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Future Trend and Perspectives
14 Future Trend and Perspectives 1
Roger H. French 1, Hsinjin Edwin Yang 2 and Laura S. Bruckman 1 Case Western Reserve University, Case School of Engineering, SDLE Research Center, Materials Science and Engineering, Cleveland, OH, United States 2 Pioneer Scientific Solutions, LLC., Long Grove, IL, United States
14.1 Introduction Photovoltaics (PV) has been recognized as a renewable energy technology that has the potential to contribute significantly to the future energy supply and reduce carbon emissions. Therefore, the usage of solar energy has significantly increased worldwide. The science and technology driving PV will continue to be devoted to improving the efficiency of solar cells and developing new and improved materials for more durable and reliable PV modules. This book has attempted to describe especially the latter issues of all materials included in PV modules. The advancement of the research and development in PV modules and systems, efficiency upgrades, storage improvements, and equipment capabilities all contribute to a more effective power output for solar panels and lower costs for systems. The technology and opportunity in the solar industry are still growing [1]. Recently, the state of California in the United States has announced that any new house must install solar energy effective January 20, 2020. Therefore, the utilization of solar energy will be significantly increased in both commercial and residential buildings and technical efforts will focus on increasing the durability and reliability of PV modules and materials.
14.2 PV Durability and Reliability Motivators Many different sectors are interested in the durability, reliability, and safety of PV modules which will help drive improvement in these areas (Fig. 14.1). Therefore, it is of great interest and incentive to pursue the technical understanding and
to develop higher efficiency, new and improved materials, and safety standards for PV modules.
14.3 Innovations in PV Materials, Modules, and System Architectures The vast majority of PV modules are made from crystal-Si solar cells, representing more than 90% of the current market. Whereas today’s standard silicon PV panels will have somewhere in the range of 18 to more than 22% module photoconversion efficiency with either Al-BSF cell designs or the new passivated emitter rear cell (PERC) technology, thin film (TF) panels, such as cadmium telluride (CdTe), are demonstrating module efficiencies above 17% (such as for Series 6 modules from the First Solar). Module efficiencies of coppereindiumegalliumeselenide (CIGS) and coppereindiumeselenide (CIS) modules have been demonstrated to be more than 18% as shown by Solar Frontier, while single junction gallium arsenide (GaAs) module efficiency can be as high as 26% as demonstrated by Alta Devices. These module efficiencies are the focus of PV power plant system designers and owners and they also point to the important and ongoing role of cell architecture and cell material advances that continue improving the photoconversion efficiency of the cells across all photovoltaic cell technologies. In the case of crystalline silicon, the widespread commercialization of PERC cells has, in a 2 or 3 year period, completely displaced the use of Al-BSF silicon cells for monocrystalline wafers. And PERC is appearing to displace Al-BSF multicrystalline cells in the next few years, due to the improved photoconversion efficiency and beneficial cost structure of these new cell designs.
Durability and Reliability of Polymers and Other Materials in Photovoltaic Modules. https://doi.org/10.1016/B978-0-12-811545-9.00014-8 Copyright © 2019 Roger Harquail French, Hsinjin Edwin Yang & Laura Sabra Bruckman. Published by Elsevier Inc. All rights reserved.
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Figure 14.1 The outside influencers on PV module durability and reliability.
The multiple fronts of the technological advances in photovoltaics can be tracked through technology benchmarking processes, such as the International Technology Roadmap for Photovoltaics [2] and the national and global energy forecasts of many agencies such as the US Energy Information Agency [3] and the International Energy Agency [4,5].
silicon regions of cast multicrystalline silicon bricks have been grown of sufficient quality for solar cell applications [8]. This has shown >20% cell efficiencies for PERC cells from monocast bricks [9]. If this progress continues, monocast wafers grown using cost-effective brick growth methods may well become a common technology in c-Si photovoltaics.
14.3.1 Silicon Wafers
14.3.2 Solar Cells
Typically most c-Si PV modules utilize solar cells fabricated using multicrystalline silicon wafers, due to the increased cost of monocrystalline silicon wafers, even though monowafers produce higher efficiency solar cells than do multicrystalline silicon wafers. In the past few years industrial capacity has been introduced to grow monocrystalline boules for wafer production, and this has appeared initially as PERC solar cells made on monocrystalline wafers. As PERC develops, it is being adapted to multicrystalline wafers also. Cost-effective silicon crystal growth for mono and multicrystalline wafers is a major focus of silicon crystal growth companies. Recently, a comparison of material quality of multicrystalline silicon wafers, across many manufacturers, using convolutional neural networks, has shown the large variability and the future room for improvement in this area [6,7]. Another exciting area in crystal growth is the recent progress in “mono-cast” silicon growth, in which substantially monocrystalline
In PV cell materials and architectures, we will continue to see increases in cell conversion efficiency, which, if they come to market in a costeffective implementation, can broadly impact the cost effectiveness of PV power plants in comparison to non-PV electricity generation. To follow the advances in cell conversion efficiency, the National Renewable Energy Laboratory’s (NREL) best research cell efficiency chart allows one to survey across all cell technologies and track the rate of improvement [10]. In addition, further market segmentation, which allows different cell and module technologies to be deployed in different regions and market segments, is vital to PV penetration. For example, thin film PV technologies typically have beneficial temperature coefficients, which can advantage them for hot climates. The technology roadmap for photovoltaics is not based soley on cell materials and cell architectures though. Instead, all the PV module components, the
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PV power plant system components, and even the design and capabilities of electrical grids and societal policies can impact PV penetration and growth.
