Available online at ScienceDirect. Energy Procedia 49 (2014 ) SolarPACES 2013

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1 Aville online t SieneDiret Energy Proei 49 (2014 ) SolrPACES 2013 Comprison of solr power output foresting performne of the Totl Sky Imger n the University of Cliforni, Sn Diego Sky Imger S. M. I. Gohri, B. Urquhrt*, H. Yng, B. Kurtz, D. Nguyen, C. W. Chow, M. Ghonim, J. Kleissl University of Cliforni, Sn Diego, Deprtment of Mehnil n Aerospe Engineering 9500 Gilmn Dr. #0411, L Joll, CA 92093, USA Astrt The performne of the Totl Sky Imger (TSI) n University of Cliforni, Sn Diego Sky Imger (USI) instruments hs een evlute using orreltion n ispersion metris pplie to ler sky inex. The TSI ws lote t 48MW PV plnt in Nev, USA, n the USI ws lote t the UCSD mpus in Sn Diego, USA. Distriute pyrnometer mesurements over severl squre kilometers were ville t oth lotions to provie sptil verges of irrine. Cler sky inex ws forest for 15 minute horizon t 30 se. intervls, n the orreltion n Eulien ispersion were evlute. A persistene forest ws generte for eh lotion to provie referene for omprison. For the ggregte tset, the forest performne of the USI exeee tht of persistene eyon the 10 minute forest horizon, wheres the TSI i not show skill over persistene. For the perio of stuy, the USI gve orreltions of ler sky inex etween 0.70 n 0.82, wheres the TSI provie orreltions etween 0.55 n Overll, the men vlue n the tren of the orreltion results were more onsistent for the USI thn the TSI on the ys evlute. It is onlue tht the USI is more relile solr resoure ssessment instrument for the wie rnge of tmospheri onitions evlute The Authors. Pulishe y Elsevier Lt. This is n open ess rtile uner the CC BY-NC-ND liense ( The Authors. Pulishe Elsevier Lt. Seletion n n peer peer review review y y the the sientifi sientifi onferene onferene ommittee ommittee of SolrPACES of SolrPACES 2013 uner 2013 responsiility uner responsiility of PSE AG. of PSE AG. Finl mnusript pulishe s reeive without eitoril orretions. Keywors: Solr foresting; sky imger; short term forest; solr power * Bryn Urquhrt. Tel.: E-mil ress: urquhr@us.eu The Authors. Pulishe y Elsevier Lt. This is n open ess rtile uner the CC BY-NC-ND liense ( Seletion n peer review y the sientifi onferene ommittee of SolrPACES 2013 uner responsiility of PSE AG. Finl mnusript pulishe s reeive without eitoril orretions. oi: /j.egypro

2 S.M.I. Gohri et l. / Energy Proei 49 ( 2014 ) Introution As ntions roun the worl push for lener soures of energy, the nee for tehnologies tht filitte the integrtion of renewle energy into the eletriity gri is growing. Foresting is gri integrtion tool tht n help the sheuling of power genertion n ispth. Rules n regultions for the energy inustry iffer y region, ut ll eletri gris hve multiple relevnt time sles t whih plnners, opertors, power genertors, n utilities nee vne informtion out how muh power will e proue. Solr power is one of the most slle hoies for renewle energy genertion, n so foresting of this resoure is vitl to relily integrte it into the eletriity gri. Short term power output foresting, on the sle of 1 to 30 minutes, n help to enle the eonomi ispth of solr power. Sky imging systems hve history of proviing tmospheri oservtion n monitoring, n hve the potentil to e effetive monitoring systems for solr plnt opertions y proviing oth situtionl wreness n estimtes of power output over lrge sptil extent. Beuse of their ility to ontinuously monitor lou onitions, the University of Cliforni, Sn Diego (UCSD) hs evelope short term power output foresting pility se on visile wvelength sky imgery [1]. A relte foresting proeure