DK152624B - SELECTIVE ABSORPTION SURFACE FOR A SUN AND MAKING PROCEDURE - Google Patents

Description

! DK 152624B

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a selective absorption surface for a solar panel, comprising a metal oxide surface coating having a thickness of 500-2000 Å applied to a mirror-like surface.

For capturing solar radiation, it is known to make use of the greenhouse effect, which consists in applying a base material which is opaque to infrared wavelengths and translucent to visible wavelengths, which is pre-coated with a substance having properties approaching say a black body, e.g. a black dye, which coating material exhibits a greenhouse property and reduces the convection heat loss resulting from conventional heat conduction. This well-known method works satisfactorily when the solar working temperature is lower than about 50 ° C, but when the temperature exceeds about 50 ° C, the method has the disadvantage that the efficiency of the heat collection in the solar collector is significantly reduced. To remove this disadvantage, it is known to employ a conventional selective absorption surface with such spectroscopic properties that, at an operating temperature of 100 ° C, corresponding to the operating temperature of a solar cell, for wavelengths of 0.3-2, 5 µm in the solar radiation exhibits the same energy absorption as a black body, while for wavelengths of 3-50 µm. exhibits a low emission. It is difficult to obtain a selective absorption surface for a solar panel with the above spectroscopic properties by natural means.

Since a sufficiently polished zinc plate and a naturally oxidized copper plate exhibit only the selective absorption properties of the solar radiation, even when such plates are placed on a solar panel, such a measure is insufficient to remove the disadvantages of the aforementioned known solar panel, where use is made of the greenhouse effect of black dye, which is why attempts have been made to artificially produce a selective absorption surface for a solar panel. For this, coating of the surface of a copper plate with copper oxide is used in a conical treatment, plating of the surface of galvanized iron plate with nickel sulphide and double surface coating of a mirror-like surface with a film which is opaque for visible wavelengths and translucent for infrared length, and then with a transparent film to prevent reflection of solar radiation, this application having been done by vacuum evaporation, atomization or arc discharge, e.g. an aluminum substrate which is coated with metallic silicon and then with SiC> 2 to prevent reflection of solar radiation. Vacuum 2

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the evaporation has been considered as the most reliable method of the aforementioned to produce a selective absorption surface for a solar panel with the effect of preventing reflection due to the interference effect of the applied films, the thickness of each film having to be controlled and the constituents of each film; simple substances or compounds must be freely selectable so that each film can be applied with a suitable index of refraction. Special vacuum evaporation methods such as atomization and arc discharge methods have been developed because films made by conventional vacuum evaporation do not have sufficient adhesion to each other and to the substrate. However, since evaporation methods themselves exhibit drawbacks in terms of production efficiency and cost, attempts have been made to produce a selective absorption surface as an anti-refraction surface by methods other than vacuum evaporation, namely dry and wet treatment methods and plating methods.

By plating methods, a selective absorption surface with selective absorption and prevention of the reflection-induced interference effect, for example, can be prepared by filming nickel sulfide or nickel-zinc sulfide and zinc sulfide onto an aluminum or galvanized iron plate. In the chemical treatment method, a selective absorption surface having the above effects is obtained by forming metal oxide on a steel plate or stainless steel plate by the same method as the above method of nickel sulfide.

Although films applied by chemical dry and wet treatment methods as well as films formed by vacuum evaporation and special vacuum evaporation methods exhibit the property of having selective absorption, by the chemical methods it is necessary to select a proper thickness range for the applied film to obtain a selective absorption surface having the same absorption properties as in films formed by vacuum evaporation and special vacuum evaporation methods.

FR patent specification No. 1,142,777 discloses a solar panel comprising an oxide layer with spectroscopic properties on a stainless steel substrate, the oxide layer being obtained by heat treatment of the high temperature stainless steel. Hereby unsatisfactory spectroscopic properties are obtained.

From DE Publication No. 1,467,734 a solar panel is known in which the spectroscopic layer consists of an alumina layer produced by thermal oxidation.

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It is an object of the invention to provide a selective absorption surface for a solar panel in which a coating film consisting of a predetermined metal oxide composition is adhered to a surface of mirror-like surface and predetermined roughness of a certain thickness, the metal oxide of the coating film being selected from the metal compositions having effect to prevent reflection of solar radiation by the film's interference effect.

In order to achieve this, a selective absorption surface of the type initially specified according to the invention is characterized in that the metal oxide coating is formed of steel from a metal composition consisting of a) 0.001-0.15% by weight C, b) 0.005-3.00 "Si, c ) 0.005-10.00 "Mn, d) 11.00-30.00" Cr, e) 0.005-22.00 "Ni, and / or 0.001-5.00% by weight of at least one additive selected from the group consisting of N , Cu, Al, V, Y,

Ti, Nb, Ta, U, Th, W, Zr and Hf, f) as a voluntary component 0.75-5.00% by weight Mo, and g) as residue Fe, which metal oxide has a high energy absorption factor at wavelengths of 0, 3-2.5 µm and a low energy emission factor at wavelengths of 3-50 µm.

This determination of the metal composition and the thickness of the absorption layer results in a solar panel with substantially improved spectroscopic properties, namely an absorbance of 90-94% and an emittance of about 12%.

