An Innovative Design and Cost Optimization of a Trigeneration (Combined Cooling, Heating and Power) System

Transcription

1 Int. J. Envron. Res., 8(): , Autumn 201 ISSN: An Innovatve Desgn and Cost Optmzaton of a Trgeneraton (Combned Coolng, Heatng and Power) System Abbaspour, M. * and Sarae, A.R. School of Mechancal Engneerng, Sharf Unversty of Technology, Tehran, Iran Receved 2 Dec. 2013; Revsed 22 Aprl 201; Accepted 27 Aprl 201 ABSTRACT: Load management and cost optmzaton are among the mportant factors n trgeneraton systems and combned coolng, heatng and power (CCHP) systems. In ths study, an nnovatve CCHP system uses a gas turbne as the prme mover and has a heat recovery steam generator (HRSG) n addton to an auxlary boler, electrc and absorpton chllers. The system s ted wth the bulk electrc grd; to export and sell excess power or mport power when necessary. Ths study analyzes load management and cost optmzaton of CCHP systems. A heurstc strategy to optmze the total energy cost s then presented. The optmal sze of CCHP s determned from the study results. Ths paper proposes a model for CCHP system optmzaton based on mnmzaton of energy consumpton and ntal nvestment costs. It s to be noted that the selected varables are the sze of the gas turbne, the absorpton chller capacty, and other dependent components. Key words: Modelng, Optmzaton, Energy, CCHP INTRODUCTION CCHP systems use waste heat created as a byproduct of power generaton to accomplsh requred heatng or coolng demand (Bhatt, 2001; Wang et al., 2002). Wth the avalablty of gas turbnes to span an ncreasngly wde range of capactes, t s becomng more attractve to utlze a CCHP va a combnaton of gas turbnes and absorpton chllers. CCHP systems, can be used at ndustral plants, hosptals, hotels, or busness centers to satsfy electrc, heatng, and coolng demands from a sngle energy resource; such as ol, coal, natural gas, bomass or solar. Natural gas s the most desrable for use n a CCHP system due to avalablty, reasonable cost, and less envronmental mpact. Durng peak load demand hours, t s sometmes necessary to use an auxlary boler or electrc chller for heatng and coolng. Thus, there s an electrcal servce connecton ted nto to the bulk electrc grd for purchasng defct or sellng surplus electrcty. Recoverng and usng waste heat for a relable energy source s what gves CCHP systems the advantage over other types of heatng and coolng equpment. (Martens, 1998; Wang, 2002). (Madment et al., 2002) nvestgates dfferent CCHP systems used n supermarkets wth dfferent coolng and engne technologes. (Mone et al., 2001) nvestgated the economc feasblty of mplementng of CHP systems *Correspondng author E-mal: abbpor@sharf.edu wth exstng, commercally avalable gas turbnes and sngle, double and trple effect absorpton chllers. A typcal CCHP system s able to fulfll the energy requrement for ts applcaton. There are several components n a CCHP system; such as gas turbne (or recprocatng engne) for the prme mover, a heat recovery steam generator (HRSG) and an auxlary boler to produce heatng, as well absorpton and electrcal chller to supply coolng demand. All these optons make energy management a very complex ssue. Mathematcal modelng technques are wdely used for decson makng n such problems. For nstance, Rao used LP for analyzng the steam flow balance n a fertlzer process (Rao et al., 1983). Furthermore, n CCHP system, Kong developed basc lnear programmng modelng for determnng the optmum purchased energy consumpton (Kong et al., 200). The assumptons for the demand of electrcty, heatng, and coolng n a tme varable manner, make the problem even more complex. Cardona presented a smplfed exergoeconomc methodology based on aggregate data to a trgeneraton plant servng a 300bed hosptal that s stuated n Medterranean area (Cardona et al., 2006). Thermoeconomc provdes a powerful tool for an economc and optmzaton of energy systems (Díaz et al., 2010). There are several studes carred out n the lterature about trgeneraton energy systems. (Ball et 971

