Engineering Papers⌕ Search

Engineering topics

Wohlgemuth, J.

Publications and source records attributed to Wohlgemuth, J..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Evaluation of Dynamic Mechanical Loading as an Accelerated Test Method for Ribbon Fatigue

Dynamic Mechanical Loading (DML) of photovoltaic modules is explored as a route to quickly fatigue copper interconnect ribbons. Results indicate that most of the interconnect ribbons may be strained through module mechanical loading to a level that will result in failure in a few hundred to thousands of cycles. Considering the speed at which DML may be applied, this translates into a few hours o testing. To evaluate the equivalence of DML to thermal cycling, parallel tests were conducted with thermal cycling. Preliminary analysis suggests that one +/-1 kPa DML cycle is roughly equivalent to one standard accelerated thermal cycle and approximately 175 of these cycles are equivalent to a 25-year exposure in Golden Colorado for the mechanism of module ribbon fatigue.

14 SOLAR ENERGY↗
DOE OSTI2015

Round-Robin Verification and Final Development of the IEC 62788-1-5 Encapsulation Size Change Test; NREL (National Renewable Energy Laboratory)

Polymeric encapsulation materials may a change size when processed at typical module lamination temperatures. The relief of residual strain, trapped during the manufacture of encapsulation sheet, can affect module performance and reliability. For example, displaced cells and interconnects threaten: cell fracture; broken interconnects (open circuits and ground faults); delamination at interfaces; and void formation. A standardized test for the characterization of change in linear dimensions of encapsulation sheet has been developed and verified. The IEC 62788-1-5 standard quantifies the maximum change in linear dimensions that may occur to allow for process control of size change. Developments incorporated into the Committee Draft (CD) of the standard as well as the assessment of the repeatability and reproducibility of the test method are described here. No pass/fail criteria are given in the standard, rather a repeatable protocol to quantify the change in dimension is provided to aid those working with encapsulation. The round-robin experiment described here identified that the repeatability and reproducibility of measurements is on the order of 1%. Recent refinements to the test procedure to improve repeatability and reproducibility include: the use of a convection oven to improve the thermal equilibration time constant and its uniformity; well-defined measurement locations reduce the effects of sampling size -and location- relative to the specimen edges; a standardized sand substrate may be readily obtained to reduce friction that would otherwise complicate the results; specimen sampling is defined, so that material is examined at known sites across the width and length of rolls; and encapsulation should be examined at the manufacturer’s recommended processing temperature, except when a cross-linking reaction may limit the size change. EVA, for example, should be examined 100 °C, between its melt transition (occurring up to 80 °C) and the onset of cross-linking (often at 100 °C).

14 SOLAR ENERGY↗
DOE OSTI2015

Overview of PVQAT: Update and Perspectives (Presentation)

The presentation provides an overview of PVQAT: International PV Quality Assurance Task Force - historical perspective, projects, climate - specific (use - environment -specific) durability testing, consistency of manufacturing, system verification, IECRE vs PVQAT, how to become involved, current status and multiyear targets, goals for today.

14 SOLAR ENERGY↗
DOE OSTI2014

International PV QA Task Force's Proposed Comparative Rating System for PV Modules: Preprint

The International PV Quality Assurance Task Force is developing a rating system that provides comparative information about the relative durability of PV modules. Development of accelerated stress tests that can provide such comparative information is seen as a major step toward being able to predict PV module service life. This paper will provide details of the ongoing effort to determine the format of such an overall module rating system. The latest proposal is based on using three distinct climate zones as defined in IEC 60721-2-1 for two different mounting systems. Specific stresses beyond those used in the qualification tests are being developed for each of the selected climate zones.

14 SOLAR ENERGY↗
DOE OSTI2014

Trial Run of a Junction-Box Attachment Test for Use in Photovoltaic Module Qualification (Presentation)

Engineering robust adhesion of the junction-box (j-box) is a hurdle typically encountered by photovoltaic (PV) module manufacturers during product development and manufacturing process control. There are historical incidences of adverse effects (e.g., fires), caused when the j-box/adhesive/module system has failed in the field. The addition of a weight to the j-box during the 'damp-heat', 'thermal-cycle', or 'creep' tests within the IEC qualification protocol is proposed to verify the basic robustness of the adhesion system. The details of the proposed test are described, in addition to a trial run of the test procedure. The described experiments examine 4 moisture-cured silicones, 4 foam tapes, and a hot-melt adhesive used in conjunction with glass, KPE, THV, and TPE substrates. For the purpose of validating the experiment, j-boxes were adhered to a substrate, loaded with a prescribed weight, and then subjected to aging. The replicate mock-modules were aged in an environmental chamber (at 85 deg C/85% relative humidity for 1000 hours; then 100 degrees C/<10% relative humidity for 200 hours) or fielded in Golden, Miami, and Phoenix for 1 year. Attachment strength tests, including pluck and shear test geometries, were also performed on smaller component specimens.

