Understanding Potential Induced Degradation

Transcription

1 Undersanding Poenial Induced Degradaion Inroducion Poenial Induced Degradaion (PID) is an undesirable propery of some solar modules. The facors ha enable PID (volage, hea and humidiy) exis on all phoovolaic (P) sysems, bu he effec does no occur on all or even mos P sysems. According o Dr. Peer Hacke of he Naional Renewable Energy Laboraory (NREL), All c-si [crysalline silicon] modules have elemens of reversible and non-reversible [PID] mechanisms. The key is o undersand he exen o which modules experience hese mechanisms. PID was firs recognized in he 1970 s, and has been sudied since. The rapid growh in P plan deploymens, combined wih dramaic reducion in module prices (and in some cases, module qualiy), has brough renewed ineres in he phenomenon. And because he issue is highly echnical, requiring a leas some undersanding of chemisry and physics, he renewed ineres has been accompanied by subsanial fear, uncerainy and doub in he marke oday. Alhough here are ways o miigae PID, and miigaion is no always even necessary, he focus of his whie paper is on is causes and effecs in a P power plan, and he saus of ess for measuring PID. This discussion provides a summary of he exensive research Advanced Energy conduced o encourage and furher educaion on he subjec in he indusry. Sources for his research are cied boh in he ex and in foonoes o assis anyone ineresed in learning more. CONTENTS Inroducion Page 1 Causes of PID Page 1 Environmenal Facors Page 2 Sysem Facors Page 3 Module Facors Page 4 Cell Facors Page 4 Effecs of PID Page 5 Tesing for PID Page 6 Conclusion Page 7

2 Whie Paper The Causes of PID Poenial Induced Degradaion, as he designaion implies, occurs when he module s volage poenial and leakage curren drive ion mobiliy wihin he module beween he semiconducor maerial and oher elemens of he module (e.g. glass, moun and frame), as shown in Figure 1, hus causing he module s power oupu capaciy o degrade. The ion mobiliy acceleraes wih humidiy, emperaure and volage poenial. Tess have revealed he relaionship of mobiliy o emperaure and humidiy: Planar conac wih he panel surface also causes a capaciive coupling o he cells, resuling in a capaciive leakage curren of varying srengh. 1 The P sysem and environmen inerac o cause PID. The condiions necessary for he occurrence of PID involve (i) environmenal facors, as well as facors involving (ii) he sysem, (iii) he module, and (iv) he cells. 2 3 While he environmen is se for each individual insallaion, i is possible o preven PID by properly conrolling only one of he facors (ii), (iii), or (iv). 4 All four facors are discussed, in urn, below. Figure 1 - Leakage curren and volage poenial (negaive poenial shown) cause negaive (-) (purple) ions o migrae away from he semiconducor, as posiive (+) (pink) ions migrae oward he semiconducor from he glass and package, and he module s exernal environmen. Environmenal Facors Because relaive humidiy and emperaure are known o adversely affec P plan performance in general, porions of boh Underwriers Laboraories (UL) and Inernaional Elecroechnical Commission (IEC) module esing proocols involve damp hea, emperaure cycling, and freeze/haw cycling. These same environmenal facors also affec PID, wih he degradaion being acceleraed by increases in emperaure and/or relaive humidiy. 5 I is ineresing o noe ha while high emperaures cause an increase in he degrading effec caused by PID, high emperaures have also been shown o faciliae regeneraion of he modules o reduce PID. 6 Because here is lile or nohing an operaor can do o change he P plan s environmen, PID is bes undersood and addressed by examining he sysem and is modules. 2

