Indoor and outdoor aging tests are common methods for PV module degradation investigation. But to what extend are accelerated indoor aging tests comparable to outdoor exposure tests? The impact of indoor and outdoor tests on the polymer degradation in full-size PV modules was investigated. Polymer aging within a PV module is one of the major factors influencing module performance in the course of its lifetime. Degradation phenomena like yellowing, delamination or changes in the elastic modulus of the encapsulation may lead to transmission losses, corrosion effects or cell cracks. Raman Spectroscopy has recently been reported by our group as a non-destructive, analytical method for encapsulation degradation analysis. The degradation of the encapsulation of indoor and outdoor aged crystalline silicon PV modules was examined by the means of Raman Spectroscopy with special attention to the spatial-dependency of the degradation. The investigated modules were subjected to several different accelerated aging procedures with a systematic variation of the climatic conditions temperature, humidity and UV. Identical modules were aged in different climates (arid, tropical, urban and alpine) for up to three years. The degradation of the encapsulant was observed, resulting in an increasing fluorescence background in the Raman spectra. A dependency of the aging process on the relative position to the edges of the cell was found. The aging conditions appeared to influence the spatial distribution of the fluorescence and therefore, the polymer degradation, markedly. Furthermore, correlations between accelerated aging tests and outdoor exposure tests could be found.
The degradation of the inorganic components in a PV module is, besides polymer degradation, one of the most important aspects of PV module aging. Especially the corrosion of the cell metallization may lead to significant decreases in PV module performance. But in which way the metallization corrosion is affected by the permeation of atmospheric gases is not understood, yet. In order to investigate this permeation impact, laminates with a systematic variation of back-sheet and encapsulation materials as well as different laminate set-ups were made. Two different kinds of encapsulation (EVA and PVB) and four different back-sheet materials (TAPT, PA and two different TPT foils) were used. Standard cells with a two and three bus bar set-up were used. The laminates were subjected to damp-heat aging tests with a relative humidity of 80% at 80°C and 90°C, respectively. The degradation was investigated by means of electroluminescence imaging, Raman spectroscopy and microscopy. Special attention was paid to the spatial distribution of corrosion effects on the cell. Furthermore, the occurrence of a typical damp-heat induced damage, apparent as a shaded area in the electroluminescence images, should be investigated. A corrosion of the grid and the ribbons could be observed. EDX measurements revealed the grid corrosion to go along with the formation of needles of lead compounds from the silver paste.
Accelerated testing of the durability of materials exposed to natural weathering requires testing of the UV stability,
especially for polymeric materials. The type approval testing of PV-modules according to the standards IEC 61215 and
IEC 61646 includes a so-called UV-preconditioning test with a total UV dose of 15 kWh/m2.
Fraunhofer ISE performed an Inter-laboratory comparison of UV-light sources in accredited test labs and in test centres
of major PV module manufacturers. One topic was the spectral measurement of the used UV sources. Another main
issue was the comparison of the integral measurements by the sensors used for control of the tests. Errors up to 120%
were found.
KEYWORDS: Solar cells, Climatology, Sensors, Reliability, Temperature metrology, Snow cover, 3D imaging standards, Photovoltaics, Solar energy systems, Corrosion
Fraunhofer ISE is running a PV-module outdoor testing set-up on the Gran Canaria island, one of the Canary Island
located west of Morroco in the Atlantic Ocean. The performance of the modules is assessed by IV-curve monitoring
every 10 minutes. The electronic set-up of the monitoring system - consisting of individual electronic loads for each
module which go into an MPP-tracking mode between the IV-measurements - will be described in detail.
Soiling of the exposed modules happened because of building constructions nearby. We decided not to clean the
modules, but the radiation sensors and recorded the decrease of the power output and the efficiency over time. The
efficiency dropped to 20 % within 5 months before a heavy rain and subsequently the service personnel on site cleaned
the modules.
A smaller rain-fall in between washed the dust partly away and accumulated it at the lower part of the module, what
could be concluded from the shape of the IV-curves, which were similar to partial shading by hot-spot-tests and by
partial snow cover.
KEYWORDS: Thin films, Amorphous silicon, Temperature metrology, Climatology, Aluminum, Solar cells, Polymers, Data corrections, Current controlled current source, Silicon
Various types of modules were installed on a outdoor test facility. IV-curves of all these modules are measured every ten
minutes.
