GB1558440A

GB1558440A - Solar collector

Info

Publication number: GB1558440A
Application number: GB23792/77A
Authority: GB
Original assignee: University of Sydney
Current assignee: University of Sydney
Priority date: 1976-06-10
Filing date: 1977-06-03
Publication date: 1980-01-03
Also published as: IL52213A0; NL180124B; FR2354521A1; DE2725914A1; FR2354521B1; IL52213A; CA1083452A; JPS5545815B2; NL7705467A; SE422783B; ES459670A1; DE2725914C2; IT1083135B; SE7706648L; CH624755A5; JPS52150834A; NL180124C

Abstract

The solar energy collector has a glass tube (5), through which the working medium to be heated flows. The glass tube (5) is covered with a metal substrate layer (6) on its outside. This substrate layer has a relatively low infra-red emission ratio. The substrate layer (6) is covered by a metal carbide layer (7) which has been produced by means of a reactive atomisation process. This solar energy collector has improved selective surfaces for collecting solar energy. <IMAGE>

Description

(54) SOLAR COLLECTOR (71) We, THE UNIVERSITY OF SYDNEY, a juridical person of Parramatta Road, Sydney, 2006, New South Wales, Commonwealth of Australia, 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 energy collector and is more specifically concerned with providing a surface which has a relatively high absorptance of direct or diffuse solar radiation which is incident upon it directly or after reflection by another surface.

Some solar energy collectors currently available operate on the total absorption principle which relies on the provision of a matt black surface capable of absorbing inddent energy and transferring it to a fluid medium passed through the collector. In practice, the total absorption surface operates marginally above the temperature of the medium which is to be heated and it inevitably radiates as well as absorbs heat. Attempts are made to reduce the amount of radiated heat as far as is possible, but heat losses through radiation increase with the temperature of operation, so that, in practice, total absorption surfaces are only usable up to about 100"C.

Proposals have been made to develop solar energy collectors using a "selective surface" which strongly absorbs solar energy but reflects strongly at wavelengths longer than those characteristic of solar radiation. Thus radiation from such a collector is reduced greatly.

Collectors incorporating such a selective surface are usable at temperatures above 100"C for collecting solar heat as their radiation losses are less than matt black surfaces.

A report on such surfaces appears in publication RI 8167 of the Bureau of Mines Report of Investigations 1976, entitled "Reflectance and Emittance of Spectrally Selective Titanium and Zirconium Nitrides".

Titanium and Zirconium metals are expensive and for this and other reasons involving nitride films, selective surface solar collec tors have not yet been accepted as com mercially viable.

An object of this invention is the pro vision of improved selective surfaces for col lecting solar energy.

In accordance with one aspect of the present invention, there is provided.

A solar energy collector comprising a glass tube which is coated on its exterior surface with a substrate and a composite metal film; the substrate having a thickness of at least 0.05 x 10-6 m. and being composed of a metal having a low infra-red emittance (as herein after defined), and the composite metal film comprising a metal-carbide which has a thick ness between 0.04x 10-6 m. and 0.20x 10-6 m., which is deposited on the substrate by a reactive sputtering process and which, when deposited, has an electrical resistance not greater than 100 Kn per square.

In accordance with a second aspect of the present invention, there is provided a method of making a solar energy collector, compris ing the steps of depositing onto the external surface of a glass tube to a'thickness of not less than 0.05 x 10-6 m. a substrate composed of a metal having a low infra-red emittance (as hereinafter defined), and depositing onto the substrate by a reactive sputtering process a metal carbide film, the metal carbide film being deposited to a thickness falling within the range 0.04X10-6 m. to 0.20X10-6 m.

and the relative propositions of the metal and carbon atoms being controlled during the depositing process to provide for an electrical resistance, when the metal carbide film is deposited, which is not greater than 100 Kn per square.

Technical details of some of the surfaces the subject of this invention and the techniques employed for producing them are to be found in the Journal of Vacuum Science Technology, Vol. 13 No. 5 Sept/Oct 1976 (published by the American Vacuum Society) on page 1070 in an article by the inventor entitled "Sputtered Metal Carbide Solar Selective Absorbing Surfaces", and on page 1073 of the same publication a second article by the inventor jointly with D. R. McKenzie and B. Window and entitled "The d.c. Sputter Coating of Solar Selective Surfaces onto Tubes". These two articles are hereby inserted by way of reference.

