Seasonal Variation of Internal Parameters of an Amorphous Silicon (a-si) Thin Film Photovoltaic Module

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1 NTENATONAL JOUNAL of ENEWABLE ENEGY EEACH ustu Eke et l., ol., No.4, 0 esonl rition of nternl Prmeters of n Amorphous ilicon (-i) Thin Film Photovoltic Module ustu Eke*, **, ener Oktik*** * Mugl ıtkı Kocmn University, Clen Energy eserch & Development Centre, Mugl,Turkey ** Mugl ıtkı Kocmn University, Fculty of ciences, Deprtment of Physics, Photovoltic Mteril nd Device Lbortory, Mugl,Turkey *** Turkiye ise ve Cm Fbriklri A..s Kuleleri, Kule Levent stnbul Turkiye erustu@gmil.com, seneroktik@gmil.com Corresponding Author; ustu Eke, Mugl ıtkı Kocmn University, Fculty of ciences, Deprtment of Physics, Photovoltic Mteril nd Device Lbortory, 480 Mugl,Turkey, , Fx: , erustu@gmil.com eceived:.06.0 Accepted: Abstrct- Operting prmeters of KA58 Amorphous ilicon (-i) photovoltic module is investigted s function of solr irrdition nd tmospheric conditions using current-voltge curves tken for yer from April to My t Mugl ıtkı Kocmn University P Outdoor Test ite. ingle diode model is proposed to explin the current-voltge curves of thin film solr cells nd modules. t is observed tht the operting prmeters of KA58 -i photovoltic module exhibit significntly different response to the chnges in solr rdition nd wether conditions in period of months. Correltions between internl prmeters, nmely, P, L, O nd (modified idelity fctor) of module nd solr irrdition nd tmospheric dt is investigted. Keywords- Outdoor mesurements of solr cells nd modules; morphous silicon module performnce, series resistnce, prllel resistnce, single diode model.. ntroduction Photovoltic (P) systems offer n environmentlly friendly wy to meet growing electricity demnd. The cumultive instlled cpcity of ll P systems round the world t the end of 009 grew to,878 MWp. The cumultive cpcity ws 480MWp in 995, 48MWp t 000 nd 6956MWp t the end of 006 [,].Cumultive instlltions of P cells nd modules round the world hve been growing t n verge nnul rte of more thn 30% since 995 nd it is rised by 45% up to,9gwp in 009. For the solr production industry, globl nnul instlltion of P modules is projected to rise from.9 GWp in 009 to more thn 79 GWp in 030 []. ilicon is the bsic mteril required for the production of solr cells bsed on crystlline technology. 93% of the world mrket is bsed on crystlline silicon nd the shre of thin film technology bsed solr cells is round 6% t the end of 006. t is expected to gin much lrger shre of the P mrket in the future. EPA expects growth in the thin film mrket shre to rech bout 0% of the totl production of P modules by 00 nd 40% by 030 [,]. Thin film modules re constructed by depositing extremely thin lyers of photosensitive mterils onto lowcost bcking such s glss, stinless steel or plstic. These results in lower production costs compred to the more mteril-intensive crystlline technology, price dvntge which is currently counterblnced by substntilly lower efficiency rtes. Three types of thin film modules re commercilly vilble t the moment. These re mnufctured from morphous silicon (-i), copper indium diselenide (C, CG) nd cdmium telluride (CdTe).