Measured irradiation, module temperature and air mass are used for analysis and comparison of the IV-curves. The
influence of the respective air mass was higher than expected.
Different correction procedures have been compared. Best results are achieved when the data are selected in a small
range of air mass and module temperature and corrected to the same irradiation.
Modules of the same types like the outdoor exposed modules were stored without irradiation for some years. Afterwards,
they were exposed at the outdoor test site together with the other modules. Comparison of the module data of the same
type allows the investigation of the stabilisation behaviour of the modules directly.
Stabilized performance parameters of PV-modules are necessary for energy yield prediction as well as for the
investigation of module degradation effects. The electrical parameters of thin-film modules show stabilization behaviors
which are typical for the applied technology. However, this behavior is not satisfyingly understood yet.
Different types of thin-film modules have been exposed to artificial irradiation and controlled temperatures in a climatic
cabinet with a class B solar simulator for up to 330h. The modules have been connected to electronic loads to perform
IV-curve measurements every 15 minutes and MPP-tracking between the measurements.
The stabilization of the different parameters (Uoc, Isc, FF, Pmpp) has been analyzed using this data. Temperature
correction was done with temperature coefficients which have been measured after a certain irradiation dose had been
applied.
Flasher-measurements have been used for confirmation of the DC-measurements after the relaxation of the modules after
the continuous irradiation exposure was finished.
Temperature cycling tests are part of the IEC 61215 qualification testing of crystalline silicon (c-Si) PV modules for
evaluating PV module degradation caused by the impact of thermo-mechanically induced stresses. The defined
temperature gradient and the cycle time by far exceed the actual impact of natural weathering, however. As a
contribution to comparisons between laboratory testing and natural weathering our work provides data from standard
temperature cycling tests as defined in IEC 61215 and extended from 200 (standard) to 800 cycles. The results of these
tests for seven commercial c-Si PV modules from various manufacturers are compared with results from identical
module types exposed outdoors in different climates for a period of 3 years. Degradation effects are evaluated with
respect to changes in output power, changes in insulation properties and with respect to interruptions in the electrical
interconnection circuits such as cell interconnects. Temperature gradients obtained at the different exposure locations are
used to model the thermo-mechanical stress arising from the mismatches of the thermal expansion coefficients of the
employed materials.
This paper is dealing with measuring the water ingress to PV-modules under different climatic conditions and simulating
it over an expected lifetime of 20 years. Furthermore different kinds of back sheet / encapsulant combinations were
considered.
For this purpose an own developed high sensitive test device composed of a climate cabinet and a mass spectrometer was
used to investigate the temperature dependent permeation and diffusion processes that takes place in the polymers.
Furthermore the results - permeation and diffusion coefficient - were used to simulate the mass transport through the
back sheet and inside the encapsulant materials.
In a second part the amount of absorbed water was analysed by a gravimetric method. For this study encapsulant
materials were exposed to different ambient climates to quantify the sorption isotherme. With these results - permeation
/ diffusion coefficient and absorbed water amount - the mass transport and water concentration were simulated over the
lifetime of a PV-module under different climatic conditions. Additional the influence of various tight back sheets on
water ingress will be shown.
PV modules have to have a service lifetime of more than 20 years. It is hard to follow suitable degradation indicators
during service life testing with sufficient accuracy for reliable service life estimation. Often the polymeric encapsulation
material, mostly ethylene vinyl acetate, shows degradation effects. The detection of small changes of the material in a
non-destructive manner helps to follow the changes over time during indoor testing.
PV modules with crystalline Si-cells of seven German manufacturers were analyzed after accelerated ageing tests with
Raman spectroscopy. This technology allows non-destructive measurements of the encapsulation material through the
glazing so that the degradation of the samples can be followed by measuring after different exposure times.
Samples had been exposed to damp-heat conditions for up to 4000 h.
The results show significant differences in the materials degradation above the edges and the center of the cell. With
increasing exposure times, it becomes apparent that the degradation process starts near the edges of the cells and
propagates towards the center, indicating the impact of diffusion processes.