14 SOLAR ENERGY↗
DOE OSTI2013

Examination of a Standardized Test for Evaluating the Degree of Cure of EVA Encapsulation (Presentation)

The curing of cross-linkable encapsulation is a critical consideration for photovoltaic (PV) modules manufactured using a lamination process. Concerns related to ethylene-co-vinyl acetate (EVA) include the quality (e.g., expiration and uniformity) of the films or completion (duration) of the cross-linking of the EVA within a laminator. Because these issues are important to both EVA and module manufacturers, an international standard has recently been proposed by the Encapsulation Task-Group within the Working Group 2 (WG2) of the International Electrotechnical Commission (IEC) Technical Committee 82 (TC82) for the quantification of the degree of cure for EVA encapsulation. The present draft of the standard calls for the use of differential scanning calorimetry (DSC) as the rapid, enabling secondary (test) method. Both the residual enthalpy- and melt/freeze-DSC methods are identified. The DSC methods are calibrated against the gel content test, the primary (reference) method. Aspects of other established methods, including indentation and rotor cure metering, were considered by the group. Key details of the test procedure will be described.

14 SOLAR ENERGY↗
NASA NTRS1981

Coplanar back contacts for thin silicon solar cells

The type of coplanar back contact solar cell described was constructed with interdigitated n(+) and p(+) type regions on the back of the cell, such that both contacts are made on the back with no metallization grid on the front. This cell construction has several potential advantages over conventional cells for space use namely, convenience of interconnects, lower operating temperatures and higher efficiency due to the elimination of grid shadowing. However, the processing is more complex, and the cell is inherently more radiation sensitive. The latter problem can be reduced substantially by making the cells very thin (approximately 50 micrometers). Two types of interdigitated back contact cells are possible, the types being dependent on the character of the front surface. The front surface field cell has a front surface region that is of the same conductivity type as the bulk but is more heavily doped. This creates an electric field at the surface which repels the minority carriers. The tandem junction cell has a front surface region of a conductivity type that is opposite to that of the bulk. The junction thus created floats to open circuit voltage on illumination and injects carriers into the bulk which then can be collected at the rear junction. For space use, the front surface field cell is potentially more radiation resistant than the tandem junction cell because the flow of minority carriers (electrons) into the bulk will be less sensitive to the production of recombination centers, particularly in the space charge region at the front surface.

Storti, G.↗
NASA NTRS1981

The gridded back contact and its effect on solar cell performance

The effect of using gridded back contacts on silicon solar cells made with Al baked surface field (BSF) was experimentally investigated and found to result in a significant increase in conversion efficiency due to higher short-circuit current. An increase of 5 to 8% was noted for thin cells while a smaller increase was observed for standard thickness cells. Several possible explanatory mechanisms are suggested and summarized, including stress relief, incomplete BSF formation, improved back surface recombination velocity, and improved optical reflection off the back silicon-air interface.

Giuliano, M.↗
NASA NTRS1980

Analysis of the effects of impurities in silicon

A solar cell fabrication and analysis program was conducted to determine the effects on the resultant solar cell efficiency of impurities intentionally incorporated into silicon. It was found that certain impurities such as titanium, tantalum, and vanadium were bad, even in very small concentrations. Cell performance appeared relatively tolerable to impurities such as copper, carbon, calcium, chromium, iron and nickel (in the concentration levels which were considered).

Wohlgemuth, J.↗
NASA NTRS1980

Short-circuit current improvement in thin cells with a gridded back contact

The use of gridded back contact on thin silicon solar cells 50 micrometers was investigated. An unexpected increase in short circuit current of almost 10 percent was experienced for 2 cm x 2 cm cells. Control cells with the standard continuous contact metallization were fabricated at the same time as the gridded back cells with all processes identical up to the formation of the back contact. The gridded back contact pattern was delineated by evaporation of Ti-Pd over a photo-resist mask applied to the back of the wafer; the Ti-Pd film on the controls was applied in the standard fashion in a continuous layer over the back of the cell. The Ti-Pd contacts were similarly applied to the front of the wafer, and the grid pattern on both sides of the cell was electroplated with 8-10 micrometers of silver.

Giuliano, M.↗
NASA NTRS1980

Radiation tolerance of vertical junction solar cells

Extensive radiation testing of vertical junction (VJ) solar cells demonstrated a radiation tolerance better than both planar silicon cells and at least one type of (AlGa)As-GaAs cell. Due to tradeoffs between short circuit current and open circuit voltage, the end of life (10 to the 16th power 1 MeV electrons/sq cm) maximum power point is nearly independent of bulk resistivity between 2 and 10 ohm cm, increases slightly with increasing wafer thickness between 3 and 11 mils, and increases slightly with increasing groove depth between 1 and 3 mils.

Schelnine, A.↗
NASA NTRS1980

High efficiency ultrathin coplanar back contact cells

Efforts to fabricate high efficiency, ultrathin coplanar back contact cells are described. Included is a description of design considerations, cell fabrication, and theoretical and experimental analyses of loss mechanisms. The results of these efforts has been the fabrication of a 11.8% AM0 efficient, 50 micron cell when measured at 25 C. Design and process changes required to increase the efficiency are indicated.

Storti, G.↗