DE2639388C2 - Selective absorption surface of an absorber for a solar collector and process for their manufacture - Google Patents

Description

The invention relates to a selective absorption surface of an absorber for a sornencollector with a surface coating made of metal oxide and a method for the production thereof. Thereby the absorption surface exists of a surface coating of a metal oxide on a substrate made of steel with a mirror-like surface.

Solar collectors which have a cover material which is impermeable in the infrared wavelength range and is transparent in the visible wavelength range, covers a substrate coated with a substance is, which has properties similar to those of the black body, are known So is in the DE-OS 14 67 734 a solar energy collector with solar energy applied over a thermally conductive substrate absorbent receiver layer made of an intimately fused mixture of gold and a Glass described. Solar energy collectors with selective absorption surfaces are also known a copper oxide layer applied to a copper plate or galvanized coated with nickel sulphide Iron sheet.

From US 32 10 220 it is known. To oxidize steel on its surface, the thickness of the oxide layer is chosen so that s ^; e absorbs or repels heat to the required extent.The oxidation of the stainless steel described there takes place with chromic acid ions. From US 30 00 375 it can be seen that a selective absorption surface, which is applied to a substrate with a mirror-like surface, is characterized by high absorption capacity for sunlight and low emission for infrared radiation

The present invention is based on the discovery that a specific relationship between the roughness of the Substrate with a mirror-like surface and the spectroscopic properties is available from the already mentioned DE-OS 14 67 734 was known that the surface coating there have a thickness of 180 nm can.

DE-OS 24 23 877 describes a process for coloring the surface of stainless steel by means of immersion the same in an aqueous solution of sulfuric acid and chromate (s). With the help of the aforementioned procedure The aim is to color the steel across the entire color scale without post-treatment and without corrosion of the steel or without losing the adhesiveness of the colored layer.

The object of the invention is to provide a selective absorption surface of an absorber for a solar collector with a surface coating of a metal oxide on a substrate made of steel with mirror-like To create a surface, the metal oxide layer having a certain roughness and so on is selected that solar radiation selectively absorbs and the reflection of solar radiation due to Interference effects of the film layer is prevented. Associated with this task is a simple process for the production of such a selective absorption surface.

This task is achieved by a selective absorptive surface of an absorber for a solar collector solved according to claim 1 claim 2 includes advantageous embodiments with regard to the The composition of the stainless steel and the metal oxide layer formed therefrom, and claim 3 relates to a method for producing the selective absorption surface.

F i g. 1 the correlation between the wavelength (μπι) of solar radiation and the reflectivity (%) of the selective absorption surface due to the roughness of the substrate surface;

F i g. 2 and 3 the correlation between the absorption capacity (λ), the emissivity {ε) and the efficiency (η) of the selective absorption surface and the roughness Ra (μπι) and Rz {\ im);

Fig. 4 C-'s correlation between wavelength and reflectivity of the selective surface at different roughness of the substrate;

F i g. Figure 5 is a cross-sectional view of one embodiment of the selective absorption surface of a solar collector using stainless steel as a substrate;

F i g. 6 the correlation between the wavelength (μπι) and the transmittance of the oxide film;

F i g. 7, the correlation between the wavelength (μηι) and the reflectivity of the Metallor.ids on the substrate made of stainless StEhI, unte ^r neglecting the interference effect;

F i g. 8 the correlation between the wavelength (μπι) and the reflectivity (%) of the selective Absorption surface of the solar panel;

Fi g. 9 the correlation between the wavelength (μΐη) and the reflectivity (%) of metal oxides, which are derived from iVrritic and austenitic stainless steel, respectively; Fig. 10 is a cross-sectional view of the solar collector with the selective absorption surface;

F i g. 1J the correlation between the wavelength (μπι) and the reflectivity (%) of the metal oxide the low carbon ferritic stainless steel;

Fig. 12 shows the correlation between the thickness (A) (iO- ^{| J} ni) of the coating layer and the chemical treatment time (minutes) for the formation of the metal oxide layer of the selective absorption surface of the solar collector; and

F i g. 13 shows the correlation between the thickness (A) of the coating layer and the absorption capacity (*) and the emissivity (β) of the selective absorption surface of the solar collector.

