Solar Absorbing Film: The "Soul Black Technology" of Flat Plate Collector

In the core component of the flat panel solar collector, the heat absorbing plate undoubtedly plays a crucial role. It is precisely the seemingly simple yet high-tech Selective Absorbing Coating on the surface of the heat-absorbing plate that endows it with the ability to convert energy efficiently. This special coating can be regarded as the "soul" of the flat-plate collector, directly determining the heat collection efficiency, working temperature and service life of the entire system. This article will delve deeply into the technical principles, evolution history, performance indicators of this "black suit" and its key impact on the overall efficiency of the collector.

I. Core Function: Why is "Selective" Absorption Necessary?

Solar radiation energy is mainly composed of visible light and near-infrared light, and its energy distribution is approximately equivalent to that of blackbody radiation at 5800K. When the heat-absorbing plate of the collector is in operation (usually below 150℃), it will also generate infrared radiation due to heat, resulting in heat loss. 

1. An ideal absorbent film layer must achieve two seemingly contradictory goals: 

Maximize the absorption of energy from the solar spectrum (wavelengths 0.3-2.5 μm). 

Minimize the energy of infrared radiation (wavelength >2.5 μm) generated by the heat-absorbing plate itself. 

2. This characteristic of "only in, no out" is the core essence of Selective Absorption. Its performance is defined by two key parameters: 

Solar energy absorption rate (α) : The ability of a film layer to absorb solar radiation energy. The closer the ideal value is to 1 (100%), the better. 

Thermal emissivity (ε) : The ability of a film layer to radiate heat outward at the working temperature. The closer the ideal value is to 0 (0%), the better. 

Therefore, the true core indicator for evaluating the quality of a film layer is its photothermal conversion performance ratio (α/ε). The higher this ratio is, the better its selectivity is and the higher the net benefit of the collector is.

Ii. Technological Evolution: From Ordinary Black Paint to High-tech Coatings

The development of the absorption film layer has undergone three generations of technological innovations: 

1. The first generation: Non-selective coating 

Material: Common black paint (such as asphalt paint, blackboard paint). 

Characteristics: Although the absorption rate is relatively high (α ~0.90-0.95), the emissivity is equally extremely high (ε ~0.85-0.90), and the α/ε ratio is close to 1. It is like a black stone, capable of absorbing heat but also dissipating it easily. It is inefficient and has been largely phased out. 

2. The second generation: Intrinsic selective coating 

Materials: Some materials that are inherently selective, such as Black Chromium, Black Nickel, copper oxide (CuO), iron oxide (Fe3O4), etc. 

Characteristics: These materials exhibit different optical properties for light of different wavelengths at the microstructure level. For instance, the traditional electroplated black chromium coating is renowned for its outstanding stability (high-temperature resistance and moisture resistance), with an absorption rate α of 0.93-0.97, an emissivity ε of 0.08-0.12, and a significant increase in the α/ε ratio to over 10. 

The third generation: Gradient selective coating (optical interference type coating) 

Technology: Magnetron Sputtering (magnetron Sputtering) vacuum coating technology is adopted. 

Structure: By successively depositing multiple layers of nanoscale films with different refractive indices (such as aluminum nitride, zirconium nitride, and stainless steel-aluminum nitride composites) on a metal substrate (such as copper or aluminum), a "gradient" structure with a gradually changing refractive index from the surface to the substrate is formed. 

Working principle: Utilizing the principle of optical interference. The thickness and material of each layer of film have been precisely designed to cause multiple reflections and interferences of sunlight within the film layer, ultimately being almost completely absorbed (the "trap effect"). Meanwhile, this multi-layer structure has high reflectivity to long-wave infrared radiation, thereby suppressing thermal emission. 

Feature: This is the current mainstream high-end technology. It has extremely excellent performance, with an absorption rate α > 0.95 and an emissivity ε as low as 0.04-0.06 (at 80℃), and the α/ε ratio is as high as over 15. The appearance is usually uniform blue-black or dark black.

Iii. Core Performance Indicators and Testing Standards

To evaluate the quality of an absorbent film layer, the following major performances need to be considered: 

Optical performance: α > 0.95, ε < 0.05 are the hallmarks of high-end products. 

Weather resistance (durability) 

Moisture and heat resistance: After being tested for hundreds of hours in a high-temperature and high-humidity environment (such as 85℃ and 85% relative humidity), the performance degradation should be minimal. This is the key to assessing the lifespan of the coating. 

Heat resistance: It can operate stably for a long time under working conditions of 150-200℃ without peeling, flaking or oxidation. 

Salt spray resistance: For coastal areas, the ability to resist salt corrosion is of vital importance. 

Adhesion: The coating must be firmly bonded to the metal substrate. Through tests such as cross-sectional tests, it is ensured that it will not fall off. 

Resistance to sun exposure: Under extreme sun exposure conditions, the coating performance remains stable, without aging or failure.

Iv. Mainstream Coating Technologies

Coating type: Sputtering gradient aluminum-nitrogen coating (blue film/black film

Optical performance: Excellent (α>0.95, ε<0.05)

Weather resistance: Excellent (resistant to damp heat and high temperatures)

Environmental friendliness: Environmentally friendly, free of heavy metals

Cost: Relatively high (large equipment investment)

Appearance: Uniform blue-black or dark black

Market trends: Mainstream and future directions

V. How to Choose High-Quality Film Layers for Your Collector?

Inquire about the type of technology: Give priority to the gradient aluminum-nitrogen coating using magnetron sputtering technology. 

Request performance parameters: Ask the supplier to provide a report issued by an authoritative testing institution, with a focus on the test results of absorption rate (α), emissivity (ε), and moisture and heat resistance. 

Observe the appearance: High-quality coating has uniform color, is dense, free of scratches and flaws. It can be gently touched by hand. There should be no powder falling off. 

Choose a reliable brand: Coating materials from well-known brands have more stable quality and a guaranteed lifespan.

Vi. Conclusion

Although the solar selective absorption film is thin, it is one of the core components with the highest technological content and the largest added value in the flat panel solar collector technology. From ordinary black paint to high-tech vacuum sputtering coating, each of its evolations has significantly enhanced the efficiency and economy of solar thermal utilization. 

Today, high-performance gradient coating has become the standard for high-end flat plate collectors, which determines not only the output capacity of the collector in fine weather, but also its heat collection performance in rainy days, low temperature environments in winter and the long-term reliability of the whole system. When choosing a flat-plate collector, a thorough understanding of this black film layer "inside" it is undoubtedly a crucial step in making a wise decision.To invest in a collector with a strong "heart" is to invest in the benefits of energy efficiency for decades to come.