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NREL: High-Value Silicon Is More Important Than Intact Wafers in Recycling of Solar Panels

published: 2020-07-23 10:30

Researchers at the US National Renewable Energy Laboratory (NREL) recently released a study that provides several recommendations on developing the methods for recycling solar panels (or photovoltaic modules). The study, which was published in the journal Nature Energy, estimates that the cumulative mass of discarded solar panels worldwide could reach up to 80 million metric tons by 2050. After evaluating various recycling methods, NREL has concluded that the focus of recycling solar panels should be on the recovery of high-value materials instead of the preservation of silicon wafers for future reuse. The NREL study also states that more aggressive R&D is needed to reduce the cost and environmental impact of the recycling process.

Besides crystalline silicon that is the basic material of solar (or photovoltaic) cells, a solar panel also contains the following: glass for the covering, aluminum for the frame structure, plastics for the encapsulation and the backsheet, and other common as well as precious metals for electricity generation and conduction. A standard solar panel comprises 65-75% glass, 10-15% aluminum, 10% plastics, and 3-5% silicon.

A polymer resin (i.e., ethylene-vinyl acetate or EVA) is used to hermetically encapsulate solar cells and the other key components during the panel manufacturing process. This bonding material is effective for protection and operational stability, but it makes the dismantling of a module very difficult. How to efficiently separate these components so that they be reused or further broken down for material recovery has become a major challenge for countries that have been promoting solar photovoltaics as part of their energy transition policies.

Several methods have been developed for recycling solar panels. Geltz Unmelt-Technologie, a German firm specializing in waste management, has developed a pyrolysis process that burns off EVA and breaks down the key components into particles of basic materials. Sieves and air classifiers are then used to sort and collect these particles. While this German company has adopted thermal decomposition as the main approach to recycling solar wastes, other companies in the same field have stayed with the method of mechanical separation. Veolia, a French counterpart of Geltz Unmelt-Technologie, has set up a process that uses robots to crush solar panels and sort the resulting granules. The grind is searched and picked for glass, silicon, plastics, copper, silver, and other materials that can be later used to make new solar products.

Under Taiwan’s official recycling scheme for solar wastes, every solar panel is registered and subject to a recycling fee before installation. According to the Environmental Protection Administration of Taiwan, local recyclers currently employ thermal decomposition and hot knife as the main methods for detaching solar cells from the glass covering of solar panels. Research agencies in Taiwan have also successfully developed processes derived from either pyrolysis or chemical treatment to remove EVA.

All in all, each recycler has its own process for dealing with abandoned solar panels; and at the same time, some of the countries that actively foster a solar industry are lacking clear regulations on recycling solar wastes. Garvin Heath, a senior scientist at NREL and one of the authors of the study, said that since solar photovoltaics plays a vital role in energy transition, finding cost-effective means of recovering and managing the materials from decommissioned solar panels will help push the solar industry to conform with the principles of the circular economy.

The NREL study concentrates on the recovery of crystalline silicon from solar wastes as this material is the foundation of standard solar cells. The conversion efficiency of a solar cell depends on the purity of the crystalline silicon. Since the material is made of the same silicon molecules, purity here means the structural integrity of the crystal lattice. If the lattice of a silicon wafer is composed of tightly and orderly arranged silicon molecules with few or no grain boundaries, then the wafer has a high degree of electron mobility.

Monocrystalline silicon, which has a purity of 99.999999999%, is used to make high-efficiency solar cells. The production of this material involves a costly and time-consuming process where low-grade silicon undergoes refinement in a furnace system. Hence, reusing crystalline silicon from solar wastes could result in huge cost savings for manufacturers of solar products.

Although the dumping of solar panels into landfills is a growing practice, the problem has yet to reach a scale that raises significant international concerns. NREL has pointed out that there is currently just one facility in the world specializing in the recycling of crystalline photovoltaic modules. Therefore, the top priority of the industry, government, and academia is to continue investing resources into the R&D of recycling processes. Improvements in this area will simultaneously raise the material recovery rate, lower the cost of recycling, and reduce the environmental hazards of solar wastes.

The article in Nature Energy asserts that recyclers should focus on the extraction of basic materials such as high-value silicon rather than trying to make sure that cell wafers are intact. Although there are ways to separate whole wafers from panels, recovered wafers are mostly likely to have suffered some damages during such processes and thus cannot be directly reused. On the other hand, a solution that is designed to recover high-value silicon will likely involve an additional step or sub-process for raising the purity of the recovered material.

Also, the implementation of a recycling method has to consider other environmental and economic factors. NREL suggests that the application of techno-economic analyses and life-cycle assessments will help ensure the desired outcome of the adopted method and practices.

The study lastly comments that limiting solar wastes can also be achieved on the supply side. Specifically, cell and module manufacturers can adopt more advanced technologies or materials to increase the operational lifespan and conversion efficiency of their products.

 (News source: TechNews. Photo credit: Pixabay.)

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