ALBERT

Description: A model that explains the flat-spot power loss phenomenon is presented. Evidence suggests that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the P-N junction is destroyed and replaced with a metal semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallization are shown along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Description: The NASA large flexible solar array space shuttle flight experiment is described. The 32 x 4 m wing is deployed from the shuttle bay, and experiments in electrical output, multiple deployment, and structural dynamics are planned. Both 2 x 4 cm and 5.9 x 5.9 cm cell assemblies on the array blanket are evaluated. Safety/hazards provisions are described, including emergency jettison provisions. Ground testing and hardware fabrication are summarized.

Description: A development program to produce 5.9 x 5.9 cm space quality silicon solar cells with a cost goal of 30 $/W is described. Cell types investigated include wraparound dielectric, mechanical wraparound and conventional contact configurations with combinations of 2 or 10 ohm/cm resistivity, back surface reflectors and/or fields, and diffused or ion implanted junctions. A single step process to cut cell and cover glass simultaneously is being developed. Results for cell and array tests are given. Large solar arrays that might use cells of this type are discussed.

Description: Objectives and progress in both low concentration ratio (6 to 10) and high concentration ratio ( 100) array developments are summarized. Problems encountered include: thermal control, maldistribution of concentrated sunlight, current busing, and optical surface degradation. The potential advantages over planar arrays are an order of magnitude reduction in per unit cost of power plus increased immunity to radiation damage.

Description: Current-voltage curves were calculated for each cell in a cascade structure using a solar cell diode equation and superposition. Terms for the light generated current, diffusion current, space charge recombination current and series and shunt resistance are included. Individual current voltage curves are added in series with ohmic resistance losses for the cell interconnects to obtain the cascade cell performance. Temperature was varied with concentration, using several models, and ranged from 55 C at one Sun to between 80 and 200 C at 100 Suns. A variety of series resistance and internal resistances were used. Coefficients of the diffusion and recombination terms are strongly temperature dependent. The study indicates that maximum efficiency (30%) occurs in the 50 to 100X Sun concentration range, provided series resistance is below 0.015 ohm-sq cm and cell temperature is 80 C at 100 Suns.

Description: Gallium and boron doped silicon solar cells, processed by ion implantation followed by either laser or furnace anneal were irradiated by 1 MeV electrons and their postirradiation recovery by thermal annealing was determined. During the postirradiation anneal, gallium doped cells prepared by both processes recovered more rapidly and exhibited none of the severe reverse annealing observed for similarly processed 2 ohm-cm boron doped cells. Ion implanted furnace annealed 0.1 ohm-cm boron doped cells exhibited the lowest post-irradiation annealing temperatures after irradiation. The drastically lowered recovery temperature is attributed to the reduced oxygen and carbon content of the 0.1 ohm-cm cells. Analysis based on defect properties and annealing kinetics indicates that further reduction in annealing temperature is attainable with further reduction in the silicon's carbon and/or divacancy content after irradiation.

Description: A space based orbital transfer vehicle (SBOTV) and ground based OTV's (GBOTV) are compared for debris protection, space based OTV maintenance provisions, flight performance, onorbit refueling, and launch and return operations. Debris protection has a severe impact on the SBOTV, while the penalty for the GBOTV is much less severe. A key technology issue is the protection capability of composite materials. Onorbit maintenance is critical for SBOTV. Reduction of losses during the various transfers is th maine problem with refueling a SBOTV. Zero-g propellant storage and transfer is an important technology area for SBOTV. A reusable shroud must be developed to return GBOTV's if a Shuttle derivative vehicle is used. The advantage of space basing lies in more efficient use of the launch vehicle. Since most of the mass going to LEO is OTV propellant, and the launches to deliver the SBOTV propellant are generally mass limited, substantially fewer launches are required to support the SBOTV.

Description: The NASA Lewis Research Center in cooperation with the World Health Organization, U.S.A. I.D., the Pan American Health Organization and national government agencies in some developing countries sponsored the installation of twenty photovoltaic powered medical vaccine storage refrigerator-freezer (R/F) systems. The Solar Power Corporation was selected as the contractor to perform the design, development and installation of these twenty units. Solar Power's experiences are described herein.

Description: The U.S. Coast Guard has developed a qualification test to screen photovoltaic modules for utilization on marine aids to navigation. The test is based on a combined-environment of hot and cold saltwater immersion and air pressurization. The test has demonstrated a very high acceleration factor and excellent correlation of electrical failures with modules in a concurrent real-time marine exposure.

Description: Design principles suggested here aim toward high conversion efficiency (greater than 15 percent) in polysilicon cells. The principles seek to decrease the liabilities of both intragranular and grain-boundary-surface defects. The advantages of a phosphorus atom concentration gradient in a thin (less than 50 microns) base of a p(+)/n(x)/n(+) drift-field solar cell, which produces favorable gradients in chemical potential, minority-carrier mobility and diffusivity, and recombination lifetime (via phosphorus gettering) are suggested. The degrading effects of grain boundaries are reduced by these three gradients and by substituting atoms (P, H, F or Li) for vacancies on the grain-boundary surface. From recent experiments comes support for the benefits of P diffusion down grain boundaries and, for quasi-grain-boundary-free and related structures. New analytic solutions for the n(x)-base include the effect of a power-law dependence between P concentration and lifetime. These provide an upper-bound estimate on the open circuit voltage. Finite-difference numerical solutions of the six Shockley equations furnish complete information about all solar-cell parameters and add insight concerning design.

Description: The paper describes the design of the solar array system (SAS) for the Solar Maximum Mission, the unique features of the SAS, and the results of its successful in-orbit operation. It is noted that the array was unique in that: (1) major weight concessions were made to produce a dynamically stiff array; (2) it was the first array designed to be compatible with the NASA Multimission Modular Spacecraft; (3) it is the first jettisonable solar array; and (4) it represents the first use of FEP-bonded overslides on a prime power array. It is concluded that the array performed as predicted with no evidence of the FEP causing any unusual array power degradations. In addition, the deployment and telemetry systems performed as designed.

Description: A broad overview is presented of current and near-term solar array technology that could be suitable for space use. Particular consideration is given to such advanced concepts as high power arrays, concentrator arrays, and ultrathin solar cell arrays. It is concluded that if such ambitious concepts as geosynchronous space platforms, orbital space stations, and alternate forms of propulsion are realized, the type of new technology described in this paper may find acceptance for space.

