[LCA] Results of the benchmark calculation
Author:
In the first article on the Life Cycle Assessment (LCA) titled “Presentation of Life Cycle Assessment by bifa” the roadmap for an LCA was summarised as follows:
“Starting with the definition of goal and scope under the terms of the life cycle inventory analysis (LCI) analysis, all relevant parameters are recorded and summarized in the life cycle impact assessment (LCIA) regarding their environmental impact.”
This second LCA article presents the results of the benchmark calculation (reference processes for evaluating the improvements and progress achieved in the project) along the roadmap.
Scope of the benchmark study
Subject of the study
The state-of-the art reference processes are compared to the improvements and progress made in the project for new processes/products. The various contents will be represented by the use of different scenarios. The reference processes are modelled in benchmark scenarios. The improvements and progress made in the project for new processes will be modelled later in newly defined independent scenarios.
Functional unit
The focus of an LCA is the functional unit defined as “quantified use of a product system for the application as comparative unit in an eco-balance study”. This is the reference both for the comparison of observed scenarios and the norm for the input and output data given in the study.
The functional unit that will be most suitable for the analysis is depend on the results that will be obtained and may vary within the project for the individual scenarios. For the presentation of the results of the reference processes, the functional unit is specified with a PV-module area of 1 m².
System boundaries
In addition to defining the functional unit, the individual boundaries for each scenario must also define in order to allow a comparison of the scenarios. The system boundary specifies the processes which shall be taken up into the system. In the ideal situation, the system shall be modelled such that inputs and outputs are elementary flows on their system boundaries.
Example of a product system
In the case that there are simply not enough data and means available to carry out such comprehensive task, decisions must be made which processes shall be included or which emissions must be considered and at what level of precision the processes shall be examined or emissions recorded.
The following processes are taken into account within the system boundaries:
- Manufacturing of wafers from multi-Si-ingots and mono-Si-ingots including any treatment of production residues or recyclable materials.
- All transports, from the supply of raw material and pre-products through to disposal of production residues.
- All relevant material and energy flows associated with the processes from extraction and treatment of raw materials through supply of the operating materials and starting products and as far as possible also including the disposal of residues. In ideal circumstances, the system boundaries also include the extraction of raw materials from natural deposits, making these available for technical processing, and the release of elementary flows to the environmental media of water, air and earth.
When setting the limits for the complexity of the model, it is important to ensure that there is a possibility of comparison of the specific scenarios for the process/product under investigation.
Offset of credits in equivalence systems
In addition to the analysed product (or process), which is the main benefit, there may be additional benefits. These include e.g., electricity and heat from waste incineration or utilisation of secondary raw materials from recycling processes. As a result, the appropriate energy quantities and products are not only a result of primary raw materials processing (assuming that demand remains the same). The environmental impact of conventional production of an individual additional benefit is therefore “saved” or “avoided.” To complete the scenarios, the “avoided” environmental impacts are offset and “credited” to the environmental impacts of the respective product.
Geographic and time reference
To classify the LCA results and assessing the data validity or, if applicable, maintaining the comparability with other LCA, a geographic and temporal reference must be defined.
The geographical coverage arises out of the economic context and from the product definition (e.g. production site, supply chains, etc.). The time-period coverage extensively reflects the reference year or reference period of the data collection. For durable products, the determined or estimated lifetime or operating time plays an important role. Due to the fact that disposal or reuse will take place in the future, modelling these life cycle phases could be correspondingly difficult and uncertain.
In general, the data and datasets used cover the situation in Western Europe and the World, respectively. For some materials and processes, country-specific manufacturers (i.e. Germany, China, U.S.A.) are also adopted. The datasets used originate from commercial databases in the latest version or from internal expertise of bifa or the project team.
Life cycle inventory analysis for benchmark processes
The standard production procedures and process chains for Si metal production and refining as well as Si-ingot and Wafer processing were identified. All partners contributed to the dataset with their expertise and position in the value chain. The initial results will also be used to determine the status of critical material replacement potentials throughout the project.
All data input and expert assumptions have been discussed within the project team, matching with the partner’s experience in Si processing, technology development, and market.
Model structure
The material and energy flow model for the wafer production is divided into the four production chain sections:
- Si metal production (metallurgical grade silicon)
- Refining Si (solar grade silicon)
- Si ingots
- Mono-Si (Czochralski single crystal)
- Multi-Si (Multi-crystalline ingot)
- Wafer sawing (silicon wafer)
- Mono-Si-wafer
- Multi-Si-wafer
From primary raw material to wafer: upstream process
The corresponding structure of the material and energy flow model is illustrated in the first article on LCA.
Benchmark data
The best source of data in terms of availability and timeliness is the IEA PVPS Task 12 (https://iea-pvps.org/research-tasks/pv-sustainability/). Data gaps are mainly closed with additional literature data as well as data from previous projects and databases. The data were processed and made available to the project partners for discussion via the project platform. Based on the feedback, the final LCI data for all four production chain sections were determined.
