PV Modules

In the last few years PV technology has seen continuous improvements, with significant enhancements at the cell and module levels. In addition to the requirement of high efficiency, the long-term reliability of PV modules leads to proposals for innovative module concepts and designs. Meyer Burger has developed a low-temperature wire-bonding technology, known as SmartWire Connection Technology (SWCT), with the aim of offering a cost-effective solution for high-efficiency solar cells while minimizing cell-to-module losses. The introduction of this interconnection design immediately brings new challenges, especially in the selection of an appropriate encapsulant, which must ensure a good processability as well as the required long-term module reliability. The compatibility of the most cost-effective types of encapsulant currently available on the market was analysed in the study reported in this paper. Thermoplastic polyolefin encapsulants with water absorption less than 0.1% and no (or few) cross-linking additives have proved to be the best option for long-lasting PV modules in a glass-glass (GG) configuration. The development of a laminator having a symmetrical structure (two heating plates without any vacuum membrane) has also opened the door to fast lamination processes with cycle times under eight minutes.

In anticipation of the expected increase in the use of back-contact cells in future PV modules, a number of different concepts have been proposed. This paper focuses on one approach that aims to stay close to conventional solder-based technology (tabbing - stringing) while still allowing the use of back-contact cells (which have more complex back-side metallization schemes). The advantages and disadvantages of such an approach are discussed, and the development of this technology is described in terms of process flow, materials, characterization and reliability.

With lower returns on investment in PV projects, financial institutions have an ever-increasing demand for risk mitigation. Project stakeholders are asked to provide evidence of risk-management actions and have to look for ways to guarantee an adequate level of quality for their systems. Product certification, although necessary to help qualify the design of a product, does not provide a guarantee that mass production will achieve the targeted quality level; it has therefore become necessary to find reliable methods to assess the quality of PV systems on a large scale. Production monitoring, as part of a global quality plan for a PV system, is a cost-effective way to implement real-time checks in the manufacturing facilities, providing reassurance for stakeholders and helping manufacturers to improve their manufacturing processes. This paper details the principles behind production monitoring, the methodology used and how to deploy a production-monitoring project.

With the objectives of reducing cell-to-module losses, improving module efficiency and reducing the price per watt, increasing importance is being placed on the optical properties of backsheets. It is assumed that a higher reflectance backsheet allows a better reuse of incident sunlight. However, this statement is not always true: another factor must be taken in account, namely the angular dependence of the reflected light. In this regard, backsheets with a high specular component deviate from the ideal Lambertian reflectance, resulting in a minor increase in module current. As a result, differences can be found in module power because of the use of backsheets with similar global reflectance but different angular components of reflected light. A total of 33 industrial backsheets with Tedlar, Kynar, EVA and PET layers from different suppliers were analysed. A comparison of backsheets with low and high global reflectances revealed that the power variation in a standard PV module reaches 0.54% abs. In the same vein, and for backsheets with similar global reflectances, it was experimentally found that the angular response of the reflected light was responsible for a power difference of 0.22% abs. in a standard module.

Why change a product which can be sold in high quantities with a large margin? This is one of the reasons why crystalline silicon modules look the same today as they did 30 years ago. In addition, a module has to last for more than 20 years; to change the technology, or even just the material, many complicated, long-lasting and costly tests are necessary. And even after a series of successful tests there is no guarantee of a long-lasting product. Moreover, during the PV crisis starting in 2009, module manufacturers did not have the manpower and budget for introducing novelties into the module market. All the above are reasons why module architecture and materials did not significantly change with time and did not adapt to the introduction of powerful, highly efficient solar cells. After the crisis, however, many module manufacturers became aware that in order to be able to sell modules on the market with a high margin, their products not only have to be cost effective but also must differentiate themselves from the mass product. Consequently high-power, optically nice, colourful, backcontact, transparent, bifacial, light and highly durable modules are now being developed and are gradually being introduced into today’s market. This paper reports on current trends and discusses future developments.