Description: Major advances in photovoltaic (PV) Power Conditioning (PC) with respect to performance and low-cost potential have been made. Solutions have been obtained to interface and control problems related to adapting available inverter designs to the grid-connected, residential photovoltaic experiments. A description is presented to contributing research and development activities. Attention is given to aspects of residential systems experience, conceptual design studies, questions of optimum topology development, and promising advanced designs for residential PV provided by development efforts of the private sector.

Description: Mechanical fatigue of solar cell interconnects is a major failure mechanism in photovoltaic arrays. A comprehensive approach to the reliability design of interconnects, together with extensive design data for the fatigue properties of copper interconnects, has been published. This paper extends the previous work, developing failure prediction (fatigue) data for additional interconnect material choices, including aluminum and a variety of copper-Invar and copper-steel claddings. An improved global fatigue function is used to model the probability-of-failure statistics of each material as a function of level and number of cycles of applied strain. Life-cycle economic analyses are used to evaluate the relative merits of each material choce. The copper-Invar clad composites demonstrate superior performance over pure copper. Aluminum results are disappointing.

Description: Computer analysis indicates that a substantial increase in solar cell conversion efficiencies can be achieved by using two-cell, multi-bandgap tandem structures instead of single-junction cells. Practical AM1 efficiencies of about 30 percent at one sun and over 30 percent at multiple suns are to be expected. The further increases in efficiency calculated for a three-cell tandem structure are much smaller and may not justify the added complexity. For inexpensive two-cell tandem modules, Si is preferred for the bottom cell, and the top-cell material should have a bandgap of 1.75 to 1.80 eV. The GaAs-AlAs and GaAs-GaP systems are very attractive candidates for the top cell. Significant advances have been achieved in growing GaAs on Ge-coated Si substrates (for the two-terminal, two-cell structure) and in growing free-standing ultrathin GaAs layers (for the two-terminal or four-terminal structures). These advances should be transferable to the GaAs-AlAs and GaAs-GaP systems.

Description: The electric output of flat-plate photovoltaic arrays changes constantly, due primarily to changes in cell temperature and irradiance level. As a result, array loads such as direct-current to alternating-current power conditioners must be able to accommodate widely varying input levels, while maintaining operation at or near the array maximum power point.The results of an extensive computer simulation study that was used to define the parameters necessary for the systematic design of array/power-conditioner interfaces are presented as normalized ratios of power-conditioner parameters to array parameters, to make the results universally applicable to a wide variety of system sizes, sites, and operating modes. The advantages of maximum power tracking and a technique for computing average annual power-conditioner efficiency are discussed.

Description: The feasibility and performance parameters for beam microwave power supplies from a space station to nearby orbiting satellites are examined. A 5.8 GHz transmission frequency is found suitable for beaming 1-10 kW over a distance of 1-10 km. The antenna could have a 15 m diameter, a 64 kW output, provide uniform illumination, and have a retrodirective phase control system. A LEO to ground demonstration project is described, involving power levels of 0.0025 mW/sq cm and yielding 202 W at a 100 x 100 m rectenna.

Description: A Programmable Power Processor (P3) has been developed for application in future large space power systems. The P3 is capable of operation over a wide range of input voltage (26 to 375 Vdc) and output voltage (24 to 180 Vdc). The peak output power capability is 18 kW (180 V at 100 A). The output characteristics of the P3 can be programmed to any voltage and/or current level within the limits of the processor and may be controlled as a function of internal or external parameters. Seven breadboard P3s and one 'flight-type' engineering model P3 have been built and tested both individually and in electrical power systems. The programmable feature allows the P3 to be used in a variety of applications by changing the output characteristics. Test results, including efficiency at various input/output combinations, transient response, and output impedance, are presented.

Description: (Previously cited in issue 12, p. 2037, Accession no. A81-29543)

Description: Commercially available metallurgical grade (MG) silicon has high B and P content which is not reduced significantly by directional solidification. By choosing high purity raw materials for an experimental Submerged Electrode Arc Furnace, most of the impurities are reduced to 10 ppmw. Directional solidification of upgraded metallurgical grade (UMG) silicon by the Heat Exchanger Method (HEM) has produced 16 cm x 16 cm cross section ingots with nearly single crystal structure. The main problem encountered during directional solidification was SiC impurities dispersed through the structure. Solar cells fabricated from UMG silicon that was directionally solidified twice by HEM have shown up to 12.33% (am1) conversion efficiency.

Description: A main source of high carbon levels in silicon crystals grown from melt under reduced pressures and contained in silica crucibles supported by graphite retainer/susceptor was identified by thermodynamic analysis. The calculations were verified by experimental results and the carbon level is reduced by approximately 50% with the use of molybdenum retainers.

Description: The most developed process for silicon crystal growth is the Czochralski (CZ) method which was in production for over two decades. In an effort to reduce cost of single crystal silicon for photovoltaic applications, a directional solidification technique, Heat Exchanger Method (HEM), was adapted. Materials used in HEM and CZ furnaces are quite similar (heaters, crucibles, insulation, etc.). To eliminate the cost of high purity argon, it was intended to use vacuum operation in HEM. Two of the major problems encountered in vacuum processing of silicon are crucible decomposition and silicon carbide formation in the melt.

Description: The Fixed Abrasive Slicing Techniques (FAST) is a new slicing technique that was developed to slice ingots more effectively. It was demonstrated that 25 wafers/cm can be sliced from 10cm diameter and 19 wafers/cm from 15 cm diameter ingots. Over 99% yield (222 out of a possible 224) was demonstrated during slicing of a 10 cm diameter ingots at 25 wafers/cm. The average thickness of wafers was 0.249 mm and the kerf was 0.151 mm. Slicing rates as high as 0.14 mm/min were also demonstrated for 10 cm diameter ingots.

Description: One of the major cost factors in silicon ingot technology adaptation for terrestrial photovoltaic application is in slicing boules into wafers. The most developed industrial practice is the Internal Diameter (ID) slicing. This method utilizes diamond cutting. The diamond stands up for long periods, hence, the cost of expendable materials is low. However, the ID technology as practiced today has poor material utilization. The Multiblade Slurry (MBS) method has low equipment and labor costs but its expendable material costs are high. Recently Multiwire Slurry (MWS) technology has shown very good material utilization, but its expendable material costs are even higher than MBS. The multiwire Fixed Abrasive Slicing Technique (FAST) still in advanced development stage, combines the low expendable material costs of ID method, the low labor and equipment costs of MBS and high material utilization of MWS.

