WO2012059789A1

WO2012059789A1 - Roll to roll manufacturing of solar selective sheets

Info

Publication number: WO2012059789A1
Application number: PCT/IB2010/055006
Authority: WO
Inventors: Figen Kadirgan
Original assignee: SELEKTIF TEKNOLOJI SANAYI TICARET Ltd SIRKETI
Current assignee: SELEKTIF TEKNOLOJI SANAYI TICARET Ltd SIRKETI
Priority date: 2010-11-04
Filing date: 2010-11-04
Publication date: 2012-05-10
Anticipated expiration: 2013-05-04

Abstract

In this invention, a method for continuously surface treating and coating of foils (metals such as copper, aluminium, steel or other metals or non-metal materials) to able to have selective properties against solar spectrum and to improve the productivity of selective surface fabrication is provided. The method for continuously surface treating to able to obtain the solar selective surface with high absorption and low emission coefficients contain the treatment of the metal foils by chemical and electrochemical way to clean the foil surface and then, electrochemical deposition of the thin and/or nano scale films passing the metal foil as cathode in front of anodes via rolls and prevention a liquid attached on the surface from being dragged into the next tank. The apparatus has two rolls (to wind and rewound again) with a certain larger diameter made of stainless steel and the guide rolls with smaller diameters between special surface treatment, depositing and dielectric layer coating tanks. A heat treatment performed on the surface by an oven before rewound the foil. A uniform, adherent selective deposit obtained on a conductive foil substrate.

Description

ROLL TO ROLL MANUFACTURING OF SOLAR SELECTIVE SHEETS

Technical field

This invention relates to a method of coating metal foils, continuously using electrochemical pretreatment and deposition methods and sol-gel techniques especially for high efficient solar collectors, with selectively absorbent films and a product obtained thereof. The disclosure provides an improved solar selective multilayer coating having thermal stability up to 240°C and a process therefore. According to the disclosure, a tandem stack of three layers of nickel, coloured nickel and Silane is deposited on metal and non-metal substrates using a roll to roll process. While the first two layers function as the absorber layer, the third layer function as antireflection layer. In order to impart additional durability to interference layers according to embodiments of the present invention, it is desirable to heat treat the foil.

Background of the invention

Various methods have been used to achieve thermal radiation control of objects by selectively controlling the object's reflectivity to infrared radiation. Methods generally involve applying a coating to an object. Thermal radiation controlled objects are used in solar collector absorber panels. For solar energy collection, the surface coating has to absorb radiation corresponding to the sun's solar emission spectrum that is principally from 300 to 2500 nanometers, while having a high reflectivity at longer such as above approximately 4 microns. This allows the object to absorb and retain the sun's heat because the decreased emittance at the thermal wavelengths. Selective absorption light in the solar ultraviolet and near infrared regions of the wavelength can be achieved by over coating the reflective metallic surface with materials that selectively absorb solar wavelength. It is important for solar collectors that the absorbance rate is high and the emittance rate is low in the respective regions mentioned; because, high absorbance rate ensures that most of solar energy is converted to thermal energy in the metal substrate whereas the low emittance rate prevents loss of the gained thermal energy by radiation from the surface in the far infrared region of the spectrum. Metal substrates having an absorbance to emittance ratio higher than 4 can be fabricated to provide that the heat absorbed is transferred to fluid conduits in the metal body for subsequent use in heating and cooling operations.

Aluminum, copper, low carbon steel (or the alloys of these metals) or other metals can be used as the metal substrate in high efficiency solar absorbers. Coated aluminum absorbers are especially valuable for heat exchange units because of the lightness of aluminum and the resultant decrease in the complexity and weight of structural elements required for support thereof. Moreover, the easy machining and fabrication of them add to their value. Coated copper absorbers are also very valuable for heat exchange units and more resistant to the corrosion, in order to overcome the weight problem, it is possible to use very thin layers of copper as substrate. On the other hand, coated low carbon steel absorbers are interesting from the cost point of view. The most common methods for forming such surfaces are by electrochemical deposition techniques followed by chemical oxidation of the deposit or vacuum techniques.

Black nickel coatings are known as solar selective surfaces for the conversion of solar radiation into useful heat (J. Jurisson, et al, J. Vac. Sci. Technol. 12: 1975, 1010; H.Tabur, Low Temperature Engineering Application to Solar Energy, ed. R.C. Jordan, New York: 1969, p. 41; R.B. Pettit, R.R. Sowell, J.Vac.Sci. Technd. 13: 1976, 596, Ewa Wackelgard Solar Energy Mat. and Solar Cells, 56, 1998, 35- 44). These coatings possess good optical properties but the durability shown is often poor. They degrade completely after exposure to 200°C.