2 Abbaspour, M. and Sarae, A.R. al., 2010) reported thermodynamc and thermoeconomc analyses of a trgeneraton (TRIGEN) system wth a gas desel engne. They consdered a tr-generaton system wth an output power about 6. MW based on gas-desel engne. (Al-Sulaman et al., 2011) studed the performance assessments of three dfferent tr-generaton systems usng organc Rankne cycle (ORC) to provde coolng, heatng and electrcty. In another study (Al-Sulaman et al., 2011) they performed the exergy analyss of a solar drven tr-generaton system. They consdered a parabolc through solar collector, organc Rankne cycle, and a sngle effect absorpton chller. (Hucochea et al., 2011) reported thermodynamc modelng of a trgeneraton system consstng of a gas turbne as the prme mover wth a double effect absorpton chller. They consdered a mcro-turbne wth 28 kw output power. They also conducted a parametrc study to evaluate desgn parameter effects on system performance. There are also some studes usng fuel cells (Tse et al., 2011 and Al-Sulaman et al., 2011). These studes show the mportance of energy and exergy analyss of tr-generaton energy systems. As t was dscussed earler, t s mportant for thermal systems to be economcal. An nnovatve model for CCHP system s presented n ths paper. Both captal costs and purchased energy costs of a varable demand trgeneraton plant are selected as the objectve functon. The operatonal varables n ths desgn are the absorpton chller and gas turbne szes, where the szes of other components are dependent on these two varables. In the proposed method, a set of optmal values for the captal and energy costs are determned n order to produce the lowest total cost. MATERIALS & METHODS The CCHP system conssts of an absorpton chller ( 1 ), electrc chller ( 2 ), auxlary boler ( 3 ), gas turbne generator ( ), HRSG ( ) and coolng tower ( 6 ). The gas turbne s used to meet the electrcal demand. The hgh-temperature exhaust gas of the gas turbne flows through the HRSG to produce hgh temperature steam. Steam s dvded between the absorpton chller and heat exchanger. The functon of these components s to help meet the coolng load and to heat exchanger to supply the hot water for domestc use and central heatng system separately. There s a heat recovery boler to help accommodate the heatng load f the HRSG heatng output does not completely satsfy the demanded. Smlarly, f generated electrcal power does not meet the demand, the user may purchase electrc Fg. 1. Flow dagram for CCHP system desgn 972

3 Int. J. Envron. Res., 8(): , Autumn 201 power from electrc network. Fg. 1 presents the layout of a gas turbne based CCHP scheme. The present model s developed on the bass of the above scheme and the followng assumptons (Kong et al., 200): ().The exhaust gas, absorpton chller, and heat recovery boler temperatures are kept relatvely constant. (). The effcences of equpment are assumed to be constant throughout ther operaton trajectory. An nnovatve model s formulated for mnmzng both captal cost of component and total cost of purchased energy needed to meet the coolng, heatng and electrcty demands under contnuous varable demand, all whle satsfyng the constrants mposed by the physcal requrements of the system. In ths analyss, the unt of measurement for the natural gas s m 3 /h whle the unt for electrcal energy s kw, ncludng coolng, heatng and electrcal loads. The heat value of natural gas s kwh/m 3. The capacty of all components s dependent on the sze of the gas turbne and absorpton chller and t s necessary to fnd optmum sze for the all system components. The objectve Functon s to consder the captal costs of all components and the total yearly energy consumpton costs. Captal cost depends on the sze of all components. The gas turbne cost s calculated by means of a cost factor. The energy costs are the total costs of purchased natural gas used n the gas turbne and the auxlary boler, and the cost of purchased electrcal energy. CRF s used to determne the annual cost of components, wth assumpton of 2 years operaton, for system (N=2) and %16 nterestng factor ( =.16); CRF s gven by equaton (1): N ( + 1) CRF = ( N (1) + 1 ) 1 Thus the objectve functon s defned by equaton (2): Y= CRF (Totalcaptalcost + MantenanceCost) + EnergyCost (2) Y represents the total cost of system, and the goal s to mnmze ths value.the mantenance cost s consdered as a porton of purchase cost. In ths study, mantenance costs are assumed to be 2% of the captal nvestment. As descrbed, the captal cost s the total cost of all components and s defned by equaton (3): Totalcaptalcost = Chller Abosrptoncost(1) + Electrc Chllercost (2) + Auxlary Bolercost (3) + Gasturbne() + HRSGcost () + Coolng Towercost (6) (3) Relatons for components cost are presented n Table1. The Energy Cost s defned by equaton (): EnergyCost = CE + CG () t WhereCEt andcgt represent the total electrcty and fuel cost respectvely. Total cost of electrcal consumpton s determned by the electrcty cost multple net value of electrc surplus or defct ( ζ ) for a year perod. CE t 12 t = CE () = 1 where: α ζ ζ 0 CE = (6) β ζ ζ < 0 ζ = CC + DE (7) CC (DC ) 1 = η (8) che And smlarly, the cost of natural gas consumpton s an aggregate of consumpton of Gas turbne and auxlary boler operaton over a year. Table 1. Captal cost of components Components ( ) Captal cost (US$) ( s expressed n kw) Absorpton chller Captal cost: ( ) 1 1 Electrc chller ( ) Auxlary boler ( ) 3 3 Gas Turbne K (K = 1300($ / kw )) HRSG (. ) Coolng tower 020. ( 6. 3 )

