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P HOT OVOL T AIC C ONC E NT R AT OR T E C HNOL OGY DE VE L OP ME NT *E. C. Boes

Sandia National LaboratoriesAlbuquerque, New Mexico 87185

ABSTRACTThis paper summarizes the progress that has beenmade in the past 18 months in the are a of photovoltaicconcentrator technology developmen t. A brief descriptionof the status of two new photovoltaic concentrator powersystems, the 300 kW ENT ECH-3M -Austin system, and thesingle-pedestal Alpha S olarco system is given. Consider ableprogress in the area of photovoltaic concentrator moduledevelopment is reported. Th e paper emphasizes thosemodule development activities that have resulted insignificantly higher conv ersion efficiencies or new mo dule

design concepts. Th e paper includes a brief discussion ofthe major gains in concentrator cell efficiencies, especiallythe increased efficiencies for multijunction cells, and theirimplications for the technology. Also, discussed are themost significant recent developments for PV concentratorcomponents, including refractive secondary optical elementsand a self-contained, electronic, array-tracking controller.Finally, the paper includes a discussion of the outlo ok formarkets and sales for photovoltaic concentrators and a briefdescription of a new program in the United States, thePhotovoltaic Conc entrator Initiative.

INTRODUCTIONThe field of photovoltaic power technology iscontinuing to develop rapidly, with the most important

changes in the last 18 months being th e very rapid grow th ofphotovoltaic power system marke ts. Because the marketsthat photovoltaics technology is now beginning to serve areextremely varied and a re extremely large , it is likely thatthey will grow very steadily and rapidly for at least theremainder of this decade. The se markets have certainlyrenewed interest on the part of the industrial community inphotovoltaics technology, including a greatly ex andedinterest and commitment to the development OFphotovoltaic concentrator technology.This paper summarizes the major advances inphotovoltaic concentrator technology since the 20th IE EEPhotovoltaic Specialists Conference across all fronts rangingfrom component development to the status of fielded arraysystems. Th e paper begins with a report on the status and

*Prepared by Sandia National Laboratories, Albuquerque,NM 87185 and Livermore, CA 94550; operated for th e U.S.Department of Energy under Contract DE-AC0676DP00789.

performance of the 300-kW linear Fresnel system recentlyconstructed by EN TEC H a t the 3M Center in Austin, Texas.It also includes a repo rt on Alpha S olarcos project toinstall a PV concen trator system in Pahrump , Nevada, and areport on the high-concentration photovoltaic (HCPV)array pro ram sponsored by the Electric Power ResearchInstitute tEP RI). The paper also includes a very briefdescription of two photovoltaic concentrator array systemswhich have been o perated in Japan fo r several years.The past 18 months have seen continued expansion inthe area of photovoltaic concentrator module development.At least a dozen differ ent groups in the United States arecurrently actively working on photovoltaic concentratormodules. This paper includes a listing and a briefdescription of these activities. Som e of these moduledevelopment activities are singled out for more detaileddescriptions because they eith er have achieved interestingperformance gains or because they represent promising newdesign ideas.The key to continuing to advance the conversionefficiency for photovolta ic conc entrator modules lies incontinued progress on the efficiencies of concentrator cells.Once again, significant gains have been made in the past 18months and are briefly reported.The resurgent growth in industrial interest inphotovoltaic concen trator technology is a consequence of

the commercial opportunities that are beginning to beapparent for this technology. The different industrialparticipants in this technology all have their own viewsregarding the most promising of these photovoltaicconcentrato r markets. This paper includes a discussion ofthe authors views on these markets and the technologydevelopment needs th at must be addressed in order tosuccessfully meet these m ark et demands.Finally, the photovoltaics program of the USDepartment of Energy (DOE) is beginning an initiative tosupport the technology development efforts of thephotovoltaic concentrator industry. This PhotovoltaicConcentrator Initiative is described.

PHOTOVOLTA IC CONCENTRATORPOW ER SYSTEMSA major photovoltaic concentrating power system wasinstalled and put into o pera tion in the past year. This is a300-kW linear Fresn el co ncentrator system installed byENTEC H at the 3M Company Center near Austin, Texas.

