Reducing the electrical and optical losses of PV modules incorporating PERC solar cells

By Henning Schulte-Huxel, Robert Witteck, Malte Ruben Vogt, Hendrik Holst, Susanne Blankemeyer, David Hinken, Till Brendemühl, Thorsten Dullweber, Karsten Bothe, Marc Köntges & Rolf Brendel, Institute for Solar Energy Research Hamelin (ISFH), Emmerthal, Germa

The continual increase in cell efficiency of passivated emitter and rear cells (PERCs), as well as the optimization of the module processes, has led to significant advances in module power and efficiency. To achieve the highest module power output, one important aspect to consider is the optimization of the solar cell front metallization and the cell interconnection.

Positive cell-to-module change: Getting more power out of back-contact modules

By Bas B. van Aken & Lenneke H. Slooff-Hoek, ECN – Solar Energy, Petten, The Netherlands

Cell-to-module (CtM) loss is the loss in power when a number of cells are interconnected and laminated in the creation of a PV module. These losses can be differentiated into optical losses, leading to a lower photogenerated current, and resistive losses, leading to a decrease in fill factor. However, since the application of anti-reflection (AR) coatings and other optical ‘tricks’ can sometimes increase the Isc of the module with respect to the average cell Isc, the CtM loss in such cases needs to be expressed as a negative value, which gives rise to confusion. It is proposed to use the CtM change, where a negative value corresponds to a loss in current or power, and a positive value to a gain. In this paper, the CtM changes for back-contact modules utilizing a conductive foil are described and compared with other mature module technologies. A detailed analysis of the CtM change for a full-size metal-wrap-through (MWT) module is presented.

Electroluminescence (EL) studies of multicrystalline PV modules

By Sreenivasa Murty Dasari, Chandra Mauli Kumar, Amresh Mahajan & Nagesh C, Tata Power Solar, Bengaluru, India

Higher power generation yield is the prime objective of any solar power plant developer. The quality and reliability of the modules used are therefore a key aspect, with customers placing stringent criteria on cell and module manufacturers with regard to product quality. Electroluminescence (EL) image monitoring, which gives a clear picture of defect distribution across a module, is an increasingly popular quality criterion.

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.