Cracking of solar cells is a serious issue for product safety and module performance. Cracks may result in power loss, hot spots or arcing, and are caused by exceeding the strength limit of silicon. During the last few years, various studies have shown that fracture of encapsulated solar cells can be influenced by the manufacturing processes, which lead to residual stresses in solar cells. The results presented in this paper will give insights into the stresses generated by soldering and lamination. Furthermore, mechanisms of stress generation will be explained. On the basis of these findings, recommendations are made as to how to mitigate stresses, for example by means of alternative soldering processes, different soldering parameters or material optimization of the copper ribbon or the encapsulant.

The main objective of this study was to evaluate the suitability of thermal analysis for characterizing the stage of hydrolytic degradation of PV backsheets containing polyethylene terephthalate (PET) as a core layer. Additionally, the ageing behaviour of single backsheets was compared with that of backsheets incorporated within PV modules. Test modules using identical components (glass, encapsulant, solar cells, etc.), varying only in the type of backsheet used, were fabricated and artificially aged (damp heat:85°C / 85% relative humidity storage up to 2000h). The material characteristics of the single backsheets and module-incorporated backsheets before and after artificial ageing were determined by thermal analysis. It was shown that the most significant changes between unaged and aged sheets can be observed in the cooling curve of the differential scanning calorimetry (DSC) runs. For all materials, a significant increase in the crystallization temperature was found. Furthermore, the results revealed no influence of the PV module lamination procedure on the thermal characteristics of the polymeric backsheets. Even after artificial ageing under damp-heat conditions, differences between single and module-incorporated laminated backsheets were negligible. The ageinginduced degradation effects of PET could be detected by DSC for all the aged sheets. It is therefore proposed that the testing of single PET-based backsheets under accelerated ageing conditions may be a practicable way to investigate the applicability of a new backsheet material for use in reliable PV modules.

The potential for PV modules to fail before the end of their intended service life increases the perceived risk, and therefore the cost, of funding PV installations. While current IEC and UL certification testing standards for PV modules have helped to reduce the risk of early field (infant mortality) failures, they are by themselves insufficient for determining PV module service life. The goal of the Fraunhofer PV Durability Initiative is to establish a baseline PV durability assessment programme. PV modules are rated according to their likelihood of performing reliably over their expected service life. Modules are subjected to accelerated stress testing intended to reach the wear-out regime for a given set of environmental conditions. In parallel with the accelerated tests, modules are subjected to long-term outdoor exposure; the correlation between the accelerated tests and actual operation in the field is an ultimate goal of the programme. As understanding of PV module durability grows, the test protocols will be revised as necessary. The regular publication of durability ratings for leading PV modules will enable PV system developers and financiers to make informed deployment decisions. This paper provides summary data for eight module types from the two rounds of testing to date.

Recent advances at the cell level and in tabber-stringer equipment have led to the development of the next generation of cell interconnection architecture, resulting in an increase in cell and module performance. The multi-busbar (MBB) concept discussed in this paper delivers the benefits of a saving in material costs, a reduction in total series resistance and an improved light utilization for higher performance at lower cost. The combination of the cell and module concept and the stringer equipment works for a wide variety of cell types and enables an appreciable decrease in cost per watt and module size per watt.

The SPF solar glass certification was developed in 2002 to guarantee the quality of glazing for use as a transparent cover for solar thermal collectors. More than 200 glass types from leading manufacturers have been measured and certified to date. Despite the certification having been explicitly developed for solar thermal applications, it became widely used in the PV module industry, even though the results are not transferable and may lead to erroneous conclusions in some cases. In 2012 the certification was therefore adapted to the needs of the PV industry, and a dedicated PV solar glass certification has since been available. This paper explains the fundamentals of the certification process, which consists of three performance characterizations: 1) transmissivity, 2) incident angle modifier (IAM), and 3) UV degradation. Results are discussed for different representative glass types, including float glass, anti-reflective-coated glass and rolled glass with different structures. Furthermore, the performance of these glass types when used as covers of crystalline silicon PV modules is compared. The examples presented also highlight the advantages of the adapted characterization methods compared with standard glass measurements.