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their interconnects, and protect them from degradation due to exposure to environmental conditions.
14.3.4.1 Frontsheet
14.3.3 Interconnect Materials Cell interconnection technologies have advanced little since the early days of the development of crystalline silicon PV modules by the US Dept. of Energy research and block grant program. Tinned copper ribbons, soldered to the front and backside of the PV cells for crystalline silicon modules, have only recently been successfully challenged by novel approaches. These have included attempts for costeffective commercialization of back-side contacted cells, such as metal wrap through cells, which did not reach large-scale utilization because of cost concerns. More recently the focus has been on shingled cells, in which electrically conductive adhesives interconnect the frontside of one cell that is overlapping the back surface edge of the adjacent cell. This shingling is often combined with half-cell architectures, or even sliver cells which can be onesixth of the size of a typical silicon cell wafer. These shingled cell approaches provide the possibility of more cost-effective cell interconnection, even though long-term reliability is still to be demonstrated in the field. Alternatively, traditional ribbon interconnection is also being used with half-cell crystalline silicon PV cells to reduce the resistive losses arising from the high currents in current high powered modules. With this design, a single 72-cell PERC module can produce 400 W of electricity. By configuring the module using half-cells and a series/parallel interconnection, the I2R resistive losses can be reduced. This approach could be extended to sliver cells, so as to reduce module DC current and increase the operating voltage of modules. In the case of building integrated PV modules, another new market segment, novel interconnection strategies are being pursued to address the application demands for novel installation opportunities and appealing visual appearance.
14.3.4 Module Packaging Materials The long lifetime performance and reliability of PV modules arise almost completely from the packaging materials used to encapsulate the solar cells and
The sun-facing frontsheet of a PV module is typically made of a low iron glass, even though some flexible PV modules use a polymeric frontsheet (e.g., a fluoropolymer such as ethylene tetrafluoroethylene (ETFE)). Due to the easy breakage of glass under mechanical impact, glass frontsheets are typically either tempered or heat-strengthened. Tempered glass frontsheets are used in glass/backsheet module constructions where the backsheet is polymeric. In these constructions, the glass is typically 3.2 mm thick allowing the glass to be fully tempered. This tempered glass, if broken, will break into small glass pieces of a few millimeter dimensions. In a module with glass used for both the frontsheet and backsheet, sometimes referred to as a double glass module, thinner glass can be used, due to the improved mechanical strength of this “laminated glass” structure. A typical glass front or backsheet is 2.5 mm thick and is heat-strengthened, since it is not possible to temper large glass sheets of this thickness. Advances in frontsheet glass include texturing the inner surface of the glass to improve glass/encapsulant adhesion and reduce reflectance losses from that interface. In addition, on the outside, surface texturing of the glass or deposition of an antireflection coating has been used to improve light coupling and reduce reflectance. But many times these glass frontsheets are subject to increased soiling and power loss increases with soiling. In addition, organic antireflection coatings applied to glass frontsheets have not demonstrated good lifetime performance, since they can degrade under harsh real-world conditions. One of the most aggressive conditions has been the impact of wind-blown sand in desert conditions on coated frontsheets. The increased power output of utility-scale power plants after cleaning has lead to soling research and the development of metrology tools to assess soling as a function of time. These tools provide information on when operation and maintenance (O&M) personnel should clean soiled modules or let cleaning happen by natural rain storms. Automated module cleaning robots are being developed for large-scale systems when the increased power output can help afford these O&M costs.
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14.3.4.2 Encapsulant Ethylene vinyl acetate (EVA) has been the standard encapsulant used in c-Si PV modules since their initial development, due to its low cost. But EVA has the undesirable side effect that hydrolysis of EVA leads to production of acetic acid, which corrodes the interconnect and cell metallization, leading to power loss [11]. A variety of polymers have been used in the laminated glass industry for automotive windshields (polyvinyl butyral) and ionoplast ionomeric polymers used in laminated glass windows. In addition, silicones were studied initially for cell encapsulation, but have not achieved market acceptance, due to the long cure times which have a negative impact on factory production rates. These alternative materials have many desirable properties but have not achieved the necessary price point to displace EVA encapsulants in the market. More recently, polyolefin elastomers (POEs) have been studied extensively, and have entered production for PV modules, due to their similarity to EVA encapsulants, while removing the hydrolytic production of acetic acid. POE encapsulants are considered by many as an opportunity to increase the lifetime of PV modules, without excessive cost impact. Their reliability and performance will become more clear after some years of usage in the field.
14.3.4.3 Substrate (Backsheet) The most popular polymeric backsheets used in PV modules are the nontransparent multilayer films of polyvinyl fluoride/polyethylene terephthalate/ polyvinyl fluoride (PVF/PET/PVF) and PVF/PET/ ethylene vinyl acetate (PVF/PET/EVA). Recently Dupont introduced the first transparent Tedlar PVFbased PV module backsheet, specifically for bifacial solar modules, a market segment that would typically use a double glass module construction. The polymeric backsheets have seen many new material introductions over the past 10 years, due to the strong cost-down push in the industry, and their cost position in the PV module bill of materials (BOM). This led to the introduction of more backsheets with only two polymer types, instead of three polymer types such as PET/PET/EVA backsheets in which the air-side layer is not a fluoropolymer. While innovation drives technological advances and increased market growth for PV, there have been
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cases where backsheet innovation has turned out to be a fiasco for the industry, as arose with the introduction of polyamide 12 backsheets. These polyamide backsheets, in which the air-side layer is polyamide, were able to pass all the required qualification tests during product development and acceptance. But once deployed in the field, polyamide backsheets exhibited extensive macro-scale cracking after only 5 years in the field, long short of the typical 25 year power warranty on current PV modules. It is believed there are currently on the order of 5 GW of PV power plants in the field that will require repair and remediation due to the industry, and our standards are inadequate to identify this issue prior to their being installed in large volumes [12].
