CH620761A5 - Solar collector absorption surface - Google Patents

Description

The present invention relates to a surface capable of selectively absorbing solar energy. More specifically, the present invention relates to a surface capable of selectively absorbing solar energy (hereinafter designated by the terms selective absorption surface of solar collector)

wherein a metal oxide coating layer is formed from a particular metallic composition fixed on a substrate having a surface like a mirror, the layer having a predetermined thickness.

A method is well known for collecting solar radiation using the greenhouse effect in which a coating material, having opacity properties in the infrared wavelength and transparency in the visible wavelength, covers a base surface previously coated with a substance having properties close to those of the black body, for example a black pigment, said coating surface exhibiting the greenhouse effect and an effect of reducing the heat losses by convection originating from the classical conduction. The conventional prior method gives a satisfactory result when the operating temperature of the collector is less than 50 ° C; but when said temperature exceeds about 50 ° C, the method has the drawback of considerably reducing the heat-taking efficiency of the solar collector. To avoid this drawback, it is well known to use a conventional selective absorption surface having the spectroscopic properties of exhibiting the same energy absorption rate as the black body in the wavelengths 0.3-2, 5 microns of solar radiation and a low emission rate in the wavelengths 3-50 microns at an operating temperature of 100 ° C which is the operating temperature of the solar collector. It is difficult to obtain a surface for selective absorption of solar radiation naturally having the characteristics mentioned above.

While a sufficiently polished zinc plate and a copper plate naturally oxidized in air have a property of selectively absorbing solar radiation,

their application to a solar collector is insufficient to remove the drawbacks of known solar collectors in which the greenhouse effect of the black pigment is used; this is the reason why an attempt was made to artificially form the selective absorption surface of the solar collector.

As in the case of the selective absorption surface of an artificially formed anterior solar collector, a copper plate covered with a copper oxide formed by chemical treatment, a sheet of galvanized iron plated with nickel sulphide and a double substrate were used. coated, the substrate having the mirror polish being first coated with a film having the properties of opacity in the visible and of transparency in the infrared, then being coated with a transparent film having the property of preventing reflection of solar radiation, this film being deposited by vacuum evaporation, bombardment and arc discharge; for example an aluminum substrate coated with silicon, then with silica SÌO2 to prevent the reflection of solar radiation.

The vacuum evaporation method has been considered as one of the safest methods among the methods for forming a selective absorption surface of the solar collector having the property of preventing reflection thanks to the interference effect of thin layers. , because the thickness of each film must be controlled and the substances (single or composite body) of each film must be chosen so that each film with the appropriate refractive index can adhere to the base surface or with another movie.

Special methods of vacuum evaporation, bombardment and arc have been developed, since each film produced by normal vacuum evaporation does not allow this adhesion. However, since the evaporation method itself has drawbacks due to the yield and the production cost, attempts have been made to form the selective absorption surface with anti-refraction surface, namely methods dry and wet chemical and a plating method.

For example, in the plating method, the selective and anti-reflective absorption surface by layer interference effect is produced by forming a film of nickel sulfide or films of nickel-zinc sulfide or sulfide zinc on an aluminum or galvanized iron plate. In the chemical treatment method, the selective absorption surface having the above-mentioned effects, is produced by forming a metal oxide on the steel or stainless steel plate according to the same method as that mentioned above. - above for nickel sulfide.

Although the film formed by dry and wet chemical methods as well as that formed by vacuum evaporation or special vacuum evaporation have the property of selective absorption, it is necessary to choose the appropriate thickness range of the film formed by method chemical to obtain a selective absorption surface having the same selective absorption properties as those formed by vacuum evaporation or special vacuum evaporation.

The object of the present invention is to provide a selective absorption surface of a solar collector in which the coating film having a predetermined composition of metal oxide adheres securely to a substrate having a surface like a mirror, of determined roughness and of given thickness, said metal oxide of the coating film being chosen from those which have the property of selective absorption and the property of preventing the reflection of solar radiation by interference effect of the film.

Figure 1 shows the relationship between the wavelengths (in microns) of solar radiation and the reflection coefficient (in%) of the selective absorption surface as a function of the roughness of the substrate surface.

Figures 2 and 3 show the relationships between the absorption coefficient (a), the emission coefficient (e) and the efficiency (r)) of the selective absorption surface as a function of the roughness Ra (microns) and Rz (microns) respectively.

FIG. 4 shows the relationship between the wavelengths and the reflection coefficient of the selective surface for various roughnesses of the substrate.

FIG. 5 shows an example of the cross section of the selective absorption surface of the solar collector using stainless steel as a substrate.

FIG. 6 shows the relationship between the wavelengths (microns) and the transmission coefficient of said oxide film.

Figure 7 shows the relationship between wavelengths (microns) and the reflection coefficient of the metal oxide on stainless steel substrate, neglecting the interference effect.

Figure 8 shows the relationship between the wavelengths (microns) and the reflection coefficient (%) of the selective absorption surface of the solar collector.

Figure 9 shows the relationship between the wavelengths (microns) and the reflection coefficient (%) of the oxides of

5

10

15

20

25

30

35

40

45

50

55

60

65

5

620,761

metal from ferritic and austenitic steels respectively.

Figure 10 is a cross section of the solar collector with selective absorption surface.

Figure 11 shows the relationship between wavelengths (microns) and the reflection coefficient of ferritic stainless steel metal oxide having a low carbon content.

Figure 12 shows the relationship between the thickness (angstroms) of the coating layer and the chemical treatment time (minutes) to form the metal oxide layer of the selective absorption surface of the solar collector.

Figure 13 shows the relationship between the thickness (angstroms) of the coating layer and the absorption coefficient (a) and the emission coefficient (e) of the selective absorption surface of the solar collector.

One of the metallic compositions used in the present invention is as follows:

0.001 to 0.15% by weight of carbon,

0.005 to 3.00% by weight of silicon,

0.005 to 10.00% by weight of manganese,

11.00 to 30.00% by weight of chromium,

0.005 to 22.00% by weight of nickel optionally 0.75 to 5% by weight of molybdenum, the rest being iron.

