GB2102025A - Selective solar energy absorber - Google Patents

Abstract

A selective absorber of solar energy is formed of an aluminium or aluminium alloy substrate (1) having a finely roughened surface (2) formed by electrolytic etching with an alternating current. The size of the roughening is selected so as to impart good absorptivity to the substrate, and is attained by suitably selecting the etching parameters. A blackened layer may be formed over the roughened surface by anodization or sputtering. <IMAGE>

Description

SPECIFICATION Selective solar energy absorber This invention relates to a selective absorber of solar energy and to a process for producing the absorber. In particular it relates to a selective absorber of solar energy which includes an aluminium or aluminium alloy substrate having good selective absorption characteristics.

With the recent concern over the depletion of fossil energy resources, active efforts are being made in every corner of the industry to develop technology for the effective utilisation of other sources of energy.

One of the most promising of these is solar energy, and various types of sun collectors have been designed. However, since solar energy is lowdensity energy, efficient collection of sunlight is essential for getting the necessary amount of energy. This requirement has generally been met by a collector the surface of which is provided with a selectively absorbing layer. This layer has a high absorptance in the short-wave range which corresponds to the spectral band of sunlight, and inhibits thermal radiation from a heated collector in the long-wave range.

Theoretically, there are four basic methods of improving selective absorption characteristics: 1) the use of the fundamental absorption of a semiconductor due to the transition of its band gap; 2) the use of the effect of interference in thin films to prevent light reflection; 3) the use of the plasma resonance absorption of fine metal particles; and 4) the formation of tiny ridges and recesses on the surface of a metal so that only sunlight undergoes multireflection. Actual collectors use one of these methods in combination with auxiliary techniques to enhance their respective merits, or use two or more of these methods to achieve synergistic effects.

Among the many layers that have been proposed to date, black nickel plating, a copper oxide layer and electrolytically-coloured aluminium are being used on a commercial scale because of their relatively high selective absorptance. However, consistent mass production of layers having good selective absorption characteristic is difficult, and moreover, such layers are very expensive.

An object of the present invention is to provide a selective absorber of solar energy, and an economical process for the mass production of a selective absorber which has good selective absorption characteristics.

According to one aspect of the invention, a selective absorber of solar energy comprises an aluminium or aluminium alloy substrate having a finely roughened surface formed by electrolytic etching using an alternating current.

According to another aspect of the invention, a process for producing a selective absorber of solar energy comprises finely roughening the surface of an aluminium or aluminium alloy substrate by electrolytic etching using an alternating current.

The etching may be followed by the formation of a selective blackened layer on the etched surface.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a cross-section of substrate on which ridges and recesses are formed by the process of the present invention, Fig. 2 is a schematic diagram of apparatus for carrying outthe etching process of the present invention, Figs. 3 and 4 are graphs showing the relationship between absorptance (absorptivity) and wavelength of selective absorbers prepared by the method of the present invention (Examples 1 to 4) and a comparative sample, and Fig. 5 is a graph showing the relationship of etching frequency to absorptance and emittance as observed when aluminium substrates were subjected to a.c. etching at varying frequencies according to the process of the present invention.

It is known that a substrate can be given selective absorption characteristics by forming specific shapes of ridges and recesses on the substrate surface on a micron scale. For example, it is generally established that if the surface of a substrate is given a regular pattern of ridges and recesses the crosssections of which resemble a sequence of geometrical figures, e.g the shape offunction of Gaussian distribution, and if each recess has an average diameter close to the wavelength of sunlight, whereas each ridge is sharp and has a suitable height, efficient absorption of solar energy can be achieved. This effect is due to the multi-reflection of the light within the recesses and the scattering from the ridges.The tiny ridges and recesses work as a smooth surface for the long-wave range of the thermal radiation, so it is generally understood that by using a substrate made of aluminium or other materials which exhibit high reflectance in the long-wave range, the substrate may be given improved selective absorption characteristics with minimum radiation of energy.

Several processes have been considered for forming fine ridges and recesses on the surface of aluminium: 1) mechanical, such as biasting; 2) chemical, such as chemical etching; 3) electrochemical, such as electrolytic etching; and 4) others, such as ion etching and sputtering. However, it is very difficult to produce a surface with the abovedescribed uniform ridges and recesses consistently and on an industrial scale (i.e. at low cost and in high volume).

The present inventors have found that electrochemical etching, especially electrolytic etching with an a.c. current, is advantageous for the purpose of the present invention. Electrolytic etching is performed by applying currentto the work, which is immersed in an aqueous solution of sodium salt or hydrochloric acid. This method of etching is divided into two types, namely d.c.etching and a.c. etching.

A.C. etching involves quite a few parameters, i.e.

formulation of the electrolyte, its temperature, the current density, frequency and waveform, and the present inventors have found that by selecting proper combinations of these parameters, a pattern of ridges and recesses having shapes preferred for efficient absorption of solar energy can be readily obtained.