14.3.5 Module Architectures Innovations in module architectures include many approaches including split cells, serieseparallel interconnection strategies, shingled cells, the use of electrically conductive adhesives, and many others. Each of these needs to both bring sufficient value to the module and do this in a cost-effective way. Typically, scientists and technologists have focused on increases in various performance metrics, such as cell efficiency, without explicitly considering the additional costs for the mature technology, and this has led to many efforts, such as back-contact cells, that are not economically viable, even if they are technologically demonstrated. In addition, many technology efforts focus on improving instantaneous performance, for example by application of an antireflection (AR) coating to the frontsheet of a module. But the performance over lifetime is not considered. A large number of organic AR coatings have been developed and sold, but these AR coatings have not yet succeeded at scale because the lifetime of the coating is shorter than the module’s lifetime. The rapid acceptance of PERC c-Si cells, and the design flexibility that PERC brings, means that it is relatively easy to produce both monofacial and bifacial PERC cells, without much additional cost. Therefore the ability of a bifacial PERC cell to collect more light, and produce more electricity, is an opportunity that can be exploited by developing new module architectures that take advantage of the PERC cell’s bifacial nature. This can be achieved by either producing bifacial modules to allow light capture
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from the module’s back side or the introduction of white encapsulants behind the cell that serve to reflect light in the module back towards the cell.
14.3.6 PV Systems As PV modules’ lifetime performance increases and their useful lifetimes become longer, other components of the PV power plant are getting renewed attention to their relative lack of progress in lifetime performance. Two areas of special concern are wires and connectors and the DC to AC inverters. Wires and connectors represent one of the larger reliability problems of PV power plants, which represent a significant contributor to O&M expenses. The inverter reliability is such that most power plant owners pencil in periodic inverter replacement on a typically 10-year timeframe. If these two areas were able to develop products with 25-year lifetimes, in a cost-effective manner, then a major ongoing expense in PV power plants could be reduced.
14.4 Data Science Approach and Improved Reliability Studies As the PV industry scales to being a major, multiterawatt contributor to the world’s electricity, we need to dramatically improve our ability to assure reliability of products whose lifetimes are extending to 50 years [13]. The historical way in which reliability studies have been performed, typically in the laboratory, using standards-based accelerated exposure protocols, is incapable of providing technological guidance for products with 25 to 50-year lifetimes. The results of these small lab-based studies tend to be observational, instead of statistically significant. They tend to only observe the initial steps in what are in reality complex, multistep degradation pathways that play out over decades. And many times, either the accelerated test conditions are too aggressive, activating mechanisms that are never observed in the real-world [14] and leading to an increase in manufacturing costs to address a nonexistent problem, or the accelerated exposures are too weak to activate important mechanisms. Lastly these standards-based accelerated tests are incapable of representing the diverse spectrum of real-world climatic zones and exposure conditions, and many times neglect the characteristic cycling that arises in the real-world as day goes to night and as summer goes to winter. It therefore is not surprising that lab-
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based studies for PV, which can work for consumer products with 2 or 5 year lifetimes, have many time misled the community in what are substantial ways, as seen in the fateful introduction of polyamide backsheets in c-Si PV modules. One advantage for the PV community is that over the next few years, as we approach a terawatt of deployed PV, we have a much more realistic fleet of PV systems to use for our reliability research. Consider that the electricity produced is continuously measured, as a time-series by the plant owner who is selling that electricity to the market. Today, with satellite-based weather and irradiance known on a 3.5 km pixel size, from companies such as SolarGIS, the temporal exposure conditions that each of these systems experiences are then known [15]. In order to continue to improve the lifetime and reliability of commercial PV modules, data need to be shared between manufacturers and commercial power plant owners. This enables the use of engineering epidemiological approaches for reliability, degradation, and lifetime performance [16,17]. This engineering epidemiological approach provides an opportunity to better design materials for stress conditions and to improve accelerated testing of PV modules and materials. This is the robust direction that PV degradation science and research should pursue [18]. And combining this with advances in data science [19] and big data analytics [20] suggests that a reliable path is ahead for PV innovation and research.
14.5 PV Standards Activities for Qualification, O&M, and System Rating Modules will need a climate zone rating schema in order to achieve 50-year lifetimes all over the world. Ko¨ppen Geiger climate zones [21,22] are a good base. Changing the packaging material, cell type, and module architecture to better fit the stress conditions that a module experiences can reduce the price in more temperate climates and increase lifetime in the harsher climates. This requires a different type of PV qualification and system rating. In order to distinguish the durability and reliability of PV modules, it will be very important to develop a rating system. The rating of PV modules depends on the years of service, and it may be classified as: Class I (40 years), Class II (25 years), and Class III (10 years). Such a system will offer potential
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advantages, for example, it will help customers select better quality and safety of modules installed in a house or building with lower risk of causing fire and lower insurance rate.
