IEEE Transactions on Energy Conversion, Vol. EC-1, No. 1, March 1986 47

POWER CONDITIONING SUBSYSTEMS FOR PHOTOVOLTAIC CENTRAL-STATION POWER PLANTS:STATE-OF-THE-ART AND ADVANCED TECHNOLOGY

Alec Bulawka Stan Krauthamer, Member, IEEE Radhe DasU.S. Department of Energy Jet Propulsion Laboratory Jet Propulsion LaboratoryWashington, D.C. 20585 California Institute of Technology California Institute of Technology

Pasadena, California 91109 Pasadena, California 91109

ABSTRACT cent-efficient CS-PCS that in volume produc-tion would cost $0.07/W (1). The realiza-

Through the combined efforts of governmen- tion of this goal would help make PV powertal agencies and laboratories, private organi- economically competitive with electric powerzations, universities and electric utilities, generated from conventional energy sources.substantial progress has been made in bringingphotovoltaic (PV) systems closer to commercialfeasibility.

PCS Cost, Efficiency Energy CostThe future availability of central station Size $/Peak W % $/kWhr*

power conditioner subsystems (CS-PCS) that areefficient, safe, reliable, and economical will _e_ -play a key role in the acceptance of large- Centralscale PV energy by electric utilities. To Station 0.07 98 0.108accelerate the development of such CS-PCS hard-ware, the Department of Energy's Photovoltaic Inter-Division, as part of its Five Year Plan, has mediate 0.16 96 0.117established a goal of a reliable, 98 percent-efficient CS-PCS that in volume productionwill cost $0.07/Wp. This will help make PV *Energy costs are based upon applications inpower economically competitive with electric the southwestern region of the United Statespower derived from conventional energy sources. and upon the following far-term considera-Various governmental agencies and private or- tions: modules, PCS and system costs, andganizations are cooperating to try to achieve 2-axis tracking systems , and 30-year lifethis proposed goal. expectancy

The combination of a dwindling federal bud- Table 1. U.S. Department of Energy Photovol-get for PV research and development (R&D), and taic Division 5-Year Plan Cost andthe reluctance of American industry to accept Efficiency Goalsthe high economic risks involved in its own PVR&D efforts, could shift the leading edge of Examination of the cost and efficiency para-PV-PCS technology development to foreign com- meters associated with state-of-the-art PCSpetition, in general, and to Japanese industry, technologies, reveals that the PCSs now avail-in particular. able fall short of the DOE goals. To meet

these goals will require novel and innovativedesigns that incorporate the new and advanced

INTRODUCTION power semiconductors now becoming available.Thus, this paper also reviews federal/industry

This paper is an overview of the technical activities in research and development (R&D)and near-term cost requirements that must be met of advanced PCSs that will contribute to theto develop economically viable power conditioning attainment of fully competitive, large-scalesubsystems (PCS) for large-scale, central, PV PV power stations (Table 2).power stations. The paper also surveys vari-ous already commercially available PCS-hardwaresuitable for use in today's central PV power PerCo ningSbyts

stations. ~~~~~~~~~~PowerConditioning Subsystemsstations.

A central PV generating plant, like itsTo accelerate the development of efficient, residential counterpart, is an example of asafe, reliable, and economical central-stationpower conditioning subsystems (CS-PCS), theU.S. Department of Energy (DOE), through the tem (2). Thus, to understand the design re-Five Year Plan of its PV Division, has estab- quirements of a CS-PCS, it is essential toFished eargal(Table 1) of a reliable, 98 per- review the technical requirements of a utility-lished a goal (Table 1) of a reliable, 98 per- interactive PV system and its subsystems.

85 SM 431-2 A paper recommended and approved A utility-interactive PV system (Figure 1)by the IEEE Power Generation Committee of the IEEE consists of a variety of subsystems: a PVPower Engineering Society for presentation at the array subsystem, a power conditioning subsys-IEEE/PES 1985 Summer Meeting, Vancouver, B. C., tem, a utility interconnection subsystem, andCanada, July 14 - 19, 1985. Manuscript submitted a control subsystem (3). The PV array subsys-January 31 , 1985 ; made available for printing May tem converts solar energy into direct current20, 1985. (dc) electrical power and delivers it to the

PCS through the dc interface. The array sub-system also provides protection and necessary

U.S. Government work not protected by U.S. copyright.

