Sep 15, 2009 - silicone in many industries, and silicones 203 and 205 are proprietary Dow Corning formulations that have a higher refractive index than PDMS ...
THE EFFECT OF ACCELERATED AGING TESTS ON THE OPTICAL PROPERTIES OF SILICONE AND EVA ENCAPSULANTS
Keith R. McIntosh1, James N. Cotsell1, Jeff S. Cumpston1 Ann W. Norris2, Nick E. Powell2 and Barry M. Ketola2 1
Centre for Sustainable Energy Systems, Australian National University, Canberra, ACT 0200, AUSTRALIA 2 Dow Corning Corporation, Midland, Michigan 48686, USA
THE EFFECT OF ACCELERATED AGING TESTS ON THE OPTICAL PROPERTIES OF SILICONE AND EVA ENCAPSULANTS Keith R. McIntosh1, James N. Cotsell1, Jeff S. Cumpston1 Ann W. Norris2, Nick E. Powell2 and Barry M. Ketola2 1
Centre for Sustainable Energy Systems, Australian National University, Canberra, ACT 0200, AUSTRALIA 2 Dow Corning Corporation, Midland, Michigan 48686, USA
ABSTRACT: The absorption coefficient of three silicones and EVA is measured before and after exposure to three accelerated aging tests: (i) ~2000 hours under Xe-arc lamp exposure at room temperature, (ii) 1200 hours at 85% relative humidity and 85 °C, and (iii) six months at the focal point of a 30× linear tracker. The first exposure satisfied the IEC’s UV conditioning test but with the samples at room temperature rather than at 60 °C (approximate IEC test temperature), the second satisfied the IEEE’s damp-heat test and the damp-heat component of the UL and IEC’s humidity-freeze tests, and the third is a stringent but non-conventional test available at the Australian National University. The most stable encapsulant was found to be polydimethylsiloxane (PDMS) silicone. The EVA was stable under the UV illumination but yellowed under damp-heat exposure and was decimated by the 30× concentrated sunlight. We found the EVA to have the greatest tendency to absorb moisture, a feature that increases scattering and reduces transmission.
Keywords: Encapsulation, PV Module, Reliability
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INTRODUCTION
Photovoltaic modules are submitted to accelerated aging tests to predict how 20–30 years of operation and environmental exposure will affect their performance. Accelerated tests are also used to qualify modules, such as those in the IEC 61215, UL 1703 and IEEE 1262 standards [1–3]. One way a module can fail an accelerated test is if the optical transmission of its encapsulant decreases significantly. The tests most likely to cause a reduction in transmission involve exposure to UV and damp-heat, which can alter the chemical composition of an encapsulant. In regards to UV exposure, the most common encapsulant, ethylene vinyl acetate (EVA), tended to yellow and brown under UV until the late 1990s [4]. Since then, improved antioxidants and UV absorbers have greatly slowed the rate of yellowing, and UV absorbing glass (such as Ce-doped glass) slows it further [4, 5, 20]. EVA also loses adhesion when exposed to UV [6]. While modern modules pass the UV exposure tests, it is worth noting that (i) there might still be a decrease in optical transmission, and (ii) the standard tests represent less than half a year of exposure to the AM1-5g spectrum [6], much less than the desired 20–30 years! Exposure to damp-heat can also reduce the optical transmission of an encapsulant. Such degradation has been observed for EVA and silicones, most notably at wavelengths less than 500 nm [8], though it was not investigated whether this was caused by an increase in absorption or an increase in scattering from water molecules. EVA also reacts with water vapour to produce acetic acid, which assists corrosion (either directly or as a catalyst) particularly when the module has an impermeable backsheet [7]. This paper presents the optical absorption coefficient a (l) (from Beers’ law) of EVA and three silicones measured before and after their exposure to the following conditions: (i) 5.6 kWh/m2 between 280 and 320 nm of UV, (ii) 1200 hours at 85% relative humidity and 85 °C,
and (iii) six months at the focal point of a 30× linear tracker. The first exposure exceeds that required by the IEC’s UV conditioning test (61215), which exposes modules to at least 15 kWh/m2 between the wavelengths of 280 and 385 nm, with at least 5 kWh/m2 between 280 and 320 nm [1]. In our experiment, however, the sample temperature is not controlled whereas the IEC test requires the samples be held at 60 ± 5 °C. The second exposure has the same relative humidity and temperature required of the IEEE’s damp-heat test [3] and the damp-heat component of the UL and IEC’s humidity-freeze tests [1, 2]. The duration of the exposure exceeds that required of the qualification tests (e.g., 1000 hrs [3] or 10 cycles of 20 hrs [1]). The third exposure does not represent a standard aging condition, but is a stringent reliability test available at the Australian National University (ANU) [30]. All samples consisted of a quartz-encapsulant-quartz structure which amplified the UV exposure over and above what is typically experienced by an encapsulant in a PV module. For each exposure, the a (l) of EVA and three silicones is monitored as a function of time. The silicones were prepared by Dow Corning and are promising alternatives to EVA due to their higher optical transmission and environmental stability [11, 12].
