The Value of Disribued Phoovolaics o Ausin Energy and he Ciy of Ausin This repor was prepared as par of a response o SOLICITATION NUMBER: SL04300013 Sudy o Deermine Value of Solar Elecric Generaion To Ausin Energy Prepared for: Ausin Energy Prepared by: Clean Power Research, L.L.C. Thomas E. Hoff Richard Perez Gerry Braun Michael Kuhn Benjamin Norris Final Repor March 17, 2006
Execuive Summary Inroducion Ausin Energy (AE) has a srong commimen o inegraing solar elecric generaion ino is power generaion and disribuion sysem. This is made clear no only by he inroducion of is recen incenive for cusomer-owned phoovolaic (PV) sysems, bu even more so by is goal of insalling 15 MW of solar generaion by he end of 2007 and 100 MW by 2020. AE wans o ensure ha he cos of solar generaion is commensurae wih is value. As such, AE issued wo reques for proposals (RFPs) o perform value sudies. One RFP was o deermine he value of he economic developmen benefis of solar. The oher sudy was o deermine he value of solar generaion o AE. Clean Power Research (CPR) was seleced for he second RFP. Objecive There are several alernaive approaches o deermining he comprehensive value of solar generaion. One approach is o perform an in-deph analysis of a single candidae echnology. This has he benefi of clearly illusraing evaluaion mehodologies. Anoher approach is o perform a less comprehensive analysis for a wide variey of solar echnologies. This approach has he benefi of providing resuls for muliple solar echnologies, such as disribued PV, cenral saion PV, cenral saion solar hermal roughs, solar dishes, or cusomer solar ho waer heaing o displace elecric waer heaers. AE saed in is RFP ha he work should provide evaluaion mehodologies. Thus, in order o maximize he effeciveness of AE s financial invesmen in his sudy, he firs approach was seleced. I performs an in-deph evaluaion of a single solar echnology ha requires a relaively complicaed analysis effor (disribued PV) and places special emphasis on documening evaluaion mehodologies. There are wo primary objecives of his sudy: 1. Quanify he comprehensive value of disribued PV o AE in 2006 2. Documen evaluaion mehodologies o assis AE in performing he analysis as condiions change and applying i o oher echnologies ES-1
Scenario Definiion The sudy assumed ha he comprehensive value of disribued PV includes he following benefis: Energy producion Generaion capaciy T&D capaciy deferrals Reduced ransformer and line losses Reacive power conrol Environmen Naural gas price hedge Disaser recovery For each of hese benefis, he analysis considers following configuraions. Fixed configuraions o Horizonal (fixed PV wih no il) o Souh-30º (souh-facing fixed PV iled a 30º) o SW-30º (souhwes-facing fixed PV iled a 30º) o Wes-30º (wes-facing fixed PV iled a 30º) o Wes-45º (wes-facing fixed PV iled a 45º) Tracking configuraions o 1-Axis (norh-souh 1-axis racking PV wih no il) o 1-Axis 30º (norh-souh 1-axis racking PV wih 30º il) A number of he benefis are a funcion of he size of he PV sysem. The analysis is performed for 15 MW of PV unless oherwise specified. Muliple evaluaion mehods and inpu assumpions can be used for almos all of he benefi calculaions. Aemping o include all possible combinaions of evaluaion mehodologies and inpu daa ses would resul in an excessive number of scenarios. A variey of evaluaion mehods and assumpions were considered during preliminary phases of he sudy. Afer furher consideraion, i was decided ha a single scenario reflecing he join opinions of AE and CPR would bes serve he purposes of his sudy. As a resul, he resuls are neiher he highes hey could be nor he lowes hey could be bu raher represen a middle ground. ES-2
Resuls The value for 15 MW of disribued PV o AE is summarized in he op par of Table ES- 1. The boom par of he able presens a facor o adjus for PV size. 1 15 MW of PV The value of 15 MW of PV is $2,312 per kw (11.3 per kwh) for he bes fixed configuraion. The bes fixed configuraion is SW-30º and is only slighly higher ha a Souh-30º configuraion. The sysem wih he highes value overall is he 1-Axis 30º racking sysem and is worh $2,938 per kw (10.9 per kwh). This sysem has a 27 percen value premium over he bes fixed configuraion. 100 MW of PV AE can use hese resuls of his sudy o deermine he value of a larger amoun of PV. For example, AE s size adjusmen facor for 100 MW is 95 percen as presened in he boom of Table ES-1. The bes fixed and racking configuraions a he 100 MW peneraion level are worh $2,196 per kw (10.7 per kwh) and $2,791 per kw (10.4 per kwh), respecively. Table ES-1. Value of 15 MW of disribued PV and size adjusmen. 2 Value of 15 MW of PV Presen Value ($/kw) Levelized ($/kwh) Fixed Sysems Horizonal $2,154 $0.111 Souh 30º $2,299 $0.108 SW 30º $2,312 $0.113 Wes 30º $2,127 $0.117 Wes 45º $1,983 $0.118 Tracking Sysems 1-Axis $2,813 $0.110 1-Axis 30º $2,938 $0.109 15 MW 25 MW 50 MW 75 MW 100 MW Size Adjusmen 100% 99% 98% 96% 95% Breakdown of Value by Componen In addiion o oal value, i is useful for AE o undersand he source of value. The resuls are presened by benefi componen in Figures ES-1 and ES-2. Figure ES-1 presens he resuls in absolue or capaciy erms (presen value in $ per kw-ac). Figure ES-2 presens he resuls in energy erms (levelized value in $ per kwh). As can be seen in he figures, he energy producion benefi accouns for wo-hirds of he oal value. 1 The reducion in value due o size increase is primarily due o placing a greaer weigh on AE s 100 MW marginal cos forecas han is 1 MW marginal cos forecas. 2 While here is some variaion in he size adjusmen facor based on he sysem configuraion, he variaion is small and he size adjusmen facor presened in he able is accurae wihin ½ percen. ES-3
PV Sysem Value (presen value - $/kw) $3,500 $3,000 $2,500 $2,000 $1,500 $1,000 $500 $0 Horizonal Souh 30º SW 30º Wes 30º Wes 45º 1-Axis 1-Axis 30º Energy Gen. Capaciy Environmen T&D Deferal Loss Savings Figure ES-1. Presen value for 15 MW of PV by configuraion ($/kw-ac). PV Sysem Value (levelized $/kwh) $0.140 $0.120 $0.100 $0.080 $0.060 $0.040 $0.020 $0.000 Horizonal Souh 30º SW 30º Wes 30º Wes 45º 1-Axis 1-Axis 30º Energy Gen. Capaciy Environmen T&D Deferal Loss Savings Figure ES-2. Levelized value for 15 MW of PV by configuraion ($/kwh). ES-4