For instance, the invention disclosed in US patent no. 2,917,817 consists in a receiver for solar heaters, which is basically a composite body comprising a metal base and a selectively absorbent thin coating applied to the said base. The nickel base is immersed into an aqueous electrolytic bath containing nickel sulphate, zinc sulphate, ammonium sulphate, ammonium tiocyanate, and citric acid. The bath temperature is about 30°C. When the base is aluminium, it is first covered with an oxide layer; then immersed in a solution of copper nitrate, nitric acid, and potassium permanganate at 85-90°C; and finally dried and heated to 450°C so that the surface colour becomes almost black. US patent no. 4,177,325 discloses a panel for selectively absorbing solar energy and a method of producing thereof. This panel comprises an aluminium substrate, a layer of zinc thereon, a layer of nickel over the zinc layer, and a layer of nickel oxide. Copper substrate may well be used for the same purpose. The chemical oxidation of the nickel layer is performed at temperatures as high as 900-950°F. US patent no. 3,920,413 discloses that an aluminium metal substrate can be cleaned, prepared to receive a brightening layer, coated with the brightening layer and further coated with a very thin solar thermal energy absorbing coating of black nickel. The bath temperature is in the range from about 115° to about 140°F.

US patent no. 4,088,547 explain a dendrite copper deposition on copper surface and black nickel deposition as directed in Metal Finishing Handbook, 1974 Ed., p.350 for 5 min. The emissivity of the surface was 29 for this coating.

US patent no. 4,244,790 describe a process and aqueous composition for electrodepositing black nickel deposit directly on conductive bright copper, nickel, brass, cadmium and the like. The aqueous solution is of a pH ranging from about to 8 to 12 and contains nickel ions in combination with soluble amines at 25-50°C between 1-10 minutes over a current density range of 2-25 amperes. No any explanation on spectral properties and durability of the surface is given in the invention.

European patent no. 0029257 explains a black, selectively absorbing layer consisting of finely comminute nickel, nickel-zinc or nickel-lead on a substrate, particularly a solar collector plate, which is protected from corrosion by a thin passivating chromic-oxide layer. Corrosion resistance in this invention is mainly achieved by the application of the chromate-layer.

In another patent PCT/TR 2003/000081 and TR 2006/02074 metal substrate panel is coated with a layer of nickel and an outermost layer of absorbent nickel layer by electrochemical batch process without any coating to provide the corrosion protection. Such processes are well-known form prior art documents by discontinuous electrochemical batch process. Additionally, it is mentioned in the literature that nickel black selective surfaces deteriorate in time and lose their optical properties under heat and moisture. Especially, surfaces heated up to 200°C lose their selectivity completely by the oxidation of copper or other metals. These effects considerably reduce the service lives of solar collectors.

A roll unit for use of surface treatment of copper foil defined in US patent no: 2010/0018273. Generally, the electrochemical surface treatment of a copper foil is continuously performed by using a copper foil winded around a coil passing in front of opposed anodes via several upper and lower rolls arranged inside and outside of electrolytic tanks by being rewound and subject to surface treatment. Current for surface treatment is flowed between anode and the copper foil as the cathode via the electrolytic solution. Electrolytic solutions such as copper sulphate and chromic acid used in the surface treatment may causes a corrosive

environment with the rolls used inside and outside of electrolytic baths infiltrating into the bearing and causing the corrosion. Moreover, as the small diameter squeeze rolls used before the foil is winded around the coil, the copper powder deposited on the squeeze rolls may cause the contamination of the surface.

According to this invention, a roll unit capable of inhibiting abrasion and corrosion of the roll shaft and the bearing and capable of simple replacement of bearing box, bearing and other components is provided. This roll unit can be used under a corrosion environment caused by the adhesion of treatment liquid and the corrosive mist generated from the surface treatment liquids are developed.

Summary of the invention

The objective of the present invention is to obtain a highly efficient, corrosion resistant high efficient solar collectors surface with a long service life and low costs. Another objective of the present invention is to provide efficient solar energy collection absorbing material with low thermal emittance in color (other than black) under the electrochemical deposition conditions.

For practical purposes, a selective absorber should have good optical properties, good thermal stability, low cost and ease of large-scale production. Keeping these considerations in view, in the present invention continuous roll to roll electrochemical deposition techniques are proposed for the preparation of nickel based absorber surfaces, at different stages.

