Selective Absorption Film of a Solar Heat Collector

The present invention relates to a solar heat collector, and more particularly to a selective absorption film formed on a heat collection plate of a solar heat collector.

Conventionally, metal members such as aluminum, copper, iron, and the like, which are good in thermal conductivity, have been used as materials for heat collection fins of solar heat collectors, and metal members having a black film such as black nickel, black chromium, copper oxide, iron oxide, and the like have been practically employed.

In a solar heat collector that includes a heat collection tube and a heat collection fin sealed in a hollow cylinder of transparent glass, a heat collection fin having both a high absorption rate .alpha. of solar rays and a surface with a low radiation rate .epsilon. is generally necessary so as to collect the solar heat effectively. Particularly, a surface with a lower radiation rate .epsilon. and a selective absorption film with a higher absorption rate .alpha. are required for conventionally used vacuum hollow cylinder type solar heat collectors.

Further, it is desirable that the absorption rate .alpha. is high in air which is not vacuum as conventional flat plate type solar heat collectors, and that the radiation rate .epsilon. is low. However, in order to mount a heat collection fins having a radiation rate .epsilon. of below 0.10 and a absorption rate .alpha. of above 0.90 on a vacuum hollow cylinder type solar heat collector, since the selective absorption film of black nickel, which is produced by electrolytically coloring an anodic oxidation film of an aluminum plate conventionally employed, has the absorption rate .alpha. of 0.87 to 0.92 and the radiation rate .epsilon. of 0.12 to 0.20 under mass production, these performances are unsatisfactory as vacuum hollow cylinder type solar heat collectors.

One object of the present invention is to provide a selective absorption film having excellent heat resistance and superior heat absorption which is applied to a heat collection plate for a solar heat collector.

Another object of the present invention is to provide a method of producing a vacuum glass tube type heat collector having a high absorption rate and lower manufacturing cost by forming a selective absorption film of cobalt oxide film on a surface of a heat collection body composed of a copper material.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

According to one embodiment of the present invention, a solar heat collector comprises a heat collection member including a heat collection tube jointed to a heat collection fin and being vacuum sealed in a transparent hollow cylinder, and a selective absorption film for selectively absorbing solar heat on a heat collection surface of the heat collection fin, the selective absorption film being composed mainly of cobalt oxide.

According to another embodiment of the present invention, a heat collection film of a solar heat collector is constituted by covering one surface of a metal plate of iron or copper with cobalt, placing the metal plate in an oxidation furnace, and oxidizing the cobalt film in an oxidizing atmosphere of water vapor at a temperature of 350.degree. to 400.degree. C. for 30 to 120 minutes, thereby producing a blue system selective absorption film having good heat resistance at high temperature.

The present invention will be better understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein:

FIG. 1 is a cross-sectional view of a solar heat collector showing one embodiment of the present invention;

FIGS. 2(a)(b) and (c) are respective views of heat collectors comprised of a heat collection fin and a heat collection tube;

FIG. 3 is a cross-sectional view of the heat collection fin;

FIG. 4 is a view of a heat-treating furnace for explaining a method of making a selective absorption film according to the present invention;

FIG. 5 is a view for another manufacturing process for a selective absorption film of a heat collection fin; and

FIG. 6 is a view for explaining a method of making a selective absorption film in a vacuum glass tube type heat collector.

A solar heat collector according to one embodiment of the present invention is shown in FIG. 1. A heat collection member 3, comprised of a heat collection tube 2 joined to a heat collection fin 1, is vacuum sealed in a transparent hollow cylinder 4. A selective absorption film 5 for selectively absorbing solar rays is provided on a heat collection surface 1a of the heat collection fin 1. The selective absorption film 5 is composed mainly of cobalt oxide.

The heat collection tube 2 is bent in the form of U-shaped structure, one side of which is welded to an inner wall of a cylindrical heat collection fin 1 which is received through supports 6a in a hollow cylinder 4 made of transparent glass. The hollow cylinder 4 is enclosed at one end 4a by welding and at the other end 4b by a sealing metal member 6, thereby vacuum sealing the hollow cylinder 4.