14.6 Module and PV System Recycling, and Repowering PV Power Plants While PV systems provide a more environmentally friendly method of producing electricity for many years, modules will eventually come to the end of their lifetime and need to be replaced in the next 15e20 years [23]. Globally it is estimated that by 2050, w60e70 Mt of modules at the end of life will need to be disposed [24]. Older PV plants/sites can be repowered by replacing the modules with new modules without necessarily needing to replace the whole infrastructure around the PV module (e.g., racking, transmission). However, there is no clear standard on when a site should be decommissioned from a technology standpoint. A difficulty in this is that module sizes have changed over the years so it might not be a one for one replacement, but modules are producing more power per area than previously. The removed modules need to be recycled to recover materials and to reduce the load in the landfill. c-Si PV modules
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contain large amounts of glass, silicon, various metals, and polymers. The difficulty in recycling includes deconstructing the module into its different material types for recycling and accounting for the changing materials used in encapsulation. While the European Union has implemented policies around recycling [25], the United States does not yet require any recycling of PV modules. As renewable energy is a rapidly growing industry sector, the recycling of PV modules will provide workforce opportunities and will decrease the need to mining new metals.
14.7 Future Perspectives By conducting systematic durability and reliability studies for PV technologies, improved safety and/or performance standards can be recommended and/or established (Fig. 14.2). This requires a systematic approach to looking at real-world power plants (e.g., power data, field surveys, field retrieval) to understand the actual degradation mechanisms occurring in PV sites in multiple different climatic zones. Then accelerated qualification tests need to be performed that better mimic these degradation mechanisms to get accurate lifetimes for PV modules, cells, and materials. Such studies would examine the chemical, optical, mechanical, electrical, and flammability properties
Figure 14.2 Systematic studies for long-term durability and reliability (for final safety), from materials and components to the PV modules/system.
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and functional performance of individual materials and components and the PV modules/system as a whole. Finally, failure modes and/or mechanisms related to PV’s long-term durability and reliability induced by the change of materials, interfacial and module/ system properties, and performance degradation when exposed to weathering, aging, and water/ moisture can then be identified and/or proposed, to help establish a fundamental understanding of materials science and technology involved in the longterm reliability and safety of solar PV panels.
References [1] P. Choudhary, R.K. Srivastava, Sustainability perspectives e a review for solar photovoltaic trends and growth opportunities, Journal of Cleaner Production (2019). [2] International Technology Roadmap for Photovoltaics, tenth ed., 2019. https://itrpv.vdma.org/. [3] EIA - Annual Energy Outlook 2018, 2018. https://www.eia.gov/outlooks/aeo/. [4] International Energy Agency, World Energy Outlook 2018, Organization for Economic Cooperation and Development (OECD)/IEA, Paris, France, 2018. [5] A. Guterres, The Sustainable Development Goals Report 2018, United Nations, Department of Economic and Social Affairs, 2018. https:// www.un.org/development/desa/publications/thesustainable-development-goals-report-2018.html. [6] M. Demant, P. Virtue, A. Kovvali, S.X. Yu, S. Rein, Learning quality rating of as-cut mc-Si wafers via convolutional regression networks, IEEE Journal of Photovoltaics (2019) 1e9, https://doi.org/10.1109/ JPHOTOV.2019.2906036. [7] M. Demant, P. Virtue, A. Kovvali, S.X. Yu, S. Rein, Visualizing material quality and similarity of mc-Si wafers learned by convolutional regression networks, IEEE Journal of Photovoltaics (2019) 1e8, https://doi.org/10.1109/ JPHOTOV.2019.2906037. [8] Kutsukake, Growth of crystalline silicon for solar cells: mono-like method, in: D. Yang (Ed.), Handbook of Photovoltaic Silicon, Springer Berlin Heidelberg, Berlin, Heidelberg, 2018, pp. 1e20, https://doi.org/10.1007/978-3-66252735-1_35-1.
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[9] E. Fornie´s, B. Ceccaroli, L. Me´ndez, A. Souto, A. Pe´rez Va´zquez, T. Vlasenko, J. Dieguez, Mass production test of solar cells and modules made of 100% UMG silicon. 20.76% record efficiency, Energies 12 (2019) 1495, https://doi.org/ 10.3390/en12081495. [10] Best Research-Cell Efficiency Chart j Photovoltaic Research j NREL, n.d. https://www.nrel. gov/pv/cell-efficiency.html. [11] L.S. Bruckman, N.R. Wheeler, J. Ma, E. Wang, C.K. Wang, I. Chou, J. Sun, R.H. French, Statistical and domain analytics applied to PV module lifetime and degradation science, IEEE Access 1 (2013) 384e403. https://doi.org/10. 1109/ACCESS.2013.2267611. [12] Y. Lyu, J.H. Kim, A. Fairbrother, X. Gu, Degradation and cracking behavior of polyamide-based backsheet subjected to sequential fragmentation test, IEEE Journal of Photovoltaics 8 (2018) 1748e1753. https://doi. org/10.1109/JPHOTOV.2018.2863789. [13] R. Jones-Albertus, D. Feldman, R. Fu, K. Horowitz, M. Woodhouse, Technology advances needed for photovoltaics to achieve widespread grid price parity, Progress in Photovoltaics: Research and Applications 24 (2016) 1272e1283. https://doi.org/10.1002/pip.2755. [14] A. Gok, D.A. Gordon, D.M. Burns, S.P. Fowler, R.H. French, L.S. Bruckman, Reciprocity and spectral effects of the degradation of poly(ethyleneterephthalate) under accelerated weathering exposures, Journal of Applied Polymer Science 9 (2019). https://doi.org/10.1002/app.47589. [15] T.J. Peshek, J.S. Fada, Y. Hu, Y. Xu, M.A. Elsaeiti, E. Schnabel, M. Ko¨hl, R.H. French, Insights into metastability of photovoltaic materials at the mesoscale through massive IeV analytics, Journal of Vacuum Science & Technology B 34 (2016) 050801. https:// doi.org/10.1116/1.4960628. [16] M.P. Murray, L.S. Bruckman, R.H. French, Photodegradation in a stress and response framework: poly(methyl methacrylate) for solar mirrors and lens, JPE 2 (2012) 022004. https:// doi.org/10.1117/1.JPE.2.022004. [17] H.M. Mirletz, K.A. Peterson, I.T. Martin, R.H. French, Degradation of transparent conductive oxides: interfacial engineering and mechanistic insights, Solar Energy Materials and Solar Cells 143 (2015) 529e538. https://doi. org/10.1016/j.solmat.2015.07.030.
