NO144365B - SELECTIVE ABSORPTION SURFACE, FOR SOLAR COLLECTOR, AND PROCEDURES IN THE PREPARATION OF THIS - Google Patents

Description

The present invention relates to an absorbent surface

of the kind specified in claim l's preamble. The invention also relates to a method for producing the absorbent surface.

It is a well-known method for collecting solar radiation to exploit the "greenhouse" effect, where a covering material that is opaque in the infrared wavelength range and transparent in the visible wavelength range is laid over a base surface that is previously coated with a substance whose properties closely correspond to that of a black body, for example a black pigment, where the coating substance has the necessary greenhouse effect by reducing heat convection loss as a result of a normal heat conduction. The previously known method can be carried out satisfactorily when the operating temperature for the solar collector is less than approx. 50°C, but when this temperature rises above 50°C, the method will be affected by a significantly reduced efficiency with regard to the collection of heat in the solar collector.

To avoid these disadvantages, it is well known to use conventional selective absorption surfaces whose spectroscopic properties

per exhibits the same energy absorption as a black body in the wavelength range "0.3 - 2.5 µm for the solar radiation and with a low degree of emission in the wavelength range 3 - 50 µm at an operating temperature of 100°C, which is the same temperature as the operating temperature of the solar collector It is difficult to achieve a selective absorption surface for a solar collector, which by its nature

shows the aforementioned spectroscopic features.

Then a sufficiently polished zinc plate and a copper plate which

are naturally air-oxidized, they only exhibit selective absorption properties for solar radiation, and even if such plates are used in solar collectors, this is not sufficient to eliminate the disadvantages of the previously known solar collectors, in which the greenhouse effect for the black pigment or the like is utilized, consequently, attempts have been made to artificially produce selective absorption surfaces for solar collectors.

As an artificially produced selective absorption surface for solar collectors, a surface with a coating of chemically applied copper oxide, a surface of galvanized iron with precipitated nickel sulphide and a double-coated surface where a substrate with a mirror-like surface is provided with a film that is opaque in the visible wavelength range and transparent in the infrared range and then a transparent film that prevents the reflection of solar radiation, which film is applied by vacuum evaporation, spraying or arc discharge methods, for example a surface-coated aluminum substrate, which is first coated with silicon metal and then with SiC> 2 to prevent reflection of solar radiation. The vacuum evaporation method has been considered one of the most reliable of the coating methods to form the selective absorption surface for the solar collector, whereby the reflection is prevented by an interference effect of the coating films, as a result of which the thickness of each film must be controlled and the constituents (single constituents or compounds) in each film must be chosen optimally, namely each film must exhibit a suitable refractive index and must adhere to each other on the base surface.

Special vacuum evaporation, e.g. sputtering and arcing methods have been developed as each film obtained by a normal evaporation method will not adhere sufficiently to each other or to the substrate. Because the evaporation method itself exhibits certain disadvantages with respect to production efficiency and costs, attempts have been made to produce the selective absorption surface with anti-reflective surface using methods other than the vacuum method, namely a chemical dry method and wet methods, as well as a plating method.

E.g. by the plating method, a selective absorption surface has been produced which exhibits selective absorption and prevents reflection as a result of the interference effect of the coating film

mean, by applying a coating film of nickel-

sulphide or by means of coating films of nickel-zinc sulphide and zinc sulphide on an aluminum sheet or a galvanized iron sheet. In the chemical treatment method, selective absorption surfaces are produced, with the above-mentioned properties, by forming metal oxide on a steel plate or stainless steel plate on

the same way as the nickel sulphide method mentioned above.

Although the coating film formed by dry and wet chemical methods, as well as obtained by vacuum evaporation and special vacuum evaporation methods exhibit the selective absorption properties, it is necessary to choose the right thickness range of the coating film obtained by the chemical methods in order to obtain a selective absorption surface with the same selective absorption properties

per as they were obtained by vacuum deposition or special vacuum deposition methods.

The purpose of the present invention relates to provision

of a selective absorption surface for a solar collector, in which the coating film consists of a predetermined mixture of metal oxide, which is strongly adherent to a substrate with a mirror-like surface, of predetermined roughness and in a certain thickness, the metal oxide of the coating film being selected from the which exhibits the properties of selective absorption, as well as the effect of preventing the reflection of solar radiation as a result of the interference effect of the film. The invention is characterized by what is stated in the characterizing part of claim 1.

In fig. 1 shows the relationship between wavelengths (in um) of solar rays and % reflection for the selective absorption surface as a result of the roughness of the substrate.

Fig. 2 and 3 show the relationship between absorption (a), emission

( t) and efficiency (*>?) of the selective absorption surface,

deposited respectively against roughness Ra (um) and Rz (um).