4 Desgn and Cost Optmzaton of a CCHP 12 CG = (9) t CG = 1 where the left hand sde s defned as follows: CG = κ CB (10) CB NDH NDH η = + (11) bo η GT 0 TDH < = ( TDH ) TDH > (12) The component cost factor for the gas turbne (CF GT ) s represented n US$/kW and the component sze s n kw. Thus the combnaton of each K wth related s n US$. Smlarly, the CE t and CG t are n US$. The objectve functon s n US$ unt. Thus, Eqs. (1) (12) provde the tools for a mathematcal programmng model of the CCHP system. The method used for ths energy optmzaton model s a smple optmzaton loop wrote n MATLAB software. Usng the above optmzaton algorthm, the optmum sze of components for the CCHP system can be determned. In fact, the optmzaton problem s lmted to determnaton of the sze of the absorpton chller (1) and the gas turbne(). The other szes of the components are as functons of these two parameters. RESULTS & DISCUSSION After runnng the model wrtten n MATLAB, the mnmum value of objectve functon (Y) can be calculated. For ths problem, there are three knds of energy demands for a 12 month perod. They are coolng, heatng, and electrcty needs. These values nclude 36 data nputs. In partcular, the followng yearly demand peaks for a 300-bed hosptal stuated n a Medterranean area were calculated (Cardona et al., 2006): -Coolng demand peak = 100 kw; -Thermal demand peak = 1600 kw; -Electrc demand peak = 170 kw; There are some fxed parameters, such as the effcency of components as well as electrcty and natural gas costs. The values of 0.67 and 1.7 are COPs of an absorpton chller and electrc chller respectvely. The effcences of the HRSG and the auxlary boler are respectvely 0.7 and 0.8. Table2 ndcates the cost of natural gas and electrcty. Fgure 2 Shows the cost functon of system (Y) versus absorpton chller sze ( 1 ) and gas turbne Sze ( ). As t s obvous for the present condton of = and = 1 kw, the total cost for the plant 1100kW 1 s mnmum. Therefore, the capacty of the electrc chller ( 2 ), auxlary boler ( 3 ), and coolng tower ( 6 ) are easy to fnd. = DC (13) 2 Max 1 3 a year. DC Max s the maxmum demand of coolng n a year. 0 TDH Max < = ( TDH Max ) TDH > TDH Max Max (1) s maxmum demand of total heatng durng = ( η ) η (1) 1 GT HRSG ηgt 1 1 = (1 + ) + (1 + ) (16) COPcha COPche Where COP cha and COP che are the coeffcent of performances for the absorpton and electrcal chllers, respectvely. Thus for 1 = 1100kW, 2 = 300kW, 3 = 168kW, = 1kW, = 237kW and 6 = 3383kW both captal and energy costs of the CCHP system s mnmum. For these components szes, the annual cost of the entre plant s 77,000 dollars. It s worth mentonng that the optmum value s not the maxmum value of demand, but ndeed the optmum sze of components closely depend on energy demand and energy costs. Fg. 3 represents the total cost of the CCHP system versus the absorpton chller sze for gas turbne szes 80, 1 and 170 kw. Remember that Electrc demand peak for ths study s 170 kw. As t s shown, total cost for = 80 kw s too hgh, wth ncreasng the gas turbne sze ( ), total cost of the system decrease snce = 1 kw that s mnmum. After ths pont, total cost ncreases wth Table 2. Natural gas and electrcty Tarff Fuel or electrcty Natural gas Purchased electrcty Sale electrcty Cost (US$/kW)

5 Int. J. Envron. Res., 8(): , Autumn 201 Fg. 2. Cost functon of system (Y) vs. absorpton chller sze ( 1 ) and gas turbne sze ( ) Fg. 3. Cost functon (Y) vs. the absorpton chller sze ( 1 ) wth gas turbne sze ( ) = 80, 1 and 170 kw 97

6 Abbaspour, M. and Sarae, A.R. Fg.. Cost functon (Y) vs. the gas turbne sze ( ) wth absorpton chller Sze ( 1 ) = 900, 1100 and 100 kw Fg.. Total cost of optmum layout (Y1=77,000 dollars) n comparsonwth non-optmum layout (Y2=781,00 dollars) 976