ws evelope in [5]. Initil results on power output foresting for 48MW of photovolti genertion using the UCSD metho were reporte in [8]. Experiene with their use t lrge solr power plnts, however, hs highlighte shortomings in ommon imging hrwre [7]. The Totl Sky Imger (TSI) is the most ommon evie use for sky imging pplitions, ut no signifint improvements hve een me in the lst 15 yers. One of the key onlusions in [7,8] is tht the Totl Sky Imger (TSI) instrument h shortomings for the purposes of solr foresting. The University of Cliforni Sn Diego (UCSD), leverging expertise in sky imging tehnology [6], hs esigne system speifilly for solr resoure ssessment work [8]. This system, lle the USI, exees the TSI in oth sptil n intensity resolution (i.e. numer of pixels n it epth, respetively). Here, omprison of the two instruments is me to quntify the foresting performne in oth reltive n solute sense. 2. Experimentl methos 2.1. TSI site: Copper Mountin Solr 48 MW To ssess forest performne, 1 se rel AC power t from 48MW setion of Sempr Genertion's Copper Mountin Solr 1 ws use (Fig. 1). This 1.3 km 2 plnt setion h 96 inverters reeiving power from mium tellurie thin film pnels tilte t 25 with ue south zimuth. This setion of the plnt ontine fifteen lirte referene ells proviing plne-of-rry glol irrine ( ) t 1 se n five wether sttions proviing stnr meteorologil mesurements inluing plne-of-rry n Glol Horizontl Irrine ( ) from Kipp & Zonen CMP11 ron pyrnometers t 1 se. Two TSIs were instlle t N, W, 572 m MSL, 1000 m EBU2 CMRR MOCC POSL BMSB HUBB U S I UCSD Sky Imger Wether Monitoring Sttion Fig. 1 () Footprint of the 48MW setion of the Sempr Genertion Copper Mountin Solr power plnt. TSI lotions re inite with irle ions. The sptil resolution of the power t is inite y the gry level shing. () University of Cliforni, Sn Diego mpus with USI n wether sttions inite y terrop ions.

3 2342 S.M.I. Gohri et l. / Energy Proei 49 ( 2014 ) n N, W, 580 m MSL, whih re t the northwest n southest orners, respetively. The imgery for the northwest unit ws use to generte forests. The forest intervls selete mth the 30 se imge pture frequeny; forests were issue every 30 seons out to 15 minutes. Forests were generte for Novemer 1 st to the 21 st, 2011, n July 12 th to the 17 th, 2012 (27 ys totl). Mesure power output t is ompre to the forest y pplying 30 se moving verging to plnt mesurements USI site: UCSD solr energy teste The forest omin for UCSD ws efine s km gri t resolution of 2.5 m per gri point. Eh gri point ws mppe to ltitue n longitue, s well s n ltitue otine from the SRTM1 tset [2]. Six Li-COR 200SZ pyrnometers, smpling t 1 se, re lote within this forest omin (Fig. 1), with positions given in Tle 1. The USI ws lote on rooftop t N, W, 145 m MSL. The USI ptures high ynmi rnge imges [8] every 30 seons uring times when the sun is ove n elevtion of -3. Forests were generte from Novemer 1 st to the 29 th, 2012, exluing Novemer 12 th (28 ys totl). A single omin verge irrine mesurement is otine y verging irrine mesurements from the six sttions. Mesure irrine t is ompre to the forest y pplying 30 se moving verging to the omin-verge time series. Tle 1. Lotions of the pyrnometers in the UCSD network n their istne to the USI. Sttion Ientifier Longitue [eg] Ltitue [eg] Altitue [m] Distne to USI [m] BMSB CMRR EBU HUBB MOCC POSL Forest methoology For revity, this work fouses on the omprison of TSI n USI forest performne. Only n overview of the forest proeure is provie here. The intereste reer is referre to [1,7-9] whih ontin etils of forest lgorithm. The si priniple of the forest is to lote lous in the sky using geometrilly lirte mer