Reduced optical transmittance of encapsulants resulting from ultraviolet (UV) degradation is frequently identified as a cause of decreased performance through the service life of photovoltaic modules. However, the present module safety and qualification standards apply short UV doses, only capable of examining design robustness and “infant mortality” failures. Furthermore, essential information remains unknown that might be used to screen encapsulants through product lifetime. We conducted an interlaboratory study to provide the understanding that will be used toward developing a higher‐fidelity, more‐rigorous UV weathering test. Five representative known formulations of poly (ethylene‐co‐vinyl acetate) were studied, in addition to one thermoplastic polyurethane material. Replicate laminated silica/polymer/silica specimens were examined at seven institutions using a variety of indoor chambers (including xenon, UVA‐340, and metal‐halide light sources). Specimens were artificially weathered for 180 cumulative days at steady‐state accelerated test conditions, predesignated relative to the default irradiance of 1.0 W·m−2·nm−1 at 340 nm, chamber temperature of 60°C, and chamber relative humidity of 30%. The solar‐weighted transmittance, yellowness index, and the UV cut‐off wavelength—each determined from the measured hemispherical transmittance—are examined to provide understanding and guidance for the UV light source (type lamp and filters), temperature, and humidity used in accelerated UV aging tests. The relative efficacy of xenon‐arc and UVA‐340 fluorescent sources and the typical range of activation energy for degradation is quantified from the experiments.
Heterophasic propylene−ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were thermally aged at 393
and 433 K. Two types of HPEC were examined, containing 25 and 10% ethylene (E), respectively, as
ethylene/propylene rubber (EPR). Spatial and temporal effects of the aging process were studied by electron
spin resonance (ESR) and ESR imaging (ESRI) of HAS-derived nitroxide radicals and FTIR of films
prepared by compression molding. The spatial distribution of the HAS-derived nitroxide radicals was
obtained by 1D ESRI. These experiments enabled the visualization of an outer region of thickness ≈100
μm that contained a lower amount of nitroxides and believed to arise from the loss of the stabilizer by
diffusion (“blooming”) and in chemical reactions during aging. Nondestructive (“virtual”) slicing of the
2D spectral spatial ESR images resulted in a series of ESR spectra that indicated the presence of nitroxide
radicals in two amorphous sites, fast and slow; the corresponding relative intensity varied with sample
depth. 1D and 2D ESRI allowed the detection of faster degradation of EPR in the amorphous phase
represented by the fast spectral component. Both ESRI and FTIR experiments suggested a faster
degradation rate in HPEC containing 25% E compared to 10% E; moreover, a larger Tinuvin 770 content
in the polymers led to less efficient stabilization. FTIR spectra indicated increased ordering of
polypropylene segments in HPEC during aging at 433 K.
The optical transmittance of encapsulation materials is a key characteristic for their use in photovoltaic (PV) modules. Changes in transmittance with time in the field affect module performance, which may impact product warranties. Transmittance is important in product development, module manufacturing, and field power production (both immediate and long-term). Therefore, an international standard (IEC 62788-1-4) 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 optical performance of PV encapsulation materials. Existing standards, such as ASTM E903, are general and more appropriately applied to concentrated solar power than to PV. Starting from the optical transmittance measurement, the solar-weighted transmittance of photon irradiance, yellowness index (which may be used in aging studies to assess durability), an d ultraviolet (UV) cut-off wavelength may all be determined using the proposed standard. The details of the proposed test are described. The results of a round-robin experiment (for five materials) conducted at seven laboratories to validate the test procedure using representative materials are also presented. For example, the Encapsulation Group actively explored the measurement requirements (wavelength range and resolution), the requirements for the spectrophotometer (including the integrating sphere and instrument accessories, such as a depolarizer), specimen requirements (choice of glass-superstrate and -substrate), and data analysis (relative to the light that may be used in the PV application). The round-robin experiment identified both intra- and inter-laboratory instrument precision and bias for five encapsulation materials (encompassing a range of transmittance and haze-formation characteristics)
The thermal degradation products of poly(methyl meth acrylate) model compounds in an ambient atmosphere were mapped via high‐resolution electrospray ionization quadrupole ion trap time of flight (Q‐ToF) mass spectro metry.
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