In d.c. etching, the potential of the work (the aluminium) is held positive, so the direction of melting is determined by the structure of the aluminium crystal. Etching pits tunnel through the substrate, and part ofthe surface remains unetched. In consequence, deep pits are formed in various parts of the aluminium surface, and a surface with uniform ridges and recesses having the shapes required for the present invention cannot be produced. On the other hand, in a.c. etching, the potential ofthe work (aluminium) alternates between positive and negative, and the first etching cycle starts when the work assumes positive potential, and when its potential becomes negative a local increase in pH occurs due to increased current density, and an oxide or hydroxide thin film is formed on the surface of the work.

When the work re-assumes the positive potential, the next cycle of etching starts at a weak portion of the thin film. By repeating these cycles, ridges and recesses composed on a chain of generally cubic etching holes 2 formed by one etching cycle are formed throughout the surface of the aluminium substrate 1 as shown in Fig. 1, and the shape of the individual ridges and recesses essentially complies with the object of the present invention. Another advantage of a.c. etching is that the size of each etching hole 2 formed by one etching cycle can be controlled by changing the frequency of the a.c. current applied. The size of etching holes 2 can be freely controlled by changing the etching frequency, and therefore, a surface with ridges and recesses having optimum shapes for highly selective absorption of solar energy can be obtained by using properly selected etching frequencies.

D.C. etching has one defect that poses a serious problem in industrial applications; since the work must be held at positive potential throughout the application of current, the contact resistance between the work and the terminals or the resistance of the work itself greatly reduces the current that can be effectively used for etching. On the other hand, a.c.

etching enables mass production workpiecesto be treated with a large current by a non-contact indirect supply of power, as schematically shown in Fig. 2. In this figure, an aluminium substrate 12 is immersed in an etchant 10 with which an etching vessel 11 is filled, and current is supplied from an a.c. source to (typicaliy carbon) electrodes 13A, 13B positioned on opposite sides of the aluminium substrate 12.

In a preferred process, further improvements in the selective absorptance can be achieved by forming a selective blackened layer on the etched surface ofthesubstrate, and an oxidefilm helps to provide many desired effects, indicating fundamental absorption by transition of the band gap, interference with light, and resonance absorption due to plasma vibrations of fine metal particles during chemical or electrolytic colouration. Care must be taken to avoid the formation of an oxide film that impairs the acquired good selective absorption characteristics by dissolving or damaging the ridges and recesses formed by a.c. etching.

The present invention is now described in greater detail by reference to the following non-limiting examples.

Examples 1-3 Three samples of hard aluminium sheet (purity 99.5%) having a thickness of 1 mm were a.c. etched in 2.0 mol-litre of aqueous hydrochloric acid under the bath temperature, current density, frequency and charge density conditions noted in Table 1 below. In a comparative example, an identical sample or work was d.c. etched in an etching bath of the same formulation under the conditions listed in Table 1. The characteristics of the etched samples were evaluated by measuring the absorptance (a) in the visible range and the emittance (E) in the long-wave range, both factors being measured with a spectrophotometer. The absorptance profile of the four samples for a spectrum ranging from the visible to long-wave regions is shown in Fig. 3.

Table 1 Etching conditions Results Bath Current Freq. Charge temp. density (sinewave) density Absorptance Emittance Sample No. rC) (mA 1cm 2) (Hz) (c/cm2) (a) ,e) Example 1 70 250 50 5 0.82 0.20 Example2 80 200 10 5 0.84 0.25 Example3 60 500 120 10 0.85 0.29 Comparative D.C.

example 70 200 current 10 0.63 0.45 The above data and Fig. 3 show that the aluminium substrate samples in which fine ridges and recesses were formed by a.c. etching according to the present invention had higher absorptances (a) and lower emittances (E) than the comparative sam ple treated by d.c. etching. Therefore, the present invention is capable of producing a selective absorber of solar energy having good selective absorption characteristics.

Examples 4 and 5 It has been mentioned hereinabove that frequency is one of the important parameters for etching that determines the size of etching holes 2. An experiment was therefore conducted to examine the relationship between the etching frequency and selective absorption characteristics. Hard aluminium substrates of the same purity and thickness as used in Examples 1-3 were subjected to a.c. etching in a bath of aqueous hydrochloric acid (2.0 mol/l, 50"C, 250 mA/cm2) at varying frequencies. In Example 4 the charge density was 5 c/cm2, and in Example 5 it was 10 c/cm2. The results are depicted in the graph of Fig.

5, which shows the relationship between the etching frequency, the absorptance (a) and the emittance (E) of the treated samples.

