WO2009140051A2

WO2009140051A2 - Methods and compositions for coating devices

Info

Publication number: WO2009140051A2
Application number: PCT/US2009/041845
Authority: WO
Inventors: Michael D. Newman; Glen Newman; Stan Pozehl; Ian Newman
Original assignee: Ausra, Inc.
Priority date: 2008-05-14
Filing date: 2009-04-27
Publication date: 2009-11-19
Also published as: WO2009140051A3

Abstract

Methods and apparatus for coating metallic substrates (such as substrates used as receivers in solar energy collection systems) are disclosed herein. The substrate is coated with a barrier layer, a solar absorption layer, and an optional overcoat layer.

Description

METHODS AND COMPOSITIONS FOR COATING DEVICES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to U.S. Application Serial Number 61/127,782 filed May 14, 2008 and entitled "METHODS AND COMPOSITIONS FOR COATING DEVICES", inventors Michael D. Newman, et al; U.S. Application Serial Number 61/127,890 filed May 16, 2008 and entitled "METHODS AND COMPOSITIONS FOR COATING DEVICES", inventors Michael D. Newman, et al.; and U.S. Application Serial Number 61/131,816 filed June 11, 2008 and entitled "METHODS AND COMPOSITIONS FOR COATING DEVICES", inventors Michael D. Newman, et al., the contents of which are incorporated by reference in their entirety herein as if put forth in full below.

BACKGROUND OF THE INVENTION

[0002] Solar selective absorption coatings applied to solar absorber components are useful in the production of receivers for solar energy collection systems. Desirably, the solar coatings have a high solar absorptivity (e.g., high amount of solar energy collected) and a low emissivity (e.g., low amount of energy lost). Such coatings significantly improve the thermal conversion efficiency of receivers by reducing radiative energy losses from the absorbing components.

[0003] Such coatings tend to be mechanically fragile, leading to manufacturing and maintenance difficulties. Improved methods and apparatus are needed for minimizing contact with fragile solar selective absorption coatings and for drying such coatings without cracking them. Economical methods that minimize the amount of time and the amount of chemicals required to coat substrates are desirable. Solar receivers that have less migration of metal from the substrate into the solar absorption layer are also desirable since this migration can degrade the solar absorption layer.

BRIEF SUMMARY OF THE INVENTION

[0004] In one aspect, the invention features methods of applying a solar absorption layer onto a substrate. In some embodiments, the method includes (a) applying a solar absorption layer onto at least a portion of a metallic substrate and (b) applying an overcoat layer (such as a sol-gel layer) onto at least a portion of the solar absorption layer. In some embodiments, the substrate changes orientation between step (a) and step (b). In some embodiments, the substrate is orientated substantially vertically during step (a) and oriented substantially horizontally during step (b). In some embodiments, two substantially vertical rollers (e.g., vertical rollers) and one substantially horizontal roller (e.g., a horizontal roller) are used to change the orientation of the substrate from a substantially vertical to a substantially horizontal orientation. In some embodiments, the method also includes bipolar electrocleaning the substrate prior to step (a). In some embodiments, the method also includes activating the substrate (i) to promote adherence of a barrier layer to the substrate, (ii) to promote nucleation of the barrier layer, or (iii) to promote both adherence and nucleation of the barrier layer prior to adding the barrier layer. In some embodiments, the method also includes applying the barrier layer onto at least a portion of the substrate prior to step (a). In some embodiments, the substrate is orientated substantially vertically during the bipolar electrocleaning, activation of the substrate, or application of the barrier layer.

[0005] In some embodiments, the method includes (a) activating a metallic substrate (i) to promote adherence of a barrier layer to the substrate, (ii) to promote nucleation of the barrier layer, or (iii) to promote both adherence and nucleation of the barrier layer, (b) applying a barrier layer onto at least a portion of the substrate, and (c) applying a solar absorption layer onto at least a portion of the barrier layer. In some embodiments, the method also includes the step of (d) applying an overcoat layer (such as a sol-gel layer) onto at least a portion of the solar absorption layer.

[0006] In some embodiments, the method includes (a) incubating a metallic substrate in a solution comprising nickel sulfamate under conditions sufficient to apply a nickel layer onto at least a portion of the substrate, and (b) applying a solar absorption layer onto at least a portion of the nickel layer. In some embodiments, the method also includes the step of (c) applying an overcoat layer (such as a sol-gel layer) onto at least a portion of the solar absorption layer.

[0007] In some embodiments, the method includes (a) applying a barrier layer comprising at least one of the group consisting of a brightening agent and a leveling agent onto at least a portion of a metallic substrate, and (b) applying a solar absorption layer onto at least a portion of the barrier layer. In some embodiments, the method also includes the step of (c) applying an overcoat layer (such as a sol-gel layer) onto at least a portion of the solar absorption layer.

[0008] In one aspect, the invention features apparatus, such as apparatus useful for applying a solar absorption layer onto a substrate. In some embodiments, the apparatus includes (a) a metallic substrate, (b) a barrier layer comprising at least one of the group consisting of a brightening agent and a leveling agent on at least a portion of the substrate, and (c) a solar absorption layer on at least a portion of the barrier layer.

[0009] The invention features additional methods of applying a solar absorption layer onto a substrate. In some embodiments, the method includes (a) inserting a metallic substrate comprising a substantially cylindrical structure into a cell comprising a solution capable of generating a solar absorption layer, and (b) applying an electric current between the substrate and an anode encircling at least a portion of the substrate such that the solar absorption layer is deposited on at least a portion of the barrier layer. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ^CC for at least about 200 hours. In some embodiments, the solar absorption layer does not comprise chromium, black chrome, or a solar paint. In some embodiments, the method includes applying a barrier layer onto at least a portion of the substrate prior to step (a). In some embodiments, the method includes using an air knife (such as an air wipe or a stream of air) to remove a portion of the solution from the substrate. In some embodiments, the method includes using a forced hot air dryer to dry (e.g., to partially or completely dry) at least a portion of the substrate. In some embodiments, the method includes applying an overcoat layer (such as a sol-gel layer) onto at least a portion of the solar absorption layer.

[0010] In some embodiments, the method includes (a) inserting a metallic substrate comprising a substantially cylindrical structure into a cell comprising a solution capable of generating a solar absorption layer, (b) electroplating the solar absorption layer onto at least a portion of the barrier layer, and (c) removing the substrate from the cell through an outlet in the cell, wherein the outlet comprises a hydraulic seal. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ^CC for at least about 200 hours. In some embodiments, the method includes applying a barrier layer onto at least a portion of the substrate prior to step (a). In some embodiments, the method includes using one or more air knives to remove a portion of the solution from the substrate used to form a strike layer, a barrier layer, a solar absorption layer, and/or an overcoat layer. In some embodiments, the method includes using a forced hot air dryer to dry (e.g., to partially or completely dry) at least a portion of the substrate after depositing e.g. a barrier layer, a solar absorption layer, and/or an overcoat layer. In some embodiments, the method includes applying an overcoat layer (such as a sol-gel layer) onto at least a portion of the solar absorption layer.

[0011] In some embodiments, the method includes (a) inserting a metallic substrate comprising a substantially cylindrical structure into a cell comprising a solution capable of generating a solar absorption layer, and wherein the cell has a device (e.g., a liquid level control system or overflow system) for maintaining a substantially constant amount of solution in the cell, and (b) electroplating the solar absorption layer onto at least a portion of the barrier layer. In some embodiments, the device for maintaining a substantially constant amount of solution in the cell keeps the cell full of solution. In some embodiments, the device keeps air out of the cell. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the method includes removing the substrate from the cell through an outlet in the cell, wherein the outlet comprises a hydraulic seal. In some embodiments, the method includes applying a barrier layer onto at least a portion of the substrate prior to step (a). In some embodiments, the method includes using an air knife to remove a portion of the solution from the substrate. In some embodiments, the method includes using a forced hot air dryer to dry (e.g., to partially or completely dry) at least a portion of the substrate after depositing, e.g., a barrier layer, a solar absorption layer, and/or an overcoat layer. In some embodiments, the method includes applying an overcoat layer (such as a sol-gel layer) onto at least a portion of the solar absorption layer.

[0012] In some embodiments, the method includes (a) inserting a metallic substrate comprising a substantially cylindrical structure through an inlet in a cell comprising a solution capable of generating a solar absorption layer, wherein the inlet comprises a first seal that reduces the amount of solution that leaves the cell through the inlet, (b) electroplating the solar absorption layer onto at least a portion of the barrier layer, and (c) removing the substrate from the cell through an outlet in the cell. In some embodiments, the outlet comprises a hydraulic second seal that allows at least a portion of the solution to leave the cell through the outlet, and at least a portion of the solution that leaves the cell through the outlet is reintroduced into the cell. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the method includes applying a barrier layer onto at least a portion of the substrate prior to step (a). In some embodiments, the method includes using an air knife to remove a portion of the solution from the substrate. In some embodiments, the method includes using a forced hot air dryer to dry (e.g., to partially or completely dry) at least a portion of the substrate. In some embodiments, the method includes applying an overcoat layer (such as a sol-gel layer) onto at least a portion of the solar absorption layer.

[0013] In one aspect, the invention features systems, such as systems useful for applying a solar absorption layer onto a substrate. In some embodiments, the system includes (a) a cell comprising a solution capable of generating a solar absorption layer, and (b) an anode capable of encircling at least a portion of a metallic substrate comprising a substantially cylindrical structure. In some embodiments, the system further includes the substrate. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the solar absorption layer does not comprise chromium or black chrome. In some embodiments, the system includes an air knife for removing a portion of the solution from the substrate. In some embodiments, the system includes a forced hot air dryer for drying (e.g., partially or completely drying) at least a portion of the substrate. In some embodiments, the system includes a fog nozzle for applying the overcoat layer (such as a sol-gel layer).

[0014] In some embodiments, the system includes (a) a cell comprising a solution capable of generating a solar absorption layer, and (b) an outlet in the cell for removing a metallic substrate comprising a substantially cylindrical structure from the cell. In some embodiments, the outlet comprises a hydraulic seal. In some embodiments, the system further includes the substrate. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the system includes an air knife for removing a portion of the solution from the substrate. In some embodiments, the system includes a forced hot air dryer for drying (e.g., partially or completely drying) at least a portion of the substrate. In some embodiments, the system includes a fog nozzle for applying the overcoat layer (such as a sol-gel layer).

[0015] In some embodiments, the system includes (a) a cell comprising a solution capable of generating a solar absorption layer, (b) an inlet in the cell for inserting a metallic substrate comprising a substantially cylindrical structure into the cell, (c) an outlet in the cell for removing the substrate from the cell, and (d) a device (e.g., a liquid level control system or overflow system) operably connected to the cell that is capable of maintaining a substantially constant amount of solution in the cell. In some embodiments, the system further includes the substrate. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the system includes an air knife for removing a portion of the solution from the substrate. In some embodiments, the system includes a forced hot air dryer for drying (e.g., partially or completely drying) at least a portion of the substrate. In some embodiments, the system includes a misting or fog nozzle for applying the overcoat layer (such as a sol-gel layer). In some embodiments, the system includes an anode encircling at least a portion of the substrate.

[0016] In some embodiments, the system includes (a) a cell comprising a solution capable of generating a solar absorption layer, (b) an inlet in the cell for inserting a metallic substrate comprising a substantially cylindrical structure into the cell, wherein the inlet comprises a first seal that reduces the amount of solution that leaves the cell through the inlet, and (c) an outlet in the cell for removing the substrate from the cell. In some embodiments, the outlet comprises a hydraulic second seal that allows at least a portion of the solution to leave the cell through the outlet, and at least a portion of the solution that leaves the cell through the outlet is capable of being reintroduced into the cell. In some embodiments, the system further includes the substrate. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the system includes an air knife for removing a portion of the solution from the substrate. In some embodiments, the system includes a forced hot air dryer for drying (e.g., partially or completely drying) at least a portion of the substrate. In some embodiments, the system includes a fog nozzle for applying the overcoat layer (such as a sol-gel layer). In some embodiments, the system includes an anode encircling at least a portion of the substrate.

[0017] In one aspect, the invention features methods of drying an apparatus. In some embodiments, the method includes using predominantly convective heat to dry (e.g., partially or completely dry) at least a portion of an apparatus that comprises a solar absorption layer on at least a portion of the substrate. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ^CC for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the method also includes using radiant heat to dry (e.g., partially or completely dry) at least a portion of the apparatus. In some embodiments, the method includes heating the solar absorption layer until the surface of the substrate is about 150 ⁰F. In some embodiments, the method also includes applying an overcoat layer (such as a sol-gel layer) onto at least a portion of the solar absorption layer.

[0018] The invention features additional systems, such as systems for drying an apparatus. In some embodiments, the system includes (a) a cell comprising a solution capable of generating a solar absorption layer, (b) an inlet in the cell for inserting a metallic substrate into the cell, (c) an outlet in the cell for removing the substrate from the cell, and (d) a blower for supplying predominantly convective heat to the substrate. In some embodiments, the system further includes the substrate. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the system includes a radiant heater.

[0019] In one aspect, the invention features methods of applying an overcoat layer (such as a sol-gel layer) to an apparatus. In some embodiments, the method includes using a pressurized fog nozzle (such as a direct pressure fog nozzle) to apply an overcoat layer (such as a sol-gel layer) to at least a portion of a solar absorption layer on a metallic substrate. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the method includes heating the overcoat layer (such as a sol-gel layer) until the surface of the substrate is about 150 ⁰F.

[0020] In one aspect, the invention features methods of heating an overcoat layer (such as a sol-gel layer) on an apparatus. In some embodiments, the method includes heating an apparatus at a rate sufficient to cure an overcoat layer (such as a sol-gel layer) but insufficient to crack the overcoat layer, wherein the apparatus comprises (i) a solar absorption layer on at least a portion of the substrate and (ii) the overcoat layer (e.g., a sol-gel layer) on at least a portion of the solar absorption layer. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours.

[0021] The invention features additional methods of applying a solar absorption layer onto a metallic substrate. In some embodiments, the method includes (a) bipolar electrocleaning a metallic substrate, and (b) applying a solar absorption layer onto at least a portion of the substrate. In some embodiments, the method includes (a) bipolar electrocleaning a metallic substrate, (b) applying a barrier layer onto at least a portion of the substrate, and (c) applying a solar absorption layer onto at least a portion of the barrier layer. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the method includes activating a metallic substrate after step (a) and before step (b) (i) to promote adherence of a barrier layer to the substrate, (ii) to promote nucleation of the barrier layer, or (iii) to promote both adherence and nucleation of the barrier layer. In some embodiments, the method includes applying an overcoat layer (such as a sol-gel layer) onto at least a portion of the solar absorption layer. In some embodiments, the method includes heating the overcoat layer (such as a sol-gel layer) until the surface of the substrate is about 150 ⁰F.

[0022] In one aspect, the invention features an apparatus for coating a tubular substrate. In some embodiments, the apparatus includes (i) an electrolytic coating chamber, (ii) an overcoat station positioned downstream of the electrolytic coating chamber, (iii) a first roller configured to guide the substrate into the electrolytic coating chamber, and (iv) a second roller downstream of the overcoat station and configured to guide the substrate. In some embodiments, there are no additional rollers or supports for a substrate intermediate of the first roller and the second roller.

[0023] In one aspect, the invention includes methods of collecting solar energy. In some embodiments, the method involves reflecting solar energy from a reflector onto a solar energy receiver. In some embodiments, the receiver includes (a) a metallic substrate, (b) a barrier layer comprising a brightening agent or leveling agent on at least a portion of the substrate, and (c) a solar absorption layer on at least a portion of the barrier layer.

[0024] In one aspect, the invention includes a solar collection system. In some embodiments, the system includes (a) a receiver and (b) a reflector capable of reflecting solar energy onto the receiver. In some embodiments, the receiver includes (i) a metallic substrate, (ii) a barrier layer comprising a brightening agent or leveling agent on at least a portion of the substrate, and (iii) a solar absorption layer on at least a portion of the barrier layer. In some embodiments, the system includes a drive means for rotating the reflector about an axis of rotation parallel to the longitudinal axis of the reflector. In some embodiments, the substrate comprises a liquid capable of absorbing heat from the substrate.

[0025] In one aspect, the invention includes a product made by any of the methods, systems, or apparatus described herein.

[0026] The following embodiments apply to any of the methods, systems, or apparatus of the invention.

[0027] In some embodiments, the solar absorption layer comprises a nickel-tin alloy. In some embodiments, the solution capable of generating a solar absorption layer comprises a nickel compound and a tin compound. In some embodiments, the nickel compound is NiCl₂ and the tin compound is SnCl₂ In some embodiments, the solution capable of generating a solar absorption layer comprises NH₄OH and NH₄F₂. In some embodiments, electroplating the solar absorption layer comprises applying an electric current between the substrate and an anode encircling at least a portion of the substrate such that the solar absorption layer is deposited on at least a portion of the substrate. In some embodiments, the temperature of the solution capable of generating a solar absorption layer is between about 65 and about 75 ⁰F. [0028] In some embodiments, the substrate is activated to promote adherence of a barrier layer to the substrate, (ii) to promote nucleation of the barrier layer, or (iii) to promote both adherence and nucleation of the barrier layer. In some embodiments, the substrate is activated by incubation in an acidic solution of metal. In some embodiments, the solution comprises a Woods nickel formulation. In some embodiments, the solution comprises nickel sulfamate.

[0029] In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate. In some embodiments, the barrier layer comprises nickel, platinum, gold, silver, tantalum, titanium, rhodium, tungsten, an alloy thereof, or any combination thereof. In some embodiments, the barrier layer comprises an oxide, such as silicon oxide, aluminum oxide, or nickel oxide. In particular embodiments, the barrier layer comprises nickel. In some embodiments, at least a portion of the barrier layer is applied by electroplating an acidic solution of metal, such as nickel. In some embodiments, at least a portion of the nickel layer is applied by electroplating an acidic solution of nickel. In some embodiments, applying the barrier layer comprises incubating the substrate in a solution comprising nickel sulfamate under conditions sufficient to apply the barrier layer onto at least a portion of the substrate. In some embodiments, the current density for the electroplating is about 150 amps per square foot. In some embodiments, the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent. In some embodiments, the barrier layer comprises both a brightening agent and a leveling agent. In some embodiments, the brightening agent is sodium saccharin. In some embodiments, the leveling agent is sodium lauryl alcohol sulfate.

[0030] In some embodiments, an overcoat layer (such as a sol-gel layer) is applied onto at least a portion of the solar absorption layer. In some embodiments, the method includes heating the overcoat layer (such as a sol-gel layer) until the surface of the substrate is about 150 ⁰F.

[0031] In particular embodiments, the substrate is rinsed between one or more steps. Any solvent (such as water) can be used for this rinse. In some embodiments, a double reverse cascade rinse is used. In particular embodiments, water from the second rinse is used for the first rinse of the double reverse cascade rinse. In some embodiments, a triple reverse cascade rinse is used. In particular embodiments, water from the third rinse is used for the second rinse, and water from the second rinse is used for the first rinse of the triple reverse cascade rinse. In some embodiments, a static rinse is used. For example, the substrate may be immersed in a solvent (such as water) or dragged through a bath of a solvent. In some embodiments, the substrate is mechanically or chemically (e.g., acid cleaning) cleaned prior to bipolar electrocleaning and/or prior to addition of a barrier layer or solar absorptive layer.

[0032] In some embodiments, the cell, apparatus, or system comprises electrical contact rollers that supply electric current to the substrate. In some embodiments, a device maintains a substantially constant ratio of nickel to tin in the solution capable of generating a solar absorption layer. In some embodiments, the deposition chamber, cell, apparatus, or system comprises a first seal and/or a second hydraulic seal. In some embodiments, the first seal is a rubber seal. In some embodiments, the hydraulic seal allows at least a portion of the solution to leave the deposition chamber or cell through the outlet of the chamber or cell. In some embodiments, the deposition chamber or cell reduces or substantially eliminates the introduction of air into the deposition chamber or cell (such as a deposition chamber or cell for applying a barrier layer or solar absorption layer). In some embodiments, an air knife is used to remove a portion of the solution from the substrate. In some embodiments, a forced hot air dryer is used to dry (e.g., partially or completely dry) at least a portion of the substrate. In some embodiments, a forced hot air dryer supplies the predominantly convective heat. In some embodiments, radiant heat is used to dry (e.g., partially or completely dry) at least a portion of the apparatus. In some embodiments, the apparatus or system includes a radiant heater.

