Abstract
With the rapid development of the photovoltaic (PV) industry, China is poised to experience a peak period of PV module retirement. The recycling market for waste crystalline silicon solar panel presents vast prospects. The recovery of intact silicon cells not only holds environmental significance but also offers economic benefits. Existing recycling methods, such as pyrolysis, mechanical disassembly, and chemical dissolution, face numerous challenges. This article investigates the pre-treatment of N,N-dimethylpropenylurea (DMPU) coupled with pyrolysis for recycling waste crystalline silicon solar panel. Through experimentation, the effects of different DMPU pre-treatment conditions on silicon cell integrity, backplate removal rates, and the impact of various pyrolysis conditions on silicon cell integrity were explored. The optimal experimental conditions for achieving the highest silicon cell integrity rate were found to be DMPU treatment at 200°C for 60 minutes, followed by pyrolysis at 480°C for 60 minutes. The best condition for backplate removal was treating the solar panel in DMPU at 200°C for 30 minutes. Finally, an analysis of energy consumption and carbon emissions for the practical application of DMPU coupled pyrolysis for recycling waste crystalline silicon solar panel revealed that the method used in this article offers significant energy-saving and emission reduction benefits.
1. Introduction
1.1 Background and Significance
Photovoltaic power generation is recognized for its safety, cleanliness, and sustainability. Among various PV technologies, crystalline silicon solar cells dominate the market due to their mature process technology, high photoelectric conversion efficiency, and low production costs, accounting for over 95% of the PV market. Considering China’s current situation and the actual service life of solar panel, it is anticipated that the first wave of PV module retirement will peak around 2030. The estimated recoverable amount of PV modules is as high as 1.4 million tons, with a cumulative value of recyclable raw materials reaching 7.683 billion yuan. This recycling could reduce carbon dioxide emissions by 7.6 million tons.
1.2 Structure and Recyclable Components of Solar Panel
The main recyclable components of crystalline silicon PV modules include aluminum frames (10%-15% of the total mass), glass (65%-75%), plastics (EVA, 10%), and valuable elements present in the silicon cells. The service life of encapsulant materials such as EVA and backplates is significantly shorter than that of silicon cells. Recovering intact silicon cells for direct use in manufacturing new PV modules can significantly reduce production costs and energy consumption. Silicon cells, the core component of PV modules, contain valuable elements such as silicon, silver, copper, and tin, making them highly valuable for recycling. Furthermore, silicon wafer raw materials and manufacturing costs account for over 60% of the total cost of producing solar cells. For PV module manufacturers, the main raw material for solar cells is silicon sand, which undergoes multiple purification steps to obtain semiconductor-grade silicon. If recycled intact silicon cells can be directly reused in production, it would significantly reduce costs, energy consumption, and carbon emissions associated with silicon purification and processing.
1.3 Current Recycling Methods and Challenges
The current mainstream recycling methods for solar panel primarily focus on softening or decomposing EVA to weaken its adhesive effect on the various layers of the PV module, enabling their separation. These methods include:
- Pyrolysis: Utilizes the softening and decomposition of EVA at high temperatures to achieve separation of layer components. However, issues such as easy fragmentation of solar cells and residual organic matter on the surface of silicon cells persist.
- Mechanical Disassembly: Involves preliminary crushing of solar panel using mechanical equipment to separate different layer components through physical forces. While cost-effective and scalable, this method generates fine powder with complex compositions that are difficult to separate, reducing economic efficiency. Additionally, the crushing process can cause dust and noise pollution.
- Chemical Dissolution: Uses solvents to remove EVA or etch metal from the surface of solar cells through physical or chemical reactions during soaking. Traditional solvents like toluene and trichloroethylene, while effective, pose health risks and environmental pollution concerns. N,N-dimethylpropenylurea (DMPU), as a non-volatile and low-toxicity organic solvent, remains chemically stable before and after reacting with EVA and can be recycled.
2. Materials and Methods
2.1 Materials
To prevent the size of solar panel from affecting experimental results, artificially cut solar panel were used, with a controlled mass of 3.5-7 g and an area of 4-9 cm². The solar panel was provided by Hunan Qianyan Technology Co., Ltd., and their performance characteristics are presented in Table 1. N,N-dimethylpropenylurea (DMPU) reagent was obtained from MACKLIN. Other equipment included a heating device (DF-101S magnetic stirrer), thermal medium (dimethyl silicone oil), clamping devices, a high-temperature furnace (KF1200), and a Fourier Transform Infrared Spectrometer (FTIR, Vertex 70).