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[18] R.H. French, R. Podgornik, T.J. Peshek, L.S. Bruckman, Y. Xu, N.R. Wheeler, A. Gok, Y. Hu, M.A. Hossain, D.A. Gordon, P. Zhao, J. Sun, G.-Q. Zhang, Degradation science: mesoscopic evolution and temporal analytics of photovoltaic energy materials, Current Opinion in Solid State & Materials Science 19 (2015) 212e226. https://doi.org/10.1016/j.cossms. 2014.12.008. [19] L.S. Bruckman, Transformative opportunities from data science and big data analytics: applied to photovoltaics, Electrochemical Society Interface 28 (2019) 57e61. https://doi.org/10. 1149/2.F07191if. [20] Y. Hu, V.Y. Gunapati, P. Zhao, D. Gordon, N.R. Wheeler, M.A. Hossain, T.J. Peshek, L.S. Bruckman, G.-Q. Zhang, R.H. French, A nonrelational data warehouse for the analysis of field and laboratory data from multiple heterogeneous photovoltaic test sites, IEEE Journal of Photovoltaics 7 (2017) 230e236. https://doi. org/10.1109/JPHOTOV.2016.2626919.
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[21] M. Kottek, J. Grieser, C. Beck, B. Rudolf, F. Rubel, World Map of the Ko¨ppenGeiger climate classification updated, Meteorologische Zeitschrift 15 (3) (2006) 259e263. https://doi.org/10.1127/0941-2948/ 2006/0130. [22] C. Bryant, N.R. Wheeler, F. Rubel, R.H. French, KGC: Koeppen-Geiger Climatic Zones, 2017. https://cran.r-project.org/web/packages/kgc/index.html. [23] F.C.S.M. Padoan, P. Altimari, F. Pagnanelli, Recycling of end of life photovoltaic panels: a chemical prospective on process development, Solar Energy 177 (2019) 746e761. https://doi. org/10.1016/j.solener.2018.12.003. [24] K. Komoto, et al., IEA PVPS Task 12, Report IEA-PVPS T12, 10, 2018. [25] Directive 2012/19/EU of the European Parliament and of the Council on Waste Electrical and Electronic Equipment (WEEE).
Roger H. French 1, Hsinjin Edwin Yang 2 and Laura S. Bruckman 1 Case Western Reserve University, Case School of Engineering, SDLE Research Center, Materials Science and Engineering, Cleveland, OH, United States 2 Pioneer Scientific Solutions, LLC., Long Grove, IL, United States
14.1 Introduction Photovoltaics (PV) has been recognized as a renewable energy technology that has the potential to contribute significantly to the future energy supply and reduce carbon emissions. Therefore, the usage of solar energy has significantly increased worldwide. The science and technology driving PV will continue to be devoted to improving the efficiency of solar cells and developing new and improved materials for more durable and reliable PV modules. This book has attempted to describe especially the latter issues of all materials included in PV modules. The advancement of the research and development in PV modules and systems, efficiency upgrades, storage improvements, and equipment capabilities all contribute to a more effective power output for solar panels and lower costs for systems. The technology and opportunity in the solar industry are still growing [1]. Recently, the state of California in the United States has announced that any new house must install solar energy effective January 20, 2020. Therefore, the utilization of solar energy will be significantly increased in both commercial and residential buildings and technical efforts will focus on increasing the durability and reliability of PV modules and materials.
14.2 PV Durability and Reliability Motivators Many different sectors are interested in the durability, reliability, and safety of PV modules which will help drive improvement in these areas (Fig. 14.1). Therefore, it is of great interest and incentive to pursue the technical understanding and
to develop higher efficiency, new and improved materials, and safety standards for PV modules.