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SAMPLE PREPARATION
Three silicones were prepared for the experiments and are dubbed 201, 203 and 205 in this paper. Silicone 201 is polydimethylsiloxane (PDMS), a commonly used silicone in many industries, and silicones 203 and 205 are proprietary Dow Corning formulations that have a higher refractive index than PDMS. The experiments also include a conventional EVA obtained from a major EVA supplier. The optical properties of these materials were measured and compared in another work [12].
Figure 1 depicts the structure of the samples subjected to the accelerated aging tests. For these tests, the encapsulants are encased between two sheets of quartz, which provides environmental protection from dust and other particles that can be washed from the quartz, but not the encapsulant. The quartz also permits a more accurate optical measurement because its surfaces are polished and scatter little of the incident beam. Table I lists the thickness of the encapsulant in each sample. The encapsulant is thicker than would be used in a regular module (~0.45 mm), particularly for the EVA samples, which consist of multiple sheets of EVA laminate. The encapsulants were made deliberately thick to attain greater accuracy in a (l). Similar results were shown with EVA samples of 1.6 mm in separate 85/85 studies that were conducted (24). We emphasise that our experiments do not replicate the module qualification standards. We do not follow the exact procedure, nor do we use complete modules (a rather expensive process). Instead, we focus on the relative differences observed between the encapsulants under stringent test conditions. The tests are stringent because the UV component of the incident light is not partially absorbed in any overlying materials—such as the glass of a PV module—and because the encapsulants are thicker than normal. Table I: Thickness of encapsulant (mm) submitted to each exposure. DH: damp-heat, T: 30× tracker. Encapsulant EVA Silicone 201 Silicone 203 Silicone 205
UV 1.8 1.6 1.7 1.7
Exposure DH(1) DH(2) 6.7 – 1.6 1.6 1.6 1.6 1.6 1.6
T(1) 3.0 1.6 1.9 1.9
T(2) 2.9 1.9 1.9 1.6
1.6 mm quartz 1.6–6.7 mm encapsulant (EVA or silicone) 1.6 mm quartz
Figure 1: Schematic diagram of experimental samples. 3
MEASUREMENT
The absorption coefficient a (l) of the encapsulants was determined as a function of wavelength by measuring the hemispherical transmission and reflection of the samples with a Varian Cary 5000 spectrophotometer and analysing the results as described in the appendix of [13]. This analysis assumes the samples to be non-scattering, semi-transparent, single layers. Although the quartz–encapsulant–quartz samples have three layers, rather than one, we believe the method still provides an accurate measurement of the encapsulant’s a (l). This requires the following to be negligible: (i) absorption in the quartz, (ii) reflection at the quartz–encapsulant interfaces, and (iii) scattering within the encapsulant or at the interfaces. Before discussing these requirements in more detail, we compare a (l) determined from single-layer silicone samples (S1 and S2) to those of quartz–silicone–quartz sandwiches (QSQ1 and QSQ2), as plotted in Figure 2. The figure shows that a (l) of the S and QSQ samples are nearly identical when a > 0.1 cm–1. We observed such consistency for the many S and QSQ samples measured over the course of the project, and for all silicone formulations. (One exception to this conclusion occurs occasionally for silicone 201 in the range, 250–300 nm, where some samples exhibit shoulders while others do not; this exception is apparent in Figure 2. The reason for the appearance of this shoulder is unknown at this time. Thus, the results of Figure 2 suggest requirements (i), (ii) and (iii) are all satisfied when a > 0.01 cm–1. The first requirement is further justified by the measurements of a (l) for a single layer of polished quartz (Q), also plotted in Figure 2. The figure shows that to the accuracy of the experiment, a (l) of quartz is negligible at l > 280 nm, and is an order of magnitude less than a (l) of the single-layer samples at l < 280 nm. This means that absorption in the quartz can be neglected from the calculations of a (l) for the QSQ samples. We note that measurements on other polished quartz samples yielded the same a (l). The second requirement is further justified by Figure 3, which plots reflection at the quartz–encapsulant interfaces. This data has been calculated from measurements of the refractive index n(l) [12] using the Fresnel equations. The figure shows that except for silicone 205, reflection at the internal interfaces is less than 0.05%, which is too small to affect the
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Wavelength (nm) Figure 2: Absorption coefficient a (l) determined for one piece of quartz (Q), two samples of free-standing silicone 201 (FS1 & FS2), and two quartz–201–quartz sandwiches (QSQ1 and QSQ2). The dotted lines show the fraction of light that would be absorbed within 0.45 mm of encapsulant, the typical thickness of EVA. Best viewed in colour.
