**1. Introduction**

96 Efficiency, Performance and Robustness of Gas Turbines

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the Geometry of Continuous Fins on an Array of Tubes of a Refrigeration Air

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optimization of a refrigeration evaporator coil with continuous fins. Trans, ASME,

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The output of Gas turbine (GT) power plants operating in the arid and semiarid zones is affected by weather conditions where the warm air at the compressor intake decreases the air density and hence reduces the net output power far below the ISO standard (15 oC and 60% relative humidity). The power degradation reaches an average of 7% for an increase in temperature by only 10oC above the 15 oC ISO standard. Furthermore; in hot summer days the plants are overloaded due to the increase in demand at peak periods, to meet the extensive use of air-conditioning and refrigeration equipment. The current techniques to cool the air at the compressor intake may be classified into two categories; direct methods employing evaporative cooling and indirect methods, where two loops refrigeration machines are used. Erickson (2003) reviewed the relative merits, advantages and disadvantages of the two approaches; Cortes and Willems (2003), and Darmadhkari and Andrepont (2004) examined the current inlet air cooling technology and its economic impact on the energy market.

In direct cooling methods water is sprayed at the compressor inlet bell mouth either through flexuous media (cellulose fiber) or fogging (droplets size in the order of 20 micron) into the air stream, Ameri *et al*. (2004). All spray cooling systems lower the intake temperature close to the ambient wet bulb temperature; therefore the use of the spray cooling is inefficient in coastal areas with high air humidity. Ameri *et al*. (2004) reported 13% power improvement for air relative humidity below 15% and dry bulb temperature between 31oC and 39oC. In addition to the effect of the ambient air humidity, the successful use of the direct method depends on the spray nozzles characteristics, Meher-Homji *et al* (2002) and droplets size, Bettocchi *et al*. (1995) and Meher-Homji and Mee, (1999). In evaporative cooling there is, to some extent, water droplets carry over problem, addressed by Tillman *et al* (2005), which is hazardous for compressor blades. Therefore, evaporative cooling methods are of limited use in humid coastal areas. Alhazmy *et al* (2006) studied two types of direct cooling methods: direct mechanical refrigeration and evaporative water spray cooler, for hot and humid weather. They calculated the performance improvement for ranges of ambient temperature and relative humidity, and their results indicated that the direct mechanical refrigeration increased the daily power output by 6.77% versus 2.5% for spray water cooling.

Energy and Exergy Analysis

**2. Energy analysis 2.1 Brayton power cycle** 

the compressor.

temperature as;

and

1. Portion of the compressed air

the exergy destruction terms are evaluated.

of Reverse Brayton Refrigerator for Gas Turbine Power Boosting 99

The objective of the present analysis is to investigate the potential of boosting the power output of gas turbine plants operating in hot humid ambiance. The previously proposed coupled Brayton and reverse Brayton refrigeration cycles is analyzed employing both the energy and exergy analysis. Both the thermal and exergetic efficiencies are determined and

Details of the first law energy analysis have been presented in a previous study by Zaki *et al* (2007) but the basic equations are briefly given here. Figure 1 shows the components of a coupled gas turbine cycle with Brayton refrigeration cycle. The power cycle is represented by states 1-2-3-4 and the reverse Brayton refrigeration cycle is represented by states 1-6-7-8-

cooled in a heat exchanger to *T7* then expands to the atmospheric pressure and *T8* as seen in Fig.1. The ambient intake air stream at *To* mixes with the cold stream at *T8* before entering

Figure 2 shows the combined cycle on the T-s diagram; states *o*- 2 -3-4 represent the power cycle without cooling, while the power cycle with air inlet cooling is presented by states 1-2-

α

The mass and energy balance for the mixing chamber gives the compressor inlet

( ) <sup>8</sup> 1 *poo p 8*

*c*

Air leaves the compressor at *P xP* 6 1 = flows through the Brayton refrigerator and the rest at *P rP* 2 1 = as the working fluid for the power cycle, *x* is defined as the extraction pressure

The temperature of the air leaving the compressor at states 6 and 2 can be estimated assuming irreversible compression processes between states 1-2 and 1-6 and introducing

> *cx <sup>T</sup> TT x 1 η*

*cr <sup>T</sup> TT r 1 η*

=+ −

=+ −

*6 1*

*2 1*

*γ 1 1 γ*

*γ 1 1 γ*

<sup>−</sup>

<sup>−</sup>

*p 1 α c T α c T*

ω

*m*<sup>1</sup> at pressure *P6* is extracted from the mainstream and

*<sup>o</sup>* enters the chamber and mixes with the

(2)

(3)

*m*<sup>1</sup> at *T8*. Air leaves the chamber at *T1*, which

α.

