KR820001483B1 - Method for manufacturing surface of absorption for solar collector - Google Patents

Abstract

This invention relates to a selective absorption surface of a solar collector in which a coating layer consisting of the predetermined composition of metal oxide is tightly adhered to a substrate having the mirror-like surface in the predetermined thickness. The composition of metal oxide consists of 0.001-0.15 % C, 0.005-3.00 % Si, 0.005-10.0 % Mn, 11.0-30% Cr, 0.005-22.0 % Ni, optionally 0.75-5.00 % Mo and the balance being Fe. The metal plate is chemically oxidized in an acidic bath consisting of 150-800 g/l of sulfuric acid at the temp. of 50 ≰C to the boiling point, a dipping time of 3-40 min. to form the oxide film at the thickness of 500 to 200∦.

Description

Manufacturing method of solar collector absorption surface

1 is a diagram showing the relationship between the wavelength of the sunlight (μm) and the reflectance (%) of the selective absorption surface according to the roughness of the substrate surface.

2 and 3 are diagrams showing the relationship between the roughness Ra (µm) and Rz (µm), respectively, the absorption rate (α), the emissivity (ε), and the efficiency (η) of the selective absorption surface.

4 is a diagram showing the relationship between reflectance and wavelength of selective absorption according to various illuminances of the substrate.

5 is a cross-sectional view showing an example of the selective absorption surface of the solar collector using stainless steel as a substrate.

6 is a diagram showing the relationship between the conductivity of a metal oxide film and a wavelength (占 퐉).

7 is a diagram showing the relationship between reflectance and wavelength (μm) of a metal oxide coated on a stainless steel substrate ignoring interference effects.

8 is a diagram showing the relationship between the reflectance (%) and the wavelength (μm) of the selective absorption surface of the solar collector.

9 is a graph showing the relationship between reflectance (%) and wavelength (μm) of metal oxides of ferritic stainless steel and austenitic stainless steel.

10 is a cross-sectional view of a solar collector having a selective absorbing surface.

FIG. 11 is a graph showing the relationship between reflectance (%) and wavelength (μm) of metal oxide of low carbon ferritic stainless steel.

12 is a diagram showing the relationship between the chemical treatment time (minutes) and the thickness of the coating layer in the preparation of the metal oxide layer on the selective absorption surface of the solar collector.

FIG. 13 is a diagram showing the relationship between the emissivity (?) And the absorptance? Of the selective absorption surface of the solar collector and the thickness of the coating layer.

The present invention relates to a method for producing a solar collector absorption surface for the selective absorption of solar radiation heat.

In particular, the present invention is a light-receiving surface (selective absorption surface of the solar collector hereinafter) for the selective absorption of the solar radiation heat, the surface of the metal oxide coating layer formed from a constant metal composition attached to a substrate of a constant thickness, such as mirror surface It refers to a manufacturing method of).

It is well known to use the greenhouse effect as a method of absorbing solar radiation heat.

That is, a coating material having an opacity at an infrared wavelength and a transparency at a visible wavelength is coated on a substrate coated with a material having a characteristic close to that of a black body, for example, a black paint, by a conventional thermal conductivity as a greenhouse effect of the electric material. It is a method of solar heat collection that can have the effect of reducing the conventional heat loss.

The conventional conventional method described above is satisfactorily used when the operating temperature of the solar collector is about 50 ° C. or lower, but when the operating temperature rises above about 50 ° C., the heat collecting efficiency of the solar collector is greatly improved by the above method. There is a drawback to being reduced.

In order to eliminate the above-mentioned defects, it shows low emissivity at the wavelength of 3-5㎛ at the operating temperature of 100 ℃ which is the same as the operating temperature of the solar collector, and black body at the wavelength of the solar radiation (0.3-2.5㎛) It is known to use a conventional selective absorption surface having spectroscopic properties indicating energy absorption such as

It is difficult to obtain a selective absorption surface of a solar collector which naturally exhibits such spectroscopy.

Fully polished zinc plates and naturally oxidized copper plates simply show the selective absorption of solar radiation, so even if these plates are used in solar collectors, they can be used to avoid the disadvantages of electric solar collectors that use the greenhouse effect such as black paint. Insufficient to remove.

Therefore, it has been attempted to artificially manufacture the selective absorption surface of the solar collector.

Copper plate coated with copper oxide or tin plate coated with nickel sulfide by chemical treatment, transparent at infrared wavelength, opaque thin film at visible wavelength, vacuum evaporation method, spattering and arc discharge method A double-coated surface coated with a mirror-like surface with a transparent thin film having a property of preventing reflection of aluminum, for example, an aluminum substrate is covered with silicon metal to prevent reflection of solar radiation, and then coated with oxidative silicon. It has been used as a selective absorbing surface of solar collectors manufactured artificially from cotton.