Preferred stainless steels for the substrate with mirror-like surface; · on which then the corresponding Metal oxide layer is formed are so

(a) 0.005-0.08% by weight C, 0.005-1.00% by weight Si, 0.005-2.00% by weight Mn, 8.00-10.50% by weight Ni, 18.00-20, 00 wt% Cr, balance Fe (683 / XIII 11 (ISO), 304 (AISI));

(b) 0.005-0.08 wt% C, 0.05-1.00 wt% Si, 0.005-2.00 wt% Mn, 10.00-14.00 wt% o Ni, 16.00-18.00 wt% Cr, 2.00-3.00 wt% Mo, balance Fe (683 / ΧΙΠ 20 (ISO), 316 (A'Sl));

(c) 0.05-0.12% by weight C, 0.005-0.75% by weight Si, 0.005-1.00% by weight Mn, 0.005-0.60% by weight Ni, 16.00-18.00 % Cr, balance Fe (683 / XIII 8 (ISO), 430 (AISI));

(d) 0.05-0.12 wt% C, 0.05-1.00 wt% Si, 0.005-1.00 wt% Mn, 0.005-0.60 wt% Ni , 16.00-18.00 O / o by weight Cr, 0.75-1.25 o / o by weight Mo, remainder Fe (434 (AISI));

(e) 0.005-0.03 wt% C, 0.005-0.75 wt% Si, 0.005-1.00 wt% Mn, 16.00-18.00 wt% Cr, 0.1 -1.0 Wt .-% Ti, remainder Fe and

(f) 0.005-0.03 wt% o / o C, 0.005-0.75 wt% Si, 0.005-1.00 wt% Mn, 16.00-18.00 wt% Cr, 0 , 1 - 1.0 O / o Ti by weight,

The selective absorption surface according to the invention is produced using acidic or alkaline methods Oxidation process, whereby the following conditions are observed:

(a) acidic oxidation process

Sodium dichromate or potassium dichromate 100-400 g / l H2O

or chromium trioxide 40-700 g / l H ₂ O

Sulfuric acid 150-800 g / l H ₂ O

preferably 400-800 g / l H ₂ O

Temperature 50 ° C

up to boiling point
of the mixture
preferably 70-12O ⁰ C

Immersion time 3–40 minutes

(b) alkaline oxidation process

Sodium hydroxide or potassium hydroxide 130-200 g / 1 H ₂ O

Trisodium or tripotassium phosphate 30-40 g / l H2O
Sodium or potassium nitrite or

Sodium or potassium nitrate 20-30 g / l H ₂ O

Iron hydroxide Fe (OH) ₃ 1-3 g / l H ₂ O

Lead peroxide (PbO ₂ ) 20-30 g / l H ₂ O

Temperature 100-15 ° C

Immersion time 3–50 minutes

The surface of the substrate is preferably pretreated before the oxidation treatment. The preferred pre-treatment method are dipping the substrate in either an aqueous mixture of one part by weight nitric acid and one part by weight water for one hour or in an aqueous mixture of 30 Oew.-tyb perchloric acid and 1 wt.-9 / o potassium chloride for 2-3 minutes " .

The metal oxide derived from stainless steel has the chemical formula FeO (FeCr) ₂ Oj for ferritic stainless steel and (Fe, Ni) O (Fe · Cr) ₂ O3 for austenitic stainless steel, both metal oxides having a spinel structure with a lattice defect.

In the context of the invention, there was also the task of improving the roughness of the substrate surface determine when stainless steel metal oxide is formed on the substrate surface. To determine the Roughness of the substrate, the following experiment was carried out:

Stainless steel of 0.005-0.12 wt% C, 0.005-0.75 wt% Si, 0.005-1.00 wt% Mn. 16.00-18.00 Wt .-% Cr, a small amount of additional metal and Fe (430 (AISI), 683 / XII1 8 (ISO)) was in one acidic aqueous bath with 100 g / l sodium dichromate and 400 g / l sulfuric acid at a temperature of 106-108 ° C for 30-35 minutes to form the metal oxide layer on the surface of the stainless To form steel.