2 NTENATONAL JOUNAL of ENEWABLE ENEGY EEACH ustu Eke et l., ol., No.4, 0 Among the three commercilly vilble thin film technologies, -i is the most importnt in terms of production nd instlltion. ome nlyticl nd numericl methods re developed for the chrcteristics of solr cells [3,4]. The performnce prmeters of P modules re optimized t some reference condition, usully t stndrd test conditions (TC) of 000 W/m of irrdince, 5 C cell temperture nd ir mss.5 globl spectrums. And internl prmeters of P modules re lwys given t these conditions by mnufcturers. However, these conditions rrely occur outdoor nd thus vritions of electricl prmeters of P devices need to be investigted nd modeled. There re few models, both in numericl nd nlyticl, in the literture for simulting the behvior of P devices for specific irrdince nd temperture conditions [5,6]. The purpose of this study is to evlute the effect of sesonl chnges in the operting prmeters of KA58 Amorphous ilicon (-i) thin film module s function of solr irrdition nd tmospheric conditions.. P Model nd Methodology The electricl power vilble from P cell/module cn be modeled with the circuit shown in Fig. [6]. ingle diode (single exponentil) model is considered s the equivlent P model. Five prmeters must be known in order to determine the current nd voltge, nd thus the power delivered to the lod. These prmeters re L, the light generted current or photovoltic current, O, drk sturtion current,, series resistnce, H or P, shunt or prllel resistnce nd, is coefficient clled s modified idelity fctor. Herein, the modified idelity fctor is clculted s ; herein, N C represents number of the solr cells or P modules, n represents diode idelity or qulity fctor, k B represents Boltzmnn constnt, q represents electron chrge nd T C represents temperture of the solr cell or P module in Kelvin [6]. The following Eqution my be written for the current flowing through the circuit t ny chosen point ccording to current rule of Kirchoff. Wherein, L represents the light generted current by in the solr cell or P module, D represents diode current nd H represents lekge currents. () L D H When H nd D is inserted in Eq. the current-voltge reltionship t fixed cell/module temperture nd irrdition for the circuit will be expressed s: ( ) L O exp () Where is the voltge cross the lod. Drk sturtion current ( O ) depends on bnd gp nd temperture of the mteril used in mnufcturing the solr cell [7]. H Fig.. Equivlent circuit of solr cell or P module. eries resistnce ( ) nd shunt or prllel resistnce ( H ) cn be determined by defining O round open circuit voltge nd HO round short circuit current with the help of grdients of the current-voltge curve round open circuit voltge nd short circuit current. O HO d (3) d d (4) d C A set of equtions is obtined by rrnging Eq., which is given for the current, round open circuit voltge, short circuit current nd the mximum power point t which mximum power is obtined from the device to determine five importnt prmeters for the solr cell or P module. Certin ssumptions re required to be mde to solve the five prmeters nlyticlly from the obtined Eqution set [8,9]. Becuse exp >> C exp nd << H in the obtined equtions, then. Thus, such prmeters of solr cell or P module,, L, O nd my be clculted s seen below: L O O HO C HO O C H C H O (5) exp (6) H exp H (7) 550

3 NTENATONAL JOUNAL of ENEWABLE ENEGY EEACH ustu Eke et l., ol., No.4, 0 ln C f prllel resistnce is H HO H C H H H (8) (9) Then, the five importnt prmeters for solr cell nd P module cn be clculted by using single currentvoltge curve. A vlue is ssigned for series resistnce ( ) s initil vlue. After prllel resistnce vlue ( H ) nd modified idelity fctor ( ) given by Eqution (8) re clculted, current generted by light ( L ) is clculted by using the Eq.6 nd, which is expressed by Eq.5 is reclculted. The new clculted vlue of is used in Eq.7 to find the drk sturtion current ( O ). Thus, ll required prmeters for solr cell or P module re determined... Mesurement ystem A multi-chnnel mesurement system in Mugl ıtkı Kocmn University Clen Energy esources eserch nd Development Center (MUTEK &D) developed by Aescusoft cn scn the whole current-voltge curve of solr cell or photovoltic module in 0 seconds [0]. oltge of solr cell or P module under test is scnned t 80 points from the vlue round short circuit current (0) to the open circuit voltge ( ) nd current vlue corresponding to ech voltge vlue is mesured nd sved in file. These mesurements re performed in minute period nd mde vilble during dy. ritions in the internl prmeters of the solr cell or P module cn be observed within dy or between sesons by using the obtined current-voltge curves. Fig. shows the connection box of the multi-chnnel mesurement system. 