The vacuum evaporation method has been considered as one of the most reliable methods of manufacturing the selective absorption surface of the solar collector having the antireflection effect by the interference effect of the film.

This is because the thickness of each film is controlled, the material (one material or mixture) of each film must be freely selected, and each film having a suitable refractive index must adhere to each other on the substrate surface.

For example, the sputtering method, the arc method, and the like have been developed since the respective films produced by the special vacuum evaporation, for example, by normal vacuum evaporation are not sufficient to adhere to the substrate.

However, since the evaporation method itself has some disadvantages with respect to the production efficiency and the production cost, the production of the selective absorption surface having the antireflection surface by the vacuum evaporation method, that is, the chemical wet and dry method and the plating method has been tried in several ways.

For example, in the plating method, the selective absorption surface having the antireflection effect and the selective absorption effect due to the interference effect of the film is produced by forming a zinc sulfate and nickel sulfide-zinc or zinc sulfide film on an aluminum plate or a tin plate. .

In the chemical treatment method, the selective absorbing surface having the above-mentioned effect is produced by coating a metal oxide on a steel sheet or a stainless steel sheet according to the same method as the nickel sulfide method described above.

The films prepared by vacuum evaporation method and special vacuum evaporation method, as well as the films prepared by wet and dry chemical method, exhibit selective absorbent properties, but the same selective absorbent properties as the films prepared by vacuum evaporation method and special vacuum evaporation method. In order to produce a selective absorbing surface having a, it is necessary to select a film thickness prepared by a chemical method in an appropriate range.

It is an object of the present invention that a film composed of a metal oxide of a predetermined metal composition selected from among the metal oxide of the film having a selective absorbent property and having an effect of preventing reflection of solar radiation as an interference effect of the film has a predetermined thickness. A method of producing a solar collector selective absorption surface in close contact with a substrate having a mirror-like surface of roughness.

Metal compound

An example metal composition used in the present invention is 0.001-0.15% (weight) of carbon, 0.005-3.00% (weight) of silicon, 0.005-10.00% (weight) of manganese, 11.00-30.00% (weight) of chromium, 0.005 -22.00% by weight nickel, optionally 0.75-5.00% by weight molybdenum and the remainder is a compound of iron.

Such metal compositions are consistent with commercially available stainless steel compounds such as 0.005-0.08% (wt) carbon, 0.005-1.00% (wt) silicon, 0.005-2.00% (wt) manganese, 8.00- 10.50% by weight nickel, 18.00-20.00% by weight chromium and the rest being iron (683 / XIII 11 (ISO), 304 (AISI): 0.005-0.008% by weight carbon, 0.005-1.00 % By weight silicon, 0.005-2.00% by weight manganese, 10.00-14.00% by weight nickel, 61.00-18.00% by weight chromium, 2.00-3.00% by weight molybdenum and the remainder 683 / XIII 20 (ISO), 316 (AISI): 0.005-0.12% (weight) of carbon, 0.005-0.75% (weight) of silicon, 0.005-1.00% (weight) of manganese, 0.005-0.60% ( Weight) nickel, 16.00-18.00% (weight) chromium and the remainder iron (683 / XIII 8 (ISO), 430 (AISI):

0.005-0.12% (weight) carbon, 0.005-1.00% (weight) silicon, 0.005-1.00% (weight) manganese, 0.005-0.60% (weight) nickel, 16.00-18.00% (weight) chromium, 0.75-1.25% (by weight) molybdenum and the remainder are iron (434 (AISI)) and other stainless steels, compounds similar to those listed above.

When stainless steel is used as the metal composition, martensitic stainless steel is not suitable, and in view of the weldability of ferritic stainless steel and austenitic stainless steel, it is suitable to be used as a metal composition of a solar collector.

Another metal composition used in the present invention is a low carbon stainless steel in which other metals are combined to improve corrosion resistance, formability and weldability. For example, 0.001-0.15% (weight) of carbon, 0.005-3.00% (weight) Of silicon, 0.005-10.00% by weight of manganese, 11.00-30.00% by weight of chromium and 0.001-5.00% by weight of nitrogen, copper, aluminum, vanadium, yttrium, titanium, naviorium, tantarum, At least one element from the group of uranium, torium, tungsten, zirconium and afnium, optionally 0.75-5.00% by weight of molybdenum and the remainder is iron, and the metal / carbon + nitrogen ratio is at least 5.0 while coming out as an additional element In the stainless steel mixed with empty, titanium or titanium, the ratio is 8.0 or more.

The metal composition is consistent with the composition of commercially available stainless steels, which are, for example, 0.005-0.03% (weight) carbon, 0.005-0.75% (weight) silicon, 0.005-1.00% (weight) Of manganese, 16.00-18.00% by weight of chromium, 0.1-1.0% by weight of titanium and the rest being iron:

0.005-0.03% (weight) carbon, 0.005-0.75% (weight) silicon, 0.005-1.00% (weight) manganese, 16.00-18.00% (weight) chromium, 0.1-1.0% (weight) titanium, 0.75-1.25% (by weight) molybdenum and the remainder are iron.

