Martin Sander

Mechanical and thermomechanical assessment of encapsulated solar cells by finite-element-simulation

Within the following work mechanical and thermo-mechanical studies on embedded solar cells were carried out. Temperature dependant material properties such as shear modulus and coefficient of thermal expansion of an EVA encapsulant were determined by dynamic mechanical analysis (DMA) and thermo mechanical analysis (TMA). Those parameters were integrated into various simulation models such as the lamination process starting from the curing temperature at 150 °C and thermo cycling. Parameter studies were carried out concerning the cell thickness to assess the thermo-mechanical behavior of the cell string and the stress distribution in the silicon. Within a second study the mechanical behavior of the laminate was investigated. As a result it is shown that the solar cells have a significant impact on the deflection of the laminate, whose behavior over a temperature range is dominated by the stiffness properties of the encapsulant. By means of a combination of global models and submodels it was possible to assess the stress distribution in the solar cells with particular interest in the interconnection region between the cells. The magnitude of the stress depends strongly on the stiffness of the encapsulant. Especially for thin cells the stress can increase critically.

Interdependency of mechanical failure rate of encapsulated solar cells and module design parameters

In recent studies the mechanical reliability of encapsulated solar cells was numerically investigated. A finite element model of a solar module with all essential components, such as cells, polymer layers and frame was created. The principle stress field in each solar cell was calculated by exposing the module to distributed pressure loads on the glass surface. By means of a probabilistic approach based on the Weibull distribution function and the size effect the stress field was evaluated and the probability of failure of each solar cell was calculated. This approach is new in the reliability evaluation of encapsulated solar cells and can enhance the module design process. Two fundamental studies were carried out varying the mounting and frame as well as the encapsulant and its thickness. The results show that there is an interdependency between the stiffness of the frame section and the type of mounting. Furthermore the recommendation for an appropriate frame and mounting selection can change if the magnitude of the load changes. It was found that there is a correlation between the stiffness of the encapsulant and the fundamental mechanical behavior of the module laminate. For high stiffness values a sandwich behavior is dominant whereas for small stiffness values a laminate behavior with shear deformation is dominant. This results in contrary thickness recommendations for different encapsulants as well as temperatures. For high stiffness values respectively low temperatures a thin encapsulant is advantageous whereas for low stiffness values at high temperatures a thick encapsulant would be better.

Investigations on crack development and crack growth in embedded solar cells

In recent investigations using various analysis methods it has been shown that mechanical or thermal loading of PV modules leads to mechanical stress in the module parts and especially in the encapsulated solar cells. Cracks in crystalline solar cells are a characteristic defect that is caused by mechanical stress. They can lead to efficiency losses and lifetime reduction of the modules. This paper presents two experiments for systematic investigation of crack initiation and crack growth under thermal and mechanical loading using electroluminescence. For this purpose PV modules and laminated test specimens on smaller scales were produced including different cell types and module layouts. They were exposed to thermal cycling and to mechanical loading derived from the international standard IEC 61215. Cracks were observed mainly at the beginning and the end of the busbars and along the busbars. The cracks were analyzed and evaluated statistically. The experimental results are compared to results from numerical simulations to understand the reasons for the crack initiation and the observed crack growth and to allow module design optimization to reduce the mechanical stress.

PV module defect detection by combination of mechanical and electrical analysis methods

The lifetime and reliability of photovoltaic modules (PV modules) is influenced by defects which have their origin either in manufacturing processes or in operation exposure. Characterization of PV modules is necessary for manufacturers to assure their warranty and to observe process difficulties during production process and for improving their modules during development processes. For customers PV module characterization is important to observe output performance of their PV system and to proof intactness of single modules. For this purpose reliable and nondestructive testing methods are desirable.

Characterization of PV modules by combining results of mechanical and electrical analysis methods

Photovoltaic modules (PV modules) are supposed to have a lifetime of more than 20 years under various environmental conditions like temperature changes, mechanical loads, etc. Common outdoor exposure may influence efficiency and lifetime which necessitates assessment of PV module performance and detection of output deficits. For this purpose reliable and nondestructive testing methods are desirable. Commercially available PV modules were tested by different analysis methods. The PV modules electrical properties were investigated by thermography and electroluminescence measurements. The combination of these two techniques is well-suited to detect many cell and module defects. A crystalline module showed significant cell breakage after temperature cycle test. To observe the mechanisms of this specific defect type laminated test specimens on smaller scales were produced and analyzed over production process and during temperature cycles derived from the international standards IEC 61215 and IEC 61646. The defect study on small scales allows conclusions about the defects influence on larger PV modules. Further methods capable for mechanical characterization like Laser Doppler vibrometry, surface geometry scan and digital image correlation are presented briefly. The combination of the methods mentioned above allows a very precise assessment of the mechanical and electrical capability which is essential for reliability and lifetime concepts.

Stress analysis of encapsulated solar cells by means of superposition of thermal and mechanical stresses

Within this contribution several 3D nite- element- models have been created in order to simulate processing of solar cells (lamination, soldering) as well as mechanical bending. The stress state for each load case was analysed with respect to magnitude and direction of principal stresses. For the process steps there are di erent mechanisms that induce stresses in the silicon. For soldering the mismatch in CTE is dominant. For lamination, bending around the ribbon is the dominant mechanism, which is due to the contraction of the encapsulant. Furthermore, it was found that cooling during lamination applies the highest loads into a solar cell. Mechanical bending was simulated and investigated experimentally by 4-point-bending with di erent load ramps. Due to strain-rate dependent properties of the encapsulant EVA there is a minor in uence on the load de ection behaviour but a large in uence on the reliability of a solar cell. By means of a parameter study the in uence of the cell distance on mechanical reliability was investigates. It was shown that a small cell distance (here < 3mm) increases the probability of failure of the solar cell signi cantly.