GB1588936A

GB1588936A - Solar heat collector

Info

Publication number: GB1588936A
Application number: GB31236/77A
Authority: GB
Original assignee: Showa Aluminum Corp
Current assignee: Showa Aluminum Can Corp
Priority date: 1976-07-29
Filing date: 1977-07-26
Publication date: 1981-04-29
Also published as: FR2360051A1; AU2724677A; AU504928B2; IT1079399B; DE2734032A1; SE7708650L

Description

(54) SOLAR HEAT COLLECTOR (71) We, SHOWA ALUMINIUM KABUSHIKI KAISHA, a joint stock com- pany organised under the laws of Japan of No. 224, 6-cho, Kaizan-cho, Sakai, Osaka, Japan do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a solar heat collector for use in water heating systems and to a process for forming a coating for selectively absorbing solar heat energy on a heat collecting member for use in the collector.

The term "aluminium" as used herein and in the claims includes pure aluminium commercial aluminium containing small amounts of impurities and aluminium alloys.

In recent years, it is predicted that the world will Ibe drained of energy resources, especially fossil fuels, in the near future. Thus much attention has been focussed on the development of new energy resources as substitutes which are free of pollution. Typical of such substitutes is solar heat energy which can be utilised for a wide variety of applications including water heating systems, air conditioning systems, power generators, etc.

These applications require the use of heat collectors for absorbing solar radiation. Ideally, it is desired that the surface of the heat collecting member have the characteristics of selectively absorbing the solar heat energy, such that its absorptivity a of solar rays in wave- lengths of up to about 2 Fm is as high as nearly 1 and that its emissivity e of radiation of infra-red wavelengths greater than 2 sm is as low as about zero.

According to the present invention there is provided a process for forming a coating for selectively absorbing solar heat energy on a heat collecting aluminium member at least over the surface thereof to be exposed to the sun, comprising the steps of subjecting the aluminium member to anodic oxidation with an aqueous phosphoric acid solution as an electrolyte and with a direct current to form a porous oxide coating thereon and electrolysing the resulting member in a solution containing a salt of a metal with an alternating current to cause the metal to be deposited in the pores of the oxide coating.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a front view of a heat collector; Fig. 2 is an enlarged view in section taken along the line II--II in Fig. 2; and Fig. 3 is a sectional view corresponding to Fig. 2 and showing a modified embodiment of a heat collecting member.

With reference to Figs. 1 and 2, a solar heat collector for use in systems for heating water comprises a heat collecting aluminium member 2 in the form of a panel having a plurality of conduit retaining parallel tubular portions 1, copper conduits 3 respectively inserted in and integrally joined to the conduit retaining portions 1 with their opposite ends projecting from the retaining portions 1, a tubular inlet header 4 joined to the lower ends of the conduits 3 and having a closed end at one side of the heat collecting member 2, and a tubular outlet header 5 joined to the upper ends of the conduits 3 and closed at its one end on the other side of the member 2. The copper conduits 3 inserted in and joined to the conduit retaining portions 1 serves to prevent corrosion of the aluminium member 2 to be caused by the water flowing through the member 2.The conduits 3 may be made from stainless steel in place of copper.

The heat collecting member 2 comprises extruded shaped aluminium segments 6 arranged side by side and joined to one another. The extruded segment 6 has one straight tubular portion for retaining the conduit 3 and a plate portion 7 integral with the tubular portion 1. The plate portion 7 has one side edge 7a extending straight sidewise and the other side edge 7b laterally U-shaped in cross section and fittable to the straight side edge 7a of the adjacent extruded segment 6.

The conduit retaining portion 1 so extends as to divide the plate portion 7. The extruded segments 6 are arranged side by side il a direction perpendicular to the longitudinal direction of the conduit retaining portions 1.

The heat collecting member 2 is provided with a coating 8 for selectively absorbing solar heat energy on the surface thereof to be exposed to the sun. The selective absorbing coating 8 is obtained by subjecting only the front face of the heat collecting member 2 to anodic oxidation to form a porous oxide coating and treating the resulting coated member 2 in an electrolyte containing a salt of a metal to cause the metal to be deposited in the pores of the oxide coating. The metal may possibly be deposited in the form of oxidized metal.

The term "metal" as used herein and in the claims includes oxidized metal. For the convenience of the anodic oxidation treatment, the selective absorbing coating may be formed over the entire surfaces of tlle heat collecting member. Useful metals are copper, iron, cobalt, nickel and tin.