In addition, another metal composition used in the present invention is a low carbon stainless steel mixed with other metals to improve corrosion resistance, formability and weldability, for example, 0.001-0.15% (weight) of carbon, 0.005-22.00% (weight) ), 11.00-30% (by weight) of chromium and 0.001-5.00% (by weight) are nitrogen, copper, aluminum, vanadium, tannium, nibadium, tantalum, uranium, torium, tungsten, zirconium and hafnium At least one element selected from the crowd, optionally 0.75-5.00% by weight of molybdenum and the remainder is iron, with a metal / carbon + nitrogen ratio of at least 5.0, the ratio being an additive element mixed with navadium, tantalum or titanium In one stainless steel, the above ratio is at least 8.0.

Preparation of Metal Oxides

The method for producing a metal oxide from the metal composition is as follows.

1) Method of manufacturing metal oxide by wet and dry chemical treatment

2) A chemical treatment method of manufacturing a metal oxide of stainless steel by bringing a stainless steel of a predetermined metal composition into close contact with a substrate having a mirror surface and other stainless steel.

3) A method of producing a metal oxide of stainless steel by vacuum evaporation, sputtering and arc discharging method after closely contacting stainless steel having a predetermined metal composition on a substrate having a mirror surface different from that of the electroplated stainless steel.

4) A method for producing a metal oxide of stainless steel by oxidizing and adhering stainless steel having a predetermined metal composition on a mirror-surface substrate other than stainless steel.

The best method among the above production methods is the following acidic and basic oxidation method.

(a) acidic oxidation

Oxidation conditions are as follows.

Sodium dichromate or potassium dichromate 100-400 g / l,

Or chromium trioxide 40-700 g / l,

Sulfuric acid 150-800 g / l, preferably 400-800 g / l,

Temperature from 50 ° C. to boiling point, preferably 70-120 ° C.

Immersion time 3-40 minutes

(b) basic oxidation

Oxidation conditions are as follows.

Sodium hydroxide or potassium hydroxide 130-200g / l,

Trisodium phosphate (Na ₃ PO ₄ ) or tripotassium phosphate (K ₃ PO ₄ ) 30-40 g / 10

Sodium nitrite (NaNO ₂ ) or potassium nitrite (KNO ₂ ) or

20-30 g / l sodium nitrate (NaNO ₃ ) or potassium nitrate (KNO ₃ ),

Ferric hydroxide (Fe (OH) ₃ ) 1-3g / ℓ,

20-30 g / l of lead peroxide (PbO ₃ ),

Temperature 100-150 ℃, immersion time 3-50 minutes

It is a good idea to pretreat the substrate before chemical treatment. A preferred pretreatment method is a method of immersing a substrate in a mixed solution having a weight ratio of nitric oxide of 1: 1 for 1 hour, or immersing the substrate in a mixed solution of 30 wt% perchloric acid and 1 wt% potassium chloride for 2-3 minutes.

The above-described metal oxides from stainless steel are composed of _e FO (F _e C _r ) ₂ O ₃ for ferritic stainless steel and (F _e , N _i ) 0 (for austenitic stainless steel). It is composed of F _e , C _r ) ₂ O ₃ and both metal oxides are spinel structures with lattice defects.

Board roughness

When the surface state of a substrate having a mirror-like surface satisfies the following requirement, the kind of material of the substrate is not limited only to attaching a metal oxide of a predetermined composition thereon.

1) In order to provide high reflectivity at the selective absorption surface, that is, in the infrared wavelength, the metal oxide layer is transparent to infrared rays, which are reflected on the substrate through the metal oxide layer adhered to the substrate.

2) The adhesion of the metal oxide layer to the substrate depends on the roughness of the substrate surface. The coating layer adhered to the smooth surface of the substrate becomes dense.

3) When the surface of the substrate has a mirror-like surface at the wavelength of visible light and near infrared light, the interference effect is not reduced, and thus the antireflection effect is apparent.

If the substrate surface is coarse, the absorbance of solar radiation increases. It is up to the producer to choose the increase in absorbency or the anti-reflective effect.

4) It is good to manufacture the mirror-like surface of the substrate smoothly to suppress the radiation of infrared rays. If the substrate is too coarse, such a coarse substrate is not suitable because the spectral characteristics of the selective surface are suppressed to the extent that the absorption wavelength of the sunlight at the selective absorption surface reaches an infrared wavelength of 3-8 占 퐉. Various kinds of metal plates, stainless plates and plastic plates can be used as the substrate material.