Description: Directional solidification by the Heat Exchanger Method (HEM) is a viable approach for directional solidification of silicon ingots used for terrestrial solar cell applications. Some of the significant advances made during this phase of the program were: ingot size, 34 cm x 34 cm x 17 cm - 45 kg, usable material yield - 90%, solar cell efficiency averaged over the whole ingot - 85% of control CZ (35 kg ingot), very uniform resistivity over the boule, and cycle time for 36 kg ingots - 56 hours. Some of the problems encountered were growth rate decrease with increased ingot height, silicon carbide precipitates, and high dislocation density in HEM material. The silicon carbide is attributed to backstreaming of oil vapors from the vacuum pump; the high dislocation density is associated with the thermal history of the boule.

Description: The group of techniques that as a class are referred to as synthetic battery cycling are described with reference to spacecraft battery systems. Synthetic battery cycling makes use of the capability of computer graphics to illustrate some of the basic characteristics of operation of individual electrodes within an operating electrochemical cell. It can also simulate the operation of an entire string of cells that are used as the energy storage subsystem of a power system.

Description: Rechargeable nickel-hydrogen systems are described that more closely resemble a fuel cell system than a traditional nickel-cadmium battery pack. This was stimulated by the currently emerging requirements related to large manned and unmanned low Earth orbit applications. The resultant nickel-hydrogen battery system should have a number of features that would lead to improved reliability, reduced costs as well as superior energy density and cycle lives as compared to battery systems constructed from the current state-of-the-art nickel-hydrogen individual pressure vessel cells.

Description: The economic advantages of solar dish collectors are evaluated and compared with other energy systems. Labor, inflation and energy deregulation are considered.

Description: Solar energy systems were investigated which complement nuclear and coal technologies as a means of reducing the U.S. dependence on imported petroleum. Solar Thermal Energy Systems (STES) represents an important category of solar energy technologies. STES can be utilized in a broad range of applications servicing a variety of economic sectors, and they can be deployed in both near-term and long-term markets. The net present value of the energy cost savings attributable to electric utility and IPH applications of STES were estimated for a variety of future energy cost scenarios and levels of R&D success. This analysis indicated that the expected net benefits of developing an STES option are significantly greater than the expected costs of completing the required R&D. In addition, transportable fuels and chemical feedstocks represent a substantial future potential market for STES. Due to the basic nature of this R&D activity, however, it is currently impossible to estimate the value of STES in these markets. Despite this fact, private investment in STES R&D is not anticipated due to the high level of uncertainty characterizing the expected payoffs.

Description: The integration and check-out of a complete Dish Solar Stirling Thermal Power System is described. The preliminary results of the tests conducted thus far are presented. The results are very encouraging and show promise of high performance and efficiency. The outstanding performance and durability of the 4-95 Stirling engine was the highlight of this 6 month integration and test activity. Exposure to severe heat, dust, sand and wind during the summer months and heavy rains, high winds, including sand storms and freezing cold in recent months did affected the engine or the receiver in any noticable manner.

Description: The Jay Carter Enterprises (JCE) Paratransit Vehicle steam engine was tested over a range of conditions which might be experienced by the power converter subsystem of the Small Community Solar Thermal Power Experiment. Some difficulties were encountered getting the engine ready for testing. These difficulties were related to the five year dormancy of the entire system and to incomplete development work that had been going on at the time of cessation of steam engine work at JCE. Other difficulties were related to the fact that the particular expander being tested never ran before and possessed some manufacturing defects. Nevertheless, the engine was operated successfully and results of testing do verify results of computer simulations of the engine in regard to the effect of temperature and power level variations. Engine efficiency was good but generally lower than expected and performance dropped as testing continued. The effect of change in expansion ratio was not demonstrated because of deterioration in engine performance. Post-test inspection revealed numerous correctable defects.

Description: A study was performed by AiResearch Manufacturing Company, a division of The Garrett Corporation, on the development a high-temperature ceramic heat exchanger element to be integrated into a solar receiver producing heated air. A number of conceptual designs were developed for heat exchanger elements of differing configuration. These were evaluated with respect to thermal performance, pressure drop, structural integrity, and fabricability. The final design selection identified a finned ceramic shell as the most favorable concept. The shell is surrounded by a larger metallic shell. The flanges of the two shells are sealed to provide a leak-tight pressure vessel. The ceramic shell is to be fabricated by an innovative combination of slip casting the receiver walls and precision casting the heat transfer finned plates. The fins are bonded to the shell during firing. The unit is sized to produce 2150 F ar at 2.7 atm pressure, with a pressure drop of about 2 percent of the inlet pressure. This size is compatible with a solar collector providing a receiver input of 85 kw(th). Fabrication of a one-half scale demonstrator ceramic receiver has been completed.

Description: The modifications and testing of a standard Stirling engine, required for connection to a 25 kW induction alternator, for use with a solar thermal parabolic dish electric module is described. Power was absorbed by a GE induction alternator connected to the utility grid. Also included are the results from recent testing of another solar engine at the DOE-Georgia Tech solar site. It was done in parallel with the testing at Edwards for the purpose of comparing performance of two solar-only receivers, which were based on the standard 4-95 involute heat exchanger.

Description: Verification testing of a solar collector was undertaken prior to its operation as part of an industrial process heat plant at Capitol Concrete Products in Topeka, Kansas. Testing was performed at a control plant installed at Sandia National Laboratory, Albuquerque, New Mexico (SNLA). Early results show that plant performance is even better than anticipated and far in excess of test criteria. Overall plant efficiencies of 65 to 80 percent were typical during hours of good insolation. A number of flaws and imperfections were detected during operability testing, the most important being a problem in elevation drive alignment due to a manufacturing error. All problems were corrected as they occurred and the plant, with over 40 hours of operation, is currently continuing operability testing in a wholly-automatic mode.

Description: The development and preliminary test results for an air-cooled, hermetically sealed 20 kW sub E organic Rankine cycle engine/alternator unit for use with point focussing distributed receiver solar thermal power system. A 750 F toluene is the working fluid and the system features a high speed, single-stage axial flow turbine direct-coupled to a permanent magnet alternator. Good performance was achieved with the unit in preliminary tests.

Description: The development of a point-focus Fresnel lens solar concentrator for high-temperature solar thermal energy system applications is discussed. The concentrator utilizes a transmittance-optimized, short-focal-length, dome-shaped refractive Fresnel lens as the optical element. This concentrator combines both good optical performance and a large tolerance for manufacturing, deflection, and tracking errors. The conceptual design of an 11-meter diameter concentrator which should provide an overall collector efficiency of about 70% at an 815 C (1500 F) receiver operating temperature and a 1500X geometric concentration ratio (lens aperture area/receiver aperture area) was completed. Results of optical and thermal analyses of the collector, a discussion of manufacturing methods for making the large lens, and an update on the current status and future plans of the development program are included.