This metallic composition corresponds to that of stainless steel commonly placed on the market, for example:

0.005 to 0.08% by weight of carbon,

0.005 to 1.00% by weight of silicon,

0.005 to 2.00% by weight of manganese,

8.00 to 10.50% by weight of nickel,

18.00 to 20.00% by weight of chromium, the remainder being iron (683 / XIII 11 (ISO);

0.005 to 0.08% by weight of carbon,

0.005 to 1.00% by weight of silicon,

0.005 to 2.00% by weight of manganese,

10.00 to 14.00% by weight of nickel,

16.00 to 18.00% by weight of chromium,

2.00 to 3.00% by weight of molybdenum,

the remainder being iron (683 / XIII 20 (ISO);

0.005 to 0.12% by weight of carbon,

0.005 to 0.75% by weight of silicon,

0.005 to 1.00% by weight of manganese,

0.005 to 0.60% by weight of nickel,

16.00 to 18.00% by weight of chromium, the remainder being iron (683 / XIII 8 (ISO);

0.005 to 0.12% by weight of carbon,

0.005 to 1.00% by weight of silicon,

0.005 to 1.00% by weight of manganese,

0.005 to 0.60% by weight of nickel,

16.00 to 18.00% by weight of chromium,

0.75 to 1.25% by weight of molybdenum, the rest being iron (434 [AISI])

and other stainless steels having a metallic composition similar to those mentioned above.

When stainless steel is used as a metallic composition, martensitic stainless steel is not suitable; but austenitic and ferritic stainless steels are suitable for use in the composition of the solar collector, because of their welding properties.

Another of the metallic compositions used in the present invention is stainless steel having a low carbon content and comprising another metal to improve anticorrosion, shaping and welding, for example:

0.001 to 0.15% by weight of carbon,

0.005 to 3.00% by weight of silicon,

0.005 to 10.00% by weight of manganese,

11.00 to 30.00% by weight of chromium and 0.001 to 5.00% by weight of one of the elements chosen from the group of nitrogen, copper, aluminum, vanadium, yttrium, titanium, niobium, tantalum, uranium, thorium , tungsten, zirconium and hafnium, optionally 0.75 to 5.00% by weight of molybdenum, the rest being iron, the Me / C + N ratio being greater than 5.0 or greater than 8.00 in steel stainless steel comprising as an additional element niobium, tantalum or titanium.

Said metallic compositions correspond to those of stainless steels commonly placed on the market, for example:

0.005 to 0.03% by weight of carbon,

0.005 to 0.75% by weight of silicon,

0.005 to 1.00% by weight of manganese,

16.00 to 18.00% by weight of chromium,

0.1 to 1.00% by weight of titanium, the remainder being iron;

0.005 to 0.03% by weight of carbon,

0.005 to 0.75% by weight of silicon,

0.005 to 1.00% by weight of manganese,

16.00 to 18.00% by weight of chromium,

0.1 to 1.00% by weight of titanium,

0.75 to 1.25% by weight of molybdenum, the rest being iron.

Another metallic composition used in the present invention is low carbon stainless steel with another metal to improve anti-corrosion, forming and welding, for example:

0.001 to 0.15% by weight of carbon,

0.005 to 3.00% by weight of silicon,

0.005 to 10.00% by weight of manganese,

0.005 to 22.00% by weight of nickel,

11.00 to 30.00% by weight of chromium and 0.001 to 5.00% by weight of one of the elements of the nitrogen, copper, aluminum, vanadium, yttrium, titanium, niobium, tantalum, uranium, thorium, tungsten group , zirconium and hafnium, optionally 0.75 to 5% by weight of molybdenum, the rest being iron, the Me / C + N ratio being greater than 5.0, greater than 8.00 if the stainless steel contains niobium , tantalum or titanium as an additional element.

The methods of manufacturing the metal oxide from the metal composition are as follows:

1] Methods of manufacturing metal oxide by wet and dry chemical methods.

2] Chemical treatment method for producing the metal oxide of stainless steel after the stainless steel of predetermined metallic composition has fixedly adhered to the substrate having a surface like a mirror other than said stainless steel.

3] Methods for developing the metal oxide of stainless steel by vacuum evaporation, bombardment and arc discharge after the stainless steel having the predetermined metallic composition has fixedly adhered to the substrate having a surface like a mirror other than said stainless steel.

5

10

15

20

25

30

35

40

45

50

55

60

65

620761

6

4] Methods for developing the metal oxide of the steel Among the methods mentioned above, those which are stainless by simultaneously adhering and oxidizing the preferable steel are the acid and alkaline oxidation methods,

stainless having the predetermined metallic composition on as follows:

the substrate having mirror polish other than stainless steel.

a) Acid oxidation method:

The oxidation conditions are as follows:

Sodium bicarbonate or potassium dichromate 100 to 400 g / 1

or Chlorine Trioxide 40 to 700 g / 1

sulfuric acid 150 to 800 g / 1

preferably 400 to 800 g / 1 Temperature 50 ° C at the boiling point, preferably 70 to 120 ° C Soaking time 3 to 40 minutes.

b) Alkaline oxidation method:

The oxidation conditions are as follows:

Sodium hydroxide or potassium hydroxide Trisodium phosphate (Na3P04) or tripotassium phosphate (K3P04)

Nitrite of sodium (NaN02) or potassium (KN02):

sodium nitrate (NaN03) or potassium nitrate (KN03)

Ferric hydroxide Fe (OH) 3 Lead peroxide PbÓ2 Temperature Soaking time

130 to 200 g / 1

30 to 40 g / 1

20 to 30 g / I there 3 g / 1 20 to 30 g / 1 100 to 150 ° C 3 to 50 minutes

It is preferable to pretreat the surface of the substrate before carrying out the above chemical treatment. Preferable pre-treatment methods are those of dipping the substrate either in an aqueous mixture containing one part by weight of nitric acid and one part by weight of water, for one hour, or in an aqueous mixture containing 30% by weight of perchloric acid and 1% by weight of potassium chloride for two to three minutes.

