JPS58200954A - Manufacture of selective absorptive heat receiving surface of heat collector utilizing solar heat - Google Patents

Abstract

PURPOSE:To obtain the heat receiving surface of a high corrosion-resisting heat collector with an excellent selective absorptive surface by forming an oxide layer of specific film thickness to the mirror surface of high chromium stainless steel when manufacturing the selective absorptive heat receiving surface of the heat collector utilizing solar heat. CONSTITUTION:High chromium stainless steel with the mirror surface of 20- 35% weight content chromium is immersed in a chromic acid-solfuric acid solution of a mol ratio of chromic acid to sulfuric acid of 0.003-0.24, and the oxide layer of film thickness of 500Angstrom -2,000Angstrom is formed to the surface of the stainless steel. A selective absorptive surface of a spectral reflecting curve shown in a curve 1 is obtained by the presence of such an oxide layer. Said coating indicates high energy absorptance in a wavelength band of 0.3-2.5mum, and indicates energy emissivity in a wavelength band of 3-50mum.

Description

[Detailed description of the invention] The present invention relates to a method of manufacturing a heat-receiving surface.

More specifically, the present invention relates to a method for producing selectively absorbing heat receiving surfaces for high chromium content ferritic and austenitic stainless steels.

A well-known method for collecting solar heat is to apply a material similar to a black body, such as black paint, to the heat-receiving surface of a heat exchanger and use it as a water filter. Methods like Shiriki and Shiko have a heat collection temperature of approx. ; This method is used when the temperature is below O''C, and is not preferable because it reduces efficiency in systems that require the entire temperature of the tube to collect heat, such as heating and cooling using solar heat.

As a method to eliminate the drawbacks mentioned in L, we have developed a T/C that has a high energy absorption rate in the solar radiation wavelength range (0.3 to 2.5 μm) and has a black color that is the same as the operating temperature of the collector. Wavelength band of body radiation (wavelength 3 to 50 μm at operating temperature ioo°C)
It is known to use a heat-receiving surface, that is, a selective absorption surface, which has an optical spectral characteristic M that results in a low energy emissivity in ).

- A surface having spectral characteristics as shown in item h, that is, a selective absorption surface, is difficult to obtain naturally and is created artificially.

Conventionally, as a selective absorption surface having the above-mentioned spectral characteristics, a metal substrate such as aluminum, copper, or lead, which has a high reflectance, or a transition metal such as nickel, chromium, etc., has been used. sulfides,! There are things that have leprosy, such as chemical compounds and semiconductor substances.

However, the selective absorption surface K as described above has the following drawbacks and drawbacks.

(1) - Poor durability (weather resistance, heat resistance).

(2) Manufacturing costs and material costs are high.

However, it has many drawbacks such as drawbacks, and there are practical problems when using it in harsh natural environments such as solar collectors. In addition, there are only a few that have been put to practical use because they do not have a heat-receiving surface with excellent spectral characteristics.

As is clear from the above description, it is important to have the following characteristics when using the electrodeposition absorbing surface as the heat receiving surface of the Tainobu East Heater, and if these points are not solved, it will not be put into practical use. It is difficult. In other words, +11: Excellent spectral characteristics.

(2) Long-term durability (weather resistance, heat resistance,
moisture).

(3) Excellent corrosion resistance against the heat medium used in the heat collector.

(4) Costs (material costs, manufacturing costs) are low.

B. From the above perspective, the present inventors have previously provided a selective absorption surface having excellent selective absorption characteristics by oxidizing ferritic stainless steel and austenitic stainless steel using a black oxidation method, and a method for producing the same. (Special Publication No. 5J-316si, Japanese Patent Publication No. 52-.3
g6! ;2).

In addition, when using stainless steel '44 as the heat receiving surface of a solar heat collector, it is necessary to take into account its corrosion resistance against heat media (e.g. water).The heat receiving surface of the heat collector simply has excellent spectral characteristics. It is well known that durability, in addition to corrosion resistance, is a major factor in practical application.

In fact, there is a risk of corrosion in stainless steel heat receiving plates under extremely poor water quality conditions (for example, when it contains hundreds to thousands of pp+n of corrosive anions such as 'll-, So, -, etc.). In such cases, stainless steel with excellent corrosion resistance is required.

When it comes to stainless steel, the chromium content in the steel greatly affects its corrosion resistance. Corrosion resistance improves as the chromium content increases, but when producing a selectively absorbing surface, the reactivity to the chemical conversion solution actually decreases. Therefore, it is difficult to obtain a selective absorption surface with excellent spectral characteristics using conventional chemical conversion treatment methods.

