The use of drones in the maintenance of photovoltaic fields

. Enel Green Power is an Italian company which realises and manages large photovoltaic (PV) plants around the world. The company is involved in a huge number of research projects and one of them involves the use of drones for PV inspection in order to guarantee the correct operation of the plants. In this paper, the use of drones the maintenance of PV plants is analysed from a technical and economic point of view, as function of plant size. First, the use of drones for thermography and visual inspection is briefly described, then, on the basis of on-field activities, the technical specifications of drones and the procedure for the inspection are defined. The results of a thermal infrared analysis performed on six PV plants are analysed. In particular, the failure rates of PV modules are estimated, describing each failure. Finally, an economic analysis is performed in order to evaluate the competitiveness of drones for PV inspection compared to manual maintenance.


Innovation in the maintenance of photovoltaic fields
Ee Gree Pwer is wrig  ivati ad sustaiabiity i severa sectrs I particuar the eergy sectr is experiecig a phase f rbtisati i rder t icrease the sustaiabiity f prcesses by aig daiy peratis cheaper ad faster It is fudaeta t iit the perati ad aiteace (&) csts f argescae phtvtaic (PV) pats (up t severa hudreds f istaed egawatts) aitaiig gd prductivity I recet decades ivatis such as dres exseets sart gasses ceaig ad grass cuttig achies have bee itrduced t iprve & activities Dres ca perfr tass such as gegraphica appig the itrig f cstructi sites ad precisi agricuture 12 The exseet is a passive upper ib techgy that assists the shuder fexi extesi f peratrs i case f verhead aipuatis The use f sart gasses by fied peratrs aws cuicati whe PV geeratrs are far fr cetra headquarters ad ca receive ifrati whie wrig r repairig a faiure Dust ad dirt ca draaticay reduce PV due pwer utput 34 thus ceaig is a fudaeta aiteace perati bviusy aua ceaig is t expesive i the case f utiegawatt pats ad thus autated sutis such as rbts reduce ceaig tie ad the use f water Ceaig is geeray perfred usig rtatig brushes that are prgraabe i ters f rtatia speed ad the weight f the rbt is reduced i rder t avid highy ccetrated ads  the PV dues Ccetrated ads such as thse ivved i waig  the dues 5 ca cause icrcracs ad cracs i the ces with upredictabe reductis i prducti Ather aiteace tas ivves reducig the shadws that activate the bypass dides f the PV dues The uber f prductive PV ces decreases ad the axiu pwer pit tracer (PPT) iside the DCAC cverter ay t be abe t wr prpery 67 I freestadig pats it is ecessary t peridicay cut the grass t avid shadws  the btt dues arge PV pats are usuay cated i rete es  ueve terrai ad seties with se spe A pssibe suti is the use f rete ctred rbts which cut the grass uder the supervisi f a peratr A re advaced suti is the use f autatic devices they erise the pathway ad ca repeat it withut a supervisr bviusy they have t be equipped with hardware ad sftware t avid exceptia bstaces ad safeguard pepe theseves ad ther assets I the preset paper prpery seected dres are used t ap argescae PV pats ad chec fr pssibe defects r htspts by perfrig visua ispecti ad thera ifrared aaysis The ecic effectiveess f these devices at the ed f the pat ifetie is evauated as a fucti f the rated pwer f PV array 2 Innovation in the maintenance of photovoltaic fields Accrdig t fight architecture there are tw types f uaed aircraft syste (UAS) rtrbased ad fixed wig The first type f dre is sa ight ad easy t ctr whie the secd devices are faster ca reach higher atitudes ad carry heavy ads Dres ca currety perfr a variety f PVreated tass They are geeray equipped with caeras fr the visua ispecti f dues it is pssibe t rapidy chec the presece f dues with visibe defects (eg dues with bre frtgass) Dres ca as be equipped with thera ifrared caeras Thera caeras 89 are used t chec the presece f abra teperatures i dues (ht spts) geeray due t cracs i the sar ces r ther types f defects The test is perfred durig the perati f the pats aiy arud  whe the teperature f the PV dues is the highest These caeras are as used t chec the presece f dagerus teperatures i ther cpets such as strig bxes cabes ad cverters Sice 2018 dres have as bee equipped with ear ifrared caeras which are used t perfr eectruiescece tests 10 I this case the device prvides a high quaity iage f the defects ad ifrati reated t their causes Figure 1 shws a exape f a dre perfrig a visua ispecti f a PV pat Aayses f visua ad thera ifrared iages are perfred after the ispecti usig apprpriate patter recgiti agriths 11 I the future dres wi perfr data aaysis thrugh patter recgiti ad chage detecti Predictive ethds wi be used t itr the prductivity f PV pats ptiisig strategic & respses 12 The ai advatages f these devices with respect t traditia appraches are the fwig • Efficiet fast ad iexpesive ispecti f arge scae PV pats • Safety ad icreased accessibiity t sites that are difficut t reach r with safety riss fr huas eg rftps • Fexibe dispatch thas t their w cst • arger types f data cecti at a faster rate • Higher accuracy f easureets fr rftp ad grud assessets