Description: Partitioning of hydrogen chloride between the aerosol and gaseous phases in the first Space Shuttle exhaust cloud was experimentally investigated as the exhaust cloud was diluted with ambient air. Airborne measurements were obtained of gaseous hydrogen chloride (HCl), total HCl, relative humidity, and temperature to determine the conditions controlling HCl aerosol formation in the Shuttle exhaust cloud. Two segments of the cloud, each at a significantly different relative humidity, were monitored. Equilibrium predictions of HCl aerosol formation agreed with the measured HCl partitioning at the higher and lower relative humidity conditions, but do not agree at the aerosol formation threshold region. Measurements were taken in the Shuttle exhaust cloud from 8.6 min until 2 h and 8 min after launch. HCl concentrations ranged from 17.5 to 0.9 ppm and relative humidity from 86% to less than 10%.

Description: Possible damages to wind turbine components due to lightning strikes are discussed and means to prevent the damage are presented. A low resistance path to the ground is noted to be essential for any turbine system, including metal paths on nonmetal blades to conduct the strike. Surge arrestors are necessary to protect against overvoltages both from utility lines in normal operation and against lightning damage to control equipment and contactors in the generator. MOS structures are susceptible to static discharge injury, as are other semiconductor devices, and must be protected by the presence of static protection circuitry. It is recommended that the electronics be analyzed for the circuit transient response to a lightning waveform, to induced and dc current injection, that input/output leads be shielded, everything be grounded, and lightning-resistant components be chosen early in the design phase.

Description: A computer-aided-design study on the dependence of the efficiency peak of a back-surface field solar cell on the concentrations of the recombination and dopant impurities is presented. The illuminated current-voltage characteristics of more than 100 cell designs are obtained using the transmission line circuit model to numerically solve the Shockley equations. Using an AM 1 efficiency of 17% as a target value, it is shown that the efficiency versus thickness dependence has a broad maximum which varies by less than 1% over more than a three-to-one range of cell thicknesses from 30 to 100 microns. An optically reflecting back surface will give only a slight improvement of AM 1 efficiency, about 0.7%, in this thickness range. Attention is given to the dependence of the efficiency on patchiness across the back-surface field low-high junction in thin cells.

Description: An alternative propulsion subsystem for MSAT is presented which has a potential of reducing the satellite weight by more than 15%. The characteristics of pulsed plasma and ion engines are described and used to estimate of the mass of the propellant and thrusters for attitude control and stationkeeping functions for MSAT. Preliminary estimates indicate that the electric propulsion systems could also replace the large momentum wheels necessary to counteract the solar pressure; however, the fine pointing wheels would be retained. Estimates also show that either electric propulsion system can save approximately 18% to 20% of the initial 4,000 kg mass. The issues that require further experimentation are mentioned.

Description: Priorties are identified for spacecraft propulsion system development and for the integration of the propulsion system with various subsystems. Near-term and long-term propulsion technology needs are identified.

Description: Orbital construction, demonstration, space construction system analysis, solar power satellite, and space operations are reviewed. Satellite services, holding and positioning aids, and space construction experiments are discussed.

Description: The workings of systems integration, its accomplishments, the influences of its character changes on the STS, propulsion out of the orbiter and LSS, and technological demands are discussed. The task of systems integration is to define, understand, and account for interactions between the major systems on a space mission. The safety and propulsion systems and their reliability are outlined.

Description: The functions and requirements of auxiliary propulsion systems are reviewed. None of the three major tasks (attitude control, stationkeeping, and shape control) can be performed by a collection of thrusters at a single central location. If a centralized system is defined as a collection of separated clusters, made up of the minimum number of propulsion units, then such a system can provide attitude control and stationkeeping for most vehicles. A distributed propulsion system is characterized by more numerous propulsion units in a regularly distributed arrangement. Various proposed large space systems are reviewed and it is concluded that centralized auxiliary propulsion is best suited to vehicles with a relatively rigid core. These vehicles may carry a number of flexible or movable appendages. A second group, consisting of one or more large flexible flat plates, may need distributed propulsion for shape control. There is a third group, consisting of vehicles built up from multiple shuttle launches, which may be forced into a distributed system because of the need to add additional propulsion units as the vehicles grow. The effects of distributed propulsion on a beam-like structure were examined. The deflection of the structure under both translational and rotational thrusts is shown as a function of the number of equally spaced thrusters. When two thrusters only are used it is shown that location is an important parameter. The possibility of using distributed propulsion to achieve minimum overall system weight is also examined. Finally, an examination of the active damping by distributed propulsion is described.

Description: Orbit transfer vehicles (OTV) are evaluated against mission model requirement. Mission suitability of OTVs using storable propellant, cryogenic propellant, and electric propulsion systems is outlined. Energy required, g-level, spacecraft deploy/return, operational constraints, mission duration, and packaging are considered.

Description: Possible uses of satellite technology up to the year 2000 are suggested and discussed. Included are electronic mail transmission, a personal communications capability, quick location of vehicles or shipments, monitoring of disputed territorial borders, upgraded scientific exploration of the universe, providing better maps of the Earth by remote sensing, space solar power stations and the safe transmission of the electrical energy to Earth, night lighting, and a small personal navigation capability. Support requirements are outlined.

Description: Three types of cast silicon, Silso, HEM (Heat-Exchanger Method) and UCP (Ubiquitous Crystallization Process) were studied for their use as solar cells. Optical microscopy after etching revealed a high density of uniform dislocations (approaching 1,000,000/sq cm), lines of dislocations indicating stress during crystal growth, and precipitates, some of which generate dislocations. Solar cells were fabricated by three processes. Results of solar cell processing revealed that these materials produce cells of lower efficiency than Czochralski control cells, and that the efficiencies of the three materials were quite close. Diffusion length and spectral response data are shown. Certain structural features are correlated with solar cell efficiency, diffusion length, and spectral response. Electron-beam induced current (EBIC) and light spot scanning are used to back up other measurements.

Description: Silicon solar cells manufactured for the terrestrial market are examined as a potential low cost option for low earth orbit (LEO) space flight use. The results of simulated space environmental testing of representative samples are reported and discussed. It is shown that although the terrestrial cells are compatible with most space use requirements significant deficiencies still exist. Cell modifications are discussed which would enhance the space applicability of the various cells examined. In most cases these are expected to be of minimal cost impact. Concern for contact/interconnector designs capable of surviving 30,000 thermal cycles (corresponding to five years in LEO) however, needs to be resolved for the large area terrestrial devices.

