State-of-the-art bifacial module technology

By Dr. Hartmut Nussbaumer; Dr. Markus Klenk; Andreas Halm; Prof. Dr. Andreas Schneider

Bifacial PV promises a significant reduction in the levelized cost of electricity (LCOE) for PV systems, which, compared with efficiency improvements at the cell level, is still achievable with comparatively moderate effort. Almost all major PV module suppliers have bifacial modules in their product portfolios or have announced production. This paper gives an overview of the currently available bifacial modules and cell technologies and the performance of these modules. Special attention is given to the cells and the layout of the modules, including light trapping and interconnection technologies, the encapsulation materials and the adapted mounting solutions. Finally, an outlook is given on the basis of the compiled information.

Advances in module interconnection technologies for crystalline silicon solar cells

By Jan Kroon; Dr. Bonna Newman; Jonathan Govaerts; Dr. Eszter Voroshazi; Tom Borgers

In the evolution towards higher cell efficiencies, new cell concepts (twosided and back contacted) have been introduced and for each of these concepts, new module materials and interconnection technologies have to be developed to fulfil all the demands of a good end product in terms of lowest costs, highest yield and power and above all superior quality (reliability and durability). There is no single module concept that fits all cell concepts or module application type so existing module concepts need to be adapted or innovative module technologies are required to fit the aforementioned requirements. This paper provides an overview summarizing the recent developments of integrated cell to module manufacturing approaches such as multi-busbar, multi-wire, half-cell and shingling technologies for two-side contacted cells and advanced soldering, woven fabric and foil based module technologies for back contacted cells aiming for the highest power outputs, lowest costs and longest lifetimes.

Power rating and qualification of bifacial PV modules

By Xiaoyu Zhang; Christos Monokroussos; Markus Schweiger; Matthias Heinze, TÜV Rheinland Group

The extra energy gain offered by bifacial PV modules has helped make them an increasingly popular choice in the global PV industry. But the question of how to define, measure and rate the electrical output from bifacial modules is a hotly debated topic, given the extent to which the rear-side contribution is dependent on a range of variable factors relating to local environmental conditions and system configurations. Drawing on in-house modelling and simulation software developed at TÜV Rheinland, this paper explores the power rating issue for bifacial devices, examining the definitions of rear irradiance, measurement test method, power stabilization and verification for type approval. Relevant reliability and safety tests are discussed, with additional modifications and suggestions for bifacial PV modules.

Challenges for the interconnection of crystalline silicon heterojunction solar cells

By Angela De Rose; Torsten Geipel; Denis Erath; Achim Kraft; Ulrich Eitner, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany

Crystalline silicon heterojunction (HJT) solar cells and modules based on amorphous silicon on monocrystalline wafers offer advantages over established wafer-based technologies in terms of efficiency potential, complexity of the manufacturing process, and energy yield of the modules. The temperature sensitivity of these solar cells, however, poses considerable challenges for their integration in modules. Currently, there exist three approaches for the interconnection of HJT solar cells, each with its own strengths and weaknesses: 1) ribbon soldering with low-meltingpoint alloys; 2) gluing of ribbons by using electrically conductive adhesives (ECAs); 3) SmartWire Connection Technology (SWCT).

The Most Efficient and Adaptable Solution Design for Bifacial Modules

Leading PV inverter manufacturer Huawei discusses recent technical developments to a better understanding of bifacial solar module PV power plants, using three recent case studies. These efficient PV modules need to be used with devices such as inverters to maximize value. Recently, many inverters and solutions that match bifacial modules have appeared in the industry. Which solution is the best match for bifacial modules? Based on a large amount of experimental data, this article describes the solution needed for bifacial modules.

Guidelines for accurate currentvoltage measurement of highefficiency c-Si solar cells

By Jacques Levrat; Jonas Geissbühler; Bertrand Paviet-Salomon; Christophe Ballif; Matthieu Despeisse

The market for commercial crystalline silicon (c-Si) solar modules has been ruled for decades by the well-established ribbon-interconnected Al-BSF solar cells, making their metrology and in particular the current-voltage measurement well defined and reproducible.