9440160-8371/90/0000-0944 0 $1.00 1990 IEEE

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TABLE 1. SUMMARY O F PV CONCEN TRATOR MODU LE DEVELOPM ENT ACTIVITIES IN TH E U.S.Dcvcloper

AESI

Alpha Solarco

ENTECH

EPRI

Midway Labs

SAIC

Sandia Base-line, SB M 3SandiaConcept 90

Sandia 1OOXExperimental

SEA Corp.

Sci Tcch

SKIVarian

Wattsun

Module D esign Description2 x 5 point-focus Fresnel,0.25 m2 aper ture , plastichousing, TI R secondary, 350X2 x 12 oint-focus Fresnel,glass silo secondary, 492 X1.25 m? aperture, Al housing,Several linear Fresnel designs,arched lenses, 10 to 40X, 1.5to 3 m2 module apertu re4 x 12 point-focus Fresnelmodule, SOOX, point-contactSi cellsPoint-focus Fresnel, Al housing,TIR secondary, f 4" trackingtolerance allows passivetracking, 91XReflective dish concentrator,PV receiver a dense array ofcells, actively cooled2 x 12 Doint-focus Fresnel.0.70 m2 apertur e, AI housing,185XPoint-focus Fresnel lenses,10-cm square, com pletely encap-sulated receiver s heet, plastichousing2 x 5 point-focus F resnelmodule, lOOX, designed forexperimentationLinear Fresnel design usingextruded acrylic lens-and-sidewall, 1OXT_ow concentra tion designs usingbifacial cclls, tracking notrequired.This design is very similar tothe SBM 3, abovePoint-focus Fresnel, lOOOX,AI housing, glass secondary

A thin-profile m odule w ith11-mm square lenses and 2.5-mmdiameter Si cells

StatusSeveral prototypes have beenfabricated.100 first generation modulesinstalled. Several secondgeneration m odules currentlyunder test.Several hundred modules in-stalled in several systems,peak dc efficiencies above15%.Several prototypes tested.

Several modules built andcurrently under test.

Dish and first experimentalreceiver currently undertest.Components and two prototypemodules under test.A sm all prototype is undertest.

Used to establish conversionefficiency marks of 17% in1984 and 20% in 1989 usingSi cells.Initial lenses and a mini-module have been made andtested.First prototypes are beingfabricated.A prototype is currentlybeing fabricated.A mini-module tested with apeak efficiency above 22%.A prototype being fabricatednow.Several prototypes have beentested at -11% efficiency.

ContactsW. Todoroff(714) 646-8900E. Schmidt(513) 771-1690

R. Walters(214) 456-0900

E. DeMeo(415) 855-2159P. Collard(312) 933-2027

K. Beninga(619) 546-5787M. Whipple(505) 844-2482T. Hund(505) 844-8236

A. Maish(505) 844-8771

D. Curchod(408) 954-1250U. Ortabasi(619) 455-1650G. Hutchinson(214) 556-2376L. Partain(415) 424-6386

J. Doherty(505) 255-8909

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bccause they have resulted in higher conversion efficiencies,or bccause they incorporate interesting new designconcepts.Sandias 20% ExDerimental Module

In 1989 Sandia measured a record 20% conversioncfficiency for an experimental pho tovoltaic concentratormodulc using silicon solar cells. This modu le uses a 2 X 6matrix of point-focus Fresnel lenses and silicon solar cellsand has a geometric concentration ratio of 1OOX. The mostcritical design feature s for a chieving this efficiency markwcrc the high-efficiency silicon solar cells fabricated by theUnivcrsity of New South Wales (UNSW),which weredcsigncd to be used with prismatic cell covers designed anddcvcloped with EN TE CH Corpo ration. The covered cellcfficicncics were approximately 24%.Figure 3displays the actual performancenicasurcmcnts taken on this module over a pe riod of twodays. During thcsc measureme nts, the cell temp eratur e washcld at 25 C per the usual specification for a peakcfficicncy. Note th at the efficiency is approxim ately 20.0%a t 1,000 watts per squa re me ter DNC nd increases to 20.3%as thc dircct normal insolation drops to 800watts persquarc m eter. (This fall-off in efficiency with increasinginsolation is consistent with the common design philosophyfor photovoltaic concentrators that attem ts to maximize