system, to trk the motions over few imges, n then to vet the lous forwr in time. Ry tring is performe over the forest omin to estimte the positions of the shows for every vetion. A show frtion for relevnt forest omin gripoints (i.e. those with solr olletors) is then ompute. Power n/or irrine is then prmeterize using the show frtion. The ompliment to the show frtion is the ler frtion, whih is simply one minus show frtion. The prmeteriztion of power or irrine requires simultneous mesurements from the PV rry or n irrine sensor, where the former will provie power output forest n the ltter will provie n irrine forest. The power or irrine prmeteriztion is se on the onstrution of histogrm of ler sky normlize mesurements [8]. The ler sky normlize power or irrine n e ompute s or (1), (1) where the susript inites ler sky moele vlue pproprite for the time, lotion, n orienttion of the solr olletors or irrine sensor. The following text will omit the or from, n refer to it s the ler sky inex. In this work, the ler sky inex for the TSI is ompute using (1) n ompute for the USI using (1). It shoul e note tht in this work, only glol irrine is onsiere. The metho of using the ler sky inex

4 S.M.I. Gohri et l. / Energy Proei 49 ( 2014 ) histogrm to physilly onstrin power output etween lower n upper ouns n inste use iret norml irrine (DNI) s input. The frtionl lou over metho evelope in [7-9], whih is similr to tht pplie in [5] for DNI foresting, n then e use to etermine the forest DNI etween these two ouns. There is n importnt ifferene etween the eployment onfigurtions of the TSI (setion 2.1) n the USI (setion 2.2): to generte forest performne results, the USI uses only six 6.25 m 2 forest gripoints, wheres the TSI uses over 190,000 forest gri points (lso 6.25 m 2 ). Sptil errors in lou n show positions re verge over the entire plnt for the TSI wheres for the USI, it is verge over the six point mesurements. To provie referene for ompring instrument performne, persistene forest is use Persistene forest Any forest tehnique must outperform persistene (i.e. using the urrent vlue s the future estimte) to e onsiere to hve forest skill. For omprison, persistene forest is generte for eh forest issue y verging the previous 60 seons of groun-mesure ler sky inex Comprison metris This setion provies efinitions of the sttistil metris use to ompre the forest performne of the TSI n USI. The utility of eh metri for ssessing forest performne will e isusse. The three ifferent metris use for omprison re the Person orreltion oeffiient, the Spermn orreltion oeffiient n Eulien ispersion Person orreltion oeffiient Person Correltion Coeffiient (PCC; ) is mesure of the liner epenene etween set of two vriles n, giving n vlue etween 1 n +1. PCC is formulte for smples y using the vrine n ovrine of two vetors n s (2) where the overr inites the men of the set. PCC is normlize ssessment of the ovrine of the forest n mesure, n inites how well the two signls mth, irrespetive of mgnitue Spermn orreltion oeffiient Spermn orreltion oeffiient (SCC; ) ssesses the monotoniity of the reltionship etween two vriles. When eh vrile is perfet monotone funtion of the other, SCC hs vlue of +1 or 1. SCC, when pplie to set of two vetors is efine s the PCC etween the rnke vetors. For smples, the vetors n re onverte to rnke vetors n, respetively, n is ompute (3) where the overr inites the men of the set, whih here is set of rnks. In this work, frtionl rnking is use euse repete vlues for or show frtion re ommon. SCC evlutes the overll tren of the forest signl ompre to the mesure. The SCC vlue is not ffete y the linerity of the reltionship etween n like the PCC vlue.