The interrelationships between the absorption capacity (λ) integrated over the solar spectrum, the emissivity [ß) integrated over the radiation of the black body and the efficiency {η) were examined in each case.

The efficiency (η) is represented by the following equation:

στ *
n-B-e-f.

where σ is the Boltzmann constant 138x 10 ^-23 J / ° Kund τ is the corrective working temperature. Here it is assumed to be 373 ° K. / represents the power of solar radiation (930 KW / m ² ). The roughness of the base surface is represented by the arithmetic mean of the deviation (Ra) and the ten-part height (Rz) in accordance with ISO recommendation R 468

The experimental results are shown in FIGS. 1 and 2 shown in FIG. 1 shows curve I the correlation between the reflectivity and the wavelength when the surface roughness is Ra = 036 μπι or Äz = 3.5 μπι; Curve 2 shows the values for /? A = 0.19 μπι or Rz = OJo μπι; Curve 3 for Ra = 0.12 μπι or Rz = 0.5 μπι; Curve 4 for Ra = 0.08 μπι or Rz = 03 μπι and curve 5 for Ra = 0.04 μπι or Rz = 0.1 μπι.

It has been found that the change in the refiection curve is small compared to the change in the roughness of the selective absorption surface of the solar collector and in the wavelength range of visible radiation, while the change in the wavelength range of infrared radiation is large, the smaller the ratio Ra / Rz , the larger becomes the reflectivity.

F i g. 2 shows the interrelationships between the Rn- Wen, the absorptivity (λ), the emissivity (/?) And the efficiency (//). In Fig. 2 the value of the absorption capacity (λ) of the selective absorption surface is not influenced too much by the value Ru. The value of the emissivity (* ·) suddenly drops at a value of Ra below 0.07 μm, while this value becomes proportionally larger at a value of Rn greater than 0.07 μm. The value of the efficiency (//) suddenly becomes greater at a value of ■>R; i less than 0.07 μπι and shows a value of more than 75%.

From Fig. 2 it can be seen that this superior result ben on the selective absorption surface; ¹ :!, Which was produced by chemically oxidizing the surface of a plate made of stainless steel with a value Ra smaller than 0.07 μπι.

As shown in FIG. 3, the absorbance value (λ) of the selective absorbing surface is not influenced too much by the value Rr . The value of the emissivity (ε) suddenly drops at a value of Rz less than 0.2 μm, and the efficiency (7) shows a high value of over 75% at a value of Rz less than 0.2 μm.

It has been shown that this superior result is based on a selective absorption surface which was produced by chemically oxidizing the surface of stainless steel with a roughness Rz of less than 0.2 μm. The selective absorption surface with a roughness Ra less than 0.07 μm or Rz less than 0.2 μm results in a completely smooth surface at the wavelengths of infrared radiation and results in a small ratio of scattered reflection to hemispherical reflection (the sum of mirror reflectivity Kcn and scattered Reflectivity) and prevents an increase in reflectivity due to multiple reflections, with a value of more than 80% for the hemispherical reflection at an infrared wavelength of more than 7 μm, as well as an excellent improvement in the selective absorption properties of this solar collector surface.

In general, the surface of stainless steel is not homogeneous due to the metallographic structure, the composition, the working process, the local heat treatment and the distribution of the internal Tensions. As long as the surface of the stainless steel plate is not homogeneous, uniformity can be achieved Oxide film cannot be formed.

One of the effects of the invention is that the selective absorption properties of selective absorption surfaces for solar collectors by finishing the surface of the stainless steel substrate with a roughness of Ra less than 0.07 μm or Rz less than 0.2 μm by mechanical polishing, chemical ablation and electrolytic polishing can be improved, thereby revealing the many disadvantages resulting from an inhomogeneous surface of the metal plate. An example to illustrate the effectiveness of the selective absorption surface in a solar collector with the appropriate roughness is shown in Fig. 4. In this example, stainless steel (304 (AlSl) 683 / XIII 11 (ISO)) was treated by a liquid-return process using glass powder with a particle size of 20-100 μm to produce a clean surface with a surface roughness of /? A = 0.2 μm or Rz = 1.0 μm, and then this surface was oxidized by the acidic oxidation process.

