Numerical simulation of a solar chimney power plant in the southern region of Iran

Transcription

1 Energy Equp. Sys./ Vol. 5/No.4/December 2017/ Energy Equpmen and Sysems hp://energyequpsys.u.ac.r Numercal smulaon of a solar chmney power plan n he souhern regon of Iran Auhor Moreza Bayareh a * a Deparmen of Mechancal Engneerng, Faculy of Engneerng, Shahrekord Unversy, Shahrekord, Iran Arcle hsory: Receved : 8 Augus 2017 Acceped : 11 Sepember 2017 ABSTRACT Three-dmensonal numercal smulaons are performed o nvesgae he effecs of pressure drop across he urbne and solar radance on he performance of a solar chmney power plan (SCPP). The SCPP sysem expeced o provde elecrc power o a cy s locaed n souhern regon of Iran (cy of Lamerd, Fars provnce). Is dmensons are smlar o he Manzanares prooype (bul n Span, 1970s). The resuls demonsraed ha he SCPP can provde up o KW of power, dependng on he season. I was found ha he urbne pressure drop and he solar radaon had sgnfcan effecs on he frs and second law effcences. Keywords: Solar Chmney Power Plan, Turbne Pressure Drop, Performance Analyss, Souh of Iran, Oupu Power. 1. Inroducon Solar energy s a clean and renewable energy source, whch s of neres o many researchers. A solar chmney power plan (SCPP) sysem s one of he praccal applcaons of solar energy. Fgure 1 shows a ypcal SCPP sysem, wh hree componens: a solar collecor o collec solar radaon, a wnd urbne o generae elecrc power, and a chmney. The solar chmney concep was orgnally proposed by an engneer from Span, Isdoro Cabanyes, n The prooype solar chmney power plan was bul n Manzanares, Span n he lae 1970s [1]. The hegh of he chmney, he dameer of he chmney, and he radus of he collecor were abou 195 m, 107 m, and 5 m respecvely. Focusng on he Manzanares prooype SCPP, Haff e al. [2] analyzed he energy balance, sysem coss, and he energy generaon of an * Correspondng auhor: Moreza Bayareh Address: Deparmen of Mechancal Engneerng, Faculy of Engneerng, Shahrekord Unversy, Shahrekord, Iran E-mal address: m.bayareh@eng.sku.ac.r SCPP sysem. Consderable nvesgaons have been conduced o demonsrae s effecveness ([3], [4], and [5]). These nvesgaons showed ha solar energy has a val role n he desgn and consrucon of an SCPP sysem. Larb e al. [6] analyzed s performance n Adrar, he souhern area of Algera. They repored ha he SCPP sysem could provde up o 140 o 200 KW of elecrcy durng a year. They revealed he effecs of solar radaon, he amben emperaure, he chmney hegh, and he collecor dameer on elecrcy producon. Sang e al. [7] examned a numercal smulaon of an SCPP sysem, based on he Manzanares prooype SCPP, and modeled he sysem, usng he sandard k-ε urbulence model. They showed ha he numercal resuls are n good agreemen wh he expermenal daa. L e al. [8] nvesgaed he effecs of he collecor dameer and he chmney hegh on he power oupu of an SCPP sysem, heorecally. The auhors demonsraed ha here s an opmum magnude for he collecor dameer o acheve

