DK153248B - Coating for selective absorption of solar energy and method for manufacturing such coating - Google Patents

Description

in

DK 153248B

The invention relates to a coating of the kind specified in the preamble of claim 1.

In order to be good solar collectors, selective coatings must have as little light reflection as possible and little transparency for light, ie.

5 be as black as possible. However, it is thus purely physical that the larger and darker a body, the greater the energy loss from heat radiation.

Black paint has long been used as a non-selective coating, the absorption and emission of which remains constant over the full spectrum of solar radiation and thermal radiation. Paint can work up to approx. 100 degrees Celsius, but above this temperature, the collector's efficiency decreases greatly due to the thermal losses caused by re-radiation from the collector.

Furthermore, paints have the serious flaw to be based on organic materials which decompose under normal solar and operating conditions.

These problems have been tried to avoid by using various electrolytic coatings.

Previously described methods for making selective coatings have employed methods and processes based on U.S. Pat. pat.

20,917,817 and 3,129,703.

DK 153248B

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The methods used so far are based on coatings of black-nickel, black-copper, black-zinc, black-silver and black-chrome, as well as combinations of chemical, electrolytic and heat-technical treatments.

Other embodiments of selective coatings, in oxidation 5, consist of steel alloys in acidic oxidizing solutions, such as oxidizing acids, whereby the upper metal layer is converted into an oxide layer with selective properties, directly linked to the steel alloy used.

A disadvantage of this method is that the selectivity is related only to the steel used. Furthermore, diffusion 10 can easily occur between the oxide layer and the steel as a separating layer of another material is missing.

This provides a less stable coating for long-term operation, because of vacuum evaporation, interference coatings based on semi-transparent thin metal layers coated with semiconductor interference layers can be produced. These two layers are deposited on a polished aluminum substrate, with the semiconducting layer at the top.

Due to the repeated reflections through the layers, selectivity is achieved.

However, these manufacturing methods are slow and expensive at large 20 surfaces, as the materials used must be of high purity. Therefore, the method is preferably used for optical filters and in aerospace engineering.

These processes have been difficult to reproduce, overly expensive, or the relationship between solar absorption ύζ and emission 25 has been unsatisfactory.

DK 153248B

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The invention differs from known principles and processes in which the selectivity is linked to the substrate, as known from Danish Patent Application No. 4184/76, and German Publication No. 2,702,079, wherein the selectivity is produced only by interference effects in one or more layers. and the methods of preparation are chemical oxidation and high vacuum evaporation respectively.

The invention breaks with the above principles and processes of introducing electrolytic dispersion coatings in solar technology.

It thus becomes more possible to control the pore structure 10 by adding insoluble substances with well-defined grain size which are precipitated together with the electrochemical coating consisting of metal oxides, metal sulfides and metal.

This makes it possible to make a controlled coating with so-called 'black holes1. By a combination of semiconductor properties 15 in both the electrochemical primer and in the solids embedded, and selective structural properties, the selectivity is significantly increased.

The manufacturing methods are inexpensive and suitable for mass production.

The coating of the type initially specified is peculiar to the characterizing part of claim 1.

This results in a surface having semiconducting dielectric properties and having a dendritic surface structure and micro cavities of the order of 2 - 2 µm. Furthermore, it is characteristic that the layer thickness is well defined, and in the order of 500-3000 Angstroms.

Thus, this can be achieved on a metallic or metallized substrate with small hemispherical radiance, £ Z 0.1, and at wavelengths in the range of 0.35-12 µm.

DK 153248B

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Thus, the ratio between the absorption factor of solar energy θ <and the emission factor £ becomes greater than 9 and the difference o ^ - £ becomes greater than or equal to 0.7.

It is obtained so that the surface absorbs as much solar energy as possible 5 and radiates as little heat as possible. This results in the maximum efficiency of a collector at the highest operating temperature.

The increased selectivity is achieved by a combination of structural and semiconductor properties. The micro-cavities can be formed by a combined technique consisting of etching the surface with acid and coating 10 with an electrolytic dispersion coating. By a coating dispersion is meant a precipitation of metal, plus solid insoluble matter, which is simultaneously precipitated, for example, by electrolysis as in this case.

Previously, nickel abrasive discs with diamond powder have been manufactured, as well as self-lubricating bearings of nickel with graphite powder by dispersion coating. The invention also relates to a process for making such a coating, which is characterized by the characterizing part of claim 4, which is further illustrated by the following examples.

The improved coating is obtained with thin coatings of colloidal nature with a composition shown by the following examples.

EKS. 1: Nickel - nickel oxide- nickel sulphide - zinc sulphide r .. lead sulphide.

EKS. 2: Nickel-nickel oxide-cobalt oxide.