The spectral reflectance of the stainless steel oxide film is shown in curve (a) of FIG. 4 shown. In another example, stainless steel was immersed in the aqueous solution with 10 wt .-% nitric acid and 2 wt .-% hydrofluoric acid in order to obtain a clean surface with a surface roughness of Ra = 0.14 μm and Rz = 0.6 μm generate, and then this surface was oxidized

The spectral reflectance of the stainless steel oxide film is shown in curve (b) of FIG. 4 shown. In this exemplary embodiment of the invention, the stainless steel was polished after carrying out with or without mechanical and / or chemical treatments, as was shown in the example, to form a treated surface with a surface roughness Ra less than 0.07 μm or Rz less than 0 , 2 μπι and then oxidized by means of the acidic oxidation process. The test results are shown in curve (c) of FIG. It has been found that the oxide film of the present invention (curve c) has a high reflectivity at wavelengths of infrared radiation as compared with the curves (ajund ^.

Fig.5 shows a cross-sectional view of the absorber of the solar collector, bii which the oxide film on the Substrate with mirror-like surface adhered to represent the reflection of an incident beam the interfaces between the air and the film and between the film and the substrate.

According to the illustration in FIG. 5, the beam coming from the air 1 is partially reflected on the Intermediate surface between the air 1 and the oxide film 2 and forms a reflected beam 4. The remaining incident ray passes through the oxide film 2, is diffracted and on the interface is reflected between the film 2 and the substrate 3 and forms the reflected beam 5. The interference between rays 4 and 5 depends on the thickness of the oxide film, so that the thickness of the oxide film is chosen becomes that interference occurs and the effect of preventing reflection is obtained. The curve 6 in F i g. 7 shows the spectroscopic character of the selective absorption surface in which the metal oxide des stainless steel adheres to the surface of a substrate with a mirror-like surface, namely under Neglect of the interference effect. F i g. 6 shows the spectroscopic transmittance of the metal oxide of stainless steel. This stainless steel has a metal composition corresponding to 683 / XIII 8 (ISO) or 430 (AISI).

The oxide film, which mainly contains chromium oxides and Fe3 © 4 (Fe2O3 - FeO or Fe3O4), is made by immersion in an acidic solution of sodium dichromate with 100 g / l H ₂ O and acid with 400 g / l H2O at a temperature manufactured by 106- 108 ⁰ C for 30-35 minutes

The stainless steel oxide film of suitable thickness that of the mirror-like surface substrate adherent coating layer shows a considerable selective absorption character under neglect the interference effect. Curve 7 in FIG. 7 shows the superior spectroscopic properties of the selective Absorption surface with such a thickness of the top layer that the interference effect

ϊξ

xion power decreases at the wavelength of solar radiation.

In general, a coating layer of dielectric material is provided which has an intermediate value of the Refractive index of materials with different optical properties has to reduce the Reflectivity at the interface between the materials. When these materials are complete are transparent, the absorption band appears sharply due to the interference effect. Self when these materials have properties intermediate between those of dielectric materials and those of electrical conductors lie, the interference effect occurs due to the presence of the penetrating Ray in appearance. Stainless steel metal oxides do not have perfect dielectric properties, but themselves have a considerable selective absorption property.

ίο Therefore, the oxide film can be used as a surface with selective absorption property, under consideration the interference effect. It is possible to change the reflectivity of the selective absorption surface to a minimum if the following equations are fulfilled:

/ J? = no n ₂ = n ₂ (1)

, λ 3A 5A U.

u / nnn n. Hpn Rrpfhnncrcinripv rlpc 1 IKpr ^ ticrcmaiprialc n »Hpn RrpphimercinHpv vnn I lift Zn _n _ W n- Hpfi Rrpfhnnoc-

index of the substrate, d is the thickness of the film and n \ d = - ^ is the wavelength of the primary absorption band. When stainless steel is used as the substrate (3), as shown in FIG. 5, the refractive index 2 = 3.5 to 3.9, while the refractive index / i = 2.0 to 2.5, based on the measurement by means of an ellipsometric analyzer. Although the refractive index (2.0 to 2.5) of the metal oxide film of stainless steel does not satisfy the equation (1), the refractive index becomes at the wavelength for primary absorption

not zero if the optical thickness of the film is -, which makes this film the superior selective