2 432 Moreza Bayareh / Energy Equp. Sys. / Vol. 5/No. 4/Dec he maxmum power oupu, bu here s no lmaon for he hegh of he chmney ower. Xu e al. [9] also examned numercal smulaons on he arflow, hea ransfer, and power oupu characerscs of an SCPP sysem, smlar o he Manzanares prooype. They revealed ha energy loss n he sysem ncreases wh an ncreasng mass flow rae of he arflow. Energy and exergy analyss of an SCPP sysem were performed by Maa e al. [10]. They showed ha as he dead sae emperaure decreases, he exergy losses decrease, whle he hermal effcency ncreases. The geomercal effec on he arflow behavor, hrough a solar chmney, was nvesgaed by Lebb e al. [11]. They revealed ha he geomery of he chmney ower has a val role on he performance of an SCPP sysem, due o an ncrease or decrease n he mass flow rae. Lee e al. [12] denfed he opmal confguraon of a solar chmney, expermenally. They used an organc Rankne cycle o generae elecrcy and demonsraed ha he maxmum oule ar emperaure s approxmaely 125 C. The nfluence of ar humdy on he performance of an SCPP sysem s suded numercally by Sudpraser e al. [13]. They found ha he overall ar emperaure n he case of mos ar s hgher han ha n he case of dry ar. Fg. 1. A schemac represenaon of an SCPP sysem Iran s close o he equaor and receves a lo of solar energy. The annual average radaon s abou 1825 kwh/m², whch s abou 2500 mes he power consumed n he counry. Iran's souhern regons receve abou 2200 kwh/m² per year; herefore, he use of SCPP n hese areas s jusfed. In he presen sudy, he performance of an SCPP sysem n Lamerd cy, locaed n he souhern par of Iran, s nvesgaed numercally. The dmensons of he Spansh prooype [1] were seleced. The effecs of he urbne pressure drop and solar radaon on he upward velocy and oule emperaure are suded for dfferen monhs. Moreover, he monhly average performance of he SCPP sysem s analyzed under dfferen condons, based on he frs and he second law effcences. 2. Governng equaons The governng equaons for ar flow hrough he sysem, ncludng connuy equaon, Naver-Sokes equaons, and energy equaon can be wren as follows: Connuy equaon: x ( u ) 0 Naver-Sokes equaons: p (1) j ( u ) ( uu j ) g (2) x j x x j Energy equaon: T u pcpt pcpu jt k j x x x x T p T u x (3) where s he densy, u s he velocy, p s he pressure, and T s he emperaure. Β denoes he hermal expanson coeffcen and τ s he sress ensor. For he naural convecon hea ransfer, he Raylegh number, Ra gth 3 /( ), s used o deermne he flud flow regme. Here, H s he hegh of he collecor, α s he hermal dffuson coeffcen, and υ s he knemac vscosy. I s found ha he Raylegh number for he presen case s hgher han he crcal 9 value, 10, whch means ha he flud flow regme n he sysem s urbulen. Hence, he sandard κ-ε model s used o smulae he urbulen ar flow: k pk pku Gk p x k x p p u x x x C1G k C2 k (4) (5)

3 Moreza Bayareh / Energy Equp. Sys. / Vol. 5/No. 4/Dec where G s he generaon of urbulence K knec energy, due o he mean velocy graden. Turbulen vscosy s defned as 2 C k /. The consans for he above equaons are as follows: C , C , C 0. 09,, Numercal mehod The governng equaons for he urbulen ar flow n he curren SCPP sysem are solved by he fne volume mehod, usng ANSYS FLUENT 15. Convecon erms are dscrezed, usng he upwnd second order scheme. Also, an algorhm called SIMPLEC s used for couplng he velocy and pressure felds. Numercal smulaons are performed, based on he followng assumpons: 1) Envronmenal condons are consan. 2) Three-dmensonal smulaons are done upon neglecng he effecs of he alude angle of he sun. 3) The chmney wall s assumed o be solaed. 4) The Boussnesq approxmaon s assumed o be vald for he densy varaon of he ar. 5) Collecor surface reflecon coeffcen s equal o ) A reverse fan s consdered as he urbne. 4. Resuls and dscussons 4.1. valdaon To valdae he numercal mehod used for our smulaons, he curren resuls are compared wh he numercal resuls of Xu e al. [9]. Fg. 2 shows he averaged velocy of he chmney oule, as a funcon of he urbne pressure drop for a solar radaon of 600 w/m². Ths fgure demonsraes ha he ex velocy decreases wh an ncrease n he urbne pressure drop. I s observed ha our resuls are n good agreemen wh he resuls of Xu e al. [9] Effec of solar radaon The effecs of solar radaon and pressure drop across he urbne, on he emperaure, have been shown n Fg. 3, for four seasons. I should be noed ha we used he average amoun of solar radaon n each season. I s observed ha he chmney oule emperaure ncreases as he urbne pressure drop ncreases. Ths s due o an ncrease n he urbne pressure drop, whch furher decreases he ar mass flow rae, hrough he sysem. Therefore, he avalable me for hea ransfer from he collecor surface o he ar flow, ncreases. Fg. 4 confrms ha he chmney oule velocy decreases as he urbne pressure drop ncreases. I s noable ha he ar flow velocy and ar emperaure of he chmney oule ncrease wh he amben emperaure (solar radaon) for consan values of he urbne pressure drop. Fg. 2. The averaged velocy of he chmney oule versus he urbne pressure drop for a solar radaon of 600 w/m².

