The global transition towards renewable energy has positioned solar power as a cornerstone of clean electricity generation. This proliferation is built upon millions of photovoltaic modules, or solar panels, deployed worldwide. A typical crystalline silicon solar panel has a service life of 20 to 25 years, leading to an impending wave of end-of-life (EoL) solar panel waste. Conventional disposal methods like landfilling and incineration are unsustainable, causing significant environmental pollution and a substantial waste of valuable resources embedded within the panels, such as glass, silicon, and metals like silver and copper. Consequently, developing efficient and environmentally sound recycling technologies for EoL solar panels is of paramount importance.
Among various recycling methods, pyrolysis has emerged as a promising thermal-chemical treatment. It involves heating the solar panel material in an oxygen-deficient or inert atmosphere. This process decomposes the polymer encapsulant, typically ethylene-vinyl acetate (EVA), which binds the glass, cells, and backsheet together. Through pyrolysis, the EVA is converted into gaseous and condensable hydrocarbons, thereby liberating the glass, silicon wafers, and metallic conductors for subsequent recovery. Effective pyrolysis of the solar panel encapsulant is known to occur substantially above 450°C, with temperatures around 500°C or higher required for complete decomposition, often needing a reaction duration of approximately 30 minutes.
Rotary kilns are widely adopted for pyrolysis processes due to their high processing capacity, continuous operation, and effective solids mixing. However, designing and optimizing such a kiln for EoL solar panels requires a deep understanding of the internal thermal environment. The feedstock enters the kiln at ambient temperature and must be heated to the reaction temperature; the time required for this heating phase is difficult to measure directly but is crucial for determining the total residence time needed for complete pyrolysis. Oversizing the kiln to ensure sufficient time leads to unnecessary capital and operational costs, while undersizing results in incomplete processing.
Computational Fluid Dynamics (CFD) simulation provides a powerful tool to investigate the complex heat transfer and temperature distribution inside a rotary kiln, which are challenging to obtain through experimental measurements alone. In this study, a CFD-based thermal model of a rotary kiln is developed to simulate the heating profile of shredded EoL solar panel material. The model aims to predict the effects of key operating parameters—such as wall temperature, residence time, and filling degree—on the material’s temperature evolution. Subsequently, the simulation findings are validated against data obtained from a pilot-scale rotary kiln pyrolysis experiment. The combined approach seeks to establish reliable guidelines for the design and operation of industrial-scale pyrolysis units for EoL solar panel recycling.
1. Development of the Rotary Kiln Thermal Model
The primary objective of the model is to simulate the steady-state temperature field within the kiln’s heating section, focusing on the solid feedstock composed of shredded EoL solar panel material. To make the problem computationally tractable while retaining physical fidelity, several simplifying assumptions are adopted:
- The kiln operates at a steady state with constant wall temperature.
- The solid material moves at a constant, plug-flow-like velocity along the kiln’s length, induced by the rotation and inclination.
- Heat generation from the pyrolysis reaction is treated as a uniform volumetric source term within the material bed.
- The gas phase (inert atmosphere like nitrogen) flows counter-currently to the solids, but for the primary heat transfer analysis focused on solid heating, its detailed flow is simplified.
- The material is considered as a continuous, homogeneous medium with effective thermal properties.
1.1. Model Geometry and Parameters
The modeled rotary kiln has a cylindrical heating section with an internal diameter (D) of 0.4 m and a length (L) of 3 m. The solids occupy the bottom portion of the cross-section, with the fill level (or filling degree, f) defined as the fractional area occupied by the solids. The gas occupies the remaining space above the bed. Heat is transferred from the constant-temperature kiln wall to the internal gas and solids. The key geometrical and operational parameters are summarized below.
| Parameter | Symbol | Value / Range | Unit |
|---|---|---|---|
| Kiln Internal Diameter | D | 0.4 | m |
| Heating Section Length | L | 3.0 | m |
| Wall Temperature | Tw | 500 – 600 | °C |
| Total Residence Time | tm | 30 – 120 | min |
| Solid Material Velocity | u | L / tm | m/s |
| Filling Degree | f | 5% – 20% | – |
| Solid Bulk Density | ρs | 1300 | kg/m³ |
| Solid Specific Heat Capacity | cp,s | 0.9 | kJ/(kg·K) |
| Solid Thermal Conductivity | ks | 0.15 | W/(m·K) |
| Gas Density (N₂) | ρg | ~1.25 | kg/Nm³ |
| Gas Specific Heat Capacity | cp,g | ~1.04 | kJ/(kg·K) |
| Gas Thermal Conductivity | kg | ~0.026 | W/(m·K) |
| Pyrolysis Reaction Enthalpy | Q | 30 (Endothermic) | kJ/kg |
1.2. Governing Heat Transfer Equation
The temperature distribution within the kiln is governed by the energy conservation equation. For a steady-state system with convection and conduction, the equation simplifies to the following form, applied separately to the solid and gas phases with appropriate coupling at the interface:
$$ \rho c_p (\mathbf{u} \cdot \nabla T) = \nabla \cdot (k \nabla T) + S $$
Where:
- $ \rho $ is the density (kg/m³).
