Photovoltaic (PV) solar energy recorded an exponential growth, in worldwide scale, over the last decade. Inevitably, mature PV markets are becoming highly competitive, boosting the need for research and development (R&D) on efficiency and reliability optimization, maintenance and fault diagnosis of key components, such as the PV modules. Indeed, a significant number of studies and technical papers have been published up today, based on an extensive feedback from both laboratory and real (field) investigations of faults and advanced diagnosis applications, especially for crystalline silicon (c-Si) PV modules. Undoubtedly, such experience is of particular interest for current PV plant operators, future investors, maintenance engineers and the R&D sector of PV industry. However, up today, such research, published in the form of reports, technical papers or even books, remains mostly dispersed and unclassified. This paper represents a comprehensive effort to review and highlight recent advances, ongoing research and future prospects, as reported in the literature, on the classification of faults in c-Si PV modules and advanced diagnosis in the field, by means of the increasingly popular method of infrared thermal (IRT) imaging. In particular, the first main part of this paper, reviews the characteristics of the most common fault types of operating PV modules, in terms of electrical and thermal response. Then, the second part gives a thorough review of recently published research, as well as the state-of-the-art, in the fields of IRT-based fault diagnosis and thermal image processing. On the basis of these two individual though supplementary review parts, an overview table is presented, followed by a discussion on the future prospects and challenges, towards the understanding and diagnosis of faults and their propagation in operating PV modules.

Faults and infrared thermographic diagnosis in operating c-Si photovoltaic modules: A review of research and future challenges

Determining the degree of crosslinking of ethylene vinyl acetate photovoltaic module encapsulants—A comparative study

CNN based automatic detection of photovoltaic cell defects in electroluminescence images

Review of technological design options for building integrated photovoltaics (BIPV)

A layer-wise theory for laminated glass and photovoltaic panels

Systematic investigation of cracks in encapsulated solar cells after mechanical loading

Analysis of laminated glass beams for photovoltaic applications

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.

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

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

Investigations on crack development and crack growth in embedded solar cells

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.

Matthias Pander

Papers

Systematic investigation of cracks in encapsulated solar cells after mechanical loading

Analysis of laminated glass beams for photovoltaic applications

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

Investigations on crack development and crack growth in embedded solar cells

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