Has absorption properties as shown in curves 8 and 9 of FIG. 8 are shown.
In Fig. 8, curves 8 and 9 show the spectral reflectivity when the wavelength of the primary absorption is about 0.5 μm, where the spectral emissivity has a maximum value, or at 0.8 μm. Although it can be deduced from curve 8 that the best selective absorption properties appear when the wavelength (11) for primary absorption is 0.5 μm, a maximum absorption capacity of the selective absorption surface is shown at a wavelength (11) for primary absorption of about 0.8 μπι taking into account the spectral distribution of solar radiation.

In curves 8 and 9 of FIG. 8 the emissivity (ε) has the same value of about 0.12 at long wavelengths. The values for minimum reflectance at primary absorption (11) and the primary peak

of the spectral reflection magnitude (12) with an optical thickness of the film of -r- are shown in curves S and 9

slightly different because of the optical scattering constants in the metal oxide coating layer and the Base plate are slightly different at certain wavelengths. A better selectivity is obtained with

a wavelength for primary absorption of 0.8 μm instead of 03 μm.
The minimum reflectivity is namely at wavelength (11) for primary absorption at curve 9

smaller than that of curve 8, while at wavelength (12) for the primary peak or the primary maximum, the maximum reflectivity of curve 9 is smaller than that of curve 8.
The line 10 in FIG. 8 shows an ideal spectral curve for the reflectivity of the selective absorption

tion surface at a working temperature of 100 ° C.
Regarding the refractive index of the metal oxide of stainless steel, it should be specifically stated that it is

builds up porous in a certain direction on the surface of the stainless steel.

The greater the porosity, the closer the refractive index to that of air, while the value of the refractive index becomes more and more that of the metal oxide as the porosity becomes smaller

approximates
The refractive index of magnetic stone (Fe ₃ O ₄ ) is 2.4 to 23 in the visible wavelength range, while

the refractive index of the metal oxide of stainless steel is 2.0 to 23 based on the measurement by the ellipsometric analyzer.
From these circumstances, it is concluded that the porosity of the metal oxide of stainless steel corresponds to 0 to 20% of the volume basis of the metal oxide layer

Transmission microscope confirmed
The appropriate thickness (de) of the coating layer of stainless steel metal oxide with anti-reflective effect

is between 50 and 125 nm if the optical thickness (n \ d) of the layer satisfies the relationship 125 nm; £ /? i </ S 250 nm and the refractive index (m) satisfies the relationship 2.0 <Πι <23 Self if the thickness of the layer is outside of this range, the selective absorption property of the surface becomes considerable

Extent in appearance so that it must be concluded that the appropriate thickness of the coating layer 50 to Is 200 nm

example 1

Both types of stainless steel, ferritic and austenitic, with the metal compositions after 683 / X1II J (ISO), 430 (AISI) and 638 / XIII 11 (ISO), 304 (AISI) are oxidized with the formation of a thin oxide layer on the stainless steel surface.

Oxidation condition

Sodium dichromate 100 g / l H2O

(Na ₂ Cr ₂ O?)

Sulfuric acid 400 g / l H ₂ O

(H ₂ SO ₄ )
Immersing for 30-35 minutes at a temperature of 106-108 ⁰ C.

