JPS6331712B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention applies black oxidation treatment to rustless steel to produce a selective absorption surface suitable as a solar heat collecting plate, and the surface condition of the rustless steel before the black oxidation treatment is made to have a specific roughness, and the rustless steel is subjected to the black oxidation treatment. The present invention relates to a method for producing a selective absorption surface which is subjected to treatment (by black acid oxidation method or black alkali oxidation method) to further improve selective absorption properties as a heat collecting plate. It is known that stainless steel, which is subjected to black oxidation treatment to form a film of oxide of rustless steel on a stainless steel substrate, has excellent selective absorption properties as a solar heat collecting plate. The object of the present invention is to further improve the selective absorption properties of a solar heat collector plate having such an oxide film of stainless steel. The present inventors conducted various experiments for this purpose, and as a result, the surface of stainless steel was polished by emery polishing, fabric polishing, chemical polishing, etc.
The present inventors have discovered that selective absorption as a solar heat collecting plate can be improved by finishing it to a uniform, mirror-like surface using a polishing method such as electrolytic polishing, and then subjecting it to black oxidation treatment, thereby achieving the present invention. In other words, when performing black oxidation treatment on a stainless steel substrate to form an oxide film on the stainless steel, by making the surface of the stainless steel mirror-finished before the treatment, it is possible to improve the Ra (center line average roughness) specified in JIS-B-0601. 0.07μ or less or the same Rz
(+ point average roughness) By finishing to 0.2 μ or less, it is possible to obtain a heat collecting surface with high reflectance in the infrared region, which is a characteristic of selective absorption. In addition, the growth of an oxide film on a stainless steel surface depends on the surface condition, but because the surface is mirror-finished, the oxide film formed by oxidation treatment becomes smooth, making it dense and with good adhesion. There are benefits to be gained. Next, the reason why the surface roughness is limited to the above-mentioned value in the present invention will be explained in detail. Identical commercially available stainless steel pieces were prepared and mirror-finished to have the surface roughness shown in the following table.