Discussion This subsecion describes each benefi in more deail. Energy Producion Disribued PV sysems produce elecriciy a he poin of consumpion a a sable price over he duraion of he life of he sysem. There are hree aspecs associaed wih he value of his energy producion. PV sysems produce elecriciy. The basic energy producion value occurs because he amoun of elecriciy ha needs o be generaed a oher plans is reduced by he amoun of PV producion, hus decreasing he amoun of fuel ha is consumed and he O&M coss associaed wih he elecriciy-generaion equipmen. PV sysems produce energy a he poin of consumpion. There are reduced losses in he T&D sysem because he energy produced by PV sysems does no have o pass hrough he ransmission and disribuion sysems o reach he poin of use. This is he energy loss savings value. PV sysems produce elecriciy a a sable price. PV cos is almos enirely capial relaed, wih nearly negligible O&M coss and no fuel coss. PV energy prices are herefore fixed and known over he life of he sysem. In conras, elecriciy prices from fossil-based generaion are subjec o poenially large fuel price flucuaions. Jus as insurance or cerain financial producs provide hedge value agains undesirable oucomes under uncerain fuure condiions, PV provides a hedge agains naural gas price uncerainy. This is he value of he reducion in fuel price uncerainy. The evaluaion mehodology includes benefis from AE s mehod of marginal cos analysis wih a mehodology aken from he esablished risk-free evaluaion approach from financial economics o capure boh he basic energy value and he value of he naural gas price hedge. The energy loss savings value is included as par of he loss savings benefi. A criical inpu ino he analysis is he naural gas price forecas. The ideal daa se is one in which here is cerainy in he naural gas prices over he life of he PV sysem. While his cerainy is obained for he firs 5 years using naural gas fuures prices for eniies ha hedge 100 percen of heir gas requiremens a he very sar of he 5 year imeframe, such a daa se was unavailable for he remaining years. The nex 25 years of daa are based on AE s consulan s forecas of naural gas prices. Generaion Capaciy Disribued PV effecively provides generaion capaciy by reducing demand-side consumpion. Generaion capaciy value is he produc of an economic value of an ideal resource (as represened by a naural gas urbine) and a echnical adjusmen o reflec PV s acual peak load reducion value o he AE sysem. The echnical adjusmen is ES-5
made using he Effecive Load Carrying Capabiliy (ELCC) mehod. Resuls indicae ha PV is worh beween one-half o wo-hirds he value of an equivalenly sized ideal resource, depending upon sysem configuraion. The generaion capaciy loss savings value is included in he loss savings secion. Environmen PV sysems provide an environmenal benefi by eliminaing emissions associaed wih non-renewable resources. While he quanificaion of reduced emissions is undispued, heir economic value is he subjec of ongoing debae. The mehod used in his analysis is o calculae he value by examining marke daa ha indicae cusomer willingness o pay premium prices for green power. In addiion o he environmenal benefis of he energy produced by he PV sysem, here are benefis associaed wih he avoided energy losses, and hese are covered in he loss savings secion. T&D Capaciy Targeed deploymen of PV relieves loads on he uiliy s ransmission, sub-ransmission, and disribuion sysems, effecively increasing available T&D capaciy. This relief allows uiliy T&D planners o defer capial invesmens in he T&D sysem. The economic value of hese deferrals includes boh he ime value of money and he reducion in T&D sysem O&M coss. The evaluaion is performed by firs calculaing he economic value of an ideal T&D resource and hen adjusing his value according o he effecive capaciy provided by PV. As a key se of inpus, AE provided esimaes of deferrable invesmens by disribuion area and year. Resuls indicae ha he T&D deferral value is relaively uniform hroughou he AE service erriory. One excepion is he downown area (no shown in Figures ES-1 and ES-2) which poenially has higher value due o slow load growh and expensive underground lines nearing capaciy. The T&D deferral benefi is locaion-specific. The value of his benefi is included only in cases where AE is able o acually defer T&D capaciy invesmens. Disaser Recovery Sixy weaher-relaed disasers over he pas 25 years have affeced a quarer of a billion U.S. ciizens and cos almos a half a rillion dollars. Hurricane Karina alone has reminded us of he saggering cos of weaher-relaed disasers and how he resuling power ouages compound he cos and slow he speed of disaser recovery. Significan deploymen of solar secure PV sysems (i.e., PV coupled wih sufficien elecric sorage) in he Ausin area would change he region s energy securiy profile. Even he small amoun of power produced by hese sysems could suppor coninuing use of homes, reail businesses, and seleced public buildings for exended periods of ime. However, deploying solar for his purpose requires ha PV sysems include energy sorage and sand-alone inverer capabiliy, and hese come a addiional capial cos. ES-6
Preliminary analysis suggess ha he disaser recovery benefi could increase he value of solar by more han 50 percen. This is firs known aemp a quanifying he disaser value benefis. As such, he valuaion mehod requires furher refinemen. A discussion abou disaser recovery is included in his repor, however, due o he uncerainies of quanificaion, i was decided ha he resuls would no be included in he numerical value calculaion. Insead, i is recommended ha AE furher consider he disaser recovery benefi when combined wih baery sorage, in paricular as i can be joinly implemened wih a plug-in hybrid elecric vehicle program. Reacive Power The reacive power value is he benefi ha would accrue if PV inverers were modified o provide volage regulaion suppor. This benefi is realized by adding a new echnical capabiliy o he inverer. Even wih such a modificaion, however, he value was found o be minimal. The value is no included in he final resuls. Loss Savings Loss savings is an indirec benefi because i increases he value of oher benefis, including energy producion, generaion capaciy, environmenal, and T&D capaciy. For example, if he energy producion value is $1,000 per kw and he echnical loss savings is five percen, he energy producion loss savings benefi is $50 per kw. The wo ses of inpus needed o calculae he loss savings value are he benefi-specific loss savings percenage and he corresponding benefi value. The resuls are based on he marginal sysem losses raher han he average sysems losses (marginal losses are abou wo imes he average losses). ES-7