[0033] In some embodiments, the fog nozzle atomizes a liquid stream under a pressure of about 20 to about 50 psi. In some embodiments, the overcoat layer (such as a sol-gel layer) is heated until the surface of the substrate is about 150 ^CF. In some embodiments, the overcoat layer (such as a sol-gel layer) is heated for less than or about 30, 20, 10, 5, or 3 seconds.

[0034] In some embodiments, the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises a barrier layer on at least a portion of the substrate, and the substrate with the barrier layer and the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours. In some embodiments, the substrate comprises steel, such as stainless steel. In some embodiments, the substrate comprises a substantially cylindrical structure. [0035] In some embodiments, the substrate maintains a substantially constant orientation throughout the coating process. In some embodiments, the substrate maintains a substantially constant elevation throughout the coating process. In some embodiments, a substantially non- cylindrical substrate is orientated substantially horizontally during one or more steps or during all the steps of the method. In other embodiments, the orientation of the substrate changes during the method. For example, a substantially non-cylindrical substrate may initially be orientated substantially vertically and then change orientation so that it is oriented substantially horizontally. In some embodiments, a substantially non-cylindrical substrate is orientated substantially vertically during one or more steps or during all the steps of the method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. IA is a cross- sectional view of a coated substrate, according to one variation.

[0037] FIG. IB shows a process flow for the production of a coated substrate without a barrier layer, according to one variation.

[0038] FIG. 1C shows a process flow for the production of a coated substrate with a barrier layer, according to one variation.

[0039] FIG. ID shows a process flow for the production of a coated substrate without a barrier layer that involves a change in orientation of the substrate from a vertical orientation to a horizontal orientation, according to one variation.

[0040] FIG. IE shows a process flow for the production of a coated substrate without a barrier layer that involves a change in orientation of the substrate from a vertical orientation to a horizontal orientation, according to one variation.

[0041] FIG. IF shows a portion of a process flow for the production of a coated substrate with a barrier layer, according to one variation. In this variation, one or more rollers are used to move the substrate between stages prior to the solar absorption layer coat stage, and one or more rollers are used to move the substrate between stages after the overcoat stage. [0042] FIG. IG shows a process flow for the production of a coated substrate and shows various containment areas, according to one variation. If desired, containment areas of other sizes can be used instead of the exemplary sizes listed in FIG. IG.

[0043] FIG. 2A shows a stage for applying a solar absorption layer, according to one variation.

[0044] FIG. 2B shows a stage for applying a solar absorption layer, according to one variation.

[0045] FIG. 2C shows a side-view of a stage for applying a solar absorption layer, according to one variation.

[0046] FIG. 2D shows a stage for applying a solar absorption layer to a substantially non- cylindrical substrate, according to one variation.

[0047] FIG. 3 A is a list of exemplary solar coatings and substrates. This list is taken from NREL Technical Report TP-520-31267 "Review of Mid- to High-Temperature Solar Selective Absorber Materials" by CE. Kennedy (July 2002) (which is incorporated by reference in its entirety, particularly with respect to the production and use of substrates with a solar absorption coating).

[0048] FIG. 3B is a list of exemplary solar coatings and substrates. This list is taken from NREL Technical Report TP-520-31267 "Review of Mid- to High-Temperature Solar Selective Absorber Materials" by CE. Kennedy (July 2002).

[0049] FIG. 4 shows a pipe-to-pipe connector, according to one variation.

[0050] FIG. 5A shows an anode basket in a barrier layer coat stage, according to one variation.

[0051] FIG. 5B shows an anode basket in a barrier layer coat stage, according to one variation.

[0052] FIG. 6A shows a stage for applying an overcoat layer, according to one variation.

[0053] FIG. 6B shows a side-view of a stage for applying an overcoat layer, according to one variation.

[0054] FIG. 6C shows an ortho-view of a stage for applying an overcoat layer, according to one variation. [0055] FIG. 6D shows a stage for applying an overcoat layer and a stage for drying the overcoat layer, according to one variation.

[0056] FIG. 7 is a table that lists conditions used to coat several exemplary substrates and the emmisivity and absorbance values of these coated substrates, according to one variation.

[0057] These and other embodiments, features, and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following more detailed description of the invention in conjunction with the accompanying drawings that are first briefly described.

DETAILED DESCRIPTION OF THE INVENTION

[0058] The following detailed description should be read with reference to the drawings, in which identical reference numbers refer to like elements through the different figures. The drawings, which are not necessarily to scale, depict selective embodiments and are not intended to limit the scope of the invention. The detailed description illustrates by way of example, not by way of limitation, the principles of the invention. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives, and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. It should also be noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly indicates otherwise. Reference to "about" a value or parameter herein includes (and describes) embodiments that are directed to that value or parameter per se. For example, description referring to "about X" includes description of "X." Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. If a definition set forth in this section is contrary to or otherwise inconsistent with a definition set forth in patents, published patent applications, and other publications that are herein incorporated by reference, the definition set forth in this section prevails over the definition that is incorporated herein by reference.

[0059] Methods and apparatus for coating substrates 100 are disclosed herein. These coated substrates are useful, for example, as receivers in solar collection systems. FIG. IA illustrates a substrate 100 coated with an optional barrier layer 300, a solar absorption layer 400, and an optional overcoat layer 500 (e.g., a protective layer such as a sol-gel layer). In some embodiments, the substrate 100 is only coated with a solar absorption layer 400. In some embodiments, the substrate 100 is coated with a barrier layer 300 and a solar absorption layer 400. In some embodiments, the substrate 100 is coated with a solar absorption layer 400 and an overcoat layer 500. In some embodiments, the substrate 100 is coated with a barrier layer 300, a solar absorption layer 400, and an overcoat layer 500. In various embodiments, the invention includes any combination of one or more substrates 100, optional barrier layers 300, solar absorption layers 400, and optional overcoat layers 500 described herein. In some embodiments, the coated substrate 100 includes one or more additional layers, such as an optional layer between the solar absorption layer 400 and the overcoat layer 500 and/or an optional layer on top of the overcoat layer 500. In some embodiments, the optional layer on top of the overcoat layer 500 includes a layer that increases the strength of the coated substrate 100 or a layer that reflects light into the coated substrate 100. In some embodiments, the coated substrate 100 includes more than one barrier layer 300, such as (i) two or more layers of the same material applied under different operating conditions or (ii) two or more layers of different materials applied under the same or different operating conditions. In some embodiments, the coated substrate 100 includes more than one solar absorption layer 400, such as (i) two or more layers of the same material applied under different operating conditions or (ii) two or more layers of different materials applied under the same or different operating conditions. In some embodiments, the coated substrate 100 includes more than one overcoat layer 500, such as (i) two or more layers of the same material applied under different operating conditions or (ii) two or more layers of different materials applied under the same or different operating conditions.

[0060] In the following sections, exemplary substrates 100, barrier layers 300, solar absorption layers 400, and overcoat layers 500 are first described. Then, the process for coating substrates 100 is summarized. Next, a more detailed description of exemplary steps and apparatus for coating substrates 100 is provided. Exemplary coated substrates 100 and methods of using them as receivers in solar collection systems are then described.

Exemplary Substrates 100 [0061] A variety of substrates 100 can be used in the methods and apparatus of the invention. In some embodiments, the uncoated substrate 100 has an emissivity between about 0.1 and about 1.0, such as between about 0.1 and about 0.5, or such as between about 0.08 and about 0.5. Exemplary substrates 100 include metallic substrates, such as copper, carbon steel, black iron, stainless steel, platinum, gold, silver, molybdenum, tungsten, nickel, tantalum, titanium, rhodium, silicon, aluminum, nickel-chrome, nickel-molybdenum, zirconium bromide, and alloys thereof. In some embodiments, the substrate 100 includes a metallic layer (such as a layer that includes copper, carbon steel, black iron, stainless steel, platinum, gold, silver, molybdenum, tungsten, nickel, tantalum, titanium, rhodium, silicon, aluminum, nickel-chrome, nickel- molybdenum, zirconium bromide, and alloys thereof) over a base material (e.g., a metallic material such as brass, bronze, copper, carbon steel, black iron, stainless steel, platinum, gold, silver, molybdenum, tungsten, nickel, tantalum, titanium, rhodium, silicon, aluminum, nickel- chrome, nickel-molybdenum, zirconium bromide, and alloys thereof, or a non-metallic material such as glass). In particular embodiments, the substrate 100 includes glass coated with a layer of copper, carbon steel, black iron, stainless steel, platinum, gold, silver, molybdenum, tungsten, nickel, tantalum, titanium, rhodium, silicon, aluminum, nickel-chrome, nickel-molybdenum, zirconium bromide, or an alloy thereof. In some embodiments, the substrate includes a non- electrically conductive base material (such as glass) that has an electrically conductive layer such as a nickel, aluminum, iron, steel, or copper layer on at least a portion of the base material. In some embodiments, substrate 100 is an oxide (e.g., silicon oxide, aluminum oxide, or nickel oxide) or a layer of an oxide (e.g., silicon oxide, aluminum oxide, or nickel oxide) over a base material. In particular embodiments, the substrate 100 includes a nickel-chrome alloy or a nickel-molybdenum alloy. In some embodiments for low temperature applications (e.g., below 150 ⁰C), a copper substrate 100 or a layer of copper over a base material is used. In some embodiments for high temperature applications (e.g., greater than 150 ⁰C), a carbon steel, black iron, or stainless steel substrate 100 or a layer of carbon steel, black iron, or stainless steel over a base material is used. Other exemplary substrates are listed in FIGS. 3 A and 3B.

[0062] In particular embodiments, the substrate 100 is 304 stainless steel. In particular embodiments, the substrate 100 is A106, PI l, or A210 carbon steel. [0063] In some embodiments, the substrate 100 is not a copper substrate (i.e., the substrate is other than a copper substrate). In some embodiments, the substrate 100 does not include a layer of copper. In some embodiments, the substrate 100 includes less than or about 10%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.01%, or 0.001% copper by weight.

[0064] As used herein, a "substrate" may therefore be an uncoated or coated material that conducts heat readily, such as a coated or uncoated tube, pipe, sheet, plate, or film of metallic material. The term "substrate" may apply to the material prior to any of the treatment steps discussed herein are performed, or the term "substrate" may refer to material that has undergone one or more of the treatments discussed herein. A substrate may be flexible or rigid and tubular or non-tubular - e.g., a plate (flat or curved), foil, sheet, or other form suitable for coating. The substrate is generally not a semiconductor substrate or silicon wafer.

[0065] Exemplary substrate configurations include (i) substantially cylindrical configurations such as tubes or pipes and (ii) substantially non-cylindrical configurations such as flat substrates, plates, sheet metal, or fins. Either of these configurations can be used in low (e.g., below 150 ⁰C) or high temperature (e.g., greater than 150 ⁰C) solar systems. In particular embodiments, the substrate 100 is alumina bead blasted or lathe turned.

[0066] The methods and apparatus described herein can be used with a substrate 100 with any suitable outer and inner dimensions. In some embodiments, the nominal outer diameter of the substrate 100 is between about 0.25 to about 15 inches. In some embodiments, the nominal outer diameter of the substrate 100 is about any of 0.25, 0.50, 0.75, 1, 1.5 2, 2.375, 2.5, 2.75, 3, 3.5, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 inches. In some embodiments, the thickness of the wall of the substrate 100 is between about 0.035 to about 0.75 inches, such as about any of 0.035, 0.05, 0.1, 0.15, 0.2 ,0.25, 0.3, 0.4 ,0.5, 0.6, or 0.7 inches. In some embodiments, the nominal inner diameter of the substrate 100 is between about 0.1 to about 14 inches, such as about any of 0.1, 0.25, 0.50, 0.75, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or 14 inches.

[0067] In some embodiments, the dimensions of the substrate 100 have sufficient uniformity to prevent damage to any of the coating stages (such as damage to a coating stage due to a section of substrate 100 having an outer dimension that is large enough for the substrate 100 to contact and damage part of a coating stage.). Exemplary Barrier Layers

[0068] The substrate 100 may have an optional barrier layer 300 between the substrate surface and the solar absorption layer 400. The barrier layer 300 forms a diffusion barrier under the solar absorption layer 400 (i) to reduce or prevent migration of metal from the substrate 100 into the solar absorption layer 400 (e.g., the migration of metal into the solar absorption layer 400 at elevated temperature that may decrease the solar absorptivity of the solar absorption layer 400 by interrupting crystalline structure or that may delaminate the solar absorption layer 400) and/or (ii) to reduce or prevent migration of molecules from the solar absorption layer 400 into the substrate (e.g., the migration of molecules such as oxygen from the solar absorption layer 400 or oxygen or water from the atmosphere into the substrate 100 that may oxidize or delaminate the substrate 100). The barrier layer may provide the final coating resistance to higher temperatures. Desirable barrier layers 300 may have a naturally low thermal emissivity (such as an emissivity between about 0 to about 0.3) and thus form a low emissivity surface which enhances the solar selectivity. In some embodiments, the barrier layer 300 decreases the migration of metal from the substrate 100 into the solar absorption layer 400 by at least or about 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, or 100% compared to the amount of migration of metal from the substrate 100 into the solar absorption layer 400 in the absence of the barrier layer 300 at a particular operating temperature (such as about 200 ⁰C) and after a particular amount of time (such as about 200 hours). Conventional analytical techniques can be used to measure the composition of the solar absorption layer 400 to determine the amount of migration of molecules into the solar absorption layer 400.

[0069] In some embodiments, the substrate 100 with a barrier layer 300 and a solar absorption layer 400 maintains a solar absorptivity between about 0.85 and about 1.0 at a particular operating temperature (such as about 200 ⁰C) for at least or about 200, 300, 400, 500, 600, 800, 1,000, 1,200, 1,400, 1,600, 1,800, 2,000, or more hours. In some embodiments, the barrier layer 300 increases the amount of time the substrate 100 with a solar absorption layer 400 maintains a solar absorptivity between about 0.85 and about 1.0 at a particular operating temperature (such as about 200 ⁰C) by at least or about 2, 5, 10, 20, 50, 60, 70, 80, 90, or 100-fold compared to the amount of time the substrate 100 maintains a solar absorptivity between about 0.85 and about 1.0 in the absence of the barrier layer 300. To measure the effect of the barrier layer on the time the coated substrate maintains a solar absorptivity value within a desired range, standard methods can be used to heat the coated substrate 100 to a particular temperature (such as about 200 ⁰C) for a particular number of hours (such as about 200 hours) and then measure the solar absorptivity of the coated substrate. The solar absorptivity value may be determined by integrating the absorbance of the coated substrate 100 over the solar spectrum at an air mass of 1.5 using conventional analytical techniques, such as spectroscopy.

[0070] In some embodiments, the barrier layer 300 decreases the migration of one or more molecules from the solar absorption layer 400 or atmosphere into the substrate 100 by at least or about 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, or 100% compared to the amount of migration of one or more molecules from the solar absorption layer 400 or atmosphere into the substrate 100 in the absence of the barrier layer 300 at a particular operating temperature (such as about 200 ⁰C) and after a particular amount of time (such as about 200 hours). In some embodiments, the barrier layer 300 decreases the amount of the substrate 100 that is oxidized by at least or about 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, or 100% compared to the amount of the substrate 100 that is oxidized in the absence of the barrier layer 300 at a particular operating temperature (such as about 200 ⁰C) and after a particular amount of time (such as about 200 hours). Conventional analytical techniques can be used to measure the composition of the substrate 100 to determine the amount of one or more types of molecules that have migrated into the substrate and/or to determine the amount of the substrate 100 that has been oxidized.

[0071] Exemplary barrier layers 300 include, but are not limited to, nickel, platinum, gold, silver, tantalum, titanium, rhodium, tungsten, alloys thereof, and any combination thereof. In some embodiments, the barrier layer 300 is or includes a high temperature refractory metal, such as tungsten, titanium, or rhodium. In some embodiments, barrier layer 300 is or includes an oxide, such as silicon oxide, aluminum oxide, or nickel oxide. In some embodiments, a Cr₂O₃ barrier layer 300 is used (NREL Technical Report TP-520-31267 "Review of Mid- to High- Temperature Solar Selective Absorber Materials" by CE. Kennedy (July 2002)).

[0072] In some embodiments, the barrier layer 300 is not oxidized (such as by exposure of the barrier layer 300 to an oxygen-containing gas). In some embodiments, the barrier layer 300 such as a nickel barrier layer includes an outer layer of nickel oxide that is about or less than 100, 50, 25, 10, 15, or 10 angstroms thick. In some embodiments, the barrier layer 300 does not include nickel oxide.

[0073] In some embodiments, all or essentially all of the outer surface of a substantially cylindrical substrate (such as a tube or pipe) is coated with the barrier layer 300. In some embodiments, all or essentially all of the outer surface of one side of a substantially non- cylindrical substrate (such as a flat substrate, plate, sheet metal, or fin) is coated with the barrier layer 300.

Exemplary Solar absorption layers 400

[0074] The methods and apparatus of the present invention can be adapted using standard techniques for the deposition of a variety of solar absorption layers 400. FIGS. 3A and 3B include exemplary solar absorption layers 400. Other solar absorption layers 400 are known in the art, such as those disclosed in NREL Technical Report TP-520-31267 "Review of Mid- to High-Temperature Solar Selective Absorber Materials" by CE. Kennedy (July 2002).

[0075] As described further below, in some embodiments, the solar absorption layer 400 is an alloy of nickel and tin, such as the nickel-tin alloy in a Black Crystal® coating. The trade name Black Crystal® refers to Crystalographic Metal Alloys with Sol-gel. The coatings (such as thin films or nano-crystalline coatings) formed by this technology are designed to maximize solar selectivity of a substrate element at a low cost. As the name implies, the coating is an alloy of nickel and tin of certain morphologies (such as a dendritic or pinnacle structure) that create a surface which absorbs a high percentage of light in the visible range yet emits a fractional percentage of the energy absorbed as radiated heat (U.S. Patent No. 6,783,653, which is hereby incorporated by reference in its entirety as if put forth fully below, and particularly with respect to solar coatings). As described further below, the Black Crystal® coating includes a sol-gel overcoat layer 500.

[0076] In some embodiments, the solar absorption layer 400 is a cermet coating. A cermet is a composite material composed of ceramic and metallic materials. FIGS. 3 A and 3B include several exemplary cermet coatings. Standard methods may be used to apply a cermet coating, such as those disclosed in NREL Technical Report TP-520-31267 "Review of Mid- to High- Temperature Solar Selective Absorber Materials" by CE. Kennedy (July 2002). [0077] In some embodiments, the solar absorption layer 400 is a black organic coating or a structured metallic coating (see, for example, U.S. Patent No. 6,783,653, which is hereby incorporated by reference in its entirety, particularly with respect to solar absorption layers). In some embodiments, the solar absorption layer 400 is black nickel or black chrome. In some embodiments, the solar absorption layer 400 is not black chrome. In some embodiments, the solar absorption layer 400 is oxidized (e.g., a metal oxide such as nickel oxide or an oxidized barrier layer). In some embodiments, the solar absorption layer 400 is not oxidized. In some embodiments, the solar absorption layer 400 is not an electrically conductive layer. In some embodiments, the solar absorption layer 400 is not a photoelectric conversion layer. In some embodiments, the solar absorption layer 400 does not convert solar energy into electricity. In some embodiments, the substrate with a solar absorption layer 400 does not function as a photovoltaic element. In some embodiments, the solar absorption layer 400 is not nickel oxide. In some embodiments, the solar absorption layer 400 dose not comprise chromium.

[0078] An exemplary measure of the thermal effectiveness of a solar selective absorption coating is the ratio of its absorption of solar energy and its thermal emission at the system operating temperature (U.S. Patent No. 6,783,653, which is hereby incorporated by reference in its entirety, particularly with respect to measuring absorptivity and emissivity values). In some embodiments, the absorptivity of solar energy (e.g., the solar absorptivity at a particular operating temperature such as about 200 ⁰C) by the combination of a solar absorption layer 400 and a barrier layer 300 is between about 0.8 and about 1.0. The solar absorptivity value may be determined by integrating the absorbance of the combined solar absorption layer 400 and barrier layer 300 over the solar spectrum at an air mass of 1.5 using a conventional analytical instrument, such as a spectrophotometer or integrated optical instrument.

[0079] In some embodiments, the emissivity of the combination of a solar absorption layer 400 and a barrier layer 300 is between about 0 and about 0.4 at a particular operating temperature such as about 200 ⁰C. The emissivity can be measured using a conventional analytical instrument, such as a spectrophotometer or integrated optical instrument.