In this invention the metal substrate foil winded around a coil, subject to surface treatment such as cleaning, electrochemical degreasing and brightening. Then, it is continuously coated with a layer of nickel and an outermost layer of absorbent nickel oxide-nickel zinc sulphide mixture in front of nickel anodes via several squeeze roll arranged outside of electrolytic tanks. Selective coating coated copper foil is immersed in a tank containing a dielectric material to provide the environmental protection and after the thermal treatment the foil is winded around the coil once again via a guide roll. Copper, low carbon steel or steel foils may treat according to this procedure. However aluminium foils are first coated with a layer of zinc before the nickel layer, and the nickel layer is applied thereon. The solar absorbance and emittance of the resulting selective surfaces were found to be 0.95and 0.06, respectively. The color may change from the dark blue to black according to the deposition conditions.

Detailed description of the invention

The present invention was contrived in view of the problems relating the low cost selective surface in mass production with easy machining and relates to a roll to roll unit to be used in a process that continuously perform chemical and electrochemical surface treatments and selective surface texture on a surface of rolled copper, aluminium or steel foil. The electrochemical surface treatment and formation of selective layers by electro deposition is continuously performed by using an apparatus as shown in Figure 1. Figure 1 shows a simple schematic diagram from the top of the continuous roll to roll process. The electrolytic and chemical treatments tanks are provided with a reserve tanks and circulation pumps to assure the constant level of electrolyte in the tanks with circulation of the solutions that was flow in during the process from reserve tank to electrolytic tank. Therefore, electrolytic bath composition in the electrolytic tanks was kept as that of their initial composition. The electrolytic tanks are provided with a slot corresponding to the width of the foil to ensure the foil crossing from one bath to the other. In order to prevent the solution in a former bath from getting into a subsequent bath between mutual surface treatment or selective film formation baths or between these baths and rinsing baths, or between the foregoing baths and winding unit or the solution adhered to the surface of treated foil, the treated foil is passed through the rubber pieces, fitted to the slots in electrolytic tanks. The width of anodes both for electrochemical degreasing and selective surface preparation baths are chosen according to the surface treatment width of the copper foil. Current for surface treatment and selective film formation is flowed between the anode and copper foil as the cathode via electrolytic solution. Between each electrolytic bath a pair of rolls is supplied as guide roll. Selective film coated copper foil is passed through a triple small diameter rolls to roll up and wind around the coil once again. The rotating guide rolls having slight convex and concave shape is necessary to provide not only the straightening of the metal sheet but also to avoid the falling down of the metal foils because of the gravity effect. The distance between two rolls is adjusted to keep away the structural deformation of the surface relatively to the foil thickness. A geared motor gives the rotating movement to the rolls.

The invention provides an optical structure having an infrared reflective layer and a nano layer with low emissivity in the infrared region. These layers are separated by a thin film spacer of a dielectric or semiconductor material. The reflectivity and transmission of the layer are selectively controlled through the thickness of the layers such that color may be independent of the infrared properties of the absorber and reflector layer. Some semiconductors are highly desirable as thin film spacer because these materials have low enough refractive indices to keep surface reflectivity at a minimum. Among such useful semiconductor materials are copper oxide, iron oxides, chromium oxides, nickel oxide, complexes of nickel-zinc sulfide, lead sulfide and so forth. One can achieve the dark colors by changing the thickness of the interference design. It may be advisable to overcoat the optical stack with a dielectric layer that does not significantly alter the optical properties of the stack, but provides environmental protection to the stack. When the thickness of such an overcoat dielectric layer is less than about 8-10 times of layer of the semiconductor, this layer can affect the color. As the dielectric thickness increases, the color may change from black dark blue.

All the metallic foils must be clean before the electrochemical deposition processes. They could clean by the following steps:

· Degreasing in an appropriate aqueous solution of alkaline cleaner containing NaOH, sodium silicates or sodium carbonates at 60-90°C for 10 minutes;

• Electrochemical degreasing in an appropriate aqueous solution of alkaline cleaner containing NaOH, sodium silicates or sodium carbonates at 60°C,

• Immersing in an appropriate brighter dip solution such as phosphoric acid 50%, nitric acid 10% and sulphuric acid 40% at 90-95°C;

• Soaking at 50-70°C for 30 seconds in an alkaline cleaner;

• Getting into diluted acid solution (e.g. 40-55 % nitric acid) at room

temperature.

During the cleaning process, the metal foils are washed with deionized water between each of the steps.

Having cleaned, the metal substrates are treated according to the following method:

• Nickel plating with a solution containing nickel sulfamate, sulphate,

phosphate or chloride at 1-20 A/dm at 40-60°C for 10 - 30 minutes.