Fig. 4 shows the relationship between wavelengths and reflection for the selective surface for varying roughness in the substrate. Fig. 5 shows as an example the cross-section of the selective absorption surface for solar collectors, in which stainless steel is used as substrate. Fig. 6 shows the relationship between wavelengths (µm) and the transmission for the oxide film. Fig. 7 shows the relationship between wavelengths (µm) and the reflection for the metal oxide on a stainless steel substrate, disregarding the interference effect. Fig. 8 shows the relationship between wavelengths (µm) and the reflection (%) for the selective absorption surface in the solar collector. Fig. 9 shows the relationship between wave lengths (um) and the reflection (%) for metal oxides respectively obtained from ferritic and austenitic stainless steel. Fig. 10 shows a cross-section of a solar collector with a selective absorption surface. Fig. 11 shows the relationship between wavelengths (um) and the reflection (%) for metal oxide obtained from ferritic stainless steel with

a low carbon content.

Fig. 12 shows the relationship between the thickness (Å) of the coating layer and the chemical treatment time (min.) to obtain the metal oxide layer for the selective absorption surface in the solar collector. Fig. 13 shows the relationship between the thickness (Å) of the coating layer and the absorption (a), as well as the emission (p) of the selective absorption surface in a solar collector.

One of the metal compositions used according to the present invention has the following composition 0.001 - 0.15% by weight.

C, 0.005 - 3.00 wt% Si, 0.005 10.00 wt% Mn, 11.OO - 30.00 wt% Cr, 0.005 - 22.00 wt% Ni, optionally 0.75 -

5.00 wt% Mo, Fe ad 100%. This metal has a composition corresponding to a commercially available stainless steel with, for example, the following composition 0.005 - 0.08% by weight C,

0.005 - 1.00 wt% Si, 0.005 2.00 wt% Mn, 8.00 - 10.50 wt% Ni, 18.00-20.00 wt% Cr, Fe ad 100% (683/ XIII 11 (ISO), 304 (AISI))'; 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 Fe ad 100% (683/XIII 20 (ISO), 316 (AISI)); 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 weight-% Cr and Fe at 100% (683/XIII 8 (ISO),

430 (AISI))$ 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.OO weight-% Cr, 0.75 - 1.25 weight-% Mo and Fe ad 100% (434 (AISI)), as well as other stainless steels with a similar composition to those mentioned above. When stainless steel is used as the metal, martensitic stainless steel is not suitable, but ferritic and austenitic stainless steel are suitable for use in solar collectors as a result of their weldability.

A further metal for use in the present invention is stainless steel with a low carbon content, containing other metals to improve anti-corrosion resistance, as well as formability and weldability, for example 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 0.001 - 5.00 wt% of at least one of the following elements N, Cu, Al, v, Y, Ti, Nb, Ta, U, Th, W, Zr and Hf, optionally 0.75 - 5.00 wt% Mo and Fe ad 100%, and where the metal/c + N ratio is greater than 5.0, while this ratio is greater than 8.0 in stainless steel comprising Nb, Ta or Ti as the additional element. These steels correspond to commercially available stainless steels, for example containing 0.005 - 0.03 wt% C, 0.005 - 0.75 wt% Si, 0.005 - 1.00 wt%

Mn, 16/)0 - 18.00 wt-% Cr, 0.1 -1.0 wt-% Ti and Fe ad 100%;

0.005 - 0.03 wt% C, 0.005 - 0.75 wt% Si, 0.005 - 1.00

weight-% Mn, 16.00 - 18.00 weight-% Cr, 0.1 - 1.0 weight-% Ti, 0.75 - 1.25 weight-% Mo and Fe ad 100%.

A further steel that can be used in the present invention

are stainless steels with a low carbon content, as well as containing additional metals to improve corrosion resistance, formability and weldability, for example 0.001 - 0.15% by weight

C, 0.005 - 3.00 wt% Si, 0.005 - 10.00 wt% Mn, 0.005 -

22.00% by weight Ni, 11.00 - 30.00% by weight Cr, as well as 0.001 - 5.00% by weight of at least one of the elements N, Cu, Al, V, Y, Ti, Nb, Ta, U, Th,

W, Zr and Hf, possibly 0.75 - 5.00 wt% Mo, and Fe ad 100%,

and wherein the metal/C + N ratio is greater than 5.0, and wherein this ratio is greater than 8.0 in stainless steels comprising Nb, Ta or Ti as the additional element.

Procedures for the production of metal oxide from the mentioned metals are as follows: (1) Procedures for the production of metal oxide by wet and dry chemical treatments respectively. (2) Chemical treatment for the production of metal oxide on the stainless steel, with a predetermined composition, which layer is firmly adherent to a substrate with a mirror-like surface and where this substrate is not stainless steel. (3) Methods of producing metal oxide on the stainless steel by the vacuum evaporation method, the spraying method, and the arc discharge method, after the stainless steel with the predetermined composition is firmly adhered to a substrate having a mirror-like surface, different from the stainless steel. (4) Methods for producing metal oxide on the stainless steel by simultaneous adhesion and oxidation of the rust

free steels with a predetermined composition on the substrate with a mirror-like surface, different from the stainless steel.

