Solar Absorber Coatings: The Key to High-Efficiency Solar Thermal Collection

Solar absorbing coatings: The key to efficient light and heat harvesting

In the global wave of pursuing sustainable energy, solar thermal utilization technology, with its direct and efficient energy conversion method, has always occupied an indispensable position. At the core of this technology lies a key element that determines the performance of the entire system - the solar absorption coating. Although it is hidden inside the collector, it is the "heart" of the entire energy conversion process. Every tiny leap in its performance leads to a leapfrog improvement in the efficiency of solar thermal utilization. This article will delve deeply into the working principle of solar absorption coatings, comprehensively explore their extensive application scenarios, objectively analyze their current development status, and look forward to their promising future prospects.

Specification And Data

1) The laser welding Line developed a small melting pool, it means enhanced the connection strength between absorber and riser pipes, the result is promise longer life of solar absorber; 

2) The appearance of solar collector without welding line is more beautiful and integrated well with building’s roof; 

3) The is no destroy of the surface of blue coating film, in the flat plate solar collector; 

4) The distance between two welding spots is 3mm, more welding spots take high transmission to system.

1. Spectral selective absorption: Intelligent "Light Sieve"

The solar absorber is a key component that absorbs solar radiation energy into heat energy and transfers heat to water. The choice of material, structure and surface coating of the heat-absorbing core have a decisive influence on the performance of the water heater. The requirement for the heat absorption plate is to have a certain pressure bearing capacity, good compatibility with water, and high thermal performance.

Solar radiation is a wide-spectrum energy source, with its main energy concentrated in the wavelength range of 0.3 to 2.5 micrometers (including ultraviolet rays, visible light and near-infrared rays). Any object with a temperature above absolute zero will also lose energy through thermal radiation, and its wavelength is usually greater than 2.5 micrometers (mid-far-infrared rays).

1. Working principle: the magic of light and heat in the micro world

The mission of solar absorption coatings seems simple: to capture solar radiation to the greatest extent and convert it into thermal energy, while minimizing the loss of its own heat to the greatest extent. However, achieving this goal requires ingenious physicochemical design, with the core lying in "spectral selectivity".

Ordinary black paint absorbs light of all wavelengths equally, but when heated, it also radiates heat outward with the same efficiency. The spectral selective absorption coating, on the other hand, is an intelligent "gatekeeper", having two completely different sides:

High absorption rate (α ≥ 0.94) : It shows an extremely strong "devouring" ability for the short-wave band of solar radiation (0.3-2.5µm), reflecting as little sunlight as possible.

Low emissivity (ε ≤ 0.05) : It exhibits an extremely high "suppression" ability for the long-wave band (>2.5µm) of its own thermal radiation, preventing the precious heat from being radiated out in the form of infrared rays.

This "high absorption and low emission" characteristic enables the coating to heat up rapidly and firmly "lock" the heat. Its performance is usually measured by the "photothermal conversion efficiency" (α/ε) ratio, and the larger this ratio, the more outstanding the coating performance.

2. Achieve selective technical approaches

This magical property is not accidental but is achieved through the following several main micro-mechanisms:

Intrinsic absorption: The inherent properties of the coating material determine that it only strongly absorbs energy in specific bands. By carefully selecting composite materials such as metal oxides, nitrides or carbides, their absorption bands can be made to overlap with the main energy regions of the solar spectrum.

Interference effect: The coating is designed as a multi-layer film structure (such as a metal-dielectric composite film), with each layer having a thickness of only the nanometer level. When sunlight is reflected at different interfaces, constructive interference and destructive interference occur, thereby enhancing absorption at short waves and reflection at long waves.

Surface texture and scattering: By fabricating the coating surface into a microscopic rough structure or nanoparticles, the incident light undergoes multiple reflections and scatters within it, thus being fully absorbed, as if trapped in a "light trap".

3. Advanced preparation process

To achieve these microstructures, the following advanced preparation techniques are mainly adopted:

Physical vapor deposition (PVD)/magnetron sputtering: This is a process in which, in a vacuum environment, plasma is used to bombard a metal target, causing its atoms to deposit on the substrate to form a dense and uniform film. This is the most advanced technology for manufacturing high-performance and long-life commercial coatings, featuring high purity and strong adhesion.

Electrochemical methods (such as anodizing and electroplating) : A conversion film or deposition layer is formed on the surface of a metal substrate (such as aluminum or copper) through chemical reactions in the electrolyte. The cost is relatively low, but its performance is usually inferior to that of PVD coatings.

Sol-gel method: A wet chemical method that forms a thin film through chemical reactions in a solution. It has potential in the preparation of coatings with special components and structures, but the stability and durability for large-scale applications are still being continuously optimized.

Ii. Application Scenarios: A Vast World Beyond Water Heaters

The application of solar energy absorption coatings has long transcended traditional household water heaters and permeated into various fields ranging from civilian, commercial to industrial use.

1. Low-temperature applications (<100°C) : domestic hot water and building heating

Household solar water heaters: Whether they are vacuum tube collectors or flat plate collectors, their core is the heat-absorbing body coated with a selective absorption coating. It provides low-cost domestic hot water for millions of families around the world.

Building Integrated Solar System (BIST) : Coatings are integrated into building facades, balcony railings or roofs, collecting heat energy while also serving as building components to provide energy for space heating and hot water.