Table 1. Performance Characteristics of Solar Panel
| Parameter | Symbol | Value |
|---|---|---|
| Maximum Power | P_max | 10 W |
| Maximum Power Current | I_mp | 0.55 A |
| Maximum Power Voltage | V_mp | 21.5 V |
| Open Circuit Voltage | V_oc | 21.5 V |
| Short Circuit Current | I_sc | 0.62 A |
Note: Standard test conditions: 1000 W/m², 25°C, AM1.5
2.2 Methods
A certain amount of DMPU was weighed and placed in a beaker, which was then preheated in an oil bath. Once the specified temperature was reached, the solar panel was added. After a certain heating time, the solar panel was removed using tweezers, and its morphology was recorded. The solar panel was then cleaned with absolute ethanol to remove any residual organic solvent. The final state of the DMPU was recorded and samples were preserved. Subsequently, the DMPU-treated solar panel was placed in a crucible and subjected to pyrolysis in a high-temperature furnace. The furnace was set to the desired temperature and held for a specified time before cooling to room temperature. The integrity of the silicon cells was recorded. The experimental flowchart, and the flowchart for direct pyrolysis without DMPU pretreatment.
The integrity rate of silicon cells and backplate removal rate were calculated using AutoCAD software for area measurement. The maximum intact area of silicon cells after processing was considered the effective area, while fragmented areas were excluded. The calculation formulas are as follows:
Silicon Cell Integrity Rate Calculation:
textSiliconCellIntegrityRate=Total Silicon Cell Area Before TreatmentMaximum Intact Silicon Cell Area After Treatment×100%
Backplate Removal Rate Calculation:
textBackplateRemovalRate=(1−Original Backplate AreaRemaining Backplate P Layer Area After Treatment)×100%
To investigate the effects of different DMPU treatment conditions on silicon cell integrity and backplate removal rates, four treatment temperatures (160°C, 170°C, 180°C, 200°C) and four treatment times (30 min, 40 min, 50 min, 60 min) were set. Additionally, to explore the impact of different pyrolysis conditions on silicon cell integrity, four temperatures (450°C, 480°C, 510°C, 540°C) and four holding times (45 min, 60 min, 75 min, 90 min) were tested.
3. Results and Discussion
3.1 Effect of DMPU Temperature and Treatment Time on Silicon Cell Integrity
When solar panel were pretreated in DMPU at 160°C followed by pyrolysis, the silicon cells exhibited severe fragmentation, making it impossible to calculate the recovery rate. Therefore, further exploration at this temperature was not conducted. Instead, the results obtained at 170°C, 180°C, and 200°C (with a pyrolysis temperature of 480°C and a holding time of 60 minutes) were analyzed. To avoid experimental randomness, the average values of three experiments were taken as the final results, with error analysis performed.
The impact of different DMPU temperatures and treatment times on silicon cell integrity. The highest silicon cell integrity rate was achieved when the solar panel were treated in DMPU at 200°C before pyrolysis. In contrast, the lowest integrity rate, averaging only 46%, was observed after treatment at 170°C. When the temperature was increased to 200°C, the silicon cell integrity rate showed an upward trend with increasing treatment time, reaching a maximum of 98%. Treatment at 200°C for 60 minutes not only preserved the largest intact silicon cell area but also minimized edge detachment. At 180°C, the silicon cells also exhibited a high degree of integrity, with an average integrity rate of 73%. Within the temperature range of 170-200°C, the silicon cell integrity improved with increasing temperature. At 170°C, severe fragmentation occurred, with the lowest integrity rate of only 31%. This is likely due to insufficient EVA swelling at lower temperatures, preventing the full opening of gas lateral release channels.
Analyzing the silicon cell integrity rates at different heating times during pretreatment revealed that at 170°C and 200°C, the integrity rates increased with time from 30 to 40 minutes. However, at 180°C, the integrity decreased slightly over this period. As the heating time continued to increase beyond 40 minutes, the integrity rates at 180°C and 200°C followed a similar trend. Specifically, at 200°C for 60 minutes, the silicon cell integrity rate reached 97.52%, nearly intact, establishing the optimal treatment condition.