[0080] In some embodiments, the solar absorption layer 400 is applied using standard electrochemical deposition, physical vapor deposition, chemical vapor deposition, or oxidation methods. In some embodiments, the solar absorption layer 400 is formed by electroplating a metal oxide, oxidizing a metal on the surface of substrate 100, or oxidizing a metal in barrier layer 300. In some embodiments, the solar absorption layer 400 is painted on the substrate 100.

[0081] In some embodiments, all or essentially all of the outer surface of a substantially cylindrical substrate (such as a tube or pipe) is coated with the solar absorption layer 400. In some embodiments, all or essentially all of the outer surface of one side of a substantially non- cylindrical substrate (such as a flat substrate, plate, sheet metal, or fin) is coated with the solar absorption layer 400.

Exemplary Overcoat layers 500

[0082] The substrate 100 may have an optional overcoat layer 500 (such as a sol-gel layer applied from a solution of tetraorthosilicate in alcohol) on top of the solar absorption layer 400, either directly upon the solar absorption layer or upon one or more layers on the solar absorption layer. In some embodiments, the overcoat layer 500 protects the solar absorption layer 400 (such as by reducing or preventing damage to the three-dimensional structure of the solar absorption layer 400 or by reducing or preventing degradation of the solar absorption layer 400). In some embodiments, an amorphous glass sol-gel layer is applied onto a substrate 100.

[0083] In the Black Crystal® coating, the sol-gel layer provides structural stability to the bimetallic matrix of the Black Crystal® coating that creates the optical properties yet renders the side benefits of enhancement of the optical properties. In particular, the sol-gel layer protects the fragile dendritic structure of the Black Crystal® layer. The sol-gel layer also provides an optical index transition layer to prevent light rays from leaving the crystallographic metallic alloy matrix after they have entered. The difference in refractive index between the sol-gel layer and its environment causes some light emitted and/or reflected by the substrate to reflect back to the substrate.

[0084] In some embodiments, the overcoat layer 500 is a transparent oxide, such as a magnesium or titanium oxide. Exemplary overcoat layers also include carbides and nitrides (such as silicon nitride). In some embodiments, the overcoat layer is applied using standard physical vapor deposition or chemical vapor deposition methods. [0085] In some embodiments, all or essentially all of the outer surface of a substantially cylindrical substrate (such as a tube or pipe) is coated with the overcoat layer 500. In some embodiments, all or essentially all of the outer surface of one side of a substantially non- cylindrical substrate (such as a flat substrate, plate, sheet metal, or fin) is coated with the overcoat layer 500.

[0086] In some embodiments, the substrate 100 includes an optional layer on top of the overcoat layer 500. In some embodiments, the optional layer on top of the overcoat layer 500 includes a layer that increases the strength of the coated substrate 100 or a layer that reflects light into the coated substrate 100.

Exemplary Combinations of Layers

[0087] In various embodiments, the invention features any combination of any of the exemplary substrates 100, barrier layers 300, solar absorption layers 400, and overcoat layers 500 described herein. In some embodiments, the coated substrate 100 includes a stainless steel substrate 100 with a nickel barrier layer 300, a nickel-tin alloy of a Black Crystal® coating as a solar absorption layer 400, and a silica sol-gel layer as an overcoat layer 500. In some embodiments, the coated substrate 100 includes a carbon steel or black iron substrate 100 with a nickel barrier layer 300, a nickel-tin alloy of a Black Crystal® coating as a solar absorption layer 400, and a silica sol-gel layer as an overcoat layer 500. In some embodiments, the nickel barrier layer is applied using a nickel strike step and a nickel plating step.

Overview of the Process for Coating Substrates

[0088] FIG. IB illustrates a process for coating a substrate 100 with a solar absorption layer 400 and an optional overcoat layer 500 (e.g., a sol-gel layer). The substrate 100 enters an optional pre-cleaning stage 101 where it may be washed with a solvent (such as ethanol). A hot spray cleaning stage 102 may be used to loosen or detach dirt and other contaminants from the substrate 100. An anodic or bipolar cleaning stage 103 removes small contaminants trapped on the surface of the substrate 100. The substrate 100 may be rinsed in stage 104. In some embodiments, rinse stage 104 consists of two stations: a drag-out rinse 104A that uses solution from the anodic or bipolar cleaning stage 103 and a reverse cascade rinse 104B with water (e.g., city or DI water). A surface activation stage 111 uses an acidic solution to activate the surface of the substrate 100. Rinse stage 112 uses water (e.g., city or DI water) to wash the substrate 100. A solar absorption layer coat stage 113 applies a solar absorption layer 400 (such as the nickel-tin alloy of a Black Crystal® coating) to the surface- activated substrate 100. Rinse stage 114 may be used to wash the coated substrate 100. A first drying stage 115 may be used to dry the coated substrate 100. In some embodiments, first drying stage 115 may be divided into two stations: first drying stage 115A and preheat/drying stage 115B (FIGS. ID and IE). Overcoat stage 116 may be used to apply an optional overcoat layer 500, such as a sol-gel layer, to the substrate 100 coated with the solar absorption layer 400. A second drying stage 117 may be used to dry and cure the overcoat layer 500.

[0089] The process may include one or more additional steps. For example, the process may include, for example, the application of one or more additional layers, such as an optional layer between the solar absorption layer 400 and the overcoat layer 500 and/or an optional layer on top of the overcoat layer 500. The process may also include combining (e.g., welding) multiple substrates 100 into a longer combined substrate 100 so that multiple substrates 100 can be pushed through the coating line at the same time. The process may also include separating the combined substrate 100 into multiple individual substrates 100 after the combined substrate 100 has been coated. Additionally, the process may include additional treatment of the coated substrate, such as the removal of a portion of the coating near the ends of individual substrates 100 to facilitate subsequent welding of the individual coated substrates 100 after they are transported to a desired location for a solar energy collection system.

[0090] FIG. 1C illustrates a process for coating a substrate 100 with a barrier layer 300, a solar absorption layer 400, and an optional overcoat layer 500. In some embodiments, this process uses stages that are similar to or identical to the stages illustrated in FIG. IB for applying the solar absorption layer 400 and overcoat layer 500. In addition, stages may be added for applying the optional barrier layer 300 between the substrate 100 surface and the solar absorption layer 400. As illustrated in FIG. IB, the substrate 100 enters an optional pre-cleaning stage 101, a hot spray cleaning stage 102, anodic or bipolar cleaning stage 103, and a rinse stage 104. Compared to FIG. IB, additional stages are added for applying the barrier layer 300 to the cleaned and rinsed substrate 100. In particular, a surface activation stage 105 uses an acidic solution to activate the surface of the substrate 100. A rinse stage 106 washes the substrate 100 with water (e.g., fresh or DI water). An optional barrier strike stage 107 applies a small amount of the barrier layer 300 to the substrate 100 to facilitate applying the rest of the barrier layer 300 to the substrate 100 in a subsequent stage. A rinse stage 108 rinses the substrate 100. A barrier layer coat stage 109 may be used to add the barrier layer 300 to the substrate 100. In some embodiments, the barrier layer coat stage 109 includes more than one station for applying the barrier layer 300. The substrate 100 with the barrier layer 300 may be rinsed in rinse stage 110. As illustrated in FIG. IB, the substrate 100 with the barrier layer 300 then enters a surface activation stage 111, a rinse stage 112, a solar absorption layer coat stage 113, a rinse stage 114, a first drying stage 115, an overcoat stage 116, and a second drying stage 117. The process may include one or more additional steps. For example, the process may include, for example, the application of one or more additional layers, such as an optional layer between the solar absorption layer 400 and the overcoat layer 500 and/or an optional layer on top of the overcoat layer 500. The process may also include combining (e.g., welding) multiple substrates 100 into a longer combined substrate 100 so that multiple substrates 100 can be pushed through the coating line at the same time. The process may also include separating the combined substrate 100 into multiple individual substrates 100 after the combined substrate 100 has been coated. Additionally, the process may include additional treatment of the coated substrate, such as the removal of a portion of the coating near the ends of individual substrates 100 to facilitate subsequent welding of the individual coated substrates 100 after they are transported to a desired location for a solar energy collection system.

[0091] The overall process (such as the process illustrated in FIGS. IB- IF) can be performed on substrate 100 as a continuous process, a batch process, or a process that has some continuous steps and one or more non-continuous steps. These stages for coating a substrate 100 can be included in one or more apparatus. In some embodiments, multiple apparatus are configured to cooperate to coat a substrate 100. In some embodiments, pre-cleaning stage 101 is part of the same apparatus as one or more other stages for coating substrate 100. In some embodiments, pre-cleaning stage 101 is part of a different apparatus than the other stages for coating substrate 100. For example, the substrate 100 can be pre-cleaned in one location and then transported to another location (such as the location of a coating line). In some embodiments, a stage for welding multiple substrates 100 into a longer combined substrate 100 is part of the same apparatus as one or more other stages for coating substrate 100. In some embodiments, this welding stage is part of a different apparatus than the other stages for coating substrate 100. In some embodiments, a stage for separating a coated substrate 100 into multiple individual substrates 100 is part of the same apparatus as one or more other stages for coating substrate 100. In some embodiments, this separation stage is part of a different apparatus than the other stages for coating substrate 100. In some embodiments, a stage for treating a coated substrate 100 (such as removing a portion of the coating near the ends of individual substrates 100 to facilitate subsequent welding of the individual coated substrates 100) is part of the same apparatus as one or more other stages for coating substrate 100. In some embodiments, this treatment stage is part of a different apparatus than the other stages for coating substrate 100. In some embodiments, one stage is divided into one or more stations. Any of the stages or stations may contain a cell.

[0092] In some embodiments, the substrate 100 maintains a constant orientation throughout the coating process illustrated in FIG. IB or 1C. For example, a substantially non-cylindrical substrate 100 (e.g., sheet metal or fin) may be orientated substantially horizontally (e.g., horizontally) throughout the entire coating process.

[0093] In other embodiments, the orientation of the substrate 100 changes during the coating process illustrated in FIG. IB or 1C. For example, a substantially non-cylindrical substrate 100 (e.g., sheet metal or fin) may initially be orientated substantially vertically (e.g., vertically) and then change orientation so that it is oriented substantially horizontally (e.g., horizontally) (FIGS. ID and IE). In some embodiments, the substrate 100 changes from a substantially vertical to a substantially horizontal orientation between solar absorption layer coat stage 113 and overcoat stage 116. The initial vertical orientation of the substrate 100 reduces the amount of chemicals needed for pre-cleaning stage 101, hot spray cleaning stage 102, anodic or bipolar cleaning stage 103, surface activation stage 105, barrier strike stage 107, barrier layer coat stage 109, solar absorption layer coat stage 113, or any combination of two or more of the foregoing. This results in less waste being generated by the process. The vertical orientation also reduces the amount of water needed for rinsing since gravity helps remove solution from the substrate 100. In some embodiments, an air knife is not used to remove solution from substrate 100 since gravity facilitates the removal of solution from substrate 100. Because of these improvements, the substrate 100 can be moved through the coating line at a faster speed than the corresponding coating line with the substrate 100 always in a horizontal orientation. Applying the overcoat layer 500 with the substrate 100 in a substantially horizontal orientation facilitates the generation of an even overcoat layer 500 on the substrate 100. For example, applying the overcoat layer 500 to the top side of a substantially non-cylindrical substrate 100 in a substantially horizontal orientation helps prevent gravity from pulling the coating towards the bottom of the substrate 100.

[0094] A shown in FIGS. ID and IE, two substantially vertical rollers (e.g., vertical rollers)

701 and one substantially horizontal roller (e.g., a horizontal roller) 702 can be used to change the orientation of the substrate 100 from a substantially vertical to a substantially horizontal orientation. The rollers may contact the side of substrate 100 that is not being coated so the rollers do not interfere with or damage the coating on the substrate 100. As one skilled in the art would appreciate, the two substantially vertical rollers 701 and one substantially horizontal roller

702 can be placed at various locations in the coating line so that the orientation of the substrate 100 changes at a desired location. In some embodiments, the substrate 100 changes from a substantially vertical to a substantially horizontal orientation between solar absorption layer coat stage 113 and overcoat stage 116. FIG. ID illustrates one such embodiment in which the first substantially vertical roller 701 is located after rinse stage 114, the second substantially vertical roller 701 is located after the first drying stage 115 A, and the substantially horizontal roller 702 is located after the second substantially vertical roller 701. The coating line illustrated in FIG. ID has an overall horseshoe or U-shaped pattern.

[0095] FIG. IE illustrates an alternative coating line with a straighter overall pattern. In some embodiments, there is an approximately 178 degree angle between the substrate 100 before the first substantially vertical roller 701 and the substrate 100 after the first substantially vertical roller 701, resulting in a 2 degree offset. This offset allows the substrate 100 to pull against the first substantially vertical roller 701 so that the orientation of the substrate 100 can be changed from substantially vertical to substantially horizontal. Having substrate 100 pull against the first substantially vertical roller 701 helps keep substrate 100 in the desired position so that it does not move in such a way that the coating on substrate 100 is damaged by contact with part of the coating line. Various other configurations can be used to change the orientation of substrate 100. For example, the offset (due to the angle between the substrate 100 before the first substantially vertical roller 701 and the substrate 100 after the first substantially vertical roller 701) can be between about 2 degrees to about 90 degrees.

[0096] In various embodiments in which the orientation of substrate 100 changes between solar absorption layer coat stage 113 and overcoat stage 116, the first substantially vertical roller 701 is located (i) between solar absorption layer coat stage 113 and rinse stage 114, (ii) between two rinse stations of rinse stage 114, (iii) between rinse stage 114 and first drying stage 115A, (iv) between first drying stage 115A and preheat/drying stage 115B, or (iv) between preheat/drying stage 115B and overcoat stage 116. In various embodiments in which the orientation of substrate 100 changes between solar absorption layer coat stage 113 and overcoat stage 116, the second substantially vertical roller 701 is located (i) between solar absorption layer coat stage 113 and rinse stage 114, (ii) between two rinse stations of rinse stage 114, (iii) between rinse stage 114 and first drying stage 115A, (iv) between first drying stage 115A and preheat/drying stage 115B, or (iv) between preheat/drying stage 115B and overcoat stage 116. In various embodiments in which the orientation of substrate 100 changes between solar absorption layer coat stage 113 and overcoat stage 116, the substantially horizontal roller 702 is located (i) between solar absorption layer coat stage 113 and rinse stage 114, (iii) between rinse stage 114 and first drying stage 115 A, (iv) between first drying stage 115A and preheat/drying stage 115B, or (iv) between preheat/drying stage 115B and overcoat stage 116.

[0097] In various embodiments in which the orientation of substrate 100 changes between solar absorption layer coat stage 113 and overcoat stage 116, the barrier strike, barrier layer 300, solar absorption layer 400, and overcoat layer 500 are applied to only one side of a substantially non-cylindrical substrate 100. In various embodiments in which the orientation of substrate 100 changes between solar absorption layer coat stage 113 and overcoat stage 116, two sides of a substantially non-cylindrical substrate 100 are treated in one or more of the following stages: pre-cleaning stage 101, hot spray cleaning stage 102, anodic or bipolar cleaning stage 103, rinse stage 104, surface activation stage 105, rinse stage 106, rinse stage 108, rinse stage 110, surface activation stage 111, rinse stage 112, rinse stage 114, first drying stage 115, overcoat stage 116, and second drying stage 117.

[0098] In some embodiments, the substantially vertical rollers 701 are located between the same two stages. In some embodiments, the substantially vertical rollers 701are located between different stages. In some embodiments, the second substantially vertical roller 701 and the horizontal roller 702 are located between the same two stages. In some embodiments, the second substantially vertical roller 701 and the horizontal roller 702 are located between different stages. In some embodiments, the two substantially vertical rollers 701 and the horizontal roller 702 are located between the same two stages. In some embodiments, the two substantially vertical rollers 701 and the horizontal roller 702 are each located between different stages.

[0099] This change in orientation from a substantially vertical to a substantially horizontal orientation can be used for any of the coating lines described herein (such as those illustrated in FIGS. IB and 1C). Thus, stages can be added or removed compared to those shown in FIGS. ID and IE as described herein. In some embodiments in which the substrate 100 changes orientation during the coating process, a barrier layer 300 is added to the substrate 100. For example, the surface activation stage 105, rinse stage 106, optional barrier strike stage 107, rinse stage 108, barrier layer coat stage 109, and rinse stage 110 can be added to the process illustrated in FIGS. ID and IE as described herein. Any of the substrates 100, optional barrier layers 300, solar absorption layers 400, and optional overcoat layers 500 described herein can be used for the methods in which the orientation of the substrate changes. For substrates 100 that are relatively stiff (such as stainless steel), a larger distance between rollers can be used to change the orientation of the substrate 100 compared to substrates 100 that are easier to bend (such as copper). If desired, the distance between the rollers can be chosen such that the shortest distance is used that is sufficient to change the orientation of the substrate 100. In some embodiments, a plastic barrier (such as a polyproplyene backing) is attached to the side of the substrate 100 that is not being coated to minimize or prevent current from traveling to the uncoated side of the substrate 100.

[0100] In some embodiments, a solar coating is created in a wet chemical processing line that processes substantially non-cylindrical (e.g., sheet metal) or substantially cylindrical substrates 100 (e.g., tubes) on a continuous basis. In some embodiments, every stage is a separate station in an apparatus for the continuous processing of a substrate 100. In some embodiments, the substrate 100 is moved through each stage at the same rate (such as about 4 meters per minute). In some embodiments, the substrate 100 is moved through one or more stages at a rate of about 2 to about 50 feet per minute, such as about 35 feet per minute. In some embodiments, each stage includes one or more cells that are positioned above one or more baths containing solution used in the cell above. In some embodiments, one continuous coating line is used to add a barrier layer 300 to the substrate, and another continuous coating line is used to add a solar absorption layer 400 and overcoat layer 500.

[0101] In some embodiments, one or more batch steps are performed. For example, the substrate may be dipped into the baths in any of the stages. In some embodiments, a coating collar is used to apply solution from one or more of the stages.

[0102] FIG. IG shows several optional containment areas, such as containments for toxic chemicals, flammable materials, or spills.

[0103] In some embodiments, one or more of the stages described herein are omitted. In various embodiments, one or more of the following stages are omitted: pre-cleaning stage 101, a hot spray cleaning stage 102, anodic or bipolar cleaning stage 103, rinse stage 104, surface activation stage 105, rinse stage 106, barrier strike stage 107, rinse stage 108, barrier layer coat stage 109, rinse stage 110, surface activation stage 111, rinse stage 112, rinse stage 114, first drying stage 115, overcoat stage 116, and second drying stage 117.

Exemplary Methods and Apparatus

[0104] A pusher drive (not shown) may be used to push a rigid substrate 100 such as pipes through the coating line (such as a continuous version of the coating line illustrated in FIGS. IB or 1C). Rollers 800 can be used between stages to help move the substrate 100 between stages (FIG. IF). In some embodiments, one or more rollers 800 are used to move the substrate between stages prior to the solar absorption layer coat stage, and one or more rollers 800 are used to move the substrate between stages after the overcoat stage. In some embodiments where a fragile absorption layer is applied, rollers 800 are not used in later stages after the solar absorption layer 400 is applied until the overcoat layer 500 is cured because rollers may damage the solar absorption layer 400 (such as the three-dimensional structure of the solar absorption layer 400) (FIG. IF). In some embodiments, rollers 800 are located above and below substrate 100 to push the substrate 800 through the coating line. Rollers 800 of any suitable size may be used, such as rollers 800 with a shaft of about 0.75 inches. In some embodiments, the rollers 800 are sufficient to minimize or prevent the sagging of substrate 100.

[0105] In some embodiments, substrates 100 (such as substantially cylindrical substrates or substantially non-cylindrical substrates) are welded together using standard methods so that multiple substrates 100 can be pushed through the coating line at the same time. Desirably, the material used for the welding is electrically conductive so that an electric current can travel down the welded substrate 100.