In view of the above problems, the surface state of the substrate in which the metal oxide of stainless steel is in close contact has a high absorption rate (i.e. a low reflectance) at the wavelength of solar radiation and a low absorption rate (i.e. a high reflection rate) at the wavelength of solar radiation, It can be seen that it is the most important factor for showing sufficient light. In order to improve the efficiency of the selective absorption surface, it is known that the surface roughness of the substrate is better than the wavelength of solar radiation and smaller than the infrared wavelength.

However, this has not been confirmed by experimental data. In general, the surface of the roughly finished substrate has an effect of increasing the absorption rate by multiple reflections and suppresses the anti-reflection effect from the interference effect, which is not limited to the roughness of the actual absorption surface. Because it follows.

Another object of the present invention is to determine the roughness of a substrate when forming a metal oxide of stainless steel on the surface of the substrate. The following experiments were effective to determine the roughness of the substrate.

0.005-0.12% (weight) of carbon, 0.005-0.75% (weight) of silicon, 0.005-1.00% (weight) of manganese, 16.00-18.00 (weight) of chromium, the remainder being a small amount of addition metals and iron (430 ( AISI), a metal compound of 6831XIII 8 (SIO)), was oxidized in an acid bath consisting of 100 g / l sodium dichromate and 400 g / l sulfuric acid at a temperature of 106-108 ° C. for 30-35 minutes. Metal oxides were formed on the surface of the steel.

The relationship between the absorbing wool (α) integrated in the solar tube and the radiant heat (ε) integrated in the radiant heat of the black body efficiency (η) was investigated. The efficiency η is represented by the following equation.

Where σ represents Stefan Boltzmann's constant 4.88 × 10 ⁻⁸ Kcal / m ² h ^O K, and γ represents the crystallization temperature, where we assume 373 ° K. J represents solar radiation (800 Kcal / m ² h).

The roughness of the substrate surface is represented by an arithmetic mean deviation (R _a ) and a ten-point height (R ₂ ) according to the standard R468 determined by ISO. Experimental results are shown in Figures 1 and 2.

In FIG. 1, curve (1) shows R _a 0.36 μm or R ₂ 3.5 μm, while curve (2) shows R _a 0.19 μm or R ₂ 0.6 μm, and curve (3) shows R _a 0.12. In the case of μm or R ₂ 0.5 μm, the curve 4 is R _a 0.08 μm or R ₂ 0.3 μm, and the curve 5 shows the relationship between reflectance and wavelength when R _a 0.04 μm or R ₂ 0.1 μm. It is shown.

Here, it can be seen that the change in reflectance is small compared to the change in illuminance of the selective absorption surface of the solar collector with respect to the wavelength of visible light, while the change is large at the infrared wavelength.

As the ratio of R _a / R ₂ decreases, the reflectance increases.

2 shows the relationship between the R _a value, the water absorption rate α, the emissivity ε and the efficiency η.

In FIG. 2, the absorbance α value of the selective absorption surface is not much influenced by the _Ra value.

The emissivity (ε) decreases rapidly when the value of R _a is less than 0.07, while the emissivity increases proportionally when the value of R _{a is} greater than 0.07. The efficiency (η) value suddenly rises when the R _a value is 0.07 or less, showing an efficiency value of 75% or more.

In Figure 2, the best results are obtained from the selective absorption surface produced by chemically oxidizing the surface of the stainless steel sheet having _a Ra value of 0.07 or less.

As shown in FIG. 3, the absorbance α value of the selective absorption surface is not much influenced by the R ₂ value.

The emissivity? Decreases abruptly when the R ₂ value is 0.2 or less, and the efficiency η represents a high efficiency value of 75% or more when the R ₂ value is 0.2 or less. The best result is that R ₂ comes from selective absorption produced by chemically oxidizing the surface of stainless steel with roughness below 0.2. Selective absorbing surfaces, each with an R _{a of} less than 0.07 or an R ₂ of less than 0.2, are almost smooth at infrared wavelengths and exhibit a small proportion of diffuse reflectance (sum of mirror reflectance and diffuse reflectance) with respect to hemispheric reflectivity. The reduction in reflectance is prevented from the doubled reflectance, which indicates a hemispheric reflectance value of 80% or more at infrared wavelengths of 占 퐉 or more, which significantly enhances the selective absorption of the surface of the solar collector.

When the oxidation treatment of the surface of stainless steel becomes effective, it is necessary to uniformly complete the surface of the metal plate in order to produce a uniform and stable oxide film.

In general, stainless steel surfaces are not homogeneous due to the metallographic structure, mixing rate, work process, localized heat treatment and internal load distribution. If the surface of the stainless steel sheet is not uniform, no uniform oxide film is formed.

