WO2016063255A1

WO2016063255A1 - High-temperature solar-absorptive coatings with high thermal conductivity and low emissivity, and methods for use thereof

Info

Publication number: WO2016063255A1
Application number: PCT/IB2015/058176
Authority: WO
Inventors: Yaniv BINYAMIN; Mubeen Baidossi; Anna NIRENBERG; Avi KENIGSBERG
Original assignee: Brightsource Industries (Israel) Ltd.
Priority date: 2014-10-23
Filing date: 2015-10-23
Publication date: 2016-04-28
Also published as: IL251802A0; CN106062095A

Abstract

Coatings for insolation-receiving surfaces, for example, in a power plant or industrial systems that convert solar energy to other forms of energy such as heat or electricity, are disclosed herein. The coatings can include planar particles of a material having thermal conductivity of at least 3 Watts per meter per degree Kelvin. The disclosed coatings can have high absorptivity in the solar portion of the electromagnetic spectrum (for example, the AM 1.5 spectrum), low emissivity with respect to blackbody radiation, high resistance to heat, and good thermal conductivity. Additional characteristics of the disclosed coatings include good adhesion to the metal substrate upon which they are applied, mechanical and environmental durability, and protection of corrosion of both the metal substrate and of the coating itself.

Description

HIGH-TEMPERATURE SOLAR-ABSORPTIVE COATINGS WITH HIGH THERMAL CONDUCTIVITY AND LOW EMISSIVITY, AND METHODS FOR USE THEREOF

FIELD

The present disclosure relates generally to formulations for coatings, and, more particularly, to solar-radiation-absorbing, heat-resistant, thermally conductive coatings for use in components of a solar tower system.

SUMMARY

Coatings for insolation-receiving surfaces, especially in a power plant or industrial system that converts solar energy to other forms of energy such as heat or electricity, can be especially useful if they have one or more desirable characteristics such as high absorptivity in the solar portion of the electromagnetic spectrum (for example, the AM 1.5 spectrum), low emissivity with respect to blackbody radiation, high resistance to heat (e.g., remaining in solid phase and chemically stable in air over long periods of time, for example 1000 hours, 2000 hours or even more than 2000 hours, at high temperatures, for example more than 550 degrees Celsius, or more than 650 degrees Celsius, or more than 750 degrees Celsius), and good thermal conductivity. Additional desirable characteristics can include good adhesion to the metal substrate upon which they are applied, mechanical and environmental durability, and protection of corrosion of both the metal substrate and of the coating itself.

In one or more embodiments, a coating for an insolation-receiving metallic surface can comprise a binder, an organic solvent, and a filler material characterized as including particles in the form of substantially plate-like or planar particles which have thermal conductivity of at least 3 Watts per meter per degree Kelvin. The coating can have emissivity of less than 80%, or less than 70%. In some embodiments, the planar particles can include a metal or metal alloy or a ceramic material with a metallic additive or coating, and can have a melting point or softening point greater than 550 degrees Celsius or greater than 650 degrees Celsius or greater than 750 degrees Celsius. The planar particles can include an alloy or super- alloy of a metal selected from, but not limited to, the group consisting of iron, nickel, cobalt, chromium, silver and gold.

For clarity, the term 'substantially planar' is used herein to describe particles wherein one dimension is smaller than either of the other two dimensions by at least 50%, or by at least 85% or by at least 95%, i.e., where a thickness (even if variable) is the smallest dimension and is at least 50% or at least 85% or at least 95% smaller than the maximum breadth and width of a particle, although particles need not be uniform or regular in shape.

In some embodiments, the coating also comprises an oxide-based pigment or a precursor thereof. The coating can also comprise at least one additive selected from the group consisting of a wetting agent, a dispersing agent, a thickening agent, a de-foaming agent, a rheological additive, an agent to improve electrostatic or other types of spraying, and an agent to prevent settling.

