ANALYSIS OF ALTERNATIVES FOR GAS TURBINES INLET AIR COOLING

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1 ANALYSIS OF ALTERNATIVES FOR GAS TURBINES INLET AIR COOLING Waldir Guirardi, Maua Institute of Tecnology Praça Mauá 01, S. Caetano do Sul, SP, Brasil Pone: Fax: Roberto Peixoto (corresponding autor) Maua Institute of Tecnology Praça Mauá 01, S. Caetano do Sul, SP, Brasil Pone: Fax: Abstract: In te coming years, it is expected an increase of te electricity consumption In Brazil. Tis fact ill collaborate to te diffusion of distributed generation of electricity strategy and to te searc for optimized systems for energy conversion systems. Te natural gas availability in Brazil is expanding, and as motivated investors for electricity generation using systems based on gas turbine associated it te production of eat/cold. Te goal is to reac consumers it demand loer tan 5 MW, suc as commercial centers, condominiums, and also some industries. Energy efficiency of combustion turbines, is negatively affected by te increment of te inlet air temperature. Te entrance aircooling in gas turbines can promote a more energy efficient and economical operation, and also a better performance in te environmental point of vie. Tis paper presents te influence of inlet atmosperic air conditions in gas turbine efficiency and operational parameters, and te benefits of air-cooling systems. It makes a comparison beteen to cooling tecnologies, evaporative cooling and refrigeration by absorption systems. Te absorption system is driven by te eat contents of te turbine effluent gases. Keyords: Combustion turbines, Evaporative cooling, Absorption cooling. NOTATION AC absorption cooler CP compressor DB dry bulb temperature [ o C] EC evaporative cooler GT Gas turbine specific entalpy [kj/kg] m mass flo [kg/s] P pressure, kpa Q eat [KW] RH relative umidity s specific entropy [kj/kg-c] T temperature[ o C] TB turbine W Poer [KW] WB et bulb temperature [ o C] X LiBr concentration, % Subscripts 0 dead state a air abs absorber cc combustion camber cond condenser cp compressor evap evaporator gen generator i inlet o outlet r real sb saturation temperature on WB curve t turbine ater Greek symbols η energetic efficiency η ex exergetic efficiency ω umidity ratio 1. Introduction In Brazil tere is a massive use of ydraulic sources for electricity generation. Te ydro-electrical energy is generated in distant places due to te decrease of ydraulic sources near te big consumer centers. Tis fact creates te need for long transmission lines, and, as a consequence, te increase of losses and te reduction of te safety level for te energy supply. Despite te periods of turbulence in te global economy, Brazil is expected to gro in te near future, at ill increase te electricity consumption. Tis scenario associated it difficulties for ydraulic electricity generation, as raised te interest in te small-scale energy production. Te diffusion of te strategy electricity distributed generation as an alternative for te demand increasing, as originated te searc for optimized cogeneration systems, it electricity and integrated eating and/or cooling production. Tis alternative is expected to expand for small and medium size consumers like commercial and residential centers, and also in some industries.