The selective absorption surface of a solar panel is prepared according to the invention by chemically oxidizing the stainless steel mirror-like surface with the predetermined metal composition by means of an acidic or alkaline bath under predetermined oxidation conditions in terms of time and temperature.

According to the invention, a first method of preparing the selective absorption surface is characterized in that a metal composition such as stainless steel consisting of

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4 a) 0.001-0.15 weight percent C, b) 0.005-3.00 "Si, c) 0.005-10.00" Mn, d) 11.00-30.00 "Cr, e) 0.005-22.00 "Ni, and / or 0.001-5.00% by weight of at least one additive selected from the group consisting of N, Cu, Al, V, Y,

Ti, Nb, Ta, U, Th, W, Zr and Hf, f) as a voluntary constituent, 0.75-5.00% by weight Mo, and g) as residual part Fe, to form oxide films with a thickness of 500-2000 To be chemically oxidized in an acid. bath consisting of 150-800 g / l sulfuric acid and 100-400 g / l sodium or potassium bichromate or 40-700 g / l chromium trioxide at a temperature between 50 ° and the boiling point and for a dipping time of 30-40 minutes.

Alternatively, the selective absorption surface can be prepared by a process which according to the invention is characterized in that a metal composition such as for stainless steel consisting of a) 0.001-0.15% by weight C, b) 0.005-3.00 "Si, c) 0.005- 10.00 "Mn, d) 11.00-30.00" Cr, e) 0.005-22.00 "Ni, and / or 0.001-5.00% by weight of at least one additive selected from the group consisting of N, Cu, Al, V, Y,

Ti, Nb, Ta, U, Th, W, Zr and Hf, f) as a voluntary constituent, 0.75-5.00% by weight Mo, and g) as residual part Fe to form oxide films with a thickness of 500-2000 Å are chemically oxidized in an alkaline bath consisting of 130-200 g / l sodium or potassium hydroxide, 30-40 g / l trisodium or potassium phosphate, 20-30 g / l sodium or potassium nitrate or sodium or potassium nitrite, 1- 3 g / l ferric hydroxide and 20-30 g / l lead peroxide at a temperature of 100-150 ° C and for a immersion time of 3-50 minutes.

The major advantage of the chemical oxidation lies in the easy control of coating layer thickness and composition as well as obtaining a uniform and dense oxide layer without adverse effect on the substrate metal due to high temperature heat treatment.

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The invention is explained in the following with reference to the drawing, in which fig. Figure 1 shows the relationship between the wavelengths of solar radiation in μm and the reflectance of a selective absorption surface resulting from the roughness of the substrate surface; 2 and 3, the relationship between the absorbance (oc) of the selective absorption surface, the emittance (£) and the efficiency (*?) And the roughness of Ra and Rz, respectively, in both cases in μτα; 4 shows the relationship between the wavelengths and the reflectance of the selective absorption surface at different roughnesses of the substrate; FIG. 5 shows an example of the cross section of a selective absorption surface for a stainless steel solar panel as a backing; FIG. 6 shows the relationship between the wavelengths (μτα) and the transmittance of the oxide film; FIG. 7 shows the relationship between the wavelengths (μτα) and the reflectance of metal oxide on the stainless steel substrate, disregarding the interference effect; 8 shows the relationship between the wavelengths (μτα) and the reflectance of the selective absorption surface in the solar panel; FIG. 9 shows the relationship between the wavelengths (μτα) and the reflectance of metal oxides formed by ferritic and austenitic stainless steels, respectively; 10 is a cross-sectional view of a solar panel with a selective absorption surface; FIG. 11 the relationship between the wavelengths (μια) and the reflectance 6

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of low carbon ferritic stainless steel metal oxide, fig. 12 shows the relationship between the thickness of the cover layer (Å) and the chemical treatment time (minutes) to produce the metal oxide layer in the selective absorption surface of a solar panel; and FIG. 13 shows the relationship between the thickness of the slab layer (Å) and the absorbance (oc) and the emitter (£) of the selective absorption surface in a solar panel.

metal Composition

One of the metal compositions used in the invention consists of 0.001-0.15 wt% C, 0.005-3.00 wt% Si, 0.005-10.00 wt% Mn, 11.00-30.00 wt% Cr, 0.005-22.00 wt% Ni , a voluntary constituent of 0.75-5.00% by weight Mo and for the remaining Fe. This metal composition corresponds to the composition of conventional stainless steel e.g. of type 683 / XIII 11 (ISO), 304 (AISI) having the composition 0.005-0.08 wt% C, 0.005-1, wt% Si, 0.005-2.00 wt% Mn, 8.00-10.50 wt% Nine, 18.00-20.00 wt% Cr and for the remaining Fe, of type 683 / XIII 20 (ISO), 316 (AISI) having the composition 0.005-0.08 wt% C, 0.005-1.00 wt% 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 and for the remaining part Fe, of type 683 / XIII 8 (ISO), 430 (AISI) having the composition 0.005-0.12 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 and for the remaining part Fe, of type 434 (AISI) having the composition 0.005-0.12 wt.% C, 0.005-1.00 wt.% Si, 0.005-1.00 wt.% Mn, 0.005-0, 60 wt% Ni, 16.00-18.00 wt% Cr, 0.75-1.25 wt% Mo and for the remaining Fe, and other stainless steel forms with compositions sv eagle to the above. When using stainless steel as the metal composition, martensitic stainless steel is not suitable, while ferritic and austenitic stainless steel are suitable because of their weldability.