Description: A configuration tradeoff study was conducted to determine optimum solar thermal parabolic dish power systems for isolated load applications. The specific application of an essentially constant power demand as required for MX missile shelters is treated. Supplying a continuous level of power with high reliability is shown to require a power system comprising modular parabolic dish power units where the heat engines of the modular power units can be driven by fossil fuels as well as solar-derived heat. Since constraints on reliability result in the provision of a power generating capability that exceeds the constant demand level, efficient utilization of the power system requires battery storage. Tradeoffs regarding the optimum size of storage are investigated as a function of the number of power modules and the cost of the fossil fuel.

Description: The High Temperature Solar Thermal Receiver, which was tested a Edwards AFB, CA during the winter of 1980-1981, evolved from technologies developed over a five year period of work. This receiver was tested at the Army Solar Furnace at White Sands, NM in 1976. The receiver, was tested successfully at 1768 deg F and showed thermal efficiencies of 85%. The results were sufficiently promising to lead ERDA to fund our development and test of a 250 kW receiver to measure the efficiency of an open cavity receiver atop a central tower of a heliostat field. This receiver was required to be design scalable to 10, 50, and 100 MW-electric sizes to show applicability to central power tower receivers. That receiver employed rectagular silicon carbide panels and vertical stanchions to achieve scalability. The construction was shown to be fully scalable; and the receiver was operated at temperatures up to 2000 deg F to achieve the performance goals of the experiment during tests at the GIT advanced components test facility during the fall of 1978.

Description: The testing and development of parabolic dish solar thermal power plants to produce, thermal mechanical, or electrical energy are discussed. The design, construction, and experiments of prototype collectors to prove the feasibility of such collectors is described.

Description: The development status and test results for the Small Community Solar Thermal Power Experiment are presented. Activities on the phase 2 power module development effort are presented with emphasis on the receiver, the plant control subsystem, and the energy transport subsystem. The components include a single prototype power module consisting of a parabolic dish concentrator, a power conversion assembly (PCA), and a multiple-module plant control subsystem. The PCA consists of a cavity receiver coupled to an organic Rankine cycle engine-alternator unit defined as the power conversion subsystem; the PCA is mounted at the focus of a parabolic dish concentrator. At a solar insolation of 100 W/sq m and ambient temperature of 28 C (82 F), the power module produces approximately 20 kW of 3-phase, 3 kHz ac power, depending on the concentrator employed. A ground-mounted rectifier to the central collection site where it is supplied directly to the common dc bus which collects the power from all modules in the plant.

Description: The design approach, rationale for the selected configuration, and the development status of a cost effective point-focus solar concentrator are discussed. The low-cost concentrator reflective surface design is based on the use of a thin, backsilvered mirror glass reflector bonded to a molded structural plastic substrate. The foundation, support, and drive subassembles are described. A hybrid, two-axis, Sun tracking control system based on microprocessor technology was selected. Coarse synthetic tracking is achieved through a microcomputer-based control system to calculate Sun position for transient periods of cloud cover as well as sundown and sunrise positioning. Accurate active tracking is achieved by two-axis optical sensors. Results of the reflective panel demonstration tests investigating slope error, hail impact survivability, temperature/humidity cycling, longitudinal strength/bending stiffness, and torsional stiffness are discussed.

Description: The status of development of the 12 m diameter parabolic dish concentrator which is planned for use with the Small Community Solar Thermal Power System. The PDC-1 unit features the use of plastic reflector film bonded to structural plastic gores supported by front-bracing steel ribs. An elevation-over-azimuth mount arrangement is employed, with a conventional wheel-and-track arrangement; outboard trunnions permit the dish to be stored in the face down position, with the added advantage of easy access to the power conversion assembly. The control system is comprised of a central computer (LSI 1123), a manual control panel, a concentrator control unit, two motor controllers, a Sun sensor, and two angular position resolvers. The system is designed for the simultaneous control of several concentrators. The optical testing of reflective panels is described.

Description: A model is presented that explains the 'flat-spot' power-loss phenomenon observed in silicon solar cells operating under deep space (low temperature, low intensity) conditions. Evidence is presented suggesting that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the PN junction is destroyed and replaced with a metal-semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallization are presented along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Preliminary data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Description: The impact of recent federal tax and regulatory legislation on the commercialization of parabolic solar reflector technology is assessed. Specific areas in need of technical or economic improvement are noted.

Description: A system feasibility test to determine the technical and operational feasibility of using a solar collector to provide industrial process heat is discussed. The test is of a solar collector system in an industrial test bed plant at Capitol Concrete Products in Topeka, Kansas, with an experiment control at Sandia National Laboratories, Albuquerque. Plant evaluation will occur during a year-long period of industrial utilization. It will include performance testing, operability testing, and system failure analysis. Performance data will be recorded by a data acquisition system. User, community, and environmental inputs will be recorded in logs, journals, and files. Plant installation, start-up, and evaluation, are anticipated for late November, 1981.

Description: the objectives and current status of the Small Community Solar Thermal Power Experiment are discussed. The adjustments in programs goals made in response to the changing emphasis in the area of solar energy in national policy are addressed. Planned fabrication and testing activities for the test bed concentrator, power conversion assembly, and control system are outlined.

Description: The design and manufacturing of a solar thermal collector is discussed. The collector has three primary subsystems: concentrator, receiver/fluid loop, and controls. Identical curved reflective columns are utilized in a faceted Fresnel design to support 864 one foot square flat inexpensive second-surface, silvered glass mirrors. The columns are ganged together and rotated through their centers of gravity to provide elevation tracking. The concentrator is supported by a lightweight spaceframe structure which distributes all wind and gravity loads to the base supports. The base of the structure is a track which rotates on wheels mounted on concrete piers. A parallel tube steel heat exchanger is mounted at the concentrator focal area in a well insulated, galvanized steel housing. Two rows of vertical close-packed, staggered tubes connect a mud header and a steam header. Automatic two axis tracking and operational control is provided with a microprocessor based package. Concentrator-mounted shadowbands are the basis for active tracking. A software program provides azimuthal tracking during cloudy periods.

Description: The solar advanced gas turbine (SAGT-1) is being developed by the Garrett Turbine Engine Company, for use in a Brayton cycle power conversion module. The engine is derived from the advanced gas turbine (AGT101) now being developd by Garrett and Ford Motor Company for automotive use. The SAGT Program is presently funded for the design, fabrication and test of one engine at Garrett's Phoenix facility. The engine when mated with a solar receiver is called a power conversion module (PCU). The PCU is scheduled to be tested on JPL's test bed concentrator under a follow on phase of the program. Approximately 20 kw of electrical power will be generated.

