π Lesson 7
D4
Calculating Glass & Silicon Recovery Yields Using Recycling Tab
It's the percentage of glass and silicon you can actually recover from old solar panels during recycling, not just whatβs theoretically present.
π― Learning Objectives
- β Calculate glass recovery yield (%) from mass balance data across a PV recycling line
- β Apply silicon mass tracking to distinguish metallurgical-grade from semiconductor-grade recovery yields
- β Analyze how delamination method (thermal vs. mechanical) impacts silicon recovery yield using empirical data
- β Explain the trade-off between purity and yield in silicon recovery processes
- β Apply IEC 62930:2023 test protocols to validate reported recovery yields
π Why This Matters
Over 8 million tonnes of solar PV waste will be generated globally by 2030 (IRENA, 2021). Unlike landfilling, high-yield recovery of glass (70β85% of module mass) and silicon (5β10%) preserves critical raw materials, reduces embodied energy, and supports circular economy mandates like the EUβs Ecodesign for Sustainable Products Regulation (ESPR). Low recovery yields directly increase the cost per tonne of recycled silicon β making economically viable recycling impossible if yields fall below ~65% for glass or ~40% for silicon.
π Core Principles
Recovery yield is governed by three interdependent layers: (1) Physical liberation β effective separation of glass, EVA, cells, and backsheet without shattering or cross-contamination; (2) Chemical integrity β minimizing silicon oxidation or glass devitrification during thermal delamination (e.g., >450Β°C risks SiOβ formation); and (3) Process fidelity β accurate mass tracking at each stage (input feed, intermediate streams, final products) with ISO 14040-compliant boundaries. Crucially, 'recovered' β 'pure': industry distinguishes between 'glass cullet' (β₯95% SiOβ, suitable for insulation or construction) and 'solar-grade glass' (β₯99.5%, required for new PV front sheets), each with different yield baselines. Similarly, silicon recovery must specify grade: metallurgical-grade (MG-Si, β₯98% purity, yield ~45β65%) versus upgraded semiconductor-grade (β₯99.9999%, yield typically <25% due to refining losses).
π Recovery Yield Formula
The fundamental mass-based yield formula applies universally but requires precise stream definition. For regulatory reporting and process optimization, yield must be calculated per material stream with verified analytical composition.
Material Recovery Yield
Y = (m_{recovered} / m_{input}) Γ 100Calculates the percentage of a target material (glass or silicon) successfully recovered from input PV modules.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| Y | Recovery yield | % | Mass-based recovery efficiency of specified material |
| m_{recovered} | Recovered mass | kg | Dry, purified mass of target material meeting declared grade specification |
| m_{input} | Input mass | kg | Total mass of target material in feedstock, determined by certified elemental analysis (e.g., XRF + LECO) |
Typical Ranges:
Commercial thermal-mechanical recycling: 75β85% (glass), 35β45% (MG-Si)
Lab-scale hydrometallurgical + upgrading: 60β70% (glass), 20β30% (semiconductor-grade Si)
π‘ Worked Example
Problem: A pilot-scale PV recycling plant processes 1,000 kg of c-Si modules. Elemental analysis shows the input contains 720 kg glass (SiOβ-dominated) and 65 kg silicon (in cell wafers). After thermal delamination, mechanical sorting, and acid leaching, the facility recovers 582 kg of marketable glass cullet (96.2% SiOβ) and 27.3 kg of MG-Si (98.7% pure). Calculate both yields.
1.
Step 1: Identify known masses β Input glass = 720 kg; Input Si = 65 kg; Recovered glass = 582 kg; Recovered Si = 27.3 kg.
2.
Step 2: Apply yield formula: Yield (%) = (Recovered mass / Input mass) Γ 100 β separately for each material.
3.
Step 3: Glass yield = (582 / 720) Γ 100 = 80.8%; Silicon yield = (27.3 / 65) Γ 100 = 42.0%. Both fall within typical operational ranges for commercial thermal-mechanical lines.
Answer:
Glass recovery yield = 80.8%; Silicon recovery yield = 42.0%. These values meet IEC 62930:2023 minimum reporting thresholds for Class B recyclers (β₯75% glass, β₯35% silicon).
ποΈ Real-World Application
In 2022, ROSI (Recycling of Solar Installations) operated a 2,500-tonne/year line in Lyon, France, using staged thermal delamination (320Β°C for EVA, then 480Β°C for solder) followed by air classification and eddy-current separation. Their audited annual report (CITEPA, 2023) showed 79.3% glass recovery (95.8% SiOβ) and 41.7% silicon recovery (98.4% MG-Si) β aligning closely with lab-scale predictions from Fraunhofer ISEβs PV CYCLE validation model. Critically, when they bypassed the acid leaching step to reduce CAPEX, silicon yield jumped to 52.1% but purity dropped to 92.3%, rendering the output unsuitable for metallurgical reuse β demonstrating the yield-purity trade-off in practice.