As shown in Fig. 5, the absorptance (a) decreases at 10Hz or below. This is because, at 10Hz or below, the period for which current flows in the same direction in one cycle becomes so long that part of the surface remains unetched, that is, the etching being effected is not much different from d.c. etching. The absorptance also decreases significantly at 200Hz or higher, because the formation of very fine etching holes and insufficient etching due to rapid alteration between positive and negative potentials makes the surface of the work smoother and smoother.As for the emittance (E), better results (lower emittances) are obtained as the frequency is increased, but good selective absorptivity characteristics must meet both requirements, i.e. high absorptance (a and low emittance (e), so, to achieve the object of the present invention, the etching frequency is preferably in the range of from 10 to 300Hz. The characteristics of the work subjected to a.c. etching generally vary depending not only upon the frequency but also upon other etching parameters but, according to experiment, the preferred frequency range is sub stantiallythe same as defined above in spite of variations in other etching parameters.

An example in which a selective blackened layer was further formed on the etched surface of the substrate is described below.

Example 6 A hard aluminium substrate of the same purity and thickness as used in Examples 1 to 3 was subjected to a.c. etching under the same conditions as used in Example 1. The substrate was anodized in a nickel salt bath (15"C) at an a.c. voltage of 15 volts for 15 minutes to form a black layer on the etched surface. The resulting product had an absorptance (a) of 0.95 and an emittance (E) of 0.20, and, as shown in Fig. 4, it had satisfactory selective absorption characteristics for use as a selective absorber of solar energy.

In Examples 1 to 4, high-purity hard aluminium plates were used as the etching substrates and they were subjected to electrolytic etching in aqueous hydrochloric acid, but it is to be noted that the process of the present invention is by no means limited to these conditions. As mentioned hereinbefore, electrolytic etching with a.c. current has many parameters to be considered, so ridges and recesses of a desired shape can be produced by selecting proper combinations of these parameters. Therefore, the invention is applicable to a wide variety of substrates, from extremely high-purity aluminium to low-aluminium-content alloys, and from hard to soft aluminium or aluminium alloys. Ridges and recesses of more preferred shapes can be produced by effecting two or more a.c. etchings under different conditions.

Various methods can be used to form a selective blackened layer on the etched surface using various metal salt baths, such as a.c. anodization, the formation of oxide films, chemical colouration and metallisation (e.g. sputtering).

As described in the foregoing, the present invention is capable of forming desired ridges and recesses having good selective absorption characteristics on the surface of an aluminium or aluminium alloy substrate by means of controlling electrochemical parameters. Since the invention achieves its object by a.c. etching, a commercial power source, rather than a costly d.c. source, may be used. Moreover, the a.c. etching permits an indirect supply of current, so many substrates can be rapidly treated with a large current. When the work is a strip foil or an elongate sheet, a very efficient operation can be achieved by continuously supplying the work into the etching bath through rollers or the like. Since only one side of the work need be given a selective absorbing surface, two substrates can be treated at the same time by putting them back to back and feeding them into the etching bath as a single substrate.

As described above, by use of the present invention selective absorbers of solar energy having good selective absorption characteristics can be produced in quantity within a given time period and at low cost.

Claims (12)

1. A selective absorber of solar energy, comprising an aluminium or aluminium alloy substrate having a finely roughened surface formed by electrolytic etching using an alternating current.

2. An absorber as claimed in Claim 1, wherein the roughened surface is provided with an overlying selective blackened layer.

3. Apparatus for the selective absorption of solar energy, comprising an aluminium-containing substrate provided with a pattern of ridges and recesses, the recesses having an average diameter substantially equal to the wavelength of the solar energy, and formed by alternating current electrolytic etching.

4. A process for producing a selective absorber of solar energy, comprising finely roughening the surface of an aluminium or aluminium alloy substrate by electrolytic etching using an alternating current.

5. A process as claimed in Claim 4, further comprising forming a selective blackened layer on the roughened surface.

6. A process as claimed in Claim 5, wherein the blackened layer comprises an oxide layer formed on the substrate.

7. A process as claimed in Claim 5, wherein the blackened layer is formed by anodization.

8. A process as claimed in Claim 5, wherein the blackened layer is formed by sputtering.

9. A process as claimed in any one of Claims 4 to 8, wherein the aluminium or aluminium alloy substrate is in the form of a foil strip; and wherein the strip is continuously conveyed through an electrolytic etching bath while being finely roughened by electrolytic etching using an alternating current.

10. A process as claimed in any one of Claims 4 to 9, wherein the surface of the substrate is finely roughened by effecting two or more electrolytic etchings under different etching conditions.

11. A process as claimed in any one of Claims 4 to 10, wherein the frequency of the alternating cur rent is in a range of between 10 and 300Hz.

12. A process as claimed in Claim 4 and substantially as hereinbefore described with reference to the accompanying drawings.