One of the effects of the present invention eliminates many of the disadvantages caused by the inhomogeneity of the metal plate, thereby completing the substrate surface of stainless steel with a roughness of R _a below 0.07 or R ₂ below 0.2 by mechanical polishing chemical abrasion and electropolishing. This is to enhance the selective absorption of the surface of the solar collector of the selective absorption. One example showing the efficiency of the selective absorption surface of a solar collector of appropriate illuminance is shown in FIG.

In this example, stainless steel 304 (AISI) 6831XIII 11 (ISO) is _a liquid honing using glass powder with a powder size of 20-100 to form a clean surface with a surface roughness of R _a 0.2 μm or R ₂ 1.0 μm. And the surface was oxidized according to the acidic oxidation method described above. The spectral reflectance of the oxide film of stainless steel is shown by the curve (a) of FIG.

In another example, stainless steel is immersed in a solution containing 10% by weight of nitric oxide and 2% by weight of fluoric acid to form a clean surface with a surface roughness of Ra0.14 μm or R ₂ 0.6 μm, and then the surface is subjected to acidic oxidation. Oxidized.

Curve b of FIG. 4 also shows the spectral reflectance of the oxide film of stainless steel.

In the examples of the present invention, the above-described stainless steel is mechanically or chemically treated or each treated as shown in the above examples to form a treated surface having a roughness of R _a of 0.07 μm or less, or R ₂ of 0.2 μm or less. After or without it was polished and oxidized by the acid oxidation method described above.

The test result is shown by the curve (c) of FIG. The oxide film of the present invention (curve (c)) exhibited higher reflectance at infrared wavelengths than the oxide films of curves (a) and (b).

Thickness of oxide film

The problem arises how to determine the thickness of the oxide film when the metal oxide of the metal composition adheres to the substrate surface according to the following process. In other words,

1) A method of effectively acidic or basic oxidation of a surface of a stainless steel of a predetermined metal composition.

2) Molecular reaction vacuum evaporation, e.g. sputtering and arc discharge, to enhance the adhesion between the oxide film and the substrate.

3) A method in which a metal oxide powder having a predetermined metal composition is brought into close contact with a substrate by using a relatively light-transmitting adhesive such as polyethylene or a silicone resin.

4) A method of oxidizing stainless steel in close contact with a substrate such as oxidized metal chromium or an alloy except for stainless steel. Selective absorption surface of solar collector and the antireflection effect of oxide film are as follows. In other words, the spectral characteristics of the selective absorption surface should reflect less at the wavelength of solar radiation (0.3-2.5 μm) and more at the infrared wavelength (3-50 μm).

5 shows a cross-sectional view of an absorber of a solar collector in which an oxide film is adhered on a mirror-like substrate to show reflection of particle rays on the contact surface between the atmosphere and the oxide film and between the film and the substrate, respectively.

In FIG. 5, incident light coming from the atmosphere 1 is partially reflected at the contact surface between the atmosphere 1 and the oxide film 2 to form the reflected light 4. The remaining incident light is attenuated to pass through the oxide film 2, and is reflected by forming the reflected light 5 at the contact surface between the oxide film 2 and the substrate 3. Since the interference between the light rays 4 and 5 depends on the thickness of the oxide film, the thickness of the oxide film is selected in such a degree as to cause interference in order to obtain an antireflection effect.

Curve 6 in FIG. 7 shows the spectral properties of a selective absorption surface in which a metal oxide of stainless steel is adhered to a substrate on a mirror-like surface ignoring interference effects.

6 shows the spectral conductivity of metal oxides of stainless steel. The stainless steels described above have metal compositions such as 683 / XIII 8 (ISO) and 430 (AISI).

An oxide film composed mainly of chromium oxide and iron oxide (Fe ₃ O ₄ ) (Fe ₂ O ₃ / FeO or Fe ₃ O ₄ ) was used at an acidic solution of 100 g / l sodium dichromate and 400 g / l acid at a temperature of 106-108 ° C. Produced by soaking for 30-35 minutes. An oxide film of stainless steel having an appropriate thickness of the coating layer attached on the mirror-like substrate shows good selective absorbency ignoring the interference effect.

Curve 7 in FIG. 7 shows the excellent spectral properties of the selective absorbing surface with the thickness of the coating layer in the range in which the interference effect reduces the reflectance at the wavelength of solar radiation.

Generally, in order to suppress the reflectance in the contact surface between dielectric materials, the coating layer of the dielectric material which has the median of the refractive index of the said material with a different optical property is provided. If the materials possess complete permeability, the absorption band resulting from the interference effect becomes apparent. Although the materials have intermediate properties between the dielectric and the conductor, the effect of interference is caused by the presence of transmitted light.

Metal oxides in stainless steel do not have complete dielectric properties, but rather have some degree of selective absorption. Therefore, the oxide film described above can be used as a surface exhibiting selective absorbency having an interference effect to some extent. It is possible to minimize the reflectance of the selective absorption surface if the following equation is satisfied.