Further, the heat collection fin 1 is made of iron and the plate thickness is about 0.4 to 0.6 mm. The selective absorption film 5 is constituted by a laminate which comprises a nickel plated layer 9 having a thickness of several microns to several tens microns and formed on the heat collection surface 1a of the heat collection fin 1, a cobalt plated layer 10 containing an iron component and having a thickness of several microns, and a cobalt oxide layer 11 (referring to FIG. 3). The heat collection fin 1 may be also made of copper.

As the heat collection fin 1, either a metal plate having at a center a recessed portion for attaching the heat collection tube thereto as shown in FIG. 2(b) or a metal plate as shown in FIG. 2(c) may be employed except the cylindrical structure to which U-shaped heat collection tube 2 is fixed at one side by welding as shown in FIG. 2(a).

A method of making the selective absorption film according to the present invention is given by forming a nickel plated layer on a surface of a metal plate of iron or copper, forming a cobalt plated layer on the nickel plated layer, or covering one surface of the metal plate of iron or copper with a cobalt plated layer, and oxidizing the cobalt layer by a heat-treatment to obtain a selective absorption film of another metal oxide mixture which is composed mainly of CoO and Co.sub.3 O.sub.4, or CoO and Fe.sub.2 O.sub.3 and which endures a high temperature up to 400.degree. C.

In order to produce the selective absorption film having good heat resistance at high temperature described above, the oxidation treatment is a key point. Of course, the selective absorption film is affected by the finished condition of the cobalt plating.

When the metal plate of iron, for example, is subjected to the plating treatment, it is accomplished by the procedure of treating processes for the selective absorption film of iron plate according to the embodiment of the present invention described below.

In this case, the formed article of iron plate is applied to the heat collection fin. Further, in this case nickel plating and cobalt plating are carried out, but the process of nickel plating may be omitted when only the cobalt plating is accomplished. However, the nickel plating may be usually provided for achieving durability and the corrosion resistance of the selective absorption film.

With respect to the cobalt plating, the composition of the cobalt plating bath and the treating condition, as shown in Table 2 to be described later as an embodiment of the present invention, may be preferable. Particularly, in order to obtain a selective absorption film having good heat resistance at high temperature and rendering the absorption rate .alpha. of solar rays of 93 to 96% and the radiation rate .epsilon. of 10 to 6%, which ensure the absorption rate .alpha. of above 90% and the radiation rate .epsilon. of below 10%, a bath containing an iron component as an additive, for example, Fe.sub.2 (SO.sub.4).sub.3.7H.sub.2 O in the composition of cobalt plating bath may be preferable. When an iron component additive is not used, the oxide film formed by the oxidation treatment during mass production may be widely varied, and the performances of the selective absorption film may not be stabilized. Therefor, it may be difficult to ensure a selective absorption film having good .alpha. and .epsilon. values. That is, a cobalt blue coloration of the selective absorption film may not often be seen by the oxidation treatment of the cobalt plated layer without using the iron component. In this case, .alpha. of the thermal absorption characteristic may not exceed 90% and may become 85 to 87% at the most. A brown uniform film containing Fe component can be obtained on the surface which is subjected to the cobalt plating treatment using the composition of the cobalt plating bath containing the iron component, thereby providing the blue system colored selective absorption film having good heat resistance at high temperature by means of the oxidation treatment according to the present invention. The similar absorption film can also be obtained when the copper plate is employed.

One embodiment of the present invention for making the selective absorption film will be described with a drawing and Tables in case of forming the selective absorption film on the heat collection fin of the solar heat collector made of iron or copper plate.

The heat collection fin 1 as shown in FIGS. 2(a) to (c) is treated until the final thermal oxidation process according to the procedure of treating processes for the selective absorption film using iron or copper plate.