[0106] In some embodiments, the pipe-to-pipe connector 600 shown in FIG. 4 is used to connect substantially cylindrical substrates 100 (such as pipes or tubes) so that multiple substrates 100 can be pushed through the coating line at the same time. The connector 600 includes a cylinder that contains a left portion 601, a right portion 602, and a flange 603 between the left portion 601 and the right portion 602. The left portion 601 of the connector 600 fits inside a first substrate 100 and the right portion 602 of the connector 600 fits inside a second substrate 100. Both of these substrates 100 may be adjacent to either side of flange 103, which helps hold the substrates 100 in place. In some embodiments, the diameters and lengths of the left portion 601 and right portion 602 are chosen so that the left and right portions hold the substrates 100 in place and prevent solution from getting inside of the connector 600 or substrates 100 but not so long that is hard to separate the connector 600 from the substrates 100 after they are coated. In some embodiments, the left portion 601 and right portion 602 are each between about 18 to about 30 inches in length. In some embodiments, the substrate 100 has a nominal inner diameter of about 0.1 to about 14 inches, such as about 2 inches. In some embodiments, the flange 603 is about an inch in length. In some embodiments, the left portion 601, a right portion 602, and a flange 603 are not electrically conductive. In some embodiments, the left portion 601, a right portion 602, and a flange 603 are made of a polymer, such as a non-swellable polymer, a high temperature nylon, polyvinyl chloride, or polypropylene. In some embodiments, the left portion 601, a right portion 602, and a flange 603 are composed of a material, such as a polymer, that is chemically resistant to the solutions used in the coating line. Desirably, the material does not degrade or corrode when incubated in any of the solutions used in the coating line. [0107] The cylinder may surround an electrically conductive rod 604 that has one or more electrically conductive bristles 605 on each end of the rod 604. The bristles 605 contact the substrate 100 that encircles them. In some embodiments, the bristles 605 are reversibly attached to the rod 604 to facilitate removal and replacement of the bristles 605 when they are damaged or no longer functionally adequately. For example, the bristles 605 may be screwed onto the ends of rod 604. The electrically conductive rod 604 and bristles 605 allow an electric connection to be maintained between the first and second substrates 100 that are connected by the connector 600. In some embodiments, the length and/or number of bristles 605 are chosen to provide a sufficient electrical connection between the first and second substrates 100 for electroplating of a barrier layer 300 and/or solar absorption layer 400 onto the substrates 100 as described herein. In some embodiments, the diameter, length and/or number of bristles 605 are sufficient to prevent the electric current applied during electroplating from substantially heating or melting one or more of the bristles 605 (such as preventing the melting of a small bristle that is not sufficiently large to absorb heat generated by the electric current without melting). In some embodiments, the size of the rod 604 is sufficient to provide a sufficient electrical connection between the first and second substrates 100 for electroplating of a barrier layer 300 and/or solar absorption layer 400 onto the substrates 100 as described herein. In some embodiments, the rod 604 and bristles 605 are made of stainless steel, brass, copper, aluminum, or any other highly conductive metal.

[0108] As an alternative to connector 600, any standard connector can be used to connect multiple substrates 100. In some embodiments, single substrates 100 are coated without bein ¹gO connected to another substrate 100.

Optional pre-cleaning stage 101

[0109] In some embodiments, the surface of the substrate 100 is optionally pre-cleaned with a solvent such as ethanol using standard methods in the pre-cleaning stage 101. In some embodiments, the surface of the substrate 100 (such as a carbon steel or black iron substrate 100) is optionally pre-cleaned using a mechanical cleaning method. Exemplary standard mechanical cleaning methods include mechanical bead blasting (such as blasting with glass beads), sandblasting, buffing, sanding, wire wheel cleaning, wire brush cleaning, grinding, polishing {e.g., multi- wheel polishing), or any combination of two or more of the foregoing. In some embodiments, polishing involves using one or more wheels to abrade a portion of the surface of substrate 100. In some embodiments, polishing is more desirable than bead blasting because bead blasting can leave a portion of the beads on the surface of the substrate 100 (such as part or all of a bead remaining in a scratch on the surface of the substrate 100). In some embodiments, the surface of the substrate 100 (such as a carbon steel or black iron substrate 100) is optionally pre-cleaned using a chemical cleaning method (e.g., treatment with an acid such as a hydrochloride-based acid (e.g., hydrochloric acid), hydrochloric salt, or phosphoric acid). In some embodiments, a chemical (such as an acid) is applied using a high pressure sprayer. In some embodiments, the surface of an A210 substrate 100 is mechanically cleaned (such as bead blasted or polished) or chemically cleaned to remove part or all of a zinc coating on the A210 substrate 100 prior to entering hot spray alkaline cleaning stage 102. In some embodiments, the substrate 100 is mechanically or chemically cleaned just prior to entering hot spray alkaline cleaning stage 102.

[0110] Pre-cleaning stage 101 is not required if the substrate 100 is already sufficiently clean. As an alternative to pre-cleaning stage 101, any standard method can be used to clean the substrate 100. In particular, the substrate 100 can be cleaned using known methods for cleaning materials of the same or identical composition as the substrate 100. In some embodiments, pre-cleaning stage 101 is omitted.

Hot spray cleaning stage 102

[0111] In some embodiments, a hot spray alkaline cleaning stage 102 is used to clean the substrate 100. This step uses high pressure to knock contaminants off the substrate 100. In some embodiments, the spray nozzle is above the substrate 100 and sprays the substrate 100 as the substrate passes below the nozzle. In some embodiments in which the substrate 100 is in a substantially vertical orientation and moving from left to right, the spray nozzle is angled downward (such as about 35 to about 45°) and sprays the solution from right to left. This cleaning step may be performed in a chamber that is elevated above a heated sump of an alkaline bath design to clean non-ferrous metals, including stainless steels. Exemplary bath conditions include an alkaline solution of METEX 662 by MacDermid Chemical or Enrpep 294 by Enthone that is heated to 15O⁰F. The solution may be sprayed on the substrate 100 (e.g., a tube or fin) under pressure to create impingement of the surface contaminants. The hot spray may be applied using a high speed pump and the solution then flows back into the sump. Water may be added daily to the hot spray cleaning stage 102. The bath may be changed about every six weeks.

[0112] As an alternative to the hot spray cleaning stage 102, any standard method can be used to clean the substrate 100. In particular, the substrate 100 can be cleaned using known methods for cleaning materials of the same or identical composition as the substrate 100. For example, any cleaner that cleans materials of the same or identical composition as the substrate 100 can be used, such as a cleaner that removes soils, oils, and/or dirt from a metal. In some embodiments, hot spray cleaning stage 102 is omitted.

Anodic or bipolar cleaning stage 103

[0113] In some embodiments, another heated alkaline cleaning step is performed using the same chemical composition {e.g., an alkaline solution of METEX 662 by MacDermid Chemical or Enrpep 294 by Enthone that is heated to 15O⁰F) as the hot spray cleaning stage 102 so that no rinsing is needed between these stages. This step may be an anodic or bipolar cleaning stage 103 (also called a reverse current cleaning step) that removes contaminants trapped on the surface of the substrate 100 (such as 1 μm or smaller contaminants). This step may be similar to electropolishing, but may not affect the surface of substrate 100 as much as electropolishing. In some embodiments, the reaction conditions are more aggressive for cleaning stainless steel than for cleaning copper substrates 100. In this process, the substrate 100 enters a flooded chamber that has a metal cathode surrounding the substrate 100 as it passes through the bath. Either anodic or bipolar {i.e., both anodic and cathodic) cleaning can be performed. For anodic cleaning, the substrate 100 has positive polarity, which is opposite to the polarity used in the later plating step. For bipolar cleaning, cathodic cleaning may also be performed using the same solution as used for the anodic cleaning. In some embodiments in which the substrate 100 is in a substantially horizontal orientation, the other electrode is also oriented substantially horizontally. In some embodiments in which the substrate 100 is in a substantially vertical orientation, the other electrode is also oriented substantially vertically. In some embodiments in which a substantially cylindrical substrate 100 is used, a cylinder (such as a pipe or tube) surrounds but does not touch the substrate 100 in anodic or bipolar cleaning stage 103. This outer cylinder has multiple electrodes (such as alternating cathodes and anodes) spaced apart from one another along the length of the cylinder. Current flows through the bath solution from one electrode to the next one.

[0114] The process bath may be also suspended above a heated sump. Solution may be carried to the flooded cell using a small pump. The solution overflows the bath through a filter and back into the sump. Residual solution on the substrate 100 may be stripped away using an air knife designed for the configuration of the substrate 100 (such as a substantially cylindrical or substantially non-cylindrical substrate 100). Water may be added daily to the anodic or bipolar cleaning stage 103. The bath may be changed about every six weeks.

[0115] If desired, the current density and/or the speed of the substrate 100 moving through the cell can be changed to optimize the anodic or bipolar cleaning step. For example, as the chemical solutions get older, the current density can be adjusted to compensate for changes in the solutions (such as changes that reduce the effectiveness of the solutions). Based on the analysis of a portion of the substrate 100, the current density and/or the speed of the substrate 100 moving through the cell can also be changed to optimize the cleaning during the coating of the substrate 100 or for subsequent uses of the coating line to coat other substrates 100.

[0116] As an alternative to the anodic or bipolar cleaning stage 103, any standard method can be used to clean the substrate 100. In particular, the substrate 100 can be cleaned using known methods for cleaning materials of the same or identical composition as the substrate 100. In some embodiments, anodic or bipolar cleaning stage 103 is omitted.

Drag-out rinse stage 104 A

[0117] In some embodiments, an ambient temperature, drag-out rinse stage 104A is used to remove most of the solution from the substrate 100. In particular, a single stage static rinse (no fresh water feed) uses a pump to spray the substrate 100 as it passes through the upper chamber. The water in this tank may be used on a daily basis to replenish the spray clean and anodic clean sumps for the prior two steps. Because they are heated, the two cleaning tanks lose a small amount of volume on a daily basis that may be replaced each day to ensure that the concentration is maintained sufficiently low for effective rinsing. Water may be transferred to the cleaner tanks using a transfer pump. In some embodiments, a collar that surrounds a portion of substrate 100 is flooded with water and used to rinse substrate 100. The collar may provide more thorough rinsing than a spray rinse.

[0118] As an alternative to the drag-out rinse stage 104A, any standard method can be used to rinse the substrate 100. In some embodiments, drag-out rinse stage 104A is omitted.

Reverse cascade rinse stage 104B

[0119] In some embodiments, a reverse cascade rinse stage 104B (such as a double or triple stage reverse cascade rinse) with an ambient, fresh water feed is used to rinse the substrate 100. This rinse may be a reverse cascade type rinse that directs the flow of water in the opposite direction in which the substrate 100 is moving. The rinse chambers may be suspended above sumps and have seals between the stages. Rinsing may be accomplished by spraying water on the substrate 100 using medium pressure pumps. The last rinse uses clean filtered water from city mains or DI water. The second to the last rinse (e.g., a drag-out rinse) may use water from the last rinse to reduce the consumption of water. In particular embodiments in which a double stage reverse cascade rinse is used, the first rinse may be a drag-out rinse using water from the second rinse and the second rinse is a DI water rinse. The final rinse may employ an air knife to minimize drag out of the rinse water.

[0120] As an alternative to the reverse cascade rinse stage 104b, any standard method can be used to rinse the substrate 100. In some embodiments, reverse cascade rinse stage 104b is omitted.

[0121] For substrates 100 to which a barrier layer 300 is applied, stages 105 to 110 may be used. If a barrier layer 300 is not used, then these stages can be omitted.

Surface activation stage 105

[0122] In some embodiments in which a barrier layer 300 is to be applied, the surface of the substrate 100 is activated using a slightly heated, acidic solution in surface activation stage 105. Desirably, the surface activation stage 105 leaves the surface of substrate 100 acidic to promote subsequent adhesion of barrier layer 300 to substrate 100. In some embodiments, the surface activation does not etch the substrate 100. This spray process may utilize dissolved dry acid salts in water. An acid treatment station may include a 110 gallon sump of an acidic solution of METEX 639 by MacDermid Chemical or Actane 340 by Enthone. High pressure may be used from a single pump to render an acidic surface condition prior to the barrier strike stage 107. In some embodiments, an air knife is used to remove residual solution from the substrate 100.

[0123] If the bath is not sufficiently acidic, the barrier layer 300 may not adhere to the substrate 100 (producing a visual change in the solar coating once it is applied). If the bath is too acidic, then the substrate 100 may corrode during use. If necessary, the composition of the solution can be adjusted to increase the adherence of the barrier layer 300 and to minimize or prevent corrosion of the substrate 100. If the pH of the bath increases over time, this is a sign that the bath should be changed. In some embodiments, the bath is changed about every four weeks.

[0124] As an alternative to the surface activation stage 105, any standard method can be used to activate the substrate 100. In particular, the surface of the substrate 100 can be activated using known methods for activating materials of the same or identical composition as the substrate 100. In some embodiments, surface activation stage 105 is omitted.

Rinse stage 106

[0125] In some embodiments in which the surface of a substrate is treated with acid as discussed above prior to applying a barrier layer 300, a rinse stage (such as a rinse with DI water, city water, or solution form surface activation stage 105) at ambient temperature is used to rinse the surface- activated substrate 100. A single rinse chamber suspended above a sump of water may be used to spray sump water on the substrate 100. The substrate 100 may be also rinsed with fresh DI water made from carbon filtered city water. In some embodiments, a collar that surrounds a portion of substrate 100 is flooded with water and used to rinse substrate 100. In some embodiments, an air knife is used to remove residual solution from the substrate 100. Desirably, a portion of the acid from surface activation stage 105 remains on the surface of substrate 100 after rinse stage 106.

[0126] Alternatively, any standard method can be used to rinse the substrate 100. In some embodiments, rinse stage 106 is omitted. Optional barrier strike stage 107

[0127] In some embodiments, a barrier strike stage 107 is used to increase the ability of the substrate 100 to receive the barrier layer 300 in the subsequent barrier layer coat stage 109. A thin and not necessarily complete layer of barrier strike material can provide nucleation sites or otherwise allow for better deposition on a substrate on which it is difficult to deposit the desired barrier layer. In some embodiments, the barrier strike uses the same metal as later applied in the barrier layer coat stage 109 (such as a nickel strike before a nickel plating step). In some embodiments, little metal (such as less than a thin layer of metal) is deposited on the substrate 100 during the barrier strike. Nickel may be the metal deposited from an ionic solution. In some embodiments in which a nickel barrier layer 300 is applied to the substrate 100, less than one millionth of an inch of nickel is deposited on the substrate 100 during the nickel strike deposition step. When a nickel barrier layer 300 is used, this barrier strike step is not needed (but can still be performed) for copper substrates 100 or substrates 100 with a copper layer because nickel adheres well to copper.

[0128] In some embodiments in which a nickel barrier layer 300 is applied to the substrate 100, a heated acidic nickel solution is used (such as a solution heated to 110 ⁰F). The composition of this bath depends on the substrate 100 base material. For stainless steel or black iron substrates 100, a Woods nickel or nickel sulfamate (e.g., Barrett nickel) formulation is desirable. A woods nickel formulation allows greater activation of stainless steel substrates 100 than a nickel sulfamate formulation. Since nickel adheres readily to black iron, nickel sulfamate or the stronger activating woods nickel formulation can be used to activate black iron substrates 100. In some embodiments, the nickel solution includes 4-8 ounce/gallon of nickel chloride, 0-15% by volume of HCl (v/v), and DI water for the remainder of the volume. In some embodiments, about 10 to about 15% by volume of HCl is used, such as about 10 to about 12% or about 12 to about 15% of HCl.

[0129] In some embodiments, electrical contact rollers supply electric current to the substrate 100 as it enters a plating cell. The plating cell is suspended above a heated sump. A high speed pump delivers the solution from the sump to the plating cell thereby providing solution agitation. Turbulence may be desirable to expose the surface of the substrate 100 to the solution and to ensure good mixing of the solution so that it is uniform. The upper plating cell may be sealed at both ends so little plating solution exits with the substrate 100. Exemplary seals include urethane and silicone seals in a polypropylene removable holder. Two wipers and an exit air knife strip residual solution from the substrate 100 being coated. The air knife blows compressed air onto the substrate 100 to push solution off the substrate 100. Water may be added daily to the barrier strike stage 107.

[0130] Alternatively, any standard method can be used to increase the ability of the later applied barrier layer 300 to adhere to the substrate 100. In some embodiments, barrier strike stage 107 is omitted.

Rinse stage 108

[0131] In some embodiments in which a barrier layer 300 is applied, a reverse cascade rinse (such as a double or triple stage reverse cascade rinse) with water at ambient temperature is used to wash the substrate 100 after the barrier strike stage 107. Desirably, enough rinse steps are used to minimize or prevent corrosion f the barrier layer 300 that is applied in the next stage. This reverse cascade rinse directs the flow of water in the opposite direction in which the substrate 100 is moving. Rinse chambers may be suspended above sumps and have seals between the stages. Rinsing may be accomplished by spraying water on the substrate 100 using medium pressure pumps. The last stage may be DI or city water to ensure complete rinsing. The final stage employs an air knife to minimize drag out of the rinse water. In particular embodiments in which a triple stage reverse cascade rinse is used, the first rinse may be drag-out rinse using water from the second rinse; the second rinse may be a drag-out rinse using water from the third rinse, and the third rinse may be a DI water rinse. In some embodiments, a collar that surrounds a portion of substrate 100 is flooded with water and used to rinse substrate 100.

[0132] As an alternative, any standard method can be used to rinse the substrate 100. In some embodiments, rinse stage 108 is omitted.

Barrier layer coat stage 109

[0133] In some embodiments in which a nickel barrier layer 300 is used, the barrier layer 300 is added using a nickel plating bath 503 in barrier layer coat stage 109 (FIGS. 5A and 5B). In various embodiments, one or more cells (such as 2, 3, 4, 5, 6, or more cells) comprising a length sufficient to ensure proper thickness of the nickel barrier layer 300 are used in barrier layer coat stage 109. In some embodiments, identical or substantially identical conditions are used for each plating cell. In some embodiments, 2, 3, 4, 5, 6, or more plating cells with identical nickel plating solutions 503 are used for the barrier layer coat stage 109. Using multiple cells allows a lower current density to be used than is required if only one cell is used for the barrier layer plating. In some embodiments, an air wipe is used between cells to reduce or prevent pitting of the barrier layer.

[0134] In some embodiments, the following nickel sulfamate solution 503 is used: 60 ounce/gallon of nickel sulfamate, 4 ounce/gallon of boric acid, and DI water for the remainder of the volume. In some embodiments, these concentrations vary by up to about 5% as the solution is used and replenished. As the solution 503 is used, boric acid may be added frequently in small amounts, such as about one to about two pounds. Nickel sulfamate may be added as needed, such as less than 5 gallons being added at a time about twice a week. In some embodiments, a sulfamex nickel solution from Enthone is used. Nickel sulfamate may be desirable because it allows nickel to be deposited quickly. However, other nickel formulations can also be used. The nickel plating solution 503 may be changed if it becomes contaminated. In some embodiments, the pH is between about 3 to about 5, and the temperature is about 110 to about 120 F. In some embodiments, the current density is about 50 to about 300 amps per square foot (ASF), such as about 150 ASF. In some embodiments, the plating time is about 1.5 minutes or less. In some embodiments, the nominal thickness of the barrier layer 300 is between about 100 to about 200 microinches. In some embodiments, the substrate 100 moves through the barrier layer coat stage 109 at a rate of between about 13 to about 26 feet per minute.

[0135] Each cell has a separate upper chamber and sump and its own rectifier and controls. An interconnecting piping ties the sumps together so that the same plating solution is available to all cells to generate a more consistent barrier layer 300 on the surface of substrate 100. In some embodiments, multiple cells are connected to the same sump to further minimize variability in the plating solution. A filter bank and chemical additive station allow real time analysis and additions. [0136] In some embodiments, an anode basket 502, such as a rectangular or square basket 502 with a mesh structure that contains nickel particles, is used in each plating cell (FIGS. 5 A and 5B). To assemble the anode basket 502, the nickel particles may be poured into the mesh basket 502 so that the mesh basket 502 retains the nickel particles. The nickel particles fill the space defined by the mesh basket 502. The nickel particles, such as S-rounds or electrolytically deposited pure nickel particles, serve as the anode. In some embodiments, the nickel particles are about 0.25 inches thick and about 0.75 inches in diameter. In some embodiments, the mesh structure of anode basket 502 is composed of a material other than nickel, such as titanium. The substrate 100 moves inside an internal wall 501 of the anode basket 502. Anode basket 502 may be suspended in a cell containing nickel plating solution 503 such that the nickel plating solution 503 fills the area between the substrate 100 and the internal wall 501. In some embodiments, one or more spargers 505 (such as two spargers) are used to keep the plating solution 503 moving in the area between the substrate 100 and the internal wall 501. A final air knife strips away the plating solution to ensure low discharge and waste treatment flow rates. An air scrubber (such as a 2500 cfm air scrubber) may be used to reduce emissions. If desired, an absorption media can be used prior to the air scrubber to further reduce emissions.