48

Ancillarydc/dc dc/ac Method of Commutation

Manufacturer Converter Inverter Power Control Components

State-of-the-Art Configurations:

Windworks N/A 12-Pulse Line-Commutated N/A, due to(not appli- Line-Commutated Phase-Angle Control line commuta-cable) SCR Inverter tion

Helionetics N/A 12-Pulse Phase-Shift RequiredSelf-Commutated ModulationSCR Inverter

Toshiba GTO 12-Pulse dc/dc Converter N/A, due toThyristor Self-Commutated for Power GTO usage

GTO Converter Control

DOE/SNL Studies of Central Station - Power Conditioning Subsystem Configurations:

United N/A 12-Pulse PWM N/A for GTOTechnologies Self-Commutated (Pulse-width DesignsCorporation Inverter (Either modulation) Required for

GTO's or SCR's SCR DesignsImplementation)

General N/A 12-Pulse PWM N/A, due toElectric Self-Commutated (Pulse-width GTO usage

GTO Inverter modulation)

Westinghouse GTO 12-Pulse dc/dc Converter N/A, due toThyristor SCR Inverter for Power Control line commuta-

tion

Table 2. Comparison of Central-Station Power Conditioning Subsystem Configurations (5,6,7,8)

UTILITY CONTROL ANDINFORMATION SIGNALS

SUBSYSTROL In operation, the PCS converts dc powerfrom the array into ac power, provides for anoptimum amount of power to be extracted fromthe PV array for any given insolation and en-vironmental conditions, matches frequency and

PV_POWER CON- UTILITYphase of the voltage desired by the utility,

ARRAY _ DITIONING- INTERCON. and provides protection not only for its inter-SOLAR SUBSYSTEM SUBSYSTEM SUBSYSTEM UTILITY nal components but also for the equipment ex-ENERGY ternal to the PCS (3,4).

Figure 1. Block Diagram of a Utility-Interac- To achieve a compatible integration of thetive Photovoltaic System PV system with the utility, it is essential

that the design of the PCS accommodate the

electrical isolation between the PCS and the dynamic range of interactions between the PVarray, and may include experimental instrumen- system and the utility grid. These arise fromtation for monitoring the performance of the changes in both grid conditions and the output

array. The utility interconnection subsystem, of the PV array. The proper and safe intercon-array. ~~~~~~~~~~nectionsof PV subsystems require not only anthrough the alternating current (ac) interface identification of their mutual functional con-with the PCS, provides for synchronization ietfcto ftermta ucinlcnwith the utClitypridesifonesyhrozactiton straints, but also a knowledge of how to selectwith the utility and, if necessary, acts toelectrically isolate the PV system from the or design the PV subsystems within such con-utility. The control subsystem, operating straints. These constraints, therefore, are

through the PCS, oversees the performance of important in the selection or evaluation of athe entire PV system. It also enables overall PCS that is suitable for central station PVcoordination of the protection of the system, systems.co.mmunicates status information to the utilitydispatch center, and, if desired, provides aninformation and tracking feedback 1oop with Energy considerations will vary in differentthe PV array. In central PV stations, the PCS parts of the U.S. The data used in this papermay also process operational commands from the refers to the Southwest region of the U.S. andutility dispatch center. nay differ in other regions .

49

COST AND EFFICIENCY REQUIREMENTS It is clear from Table 3 that the projectedefficiencies, of the intermediate PCSs areAlong with taking into account the compati- less than the proposed DOE goal of 96 percent.bility of the PCS with the utility, it is

necessary to consider the contribution of thePCS to the cost of PV-generated electricity.The levelized busbar cost of the energy thatthe PV plant produces is a figure of merit thatbest characterizes the performance of a PV I 00system. Obviously, since the PCS is an inte- o ! 90gogral component of the overall PV system, its X >cost and performance will affect the price of Z0 75unit energy.

O60U~_z

I- -

Cost and Efficiency of Central Station Designs u>0

There are several approaches to CS-PCS de- 50signs that are described in the following sec- otions. The cost and energy efficiency data for 010 100 1000proposed and state-of-the-art designs are shown ANNUAL QUANTITYin Tables 3, 4, and 5.

Figure 2. Relative FOB Cost of IntermediatePower Conditioning SubsystemIntermediate Power Conditioning Subsystems. versus Annual QuantityA CS-PV system may include a large PCS or mul-

tiple intermediate-type PCSs. The cost andefficiency of both of these types of PCSs are Large Power Conditioning Subsystemsreviewed in this section.