Bifacial PV: comparing apples with apples sometimes does not make sense

By Radovan Kopecek & Joris Libal, ISC Konstanz, Germany

Bifaciality can be implemented by varieties of architectures for solar cells, modules and in addition there are even many more applications on system level.This makes bifaciality a complex technology. Currently there is some confusion in the PV community what bifacial gains can be expected and how these transfer to the cost reduction and lowering the LCOE of the system. In this article we will describe how bifacial gains are defined, what bifacial gains can be expected and what this means for real applications.

From bifacial PV cells to bifacial PV power plants – the chain of characterization and performance prediction

By Christian Reise, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany; Michael Rauer, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany; Max Mittag, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany; Alexandra Schmid, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany

Bifacial PV technology raises new challenges for the characterization and modelling of solar cells and modules, as well as for the yield predictions of power plants, as the contribution of the rear side can significantly affect the performance of these types of device.

Complex problems require simple solutions: How to measure bifacial devices correctly?

By Dr. Radovan Kopecek, is one of the founders of ISC Konstanz. He has been working at the institute as a full-time manager and researcher since January 2007 and is currently the leader of the advanced solar cells department.; Dr. Joris Libal, works at ISC Konstanz as a project manager, focusing on business development and technology transfer in the areas of highefficiency n-type solar cells and innovative module technology.

LONGi, Jolywood and many other large PV manufacturers claim that bifacial mono c-Si technology is the future. Since 2015, bifacial PV installations have been entering multi- MW installation levels, and are expected to enter multi-GW levels in 2018.

Power loss/gain characterization: Modules with multi-busbar, half-cut cells & light-trapping ribbon

By Jai Prakash Singh , received his Ph.D. in electrical and computer engineering from NUS. He works as a research scientist with SERIS at NUS, and has more than 10 years’ experience in solar PV.; Yong Sheng Khoo, is the head of module development group at SERIS, and has more than five years’ experience in PV module development and testing.; Cai Yutian, received his B.Eng. (Hons) in materials science and engineering. He is currently a research engineer at SERIS, where his research focuses on the prototyping of PV modules; Srinath Nalluri, obtained his B.Eng. from NUS and has around three years’ experience in the PV industry.; Sven Kramer, studied management and engineering at the Cooperative University in Stuttgart, Germany.; Axel Riethmüller, studied mechanical engineering at the University of Stuttgart, Germany.; Yan Wang, is the director of the PV module cluster at SERIS, and has profes siona l knowledge of PV te chnolog y and hands-on manufacturing experience spanning various PV products.

To guarantee the long-term competitiveness of the PV industry, the cost of PV power generation ($/kWh) must be continuously reduced. Such reduction can be achieved in two ways: 1) by improving PV module performance (efficiency, annual energy yield, reliability); 2) by reducing manufacturing costs ($/Wp).

Systematic PV module optimization with the cell-to-module (CTM) analysis software

By Max Mittag, Max Mittag studied industrial engineering and management at the Freiberg University of Mining and Technology. In 2010 he completed his diploma thesis at Fraunhofer ISE and joined the department for photovoltaic modules. His current work includes the cell-to-module efficiency analysis and the development new photovoltaic module concepts.; Matthieu Ebert, Matthieu Ebert holds a ma s t er de g r e e i n r e n ewa b l e en e r g y s y s t ems f rom t h e University of Applied Science, Berlin. Before joining Fraunhofer ISE in 2011 he completed research stays at the Fraunhofer CSE in Boston and at the Australian National University in Canberra. Since 2011 he has been undertaking research on PV module technology. Since 2015 he has led the module efficiency and new concepts team. His main areas of research are module efficiency and CTM analysis, building-integrated PV and PV for automotive applications.

Understanding power losses in technical systems is vital to improve products in every industry and photovoltaic modules present no exception. Losses in solar modules are caused by optical and electrical effects or are determined by simple module geometry through inactive areas.