The preferred metal, from a cost viewpoint, used in the formation of the film is iron although one can also use other more expensive metals such as molybdenum, chromium, tungsten, tantalum or titanium or a mixture of them. For example, the sputter electrode may be made of stainless steel to provide iron chromium and nickel atoms in the film. The carbon atoms are preferably obtained by employing methane (CH+) or other hydrocarbon as an impurity in an inert gas (e.g. argon) in the reactive sputtering process.

The preferred thickness of the metal carbide film is 0.09 x 106 m.

To obtain a uniform film composition on a large substrate it is preferred to use a reactive sputtering process in which the impurity gas is prevented from travelling from one point where, simultaneously, reactive sputtering is taking place to another point where reactive sputtering is also taking place. This may be achieved by using the process described in the aforesaid publication. An advantage of the invention is that stable selective films can be produced from cheap metals such as iron, rather than expensive metals such as molybdenum, tungsten or zirconium. However, the invention is also usable with more expensive metals if desired.

The invention operates by using the interference principle. Although the interference principle is well known, the selective surfaces previously suggested have involved metal oxides (such as copper oxide or chromium oxide) which are not particularly stable at high temperatures in vacuum, or relatively expensive refractory metals and metal compounds (such as an "AMA" surface which consists of alternate layers of Aluminium oxide and Modybdenum oxide).

Thus the selective surfaces so far suggested make the commercial production of high temperature solar collectors unattractive. The selective surfaces of the invention have the advantage that they can be made using inexpensive metals, such as iron and chromium and are stable at temperatures above 100"C and they can be applied by an inexpensive sputtering process. The thickness of the film can be easily controlled by controlling the time for which sputtering occurs.

The expression 'low infra-red emittance" is to be understood in the context of this specification as meaning that the emittance at room temperature is less than 0.10 and preferably in the range 0.02 to 0.05. Substrates which exhibit such low emittance are, for example, copper, silver, and gold.

The expression "a high absorptance for solar radiation" is to be understood in the context of this specification as meaning that the film absorbs at least 75 % and preferably 76% or more of solar radiation which is incident normal to the surface of the film irrespective of whether such radiation is applied directly or after reflection by another surface such as a cusp or cylinder reflector.

Single layer metal-carbide films have been produced routinely on copper substrates by the method of the invention to provide a room temperataure emittance of 0.03 and absorptance of 80%.

The invention will now be described in more detail, by way of example, with refer- ence to the accompanying diagrammatic drawing in which: Figure 1 shows a solar energy collector; Figure 2 is a cross-section through an energy collecting tube in the collector; and, Figure 3 shows a solar heating panel employing such collectors.

Figure 1 shows a solar energy collector comprising a cusp reflector 1 surrounding one side of an energy collecting tube 2 through which a fluid medium is continuously passed in the direction of the arrow 3 to extract heat conducted through the wall of the tube and which provides the working medium. In practice, a bank of such collectors would be arranged side by side and the working fluid would be passed through them serially. The aperture of the cusp reflectors is directed towards the sun. A glass envelope 10 encloses the reflector 1.

Figure 2, like Figure 1 is diagrammatic.

The light falling on the tube 2 directly and by reflection from the reflector 1 is absorbed by a heat collecting layer 4 which coats a glass cylinder 5 through which the working fluid is passed.

The layer 4 comprises an inner substrate 6 which is sputtered onto the surface of the glass cylinder 5. The technique used involves sputtering in a low pressure atmosphere of an inert gas such as argon to build up a copper substrate on the cylinder 5 having a low infra-red emittance as defined. The sputtering is discontinued when the substrate thickness is at least 0.5x10-6 m. and preferably 0.2 x 10-6 m. thick.

The copper substrate is then coated with a film 7 of iron carbide having a thickness lying between 0.04x10-'i m. and 0.20x10-B m., but preferably 0.09x10-6 m. thick. The required thickness of the film and its quality can be obtained from the electrical and other parameters of the reactive sputtering process used. It will be understood that the desired electrical resistance is determined by the proportions of the metal and carbon atoms in the film. In practice it has been found that acceptable results are obtained when using the sputtering process described in the Journal of Vacuum Science Technology referred to above and if the metal carbide film has an electrical resistance per square of 10 kilohms to 1 megohm.The deposition process may be controlled to decrease the metal component proportion of the film with increasing thickness and to increase the carbon component proportion with increasing thickness.