Other metal compositions which may be used in the invention are low carbon stainless steel and containing an additive to improve the anti-corrosion, moldability and weldability, e.g. with the composition 0.001-0.15 wt% C, 0.005-3.00 wt% Si, 0.005-10.00 wt% Mn, 11.00-30.00 wt% Cr and 0, ool-5.00 wt% of at least one substance selected from the group consisting of N, Cu, Al, V, Y, Ti, Nb, Ta, U, Th, W, Zr and

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Hf, a voluntary constituent of 0.75-5 / 00% by weight of Mo and for the remaining part Fe, the atomic ratio of metal to carbon plus additive being higher than 5.0, and for stainless steel containing Nb, Ta or Ti as additive exceeding 8.0. Metal compositions of this kind are similar to conventional stainless steel, e.g. may have the composition 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 and the remaining part Fe, or the composition 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, 0, 75-1.25% by weight Mo and for the remaining Fe.

further, the invention may make use of other low-carbon stainless steel metal compositions comprising another metal for improving the anti-corrosion, formability, and weldability, such as e.g. the composition 0.001-0.15 wt% C, 0.005-3.00 wt% Si, 0.005-10.00 wt% Mn, 0.005-22.00 wt% Ni, 11.00-30.00 wt% Cr and 0.001-5.00 wt% at least one substance selected from the group consisting of N, Cu, Al, V, Y, Ti, Nb, Ta, U, Th, W, Zr and Hf, a voluntary component of 0.75-5.00% by weight Mo and for the residue Fe, the metal to carbon plus additive atomic ratio being higher than 5.0, and for stainless steel containing Nb, Ta or Ti as an additive exceeding 8.0.

Formation of metal oxide

The formation of metal oxide from the metal composition can take place in the following ways: (1) Methods for producing metal oxide by chemical wet and dry treatments, (2) a chemical treatment method for producing stainless steel metal oxide, after stainless steel having the predetermined metal composition (3) methods of producing stainless steel metal oxide by vacuum evaporation, atomization and arc discharge, after the stainless steel having the predetermined metal composition is attached to the substrate; the substrate having a mirror-like surface of a material other than the stainless steel concerned, and (4) methods of producing stainless steel metal oxide by simultaneously adhering and oxidizing the stainless steel with the preceding 8.

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certain metal composition on the substrate with the mirror-like surface and of material other than the stainless steel concerned.

Among the above processes, the most preferred methods are the following acidic and alkaline oxidation methods: (a) Acid oxidation method.

The oxidation state is determined as follows:

Sodium 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 50 - boiling point

preferably 70-120 ° C

immersion time 3-40 minutes (b) Alkaline oxidation method.

The oxidation state is determined as follows:

Sodium or potassium hydroxide 130 - 200 g / l trisodium or tricalium phosphate (Na3 PO4) (K3E04) 30 - 40 g / l sodium or potassium nitrite or sodium or potassium nitrate (NaNo2) (KN02) (NaNo3) (KN03) 20-30 g / l ferric hydroxide Fe (OH) 3 1-3 g / l lead peroxide (PbO2) 20 - 30 g / l

temperature 100 - 150 ° C

immersion time 3-50 minutes.

Prior to the chemical treatment, it is preferable to pretreat the substrate surface. The preferred pretreatment methods consist in immersing the substrate either in an aqueous solution of one part by weight of hydrochloric acid and one part by weight of water for one hour or in an aqueous solution of 30% by weight perchloric acid and 1% by weight potassium chloride for 2-3 minutes.

The metal oxide obtained from stainless steel consists of ferritic stainless steel of oxide of the chemical formula Fe0 (FeCr) 203 and of austenitic stainless steel of oxide of the chemical formula (Fe, Ni) O (FeCr) 2C> 3, both of which have metal oxides. spinel structure with lattice defects.

Surface roughness.

If the surface condition of the substrate with the mirror-like surface meets the following conditions, there are no restrictions as to the nature of the substrate material as far as the

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the adhesion of metal oxide of the predetermined composition to the substrate.

(1) To achieve the characteristic of the selective absorption surface that it has high reflectance for infrared wavelengths, the metal oxide layer must be translucent to infrared radiation so that infrared radiation is reflected onto the substrate and passes through the attached metal oxide layer.

(2) The adhesion of the metal oxide layer to the substrate depends on the roughness of the substrate surface. By attaching the cover layer to a smooth substrate surface, a dense cover layer is obtained.

(3) When the surface of the surface exhibits mirror-like properties of wavelengths in the visible and near the infrared region, the interference effect does not decrease, whereby the effect of reflection prevention is clearly evident. If the substrate surface is roughened, the absorption of solar radiation increases. Therefore, there is free choice in obtaining either the reflection prevention effect or the absorption increase effect.

(4) It is preferred to smooth the mirror-like surface of the substrate to suppress radiation in the infrared region. At too high a roughness of the substrate, the spectroscopic nature of the selective coating will be suppressed to such an extent that the wavelengths absorbed in the selective absorption surface reach the infrared wavelength range 3-8 µm, such that a substrate having such a roughness is not suitable. Various types of metal sheet, stainless steel sheet and plastic sheet can be used as substrate material.