convcrsion efficiency at approximately 80 1watts per squaremctcr. Most designers pick a concentration ratio th at isslightly above the con centration ratio at which a cellspcrformance peaks in order to minimize cell area and,thcrcforc, cell cost.)i t dcmonstrates that 20% conversion efficiencies are likelyto bc a commercial reality in the very near future. Th edcsign concepts in this module a re not particularly exotic.As indicatcd above, the m ost critical achievement was tha tof ccll convcrsion efficiencies of 24% . W e fully expect thatsuch conversion cfficiencies using silicon or G aAs cells willbc routincly achieved in the future, an d they will notncccssarily depcnd on the application of the prism covertcchnology. Fo r morc information see (4).

The m ost important aspect of this achievement is that

Varians l000X Mini-ModuleVarian has also reported an exciting conversioncfficicncy achievement in the past year. Th e companynicasurcd a conversion e fficiency of 22% with a tw o-lens,two-cell m ini-module using GaAs cells. This mini-modulercprcsents a scction of a prototy pe lOOOX mod ule th at isundcr dcvclopmcnt at Varian. Because this measurementwas taken at an ambient temperature of 27 C, the peakconvcrsion efficiency, corresponding to a cell tem peratu reof 25OC, would have bc en significantly higher. Th e DNIwas recordcd at approximately 800 watts per square meter,and thc measurcd heat sink temp erature was 54C for thistcst. This achievement also indicates that commercialmodulc cfficiencies above 20% are possible today.

SEAS 10X Linear Fresnel M oduleSE A Corp. is pursuing the development of a linearFrcsncl module that is approximately 25 cm wide and isdcsigncd to oper ate at a geometric concentration ratio ofapproxim ately 1OX. T he most interesting aspect of SE ACorp.s dcsign is the p roposed manufacturing method forthc Icns. Th e company is planning to have the linear

S a n d l a E x p e r i m e n t a l M o d u l e4117 ,89, 220 I

21 5 12 1 0

20 5 -

,* 200

. 0 000 0 04 0 a0 0 92 0 960 I000

Direct Normal 1rradt.n~.m a t t r I 1-2Figure 3. E fficiency Measurem ents to r tne 2U% Sandia1OOX Experim ental Mo dule.

Fresnel lens and the modu le sidewalls manufactured in asingle acrylic extrusion process. This has th e advantage ofproviding a lens via a w ell-known an d low-costmanufacturing process. It also has the advantage ofproviding a lens and m odule sidewall design that is veryeffectively sealed against rain penetration and doesntrequire a separ ate joining step during module fabrication.SE A Corp. is currently placing heavy emphasis ondemonstrating that th e plastic extrusion process can provide

linear lenses of adequ ate quality for its design. Because thecompany expects its module design to be particularly lowcost, it believes that an optical efficiency of 70% is adequa teto m eet its design goals. SE A has already achievedapproximately 70 % on som e of the trial runs in its lensdevelopment program. Note tha t while the nominal modulewidth is currently 25 cm and the nominal concentrationratio is currently 10X, neither of these parameters has yetbeen optimized. SE A Corp. does plan to stay at a relativelylow concentration ratio, in the neighborhood of lo x , and touse concentrator cells that are fabricated largely using 1-suncell processing. For m ore details see (5 ) .Sandias ConceDt 90 Mo dule