F i g. 9 * shows the spectral reflectivity of the selective absorption surface, which is found in stainless Steel results in comparison to that of an ordinary selective absorption surface of a solar collector. In Fig. Fig. 9 shows curve 1 the reflectivity of the selective absorption surface of an oxide film made of ferritic stainless steel, curve 2 shows the reflectivity cir.es oxide film formed from austenitic stainless steel, curve 3 shows the reflectivity a copper oxide coating surface, which was obtained by the aican oxidation of a copper plate, Curve 4 shows the reflectivity of nickel sulfide on embossed nickel, both of which are plated on the steel and curve 5 shows the ideal spectral reflectance of the selective absorption surface of the solar collector at an operating temperature of 100 ° C. The selective ones coated with copper oxide Absorbent surface shows an excessively large reflectivity at long wavelengths, the larger are as 4 μΐη. This corresponds to a reflectivity of 3 to 5% more than with the selective absorption surface of the stainless steel oxide film as shown in curves 1 and 2 at wavelengths the solar radiation from 03 to 2.5 μπι, taking into account diffuse reflection, while with the selective Absorbent surface made of ferritic stainless steel as shown in curve 1, the reflectivity is excessively small at wavelengths that are smaller than 2.0 μπι and of considerable Size at wavelengths that are greater than 2.0 μm. The selective one made of ferritic stainless steel The absorption surface is to the same extent better than the selective surface obtained from copper oxide was produced. As in curve 2 of FIG. 9, the reflectivity is the selective Absorbent surface made of austenitic stainless steel, slightly inferior to that of made of ferritic stainless steel at wavelengths of radiation des black body and at the working temperature of the solar panel. Although the selective absorption surface, made of austenitic stainless steel, somewhat inferior in spectroscopic properties Properties it deserves as a selective absorption surface of a commercial solar collector « to be used., with the better corrosion resistance and welding properties of the austenitic stainless steel are taken into account.

As already mentioned, the selective absorption surface according to the invention, made of ferritic or austenitic stainless steel, advantageously as a selective absorption surface be used for solar panels because these surfaces have good spectroscopic properties, superior Have corrosion resistance and heat resistance, which are the specific merits of stainless Are steel. The metal oxide coating surfaces consisting of the ferritic and austenitic stainless Steel made by the chemical oxidation process are uniform and stable without the Corrosion resistance internal to the stainless steel is impaired.

The heat resistance of the selective absorption surface made according to the invention possesses same size as stainless steel even if a substrate other than stainless steel is used Stole.

F i g. 10 shows a cross-sectional view of a suitable solar collector using the selective Absorbent surface made of ferritic or austenitic stainless steel. In the Illustration according to FIG. 10, a ray of sunlight represented by an arrow is converted into heat by it runs through transparent cover material (one to three panes of glass la or a resin plate), which is provided as protection against convection heat losses, then passes through a layer of air 2a and is absorbed in the oxide film 3a of ferritic or austenitic stainless steel. The absorbed Heat is transferred to a heating medium 8a, for example air, water, etc., through the substrate 4a or other conventional materials 5a, which can be bonded to the substrate 4a by plating processes or Diffusion bonding processes are connected. In FIG. 10 there is also an air layer 6a as a heat insulator provided, as well as insulating material 7a, which comprises glass wool, asbestos or a honeycomb structure It has been observed that the selective absorption surface obtained by the chemical oxidation process of ferritic or austenitic stainless steel using this surface in one Solar collector shows an excellent heat collection effect

Example 2

It is true that the selective absorption surface, made of ferritic rustproof. Steel with a composition manufactured according to Example 1, has superior spectroscopic properties and is inexpensive to them however, it has minor disadvantages in terms of weldability, formability and corrosion resistance.

In order to alleviate these disadvantages, stainless steel with a low carbon content was treated under the chemical oxidation conditions as in Example 1. In Fig. Figure 11 shows Curve 1 of the correlation between the wavelength and the reflectivity of the selective absorbing surface made from this low carbon stainless steel containing Ti, Mo and additional metals. Curve 2 shows the relationship of selective absorption surface made of ferritic stainless steel mil low carbon, which does not contain Ti, Mo and additional metals, and curve 5 shows its ideal curve.

From Fig. 11 it can be seen that the selective absorption surface, which is made of stainless steel with additional Metals, has better spectroscopic properties, as well as those of conventional

ίο chem ferritic stainless steel that does not contain additional metals

Example 3.

The following experiment was carried out to prove that the selective absorption surface of the Solar collector with anti-reflection effect due to the interference effect and superior spectral reflectivity is produced by selecting the oxidation conditions for forming the oxide film with one suitable thickness of 50 to 200 nm on the surface of the stainless steel.

The stainless steel plate corresponding to the metal composition 683 / XIII 8 (ISO), 430 (AISI) was made chemically oxidized by immersion in an aqueous solution having the compositions (A) and (B) at each changed immersion time to form the oxide film on the surface of the stainless steel.