The thickness of the carbide film is such that the graph of reflectance (at near normal incidence of light) exhibits a minimum at wavelengths between 0.80X10-6 m. and 1.0 x 10-6 m. This corresponds to a metal carbide film thickness of about 0.09 X 10-6 m.

in practice.

The substrate can be manufactured on a small scale on a plane surface by placing a plane copper disc electrode parallel to and spaced from the surface in an inert gas such as argon. The surface is supported on a second plane disc electrode which is at zero electrical potential. The electrode separation from the surface is 17 mm and the electrode area is 7.5X10-3 m2. In these circumstances the following conditions are suitable for the deposition of the copper substrate.

Electrode Material Copper Electrode Voltage -1200 V Gas Pure Argon Flow Rate As Fast As Possible Gas Pressure 0.2 torr Sputtering Time 5 Minutes For an electrode area of 7.5x 10-3 m2, the following conditions are suitable for the deposition of the iron carbide film.

Electrode Material Iron (or stainless steel) Electrode Voltage - 1200 V Gas 1.6% (by volume) of methane in argon Flow Rate 0.25 cm3s-l at 1 atmosphere Gas Pressure 0.2 torr.

Sputtering Time 4 Minutes Film Thickness 0.09x10- 10-' m.

The coating produced under the above conditions has absorptance of 82% and emittance of 0.03 at room temperature.

For coating long glass tubes, the formation of the metal carbide film may be carried out by reactive sputtering using a long metal (e.g. stainless steel) electrode extending parallel to the axis of the tube and spaced about 10 mm from its surface. During sputtering, the tube is slowly rotated about its axis.

Argon and methane is caused to flow into the sputtering chamber in such a way that the same sputtering conditions are maintained at all points in a plane of sputtering through which simultaneous electrical discharge is taking place. This is achieved by arranging for the gas to flow at right angles to the plane from a perforated inlet pipe to a perforated outlet pipe both of which extend parallel to the electrode and which lie, respectively, on opposite sides of the sputtering plane. By carrying out the sputtering under these conditions the film deposited on the tube is of uniform thickness along its length.

The total film thickness is determined by the time during which sputtering occurs and the quality of the film by monitoring the electrical resistance of the film.

The formation of the copper substrate may be carried out by sputtering using a copper electrode extending parallel to the axis of the tube and using pure argon in the gas flow.

After formation of the copper substrate 6 and the metal carbide film 7, the collector tube so formed is sheathed in the permanentlv sealed vacuum envelope 10, which is made of baked pyrex or soda glass, together with the reflector 1. The reflector 1 is not, incidentally, essential.

The working fluid may comprise oil or another fluid which can be passed into a collector through a conduit extending into the collecting tube. The fluid then passes along the annular space between the conduit and the tube and out of the tube.

A number of such tubes are mounted side by side to form a heating array of a solar heating panel 20 as shown in Figure 3, which is mounted facing the sun and has the tubes hydraulicallv connected in series or in parallel in a closed circuit 21 containing a pump 22 and a heat exchanger 23.

WHAT WE CLAIM IS:- 1. A solar energy collector comprising a glass tube which is coated on its exterior surface with a substrate and a composite metal film; the substrate having a thickness of at least 0.05 X 10-6 m. and being composed of a metal having a low infra-red emittance (as hereinbefore defined), and the composite metal film comprising a metal-carbide which has a thickness between 0.04 x 10-6 m. and 0.20 X 10' m., which is deposited on the substrate bv a reactive sputtering process and which, when deposited, has an electrical resistance not greater than 100 KQ per square.

2. A solar energy collector as claimed in claim 1, wherein the composite metal film comprises atoms of iron, chromium, nickel and carbon which are deposited to form a film thickness of about 0.10x10-6 m.

3. A solar energy collector as claimed in claim 1 or claim 2 wherein the substrate comprises copper which is deposited on the glass tube surface by a sputtering process.