· Absorbent nickel oxide-sulphide plating with a solution containing nickel sulfamate, sulphate, chloride, phosphate or nitrate of the following

composition:

nickel salt 75-150 g/1,

- ammonium chloride, sulphate, phosphate or nitrate 11-35 g/1,

- zinc chloride, sulphate, phosphate or nitrate 8-35 g/1,

- sodium or potassium tiociyanide or sulphide 3-18 g/1,

Electroplating is performed for 0.1-15 min., at 20-35°C, under a potential difference of 1-20 A/dm .

For the environmental protection of the stack, water soluble Silan solution is used. The foil coated with nickel stack is immersed in 1 to 15 % Silan containing solution. Then, a heat treatment between 180 -300°C is performed. The resulting coating is deep blue-black in color and the absorptivity of the specimens is 0.93-0.97 whereas the emissivity is 0.05-0.12.

If the metal substrate is aluminium, between the cleaning and nickel plating steps, a zincating step is performed.

The invention will now further be explained with reference to the following specific examples for a clearer understanding of this invention, which are merely illustrative and are not to be understood as limiting the scope and underlying principles of this invention in any way.

Example 1 :

Aluminium substrates or its alloys are converted to selective absorbers for solar energy by the following steps:

degreasing in an alkaline solution for 10 minutes at 60-70°C

neutralising with 40-50% nitric acid at room temperature

immersing in bright dip solution at 90-95°C for 8-10 minutes

immersing in alkaline solution at 50-70°C for 30 seconds

• neutralising with 40 or 50% nitric acid at room temperature

• zincating with a solution containing zinc oxide (30-60 g/1), sodium cyanide (40- 80 g/1) and sodium hydroxide (60-90 g/1) at 25-50 °C

• nickel plating with a solution containing nickel sulfamate, sulphate, phosphate or chloride of following composition:

- nickel content 65-90 g/1,

- boric acid 30-45 g/1,

- anti-pitting agent 0.5 - 1 g/ 1 ,

Electroplating is done at 1-20 A/dm² at 40-60°C for 10-30 minutes.

· Absorbent nickel plating with a solution containing nickel sulfamate, sulphate, chloride, phosphate or nitrate of the following composition:

- nickel salt 75-150 g/1,

- ammonium chloride, sulphate, phosphate or nitrate 11-35 g/1,

- zinc chloride, sulphate, phosphate or nitrate 8-35 g/1,

- sodium or potassium tiociyanide or sulphide 3-18 g/1,

Electroplating is done at 20-35°C, under 1-20 A/dm . The foil coated with nickel stack is immersed in 1 to 15 % Silan containing solution. Then, a heat treatment between 180 -300°C is made.

The resulting selective coating is deep blue-black in color and the absorptivity of the specimens is 0.93-0.97 and the emissivity is 0.05-0.12.

Example 2:

Copper substrates or its alloys are converted to selective absorbers for solar energy by

the following steps:

• degreasing in an alkaline solution for 10 minutes at 60-70°C

· neutralising with 40-50% nitric acid at room temperature

• immersing in bright dip solution at 90-95°C for 8-10 minutes

• immersing in alkaline solution at 50-70°C for 30 seconds

• neutralising with 40 or 50% nitric acid at room temperature

- nickel content 65-90 g/1,

- boric acid 30-45 g/1,

- anti-pitting agent 0.5 - 1 g/ 1 ,

Electroplating is done at 1-20 A/dm² at 40-60°C for 10 - 30 minutes.

- nickel salt 75-150 g/1,

- ammonium chloride, sulphate, phosphate or nitrate 11-35 g/1,

- zinc chloride, sulphate, phosphate or nitrate 8-35 g/1,

- sodium or potassium tiociyanide or sulphide 3-18 g/1,

Electroplating is done at 20-35°C, under 1-20 A/dm².

The foil coated with nickel stack is immersed in 1 to 15 % Silan containing solution. Then, a heat treatment between 180 -300°C is made.

The selective coating is deep blue-black in colour and the absorptivity of the specimens is 0.93-0.97 and the emissivity is 0.05-0.12.

