A photovoltaic (PV) module is a complex structure where each component has a fundamental role. The polymers that are part of a PV module are responsible for several degradation modes that happen in the field. The materials are stressed by the climatic characteristics of the surrounding environment and by the microclimate. This latter corresponds to the actual boundary conditions that the polymers in a PV module configuration experience during operation. Temperature, relative humidity, oxygen and ultraviolet (UV) radiation can strongly affect the stability of the polymers and cause their degradation. Understanding how the single factors and their combination impact on polymers on a molecular lever, transfer this knowledge to the changes that we can see on a macro scale and eventually link those changes to PV module performances is a challenging task.
The objectives of this work are to (I) define the relevant factors that characterize the polymer microclimate during the exposure in a PV module configuration, (II) identify the characteristic of the encapsulant materials that mostly change during operation and how those properties are influenced by the microclimate, (III) correlate the polymer changes to PV module performances, (IV) identify characterization methods able to effectively describe polymer changes.
After a brief introduction regarding the deployment of PV technologies and the main issues concerning reliability, a literature review is carried out to determine what the state-of-the-art is about analysis of polymer encapsulants in PV modules. In particular, the PV module reliability and above all the role of the polymer encapsulants is discussed. The second chapter gives additional information how to study reliability and compatibility of materials in PV modules, and focuses on the polymer properties that are more significant for PV applications and how to characterize them.
In the fourth chapter, the performances of the state-of-the-art encapsulant material (i.e. ethylene vinyl acetate, EVA) are compared with two newly developed materials to verify their suitability in replacing EVA. Qualitative additive analysis, characterization of thermal properties, optical properties and chemical structure are carried out to assess the impact of two artificial ageing tests on the materials performances. The results showed that UV irradiation is the most detrimental stress factor when standalone films are exposed. A good stabilization recipe is the key factor to ensure long term stability. Additionally, a good correlation is found between the depletion of stabilizers, increase of formation of oxidation products and decrease of polymer thermal stability. The results of this study showed that the polyolefin elastomer (POE) encapsulant could be a replacement for EVA because of the very similar performances and the advantage of not producing acetic acid upon degradation.
The fifth chapter is focused on understanding how different artificial ageing tests and different microclimates influence polymer degradation and eventually PV mini-module performances using EVA as encapsulant. The polymers were extracted from three different areas of the mini-module and characterized and the electrical performances of the mini-modules were evaluated. Among the test used, the combined UV-damp heat (UV-DH) showed the most relevant impact on PV module performances, causing about 5% power loss with respect to the reference value. The power loss was mostly associated with encapsulant yellowing, caused by additive degradation. The microclimate played a fundamental role because the excess of encapsulant directly exposed to the environment experienced a much more severe degradation with respect to the material encapsulated within the mini-module.
Finally, the sixth chapter shows the effect of the microclimate on polymer degradation and PV module performances of full scale PV modules operating in two different climates for about eight years. Encapsulant samples were extracted from different areas of the PV module: from the front side, from the back side and from the back side in correspondence of the junction box. The results showed that the tropical climate had the strongest impact on the electrical performances causing power losses between 10% and 45%. The power degradation was mostly associated with corrosion of metallization and interconnections that were caused by encapsulant degradation, with formation of acetic acid, and humidity ingress. In this study, the additive analysis proved itself again to be a valid tool to better understand the mechanisms behind polymer degradation. The acetic acid production, indeed, was mostly associated with a depletion of the UV absorber that had a dramatic impact on the EVA stability. Additionally, the local higher temperatures in correspondence of the junction box caused a non-homogeneous depletion of the primary antioxidant for the modules exposed in the tropical climate.