는 1차 흡수대의 파장을 나타낸다. 만일 스테인레스강을 제5도에 표시한 바와 같이 기판(3)으로 사용하면 타원 계측 분선기의 측정에 의하여 굴절율 n₂는 3.5-3.9이며, 굴절율 n₁는 2.0-2.5이다.Where n ₁ is the refractive index of the coating material: n ₀ is the refractive index of the atmosphere (n ₀ = 1), n ₂ is the refractive index of the substrate, d is the thickness of the film, n ₁ d =

Represents the wavelength of the primary absorption band. If stainless steel is used as the substrate 3 as shown in Fig. 5, the refractive index n ₂ is 3.5-3.9, and the refractive index n ₁ is 2.0-2.5, as measured by the elliptical measuring divider.

일때, 0이 되지 않더라도 제8도의 곡선(8)과 (9)로써 표시한 바와 같이 상기 산화물막은 우수한 흡수 선택성을 나타낸다. 제8도에서 곡선(8)과 (9)는 일차 흡수파장이 각각 분광 방사력이 최고치인 약 0.5㎛와 약 0.8㎛인때, 분광반사율을 표시한 것이다. 비록 가장 우수한 선택 흡수성이 일차 흡수파장(11)이 0.5㎛인때 나타나고, 선택 흡수표면의 최고 흡수율이, 태양 복사선의 분광분포를 고려하여 0.8㎛의 일차 흡수파장(11)에서 나타난다는 것에 곡선(8)로부터 추정되더라도, 현재 곡선(8)과 (9)의 태양 복사선의 흡수율(α)이 공기 질량이 2라는 가정하에서 계산될때, 곡선(8)과 (9)의 상기 흡수율(α)치는 각각 0.90과 0.94로 측정된다.Although the refractive index (2.0-2.5) of the metal oxide film of stainless steel does not satisfy the first equation, the refractive index of the primary absorption wavelength is

In this case, the oxide film exhibits excellent absorption selectivity as indicated by the curves 8 and 9 in FIG. 8 even if it is not zero. In Fig. 8, curves 8 and 9 show spectral reflectances when the primary absorption wavelengths are about 0.5 mu m and about 0.8 mu m, respectively, with the highest spectral radiative powers. Although the best selective absorptivity appears when the primary absorption wavelength 11 is 0.5 [mu] m, the highest absorption rate of the selective absorption surface appears at the primary absorption wavelength 11 of 0.8 [mu] m considering the spectral distribution of solar radiation. Although estimated from 8), when the absorption rate α of the solar radiation of the current curves 8 and 9 is calculated under the assumption that the air mass is 2, the absorption rate α values of the curves 8 and 9 are respectively Measured as 0.90 and 0.94.

In curves (8) and (9) in FIG. 8, the emissivity ε reaches an equivalent value of about 0.12 at long wavelength.

Since the optical constants of the dispersion are different for the substrate and the metal oxide cladding layer at a given wavelength, the wavelengths corresponding to the lowest reflectance 11 at the first absorptance and the primary peak 12 of the spectral reflectance at the optical thickness of the cladding layer of λ / 2. Are different in curves (8) and (9), respectively.

The best selectivity is obtained at the primary absorption wavelength of 0.8 µm rather than the primary absorption wavelength of 0.5 µm.

This is because the minimum reflectance 11 of the curve 9 is less than the minimum reflectance of the curve 8, while the highest reflectance of the curve 9 is less than the highest reflectance of the curve 8.

Line 10 in FIG. 8 shows an ideal spectral reflectance curve of the selective absorption surface at an operating temperature of 100 ° C.

When the refractive index of the metal oxide of the stainless steel is described in detail, the metal oxide grows porous on the surface of the stainless steel in some directions.

In general, the greater the porosity, the closer the refractive index approaches the refractive index of the atmosphere. The refractive index of magnetite (Fe ₃ O ₄ ) is 2.4-2.5 at visible wavelength, while the refractive index of metal oxide of stainless steel is 2.0-2.5 as measured by ellipsoidal analyzer.

Therefore, it can be concluded that the porosity of the metal oxide of stainless steel is 0-20% of the base volume of the metal oxide layer.

The above facts were confirmed by precision microscopic measurements.

The proper thickness (dc) of the metal oxide cladding layer of the stainless steel having the antireflection effect is 500 mW-1,250 mW when the optical thickness (n ₁ d) of the coating layer described above is 1,250 m −2,500 m and the refractive index (n ₁ ) is 2.0-2.5. .

Although the thickness of the coating layer is out of the above-mentioned range, the selective absorbency of the surface appears to some extent, so that the appropriate thickness of the coating layer is 500 kPa-2000 kPa.

The above-mentioned appropriate thickness of the oxide coating layer can be applied not only to stainless steel but also to other substrates. If the substrate is selected from a material having a high refractive index of 4.0 or higher, the selective absorption surface of the solar collector made of the substrate is better than that made of stainless steel substrate.