3 Whie Paper Sysem Facors A he sysem level, he mos significan impacs are he module s volage poenial and sign, which depend on boh he module s posiion in he array and he sysem grounding opology. There are numerous consideraions affecing sysem and inverer classificaion, bu for he purposes of PID, he inverer(s) can be classified based on he volage experienced by he arrays. As shown in Figure 2, four basic classificaions are possible. +½ +½ +½0 SG -½0 SG -½ +½0 SG -½0 SG -½ Symmeric, ungrounded or ground-referenced Symmeric, ungrounded or ground-referenced Ungrounded + AC Ungrounded + AC Negaive Grounding + Posiive Grounding Figure 2 olage poenial depends Negaive on Grounding he grounding opology of he P sysem, which Posiive can have Grounding hese four basic classificaions. Figure 2 shows ha he array volage poenial can vary based on he grounding opology. PID is mos ofen associaed wih a negaive volage poenial o ground, alhough issues arising from a posiive reference o ground have been documened by SunPower. 7 Neverheless, [m]any years of experience wih numerous sysems provide a clear and reassuring answer: for panels wih crysalline solar cells, here is no inerrelaionship beween poenial panel degradaion and he inverer principle used. [1] Addiional research has revealed ha, [a]n inerpreaion of he volage dependency migh be capaciive effecs. Ionic migraion caused by a cerain elecric force according o an applied volage leads o a sauraion of elecric charge keeping all forces in hermodynamic equilibrium. These elecrical charges influence he semiconducing properies [r]egardless of he volage level iself he modules degradaion processes sabilize on cerain levels which seem o be characerisic for each module ype. 8 Furher research will be needed o undersand he impac of higher array volages on PID. The U.S. is beginning o adop 1000 arrays, and hroughou he indusry he use of 1500 and even 2000 arrays is being considered as a means of reducing sysem coss in large commercial and uiliy-scale P plans. Resisance o PID will become increasingly criical as array volages increase. One heory, for example, posulaes ha in arrays of 1500 or more, a high posiive poenial is capable of causing new failure mechanisms. 9 3

4 Whie Paper Module Facors The choice of glass, encapsulaion, and diffusion barriers have all been shown o have an impac on PID. For he fron glass, several sudies have shown sodium o exhibi a causaive facor. According o one sudy, [a]n ingredien conained in soda-lime glass bu no in Quarz glass is required for he effec o occur I was suggesed ha his species migh be sodium. [5] While sodium is he prime suspec due o is availabiliy and high mobiliy, aluminum, magnesium, and calcium are presen in smaller concenraion in soda-lime glass bu no in Quarz glass and migh conribue o he difference. [4] The various means for encapsulaing modules have significanly differen properies, and hese have been shown o have an equally significan effec on PID: EA [ehylene vinyl aceae] appears o also play a vial role in PID since all he differen subsiues were able o preven PID. I was proposed ha his finding migh be linked o differences in conduciviy. [3] Furhermore, aceic acid conained in EA in conjuncion wih moisure migh be responsible for dissoluion of meal ions a he glass inerface, known as glass corrosion. The resuls indicae ha PID is associaed wih a ranspor process hrough he inerface beween glass and EA as well as hrough he inerface beween EA and he cell surface. [4] Anoher es indicaed ha module samples laminaed wih polyvinyl buyral (PB) show he highes suscepibiliy o PID. [8] PB has a very low resisance o moisure inrusion, and more moisure increases conduciviy. Oher encapsulans ha have superior moisure permeabiliy properies compared wih EA have been shown o reduce suscepibiliy o PID: The use of alernaive maerials is also recommended based on he fac ha leakage currens in he module can, in principle, be curailed by using an encapsulaion maerial ha is impermeable o charge carriers. [6] Using silicon dioxide as a sodium diffusion barrier beween he glass and he elecrically acive porions of he semiconducor has been shown o work fairly well a prevening PID, bu such a barrier is no immune o pin-hole leaks. Addiionally, laser-ablaion of he fron conac for hin film applicaions can leave gaps in he barrier layer, which can become problemaic wihou pos-processing seps o fill hese gaps. Cell Facors An ani-reflecive coaing (ARC) increases he capure of ligh and, herefore, increases module power conversion. Bu research has shown ARC properies o be a causaive facor in PID: ARC is anoher prerequisie for he PID process. This is in agreemen wih he repored dependency of PID on ARC properies. [2][3] I was recenly found from SIMS [secondary ion mass specromery] measuremens ha sodium originaing from he glass can be readily found in he op layers of he [4] 10 cell. 4