5 2344 S.M.I. Gohri et l. / Energy Proei 49 ( 2014 ) Eulien ispersion The eqution of line in two imensions is, where the line is efine y the vetor. The Eulien istne vetor ssesses how fr eh orere pir is lote from the line efine y : (4) The Eulien istne vetor n e onense to slr metri, whih we ll Eulien ispersion, y looking t the men solute istne (MAD) or root men squre istne (RMSD). For evluting the mesure versus forest ler sky inex, one-to-one line is use for euse the reltion etween oth ler sky inex sets shoul hve unity sling n zero is. For ompring forest ler frtion (setion 2.3) n mesure ler sky inex, referene line efine y the points n is use, where n represent the 5 th n 95 th perentile of the mesure ler sky inex set. These points were selete euse the working hypothesis of the sky imger forest is tht ler sky inex is linerly proportionl to ler frtion through the rnge mesure ler sky vlues for given y. Eulien ispersion ssesses the vliity of this hypothesis. 3. Results Forest performne for eh instrument ws hrterize for one month of t. The ggregte tset is useful in ssessing the overll forest skill of the forest metho pplie to oth instruments, ut eeper unerstning of performne is otine only when nlyzing iniviul ys. This setion is ivie into two setions: one whih presents results for single y, n one whih presents results for the ggregte tset Forest performne on single y To evlute single y forest performne, July 12, 2012 ws selete for the TSI, n Novemer 8, 2012 ws selete for the USI. These ys were hosen euse they h high vriility, whih is the most interesting onition in whih to evlute forest skill. Stter plots of mesure versus forest ler sky inex for the 0-min (nowst) n 10-min horizon for oth instruments re shown in Fig. 2. The stter for oth instruments t eh time sle whih inites tht there is signifint error in the forest. In Fig. 2, the TSI nowst is prtiulrly poor. This is ue to the shown on the TSI mirror tht loks the view of the sky ome ner the sun. Beuse the TSI is ollote with the solr olletors, this region of sky is where lous using urrent shing exist (e.g. lou etween n oserver n the sun sts show on the oserver). Even for plnt of 1.3 km 2 in re, the shown n eliminte ll (or nerly ll) sky imge t neee for 0 to 5 minute forest. The size of the plnt re impte y the shown sles linerly with lou height (in the positive sense). The verge lou height on July 12, 2012 ws 4.6 km. When the projetion of the shown (in the lou mp [1,8]) vets wy from the plnt, vli imge t n e use to generte forest, suh s tht in Fig. 2 whih shows muh etter result thn Fig. 2. The USI, with no oultor loking the sun, oes not hve this prolem s is eviene in Fig. 2 n 2. Stter plots of mesure ler sky inex versus ler frtion re shown in Fig. 3. From single y of mesurements, it is not pprent tht this reltion is exhiite y either instrument. The results for the TSI (Fig. 3) re ounter intuitive in tht it ppers tht the more shing the plnt reeives, the higher the irrine. This is not rel result, ut rther n rtift of the imge qulity, the shown, n the proessing lgorithms pplie. It shoul e note tht the vertil lines in Fig. 3 n 3 (USI) re ue to the six sttions use for generting ler frtion whih nominlly hs seven possile ler frtion vlues unless t is missing or the lous vet too fr from mesurement sttion to etermine if it is she. For the TSI (Fig. 3 n 3), the gri points re 6.25 m 2 squres overing 1.3km 2 plnt, giving mny possile vlues for ler frtion.

6 S.M.I. Gohri et l. / Energy Proei 49 ( 2014 ) Fig. 2 Stter plots of mesure vs. forest ler sky inex for nowst (, ) n 10-min horizon (, ) for the USI on Novemer 8, 2012 (, ) n the TSI on July 12, 2012 (, ). Fig. 3 Stter plots of mesure ler sky inex vs. ler frtion for nowst (, ) n 10-min forest (, ) for the USI on Novemer 8, 2012 (, ) n the TSI on July 12, 2012 (, ).