4 points mesurement method is used ( cbles for lod nd cbles for sense) for the current nd voltge mesurements of the tested P modules. Fig.. Connection box of the multi-chnnel mesurement system. Prmeters of solr cell nd P module vry under operting conditions nd lower performnces re obtined compred those given under TC. These conditions combine solr irrdition on cler summer dy, temperture of solr cell or photovoltic module on cler winter dy nd solr spectrum on cler spring dy. However, these conditions re encountered rrely. High irrdition levels occur on surfces of P modules exceeding 000 W/m in summer months; however, becuse mbient temperture is high, operting tempertures of P modules my rech up to ºC. n winters, lthough mbient temperture nd the consequent operting temperture of P modules re lower (it is close to 35 ⁰C t noon time), solr irrdition, which cn be obtined on surfces of P modules, is below 000 W/m... KA58 Amorphous ilicon (-i) Photovoltic Module Electricl power conversion efficiency of the KA58 Amorphous ilicon (-i) Photovoltic Module, whose performnce hs been observed since April in Mugl ıtkı Kocmn University s photovoltic test re, is defined s 6.6% under TC. The mnufcturer s electricl vlues for the KA58 -i photovoltic module with power of 58W under TC re given in Tble. The tested P module consisting of 00 seriesconnected -i solr cells covers n re of 0.876m. According to the dt provided by the mnufcturer, the potentil power, which cn be generted t the mximum power point, decreses by % for n increse of C in temperture depending on operting conditions. delity fctor of the KA58 -i photovoltic module under TC is given s.. Tble shows vlues of some physicl prmeters nd coefficients for vrition of current nd voltge depending on temperture. Tble. Prmeters of KA58 Amorphous ilicon (-i) Photovoltic Module under TC C (A) () (A) () (Ω) H (Ω) 0,ref (na) μ (m/ºc) μ C (ma/ºc) esults nd Discussion For estblish the current-voltge curve of the photovoltic module, 80 voltges nd the corresponding currents vlues tken with the help of electronic lod in 0-0 s were mesured. hort circuit current ( C ), open circuit voltge ( ), the current t the mximum power point ( ) nd the voltge t the mximum power point ( ) were determined for the P module under test. New current vlues corresponding to the mesured voltge vlues were clculted by using Bls Model bsed on single diode model. Although smll vrition exist between the vlues clculted by using Phng model nd empiricl vlues obtined for the KA58 -i P module, it is seen tht Bls model is in good correltion with empiricl vlues for the P module (Fig. 3 nd Fig.4). th dy of July tht ws cler dy ws chosen to represent behvior of the KA58 -i P module in summer seson. On tht dy, 090W/m plne of rry irrdition is 55

4 NTENATONAL JOUNAL of ENEWABLE ENEGY EEACH ustu Eke et l., ol., No.4, 0 mesured t noon nd temperture of the P module rised up to 6ºC. Fig. 3 shows vrition in current-voltge for four different irrdition levels for the KA58 -i photovoltic module. As seen in Fig. 3, currents mesured t noon re higher thn those provided by the mnufcturer under TC. The reson is the fct tht irrdition is higher thn tht in TC by 9% s well s temperture of the P module is mesured s 36ºC, higher thn 5⁰C. under TC, the generted current is low due to the vrince in the solr spectrum. Therefore, power, which might be generted from the P module under 950W/m plne of irrdition, is lso low. n fct, it is only hlf of the vlue provided by the mnufcturer under TC. As seen in Fig. 3 nd Fig.4, becuse the vlues, which re obtined by using Bls method, re consistent with the empiric vlues under winter nd summer conditions, prmeters for the solr cell nd P module my be clculted by using Eqution sets from Eq.5 to Eq.9 closer to the vlues under the operting conditions. 