2 Natural gas availability in Brazil is increasing, eiter due to te discovery of ne fields and to te possibility of importation from Bolivia and Argentina. Tis fact motivates te investors for electricity generation. Consumers it demand loer tan 5 MW constitute te main market, and gas turbine is an available tecnology. Energy efficiency of combustion turbines, is negatively affected by te increment of te inlet air temperature, and tis generally occurs in te period of electricity consumption peak. During tis period te cost of te bougt energy is very ig and it becomes necessary to increase te proper production. Te entrance air-cooling in gas turbines can promote a more energy efficient and economical operation, and also a better performance in te environmental point of vie. Tis paper presents te influence of inlet atmosperic air conditions in gas turbine efficiency and operational parameters, and te benefits of air-cooling systems. It makes a comparison beteen to cooling tecnologies, evaporative cooling and refrigeration by absorption systems. Te absorption system is driven by te eat contents of te turbine effluent gases Te study is based on te numerical simulation of a straigtforard matematical model developed using te conservation principles. Te softare EES as used for te simulation. Operational parameters, suc as produced or consumed poers, fuel consumption, outflos, temperatures, etc ere calculated. Te simulations ad varied te altitude, te air relative umidity and temperature using atmosperic data for to cities in Brazil Belém (nort region) and Curitiba (sout region). Curitiba being a relatively ig city, distant of te sea and more to te sout, as a colder and drier climate tan Belém, tat is a city to te level of te sea, close to a great river, and nearby to te equator. 2. GAS TURBINE Basically, combustion turbines or gas turbines (GT) are termal engines, under te point of vie of te Termodynamics, for production of mecanical energy using te energy of te gases produced during te fuel combustion. Te Brayton cycle, using air as orking fluid is te ideal termodynamic cycle tat serves of model for te GT. Tis cycle is son in Fig. (1a). It consists basically of compressor, eater, turbine and cooler. Te Brayton cycle is formed by four ideal processes, initiating by an isentropic compression (1 2), folloed by isobaric eating (2 3) and posterior isentropic expansion (3 4). Te fourt stage is te air isobaric cooling of (4 1). A real GT consists of compressor, turbine and combustion camber. Te last stage (effluent gases cooling) is made in te atmospere, itout te existence of a eat excanger. Te real engine does not operate in closed cycle, terefore, air does not return to te equipment being discarded it te combustion gases. Figure (1b) sos te sceme of a GT and Fig. (2) sos te diagram x s for air in te Brayton cycle. Heater 3 Combustible 2 CC 3 CP TB G 2 1 Cooler 4 CP 4 TB G 1 Gases a b Figure 1 a) Basic sceme of Brayton cycle. b) Basic sceme of gas turbine 2.1 Gas turbine analyzed. Tere are several types of GT configurations, varying from te basic design to regenerative, it reeat and intercooling. Te study as developed considering te basic design GT, ic is te most used for poer belo 5 MW. In a real GT te processes are irreversible and compression and expansion ave a significant entropy increase as it is soed by te dotted lines in Fig. (2). Te GT matematical modeling is based on te conservation equations, mass and energy balances. To simplify te analysis te folloing assumptions ere adopted: 1. Closed cycle it umid air as a orking fluid, based on Brayton cycle, it irreversible processes (as described above) 2. Steady state conditions.

3 3. Negligible pressure drop in te pipes 4. Negligible eat losses to te surrounding. entalpy Q cc entropy Figure 2 Mollier diagram for air Te equations are: Compressor m m + W a 1 a 2' cp Combustion camber m a 2' + Qcc ma3 Turbine m m + W a 3 a 4' t Te efficiency of te real equipment is calculated by: η η m r cc a ( 3 4) 2 1 ηt η Q Were te efficiencies are defined by: Combustion camber: η Q Q cc cc r Q r Fuel energy Compressor: cc cp η cp 2 ' 2 1 1

4 Turbine: η t 3 ' INLET AIR COOLING To alternatives for inlet air-cooling ere studied: evaporative cooling and absorption cooling systems driven by te eat contents of te turbine effluent gases Evaporative Cooling (EC) Wen not saturated air enters in direct contact it ater, part of te ater evaporates, increasing te air relative umidity and decreasing its temperature. Tis is a process ere te variation of te air temperature and umidity is caused only by te ater evaporation. It is considered tat te process is adiabatic, not aving eat excange it te ambient. In te teoretical process, te air is considered saturated at te exit, and its temperature is called adiabatic-saturation temperature tat is very close to te et bulb temperature WB. In te real process, te air final temperature can be around 1 o C iger tan te WB. Te matematical modeling for te EC system simulation as developed using te mass and energy balance equations: m + m m ai ai ai m + m m m ao ω ai + ωao mai ao ao Te EC effectiveness is defined by te equation belo and it is adopted equal to 0,85 in te matematical modeling. η EC Ti T T T i o sb In Fig. (3a) it is presented a sceme of an evaporative cooling system and Fig. (3b) sos te psycometric cart of evaporative cooling of te umid air. Figure 3 a) Evaporative Cooling (EC) sceme. b) Evaporative cooling diagram of psycometric cart Absorption Cooling (AC) AC is a cooling system tat orks based on te absorption cycle ic transfers eat from a cold source to a ot source. Te cycle utilizes to fluids: an absorbent and a refrigerant. Figure (4) sos AC ere, inside te atced