[0137] In some embodiments, a substantially non-cylindrical substrate 100 is orientated substantially horizontally during barrier layer coat stage 109. In some embodiments, a substantially non-cylindrical substrate 100 is orientated substantially vertically during barrier layer coat stage 109. For example, a substantially non-cylindrical substrate 100 that is substantially vertical or horizontal may be electroplated using essentially the same method as for a substantially cylindrical substrate 100.

[0138] If desired, the temperature, current density, and/or the speed of the substrate 100 moving through the cell can be changed to optimize the barrier layer plating step. For example, as the chemical solutions get older, these parameters can be adjusted to compensate for changes in the solutions (such as changes that reduce the effectiveness of the solutions). Based on the analysis of a portion of the substrate 100, the temperature, current density, and/or the speed of the substrate 100 moving through the cell can also be changed to optimize the deposition of the barrier layer 300 during the coating of the substrate 100 or for subsequent uses of the coating line to coat other substrates 100.

[0139] A variety of brightness levels can be achieved for the barrier layer 300 (such as a nickel barrier layer) ranging from, for example, a dull matt finish to the brightness of the mirror. If desired, a brightening agent such as an organic additive (e.g., sodium saccharin) can be added to the plating solution. If too much brightening agent is added, the resulting barrier layer 300 (such as a nickel barrier layer) may become too brittle or more susceptible to cracking over time at elevated temperatures. If not enough brightening agent is added, the resulting barrier layer 300 (such as a nickel barrier layer) may have an undesirably high emissivity. In some embodiments, an amount of brightening agent that results in a semi-bright barrier layer 300 with a smooth surface is used. In some embodiments, the least amount of brightening agent that results in a coated substrate 100 with desirable optical properties (such as a desirable emissivity value) is used since less bright metal is more ductile and easier to work with than brighter metal. In some embodiments, the concentration of brightening agent is about 0.1 ounce per gallon of plating solution. In some embodiments, no brightening agent is used.

[0140] If desired, a leveling agent such as an organic additive (e.g., sodium lauryl alcohol sulfate) can be added to the plating solution. If too much leveling agent is added, the resulting barrier layer 300 may not adhere sufficiently to the substrate 100. If not enough leveling agent is added, the resulting barrier layer 300 may have an undesirably high emissivity. In some embodiments, the least amount of leveling agent that results in a coated substrate 100 with desirable optical properties (such as a desirable emissivity value) is used. In some embodiments, the concentration of leveling agent is about 0.05 ounce per gallon of plating solution. In some embodiments, no leveling agent is used. In some embodiments, the brightening agent and the leveling agent may be consumed from the plating solution at the same rate.

[0141] In some embodiments, a brightening agent and/or a leveling agent is chosen based on the desired emissivity of the coated substrate 100.

[0142] In some embodiments, the thickness of the barrier layer 300 (such as a nickel barrier layer) is between about 75 to about 180 microinches, such as about 75 to about 100 microinches, about 100 to about 125 microinches, about 125 to about 150 microinches, or about 150 to about 180 microinches. In some embodiments, the thickness of the barrier layer 300 may depend on the type of material from which the barrier layer is formed. If desired, thicker barrier layers 300 can be used to create a stronger coating. In some embodiments, the emissivity of the stainless steel substrate 100 at a particular operating temperature (such as about 200 ⁰C) is between about 0.2 and about 0.4 (such as between about 0.23 and about 0.25), and the emissivity of the stainless steel substrate 100 with the nickel barrier layer at the same operating temperature is between about 0.3 and about 0.15 (such as about or less than 0.08 or 0.06).

[0143] Alternatively, any standard method can be used to apply a barrier layer 300 to the substrate 100. For example, the methods disclosed herein can be adapted using known techniques to deposit other barrier layers 300. In some embodiments, barrier layer coat stage 109 is omitted.

Rinse stage 110

[0144] In some embodiments in which a barrier layer 300 is used, a reverse cascade rinse (such as a double or triple stage reverse cascade rinse) with water at ambient temperature is used to wash the substrate 100 after barrier layer coat stage 109. This reverse cascade rinse may direct the flow of water in the opposite direction in which the substrate 100 is moving. The last stage may be DI or city water to ensure complete rinsing. This rinse may be essentially the same as the prior rinse stage 108. The final stage employs an air knife to minimize drag out of the rinse water. In particular embodiments in which a triple stage reverse cascade rinse is used, the first rinse is a drag-out rinse using water from the second rinse; the second rinse is a drag-out rinse using water from the third rinse, and the third rinse is a DI water rinse. In some embodiments, a collar that surrounds a portion of substrate 100 is flooded with water and used to rinse substrate 100.

[0145] As an alternative, any standard method can be used to rinse the substrate 100. In some embodiments, rinse stage 110 is omitted.

Surface activation stage 111 [0146] In some embodiments, a surface activation step is performed using an acidic solution at ambient temperature in surface activation stage 111. Desirably, the surface activation stage 111 leaves the surface of substrate 100 acidic to promote subsequent adhesion of solar absorption layer 400 to substrate 100. In some embodiments, the surface activation does not etch the substrate 100. Exemplary bath conditions include a 140 gallon sump of a solution of 10% sulfuric acid by volume in water. This spray process uses a gasketed or sealed chamber that may be above an ambient temperature sump. The activation solution may be applied through nozzles under high pressure. The chamber drains to the sump below. A final stage air knife minimizes drag out. If desired, more than one air knife can be used (use as 2, 3, or more air knives).

[0147] If the bath is not sufficiently acidic, then the desired amount of the solar absorption layer 400 may not adhere to the substrate 100 (producing a visual change in the solar coating). If the bath is too acidic, then the substrate 100 may corrode. If necessary, the composition of the solution can be adjusted to increase the adherence of the solar absorption layer 400 and to minimize or prevent corrosion of the substrate 100. If the pH of the bath increases over time, this is a sign that the bath should be changed. The bath may be changed about every four weeks.

[0148] As an alternative to the surface activation stage 111, any standard method can be used to activate the substrate 100. In particular, the surface of the substrate 100 can be activated using known methods for activating materials of the same or identical composition as the substrate 100. In some embodiments, surface activation stage 111 is omitted.

Rinse stage 112

[0149] In some embodiments, the substrate 100 is rinsed using a single stage rinse with an ambient temperature DI water feed. A single rinse chamber suspended above a sump of water may be used to spray sump water on the substrate 100. The exit stage may be fresh DI water made up from carbon filtered city water. In some embodiments, a collar that surrounds a portion of substrate 100 is flooded with water and used to rinse substrate 100. In some embodiments, only a single stage is used to ensure the surface is acidic when it enters the solar absorption layer 400 (e.g., a nickel-tin alloy of a Black Crystal® coating) coat stage 113. In some embodiments in which a Black Crystal® coating is applied, no air knife is used in the rinse stage 112 because the substrate 100 needs to be wet when it enters the Black Crystal® bath in the next stage. In some embodiments, an air knife is used in this stage. In some embodiments, the substrate 100 does not need to be wet when it enters solar absorption layer coat stage 113. Overflow from this rinse may be used to neutralize rinses from the hot spray cleaning stage 102 or the anodic or bipolar cleaning stage 103. Desirably, a portion of the acid from surface activation stage 111 remains on the surface of substrate 100 after rinse stage 112.

[0150] As an alternative, any standard method can be used to rinse the substrate 100. In some embodiments, rinse stage 112 is omitted.

Solar absorption layer coat stage 113

[0151] In some embodiments in which a Black Crystal® coating is applied to the substrate 100, a chilled, acidic solution is used to electrolytically apply the nickel-tin alloy of the Black Crystal® coating in the solar absorption layer coat stage 113. Physical contact with the nickel-tin alloy of the Black Crystal® coating should be minimized until an overcoat layer 500 is applied due to the fragile dendritic structure of the Black Crystal® coating.

[0152] As shown in FIG. 2A and side-view FIG. 2C, a multi compartment cell may be used to minimize oxidation of the bath solution 211. In some embodiments for the coating of substantially cylindrical substrates 100 such as tubes, this station is a straight pass through employing a gasket seal 205 (such as rubber seal) at the entrance and a hydraulic seal 210 at the exit. In some embodiments, hydraulic seals are used at both the entrance and exit of the cell. For the coating of a substantially non-cylindrical substrate 100, only one side is coated, so rollers (such as three orienting rollers) can contact the opposite (uncoated) side of the substrate 100 to move the substrate 100 through the upper cell 202. For the coating of substantially non-cylindrical substrate 100, no hydraulic seal is needed (but may still be used) at the exit of the upper cell 202 (FIG. 2D).

[0153] This stage may include an upper plating cell 202 housed in a structure that makes up a lower sump where plating solution 211 is stored. Exemplary formulations for the nickel-tin alloy of a Black Crystal® coating are disclosed in U.S. Patent No. 6,783,653, which is hereby incorporated by reference in its entirety as if put forth fully below, and particularly with respect to solar coatings. A exemplary formulation of one liter of solution 211 for applying the nickel-tin alloy of a Black Crystal® coating is made by combining 254 mL of NiCl₂ (178.49 g/L Ni²⁺), 200 mL of H₂O, 25 g NH₄HF₂, and 5 g SnCl₂. DI water may be added until the volume reaches 900 mL, then the pH may be adjusted to between about 5.8 to about 6.0 (such as about 5.8, about 5.9, or about 6.0) with NH₄OH. The solution may be then brought to a total volume of exactly one liter using DI water. An exemplary 100 gallon solution for applying the nickel-tin alloy of a Black Crystal® coating is made from 285 lbs. of nickel chloride, 44.5 lbs. ammonium hydroxide, 2.1 kg tin chloride, 9.6 kg ammonium bi- fluoride, and enough DI water to bring the final volume to 100 gallons. The pH may be adjusted to between about 5.8 to about 6.0 (such as about 5.8, about 5.9, or about 6.0) with NH₄OH. Continuous or batch (e.g., periodic additions such as daily or hourly additions) additions may be made of each component to keep the desired ratios of the components in the bath. In some embodiments, a 110 gallon sump is used.

[0154] In some embodiments, the current density is between about 15 to about 25 ASF, such as about 20 ASF. In some embodiments, the plating time is about 7.5 seconds, which is the residence time for a point on the substrate 100 as it moves through the upper cell 202. In some embodiments, the temperature is maintained at about 21 ⁰C but may increase or decrease few degrees throughout the production cycle.

[0155] The plating solution 211 may be lifted to the upper cell 202 by pump 207 with flow control 206 or it may be pumped to a gravity feed tank separate from the sump and drained into the cell at a controlled rate. The solution 211 fills the plating cell 202 when a substrate 100 (e.g., tube or pipe) may be inserted for coating. As show in FIG. 2B, multiple hoses 303- 306 (such as 2, 3, 4, or more hoses) can be connected to the same pump 207 to allow solution 211 to be applied to multiple areas near the entrance of the plating cell 202. Having the solution 211 enter the plating cell 202 in multiple locations may allow a more uniform movement of the solution 211 through the plating cell 202 and a more even deposition of the solar absorption layer 400 on the substrate 100. In some embodiments, one or more baffles are included in plating cell 202 so that solution flows through plating cell 202 more smoothly. Take-off hoses at the top 301 and bottom 320 of the plating cell 202 remove some of the solution 211 from the plating cell 202 and help prevent gradients and an undesirable flow of solution between the top and bottom of the plating cell 202. Referring again to FIG. 2A, a seal 205 at the inlet end keeps solution from leaking out the back. The seal 210 at the exit may be a pure hydraulic seal to minimize contact with the surface of the substrate 100 after the electroplating that forms the solar absorption layer 400. The hydraulic seal 210 allows a controlled amount of solution to flow out of the upper cell 202 which may be directed using a spillway 212 for a gentle return back to the sump. This solution leaks out around the entire circumference of the substrate 100 as if flows out of the upper cell 202. This overall design reduces the amount of agitation and the amount of air the plating solution is exposed to in order to reduce oxidation of the tin component out of the solution. Reducing the amount of air can extend the life of the solution 211, for example, from about 2 weeks to between about 4 to about 6 weeks. The solution 211 may be changed about every four weeks.

[0156] Solar absorption layer coat stage 113 has a number of features that minimize both contamination with oxygen and bath loss. The substrate 100 enters and exits above the surface of the plating bath 211, and the upper cell 202 discharges back to the bath. The air knife also releases excess solution from the pipe back to the bath. If the substrate 100 entered the bath directly, the seals would leak plating solution (especially the hydraulic seal 210). The only plating solution 211 lost is the plating solution 211 retained on the surface of the substrate 100. The low flow rate of the solution past the substrate 100 and the hydraulic seal 210 also minimizes lose of the plating solution 211. Air or an inert gas (such as nitrogen or argon) enters through the air knife, but its flow-rate is maintained as low as is practical to remove excess plating solution 211. In some embodiments, the sealed case otherwise prevents air from entering. The spillway 212 gently reintroduces plating solution from the hydraulic seal 210 back into the bath, keeping intimate mixing of air and bath solution to a low level. Spillway 212 might be a tube or a plate, for instance.

[0157] The device has electrical contact rollers 204 at the in-feed to the upper cell to apply current to the substrate 100 as it enters the cell. A cylindrical nickel anode 203 encircles the substrate 100 anode to ensure an even distribution of the coating as current may be applied to the substrate 100 through the contacts 204 and directly to the anode 203. In some embodiments in which the substrate 100 is in a substantially horizontal orientation, the anode is also oriented substantially horizontally. In some embodiments in which the substrate 100 is in a substantially vertical orientation, the anode is also oriented substantially vertically. The substantially vertical orientation may minimize or prevent gradients and undesirable current flows in plating cell 202. In some embodiments, the current density is about 15 to about 20 ASF. The length of the coating cell 202 may be a function of the diameter of the substrate 100 being coated and the speed at which the line is designed to operate.

[0158] As the substrate 100 leaves the upper coating cell 202, it may be coated with solution. An air knife 201 wipes the liquid from the surface as the substrate 101 moves to eliminate a large amount of drag-out of the solution into the rinse tanks. This greatly reduces the amount of energy and chemicals needed to treat waste from the process. The air knife 201 can be used to ensure that the solar absorption layer 400 has the desired thickness and uniformity. In some embodiments, the thickness of the solar absorption layer 400 is between about 900 to about 1200 angstroms. In some embodiments, the solar absorption layer 400 does not streaks or a pattern based on visual inspection. The air knife 201 also uses as little air as possible when removing solution off the substrate 100. The air knife 201 may be adjusted to eliminate the surface liquid but not dry out the bi-metallic matrix of the solar absorption layer 400. If the solution on the substrate becomes too dry or is over heated, the solution may crystallize, forming contaminants in the matrix structure of the solar absorption layer 400 which cannot be removed by rinsing. In some embodiments, multiple air knives, such as 2, 3, or more air knives are used.

[0159] A chilling coil 208 in the sump keeps the bath at a low temperature to reduce oxidation of the tin in solution. Low temperatures (e.g., temperatures lower than or about 75 ⁰F, such as temperatures lower than or about 70 ⁰F, 65 ⁰F, 60 ⁰F, or 55 ⁰F, or temperatures between about 65 ⁰F and about 75 ⁰F) result in a more consistent coating because the tin stays in solution. A thermostat may be used to control a pump 207 moving chilled water from a mechanical chiller to the internal coil 208 and back to maintain the temperature.

[0160] A chemical feed tank provides constant bath replenishment to ensure consistent surface properties. A metering pump (not shown) makes continuous additions to the solution 211. The metering pump adds nickel and tin in essentially the same ratio (e.g., 80% nickel to 20% tin) as they are in solution 211. Thus, the metering pump may be used to maintain the nickel and tin in the solution 211 (which is later added to the upper cell 202) in a desired ratio. About 4 liters of bath solution may be added to the tank daily. The stage also has a level sensor (not shown). If the level of solution is too high, the metering pump may be turned off so that it temporarily stops adding more solution to the bath.

[0161] The device has a cover lid (not shown) sealed with a gasket to minimize air circulation since air oxidizes tin. The lid also keeps out condensing moisture, which over time would dilute the bath solution.

[0162] From the exit of the solar absorption layer coat stage 113, the substrate 100 may be desirably suspended without support until after the overcoat layer 500 is cured.

[0163] In some embodiments, a substantially non-cylindrical substrate 100 is orientated substantially horizontally during solar absorption layer coat stage 113 (FIG. 2D). In some embodiments, a substantially non-cylindrical substrate 100 is orientated substantially vertically during solar absorption layer coat stage 113. For example, a substantially vertical substrate 100 may be electroplated using essentially the same method as described for a substantially cylindrical substrate 100 or a substantially non-cylindrical substrate 100 that is orientated substantially horizontally.

[0164] If desired, the current density and/or the speed of the substrate 100 moving through the upper cell 202 can be changed to optimize the application of the solar absorption layer 400. For example, as the chemical solutions get older, the current density can be adjusted to compensate for changes in the solutions (such as changes that reduce the effectiveness of the solutions). Based on the analysis (such as the solar absorptivity measurement) of a portion of the substrate 100, the current density and/or the speed of the substrate 100 moving through the upper cell 202 can also be changed to optimize the deposition of the solar absorption layer 400 during the coating of the substrate 100 or for subsequent uses of the coating line to coat other substrates 100.

[0165] In some embodiments, the solar absorptivity of the solar absorption layer 400 is between about 0.85 to about 1.0, such as between about 0.935 and about 1.0, between about 0.920 and about 0.930, between about 0.935 and about 0.980, or between about 0.935 and about 0.940. If the solar absorptivity value is too low, a thicker solar absorption layer 400 can be applied in some embodiments. Current density and/or speed that the substrate 100 travels through the bath can be adjusted to apply a thicker solar absorption layer 400 layer during processing. If the solar absorptivity value is higher than desired for a particular application, a thinner solar absorption layer 400 can be applied in some embodiments. For example, a solar absorptivity value above about 0.97 may result in an emissivity that is higher than desired for certain applications. Current density and/or speed that the substrate 100 travels through the bath can be adjusted to apply a thinner solar absorption layer 400 layer during processing. Alternatively, if the desired absorptivity range is not obtained, then the solutions may be contaminated and thus should be replaced.

[0166] As an alternative to the solar absorption layer coat stage 113, any standard method (such as any vacuum deposition method) can be used to apply the solar absorption layer 400 to substrate 100. For example, the methods disclosed herein can be adapted using known techniques to deposit other solar absorption layers 400.

Rinse stage 114

[0167] In some embodiments, the substrate 100 coated with a solar absorption layer 400 is then rinsed using a rinse, such as a reverse cascade rinse (e.g., a double or triple stage reverse cascade rinse), with water (such as DI water) at ambient temperature.

[0168] In particular embodiments in which a triple stage reverse cascade rinse is used, the first rinse is a drag-out rinse using water from the second rinse; the second rinse is a drag-out rinse using water from the third rinse, and the third rinse is a DI water rinse. Three pumps supply water to spray bars from a corresponding sump below each of the three stages. Overflow from this rinse flows to the main waste treatment sump. At the end of each stage there may be air wiping seals to prevent drag out. A final high shear air knife strips the rinse water at the exit of the last stage.

[0169] As an alternative, any standard method can be used to rinse the substrate 100. In some embodiments, rinse stage 114 is omitted.

Drying stage 115

[0170] In some embodiments, one or more stations that use convective heat and/or one or more stations that use radiant heat are used to partially or completely dry the coated substrate 100 in drying stage 115. In particular embodiments, a two or three station electric powered infrared drying stage is used. In some embodiments, the heaters are quartz tubes with parabolic reflectors behind them. If a substantially cylindrical substrate 100 (e.g., a tube) is being coated, the dryers may be mounted below the substrate 100 with parabolic reflectors above forming an elliptical chamber where all energy is focused on the substrate 100. If a substantially non-cylindrical substrate 100 (e.g., sheet metal) is being coated, the dryers may be above radiating on the substrate surface. In some embodiments, one dryer (e.g., one radiant source) is used for each parabolic reflector. Fans may be used to mitigate heat build up. A short cool down section may be at the end of the heater array.

[0171] In some embodiments, predominantly convective heat is used to partially or completely dry at least a portion of substrate 100. For example, a forced hot air dryer (such as a Hotwind 2 440V dryer by Leister) can be used to supply the convective heat. In some embodiments, the use of a forced hot air dryer instead of an infrared dryer allows a shorter drying stage 115 to be used (such as a drying stage 115 of about 110 feet or less in length). A forced hot air dryer may also be less susceptible to corrosion than an infrared dryer. A forced hot air dryer may also be less likely to overheat the substrate 100 than an infrared dryer. If desired, one or more infrared drying sections can be used instead of or after the forced hot air drying section.