5 Whie Paper The Effecs of PID As shown in Figure 3, he reducion in shun resisance (R sh ) caused by PID reduces boh he module s maximum power poin (MPP) and is open circui volage ( oc ). [4] TÜ Rheinland Group idenifies he problem using hree facors: yield reducions; power and volage losses; and infrared (IR) imaging. [6] Figure 3 As shown here by he reducions in shun resisance (Rsh), maximum power poin (MPP) and open circui volage, PID can significanly reduce yield in P plans. [Source: Schueze, e al, Laboraory Sudy of Poenial Induced Degradaion of Silicon Phoovolaic Modules] Unexplainable yield losses can be a sign of PID. Because measuring R sh, MPP and IR signaures require expensive equipmen, he easies way o deec PID in he field is o use an ordinary volmeer o measure module-level oc. While he shape of he diode curve shown in Figure 3 canno be inferred by oc alone, he exen of PID can be revealed by ploing oc measuremens by sring posiion, or comparing he oc measuremens from opposie ends of he array. The PID effec can be eiher irreversible or reversible, depending on he cause. Obviously an irreversible effec is far more serious, requiring immediae deecion and miigaion. Irreversible PID is ypically caused by elecrochemical reacions ha leads o elecro-corrosion and/or film delaminaion in he modules. These irreversible characerisics have been documened primarily in hin film echnologies, and are herefore no a focus of c-si panel manufacurers. The reversible form of PID, also known as Surface Polarizaion or he Polarizaion Effec, was discovered by SunPower in [7] According o SunPower s announcemen of he discovery, []his new effec, called surface polarizaion, creaes he non-desrucive and reversible accumulaion of saic charge on he surface of high-efficiency solar cells. The Polarizaion Effec has since been sudied and documened by Solon [2] and many ohers and is generally associaed wih c-si modules. As a resul of hese sudies, he specific effec occurring beween he glass and he semiconducor is now well undersood: Models are based on he fac ha mobile sodium ions can diffuse from he fron glass o he cell surface due o a force caused by poenial induced sress. The velociy of he posiive charged ions are mainly influenced by he encapsulae maerial, he emperaure, he humidiy and he applied volage. [8] The specific mechanisms occurring once sodium reaches he semiconducor are no as well undersood, and several differen heories have been proposed: On he one hand he charged ions concenrae on he surface of he layer building up an elecric field leading o an ani-passivaing effec resuling in an increasing surface recombinaion rae. On he oher hand he sodium may diffuse ino he bulk and ac as a donaor aom. This leads o a rising concenraion of sodium ions in he emier, so he negaive doping will be neuralized, he p-n-juncion will be diminished, [and] so [is] he phoovolaic effec of he cell. [8] 5

6 Whie Paper Regardless of he specific mechanisms, exensive esing has demonsraed he abiliy o reverse he polarizaion effec, and hereby fully resore he power oupu of he modules. One such es was conduced a Arizona Sae Universiy. [9] Anoher es, he resuls of which are shown in Figure 4, found ha reversing polariy fully miigaes he polarizaion effec. [2] Figure 4 The Polarizaion Effec on c-si modules can be fully miigaed by reversing he poenial volage applied. [Source: S. Pingel, e al, Poenial Induced Degradaion of Solar Cells and Panels] Tesing for PID Tesing modules for suscepibiliy o PID is now imporan in any large-scale P projec, and is ofen needed o obain financing. Tesing will also deermine if he mechanism causing PID is reversible, which deermines wheher miigaing measures are required and wha measures are appropriae. A full lis of esing agencies and he modules esed o dae is provided in he November 2012 issue of Phoon Inernaional Magazine. [6] Many esing organizaions currenly offer PID esing, including: NREL (hp:// Fraunhofer (hp:// Inerek (hp:// CF Solar Tes Laboraory ( P Evoluion Labs ( TÜ Rheinland PTL ( TÜ SÜD America ( Pearl Laboraories (hp:// PI Berlin (hp:// 6