5 line, it is presented te system for te refrigerant pressure rising. Te main caracteristic of tis refrigeration system is to be driven basically by eat and not by ork as it occurs in a vapor compression cycle (tere is a small conumption of ork to move te solution pump). In te generator, a concentrated solution of refrigerant receives eat, at provokes te formation of refrigerant overeated vapor tat is directed to te condenser. Liquid refrigerant from te condenser passes troug te expansion device and enters te evaporator ere it vaporizes creating te cooling effect. Refrigerant liquid/vapor is moved to te absorber ere it dissolves in te concentrated solution tat comes from te generator. generator Q gen condenser Q cond HEX expansion valve evaporator Q abs absorber Q ev ap pumb Figure 4 Absorption cooling (AC) sceme. Fig. (4) sos te presence of a internal eat excanger used to increase te coefficient of performance preeating te solution tat leaves te absorber. Te matematical modeling of te absorption cycle as developed troug te use of mass and balance equations based on Herold ET all (1996). Generator m m + m ogen m + Q m + m gen ogen ogen gen Condenser m Q m gen cond cond Water expansion valve cond ev Evaporator m + Q m ev evap evap

6 Absorber m m + m oabs ogen iabs evap Internal eat excanger m ( ) m ) oabs Solution expansion valve iev iabs ogen( ogen iev 3.3. Association of gas turbine it air-cooling systems. Figure (5a) sos te GTEC system formed by GT associated it EC. Figure (5b) sos te association of a GT it an AC called GTAC. Tis system, tat is more complex tan te GTEC, involves te use of a eat recovery steam generator (HRSG) using GT exaust gases. Tis component is necessary due to te fact tat commercially available eat recovery absorption systems are driven by steam or ot ater. Figure 5 a) GTEC sceme. b) GTAC sceme Figure 6 Control volumes 3.4. Exergetic analysis. A simplified exergetic analysis as developed considering a control volume tat contains all te equipment and te entire atmospere, ose conditions of air are different from tat one in te equipment inlet. Tis assumption means tat all eat excanges needed to return te air to its original conditions are inside of te control volume, as son in Fig. (6). In tis case e ave an adiabatic system, and te exergetic efficiency can be defined as:

7 η ex oulet _ exergy W inlet _ exergy T Q 1 T 0 cc It as considered in te study te folloing parameters tat caracterize te dead state for te exergy analysis: T 0 293,15K P 0 101,325 kpa UR 0 50 % X 0 50 % T cc 1273,15 K 4. RESULTS AND DISCUSSION As it is soed in Fig. (7), te GT energy production is affected by te location ere it is installed, due to te ambient air different termodynamic conditions. Te poer is directly related to te air inlet pressure in te compressor, and indirectly to te temperature and relative umidity. Tus, keeping te dry air bulb temperature constant, te produced poer decreases it te increase of te relative umidity. Tis influence is more pronounced at igest temperatures. Te altitude of te GT installation affects te barometric pressure and te air relative umidity and, consequently, te produced poer. Tis can be observed in Fig. (8) ere te GT poer produced in Curitiba is loer compared to te poer produced in Belém, for te same conditions of DB. Te reason for tat is te loer air density in Curitiba, due to te loer barometric pressure. Te loer RH in Curitiba reduces tis effect. As te volumetric flo is te same for bot cities, te air mass flo is loer in Curitiba and te produced poer is loer too. Te specific-energy produced in Curitiba is greater but te poer is loer. Figure (9) sos te influence of te cooling system in te turbine performance. Te poer increase obtained it GTEC in Curitiba is greater tan in Belém. Tis is due to te loer relative umidity in Curitiba, at makes te aircooling more effective. GTAC as better performance, considering poer production, tan GTEC. Te reason is te loer inlet air temperature and absolute umidity obtained by te GTAC. Te influence of te GTAC is muc greater in te city of Belém tan in Curitiba. Te reason is tat te temperatures obtained after cooling in te to cities is nearly equal, but te temperature decrease is larger in Belém. Te cange in produced poer is caused by climatic conditions variation in te cities, as can be seen in Fig. (9). Figure (10) sos te temperature decrease using EC. Figure (11) sos te GTAC poer variation. In contrast to te GTEC, te iger is te ambient air temperature and te relative umidity te larger is te increment in te poer production. In te same ay, for te GTEC te poer production is directly affected by barometric pressure. Anoter GTAC advantage is te exaust gases temperature tat is loer tan te obtained in GTEC, due to te use of te exaust gas energy to generate te vapor for te absorption cycle. Figure (12) sos te exergetic efficiency variation caused by reduction of te temperature and umidity of air it te use of GTEC and GTAC. CONCLUSIONS Te air-cooling in te entrance of gas turbines is a good option for poer production increase mainly during te period of energy consumption peak. Te performance improvement obtained it te cooling of te entrance air depends on te process used. Hotter and more umid places ave better results it te GTAC tan colder and driery regions. GTEC presents better results in dry tan in umid places. Te increase of poer obtained it te GTAC varies beteen 8 to 37 % more tan te presented by GTEC. Based on te simplified exergy analysis performed, te exergetic losses it te GTAC are 20 % loer tan te values presented by GTEC. One improvement in te GTAC system is te exclusion of te boiler for vapor, or ot ater, generation. In tis case, using an absorption system tat ould employ directly in its generator te turbine exaust gases, as analyzed by Varani, More ork is being carried out at Instituto Mauá de Tecnologia in order to improve te matematical modeling and also to include oter variables, including te system cost. REFERENCES AMELL, A, A., CADAVID, F.J., Influence of te relative umidity on te air cooling termal load in gas turbine poer plant, Applied Termal Engineering 22 (2002) BASSILY,A.M., Effects of evaporative inlet and aftercooling on te recuperated gas turbine cycle.applied Termal Engineering 21 (2001) HEROLD, K.E., RADERMACHER, R., KLEIN, S.A, Absorption Cillers and Heat Pumps. CRC Press, Inc EES Engineering Equation Solver, F-Cart Softare, Middleton, Wisconsin, GOULART, S.V.G, LAMBERTS, R, FIRMINO, S, Climatic data of 14 Brazilian cities. ABRAVA, MONE, C.D.,CHAU, D.S.,PHELAN, P.E., Economic feasibility of combined eatand poer and absorption refrigeration it commercially available gas turbines, Energy Conversion and Management 42 (2001)