Example 3: Low carbon steel or steel substrates or its alloys are converted to selective absorbers for solar energy by the following steps:

• degreasing in an alkaline solution for 10 minutes at 60-70°C

• neutralising with 40-50% nitric acid at room temperature

• immersing in bright dip solution at 90-95°C for 8-10 minutes

• immersing in alkaline solution at 50-70°C for 30 seconds

• neutralising with 40 or 50% nitric acid at room temperature

- nickel content 65-90 g/1,

- boric acid 30-45 g/1,

- anti-pitting agent 0.5 - 1 g/ 1 ,

Electroplating is done at 1-20 A/dm² at 40-60°C for 10 - 30 minutes,

•nickel plate with a solution containing nickel sulfamate, sulphate,

chloride,phosphate or nitrate of the following composition:

. nickel salt 75-150 g/1,

ammonium chloride, sulphate, phosphate or nitrate 11- 35 g/1,

zinc chloride, sulphate, phosphate or nitrate 8-35 g/1, sodium or

potassium tiociyanide or sulphide 3-18 g/1,

Electroplating is done at 20-35°C, under 1-20 A/dm .

The selective coating is deep blue-black in color and the absorptivity of the specimens is 0.93-0.97 and the emissivity is 0.05-0.12.

Claims

A roll to roll, continuous method comprising:

a. The metal substrate foil winded around a coil, subject to surface treatment and selective layer deposition in front of anodes via several guide rolls arranged outside of electrolytic tanks.

b. The electrolytic and chemical treatments tanks are provided with a reserve tanks and circulation pumps to assure the constant level of electrolyte in the tanks with circulation of the solutions that was flow in during the process from reserve tank to electrolytic tank. c. The electrolytic tanks are provided with a slot corresponding to the width of the foil to ensure the foil crossing from one bath to the other

d. The treated foil is passed through the rubber pieces, fitted to the slots in electrolytic tanks.

e. The rotating guide rolls having slight convex and concave shape is necessary to provide not only the straightening of the metal sheet but also to avoid the falling down of the metal foils because of the gravity effect. The distance between two rolls is adjusted to keep away the structural deformation of the surface relatively to the foil thickness.

A method of depositing a selectively absorbent film on a metal substrate panel, comprising the steps of

a. cleaning, degreasing and brightening the metal substrate b. coating with a first layer of nickel

and characterised in that the method further includes the step of:

c. coating with an outermost layer of absorbent nickel oxide-nickel zinc sulphide layer by electrochemical deposition at relatively low temperatures.

The method of Claim 1, characterized in that the metal substrate is selected from the group consisting of copper, low carbon steel, steel and their alloys.

The method of Claim 1, characterized in that the metal substrate is aluminium.

The method of Claim 3, characterized in that the substrate is coated with a layer of zinc before the nickel plating step.

One can achieve the dark colors by changing the thickness of the interference design. It may be advisable to overcoat the optical stack with a dielectric layer that does not significantly alter the optical properties of the stack, but provides environmental protection to the stack. When the thickness of such an overcoat dielectric layer is less than about 8-10 times of layer of the semiconductor, this layer can affect the color. As the dielectric thickness increases, the color may change from black dark blue.

The method of any of the Claims above, characterized in that the resulting coating is deep blue-black in color and the absorptivity of the specimens is 0.93-0.97 whereas the emissivity is 0.05-0.12.

The method according to any of the claims above, characterized in that the cleaning step comprises the steps of:

a. Degreasing in an appropriate aqueous solution of alkaline cleaner containing NaOH, sodium silicates or sodium carbonates at 60- 90°C for 10 minutes;

b. Immersing in an appropriate brighter dip solution such as phosphoric acid 50%, nitric acid 10% and sulphuric acid 40%> at 90-95°C;

c. Soaking at 50-70°C for 30 seconds in an alkaline cleaner; d. Dipping in a diluted acid solution (e.g. 40-50 % nitric acid) at room temperature.

9. The method of Claim 6 above characterized in that the metal substrates are washed with de-ionized water between each of the steps of the cleaning process.

10. The method of any of the claims above, characterized in that the nickel plating is performed with a solution containing nickel sulfamate, sulphate, phosphate or chloride at 1-20 A/dm at 40-60°C for 10 - 30 minutes.

11. The method of any of the claims above, characterized in that the absorbent nickel oxide-nickel sulphide plating is performed with a solution containing nickel sulfamate, sulphate, chloride, phosphate or nitrate of the following composition:

a. nickel salt 75-150 g/1,

b. ammonium chloride, sulphate, phosphate or nitrate 11-35 g/1, c. zinc chloride, sulphate, phosphate or nitrate 8— 35 g/1, d. sodium or potassium tiociyanide or sulphide 3-18 g/1, wherein the electroplating is performed for 0.1-15 min., at 20-35°C, under a potential difference of 1-20 A/dm .

12. The method of any of claims above, characterized in that the nickel stack is immersed in aqueous Silan (1-15 % ) solution. Then, a heat treatment between 180 -300°C is made.