Among the above-mentioned methods, the most preferred acid and alkaline oxidation methods are namely: (a) The acid oxidation method. The oxidation conditions are as follows: (b) The alkaline oxidation method.

The oxidation conditions are as follows: Sodium or potassium nitrite: Sodium or potassium nitrate

It is preferred to pre-treat the substrate surface before the chemical treatment is carried out. The preferred pre-treatment methods are those which involve immersing the substrate in either an aqueous mixture of one part by weight of nitric acid and one part by weight of water for 1 hour, or in an aqueous mixture of 30% by weight perchloric acid and 1% by weight of potassium chloride for 2 - 3 min..

The metal oxide obtained from stainless steels include those with the formula FeO (FeCr^O^ obtained from ferritic stainless steel and (Fe, Ni) 0 (Fe.Cr^O^ obtained from austenitic stainless steel, and where both metal oxides have a spine11 structure with

lattice defect.

When the surface condition of the substrate with the mirror-like surface satisfies the following requirements, the substrate material type is not limited in those cases where the adhering metal oxide of predetermined composition is applied to the substrate. 1) In order to provide the characteristic features for the selective absorption surface, namely high reflection in the infrared wavelength range, the metal oxide layer is transparent in the infrared range, whereby the infrared rays are reflected on the substrate when passing through the metal oxide layer which is attached to the substrate. 2) Adhesion of the metal oxide layer on the substrate is dependent on its roughness. The coating layer adhering to the smooth surface of the substrate becomes dense. 3) When the surface of the substrate exhibits a mirror-like surface for wavelengths in the visible and near-infrared range, the interference effect does not decrease, whereby prevention of the reflection clearly takes place. In the case where the substrate surface becomes rough, the absorption of solar radiation increases. It is thus up to the manufacturer to choose either to emphasize the effect by preventing reflection or achieve an increased absorption. 4) It is preferred to make the mirror-like surface of the substrate smooth in order to reduce the radiation of infrared rays. If the substrate becomes very rough, the spectroscopic properties of the selective surface will be reduced to such an extent that the selective absorption surface absorbs in the infrared wavelength range of 3 - 8 µm, which makes the surface unsuitable. Of different plate materials, both a stainless steel plate can be used

and a plastic sheet is used as substrate material.

As a result of what has been mentioned above, it has been found that the surface condition of the substrate, to which metal oxides of stainless steel are to be attached, is the most important factor with regard to achieving the spectroscopic properties of the selective absorption surface, where the absorption of wavelengths in the solar radiation is large (i.e. the reflection is small), while the emission of wavelengths in the infrared range is small, (i.e. the reflection from there is large).

It is known that in order to improve the efficiency of the selective absorption surface it is preferred that the roughness of the surface is large compared to the wavelength range of the solar radiation and is small compared to the wavelength range of infrared radiation. However, this has not been confirmed with experimental data.

In general, we 1 a substrate with a rough surface have a tendency

to increase absorption by repeated reflections, as well as suppress the effect of preventing reflection as a result of the interference effect, because both of these effects are dependent on the roughness of the selective absorption surface.

The second purpose of the present invention relates to determining the roughness of the substrate surface when metal oxide of stainless steel is formed on the surface of the substrate. In order to determine the surface roughness of the substrate, the following experiment was carried out.

Stainless steel with the following 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, the rest being made up of small amounts of additional metal, as well as iron (430 (AISI), 683/XIII 8 (ISO)), was oxidized in an acidic aqueous bath comprising 100 g/l sodium bichromate and 400 g/l sulfuric acid at a temperature of 106 - 108 °c for 30 35 min. to produce a metal oxide layer on the surface of the stainless steel.

The relationships between the absorption (a) integrated over the solar spectrum, the emittance ((3) integrated over the radiation efficiency of a black body (r) were investigated.

The efficiency (17) is given by the following equation:

Where 6 means the Stefan Boltzmann constant: 4.88 x 10" Kcal/m /h K , 7" represents the corrected operating temperature, which here is assumed to be 373°K. J represents the solar radiation effect (800 Kcal/m/h). The roughness of the base surface is expressed by it

arithmetic mean deviations (Ra) and "ten point height" (Rz) according to ISO Recommendation R 468.

The experimental data are shown in fig. 1 and 2. In fig. 1, curve 1 shows the relationship between the reflection and the wavelength when the surface roughness Ra is 0.36 um or Rz is 3.5 um, curve 2 shows the corresponding relationship for Ra 0.19 um or Rz 0.6 um, curve 3 shows that same ratio for Ra 0.12 um or Rz 0.5 um and curve 4 shows the same ratio for Ra 0.08 um or Rz 0.3

um and curve 5 shows the same relationship for Ra 0.04 um or Rz 0.1 um.