7 Whie Paper A es sandard is being developed by a consorium of indusry expers led by Dr. Peer Hacke of NREL. The eam is expecing o have he Final Draf Inernaional Sandard (FDIS) for IEC in The sandard is expeced o be adoped by esing organizaions, and many are already using is preliminary provisions. The IEC sandard will prescribe a very specific es procedure and he basic condiions for conducing he es, including: Module-raed sysem volage and polariies Chamber air emperaure 60 C ± 2 C Chamber relaive humidiy 85% ± 5% Tes duraion of 96 hours a above saed emperaure and relaive humidiy wih applied saed volage Under he IEC sandard, modules will be deemed o be PID-resisan if: Power loss is less han 5% There is no evidence of any major defec as defined in IEC clauses 10.1, 10.2, 10.7, and Conclusion Poenial Induced Degradaion can have profound adverse impac on he financing and operaing of P plans. While he enire P sysem ineracs o cause PID, he failure mode occurs in he modules. Forunaely PID does no occur in all modules, and ess are available o deermine wheher modules are suscepible or resisan o he effec. Many module manufacurers have aken seps o produce PID resisan modules. And for exising c-si modules ha do experience PID, he effec is usually reversible wih cos-effecive miigaion measures. Because miigaing PID in he P plan can increase iniial sysem coss, a judicious choice of resisan modules and oher prevenaive effors, wihin he consrains of each individual sysem, may be warraned. Bu i is also possible, of course, o inenionally design a new P plan wih modules suscepible o PID if he savings from using such modules is greaer han he miigaion measures required. Refer o he Miigaing Panel Polarizaion applicaion noe for opions using Advanced Energy inverers. The bes soluion for he indusry long-erm is o minimize or eliminae PID by making design changes a he sysem, module and/or cell levels. Unil hen, i will remain imporan for operaors o overcome any fear, uncerainy, and doub by becoming more knowledgeable abou PID, and hopefully he informaion and references provided here help achieve ha objecive. 1 Heriber Schmid and Bruno Burger, Ineracions beween Solar Panels and Inverers, Fraunhofer-Insiu für Solare Energiesyseme (ISE), December S. Pingel, e al, Poenial Induced Degradaion of Solar Cells and Panels, 35TH IEEE PSC, J. Berghold, e al, Poenial Induced Degradaion of Solar Cells and Panels, 25TH EUPSEC, 2010, pp Schueze, e al, Laboraory Sudy of Poenial Induced Degradaion of Silicon Phoovolaic Modules, Q-Cells SE, Sonnenallee 17-21, Bierfeld-Wolfen 5 P. Hacke, e al, Sysem olage Poenial-Induced Degradaion Mechanisms in P Modules and Mehods for Tes, NREL, 37TH IEEE Phoovolaic Specialiss Conference (PSC 37), Seale, Washingon, June Ruschman, Ines, Power Losses below he Surface, Phoon Inernaional Magazine, November 2012, pp R. Swanson, e al, The Surface Polarizaion Effec in High-Efficiency Silicon Solar Cells, 2005, SunPower Corporaion 8 S Koch, e al, Polarizaion Effecs and Tess for Crysalline Silicon Cells, 26TH European Phoovolaic Solar Energy Conference and Exhibiion, 5-8 Sepember 2011, Hamburg, Germany 9 Ebniali, Faraz, Poenial Induced Degradaion of Phoovolaic Modules: Influence of Temperaure and Surface Conduciviy, Arizona Sae Universiy, April P. Hacke, e al, Characerizaion of Mulicrysalline Silicon Modules wih Sysem Bias olage Applied in Damp Hea, 25TH EUPSEC, 2010, pp

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