Double-glass PV modules with silicone encapsulation

By Shencun Wang & Xiang Sun, BYD; Yujian Wu & Yanxia Huang, Dow Corning (China) Holding Co; Nick Shephard, Dow Corning Corporation; Guy Beaucarne, Dow Corning Europe

Double-glass PV modules are emerging as a technology which can deliver excellent performance and excellent durability at a competitive cost. In this paper a glass–glass module technology that uses liquid silicone encapsulation is described. The combination of the glass–glass structure and silicone is shown to lead to exceptional durability. The concept enables safe module operation at a system voltage of 1,500V, as well as innovative, low-cost module mounting through pad bonding.

Cell-to-module power loss/gain analysis of silicon wafer-based PV modules

By Jai Prakash Singh, Yong Sheng Khoo, Jing Chai, Zhe Liu & Yan Wang, Solar Energy Research Institute of Singapore, National University of Singapore

We are always hearing about champion cells demonstrating efficiencies of 24% or higher, yet only 20 or 21% can be obtained at the module level. This paper highlights the different loss mechanisms in a module, and how they can be quantified. Once it is known where photons and electrons are lost, it is possible to develop strategies to avoid this happening.

Reliability and durability comparison of PV module backsheets

By Haidan Gong & Guofeng Wang, Wuxi Suntech Power Co

The backsheet is the first barrier for ensuring the reliability and durability of PV modules for 25+ years. To reduce cost, backsheets with a variety of compositions and constructions have been developed and introduced in PV modules. For PV module manufacturers, a major challenge is choosing a low-cost backsheet that can maintain the current levels of high reliability and durability performance. In the work reported in this paper, the properties of several backsheets of various compositions and constructions were compared.

Cost/kWh thinking and bifaciality: Two allies for low-cost PV of the future

By Radovan Kopecek, Ismail Shoukry & Joris Libal, ISC Konstanz

This paper demonstrates that the future of the lowest-cost electricity generation from PV is not all about increasing cell and module efficiencies and minimizing cost/Wp, but rather squeezing the best out of a system using a few simple tricks, such as bifaciality, tracking and ground reflection improvements, to achieve the lowest cost/kWh.

Reliability and durability impact of high UV transmission EVA for PV modules

By Haidan Gong, Wuxi Suntech Power Co., Ltd., Wuxi, China; Guofeng Wang, Wuxi Suntech Power Co., Ltd., Wuxi, China

Newly developed high UV light transmission ethylene vinyl acetate (EVA) has recently been extensively introduced for use in PV modules. It has been proved that this type of EVA can result in potential power gain because of the better blue light response of the solar cell, which in turn can further reduce the cost per watt of the PV module. However, if only high UV transmission EVA is used as an encapsulant, too much UV light irradiates the backsheet, which can cause the backsheet to yellow. In order to improve the reliability and durability of the modules, SUNTECH, as a module manufacturer, therefore uses combined EVA, i.e. high UV transmission EVA as the front encapsulant and conventional UV cut-off EVA as the rear encapsulant, to protect the UV-sensitive backsheet. This paper presents the results of an investigation of the reliability and durability of high UV transmission EVA in PV modules, through an enhanced UV test which exceeds IEC standards.

Cell-to-module losses in standard crystalline PV modules – An industrial approach

By Eduardo Forniés, Aurinka PV Group SL, Madrid; José Pedro Silva, CIEMAT – División de Energías Renovables, Madrid, Spain

One of the main concerns of module manufacturers is the power loss that takes place when the solar cells are incorporated in PV modules. This power loss, known as cell-to-module (CTM) loss, results from the influence of many factors which occur during module production. Some of these factors lead to a gain in power at the end of the process; on the other hand, some are responsible for a loss of power and offset the positive effects of other ones, resulting in a net power loss. In this paper the CTM losses will be addressed from an industrial point of view and for standard crystalline PV modules. The focus will first be on some of the most frequent issues detected in production lines and their influence on module power loss. More extensive research is then carried out to arrive at an explanation of their origin. This paper describes some of the mentioned factors along with the different ways of detecting them.

Operational sustainability in the field versus the laboratory: PV modules and insulation resistance

By Jim Zhu , Wuxi Suntech Power Co., Ltd., Wuxi, China; Jeff Wang, Wuxi Suntech Power Co., Ltd., Wuxi, China

Poor insulation resistance in modules is one of the primary contributors to module failure. Regimes currently in place to test the insulation resistance of crystalline silicon modules have proved problematic, as the conditions found in a laboratory are not on a par with environmental conditions at installation sites. This paper explores the shortcomings of current testing standards and recommends further tests that should be introduced to prevent module failures in the field.

Minimizing measurement uncertainties: Challenges for power measurement of high-efficiency c-Si PV modules

By Christos Monokroussos, TÜV Rheinland, Shanghai, China; Johannes Stang, TÜV Rheinland, Cologne, Germany

High-efficiency (HE) PV technologies, such as heterojunction, back-contact or n-type, can be affected by significant measurement errors compared with conventional technologies; the power measurement of HE crystalline silicon PV modules and cells has therefore been a challenge for the PV industry for at least two decades. To deal with the internal capacitance and the spectral mismatch errors of HE cells and modules, various measurement techniques are currently used: steady-state, multi-flash, dynamic I–V, DragonBack™ and dark I–V and reconstruction methods, to name a few. This paper discusses the challenges and provides guidance for best practice for acquiring accurate measurements.