It can be seen that the variation in the reflection is small compared

net with the variation in the roughness of the selective absorption surface for the visible wavelength range, while the variation is large in the infrared range. When the ratio Ra/Rz becomes small, the reflection rises.

Fig. 2 shows the relationship between the Ra value, the absorption (a), the emittance (£) and the efficiency (T)). From fig. 2 it appears that the absorption (a) for the selective absorption surface is not greatly affected by the value Ra. The value for the emittance (fc) falls rapidly for values of Ra less than 0.07, while ver-

dien rises proportionally with values for Ra greater than 0.07. The value of the efficiency (nrj_) increases rapidly for values of Ra greater than 0.07 and shows a value of more than 75%.

It can be seen from fig. 2 that outstanding results are obtained

with the selective absorption surface produced by chemical oxidation of the surface of stainless steel with a Ra value of less than 0.07.

From fig. 3 it appears that the absorption (a) for the selective absorption surface is not greatly affected by the value Rz. Ver-

dien of the emittance ( t) decreases rapidly for values of Rz smaller

than 0.2, while the efficiency shows values higher than 75%

for values of Rz less than 0.2.

It has thus been found that improved results are obtained from

the selective absorption surface obtained by chemical oxidation of a stainless steel surface with a roughness Rz less than 0.2. A selective absorption surface with a roughness of Ra less than 0.07 provides a completely smooth surface for wavelengths in the infrared region and provides a small ratio of diffuse reflection to hemispheric reflection (the sum of the specular reflection and diffuse reflection) and prevents a suppression of the reflection as a result of multiple reflection and thereby exhibits a value of more than 80% for hemispherical reflection in the infrared wavelength range of more than 7 µm, as well as significant improvement of the selective absorption property of the surface in the solar collector. It is necessary to give the surface of the metal plate an even "finish" in order to be able to produce an even and stable oxide film, when this is to be produced by oxidation treatment of one stainless steel surface.

Generally speaking, the surface of stainless steel is inhomogeneous as a result of its metallographic structure, composition, method of treatment, local heat treatment and distribution of internal stresses. If the surface of the stainless steel sheet is inhomogeneous, a uniform oxide film will not form.

One of the purposes of the present invention is to improve

the selective absorption properties of the selective absorption surface in the solar collector, by processing the surface of the stainless steel substrate to a roughness Ra less than 0.07 or Rz less than 0.2 by means of mechanical polishing, chemical etching and electrolytic polishing, thereby removing many of the disadvantages that arise due to inhomogeneities in the surface of the metal plate. An example showing the effectiveness of the selective absorption surface for the solar collector, where the surface has a suitable roughness is shown in fig. 4. In this example, stainless steel (304 (AISI) 683/XIII 11 (ISO)) was treated with a liquid grinding method where glass powder with a particle size in the range of 20 - 100 µm was used to give a clean surface

surface with a roughness Ra of 0.2 µm or Rz of 1.0 µm, after which the surface was oxidized according to the acidic oxidation method mentioned under point (3a).

The spectral reflection for the oxide film on the stainless steel plate is shown in curve (a) in fig. 4. In the second example, the stainless steel was dipped in an aqueous solution containing 10 wt% nitric acid and 2 wt% hydrofluoric acid to give a clean surface with a roughness Ra of 0.14 µm or Rz of 0.6 µm. and then surface oxidized according to the method specified in point (3a).

The spectral reflectance for the oxide film on the stainless steel is

shown in curve (b) in fig. 4. In the example according to the present invention, the stainless steel was polished, possibly after a previous mechanical and/or chemical treatment, as mentioned in the previous examples, to give a treated surface with a roughness Ra of less than 0.07 µm or Rz of less than 0.2 µm and then oxidized using the acid oxidation method according to point (3a). The results obtained are shown in curve (c) in fig. 4. It can be seen that the oxide film according to the present invention (curve (c)) shows a high reflection in the infrared wave range, compared to what appears from curves (a) and (b).

It causes certain problems with regard to the determination of the thickness of the oxide layer when the metal oxide of the metal mixture is to be adhered to the surface of the substrate according to the following procedures, namely: (1) The method of obtaining an acid or alkaline oxidation on the surface of the stainless steel with a predetermined composition. (2) The particularly reactive vacuum deposition method, for example sputtering and arc discharge methods to improve the adhesive properties between the oxide film and the substrate. (3) , The method for adhering metal oxide powder of a predetermined metal mixture to the substrate using a binder which is relatively transparent in the infrared wavelength range, for example polyethylene-bg silicone resin, etc. (4) The method for carrying out the oxidation treatment of stainless steel which is firmly adherent to the substrate, excluding stainless steel, for example oxidised,<:> colored (chromalyzing) metal or coated metal etc..