Summary and Recommendaions Sudy Uses The firs objecive of his sudy is o quanify he comprehensive value of disribued PV o AE in 2006. The preceding paragraphs describe he resuls of his analysis. There are several ways ha AE migh consider how o use hese resuls in advancing is 2020 goals: Assis in srucuring an RFP for uiliy-owned sysems or power purchase agreemens Provide inpu ino incenive design for cusomer-owned sysems Help o assess he meris of kw-based buydown incenives vs. kwh-based performance incenives for cusomer-owned sysems Evaluae oher PV applicaions (e.g., a cenral saion PV could be screened by deleing he disribued benefis loss savings, and T&D deferral) Invesigae synergies wih oher AE programs such as demand managemen and plug-in hybrid vehicles Assis in evaluaing opporuniies relaed o AE s new Non-Tradiional Energy Business Planning process Mehodology Advances The second objecive of his sudy is o documen evaluaion mehodologies o assis AE in repeaing he analysis as condiions change and/or expand he analysis o include oher echnologies. These mehodologies are documened hroughou his repor. In he process of he analysis, CPR developed new mehodologies and enhanced exising mehodologies. More specifically, CPR: Applied financial economics risk-neural valuaion mehodology o accoun for he naural gas price hedge benefi Demonsraed ha loss savings calculaions should be performed on a marginal, raher han an average, basis and hen used he resuls o esimae hourly loss savings Developed a preliminary mehod o quanify he disaser recovery benefi Developed a mehod of capuring echnology synergies by convering a non-firm resource ino a firm resource by bundling i wih load conrol, hereby capuring addiional capaciy-relaed benefis ES-8
Sudy Enhancemens There are a number of ways ha his sudy can be furher enhanced. This disaser recovery benefi could increase he value of solar by 50 percen. Furher invesigae he disaser recovery benefi and assess how disribued PV could be incorporaed wih AE s plug-in hybrid vehicle program and disaser recovery services (Ciy and Couny) o provide he required sorage a a minimal cos; in paricular, evaluae implemenaion on public buildings, such as schools. The naural gas price forecas is he mos criical assumpion in deermining energy value in fuure years. However, only he firs 5 years of AE s naural gas forecas reflec cerainy in he naural gas price esimaes. I would be beneficial o exend he duraion over which cerainy could be obain for naural gas prices. PV could enable AE o offer a new produc: long-erm (20 o 30 years) fixed price elecriciy. Assess he feasibiliy of using PV o offer a long erm, fixed price elecriciy conrac o AE s cusomers, covering issues such as he cusomer s willingness o pay and he solar oupu inermiency risks o AE. AE s cusomers who paricipae in he GreenChoice program receive environmenal benefis plus fuel price risk proecion a a price ha is significanly below wha oher marke eniies charge for he environmenal benefis alone. Confirm ha AE is saisfied wih is GreenChoice program pricing. The T&D deferral benefi is lower a AE han a oher municipal uiliies wih which CPR has worked (AE s poenially-deferrable T&D invesmens represen slighly more han ¼ percen of AE s annual revenues). Confirm ha he cos of poenially-deferrable T&D capial invesmens is no arificially low due o AE s budge reporing pracices. Evaluae how he benefis idenified in his sudy could be applied from perspecives oher han AE (e.g., cusomer-ownership, and local, sae, and federal governmens) AE is a leader in is commimen o renewable energy. I is hoped ha his sudy will help o suppor and expand AE s vision and leadership. ES-9
Acknowledgemens The auhors acknowledge Ausin Energy s Roger Duncan (Depuy General Manager, Disribued Energy Services), John H. Baker Jr., P.E. (Chief Sraegy Officer, Sraegic Planning and Business Developmen), Mark Kapner (Senior Sraegy Planner, Sraegic Planning Group), and Leslie Libby (Projec Manager, Commercial Energy Managemen) for iniiaing and supporing his sudy on he Value of Disribued Phoovolaics o Ausin Energy and he Ciy of Ausin. This repor would no have been possible wihou he dedicaion and cooperaion of Ausin Energy s personnel across many differen deparmens. The following deserve high praise for providing he source daa required o complee he analysis, for heir ineres and enhusiasm for learning he mehodologies, and for he assignmen of valuable resources o he sudy. Michael McCluskey, P.E. (Senior Vice Presiden, Wholesale and Reail Markes) Lewis De La Rosa, P.E. (Manager, Energy Marke Analysis) Joe Lauer (Producion Specialis, Energy Marke Analysis) Seven Havemann, P.E. (Producion Specialis, Energy Marke Analysis) Madjid Zehani, P.E. (Marke Sysems) Charles J. Robinson, P.E. (T&D Planning) Jerrel Wallace, P.E. (T&D Planning) Ann Lile, CPA (Long Range Planning) Gordon Alexander (Regulaory Planner) Bob Breeze, P.E. (Air Program Coordinaor) 1
Table of Conens Inroducion... 8 Evaluaion Frameworks... 8 Energy Conservaion Evaluaion Framework... 9 Disribued PV Evaluaion Framework... 10 Framework for Curren Sudy... 11 Scenario Specificaion... 12 PV Performance Esimaes... 14 Inroducion... 14 Convenions... 14 Raing Convenion... 14 Naming Convenion... 16 Orienaion Selecion... 16 Oupu Esimaion... 18 Precision of Saellie-Derived Esimaes... 18 Oupu Simulaion... 20 Resuls... 22 Fuure Work... 26 Loss Savings... 27 Inroducion... 27 Mehodology... 27 Resuls... 28 Energy Producion... 29 Inroducion... 29 Mehodology... 29 Overview... 29 Deailed Seps... 31 Resuls... 32 Annual Energy Value... 32 Adjusmen for Revised Naural Gas Prices... 33 Energy and Loss Savings Value... 35 Discussion... 35 Generaion Capaciy... 36 Inroducion... 36 Mehodology... 36 Economic Inpu... 36 Technical Inpu... 37 Resuls... 37 Technical Resuls... 37 Combined Resuls... 39 Disribuion Sysem... 40 Inroducion... 40 Mehodology... 40 Economic Evaluaion... 41 Area-Specific Expansion Plan Cos... 41 2