14.3 Innovations in PV Materials, Modules, and System Architectures The vast majority of PV modules are made from crystal-Si solar cells, representing more than 90% of the current market. Whereas today’s standard silicon PV panels will have somewhere in the range of 18 to more than 22% module photoconversion efficiency with either Al-BSF cell designs or the new passivated emitter rear cell (PERC) technology, thin film (TF) panels, such as cadmium telluride (CdTe), are demonstrating module efficiencies above 17% (such as for Series 6 modules from the First Solar). Module efficiencies of coppereindiumegalliumeselenide (CIGS) and coppereindiumeselenide (CIS) modules have been demonstrated to be more than 18% as shown by Solar Frontier, while single junction gallium arsenide (GaAs) module efficiency can be as high as 26% as demonstrated by Alta Devices. These module efficiencies are the focus of PV power plant system designers and owners and they also point to the important and ongoing role of cell architecture and cell material advances that continue improving the photoconversion efficiency of the cells across all photovoltaic cell technologies. In the case of crystalline silicon, the widespread commercialization of PERC cells has, in a 2 or 3 year period, completely displaced the use of Al-BSF silicon cells for monocrystalline wafers. And PERC is appearing to displace Al-BSF multicrystalline cells in the next few years, due to the improved photoconversion efficiency and beneficial cost structure of these new cell designs.
Durability and Reliability of Polymers and Other Materials in Photovoltaic Modules. https://doi.org/10.1016/B978-0-12-811545-9.00014-8 Copyright © 2019 Roger Harquail French, Hsinjin Edwin Yang & Laura Sabra Bruckman. Published by Elsevier Inc. All rights reserved.
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Figure 14.1 The outside influencers on PV module durability and reliability.
The multiple fronts of the technological advances in photovoltaics can be tracked through technology benchmarking processes, such as the International Technology Roadmap for Photovoltaics [2] and the national and global energy forecasts of many agencies such as the US Energy Information Agency [3] and the International Energy Agency [4,5].
silicon regions of cast multicrystalline silicon bricks have been grown of sufficient quality for solar cell applications [8]. This has shown >20% cell efficiencies for PERC cells from monocast bricks [9]. If this progress continues, monocast wafers grown using cost-effective brick growth methods may well become a common technology in c-Si photovoltaics.
14.3.1 Silicon Wafers
14.3.2 Solar Cells
Typically most c-Si PV modules utilize solar cells fabricated using multicrystalline silicon wafers, due to the increased cost of monocrystalline silicon wafers, even though monowafers produce higher efficiency solar cells than do multicrystalline silicon wafers. In the past few years industrial capacity has been introduced to grow monocrystalline boules for wafer production, and this has appeared initially as PERC solar cells made on monocrystalline wafers. As PERC develops, it is being adapted to multicrystalline wafers also. Cost-effective silicon crystal growth for mono and multicrystalline wafers is a major focus of silicon crystal growth companies. Recently, a comparison of material quality of multicrystalline silicon wafers, across many manufacturers, using convolutional neural networks, has shown the large variability and the future room for improvement in this area [6,7]. Another exciting area in crystal growth is the recent progress in “mono-cast” silicon growth, in which substantially monocrystalline
In PV cell materials and architectures, we will continue to see increases in cell conversion efficiency, which, if they come to market in a costeffective implementation, can broadly impact the cost effectiveness of PV power plants in comparison to non-PV electricity generation. To follow the advances in cell conversion efficiency, the National Renewable Energy Laboratory’s (NREL) best research cell efficiency chart allows one to survey across all cell technologies and track the rate of improvement [10]. In addition, further market segmentation, which allows different cell and module technologies to be deployed in different regions and market segments, is vital to PV penetration. For example, thin film PV technologies typically have beneficial temperature coefficients, which can advantage them for hot climates. The technology roadmap for photovoltaics is not based soley on cell materials and cell architectures though. Instead, all the PV module components, the
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PV power plant system components, and even the design and capabilities of electrical grids and societal policies can impact PV penetration and growth.
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their interconnects, and protect them from degradation due to exposure to environmental conditions.
14.3.4.1 Frontsheet
14.3.3 Interconnect Materials Cell interconnection technologies have advanced little since the early days of the development of crystalline silicon PV modules by the US Dept. of Energy research and block grant program. Tinned copper ribbons, soldered to the front and backside of the PV cells for crystalline silicon modules, have only recently been successfully challenged by novel approaches. These have included attempts for costeffective commercialization of back-side contacted cells, such as metal wrap through cells, which did not reach large-scale utilization because of cost concerns. More recently the focus has been on shingled cells, in which electrically conductive adhesives interconnect the frontside of one cell that is overlapping the back surface edge of the adjacent cell. This shingling is often combined with half-cell architectures, or even sliver cells which can be onesixth of the size of a typical silicon cell wafer. These shingled cell approaches provide the possibility of more cost-effective cell interconnection, even though long-term reliability is still to be demonstrated in the field. Alternatively, traditional ribbon interconnection is also being used with half-cell crystalline silicon PV cells to reduce the resistive losses arising from the high currents in current high powered modules. With this design, a single 72-cell PERC module can produce 400 W of electricity. By configuring the module using half-cells and a series/parallel interconnection, the I2R resistive losses can be reduced. This approach could be extended to sliver cells, so as to reduce module DC current and increase the operating voltage of modules. In the case of building integrated PV modules, another new market segment, novel interconnection strategies are being pursued to address the application demands for novel installation opportunities and appealing visual appearance.