7 2346 S.M.I. Gohri et l. / Energy Proei 49 ( 2014 ) The orreltion (PCC n SCC) etween the forest n mesure ler sky inex is shown in Fig. 4 for the USI n Fig. 4 for the TSI. The orreltion of the persistene forest with the mesurements is lso given for omprison. Persistene is etter orrelte with the mesurements thn the USI forest until out 5 minutes. After this time, the USI n persistene re omprle until out 10 minutes where the USI egins outperforming persistene. The TSI forest performne is poor (even nti-orrelte) until the projetion of the shown vets wy from the plnt, fter whih the forest performne improves, ut is still not s high s persistene. Both sky imger forests n persistene show the expete erese in ury with forest horizon. The orreltion (PCC n SCC) of ler frtion n mesure ler sky inex is shown in Fig. 4 for the USI n Fig. 4 for the TSI. For the USI, ler frtion is positively orrelte with ler sky inex, ut it shows mrke reution in orreltion with forest horizon. In Fig. 4 the effet of the shown n gin e seen until pproximtely the 6 minute forest horizon. The persistene forest for UCSD n for Copper Mountin (the USI n TSI, respetively) ey t muh ifferent rtes, with UCSD experiening muh poorer performne when using persistene thn for Copper Mountin. The onitions t Copper Mountin re no less vrile thn for UCSD on the y selete, ut quite pprently, the mesurements me within the lst 15 minutes hve higher orreltion with urrent mesurements t Copper Mountin. There is sptil smoothing ftor t Copper Mountin tht oes not exist t UCSD; s the re inreses, the timesle of orreltion lso inreses [3,4] (i.e. the time it tkes for orreltion to ey to given vlue inreses). This results in longer time sles hving higher orreltions, the onverse of whih is tht point sensors suh s those t UCSD hve the sme level of orreltion on shorter timesles. This is lerly exhiite y the persistene forests in Fig. 4 n 8. The Eulien ispersion for the USI (Fig. 5) is onsistent throughout the 15 minute forest horizon, wheres for persistene it inreses shrply, n fter 7-min is higher thn for the USI. The ispersion for the TSI (Fig. 5) is tully lower thn for the USI, whih ours euse of the smller rnge of ler sky inex for the TSI on the y selete (0.2 to 1.6 for the USI vs. 0.1 to 1.1 for the TSI). The Eulien ispersion etween ler frtion n mesure ler sky inex for the USI (Fig. 5) is lower thn tht of the TSI (Fig. 5), even with the rnge isprity of mesure ler sky inex, initing it performs etter when viewing this metri. Fig. 4 Person (PCC) n Spermn (SCC) orreltion oeffiients for forests with the USI on Novemer 8, 2012 (, ) n TSI on July 12, 2012 (, ). Forest vs. mesure ler sky inex is shown in (, ), n ler frtion vs. mesure ler sky inex in (, ). Performne of the persistene forest is shown in (, ) s she line (susript p in legen).

8 S.M.I. Gohri et l. / Energy Proei 49 ( 2014 ) Fig. 5 Eulien ispersion of mesure vs. forest ler sky inex (, ) n ler frtion vs. mesure ler sky inex (, ) for the USI on Novemer 8, 2012 (, ) n the TSI on July 12, 2012 (, ). Performne of the persistene forest is shown in (, ) s she line (susript p in legen) Forest performne for one month Reviewing forest results for single y highlights performne in prtiulr onitions, ut to view the overll performne ssessment, longer tset is require. Stter plots of mesure versus forest ler sky inex uring the one month stuy perio is shown in Fig. 6. For oth the nowst (left olumn) n the 10-min forest horizon (right olumn), the USI (top row) shows less (ut still onsierle) stter thn the TSI (ottom row). For the USI, there pper to e less frequent ourrenes of inverte forests (points in the top left or ottom right). Stter plots of mesure ler sky inex versus ler frtion for the one month stuy perio re shown in Fig. 7. There is gin no ler reltion etween the ler frtion n the ler sky inex for either the USI (top row) or the TSI (ottom row). It is lso not pprent from the stter plots tht the nowst ler frtion (left olumn) proues etter results thn the 10-min forest ler frtion (right olumn). The TSI hs the inherent vntge of the lrge sptil verge over the power plnt, ut still oes not show signifint improvement. The higher ensity of points ove the referene line in the ottom row of Fig. 7 inites tht the TSI ises forests to ontin lou when the ler sky inex inites power output is ner 100% of ler sky levels. The orreltion results for the one month t set re shown in Fig. 8. Reviewing the mesure versus forest ler sky inex, the verge PCC ws 0.78 n the SCC ws 0.72 for the USI, wheres for the TSI, the respetive vlues were 0.58 n The SCC for the USI n TSI re similr, whih inites oth systems re pturing the trens of the irrine or power flututions. The signifintly higher PCC vlue for the USI inites tht the reltion etween forest n mesure ler sky inex is muh more liner thn for the TSI, whih is positive result for the USI euse the reltionship shoul e one-to-one. The effets of the shown on the TSI n lerly e seen in Fig. 8, where orreltion inreses nerly monotonilly with time. The TSI tully exees performne of the USI with the SCC vlue for ler frtion versus mesure ler sky inex t the longest forest horizons.