It can be beneficial if the planar particles are larger in one or two dimensions than the thickness of each of the one or more layers of the coating after application or after curing. This tends to 'encourage' the particles to 'lie down' in overlapping strata that are parallel to the metallic substrate, or within 10 or 20 or 30 degrees of parallel, and thus provide enhanced abrasion resistance, corrosion resistance and high temperature oxidation protection to the substrate and/or the coating itself, as well as reduce emissivity of the coated object and enhance the thermal conductivity of the coating. In embodiments, a layer of coating can be between 30 microns and 200 microns in thickness after application or after curing.

In some embodiments, at least 95% of the planar particles are larger than 30 microns in at least one dimension. Additionally or alternatively, at least 50% of the planar particles are larger than 150 microns or larger than 200 microns in at least one dimension. In some embodiments, at least 95% of the planar particles are larger than 30 microns in each of two dimensions. Additionally or alternatively, at least 50% of the planar particles are larger than 150 microns or larger than 200 microns in each of two dimensions.

With respect to the thickness of the planar particles, in some embodiments the smallest dimension (generally the thickness) of each of at least 95% of the planar particles is between 0.5 micron and 20 microns, inclusive. Additionally or alternatively, the smallest dimension of each of at least 50% of the planar particles is between 1 micron and 10 microns, inclusive.

Enough of the filler material can be provided so that there is at least partial overlap among the planar particles when they 'lie down' roughly in parallel to the metallic substrate. For clarity, it is pointed out that 'partial overlapping' and 'overlapping' should be construed herein as being synonymous, and that overlap or partial overlap (or partially overlapping or at least partially overlapping) means that at least one percent or at least 10 percent or at least 50 percent of a particle or even all of a particle extends on a projected x-y plane so as to 'cover' a corresponding part of at least one other particle that is closer to the substrate if viewed from a perspective removed any distance from the outermost surface of the applied coating. Preferably there is enough of the filler material, and it is distributed evenly enough, so that the planar particles form overlapping strata when the coating is applied to the metallic surface. For example, the combined surface area of one face of each of the planar particles (or the projection of one face on an x-y plane parallel to a substrate) can be greater than or equal to the surface area of the coated metallic surface when the coating is applied to the metallic surface.

In some embodiments, the coating is in liquid form and can include at least one of a dispersion and a solution.

In some embodiments, a heat transfer member with an insolation-receiving surface comprises a metallic substrate and a coating that comprises at overlapping strata of substantially planar particles that increase the thermal conductivity of the coating and lower its emissivity. This coating can consist of a single layer, and additionally comprise an oxide-based pigment. Alternatively, this coating can consist of at least two layers, in which case at least the layer furthest from the metallic substrate comprises an oxide-based pigment, and at least the layer closest to the metallic substrate comprises at least partially overlapping strata of planar particles that increase the thermal conductivity and reduce the emissivity of the coating. When the coating consists of at least two layers, the layer closest to the insolation-receiving metallic surface can act as a primer layer, which can enhance adhesion and can enhance abrasion resistance, corrosion resistance, and/or hot oxidation protection for the substrate and/or for the coating itself. When coated, the heat transfer member can have high absorptivity in the solar portion of the electromagnetic spectrum, for example as measured with respect to the AM 1.5 spectrum. This high absorptivity can be at least 90% or at least 95%. According to an embodiment, the coated heat transfer member has emissivity of less than 80%, and in some embodiments the coated heat transfer member has emissivity of less than 70%. Emissivity can be measured at 20 degrees Celsius. Alternatively emissivity can be measured at a higher temperature, for example at more than 200 or 300 or 400 or 500 or 600 or 700 degrees Celsius.

In some embodiments, the metallic substrate of the heat transfer member can comprise a metal alloy selected from the group consisting of steels, alloy steels and nickel superalloys. The heat transfer member can have an inner volume that comprises a conduit for a fluid. For example, the inner volume can be the inside of a pipe or of a tube or of a channel.

In some embodiments, the planar particles in the coating of the heat transfer member include metallic particles, and in some embodiments include ceramic-containing particles with thermal conductivity of at least 3 W per meter per degree Kelvin, for example talc with a metallic (e.g., gold alloy) coating. The metallic particles can include an alloy or a super- alloy of a metal selected from the group consisting of iron, nickel, cobalt, chromium, silver and gold. The metallic particles can include a metal or metal alloy and have a melting or softening point greater than 550 degrees Celsius or greater than 650 degrees Celsius or greater than 750 degrees Celsius.