The cost and efficiency of both prototypesThrough contracts awarded by the Sandia and production hardware of large, state-of-National Laboratory (SNL), innovative ap- the-art PCSs are depicted in Table 4. Theproaches to the intermediate power condition- efficiency figures represent estimated rathering subsystems were studied in 1981 (9,10).Based on results of these studies, Table 3 than measured values. Although several ofshows the cost and efficiency projections for thesepesetly avaib largeP OEhav eboth 10 and1000 nits o thes PCSs whenficiencies that approach that of the DOE goal,both 100 and 1000 uaits of these PCSs, whenconsidered on the basis of 1981 dollars. Anannual production of 100 PCSs, each capable ofhandling 50 kW, will be required for a 5-MW Maximumplant. Estimated prices are obtained from MaximumFigure 2, in which relative price is plotted oEnergyagainst annual production quantity. The pro- Effi-jected costs for 100 50-kW PCSs do not include Research PCS ciency, %the additional costs of external weather-proof- Organization Status Sizeb (Estimated)ing, enclosures, transformation, and protec-tion of the utility interface. On that basis,the projected prices for these proposed de- Windworks Production 1000 kWsigns appear to be very competitive in the Hardware 300 kW 97.5near term to the large, prototype, state-of- H 3 k 97-5the-art PCSs now available. Helionetics Production 75 kW 93.0

Hardware

Production 750 kW 96.0PCS 100 1000 Hardware

Power Effi- Units -Units Toshiba Prototype 750 kW 97.0Rating, ciency, Cost, Cost, ProductionManufacturer kW $/W $/WlPrototype 1000 kW 97.0ProductionGeneral

Electric 92.0 95.3 0.146 0.117

aEnergy cost varies from $0.14 to $0.15/kWhr.Westinghouse 90.0 94.2 0.188 0.15 Calculations are based upon applications inthe southwestern region of the United States,l 1150.0 94.8 0.136 00.109 | present PCS costs, and the far-term costs ofmodules and systems.United

Technologies ll l|bApproximate cost of CS PCS is between $0.45LCorporation 80.0 95.4 0.136 0.109 |to $0.5'5/Wp.l

Tahle 3. Cost and Efficiency Projections for Table 4. Industry Central-Station HardwareProposed PCS Designs tus

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they neither satisfy the cost goal (Table 1) LARGE, STATE-OF-THE-ART POWER CONDITIONERS:nor the energy cost projections. Thus, while TECHNICAL CHARACTERISTICSthe energy cost of these systems is S0.15/kWhr,the proposed DOE goal is $0.108/kWhr.

This section deals with a comparative analy-Table 5 shows the cost, efficiency, and sis of three, presently available, large power

energy costs of several conceptual designs of conditioners suitable for use in central-PCSs for central stations. These conceptual station PV applications. A technical summarydesigns meet neither cost and efficiency goals is provided in Table 2.nor the projections of energy costs. The en-ergy cost of these conceptual designs , ba- 1. 500-kW and 750-kW self-commutated Thyristorsed on a life expectancy of 30 years, more Inverter (Manufactured by the DECC Divisionclosely reflects the energy cost in the DOE of Helionetics Corporation).projections rather than the energy cost forboth large-and intermediate-sized, state-of- Two 500-kW, self-commutated inverters havethe-art PCSs. Thus, while the energy cost of been operating in a 1-MW PV installation atthese conceptual systems is 0.114/kWhr, the the ARCO Solar Lugo Installation in Hes-DOE proposed goal is $0.108/kWhr. peria, California. Multiple 750-kW units

have been installed and are operational atthe ARCO Solar Carrisa Plains Installation,California (Figure 3). The installed capa-

Pro- city is 6 MW. Except for being packaged inPCS jected Pro- a self-contained, outdoor enclosure, thePower Effi- jected Energy 750-kW units are electrically similar to

Rating, ciency, Costs, Cost the 500-kW units. Harmonic reduction isManufacturer MW l VW $/kWhr accomplished by waveform synthesis.