3.2 Effect of DMPU Temperature and Treatment Time on Backplate Removal Rate
The backplate contains fluorine elements, and direct pyrolysis of solar panel can release toxic fluorine-containing gases, causing severe environmental pollution. Furthermore, the backplate acts as a barrier, preventing DMPU molecules from penetrating EVA for chemical reactions, which hinders EVA swelling and the opening of gas lateral release channels, adversely affecting gas expulsion. Therefore, synchronous backplate removal during DMPU treatment is crucial for reducing pollution and enhancing silicon cell integrity during the recycling process. Similar to Section 3.1, the average values of three experiments were taken as the final results, with error analysis performed.
The impact of DMPU treatment conditions on backplate removal rates. Within the temperature range of 170-200°C, the backplate removal rate increased with rising temperature. At 170°C, the backplate removal rate fluctuated significantly over time, with the lowest removal rate of only 26%. When the DMPU treatment temperature was increased to 180°C, the backplate removal effect was significantly enhanced, with an average removal rate of 98%. After 40 minutes of heating, the backplate was completely removed. At 200°C, the backplate removal effect was optimal, with complete removal achieved within 30 minutes, making it the fastest condition for backplate removal.
3.3 Effect of Pyrolysis Temperature and Treatment Time on Silicon Cell Integrity
After treating solar panel in DMPU at 200°C for 35 minutes, the silicon cell integrity rates under different pyrolysis temperatures and times. At 450°C, due to the low temperature and insufficient holding time, the layers of the solar panel could not be separated, resulting in poor silicon cell integrity. At 480°C, the integrity rate first increased and then decreased. At 510°C and 540°C, the average integrity rates were lower. When the pyrolysis temperature is too low, EVA cannot be fully decomposed, retaining some adhesive properties that prevent layer separation. Conversely, at excessively high temperatures, the rapid EVA decomposition generates gas faster than it can be released, causing gas buildup that may rupture the silicon cells. Additionally, the increased thermal stress on the silicon cells exacerbates fragmentation.
The silicon cell integrity rates varied significantly with different pyrolysis times. The peak integrity rate occurred at 60 minutes, with the highest rate reaching 97%. When the pyrolysis time exceeded 60 minutes, the integrity rates declined, indicating that 60 minutes is the optimal pyrolysis time.
3.4 Feasibility Verification of DMPU Cycled Coupled Pyrolysis for Silicon Cell Recycling
A comparison of the silicon cell integrity rates between direct pyrolysis without DMPU treatment and DMPU-pretreated coupled pyrolysis. the calculation of silicon cell integrity after direct pyrolysis, revealing significant fragmentation. Therefore, the maximum intact area was used for integrity calculation, resulting in a rate of only 30.75%. In contrast, after DMPU pretreatment followed by coupled pyrolysis, the silicon cells maintained good integrity. The lowest integrity rate was 42%, and the highest reached 97.52%, significantly higher than that of the control group. Hence, DMPU pretreatment significantly reduces EVA decomposition-induced damage to silicon cells, enhancing their integrity.
Five cyclic experiments were conducted following the experimental procedure, and FTIR testing was performed on the DMPU used in each cycle. The infrared absorption spectra, showing absorbance variations with wavenumbers. Analysis of these spectra revealed that the characteristic bands and peak values in the absorption spectra of DMPU remained essentially unchanged after different cycles. Therefore, DMPU does not generate other organic compounds during the reaction process with EVA, and its structure and properties remain stable. This indicates that DMPU can be recycled when used to treat EVA, preliminarily confirming the feasibility of the DMPU cycled coupled pyrolysis method for recycling waste solar panel.
4. Conclusion
- Feasibility of DMPU as a Novel Green Solvent: DMPU exhibits strong solubility, stability, and low toxicity. This study experimentally verified the feasibility of DMPU cycled coupled pyrolysis for recycling waste crystalline silicon solar panel, providing theoretical guidance for efficient and green recycling methods.
- Optimal Recycling Conditions: The optimal conditions for DMPU coupled pyrolysis recycling of waste crystalline silicon solar panel is 200°C DMPU treatment for 60 minutes, followed by pyrolysis at 480°C for 60 minutes. Under these conditions, the silicon cell integrity rate is maximized, and the backplate is completely removed (achievable with 30 minutes or more of DMPU treatment at 200°C).