Description: The utilization potential of the point focusing parabolic dish is identified. Its main design parameters are summarized. Performance tests and the utilization of the collector as primary energy source in a food-water-power complex are described. Process heat, heat storage, heat transfer, and cogeneration are discussed.

Description: A discrete gust model has been designed which includes an expression for the number of times per unit time thy wind exceeds a specific value. This expression, based on Rice's (1944, 1945) number-of-crossings model, assumes that the yearly mean wind speed is averaged over a period of 10 minutes to 1 (one) hour. Vertical and lateral coherence functions are the basis for a mathematical filter which isolates atmospheric disturbances of a characteristic size (e. g., those which would completely engulf a rotor). Predictions are calculated usising the given definition of cut-off frequency, then they are compared with actual data, showing that the model is reliable. The expression is provided in a format such that it may be used for engineering design calculations.

Description: A comprehensive state-of-the-art review of solar/hydrogen technologies has been conducted. From this, solar/hydrogen production systems which could be commercialized by the year 2000 have been characterized technically and economically. Incentives and disincentives for the early commercialization of four solar/hydrogen systems have been explored, conclusions drawn and recommendations made.

Description: A new innovative planar multijunction solar cell chip for concentrated sunlight applications is proposed. The chip consists of many voltage-generating regions, called unit cells, which are connected in series within a single silicon wafer, thereby providing a high open-circuit voltage at multiple sun illumination levels. The unit cells are fabricated on 75 micron thick p-type single crystal silicon substrate. Each chip consists of 1.42 x 9.63 mm n(+)/p collecting junctions on the back of the wafer, while the illuminated front surface area is divided into 0.3 micron deep n(+) regions. The fabrication sequence includes standard degreasing and cleaning procedures, double-sided alignment photomasking, introduction of boron and phosphorus impurities, and photolithography. The open circuit voltage of the chip increased rapidly with illumination up to about 4 AM1 suns, and then began to saturate at the sum of the individual unit cell voltages of 3.5 above 4 AM1 suns. A short circuit density per unit cell of 300 mA/sq cm at 20 AM1 suns was observed.

Description: (Previously announced in STAR as N82-23711)

Description: The calibration of solar cells by means of solar simulators and calibrated reference cells has always been faced with the problem of errors due to source spectral irradiance mismatch and cell spectral response mismatch. A general expression is derived in this paper to enable calculation of mismatch error. The information required to make this calculation is the relative spectra responses of the test and reference cells, and the relative spectral irradiance of the simulator (over the spectral range defined by cell response). The spectral irradiance of the solar standard is assumed given. Experimental results are presented.

Description: A flat-plate solar collector is conceived where energy cascades through thermoelectric power modules generating direct-current electricity. The intent of this work was to choose a collector configuration and to perform a steady-state thermal performance assessment. A set of energy balance equations were written and solved numerically for the purpose of optimizing collector thermal and electrical performance. The collector design involves finned columns of thermoelectric modules imbedded in the absorber plate (hot junction) over a parallel array of vertical tubes. The thermoelectric power output is limited by the small hot-junction/cold-junction temperature difference which can be maintained under steady-state conditions. The electric power per unit tube pass area is found to have a maximum as a function of a geometric parameter, while electric power is maximized with respect to an electric resistance ratio. Although the electric power efficiency is small, results indicate that there is sufficient electric power production to drive a coolant circulator, suggesting the potential for a stand-alone system.

Description: (Previously cited in issue 19, p. 3284, Accession no. A81-40931)

Description: Results for the energy consumption analysis at the Mars deep space station are presented. It is shown that the major energy consumers are the 64-Meter antenna building and the operations support building. Verification of the antenna's energy consumption is highly dependent on an accurate knowlege of the tracking operations. The importance of a regular maintenance schedule for the watt hour meters installed at the station is indicated.

Description: The performance of a simple capacitive heat exchanger was analyzed. The shooting method for solving the governing equations is presented. Simplified linear and nonlinear models are analyzed using this method and it is shown that capacitive heat exchangers are more efficient than conventional ones.

Description: As part of the energy conversion/self sufficiency efforts of DSN engineering, it was necessary to have a simplified computer model of a solar photovoltaic (PV) system. This article describes the analysis and simplifications employed in the development of a PV cell array computer model. The analysis of the incident solar radiation, steady state cell temperature and the current-voltage characteristics of a cell array are discussed. A sample cell array was modelled and the results are presented.

Description: Some performance requirements and development needs for the design of large space structures are described. Areas of study include: (1) dynamic response of large space structures; (2) structural control and systems integration; (3) attitude control; and (4) large optics and flexibility. Reference is made to a large space telescope.

Description: The constraints placed on the design of large space structures by acceleration, attitude control, and stationkeeping forces are discussed. Stiffness requirements for the structures are derived. The use of active versus passive accuracy control methods is also addressed.

Description: The low Earth orbit (LEO) versus the geosynchronous Earth orbit (GEO) deployment systems were discussed. The following items are emphasized: (1) large area systems, such as deployment, altitude, orbit transfer, and on orbit operation; (2) propulsion oriented issues such as the orbit transfer vehicle and the auxiliary propulsion systems; (3) programmatic issues. The LEO versus GEO deployment is a significant driver on propulsion requirements for an orbit transfer vehicle. It is suggested that early systems will be deployed at LEO. The Shuttle remote manipulator system (RMS) will not be involved in early demonstration activities. It is concluded that an integrated large space system/propulsion approach is essential and that propulsion development requirements need to be established.

Description: The effects of low-thrust primary propulsion system characteristics on the mass, area, and orbit transfer characteristics of large space systems (LSS) were determined. Three general structural classes of LSS were considered, each with a broad range of diameters and nonstructural surface densities. While transferring the deployed structure from LEO and to GEO, an acceleration range of 0.02 to 0.1 g's was found to maximize deliverable payload based on structural mass impact. After propulsion system parametric analyses considering four propellant combinations produced values for available payload mass, length and volume, a thrust level range which maximizes deliverable LSS diameter was determined corresponding to a structure and propulsion vehicle. The engine start and/or shutdown thrust transients on the last orbit transfer (apogee) burn can impose transient loads which would be greater than the steady-state loads at the burnout acceleration. The effect of the engine thrust transients on the LSS was determined from the dynamic models upon which various engine ramps were imposed.