3.. ritions in eries esistnce Fig. 3. The current-voltge chrcteristics of the photovoltic module t nd of July The mesured open circuit voltge is lower tht provided by the mnufcturer under TC becuse temperture coefficient of open circuit voltge is negtive. t ws clculted tht power of 47.79W could be drwn from the photovoltic module on nd of July t 3.4 during noon time in which the highest irrdition ws mesured. This is 7% below the vlue provided by the mnufcturer under TC. 8th of Jnury tht ws cler dy ws chosen to represent behvior of the KA58 -i photovoltic module in winter seson. t is seen tht the experimentlly mesured current-voltge vlues re correlted with the obtined vlues by the used models (Fig. 4). eries resistnce vlues for the KA58 -i thin film P module decrese logrithmiclly ccording to Eq.0 with irrdition. Herein, r nd r re coefficients representing vrition in series resistnce, which vry depending on months ( r is clculted s positive only for Jnury). r r lng (0) Lower vlues re preferred for series resistnce. Therefore, series resistnce vlue decreses under high irrdition levels with the contribution of the incresing temperture of the P module nd the incresing irrdition contributes to performnce of the P module. Fig. 5 shows vrition of series resistnce with irrdition clculted on the selected cler dys during yer for the KA58 -i P module. The series resistnce vlues higher thn 0Ω were clculted under low irrdition levels up to 500W/m. n the KA58 -i P module, the series resistnce vlues, which were clculted under irrdition vlues higher thn 300W/m, re lower tht (8.39Ω) provided by the mnufcturer (Tble ). For the KA58 -i P module, the coefficients given in Eq.0 vry depending on months ccording to the result of the regression nlysis conducted by using the lest-squre method for the vlues given in Fig.5; the generl expression ws clculted s the following lng () Fig. 4. The current-voltge chrcteristics of the photovoltic module t 8 th of Jnury Although the P module temperture mesured under the highest irrdition is very close to the vlue obtined Fig. 5. rrdition dependencies of the mesured nd clculted series resistnce vlues 55

5 NTENATONAL JOUNAL of ENEWABLE ENEGY EEACH ustu Eke et l., ol., No.4, ritions in eries esistnce Prllel resistnce ( H ) expresses lekge currents in solr cell or photovoltic module nd they re prmeters ffecting performnce. Deprting from the grdient of current-voltge curve round the short circuit current, the prllel resistnce vlue my be clculted by using Eq.9. t is seen ccording to the clcultions mde under different conditions tht the grdient of current-voltge curve round the short circuit current decreses s prllel resistnce vlue of the photovoltic module increses. Becuse lekge currents re not desired in solr cell or photovoltic module, prllel resistnce should be kept higher s much s possible. is ccepted for n idel solr cell. t is H seen in the photovoltic module under test tht the prllel resistnce, H, depends on the irrdition level G nd temperture of the solr cell/module, T C, logrithmiclly ccording to Eq.. H rh rh ln G () Herein, r H nd r H re coefficients representing vrition in prllel resistnce, which vry depending on months. rition of prllel resistnce vlue is seen on current-voltge curve more clerly. n the KA58 -i photovoltic module, decrese occurs in voltge s soon s the current t the mximum power point increses nd the plce of this point moves towrd lower voltges nd bit higher currents on the current-voltge curve. n the KA58 -i photovoltic module, the clculted resistnce vlues re in the intervl of Ω. t is in the clcultions mde up to irrdition level of 400W/m tht prllel resistnce vlues re clculted between 900 nd 300Ω while they re lower thn 700Ω for the irrdition higher thn 600W/m. The mnufcturer s prllel resistnce vlue for the KA58 -i photovoltic module is 050Ω (Tble ). Fig. 6 shows vrition in the vlues clculted under different irrdition levels during yer. The highest vlues for prllel resistnce were clculted in August in which mbient temperture elevted to 40ºC nd temperture of the photovoltic module reched to 70ºC. They increse up to 