7 Int. J. Envron. Res., 8(): , Autumn 201 Table 3. Verfcaton of the optmzaton method Total Cost (USD) Present Study $77,000 Cardona et al., 2006 $80,000 ncreasng the value of. For example n = 170 kw (electrc demand peak) total cost of plant s more than prevous sze. Smlarly objectve functon s presented versus gas turbne sze for three varous szes of absorpton chller. It s worth mentonng that the optmum value for the sze of absorpton chller s not the peak n the coolng demand. Results show that for provdng a refrgeraton of 100kW, the mnmum cost value s acheved when refrgeraton of 1100kW s provded va absorpton chller and 300kW s provded va electrc chller. For a gas turbne wth sze of 1kW and an absorpton chller wth sze of 1100kW, the total cost wll be mnmum. Selectng the optmum sze of the gas turbne and absorpton chller wth respect to the demanded electrc and refrgeraton power would save over $6,300 dollars per year and $10,000 dollars for a perod of 2 years. Fgure shows optmum layout (Y1) and non-optmum layout (Y2) n one dagram. For evaluatng the accuracy of the proposed method, a comparson has been made between the present study and the results of the Cardona et al., 2006 (Table 3). For both models, the values of the decson varables are 1=1100kW and =1kW. CONCLUSIONS A new modelng approach s presented to optmze the CCHP system, t has been shown that for present case study wth 1 =1100 kw, 2 =300 kw, 3 =168 kw, =1 kw, =237 kw and 6 =3383 kw, both captal and energy cost of CCHP system are mnmzed. Optmzaton of CCHP systems wth varable demand s a complex task, because of the role of many components nvolved. It was found that the cost parameters especally cost of fuel and purchased electrcty, are hghly mportant for fndng the optmum operaton condton of CCHP plant. On the other hand, the optmum sze of the CCHP system depends on the energy demands and energy consumpton costs, as well as the captal cost of plant components. Regardng to the mentoned ssues, for a gas turbne wth the sze of 1kW and an absorpton chller wth the capacty of 1100kW, the total cost would be mnmum. Selecton of the optmum sze of gas turbne and absorpton chller, wth respect to the demanded electrc and refrgeraton power, would save over than $6,300 dollars per year and $10,000 dollars for a perod of 2 years. ACKNOWLEDGEMENTS The authors would lke to express ther sncere thanks to Dr. Mohammad Zabetan for hs valuable support durng ths work. REFERENCES Al-Sulaman, F. A., Dncer, I. and Hamdullahpur, F (2011). Exergy modelng of a new solar drven trgeneraton system. Solar Energy, 8, Al-Sulaman, F. A., Hamdullahpur, F. and Dncer, I. (2011). Performance comparson of three trgeneraton systems usng organc rankne cycles. Energy, 36, Al-Sulaman, F. A., Dncer, I. and Hamdullahpur, F. (2010). Exergy analyss of an ntegrated sold oxde fuel cell and organc Rankne cycle for coolng, heatng and power producton. Journal of Power Sources, 19, Ball, O., Aras, H. and Hepbasl, A. (2010). Thermodynamc and thermoeconomc analyses of a trgeneraton (TRIGEN) system wth a gas desel engne: Part I Methodology. Energy Converson and Management, 1, Bhatt, M. S. (2001). Mappng of general combned heat and power systems. Energy Converson and Management, 2, Cardona, E. and Pacentno, A. (2006). A new approach to exergoeconomc analyss and desgnof varable demand energy systems. Energy, 31, Díaz, P. R., Bento, Y. R. and Parse, J. A. R. (2010). Thermoeconomc assessment of a mult-engne, mult-heatpump CCHP (combned coolng, heatng and power generaton) system -A case study. Energy, 3, Hucochea, A., Rvera, W., Gutérrez-Urueta, G., Bruno, J. C. and Coronas, A. (2011). Thermodynamc analyss of a trgeneraton system consstng of a mcro gas turbne and a double effect absorpton chller. Appled Thermal Engneerng, 31, Kong,. Q.,Wang, R. Z. and Huang,. H. (200). Energy optmzaton model for a CCHP system wth avalable gas turbnes. Appled Thermal Engneerng, 2, Madment, G. G. and Tozer, R. M. (2002). Combned coolng heat and power n supermarkets. Appled Thermal Engneerng, 22, Martens, A. (1998).The energetc feasblty of CHP compared to the separate producton of heat and power. Appled Thermal Engneerng, 11,

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