Annual Load Growh... 46 Deferral Value... 46 Technical Evaluaion... 46 Resuls... 47 Discussion... 47 Reacive Power Conrol... 48 Inroducion... 48 Mehodology... 48 Power Facor... 48 Availabiliy of Reacive Power... 49 Resuls... 49 Summary... 50 Environmen... 51 Inroducion... 51 Mehodology... 51 Renewable Porfolio Sandards... 51 New Jersey Solar Renewable Energy Credi (SREC)... 51 Cusomer Choice Programs... 52 AE Green Choice Program... 52 Marke-Based Green Power Prices in Texas... 52 Resuls... 53 Disaser Recovery... 55 Inroducion... 55 The Ciy of Ausin s Vulnerabiliy... 55 Disaser Recovery Value Model... 58 General Inpus... 59 Disaser Cos... 59 Disaser Propensiy... 59 Base Case Disaser Scale and Timing... 60 Solar Specific Inpus... 60 Emergency Power... 60 Solar Emergency Power... 60 Emergency Managemen Consideraions... 61 Greaer Ausin and Travis Couny... 61 Emergency Managemen Plans and Experience... 61 Faciliies... 62 Value Analysis... 62 Resuls... 65 Conclusions and Recommendaions... 66 Fuure Work... 66 Conclusions... 68 Value of PV o AE in 2006... 68 Resuls Represen a Middle Ground Scenario... 70 Summary and Recommendaions... 70 Sudy Uses... 70 Mehodology Advances... 71 3
Sudy Enhancemens... 71 References... 73 Appendix B. Marginal Loss Savings... 76 Absrac... 76 Inroducion... 76 Compeiive Elecriciy Markes... 76 Uiliy Marginal Coss... 77 Objecive... 77 Analysis... 77 Insananeous Poin Losses... 77 Problem Formulaion... 77 Change in Losses... 79 Discussion... 79 Numerical Example... 79 Loss Definiion... 80 Technical and Non-Technical Losses... 80 Capaciy versus Energy Losses... 81 Insananeous Line Losses... 82 Discussion... 84 Preliminary Verificaion... 87 AE Verificaion... 87 Mehodology Implemenaion... 88 Parameer Deerminaion... 88 Annual Energy Value... 88 Conclusions and Fuure Work... 89 Appendix C: Naural Gas Price Uncerainy... 90 Inroducion... 90 Mehodology... 90 Scenario Analysis... 90 Risk-Neural Valuaion... 91 Forward Conrac Example... 92 Fuures Marke Characerisics... 92 Applicaion o PV Value... 92 AE Reains NG Price Risk Hedge Benefis... 93 AE Sells NG Price Risk Hedge Benefis... 93 Required Inpus... 94 Naural Gas Price... 94 Risk-Free Discoun Rae... 94 Appendix D: Energy Value... 96 Specificaion... 96 Energy Value Wih Loss Savings... 97 Calculae Value Using 1 MW Marginal Coss... 97 Calculae Value Using 100 MW Marginal Coss... 98 Inerpolae Beween 1 MW and 100 MW Marginal Coss... 99 Adjus Value Based on Naural Gas Prices And Calculae Resul... 100 Energy Value wihou Loss Savings... 101 4
Calculae Value Using 1 MW Marginal Coss... 101 Calculae Value Using 100 MW Marginal Coss... 102 Inerpolae Beween 1 MW and 100 MW Marginal Coss... 103 Adjus Value Based on Naural Gas Prices And Calculae Resul... 104 Appendix E: Effecive Load Carrying Capabiliy... 105 Appendix F: Disaser Recovery... 106 Solar Deploymen Consideraions... 106 Iniial Solar Secure Deploymen... 106 Fuure Solar Secure Deploymen... 107 PV Deploymen Paern... 107 Disaser Recovery Model... 107 5
Lis of Figures Figure 1. Esimaed annual energy producion in Ausin (Clean Power Esimaor)... 17 Figure 2. Comparison of saellie-derived o ground measured global irradiance (A) and relaive PV oupu a wo locaions (B).... 19 Figure 3. Saellie-simulaed compared o measured PV oupu (2002 peak day)... 21 Figure 4. Saellie-derived compared o measured global irradiance on peak days... 22 Figure 5. Average summer (op) and winer (boom) daily PV oupu... 23 Figure 6. Average monhly PV capaciy facors... 24 Figure 7. Comparison of seasonal clearness o long-erm (2002-2004 vs. NSRDB).... 25 Figure 8. Average 2002-2004 ELCC versus load peneraion... 38 Figure 9. Disribuion capaciy expansion plan coss by area and year... 44 Figure 10. U. S. Disaser Declaraions,... 56 Figure 11. Billion Dollar Weaher Disasers 1980-2004... 57 Figure 12. Disaser Risk Hospos... 58 Figure 13. Disaser recovery value flow char... 64 Figure 14. Relaionship beween load me and disaser recovery value... 65 Figure 15. Toal value by PV configuraion ($/kw)... 69 Figure 16. Levelized value by PV configuraion ($/kwh).... 69 Figure 19. Hypoheical resuls for wo mehods o calculae marginal losses... 86 Figure 20. Hisorical and forecas naural gas prices for PG&E and Henry Hub... 91 Figure 21. Treasury yield curve (January 9, 2006)... 95 6
Lis of Tables Table 1. Benefis (+) and coss (-) using Sandard Pracice ess... 9 Table 2. Benefis (+) and coss (-) using comprehensive evaluaion... 11 Table 3. PV Sysem deraing facors according o NREL.... 15 Table 4. PV sysem raing mehods a peak weaher condiions... 16 Table 5. Saellie model validaion resuls (Ausin, TX).... 18 Table 6. Saellie-derived vs. measured PV capaciy facors (Bergsrom Airpor 2002). 20 Table 7. Annual PV sysem oupu (kwh per kw)... 26 Table 8. AE T&D sysem marginal losses and loss savings... 28 Table 9. Loss savings value.... 28 Table 10. Energy value (15 MW plan, wih loss savings and degradaion) - $/kw/yr... 32 Table 11. Adjused energy value (15 MW plan, wih loss savings) - $/kw/yr.... 34 Table 12. Energy value and energy loss savings value ($/kw).... 35 Table 13. Annual and average ELCC versus load peneraion.... 39 Table 14. Generaion capaciy value and loss savings value ($/kw).... 39 Table 15. AE disribuion budge due o new load growh... 42 Table 16. Toal disribuion cos due o new growh (budgeed and equivalen cos)... 43 Table 17. Disribuion capaciy expansion plan coss by area and year ($M).... 45 Table 18. T&D area-specific annual load growh... 46 Table 19. Deferral value for ideal resource.... 46 Table 20. T&D deferral value and loss savings resuls ($/kw)... 47 Table 21. Scenarios for reacive power availabiliy... 49 Table 22. Reacive power benefi.... 50 Table 23. Green-e cerified producs in Texas... 53 Table 24. Environmenal value... 54 Table 25. Disaser recovery value resuls on energy and capaciy basis ($/kw)... 65 Table 26. Breakdown by benefi componen ($/kw and $/kwh)... 70 Table 28. Hypoheical example: 10 MW PV has 24.9 percen loss savings.... 80 Table 29. Hypoheical example: 100 MW PV has 24.2 percen loss savings.... 80 Table 30. 100 MW of new cusomer loads requires 124.2 MW of generaion capaciy. 82 Table 31. On-peak marginal losses... 88 Table 32. Energy value (1 MW marginal coss, wih loss savings) - $/kw/yr.... 97 Table 33. Energy value (100 MW marginal coss, wih loss savings) - $/kw/yr.... 98 Table 34. Energy value (15 MW plan, wih loss savings) - $/kw/yr.... 99 Table 35. Adjused energy value (15 MW plan, wih loss savings) - $/kw/yr.... 100 Table 36. Energy value (1 MW marginal coss, wihou loss savings) - $/kw/yr... 101 Table 37. Energy value (100 MW marginal coss, wihou loss savings) - $/kw/yr... 102 Table 38. Energy value (15 MW plan, wihou loss savings) - $/kw/yr.... 103 Table 39. Adjused energy value (15 MW plan, wihou loss savings) - $/kw/yr.... 104 Table 40. Disaser recovery model inpus... 108 Table 41. Disaser recovery model calculaions.... 109 7