700Ω under low irrdition levels. Fig. 6. rrdition dependencies of the mesured nd clculted shunt resistnce vlues t is determined ccording to the clcultions tht the generl Eqution showing vrition of prllel resistnce vlue with irrdition in Eq.3 for the KA58 -i photovoltic module. H ln G (3) 3.3. ritions in Modified delity Fctor Modified idelity fctor ( ) is one of the importnt prmeters ffecting performnce of solr cell nd P module. Modified idelity fctor my be clculted by using Eq.8 with the help of current-voltge curve. rition in modified idelity fctor does not chnge short circuit current of the solr cell or P module; however, open circuit voltge vry linerly with modified idelity fctor. According to the clcultions mde for the KA58 -i P module, the modified idelity fctor intends to increse with the irrdition level (Fig. 7). According to the monthly regression nlysis, it is seen tht vrition of modified idelity fctor with irrdition level is consistent with Eq.4 provided tht coefficients of nd tke different vlues depending on months. Fig. 7. rrdition dependency of the mesured nd clculted modified idelity fctor G e (4) However, in the nlysis conducted for period of months, the generl expression of the modified idelity fctor with irrdition for the KA58 -i P module is expressed in Eq.5 nd irrdition dependency is shown in Fig.7. G 0 e (5) 0, e G (6) Modified idelity fctor for the KA58 -i P module ws clculted s 5.7 t 5⁰C by using the diode idelity fctor provided by the mnufcturer s. under TC. The modified idelity fctor vlue vries between 4 nd 4 during the test period of months. Fig. 7 shows its vrition with the irrdition. t decreses with the irrdition in December, 553

6 NTENATONAL JOUNAL of ENEWABLE ENEGY EEACH ustu Eke et l., ol., No.4, 0 Jnury, Mrch nd April ( <0) while it increses in other months ( >0) ritions in Drk turtion Current rition in drk sturtion current does not chnge the short circuit current in solr cell or P module; however, it cuses the solr cell or P module to work t lower voltges. The voltge on the mximum power point nd the open circuit voltge decreses. As the drk sturtion current increses, the mximum power point moves towrd lower voltges; consequently, fill fctor (FF), which equls to the mximum power over the product of short circuit current nd open circuit voltge, decreses. Temperture dependency of the P module depends on the solr cell mteril s given in Eq.7. Drk sturtion current ( O ), of solr cell or P module is given s the following ccording to the dt provided by the mnufcturer under TC. m O T C EG TC, ref exp O, ref T C, ref ref TC (7) Herein, it is T C,ref =5ºC solr cell/module temperture, O,ref is the drk sturtion current under TC nd E G is the bnd gp of the semi-conductor of which the solr cell is mde nd m is coefficient, which might tke vlues between nd 4 nd shows temperture dependency of drk sturtion current []. According to the clcultions, drk sturtion current is within different intervls between 97 nd 303 K in summers nd winters. Tble shows the vlues obtined for the KA58 -i P module. ery high drk sturtion current ws clculted for the KA58 -i P module compred with tht provided by the mnufcturer under TC. This my increse up to ma in winter while the vlue provided by the mnufcturer under TC is na tht is pproximtely /5000 of the clculted vlue. Tble. Clculted drk sturtion current intervl vlues in summer nd winter months for KA58 -i module t K December, Jnury, June, July, lue t Februry August TC ma μa 9 na 3.5. ritions in Light Generted Current Light generted current, L, is proportionl to irrdition level on the solr cell/module. Operting temperture of the P module, T C, nd the ir mss (AM) due to its effect on solr spectrum. Light generted current under TC my be clculted t ny irrdition G nd operting temperture T C s [] G L L, ref C TC TC, ref G ref (8) hort circuit current nd open circuit voltge of the P module will increse with incresing light generted current. Becuse open circuit voltge depends on the current logrithmiclly; n increse in open circuit voltge will be lower thn tht in the short circuit current. Light generted current vlues for different irrdition levels for the KA58 - i P module ws clculted nd the vrition is shown in Fig. 