During the past 3 years crystalline PV modules fabricated by various German manufacturers have been exposed to
outdoor conditions in four different climates: warm moderate climate (Cologne, Germany), tropical climate (Serpong,
Indonesia), cold high mountain climate (Zugspitze, Germany) and arid conditions (Sede Boqer, Israel). Annual
inspections and measurements examined the degradation of these modules with respect to electrical performance,
mechanical condition and visible alterations. We give a detailed report of the results after 3 years of weathering, along
with the outlook for an extension to new worldwide test locations and for the enhancement of measurement options for
long-term characterizations.
KEYWORDS: Amorphous silicon, Thin films, Temperature metrology, Climatology, Silicon, Crystals, Thin film solar cells, Photovoltaics, Optical filters, Linear filtering
Stabilized electrical performance data are necessary to compare different types of modules in the emerging thin-film-PV
market and as basic information for energy yield calculations. The problems with accurate power measurements of thin
film modules are well known. The module test-standard IEC 61646 ed. 2 tries to take this into account by demanding
light-soaking and repeated STC-measurements which leads to time analysed procedures.
The stabilisation behaviour over time of short circuit current, open-circuit voltage, efficiency and filling-factor is
compared under the influence of different illumination conditions for various types of CdTe, CI(G)S and a-Si modules.
Therefore, I-V-curves are measured with high frequency during outdoor exposition and indoor exposition to 1000 W/m2
irradiation from a class B solar simulator in a climatic cabinet under temperature-controlled conditions. The different
modules are held in MPP conditions between the measurements. The results are compared with STC measurements
according to IEC 61646 procedures. To describe the development of the performance of the different types of thin film
modules, suitable mathematical approaches are taken to describe the different developments during the process of light
soaking.
It turns out that the different module types behave very differently and some types need very long times until a stabilised
state is reached.
The dynamic behaviour of modules with different designs and sizes is analyzed with different methods. Outdoor measurements
of the deflection show their dynamic behaviour under wind loads and the correlation between wind velocity
and deflection. Indoor tests were performed with acoustic excitation of the modules with monitoring the deflection. Numerical
calculations, based on FEM-modelling, showed that their resonance frequencies are typically in the range from 1
to 100 Hz.
Results of the indoor and outdoor measurements are reported and compared with the numerical results of the FEM-simulation.
The estimation of PV-modules lifetime facilitates the further development and helps to lower risks for producers and
investors. One base for this extensive testing and simulation work is the knowledge of the chemical degradation
processes and their kinetics, as well as of the permeation of water and oxygen into the module, especially of the
encapsulant. Besides ethylen-vinylacetate copolymer (EVA), which is the dominant material for encapsulation, new
materials become available and need the assessment of their properties and the durability impact.
Accelerated durability tests were performed on different EVA materials. The paper reports on several measurement
methods for analysis of the polymers that were used, FT-IR with attenuated total reflection (ATR), and Raman
microscopy, e.g. It is very important to identify degradation products and intermediates in order to identify the leading
degradation processes and their kinetics as well as potential interactions between different processes.
Another important factor for the degradation of the PV-modules and the concerned polymers in particular is the
permeation of reactive substances, especially of water vapor, into and inside the modules. The paper shows results of
permeation measurements of the new materials, as well as FEM-based numerical simulations of the humidity diffusion
within a PV-module what is an important step towards the calculation of the chemical degradation using numerical
simulation tools in the future.
Manufacturers of PV-modules usually give a warranty for at least 20 years. There is still only little knowledge about the
lifetime of newly developed modules, however. How do they cope with snow, desert-climate or tropical humidity? In
order to answer this question the Fraunhofer-Institute for Solar Energy Systems and TUV Rheinland have installed
different outdoor exposure sites where modules have to stand extreme climates: high temperatures with high differences
between day and night in the Negev desert at Israel, snow, wind and changing irradiation in the German Alps, and high
humidity at warm temperatures at Indonesia.
Commercial modules from industrial partners as well as innovative modules with different combinations of encapsulants
and back-sheets were exposed. UV-irradiation, solar-irradiation, ambient- and module temperatures, ambient humidity
and wind speed is measured and collected at a central server in Germany. These data are the basis for the calculation of
integral loads for the comparison of different climatic regions and for an estimation of the service life, an exciting field
of work since decades. Results from the evaluation of the monitoring during the fist 12 months of exposure are
compared.
Fluorescent lamps are chosen for accelerated UV-testing, since they simulate the UV-irradiation of the sun well while
emitting less thermal radiation than Xenon-lamps. The UV-source is designed for use in climatic cabinets for damp-heat testing with UV.
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