Thermomechanical degradation mechanisms of silicon photovoltaic modules

The durability and lifetime of photovoltaic (PV) modules is one of the chief concerns for an industry which is rapidly approaching maturity. Guaranteeing the economic viability of potential PV installations is paramount to fostering growth of the industry. Whilst certification standards have helped to improve the reliability of modules, with a significant reduction in early failures, long-term performance degradation and overall lifetimes are yet to be addressed in a meaningful way. For this, it is necessary to quantify the effects of use-environment and module design. Long-term degradation of the solder bonds in PV modules causes steady power loss and leads to the generation of more devastating, secondary mechanisms such as hot-spots. Whilst solder bond degradation is well-recognised and even tested for in certification protocols, the potential rate of degradation is not well understood, particularly with respect to different environmental conditions and material selection. The complex nature of a standard silicon PV module construction makes it difficult to observe the stresses experienced by the various components during normal operation. This thesis presents the development of a finite-element model which is used to observe the stresses and strains experienced by module components during normal operating conditions and quantifies the degradation of solder bonds under different environmental conditions. First, module operating temperatures are examined across a range of climates and locations to evaluate the thermal profiles experienced by modules. Using finite-element techniques, the thermomechanical behaviour of modules is then simulated using the same thermal profiles and a quantification of solder bond degradation potential in each location is achieved. It is shown that hot climates are responsible for the highest degradation potential, but further to this, hot environments with many ii clear sky days, allowing for large swings in module temperature, are significantly more damaging. A comparison is drawn between indoor accelerated stress procedures and outdoor exposure, such that an equivalence between test duration and location-dependent outdoor exposure can be determined. It is shown that for the most damaging climate studied, 86 standard thermal cycles is appropriate for one-year of outdoor exposure whereas the least damaging environment would require 11 standard thermal cycles. However, these conclusions may only be applicable to the specific module design which was modelled as the material selection and interaction within a device plays a major role in the thermomechanical behaviour and degradation potential. In addition to a study on the influence of use-environment, a study on the influence of the encapsulating material is conducted with a particular focus on the effects of the viscoelastic properties of the materials. It is shown that the degradation of solder bonds can vary depending on the encapsulating material. Furthermore, the intended use-environment could inform the selection of the encapsulating material. The temperature-dependency of the material properties means that some materials will mitigate thermomechanical degradation mechanisms more than others under certain conditions i.e. hotter or colder climates

Thermo-mechanical stresses of silicon photovoltaic modules

Modelling and analysis of the thermomechanical behaviour of silicon photovoltaic (PV) modules has been conducted using finite-element numerical methods (FEM). Experimentally determined material properties have been implemented in the model to represent the 6-cell mini-modules fabricated at the Centre for Renewable Energy Systems Technology (CREST). The stresses generated during indoor accelerated ageing tests and real outdoor conditions have been compared. It is found that the thermo-mechanical stresses are highest at the extreme temperatures during indoor testing. The outdoor accumulated stress generated within the interconnecting ribbons is greater than the stress generated during indoor thermal cycling programs for the same amount of temperature travelled. The results shed light on the relevance of indoor accelerated ageing programs to real outdoor conditions

The role of EVA encapsulation in the degradation of wafer based PV modules

This paper investigates the effects of ethylene vinyl acetate (EVA) encapsulation on PV module ageing and how crosslinking degree of encapsulation influences module durability under damp-heat (DH) and thermal cycle (TC) stresses. Results show that the high crosslinking samples favoured TC stresses, while the low crosslinking samples performed better under DH stresses. The primary mechanism of DH-induced degradation is series resistance (RS) increase and parallel resistance (RP) decrease due to moisture ingress and grid/contact corrosion. Comparison analysis of the result indicates that the lower cross-linked EVA appeared to be able to accommodate a smaller amount of the generated acetic acid and thus resulted in a lower corrosion rate. The primary effect of TC is to impose thermal expansion/compression on device. The EVA with lower crosslinking degree is less compact and the freedom of motion of EVA macromolecules is higher, which appeared to be less resistant to the effect of expansion/compressio

Effect of viscoelasticity of EVA encapsulants on photovoltaic module solder joint degradation due to thermomechanical fatigue

The solder joint degradation due to thermomechanical fatigue is investigated in this paper for PV mini-modules with EVA of different viscoelastic properties. The mini-modules were laminated at different curing temperatures in order to obtain EVA encapsulation with different viscoelastic properties. The influence of viscoelasticity of EVA on the thermomechanical fatigue generated on solder joint is analyzed based on a 2D finite-element model. Based on simulation of thermomechanical stresses accumulation, mini-modules with EVA cured at lower temperatures accumulated approximately 40% more stresses during the thermal cycle testing than mini-modules with optimal cured EVA. The tested mini-modules with EVA cured at lower temperature showed greater power degradation than the optimal cured mini-modules. An apparent increase in equivalent series resistance is the primary factor the power loss. A good correlation between the accumulated thermomechanical fatigue and the increase in equivalent series resistance is demonstrated with the tested samples

Influences of lamination condition on device durability for EVA-encapsulated PV modules

PV modules rely on their encapsulation to provide durability. The pottant is predominantly ethylene vinyl acetate (EVA). It is protected by foils and glass to minimise encapsulant related degradations such as delamination, decomposition and corrosion. Types of EVA and/or backsheet will influence overall durability, as has been reported frequently. The lamination process as well as material handling also contributes to overall durability, but the impact is not always obvious. This paper investigates the effect of lamination temperature on encapsulation quality and its impact on module durability in accelerated ageing tests. A series of laminations is carried out at different conditions within the typical window suggested by the manufacturer as well as slightly off specifications as could occur due to insufficient temperature control. The samples were exposed to prolonged standard ageing tests for up to 7000 hours. Use of subtractive electroluminescence (EL) images demonstrates a minimum of two different ageing mechanisms are active at different time constants and of different activation energies (Ea)