004, 2007, Johnson 2005, Alhazmy 2004, 2006]. This cooling method is not only simple and inexpensive, but the water spray also reduces the NOx content in the exhaust gases. Recently, Sanaye and Tahani (2010) investigated the effect of using a fog cooling system, with 1 and 2% over-spray, on the performance of a combined GT; they reported an improvement in the overall cycle heat rate for several GT models. Although evaporative cooling systems have low capital and operation cost, reliable and require moderate maintenance, they have low operation efficiency, consume large quantities of water and the impact of the non evaporated water droplets in the air stream could damage the compressor blades [Tillman *et al* 2005]. The water droplets carryover and the resulting damage to the compressor blades, limit the use of evaporative cooling to areas of dry atmosphere. In these areas, the air could not be cooled below the wet bulb temperature (WBT). Chaker *et al* (2002, 2003), Homji-meher *et al* (2002)

Energy, Exergy and Thermoeconomics

discussed.

thermodynamics.

gases;

**2.1 Gas turbine cycle** 

in a smaller profit than inlet air chilling [Yang *et al* 2009].

**2. GT-air cooling chiller energy analysis** 

Analysis of Water Chiller Cooler for Gas Turbines Intake Air Cooling 73

payback period and the efficiency ratio for off-design conditions of both the GT and cooling system. Investigations of evaporative cooling and steam absorption machines showed that inlet fogging is superior in efficiency up to intake temperatures of 15-20oC, though it results

In the present study, the performance of a cooling system that consists of a chilled water external loop coupled to the GT entrance is investigated. The analysis accounts for the changes in the thermodynamics parameters (applying the first and second law analysis) as well as the economic variables such as profitability, cash flow and interest rate. An objective of the present study is to assess the importance of using a coupled thermo-economics analysis in the selections of the cooling system and operation parameters. The developed algorithm is applied to an open cycle, HITACH MS-7001B plant in the hot weather of KSA (Latitude 24o 05" N and longitude 38o E) by The result of this case study are presented and

Figure 1.a shows a schematic of a simple open GT "Brayton cycle" coupled to a refrigeration system. The power cycle consists of a compressor, combustion chamber and a turbine. It is presented by states 1-2-3-4 on the T-S diagram, Fig. 1.b. The cooling system consists of a refrigerant compressor, air cooled condenser, throttle valve and water cooled evaporator. The chilled water from the evaporator passes through a cooling coil mounted at the air compressor entrance, Fig. 1.a. The refrigerant cycle is presented on the T-S diagram, Figure 1.c, by states *a, b, c* and *d*. A fraction of the power produced by the turbine is used to power the refrigerant compressor and the chilled water pumps, as indicated by the dotted lines in Fig. 1.a. To investigate the performance of the coupled GT-cooling system the different involved cycles are analyzed in the following employing the first and second laws of

As seen in Figures 1.a and 1.b, processes 1-2s and 3-4s are isentropic. Assuming the air as an

*T T <sup>P</sup> PR*

The first term of the RHS is the power produced by the turbine due to expansion of hot

( ) *W mc T T t t pg t* = − η

1

− <sup>−</sup> == =

*k <sup>k</sup> <sup>k</sup> <sup>s</sup> <sup>k</sup>*

1

, ( ) *W WW W net t comp el ch* =− + (2)

3 4*<sup>s</sup>* . (3)

(1)

ideal gas, the temperatures and pressures are related to the pressure ratio, *PR*, by:

2 3 2 14 1

*s*

The net power output of a GT with mechanical cooling system as seen in Fig. 1.a is

*TT P*

and Gajjar *et al* (2003) have presented results of extensive theoretical and experimental studies covering aspects of fogging flow thermodynamics, droplets evaporation, atomizing nozzles design and selection of spray systems as well as experimental data on testing systems for gas turbines up to 655 MW in a combined cycle plant.

In the indirect mechanical refrigeration cooling approach the constraint of humidity is eliminated and the air temperature can be reduced well below the ambient WBT. The mechanical refrigeration cooling has gained popularity over the evaporative method and in KSA, for example, 32 GT units have been outfitted with mechanical air chilling systems. There are two approaches for mechanical air cooling; either using vapor compression [Alhazmy (2006) and Elliott (2001)] or absorption refrigerator machines [Yang *et al* (2009), Ondryas *et al* (1991), Punwani (1999) and Kakarus *et al* (2004)]. In general, application of the mechanical air-cooling increases the net power but in the same time reduces the thermal efficiency. For example, Alhazmy *et al* (2004) showed that for a GT of pressure ratio 8 cooling the intake air from 50oC to 40oC increases the power by 3.85 % and reduces the thermal efficiency by 1.037%. Stewart and Patrick (2000) raised another disadvantage (for extensive air chilling) concerning ice formation either as ice crystals in the chilled air or as solidified layer on air compressors' entrance surfaces.

Recently, alternative cooling approaches have been investigated. Farzaneh-Gord and Deymi-Dashtebayaz (2009) proposed improving refinery gas turbines performance using the cooling capacity of refinerys' natural-gas pressure drop stations. Zaki *et al* 2007 suggested a reverse Brayton refrigeration cycle for cooling the air intake; they reported an increase in the output power up to 20%, but a 6% decrease in thermal efficiency. This approach was further extended by Jassim *et al* (2009) to include the exergy analysis and show that the second law analysis improvement has dropped to 14.66% due to the components irreversibilities. Khan *et al* (2008) analyzed a system in which the turbine exhaust gases are cooled and fed back to the compressor inlet with water harvested out of the combustion products. Erickson (2003, 2005) suggested using a combination of a waste heat driven absorption air cooling with water injection into the combustion air; the concept is named the "*power fogger cycle*".

Thermal analyses of GT cooling are abundant in the literature, but few investigations considered the economics of the cooling process. A sound economic evaluation of implementing an air intake GT cooling system is quite involving. Such an evaluation should account for the variations in the ambient conditions (temperature and relative humidity) and the fluctuations in the fuel and electricity prices and interest rates. Therefore, the selection of a cooling technology (evaporative or refrigeration) and the sizing out of the equipment should not be based solely on the results of a thermal analysis but should include estimates of the cash flow. Gareta *et al* (2004) has developed a methodology for combined cycle GT that calculated the additional power gain for 12 months and the economic feasibility of the cooling method. From an economical point of view, they provided straight forward information that supported equipment sizing and selection. Chalker *et al* (2003) have studied the economical potential of using evaporative cooling for GTs in USA, while Hasnain (2002) examined the use of ice storage methods for GTs' air cooling in KSA. Yang *et al* (2009) presented an analytical method for evaluating a cooling technology of a combined cycle GT that included parameters such as the interest rate, payback period and the efficiency ratio for off-design conditions of both the GT and cooling system. Investigations of evaporative cooling and steam absorption machines showed that inlet fogging is superior in efficiency up to intake temperatures of 15-20oC, though it results in a smaller profit than inlet air chilling [Yang *et al* 2009].

In the present study, the performance of a cooling system that consists of a chilled water external loop coupled to the GT entrance is investigated. The analysis accounts for the changes in the thermodynamics parameters (applying the first and second law analysis) as well as the economic variables such as profitability, cash flow and interest rate. An objective of the present study is to assess the importance of using a coupled thermo-economics analysis in the selections of the cooling system and operation parameters. The developed algorithm is applied to an open cycle, HITACH MS-7001B plant in the hot weather of KSA (Latitude 24o 05" N and longitude 38o E) by The result of this case study are presented and discussed.