[Table] Each sample was then subjected to black oxidation treatment under the following conditions. Lithium dichromate 100-400g / Sulfuric acid 400-800g / Temperature 50-150℃ Treatment time 3-40 minutes In this experiment, coloring was carried out by the acidic oxidation method described above, but other methods such as the following alkali oxidation method were used. You can color it even if it is twisted. Sodium hydroxide 130-200g / Trisodium phosphate 30-40g / Sodium nitrite 20-30g / Ferric hydroxide 1-3g / Lead peroxide 20-30g / Temperature 100-110℃ Processing time 3-30 minutes FIG. 1 shows the spectral reflectance curve of the selective absorption surface obtained as described above. As is clear from Figure 1, changes in surface roughness do not have much effect in the visible region, but in the infrared region, they greatly affect the reflectance, and the smaller the Ra and Rz values, the higher the reflectance. I can understand what it shows. FIG. 2 is a graph showing the relationship between the Ra value and absorption rate (α), emissivity (ε), and efficiency (η) for the above sample.
Here, the efficiency (η) assumes that the operating temperature is 100℃, and the efficiency is 100℃.
The radiation energy of the black body Eb [Eb = σT ⁴ , σ (Stephan-Boltzmann constant) = 4.88×10 ^-8 Kcal/
m ²・h 〓 ⁴ ; T=absolute temperature] is 944.6Kcal/m ²・
h, and the amount of solar radiation (J) was 800 Kcal/m ² ·h, and was calculated using the formula η=α−Eb/Jε. As the graph shows, the absorption rate (α) is not affected much by the Ra value, but the emissivity (ε) decreases rapidly when the Ra value is 0.07μ or less, and increases proportionally when the Ra value is 0.07μ or more. In addition, the efficiency (η) increases rapidly from around 0.07 μ, reaching 75
% or more. As described above, the selective absorption surface obtained by black oxidation treatment (hereinafter simply referred to as black oxidation treatment) using the black acid oxidation method or the black alkali oxidation method on stainless steel having a surface condition with an Ra value of 0.07μ or less is extremely excellent. ing. Figure 3 shows the R value, absorption rate (α), emissivity (ε), and efficiency (η) for the above sample (same conditions as above).
This is a graph showing the relationship between As the graph shows, the absorption rate (α) is not affected much by the Rz value, whereas the emissivity decreases rapidly when the Rz value is less than 0.2μ.
It can be seen that the efficiency (η) also shows a high value of 75% or more at 0.2 μ or less. As described above, the selective absorption surface obtained by black oxidation treatment of stainless steel having a surface condition with an Rz value of 0.2 μ or less is extremely excellent. The mirror surface having the above-mentioned surface roughness is sufficiently smooth in the infrared region, reduces the ratio of diffuse reflection to hemispherical reflection (reflection that combines specular reflection and diffuse reflection), and prevents a decrease in reflectance due to repeated reflections. As a result, it shows a hemispherical reflectance of more than 80% in the infrared region with a wavelength of 7 μm or more, which greatly contributes to improving the selective absorption of solar heat collectors. Furthermore, when performing black oxidation treatment on rustless steel, it is necessary that the surface condition of the metal base be uniform in order to obtain a stable and uniform oxide film. Strictly speaking, commercially available stainless steel has a nonuniform structure, composition, processing method, local heat treatment, and internal stress distribution, and the surface condition is usually nonuniform.If these points are not resolved, it will be difficult to form a uniform oxide film. However, the present invention not only improves the selective absorption properties of a solar heat collecting plate by mirror-finishing the stainless steel surface with the above-mentioned surface roughness, but also improves the selective absorption by polishing. By making the surface uniform,
Its effects are of great importance, such as eliminating the drawbacks of non-uniformity mentioned above. The type of stainless steel that can be used in the method of the present invention is not particularly limited, but stainless steels that can be suitably used in the method of the present invention include SUS 304, 316, 430, and 434. Preferred specific examples of the present invention will be described in detail below, but the present invention is not limited thereto. FIG. 4 shows an example of a heat collecting member made of stainless steel. When performing black oxidation treatment on the surfaces of two pieces of stainless steel that are locally joined by welding to have a hollow part through which the heat medium flows, it is necessary to perform black oxidation treatment without polishing the surface as in conventional methods. However, there is a time delay in the black oxidation treatment locally, and the heat collecting surface becomes non-uniform, so that its performance cannot be fully demonstrated. To explain this in detail with reference to Figures 4 and 5, when manufacturing the heat collection plate of a solar heat collector using stainless steel, in order to increase the heat exchange efficiency, the heat exchanger as shown in Figure 4 is used. Preferably, a medium flow path is defined. Fourth
As shown in the figure, in a heat collecting plate with this shape, the heat medium follows the flow path in the direction of the arrow. In order to manufacture a heat collecting plate having such a heat medium flow path, two pieces of stainless steel are locally joined to each other so as to have a hollow portion through which the heat medium flows.
That is, as shown in FIG. 5, stainless steels 6 and 7 are overlapped and joined by welding at several predetermined joining points 8, and then a hollow part 9 is formed through which a heat medium flows under pressure of gas or fluid. Alternatively, after press working the stainless steels 6 and 7 in advance, they are joined by welding at the overlapping portion 8 to form the hollow portion 9. Therefore, when welding is performed at the joint 8 in manufacturing such a heat collecting member, a stable oxidized film of stainless steel, generally referred to as a temper color, is formed near the weld. During black oxidation treatment of the localized oxide film generated during welding, there is a significant difference in processing time between the non-welded area and the heat collecting surface becomes uneven, resulting in poor performance. The problem was that it was not fully demonstrated. In order to fully demonstrate its performance as a heat collecting part, it is necessary to remove this local oxide film created during welding before applying black oxidation treatment, but conventional methods for removing this local oxide film Mechanical removal methods such as blasting and chemical removal methods such as pickling were used. However, when these methods are used, the surface of the stainless steel is excessively roughened, and the reflectance, particularly in the infrared region, is extremely reduced, so that the selective absorption property is significantly reduced. In this case, in order to compensate for the above-mentioned drawbacks, either the above-mentioned removal method is applied and the surface is polished to a mirror finish as specified in the present invention, or the above-mentioned removal method is not used and this localized area is removed from the beginning. An excellent selective absorption surface can be obtained by removing the oxide film by polishing and then subjecting the surface to a finishing black oxidation treatment as specified in the present invention. In Figure 6, curve a is an example of a mechanical removal method with a particle size of 20 to 100μ.
After removing this local oxide film using the liquid homing method using glass powder, the Ra value
This is a spectral reflectance curve when a surface roughness of 0.2μ and an Rz value of 1.0μ is subjected to black oxidation treatment. Similarly, curve b shows 10% nitric acid and 2% hydrofluoric acid as chemical removal methods.
This is a spectral reflectance curve obtained when a surface roughness having an Ra value of 0.14μ and an Rz value of 0.6μ was subjected to black oxidation treatment after the local oxide film was removed by immersion in an aqueous solution containing %. Curve c shows that the local oxide film is removed by mechanical or chemical removal and then polished, or by polishing without using the mechanical or chemical removal method. This is a spectral reflectance curve when the film is removed and the surface is finished to the surface condition defined by the present invention, and then the surface is subjected to black oxidation treatment. As is clear from FIG. 6, the present invention eliminates the drawbacks of mechanical and chemical removal methods for localized oxide films, maintains and improves high reflectance in the infrared region, and improves the performance of heat collecting plates. It is possible to perform at 100%. As described above, the present invention provides an industrially extremely useful method for manufacturing a solar heat collector.

[Brief explanation of the drawing]

Figure 1 is a graph showing the spectral reflectance curves of selective absorption surfaces of stainless steel with various surface roughnesses, and Figure 2 is a graph showing the spectral reflectance curves of selective absorption surfaces of stainless steel with various surface roughnesses.
Graph showing changes in absorption rate (α), emissivity (ε), and efficiency (η) of the selected absorption surface due to changes in values, 3rd
The figure is a graph showing changes in absorption rate (α), emissivity (ε), and efficiency (η) of a selected absorption surface due to changes in Rz value. FIG. 6, a diagram showing a specific example, is a graph showing the influence of the spectral characteristics depending on the surface finish state when manufacturing the solar heat collecting member shown in FIGS. 4 and 5. Figure 5; 6, 7... stainless steel, 8... joint, 9
...Hollow part.

Claims (1)

[Claims] 1. In the method of manufacturing selective absorption surfaces of solar heat collector plates by subjecting the surface of rustless steel to black oxidation treatment, the surface roughness of the rustless steel is polished to a surface roughness Ra value of 0.07 μ or less or Rz value specified in JIS B0601.
A method for producing a selective absorption surface, which comprises mirror-finishing the surface to 0.2μ or less and then subjecting it to treatment using a black acid oxidation method or a black alkali oxidation method.