Long-term durability and reliability is a major issuefacing photovoltaic concentrator m odule developm ent. Akey part of this issue is how the electrical circuit should beprotected, both from oth er system compone nts and from thepossibility of electrical current leakage during wetconditions. Sandia recently embarked on a concentratormodule design project specifically aimed a t resolving theseissues while maintaining the low-cost and high-efficiencypotential that is recognizcd for photovoltaic concentrators.The approach is to try to incorporate som e of the samedurability and reliability into a concentrator module designthat has been proven in the field for flat-plate, crystalline-silicon modules.The result of this design effort is the Sand ia Concept90 Modu le. This is a point-focus Fresnel module using amatrix of lenses and silicon concentrator cells. Th emodules most important design feature is that its electriccircuit is completely encapsulated in a flat, sealed packagethat is very similar to a convention al, crystalline-silicon, flat-plate module. Th e solar cells are mounted on thin copperheat spreaders, which serve as the rear con tact to the cell

and which are nearly as large as the Fresnel lenses. Thecells top interconnect is isolated from its bottominterconnect with a thin insulating material, such as kapton.

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The en tire solar cell and interconnect package is sealedfrom the elements on th e back in essentially the sam e wayas flat-plate crystalline silicon modules, and the front side ofthis circuit/receiver sheet is potted in another plasticcompound. This completely encapsulated receiver sheetserves as the bottom of an acrylic module housing. Th e firstprototype which we have built also incorporates glass silosecondary elements which ar e partially submerged in thesame pottant material.The status of this development program is that a small2 X 3 lens and cell mini-module has been fabricated, andtesting is underway. Initial performance tests indicate th atthe module's peak efficiency is above 19%. On e interestingissue with this design is that of operatin g cell temp eratures .Our analyses suggest that operating cell temperaturesshould be about the same as for other concen tratormodules. Our first day's testing confirms that the op eratingcell temperatures w ere between 25 to 30 C above ambient,under test conditions of abo ut 950 W/m3 DNI and a windspeed of about 5 m/sec.The result of this development effort should be aconcentrator mod ule that still has excellent conversionefficiency and als o has electrical du rability and reliabilitycharacteristics similar to those of flat-plate crystallinesilicon modules. For mo re details see (6).CONCENTRATOR CELL DEVELOPM ENTTh e key componen t for photovoltaic concentratormodules is the concentrator solar cell, and it has been theexciting progress in concentrator cell research that has beenlargely responsible for the rapid resurgence in interest inthis technology. These exciting research results havecontinued over the past 18 months. Indeed, the 20th IE E EPhotovoltaic Specialists Conference included a repo rt onthe world's first concentrator device with an efficiencyabove 30%. This was a mechanically stacked galliumarsenide-on-silicon cell rep orted by J. G ee (7). Thatimportant milestone has already been topped with theachievement of an efficiency of 34% for a mechanically-stacked GaAs-on-GaSb cell operated a t lOOX by the BoeingCorporation in the summ er of 1989. Moreover, Boeing isexpected to announce an efficiency above 35% at thisconference (8).In addition t o these very substantial laboratoryresearch results, there have also been importantachievements in silicon concentrator cell development overthe past two years. For example, UNSW has providedsilicon cells to Sandia that have efficiencies above 26% withprism covers attached. UNSW has also provided low-cost,laser-grooved Si cells suitable for linear conce ntration th athave efficiencies close to 23% with prism covers. Similarly,Stanford University has provided back-contact silicon cellsto Sandia that have had stable efficiencies up to 26% aftermounting to copper substrates. Finally, Stanford hasidentified several solutions to th e UV degradation problemthat the University was experiencing on its point-contactsilicon cells at the time of th e 20th IEEE PhotovoltaicSpecialists C onference. For details on these developm entssee (10, 11).With the adven t of concentrator cell efficiencies above30%, it is appropriate to look again at the trade-off betweenconcentrator cell efficiency, concentration ratio, andconcentrator cell cost. Figure 4 ffers a vehicle for

30

25

20

15

j0

5

0 30 35 4025Cell E f f i c i e n c y . %4llowable Cell Cost Versus Efficiency for aigure 4.$2/Wp Installed System.