In view of the above, it has been found that the surface state of the substrate to which stainless steel metal oxides are to be attached is the most important factor in obtaining a spectroscopic nature of the selective absorption surface with high absorption of the wavelengths of solar radiation, ie. low reflectance, and small wavelength emittance in the infrared region, i.e. great reflectance.

It is known that in order to improve the efficiency of a selective absorption surface, it is preferable that the surface roughness is large relative to the wavelengths of solar radiation and small relative to the wavelengths of the infrared region. But this is not confirmed by experimental data.

In general, the surface of a roughly finished substrate will tend to cause an increase in absorption due to repeated reflection, but suppress the reflection-inhibiting effect of the interference effect, both of

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practice depends on limitations in the roughness of the selective absorption surface.

Another object of the invention therefore relates to the determination of the roughness of the substrate surface upon which stainless steel metal oxide is to be formed. To determine the roughness of the substrate, the following experiments have been carried out.

Stainless steel with the metal composition 0.005-0.12 wt% C, 0.005-0.75 wt% Si, 0.005-1.00 wt% Mn, 16.00-18.00 wt% Cr and for the remaining part a small amount of additive metal and Fe, 430 (AISI), 683 / XIII 8 (ISO), was oxidized in an aqueous acid bath containing 100 g / l sodium bichromate 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 steel.

The relationship between the absorption (° c) integrated over the solar radiation spectrum, the emitter (£) integrated over the radiation spectrum and the blackbody efficiency () were investigated.

The efficiency (C *)) is determined by the following expression

where S represents Boltzmann's constant 4.88 x 10-oKcal / ir R °, while 7 represents the corrected operating temperature assumed here to be 373 ° K, and J represents the solar radiation power, 800 K

2 cal / m.h). The surface roughness is expressed by arithmetic mean deviation, Ra, and peak point height, Rz, in accordance with ISO re-recommendation R 468.

The test results are shown in FIG. 1 and 2. In FIG. 1, curve 1 indicates the relationship between reflectance and wavelength at a surface roughness in Ra measure of 0.36 µm or in Rz measure of 3.5 µm, curve 2 applies to the values 0.19 µm in Ra measure or 0.6 µm in Rz measure, curve 3 applies to the values 0.12 µm in Ra measure or 0.5 µm in Rz measure, curve 4 applies to the values 0.08 µm in Ra measure or 0.3 µm in Rz measure and curve 5 apply to the values 0.04 µm in Ra measure or 0.1 jbm in Rz measure.

It has been observed that in the wavelength range of visible radiation, the variation of the reflectance is small compared to the variation in roughness of the selective absorption surface in the solar panel, while the reflectance variation is large for wavelengths in the infrared region. The smaller the ratio Ra / Rz, the greater the reflectance.

FIG. Figure 2 shows the relationship between the Ra value, the absorbance (cC), the emittance (£) and the efficiency (^). This figure shows the value

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of the absorbance (oc) of the selective absorption surface not to be particularly affected by the Ra value. The value of the emission (g) suddenly decreases for values of Ra less than 0.07 and increases proportionally for values of Ra higher than 0.07. For Ra values lower than 0.07, the value of efficiency (*)) increases suddenly and exceeds 75%.

In FIG. 2 shows that the best result is obtained by preparing the selective absorption surface by chemically oxidizing the surface of stainless steel plate with a Ra value lower than 0.07.

In FIG. 3, it is seen that the value of the absorbance (ot) of the selective absorption surface is not particularly affected by the R 2 value. The value of the emission (ε) suddenly decreases at an Rz value lower than 0.2, and the efficiency ('J) exhibits high values above 75% at Rz values lower than 0.2.

From this it can be seen that the best result is obtained by preparing the selective absorption surface by chemically oxidizing the stainless steel surface with a roughness in Rz target lower than 0.2. Thus, a selective absorption surface having a roughness in Ra target lower than 0.07 or in Rz target lower than 0.2 will result in a completely smooth surface for wavelengths in the infrared region and a poor ratio of diffuse reflectance to hemispheric reflectance, ie. the sum of the mirror reflectance and the diffuse reflectance so as to prevent the reflection reflectance suppression, and for infrared wavelengths above 7 µm, a value of more than 80% for the hemispherical reflex ion is obtained and thus a significant improvement in the selective absorption properties of the surface of the collector. In order to obtain a uniform and stable oxide film by conducting oxidation treatment on the stainless steel surface, it is necessary to finalize the metal plate to obtain a uniform surface.

In general, the stainless steel surface is inhomogeneous due to the metallographic structure, composition, machining operation, local heat treatment and the distribution of internal stresses. To the extent that the surface of a stainless steel plate is inhomogeneous, no uniform oxide film can be produced.

One of the objects of the present invention is to improve the selective absorption properties of the absorption surface in a solar panel by finishing the surface of a stainless steel substrate to a roughness in Ra measure less than 0.07 or in Rz measure less than 0.2 by mechanical polishing, chemical abrasion and electrolytic polishing to remove the many disadvantages arising from the inhomogeneities of the metal plate surface. In FIG. 4 is an example of

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the degree of selectivity of the selective absorption surface in a solar panel with appropriate roughness. In this example, stainless steel of type 304 (AISI), 683 / XIII 11 (ISO) was treated by a liquid grinding method using glass powders having a particle size of 20-100 µm to obtain a clean surface having a surface roughness in Ra scale of 0.2 μια or in Rz target of 1.0 μm and subsequent oxidation of this surface by an acid oxidation method.