Example 1

Ferritic stainless steel and austenitic stainless steel of 683 / XIII 8 ((ISO), 430 (AISO) and 638 / XIII 11 (ISO), 304 (AISI) are composed of 100 g / l sodium dichromate and 400 g / l It was chemically oxidized by immersing in a bath at a temperature of 106-108 ° C. for 30-35 minutes to form an oxide layer on the stainless steel surface.

9 shows the spectral reflectance of the selective absorption surface of the oxidized stainless steel as compared with the selective absorption surface of the conventional solar collector.

In FIG. 9, curve (1) shows the reflectance of the surface of oxidized ferritic stainless steel, curve (2) shows the reflectance of the surface of oxidized austenitic stainless steel, and curve (3) shows the copper plate. The reflectivity of the copper oxide coating layer prepared by basic oxidation is shown, and the curve (4) shows the reflectance of nickel sulfate plated on nickel and replated thereon, and the curve (5) shows an operating temperature of 100 ° C. Shows the ideal spectral reflectance of the selective absorption surface of the solar collector.

The selective absorption surface coated with copper oxide exhibits excessively high reflectance at long wavelengths of 4 mu m or more.

It has a reflectance of 3-5% higher than the reflectance (curves 1 and 2) of the selective absorption surface of the oxide film of stainless steel in consideration of the scattered reflectance at a wavelength of 0.3-2.5 mu m.

In the case of the absorption surface (curve 1) formed of ferritic stainless steel, the reflectance is extremely small at a wavelength of 2.0 mu m or less, and is quite high at a wavelength of 2.0 mu m or more.

The selective absorption surface made of ferritic stainless steel is excellent in the same range as that made of copper oxide selective surface.

As indicated by the curve (2) of FIG. 9, the reflectance of the selective absorption surface made of austenitic stainless steel was made of ferritic stainless steel at the same temperature as the operating temperature of the solar collector. Inferior to that

Although the selective absorption surface made of austenitic stainless steel is inferior in spectral characteristics, this surface is useful as an economical solar collector in view of weldability and excellent corrosion resistance.

As described above, the selective absorption surface of the present invention made of ferritic or austenitic stainless steel is advantageous for use in solar collectors because it has excellent heat resistance, corrosion resistance and good breakdown characteristics, which are characteristics of stainless steel.

The metal oxide coating layer made of ferritic and austenitic stainless steel by the chemical oxidation process is a uniform and stable surface in which the original corrosion resistance of stainless steel is not impaired.

The heat resistance of the selective absorbing surface produced by the present invention is in the same category even if a substrate other than stainless steel is used.

10 is a cross-sectional view of a solar collector equipped with a selective absorption surface made of ferritic and austenitic stainless steels.

In FIG. 10, the incident light, as indicated by the arrow, uses a transparent covering (1-3 glass or synthetic resin plates) installed to protect the solar radiation from being separated from the atmosphere 2 and to protect the convective heat loss. It is conducted through and absorbed into the oxide film 3 of ferritic and austenitic stainless steel and converted to heat.

The absorbed heat is conducted through the substrate 4 to a heating medium such as water or air, or to a conventional material 4 adhered to the substrate 4 by plywood or dispersion bonding.

In FIG. 10, 6 is an air layer provided as a heat sink and 7 is an insulating material composed of glass wool, asbestos, or honeycomb structure.

As described above, the selective absorption surface prepared by the chemical oxidation method from ferritic and austenitic stainless steels has an excellent effect of collecting heat when used in solar collectors.

Example 2

The selective absorption surface formed of the ferritic stainless steel according to Example (1) has excellent spectral characteristics and low cost, but has some disadvantages in weldability, formability and corrosion resistance.

To remedy this drawback, stainless steels were treated under the same chemical oxidation conditions as in Example 1.

In FIG. 11, curve (1) shows the relationship between the reflectance and the wavelength of the selective absorption surface made from low carbon stainless steel containing titanium, bolybdenum and additional metals, and curve (2) shows titanium, molybdenum And the above-mentioned relationship of the selective absorption surface produced from low carbon ferritic stainless steel containing no additional metals, and curve (3) shows its ideal curve.

As can be seen from FIG. 11, the selective absorbing surface made of stainless steel containing additional bodies, as well as the absorbing surface of universal ferritic stainless steel containing no additional metals, has excellent spectral characteristics.

Example 3

The following experiments demonstrate that the selective absorption surface of solar collectors with antireflection capability and good spectral reflectance due to interference effects is produced by selecting the oxidation conditions to form an oxide film with a direct thickness of 500-2000Å on the surface of stainless steel. It was effective to

683 / XIII 8 (ISO) and 430 (AISI) stainless steel sheets were chemically oxidized in the following aqueous solutions (A) and (B) while changing the immersion time to form an oxide coating layer on the stainless steel surface.

