ENVIRONMENTAL BENEFITS OF SILICON KERF SECONDARY PRODUCTS IN PILOT PROCESSES OVER CONVENTIONAL PRODUCTION OF EQUIVALENT PRODUCTS WITH PRIMARY RAW MATERIALS IN CHINA AND EUROPE

ENVIRONMENTAL BENEFITS OF SILICON KERF SECONDARY PRODUCTS IN PILOT PROCESSES OVER CONVENTIONAL PRODUCTION OF EQUIVALENT PRODUCTS WITH PRIMARY RAW MATERIALS IN CHINA AND EUROPE René Peche, Matthias Seitz, Markus Schönheits, Karsten Wambach bifa Umweltinstitut GmbH Am Mittleren Moos 46, 86167 Augsburg, Germany ABSTRACT: bifa Umweltinstitut GmbH is evaluating four pilot units newly developed in the ICARUS project for processing silicon kerf waste into secondary materials and marketable products and shows whether an ecological improvement compared to the conventional Chinese and European supply of functionally equivalent materials from primary raw material can be achieved. The evaluation is done using the method of life cycle assessment (LCA). The turning of PV waste silicon kerf into secondary raw materials or marketable products eliminates a large part of the environmental impact associated with the conventional production of equivalent raw materials or products. The reduction in environmental impact ranges from more than 85% for the production of secondary metallurgical grade and solar grade silicon compared to the conventional production in China over more than 75% for the production of secondary metallurgical grade and solar grade silicon compared to the conventional production in Europe to more than 30% for the production of hydrogen and water glass from silicon kerf waste compared to the conventional production in Europe. Thus, the newly developed processes in the ICARUS project not only reduce dependence on supplies from Asia, but also help to reduce harmful emissions into the environment during the production of raw materials. Keywords: LCA; ecology index; secondary materials; silicon; recycling; PV modules 1 INTRODUCTION In the ICARUS project, 17 European partners are collaborating to develop innovative methods for processing and refining secondary raw materials from silicon PV manufacturing. This involves transforming the process wastes Si-kerf waste, graphite waste and silica waste from silicon (Si)ingot and wafer production into valuable secondary resources (Figure 1). Figure 1: ICARUS project For filtered Si-Kerf, which is produced during the diamond wire sawing of silicon blocks into wafers, four pilot processes have been developed and operated, demonstrating the relevance of the different technological options and bringing modularity in: (a) Si-kerf recycling and refining (PILOT A, B and C) considering different inputs and purity grades for diverse applications; and (b) revalorisation of Si (PILOT D), transforming Si-kerf into the valuable commodities green hydrogen and water glass. The energy intensity of silicon is strongly related to its purity. Therefore, the ICARUS project will take advantage of the silicon content of the kerf, invested with a lot of energy, and reuse it as secondary raw material. For example, reusing secondary silicon in wafer production can significantly reduce the high energy demands associated with processing primary raw Si into wafers, which accounts for about 75 % of the total energy used in PV module manufacturing. The LCA conducted in the ICARUS project supports and quantifies the process developments. 2 METHOD An LCA is a system analysis method for the integrated, media-wide acquisition and evaluation of environment-related matters in connection with products, processes and services. LCA are characterized by the analyses of environmental influences in association with prior or subsequent life cycle stages. Also considered here are inputs and withdrawals of raw materials and energy to and from environmental media, namely water, air and earth. Overall, LCA can make a comprehensive statement on the relevance to the environment of the systems investigated and are therefore optimally suited for environment-related comparison of various systems. The LCA in the ICARUS project is carried out under the norm specifications for the execution of eco-balances DIN EN ISO 14040 [1] and DIN EN ISO 14044 [2], taken into account the Product Environmental Footprint Category Rules (PEFCR) for photovoltaic modules used in photovoltaic power systems for electricity generation [3,4]. Starting with the definition of goal and scope under the terms of the Life cycle inventory analysis, all relevant parameters are recorded and summarized in the life cycle impact assessment regarding their environmental impact. Figure 2 shows a schematic overview of the basic compilation of an LCA with fields of application. Figure 2: Schematic overview of the basic compilation of an LCA with fields of application Table 1 shows the thirteen impact categories that are assessed and interpreted in the LCA of the ICARUS project. . Table 1: Applied environmental impact category used in the ICARUS project Impact category Unit Climate change kg CO2 eq. Ozon depletion kg CF-11 eq. Particulate matter Disease Incidence Ionizing radiation, human health kBq U235 eq. Photochemical ozone formation, human health kg NMVOC eq. Acidification mole H+ eq. Eutrophication, terrestrial mole N eq. Eutrophication, freshwater kg P eq. Eutrophication, marine kg N eq. Land use Pt Water use kg world eq. deprived Resource use, minerals and metals kg Sb eq. Resource use, energy carriers MJ The individual results of the impact categories are combined using the normalization and weighting factors published by the European Commission’s Joint Research Centre [5,6], to create a dimensionless single ecological indicator – the so-called ecology index. 