In some embodiments, the coating of the heat transfer member has a thermal conductivity of at least 0.5 Watt per meter per degree Kelvin, or at least 1.0 Watt per meter per degree Kelvin. The coating can be formulated so that after application and curing on the heat transfer member it is chemically stable in air for extended periods of time, for example 1000 hours or more than 2000 hours, at a temperature of at least 650 degrees Celsius or at least 750 degrees Celsius.

In some embodiments, the overlapping strata of planar particles cover at least 90% or at least 95% or at least 99% of the surface area of the metallic substrate of the heat transfer member that is coated with the coating. The coverage of the substrate can contribute to the reduction in emissivity, or the reduction in corrosion, or an increase in abrasion resistance, or the reduction of high-temperature oxidation, or any combination of these four. At least 50%, or at least 75%, or at least 95% of the planar particles can be larger in at least one dimension than the thickness of the respective coating layer comprising the particles. Additionally or alternatively, at least 50%, or at least 75%, or at least 95% of the planar particles can be larger in two dimensions than the thickness of the respective coating layer comprising the particles.

In some embodiments a method is provided for collecting solar energy in a fluid, where the method includes reflecting insolation at a high concentration, for example, at least 100 suns, on an insolation-receiving surface of a heat transfer member with a metallic substrate that has been coated with a coating that has an absorptivity of at least 90% with respect to the AM 1.5 spectrum and an emissivity of less than 80%. Emissivity can be measured at 20 degrees Celsius or at some higher temperature. In some embodiments the absorptivity is at least 95%. In some embodiments the emissivity is less than 75% or less than 70%. The heat transfer member can comprise a metal alloy selected from the group consisting of steels, alloy steels and nickel superalloys. The method can additionally comprise conveying the fluid through an inner volume of the heat transfer member. The coating can include one layer or more than one layer, where at least the layer closest to the metallic substrate comprises a binder, an organic solvent, and overlapping strata of planar particles that increase the thermal conductivity and reduce the emissivity of the coating. The layer closest to the metallic substrate can additionally comprise an oxide-based pigment. In a multi-layer coating, at least the layer furthest from the metallic substrate can include an oxide-based pigment.

Objects and advantages of embodiments of the disclosed subject matter will become apparent from the following description when considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

Embodiments will hereinafter be described with reference to the accompanying drawings, which have not necessarily been drawn to scale. Where applicable, some features may not be illustrated to assist in the illustration and description of underlying features.

Throughout the figures, like reference numerals denote like elements.

FIG. 1A is a simplified diagram illustrating an elevation view of a solar thermal system with a single solar tower, according to embodiments of the disclosed subject matter.

FIG. IB is a simplified diagram illustrating an elevation view of a solar thermal system with multiple solar towers, according to embodiments of the disclosed subject matter.

FIG. 2A is a simplified diagram illustrating a top view of pipes in a receiver of a solar tower, according to embodiments of the disclosed subject matter.

FIG. 2B is a simplified diagram illustrating an isometric view of the receiver pipes of FIG. 2A, according to embodiments of the disclosed subject matter.

FIG. 3 is a simplified diagram illustrating an isometric cutaway view of a section of a coated heat transfer member, according to embodiments of the disclosed subject matter.

FIGS. 4A, 4B, 4C and 4D are exemplary plan-view and elevation- view illustrations of planar particles, according to embodiments of the disclosed subject matter.

FIG. 5 is a cross-sectional view of a longitudinal section of a coated heat transfer member according to embodiments of the disclosed subject matter.

FIG. 6 is a plan-view illustration of overlapping planar particles, according to embodiments of the disclosed subject matter.

FIG. 7 shows exemplary plan-view and elevation- view illustrations of a planar particle with an exemplary angular orientation, according to embodiments of the disclosed subject matter.