Iron material or formed article of copper material (heat collection fin).fwdarw.defatting.fwdarw.washing with water.fwdarw.pickling.fwdarw.washing with water.fwdarw.removal of water.fwdarw.(2) luster nickel plating.fwdarw.washing with water.fwdarw.removal of water.fwdarw.(3) cobalt plating.fwdarw.washing with water.fwdarw.removal of water.fwdarw.drying.fwdarw.(4) thermal oxidation treatment (production of dry type oxide film for selective absorption film).

The luster nickel plating described above is accomplished by a composition of luster nickel plating bath and a condition shown in the following Table 1.

The cobalt plating (3) described above is accomplished by using a composition of cobalt plating bath and a condition shown in the following Table 2.

The thermal oxidation treatment (4) described above is carried out through a heat-treating furnace shown in FIG. 4. That is, cobalt plated finished articles 14, on which a selective absorption film is produced, are transferred through the furnace in the direction of an arrow shown in FIG. 4. While supplying H.sub.2 O vapor from a superheated water vapor generator 12 to nozzles 15 arranged under the articles 14, H.sub.2 O vapor is applied to the cobalt plated finished articles 14 to be treated, which are placed on a conveyor 13. The furnace is maintained in an air heating type opened normal pressure, air oxidation atmosphere at 350.degree. to 400.degree. C.

The following shows the embodied treating condition described above.

Atmosphere: Air oxidizing atmosphere: 1 atmosphere Opened air heating type (gas furnace generating H.sub.2 O may be preferable)

Temperature: 350.degree. to 400.degree. C. (350.degree. to 370.degree. C. for 1 Hr)

Time: 30 minutes to 2 hours. For the purpose of uniform oxidation treatment, it is necessary that dusts, finger marks and the like do not adhere to the articles.

In the case in order to obtain the heat resisting selective absorption film with stabilized quality, the treating temperature may preferably be about 350.degree. to 370.degree. C., and the treating time may be about 1 hour.

The following Table 3 shows the valuation of .alpha. and .epsilon. for selective absorption films produced by various treating conditions.

It will be apparent from Table 3 that the cobalt-plated blue system colored selective absorption film, treated in the oxidizing air atmosphere of water vapor at 350.degree. to 400.degree. C. for 30 to 120 minutes, is good.

Since the selective absorption film is produced by the described method, it can be manufactured in lower cost. In addition, the selective absorption film has good durability at a high temperature and is suitable for improving the heat collection efficiency of the heat collection fin for solar heat collectors.

Another embodiment for making a heat collection fin of the present invention will be described.

Referring to FIG. 5, a coil-shaped material 19 of copper or steel plate is plated with nickel or copper with unit of several tons, thereby producing a pre-plating layer 17 thereon. Although the pre-plating is generally carried out by electrical plating, chemical plating, vacuum deposition, sputtering, metallizing and the like can also be employed. After the material 19 with the pre-plating layer is cut into the size corresponding to one heat collection fin, the resulting strip is subjected to mechanical machining such as roll forming, curring, pressing, and the like, or welding, thereby providing a strip member 16 used as the heat collection fins shown in FIGS. 2(a)(b) and (c). Thereafter, processes of defatting, washing with water, pickling with weak acid bath, washing with water and removal of water are successively accomplished. A cobalt plating treatment is then carried out under the composition of the bath and the condition shown in Table 2, thereby forming a cobalt plated layer 10 on the pre-plating layer as shown in FIG. 3.

The strip member 16 with the cobalt plated layer 10 thus obtained are successively subjected to processes of washing with water, removal of water and drying, and then subjected to a thermal oxidation treatment to produce a selective absorption film 5 thereon. The thermal oxidation treatment is carried out in air atmosphere at 300.degree. to 400.degree. C. at one atmospheric pressure for 30 minutes to 2 hours by using an opened air heating furnace (a gas furnace for generating H.sub.2 O may be employed). In this case, in order to accomplish a uniform oxidation treatment, it is necessary that dusts, finger marks and the like do not adhere to the plated layer. After the heat collection fin thus obtained is joined to the heat collection tube 2, the assembly is received in the hollow cylinder 4 to complete the solar heat collector.