Said metal oxide originating from stainless steel is one of those having the chemical formula FeO (FeCr) 203 in ferritic stainless steel and (Fe, Ni) 0 (FE.Cr) 203 in austenitic stainless steel, the two metal oxides being spinel structures having network defects.

When the surface condition of the substrate having a surface like a mirror satisfies the following requirements, the kind of the material of the substrate is not limited to the case of metal oxide of predetermined composition adhering to the substrate.

1) To obtain the characteristic of selective absorption surface, i.e. the high reflection coefficient in the infrared, the metal oxide layer is transparent in the infrared radiation, the infrared -red reflecting on the substrate by crossing the metal oxide layer fixed to the substrate.

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

3) When the surface of the substrate has a surface like a mirror in the wavelengths of the visible and the near infrared, the interference effect does not decrease while the reflection prevention effect appears clearly. If the surface of the substrate becomes rough, the absorption coefficient of solar radiation increases. Thus, the manufacturer is left to choose either the effect of preventing reflection or that of increasing the absorption coefficient.

4) It is preferable to make the polished surface soft like a mirror of the substrate to lower the infrared radiation. If the substrate becomes too rough, the spectroscopic character of the selective surface will decrease so much that the wavelengths absorbing solar radiation in the selective absorption surface will reach the infrared wavelengths 3 to 8 microns; such a rough substrate is not suitable.

Various kinds of metal plates, stainless plate and plastic plate can be used as the substrate material.

In view of the foregoing, it has been found that the surface condition of the substrate on which the metal oxides of stainless steel must adhere, is the most important factor for the spectroscopic character of the surface to be fully filled. selective absorption in which the absorption coefficient of solar radiation is high (the reflection coefficient is low), while the emission coefficient in the infrared is low (large reflection coefficient).

It was recognized as preferable, in order to improve the efficiency of the selective absorption surface, to choose a surface roughness large with respect to the wavelength of solar radiation and low with respect to the length of infrared wave. But this was not confirmed by the experimental results. Generally speaking, a roughly finished substrate surface tends to increase the absorption coefficient by repeated reflections, but to decrease the anti-reflection effect by interference, since the two effects depend on the limitations of the roughness of the surface d 'selective absorption.

Another object of the present invention relates to the determination of the roughness of the surface of the substrate when the metal oxide of stainless steel is formed on the surface of the substrate. To determine the roughness of the substrate, the following experiment was carried out.

Stainless steel having the following metallic composition:

0.005 to 0.12% by weight of carbon,

0.005 to 0.75% by weight of silicon,

0.005 to 1.00% by weight of manganese,

16 to 18.00% by weight of chromium, the rest being a small amount of additional metal and iron (430 [AISI], 683 / XIII8 [ISO]), was oxidized in an acidic aqueous bath containing 100 g / 1 of sodium dichromate and 400 g / 1 of sulfuric acid, at a temperature of 106-108 ° C, for 30 to 35 minutes to form the layer of metal oxide on the surface of said stainless steel.

35

40

45

50

55

7

620761

The relationships between the absorption coefficient (a) integrated on the solar spectrum, the emission coefficient (e) integrated on the black body and the efficiency (r)) were examined respectively.

The yield (rç) is represented by the equation:

ox4

r) = a — e

J

in which:

o is Stefan Boltzmann's constant (4.88 10 ~ 8Kcal / m2h K °)

r is the operating temperature factor.

Here we assume 373 ° K.

J represents the power of solar radiation (800 Kcal / m2h).

The roughness of the base surface is determined by the mean arithmetic deviation (Ra) and the height of ten points (Rz) according to the ISO RECOMENDATION R468 standard.

The experimental results are shown in Figures 1 and 2.

In Figure 1, curve 1 shows the relationship between the reflection coefficient (on the ordinate in%) and the wavelength (on the abscissa in microns) with Ra = 0.36 microns or Rz = 3.5 microns;

Curve 2 with Ra = 0.19 microns or Rz = 0.6 microns; Curve 3 with Ra = 0.12 microns or Rz = 0.5 microns; Curve 4 with Ra = 0.08 microns or Rz = 0.3 microns; Curve 5 with Ra = 0.04 microns or Rz = 0.1 microns.

It is observed that the variation of the reflection coefficient with respect to the variation of the roughness of the selective absorption surface of the solar collector is small in the visible, and large in the infrared.

The lower the Ra / Rz ratio, the greater the reflection coefficient.

Figure 2 shows the relationships between the Ra values, the absorption coefficient (a), the emission coefficient (e) and the efficiency (r |).

For (a) the ordinate is graduated from 0.5 to 1.

For (e) from 0, to 0.25, for r] from 50 to 100.

In Figure 2, the value of the absorption coefficient (a) of the selective absorption area is not very affected by the value of Ra. The value of the emission factor (e) decreases sharply when Ra becomes less than 0.07 and increases proportionally to Ra for values of Ra greater than 0.07.

The value of the yield rj decreases suddenly when Ra becomes greater than 0.07 and shows values greater than 75%.

It is observed by curve 2, that the best result is obtained for a selective absorption surface produced by chemical oxidation of the surface of the stainless steel plate having a Ra value of less than 0.07.

In Figure 3, the value of the absorption coefficient (a) of the selective absorption surface is not too affected by the value of Rz. The value of the emission factor (e) suddenly decreases for the values of Rz less than 0.2 and the efficiency r) has a value greater than 75% for Rz less than 0.2.

It has been observed that the best result is obtained for the selective absorption surfaces produced by chemical oxidation of the stainless steel surface having a roughness Rz of less than 0.2. The selective absorption surface having a roughness Ra of less than 0.07 or Rz of less than 0.2 respectively gives a completely soft surface at the infrared wavelength and gives a low ratio of the reflection coefficient by diffusion. hemispherical reflection (sum of the mirror-type reflection coefficient and the diffusion reflection coefficient) and prevents a decrease in the reflection coefficient from multiple reflections, thus showing a value greater than 80% of the hemispherical reflection at lengths infrared wave greater than 7 microns and remarkably improving the selective absorption property of said surface of the solar collector. It is necessary to complete making the surface of the metal plate uniform so as to produce a uniform and stable oxide film when the oxidation treatment of the stainless steel surface is carried out.

