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)

Vacuum lamination of photovoltaic modules

Vacuum lamination of terrestrial photovoltaic modules is a new high volume process requiring new equipment and newly develop materials. Equipment development, materials research, and some research in related fields and testing methods are discussed

Investigation of the reliability of the encapsulation system of photovoltaic modules

Good reliability of the encapsulation system of Photovoltaic (PV) modules is crucial to ensure the long-term performance of PV modules. A carefully controlled lamination process is required to produce a reliable encapsulation system. To date, the influences of different lamination conditions on the reliability of the encapsulation system are poorly understood. To predict the performance of the encapsulation system, the correlation of the reliability of the encapsulation system with various stress levels is required, which is poorly developed. This thesis improves the understanding of these issues by investigating the correlation of different lamination conditions with the reliability of the encapsulation system and the degradation of adhesion strength under variable damp-heat conditions. The influence of the curing temperature and curing time on the long-term reliability of the encapsulation system is investigated from various viewpoints such as curing level of EVA, chemical and optical stability of EVA and adhesion strength within the encapsulation system. The correlation of curing level and lamination quality has been identified. The effects of over-curing are demonstrated. Results show that the chemical stability, optical stability and the adhesion strength between encapsulant and backsheet increases with the increasing curing level. However, the best long-term adhesion performance at the glass-encapsulant interface is obtained at lower gel content. Too high curing can cause problems of bubble generation, discoloration and unstable interfaces. Among those identified degradation phenomena, interfacial adhesion strength demonstrates the fastest and the largest degradation. The reliability of the adhesion strength is further examined under different stress levels. Among different environmental stress factors, moisture is considered to cause the greatest problems of adhesive interfacial stability. Therefore, the adhesion strength is investigated under different damp-heat conditions. A methodology is developed that can be used to model the adhesion degradation induced by moisture at different humidity and temperature conditions. To do so, a stress model is established which enables quantitative description of the moisture related stresses on PV modules. Based on this model, an exponential correlation is established between the adhesion strength and the humidity and temperature levels. This enables the comparison of adhesion strength of PV modules operating at different humid environments

Effects of lamination condition on durability of PV module packaging and performance

Ten mini-modules of glass /encapsulant /backsheet structure were laminated under the condition of the same curing time and pressure, but different curing temperatures and aged in damp-heat accelerated ageing test in order to investigate the effect of temperature on the durability of PV module packaging and performance. Results show that the mini-modules using EVA as encapsulant were affected more by the laminating temperature compared to the mini-modules using modified ionomer. For EVA modules, samples with relatively low curing temperatures at 135-140ºC appeared to have higher adhesion between EVA and glass, lower moisture permeability into module and better dielectric of cells than those with high curing temperatures

Selection of paste and glue elements for CPV modules

Tese de mestrado em Engenharia da Energia e do Ambiente, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2010This thesis reports on the progress of the development of encapsulation methods and materials to use in the HSUN PV receiver. The HSUN is a concentration photovoltaic (CPV) system concept under development by WS Energia. After a thorough description of the encapsulation state of the art, two main approaches were tested: Ethylene Vinyl Acetate (EVA) laminates and Silicone stacks. Experimental results were unsatisfactory regarding the EVA laminates. The curing process was not fully optimized which lead to (i) the appearance of yellowness after exposing the laminate to concentrated irradiation for a few days; and (ii) the increase of the series resistance of the solar cell during the curing process, probably associated to stretching of the soldered contacts. Numerical thermal modelling of the EVA laminate has also shown the need to introduce active cooling of the PV receiver in order to prevent thermal damage to the cell. The silicone stacks tests were satisfactory regarding the optical, mechanical and electrical properties of the PV receiver. Even after a few days of concentrated irradiation there was no evidence of the development of yellowness or moisture. Thermal modelling showed that further optimization of the HSUN receiver concept is required but suggest that passive cooling approaches are probably sufficient to warrant safe thermal conditions for the solar cell even under concentrated irradiation.O desenvolvimento da presente tese é baseado na pesquisa de materiais e desenvolvimento de métodos de encapsulamento. Tem como principal objectivo, uma aplicação no projecto HSUN, que visa o desenvolvimento de um módulo fotovoltaico de concentração (CPV) na WS Energia. Após uma descrição pormenorizada do estado de arte do encapsulamento, foram testadas duas abordagens: Laminação de amostras com Acetato de Etileno Vinil (EVA) e Silicone. Os resultados experimentais podem ser considerados satisfatórios atendendo à qualidade dos laminados de EVA. O processo de cura não estava completamente optimizado o que poderá ter conduzido a (i) aparecimento de amarelamento após exposição das amostras à luz concentrada durante alguns dias; e (ii) o aumento das resistências de série da célula durante o processo de cura. Este facto poderá ser associado ao alongamento dos contactos soldados. A modelação do modelo térmico para o laminado de EVA também demonstrou a necessidade de introdução de um arrefecimento activo no módulo PV de modo a evitar que a temperatura provoque danos nas células. As amostras de silicone revelaram resultados satisfatórios em relação às propriedades ópticas, mecânicas e eléctricas do módulo PV. Após exposição da amostra à concentração solar durante alguns dias, não foi evidenciando o aparecimento de amarelamento ou humidade. O modelo térmico revelou que é necessária uma optimização do conceito HSUN. As aproximações efectuadas sugerem que um modelo de arrefecimento passivo será suficiente para garantir as condições óptimas para a célula quando submetida à irradiação concentrada