[0172] In some embodiments, first drying stage 115 is divided into two stations: first drying stage 115A and a preheat/drying stage 115B (FIGS. ID and IE). In some embodiments as illustrated in FIGS. ID and IE, the substrate 100 changes from a substantially vertical to a substantially horizontal orientation between first drying stage 115A and preheat/drying stage 115B. In some embodiments, substrate 100 is completely dried by first drying stage 115A so that it is dry before contacting rollers 701 or 702 that are after first drying stage 115. Because of the distance between first drying stage 115A and preheat/drying stage 115B that is needed to change the orientation of the substrate 100, preheat/drying stage 115B may be useful to prevent the temperature of the substrate 100 that has been dried in the first drying stage 115A from cooling to a temperature that is too low prior to entering overcoat stage 116. In some embodiments, preheat/drying stage 115B contains a single dryer, such as an infrared dryer or forced hot air dryer. In some embodiments, first drying stage 115A is omitted, and at least a portion of the substrate is partially or completely dried in preheat/drying stage 115B. [0173] If the temperature of the drying stage 115 is too low, then the substrate 100 may be too damp when it enters the overcoat stage 116. If the substrate 100 is too damp, the liquid may dilute the solution for forming over coat layer 500. Desirably, substrate 100 is completely dry when it enters the overcoat stage 116. If the temperature is too high, the solvent for the solution used to deposit overcoat layer 500 may flash or evaporate before contacting the substrate 100, or the overcoat layer 500 may crack. In some embodiments, the temperature of the substrate 100 entering the next overcoat stage 116 is between about 90 to about 150 ⁰F, such as between about 90 to about 110 ⁰F or between about 100 to about 120 ⁰F.

[0174] If desired, the substrate 101 can be inspected (such as by measuring its temperature, emissivity, solar absorptivity, or infrared reflectivity) prior to entering overcoat coat stage 116. Desirably, the infrared reflectivity of the substrate 100 is between about 0.87 and 0.92. As an alternative, any standard method can be used to at least partially or completely dry the substrate 100. In some embodiments, drying stage 115 is omitted.

Overcoat Coat stage 116

[0175] In some embodiments, an overcoat layer 500 (such as a sol-gel layer) is applied using spray or fog nozzles in overcoat coat stage 116. An exemplary solution for forming a sol-gel layer includes about 10% to about 20% (such as about 15%) tetraethylorthosilicate (TEOS) by volume in a solvent (e.g., ethanol) at ambient temperature. Hydrochloric acid may be used as a catalyst. Another exemplary solution for forming a sol-gel layer is made by combining (i) 140 milliliters (ml) (0.622 moles) of tetraethyl orthosilicate; (ii) 140 ml (2.38 moles) of ethanol; (iii) 10.93 ml (0.607 moles) of water; and (iv) 0.45 ml of IM hydrochloric acid solution {see, for example, U.S. Patent NO. 6,783,653, which is hereby incorporated by reference in its entirety, particularly with respect to solar coatings and sol-gel layers). Another exemplary solution for forming a sol-gel layer is made by generating about 4 liters of concentrate solution that is then diluted as needed. To create a base stock, 1745 ml TEOS, 1745 ml ETOH, 5.75 ml HCL, and 141 ml distilled water are combined in a heated reactor with stirring and water cooled condensing. While stirring, the temperature may be increased to 60 ^CC for 1 hour and 30 minutes. This base stock may be then converted to the concentrated sol gel solution by combining 1000 ml base stock, 2438 ml ETOH, and 174 ml distilled water in a similar reactor system. While stirring, the temperature may be increased to 40⁰C for 2 hours. The resulting concentrated solution may be then combined at the time of production with straight ethanol to make the working solution. In some embodiments, the ratio of the concentrated solution to ethanol is about 50%. If desired, the working solution can be adjusted to adjust the thickness of the overcoat layer 500 on the substrate 100. For example, the final concentration of TEOS in the working solution can be adjusted to yield a desired deposition rate of the sol gel layer on the substrate 100. In some embodiments, a higher final concentration of TEOS is used to coat cylindrical substrates 100 compared to the final concentration of TEOS used to coat non-cylindrical substrates 100 to increase the surface tension of the silica around the cylindrical substrates 100. The solution may be applied at a rate of about 2 to about 3 gallons per hour (gph). The long chain polymer formed from the TEOS may further polymerize as the solvent evaporates. Over time, the sol-gel layer may contract, which improves the optical properties of this layer. Desirably, overcoat layer 500 (such as a sol-gel layer) completely covers a substantially cylindrical substrate 100. In some embodiments, overcoat layer 500 (such as a sol-gel layer) completely covers one side of a substantially non-cylindrical substrate 100. Desirably, overcoat layer 500 (such as a sol- gel layer) completely covers the solar absorption layer 400 on substrate 100 after overcoat layer 500 dries.

[0176] In some embodiments in which a substantially cylindrical substrate 100 (e.g., a tube) is coated, an array 701 of two or more nozzles (such as 3, 4, or 5 fog or spray nozzles) is arranged around the substrate 100, such as nozzles at about 25 degree increments around the substrate 100 (FIGS. 6A-6C). In certain embodiments, an array 701 of five fog or spray nozzles is used (FIGS. 6A-6C). In some embodiments, overcoat stage 116 has two arrays 701 of nozzles (such as two arrays that each have five fog or spray nozzles) and only one array of nozzles is used at a time (FIGS. 6A and 6C). The extra array of nozzles allows the coating line to be used continuously even if one array 701 of nozzles is undergoing maintenance or being repaired.

[0177] If some embodiments where a substantially cylindrical substrate 100 is coated, fog nozzles such as high volume low pressure (HVLP) fog nozzles (e.g., MAG HVLP spray gun by Binks) or pressurized fog nozzles (such as direct pressure fog nozzles) are used to apply the overcoat layer 500. As illustrated in FIG. 6A, in some embodiments, a tank 709 holds the solution for forming a sol-gel layer. The tank 708 may be in fluid communication with an application pump 708. An air pressure regulatory 705 may be used to control the amount of pressurized gas (such as pressurized nitrogen or air) from a pressure chamber that is used to force solution through pipes to the array 701 of fog nozzles. In some embodiments, a fluid pressure regulatory 707 is also used to control the amount of fluid that flows through the pipes to the array 701 of fog nozzles, where the fluid is atomized into droplets. Pressurized fog nozzles are easier to adjust than conventional pump sprayers because it is possible to vary the amount of pressure used. HVLP fog nozzles can be adjusted to alter the amount of fluid flowing through the orifice and/or to alter the width of the fan of particles generated by the fog nozzle. Fog nozzles use less solution, result in less solution dripping off the substrate 100, and generate a more even coating than conventional pump sprayers.

[0178] In some embodiments, an air knife 712 using nitrogen gas evens the distribution of the overcoat layer 500 around the surface of the tube (FIG. 6D). The air wipe step also minimizes the effect of gravity pulling the coating towards the bottom of the substrate 100. This air wipe may be performed in a separate chamber than the application of the overcoat layer 500 so the air flow for the air wipe does not affect the fog pattern created by the fog nozzle. In some embodiments, the air wipe step is omitted.

[0179] If desired, excess solution and/or silica dust from the TEOS solution can be collected using standard methods. For example, excess solution can be collected in a receptacle 702 (such as a catch tray) below the substrate 100 to minimize or prevent ethanol from entering waste streams. If desired, a silica dust filter 711 and/or a wet silica filter medium 704 can be used to remove silica dust from vapor before it is released to the atmosphere to minimize pollution (FIGS. 6B-6D). In some embodiments, vapor and/or liquid from the solution used to generate the overcoat layer 500 is removed using ducting 703 to either a wet scrubber or a dust collector or filter 711 with a carbon pack 710 (FIG. 6D). An air scrubber (such as a 2500 cfm air scrubber) may be used to reduce emissions. If desired, an absorption media can be used prior to the air scrubber to further reduce emissions. In some embodiments, any silica that collects inside of the ducting 703 is removed during periodically.

[0180] If a substantially non-cylindrical substrate 100 (e.g., sheet metal) is being coated, the spray is on one side, desirably from above, with precise adjustment to deliver the correct amount. For example, a single nozzle (such as a HVLP fog nozzle or a pressurized fog nozzles) above the substrate 100 may be used to spray the substrate as the substrate passes below the nozzle. In some embodiments, a flat or V-shaped spray pattern is used. In some embodiments, an air knife is not used to remove excess solution since the process is self- leveling. An alternate method uses a flood bath and a leveling nitrogen gas knife.

[0181] In some embodiments for coating cylindrical or non-cylindrical substrates 100, pressurized nozzle (e.g., a fog nozzle such as a direct pressure nozzle) with a small orifice is used to create small droplets. Fluid under pressure, such as between about 20 to about 50 psi, may be used. In some embodiments for coating cylindrical or non-cylindrical substrates 100, a fog nozzle with an orifice of about 0.15 mm/0.006 inches, such as a natural fog nozzle (e.g., fog nozzle #NFN-1510SS/cvl018LS with 1/8-27NPT male threads manufactured by Natural fog Company Ltd. in Feng- Yuan City, Taiwan). In some embodiments, the nozzle is a stainless steel nozzle (such as an industrial grade, stainless steel nozzle made for continuous use).

[0182] The chamber configurations for cylindrical and non-cylindrical substrates 100 are similar but not interchangeable. In some embodiments in which cylindrical substrates 100 are used, the openings where the substrate enters and leaves the chamber are substantially round holes. In some embodiments in which non-cylindrical substrates are used 100, the openings where the substrate enters and leaves the chamber are substantially rectangular openings. As discussed above, the spray head orientation and spray pattern that is used may differ depending on whether a cylindrical or non-cylindrical substrate 100 is being coated.

[0183] In some embodiments, rollers are used to apply the overcoat layer 500 (such as a sol-gel layer) onto non-cylindrical substrates 100.

[0184] If desired, the amount of pressure used for the fog nozzle, the amount of overcoat solution applied to substrate 100, the speed of the substrate 100 moving through the cell, and/or the temperature of the substrate entering overcoat coat stage 116 can be changed to optimize the application of the overcoat layer 500. Based on the analysis (such as the solar absorptivity measurement) of a portion of the substrate 100, one or more of these variables can be changed to optimize the deposition of the overcoat layer 500 during the coating of the substrate 100 or for subsequent uses of the coating line to coat other substrates 100.

[0185] As an alternative, any standard method can be used to apply the overcoat layer 500 to substrate 100. For example, the methods disclosed herein can be adapted using known techniques (e.g., physical vapor deposition or chemical vapor deposition) to deposit other overcoat layers 500. In some embodiments, overcoat coat stage 116 is omitted.

Drying stage 117

[0186] In some embodiments, the substrate 100 with an overcoat layer 500 is dried in a drying stage 117 that is similar to the drying stage 115 but contains more heating elements to cure the overcoat layer 500 (such as a sol-gel layer). In some embodiments, radiant heat and/or hot air is used to remove water from the overcoat layer 500, and radiant heat is used to cure the overcoat layer 500. In some embodiments, one or more forced air blowers (such as 1, 2, 3, 4, or more Hotwind S 440V driers by Leister) and then one or more infrared heaters (such as 1, 2, 3, 4, or more W-2024 SS heaters by Infratech) are used. In particular embodiments, three forced air blowers and two infrared heaters are used. If desired, the substrate 100 can be inspected during the drying process, such as between the first and second forced air blower to measure its emissivity, solar absorptivity, or infrared reflectivity. This section may be enclosed and vented to an air scrubber (such as a 2500 cfm air scrubber) to control alcohol vapors from the curing overcoat. The nitrogen gas must be anhydrous when a sol-gel layer is being dried. Substantially cylindrical substrates 100 are desirably heated more rapidly than substantially non-cylindrical substrates 100. Desirably, the overcoat layer 500 is heated rapidly enough to prevent gravity from substantially affecting the uniformity of overcoat layer 500 on substantially cylindrical substrates 100. In some embodiments, the overcoat layer 500 is heated until sufficient moisture is removed. Desirably, the overcoat layer 500 is heated under conditions (such as the rate or duration of heating) that do not cause the drying overcoat layer 500 to fracture. In some embodiments, a process control is used to adjust conditions for drying substrate 100 so that overcoat layer 500 does not fracture. In some embodiments, the sol-gel layer is heated until the surface of the substrate is about 140 ⁰F or 150 ⁰F. In some embodiments, the internal operating temperature of the drying stage 117 is about 300 ⁰C or less. In some embodiments, the sol-gel layer is heated for less than or about 30, 20, 10, 5, or 3 seconds.

[0187] As an alternative, any standard method can be used to dry the substrate 100. In some embodiments, drying stage 117 is omitted.

Separation of Coated Substrates

[0188] If multiple substrates were combined prior to being coated, they can optionally be separated using standard methods after they are coated. For example, substrates 100 that were welded together can be separated by cutting (such as cutting using a saw) the welded substrate 100 to remove the welds and separate the multiple substrates 100 that had been welded together. In some embodiments for substrates 100 that were connected using a pipe- to-pipe connector (such as the pipe-to-pipe connector shown in FIG. 4), the substrates 100 are separated by removing the pipe-to-pipe connector using standard methods. Before and/or after the combined substrate 100 is separated into multiple individual substrates 100, the coating near the ends of the individual substrates 100 is optionally partially or completely removed using standard methods, such as grinding. Removal of the coating near the ends of the individual substrates 100 facilitates the subsequent welding of substrates 100 together after they are transported to a desired location for a solar energy collection system.

Exemplary Coated Substrates

[0189] The substrates 100 coated using the methods and apparatus of the invention can be used as receivers in a variety of solar applications. In one such aspect, the invention features a solar collection system comprising (a) a receiver (such as any of the coated substrates 100 described herein) comprising a metallic substrate 100, a barrier layer 300, and a solar absorption layer 400 on at least a portion of the barrier layer 300, and (b) a reflector capable of reflecting solar energy onto the receiver. In another aspect, the invention provides a method for collecting solar energy. This method includes reflecting solar energy from a reflector onto a solar energy receiver. In some embodiments, the receiver (such as any of the coated substrates 100 described herein) comprises a metallic substrate 100, a barrier layer 300, and a solar absorption layer 400 on at least a portion of the barrier layer 300. In some embodiments, the substrate 100 also includes an overcoat layer 500 (such as a sol-gel layer) on at least a portion of the solar absorption layer 400. In some embodiments, the coated substrate 100 contains a heat exchange fluid (such as a heat exchange fluid disclosed in WO 2005/07360, which is incorporated by reference in its entirety, particularly with respect to solar collection systems and the transfer of energy from a coated receiver to a heat exchange fluid).

[0190] In some embodiments, the coated substrate 100 (e.g., a substrate 100 coated with a barrier layer 300, a solar absorption layer 400, an overcoat layer 500, or any combination thereof) has solar absorptivity and emissivity values that makes the substrate 100 useful for collecting solar energy. In some embodiments, the coated substrate 100 e.g., a substrate 100 coated with a barrier layer 300, a solar absorption layer 400, an overcoat layer 500, or any combination thereof) has a solar absorptivity (e.g., a solar absorptivity at particular operating temperature such as about 200 ⁰C) between about 0.900 and about 1.0, such as such as between about 0.935 and about 1.0, between about 0.920 and about 0.930, between about 0.935 and about 0.980, or between about 0.935 and about 0.940, or between about 0.950 and about 1.0. In some embodiments, the emissivity of the coated substrate 100 (e.g., a substrate 100 coated with a barrier layer 300, a solar absorption layer 400, an overcoat layer 500, or any combination thereof) at particular operating temperature such as about 200 ⁰C is between about 0 and about 0.4, such as between about 0 and about 0.3, between about 0.08 to about 0.3, between about 0.2 to about 0.3, between about 0.3 to about 0.4, between about 0 to about 0.2, between about 0 and about 0.1, or between about 0 and about 0.08.

[0191] In some embodiments, the coated substrate 100 (e.g., a substrate 100 coated with a barrier layer 300, a solar absorption layer 400, or an overcoat layer 500, or any combination thereof) is stable in air (e.g., dry air) at greater than 150 ⁰C for at least about 200 hours. In particular embodiments, the coated substrate 100 (e.g., a substrate 100 coated with a barrier layer 300, a solar absorption layer 400, an overcoat layer 500, or any combination thereof) is stable in air (e.g., dry air) at greater than 150 °C for at least or about 300, 400, 500, 600, 800, 1,000, 1,200, 1,400, 1,600, 1,800, 2,000, or more hours. In some embodiments, the coated substrate 100 (e.g., a substrate 100 coated with a barrier layer 300, a solar absorption layer 400, an overcoat layer 500, or any combination thereof) is stable in air (e.g., dry air) at a temperature between greater than 150 °C and about 500 ⁰C (e.g., about 200, 250, 300, 325, 350, 375, 400, 425, or 450 "C) for at least about 200 hours, such as at least or about 300, 400, 500, 600, 800, 1,000, 1,200, 1,400, 1,600, 1,800, 2,000, or more hours. In some embodiments, the coated substrate 100 (e.g., a substrate 100 coated with a barrier layer 300, a solar absorption layer 400, an overcoat layer 500, or any combination thereof) is stable in air (e.g., dry air) at a temperature between greater than 150 ⁰C and about 375 ⁰C for at least about 200 hours, such as at least or about 300, 400, 500, 600, 800, 1,000, 1,200, 1,400, 1,600, 1,800, 2,000, or more hours.

EXAMPLE

[0192] The example, which is intended to be purely exemplary of the invention and should therefore not be considered to limit the invention in any way, also describes and details aspects and embodiments of the invention discussed above.

Example 1: Coating of Substrates with Black Crystal®

[0193] Several exemplary cylindrical substrates were coated with Black Crystal® (with or without a nickel barrier layer) using a rotating cathode station designed to replicate conditions that are present in a continuous coating line (such as one of the continuous coating lines described herein). FIG. 7 lists the substrates that were coated. Samples Xl - X12 were alumina bead blasted ASTM A106 carbon steel. Samples BP- 12 and BP- 13 were ASTM A 106 carbon steel, lathe turned, and plated with nickel to provide a comparison of the two surface conditions on A 106 substrate. A nickel strike, nickel barrier layer 300, and Black Crystal® layer were applied to all samples listed in FIG. 7 except for samples SS-25-2 and SS-26-2. Samples SS-25-2 and SS-26-2 were 304 stainless steel pipes that were lathe turned to provide a uniform surface. A nickel strike and Black Crystal® layer were applied to samples SS-25-2 and SS-26-2. Samples SS-25-2 and SS-26-2 were used as control samples to measure repeatability of the process for applying the Black Crystal® coating.

[0194] A nickel strike was applied to all of the substrates in FIG. 7 using a Woods nickel chloride formulation. The nickel strike produced a minimal thickness deposit (such as a 1 microinch or smaller layer of nickel). For all of the samples except SS-25-2 and SS-26-2, a nickel barrier layer was applied using a nickel sulfamate solution. A custom designed rotating cathode station was used with an internal brush inserted in the pipe substrates to provide DC current in the plating circuit. This station provides movement of the pipe substrates in the plating solution to replicate conditions that are present in the continuous coating line. A hot plate and stirrers were used to control temperature and solution agitation.

[0195] The thickness of the nickel barrier layer was measuring using standard X-Ray fluorescence methods (FIG. 7). The absorptivity of the coated substrates was measured with a Devices and Services Company model SSR-E solar spectrum reflectometer using standard methods. The emissivity of the coated substrates was measured with an AZ Temp 2000 portable IR reflectometer using standard methods. The center of the Black Crystal® plating band has the most even coating; thus, this area was selected for measuring the absorbance and emissivity. The absorbance and emissivity measurements were made at room temperature centered in the critical zone in the center of the Black Crystal® plating band at four equidistant points circumferentially around the pipe substrates. The four measurements were averaged (FIG. 7).

Example 2: Exemplary Chemicals and Methods for Coating of Substrates

[0196] In some embodiments, the coating line cleans, plates, and clear coats a substrate 100, such as a metal pipe (FIG. IG). The cleaning portion of the process strips any organic contaminants, surface rust, or other impurities from the metal in preparation for plating. Following the cleaning steps, the plating portion of the line applies a metallic outer layer to the substrate 100 (such as a pipe). A final protective outer clear coat is applied in the spray booth and dried at temperature in the curing stations.