In general, the surface of stainless steel is inhomogeneous due to the metallographic structure, composition, production process, local heat treatment and the distribution of internal stresses. As long as the surface of the stainless steel plate is inhomogeneous, a uniform oxide film cannot be formed.

One of the aims of the present invention is to improve the selective absorption property of the selective absorption surface of the solar collector by finishing the surface of the stainless steel substrate having a roughness Ra of less than 0.07 or Rz of less than 0.2 by mechanical polishing, chemical abrasion and electronic polishing, to prevent the many disadvantages coming from an inhomogeneous metal plate surface. One of the examples showing the efficiency of the selective absorption surface of the solar collector, having the suitable roughness is represented in FIG. 4.

In this example, stainless steel (304 [AISI] 683 / XIII 11 [150]) has been treated by a liquid abrasion method using glass powder whose particles have a size of 20 to 100 microns to develop a clean surface with a surface roughness of Ra = 0.2 microns or Rz = 1.0 microns; the surface was then oxidized according to the acid oxidation method of paragraph (3 a).

The spectral reflection coefficient of the oxide film of stainless steel is shown in Figure 4, curve (a).

On the ordinate, the coefficient in%. On the abscissa the wavelength in microns.

In another example, said stainless steel was immersed in an aqueous solution containing 10% by weight of nitric acid and 2% by weight of hydrofluoric acid to form a clean surface having a roughness of Ra = 0.14 microns or Rz = 0.6 microns, then oxidized on the surface according to the method of paragraph (3a).

The reflection coefficient as a function of the wavelength is represented by the curve (b) in FIG. 4. In the example of the present invention, said stainless steel has been polished after possible execution of mechanical and / or chemical treatments to give the surface a roughness Ra less than 0.07 microns or Rz less than 0.2 microns, then oxidized by the acid oxidation method of paragraph (3a). The result of this test is represented by curve (c) in FIG. 4. It is noted that the oxide film of the present invention has the highest coefficient of reflection at the wavelengths of the infrared with respect to to the coefficients of curves (a) and (b).

We now address the problem of determining the thickness of the oxide film when the metal oxides of the metallic composition are fixed to the surface of the substrate according to the following methods.

(1) Method for effecting acid or alkaline oxidation on the surface of stainless steel having the predetermined metallic composition.

(2) Particular vacuum evaporation method, methods

5

10

15

20

25

30

35

40

45

50

55

60

65

620761

8

bombardment or arc discharge to improve the adhesion property between the oxide film and the substrate.

(3) Method for fixing the metal oxide powders of predetermined metallic composition on the substrate using a binder having realtive transparency for the infrared, for example polyethylene and silicone resin, etc.

(4) Method for carrying out oxidation treatments of stainless steel fixed firmly to the substrate, except for stainless steel, for example oxidized chromium plating metal or coating metal.

The spectroscopic character of the selective absorption surface of the solar collector and the anti-reflection effect of the oxide film can be explained as follows: the spectroscopic character of the selective absorption surface is to have less reflection in the wavelengths of solar radiation (0.3 to 2.5 microns) and a large reflection in the infrared wavelengths (3 to 50 microns).

FIG. 5 represents a cross-section view of the absorber of the solar collector in which the oxide film is fixed to the substrate having a surface like a mirror to show the reflection of the incident ray on the interfacial surfaces between the air and the film and between the film and the substrate respectively. In FIG. 5, the incident ray coming from the air 1 is partially reflected on the interfacial surface between the air 1 and the oxide film 2 to form the reflected ray 4. The remaining incident ray penetrates through the film d oxide 2 without being attenuated and is reflected on the interfacial surface between the film 2 and the substrate 3 to form the reflected ray 5. The interference between the rays 4 and 5 depends on the thickness of the oxide film; the thickness of the oxide film is chosen so that the interference occurs so as to avoid reflection. Curve 6 in FIG. 7 shows the variations in the reflection coefficient (in%) as a function of the wavelength (in microns) of the surface for the selective absorption of stainless steel metal oxide on the surface of the substrate having a surface like a mirror, neglecting the interference effect. Figure 6 shows the variations in the transmission coefficient (in%) of the metal oxide of stainless steel as a function of the wavelength (in microns). Said stainless steel has the metallic composition corresponding to 683 / XIII 8 (ISO) and 430 (AISI).

The oxide film mainly comprising chromium and Fe304 oxides (Fe203 FeO or Fe304) is produced by soaking in an acid solution of sodium dichromate at 200 g / 1 and acid at 400 g / 1 for 30 to 35 minutes at a temperature of 106-108 ° C.

The stainless steel oxide film having the appropriate thickness of the coating layer fixed on the substrate having a surface like a mirror exhibits a considerable selective absorption character by neglecting the interference effect.

Curve 7 in FIG. 7 shows the higher spectroscopic nature of the selective absorption surface having a coating layer thickness such that the interference effect reduces the reflection coefficient at the wavelength of solar radiation. In general, a coating layer of dielectric material having a refractive index value intermediate between those of materials having different optical characters is provided so as to reduce the reflection coefficient at the interface of said materials.

If said materials have perfect transparency, the absorption band from the interference effect will appear in a pointed manner. Even if the materials have intermediate properties between the dielectrics and the electrical conductors, the interference effect will appear through the existence of a penetrating ray. The metal oxide of stainless steel does not have the perfect dielectric property but an important selective absorption property.

However, said oxide film can be used as a surface having the property of selective absorption taking into consideration the interference effect. It is possible to make the reflection coefficient of the selective absorption surface minimal if the equations below are satisfied.

n ^ = no-n2 = n2 1

_ _X_ JU _5X_ IX 4 '4' 4 '4

In the equations, ni represents the refractive index of the coating material; no represents the refractive index of air (no = 1); n2 represents the refractive index of the substrate; d represents the thickness of the film.

X

nest =.

4

represents the wavelength of the primary absorption band.