The spectroscopic property of the selective absorption surface in the solar collector and the anti-reflection effect of the oxide film can be explained in the following way: The spectroscopic property of the selective absorption surface must show a lower reflection of the solar radiation in the wavelength range 0.3 - .2.5 um and a higher reflection in the infrared wavelength range, namely 3 - 50 um.

Fig. 5 shows a cross-section of the absorbing unit of the solar collector where the oxide film is adherent to a substrate with a mirror-like surface, and where the reflection of an incident beam between the interfaces between air and the film and between the film and the substrate is shown respectively.

In fig. 5, an incident beam is partially reflected on the interface between air 1 and the oxide film 2 and forms a reflected beam 4. The remaining part of the incident beam penetrates the oxide film 2 and is deflected and reflected on the interface between 1 the film 2 and the substrate 3 and forms the reflected beam 5. The interference between beams 4 and 5 is dependent on the thickness of the oxide film, so that the thickness of the oxide film is chosen so that the interference effect will prevent reflection. Curve 6 in fig. 7 shows the spectroscopic properties of the selective absorption surface with adhering metal oxide film on stainless steel on the substrate surface with a mirror-like surface, when the interference effect is not taken into account.

Fig. 6 shows the spectroscopic transmission properties of metal oxide based on stainless steel, which has a composition corresponding to 683/XIII 8 (ISO) and 430 (AI3I).

The oxide film essentially comprises chromium oxides and Fe^O^

(Fe20^.Fe0 or Fe^O^) is obtained by dipping in an acidic solution of 100 g/l sodium bichromate and 400 g/l sulfuric acid at a temperature in the range 106 - -108°C for 30 - 35 min..

The oxide film on the stainless steel with an appropriate thickness of the coating layer adhering to the substrate with the mirror-like surface exhibits a significant selective absorption property,

even when the interference effect is disregarded. Curve 7 in fig. 7 shows the substantially improved properties of the selective absorption surface which has such a thickness of the coating layer that the interference effect reduces the reflection in the wavelength range for solar radiation.

In general, a coating layer of a dielec-

tric material whose refractive index has an intermediate value in relation to the materials with different optical properties in order to reduce the reflection at the interface between these materials. If the materials are completely transparent, the absorption band due to the interference effect will appear very sharp. Even if the materials exhibit properties that lie between dielectric and electrical conductors, the interference effect will appear as a result of the presence of the penetrating beam. Metal oxide obtained from stainless steel does not exhibit perfect dielectric properties, but exhibits significant selective absorption by itself.

Therefore, the oxide film can be used because the surface exhibits the selective absorption property when the interference is taken into account. It is possible to give the reflection for the selective absorption surface a minimal value if the following equations are satisfied.

where n^ is the refractive index of the coating material, nQ is the refractive index of air (n = 1), n2 is the refractive index

for the substrate, d is the thickness' of the film, n^d = represents the wavelength of the primary absorption band. If stainless steel is used as the substrate (3) as shown in fig. 5 so

is the refractive index n^ 3.5 - 3.9, while the refractive index n^

is 2.0 - 2.5, measured with an ellipsometric analyzer.

Although the refractive index of 2.0 - 2.5 for the film of metal oxide obtained from stainless steel does not satisfy equation (I) and the refractive index at the primary absorption wavelength does not become zero for an optical thickness of the film of the film exhibits excellent selective absorption, as shown by curves 8 and 9 in fig. 8.

In fig. 8, curves 8 and 9 show the spectral reflection when the wavelength for the primary absorption is respectively 0.5 µm, at which the spectral emission effect is at a maximum value, and for 0.8 µm. Although it can be concluded from curve 8 that the best selective absorption property is obtained when the primary absorption wavelength (11) is 0.5 µm and that the maximum absorption for the selective absorption surface is at the wavelength (II) of the primary absorption at 0.8 µm when the spectral distribution of solar radiation is taken into account. When the absorption (a) for the solar radiation for curves 8 and 9 is calculated on the assumption that the mass of the air is 2, the values (a) for curves 8 and 9 will respectively amount to 0.90 and 0.94.

According to curves 8 and 9 in fig. 8, the emittance ( i.) constitutes the same value of approx. 0.12 for the longer wavelengths. The values showing the minimum reflection for the primary absorption (11) and the primary peak for the spectral reflection (12) for the optical thickness of the film of ^ , are somewhat different for

show curves 8 and 9, because the optical dispersion in the metal oxide coating layer and for the base plate respectively are somewhat different. Better selectivity is obtained at a wavelength for the primary absorption of 0.8 µm than for 0.5 µm.

This is because the minimum reflectance for curve 9 is smaller

than that for curve 8 in the primary absorption wavelength (11), while the maximum reflection for curve 9 is less than the

for curve 8 in the primary peak (12).

