By Sarthak Takyar
Solar power as a sustainable and environment-friendly source of clean energy has been receiving increasing attention in recent decades. Two technologies have predominantly been used to harness solar power: solar photovoltaic and concentrating solar power (CSP). Over the years, scientists have been making concerted efforts to improve CSP system efficiency. The thermal efficiency of parabolic trough solar collectors is one of the key areas of focus for research. The radiation heat transfer in parabolic trough solar collectors plays an essential role in this regard. Radiation heat is directly concentrated on solar mirrors used in the collectors. Since the density of sunlight is really low with respect to the temperature needed for the heat transferring fluid in a collector absorber tube, mirrors are used as solar thermal reflectors to concentrate high solar flux on a small area of absorber tube in order to attain higher fluid temperature.
An ideal mirror should have high reflectance, low maintenance and initial cost, and a long lifetime. There are many factors that could affect the reflectance and economic lifetime of a solar reflector. The manufacturing methods for mirrors should ensure that these properties are kept intact even as the mirrors get degraded and stressed by storms, physical abuse, and changes in temperature, water, humidity, vapour, ultraviolet radiation and dust.
Scientists have been endeavouring to achieve a solar mirror with the ideal reflectance of 1. While they have not been able to practically achieve this yet, they have come up with various materials for the reflecting surfaces of mirrors, each of which has its advantages and disadvantages. 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 corrosive agents.
The following are some of the materials and methods studied by researchers to design mirrors of better quality.
Physical vapour deposition
Physical vapour deposition (PVD) is a process wherein atoms or molecules of a material are vaporised from a solid or liquid source, transported in the form of vapour through a vacuum or low pressure gaseous environment, and then condensed on a substrate. The process is also used to deposit films of elements, alloys and compound materials as well as some polymeric materials.
The sol–gel process involves a number of stages such as gel formation for synthesising materials from solutions. The most popular type of sol–gel process is based on the phase conversion of a sol obtained from organometallic or alkoxide precursors. During the process, the sol gets polymerised at low temperature and forms a wet gel. Finally, a dry gel can be formed by eliminating the solvent through standard heating. The main advantages of the sol–gel process over conventional ones are the low synthesis temperature, high purity, high homogeneity, better yield, novel materials and low cost.
Lamination is a technique of manufacturing a material in multiple layers so that the composite material improves in terms of strength, stability, sound insulation, appearance and other properties through applying various materials. There are two types of lamination – extrusion based and adhesive based. The performance of both types depends largely on the polymeric, adhesive and surface treatment ingredients. Adhesive-based laminates can be made by dry bonding, wet bonding and hot melt adhesion processes. Extrusion-based laminates are made by extruding a thin tie-layer of a plastic material to bond two other materials such as a polymer film, paper or foil. Also, co-extrusion of materials can be done by extruding them at the same time and then pressing them together. For incompatible layers, a thermoplastic adhesive is used as a tie-layer to laminate them together.
Thermal decomposition of organometallic compounds
Thermal decomposition, or thermolysis, of organometallic compounds is a chemical decomposition process in which the compounds are decomposed by heat. Organometallic compounds are those in which the carbon atoms of organic groups are bound to metal atoms. As a result of the low electronegativity of metals, carbon–metal bonds are polarised so that carbon becomes the negative pole while metals tend to form positive ions. In reactions between organometallic compounds and compounds with electronegative substituents, attractive interactions between two carbon atoms are possible. Reactions of this type lead to many useful and important carbon–carbon bond syntheses. Accordingly, organometallic compounds are increasingly preferred as the precursors for depositing materials on various substrates via thermal decomposition of the metal compound.
Glass as a classic material has been used in solar mirrors since a long time. In first-surface solar glass mirrors, glass plays the role of a substrate, which is coated by appropriate protective films and the reflective films are positioned on the mirror front side. In second-surface solar glass mirrors, glass plays the role of a superstrate, which is coated by appropriate protective films and the reflective films are positioned at the back of the mirror and protected by other layers.
In order to prepare both first-surface and second-surface glass mirrors, glass substrates and superstrates need to be coated by appropriate reflective and protective films through deposition processes. Various types of glass are used in research including soda lime float glass, corning microsheet glass, aluminosilicate, borosilicate, microscopic glass substrate and simple glass substrates. Each glass type has its own advantages and disadvantages, depending on the technological applications and purposes that researchers and manufacturers aim for. The major optical attributes of an ideal reflector are high optical performance, that is, ideal reflectance, low maintenance, low initial costs and long life. To achieve these properties, materials and manufacturing processes have been undergoing several modifications.
In sum, there are various types of mirrors’ with different characteristics in terms of reflectance, durability and cost. Going forward, experiments will continue to be conducted to achieve solar concentrating mirrors of higher reflectance. In some ongoing experiments, the reflectance of some reflectors has been improved through creative application of materials and manufacturing techniques. But some properties of mirrors’, including their lifetime and costs, have been affected unfavourably to some extent. For this, scientists have been trying to make a trade-off between the above-mentioned factors as per their requirements.
Therefore, future works regarding the issue are expected to be focused on the application of innovative materials and manufacturing methods in order to achieve the highest reflectance with favourable effects on the mirror’s properties. n
Based on the research paper “Investigation and Review of Mirrors Reflectance in Parabolic Trough Solar Collectors”, by Hamzeh Jamali