8 for months test period. Fig. 8. rrdition dependency of light generted current Becuse the light generted current is directly proportionl to irrdition level, the photovoltic module s short circuit current nd open circuit voltge will increse. Effects of sesons on the KA58 -i P module re very cler s seen in Fig. 8. There re differences up to 40% between light generted currents under sme irrdition levels but different operting tempertures. lues clculted in eptember represent upper prt of the distribution in Fig. 8 while those clculted in Jnury represents lower prts. n this study, it is shown tht the single-diode model is successfully pplicble to P modules consisting of morphous silicon thin film solr cells successfully with the help of modified idelity fctor, which is known s curve prmeter including temperture. ritions of the prmeters ffecting performnce of the KA48 -i thin film P modules mesured during period of months with irrdition were determined. 4. Conclusion The current-voltge curves of n -i module re tken for yer from April to My t the Mugl ıtkı Kocmn University P Outdoor Test ite. ingle diode model is proposed to explin the current-voltge curves of thin film solr cells nd modules. The reltionship between the environmentl sesonl vritions nd internl prmeters were nlyzed. The following points were clrified:. n summer seson the clculted power of the selected power of the tested module is lower thn the vlue given t TC becuse of the high operting temperture. 554

7 NTENATONAL JOUNAL of ENEWABLE ENEGY EEACH ustu Eke et l., ol., No.4, 0. The series resistnce vlues higher thn 0Ω were clculted under lower irrdition levels up to 500W/m. n the KA58 -i photovoltic module, the series resistnce vlues, which were clculted under irrdition vlues higher thn 300W/m, re lower tht (8.39Ω) provided by the mnufcturer. 3. n the KA58 -i photovoltic module, the clculted resistnce vlues re in the intervl of Ω. t is in the clcultions mde up to irrdition level of 400W/m tht prllel resistnce vlues re clculted between 900 nd 300Ω while they re lower thn 700Ω for the irrdition higher thn 600W/m. The mnufcturer s prllel resistnce vlue for the KA58 -i photovoltic module is 050Ω. 4. The modified idelity fctor for the KA58 -i photovoltic module ws clculted between 4 nd 4 during the test period of months. 5. ery high drk sturtion current ws clculted for the KA58 -i photovoltic module compred with tht provided by the mnufcturer under TC. Clculted vlues re in ma scle while the given vlue is in na scle. 6. There re differences up to 40% between light generted currents under sme irrdition but different operting tempertures. eferences [] W. Hoffmnn nd. Teske, olr Genertion -,Greenpece, EPA, Hmburg; 007. [] EPA, Globl Mrket Outlook for Photovoltics until 04; My 00. [3] M.A. Mnnn, M. Anjn., M.Z. Kbir, -Modeling of currentvoltge chrcteristics of thin film solr cells, olid-tte Electronics, vol. 63, pp , eptember 0. [4] M Burgelmn., J. Poortmns,. Arkhipov, Thin film solr cells: fbriction, chrcteriztion nd pplictions. Englnd: Wiley & ons; 006, ch 7. [5] C. Oguz., Mesurement nd Anlysis of olr rrdition t Mugl Province (in Turkish), Mc Thesis, Mugl ıtkı Kocmn University, Mugl, Turkey [6] J. A. Duffie nd W. A. Beckmn., olr Engineering of Therml Processes, third ed. Wiley nterscience,hoboken, New Jersey; 006, ch 3. [7] D. K. chröder, emiconductor Mteril nd Device Chrcteriztion, third ed. Wiley nterscience, Hoboken, New Jersey; 006, ch 4. [8] D..H Chn nd J.C.H Phng, Anlyticl methods for the extrction of solr-cell single- nd double-diode model prmeters from - chrcteristics, EEE Trnsctions on Electron Devices, vol. 34 ssue:, pp , 987. [9] M.A. de Bls, J.L. Torres, E. Prieto nd A. Grcí, electing suitble model for chrcterizing photovoltic devices, enewble Energy, vol. 5, ssue: 3, pp , Mrch 00. [0] D. M. pinner, nstruction Mnuel: olr Mesuring ystem for Chrcteristion of olr Cells (.0 /003), Freiburg, Germny; 003. [] W. de oto,.a. Klein nd W.A. Beckmn, mprovement nd vlidtion of model for photovoltic rry performnce, olr Energy, ol. 80, ssue:, pp , Jnury 006. [] T.U. Townsend, A Method for Estimting the Long- Term Performnce of Direct-Coupled Photovoltic ystems, Mc Thesis, University of Wisconsin, Mdison, UA,