[0197] Exemplary chemical components used in the cleaning, plating and spray processes of the line (FIG. IG) are outlined in Table 1 below with estimated quantities in solution. If desired, other chemicals or other quantities can be used (such as other chemicals and quantities described herein). Table 1: Exemplary Chemicals and Quantities for Coating Substrates

[0198] This invention has been described and specific examples of the invention have been portrayed. While the invention has been described in terms of particular variations and illustrative figures, those of ordinary skill in the art will recognize that the invention is not limited to the variations or figures described. In addition, where methods and steps described above indicate certain events occurring in certain order, those of ordinary skill in the art will recognize that the ordering of certain steps may be modified and that such modifications are in accordance with the variations of the invention. Additionally, certain of the steps may be performed concurrently in a parallel process when possible, as well as performed sequentially as described above. Therefore, to the extent there are variations of the invention, which are within the spirit of the disclosure or equivalent to the inventions found in the claims, it is the intent that this patent will cover those variations as well. Finally, all publications, patents, and patent application cited in this specification are herein incorporated by reference in their entirety as if each publication, patent, or patent application were specifically and individually put forth herein. [0199] What is described herein by example and not by limitation includes the following:

1. A method of applying a solar absorption layer onto a metallic substrate 100, the method comprising: (a) activating a metallic substrate (i) to promote adherence of a barrier layer to the substrate, (ii) to promote nucleation of the barrier layer, or (iii) to promote both adherence and nucleation of the barrier layer; (b) applying a barrier layer onto at least a portion of the substrate; and (c) applying a solar absorption layer onto at least a portion of the barrier layer.

2. The method of paragraph 1, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

3. The method of any of paragraphs 1 and 2, wherein the solar absorption layer comprises a nickel-tin alloy.

4. The method of any of paragraphs 1-3, wherein the substrate is activated by incubation in an acidic solution of metal.

5. The method of paragraph 4, wherein the solution comprises a Woods nickel formulation.

6. The method of paragraph 4, wherein the solution comprises nickel sulfamate.

7. The method of any of paragraphs 1-6, wherein at least a portion of the barrier layer is applied by electroplating an acidic solution of metal.

8. The method of any of paragraphs 1-7, wherein the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent.

9. The method of any of paragraphs 1-8, wherein the substrate comprises steel.

10. The method of any of paragraphs 1-9, wherein the substrate comprises a substantially cylindrical structure.

11. The method of any of paragraphs 1-10, further comprising the step of (d) applying a sol-gel layer onto at least a portion of the solar absorption layer.

12. The method of paragraph 11, wherein the sol-gel layer is applied using a fog nozzle.

13. The method of any of paragraphs 11 and 12, further comprising heating the sol-gel layer until the surface of the substrate is about 150⁰F.

14. A method of applying a solar absorption layer onto a metallic substrate, the method comprising: (a) incubating a metallic substrate in a solution comprising nickel sulfamate under conditions sufficient to apply a nickel layer onto at least a portion of the substrate; and (b) applying a solar absorption layer onto at least a portion of the nickel layer.

15. The method of paragraph 14, wherein the solar absorption layer comprises a nickel-tin alloy. 16. The method of any of paragraphs 14 and 15, wherein at least a portion of the nickel layer is applied by electroplating an acidic solution of nickel.

17. The method of paragraph 16, wherein the current density for the electroplating is about 150 amps per square foot.

18. The method of any of paragraphs 14-17, wherein the nickel layer comprises at least one of the group consisting of a brightening agent and a leveling agent.

19. The method of any of paragraphs 14-18, wherein the substrate comprises steel.

20. The method of any of paragraphs 14-19, wherein the substrate comprises a substantially cylindrical structure.

21. The method of any of paragraphs 14-20, further comprising the step of ( c) applying a sol-gel layer onto at least a portion of the solar absorption layer.

22. The method of paragraph 21, wherein the sol-gel layer is applied using a fog nozzle.

23. The method of any of paragraphs 21 and 22, further comprising heating the sol-gel layer until the surface of the substrate is about 150⁰F.

24. A method of applying a solar absorption layer onto a metallic substrate, the method comprising: (a) applying a barrier layer comprising at least one of the group consisting of a brightening agent and a leveling agent onto at least a portion of a metallic substrate; and (b) applying a solar absorption layer onto at least a portion of the barrier layer.

25. The method of paragraph 24, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

26. The method of any of paragraphs 24 and 25, wherein the solar absorption layer comprises a nickel-tin alloy.

27. The method of any of paragraphs 24-26, wherein the barrier layer comprises both a brightening agent and a leveling agent.

28. The method of any of paragraphs 24-27, wherein the brightening agent is sodium saccharin.

29. The method of any of paragraphs 24-28, wherein the leveling agent is sodium lauryl alcohol sulfate.

30. The method of any of paragraphs 24-29, wherein the substrate comprises steel.

31. The method of any of paragraphs 24-30, wherein the substrate comprises a substantially cylindrical structure.

32. The method of any of paragraphs 24-31, further comprising the step of (c) applying a sol-gel layer onto at least a portion of the solar absorption layer. 33. The method of paragraph 32, wherein the sol-gel layer is applied using a fog nozzle.

34. The method of any of paragraphs 32 and 33, further comprising heating the sol-gel layer until the surface of the substrate is about 150⁰F.

35. An apparatus comprising: (a) a metallic substrate; (b) a barrier layer comprising at least one of the group consisting of a brightening agent and a leveling agent on at least a portion of the substrate; and ( c) a solar absorption layer on at least a portion of the barrier layer.

36. The apparatus of paragraph 35, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

37. The apparatus of any of paragraphs 35 and 36, wherein the solar absorption layer comprises a nickel-tin alloy.

38. The apparatus of any of paragraphs 35-37, comprising both a brightening agent and a leveling agent.

39. The apparatus of any of paragraphs 35-38, wherein the brightening agent is sodium saccharin.

40. The apparatus of any of paragraphs 35-39, wherein the leveling agent is sodium lauryl alcohol sulfate.

41. The apparatus of any of paragraphs 35-40, wherein the substrate comprises steel.

42. The apparatus of any of paragraphs 35-41, wherein the substrate comprises a substantially cylindrical structure.

43. A method of applying a solar absorption layer onto a metallic substrate, the method comprising: (a) inserting a metallic substrate comprising a substantially cylindrical structure into a cell comprising a solution capable of generating a solar absorption layer, wherein the solar absorption layer does not comprise black chrome; and (b) applying an electric current between the substrate and an anode encircling at least a portion of the substrate such that the solar absorption layer is deposited on at least a portion of the substrate.

44. The method of paragraph 43, wherein the substrate with the solar absorption layer is stable at over 150⁰C for at least about 200 hours.

45. The method of any of paragraphs 43 and 44, wherein the solar absorption layer comprises a nickel-tin alloy.

46. The method of any of paragraphs 43-45, wherein the solution comprises a nickel compound and a tin compound. 47. The method of any of paragraphs 46, wherein the nickel compound is NiCl₂ and the tin compound is SnCl₂.

48. The method of any of paragraphs 43-47, wherein the solution comprises NH₄OH and NH₄F₂.

49. The method of any of paragraphs 43-48, wherein the cell comprises electrical contact rollers that supply electric current to the substrate.

50. The method of any of paragraphs 43-49, wherein the temperature of the solution is between about 65 and about 75°F.

51. The method of any of paragraphs 43-50, further comprising applying a barrier layer onto at least a portion of the substrate prior to step (a).

52. The method of paragraph 51, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

53. The method of any of paragraphs 51 and 52, wherein the substrate is activated by incubation in an acidic solution of metal.

54. The method of paragraph 53, wherein the solution comprises a Woods nickel formulation.

55. The method of paragraph 53, wherein the solution comprises nickel sulfamate.

56. The method of any of paragraphs 51-55, wherein at least a portion of the barrier layer is applied by electroplating an acidic solution of metal.

57. The method of any of paragraphs 51-56, wherein the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent.

58. The method of any of paragraphs 43-57, further comprising using an air knife to remove a portion of the solution from the substrate.

59. The method of any of paragraphs 43-58, further comprising using a forced hot air dryer to dry at least a portion of the substrate.

60. The method of any of paragraphs 43-59, further comprising applying a sol-gel layer onto at least a portion of the solar absorption layer.

61. The method of paragraph 60, wherein the sol-gel layer is applied using a fog nozzle.

62. The method of any of paragraphs 60 and 61, further comprising heating the sol-gel layer until the surface of the substrate is about 150⁰F.

63. A method of applying a solar absorption layer onto a metallic substrate, the method comprising: (a) inserting a metallic substrate comprising a substantially cylindrical structure into a cell comprising a solution capable of generating a solar absorption layer; (b) electroplating the solar absorption layer onto at least a portion of the substrate; and (c) removing the substrate from the cell through an outlet in the cell, wherein the outlet comprises a hydraulic seal.

64. The method of paragraph 63, wherein the substrate with the solar absorption layer is stable at over 150⁰C for at least about 200 hours.

65. The method of any of paragraphs 63 and 64, wherein the solar absorption layer comprises a nickel-tin alloy.

66. The method of any of paragraphs 63-65, wherein the hydraulic seal allows at least a portion of the solution to leave the cell through the outlet.

67. The method of any of paragraphs 63-66, wherein the cell comprises electrical contact rollers that supply electric current to the substrate.

68. The method of any of paragraphs 63-67, wherein the electroplating comprises applying an electric current between the substrate and an anode encircling at least a portion of the substrate such that the solar absorption layer is deposited on at least a portion of the substrate.

69. The method of any of paragraphs 63-68, wherein the solution comprises a nickel compound and a tin compound.

70. The method of paragraph 69, wherein the nickel compound is NiCl₂ and the tin compound is SnCl₂.

71. The method of any of paragraphs 63-70, wherein the solution comprises NH₄OH and NH₄F₂.

72. The method of any of paragraphs 63-71, wherein the temperature of the solution is between about 65 and about 75 ⁰F.

73. The method of any of paragraphs 63-72, further comprising applying a barrier layer onto at least a portion of the substrate prior to step (a).

74. The method of paragraph 73, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

75. The method of any of paragraphs 73 and 74, wherein the substrate is activated by incubation in an acidic solution of metal.

76. The method of paragraph 75, wherein the solution comprises a Woods nickel formulation.

77. The method of paragraph 75, wherein the solution comprises nickel sulfamate.

78. The method of any of paragraphs 13-11 , wherein at least a portion of the barrier layer is applied by electroplating an acidic solution of metal.

79. The method of any of paragraphs 73-78, wherein the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent. 80. The method of any of paragraphs 63-79, further comprising using an air knife to remove a portion of the solution from the substrate.

81. The method of any of paragraphs 63-80, further comprising using a forced hot air dryer to dry at least a portion of the substrate.

82. The method of any of paragraphs 63-81, further comprising applying a sol-gel layer onto at least a portion of the solar absorption layer.

83. The method of paragraph 82, wherein the sol-gel layer is applied using a fog nozzle.

84. The method of any of paragraphs 82 and 83, further comprising heating the sol-gel layer until the surface of the substrate is about 150⁰F.

85. A method of applying a solar absorption layer onto a metallic substrate, the method comprising: (a) inserting a metallic substrate comprising a substantially cylindrical structure into a cell comprising a solution capable of generating a solar absorption layer, and wherein the cell has a device for maintaining a substantially constant amount of solution in the cell; and (b) electroplating the solar absorption layer onto at least a portion of the substrate.

86. The method of paragraph 85, wherein the substrate with the solar absorption, layer is stable at over 15O⁰C for at least about 200 hours.

87. The method of any of paragraphs 85 and 86, wherein the solar absorption layer comprises a nickel-tin alloy.

88. The method of any of paragraphs 85-87, wherein the device maintains a substantially constant ratio of nickel to tin in the solution.

89. The method of any of paragraphs 85-88, further comprising removing the substrate from the cell through an outlet in the cell, wherein the outlet comprises a hydraulic seal.

90. The method of paragraph 89, wherein the hydraulic seal allows at least a portion of the solution to leave the cell through the outlet.

91. The method of any of paragraphs 85-90, wherein the cell comprises electrical contact rollers that supply electric current to the substrate.

92. The method of any of paragraphs 85-91, wherein the electroplating comprises applying an electric current between the substrate and an anode encircling at least a portion of the substrate such that the solar absorption layer is deposited on at least a portion of the substrate.

93. The method of any of paragraphs 85-92, wherein the solution comprises a nickel compound and a tin compound. 94. The method of paragraph 93, wherein the nickel compound is NiCl₂ and the tin compound is SnCl₂.

95. The method of any of paragraphs 85-94, wherein the solution comprises NH4OH and NH₄F₂.

96. The method of any of paragraphs 85-95, wherein the temperature of the solution is between about 65 and about 75°F.

97. The method of any of paragraphs 85-96, further comprising applying a barrier layer onto at least a portion of the substrate prior to step (a).

98. The method of paragraph 97, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

99. The method of any of paragraphs 97 and 98, wherein the substrate is activated by incubation in an acidic solution of metal.

100. The method of paragraph 99, wherein the solution comprises a Woods nickel formulation.

101. The method of paragraph 99, wherein the solution comprises nickel sulfamate.

102. The method of any of paragraphs 97- 101 , wherein at least a portion of the barrier layer is applied by electroplating an acidic solution of metal.

103. The method of any of paragraphs 97-102, wherein the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent.

104. The method of any of paragraphs 85-103, further comprising using an air knife to remove a portion of the solution from the substrate.

105. The method of any of paragraphs 85-104, further comprising using a forced hot air dryer to dry at least a portion of the substrate.

106. The method of any of paragraphs 85-105, further comprising applying a sol-gel layer onto at least a portion of the solar absorption layer.

107. The method of paragraph 106, wherein the sol-gel layer is applied using a fog nozzle.

108. The method of any of paragraphs 105 and 106, further comprising heating the sol-gel layer to until the surface of the substrate is about 150⁰F.

109. A method of applying a solar absorption layer onto a metallic substrate, the method comprising: (a) inserting a metallic substrate comprising a substantially cylindrical structure through an inlet in a cell comprising a solution capable of generating a solar absorption layer, wherein the inlet comprises a first seal that reduces the amount of solution that leaves the cell through the inlet; (b) electroplating the solar absorption layer onto at least a portion of the substrate; and (c) removing the substrate from the cell through an outlet in the cell, wherein the outlet comprises a hydraulic second seal that allows at least a portion of the solution to leave the cell through the outlet, and wherein at least a portion of the solution that leaves the cell through the outlet is reintroduced into the cell.

110. The method of paragraph 109, wherein the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours.

111. The method of any of paragraphs 109 and 110, wherein the solar absorption layer comprises a nickel-tin alloy.

112. The method of any of paragraphs 109-111, wherein the first seal is a rubber seal.

113. The method of any of paragraphs 109- 112, wherein cell comprises a device that maintains a substantially constant ratio of nickel to tin in the solution.

114. The method of any of paragraphs 109- 113, wherein the cell comprises electrical contact rollers that supply electric current to the substrate.

115. The method of any of paragraphs 109-114, wherein the electroplating comprises applying an electric current between the substrate and an anode encircling at least a portion of the substrate such that the solar absorption layer is deposited on at least a portion of the substrate.

116. The method of any of paragraphs 109-115, wherein the solution comprises a nickel compound and a tin compound.

117. The method of paragraph 116, wherein the nickel compound is NiCl₂ and the tin compound is SnCl₂.

118. The method of any of paragraphs 109-117, wherein the solution comprises NH₄OH and NH₄F₂.

119. The method of any of paragraphs 109-118, wherein the temperature of the solution is between about 65 and about 75 ⁰F.

120. The method of any of paragraphs 109-119, further comprising applying a barrier layer onto at least a portion of the substrate prior to step (a).

121. The method of paragraph 120, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

122. The method of any of paragraphs 120 and 121, wherein the substrate is activated by incubation in an acidic solution of metal.

123. The method of paragraph 122, wherein the solution comprises a Woods nickel formulation.

124. The method of paragraph 122, wherein the solution comprises nickel sulfamate.

125. The method of any of paragraphs 120-124, wherein at least a portion of the barrier layer is applied by electroplating an acidic solution of metal. 126. The method of any of paragraphs 120-126, wherein the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent.

127. The method of any of paragraphs 109-126, further comprising using an air knife to remove a portion of the solution from the substrate.

128. The method of any of paragraphs 109-127, further comprising using a forced hot air dryer to dry at least a portion of the substrate.

129. The method of any of paragraphs 109-128, further comprising applying a sol-gel layer onto at least a portion of the solar absorption layer.

130. The method of paragraph 129, wherein the sol-gel layer is applied using a fog nozzle.

131. The method of any of paragraphs 129 and 130, further comprising heating the sol-gel layer until the surface of the substrate is about 150⁰F.

132. A system comprising: (a) a deposition cell comprising a solution capable of generating a solar absorption layer, wherein the solar absorption layer does not comprise black chrome; and (b) an anode capable of encircling at least a portion of a metallic substrate comprising a substantially cylindrical structure.

133. The system of paragraph 132, further comprising the substrate.

134. The system of any of paragraphs 132 and 133, wherein the substrate with the solar absorption layer is stable at over 15O⁰C for at least about 200 hours.

135. The system of any of paragraphs 132-134, wherein the solar absorption layer comprises a nickel-tin alloy.

136. The system of any of paragraphs 132-135, wherein the solution comprises a nickel compound and a tin compound.

137. The system of paragraph 136, wherein the nickel compound is NiCl₂ and the tin compound is SnCl₂.

138. The system of any of paragraphs 132-137, wherein the solution comprises NH₄OH and NH₄F₂.

139. The system of any of paragraphs 132-138, wherein the cell comprises electrical contact rollers that supply electric current to the substrate.

140. The system of any of paragraphs 132-139, wherein the temperature of the solution is between about 65 and about 75°F.

141. The system of any of paragraphs 132-140, wherein the substrate further comprises a barrier layer on at least a portion of the substrate. 142. The system of paragraph 141, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

143. The system of any of paragraphs 141 and 142, wherein the substrate is activated by incubation in an acidic solution of metal.

144. The system of paragraph 143, wherein the solution comprises a Woods nickel formulation.

145. The system of paragraph 143, wherein the solution comprises nickel sulfamate.

146. The system of any of paragraphs 141-145, wherein at least a portion of the barrier layer is applied by electroplating an acidic solution of metal.

147. The system of any of paragraphs 141-146, wherein the barrier layer comprises least one of the group consisting of a brightening agent and a leveling agent:

148. The system of any of paragraphs 132-147, further comprising an air knife for removing a portion of the solution from the substrate.

149. The system' of any of paragraphs 132-148, further comprising a forced hot air dryer for drying at least a portion of the substrate.

150. The system of any of paragraphs 132-149, wherein the substrate further comprises a sol-gel layer on at least a portion of the solar absorption layer.

151. The system of any of paragraphs 150, further comprising a fog nozzle for applying the sol- gel layer.

152. A system comprising: (a) a deposition cell comprising a solution capable of generating a solar absorption layer; and (b) an outlet in the cell for removing a metallic substrate comprising a substantially cylindrical structure from the cell, wherein the outlet comprises a hydraulic seal.

153. The system of paragraph 152, further comprising the substrate.

154. The system of any of paragraphs 152 and 153, wherein the substrate with the solar absorption layer is stable at over 15O⁰C for at least about 200 hours.

155. The system of any of paragraphs 152-154, wherein the solar absorption layer comprises a nickel-tin alloy.

156. The system of any of paragraphs 152-155, wherein the hydraulic seal allows at least a portion of the solution to leave the cell through the outlet.

157. The system of any of paragraphs 152-156, wherein the cell comprises electrical contact rollers that supply electric current to the substrate. 158. The system of any of paragraphs 152-157, wherein the electroplating comprises applying an electric current between the substrate and an anode encircling at least a portion of the substrate such that the solar absorption layer is deposited on at least a portion of the substrate.

159. The system of any of paragraphs 152-158, wherein the solution comprises a nickel compound and a tin compound.

160. The system of paragraph 159, wherein the nickel compound is NiCl₂ and the tin compound is SnCl₂.

161. The system of any of paragraphs 152-160, wherein the solution comprises NH₄OH and NH₄F₂.

162. The system of any of paragraphs 152-161, wherein the temperature of the solution is between about 65 and about 75°F.

163. The system of any of paragraphs 152-162, wherein the substrate further comprises a barrier layer on at least a portion of the substrate.

164. The system of paragraph 163, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

165. The system of any of paragraphs 163 and 164, wherein the substrate is activated by incubation in an acidic solution of metal.

166. The system of paragraph 165, wherein the solution comprises a Woods nickel formulation.

167. The system of paragraph 165, wherein the solution comprises nickel sulfamate.

168. The system of any of paragraphs 163-167, wherein at least a portion of the barrier layer is applied by electroplating an acidic solution of metal.