− + <sup>=</sup> (1)

α

3-4-1. States 1-6-7-8 present the reverse Brayton refrigeration cycle.

*1*

*T*

depends on the ambient air conditions and the extraction ratio

In the mixing chamber ambient air at *mo , To* and

cold air stream having mass flow rate of

ratio and *r* is the pressure ratio.

isentropic compressor efficiency so that;

Indirect cooling by mechanical refrigeration methods can reduce the air temperature to any desirable value, even below the 15oC, regardless of the ambient humidity. There are two common approaches for air chilling: a) use of refrigeration units via chilled water coils, b) use of exhaust heat-powered absorption machines. As reported by Elliot (2001), application of mechanical air-cooling increases the net power on the expense of the thermal efficiency, 6% power boosting for 10oC drop in the inlet air temperature. Using absorption machines was examined for inlet air-cooling of cogeneration plants, Ondrays *et al* (1991), while Kakaras *et al.* (2004) presented a simulation model for NH3 waste heat driven absorption machine air cooler. A drawback of the mechanical chilling is the risk of ice formation either as ice crystals in the air or as solidified layer on surfaces, such as the bell mouth or inlet guide vanes, Stewart and Patrick, (2000).

Several studies have compared the evaporative and mechanical cooling methods; with better performance for mechanical cooling. Mercer (2002) stated that evaporative cooling has increased the GT power by 10–15%, while the improvement for refrigeration chillers has reached 25%. Alhazmy and Najjar (2004) concluded that the power boosting varied between 1-7% for spray cooling but reached10-18% for indirect air cooling. In a recent study by Alhazmy *et al* (2006), they introduced two generic dimensionless terms (power gain ratio (*PGR*) and thermal efficiency change factor (*TEC*)) for assessment of intake air cooling systems. They presented the results in general dimensionless working charts covering a wide range of working conditions. Zadpoor and Golshan (2006) discussed the effect of using desiccant-based evaporative cooling on gas turbine power output. They have developed a computer program to simulate the GT cycle and the NOx emission and showed that the power output could be increased by 2.1%. In another development Erickson (2003 & 2005) suggested combination of the methods combining waste driven absorption cooling with water injection into the combustion air for power boosting; the concept was termed the "*power fogger cycle*". A novel approach has been presented by Zaki *et al* (2007), where a reverse Joule-Brayton air cycle was used to reduce the air temperature at the compressor inlet. Their coupled cycle showed a range of parameters, where both the power and thermal efficiency can be simultaneously improved.

As revealed in the above account abundant studies are concerned with the first law analysis of air intake cooling but those focuses on the second law analysis are limited. The basics of the second law analysis have been established and employed on variety of thermal systems by number of researchers. Bejan (1987, 1997), Bejan et al 1996, Rosen and Dincer (2003 a, and 2003 b) Ranasinghe, *et al* (1987), Zaragut *et.al* (1988), Kotas *et al* (1991) and Jassim (2003a, 2003b, 2004, 2005 and 2006), Khir *et al* (2007) have dealt extensively with various aspects of heat transfer processes. Chen *et al* (1997) analyzed the performance of a regenerative closed Brayton power cycle then extended the method to a Brayton refrigeration cycle, Chen *et al* (1999). The analysis considered all the irreversibilities associated with heat transfer processes. The exergy analysis of Brayton refrigeration cycle has been considered by Chen and Su (2005) to set a condition for the maximum exergetic efficiency while Tyagi *et al* (2006) presented parametric study where the internal and external irreversibilities were considered. The maximum ecological function, which was defined as the power output minus the power loss was determined for Brayton cycle by Huang *et al* (2000). Their exergy analysis was based on an ecological optimization criterion and was carried out for an irreversible Brayton cycle with external heat source.

The objective of the present analysis is to investigate the potential of boosting the power output of gas turbine plants operating in hot humid ambiance. The previously proposed coupled Brayton and reverse Brayton refrigeration cycles is analyzed employing both the energy and exergy analysis. Both the thermal and exergetic efficiencies are determined and the exergy destruction terms are evaluated.