If stainless steel is used as a substrate (3) as shown in Figure 5, the refractive index n2 is 3.5 to 3.9 while the refractive index ni is 2 to 2.5, measured at the ellipsometric analyzer. Although the refractive index (2.0-2.5) of the metal oxide film of stainless steel does not satisfy equation (1) and the refractive index at wavelength of primary absorption does not become zero when the optical thickness of the film is

K

4

said film has a selective absorption property as shown by curves 8 and 9 in FIG. 8 where the variations of the reflection coefficient in% are represented as a function of the wavelengths in microns.

In FIG. 8, curves 8 and 9 show that the spectral reflection coefficients have respectively a maximum value of emissivity when the wavelengths of primary absorption are respectively equal to 0.5 and 0.8 microns. However, it is deduced from curve 8 that the greatest property of selective absorption takes place when the wavelength of primary absorption (11) is 0.5 microns and that the maximum absorption of the surface of selective absorption is present for a primary absorption wavelength equal to about 0.8 microns, taking into account the spectral distribution of solar radiation. Now, when the absorption coefficient (a) of solar radiation of curves 8 and 9 is calculated on the assumption that the air mass is 2, said values (a) of curves 8 and 9 amount to 0, 90 and 0.94 respectively.

In curves 8 and 9 in FIG. 8, the emission coefficient (e) rises to the same value substantially equal to 0.12 at long wavelengths. The values showing the minimum reflection coefficient in the primary absorption (11) and the primary reflection coefficient 12 peak in the optical thickness of the film

X.

2

are somewhat different in curves 8 and 9, since the optical dispersion constants in the metal oxide coating layer and in the base plate at certain wavelengths are somewhat different. Better selectivity will be obtained in the primary absorption wavelength of 0.8 microns than in that of 0.5 microns.

Specifically, the minimum reflection coefficient of curve 9 is lower than that of curve 8 at the primary absorption wavelength (11) while the maximum reflection coefficient of curve 9 is smaller than that of curve 8 at the wavelength of the primary peak (12). Line 10 of the s

10

15

20

25

30

35

40

45

50

55

60

65

9

620,761

Figure 8 shows the curve of the ideal reflection coefficient of the selective absorption surface at the operating temperature of 100 ° C.

With regard to the refractive index of the metal oxide of said stainless steel, it should be noted that pores develop on the surface of the stainless steel in a certain direction.

Generally, the higher the porosity, the closer the value of the refractive index to that of air,

while the lower the porosity, the closer the value of the refractive index to that of the metal oxide.

The refractive index of magnetite (Fe304) is 2.4-2.5 in the wavelength of visible radiation while the refractive index of the metal oxide of stainless steel 15 is 2, 0-2.5 measured with the ellipsometric analyzer.

It is deduced from the above that the porosity of the stainless steel metal oxide corresponds to 0-20% of the volume of the metal oxide layer. The fact was confirmed by measurements with a transmission microscope. 20

The suitable thickness (of) of the stainless steel metal oxide coating layer having the anti-reflection effect is between 500 and 1250 angstroms, when the optical thickness (nest) of said layer is such that : 1250 angstroms C nest <2500 angstroms and when the refractive index ni 25 is such that 2.0 <nor <2.5.

Even if the thickness of the layer is outside this range, the selective absorption property of the surface manifests itself greatly, so that the suitable thickness of the coating layer will be chosen between 500 and 2000 angstroms. Said coating layer thickness can be applied both to stainless steel as a substrate and to a substrate other than stainless steel. If the substrate is chosen from materials having a refractive index greater than 4.0, the 3S solar radiation absorption surface manufactured with the substrate is improved compared to that made with stainless steel as substrate.

Example 1

The two kinds of ferritic and austenitic stainless steel 40 having the metallic compositions 683 / XIII 8 (ISO), 430 (AISI) and 638 / XIII11 (ISO) 304 (AISI) were chemically oxidized under the following conditions to form the film oxide on the surface of stainless steel.

45

Oxidation conditions:

Sodium dichromate (NA2 Cr2 07) 100 g / 1

Sulfuric acid (H2 S04) 400 g / 1

Soaking for 30 to 35 minutes at a temperature of 106-108 ° C. so

FIG. 9 shows the reflection coefficient (in%) as a function of the wavelength (in microns) for an absorption surface produced with these steels, in comparison with the selective absorption surface of an ordinary solar collector.

In FIG. 9, curve 1 shows the coefficient of reflection of the selective absorption surface with oxide film made from ferritic stainless steel; curve 2 shows that of the oxide film made from austenitic stainless steel, curve 3 shows that of the surface coated with copper oxide made by alkaline oxidation 60 of a copper plate, figure 4 that of nickel sulfide on nickel plating, both being plated on steel, curve 5 shows the ideal spectral reflection coefficient of a selective absorption surface of a solar collector operating at a temperature of 100 ° C. The selective absorption surface is coated with copper oxide and has an excessive reflection coefficient at wavelengths greater than 4 microns. This corresponds to a reflection coefficient 3 to 5% higher than that of the selective absorption surface of the stainless steel oxide film, as presented in curves 1 and 2, at the wavelengths of the solar radiation. 0.3 to 2.5 microns taking into account the diffuse reflection; on the other hand, in the selective absorption surface made from ferritic stainless steel as shown in curve 1, the reflection coefficient is very small at wavelengths less than 2.0 microns and very high at long lengths d 'waves larger than 2.0 microns. The selective absorption area made from ferritic stainless steel is greater to the same extent than the selective area made from copper oxide. As shown in curve 2 of Figure 9, the reflection coefficient of the selective absorption surface made from stainless steel is somewhat lower than that made from ferritic stainless steel in lengths of black body radiation at the same temperature as that of the operation of the solar collector. Although the selective absorption surface made from austenitic stainless steel is somewhat lower in its spectroscopic property, said surface is valid for constituting a selective absorption surface of a commercial solar collector due to the superior properties of corrosion and welding of austenitic stainless steel.

As mentioned above, the selective absorption surfaces of the present invention made from ferritic and austenitic stainless steels can advantageously be used as the selective absorption surface of a solar collector, since said surfaces have good spectroscopic properties and superior anticorrosion and heat resistance properties, which are specific to stainless steel. The coating surfaces made from ferritic and austenitic stainless steels by chemical oxidation process are uniform and stable without altering the inherent anticorrosion properties of stainless steel.