The present inventors have conducted basic research on selective absorption surface treatment methods for high chromium stainless steel, and as a result, we have developed a heat receiving collector with high corrosion resistance that has an excellent selective absorption surface, which is the object of the present invention. The present invention has been completed by being able to obtain the above-mentioned surface. That is, the present invention has a chromium content of -〇 to 3.
High chromium stainless steel with a mirror surface of 5% (superimposed) total chromic acid:sulfuric acid molar concentration ratio of 0.00 J or θ, 2
The stainless steel surface is immersed in a chromic acid-sulfuric acid solution of No. 1 to form an entire oxide layer with a film thickness of SOO to 200 OA, exhibiting a high energy absorption rate in the wavelength band of 0.3 to 2.5 μm, 3 or S Oltm
The present invention relates to a selectively absorbing heat-receiving surface of a solar heat exchanger with a coating that exhibits low energy emissivity in the wavelength range of .

The high chromium stainless steel used in the method of the present invention contains 2 chromium.
These are ferritic stainless steels and austenitic stainless steels containing 0 to 35% by weight. Table 1 shows steel types that are already commercially available, but the invention is not limited thereto.

In order for the above-mentioned high chromium stainless steel sheet to have all the selective absorption characteristics, it must exhibit a high absorption rate in the wavelength range of solar radiation energy of 0.3 to 2.5 μm, and
It is necessary to have low radiation in the long wavelength region of 0 μm, but if the surface of the steel sheet becomes extremely rough, the spectral characteristics of the selective absorption surface will deteriorate, and the absorption wavelength of sunlight will be 3 to 11 m at the absorption surface. It extends into the infrared region and is unfavorable in terms of spectral characteristics. The surface roughness of the high chromium stainless steel plate suitable as the selected absorption surface is preferably less than the ISO standard R11 μ, taking into account absorption rate, emissivity, and absorption efficiency (% Kaisho!; 2-386!;.2 (see publication).

In order to adjust the surface roughness of the above-mentioned high chromium stainless steel plate to a roughness suitable as a selective absorption surface, puff finishing, electrolytic polishing,
It is appropriate to perform surface finishing such as mechanical polishing.

'Also, the appropriate thickness of the oxide film formed on the stainless steel plate is described in detail in Japanese Patent Application Laid-Open No. 38652, The optical thickness n, d of the film is /2! ; Og≦n, d<, 2300 s, refractive index n, k2.0≦2. If it is S, the appropriate film thickness is soo
It becomes 'h to/2SO'. Even if the appropriate film thickness slightly deviates from this range, the performance as a selective absorption surface will still be strong.
Since there are various variations, the range of appropriate film thickness was set as 50OA to 100OA.

Absorption rate o by chromic acid-sulfuric acid oxidation on high chromium stainless steel surfaces In order to produce an oxide film with gs or more and emissivity of 0.2 or less, which is practical and has excellent selective absorption characteristics, it is necessary to maintain the specular characteristics of the stainless steel surface without emitting sound or reducing the thickness of the film. ;OOA to 200OA
This is a necessary condition to adjust to.

It was also found that when high chromium stainless steel is oxidized, there is a certain correlation between the chromium content (%) in the stainless steel and the molar ratio of chromic acid: sulfuric acid. The relationship is shown in Figure 2.

As can be seen in Figure 1, as the amount of chromium in the stainless steel edge increases, the molar ratio of romic acid:sulfuric acid decreases, resulting in a bath composition that is dominated by sulfuric acid. This is one of the important points that became clear for the first time in the present invention.

When making a selective absorption surface of high chromium stainless steel as described above, it is necessary to change the oxidation bath composition appropriately depending on the chromium content (steel type).

In order to fully form an oxide film having all of the above practical selective absorption characteristics, it is preferable to use an oxidation bath having the following bath composition.

Bath composition: dichromic acid (converted as Cry) o, o3s to hexamol/sulfuric acid lI to/
2. S mol/bu.

Chromic acid:sulfuric acid molar ratio 0.0 0 3 to 0.2
μ-chromic acid-based drugs include chromic anhydride (Cry3),
It can be appropriately selected from those containing all hexavalent chromium, such as chromates (especially sodium salts and potassium salts) and dichromates (especially sodium salts and potassium salts). It is also possible to use a mixture of the above-mentioned chromium-based chemical shavings. The amount of hexavalent chromium in the chromic acid-based drug used in the method of the present invention may be converted into chromic acid (Cry) and adjusted to fall within the above molar ratio range.