Cell modifications for preventing potential-induced degradation in c-Si PV systems

By Gaby Janssen, Gaby Janssen obtained her Ph.D. in quantum chemi s t r y f rom the University of Groningen in the Netherlands. At ECN she has been working as a research scientist on the simulation, characterization and optimization of materials for energy conversion technologies. Since 2011 she has been focusing on simulation and modelling of PV cells and modules; Maciej Stodolny, Maciej Stodolny received his M.Sc. in applied physics from Gdansk University of Technology in Poland. His Ph.D. research at ECN and the University of Twente dealt with Cr tolerance of solid oxide fuel cells. He now works as a solid state physicist and materials scientist at ECN Solar Energy.; Bas Van Aken, Bas Van Aken received his Ph.D. in solid state chemistry from the University of Groningen, after which he worked as a postdoc at Cambridge University and at the Max Born Institute for Nonlinear and Ultrafast Optics in Berlin. He is currently a researcher in the PV module technology group at ECN, where he focuses on fabrication, reliability and (outdoor) performance of bifacial and back-contact modules.; Jochen Löffler, Jochen Löffler holds a Ph.D. in physics from Utrecht University in the Netherlands, and has been working on PV and related topics since 1998. He joined ECN in 2005 and is currently a senior scientist and project leader in the field of crystalline silicon solar cells, with a focus on industrial high-efficiency cells and modules.; Hongna Ma, Hongna Ma holds an M.Sc. and has many years’ experience in silicon solar cell research. She joined Yingli in 2008 and is currently the leader of the group working on n-type Si solar cells. Her focus is on R&D of solar cells with ion implantation technology.; Dongsheng Zhang, Dongsheng Zhang has an M.Sc. and joined Yingli in 2004, where he currently leads the battery technology department. His work experience in silicon solar cell research spans many years, and he has acquired an in-depth understanding of all aspects of the crystalline silicon solar cell process; Jinchao Shi, Jinchao Shi holds an M.Sc. and has worked in silicon solar cell and module research for many years. He joined Yingli in 2006, where he is currently the general manager of the Technology Center. He has extensive experience in lab to fab transference, as well as in cell and module mass production.

In recent years, potential-induced degradation (PID) has been recognized as a serious reliability issue for large PV systems, potentially causing efficiency losses of more than 90%, and even failures [1–4]. Such large decreases in efficiency may require the modules in the system to be replaced after just a few years’ operation. This has motivated a substantial research effort in the PV community, leading to a better understanding of the phenomenon, as well as to a range of mitigation strategies. A recent publication by Luo et al. gives a comprehensive overview of this research [5].

Potential induced degradation (PID): a test campaign at module level

By J. Carolus, Hasselt University and imec vzw; M. Daenen, Hasselt University and imec vzw

Potential induced degradation (PID) of photovoltaic (PV) modules gets a lot of attention since 2010 when Solon published their findings about a degradation mechanism in their PV modules caused by high potential differences. When multiple PV modules are connected in series, a potential difference up to 1000 V or at some places even 1500 V is created between the cell and the grounded frame. This electrical field causes a leakage current and ion diffusion. PID is a multi-level degradation with causes and solutions at cell, module and system level. A test campaign was conducted within the frame of a feasibility study for pidbull, a curing technology for PID developed by pidbull nv. 80 PV modules were characterized whereof 49 PV modules were stressed and cured for PID. The selected set of PV modules was composed of 49 different module types of 33 brands. The test was done according to the foil-method, as described by the standard in progress IEC 62804. However, to apply higher stressing and curing rates, the modules were tested with an aluminium foil inside a climate chamber for 96 hours. After the stress test, only 22% of the tested modules passed the 5% loss criteria as described by IEC 62804. In other words, 78% out of a set of today's most installed PV modules in Flanders are PID sensitive. Remarkable is that only 16 out of the 49 PV modules have less than 20% PID after the stress test. Additionally, a linear trend for PID reversibility was shown for modules with a stress level of less than 85%. The modules which lost more than 85% due to PID showed a lower recovery rate or in worst case didn’t recover at all.

Technical progress in high-efficiency solar cells and modules

By Rulong Chen, Xi Xi, Jie Zhou, TingTing Yan & Qi Qiao, Wuxi Suntech Power Co., Ltd

This paper focuses on the technical progress of high-efficiency crystalline silicon solar cells and modules, specifically with regard to passivated emitter and rear cell (PERC) processes, module description and light induced degradation (LID) data. Through appropriate optimizations of the solar cell and module processes, the cell efficiency achieved in mass production is 21.3%, with module power exceeding 300W. To solve the LID problem, hydrogenation technology developed by UNSW is used, bringing the cell LID rate down to below 1%.