(b) (c)

General Electric 5 96.7 0.105 0.112

Westinghouse 5 96.8 0.134 0.114

United Technology 1 96.7 0.13i 0.114

(AEach 50 units/year, (b)pCS cost, (C)Entire PVplant

Table 5. Projected Results of DOE-FundedCentral-Station Power Condi-tioning Subsystem Studies

Although there are discrepancies betweenthe DOE proposed goals for cost and efficiencyand the values derived from the conceptualdesigns, the latter approximate the DOE-pro-posed energy-cost goals. Thus, further re- Figure 3. 12-Pulse, Self-Commutated, 750-kWsearch and development of new hardware are Inverter Assembly (Courtesy ofneeded to determine whether the cost and effi- DECC Division of Helionetics)ciency goals can be met.

Table 6 provides a calculated relationship 2. One-Megawatt, Line-Commutated Inverterbetween efficiency and cost for CS-PCSs. This (Manufactured by Windworks, Inc.).analysis is based upon the configuration offuture PV systems. At present, as part of the Sacramento Muni-

cipal Utility District (SMUD) Phase I pro-gram, a 1-MW, line-commutated inverter isoperational for PV central-station applica-

Relative Change in Cost tion at a SMUD installation. Harmonic re-of PCS Compared to Its Cost duction is accomplished with filters.

Efficiency X) | at 98% Efficiency Figure 4 shows various views of this inver-__________________________ lter's subassemblies.

99 $13/kW Lower 3. One-Megawatt, Self-Commutated Inverter(Manufactured by Toshiba, Inc.).

98 0At present, a 750-kW inverter is installed

97 $13/kW Higher at the ARCO Solar PV installation at Car-l_______________ I_______________________Irisa Plains, California. A 1-MW unit also

will be supplied to SMUD for its Phase IITable 6. Relative Change in Cost with Change central-station installation. Filtering of

in Efficiency the harmonics produced is accomplished by

51

Figure 4. 1-MW, Photovoltaic Inverter Manufactured by Windworks, Inc. (Courtesy of Windworks)

waveform synthesis and an output filter. of the PV-PCS-related industry. Because ofFigure 5 shows an inverter assembly similar to production volume and lack of foreign competi-the 1-MW inverter assembly. tion in this sector, there was little need for

innovative designs with their attendantreductions in cost. This situation now ischanging as both new and advancedsemiconductor devices and more advanced PCSdesigns are emerging from foreign industrialcompetition.

To date, most of the larger, multi-kilowatt,government-funded, PV-demonstration projectsuse inverters that were retrofitted for PVapplications from the designs of UPS. Thecost for power conditioning from these UPSdesigns varied between $0.50 to 1.10/Wp,depending on their size. Since the cost goalfor a CS-PCS (Table 1) is $0.07/Wp, the costrange of these UPS-derived power conditionerswas unacceptable for future PV systems. Ob-viously, significant technical innovationswere needed to improve large, PV-specific PCSB.

Figure 5. 1-MW, Photovoltaic Inverter Manu-factured by Toshiba, Inc. (Courtesy Meanwhile, recent commercial incentivesof 3T Tanakaya, Inc.) (substantial tax credits, third-party financ-

ing, etc.) opened new opportunities for thedevelopment of multi-megawatt, central-station

NEW DESIGN OPPORTUNITIES PV system power plants. This, in turn, neces-sitated re-evaluation of the designs for large

The widespread use of central-station PV PCSs for PV applications. The large-scaleplants will occur only if they are competitive ARCO PV installations at Lugo and Carrisawith other sources of electrical energy in Plains in California are recent PV successterms of busbar energy and reliability. Yet, stories. In addition, the joint SMUD-DOE ven-until recently, the concepts for grid-interac- ture shows promise on the even larger scale oftive, central-station PV systems and their 100 MW.optimized PCSs were only in the exploratoryplanning stage. In contrast, small-size (up A much needed technical innovation in PCSsto 10 kW), second-generation, process-inten- recently has come in the form of developmentssive, cost-effective, and efficient power con- of commercial, large (100-MW), gate turn-offditioners for residential systems already were (GTO) thyristors and other fast-switchingcommercially available. Similar, but larger power devices. These developments have openedpower conditioning subsystems have become the door to new approaches to cost-effective,available only recently. large PCSs for PV applications. Most impor-

tantly, these multi-megawatt PC units offerIn addition, the previous lack of appropri- promise of meeting the DOE's projected cost

ate power semiconductor devices led to mater- and performance goals.ial-intensive designs burdened with componentsthat assist in semiconductor switching (commu- In the United States, at present, there aretation) or with cumbersome techniques to accom- no commercially available intermediate- orplish harmonic filtration. Designs of unin- large-scale inverters that use gate turn-offterruptible power supplies (UPS) and adjus- (GTO) thyristors for application in UPS, PVtable dc-speed drives have become the standard and other dispersed storage-and-generation

systems.