Inroducion Ausin Energy (AE) has a srong commimen o inegraing solar elecric generaion ino is power generaion and disribuion sysem. This is made clear no only by he inroducion of is recen incenive for cusomer-owned phoovolaic (PV) sysems, bu even more so by is goal of insalling 100 MW of solar generaion by 2020. AE wans o ensure ha he cos of solar generaion is commensurae wih is value. As such, AE issued wo reques for proposals (RFPs) o perform value sudies. One RFP was o deermine he value of he economic developmen benefis of solar and he oher was o deermine he value of solar generaion o AE. Clean Power Research (CPR) was seleced for he second RFP. Objecive There are several possible approaches o deermine he comprehensive value of solar generaion. One opion is o perform an in-deph analysis of a single candidae echnology. This opion has he benefi of being able o clearly illusrae evaluaion mehodologies. Anoher opion is o perform a less comprehensive analysis for a wide variey of solar echnologies. This opion has he benefi of providing resuls for muliple solar echnologies, such as disribued PV, cenral saion PV, cenral saion solar hermal roughs, solar dishes, or cusomer solar ho waer heaing o displace elecric waer heaers. AE saed in is RFP ha he work should provide evaluaion mehodologies. Thus, in order o maximize he effeciveness of AE s financial invesmen in his sudy, he firs approach was seleced. I performs an in-deph evaluaion of a single solar echnology ha requires a relaively complicaed analysis effor (disribued PV) and places special emphasis on documening evaluaion mehodologies. Disribued PV is also seleced because i is firmly esablished in he lieraure ha he value of disribued PV is subsanially higher han he value of cenral saion PV. There are wo primary objecives of his sudy: 1. Quanify he comprehensive value of disribued PV o AE in 2006 2. Documen evaluaion mehodologies o assis AE in performing he analysis as condiions change and applying i o oher echnologies Evaluaion Frameworks There is growing ineres in cusomer-owned generaion wih a paricular ineres in PV sysems. This has resuled in a number of analyical sudies aimed a deermining he value of PV. Two difficulies ha hese sudies have encounered are he proper selecion of evaluaion perspecive and he deerminaion of which benefis and coss o include in he analysis. Wih hese difficulies in mind, CPR and he Naional Renewable Energy Laboraory (NREL) have consruced a framework o evaluae disribued PV [34]. While his 8
curren AE sudy is focused only on he issue of PV value and does no specifically address he ownership issue (i.e., wheher he sysems should be uiliy-owned or cusomer-owned), he NREL framework is relevan o he curren sudy. By way of background, hen, he highlighs of he framework are presened here. Energy Conservaion Evaluaion Framework Since he 1970s, conservaion and load managemen programs have been promoed by he California Public Uiliies Commission (CPUC) and he California Energy Commission (CEC) as alernaives o power plan consrucion and gas supply opions. In 1983, he evaluaion mehodology was formalized and documened in he California Sandard Pracice Manual. This manual has been updaed several imes; he mos recen version is available in [17]. The Sandard Pracice Manual idenifies he cos and benefi componens and coseffeciveness calculaion procedures from four major perspecives as summarized in Table 1. Those perspecives include: 1. Paricipan 2. Raepayer Impac Measure (RIM) 3. Toal Resource Cos (TRC) 4. Program Adminisraor Cos (PAC) Table 1. Benefis (+) and coss (-) using Sandard Pracice ess. Toal Resource Cos (TRC) Paricipan (PAC) Rae Impac Measure (RIM) Paricpan All Raepayers Uiliy Invesmen Equipmen - Insallaion - Sales Tax - O&M Cos - Elecric Uiliy Bill + - Incenives Incenive Paymens + - Program Adminisraion - Tax Effecs Tax Credis + Uiliy Cos Savings Energy + Capaciy + T&D Sysem + Losses + 9
Disribued PV Evaluaion Framework The NREL sudy [34] idenified wo limiaions of he framework presened in Table 1. Firs, for he given perspecives ha are defined, here are benefis and coss ha are no included in he analysis. Second, here are addiional perspecives ha are no included in he analysis. Table 2 illusraes how he marix can be expanded for cusomer-owned disribued PV generaion under he curren U.S. incenive srucure The blue secion iles are he various ess, he headers a he op are he perspecives, and he labels o he lef of he marix are he benefi/cos componens. Wihin he body of he marix, he boxes have hree possible colors: whie, yellow, or gray. Whie indicaes ha he benefi is currenly included in exising Sandard Pracice ess, yellow indicaes ha i is no ypically included (alhough some sudies may include hese componens), and gray indicaes ha he componen does no apply from ha perspecive. A + corresponds o a benefi and a corresponds o a cos for ha paricular componen and perspecive. Noe ha while a Socieal Tes is included in he ess described in he Sandard Pracice Manual, i is reaed informally. Several general observaions can be made based on a comparison of Table 1 o Table 2. Firs, he expanded framework conains many more perspecives and benefis/cos componens han Table 2. Second, he indusry and governmen perspecives have many new enries. Third, many of he componens have a yellow background and hus are no included in a ypical financial analysis even for he exising ess (RIM, TRC, PAC, and Paricipan). Finally, here are generally more pluses han minuses wih he new enries. Including hese benefis and coss increase he overall cos-effeciveness of disribued PV sysems. 10