The correlation between the thickness (1 A = 10- ' ⁰ m) of the top layer and the immersion time (treatment time) (Fig. 12) as well as the correlation between the absorption capacity («) in the solar spectrum, the emissivity (ε) integrated via the radiation of the black body at an operating temperature of 50-100 ^C C of the solar collector and the thickness of the coating layer (Fig. 13) in λ were each examined

The oxidation conditions for the surface of the stainless steel are as follows:

(,:.) Sodium dichromate (Na ₂ Cr ₂ O ₇ ) 100 g / l H ₂ O

Sulfuric acid (H ₂ SO ₄ ) 400 g / l H ₂ O Treatment temperature from 106 to 108 ⁰ C

(B) Chromium trioxide (CrO ₃ ) 250 g / l H ₂ O

Sulfuric acid (H ₂ SO ₄ ) 500 g / IH ₂ O Treatment temperature 70 ° C

Fig. 12 shows the correlation between the thickness (A) of the coating layer and the treatment line (Minutes) obtained from measuring the change in the point (wavelength) of primary absorption at which the

optical thickness ι d ~ r < where n \ represents the main value 2.2 of the already mentioned value from 2.0 to 23

The curve 13 shown in Fig. 12 results from the treatment under the conditions (A) while

Curve 14 results from the treatment according to the conditions (B). Curve 13 shows the interrelationships between the absorption capacity («) and the emissivity (e) at the operating temperature of 100 ° C of the solar collector and the thickness (A) of the top layer. 13, curve 15 shows the relationship between the absorptivity (λ) and the thickness of the coating layer, while curve 16 shows the correlation between the emissivity (ε) and the thickness of the coating layer

It can be seen from curves 15 and 16 that the value (λ) is greater than 0.80 with a thickness of the coating layer

of 500-2000 Å and that the value (λ) is equal to 034 with a thickness of the top coat layer of about 900 Å, ifr the wavelength for primary absorption due to the interference effect is 0.8 μπι, and that the value (λ) is slow

decreases if the thickness of the coating layer is greater than 1000 A.

It can also be seen from Fig. 13 that the emissivity (ε) slowly decreases until the thickness dei

so that the coating layer reaches about 1500 Å and that the value (ε) is greater than 0.2 when the thickness of the coating layer exceeds 2000 Å and finally that a selective absorption surface having the best selective absorption properties is produced when the thickness of the oxide film is stainless steel 500-2000 A reached Taking this fact into account, it has been proven that a selective absorption surface with best

Properties is obtained regardless of the oxidation process for the coating surface when the The thickness of the coating layer reaches 500-2000 Å.

This characteristic property of the selective absorption surface according to the invention can be as show as follows:

1. The selective absorption surface with the best properties in terms of durability and heat resistance Speed, corrosion resistance and adhesive effect are obtained by the method according to the invention when stainless steel is used as the substrate.

2. With the conventional "elective absorption surface with copper oxide", the more spectroscopic Properties at a temperature of 180-200 ° C (24 hours) admittedly not noticeably deteriorated

when the color of the surface is changed, at a temperature greater than 210 ⁰ C (24 hours) they will however, deteriorates because the surface structure of the metal oxide is destroyed.

3. In the conventional selective copper absorption surface due to the changes de; Surface condition and the spectroscopic character of this surface upon exposure

Air observes that the surface structure is significantly degraded when it Exposure to rainwater, thereby causing the metal oxide to peel off, and that the Emissivity in the infrared range is increased however moderately, whereby at the same time the reflectivity is deteriorated and the spectroscopic properties of the selective absorption surface get lost. In the case of the selective absorption surface according to the invention, the above mentioned 5 adverse effects were not observed

4. A selective absorption surface with the best spectroscopic properties and low manufacturing costs is obtained by the method according to the invention when ferritic stainless steel is used as Substrate material is used, but such a surface shows in comparison to the properties of austenitic stainless steel poorer weldability, formability and corrosion resistance 10 To overcome these drawbacks, the surface is made of a metal composition low carbon ferritic stainless steel or a small amount of special additional Metals in ferritic stainless steel or also from ferritic stainless steel with low Carbon content and a small amount of specific additional metals, thereby preventing the occurrence of Corrosion due to stress, which always occurs in austenitic stainless steel, prevents 15 and the same mechanical strength is achieved as with austenitic stainless steel.