The heat resistance property of the selective absorption surface of the present invention is of the same order of magnitude as that of stainless steel, even if a substrate other than steel is used.

Figure 10 shows a sectional view of a solar collector with selective absorption surface made from ferritic and austenitic stainless steel. In figure 10, an incident solar ray represented by an arrow is converted into heat by transmission through the layers of coating (one to three sheets of glass or resin) intended to protect heat losses by convection in the air (2 ), and by absorption by the film (3) of ferritic and austenitic stainless steel oxide. The absorbed heat is transmitted to an exchange medium such as air or water through the substrate (4) or all other conventional materials linked to the substrate 4 by coating method or diffusion methods. In FIG. 10, the reference 6 designates an air layer intended as a thermal insulator, the reference 7 designates the insulating material comprising glass wool, asbestos or honeycomb structure. It has been observed that the selective absorption surface made from austenitic or ferritic stainless steel by chemical oxidation exhibits a superior heat collecting effect when said surface is used for a solar collector.

Example 2

Although the selective absorption surface made from ferritic stainless steel having the metallic composition defined in Example 1 shows a superior spectroscopic property and a low cost price, it has slightly disadvantages of welding, formability and anticorrosion property. To overcome these disadvantages, low carbon stainless steel has been treated under the conditions

620,761

10

of chemical oxidation shown in Example 1. Figure 11 shows the relationship between the reflection coefficient (in%) and the wavelength (in microns) of a surface made from low grade stainless steel carbon additionally containing titanium, molybdenum and additional metals; curve 2 shows the relationship for a selective surface made from low carbon stainless steel containing no titanium, molybdenum and additional metals; curve 3 shows the ideal curve.

It can be seen from FIG. 11 that the selective absorption surface made from stainless steel containing additional metals has spectroscopic properties superior to those of a conventional stainless steel surface without additional metals.

Example 3 xs

The following experiment was made to prove the fact that the selective absorption surface of a solar collector having the property of anti-reflection by interference effect and a higher spectral reflection coefficient is fabricated by choosing the conditions of oxidation of the formation of oxide film 20 having the suitable thickness of 500 to 2000 angstroms on the surface of the stainless steel.

The stainless steel plate corresponding to the metal composition 683 / XIII 8 (ISO), 430 (AISI) was chemically oxidized by soaking in an aqueous solution having compositions (A) and (B) and varying the time soaking.

Figure 12 shows the relationship between the thickness of the coating layer (on the ordinate in angstroms) and the soaking time (on the abscissa in minutes); FIG. 13 shows the 30 variations of the absorption coefficient a (curve 15) on the solar spectrum (the weight of the air is 2), of the emission coefficient a (curve 16) integrated on the radiation of the black body and the operating temperature of the solar collector (50 to 100 ° C), depending on the thickness of the coating layer 35 (in angstroms). The scale relating to a is on the left of figure 13, that relating to e on the right.

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

40

AT)

Sodium dichromate (NA2 Cr2 07) 100 g / 1

Sulfuric acid (H2 S04) 400 g / 1

Processing temperature 106 to 108 ° C

45

B)

Chromium trioxide (Cr 03) 250 g / 1

Sulfuric acid (H2 S04) 500 g / 1

Processing temperature 70 ° C

50

Figure 12 shows the relationship between the thickness (in angstroms) of the coating layer and the treatment time (in minutes) obtained by measuring the change in the position (wavelength) of the primary absorption in where the optical thickness is nest = Ji / 4, where ni represents the mean ss value 2.2 of the above mentioned value 2.0-2.5.

In FIG. 12, the curve 13 relates to the treatment conditions (A), while the curve 14 is that obtained for the treatment conditions (B).

Figure 13 shows the relationship between the absorption coefficient - "> ° tion (a) and the thickness (in angstroms) of the coating layer, at the operating temperature (100 ° C) of the solar collector. also shown, for the same conditions, variations in the emission factor (e).

Curve 15 is the curve relating to the absorption coefficient (a), curve 16 is the curve relating to the emission coefficient (e).

Examination of curves 15 and 16 shows that the value (a) is greater than 0.80 for the thicknesses of the coating layer from 500 to 2000 angstroms and that the value (a) is 0.94 for a thickness of 900 angstroms when the wavelength of the primary absorption due to the interference effect is 0.80 microns; it is further noted that the value (a) decreases slowly for the thicknesses of the coating layer greater than 1000 angstroms.

The examination of FIG. 13 also shows that the emission coefficient (s) increases until the thickness of the coating layer reaches approximately 1500 angstroms; this value (e) is greater than 0.2 when the thickness of the coating layer reaches more than 2000 angstroms. The selective absorption surface therefore has a greater selective absorption property when the thickness of the stainless steel oxide film reaches 500-2000 angstroms.

In view of the above, it has been shown that the selective absorption surface is the best, regardless of the process of oxidation of the coating surface, when the thickness of the coating surface reaches 500 to 2000 angstroms.

The characteristics of the selective absorption surface of the invention are as follows:

(1) The selective absorption surface having the best endurance, heat resistance, anticorrosion and aeration properties is that produced according to the process of the present invention when stainless steel is used as a substrate.

[1] In the selective absorption surface comprising copper oxide, the spctroscopic property does not decrease markedly at the temperature of 180-200 ° C (24 hours) when the surface changes color, but decreases at a higher temperature at 210 ° C (24 hours) since the metal oxide surface structure is destroyed.

[2] In said conventional selective copper absorption surface, it has been observed from variations in the surface state and the spectroscopic nature of the surface that, when the surface is exposed to air, the structure of surface decreases notably, because exposure to rain causes the exfoliation of the metal oxide; the emission coefficient in the infrared increases notably which causes a reduction in the reflection coefficient and a defect in the property of selective absorption.

In the selective absorption surface according to the present invention, such poor properties have not been observed.