As a method of producing the heat collection fin 1 by subjecting the material 19 to copper plating serving as pre-plating, cobalt plating is accomplished after processes shown by dotted lines of FIG. 5 have been done. That is, the material 19 after pre-plating, is cut into the size corresponding to one heat collection fin, thereby providing the strip member 16. The strip member is successively subjected to processes of defatting, washing with water, pickling with weak acid bath, washing with water, and removal of water. A luster nickel plating treatment acting as the pre-plating is then carried out by using the composition of bath and the condition shown in Table 1. After processes of washing with water and the removal of water are carried out, cobalt plating is accomplished in the same manner as described above and the cobalt plated layer is subjected to a thermal oxidation treatment.

As described above, when several tons of the material of cold rolled steel coil or copper plate coil is subjected to the continuous nickel plating or copper plating treatment, the adhesive fault of the plated layer to the material will be eliminated and the product will be free from peeling of the plated layer due to the defatting fault. Consequently, the finished faulty rate will be lowered to below 1% while the conventional faulty rate is about 5 to 10%, thereby remarkably reducing the faulty rate. Further, as the pre-plating is accomplished under the condition of the material 19, the time required for the pre-plating treatment can be greatly decreased as compared with a known process in which the pre-plating is carried out after cutting the material, thereby remarkably reducing the cost of plating treatment. The selective absorption film 5, which is free from the plating quality, can be produced by using several tons of the coil-shaped material. Since the selective absorption film having the stable heat collection performances, high absorption rate of solar rays and low radiation rate can be obtained, heat collection fins with high heat collection performances will be provided to improve the heat collection efficiency of solar systems.

According to the present invention, instead of joining the heat collection fin to the heat collection tube 2 after producing the selective absorption film on the heat collection fin as described in the embodiment, the cobalt plating and the thermal oxidation treatment may be accomplished after the strip member is jointed to the heat collection tube. Thereafter, the assembly may be mounted to a header tube and the like to provide a solar heat collector. Further, the processes described above can be applicable to the production of a selective absorption film of black chromium except the selective absorption film of cobalt oxide.

As is evident from the explanation described above, according to another embodiment of the present invention, the pre-plating layer is formed on the surface of the material of the metal plate, and a material is cut into the size corresponding to one heat collection fin, thereby producing a strip member. Thereafter, the pre-plating layer of the strip member is subjected to the surface treatment to provide the heat collection fin for solar heat.

Another method of producing a selective absorption film on a collector of a vacuum glass tube according to the present invention will be described.

A heat collector body with a cobalt plated layer is first inserted into a glass tube as shown in FIG. 6. That is, a heat collector body 24, which is previously plated with cobalt, is introduced into a glass tube 27 having a vacuum exhaust aperture 26 at one end, and is sealed at the other end by a sealing metal member 29. Thereafter, the glass tube 27 is vertically positioned as shown in the drawing, and distilled water or water such as 2 to 3% hydrogen peroxide solution is poured into the glass tube from the vacuum exhaust aperture 26. The glass tube 27 is placed in a vacuum exhausted furnace which is raised to 350.degree. to 600.degree. C. After the heat collection plate is maintained at 350.degree. to 400.degree. C. for about 5 to 20 minutes, a vacuum pump is actuated to evacuate the glass tube. Then, a selective absorption film due to the cobalt oxidation is produced on the collector body to color the cobalt blue.

According to this embodiment, since the heat treatment of the cobalt plated selective absorption film can be accomplished by using the heat generated in the evacuation treatment, reduction of the heat-treating temperature can be prevented, and unwanted corrosion and rust due to the dew produced by high humidity may be avoided. In addition, the evacuation treatment and the oxidation treatment can be carried out at the same time.

Although solar heat collectors have been described as embodiments of the present invention, the present invention should not be limited solely to solar heat collectors.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.