9 2348 S.M.I. Gohri et l. / Energy Proei 49 (2014) Fig. 6 Stter plots of mesure vs. forest ler sky inex uring the one month stuy perio for nowst (, ) n 10-min horizon (, ) for the USI (, ) n the TSI (, ). Fig. 7 Stter plots of mesure ler sky inex vs. ler frtion uring the one month stuy perio for nowst (, ) n 10-min forest (, ) for the USI (, ) n TSI (, ).

10 S.M.I. Gohri et l. / Energy Proei 49 ( 2014 ) Fig. 8 Person (PCC) n Spermn (SCC) orreltion oeffiients uring the one month stuy perio for the USI (, ) n TSI (, ) forests. Forest vs. mesure ler sky inex is shown in (, ), n ler frtion vs. mesure ler sky inex in (, ). Performne of the persistene forest is shown in (, ) s she line (susript p in legen). Compring the performne of oth the USI n TSI to persistene t their respetive lotions shows tht the USI performe signifintly etter for the perio of stuy. As note in setion 3.1, foresting t lrge plnt like Copper Mountin shoul proue higher orreltions for the 15 minute forest winow euse of the inrese orreltion time sle, yet the TSI shows orreltion results tht re onsistently elow persistene y 0.15 to 0.30, even for longer forest horizons. The USI, however, outperforms persistene t longer time sles. The numer of ler ys t Copper Mountin my e suspet use for higher orreltions of persistene, ut oth the UCSD n Copper Mountin t sets ontine 8 ler ys. 4. Conlusions The overll forest results for oth instruments inites tht there is muh error in the eterministi foresting proess. The working hypothesis tht ler frtion is proportionl to ler sky inex ws not onlusively vlite in this work. The orreltion ws positive etween these two prmeters, ut the low orreltion vlues n stter in the t suggests this hypothesis is only prtilly vli. The ler frtion estimte for the solr olletors or irrine sensors is epenent primrily upon lou etetion, lou spee n vetion ssumptions, n the geometri lirtion of the instrument. If lou is not orretly etete, n if the positionl informtion of eh pixel is not urte, then the ry tre ler frtion will e inorret. Similrly, if the lou vetion is erroneous, ue to either n error in lou spee or in the ssumption tht lous n e vete forwr in time without ounting for onenstion/evportion, errors in ler frtion will result. These results lso inite tht more investigtion into the lou overge (i.e. ler frtion) to power output prmeteriztion is require. Compring the USI performne to tht of the TSI inites tht the USI outperforms the TSI overll, prtiulrly for the first 10 minutes of the forest winow. This is n expete result euse the USI oes not hve missing t ue to the shown, it oes not lose informtion to imge ompression, n hs oth higher sptil n intensity resolution (i.e. it hs more horizontl n vertil pixels, n provies 48-it imges s oppose