The relationship between the thickness of the coating layer and the immersion time is shown in FIG. 12. Black body radiation as the absorption rate α (air weight 2) during solar spectroscopy and the operating temperature of the solar collector (50-100 ° C.) The relationship between the emissivity ε integrated at and the thickness of the coating layer is shown in FIG.

The conditions (solution and temperature) for oxidizing the surface of stainless steel are as follows.

FIG. 12 shows the relationship between the thickness of the oxide coating layer and the processing time (minutes) obtained by measuring the change in the primary absorption position (wavelength) with an optical thickness n ₁ d = lambda / 4.

N <_1> represents the average value 2.2 of the said range 2.0-2.5 here.

In FIG. 12, curve 13 relates to the coating layer obtained from the treatment condition (A), and curve 14 relates to the coating layer obtained from the treatment condition (B). FIG. 13 shows the relationship between the emissivity (ε), the water absorption (α), and the thickness of the oxide coating layer at an operating temperature of 100 ° C.

In FIG. 12, the curve 15 is the relationship between the thickness of the coating layer and the absorbance α, and the curve 16 is the relationship between the radiation rate ε and the thickness of the coating layer. As can be seen from curves (15) and (16), the absorbance value (α) is 0.80 or more for the thickness of the coating layer of 500-2000 Hz, and the thickness of the coating layer of about 900 Hz when the wavelength of primary absorption resulting from the interference effect is 0.8 µm. Is 0.94, and gradually decreases when the thickness of the coating layer is 1000 kPa.

As can be seen from the curve 13, the emissivity (ε) gradually increases until the thickness of the coating layer reaches about 1500 kPa, and is 0.2 or more when the thickness of the coating layer is 2000 kPa or more, and the selective absorption surface with excellent selective absorption is oxide of stainless steel. Obtained when the film thickness is 500-2000 mm 3.

It can be seen that the selective absorption surface having the excellent characteristics as described above is obtained regardless of the oxidation process when the thickness of the oxide coating layer is 500-2000 mm 3.

As described above, according to the present invention, a selective absorption surface having excellent properties in durability, heat resistance, corrosion resistance and adhesion when using stainless steel as a substrate can be obtained.

In the case of the conventional selective absorption surface composed of copper oxide, the spectroscopic property is not changed at a temperature of 180-200 ° C (24 hours) by changing the surface color, but the surface structure of the metal oxide is not changed at a temperature of 210 ° C (24 hours) or more. Since it is destroyed, there is a closed end where the spectroscopy is changed.

In the conventional copper selective absorption surface described above, the surface is exposed to rain water, so that the reflectance of infrared rays by the surface is reduced.

The selective absorption surface of the solar collector produced by the method of the present invention does not have such a closed end.

According to the present invention, the selective absorption surface having excellent spectroscopic properties and low manufacturing cost is produced even when ferritic stainless steel is used as a substrate, but the above-mentioned surface is weldable and corrosion resistant as compared with the case where austenitic stainless steel is used. This is inferior.

In order to remedy the above disadvantages, the above-mentioned surfaces are prepared from low carbon ferritic stainless steel, ferritic stainless steel containing a small amount of special addition metal or low carbon ferritic stainless steel containing a small amount of special addition metal.

These steels do not have closed ends, such as stress corrosion, found in austenitic stainless steels, and have almost the same mechanical strength as austenitic stainless steels.

Claims (1)

Each contains carbon: 0.001-0.15% (weight), silicon: 0.005-3.00% (weight), manganese: 0.005-10.00% (weight), chromium: 1100-30.00% (weight), and nickel: 0.005-22.00% (Weight) and 0.001-5.00% (weight) of at least one of nitrogen, copper, aluminum, vanadium, yttrium, lithium, nibium, tannium, uranium, torium, tungsten, zirconium and hafnium A stainless steel substrate containing either or both molybdenum and with or without 0.75-5.00% (by weight), the remainder being iron and having a mirror-like surface, with 150-800 g / l sulfuric acid, 100 Acidification by immersion for 3-40 minutes at a temperature of 50 ° C. to boiling point in an acid bath composed of -400 g / l sodium dichromate or potassium dichromate or 40-700 g / l chromium trioxide, or 130-200 g / l sodium hydroxide Or potassium hydroxide, 30-40 g / L trisodium phosphate or tripotassium phosphate, 20-30 g ₃ at a temperature of 100-150 ° C. in a basic bath consisting of sodium acetate or potassium acetate or sodium acetate, potassium acetate, _1-3 g / l iron hydroxide and 20-30 g / l lead peroxide (PbO ₃ ) A method for producing a solar collector absorbing surface, which is immersed in -50 minutes for alkaline oxidation to form an oxide film having a thickness of 500-2000 kPa on the surface of the substrate.

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