The dotted line 10 in fig. 8 shows the ideal spectral-reflection curve for a selective absorption surface at an operating temperature of 100°C.

With regard to the refractive index of metal oxide obtained from stainless steel, this metal oxide is porous and grows from the surface of the stainless steel in certain directions. In general, it can be said that when the porosity increases, the refractive index will approach the refractive index of air, while if the porosity decreases, the refractive index will approach that of the metal oxide.

The refractive index of magnetite (Fe^O^) is 2.4 - 2.5 for wavelengths in the visible region, while the refractive index of metal oxide obtained from stainless steel is 2.0 - 2.5, determined with an elliptometric analyzer.

It can be concluded from what has been mentioned above that the porosity of metal oxide obtained from stainless steel corresponds to 0 - 20%, calculated on the volume of the metal oxide layer. This fact is substantiated by measurements with a light microscope.

A suitable thickness (dc) for the coating layer of metal oxide obtained from stainless steel, and which exhibits the anti-reflective effect is 500 - 1,250 Å, when the optical thickness

(n^d) of the layers is represented by 1.250 Å n^d = 2.500 Å and the refractive index (n^) is in the range 2.0 = n^ = 2.5.

Even if the thickness of the layer is outside the mentioned range

the selective absorption properties of the surface will be such that it appears that a suitable thickness of the coating layer comprises 500 - 2,000 Å. The said suitable thickness of the coating layer can be used for stainless steel as well as substrates other than stainless steel. If the substrate is chosen so that the material has a higher refractive index than 4.0, the absorption surface for a solar radiation produced from such a substrate will be improved compared to that for a stainless steel substrate.

Example 1

Two types of ferritic and austenitic stainless steel with a composition corresponding respectively to 683/XIII 8 (ISO), 430. (AISI) and 638/XIII 11 (ISO), 304 (AlSI) were chemically oxidized under the following conditions t± 1 to give an oxide film on the surface

of the stainless steel.

The oxvdasion terms were:

Dip time 30 - 35 min. at a temperature in the range 106 - 108°c.

Fig. 9 shows the spectral reflection for the selective absorption surface obtained from the treated stainless steels, compared to the reflection for a normal selective absorption surface for a solar collector. In fig. 9, curve 1 shows the reflection for the selective absorption surface where the oxide film was obtained from ferritic stainless steel, curve 2 for. the oxide film obtained from the austenitic stainless steel, curve 3 for absorption surface for copper oxide obtained by alkaline oxidation of a copper plate, curve 4 for absorption for nickel sulfide on plated nickel, both coated on steel, and curve 5 shows the ideal spectral reflection for a selective absorption surface for a solar collector that works at an operating temperature of 100°C. The selective absorption surface coated with copper oxide shows an excessively high reflection for wavelengths greater than 4 µm. This corresponds to a reflection of 3 - 5% higher than that of the selective absorption surface of the oxide film on stainless steel as shown in curves 1 and 2 for wavelengths of solar radiation in the range 0.3 - 2.5 µm, taking into account diffuse reflection, while the selective absorption surface made from ferritic stainless steel, as shown in curve 1, exhibits a very small reflection at wavelengths less than 2.0 µm and significantly higher at wavelengths greater than 2.0 µm. The selective absorption surface produced from ferritic stainless steel is to the same extent superiorly better than that produced with copper oxide.

As can be seen from curve 2 in fig. 9, the reflection for the selective absorption surface made from stainless steel is somewhat less effective than that made from ferritic stainless steel in wavelengths for radiation from a black body at the same operating temperature for the solar collector. Although the selective absorption surface produced from austenitic stainless steel is somewhat less efficient with regard to spectroscopic properties, this surface is nevertheless excellent as an absorption surface in commercial solar collectors when one takes into account the excellent anti-corrosion properties and weldability of austenitic stainless steel.

As previously mentioned, the selective absorption surfaces produced according to the invention from ferritic or austenitic steel are very suitable as selective absorption surfaces in solar collectors because these surfaces exhibit good spectroscopic properties and significantly improved anti-corrosion and heat-resistant properties, which are characteristic of stainless steel. The metal oxide coating surfaces produced from ferritic and austenitic steels using the chemical oxidation process are smooth and stable and do not impair the stainless steel's inherent anti-corrosion properties.

The heat-resistant properties of the selective absorption surface produced according to the present invention are similar to those of stainless steel, even if substrates other than stainless steel are used.

Fig. 10 shows a cross-section of a suitable solar collector, in which the selective absorption surface produced from ferritic and austenitic steel is used. In fig. 10, a sunbeam, shown as an arrow, is converted into heat when passing through a transparent cover material (1-3 glass sheets or a plastic sheet) which is arranged to prevent the loss of convection heat and the part of the heat from the solar radiation that is emitted into the air (2 ) and absorbed on the oxide film 3 on ferritic or austenitic stainless steel. The absorbed heat is transferred to a heat transport medium, such as air or water etc, through the substrate 4 or other conventional material 5, bonded to the substrate by a

coating method or a diffusion attachment process.