8 NAJJAR, Y.S.H., Enancement of performance of gas turbine engines by inlet air cooling and cogeneration system, Applied Termal Engineering Vol. 16 Nº. 2 (1996) PERRY, R.H, GREEN, D.W., Perry s Cemical Engineer s Handbook. 7 t. ed., McGra Hill Companies, TALBI, M.M., AGNEW, B., Exergy analysis: an absorption refrigerator using litium bromide and ater as te orking fluids, Applied Termal Engineering 20 (2000) VARANI, C. M. R., Energetic and Exergetic Evaluation of an Litium Bromide /Water Absorption Refrigeration System Utilizing Natural Gas. D. Sc. Tesis, Federal University from Paraíba, T - 5 o C R H 0 T - 5 o C R H 0, 5 T - 5 o C R H 1 T 0 o C R H 0 T 0 o C R H 0, 5 T 0 o C R H 1 Poer [kw] T 2 0 o C R H 0 T 3 5 o C R H 0 T 2 0 o C R H 0, 5 T 2 0 o C R H P a r [ k P a ] T 3 5 oc RH0,5 T 3 5 oc RH1 Figure 7 Influence of inlet air pressure, temperature and relative umidity in te GT poer. 10 x Specific Energy Belém Poer Belém Poer Curitiba 10 x Specific Energy Curitiba RH Belém RH Curitiba ,2 Poer [kw] or 10 x Sp Energy [kj/kg] ,8 0,6 0,4 0,2 RH DB [ºC] 0 Figure 8 Installation site influence in te GT poer.

9 Belém - mar - GTAC Belém - aug - GTAC Curitiba - nov - GTAC Curitiba - jul - GTAC Curitiba - jul - GTEC Curitiba - jul Belém - mar - GTEC Poer [kw] Belém - aug Belém - aug - GTEC Belém - mar Curitiba - nov - GTEC Curitiba - nov our Figure 9 Influence of te cooling-system in GT poer during te day. 9 C u ritib a n o v 8 7 C u ritib a ju l B e lé m m a r B e lé m a u g Temperature decrease [C] o u r Figure 10 - Temperature decrease using EC in Belém and Curitiba.

10 P 101,325 kpa P 92 kpa 40 o C 7500 Poer [KW] o C ,2 0,4 0,6 0,8 1 RH[0] Figure 11 Influence of te pressure, temperature and relative umidity in GTAC poer. 0,58 Belém mar GTAC 0,56 Belém aug GTAC Curitiba nov GTAC 0,54 ηex 0,52 Curitiba jul GTAC 0,5 0,48 Curitiba jul GTEC Curitiba nov GTEC 0,46 Belém mar Curitiba nov Belém aug GTEC 0,44 Belém aug Curitiba jul Belém mar GTEC our Figure 12 Influence of te cooling-system in exergetic efficiency.

Conventional Paper I Determine time required in minutes for a 50mm diameter steel sphere

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