The spectral reflectance of the thus obtained oxide film on the stainless steel is shown in FIG. 4 is shown in curve a. In another example, the stainless steel was immersed in an aqueous solution containing 10 wt.% Nitric acid and 2 wt.% Hydrofluoric acid to give a clean surface having a surface roughness in Ra scale of 0.14 μια or in Rz measure. of 0.6 μτα and subsequent oxidation of this surface by the same method.

The spectral reflectance of the resulting oxide film on the stainless steel is shown in FIG. 4 shown by curve b. In an example in accordance with the present invention, the stainless steel was polished with or without prior mechanical and / or chemical treatment as set forth in the above examples to obtain a treated surface having a roughness in Ra target less than 0.07 μνα or in Rz measurements less than 0.2 μm and then oxidized by the same acid oxidation method. The result of this is in FIG. 4 is shown by curve c. It will be seen that the oxide film represented by curve c obtained by the present invention exhibits a higher reflectance at the wavelengths of the infrared region compared to the oxide film represented by curves a and b.

Oxide Film Thickness In the following, the problems are determined by determining the thickness of the oxide film by the formation of metal oxides on the surface of a substrate by the following methods.

(1) Acidic or alkaline oxidation of the stainless steel surface with the predetermined metal composition.

(2) Particularly effective vacuum evaporation, e.g. atomization and arc discharge, to improve the adhesion between oxide film and substrate.

(3) Use of a binder with relative transparency for infrared radiation, e.g. polyethylene or silicone resin, etc., for adhering metal oxide powder of the predetermined metal composition to the substrate.

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(4) Performing stainless steel oxidation treatments tightly adhered to a non-stainless steel substrate, e.g. oxidation of chromalized metal or rolled metal, etc.

The spectroscopic nature of the selective absorption surface in a solar panel and the anti-reflection effect of the oxide film are explained as follows: The spectroscopic nature of the selective absorption surface should exhibit less reflection at wavelengths 0.3-2.5 fm for solar radiation and large reflection in the infrared wavelength range 3- 50 extraction.

In FIG. 5 is a cross-sectional view of the absorption body of a solar panel in which an oxide film adheres to a mirror-like surface to illustrate the reflection of incident radiation at the interfaces between air and film and between film and substrate, respectively.

In FIG. 5, the incoming radiation from the air 1 is partly reflected at the interface between the air 1 and the oxide film 2 to a reflected beam 4. The incident incident radiation penetrates attenuation through the oxide film 2 and is reflected at the interface between the film 2 and the substrate 3 to form a reflected beam 5 The interference between the rays 4 and 5 depends on the thickness of the oxide film, so that the thickness of the oxide film must be chosen in such a way that interference effect occurs to obtain the reflection prevention effect. The curve 6 in FIG. 7 illustrates the spectroscopic nature of the selective stainless steel metal oxide absorption surface on a mirror-like surface without regard to the interference effect. In FIG. 6 shows the spectroscopic transmission effect of stainless steel metal oxide. The stainless steel in question has a metal composition similar to type 683 / XIII 8 (ISO), 430 (AISI).

The oxide film, which contains mainly chromium oxides and Fe 2 O 2 (Fe 2 O 2 * Fe O or Fe 2 O 2), is produced by immersion in an acid solution of 100 g / l sodium bichromate and 400 g / l acid at a temperature of 106-108 °. C for 30-35 minutes.

A stainless steel oxide film of appropriate thickness of the cover layer on a mirror-like surface will exhibit significant selective absorption character regardless of the interference effect. The curve 7 in FIG. 7 shows the improved spectroscopic nature of the selec. tive absorption surface with such a thickness of: the coating that the interference effect reduces the reflectance at the wavelengths of solar radiation.

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Generally, the slab layer is made of dielectric material with a mean value for refractive index for materials with different optical properties with respect to the suppression of the reflectance at the interface between these materials. If the materials in question exhibit perfect transparency, the absorption band resulting from the interference effect will appear sharp. Even if the materials exhibit intermediate properties between dielectrics and electrical conductors, the interference effect will occur due to the existence of the penetrating radiation. Stainless steel metal oxide does not exhibit the perfect dielectric property, but in itself a significant selective absorption property.

Such an oxide film can therefore be used as a surface having a selective absorption property, taking into account the interference effect. It is possible to minimize the reflectance of the selective absorption surface if the following equations are met.

2 1 ηΊ = n. n "= n" ..... 1 1 o 2 2 n Λ - JL 5λ, 7λ_ ..... 2 nl 4 4 4 4 where n ^ represents the refractive index of the coating material, nQ represents the refractive index of the air, ie. n = 1, ^ represents the refractive index of the substrate, d represents the film thickness, and n ^ d = represents the wavelength of the primary absorption band. If stainless steel is used as in FIG. 5, the refractive index være will be 3.5-3.9, while the refractive index n ^ will be 2.0-2.5 when measured with an ellipsometric analyzer. Although the refractive index 2.0-2.5 for the stainless steel metal oxide film does not meet equation (1), and the refractive index for the primary absorption wavelength does not become zero when the optical thickness of the film is the film, as shown by curves 8 and 9 of FIG. . 8, exhibit improved selective absorption properties.