In fig.'10, the number 6 indicates an air layer arranged as a heat insulator, 7 is an insulating material comprising glass uU, asbestos or

a "honeycomb" structure. It has been found that the selective absorption surface produced from ferritic or austenitic stainless steel by the chemical oxidation process is particularly effective in absorbing heat when such a surface is used in a solar collector.

Example 2

Although the selective absorption surfaces made from ferritic stainless steel with a composition as shown in Example 1 exhibit excellent spectroscopic properties and are inexpensive, they exhibit certain disadvantages in terms of weldability, formability and anti-corrosion properties. To avoid these disadvantages, a low-carbon stainless steel was chemically oxidized under the same conditions as shown in Example 1.

In fig. 11, curve 1 shows the relationship between wavelength and reflection for a selective absorption surface made from this stainless steel containing Ti, Mo, as well as additional metals, while curve 2 shows the same relationship for an absorption surface made from a low-carbon ferritic stainless steel that does not contain Ti , Mo and further metals, and curve 3 shows an idealized curve.

It can be seen from fig. 11 that the selective absorption surface produced from stainless steel containing additional metals exhibits excellent spectroscopic properties corresponding to the conventional ferritic stainless steel, not containing additional metals.

Example 3

The following experiment was carried out to show that the selective absorption surface for a solar collector, which exhibits anti-reflection properties as a result of interference, and improved spectral reflection is produced by choosing the oxidation conditions so as to form an oxide film of the correct thickness in the range of 500 - 2,000 Å on the surface of stainless steel.

A steel plate whose composition corresponded to 683/XIII 8 (ISO), 430 (AISI) was chemically oxidized by immersion in aqueous solutions with the compositions A and B respectively and where the immersion time was varied to produce oxide films on the surface of the stainless steel.

The relationship between the coating thickness (Å) and the dipping time is indicated in fig. 12 and the ratio between the absorption (a) for solar air (weight of air =2) and the emittance (£) integrated over black body radiation at an operating temperature of 50 - 100°C for the solar collector, and the thickness (Å) of the coating layer shown in fig.

13, was examined.

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

Fig. 12 shows the relationship between the thickness of the coating layer (Å) and the treatment time in minutes, determined by measuring the shift (in wavelength) of the primary absorption where the optical thickness is n^d = -j-, and where n^ indicates the average value 2.2 for the previously mentioned value 2.0 - 2.5.

In fig. 12, the curve 13 shown is obtained by the treatment condition (A), while the curve 14 is obtained by the treatment condition

(B). In fig. 13 shows the relationship between absorption (a) and emittance (£) at an operating temperature of 100°C for a solar collector and the thickness Å of the coated layer. In fig. 13

curve 15 shows the relationship between the absorption (a) and the thickness of the coating layer, while curve 16 shows the relationship between emittance (£) and the thickness of the coating layer.

It can be seen from curves 15 and 16 that the value (a) is greater than

0.80 when the thickness of the coating layer is in the range 500 - 2p00 Å,

and that the value (oc) is 0.94 when the thickness of the coating layer is approx. 900 Å, when the wavelength of the primary absorption comes from the interference effect at 0.8 µm, and that the value (a) decreases slowly for thicknesses of the coating layer greater than 1.000 Å.

It can also be seen from fig. 13 that the emittance (£) slowly increases until the thickness of the coating layer reaches approx. 1,500 Å and that the value (£) is greater than 0.2 when the thickness of the coating layer becomes more than 2,000 Å, - and that the selective absorption surface with excellent selective absorption properties can be produced if the thickness of the oxide film obtained from stainless steel is in the range of 500 - 2,000 Å.

In light of the above, it has been shown that selective absorption surfaces with excellent properties are produced regardless of the oxidation method used to coat the surface if the thickness of the coating layer is in the range 500 - 2,000 Å.

The characteristic properties of the selective absorption surface according to the present invention are as follows: (1) The selective absorption surface exhibits significantly improved properties with respect to durability, heat resistance, anti-corrosion properties and adhesion if it is produced according to the present method using stainless steel as substrate.

(å) In a conventional selective absorption surface comprising copper oxide, its spectroscopic properties do not change to a particular extent at a temperature of 180 - 200°C (24 h) by changing the color of the surface, but decrease significantly at a temperature above 210°C (24 h ) because the surface structure of the metal oxide is destroyed.