Description: Studies determined that shuttle optimized design, allowing the large space system and transfer vehicle in one shuttle flight, greatly reduces transportation costs and minimizes orbital operations. Careful attention to design resulted in efficient payload packaging. A minimum volume, high energy (liquid oxygen and liquid hydrogen) transfer vehicle is described that allows maximum volume for the payload in the orbiter cargo bay.

Description: Power systems integration in large flexible space structures is discussed with emphasis upon body control. A solar array is discussed as a typical example of spacecraft configuration problems. Information on how electric batteries dominate life-cycle costs is presented in chart form. Information is given on liquid metal droplet generators and collectors, hot spot analysis, power dissipation in solar arrays, solar array protection optimization, and electromagnetic compatibility for a power system platform.

Description: A rigid body analysis of a baseline Large Space System (LSS) which is to function as a radiometer is presented. The LSS is placed in circular orbit about the Earth at an altitude of 650 km, subjected to environmental and vehicle interaction forces and torques, without an active control system of any type on board. The environment forces and torques are gravity gradient, solar radiation, and aerodynamic. Normal operation is in nadir pointing along the Z-local vertical axis. Orbital velocity is assigned to the x-axis of the spacecraft. The analysis is then used to demonstrate the ability or lack of the gravity gradient torques to stabilize the LSS over one complete orbit.

Description: Requirements of future space systems, including large space systems, that operate beyond the space shuttle are discussed. Typical functions required of propulsion systems in this operational regime include payload placement, retrieval, observation, servicing, space debris control and support to large space systems. These functional requirements are discussed in conjunction with two classes of propulsion systems: (1) primary or orbit transfer vehicle (OTV) and (2) secondary or systems that generally operate within or relatively near an operational base orbit. Three propulsion system types are described in relation to these requirements: cryogenic OTV, teleoperator maneuvering system and a solar electric OTV.

Description: The dynamics of the interaction of space structures and their propellant systems are outlined. Optimization for a platform type of space structure is discussed. Static and transient loads, propellant accelerations, tolerances, attitude control, and distributed thrust are considered.

Description: Electric propulsion systems for transferring large payload masses to geosynchronous Earth orbits and providing accurate on-orbit stationkeeping are evaluated. Orbit boosting, inclination change, attitude control, stationkeeping, relocation, disposal, and power sharing on orbits using electric propulsion are compared with the use of chemical propulsion.

Description: A workshop panel defined missions that would require the potential use of very large, advanced space systems, and ranked them on the basis of need from both a civilian and military perspective. The panel also pointed out those that would need advanced propulsion technology. Surveillance, communications, and defense were given the highest priority, followed by command and control, orbital support, terrestrial support, and space science.

Description: Potential mission opportunities outlined and illustrated are missile defense; space defense; command, control, and communications; defense suppression; force support; space transportation; and orbital support.

Description: The overall technology model is outlined and the objectives and descriptions of the primary and secondary propulsion drives presented. The primary propulsion driver missions are the geostationary platform; the coherent optical system of modular imaging collectors (COSMIC); the 100 meter thinned aperature telescope; and the orbiting deep space relay station (ODSRS). The secondary propulsion driver missions are space platform alpha, the space station, and the automated planetary station.

Description: (Previously cited in issue 14, p. 2320, Accession no. A81-32905)

Description: The Jet Propulsion Laboratory has developed a number of photovoltaic test and measurement specifications to guide the development of modules toward the requirements of future large-scale applications. Experience with these specifications and the extensive module measurement and testing that has accompanied their use is examined. Conclusions are drawn relative to three aspects of product certification: performance measurement, endurance testing and safety evaluation.

Description: Layered crystals of the transition metal thiophosphates were synthesized and characterized for use as photoelectrodes in photoelectrochemical cells. Crystals incorporating tin and manganese show n-type response while those with iron and nickel show p-type response. These materials have a measured indirect bandgap of about 2.1 eV. They show ability to photoelectrolyze water in acid solutions with onset potentials which change in a Nernstian way as the PH of the solution changes. The onset potential is near zero volts versus a saturated calomel electrode at pH 2. At n-type crystals, oxygen could be evolved upon irradiation at underpotentials of 850 mV and at p-type crystals, hydrogen could be evolved at underpotentials of 400 mV, indicating a net gain in energy conversion. All crystals were unstable in basic solution. Liquid junction photovoltaic cells in iodide-triiodide acid solution using these layered materials were also constructed and found to have low efficiences.

Description: Industrial support was the subject of a panel discussion on solar energy technology. Members of various energy-related businesses and agencies were present. Topics cofered include: (1) solar collectors; (2) solar energy policy; (3) government/industry relations; and (4) economic factors which influence the use of solar energy.

Description: The development of a natural gas-fired Brayton engine is discussed. It is intended to be the prime mover for a 10-ton commercial heat pump. This engine has many attractive features that make it an ideal candidate for solar thermal-power generation applications. The unique feature of this engine is its subatmospheric mode of operation. It operates between atmospheric pressure and a partial vacuum. This means that heat is added to the cycle at atmospheric pressure; this permits the receiver to be unpressurized, greatly simplifying its design and cost.

Description: The development and testing of a rigid panel concept that utilizes a thin film reflective surface for application to a low-cost point-focusing solar concentrator is discussed. It is shown that a thin film reflective surface is acceptable for use on solar concentrators, including 1500 F applications. Additionally, it is shown that a formed steel sheet substrate is a good choice for concentrator panels. The panel has good optical properties, acceptable forming tolerances, environmentally resistant substrate and stiffeners, and adaptability to low to mass production rates. Computer simulations of the concentrator optics were run using the selected reflector panel design. Experimentally determined values for reflector surface specularity and reflectivity along with dimensional data were used in the analysis. The simulations provided intercept factor and net energy into the aperture as a function of aperture size for different surface errors and pointing errors. Point source and Sun source optical tests were also performed.

Description: An advanced state-of-the-art steam turbine-generator developed to serve as the power conversion subsystem for the Department of Energy's Sandia National Laboratories' Solar Total-Energy Project (STEP) is described. The turbine-generator, which is designed to provide 400-kW of net electrical power, represents the largest turbine-generator built specifically for commercial solar-powered cogeneration. The controls for the turbine-generator incorporate a multiple, partial-arc entry to provide efficient off-design performance, as well as an extraction control scheme to permit extraction flow regulation while maintaining 110-spsig pressure. Normal turbine operation is achieved while synchronized to a local utility and in a stand-alone mode. In both cases, the turbine-generator features automatic load control as well as remote start-up and shutdown capability. Tests totaling 200 hours were conducted to confirm the integrity of the turbine's mechanical structure and control function. Performance tests resulted in a measured inlet throttle flow of 8,450 pounds per hour, which was near design conditions.