- $ c_p $ is the specific heat capacity (J/(kg·K)).
- $ \mathbf{u} $ is the velocity vector (m/s).
- $ T $ is the temperature (K or °C).
- $ k $ is the thermal conductivity (W/(m·K)).
- $ S $ is the source term (W/m³). For the solid phase representing the solar panel material, $ S $ incorporates the endothermic heat of the pyrolysis reaction. For the gas phase and in solid regions without reaction, $ S = 0 $.
The term on the left, $ \rho c_p (\mathbf{u} \cdot \nabla T) $, represents energy transfer due to the bulk motion of the material (convection). The first term on the right, $ \nabla \cdot (k \nabla T) $, represents energy transfer due to conduction.
To analyze the results, the “bulk solid temperature” at any axial position (x) along the kiln is defined as the cross-sectional average temperature of the solid material at that location. This metric effectively represents the thermal state of the shredded solar panel feedstock as it progresses through the kiln.
2. Simulation Results and Analysis of Influencing Factors
The CFD model was solved for various operational scenarios to understand how different parameters affect the heating profile of the EoL solar panel material.
2.1. Influence of Filling Degree (Feed Rate)
The filling degree directly relates to the kiln’s feed rate and determines the bed depth. Simulations were conducted for filling degrees of 5%, 10%, and 20% under a constant wall temperature of 550°C and a total residence time of 60 minutes. The resulting axial profiles of the average solid temperature are plotted. The results indicate that the impact of filling degree on the heating curve is relatively minor. The maximum temperature difference observed at a given position (e.g., at 30 minutes residence) was about 9°C. Given that typical furnace temperature control tolerances and inhomogeneities can exceed ±5°C, the influence of the filling degree, within the studied range, can be considered secondary for thermal design purposes when processing this solar panel material. This is advantageous as it allows for some operational flexibility in feed rate without drastically altering the core pyrolysis kinetics.
2.2. Influence of Wall Temperature
The wall temperature (Tw) is the primary driving force for heat transfer. Simulations were run with Tw ranging from 500°C to 600°C, a filling degree of 10%, and a residence time of 60 minutes. The axial temperature profiles reveal a consistent pattern: the solar panel material heats rapidly upon entry, with the most significant temperature rise occurring in the first 20 minutes. As the material temperature approaches the wall temperature, the driving force for heat transfer (the temperature difference) diminishes, leading to a slower heating rate. After approximately 50 minutes, the material temperature is within about 12°C of the wall temperature. By 60 minutes, the difference reduces to roughly 8-9°C. For practical purposes in a well-mixed kiln, the material can be considered near the wall temperature after 50-60 minutes.
To ensure complete pyrolysis of the solar panel encapsulant, two thermal criteria must be met: (1) The material must reach the major reaction initiation temperature (~450°C), and (2) it must be maintained at a sufficiently high temperature (ideally >500°C) for a duration adequate for complete decomposition (~30 minutes). Based on the simulation results, the time to reach 450°C and the estimated minimum required residence time for each wall temperature are calculated and summarized below.
| Wall Temp. Tw (°C) | Time to Reach 450°C (min) | Estimated Min. Residence Time* (min) | Final Avg. Material Temp. (°C) |
|---|---|---|---|
| 500 | 21 | ≥ 51 | ~491 |
| 525 | 14 | ≥ 44 | ~516 |
| 550 | 12 | ≥ 42 | ~540 |
| 575 | 10 | ≥ 40 | ~565 |
| 600 | 9 | ≥ 39 | ~590 |
*Estimated Min. Residence Time = Time to 450°C + 30 min (reaction hold).
The analysis shows that a wall temperature of 500°C yields a final material temperature below 500°C, which may be insufficient for complete pyrolysis. Temperatures of 525°C and above reliably bring the material above 500°C. However, excessively high temperatures (e.g., 600°C) increase energy consumption and may accelerate the degradation of the kiln shell material. Therefore, an optimal operating window for the pyrolysis of EoL solar panels is identified from the simulation to be between 525°C and 575°C.