The following table shows as an example of the LCI data for the benchmark process ” Mono-Si ingot”. The data describe the production of a monocrystalline ingot using the Czochralski (CZ) process.
LCI data for benchmark process Mono-Si ingot (Czochralski single crystal)
| Process information | ||||
| Sources: | International Energy Agency: Life Cycle Inventory and Life Cycle Assessments of Photovoltaic Systems. Report IEA-PVPS T12-19:2020, 2020 Life cycle inventory database ecoinvent v3.7: https://www.ecoinvent.org, Zürich, 2020 Eco-Solar Factory: 40%plus eco-efficiency gains in the photovoltaic value chain with minimised resource and energy consumption by closed loop systems. D23 Report Final LCA and EEA, bifa Umweltinstitut GmbH, 2018 |
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| Intermediate product: | Single-crystalline silicon | |||
| Production process: | Czochralski process: production of a monocrystalline block with a length of 150 cm | |||
| Description: | 1. Crushing of SG-silicon, etching with HNO3, HF and acetic acid.
2. Melting in a silica pot and crystallisation. |
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| Process values | Functional unit (for all input and output values of this page): 1 kg CZ single-crystalline silicon | |||
| Materials | Value | Source of supply | Transport information | Comment |
| SG-silicon | 0.7 kg | Global | 600 km container ship 210 km lorry 310 km train 25 km barge |
SG = Solar grade |
| Argon, liquid | 1 kg | Europe | 20 km lorry 5 km train |
Use: protection gas for growing |
| Ceramic tiles | 0.167 kg | Europe | 310 km lorry 70 km train 1 km LCV |
Approximation for quartz crucible for melting the silicon |
| Water, deionised | 4.01 kg | Europe | – | |
| Water, cooling | 5.09 m³ | Europe | – | |
| Hydrogen fluoride | 0.01 kg | Europe | 50 km lorry 15 km train |
Use: for etching |
| Nitric acid | 0.0668 kg | Europe | 170 km lorry 40 km train 20 km barge |
Use: for etching; Solution state: 50 % in H2O |
| Sodium hydroxide | 0.0415 kg | Europe | 210 km lorry 310 km train 25 km barge |
Use: waste gas neutralization; Solution state: 50 % in H2O; Approximation transport: global market data without consideration of container ships |
| Lime, hydrated | 0.0222 kg | Europe | 80 km lorry 10 km train 1 km barge |
Use: waste water treatment |
| Energies/Fuels | Value | Source of supply | Transport information | Comment |
| Electricity | 32 kWh | Europe | ||
| Natural gas | 68.2 MJ | Europe | Burned in industrial furnace low-NOx | |
| Wastes / Waste water | Value | Source of supply | Transport information | Comment |
| Waste from silicon production | 0.167 kg | Europe | 20 km lorry 10 km train |
Disposal: residual material landfill; Approximation transport: global market data without consideration of container ships |
| Sewage | 4.84 m³ | Europe | – | Treatment: waste water treatment class 2 |
| Air emissions | Value | Comment | ||
| Waste heat | 115 MJ | |||
| Water | 255 kg | |||
| Nitrogen oxides | 33.9 g | |||
| Water emissions | Value | Comment | ||
| Hydroxide | 3.39 g | |||
| COD | 130 g | COD: Chemical oxygen demand | ||
| BOD5 | 130 g | BOD5: Biological oxygen demand | ||
| DOD | 40.5 g | DOD: Dissolved organic demand | ||
| TOC | 40.5 g | TOC: Total organic carbon | ||
| Nitrate | 83.5 g | |||
Life cycle impact assessment of the benchmark processes
In this section, the results of the impact assessment for the impact category acidification as an example are presented in detail. The results of all impact categories analysed are then summarised.
Representations are used to evaluate the impact categories analysed, as explained in the following figure.
Sectoral evaluation of the considered impact categories
The LCA provides three sets of results. The respective bars on the left-hand side show the gross expense results (environmental burden – bar up) on one hand the credits (environmental benefits – bar down) on the other. The color-coding of the sectors makes it possible to identify the relevant contributions to the overall result.
The net impact category result is obtained from the offsetting of the environmental impact and benefit, which is displayed respectively in the grey bar on the right-hand side. It shows whether the environment suffers (bar up) or receives benefits (bar down) due to the contribution of the scenario.
Detailed evaluation of the impact category acidification
The impact category acidification describes the emission of substance that lead to acid rain and poorer quality of air, water and soil.