Table 2. Benefis (+) and coss (-) using comprehensive evaluaion. SOCIETAL Toal Resource Cos (TRC) Indusry Governmen Paricipan (PAC) Rae Impac Measure (RIM) Paricpan All Raepayers Uiliy Indusry Sae/Local Gov. Federal Gov. Invesmen Equipmen - + Insallaion - + Sales Tax - + O&M Cos - + Financing - + Elecric Uiliy Bill + - Incenives Incenive Paymens + - Program Adminisraion - + + Tax Effecs Tax Credis + - - Depreciaion + - - Loan Ineres Wrie-Off + - - O&M Coss + - - Uiliy Bill Savings - + + Tax on Tax Credis - + Uiliy Cos Savings Energy + Capaciy + T&D Sysem + Losses + Technology Synergies + + Environmenal Emissions + Waer + Healh + RECs/Green Tags + + Job Creaion - + + + Reliabiliy Blackou Prevenion + + + + Emergency Uiliy Dispach + + Caasrophe Recovery + + + Backup Power + + + Risk Facors Manage Load Uncerainy + Wholesale Price Hedge + + + Reail Price Hedge + + + Reail Price Cap + - + + Naional Energy Securiy + + + Framework for Curren Sudy The expanded benefi-cos framework described above was developed for cusomerowned disribued PV sysems. AE has no ye deermined wheher he solar generaion in his sudy will be cusomer-owned or uiliy-owned. Consequenly, some of he perspecives presened in Table 2 need o be combined. This AE sudy will examine he benefis ha accrue o he perspecives of boh All Raepayers and Uiliy. In paricular, his sudy will examine he following benefis: Energy Producion Generaion Capaciy T&D Sysem (Disribuion Deferrals) Reduced Transformer and Line Losses Environmen 11
NG Price Hedge (Wholesale Price Hedge) Disaser Recovery (Caasrophe Recovery) The Technology Synergies benefi in Table 2 refers o he siuaion when he benefi of a bundle of echnologies exceeds he sum of he benefis of he echnologies evaluaed in isolaion. The Technology Synergies benefi can be incorporaed in he capaciy benefis by combining PV wih a uiliy-adminisered load conrol program. This combinaion of echnologies may provide firm capaciy a a cos ha is lower han using eiher echnology in isolaion (see [22] for more background informaion). Four benefis are lised for he All Raepayers and Uiliy perspecives ha are no included in he analysis are: Blackou Prevenion, Emergency Uiliy Dispach, Manage Load Uncerainy, and Reail Price Cap. The Blackou Prevenion and Emergency Uiliy Dispach benefis ([18], [26], [27], and [30]) are he benefis o he uiliy of having a small amoun of baery sorage available as par of a PV sysem o respond o brief emergency siuaions encounered by he uiliy. AE decided ha baery sorage was a separae echnology from solar and ha hese benefis should be excluded from he analysis. Therefore, hese benefis are no considered furher. The Managing Load Uncerainy benefi [12] follows from he modulariy and shor-lead imes of disribued generaion in he deferral of T&D sysem invesmens. This benefi, while real, is excluded from he sudy because he T&D deferral value was very small for AE and did no jusify he effor required o collec he daa necessary o implemen he mehodology. The Reail Price Cap [23] is he benefi of having PV become a backsop echnology, i.e., one ha allows large or unlimied quaniies of a perfec or near perfec subsiue o be produced a a given price. I ensures he exisence of a choke price, he price above which he produc which i is replacing will no go. In he case of PV, i can provide long-erm elecric rae proecion o all cusomers wheher or no hey purchase he PV. The Reail Price Cap benefi is excluded because i requires he assumpion of cusomer ownership, an assumpion which his sudy does no make. One benefi no lised above bu included is Reacive Power Conrol. The sudy evaluaes such a benefi. The value is calculaed, bu i is no included in he final resul because inverer conrol modificaions required o capure his benefi would no likely be implemened. Scenario Specificaion There are muliple evaluaion mehods and inpu assumpions ha could be used for almos all of he benefis lised above. This sudy would resul in an unmanageable number of scenarios if an aemp was made o presen all possible combinaions of he various evaluaion mehodologies and inpu daa ses. 12
A variey of scenarios were developed during preliminary phases of he sudy. Afer furher consideraion, however, i was decided ha a single scenario reflecing he join opinions of AE and Clean Power Research would bes serve he purposes of his sudy. The resuls are neiher he highes hey could be nor he lowes hey could be bu raher represen a middle ground. A number of he benefis are a funcion of he size of he PV sysem. The analysis is performed for 15 MW of PV unless oherwise specified. 13
PV Performance Esimaes Inroducion Accuraely deermining he value of disribued PV requires he availabiliy of PV power oupu daa in inervals of one hour or less. Ideally, measured performance daa from exising, represenaive PV insallaions would be available for his purpose. In pracice, however, his ype of measured daa is exremely difficul o obain. Ofen, measured daa is no available for he ime period of he sudy, i does no include he range of he configuraions of ineres, or i does no cover a sufficienly broad geographical area. Such difficulies were encounered in he presen sudy for AE. The nex bes alernaive o measured PV oupu daa is o simulae PV oupu using measured global horizonal and direc irradiance daa from a geographically diverse se of well-mainained ground saions. Unforunaely, here was no even a single locaion (much less muliple locaions) for which ground-based measuremens were available ha included boh global horizonal and direc irradiance for he ime period of his sudy. Wihou measured PV sysem daa or suiable ground based irradiance daa, he nex bes alernaive is o use saellie-based measured weaher daa. This daa is available corresponding o he ime period of he sudy, i can be used o simulae a wide variey of PV configuraions, and i encompasses a broad geographic area. This is he alernaive ha was seleced o perform he sudy for AE. This secion describes he PV sysem convenions used hroughou he repor, selecs he sample PV configuraions used in he deailed analysis, describes he saellie-based weaher daa, and presens he resuling PV oupu simulaions. As he number of insalled PV sysems grows in AE s erriory, AE should consider collecing 5 or 15-minue daa on he elecric producion from a large number of wellperforming PV sysems a a variey of locaions and orienaions in order o be able o repea he analysis presened in his sudy using measured PV oupu daa, hereby improving he resuls of he analysis. Convenions Raing Convenion I is imporan o begin wih a review of common PV sysem raing mehods. As shown in Table 3, according o he Naional Renewable Energy Laboraory (NREL), here are eleven facors ha influence he amoun of energy produced by a PV sysem. 