In FIG. 8, curves 8 and 9 show the spectral reflectance of a wavelength of the primary absorption of about 0.5 µm, respectively, where the spectral emission effect is maximum, and about 0.8 µm. From curve 8, it can be deduced that the best selective absorption properties are obtained when the wavelength 11 of the primary absorption is 0.5 µm and that the maximum absorbance of the selective absorption surface is obtained when the wavelength 11 of the primary absorption is about 0.8 µm, taking into account the spectral distribution of the solar radiation. When calculating

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the absorbance (oc) of the solar radiation off curves 8 and 9 assuming that the air mass is 2, the oC values for curves 8 and 9 will be 0.90 and 0.94 respectively.

For the curves 8 and 9 of FIG. 8, the emitter (S) has the same value of about 0.12 for long wavelengths. The values for the minimum reflectance at the primary absorption 11 and for the primary peak 12 of the spectral reflectance at an optical thickness of the film of -y- are somewhat different for curves 8 and 9, the optical dispersion constants in the metal oxide coating and the base plate at the wavelength in question being somewhat different. . Better selectivity is obtained for a wavelength of the primary absorption of 0.8 µm than for a wavelength of 0.5 µm.

Namely, the minimum reflectance at wavelength 11 for the primary absorption is less for curve 9 than for curve 8, while the maximum reflectance at wavelength 12 for the primary peak is less for curve 9 than for curve 8.

The line 10 in FIG. 8 indicates the ideal spectral reflectance curve for a selective absorption surface at operating temperature 100 ° C.

For a better understanding of the refractive index of stainless steel metal oxide, it should be noted that the metal oxide grows porously on the stainless steel surface in a given direction.

The greater the porosity, the more the refractive index value approaches the refractive index of the air, and the smaller the porosity, the more the refractive index value approaches the refractive index of the metal oxide.

The refractive index of magnetite Fe ^ O ^ is 2.4-2.5 in the wavelength range of visible radiation, while the refractive index of stainless steel metal oxide is 2.0-2.5 when measured with an ellipsometric analyzer.

It can be deduced from this that the porosities in stainless steel metal oxide are 0-20% by volume of the metal oxide layer. This is confirmed by transmission microscope measurement.

A suitable thickness dc of a stainless steel metal oxide coating having anti-reflection effect will be between 500 Å and 1250 Å when the optical thickness n ^ d of this layer is 1250 Å = n ^ d = 2400 Å, and refractive index n ^ is 2 , 0 = n ^ = 2.5. Even if the thickness of the layer is outside this range, a significant selective absorption property will appear, so that a suitable thickness of the cover layer will be 500 Å - 2000 Å. A cover layer of this thickness can be applied to both stainless steel and a substrate of other material. If the substrate is 16

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of a material with a high refractive index above 4.0, an improved absorption surface for solar radiation is obtained in comparison with stainless steel substrates.

Example 1

The two types of ferritic and austenitic stainless steels with the metal compositions 683 / XIII 8 (ISO), 430 (AISI) and 638 / XIII 11 (ISO), 304 (AISI), respectively, were chemically oxidized under the following conditions to form the oxide film on the surface of the the stainless steel.

Oxide r ing sb et ing plaintiff:

Sodium Bichromate 100 g / l

Na2Cr2 ° 7

Sulfuric acid 400 g / l H2 SO4

Immersion for 30-35 minutes at a temperature of 106-108 ° C.

In FIG. 9, the spectral reflectance of the selective absorption surface thus obtained is shown in comparison with the spectral reflectance of a conventional selective absorption surface in a solar panel. Curve 1 shows the reflectance of a selective absorption surface in the form of an oxide film obtained from ferritic stainless steel, curve 2 shows the reflectance of an oxide film obtained from austenitic stainless steel, curve 3 shows the reflectance of a copper oxide coated surface obtained by alkaline oxidation of copper plate, the curve 4 shows the reflectance of steel plated with pure nickel and then with nickel sulfide, and curve 5 shows the ideal spectral reflectance of the selective absorption surface in a solar panel at the operating temperature 100 ° C. The copper oxide-coated absorption surface exhibits a very high reflectance at long wavelengths above 4 µm. This reflectance is 3-5% higher than the reflectance of a selective stainless steel oxide film surface as shown by curves 1 and 2 at the solar radiation wavelengths of 0 , 3-2.5 yrs. taking into account the diffuse reflectance, while reflecting a selective absorption surface obtained from ferritic stainless steel as shown by. curve 1 is very small for wavelengths below 2.0 µm and very high for wavelengths greater than 2.0 µm &. A selective absorption surface obtained from ferritic stainless steel has correspondingly better properties over an absorption surface obtained from copper oxide. As shown by curve 2 in FIG. 9 is the reflectance of a selective absorbent.

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on surface obtained from stainless steel somewhat inferior to the reflectance obtained from ferritic stainless steel for wavelengths corresponding to radiation from a black body at the same temperature as the solar operating temperature. Although a selective absorption surface obtained from austenitic stainless steel gasoline for the spectroscopic properties is somewhat inferior, such a surface is useful as a selective absorption surface in a commercial solar panel, given the better anti-corrosion and welding properties of austenitic stainless steel.