Fig. 3 shows a modified embodiment of the heat collecting member. The modified heat collecting member 11 has the same structure as the heat collecting member 2 except that the conduit retaining portion 12 thereof is in the form of a straight groove of circular arc cross section and that it is composed of extruded shaped segments 13 which therefore correspondingly differ from the extruded shaped segments 6 in cross section. Through out Figs. 2 and 3 like parts are referred to by like reference numerals. Before being joined to the heat collecting member, the conduit 3 of the embodiment is coated with a brazing agent consisting predominantly of zinc. The conduits 3 are forced into the retaining por tions 12 from the openings thereof, and the assembly is thereafter heated, whereby the conduits 3 are brazed to the inner surfaces of the retaining portions.

Instead of connecting the upper and lower ends of the conduits 3 by the headers 4 and 5, each two adjacent conduits 3 may be con nected by a U-shaped bend to provide a zig zag passage for water. When the conduit re taining portion is in the form of a groove as illustrated in Fig. 3, the heat collecting member can be made from a single aluminum panel. Further in the case where the conduit retaining portion is groove-shaped, a single zigzag conduit including straight tube portions arranged in parallel at the same spacing as the parallel retaining portions can be used in place of plurality of conduits. The panel shaped heating collecting member can be made by the technique disclosed in U.S.

Patent No. 2,690,002.

Given below are examples in which coat ings for selectively absorbing solar heat energy were formed on usual aluminum panels according to this invention and tested for selectivity.

Example 1.

An aluminum panel was subjected to anodic oxidation in a 20% aqueous solution of ortho phosphoric acid with a direct current at a current density of 1.0 A/dm2 for 20 minutes to form a porous oxide coating on the panel.

The aluminum panel was then subjected to electrolysis for 15 minutes in an aqueous solu tion of 30 g/l of H,BO, and 30 g/l of NiSO4 7H2O at a bath temperature of -30 C with an alternating current, whereby nickel was deposited in the pores of the oxide coating, forming a black coating on the surface of the aluminum panel for selectively absorbing solar heat energy. When tested, the coating was found to have a solar ray absorptivity a of 0.92 and an emissivity e at 100"C of 0.10, hence qCt/E = 9.2. This value indicates that the coating exhibits a high selective absorption of solar heat energy.

Example 2.

An aluminum panel was subjected to anodic oxidation in a 15% aqueous solution of orthophosphoric acid with a direct current at a current density of 1.5 A/dm3 for 15 minutes to form a porous oxide coating on the panel.

The aluminum panel was then subjected to electrolysis for 20 minutes in an aqueous solution containing 50 g/l of CoSO4 : 7H2O and 25 g/l of H3BOB at a bath temperature'of 250 C with an alternating current, whereby cobalt was deposited in the pores of the oxide coating, forming a black coating on the surface of the aluminum panel for selectively absorbing solar heat energy. When tested in the same manner as in Example 1, the coating was found to have an absorprivity a of 0.94 and an emissivity e of 0.13, hence a/s = 7.2.

This value indicates that the coating exhibits a high selective absorption of solar heat energy.

The panels in the foregoing examples were evaluated for selective absorption of solar heat energy by determining the value of absorptivity a/emissivity E thereof.

This invention may be embodied differently without departing from the basic features of the invention. Accordingly the embodiments herein disclosed are given for illustrative purposes only and are not in any way limitative.

It is to be understood that the scope of this invention is defined by the appended claims rather than by the specification and that various alterations and modifications within the definition and scope of the claims are included

Claims

in the claims. WHAT WE CLAIM IS:

1. A process for forming a coating for selectively absorbing solar heat energy on a heat collecting aluminium member at least over the surface thereof to be exposed to the sun, comprising the steps of subjecting the aluminium member to anodic oxidation with an aqueous phosphoric acid solution as an electrolyte and with a direct current to form a porous oxide coating thereon and electrolysing the resulting member in a solution containing a salt of a metal with an alternating current to cause the metal to be deposited in the pores of the oxide coating.

2. A process according to claim 1, wherein the heat collecting member is in the form of a panel having a plurality of parallel conduit retaining portions integral therewith, each of the cor,duit retaining portions retaining therein a conduit made of a metal selected from the group consisting of copper and stainless steel.

3. A process according to claim 1 or 2, wherein the metal salt-containing electrolyte is an aqueous boric acid solution.

4. A process according to claim 1 or 2, wherein the metal salt-containing electrolyte is an aqueous sulfuric acid solution.

5. A process according to claim 1 or 2, wherein the metal salt-containing electrolyte is a mixed aqueous solution of ammonium sulfate and boric acid.

6. A process according to any of claims 1 to 5, wherein the metal is a member selected from the group consisting of copper, iron, cobalt, nickel and tin.

7. A process for forming a coating of selectively absorbing solar heat energy on a heat collecting member substantially as hereinbefore described.