Multi-scale, multiphysics modelling and characterization of photovoltaic systems: a co-development approach

Abstract

Solar photovoltaics (PV) has proven and continues to be an essential asset towards tackling the energy transition. The reliability of the systems and their respective components remains essential to continue lowering the levelized cost of electricity (LCOE), lowering the carbon footprint, and enabling further penetration into emerging markets. The growth of PV has been driven mainly by a combination of cost reductions and significant efficiency improvements. These drastic cost reductions have been achieved mainly through scaling of manufacturing, increased production automation, and technological innovations from a packaging and efficiency perspective to the point where the raw materials make up an estimated 35-50% of the total module cost on average in 2024. Quantifying the reliability of PV systems, however, is becoming increasingly complex as materials used, cells, modules and mounting technologies are evolving rapidly. Emerging markets such as building-integrated PV (BIPV), vehicle-integrated PV (VIPV), floating PV and agrivoltaics, each with its specific market requirements, further complicate this and often lack quantities for statistical studies. Some typical approaches to asses reliability are field inspection, analytical methods, accelerated stress testing, labscale testing, and modelling. Finally, the use of physics-based simulations poses a unifying value proposition, having the ability to study various materials, cells and module technologies while focusing either on a single stressor or a combination thereof. Physics-based simulation tools are increasingly employed towards quantifying the lifetime; the finite element method (FEM) is particularly popular because of its flexibility and the ability to quantify stress levels for a large variety of scenarios. However, the approaches used, the various inputs considered and the obtained results are highly scattered and occasionally conflicting. Chapter 2 provides a critical review of the reported simulation approaches and resulting insights obtained through thermo-mechanical finite element simulations on both commercial and novel PV module technologies and materials. The influence and validity of various inputs, such as the used material models, boundary conditions and other assumptions, are discussed. Technological trends observed through simulation, such as the impact of transitioning to a wire-based interconnection on the process-induced stress or a comparison between a glass-glass or glass-backsheet topology from a solder joint damage accumulation perspective are The learnings, and best practices are summarized on a component basis can be leveraged by future simulations to expand on and accelerate the design-for-reliability capabilities of FE models for PV modules. With the aim of expanding insights into the multiscale nature of reliability beyond the state-of-the-art, within Chapter 3, interconnection constituents, encapsulants, back sheets and PV cells are studied experimentally as input for FE simulations. Due to the importance of accurate geometry representation at the micron scale, Scanning electron microscopy(SEM) and elemental analysis using EDX were performed on a solder joint formed using a novel interconnection method. Material models for metal and polymeric constituents are discussed with a focus on encapsulants due to their contributions to field failures. Computationally efficient linear elastic, bilinear elastic-plastic and visco-elastic material models are determined for a commercial backsheet and encapsulant material using uniaxial tensile testing, dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The links between the mathematical models and experimental observations made are discussed to establish proper limitations for their use. Studies of the bulk behaviour of encapsulants was also extended to their interface behaviour. Various factors that affect interface adhesion were examined for EVA, POE and TPO encapsulants, revealing significant differences in adhesion mechanisms employed by these various encapsulants. The use of flux within the module, typically for interconnection formation, has shown to have a significant impact on the adhesion of the encapsulant interfaces and, thereby, potentially initiate delamination. As many different cell abstractions have been reported in literature while lacking validation, cells were diced and tested mechanically to identify significant contributors to changes in the local stiffness of the Si wafer and, by extension, derive their homogenized material properties. Input gathered in chapters 2 and 3 on cells, materials, and assumptions served as input for the creation of a novel multiscale framework which enables the study of the reliability of PV modules using the FEM. The framework is built up of six discrete simulation models, namely the material level, the sub-cell level, the cell level, the submodule level, the module level and the system level. Through careful implementation of abstraction methods and boundary conditions chosen at each level, a bi-directional coupling was achieved across multiple length scales while maintaining reasonable levels of computational requirements through an iterative swept-level approach. A case study was performed using the module and system level of the framework to examine the structural integrity of two distinct commercial lightweight roof systems, one designed for bitumen-based corrugated roof constructions and the other for insulated corrugated roof sheet constructions. A custom system validation setup was designed and built on which a single module system was built for each configuration, and racking and module displacements were precisely monitored during static pressure loading at 0 °C and 30 °C roof angles. Following a benchmarking study to obtain a black-box representation of the commercial PV modules used for testing, the PV system designed for bitumen-based corrugated roof constructions showed to handle up to 1800 Pa of pressure loading until failure within the racking was predicted. Increasing the system spacing by 51% was found to reduce load-bearing capabilities by half. The second system demonstrated a maximum loading capacity of up to 1400 Pa in pressure, while simulation-driven extrapolation showed that both systems are less sensitive to homogenous suction loads. Driven by the lack of a flexible thermo-mechanical validation tool as well as limitations identified through a displacement-based validation approach, as shown in Chapter 4, a novel in-house developed sensor solution is presented in Chapter 5. The solution consists of an optical in-laminate thermo-mechanical sensing approach using fibre Bragg gratings (FBGs). By leveraging a patented, scalable packaging method, absolute temperature measurements have an accuracy of ± 0.3 °C along with the ability to detect changes in strain as low as ±0.248 με. With applications ranging from investigation process-induced stresses up to in-field degradation monitoring, it provides a valuable tool for module development, monitoring or validation of physics-based simulation. This potential is briefly shown by presenting an in-situ measurement during thermal cycling. A combination of the novel framework and the novel sensing approach forms the basis for a co-development approach through which many aspects of the thermomechanical behaviour of PV modules or systems can be further examined and quantified to improve the reliability of PV systems.