Pv Sol 7 5 Cracks
Cracking in Silicon solar cells is an important factor for the electrical power-loss of photovoltaic modules. Marathi Dabalbari Bhajan Mp3. Simple geometrical criteria identifying the amount of inactive cell areas depending on the position of cracks with respect to the main electric conductors have been proposed in the literature to predict worst case scenarios. Here we present an experimental study based on the electroluminescence (EL) technique showing that crack propagation in monocrystalline Silicon cells embedded in photovoltaic (PV) modules is a much more complex phenomenon.
In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual thermo-elastic stresses, cracked regions can recover the electric conductivity during mechanical unloading due to crack closure. During cyclic bending, fatigue degradation is reported. This pinpoints the importance of reducing cyclic stresses caused by vibrations due to transportation and use, in order to limit the effect of cracking in Silicon cells.
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Geometrical criterion showing the expected amount of potentially inactive cell areas depending on the orientation of the crack with respect to the busbars,: 6% of potentially inactive cell (a); 25% of potentially inactive cell (b). In reality, it is reasonable to expect intermediate configurations where cracks may partially conduct depending on the relative crack opening displacement at crack faces.
A complete modelling of the phenomenon should therefore consider the following steps: (i) simulation of crack nucleation and propagation according to a computational approach based on the cohesive zone model (CZM),,, where cohesive tractions opposing to the relative displacements of the crack faces are decreasing functions of opening and sliding; (ii) analysis of thermal effects by augmenting the basic mechanical CZM to take into account the additional thermal resistance of cracks; (iii) modelling of the electric response of the cell. For this last item, localized series resistances dependent on crack opening might be postulated in correspondence of cracks, in addition to the distributed series resistance. A preliminary experimental observation supporting coupling effects induced by the thermo-mechanical field on the electric one due to cracking regards the highly oscillating electrical response in time of a PV string containing a cracked cell, depending on the cell temperature. At present, electric models of solar cells do not consider this form of coupling induced by cracking.
In the most refined versions,, a discretization of the solar cell is made in the plane and a two-diode model is applied to each node of the mesh to predict the electric response of the semiconductor. In case of hot spots, however, it was indeed necessary to modify the value of the series resistance in the nodes close to a crack. Hence, to achieve a predictive stage useful for power-loss predictions and durability assessment of PV systems exposed to environmental loads, series resistance values used as input of the circuit model should be related to the thermo-elastic stress state in the solar cell. Results To make an insight into the mechanisms leading to these coupling effects, a bending test on a rectangular PV module has been performed in the laboratory by monitoring cracking at different deformation levels by EL imaging. A semi-flexible module made of 2 rows of 5 monocrystalline Si cells each is used. The dimension of the cells is 156 × 156 mm. The partially symmetric arrangement of the layers through the thickness (0.265 mm of polyethylene terephthalate, 0.600 mm of epoxy-vinyl-acetate, 0.166 mm of Silicon, 0.400 mm of epoxy-vinyl-acetate and 0.345 mm of backsheet) and the different Young moduli of the materials lead to Si cells just above the neutral axis of the cross-section (see ).