As mentioned above, the preparation of selective absorption surfaces from ferritic and austenitic stainless steel types proposed by the invention can advantageously be used in a solar panel, the surfaces thus produced having the good spectroscopic properties and the excellent anti-corrosion and heat resistance properties which are specific properties of stainless steel.

The metal oxide surface coatings produced from ferritic and austenitic stainless steels by the chemical oxidation process are uniform and stable, without reducing the inherent anti-corrosion properties of stainless steel.

The heat resistance of the selective absorption surfaces made in accordance with the invention is of the same order of magnitude as that of stainless steel, although materials other than stainless steel are used as substrates.

In FIG. 10 is a cross-sectional view of a solar panel utilizing a selective absorption surface made from ferritic and austenitic stainless steel. In FIG. 10, an incident solar beam shown by an arrow is converted to heat by transmission through transparent cover materials, one-layer glass or resin plate placed to protect against convection heat loss and against shedding in solar radiation and air (2) and absorbed in the oxide film 3 by ferritic or austenitic stainless steel. The absorbed heat is transferred to a heating medium, e.g. air or water through the substrate 4 or other conventional material 5 applied to the substrate 4 by rolling and diffusion coating. In FIG. 10, the reference number 6 denotes an air layer arranged as a heat insulator, while 7 denotes an insulating material in the form of glass wool, asbestos or a cell structure. It has been found that a selective absorption surface made from ferritic or austenitic stainless steel in the chemical oxidation process has excellent heat collection properties when used in a solar panel.

18

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Example 2

Although selective absorption surfaces made from ferritic stainless steel with the metal composition of Example 1 have excellent spectroscopic properties and are inexpensive in manufacture, they exhibit certain disadvantages in terms of weldability, malleability and anti-corrosion properties. To reduce these drawbacks, low carbon stainless steels were treated under the same chemical oxidation conditions as given in Example 1. In FIG. 11 shows the curve 1 is the relation between the wavelengths and the reflectance of a low carbon selective absorption surface made of stainless steel and containing Ti, Mo and additives, whereas curve 2 shows the same relationship for a low carbon selective absorption surface made of ferritic stainless steel, but without Ti, Mo, and additives, and curve 5 indicates the ideal curve progression.

It can be seen from FIG. 11, that the selective absorption surface made from stainless steel containing additives has the same good spectroscopic properties as conventional ferritic stainless steels which do not contain additives.

Example 3

The following experiment was carried out to verify that, by appropriate selection of the oxidation conditions in the preparation of an appropriate thickness oxide film in the range of 500-2000 Å on the stainless steel surface, a selective absorption surface for a solar with anti-reflection capability due to the interference effect and excellent spektralreflektans.

A stainless steel sheet with metal composition corresponding to 683 / XIII 8 (ISO), 430 (AIST) was chemically oxidized by immersing in aqueous solutions with the following compositions A and B respectively, varying the immersion time to produce the oxide film on the stainless steel surface.

Next, the relationship between the thickness of the coating in Å and the immersion time, as shown in FIG.

12 and the correlations between the thickness of the coating in Å and the absorbance and distribution over the solar spectrum, the weight of the air = 2, and the emittance € integrated over the blackbody radiation in the operating temperature range 50-100 ° C as shown in FIG. 13th

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

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(A) Sodium Bichromate (1 C, O 2) 100 g / l

Sulfuric acid (^ SQ ^) 400 g / l

Treatment temperature 106-108 ° C (B) Chromium trioxide (CrO.3) 250 g / l

Sulfuric acid (HjS © ^) 500 g / l

Treatment temperature 70 ° C

FIG. 12 shows the relationship between the coating thickness in Å and the processing time in minutes Obtained by measuring the variation in the position (wavelength) of the primary absorption, the optical thickness being n ^ d = λ / 4, where n ^ represents the mean 2.2 of the aforementioned value range 2.0-2.5.

The curve 13 in FIG. 12 is obtained by treatment under conditions (Ά), while curve 14 is obtained by treatment under conditions (B). FIG. 13 shows the relationships between the absorbance noc and the emitance 6 of the solar at the operating temperature 100 ° C and the thickness Å of the coating. In FIG. 13, curve 15 indicates the relationship between absorbance <*. and the thickness of the coating, while the curve 16 indicates the relationship between the emitter 6 and the thickness of the coating.

From curves 15 and 16, it is seen that the value of ot exceeds 0.80 in the thickness range of 500-2000 Å and is 0.94 at a thickness of the coating of about 900 Å, where the wavelength of the primary absorption resulting from the interference effect is 0.8 µm. whereupon the value and decreases slowly at thicknesses of the coating layer above 1000 Å.

It can also be seen from FIG. 13, that the emittance 6 decreases slowly until the thickness of the coating reaches about 1500 Å and that the value δ exceeds 0.2 when the thickness of the coating exceeds 2000 Å, so that the selective absorption surface with good selective absorption properties can be produced with a thickness of the oxide film. between 500-2000 Å.

Thus, it has been demonstrated that a selective absorption surface with good properties can be prepared independently of the oxidation process when the thickness of the coating is in the range of 500-2000 Å.

