The quality of equipment used in concentrated solar power (CSP) and solar photovoltaic (PV) plants has a major role to play in increasing the overall generation from the plant. One of the key components in a CSP plant is the solar mirror and, of a PV plant, it is solar glass. Efforts are under way to increase the efficiency of these components and ensure proper waste management of solar glasses.
Solar mirrors
According to the research paper “Investigation and Review of Mirrors Reflectance in Parabolic Trough Solar Collectors” by Hamzeh Jamali, scientists have been trying hard to improve the efficiency of CSP systems. Thermal efficiency of parabolic trough solar collectors is one of the key areas of research. As part of this, the radiation heat transfer in parabolic trough solar collectors plays an essential role. The radiation heat is directly concentrated on solar mirrors used in the collectors. Mirrors are used as solar thermal reflectors to concentrate a high solar flux on a small area of absorber tube in order to attain higher fluid temperature. Scientists are trying to achieve a solar mirror with the ideal reflectance of one. While they have not been able to achieve this yet, they have come up with various materials for the reflecting surfaces of mirrors, each of which has its advantages and disadvantages. Meanwhile, some of the mirrors designed by scientists have shown nearly ideal reflectance. However, mirrors often face a problem of corrosion, which leads to degradation and a decline in solar reflectance. It is, therefore, crucial to design mirrors that not only have ideal reflectance but are also durable and resistant to corrosion.
Solar glasses
Poor quality solar glass sheets can become cloudy over time and severely affect the level of irradiation on the cells. A possible solution to tackle this issue is the use of tempered glass. It is a toughened glass sheet and is considered than other glass sheets. When tempered glass breaks, it shatters into smaller pieces unlike normal glass plates. Moreover, tempered glass makes solar panels more durable.
An anti-reflective coating may be added to the glass before it is tempered. This helps in reducing the amount of reflected light, thereby increasing light absorption by the panel. This, in turn, improves solar panel efficiency and increases power generation from the solar plant.
Another trend is the use of glass-glass panels with the growing uptake of bifacial modules. These panels have a glass sheet at the rear end as well, similar to the dual-faced solar cells in bifacial panels. Compared to backsheets, glass-glass panels allow better transparency on both ends of the panel, thereby generating more electricity from a solar module.
Recycling solar glass
In 2019, the Ministry of New and Renewable Energy (MNRE) issued a concept note/blue print on the “Management of Antimony Containing Glass from End-of-life of the Solar PV Panels”. Antimony is a chemical element used in solar panel glasses, that can harm the environment. Although antimony-free glasses are available, their uptake is limited.
According to the blueprint, solar glasses can leach this hazardous element when exposed to wet conditions after their end of life. There are various use cases of the recycled solar glass. The recycling process of 1 tonne of PV waste is likely to produce 686 kg of clean glass and 14 kg of contaminated glass, and the recycled glass can be used to produce new solar panels. The blueprint mentions that in case there are no facilities to recycle, the option of disposal in secured landfills or safe storage till recycling facilities are developed can be considered. Countries such as Germany have developed PV recycling technology where glass containing antimony may be recycled without affecting its properties.
The blueprint provides valuable data sets on this topic. The Central Pollution Control Board (CPCB) studied two samples of used solar glasses with antimony to anaylse “antimony concentration”. The analysis indicated that the concentration of antimony in textured glass is in the range of 0.13 per cent and 0.29 per cent (the results are presented in the table). The CPCB also conducted a soluble threshold limit concentration test on solar glasses with antimony following a method specified under the Hazardous and Other Waste Management Rules (HOWM), 2016. In this test, the leached content was tested for antimony concentration and compared with standards stipulated under the HOWM Rules, 2016. Overall, the results indicated that samples of waste solar panel glass containing antimony do not fall in the category of hazardous waste as per the concentration limits stipulated for antimony in Schedule II of the HOWM Rules, 2016. Therefore, the blueprint suggested that waste glass containing antimony may be considered as a “low effect” waste and needs to be regulated for safe handling, recycling or disposal.
The blueprint gives many suggestions for the effective management of waste solar glasses. For the management of PV modules, it recommends a four-stage cradle-to-grave life cycle assessment – component production, module manufacturing, module use, and end-of-life use stage. Furthermore, it recommends the disposal of solar glasses in an environmentally safe manner following the principles of resource recovery, resource efficiency and circular economy. Recycling is key as glass is a valuable resource. However, the blueprint notes that recycling facilities for solar glasses with antimony are not yet available. Such facilities may be created by the industry once adequate quantity of PV waste is available and a policy framework is set that puts the responsibility of sending the waste for recycling on the generator/producer.
Apart from management-related suggestions, the blueprint recommends some regulatory interventions as well. Broadly, the recycling of end-of-life solar panel glasses containing antimony may be made mandatory and the responsibility for recycling be put on generators. In addition, producers may be made responsible for ensuring recycling of waste solar glasses as part of extended producer’s responsibility. This is akin to the case of e-waste, used lead acid batteries, packaging material, etc.
Regarding regulations for reuse or recycling of solar glasses, the blueprint mentions that the process is technically feasible; however, at present, it is not widely done due to high operating costs and low profitability. Also, manufacturers may be given the responsibility of recycling and reusing the waste solar glass. Furthermore, every generator may set up facilities for the safe dismantling of used solar panels or tie up with an authorised dismantling facility. Moreover, it should be ensured that glass containing antimony is not mixed with normal glasses during recycling, as it may contaminate the entire glass being produced. In addition, the possibility of utilising used solar panels in cement kilns should be explored.
Regarding handling and storage, waste solar glasses should be collected and stored safely under a covered shed till the time the material is sent for recycling or the option for recycling is available. In addition, the transportation of waste solar panels should be done in covered trucks, preferably in trucks authorised for transportation of hazardous waste as per the Motor Vehicles Act, 1988.
For the ultimate disposal in a secured landfill, it should be ensured that waste solar glass is never disposed of or dumped in open landfills to prevent the release of antimony. While the disposal of waste solar glass in a secured landfill is not the preferred option, it may be exercised in case the waste glass cannot be recycled even in the long run. Further, only the non-recyclable material in solar panels (excluding glass, aluminium and junction boxes) may be allowed to be disposed of at secured landfills.
Outlook
The installed solar capacity is increasing every year. With this, the expectations regarding efficiency gains from solar equipment are also going up. Therefore, the focus on technological improvements in solar mirrors and glasses is likely to remain. Further, with solar waste increasing and more recycling facilities coming up, the recycling of solar glasses will soon become profitable and widespread. Therefore, going forward, both efficiency and sustainability will be key trends in the solar power space.