Fig 1
Dre fyig ver a PV pat

Management of the inspections
The first ipeetati f dres fr visua ispectis ad thergraphy aaysis was i Ee Gree Pwer rth Aerica This PV pat with a ratig pwer f 179 W has bee ispected with dres aaysig abut 8 Wh

Choice of hardware and software
Dres fr the ispecti f PV pats d t require high techica specificatis because they fy  a 2D pathway ever exceedig a height f 150  Fr this reas their assciated csts are iited The seecti f a adequate dre fr PV appicati ivves severa aspects A ar issue ccerig dres is their w fight tie The batteries istaed have iited capacity ad their repaceet ay be required durig the ispecti f a pat causig deays ay dres brea because they fa dw withut warig if the batteries are discharged ad this is a ptetia ris fr pit safety The weight f the dre cat be wer tha the weight f the caera it has t withstad gusts f wid f at east 8 s ad cpatibiity with severa types f caera has t be tae it accut A uique caera with thera ad RGB spectru r tw caeras e fr thera iages detecti ad e fr visua ispecti have t be seected fr PV ispectis The first suti is preferred i rder t iit the weight  the dre ad thus aw greater battery ife It is as iprtat t csider the fwig aspects • Avaiabiity f iage etadata such as GPS cati reative atitude giba attitude ad tiestap • Resuti f 640x480 dpi r higher • Stabiised giba t eep the caera i a stabe psiti despite dre veets • Hrita fied f view i rder t capture the axiu area i the iages

Flight directives
I this wr visua ispectis ad thera ifrared aaysis f PV pats wi be carried ut by Ee Gree Pwer peratrs i fieds The peratr wi verify the fwig cditis fr the iitia prefight phase • Wid at a axiu speed f 15 ph (67 s) • Cear r sighty vercast sy with irradiace  600 W2 • Reative huidity RH  60% • existet r w due siig • The pit wi fw the guideies bew fr the fight pa • The fight budary ie wi icude a exteded area beyd the edges f the sar far • The fight path wi be parae t the rws f the sar far • The fight eevati wi be csistety abve grud eve Terrai with sigificat eevati chages ay require utipe fights adr advaced grud ctr sftware with terrai fwig capabiities • The giba pitch wi perit deviatis up t 20 t avid gare • Iage capture wi be autatic with a shutter iterva set t 2 s • The fight speed wi t exceed 25 s • The frt verap wi be 80% i the directi f fight • The side verap wi be 20% ver each pass • The iage etadata wi ctai GPS cati reative atitude giba attitude tiestap • There wi be a cstat caera headig durig the ispecti despite UAS rietati • The iages wi be saved  a high speed ery card with a UHS3 ratig • After the fight the fwig ast prescriptis are ecessary • site quaity chec f the etire dataset • Refy sectis f parts f the pat if se data is issig

Photovoltaic modules failures
The cuuative istaed PV capacity rapidy grew i the ast decade icreasig fr ≈ 92 GW i 2007 t ≈ 480 GW i 2019 13 As a csequece checig the presece f defects  the etire prducti vue is very g ad expesive therefre a restricted uber f PV dues is usuay subect t a aua idetificati f defects rever the thicess f wafers fr crystaie siic PV ces decreased fr ≈0304  t ≈02  14 i rder t reduce the wastage f siic ad the aufacturig csts Hwever if the wafer is t sa defects such as cracs ad icrcracs ca appear Fr these reass the aiteace f PV arrays is fudaeta i rder t detect with fast respse defective subarrays ad iit the ss f prducti The resuts f experieta wr are aaysed i rder t defie the type f defects that ca be detected by thergraphy ad visua ispecti The faiures detected by dres are cected i six thergraphic reprts fr pats i rth ad Suth Aerica The average uber f daaged dues is ess tha 2% The faiures are divided it three ai categries • Ce faiure i ay cases the faiure f a ce causes a ht spt If the teperature icrease is high the etire due ust be repaced There are ay ther faiures that ca ccur durig the first years f ife aiy due t pr quaity aufacturig prcesses 15) ther pheea such as ptetia iduced degradati (PID) verheatig f dides i the ucti bxes ad deaiati ca ccur durig the ife f the pat 161718 • Prtecti syste issues this faiure categry icudes cases i which etire strigs ad arrays are ffie I this case there is  daage t the dues but wires r prtective equipet (aiy fuses) are daaged • Activated bypass dide ad siig the activati f the bypass dide ca be a csequece f sig r shadig I this case the prducti f the ces prtected by the dide is st (geeray PV dues are equipped with three r fur dides) Prducti ca as be reduced due t the icrrect perati f the PPT 19 I this case prducti is recvered by ceaig the due r the reva f the shadwig bstace 29% 62% 9%