examining these tradeoffs. This figure displays theconce ntrator ce ll cost required at various efficiency levelsand concentration ratios in o rder to achieve a photovoltaicpower system installed co st of $2 per peak watt. Only directcosts are included in this analysis. Cost assumptions includebalance of m odule costs of $150/m2, balance-of-array-system costs of $125/m2, and p ower-con ditioner costs of$O.lS/W. Th e conversion from peak cell efficiency to peaksystem efficiency was m ade by multiplying by 0.70 toaccount for optical losses and other system losses.As expected, the curves in Figure 4indicate that quitehigh con centrato r ce ll costs are possible with relatively highconcentration ratios and efficiencies. For example, with anefficiency of 35% for the concentrator cell and a geometricconcentration ra tio of 500X, a cell cost as high as $9/cm2will permit an installed field cost of $2 per watt peak. Evena concentrator cell efficiency of 25% permits a cell cost over$2 per square cen timeter fo r a technology operating at500X. Thus, Figure 4 very graphically displays the strongmotivation for continuing to conduct concentrator cellresearch in order to achieve ever more competitivephotovoltaic concen trator power system prices.

PV CONCENTRATOR COM PONENT DEVELOPM ENTThe past 18 months have seen important progress in

the development of concentrator components in severalareas. Th e first of these is the con tinued developm ent ofthe microprocessor-based array tracking controller by A.Maish of Sandia Laboratories. Recent developmentsinclude the expansion of the controller capabilities, such asthe developmen t of a simple mechanism for fieldadjustment or checkout of controllers, and the developmentof a simpler and cheaper pow er supply for the controller.Just as significant is the fact tha t several prototype s of th econtroller have been installed on arrays and have operatedsuccessfully for several months. Thu s far, prototypes havebeen opera ted by Alpha Solarco on its array at Pahrump,Nevada, by EN TE CH on its array at its headquarters at theDFW A irport. In all cases, the controllers have operatedvery successfully. For example, the controller installed onthe Alpha Solarco array has maintained a tracking accuracyof less than 0.1" in both azimuth and elevation over aseveral month period.the licensing of this array con troller d esign on aSandia is currently developing procedures to permit

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sure that the products tha t it develops over the next severalyears meet the performance and economic requirements ofthese customers.THE CONCENTRATOR I NI TIATI VE

Th e photovoltaics program of th e US Departme nt ofEnergy recognizes that photovoltaic concentrators d oindeed have the potential for achieving the performanceand costs required f or US utility pow er in the m id-1990's.As a means of providing direct support to the P Vconcentrator industry to bring this potential to a reality,DO E and Sandia National Laboratories have begun aPhotovoltaic Conc entrato r Initiative. The purpose of thisInitiative is to support industry in its development ofphotovoltaic conc entrato r technology to the point that itcan provide cost-effective power in the US utility market.The Initiative will provide the support via multiyear, cost-shared contracts t o individual firms selected on acompetitive basis. Both developm ent of concentrator celltechnologies and concentrator collector technologies will besupported. At the end of these multiyear, cost-sharedcontracts, the participating firms will have developed andproven concentrator modules, and they will have installedand demonstrated the performance of photovoltaicconcentrator arrays in the app roximate size of 5 to 10 kW.The participating firms will also have developedmanufacturing facility and business plans for putting theirtechnolo into production, and it is anticipated that someof these E m s will have actually begun the construction ofthese production facilities.

support on a cost-shared basis for these technology deve%pment efforts,Sandia Laboratories will provide significant technologicalassistance to the firms in their efforts. This assistance willinclude parallel design analysis, experimentation, andevaluation activities, as well as design consultation, to th econcentrator cell manufacturers in the program. Similarly,the firms undertaking the collector development activitieswill receive design analysis, component evaluation, andmodule and array evaluation support from Sandia. Sandiawill also work with the participating companies t o ensurethat these activities are carried out with an emphasis ondesign for manufacturability, an emphasis on productquality, and w ith emphasis on meeting customerrequirements.Th e status of this Initiative is that re uest forproposals were issued in December of 1988, and proposalswere received in February of 1990. These proposals havebeen evaluated, and the top proposals have been identifiedfor program participation. Negotiations for placement ofthe multiyear, cost-shared co ntracts are now underway.More details on this program a re given in (13).