14.3.4 Module Packaging Materials The long lifetime performance and reliability of PV modules arise almost completely from the packaging materials used to encapsulate the solar cells and
The sun-facing frontsheet of a PV module is typically made of a low iron glass, even though some flexible PV modules use a polymeric frontsheet (e.g., a fluoropolymer such as ethylene tetrafluoroethylene (ETFE)). Due to the easy breakage of glass under mechanical impact, glass frontsheets are typically either tempered or heat-strengthened. Tempered glass frontsheets are used in glass/backsheet module constructions where the backsheet is polymeric. In these constructions, the glass is typically 3.2 mm thick allowing the glass to be fully tempered. This tempered glass, if broken, will break into small glass pieces of a few millimeter dimensions. In a module with glass used for both the frontsheet and backsheet, sometimes referred to as a double glass module, thinner glass can be used, due to the improved mechanical strength of this “laminated glass” structure. A typical glass front or backsheet is 2.5 mm thick and is heat-strengthened, since it is not possible to temper large glass sheets of this thickness. Advances in frontsheet glass include texturing the inner surface of the glass to improve glass/encapsulant adhesion and reduce reflectance losses from that interface. In addition, on the outside, surface texturing of the glass or deposition of an antireflection coating has been used to improve light coupling and reduce reflectance. But many times these glass frontsheets are subject to increased soiling and power loss increases with soiling. In addition, organic antireflection coatings applied to glass frontsheets have not demonstrated good lifetime performance, since they can degrade under harsh real-world conditions. One of the most aggressive conditions has been the impact of wind-blown sand in desert conditions on coated frontsheets. The increased power output of utility-scale power plants after cleaning has lead to soling research and the development of metrology tools to assess soling as a function of time. These tools provide information on when operation and maintenance (O&M) personnel should clean soiled modules or let cleaning happen by natural rain storms. Automated module cleaning robots are being developed for large-scale systems when the increased power output can help afford these O&M costs.
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14.3.4.2 Encapsulant Ethylene vinyl acetate (EVA) has been the standard encapsulant used in c-Si PV modules since their initial development, due to its low cost. But EVA has the undesirable side effect that hydrolysis of EVA leads to production of acetic acid, which corrodes the interconnect and cell metallization, leading to power loss [11]. A variety of polymers have been used in the laminated glass industry for automotive windshields (polyvinyl butyral) and ionoplast ionomeric polymers used in laminated glass windows. In addition, silicones were studied initially for cell encapsulation, but have not achieved market acceptance, due to the long cure times which have a negative impact on factory production rates. These alternative materials have many desirable properties but have not achieved the necessary price point to displace EVA encapsulants in the market. More recently, polyolefin elastomers (POEs) have been studied extensively, and have entered production for PV modules, due to their similarity to EVA encapsulants, while removing the hydrolytic production of acetic acid. POE encapsulants are considered by many as an opportunity to increase the lifetime of PV modules, without excessive cost impact. Their reliability and performance will become more clear after some years of usage in the field.
14.3.4.3 Substrate (Backsheet) The most popular polymeric backsheets used in PV modules are the nontransparent multilayer films of polyvinyl fluoride/polyethylene terephthalate/ polyvinyl fluoride (PVF/PET/PVF) and PVF/PET/ ethylene vinyl acetate (PVF/PET/EVA). Recently Dupont introduced the first transparent Tedlar PVFbased PV module backsheet, specifically for bifacial solar modules, a market segment that would typically use a double glass module construction. The polymeric backsheets have seen many new material introductions over the past 10 years, due to the strong cost-down push in the industry, and their cost position in the PV module bill of materials (BOM). This led to the introduction of more backsheets with only two polymer types, instead of three polymer types such as PET/PET/EVA backsheets in which the air-side layer is not a fluoropolymer. While innovation drives technological advances and increased market growth for PV, there have been
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cases where backsheet innovation has turned out to be a fiasco for the industry, as arose with the introduction of polyamide 12 backsheets. These polyamide backsheets, in which the air-side layer is polyamide, were able to pass all the required qualification tests during product development and acceptance. But once deployed in the field, polyamide backsheets exhibited extensive macro-scale cracking after only 5 years in the field, long short of the typical 25 year power warranty on current PV modules. It is believed there are currently on the order of 5 GW of PV power plants in the field that will require repair and remediation due to the industry, and our standards are inadequate to identify this issue prior to their being installed in large volumes [12].
14.3.5 Module Architectures Innovations in module architectures include many approaches including split cells, serieseparallel interconnection strategies, shingled cells, the use of electrically conductive adhesives, and many others. Each of these needs to both bring sufficient value to the module and do this in a cost-effective way. Typically, scientists and technologists have focused on increases in various performance metrics, such as cell efficiency, without explicitly considering the additional costs for the mature technology, and this has led to many efforts, such as back-contact cells, that are not economically viable, even if they are technologically demonstrated. In addition, many technology efforts focus on improving instantaneous performance, for example by application of an antireflection (AR) coating to the frontsheet of a module. But the performance over lifetime is not considered. A large number of organic AR coatings have been developed and sold, but these AR coatings have not yet succeeded at scale because the lifetime of the coating is shorter than the module’s lifetime. The rapid acceptance of PERC c-Si cells, and the design flexibility that PERC brings, means that it is relatively easy to produce both monofacial and bifacial PERC cells, without much additional cost. Therefore the ability of a bifacial PERC cell to collect more light, and produce more electricity, is an opportunity that can be exploited by developing new module architectures that take advantage of the PERC cell’s bifacial nature. This can be achieved by either producing bifacial modules to allow light capture
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from the module’s back side or the introduction of white encapsulants behind the cell that serve to reflect light in the module back towards the cell.