Description: Airborne measurements of two types of cloud nuclei, cloud condensation nuclei (CCN) and ice nuclei (IN), were conducted in the stabilized ground clouds which resulted from the launches of a liquid-fueled ATLAS/Centaur rocket and a solid-fueled TITAN III rocket. Results show that the concentrations of CCN in both types of clouds were greater than ambient values for the 2 hours duration of the measurements. The initial production of CCN active at 0.5% supersaturation in the ATLAS and TITAN clouds was found to be equivalent to a 20 and 700 sec emission, respectively, by the city of Denver, Colorado. After the initial production, the clouds continued to generate CCN at a rate of about 1/cu cm sec. However, concentrations of IN in the ATLAS cloud were greater than ambient values for only a short period after launch, and it appears that the nuclei resulted from entrained launch pad and ground debris. The concentrations of IN in the TITAN cloud were found to be at or below ambient levels until about 2 hours after launch when they increased substantially above ambient values.

Description: The significance of including factors such as the base resistivity loss for solar cells thicker than 100 microns and emitter and BSF layer recombination for thin cells in predicting the fill factor and efficiency of solar cells is demonstrated analytically. A model for a solar cell is devised with the inclusion of the dopant impurity concentration profile, variation of the electron and hole mobility with dopant concentration, the concentration and thermal capture and emission rates of the recombination center, device temperature, the AM1 spectra and the Si absorption coefficient. Device equations were solved by means of the transmission line technique. The analytical results were compared with those of low-level theory for cell performance. Significant differences in predictions of the fill factor resulted, and inaccuracies in the low-level approximations are discussed.

Description: Material imperfections, impurity clusters and fabrication defects across the back-surface-field junction can degrade the performance of high-efficiency solar cells. The degradation from defects appearing on the circumference of a solar cell is analyzed using a two-region developed perimeter device model. The width of the defective perimeter region is characterized by the range or the distance-of-influence of the defective edge and this width is about two diffusion lengths. The defective edge is characterized by a surface recombination velocity. Family of theoretical curves and numerical examples are presented to show that significant reduction of open-circuit voltage can occur in high-efficiency cells which are thin compared with the diffusion length. In one example, the degradation is decreased from 135 mV to 75 mV when the cell size is increased from 10 to 100 times the diffusion length in a thin cell whose thickness is 1% of the diffusion length.

Description: An analytic technique which determines the variation of solar irradiance from long term averages is presented. The technique involves computer-assisted data reduction techniques, and was designed to improve system reliability by determining the amount of storage capability required to supplement a baseline system. Variations in time intervals of up to 60 days can be determined, and 10 years of data collection are reviewed. The technique involves first calculating average monthly irradiance values, then examining the average irradiance deviation over time intervals. The calculation procedure is clarified by determining solar energy level probabilities and the long term solar energy deviation (achieved by repeatedly integrating actual irradiance figures). It is found that a 15% increase in collector area and the addition of energy storage or backup are essential contributions to achieving cost-effectiveness. In addition, one to seven no-sun day storage capacities are required to accommodate weather caused deficits.

Description: Reliability attributes are being developed on a wide variety of advanced single-crystal silicon solar cells. Two separate investigations: cell-contact integrity (metal-to-silicon adherence), and cracked cells identified with fracture-strength-reducing flaws are discussed. In the cell-contact-integrity investigation, analysis of contact pull-strength data shows that cell types made with different metallization technologies, i.e., vacuum, plated, screen-printed and soldered, have appreciably different reliability attributes. In the second investigation, fracture strength was measured using Czochralski wafers and cells taken at various stages of processing and differences were noted. Fracture strength, which is believed to be governed by flaws introduced during wafer sawing, was observed to improve (increase) after chemical polishing and other process steps that tend to remove surface and edge flaws.

Description: The placement of photovoltaic modules in various applications, in climates and locations throughout the world, results in different degrees and combinations of environmental and electrical stress. Early detection of module reliability deficiencies via laboratory testing is necessary for achieving long, satisfactory field service. This overview paper describes qualification testing techniques being used in the US Department of Energy's flat-plate terrestrial photovoltaic development program in terms of their significance, rationale for specified levels and durations, and test results.

Description: The development of an accelerated test schedule for terrestrial solar cells is described. This schedule, based on anticipated failure modes deduced from a consideration of IC failure mechanisms, involves bias-temperature testing, humidity testing (including both 85-85 and pressure cooker stress), and thermal-cycle thermal-shock testing. Results are described for 12 different unencapsulated cell types. Both gradual electrical degradation and sudden catastrophic mechanical change were observed. These effects can be used to discriminate between cell types and technologies relative to their reliability attributes. Consideration is given to identifying laboratory failure modes which might lead to severe degradation in the field through second quadrant operation. Test results indicate that the ability of most cell types to withstand accelerated stress testing depends more on the manufacturer's design, processing, and worksmanship than on the particular metallization system. Preliminary tests comparing accelerated test results on encapsulated and unencapsulated cells are described.

Description: An overview of the photovoltaic array reliability problem is presented, and a high reliability/minimum cost approach to this problem is presented. Design areas covered are cell failure, interconnect fatigue, and electrical insulation breakdown, and three solution strategies are discussed. The first involves controlling component failures in the solar cell (cell cracking, cell interconnects) and at the module level (must be statistically treated). Second, a fault tolerant circuit is designed which reduces array degradation, improves module yield losses, and controls hot-spot heating. Third, cost optimum module replacement strategies are also effective in reducing array degradation. This can be achieved by minimizing the life-cycle energy cost of the photovoltaic system. The integration of these solutions is aimed at reducing the 0.01% failure rate.

Description: During the first four years of the U.S. Department of Energy (DOE) National Photovoltatic Program, the Jet Propulsion Laboratory Low-Cost Solar Array (LSA) Project purchased about 400 kW of photovoltaic modules for test and experiments. In order to identify, report, and analyze test and operational problems with the Block Procurement modules, a problem/failure reporting and analysis system was implemented by the LSA Project with the main purpose of providing manufacturers with feedback from test and field experience needed for the improvement of product performance and reliability. A description of the more significant types of failures is presented, taking into account interconnects, cracked cells, dielectric breakdown, delamination, and corrosion. Current design practices and reliability evaluations are also discussed. The conducted evaluation indicates that current module designs incorporate damage-resistant and fault-tolerant features which address field failure mechanisms observed to date.