MODULE FAULTS
module thermal anomalies protection systtem issues activated bypass diode Fig 2 Frequecy fr the three categries f faiure Figure 2 shws the prprtis f the tta uber f faiures i the abve categries The st frequet issue is the faiure f wires r prtective equipet I this case the tta prducti f the part f the pat affected by the faiure is st but the faiure is cheap t repair (it des t require due substituti) The ther faiures cprise 40% f the tta ad i the case f thera aaies the repaceet f dues is required Visua ad thera ispecti perits the detecti f faiures but fr st f the faiures there is  crreati betwee the defect ad the pwer ss I rder t cacuate pwer sses a deteriati f its curretvtage (IV) characteristic curve by dyaic ethds is ecessary 20 This ca be perfred with adequate ad caibrated easureet systes 21 usig capacitive r eectric ads Thas t the capacitive ad 22 a sca f the etire IV curve ca be easiy perfred at due strig ad array eve up t severa hudred iwatts I ccusi the cpeetary executi f the IV characterisati f PV geeratrs ad the thera ispecti perits defects t be caised ad sses t be quatified

Economic analysis
The et preset vaue (PV) is used i budgetig ad ivestet paig t aayse the prfitabiity f a ivestet r prect Fr exape it ca be used t quatify the csteffectiveess f the istaati f a ew PV pat 23 r the repaceet f defective dues i existig pats 24 I this paper the PV is used t quatify the csteffectiveess f the use f dres fr thera ispecti cpared t aua thergraphy This prcedure is repeated fr differet sies f PV pats The frua f the PV is the fwig (1) where t is the uber f years Ct is the cash fw (psitive fr reveue r egative fr csts) at t th year ad r is the iterest rate f the ivestet (i this case it is assued equa t 8%) First the csts have bee partitied it CAPita EXpediture (CAPEX) ad Peratig EXpese (PEX) CAPEX is the cst f devepig r prvidig durabe assets fr the prduct r syste ad PEX is the cst ecessary t aage a prduct a busiess r a syste I this study CAPEX ivves tw csts • Dre batteries ad caera purchase cst • Traiig Ee Gree Pwer peratrs t bece dre pits PEX are variabe csts depedig  the pat sie I particuar they ivve the fwig ctributis • Fixed aiteace csts fr hardware • Fixed cst fr dre repaceet every five years • Csts ccerig survey ad the iage aaysis • aiteace pat cst (aiy due repaceet ad eectrica daage repair ad ther crrective actis fr PV pat) The ecic beefits were ivestigated via tw ters • PEX reducti passig fr aua t autatised ispecti that depeds  pat sie • Beefit due t higher prducti Usig dres it is pssibe t perfr tw ispectis per year istead f e ad the average tie that the daaged dues sped i the fied is haved PV pat ifetie is estiated at abut 25 years ad s the PV after 25 years has bee cacuated as a fucti f pat sie The PV icreases after 25 years with arger pat sie (Fig 3) the arger the fied the higher the gai Dres are csteffective sutis fr ispectis i PV pats with a pwer ratig greater tha 50 W (gree cus) but aua ethds are preferred fr saer pats (red cus) I this wr the ifetie f the dres is suppsed t be five years therefre after that tie iterva they have t be repaced with ew es -1'000'000

Conclusions
Future dres wi verce their curret iits further iprvig rbtisati i PVreated tass The curret versi wi be further deveped ad this dre ca fy withut hua ctr because it fws a path iserted i its ery ad ca be set ff retey rever it is abe t recharge itsef thas t its patfr ad it is pssibe t retey activate deactivate ad eve destry it It ca be used fr utipe appicatis fr exape fr surveiace purpses whe see crsses the perieter f the fied it raises the aar ad fies t the pit f viati fiig the itruder bviusy the st iterestig use f this dre is data capture if the cpay has its w sftware fr the aaysis f RGB ad ifrared iages the dre ca fy ad tae pictures ce a wee r ce a day Iages cected at shrt tie itervas are usefu fr uderstadig hw the pat degrades based  ciate years f perati ad techgy ipeeted Data ad artificia iteigece are the ies f techgica research used t autate the predictive aiteace f arge PV pats