Thermal Stress and Strain of Solar Cells in Photovoltaic Modules

Eitner, Ulrich; Kajari‐Schröder, Sarah; Köntges, Marc; Altenbach, Holm

doi:10.1007/978-3-642-21855-2_29

Cited by 67 publications

(42 citation statements)

References 9 publications

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“…This is consistent with prior observations of rapid potential-induced degradation occuring in less than half a month [6]. Silver mismatched coefficients of thermal expansion [21,22]. This tendency may also be further reinforced by the introduction of daily and seasonal thermal cycling that can cause gradual buildup of stress over time.…”

Section: Resultssupporting

confidence: 91%

A modeling framework for potential induced degradation in PV modules

et al. 2015

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Major sources of performance degradation and failure in glass-encapsulated PV modules include moisture-induced gridline corrosion, potential-induced degradation (PID) of the cell, and stress-induced busbar delamination. Recent studies have shown that PV modules operating in damp heat at -600 V are vulnerable to large amounts of degradation, potentially up to 90% of the original power output within 200 hours. To improve module reliability and restore power production in the presence of PID and other failure mechanisms, a fundamental rethinking of accelerated testing is needed. This in turn will require an improved understanding of technology choices made early in development that impact failures later.In this work, we present an integrated approach of modeling, characterization, and validation to address these problems. A hierarchical modeling framework will allows us to clarify the mechanisms of corrosion, PID, and delamination. We will employ a physics-based compact model of the cell, topology of the electrode interconnection, geometry of the packaging stack, and environmental operating conditions to predict the current, voltage, temperature, and stress distributions in PV modules correlated with the acceleration of specific degradation modes. A self-consistent solution will capture the essential complexity of the technology-specific acceleration of PID and other degradation mechanisms as a function of illumination, ambient temperature, and relative humidity. Initial results from our model include specific lifetime predictions suitable for direct comparison with indoor and outdoor experiments, which are qualitatively validated by prior work. This approach could play a significant role in developing novel accelerated lifetime tests.

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Section: Resultssupporting

confidence: 91%

A modeling framework for potential induced degradation in PV modules

et al. 2015

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“…The EVA polymer displays a strong thermo-visco-elastic constitutive response, as experimentally reported in [5, 6], with a variation of the elastic modulus of up to three orders of magnitude depending on temperature. Generalized Maxwell rheological models used so far generally provide exponential type relations for the relaxation modulus and, in order to approximate the experimentally observed power-law trend, a huge number of elements (and thus of model parameters) has to be taken into account.…”

Section: Introduction and Motivationsmentioning

confidence: 61%

A geometrical multi-scale numerical method for coupled hygro-thermo-mechanical problems in photovoltaic laminates

Lenarda

Paggi

2016

Comput Mech

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A comprehensive computational framework based on the finite element method for the simulation of coupled hygro-thermo-mechanical problems in photovoltaic laminates is herein proposed. While the thermo-mechanical problem takes place in the three-dimensional space of the laminate, moisture diffusion occurs in a two-dimensional domain represented by the polymeric layers and by the vertical channel cracks in the solar cells. Therefore, a geometrical multi-scale solution strategy is pursued by solving the partial differential equations governing heat transfer and thermo-elasticity in the three-dimensional space, and the partial differential equation for moisture diffusion in the two dimensional domains. By exploiting a staggered scheme, the thermo-mechanical problem is solved first via a fully implicit solution scheme in space and time, with a specific treatment of the polymeric layers as zero-thickness interfaces whose constitutive response is governed by a novel thermo-visco-elastic cohesive zone model based on fractional calculus. Temperature and relative displacements along the domains where moisture diffusion takes place are then projected to the finite element model of diffusion, coupled with the thermo-mechanical problem by the temperature and crack opening dependent diffusion coefficient. The application of the proposed method to photovoltaic modules pinpoints two important physical aspects: (i) moisture diffusion in humidity freeze tests with a temperature dependent diffusivity is a much slower process than in the case of a constant diffusion coefficient; (ii) channel cracks through Silicon solar cells significantly enhance moisture diffusion and electric degradation, as confirmed by experimental tests.

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“…Here it must be mentioned, however, that the pressure during lamination may not be so important, as long as it is sufficiently high to bend the cells around the contact strips. Of more relevance may be the high pressure which the cells should experience from the contraction and hardening of EVA in the cool periods of the thermal cycles [26,27]. As evidenced by the scratches, the rough surface of the bus bars causes permanent indentations on the SnPbAg coating, and these could become so deep, that in the relaxed state of EVA at STC-temperatures and above the SnPgAg coating formed only loose contact to the cells and actually decreased the contacted area.…”

Section: Snpbagmentioning

confidence: 99%

“…5c). The thermal cycles might have lead to additional shrinkage of the EVA [27,33], thereby increasing contact force. Aside from these mechanical aspects, the chemical properties of silver also favor a good performance of modules with Ag-coated contact strips.…”

Section: Snpbagmentioning

confidence: 99%

“…5c and 5d), which means that on the back sides of the cells the contact strips were pressed onto the cells only by the adhesive tape, the encapsulant, and the back sheet of the module. Most likely this pressure is quite a bit lower than that exerted on a contact strip between two overlapped cells, and it may get smaller with repeated warming and cooling of the module, due to a change of viscoelastic properties of EVA [27,32,33].…”

Section: Outdoor Tests Of Large Modulesmentioning

confidence: 99%

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Cell interconnection without glueing or soldering for crystalline Si photovoltaic modules

Summhammer

Halavani

2016

EPJ Photovolt.

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In order to maximize the power output of polycrystalline silicon PV-modules, in previous work we have already tested rectangular cells of 39 × 156 mm which are overlapped along the long sides. The low current density at the cell overlap allows interconnections which need neither soldering nor glueing, but use metallic strips inserted between the cells in the overlap region. The contact is established by the pressure applied to the module during lamination and is retained by the slightly bent cells in the solidified encapsulant. Here we report on the long term stability of different contact materials and contact cross sections applied in eight modules of the 240 W class monitored for up to 24 months of outdoor operation and in a variety of small 5-cell modules exposed to rapid ageing tests with up to 1000 thermal cycles. Cells with three different electrode designs were tested and the contact materials were Cu, Ag, SnPbAg and Sn. Focussing especially on series resistance, fill factor and peak power, it is found that Ag-coated contact strips perform equally well and have practically the same stability as soldered cell interconnections. Due to 70-90% savings in copper and simpler manufacturing the cost of PV-modules may thus be reduced further.

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Thermal Stress and Strain of Solar Cells in Photovoltaic Modules

Cited by 67 publications

References 9 publications

A modeling framework for potential induced degradation in PV modules

A modeling framework for potential induced degradation in PV modules

A geometrical multi-scale numerical method for coupled hygro-thermo-mechanical problems in photovoltaic laminates

Cell interconnection without glueing or soldering for crystalline Si photovoltaic modules

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