169. The system of any of paragraphs 163-168, wherein the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent.

170. The system of any of paragraphs 152-169, further comprising an air knife for removing a portion of the solution from the substrate.

171. The system of any of paragraphs 152-170, further comprising a forced hot air dryer for drying at least a portion of the substrate.

172. The system of any of paragraphs 152-171, wherein the substrate further comprises a sol-gel layer on at least a portion of the solar absorption layer.

173. The system of paragraph 172, further comprising a fog nozzle for applying the sol-gel layer.

174. A system comprising: (a) a deposition cell comprising a solution capable of generating a solar absorption layer; (b) an inlet in the cell for inserting a metallic substrate comprising a substantially cylindrical structure into the cell; (c) an outlet in the cell for removing the substrate from the cell; and (d) a device operably connected to the cell that is capable of maintaining a substantially constant amount of solution in the cell.

175. The system of paragraph 174, further comprising the substrate.

176. The system of any of paragraphs 174 and 175, wherein the substrate with the solar absorption layer is stable at over 150⁰C for at least about 200 hours.

177. The system of any of paragraphs 174-176, wherein the solar absorption layer comprises a nickel-tin alloy.

178. The system of any of paragraphs 174-177, wherein the device maintains a substantially constant ratio of nickel to tin in the solution.

179. The system of any of paragraphs 174-178, wherein the outlet comprises a hydraulic seal.

180. The system of paragraph 179, wherein the hydraulic seal allows at least a portion of the solution to leave the cell through the outlet.

181. The system of any of paragraphs 174-180, wherein the cell comprises electrical contact rollers that supply electric current to the substrate.

182. The system of any of paragraphs 174-181, further comprising an anode encircling at least a portion of the substrate.

183. The system of any of paragraphs 174-182, wherein the solution comprises a nickel compound and a tin compound.

184. The system of paragraph 183, wherein the nickel compound is NiCl₂ and the tin compound is SnCl₂.

185. The system of any of paragraphs 174-184, wherein the solution comprises NH4OH and NH₄F₂.

186. The system of any of paragraphs 174-185, wherein the temperature of the solution is between about 65 and about 75 ⁰F.

187. The system of any of paragraphs 174-186, wherein the substrate further comprises a barrier layer on at least a portion of the substrate.

188. The system of paragraph 187, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

189. The system of any of paragraphs 187, and 188, wherein the substrate is activated by incubation in an acidic solution of metal.

190. The system of paragraph 189, wherein the solution comprises a Woods nickel formulation.

191. The system of paragraph 189, wherein the solution comprises nickel sulfamate.

192. The system of any of paragraphs 187-191, wherein at least a portion of the barrier layer is applied by electroplating an acidic solution of metal. 193. The system of any of paragraphs 187-192, wherein the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent.

194. The system of any of paragraphs 174-193, further comprising an air knife for removing a portion of the solution from the substrate.

195. The system of any of paragraphs 174-194, further comprising a forced hot air dryer for drying at least a portion of the substrate.

196. The system of any of paragraphs 174-195, wherein the substrate further comprises a sol-gel layer on at least a portion of the solar absorption layer.

197. The system of paragraph 196, wherein the sol-gel layer is applied using a fog nozzle.

198. A system comprising: (a) a deposition cell comprising a solution capable of generating a solar absorption layer; (b) an inlet in the cell for inserting a metallic substrate comprising a substantially cylindrical structure into the cell, wherein the inlet comprises a first seal that reduces the amount of solution that leaves the cell through the inlet; and (c) an outlet in the cell for removing the substrate from the cell, wherein the outlet comprises a hydraulic second seal that allows at least a portion of the solution to leave the cell through the outlet, and wherein at least a portion of the solution that leaves the cell through the outlet is capable of being reintroduced into the cell.

199. The system of paragraph 198, further comprising the substrate.

200. The system of any of paragraphs 198 and 199, wherein the substrate with the solar absorption layer is stable at over 15O⁰C for at least about 200 hours.

201. The system of any of paragraphs 198-200, wherein the solar absorption layer comprises a nickel-tin alloy.

202. The system of any of paragraphs 198-201, wherein the first seal is a rubber seal.

203. The system of any of paragraphs 198-202, wherein cell comprises a device that maintains a substantially constant ratio of nickel to tin in the solution.

204. The system of any of paragraphs 198-203, wherein the cell comprises electrical contact rollers that supply electric current to the substrate.

205. The system of any of paragraphs 198-204, further comprising an anode encircling at least a portion of the substrate.

206. The system of any of paragraphs 198-205, wherein the solution comprises a nickel compound and a tin compound. 207. The system of paragraph 206, wherein the nickel compound is NiCl₂ and the tin compound is SnCl₂.

208. The system of any of paragraphs 198-207, wherein the solution comprises NH₄OH and NH₄F₂.

209. The system of any of paragraphs 198-208, wherein the temperature of the solution is between about 65 and about 75°F.

210. The system of any of paragraphs 198-209, wherein the substrate comprises a barrier layer on at least a portion of the substrate.

211. The system of paragraph 210, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

212. The system of any of paragraphs 210 and 211, wherein the substrate is activated by incubation in an acidic solution of metal.

213. The system of paragraph 212, wherein the solution comprises a Woods nickel formulation.

214. The system of paragraph 212, wherein the solution comprises nickel sulfamate.

215. The system of any of paragraphs 210-214, wherein at least a portion of the barrier layer is applied by electroplating an acidic solution of metal.

216. The system of any of paragraphs 210-215, wherein the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent.

217. The system of any of paragraphs 198-216, further comprising an air knife for removing a portion of the solution from the substrate.

218. The system of any of paragraphs 198-217, further comprising a forced hot air dryer for drying at least a portion of the substrate.

219. The system of any of paragraphs 198-218, wherein the substrate further comprises a sol-gel layer on at least a portion of the solar absorption layer.

220. The system of paragraph 219, wherein the sol-gel layer is applied using a fog nozzle.

221. A method of drying an apparatus, the method comprising using predominantly convective heat to dry at least a portion of an apparatus, wherein the apparatus comprises a solar absorption layer on at least a portion of the substrate.

222. The method of paragraph 221, wherein the substrate with the solar absorption layer is stable at over 15O⁰C for at least about 200 hours.

223. The method of any of paragraphs 221 and 222, wherein a forced hot air dryer supplies the predominantly convective heat. 224. The method of any of paragraphs 221-223, wherein the solar absorption layer comprises a nickel-tin alloy.

225. The method of any of paragraphs 221-224, wherein the substrate is heated until the surface of the substrate is about 150⁰F.

226. The method of any of paragraphs 221-225, further comprising using radiant heat to at dry at least a portion of the apparatus.

227. The method of any of paragraphs 221-226, wherein the substrate comprises a barrier layer on at least a portion of the substrate.

228. The method of paragraph 227, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

229. The method of any of paragraphs 221-228, wherein the substrate comprises steel.

230. The method of any of paragraphs 221-229, wherein the substrate comprises a substantially cylindrical structure.

231. The method of any of paragraphs 221-230, further comprising applying a sol-gel layer onto at least a portion of the solar absorption layer.

232. The method of paragraph 231, wherein the sol-gel layer is applied using a fog nozzle.

233. The method of any of paragraphs 231 and 232, further comprising heating the sol-gel layer until the surface of the substrate is about 150⁰F.

234. A system comprising: (a) a deposition cell comprising a solution capable of generating a solar absorption layer; (b) an inlet in the cell for inserting a metallic substrate into the cell; (c) an outlet in the cell for removing the substrate from the cell; and (d) a blower for supplying predominantly convective heat to the substrate.

235. The system of paragraph 234, further comprising the substrate.

236. The system of any of paragraphs 234 and 235, wherein the substrate with the solar absorption layer is stable at over 150⁰C for at least about 200 hours.

237. The system of any of paragraphs 234-236, wherein the substrate comprises a barrier layer on at least a portion of the substrate.

238. The system of paragraph 237, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

239. The system of any of paragraphs 234-238, wherein the blower is a forced hot air dryer.

240. The system of any of paragraphs 234-239, wherein the solar absorption layer comprises a nickel-tin alloy. 241. The system of any of paragraphs 234-240, wherein the substrate is heated until the surface of the substrate is about 150⁰F.

242. The system of any of paragraphs 234-241, further comprising a radiant heater.

243. The system of any of paragraphs 234-242, wherein the substrate comprises steel.

244. The system of any of paragraphs 234-243, wherein the substrate comprises a substantially cylindrical structure.

245. The system of any of paragraphs 234-244, wherein the substrate further comprises a sol-gel layer on at least a portion of the solar absorption layer.

246. The system of paragraph 245, wherein the sol-gel layer is applied using a fog nozzle.

247. A method of applying a sol-gel layer to an apparatus, the method comprising using a pressurized fog nozzle to apply a sol-gel layer to at least a portion of a solar absorption layer on a metallic substrate.

248. The method of paragraph 247, wherein the substrate with the solar absorption layer is stable at over 15O⁰C for at least about 200 hours.

249. The method of any of paragraphs 247 and 248, wherein the substrate comprises a barrier layer on at least a portion of the substrate.

250. The method of paragraph 249, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

251. The method of any of paragraphs 247-250, wherein the solar absorption layer comprises a nickel-tin alloy.

252. The method of any of paragraphs 247-251, wherein the fog nozzle comprises fluid under a pressure of about 20 to about 50 psi.

253. The method of any of paragraphs 247-252, wherein the fog nozzle comprises fluid under a pressure of about 30 to about 40 psi.

254. The method of any of paragraphs 247-253, further comprising heating the sol-gel layer until the surface of the substrate reaches about 150⁰F.

255. The method of any of paragraphs 247-254, wherein the substrate comprises steel.

256. The method of any of paragraphs 247-255, wherein the substrate comprises a substantially cylindrical structure.

257. A method of heating a sol-gel layer on an apparatus, the method comprising heating an apparatus at a rate sufficient to cure a sol-gel layer but insufficient to crack the sol-gel layer, wherein the apparatus comprises (i) a solar absorption layer on at least a portion of the barrier layer, and (ii) the sol-gel layer on at least a portion of the solar absorption layer.

258. The method of paragraph 257, wherein the substrate with the solar absorption layer is stable at over 150 ⁰C for at least about 200 hours.

259. The method of any of paragraphs 257 and 258, wherein the substrate comprises a barrier layer on at least a portion of the substrate.

260. The method of paragraph 259, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

261. The method of any of paragraphs 257-260, wherein the apparatus is heated for about 5 minutes or less.

262. The method of any of paragraphs 257-261, wherein the solar absorption layer comprises a nickel-tin alloy.

263. The method of any of paragraphs 257-262, wherein the substrate comprises steel.

264. The method of any of paragraphs 257-263, wherein the substrate comprises a substantially cylindrical structure.

265. A method of applying a solar absorption layer onto a metallic substrate, the method comprising: (a) bipolar electrocleaning a metallic substrate; and (b) applying a solar absorption layer onto at least a portion of the substrate.

266. The method of paragraph 265, wherein the substrate with the solar absorption layer is stable at over 15O⁰C for at least about 200 hours.

267. The method of any of paragraph 265 and 266, further comprising applying a barrier layer onto at least a portion of the substrate prior to step (b).

268. The method of paragraph 267, wherein applying the barrier layer comprises incubating the substrate in a solution comprising nickel sulfamate under conditions sufficient to apply the barrier layer onto at least a portion of the substrate.

269. The method of any of paragraphs 267 and 268, wherein the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent.

270. The method of any of paragraphs 267-279, further comprising activating the metallic substrate to increase its ability to absorb or nucleate the barrier layer after step (a) and before step (b).

271. The method of paragraph 270, wherein the substrate is activated by incubation in an acidic solution of metal. 272. The method of paragraph 271, wherein the solution comprises a Woods nickel formulation.

273. The method of paragraph 271, wherein the solution comprises nickel sulfamate.

274. The method of any of paragraphs 265-273, wherein the solar absorption layer comprises a nickel-tin alloy.

275. The method of any of paragraphs 265-274, further comprising the step of applying a sol-gel layer onto at least a portion of the solar absorption layer.

276. The method of paragraph 275, wherein the sol-gel layer is applied using a fog nozzle.

277. The method of any of paragraphs 275 and 276, further comprising heating the sol-gel layer until the surface of the substrate reaches about 150⁰F.

278. The method of any of paragraphs 265-277, wherein the substrate comprises steel.

279. The method of any of paragraphs 265-278, wherein the substrate comprises a substantially cylindrical structure.

280. A method for collecting solar energy, the method comprising reflecting solar energy from a reflector onto a solar energy receiver, wherein the receiver comprises: (a) a metallic substrate; (b) a barrier layer comprising a brightening agent or leveling agent on at least a portion of the substrate; and (c) a solar absorption layer on at least a portion of the barrier layer.

281. The method of paragraph 280, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

282. A solar collection system comprising: (a) a receiver comprising (i) a metallic substrate, (ii) a barrier layer comprising a brightening agent or leveling agent on at least a portion of the substrate, and (iii) a solar absorption layer on at least a portion of the barrier layer; and (b) a reflector capable of reflecting energy onto the receiver.

283. The system of paragraph 282, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

284. The system of any of paragraphs 282 and 283, further comprising a drive means for rotating the reflector about an axis of rotation parallel to the longitudinal axis of the reflector.

285. The system of any of paragraphs 282-284, wherein the substrate comprises steel.

286. The system of any of paragraphs 282-285, wherein the substrate comprises a substantially cylindrical structure.

287. The system of any of paragraphs 282-286, wherein the substrate comprises a liquid capable of absorbing heat from the substrate.

288. A product made by the method of any of paragraphs 1-34, 43-131,221-233, and 247-281. 289. The method of any of paragraphs 1-34,43-131,221-233, and 247-281, wherein the substrate is substantially non-cylindrical and is orientated substantially horizontally during one or more steps of the method.

290. The method of paragraph 289, wherein the substrate is orientated substantially horizontally during all the steps of the method.

291. The method of any of paragraphs 1-34,43-131,221-233, and 247-281, wherein the substrate is substantially non-cylindrical and is orientated substantially vertically during one or more steps of the method.

292. The method of any of paragraphs 1-34, 43-131,221-233, and 247-281, wherein the substrate is a substantially non-cylindrical substrate, and wherein the orientation of the substrate changes during the method.

293. The method of paragraph 292, wherein the substrate is initially orientated substantially vertically and is later orientated substantially horizontally.

294. An apparatus for coating a tubular substrate comprising: (a) an electrolytic coating chamber; (b) an overcoat station positioned downstream of the electrolytic coating chamber; (c) first roller configured to guide the substrate into the electrolytic coating chamber; and (d) second roller downstream of the overcoat station and configured to guide the substrate, wherein there are no additional rollers intermediate of the first roller and the second roller.

295. The apparatus of paragraph 294, further comprising the substrate.

296. The apparatus of paragraph 295, wherein the substrate comprises a solar absorption layer on at least a portion of the substrate.

297. The apparatus of paragraph 296, wherein the substrate with the solar absorption layer is stable at over 15O⁰C for at least about 200 hours.

298. The apparatus of any of paragraphs 295-297, wherein the substrate comprises a barrier layer on at least a portion of the substrate.

299. The apparatus of paragraph 298, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

300. The apparatus of any of paragraphs 294-299, further comprising a dryer positioned between the overcoat station and the second roller.

301. The apparatus of any of paragraphs 294-300, wherein the first roller is connected to a power supply. 302. The apparatus of any of paragraphs 294-301, wherein the electrolytic coating chamber is in fluid communication with a solar absorption coating solution to be applied to the substrate.

303. The apparatus of paragraph 302, wherein the solar absorption coating solution comprises a nickel-containing compound and a tin-containing compound.

304. The apparatus of any of paragraphs 294-303, wherein the overcoat station is in fluid communication with a coating solution.

305. The apparatus of paragraph 304, wherein the coating solution comprises a sol gel.

306. The apparatus of any of paragraphs 294-305, wherein the electrolytic coating chamber comprises a tubular electrode.

307. The apparatus of any of paragraphs 294-306, wherein the electrolytic coating chamber has a hydraulic seal at a downstream side of said coating chamber, said seal positioned to permit the substrate to pass through the hydraulic seal.

308. The apparatus of paragraph 307, wherein the electrolytic coating chamber has a mechanical seal at an upstream side of said coating chamber, said seal positioned to permit the substrate to pass through the mechanical seal.

309. The apparatus of paragraph 307, wherein the electrolytic coating chamber has a second hydraulic seal at an upstream side of said coating chamber, said seal positioned to permit the substrate to pass through the second hydraulic seal.

310. The apparatus of any of paragraphs 294-309, wherein the electrolytic coating chamber is in fluid communication with a container that holds a supply of an electroplating solution.

311. The apparatus of paragraph 310, wherein the electroplating solution is a solar absorption coating solution.

312. The apparatus of any of paragraphs 310 and 311, wherein the electrolytic coating chamber has a catch system configured to receive the electroplating solution from a hydraulic seal and reintroduce the received electroplating solution into said container without exposing the electroplating solution to excess oxygen as would occur in the absence of said catch system.

Claims

CLAIMSWhat is claimed is:

1. A method for collecting solar energy, the method comprising reflecting solar energy from a reflector onto a solar energy receiver, wherein the receiver comprises:

(a) a metallic substrate,

(b) a barrier layer comprising a brightening agent or leveling agent on at least a portion of the substrate, and

(c) a solar absorption layer on at least a portion of the barrier layer.

2. The method of claim 1, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

3. A solar collection system comprising:

(a) a receiver comprising: (i) a metallic substrate, (ii) a barrier layer comprising a brightening agent or leveling agent on at least a portion of the substrate, and (iii) a solar absorption layer on at least a portion of the barrier layer, and

(b) a reflector capable of reflecting solar energy onto the receiver.

4. The system of claim 3, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

5. The system of any of claims 3 and 4, further comprising a drive means for rotating the reflector about an axis of rotation parallel to the longitudinal axis of the reflector.

6. The system of any of claims 3-5, wherein the substrate comprises steel.

7. The system of any of claims 3-6, wherein the substrate comprises a substantially cylindrical structure.

8. The system of any of claims 3-7, wherein the substrate comprises a liquid capable of absorbing heat from the substrate.

9. A method of applying a solar absorption layer onto a metallic substrate 100, the method comprising:

(a) activating a metallic substrate (i) to promote adherence of a barrier layer to the substrate, (ii) to promote nucleation of the barrier layer, or (iii) to promote both adherence and nucleation of the barrier layer,

(b) applying a barrier layer onto at least a portion of the substrate, and

(c) applying a solar absorption layer onto at least a portion of the barrier layer.

10. The method of claim 9, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

11. The method of any of claims 9 and 10, wherein the solar absorption layer comprises a nickel-tin alloy.

12. The method of any of claims 9-11, wherein the substrate is activated by incubation in an acidic solution of metal.

13. The method of claim 12, wherein the solution comprises a Woods nickel formulation.

14. The method of claim 12, wherein the solution comprises nickel sulfamate.

15. The method of any of claims 9-14, wherein at least a portion of the barrier layer is applied by electroplating an acidic solution of metal.

16. The method of any of claims 9-15, wherein the barrier layer comprises at least one of the group consisting of a brightening agent and a leveling agent.

17. The method of any of claims 9-16, wherein the substrate comprises steel.

18. The method of any of claims 9-17, wherein the substrate comprises a substantially cylindrical structure.

19. The method of any of claims 9-18, further comprising the step of (d) applying a sol-gel layer onto at least a portion of the solar absorption layer.

20. The method of claim 19, wherein the sol-gel layer is applied using a fog nozzle.

21. The method of any of claims 19 and 20, further comprising heating the sol-gel layer until the surface of the substrate is about 150⁰F.

22. An apparatus comprising:

(a) a metallic substrate,

(b) a barrier layer comprising at least one of the group consisting of a brightening agent and a leveling agent on at least a portion of the substrate, and

(c) a solar absorption layer on at least a portion of the barrier layer.

23. The apparatus of claim 22, wherein the barrier layer comprises nickel, platinum, tantalum, tungsten, or an alloy thereof.

24. The apparatus of any of claims 22 and 23, wherein the solar absorption layer comprises a nickel-tin alloy.

25. The apparatus of any of claims 22-24, comprising both a brightening agent and a leveling agent.

26. The apparatus of any of claims 22-25, wherein the brightening agent is sodium saccharin.

27. The apparatus of any of claims 22-26, wherein the leveling agent is sodium lauryl alcohol sulfate.

28. The apparatus of any of claims 22-27 ', wherein the substrate comprises steel.

29. The apparatus of any of claims 22-28, wherein the substrate comprises a substantially cylindrical structure.