If the molar ratio of chromic acid:sulfuric acid deviates from the above range, practical selective absorption characteristics (reflectance) cannot be obtained as shown in the following examples.

Further, there is a correlation between the oxidation bath temperature and the oxidation time (immersion time), and the higher the oxidation temperature, the shorter the oxidation time.

In order to use an oxidizing bath in which the molar ratio of chromic acid: sulfuric acid is in the range of O0θ03 to 0.2q, and to maintain the oxidized skin film thickness within the range of SOO to 200OA, the temperature of the oxidation bath must be set at so'c to boiling point. An appropriate oxidation time is OoS or 70 minutes.

The method of the present invention provides the following effects.

CI+: Excellent selective absorption surface.

(2) It is possible to provide a heat receiving surface having selective absorption characteristics of a heat collector with sufficient corrosion resistance even under extremely adverse conditions in practice.

(3) The cost is low.

The present invention makes it possible to provide a heat-receiving surface having ideal selective absorption characteristics that satisfies all the conditions required for a solar heat collector, and has great industrial effects.

Next, the present invention will be explained with reference to examples, but the present invention is not limited thereto.

EXAMPLE IF Re-stainless steel was selected from JIS GtlJOj SOSχM, 27i having a mirror surface, degreased with alkali, and immersed in an oxidation bath having the composition shown below.

Sodium dichromate s o fl 7.1
Trisulfuric acid gsoy-/1
Trichromic acid (Cry): Molar ratio of sulfuric acid o, o
*<z.

Oxidation bath temperature /, 30 ~ /, 75 ° C, immersion time q minutes, the absorption rate of the obtained selected absorption surface is 0.92, the emissivity is 0.1
It was 0.

Comparative Example/ Stainless steel having the same mirror surface as above was used as the steel type, and pretreated in the same manner as above.

Next, it was immersed in an oxidation bath having the following bath composition.

Sodium dichromate /l,of/,e sulfur
Acid 3soV/13 Chromium rl (Crys): Molar ratio of sulfuric acid 0.3q
Oxidation bath temperature ioo to tos”c, immersion time 3o minutes.

The absorption rate of the obtained surface is 0.7/, and the emissivity is 0.7/. , l/.

Comparative Example - Stainless steel having the same mirror surface as above was used as the steel type, and pretreated in the same manner as above.

Next, it was immersed in an oxidation bath having the following bath composition.

Sodium dichromate 1I9-/e
・Sulfuric acid/cogll
ff/e Chromic acid (cro,): sulfuric acid nomolar ratio
0.002 Oxidation bath temperature lqs to /3
0″C immersion time: 7 minutes The resulting surface had an absorption rate of oyt and an emissivity of ./λ.

Comparison of the selective absorption characteristics (reflectance) of the selectively absorbing heat-receiving surfaces of high chromium stainless steel obtained by the method of the present invention (those described in Examples and those described in Comparative Examples 1 and 1)
Shown in the figure.

[Brief explanation of the drawing]

Figure 1 is a graph showing the spectral reflection characteristics of the selectively absorbing heat-receiving surface obtained by the method of the present invention, and Figure 2 is a graph showing the oxidation bath composition (C
2 is a graph showing the relationship between the rO5/H2SO4 molar ratio) and chromium content in stainless steel. In Figure 1, /-1 Spectral reflection curve of the selective absorption surface obtained by the method of the present invention, 2-1 Spectral reflection curve of the heat-receiving surface obtained by the method of Comparative Example 3-1 Comparative Example The spectral reflectance curve of the obtained heat-receiving surface, and the spectral reflectance curve of the l-1 ideal selective absorption surface.

Claims (1)

[Claims] (11 chromium content 20 to 35% (ton: tablet))
] A high chromium stainless steel having a temperature of 50% is immersed in a chromic acid-sulfuric acid solution having a molar ratio of sulfuric acid of 0.00 J to 0.2'I.
It is possible to produce a coating with a thick oxide lf+ of 0A to 2000A, which exhibits high energy absorption in the wavelength band of 0.3 to Sμm, and low energy emissivity in the wavelength band of 3 to 3011m. A method for manufacturing a selective absorption V-receiving surface of a solar heat collector. (2) Hiki sharp side (is TSO KI Raku R'i (f
(Measure correctly) (if a is less than 0.07μ, H2 is O
The method according to claim 1, wherein the method has a surface roughness of 1 Ωμ or less.