4. A solar energy collector substantially as hereinbefore described with reference to the accompanying drawings.

5. A method of making a solar energy collector, comprising the steps of depositing onto

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. electrical resistance per square of 10 kilohms to 1 megohm. The deposition process may be controlled to decrease the metal component proportion of the film with increasing thickness and to increase the carbon component proportion with increasing thickness. The thickness of the carbide film is such that the graph of reflectance (at near normal incidence of light) exhibits a minimum at wavelengths between 0.80X10-6 m. and 1.0 x 10-6 m. This corresponds to a metal carbide film thickness of about 0.09 X 10-6 m. in practice. The substrate can be manufactured on a small scale on a plane surface by placing a plane copper disc electrode parallel to and spaced from the surface in an inert gas such as argon. The surface is supported on a second plane disc electrode which is at zero electrical potential. The electrode separation from the surface is 17 mm and the electrode area is 7.5X10-3 m2. In these circumstances the following conditions are suitable for the deposition of the copper substrate. Electrode Material Copper Electrode Voltage -1200 V Gas Pure Argon Flow Rate As Fast As Possible Gas Pressure 0.2 torr Sputtering Time 5 Minutes For an electrode area of 7.5x 10-3 m2, the following conditions are suitable for the deposition of the iron carbide film. Electrode Material Iron (or stainless steel) Electrode Voltage - 1200 V Gas 1.6% (by volume) of methane in argon Flow Rate 0.25 cm3s-l at 1 atmosphere Gas Pressure 0.2 torr. Sputtering Time 4 Minutes Film Thickness 0.09x10- 10-' m. The coating produced under the above conditions has absorptance of 82% and emittance of 0.03 at room temperature. For coating long glass tubes, the formation of the metal carbide film may be carried out by reactive sputtering using a long metal (e.g. stainless steel) electrode extending parallel to the axis of the tube and spaced about 10 mm from its surface. During sputtering, the tube is slowly rotated about its axis. Argon and methane is caused to flow into the sputtering chamber in such a way that the same sputtering conditions are maintained at all points in a plane of sputtering through which simultaneous electrical discharge is taking place. This is achieved by arranging for the gas to flow at right angles to the plane from a perforated inlet pipe to a perforated outlet pipe both of which extend parallel to the electrode and which lie, respectively, on opposite sides of the sputtering plane. By carrying out the sputtering under these conditions the film deposited on the tube is of uniform thickness along its length. The total film thickness is determined by the time during which sputtering occurs and the quality of the film by monitoring the electrical resistance of the film. The formation of the copper substrate may be carried out by sputtering using a copper electrode extending parallel to the axis of the tube and using pure argon in the gas flow. After formation of the copper substrate 6 and the metal carbide film 7, the collector tube so formed is sheathed in the permanentlv sealed vacuum envelope 10, which is made of baked pyrex or soda glass, together with the reflector 1. The reflector 1 is not, incidentally, essential. The working fluid may comprise oil or another fluid which can be passed into a collector through a conduit extending into the collecting tube. The fluid then passes along the annular space between the conduit and the tube and out of the tube. A number of such tubes are mounted side by side to form a heating array of a solar heating panel 20 as shown in Figure 3, which is mounted facing the sun and has the tubes hydraulicallv connected in series or in parallel in a closed circuit 21 containing a pump 22 and a heat exchanger 23. WHAT WE CLAIM IS:-

1. A solar energy collector comprising a glass tube which is coated on its exterior surface with a substrate and a composite metal film; the substrate having a thickness of at least 0.05 X 10-6 m. and being composed of a metal having a low infra-red emittance (as hereinbefore defined), and the composite metal film comprising a metal-carbide which has a thickness between 0.04 x 10-6 m. and 0.20 X 10' m., which is deposited on the substrate bv a reactive sputtering process and which, when deposited, has an electrical resistance not greater than 100 KQ per square.

the external surface of a glass tube to a thick ness of not less than 0.05x10-6 1 & m. a sub- strate composed of a metal having a low infrared emittance (as hereinbefore defined), and depositing onto the substrate by a reactive sputtering process a metal carbide film, the metal carbide film being deposited to a thickness falling within the range 0.04X 10-6 m. to 0.20x 10-6 m. and the relative proportions of the metal and carbon atoms being controlled during the depositing process to provide for an electrical resistance, when the metal carbide film is deposited, which is not greater than 100 KQ per square.

6. A method of making a solar energy collector substantially as hereinbefore described with reference to the accompanying drawings.