In addition to providing moneta

SUM M ARYProgress in all area s of photovoltaics technologycontinues to proceed forward at a steady and mostencouraging pace. Th e concept of lowering the cost ofphotovoltaic power by the use of concentrators with high-efficiency cells has gained considerable mom entum in thepast 18 months. We ar e likely to see commercialphotovoltaic concentrator modules with 20% peakconversion efficiencies within the next two years. Finally,we are also likely to see several additional photovoltaicconcentrator array an d system installations within the nexttwo years. These products are likely to use modules with

excellent durability and reliability. We are also likely to seeat least the start of th e construction of significantmanufacturing facilities for p hotovoltaic concen trators,facilities capable of production levels of approximately 10MW per year. Inde ed, the most interesting question forphotovoltaic concentrator technology is whether itrepresents th e technology that provides the g reatestresponse in terms of manufacturing capacity to the rapidlyexpanding markets for photovoltaics pow er throughout theworld.REFERENCES

1. O'Neill, M. J.,&& "Fabrication, Installation, andInitial Op eration of the 2,000 SQ.M. Linear F resnel Lens,,Photovoltaic Concentrator System at 3WA ustin (Texas),21st IE EE P hotovoltaic SDehalists ConferenceProceedings. (Piscatawa? IEE E, 1990).2. C arroll, D., E. Schmidt, and B. Bailor. "Production ofthe AlDha Solarco Proof-of-Concept Array," 21st IEE EPhotoGoltaic SDecialists Confe renie Proceedings.(Piscataway: IEEE, 1990).3. Dostalek, F. "EPRI High-Concentration PhotovoltaicProgram," 1989 DOE/Sandia Crvstalline PhotovoltaicTechnolow Proiect Review M eetin . (Albuquerque:Sandia Ngiional"Laboratories, 1989 ,SAND 89-1543, p.114.4. C hiang , C., and E. Richards. "A 20% EfficientPhotovoltaic Concentrator Module," 21st IEEEPhotovoltaic SDecialists Conference Proceedings.(Piscataway: IEEE, 1990).5. Kaminar N. R., and D. Curchod. "D esign andConstruction of a n Extruded, Curved, Linear Focus,Fresnel Lens," 21st IE EE Photovoltaic SDecialistsConference Proceedings. (Piscataway: IE EE , 1990).6. Chiang, C. J. "Sandia's Concept 90 PhotovoltaicConcentrator Module," 21st IEE E Photovoltaic SDecialistsConference Proceedings. (Piscataway: IE EE , 1990).7. Gee, J., and G. Virshup. "A 3l%-Efficient GaAs/SiliconMechanically Stacked, Multijunctlon C oncentrator SolarCell," 20th I EE E P hotovoltaic SDecialists ConferenceProceedings. (Piscataway: IEE E, 1988). p. 754.8. Fraas, L. M., &&. "Tandem GaAs/GaSb ConcentratorSolar Cells with 35% Conversion Efficiency for TerrestrialApplications," 21st IE EE Photovoltaic SDecialistsConference Proceedings. (Piscataway: fEEE, 1990).9. Green, M. A. , &&. "Recent Improvemen ts in SiliconCell and Module E fficiency," 21st IEE E PhotovoltaicS~e ciali sts onference Proceedings. (Piscataway: IEE E,1990).10. Kin R. R., "Passivated Em itter s in Silicon SolarCells," !21st IE EE Photovoltaic Specialists ConferenceProceedings. (Piscataway: IEEE, 1990).11.Maish, A. B. "Perform ance of a Self-Aligning SolarArray Tracking Controller," 21st IEE E PhotovoltaicSDecialists Conference Proceedings. (Piscataway: IEE E,1990).

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12. Cham berlin, J. L., an d D. L. King. "The P hotovoltaicConce ntrator Initiative," 21st IEEE PhotovoltaicSDccialists Conference Proceedinps. (Piscataway: IEEE,1990).

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