3 RESULTS Information and process data for the pilot processes are provided by the project partners. The Chinese and European production processes are modelled using data from IEA PVPS Task 12 [6] and ecoinvent database [7]. The data sets used together with the process data and information to create the LCA models also came from the ecoinvent database. PILOT A process: Secondary dry silicon compared to conventional metallurgical grade silicon produced in China and Europe The product of the pilot A process developed by project partner Resitec AS (Norway) is secondary dry silicon with less than 1% moisture. The starting material is filtered Si-kerf containing 47 % moisture from the silicon wafer sawing process. The comparable conventional product is metallurgical grade silicon, produced from primary silica sand. Figure 3: Comparison of the ecological indices of the pilot A process and the conventional production processes in China and Europe Figure 3 demonstrates that secondary dry silicon has an ecological advantage of more than 90 % over conventional production of metallurgical grade silicon in China, and almost 80% compared to conventional production in Europe The improved ecology index can be attributed primarily to the lower quantity of energy used in the pilot A process, and secondarily to the use of materials, which is significantly reduced compared to those used in the Chinese and European production processes. For instance, the electricity consumption for the pilot A process is more than eight times lower than that required for the conventional production of metallurgical grade silicon. Furthermore, unlike the 2 conventional production processes the pilot A process does not require any thermal energy PILOT B and PILOT C processes: Secondary silicon 6N+ and 8N compared to conventional solar grade silicon produced in China and Europe The product of the pilot B process developed by project partner ROSI SAS (France) is secondary silicon of 8N purity and the product of the pilot C process developed by project partner Northern Silicon (Norway) is secondary silicon of 6N+ purity. The comparable conventional product is solar grade silicon, produced from primary metallurgical grade silicon. Figure 4: Comparison of the ecological indices of the pilot B and pilot C processes and the conventional production processes in China and Europe Figure 4 shows that the secondary silicon 8N and 6N+ have an ecological advantage of more than 85 % over the conventional production of solar grade silicon in China and more than 75% over the conventional production in Europe. The improved ecological indices are due to the same factors as described for pilot A process. The main reason is the lower quantity of energy used in the pilot processes. In addition, the consumption of primary materials is significantly lower compared to the conventional production processes because of the use of waste as a starting material. PILOT D process: Hydrogen and water glass produced from silicon kerf compared to conventional hydrogen and sodium silicate produced in Europe The products of the pilot D process developed by project partner LuxChemtech GmbH (Germany) are hydrogen and water glass. The starting material is the processed Si-kerf from the pilot A process. In the conventional production processes of hydrogen (produced via the cracking of fossil fuels and chlor-alkali electrolysis) and water glass (produced from the furnace process), the starting materials are natural gas and the silica sand as well as soda ash, respectively. Figure 5: Comparison of the ecological indices of the pilot D process and the conventional production processes in Europe Figure 5 shows that hydrogen and water glass produced from silicon kerf have an ecological advantage of more than 30 % over the conventional production of the two products in Europe. The improved ecology index results from the reduction of primary raw materials and the lower consumption of fossil fuels, which are typically required for conventional hydrogen and water glass production. 4 DISCUSSION AND CONCLUSION The turning of PV waste silicon kerf, which is energydense and rich in highly pure silicon, into secondary raw materials or marketable products eliminates a large part of the environmental impact associated with the conventional production of equivalent raw materials or products. The reduction in environmental impact ranges from more than 85% for the production of secondary metallurgical grade and solar grade silicon compared to the conventional production in China over more than 75% for the production of secondary metallurgical grade and solar grade silicon compared to the conventional production in Europe to more than 30% for the production of hydrogen and water glass from silicon kerf waste compared to the conventional production in Europe. Thus, the newly developed processes in the ICARUS project not only reduce dependence on supplies from Asia, but also help to reduce harmful emissions into the environment during the production of raw materials. 5 REFERENCES [1] DIN EN ISO 14040:2021-02: Environmental management - Life cycle assessment - Principles and framework (ISO 14040:2006 + Amd 1:2020), Deutsches Institut für Normung DIN e.V.; 2021 [2] DIN EN ISO 14044:2021-02: Environmental management - Life cycle assessment - Requirements and guidelines (ISO 14044:2006 + Amd 1:2017 + Amd 2:2020), Deutsches Institut für Normung DIN e.V.; 2021 [3] Product Environmental Footprint Category Rules (PEFCR) for photovoltaic modules used in photovoltaic power systems for electricity generation. European Commission’s Joint Research Centre - Institute for Environment and Sustainability; 2020 [4] JRC Technical Report - Updated characterisation and normalisation factors for the Environmental Footprint 3.1 method. European Commission’s Joint Research Centre - Institute for Environment and Sustainability; 2023 [5] JRC Technical Reports - Development of a weighting approach for the Environmental Footprint. European Commission’s Joint Research Centre - Institute for Environment and Sustainability; 2018 [6] Life Cycle Inventory and Life Cycle Assessments of Photovoltaic Systems. Report IEA-PVPS T1219:2020, International Energy Agency; 2020 [7] Life cycle inventory database ecoinvent v3.8: https://www.ecoinvent.org, Zürich; 2021 6 ACKNOWLEDGEMENTS This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 958365; project ICARUS. Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor the granting authority can be held responsible for them.