The selective absorption surface of the invention has the following characteristic features: (1) When prepared in accordance with the method of the invention and using stainless steel as a substrate, a selective absorption surface having good properties with respect to

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carpet strength, heat resistance, anti-corrosion and adhesion.

In usual selective absorption surfaces containing copper oxide, the spectroscopic properties do not noticeably decrease at a temperature of 180-200 ° C (24 hours) as the surface changes color, but decrease at a temperature above 210 ° C (24 hours), destroying the surface structure of the metal oxide.

(2) In these conventional selective copper absorption surfaces, due to variations in the surface state and the spectroscopic properties of the surface when exposed to air, it is observed that the surface structure is noticeably deteriorated when exposed to rainwater which causes a peel of the metal oxide and a strong increasing the emittance in the infrared region with a concomitant noticeable decrease in reflectance and reduction of the spectroscopic properties as selective absorption surface. At the selective absorption surface of the invention, such a deterioration of the properties is not observed.

(3) Using ferritic stainless steel as the backing material, the process of the invention provides a selective absorption surface with good spectroscopic properties at low manufacturing cost, but compared to austenitic stainless steel, this surface has less good properties in terms of weldability, malleability and anti-corrosion performance. . To reduce these drawbacks, the surface can be made of low carbon ferritic stainless steel or with small amounts of certain ferritic stainless steel additives or small amounts of such low carbon ferritic stainless steel, avoiding occurring stress corrosion that always occurs in austenitic stainless steel and will impair the mechanical strength to the same extent as for austenitic stainless steel.

Claims (6)

21 DK 152624 B A selective absorption surface for a solar panel, comprising a metal oxide surface coating having a thickness of 500-2000 Å, applied to a substrate having a mirror-like surface, characterized in that the metal oxide coating is formed of steel from a metal composition consisting of a) 0.001 -0.15 wt% C, b) 0.005-3.00 "Si, c) 0.005-10 ,, (00" Mn, d) 11.00-30.00 "Cr, e) 0.005-22.00" Ni and / or 0.001-5.00% by weight of at least one additive selected from the group consisting of N, Cu, Al, V, Y, Ti, Nb, Ta, U, Th, W, Zr and Hf, f) as a voluntary component 0.75-5.00 wt.% Mo, and g) as the residual Fe, which metal oxide has a high energy absorption factor at wavelengths of 0.3-2.5 µm and a low energy emission factor at wavelengths of 3-50 µm. Selective absorption surface according to claim 1, characterized in that the metal composition comprises said 0.001-5.00% by weight of at least one additive selected from the group consisting of N, Cu, Al, V, Y, Ti, Nb, Ta, U, Th, W, Zr and Hf, and that the metal to carbon plus additive atomic ratio is greater than 5.0 and exceeds 8.0 for stainless steels containing Nb, Ta or Ti as additives. Selective absorption surface according to claim 1, characterized in that the mirror-like surface of the substrate, determined according to the method according to ISO recommendation R 468, has a roughness less than 0.07y indicated as Ra or less than 0.2y indicated as Rz . Selective absorption surface according to claim 1, 2 or 3, characterized in that the optical thickness of the surface coating, determined from the wavelength of the primary absorption, is represented by the formula o ^ d = λ / 4, where n ^ represents the refractive index of the coating, d its thickness and y the wavelength in ym, is about 0.8 ypn A process for preparing a selective absorption surface for a solar panel according to one of claims 1-4, characterized in that a metal composition such as for stainless steel consisting of a) 0.001-0.15% by weight of C, b) 0.005-3.00 "Si, c) 0.005-10.00" Mn, d) 11.00-30.00 "Cr, e) 0.005-22.00" Ni, and / or 0.001-5.00 weight percent of at least one additive selected from the group consisting of N, Cu, Al, V, Y, Ti, Nb, Ta, U, Th, W, Zr and Hf, (f) as a voluntary component 0.75-5.00% by weight Mo and g) as residue Fe, to form oxide films of thickness 500-2000 Å are chemically oxidized in an acid bath consisting of 150-800 g / l sulfuric acid and 100-400 g / l sodium or potassium bichromate or 40- 700 g / l chromium trioxide at a temperature between 50 ° and the boiling point and for a immersion time of 3-40 minutes. Process for the preparation of a selective absorption surface for a solar panel according to one of claims 1-4, characterized in that a metal composition as for stainless steel consisting of a) 0.001-0.15% by weight C, b) 0.005-3.00 " Si, c) 0.005-10.00 "Mn, d) 11.00-30.00" Cr, e) 0.005-22.00 "Ni, and / or 0.001-5.00% by weight of at least one additive selected from the group consisting of N, Cu, Al, V, Y, Ti, Nb, Ta, U, Th, W, Zr and Hf, (f) as a voluntary constituent, 0.75-5.00 weight percent Mo, and (g) as the residual part Fe to form oxide films of thickness 500-2000 Å are chemically oxidized in an alkaline bath consisting of 130-200 g / l sodium or potassium hydroxide, 30-40 g / l trisodium or potassium phosphate, 20-30 g / l sodium or potassium nitrate or sodium or potassium nitrite, 1-3 g / l ferric hydroxide and 20-30 g / l lead peroxide at a temperature of 100-150 ° C and for a immersion time of 3-50 minutes.