EKS. 3: Nickel-nickel oxide-molybdenum oxide.

In addition, the coating contains one or more of the following substances: Carbides, nitrides, silicides, borides, silica, alumina, chalconenide glasses, carbon, titanates.

The percent composition is varied as follows.

DK 153248B

5 EX. 1: Nickel: 23-50%

Zinc: 7-48 "

Sulfur: 8-15 "

Nitrogen: 0-3.5 "5 Oxygen: 0-5"

Carbon: 0-4.5 "

Lead: 0-10 "EX. 2: Nickel: 0-50%

Cobalt: 0-50 "10 Oxygen: 0-50" EX. 3: Nickel: 5-25%

Molybdenum: 10-60 "

Oxygen: 20-40 "

Lead: 0-10 ”15 EX. 1: Precipitated from baths consisting of nickel sulfate, nickel ammonium sulfate, zinc sulfate, thiocyanates and lead compounds.

EKS. 2: Nickel chloride, zinc chloride, thiocyanates and lead compounds, at pH 5-6.5 and current densities of 0.1-1 Arap / dnt2 and temperatures of 18-60 degrees Celsius.

EKS. 3: Precipitated from baths containing nickel sulphates, molybdates and lead compounds under the same conditions as Examples 1 and 2.

• $ 6

DK 153248B

The invention is explained in more detail below with reference to the schematic drawing, in which fig. 1 shows a simple embodiment of a selective coating; FIG. 2 shows an enlargement of the selective coating structure.

5 FIG. 3 shows an enlargement of FIG. 2, to illustrate the reflection conditions in a so-called 'absolute black hole'.

FIG. 1 shows a sheet of metal (1) coated with a layer of electrolytically precipitated metal (2) and, further, a layer of metal or metal oxide (3) and extremely the selective black coating (4) which is irradiated by sunlight (5).

FIG. 2 shows an enlargement of the selective black coating structure (4) which forms 'absolutely black holes' (6), hereinafter this will be explained in more detail.

The size of the holes is 0.2 - 2 µm. Thereby, the absorption 15 in the region is increased up to 2 µm, and at the same time the holes function as reflectors for the infrared region above 2 µm.

FIG.3 shows an 'absolute black hole' (6).

No material surface constitutes an absolute black body, but a hole in a wall or surface can be considered an absolute black body.

20 The radiation (7) the scan penetrates through the hole is reflected from the inner walls, but only a vanishing part finds its way out again. The emission spectrum of an absolutely black body is determined entirely by the temperature of the cavity.

This results in increased selective absorption in the range of 0.35-2.5 µm (solar spectrum) and minimal emission in the infrared region for wavelengths above 2 ms.

The selectivity is characterized by the ratio, where is the absorption, and £ is the emission. The greater this ratio, the greater the selectivity. However, the absorbance must be in the order of> 0.9, since o (is included in the collector equation with greater weight than £) in order for full benefit of the effect to be obtained.

Claims (4)

A coating (4) for selective absorption of solar energy (5) characterized in that the coating consists of a dispersion of solids, scanning particles in the order of o-3yU, nine, embedded in a carrier matrix of semiconducting or di-5 electrically that the thickness of the coating is 500-300 Angstroms, that the surface structure of the coating is comprised of microhills of 0.2-2 µm 3 and that the dispersion of solids is homogeneous, that the coating has a chemical composition wherein the metal content is 0- 50% by weight of Ni, or a mixture of 10 of Ni and at least one metal and not more than three of the group consisting of: Co, Mo, Cr, W, Mn, Fe, V, Cu, Fb, Zn, Al, Si at least one of the metalloids: N, S, C, B, 0, or a chemical compound between two or more, alone or in association with a metal, precipitated as solid particles in the amount of 0-20% by weight. DK 153248B < A coating for selective absorption of solar energy according to claim 1, characterized in that the coating is applied to a metallic substrate or metallized substrate, which exhibits a small hemispherical radiation factor ≤ <0.1 within the wavelengths of 0.35 to 12 ytxm. A coating for selective absorption of solar energy according to claims 1 and 2, characterized in that the ratio between the absorption factor of solar energy OC and the emission factor £ expressed by the fraction is equal to or greater than the 9 or 10 difference of 0.7. Method for producing a coating for selective absorption of solar energy according to claims 1, 2 and 3, characterized in that the coating is produced by electrolytic application methods in a design wherein a metal plate (1) is coated with a layer of electrolytically precipitated metal (2) or an alloy consisting of one or more of the group: Ni, Co, Mo, Cr, W, Mn, Fe, V, Cu, Pb, Zn, Al, Si, additionally provided with a coating of a metal or metal oxide (3) from the above group, and extremely provided with a selective coating (4).