JPS5857666B2 - Selected absorption surface and manufacturing method for solar heat collectors - Google Patents

Description

[Detailed Description of the Invention] Many methods for selective absorption treatment of the surface of the heat absorption plate of a solar collector have already been developed and put into practical use, but a coating that achieves both low processing cost and high performance has not yet been put into practical use. I'm looking forward to its development.

A typical example is a treatment method in which a selective absorption surface is obtained by using a simple coloring method such as a chemical conversion treatment or a high-temperature oxidation method on the surface of stainless steel.

However, its absorption properties are limited.

That is, the coatings produced by these coloring methods are homogeneous single-phase chromium oxide coatings, and by controlling the thickness of this coating, the absorption characteristics can be improved in the wavelength band where solar energy is concentrated.

However, since the coating is single-phase, the reflectance will vary due to light interference effects.

This puts a limit on the characteristics.

The reflectance of the selective absorption surface of the solar heat collector is preferably as low as possible on the short wavelength side, and rises rapidly from around 2 μm.

In order to shift the rising wavelength to the longer wavelength side, it is sufficient to increase the thickness of the oxide film.

However, in the case of a homogeneous single-phase coating, the waviness on the short wavelength side cannot be forced, and the reflectance cannot be lowered due to the reasons mentioned above.

On the contrary, in order to keep the reflectance low on the single wavelength side, it is possible to make the film thinner, but it is difficult to make the rise wavelength longer than 1 μm, and the disadvantage is that the reflectance in the wavelength range of 1 μm or more increases. will occur.

For the above reasons, a chromium oxide single-phase coating cannot obtain favorable optical properties over the entire wavelength range, and its heat absorption properties cannot be said to be sufficient.

An example of a coating that avoids this problem is a black chrome coating in which a mixed coating of metallic chromium and chromium oxide is electrodeposited.

There have also been reports of cases in which good optical properties were obtained by using this coating to control the mixing state of the two and the coating thickness.

Alternatively, there have been reports of good results obtained by depositing several types of coatings to form a multilayer structure.

However, both methods require complicated treatment methods and are expensive, which has hindered their widespread use.

In order to eliminate the above-mentioned drawbacks, in the present invention, a black oxidized film (chromium oxide film) containing silicon oxide was created on the surface of a stainless steel plate.

The feature of the present invention is that the reflection of light at the short wavelength side of 1 μm or less, so-called sunlight wavelength, is reduced by an oxide film mainly composed of chromium oxide containing silicon oxide on the surface, and the rise wavelength of 1 reflectance is reduced to 2.0 μm. Adjust so that it is located close to 2.0μ
The objective is to reduce the reflectance on the wavelength side shorter than m and improve the absorption characteristics of the selective absorption surface.

Also for forming the coating of the present invention. Since the stainless steel is immersed in an aqueous solution containing mainly hydrochloric acid, oxides on the surface of the stainless steel are dissolved and fine irregularities are formed, thereby further improving the absorption characteristics of the selective absorption surface.

The coating manufacturing method of the present invention includes cleaning a stainless steel plate by removing oil, fat, dirt, etc. by well-known means, and then
Immerse it in an aqueous hydrochloric acid solution to dissolve the oxides on one surface.

When stainless steel is immersed in an aqueous hydrochloric acid solution, a silicon compound such as sodium silicate, fluorosilicic acid, silicate colloid, etc. is added thereto, or after the stainless steel is immersed in an aqueous hydrochloric acid solution, the above-mentioned silicon compound is added. is applied to the surface of stainless steel by spraying, dipping, or other means.

After that, by immersing it in a chromic acid-sulfuric acid aqueous solution, the surface of the stainless steel is coated with 1000-200% silicon oxide.
A black oxidized film (chromium oxide film) of oooA is formed.

Stainless steel is immersed in an aqueous solution of hydrochloric acid.

The presence of chlorine ions creates fine irregularities on the surface due to pitting.

The degree of surface unevenness is about 1 μm across.