3 These various losses occur a differen pars of he sysem. Some of he losses occur wihin he PV module, some in he sysem inerconnecion, some in he power conversion, and some in he oal sysem. 3 NREL liss a variey of deraing facors a hp://rredc.nrel.gov/solar/calculaors/pvwatts/derae.cgi. 14
Table 3. PV Sysem deraing facors according o NREL. Componen Derae Facors Range of Accepable Values PV module nameplae DC raing 0.80-1.05 Inverer and Transformer 0.88-0.95 Mismach 0.97-0.995 Diodes and connecions 0.99-0.997 DC wiring 0.97-0.99 AC wiring 0.98-0.993 Soiling 0.30-0.995 Sysem availabiliy 0.00-0.995 Shading 0.00-1.00 Sun-racking 0.95-1.00 Age 0.70-1.00 As a resul, a variey of raing mehods have emerged wihin he PV indusry. There are a leas five indusry raing convenions ha are in use: Nameplae (DC) Nameplae (DC) x Inverer Efficiency PTC Module, no Inverer AC-PTC (Defined by CEC) Delivered AC A simple example is useful o illusrae he difference beween hese raing mehods. Suppose ha a PV sysem consiss of BP Solar 4175 modules and a Xanrex GT 3.0 inverer. According o he California Energy Commission, a BP 4175 module has a nameplae raing of 175 Was and a PTC module raing (he raing defined a less opimal ambien condiions) of 155.2 Was. This means ha he PTC module raing is 89 percen of he nameplae raing. A Xanrex GT 3.0 inverer has 94 percen efficiency. 4 Assume ha here are an addiional 5 percen oher losses in obaining he acual AC oupu. The various raing mehods range from 79 o 100 percen of nameplae (DC) for his paricular sysem configuraion (see Table 4). 4 hp://www.consumerenergycener.org/cgi-bin/eligible_pvmodules.cgi and hp://www.consumerenergycener.org/cgi-bin/eligible_inverers.cgi. 15
Table 4. PV sysem raing mehods a peak weaher condiions. Raing Mehod Losses Raing Module Inverer Oher 5 (% of DC raing) Nameplae (or DC) 100% Nameplae x Inverer 94% 94% PTC Module 89% 89% AC-PTC 89% 94% 83% Delivered AC 89% 94% 95% 79% Throughou his repor, PV sysems are specified in erms of he Delivered AC raing. This refers o he sysem s oupu a 25ºC (77ºF) ambien emperaure and 1,000 Was/square meer plane of array irradiance. Tha is, he PV raing in his sudy is he delivered AC raing and is approximaely equal o 79 percen of he DC or nameplae raing. Naming Convenion The PV sysem naming convenion used in his repor is ha he azimuh orienaion is lised firs followed by il. Thus, a Souh-30º sysem refers o a souh-facing fixed PV sysem wih a 30º il. 6 Orienaion Selecion Two criical facors ha affec PV oupu are sunligh availabiliy (irradiance) and PV sysem configuraion (i.e., he direcion and il of he sysem). 7 While i is possible o simulae hourly PV sysem oupu daa for many conceivable orienaions, his sudy concenraes on a limied number of represenaive PV configuraions. The selecion was made by pre-screening he orienaions using a simulaion model, daa for a ypical meeorological year (as described by he 30-year Naional Solar Resource Daa Base), and applying some knowledge abou he value of PV. In Ausin, TX, a Souh-30º PV sysem has he highes annual energy producion for a fixed PV sysem. 8 The configuraion wih he highes annual energy producion, however, does no necessarily have he highes oal value. This is because wes-facing sysems bias PV oupu oward peak periods and provide corresponding enhanced capaciy o suppor peak uiliy loads. The addiional capaciy-relaed benefis may offse he reduced energy-relaed benefis. Figure 1 presens he annual energy producion for sysems ranging in direcion from eas o wes wih a il from horizonal (0º) o 60º relaive o he annual energy producion for a Souh-30º PV sysem. In order o provide a represenaive sample of PV sysem 5 Oher losses include inverer and ransformer, mismach, diodes and connecions, DC wiring, AC wiring, and soiling losses. 6 PV sysems are ofen insalled on sloped roof-ops or angled mouning brackes o poin in he direcion of he sun s pah and capure more energy. 7 A hird facor is shading. I is assumed for his analysis ha he PV sysems are no shaded. 8 The 30º opimum il angle corresponds o Ausin s 30º norh laiude. 16
orienaions for he sudy, based on his pre-screening analysis, he following fixed sysems are seleced for deailed analysis: Horizonal (fixed PV wih no il) Souh-30º (souh-facing fixed PV iled a 30º) SW-30º (souhwes-facing fixed PV iled a 30º) Wes-30º (wes-facing fixed PV iled a 30º) Wes-45º (wes-facing fixed PV iled a 45º) In addiion, single-axis norh-souh racking sysems for boh a horizonal and a 30º iled sysem will be included. 1-Axis (norh-souh 1-axis racking PV wih no il) 1-Axis 30º (norh-souh 1-axis racking PV wih 30º il) These racking sysems, while more cosly and mechanically complex, deliver greaer energy o he grid han comparably-sized fixed sysems. Noe: while he pre-screening analysis uses he 30-year Naional Solar Resource Daa Base values, he sudy is based on acual weaher daa colleced for he Ciy of Ausin. Relaive Annual Energy Producion 100% 80% 60% 40% 20% Simulaions o perform 0º Til 15º Til 30º Til 45º Til 60º Til Source: Simulaion performed using he Clean Power Esimaor 0% Eas SE S SW Wes Figure 1. Esimaed annual energy producion in Ausin (Clean Power Esimaor). 17