52

GTO thyristors, representing advanced U.S. industry, seemingly lagging in this area,devices, are commercially available in Japan. will be encouraged to stay active and competi-The Japanese are producing large, multi- tive with foreign developments in the powermegawatt power conditioners using GTOs. Since electronics marketplace. This will requireGTOs presently are not being produced by component development incentives to encourageAmerican manufacturers, the United States has better PCS hardware designs.fallen behind Japan in the manufacture andapplication of GTOs and other power semi-conductors. American semiconductor manufac- Recent Program Resultsturers and equipment designers, however, noware showing increasing interest in GTO devices. Large, multi-megawatt, second-generation

power conditioners, with potentially high ef-Based on initial and optimistic test re- ficiency and lower cost, have been studied for

sults with these newly developed technologies, PV applications by Westinghouse, General Elec-the DOE-funded program has embarked on a tric, and United Technologies. The results ofcourse of research and development aimed to- these studies are summarized in Table 2 andwards building a testable prototype of a multi- Table 5. It should be noted that the threemegawatt type inverter. It is to be specifi- design approaches suggest the use of GTO semi-cally designed to meet both the technical conductor devices in the PCS design.needs and the cost objectives of a grid-interactive PV central station. While pursuing the long-range objective of

designing large, multi-megawatt PCSs, effortsalso are being made to be responsive to pre-

THE DOE CENTRAL-STATION PCS PROGRAM sent, central-station economic perspectives.The intermediate PCSs will be re-evaluated as

Various central-station PV system and PCS a possible option now for central-station ap-hardware study programs have been initiated by plications. Within this research-orientedindustry, the U.S. government, DOE, and the activity, U.S.industry will continue to workElectric Power Research Institute (EPRI). A with smaller, high-tech designs until the con-

major thrust of these programs is to identify cept of the large, multi-megawatt PCS can beoptimal, conceptual, central-station PCS topo- fully accepted economically by both the utili-

logies that could provide improvements in en- ties and industry.ergy cost as compared to present, state-of-the-art PCSs for given power, voltage and per-formance levels. CONCLUSIONS AND WHAT NEEDS TO BE DONE

In 1983, a government-industry workshop A cost-effective PCS, with a track recorddealing with large, multi-megawatt PCS techno- of trouble-free operation, will increase util-logy for PV applications was initiated by DOE ity interest in the use of PV systems. Itand hosted at EPRI. This workshop brought also will improve the prospects of sales of PVtogether the two major areas of PV systems systems in both national and internationalexpertise: PV cell/module technology and PCS markets. To achieve the DOE's proposed costphotovoltaic balance-of-systems. The objec- and efficiency goals for PCSs requires addi-tive of the workshop was to determine the op- tional research and development efforts. Ef-timal voltage and megawatt size of future ficiencies of several state-of-the-art CS-PCSsphotovoltaic central-station PCSs. Previous already have come close to the desired effi-assessments had indicated an optimal configur- ciency goals. However, energy costs are substant-ation of 5 MW at 2000 Vdc. A 5-MW, 800-Vdc ially greater than the proposed DOE goal and

(center tap ground) arrangement was agreed milestone. Thus, it is likely that proper-upon because of the present unavilability of ly implemented second-generation designs canphotovoltaic modules that are able to with- meet both cost and efficiency goals.stand a higher voltage stress and still retaina projected lifetime of 20 to 30 years. The viability of central-station PCSs, with

respect to multiple units (greater than 500-kW)Under an SNL/DOE multi-year plan, PCS con- in parallel, has been proven with presently

figurations have been solicited from industry available designs. Multiple-kilowatt, powerfor future hardware implementation. The plan conditioner subsystems are operational at thestresses the use of innovative and process- ARCO installations at Lugo and Carrisa Plains,intensive concepts that make use of advanced as well as the SMUD installation. Theirelectronic components. To take future cost- operational track record is good. Knowledgeeffective designs into account, the plan also gained at these installations will be benefi-emphasizes flexibility for upward voltage- cial in the design and operation of large,scaling to accommodate high-voltage PV modules multi-megawatt, central-station PCS units.of the future. Concurrently, the need for aninnovative design of a PCS for multi-megawatt New power semiconductors, such as GTO thy-PV systems was identified. Contracts were ristors, offer the prospects of substantialsigned with both industry and universities to PCS cost reduction and improved delivered en-fill this void. The 5-MW block concept will ergy costs. Their use should be considered inbe developed as far as the budget will permit, future CS-PCS designs. Pulse-width modulationpossibly beyond the proof-of-concept stage and technology for large-scale PCSs has not been

to the point of a scaled-down prototype.