14.3.6 PV Systems As PV modules’ lifetime performance increases and their useful lifetimes become longer, other components of the PV power plant are getting renewed attention to their relative lack of progress in lifetime performance. Two areas of special concern are wires and connectors and the DC to AC inverters. Wires and connectors represent one of the larger reliability problems of PV power plants, which represent a significant contributor to O&M expenses. The inverter reliability is such that most power plant owners pencil in periodic inverter replacement on a typically 10-year timeframe. If these two areas were able to develop products with 25-year lifetimes, in a cost-effective manner, then a major ongoing expense in PV power plants could be reduced.
14.4 Data Science Approach and Improved Reliability Studies As the PV industry scales to being a major, multiterawatt contributor to the world’s electricity, we need to dramatically improve our ability to assure reliability of products whose lifetimes are extending to 50 years [13]. The historical way in which reliability studies have been performed, typically in the laboratory, using standards-based accelerated exposure protocols, is incapable of providing technological guidance for products with 25 to 50-year lifetimes. The results of these small lab-based studies tend to be observational, instead of statistically significant. They tend to only observe the initial steps in what are in reality complex, multistep degradation pathways that play out over decades. And many times, either the accelerated test conditions are too aggressive, activating mechanisms that are never observed in the real-world [14] and leading to an increase in manufacturing costs to address a nonexistent problem, or the accelerated exposures are too weak to activate important mechanisms. Lastly these standards-based accelerated tests are incapable of representing the diverse spectrum of real-world climatic zones and exposure conditions, and many times neglect the characteristic cycling that arises in the real-world as day goes to night and as summer goes to winter. It therefore is not surprising that lab-
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based studies for PV, which can work for consumer products with 2 or 5 year lifetimes, have many time misled the community in what are substantial ways, as seen in the fateful introduction of polyamide backsheets in c-Si PV modules. One advantage for the PV community is that over the next few years, as we approach a terawatt of deployed PV, we have a much more realistic fleet of PV systems to use for our reliability research. Consider that the electricity produced is continuously measured, as a time-series by the plant owner who is selling that electricity to the market. Today, with satellite-based weather and irradiance known on a 3.5 km pixel size, from companies such as SolarGIS, the temporal exposure conditions that each of these systems experiences are then known [15]. In order to continue to improve the lifetime and reliability of commercial PV modules, data need to be shared between manufacturers and commercial power plant owners. This enables the use of engineering epidemiological approaches for reliability, degradation, and lifetime performance [16,17]. This engineering epidemiological approach provides an opportunity to better design materials for stress conditions and to improve accelerated testing of PV modules and materials. This is the robust direction that PV degradation science and research should pursue [18]. And combining this with advances in data science [19] and big data analytics [20] suggests that a reliable path is ahead for PV innovation and research.
14.5 PV Standards Activities for Qualification, O&M, and System Rating Modules will need a climate zone rating schema in order to achieve 50-year lifetimes all over the world. Ko¨ppen Geiger climate zones [21,22] are a good base. Changing the packaging material, cell type, and module architecture to better fit the stress conditions that a module experiences can reduce the price in more temperate climates and increase lifetime in the harsher climates. This requires a different type of PV qualification and system rating. In order to distinguish the durability and reliability of PV modules, it will be very important to develop a rating system. The rating of PV modules depends on the years of service, and it may be classified as: Class I (40 years), Class II (25 years), and Class III (10 years). Such a system will offer potential
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advantages, for example, it will help customers select better quality and safety of modules installed in a house or building with lower risk of causing fire and lower insurance rate.
14.6 Module and PV System Recycling, and Repowering PV Power Plants While PV systems provide a more environmentally friendly method of producing electricity for many years, modules will eventually come to the end of their lifetime and need to be replaced in the next 15e20 years [23]. Globally it is estimated that by 2050, w60e70 Mt of modules at the end of life will need to be disposed [24]. Older PV plants/sites can be repowered by replacing the modules with new modules without necessarily needing to replace the whole infrastructure around the PV module (e.g., racking, transmission). However, there is no clear standard on when a site should be decommissioned from a technology standpoint. A difficulty in this is that module sizes have changed over the years so it might not be a one for one replacement, but modules are producing more power per area than previously. The removed modules need to be recycled to recover materials and to reduce the load in the landfill. c-Si PV modules
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contain large amounts of glass, silicon, various metals, and polymers. The difficulty in recycling includes deconstructing the module into its different material types for recycling and accounting for the changing materials used in encapsulation. While the European Union has implemented policies around recycling [25], the United States does not yet require any recycling of PV modules. As renewable energy is a rapidly growing industry sector, the recycling of PV modules will provide workforce opportunities and will decrease the need to mining new metals.
14.7 Future Perspectives By conducting systematic durability and reliability studies for PV technologies, improved safety and/or performance standards can be recommended and/or established (Fig. 14.2). This requires a systematic approach to looking at real-world power plants (e.g., power data, field surveys, field retrieval) to understand the actual degradation mechanisms occurring in PV sites in multiple different climatic zones. Then accelerated qualification tests need to be performed that better mimic these degradation mechanisms to get accurate lifetimes for PV modules, cells, and materials. Such studies would examine the chemical, optical, mechanical, electrical, and flammability properties
Figure 14.2 Systematic studies for long-term durability and reliability (for final safety), from materials and components to the PV modules/system.
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and functional performance of individual materials and components and the PV modules/system as a whole. Finally, failure modes and/or mechanisms related to PV’s long-term durability and reliability induced by the change of materials, interfacial and module/ system properties, and performance degradation when exposed to weathering, aging, and water/ moisture can then be identified and/or proposed, to help establish a fundamental understanding of materials science and technology involved in the longterm reliability and safety of solar PV panels.
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