(b) For the conventional selective absorption surface on

base of copper, it has been found from variations in the surface state and from the spectroscopic properties of the surface that when this is exposed to air the surface structure will substantially

Claims (15)

1. Selective absorption surface for solar collectors comprising a surface coating of metal oxide with a thickness in the range of 500 - 2,000 Å on a substrate with a mirror-like surface, characterized in that the oxide is then net on the surface and of steel consisting 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% by weight Ni, optionally 0.75 - 5.00% by weight Mo and Fe at 100% by weight, which oxide exhibits a high energy absorption factor in the wavelength range 0.3 - 2.5 µm and a low- energy radiation factor in the wavelength range 3 - 50 um. 2. Selective absorption surface according to claim 1, characterized in that the oxide is formed from steel consisting 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, as well as 0.001 - 5.00 wt% of at least one additional element comprising N, Cu, Al, V, Y, Ti, Nb, Ta, U, Th, W, Zr and Hf , optionally 0.75 - 5.00% by weight Mo and Fe ad 100% by weight. 3. Selective absorption surface according to claim 1, characterized in that the oxide is formed from steel consisting 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, as well as 0.001 - 5.00 wt% of at least one additional element comprising N, Cu, Al, V, Y, Ti, Nb, Ta , U, Th, W, Zr and Hf, optionally 0.7 5 - 5.00 wt% Mo and Fe ad 100 wt%. 4. Selective absorption surface according to claim 1, characterized in that the oxide is formed on stainless steel (683/XIII 11 (ISO), 304 (AISI)), consisting of 0.005 - 0.08 wt% C, 0.005 - 1.00 wt -% Si, 0.005 - 2.00 wt% Mn, 8.00 - 10.50 wt% Ni, 18.00 - 20.00 wt% Cr, and Fe ad 100 wt%. 5. Selective absorption surface according to claim 1, characterized in that the oxide is formed on stainless steel (683/XIII 20 (ISO), 316 (AISI)), consisting of 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 Fe ad 100% by weight. 6. Selective absorption surface according to claim 1, characterized in that the oxide is formed on stainless steel (683/XIII 8 (ISO), 430 (AISI)), consisting 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 and Fe ad 100 wt%. 7. Selective absorption surface according to claim 1, characterized in that the oxide is formed on stainless steel (434 (AISI)), consisting of 0.005 - 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 wt% Cr; 0.75 - 1.25 wt% Mo and Fe ad 100 wt%. 8. Selective absorption surface according to claim 2, characterized in that the oxide is formed on stainless steel consisting of 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 Fe ad 100 wt%. 9. Selective absorption surface according to claim 2, characterized in that the oxide is formed on stainless steel consisting of 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 - 5.00 wt% Mo and Fe ad 100 wt%. 10. Selective absorption surface according to claims 2, 3, 8 and 9, characterized by> that the ratio Me/C + N is greater than 5.0, while this ratio is greater than 8.0 in stainless steels comprising Nb, Ta or Ti as an additional element. 11. Selective absorption surface according to claims 1-9, characterized in that the steel surface has a roughness Ra of less than 0.07 µm and an Rz value of less than 0.2 µm, determined in accordance with ISO Recommendation R 468. 12. Selective absorption surface according to claims 1-9, characterized in that the optical thickness of the oxide layer is determined on the basis of the wavelength of the primary absorption, represented by the following formula n1d .=-^=- (where n-j^ means the refractive index of the coating layer, d means the thickness of the coated layer, X means the wavelength in um) is approx. 0.8 µm. 13. Method for producing a selective absorption surface according to claims 1-12, from stainless steel with a mirror-like surface, characterized in that the steel with a composition corresponding to 0.001 - 0.15% by weight C, 0.005 - 3.00% by weight Si, 0.005 - 10.00 wt% Mn, 11.00 - 30.00 wt% Cr, 0.005 - 22.00 wt% Ni, possibly 0.75 - 5.00 wt% Mo and Fe ad 100 % by weight is oxidized chemically for 3-50 min. in a hot, acidic or alkaline bath containing an oxidizing agent. 14. Method for producing the selective absorption ion surface according to claim 13, characterized in that the steel is oxidized in an acid bath comprising 150 - 800 g/l sulfuric acid and 100 - 400 g/l sodium or potassium ambichromate or 4 0-7 00 g/l chromium trioxide at a temperature in the range of 50°C to the boiling point of the bath, using a dipping time of 3 - 40 min. for formation of the oxide film. 15. Method for producing the selective absorption ion surface according to claim 13, characterized in that the steel is oxidized in an alkaline bath containing 130 - 200 g/l sodium or potassium hydroxide, 30 - 40 g/l trisodium or tripotassium phosphate , 20 - 30 g/l sodium or potassium nitrate or sodium or potassium nitrite, 1-3 g/l iron (III) hydroxide and 20 - 30 g/T lead peroxide at a temperature in the range 100 - 150°C and where it is used a dipping time of 3 - 50 min. for formation of the oxide film.