[3] The selective absorption surface having the best spectroscopic character and a low cost price is that produced according to the process of the present invention when ferritic stainless steel is used as substrate; but this surface has the properties of defect in weldability, formability and anticorrosion property compared to austenitic stainless steel. To overcome these drawbacks, said surface is made from a metallic composition of ferritic stainless steel with low carbon content, or of ferritic stainless steel with a small amount of specific additional metals, or of ferritic stainless steel low carbon with a small amount of specific additional metals which prevents stress corrosion which develops in austenitic stainless steel and provides mechanical strength of the same order of magnitude as that of austenitic stainless steel.

B

5 sheets of drawings

Claims (21)

620761 1. Selective absorption surface of a solar collector, characterized in that it comprises a metal oxide coating surface of thickness between 500 and 2000 angstroms on a substrate having a surface like a mirror and having a absorption factor greater than 0.80 at wavelengths from 0.3 to 2.5 microns and energy radiation factor less than 0.2 at wavelengths from 3 to 50 microns. 2.00 to 3.00% by weight of molybdenum, the rest being iron. 2. Selective absorption surface of a solar collector io according to claim 1, characterized in that the metal oxide has the following composition: 0.001 to 0.15% by weight of carbon, 0.005 to 3.00% by weight of silicon, is 0.005 to 10.00% by weight of manganese, 2 3 620,761 3. Selective absorption surface of a solar collector according to claim 1, characterized in that the metal oxide has the following composition: 25 0.001 to 0.15% by weight of carbon, 0.005 to 3.00% by weight of silicon, 0.005 to 10.00% by weight of manganese, 4 4. Selective absorption surface of a solar collector according to claim 1, characterized in that the metal oxide has the following composition: 0.001 to 0.15% by weight of carbon, 0.005 to 3.00% by weight of silicon, 0.005 to 10.00% by weight of manganese, 5. Selective absorption surface of a solar collector according to claim 1, characterized in that the metallic composition of the metal oxide coating layer is that of stainless steel (683 / XIII 11 (SIO ), consisting of: 0.005 to 0.08% by weight of carbon, 0.005 to 1.00% by weight of silicon, 0.005 to 2.00% by weight of manganese 60 6. Selective absorption surface of a solar collector according to claim 1, characterized in that the metallic composition of the metal oxide coating layer is that of stainless steel (683 / XIII 20 (ISO ), consisting of: 0.005 to 0.08% by weight of carbon, 0.005 to 1.00% by weight of silicon, 0.005 to 2.00% by weight of manganese, 7. Selective absorption surface of a solar collector according to claim 1, characterized in that the metallic composition of the metal oxide coating layer is that of stainless steel (683 / XIII 8 (ISO) , consisting of: 0.05 to 0.12% by weight of carbon, 0.005 to 0.75% by weight of silicon, 0.005 to 1.00% by weight of manganese, 0.005 to 0.60% by weight of nickel, 8. Selective absorption surface of a solar collector according to claim 1, characterized in that the metallic composition of the metal oxide coating layer is that of stainless steel consisting of: 0.05 to 0.12% by weight of carbon, 0.05 to 1.00% by weight of silicon, 0.005 to 1.00% by weight of manganese, 0.005 to 0.60% by weight of nickel, 8.00 to 10.50% by weight of nickel, 9. Selective absorption surface of a solar collector according to claim 3, characterized in that the metallic composition of the metal oxide coating surface is that of stainless steel consisting of: 0.005 to 0.03% by weight of carbon, 0.005 to 0.75% by weight of silicon, 0.005 to 1.00% by weight of manganese, 10. Selective absorption surface of a solar collector according to claim 3, characterized in that the metallic composition of the metal oxide coating surface is that of stainless steel consisting of: 0.005 to 0.03% by weight of carbon, 0.005 to 0.75% by weight of silicon, 0.005 to 1.00% by weight of manganese, 10.00 to 14.00% by weight of nickel, 11.00 to 30.00% by weight of chromium, 0.005 to 22.00% by weight of nickel, and 0.001 to 5.00% by weight of at least one additional element chosen from the group formed by nitrogen, copper, aluminum, vanadium, yttrium, niobium, tantalum, uranium, thorium, tungsten, zirconium and hafnium, optionally 0.75 to 5.00% by weight of molybdenum, the rest being iron, is chemically oxidized in an alkaline bath comprising 130 to 200 g / l of hydroxide sodium or potassium, 30 to 40 g / 1 of trisodium or tripotassium phosphate, 20 to 30 g / 1 of sodium or potassium nitrate or sodium or potassium nitride, 1 to 3 g / 1 of ferric hydroxide and 20 to 30 g / l of lead peroxide, at a temperature between 100 and 150 ° C., the soaking time being between 3 and 50 minutes so as to form the oxide film. 620,761 11.00 to 30.00% by weight of chromium, 0.001 to 5.00% by weight of at least one additional element chosen from the group formed by nitrogen, copper, aluminum, yttrium, vanadium, titanium, 60 niobium, tantalum, l uranium, thorium, tungsten, zirconium and hafnium, optionally 0.75 to 5.00% by weight of molybdenum, the rest being iron, is chemically oxidized in an acid bath consisting of 100 to 400 g / 1 of sodium or potassium dichromate or 40 to 700 g / 1 of 65 chromium trioxide and 150 to 800 g / 1 of sulfuric acid, at a temperature between 50 ° C and the boiling point, soaking time being between 3 to 40 minutes so as to form an additional element chosen from the group formed by nitrogen, copper, aluminum, vanadium, yttrium, titanium, niobium, tantalum, uranium, thorium, tungsten, zirconium and hafnium, optionally 0.75 to 5.00% by weight of molybdenum, the rest being iron, is chemically oxidized in an acid bath comprising 100 to 4 00 g / 1 of sodium or potassium dichromate or 40 to 700 g / 1 of chromium trioxide and 150 to 800 g / 1 of sulfuric acid at a temperature between 50 ° C and the boiling point, the time soaking time being between 3 and 40 minutes so as to form the oxide film. 11.00 to 