Optical properties can be improved by reducing the reflectance of sunlight having a wavelength of 1 μm or less.

In order to control the degree of unevenness on the surface of stainless steel, in addition to adjusting the bath temperature and immersion time of the hydrochloric acid aqueous solution, it is also necessary to increase the chlorine ion concentration.

This is also possible by adding chlorides such as nickel and antimony, or by adding hydrofluoric acid.

According to the present invention, a stainless steel plate is immersed in an aqueous hydrochloric acid solution and then a silicon compound is attached to the surface, or by immersing a stainless steel plate in an aqueous hydrochloric acid solution containing a silicon compound, chlorine ions are applied to the surface to prevent sunlight reflection. Effective fine irregularities are formed on the surface, and a silicon compound is also attached to the surface.

In this condition. When immersed in a chromic acid-sulfuric acid aqueous solution, the silicon compound is oxidized to silicon oxide, forming a black oxidized film mainly composed of chromium oxide.

Although the mechanism by which silicon oxide is generated is not clear, the presence of silicon oxide has been confirmed as a result of examining the formed film using a photoelectron spectroscopy apparatus (ESCA).

As described above, according to the present invention, it can be achieved at extremely low cost.

A surface coating structure with ideal optical properties can be obtained.

Example Among ferritic stainless steels, a thin plate of 19Cr-2Mo steel, which is most often used for solar heat absorption plates, was
It was immersed for 4 minutes in an aqueous solution of 1e containing 30.9 g of CglOO, L sodium silicate.

A microscopic photograph showing the surface condition of the stainless steel at that time is shown in FIG.

It can be seen that many fine pits of 1 μm or less are formed on the surface.

Thereafter, black oxidation treatment was performed under the conditions shown in Table 1.

First table treatment bath CrO3300g/e+H2SO4500g/
e Temperature: 70'C Immersion time: 4 minutes Figure 2 shows, for comparison, the optical properties of 19Cr-2Mo steel subjected to only the black oxidation treatment shown in Table 1 (a) and 1 of the present invention. This is the result.

It can be seen that the optical properties of the surface coating produced by the method of the present invention are extremely superior to those of the conventional method.

Figure 3 shows an example of the structure of a film obtained by the method of the present invention, which was investigated using an ion microanalyzer (IMA).

It shows the composition distribution in the depth direction from the surface of the coating.

The Si in this film was determined by photoelectron spectroscopy using silicon oxide (S).
i02).

In addition, S, which is a representative steel type of austenitic stainless steel,
A thin plate of US 304 steel was immersed for 4 minutes in an aqueous solution of 30% H (1e made by adding water to J'101), washed with water, blown off the moisture on the surface with compressed air, and immediately exposed to silicone. Acid colloid aqueous solution (20 g as Sin2
/(1) and dried.

Thereafter, the optical properties of the surface coating which was subjected to black oxidation treatment under the conditions shown in Table 1 are shown as C in FIG.

It was found that, like ferritic stainless steel, it has excellent optical properties, and the absorption rate α of the austenitic stainless steel coating is 0.96. The emissivity was 0.16.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a micrograph (010000x) showing the surface condition of ferritic stainless steel immersed in an aqueous hydrochloric acid solution. FIG. 2 shows the measurement results of the optical properties of the black oxidized coating only a and the surface coating 1) sC of the present invention. FIG. 3 shows the results of examining the surface coating of the present invention using an ion microanalyzer (IMA).

Claims (1)

[Claims] 1. Solar heat utilization characterized in that a black oxidized film (chromium oxide film) containing silicon oxide is formed on the surface of stainless steel so that it is concentrated on the surface and gradually decreases toward the inside. Selective absorption surface of the heat collector. 2 Stainless steel is immersed in an aqueous hydrochloric acid solution and then a silicon compound is attached to the surface, or it is immersed in an aqueous hydrochloric acid solution containing a silicon compound and then immersed in an aqueous chromic acid-sulfuric acid solution to form a black oxidized film. A method for manufacturing a selective absorption surface of a solar heat collector.