Moisture ingress in photovoltaic modules: A review

Moisture ingress in photovoltaic (PV) modules is the core of most degradation mechanisms that lead to PV module power degradation. Moisture in EVA encapsulant can lead to metal grids corrosion, delamination and discolouration of encapsulants, potential induced degradation, optical and adhesion losses. The present work is a review of literature on the causes, effects, detection, and mitigation techniques of moisture ingress in PV modules. Literature highlights on determining the diffusivity, solubility, and permeability of polymeric components of PV modules via water vapour transmission rate tests, gravimetric, and immersion methods, have been presented. Electroluminescence, photoluminescence, and ultraviolet fluorescence spectroscopy, as well as dark lock-in thermography are some techniques used to detect moisture ingress in modules. Encapsulants with excellent moisture barrier and adhesion characteristics, desiccant-stacked polyisobutylene sealants, imbedded moisture sensors, and PV designs with/without breathable backsheets are ways of preventing/detecting moisture ingression in PV modules. Areas of focus for future research activities have also been discussed.publishedVersio

Electricity from photovoltaic solar cells: Flat-Plate Solar Array Project final report. Volume VII: Module encapsulation

The Flat-Plate Solar Array (FSA) Project, funded by the U.S. Government and managed by the Jet Propulsion Laboratory, was formed in 1975 to develop the module/array technology needed to attain widespread terrestrial use of photovoltaics by 1985. To accomplish this, the FSA Project established and managed an Industry, University, and Federal Government Team to perform the needed research and development. The objective of the Encapsulation Task was to develop, demonstrate, and qualify photovoltaic (PV) module encapsulation systems that would provide 20-year (later increased to 30-year) life expectancies in terrestrial environments, and which would be compatible with the cost and performance goals of the FSA Project. The scope of the Encapsulation Task included the identification, development, and evaluation of material systems and configurations required to support and protect the optically and electrically active solar cell circuit components in the PV module operating environment. Encapsulation material technologies summarized in this report include the development of low-cost ultraviolet protection techniques, stable low-cost pottants, soiling resistant coatings, electrical isolation criteria, processes for optimum interface bonding, and analytical and experimental tools for evaluating the long-term durability and structural adequacy of encapsulated modules. Field testing, accelerated stress testing, and design studies have demonstrated that encapsulation materials, processes, and configurations are available that will meet the FSA cost and performance goals. Thirty-year module life expectancies are anticipated based on accelerated stress testing results and on extrapolation of real-time field exposures in excess of 9 years

Review of world experience and properties of materials for encapsulation of terrestrial photovoltaic arrays

Published and unpublished information relating to encapsulation systems and materials properties was collected by searching the literature and appropriate data bases (over 1,300 documents were selected and reviewed) and by personal contacts including site and company visits. A data tabulation summarizing world experience with terrestrial photovoltaic arrays (50 installations) is presented in the report. Based on criteria of properties, processability, availability, and cost, candidate materials were identified which have potential for use in encapsulation systems for arrays with a lifetime of over 20 years high reliability, an efficiency greater than 10 percent, a total price less than $500/kW, and a production capacity of 500,000 kW/yr. The recommended materials (all commercially available) include, depending upon the device design, various borosilicate and soda-lime glasses and numerous polymerics suitable for specific encapsulation system functions

Factors influencing the measured adhesive strength between glass and encapsulant of photovoltaic modules

Peel tests are used in Photovoltaic (PV) industry to check the encapsulation quality of PV modules. The test results are influenced by variable factors including peel speed, peel angle, mechanical strength of both adhesive and adherend, as well as testing environment. This paper analyses the correlation of measured peel strength with several main factors of peel speed, peel angle and curing state of encapsulant. Laminates with a structure of Glass-Ethylene Vinyl Acetate (EVA) - double Polyethylene Terephthalate (PET/PET) are fabricated at variety conditions to achieve different curing states of EVA. Peel test between glass and EVA of these laminates are examined at the angle of 90o. Peel strength at different speeds and at angles of both 90o and 180o are also evaluated at fixed lamination condition. Finite Element Model (FEM) for 90o peel test is developed using Abaqus software. The interfacial fracture energy between Glass and EVA is calculated using Virtual Crack Closure Technique (VCCT). Results show that peel strength increases with the increasing peel speed and curing state. Fracture energy calculated from FEM modelling also indicates an increasing trend with peel speed

Developing an advanced module for back-contact solar cells

This paper proposes a novel concept for integrating ultrathin solar cells into modules. It is conceived as a method for fabricating solar panels starting from back-contact crystalline silicon solar cells. However, compared to the current state of the art in module manufacturing for back-contact solar cells, this novel concept aims at improvements in performance, reliability, and cost through the use of an alternative encapsulant, namely silicones as opposed to ethylene vinyl acetate, an alternative deposition technology, being wet coating as opposed to dry lamination; and alternative module-level metallization techniques, as opposed to cell-level tabbing-stringing or conductive foil interconnects. The process flow is proposed, and the materials and fabrication technologies are discussed. As the durability of the module, translated into the module's lifetime, is very important in the targeted application, namely solar cell modules, modeling and reliability testing results and considerations are presented to illustrate how the experimental development process may be guided by experience and theoretical derivations. Finally, feasibility is demonstrated in some first proofs of the concept, and an outlook is given pointing out the direction for further research