30.00% by weight of chromium, 0.005 to 22.00% by weight of nickel optionally 0.75 to 5.00% by weight of molybdenum, the remainder 40 being iron, is chemically oxidized in an alkaline bath comprising 130 to 200 g / 1 of sodium or potassium hydroxide, 30 to 40 g / 1 of trisodium or tripotassium phosphate, 20 to 30 g / 1 of sodium or potassium nitrate or sodium or potassium 45 nitride, 1 to 3 g / 1 of ferric hydroxide and 20 to 30 g / 1 of lead peroxide, at a temperature between 100 and 150 ° C, the soaking time being between 3 and 50 minutes so as to form an oxide film. 11.00 to 30.00% by weight of chromium, 0.005 to 22.00% by weight of nickel, 0.001 to 5.00% by weight of at least 0.001 to 0.15% by weight of carbon, 0.005 to 3.00% by weight of silicon, 35 0.005 to 10.00% by weight of manganese, 11.00 to 30.00% by weight of chromium, 0.001 to 5.00% by weight of at least one additional element chosen from the group formed by nitrogen, copper, aluminum, yttrium, titanium, niobium, tantalum, uranium, thorium, tungsten, zirconium and hafnium, optionally 0.75 to 5% by weight of molybdenum, the remainder being iron, is chemically oxidized in an alkaline bath comprising 130 to 200 g / l of sodium hydroxide or potassium, 30 to 40 g / 1 of trisodium or tripotassium phosphate, 20 to 30 g / 1 of sodium or potassium nitrate, or of sodium or potassium nitride, 1 to 3 g / 1 of ferric hydroxide and 20 at 30 g / l of lead peroxide, at a temperature between 100 and 150 ° C., the soaking time being between 3 and 50 minutes so as to form the oxide film. 11. Selective absorption surface of solar collector according to one of claims 3,4, 9 and 10, characterized in that the metal / (carbon + nitrogen) ratio is greater than 5.0 or greater than 8.0 if stainless steel contains niobium, tantalum or titanium as an additional element. 11.00 to 30.00% by weight of chromium, 0.005 to 22.00% by weight of nickel 45 and 0.001 to 5.00% by weight of at least one additional element chosen from the group comprising nitrogen, copper, aluminum, vanadium, yttrium, titanium, niobium, tantalum, uranium, thorium, tungsten, zirconium and hafnium, so optionally 0.75 to 5.00% by weight of molybdenum, the rest being iron. 11.00 to 30.00% by weight of chromium 30 35 and 0.001 to 5% by weight of at least one additional element chosen from the group comprising nitrogen, copper, aluminum, vanadium, ryttrium, titanium, niobium, tantalum, uranium, thorium , tungsten, zirconium and hafnium, optionally 0.75 to 5.00% by weight of molybdenum and the remainder being iron. 11.00 to 30.00% by weight of chromium, 0.005 to 22.00% by weight of nickel optionally 0.75 to 5.00% of molybdenum, the rest being iron. 12. Selective absorption surface of solar collector according to one of claims 2 to 10, characterized in that the surface condition of the substrate having a surface like a mirror has a roughness Ra of less than 0.07 microns or Rz less than 0.2 microns determined according to the method of ISO Recommendation R 468. 13. Selective absorption surface of a solar collector according to one of claims 2 to 10, characterized in that the optical thickness of the coating layer is determined so that the absorption wavelength primary represented by the formula nest = M 4 where ni represents s the refractive index of the coating layer, d the thickness of the coating layer, ï. represents the wavelength in microns is equal to about 0.8 microns. 14. A method of manufacturing a selective absorption surface of a solar collector according to claim 1, characterized io by the fact that one oxidizes by soaking in an acidic or basic bath a stainless steel having a surface state of roughness Ra less than 0.07 micron or Rz less than 0.2 micron determined according to ISO Recommendation R 468 method. 15. The manufacturing method according to claim 14, characterized in that the stainless steel having the composition: 0.001 to 0.15% by weight of carbon, 0.005 to 3.00% by weight of silicon, 0.005 to 10.00% by weight of manganese, 20 16. Manufacturing process according to claim 14, characterized in that the stainless steel having the composition: the oxide film. 16.00 to 30.00% by weight of chromium, 0.005 to 22.00% by weight of nickel optionally 0.75 to 5% by weight of molybdenum, the remainder being iron is chemically oxidized in an acid bath consisting of 150 to 800 g / 1 of sulfuric acid and , 100 to 400 g / 1 of sodium or potassium dichromate or 40 to 700 g / 1 of chromium trioxide, at a temperature between 50 ° C and the boiling point, the soaking time being between 3 and 40 minutes to form an oxide film. 30 16.00 to 18.00% by weight of chromium, 0.1 to 1.0% by weight of titanium, 0.75 to 5.00% by weight of molybdenum, the rest being iron. 16.00 to 18.00% by weight of chromium, 0.1 to 1.0% by weight of titanium, the remainder being iron. 16.00 to 18.00% by weight of chromium, 0.75 to 1.25% by weight of molybdenum, the rest being iron. 16.00 to 18.00% by weight of chromium, the rest being iron. 16.00 to 18.00% by weight of chromium, 17. Manufacturing process according to claim 14, charac- terized in that the stainless steel having the composition: 0.001 to 0.15% by weight of carbon, 0.005 to 3.00% by weight of silicon, 0.005 to 10.00% by weight of manganese, 55 18. The manufacturing method according to claim 14, characterized in that the stainless steel having the composition: 0.001 to 0.15% by weight of carbon, 0.005 to 3.00% by weight of silicon, 0.005 to 10.00% by weight of manganese, 18.00 to 20.00% by weight of chromium, the rest being iron. 19. Manufacturing process according to claim 14, characterized in that the stainless steel having the composition: 0.001 to 0.15% by weight of carbon, 0.005 to 3.00% by weight of silicon, 0.005 to 10.00% by weight of manganese, 20. Manufacturing process according to claim 14, characterized in that the stainless steel having the composition: 0.00 there 0.15% by weight of carbon, 0.005 to 3.00% by weight of silicon, 0.005 to 10.00% by weight of manganese, 21. Manufacturing method according to one of claims 16 to 20, characterized in that a surface treatment is carried out chemically, physically and by electrolytic polishing.