4 434 Moreza Bayareh / Energy Equp. Sys. / Vol. 5/No. 4/Dec Fg. 3. The emperaure of he chmney oule versus urbne pressure drop for dfferen seasons. Fg. 4. The velocy of he chmney oule versus urbne pressure drop for dfferen seasons. The power generaed by he urbne s calculaed by he followng equaon: W.p. V. A (6) ch ch Where, represens he effcency of he urbne, V s he ar velocy n he chmney ch oule, and A s he area of he chmney ch oule. I s assumed ha he urbne effcency s equal o 80%, whch s less han he opmzed daa. The area of he chmney oule s abou m², based on he Manzanares prooype dmensons. Fg. 5 shows he urbne oupu power as a funcon of he urbne pressure drop for dfferen seasons. A a consan urbne pressure drop, as he solar radaon ncreases, he ar volume flow rae ncreases due o he buoyancy force, resulng n an ncrease n he oupu power of he sysem. On he oher hand, s observed ha here s an opmum oupu power n each season. Ths s due o he fac ha low a urbne pressure drop has more sgnfcan effecs, as he volume flow rae (he ar flow velocy) decreases. The opmal values of he pressure drop across he urbne are 185, 300, 360, and abou 400 PA for dfferen seasons. I s noable ha as he solar

5 Moreza Bayareh / Energy Equp. Sys. / Vol. 5/No. 4/Dec radaon ncreases (for example, n he summer season), he opmal value of he urbne pressure drop ncreases. 4.3.Frs and second law analyss To nvesgae he hermal performance of he SCPP sysem, s necessary o calculae s hermal effcency. Guo e al. [14] expressed he overall effcency of an SCPP sysem by gnorng he aerodynamc losses: 1 3 o C1. C2. Vch. T C1V ch 2 (7) Where, C1 Ach /( Acoll G) and C2 gh /T. 0 A represen he area of he collecor and G s coll he global radaon. Here, T 0 s he emperaure of he dead sae as a daum value. The overall effcency of he SCPP sysem was calculaed, usng he curren smulaons, as shown n Fg. 6. As we expeced, he solar radaon has a sgnfcan effec on he overall effcency and he pressure drop across he urbne. The fgure shows ha he maxmal overall effcency occurs a an opmal amoun of he urbne pressure drop n each case. I should be noed ha he overall effcency of he Spansh prooype was less han 0.2% under dfferen condons. The overall effcency of he SCPP sysem, under curren solar radaons, s slghly more han ha of he Manzanares prooype, especally for hgher solar radaons, due o neglecng he flow losses. To deermne he SCCP second law effcency, we need o defne he avalably: T s (8) h 0 where, h s he enhalpy and s s he enropy. The second law effcency or he raonal effcency s defned as follows: W a (9) o Here, W s he acual nework and a o represens he maxmum nework ha can be acheved. Fgure 7 shows he effecs of solar radaon and he urbne pressure drop on he second law effcency. As solar radaon ncreases, he raonal effcency ncreases. I s ndcaed ha he maxmum useful work avalable from he cold flow s less han ha avalable from he ho sream. In oher words, he rreversbly or he exergy loss reduces wh an ncrease n he emperaure a whch he hea s ransferred. Moreover, s observed ha he second law effcency does no have an opmal value. As he urbne pressure drop ncreases, he raonal effcency ncreases. Fg. 5. Turbne oupu power versus urbne pressure drop for dfferen seasons.

6 436 Moreza Bayareh / Energy Equp. Sys. / Vol. 5/No. 4/Dec Fg. 6. Overall effcency versus urbne pressure drop for dfferen seasons. Fg. 7. Second law effcency versus urbne pressure drop for dfferen seasons 5. Conclusons In he presen work, he performance of an SCPP sysem n he souhern regon of Iran has been suded, usng hree-dmensonal smulaons. The SCPP sysem expeced o provde elecrc power o a cy, locaed n souhern regon of Iran. Is dmensons are smlar o he Manzanares prooype. The resuls demonsraed ha he SCPP sysem can provde up o KW of power, dependng on he season. I was found ha he emperaure and he velocy of he chmney oule are affeced by he pressure drop and solar radaon, sgnfcanly. As he amben emperaure (or solar radance) ncreases, he oule emperaure and he oule velocy ncrease a a consan urbne pressure drop. Our resuls revealed ha here s an opmal value of he urbne oupu power and he overall effcency, under dfferen amouns of urbne pressure drop. However, he second law effcency ncreases wh an ncrease n he urbne pressure drop. References [1] Haaf W., Solar Chmneys, Par 2, Prelmnary Tes Resuls from he Manzanares Plo Plan, Inernaonal Journal of Solar Energy (1984) 2:

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