In addition 9 sheets of drawings

Claims (3)

Patent claims: 1. Selective absorption surface of an absorber for a solar collector with a 50 to 200 nm thick surface coating of a metal oxide on a substrate made of steel with a mirror-like surface Surface, characterized in that the metal oxide layer consists of that on the surface oxidized steel of the composition in% by weight 0.001-0.15 C, 0.005-3.00Si,
0.005-10.00Mn, 11.00 -30.00 Cr,
optionally additionally 0.005-22.00 Ni, optionally additionally at least one of the following elements
0.001 - 5.00 N, Cu, Al, V, Y, Ti, Nb, Ta, U, Th, W, Zr and / or Hf optionally in addition 0.75 - 5.00 Mon and
Remainder Fe consists of use. the mirror-like surface has an average roughness Ra of less than 0.07 μm or an average roughness in the professional! Rz before. less than 0.2 μίϊ !, according to asr. si? of the! SO recommendation R-468 given definitions for roughness values 2. Selective absorption surface according to claim 1, characterized in that the metal oxide layer that of (a) stainless steel, (683 / XlU 11 (ISO), 304 (AlSl)) from 0.005-0.08% by weight C, 0.005-1.00% by weight Si, 0.005-2.00% by weight Mn, 8.00-10.50% by weight Ni, 18.00-20.00% by weight C /, remainder Fe, or of (b) stainless steel, (683 / XlH 20 (ISO), 316 (AISl)) of 0.005-0.08% by weight C, 0.00-1.00% by weight Si, 0.005-2.00 wt% Mn, 10.00-14.00 wt% Ni, 16.00-18.00 wt% Cr, 2.00-3.00 wt% Mo, Remainder Fe, or from (c) stainless steel, (683 / XI1I 8 (ISO), 430 (AISI)) of 0.05-0.12 wt% C, 0.005-0.75 wt% Si. 0.005-1.00 wt% Mn, 0.005-0.60 wt% Ni, 16.00-18.00 wt% Cr, remainder Fe. or from (d) stainless steel, <434 (AiSI)) of 0.05-0.12 wt% C, 0.05-1.00 wt% Si, 0.005-1.00 wt% Mn ₁ 0.005-0.60 wt% Ni, 16.00-18.00 wt% Cr, 0.75-1.25 wt% Mo, remainder Fe, or of (e) stainless steel, composed of 0.0υ6-0.03 wt% C, 0.005-0.75 wt% Si, 0.005-1.00 wt% Mn ₁ 16.00-1 R ₁ OO Wt .-% Cr, 0.1-1.0 wt .-% Ti, remainder Fe, or of (f) stainless steel, comprised of 0.005-0.03 wt% C, 0.005-0.75 wt% Si, 0.005-1.00 wt% Ma. 16.00-18.00 wt% Cr, 0.1-1.0 wt% Ti, 0.75-1.25 wt% Mo, remainder Fe is 3. Process for the production of the selective absorption surface of an absorber for uinen solar collector according to claim 1, characterized in that the stainless steel with mirror-like Surface of the composition mentioned in claim 1 of an acidic or alkaline oxidation treatment subjecting the steel to a bath of the following composition for the following application is exposed: (a) acidic oxidation process Sodium dichromate or potassium dichromate 100-400 g / l H2O or chromium trioxide 40-700 g / l H ₂ O Sulfuric acid 150-800 g / l H ₂ O preferably 400-800 g / l H ₂ O Temperature 50 ⁰ C to the boiling point of the mixture, preferably 70-120 ⁰ C. Immersion time 3–40 minutes (b) alkaline oxidation process |! 60 sodium hydroxide or potassium hydroxide 130-200 g / l H ₂ O Trisodium or tripotassium phosphate 30-40 g / l H ₂ O sodium or potassium nitrite or Sodium or potassium nitrate 20-30 g / l H ₂ O Iron hydroxide Fe (OH) ₃ 1-3 g / l H ₂ O Lead peroxide (PbO ₂ ) 20-30 g / l H ₂ O Temperature 100-150 ⁰ C Immersion time 3–50 minutes