53

implemented as yet and should be examined for 2. A. Bulawka, R. Das, S. Krauthamer, "PowerCS-PCS applications. In such configurations, Conditioners for Residential Photovoltaickeeping the need for filtering of the PCS's Applications," Photovoltaics Magazine,output to a minimum will provide trouble-free Vol. 1, No. 6, pp. 22-29.utility integration. With such PCSs, the sys-tem loop stability will be at a maximum while 3. S. Krauthamer, et. al., "Photovoltaicrisks of system resonances will be at a mini- Power Conditioning Subsystem: State ofmum. Improving filtering techniques and eli- the Art and Development Opportunities,"mination of commutation components will result JPL Publication 83-81, Jet Propulsionin increased efficiency and reliability. Laboratory, Pasadena, California, January

15, 1984.Further research is required to determine

the technical and economic viability of multi- 4. "Request for Bid and Specifications forple, intermediate-size inverters for central- the Purchase of Power Conditioning Unit,"station PV systems. Bid Request No. 2505, SMUD, November 1983.

Cooperative efforts among DOE-Photovoltaic 5. "Specification for a One-Megawatt GeminiDivision, DOE-Electric Energy Systems Division, Synchronous Inverter Power ConditioningEPRI, SNL, JPL, universities, utilities, IEEE, System for Photovoltaic Applications,"and the private sector in program planning, Windworks, Inc., 1984.conceptualizing advanced designs, hardwarefabrication, test evaluation, and formulation 6. "Major Features of TOSNIC-1000 Seriesof standard guidelines, have achieved substan- UPS," Toshiba Corporation, Tokyo, Japantial progress in bringing PV systems closer to (Rep. by 3T Tanakaya, Inc.), Decembercommercialization. Continued efforts by all 1982.participants will enable photovoltaic systemsto be competitive with conventional sources of 7. "Advanced Converter Technology," Techni-energy and, therefore, be widely used. The cal Progress Report for Period July 1,government-industry partnership in the re- 1982, through June 30, 1983, FCR-5454,search and development of PV systems must con- United Technologies Power Systems, Wind-tinue. sor, Connecticut.

To date, American industry stresses high 8. S. Krauthamer, et al., "Photovoltaic Cen-economic risks as detrimental to their own tral-Station Power Conditioner Subsys-pursuit of research and development efforts. tems," Proceedings of 19th IECEC, SanAt the same time, the federal R&D budget has Francisco, California, August 1984.been dwindling. These two conditions couldlead to a shift of the leading edge of techno- 9. T.S. Key, "Power Conditioning Developmentlogy development in photovoltaics to foreign for Grid Connected Photovoltaic Applica-competition. This, in turn, could lead to a tions Less Than 250 KW," Proceedings ofsignificant setback to the PV segment of U.S. 19th IECEC, San Francisco, California,industry. Continuation of this shift will August 1984.allow the Japanese industry to become undis-puted leaders in this technology and would, 10. D. Chu and T. Key, "Assessment of Powerfor years to come, preclude American industry Conditioning for Photovoltaic Centralfrom cornering its share of the market. Power Stations," Proceedings of the 17th

IEEE Photovoltaic Specialists Conference,ACKNOWLEDGMENT Orlando, Florida, May 1984.

The work described in this paper was car-ried out by the Jet Propulsion Laboratory, Alec 0. Bulawka is the Project Manager,California Institute of Technology, and was Photovoltaic Energy Technology Division, Con-sponsored by the U.S. Department of Energy servation and Renewable Energy, U.S. Depart-through an agreement with the National Aero- ment of Energy, Washington, D.C.nautics and Space Administration.

REFERENCESStan Krauthamer and Radhe Das are members

1. National Photovoltaics Program Five-Year of the Technical Staff of the Jet PropulsionResearch Plan (1984-1988), U.S. Department Laboratory, California Institute of Technology,of Energy, Washington, D.C., May 1983. Pasadena, California.