Shunting-type potential-induced degradation: How to ensure 25 years’ service life

By Christian Taubitz, Hanwha Q CELLS GmbH, Bitterfeld-Wolfen, Germany; Matthias Schütze, Hanwha Q CELLS GmbH, Bitterfeld-Wolfen, Germany; Marcel Kröber, Hanwha Q CELLS GmbH, Bitterfeld-Wolfen, Germany; Max B. Koentopp, Hanwha Q CELLS GmbH, Bitterfeld-Wolfen, Germany

Potential-induced degradation (PID) of the shunting type (PID-s) is one of the most severe forms of PID, which is caused by the negative potential of p-type solar cells with respect to grounded frames/mounting. Although this negative potential can be completely avoided at the system level, that is not the case for a large number of modern PV systems. PV modules that are able to sustain PID-s stress for at least the duration of their service life are therefore essential. To assess whether modules fulfil this requirement, laboratory tests are currently recommended in which the modules are exposed to a certain constant level of PID-s stress for a given amount of time. These types of test with constant stress levels, however, are only feasible in the case of degradation mechanisms that are not reversible in the field, for which non-coherent stress episodes simply sum up to the total stress. Unlike other mechanisms, PID-s is reversible under field conditions; as a consequence, the level of PID-s of a fielded module is the result of an intricate interplay of phases of degradation and regeneration. This behaviour cannot be replicated in a laboratory test using a constant stress level; the currently recommended laboratory tests for PID-s with constant stress levels are therefore not appropriate for assessing the service life duration, and can only be used for differentiating the susceptibility to PID-s stress and for monitoring the stability of production processes. For monitoring the PID-s resistance of its products, Hanwha Q CELLS uses tests for PID-s with constant stress in accordance with the draft for IEC PID test method 62804. This assures that all the products of the Q CELLS brand come with Anti-PID Technology (APT). The expected service life duration with respect to PID-s is assessed by simulating the interplay of degradation and regeneration under non-constant outdoor conditions that are based on meteorological data.

Lamination process and encapsulation materials for glass-glass PV module design

By Gianluca Cattaneo, Modules and Systems Sector, CSEM PV-Center; Antonin Faes, CSEM PV-Center; Heng-Yu Li, CSEM PV-Center, PV-Lab, Institute of Microengineering (IMT), EPFL; Federico Galliano, Modules and Systems Sector, CSEM PV-Center, PV-Lab, Institute of Microengineering (IMT), EPFL; Maria Gragert, R&D Department, Meyer Burger AG; Yu Yao, R&D Project Manager, Meyer Burger AG; Rainer Grischke, R&D Department, Meyer Burger AG; Thomas Söderström, Head of Department, Transfer process and innovation module, Meyer Burger AG; Matthieu Despeisse, CSEM PV-Center; Christophe Ballif, CSEM PV-Center, PV-Lab, Institute of Microengineering (IMT), EPFL; Laure-Emmanuelle Perret-Aebi, CSEM PV-Center

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.

Adapting conventional tabbing- stringing technology for back-contact solar cells and modules

By Tom Borgers, Si PV Group, IMEC; Jonathan Govaerts, Si PV Group, IMEC; Arvid van der Heide, Development and Research, Soltech; Stefan Dewallef, Engineer, Soltech; Ivan Gordon, Si PV Group Leader, IMEC; Jozef Szlufcik, PV Department Director and Si PV Programme Manager, IMEC; Jef Poortmans, Director of the Solar and Organic Technologies Department, IMEC

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.

Production monitoring in PV: Principles, methodology and deployment

By Thibaut Lemoine, General Manager, Senergy Testing Solutions Ltd (STS)

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.

The importance of optical characterization of PV backsheets in improving solar module power

By Salvador Ponce-Alcántara, Head of the PV Module Section, Valencia Nanophotonics Technology Center, UPV; Alberto A. Vivas Arangú, Valencia Nanophotonics Technology Center, UPV; Guillermo Sánchez Plaza, Director of Technology, Valencia Nanophotonics Technology Center, UPV

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.

Module technologies for highefficiency solar cells: The move away from powerful engines in old-fashioned car bodies

By Joris Libal, Research Engineer, ISC Konstanz; Andreas Schneider, Head of the Module Development Department, ISC Konstanz; Andreas Halm, Project Manager, ISC Konstanz; Radovan Kopecek, Head of the Advanced Solar Cells Department, ISC Konstanz

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.

Stress analysis of manufacturing processes for solar modules

By Sascha Dietrich, Module Reliability Group, Fraunhofer Center for Silicon Photovoltaics CSP; Matthias Pander, Module Reliability Group, Fraunhofer Center for Silicon Photovoltaics CSP; Martin Sander, Module Reliability Group, Fraunhofer Center for Silicon Photovoltaics CSP; Rico Meier, Module Reliability Group, Fraunhofer Center for Silicon Photovoltaics CSP; Matthias Ebert, Head of the Module Reliability Group, Fraunhofer Center for Silicon Photovoltaics CSP

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.