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Wavelength (nm) Figure 3: Reflection at the quartz–encapsulant interface calculated from measured data for the refractive index. determination of a (l). Reflection from the quartz–205 interfaces is a little larger and may introduce a small error into the calculated a (l). These conclusions presuppose the quartz–encapsulant interfaces to be ideal (e.g., they contain no air bubbles). We believe the variation in a (l) below 0.1 cm–1 is a result of requirement (iii) being unjustified. Such variations in the lower a (l) regions (i.e., between absorption peaks) occurred for all encapsulants, whether free-standing or encased in quartz. The variations in the lower a (l) regions were small for silicone, with Figure 2 plotting the greatest disparity observed over the course of the project, but large for EVA (more than an order of magnitude). The most likely reason for the variation in a (l) is scattering from water molecules and, in the case of EVA, from fine paper-like contaminants that derive from the EVA’s packaging. Scattering disrupts the measurement of reflection and transmission, causing some light to escape from the sample without entering the integrating sphere. Escaped light is not recorded as reflection or transmission, and is erroneously assumed to be absorption. The role of moisture in the measurement of a (l) will become clearer in Section 5, and is further justified by the results of another work [24]. We conclude that when scattering is negligible, the measurement technique provides an accurate measurement of an encapsulant’s a (l) when the encapsulant is contained within two quartz layers. It is also concluded that the measured a (l) can be overestimated due to scattering as is seen in damp heat results when no sample dry out is performed (figures 6 and 7) .
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Figure 5 plots a (l) of the four encapsulants before and after various exposures to the Xe arc lamp. The figure shows that there was no increase in a (l) of silicone 201 or EVA. In fact, there is an initial decrease in a (l), particularly for the EVA. We attribute this decrease to a drying out of the samples under the exposure, which would lead to less scattering by water molecules during the measurement. Thus, we suspect the reduction in a (l) to be an experimental artefact rather than an actual change in a (l). The role of moisture will become more evident during the discussion of the dampheat exposure. Figure 5 shows that silicones 203 and 205 degrade under Xe-lamp exposure. This change in a (l) is visually apparent, with the samples yellowing significantly. We believe the degradation is related to an absorption peak at 270 nm associated with the chemical composition of 203 and 205. It is also felt that this composition increases the polarity and thus increases its propensity to pick up moisture. which is evident in the following section. These results do not preclude other silicones of high refractive index from exhibiting better stability under UV illumination. In a parallel experiment performed at Dow Corning, we attained identical results. Namely, that silicone 201 and EVA were stable under UV, while silicones 203 and 205 degraded by a similar degree. In this case, however, the samples were maintained at approximately 60 °C and the illumination source was an Atlas UV2000 with UVA 340 fluorescent bulbs set to 1 W m-2 nm-1 (Figure 4)
RESULTS: UV EXPOSURE
Samples were illuminated by a xenon arc lamp for 1948 hours (silicones) and 1216 hours (EVA). The spectral irradiance of the illumination incident to the samples was measured and is compared to the AM1-5g spectrum in Figure 4. The figure illustrates that the two spectra are similar though the arc lamp has a higher UV irradiance. Table II quantifies the UV irradiance in terms of the power within wavelength ranges used by the accelerated testing standards. It also shows that by all definitions, the exposure to the silicone samples exceeds all of the UV requirements, and the exposure to the EVA exceeds two of the three requirements.
Figure 4: Spectral irradiance of the xenon arc lamp compared to the AM1-5g spectrum.
Table II: UV irradiance within certain wavelength ranges. The table lists the requirements of the accelerated testing standards in terms of the wavelength range l and incident energy E, and it lists the incident power P under the AM1-5g, the time required treq to meet the standard and the equivalent time teq in operational conditions, see reference [7]. Finally, the table lists the incident power under our Xe arc lamp P, the exposure time t, and the exposure energy E. Include the Dow Corning UV results. Requirement for UV exposure Standard l E (nm) (kWh/m2) IEC 61215 280–320 5 IEC 61215 280–385 15 IEEE 1262