The following figures show the proportions of the production chain sections on the acidification potential.
| Wafer sawing (silicon wafer) | ||
| Si ingot | ||
| Refining Si (solar grade silicon) | ||
| Si metal production (metallurgical grade silicon) | ||
| Credits from waste treatment | ||
| NET RESULT | ||
Acidification potential of 1 m² mono-Si-wafer manufactured by the benchmark process
The net result of one square meter of mono-Si-wafer with a total of 0.14 mole of H+ equivalents is determined by the production of silicon ingot. 54 % of total air emissions of the gross result (mainly sulphur dioxide and nitrogen oxides) come from this section. With shares of 19 % and 17 % respectively, emissions of wafer sawing and the production of solar grade silicon play a subordinate role. Emissions of the production of metallurgical grade silicon contribute 11 % to the gross result. There are no credits from waste treatment.
| Wafer sawing (silicon wafer) | ||
| Si ingot | ||
| Refining Si (solar grade silicon) | ||
| Si metal production (metallurgical grade silicon) | ||
| Credits from waste treatment | ||
| NET RESULT | ||
Acidification potential of 1 m² multi-Si-wafer manufactured by the benchmark process
The net result of one square meter of multi-Si-wafer with a total of 0.1 mole of H+ equivalents is determined by the production of solar grade silicon and wafer sawing on a similar scale. 33 % and 31 % respectively, of total air emissions of the gross result (mainly sulphur dioxide and nitrogen oxides) come from these two sections. With shares of 21 % and 15 % respectively, emissions of the production of metallurgical grade silicon and silicon ingot play a subordinate role. There are no credits from waste treatment included.
The contributions of the most important processes in the production chain sections to the gross results are shown in the following tables.
Contributions of the most important processes to the gross result of acidification potential of 1 m² mono-Si-wafer manufactured by the benchmark process
| Production chain section | Key process | Emission category | Contribution [H+ eq.] |
Share of gross result |
| Si ingot | Electricity supply, medium voltage | Scope 2 | 0.04 mole | 27 % |
| Si ingot | Treatment of waste water | Scope 3 | 0.016 mole | 11 % |
| Si ingot | Production of silicon, single crystal, Czochralski process | Scope 1 | 0.015 mole | 10 % |
| Refining Si | Electricity supply, medium voltage | Scope 2 | 0.012 mole | 8 % |
| Si metal production | Production of silicon, metallurgical grade | Scope 1 | 0.011 mole | 8 % |
| Wafer sawing | Electricity supply, medium voltage | Scope 2 | 0.01 g | 7 % |
| Refining Si | Heat production, co-generation, natural gas | Scope 2 | 0.007 g | 5 % |
| Wafer sawing | Production of dipropylene glycol monomethyl ether | Scope 3 | 0.006 g | 4 % |
| TOTAL | 0.12 mole | 80 % |
Contributions of the most important processes to the gross result of acidification potential of 1 m² multi-Si-wafer manufactured by the benchmark process
| Production chain section | Key process | Emission category | Contribution [H+ eq.] |
Share of gross result |
| Refining Si | Electricity supply, medium voltage | Scope 2 | 0.016 mole | 16 % |
| Si metal production | Production of silicon, metallurgical grade | Scope 1 | 0.015 mole | 15 % |
| Wafer sawing | Electricity supply, medium voltage | Scope 2 | 0.012 mole | 12 % |
| Si ingot | Electricity supply, medium voltage | Scope 2 | 0.01 mole | 10 % |
| Refining Si | Heat production, co-generation, natural gas | Scope 2 | 0.009 mole | 9 % |
| Wafer sawing | Production of dipropylene glycol monomethyl ether | Scope 3 | 0.006 mole | 6 % |
| Refining Si | Production of hydrochloric acid | Scope 3 | 0.005 mole | 5 % |
| Si ingot | Treatment of waste water | Scope 3 | 0.003 mole | 3 % |
| TOTAL | 0.076 mole | 76 % |
Net results of all impact categories analysed
The following table summarises the impact indicator results of all environmental impact categories examined.
Impact indicator results for all environmental impact categories for the production of 1 m² Si-wafer by the benchmark process
| Impact category | Unit | mono-Si-wafer | multi-Si-wafer |
| Climate change | kg CO2 eq. | 29.5 | 25.7 |
| Ozon depletion | mg CF-11 eq. | 2.89 | 2.65 |
| Particulate matter | Disease incidences | 9.18 e-7 | 8.52e-7 |
| Ionizing radiation, human health | kBq U235 eq. | 7.76 | 4.7 |
| Photochemical ozone formation, human health | kg NMVOC eq. | 0.09 | 0.06 |
| Acidification | mole H+ eq. | 0.14 | 0.1 |
| Eutrophication, terrestrial | mole N eq. | 0.35 | 0.2 |
| Eutrophication, freshwater | kg P eq. | 0.02 | 0.02 |
| Eutrophication, marine | kg N eq. | 0.11 | 0.03 |
| Land use | Points | 127 | 104 |
| Water use | kg world eq. deprived | 44 | 30 |
| Resource use, minerals and metals | kg Sb eq. | 0.18 | 0.17 |
| Resource use, energy carriers | MJ | 568 | 460 |
Conclusion
The results of the benchmark processes will help assess the project progress by identifying the expected environmental benefits or burdens resulting from the project processes / products. For this purpose, scenarios are defined based on the work packages and tasks in which the project partners carry out the processes / products development. The scenarios agreed at the current stage of processing can be grouped into four groups:
- Process optimization: three scenarios
- Product manufacturing: one scenario
- Secondary materials replace primary materials: five scenarios
- Secondary materials for silicon ingot production: two scenarios
As the project progresses, scenarios can be added or omitted.