Oupu Esimaion Hourly PV oupu daa were simulaed for he seleced PV configuraions based upon solar irradiance daa exraced from hourly geosaionary saellies ([8], [16], [21], [25], and [28]), and ambien emperaures and wind-speeds exraced from he Naional Weaher Service measuremens. Simulaions were performed using PVFORM 4.0, he program ha powers NREL s PV-Was PV simulaion program. Precision of Saellie-Derived Esimaes The accuracy of irradiance daa from saellie images has been exensively validaed using ground daa from several climaically disinc locaions including Ausin, TX [28]. Saellie daa are aken as an insananeous snapsho of he cloud cover once an hour a 27 minues afer he hour. Table 5 summarizes model validaions compared o five years of measured daa from he Universiy of Texas a Ausin. Table 5. Saellie model validaion resuls (Ausin, TX). GLOBAL IRRADIANCE DIRECT IRRADIANCE Observed Average 494 W/sq.m 405 W/sq.m Mean Bias Error 13 W/sq.m -8 W/sq.m Roo Mean Square Error 89 W/sq.m 149 W/sq.m The Mean Bias Error is a measure of he overall endency of he saellie-derived irradiances o overesimae or underesimae acual measuremens. For Ausin, his endency is +2.5 percen for global irradiance and -1.8 percen for direc irradiance. This suggess ha he modeled irradiances used in his sudy should predic he acual annual value wih a bias error no exceeding ± 3 percen. Global irradiance is he oal amoun of solar energy on a horizonal surface while direc irradiance is he only he direc normal componen. The Roo Mean Square Error is a measure of he exen o which any given hourly esimaion differs from he ground measuremens. I is calculaed based on he square roo of he sum of he square of he hourly error. Reference [15] presens deailed resuls abou he effec of measuring daa a a single locaion versus averaging i over an exended area. As discussed in ha sudy [15], he shor-erm difference as refleced in he Roo Mean Square Error is largely he consequence of he fac ha he saellie and he ground saions measure differen hings: he saellie daa inegraes irradiance over an exended area (e.g., 1 pixel = 10 X 10 km) while he measuremen reflecs condiions a a pinpoin locaion. As a resul, he shorerm scaer is o be expeced when comparing saellie and ground measuremens as seen in par A of Figure 2. Noe, however, ha a similar degree of scaer occurs when comparing wo nearby pinpoin locaions such as he Ausin Bergsrom Airpor and he Howson Branch library as shown in par B of Figure 2. 9 In boh cases, he oulying poins represen parly cloudy condiions when one of he sies may be shaded while he 9 The source for he boom par of he figure is AE s Inernal Draf on PV capaciy. 18
oher is no and when he inegraed saellie pixel averages he local variaions. Therefore, i is arguable ha, when invesigaing he impac of dispersed PV spanning several 100 sq. km on he AE grid, i is preferable o selec a spaially inegraed signal (9 pixels in he presen sudy) ending o smooh shor-erm, bu highly local, variaions, raher han he noisier pinpoin signal. I may be more appropriae o use daa from acual PV sysems once AE has insalled a large number of well-mainained PV sysems (wih well mainained daa collecion sysems) a a variey of locaions and orienaions hroughou he ciy of Ausin. Figure 2. Comparison of saellie-derived o ground measured global irradiance (A) and relaive PV oupu a wo locaions (B). 19
Oupu Simulaion Hourly ime sep is he highes resoluion readily available from saellie sources for arbirary locaions. I may be argued ha hourly daa do no capure he shor-erm variabiliy ha characerizes he oupu of a PV sysem. However, if his is rue for any one sysem a one paricular locaion, i is no criical for he ype of PV oupu of concern o his analysis: he oupu of PV generaion dispersed over several hundred square miles (i.e., hroughou AE s service erriory). The saellie-derived PV oupu used here represens he average of 9 saellie pixels, each pixel covering an area of 10X10 km. PV simulaions were performed for each load daa year analyzed: 2002, 2003 and 2004. For he firs half of 2002, measured PV oupu daa a he Ausin Bergsrom Airpor were available, coinciden o he saellie-based simulaions. Table 6 compares he PV capaciy facors recorded a he airpor insallaion wih he same simulaed from saellie daa. Resuls indicae ha he sysem-wide geomery-specific numbers generaed for his analysis are comparable and are slighly on he conservaive side. Table 6. Saellie-derived vs. measured PV capaciy facors (Bergsrom Airpor 2002). Monh Measured Capaciy Facor Saellie-Derived Capaciy Facor January 15.0% 14.4% February 19.2% 18.5% March 18.9% 17.8% April 21.0% 21.3% May 24.2% 23.3% June 24.3% 23.3% July 24.1% 22.3% Augus 24.8% 24.5% Sepember missing 19.9% Ocober missing 13.2% November missing 16.0% December missing 13.3% As an addiional check of he validiy of saellie-derived PV oupus, Figure 3 compares he peak day oupu of saellie-derived irradiances and PV oupus wih he corresponding recorded daa from he Bergsrom Airpor and he Howson Branch library in 2002. The year 2002 is seleced because i is he only year wihin he sudy s 2002-2004 ime-frame when daa were available from boh he saellie-derived PV oupu and he daa measured from he wo PV power plans. The figure indicaes ha irradiance-o- PV simulaion accuraely capures conversion losses from irradiance o AC oupu. 20
1000 AUGUST 26, 2002 Saellie-Derived Horizonal PV Oupu Library Normalized PV Oupu normalized PV oupu (W/kW) 800 600 400 200 Airpor Normalized PV Oupu 0 0 6 12 18 24 Time of Day (CST) Figure 3. Saellie-simulaed compared o measured PV oupu (2002 peak day). Figure 4 provides a check on saellie-derived versus ground-based global irradiance for uiliy peak days in 1999, 2000 and 2001. The figure indicaes ha saellie-o-irradiance simulaion adequaely capures peak ime condiions (cloudless bu hazy in 1999 and wih some cloudiness impac in boh 2000 and 2001 noe ha for he reasons discussed above in Figure 2, he impacs of ligh clouds are expeced no o be perfecly in phase beween he pinpoin saion and he exended pixel). 21
Figure 4. Saellie-derived compared o measured global irradiance on peak days. Resuls Daily PV oupu profiles for summer and winer averaged over he hree-year period are presened in Figure 5. The monhly capaciy facors are presened in Figure 6. In order o gauge he consisency of he seleced period agains long-erm sandards, Figure 7 compares he 2002-2004 saellie-derived clearness indexes 10 agains he 30-year Naional Solar Resource Daa Base values. The comparison shows ha he 2002-2004 values are on he conservaive side and ha July and Sepember are below long-erm average. 10 Defined as he raio of global irradiance and clear-sky global irradiance 22