Swimming pool heating: Even simple uncovered collectors use weather-resistant absorbent coatings to raise the temperature of the pool water.

2. Medium-temperature applications (100°C - 400°C) : Industrial Thermal Energy and drive

Industrial process heating: This is a huge potential "blue ocean" market. In industries such as food processing, textile printing and dyeing, chemical engineering, and electroplating, a large amount of medium and low-temperature steam or hot water is required. Solar thermal fields based on high-performance coatings can replace fossil fuels and directly supply energy to these processes, achieving energy conservation and carbon reduction.

Solar cooling: By using the heat generated from solar heat collection to drive absorption or adsorption refrigerators, it achieves the green cooling effect of "the stronger the sun, the colder the air conditioner".

Seawater desalination: Solar thermal seawater desalination technology relies on the heat energy collected by the coating to evaporate seawater and obtain fresh water.

3. High-temperature applications (>400°C) : Concentrated solar power generation and advanced power

Concentrated solar power (CSP) : In CSP power stations such as parabolic trough and tower types, absorption coatings are applied to the heat collection tubes or receivers. Under highly concentrated light conditions, it must withstand temperatures of hundreds or even thousands of degrees Celsius and efficiently transfer heat to the heat transfer fluid (such as molten salt) to drive the steam turbine for power generation. The coating here is the lifeline for the power station to achieve high photoelectric conversion efficiency.

Iii. Current Development Status: Mature technologies coexist with fierce competition

After decades of development, the solar energy absorption coating industry has formed a pattern with a clear technological hierarchy and fierce market competition.

Technology maturity and market stratification: Coatings such as blue film and black chromium produced by magnetron sputtering technology have become standard configurations for mid-to-high-end flat plate and vacuum tube collectors. The technology is highly mature and has a lifespan of up to 15 to 20 years. The market simultaneously offers cost-effective electrochemical coatings and PVD coatings that pursue ultimate performance.

Continuous breakthroughs in performance bottlenecks: The focus of research and development has always been on enhancing the α/ε ratio and improving the thermal stability (not degrading at high temperatures) and weather resistance (the ability to resist environmental erosion such as moisture and salt spray) of the coating.

Strong competition from photovoltaic: The "cliff-like" drop in the price of photovoltaic modules has exerted a huge squeeze on the market for medium and low-temperature thermal utilization in solar energy. In scenarios where only low-grade hot water is needed, the "photovoltaic + electric heating" model poses a severe challenge to the traditional solar thermal market due to its simple installation and initial investment advantages.

The persistence of cost pressure: The investment in high-performance PVD coating production lines is huge, resulting in high costs and hindering their promotion in price-sensitive markets.

Iv. Development prospects: the future of innovation-driven and cross-border integration

Facing challenges, the future of solar absorption coatings is not bleak but rather brimming with opportunities to open up new horizons through cutting-edge material technology and system innovation.

1. Pursuing ultimate performance: New coating materials and structures

Multilayer gradient film systems and metamaterials: Through computer-aided design, more complex multilayer film systems are constructed to achieve "precise customization" of spectral absorption. By leveraging emerging concepts such as photonic crystals and plasmonic metamaterials, it is expected to fabricate "ideal" coatings with extremely wide absorption spectra and extremely low emissivity.

Breakthrough in high-temperature stability: New coating materials based on transition metal nitrides, carbides, etc. have been developed, enabling them to remain stable for a long time at temperatures above 400°C and in the air. This will significantly reduce the operation and maintenance costs of CSP power stations and enhance their efficiency.

2. Intelligent and adaptive coatings

Spectral adaptive coating: This is a cutting-edge direction. Research on "intelligent" coatings that can automatically adjust their emissivity according to the ambient temperature. Maintain a low emissivity during heat collection; When the temperature is too high and may damage the system, it can switch to a high emissivity state to enhance heat dissipation and play a self-protection role.

3. Core role in Photovoltaic and Solar Thermal (PV-T) systems

The PV-T system can generate electricity and heat simultaneously and is the star of distributed energy in the future. Here, the high-performance absorption coating plays a crucial role: as a heat sink for the backsheet of PV cells, it efficiently collects the waste heat generated by the cells. This not only cools the photovoltaic modules (enhancing their power generation efficiency and lifespan), but also acquires free thermal energy, achieving the maximization of the comprehensive utilization efficiency of solar energy per unit area.

4. Intelligent Manufacturing and Cost Optimization

By improving the utilization rate of sputtering targets, developing Roll-to-Roll continuous coating technology and enhancing the automation level of production lines, the production cost of high-performance coatings can be continuously reduced, making them competitive in a broader market.

Conclusion

The solar-absorbing coating, this thin layer with a thickness measured in micrometers, is a perfect example of the integration of materials science, optics and thermodynamics. Although it is hidden inside the collector, it is the core engine driving the development of the entire solar thermal utilization industry. Against the backdrop of the global energy structure's transformation towards carbon neutrality, solar absorption coatings play an irreplaceable role, whether it is providing warmth to families, injecting power into industries, or ultimately lighting up cities through concentrated power generation. Its continuous evolution is not only related to the competitiveness of solar thermal technology itself, but also to whether we can capture and utilize the tremendous energy of this star more efficiently and economically. In the future, with the continuous emergence of new materials and new structures, this "key" that unlocks the door to efficient solar thermal energy will surely open up a cleaner and more sustainable new world of energy for us.