3. Experimental Validation and Process Optimization
3.1. Experimental Setup and Method
Validation experiments were conducted using a pilot-scale electrically heated rotary kiln. The key specifications of the kiln included a barrel diameter of 0.4 m, a total length of 4.5 m (with a 3 m heated section matching the model), a maximum temperature of 900°C, and a variable rotational speed and tilt angle to control residence time. The feedstock consisted of de-framed, shredded crystalline silicon solar panels.
The experimental procedure was as follows: The kiln was heated to the target setpoint temperature (wall temperature) under a continuous nitrogen purge to maintain an inert atmosphere. The shredded solar panel material was then fed continuously via an airlock system. The residence time was adjusted by changing the rotational speed and inclination. The temperature of the discharged solid residue was measured at the outlet. To quantitatively assess the completeness of pyrolysis, a post-experiment analysis was performed: samples of the discharged material were placed in a nitrogen-purged tube furnace, reheated to 600°C, and held for 1 hour. The mass loss during this post-treatment was measured. A mass loss of less than 0.1% indicates that virtually all pyrolyzable organics from the original solar panel were removed during the kiln process, signifying complete pyrolysis.
3.2. Temperature Validation
The simulated and experimentally measured discharge temperatures were compared for wall temperatures of 525°C, 550°C, and 575°C across residence times from 30 to 60 minutes. The results show excellent agreement in trend. The actual discharge temperatures were consistently slightly higher (by no more than 13°C) than the simulated values. This minor discrepancy is within expected margins considering model simplifications (e.g., idealized plug flow, homogeneous properties) and measurement uncertainties. The close correlation validates the CFD model’s capability to accurately predict the thermal history of solar panel material within the rotary kiln.
3.3. Determination of Optimal Pyrolysis Parameters
Based on the post-treatment mass loss analysis, the completeness of solar panel pyrolysis under various conditions was evaluated. The results are presented in the table below.
| Wall Temp. (°C) | Residual Mass Loss after Re-heating to 600°C, w/% | |||
|---|---|---|---|---|
| 30 min | 40 min | 50 min | 60 min | |
| 500 | – | – | – | 0.6 |
| 525 | 1.9 | 0.7 | < 0.1 | < 0.1 |
| 550 | 0.8 | 0.5 | < 0.1 | < 0.1 |
| 575 | 0.3 | < 0.1 | < 0.1 | < 0.1 |
Bold indicates conditions achieving complete pyrolysis (mass loss < 0.1%).
The experimental data confirm the simulation predictions. At 500°C, even with 60 minutes of residence, pyrolysis was incomplete (0.6% mass loss). Effective pyrolysis conditions were: a wall temperature of 525°C with a residence time of 50 minutes, 550°C with 50 minutes, or 575°C with 40 minutes. These align well with the “Estimated Min. Residence Time” from the simulation (Table 2). Considering energy efficiency, equipment longevity (lower thermal stress), and process safety, the optimal operating point for recycling this specific EoL solar panel feedstock is determined to be a wall temperature of 525°C with a residence time of 50 minutes.
4. Conclusion
This study successfully demonstrates an integrated simulation and experimental approach for analyzing and optimizing the pyrolysis of end-of-life solar panels in a rotary kiln. A simplified yet effective CFD heat transfer model was developed to simulate the temperature field within the kiln. The model revealed that the heating profile of the solar panel material is predominantly governed by the kiln wall temperature and the solid residence time, while the influence of the filling degree (or feed rate) is relatively minor within practical ranges. The material undergoes rapid heating in the initial 20 minutes, approaching within 10-15°C of the wall temperature after about 50 minutes.
Experimental validation using a pilot-scale kiln showed good agreement between simulated and measured temperatures, confirming the model’s reliability. Furthermore, the experiments identified the optimal pyrolysis parameters for complete decomposition of the solar panel encapsulant: a wall temperature of 525°C and a residence time of 50 minutes. This operational window ensures effective resource recovery while balancing energy consumption and equipment considerations.
The proposed methodology, combining targeted CFD simulation with pilot-scale validation, provides a valuable tool for the design, scaling, and operational guidance of industrial rotary kiln systems dedicated to recycling waste solar panels. It helps move away from overly conservative design based on large safety margins towards more efficient, data-driven engineering for sustainable solar panel recycling.