11 2350 S.M.I. Gohri et l. / Energy Proei 49 ( 2014 ) to 24-it imges). In the experimentl onfigurtion use here, the TSI h the vntge of signifintly lrger sptil verge (35,500:1; TSI:USI) euse it ws ple t power plnt. It is resonle to expet some of the sptil errors in ler frtion to verge out in this senrio, however this result ws not relize n the USI still showe etter performne. Moving forwr with this forest work, emphsis will e ple on improving results with the USI. It hs een shown here to e superior instrument for short term foresting, n will provie higher qulity imge t for this omputer vision-se foresting work. Aknowlegements Hrwre evelopment for the USI ws fune y Pnsoni Corportion. Avne forest lgorithms n emonstrtions were fune y Pnsoni Corportion, the Deprtment of Energy High Solr PV Penetrtion Grnt 10DE-EE002055, the Cliforni Energy Commission ontrts n , n the Cliforni Puli Utilities Commission Cliforni Solr Inititive RD&D Progrm. The help of Dvi Jeon, Leslie Pill, Ron Clugh, Drrel Lopez n Shiv Bhumn from Sempr US Gs n Power, Jim Blthfor from the Cliforni Inepenent System Opertor, Dvi Zeglinski n Megn Fox from OSISoft, n Jo Fretti from the Sn Diego Superomputer Center re ppreite. This work woul hve not een possile without the ssistne of Elliot Dhlin, Cspr Hnselr, Emuno Goinez, Willim Gui, Prithvi Sunr, Dn Erez, Sott Kto, Amy Ching, Jessi Trynor, Kristen Otosh, Tyler Cpps, Sestin Shwrzfisher, Nik Truong, Jeff Yeh, Mx Twogoo, Alex Turhik, Dnielle Donnely, n Emily Dvis, whom helpe with hrwre evelopment, frition, ssemly, n eployment of the USI systems. Anthony Dominguez, Ptrik Mthiesen, Mtt Lve n Felipe Meji set up n mintine the UCSD pyrnometer network. Referenes [1] Chow CW, Urquhrt B, Lve M, Dominguez A, Kleissl J, Shiels JE, Wshom B. Intr-hour foresting with totl sky imger t the UC Sn Diego solr energy teste. Solr Energy 2011; 85: oi: /j.solener [2] Frr TG, Rosen PA, Cro E, Crippen R, Duren R, Hensley S, Korik M, Pller M, Roriguez E, Roth L, Sel D, Shffer S, Shim J, Umln J, Werner M, Oskin M, Burnk D, Alsorf D. The shuttle rr topogrphy mission. Review of Geophysis 2007; 45:RG2004. oi: /2005RG [3] Lve M, Kleissl J, Aris-Cstro E. High-frequeny irrine flututions n geogrphi smoothing. Solr Energy 2012; 86: oi: /j.solener [4] Lve M, Kleissl J. Clou spee impt on solr vriility sling - pplition to the wvelet vriility moel. Solr Energy 2013; 91: oi: /j.solener [5] Mrquez R, Coimr CFM. Intr-hour DNI foresting se on lou trking imge nlysis. Solr Energy; 91: oi: /j.solener [6] Shiels JE, Krr ME, Johnson RW, Buren AR. Dy/night whole sky imgers for 24-h lou n sky ssessment: history n overview. Applie Optis 2013; 52: oi: /AO [7] Urquhrt B, Chow CW, Nguyen D, Kleissl J, Sengupt M, Blthfor J, Jeon D (2012). Towrs intr-hour solr foresting using two sky imgers t lrge solr power plnt. Proeeings of the Amerin Solr Energy Soiety, Denver, CO, USA, [8] Urquhrt B, Ghonim M, Nguyen D, Kurtz B, Kleissl J. Sky imging systems for short-term solr foresting. In: Kleissl J, eitors. Solr Energy Foresting n Resoure Assessment. Elsevier; Chpter 9. [9] Yng H, Urquhrt B, Kurtz B, Nguyen D, Chow CW, Ghonim M, Kleissl J. Solr irrine foresting using groun-se sky imger evelope t UC Sn Diego. To e pulishe, 2013.