DETAILED DESCRIPTION

Insolation can be used by a solar thermal system to heat a fluid, for example to generate steam or heat a molten salt or a molten metal or a gas or a supercritical fluid, which may subsequently be used in the production of electricity or in industrial applications. Referring to FIG. 1A, a solar thermal system employing a single solar tower is shown. The system can include a solar tower 100, which has a target 102 that receives reflected insolation 110 from a solar field 104, which at least partially surrounds the solar tower 100. The solar tower 100 can have a height of, for example, at least 25m or at least 100m or at least 200m. The target 102 can be a solar energy receiver system, which can include, for example, an insolation receiving surface of one or more solar receivers configured to transmit heat energy of the insolation to a working fluid or heat transfer fluid flowing therethrough. The target 102 may include one or more separate solar receivers (e.g., an evaporating solar receiver and a superheating solar receiver) arranged at the same or different heights or positions. The solar field 104 can include a plurality of heliostats 106, each of which is configured to direct insolation at the target 102 in the solar tower 100. Heliostats 106 within the solar field adjust their orientation to track the sun 108 as it moves across the sky, thereby continuing to reflect insolation onto one or more aiming points associated with the target 102. The solar field 104 can include, for example, tens of thousands of heliostats deployed in over an area of several square kilometers.

FIG. IB shows a "multi-tower" version of a solar thermal system. Each tower can have a respective target, which may include one or more solar receivers. The first solar tower 100A has a target 102A thereon and is at least partially surrounded by solar field 104 for receiving reflected insolation therefrom. Similarly, a second solar tower 100B has a target 102B thereon and is at least partially surrounded by solar field 104 for receiving reflected insolation therefrom. For example, the solar receiver in one of the towers may be configured to produce steam from insolation (i.e., an evaporating solar receiver) while the solar receiver in another one of the towers may be configured to superheat the steam using insolation (i.e., a superheating solar receiver). In another example, one or more of the solar towers may have both an evaporating solar receiver and a superheating solar receiver. A limited number of components have been illustrated in FIGS. 1A-1B for clarity and discussion. It should be appreciated that actual embodiments of a solar thermal system can include, for example, optical elements, control systems, sensors, pipelines, generators, and/or turbines.

A solar receiver can include a plurality of heat transfer members such as pipes 202 with metallic surfaces which serve to transfer heat from concentrated and/or reflected insolation to heat a fluid which can be flowing through an inner volume of a heat transfer member. The receiver in each solar tower can include tens or hundreds or more of such heat transfer members, which can comprise fluid conduits or pipes configured to convey a working fluid or heat transfer fluid at high temperatures and/or pressures. For example, pipes can be configured to convey pressurized water and/or pressurized steam at temperatures in excess of 500°C and pressures in excess of 160 bar, or molten salt mixtures at temperatures between 270°C and 600°C at approximately atmospheric pressure. Referring now to FIGS. 2A-2B, an exemplary

configuration of a portion 200 of a solar receiver is shown. Heat transfer members such as pipes 202 of the receiver portion 200 can be arranged in a single row following a particular geometric configuration, for example, in the shape of a circle, hexagon, or rectangle (as shown in FIG. 2A), or in any other suitable configuration. At least a portion of the exterior surface of each pipe 202 can be arranged to receive insolation reflected by heliostats in the solar field onto the receiver. The solar insolation can heat pipes 202 and thereby heat the fluid therethrough for use in producing electricity or in other applications.

When pipes 202 or other functionally equivalent heat transfer members are constructed from metal, the native surface of the metal may be at least partially reflective to the solar radiation, thereby reducing the efficiency by which energy of the insolation is transferred as heat to the fluid flowing through the pipes 202. The metal pipes 202 can thus be treated or painted or coated to maximize or at least improve the solar absorption of the pipes 202. However, high- temperature operation of the solar thermal system (for example, at temperatures in excess of 550°C or 650°C or 750°C) and environmental exposure (for example, to a desert atmosphere where the solar thermal system is located) may adversely affect the outer layers of the metal surface of the pipes 202, including any coating applied thereto.

Coatings according to one or more embodiments of the disclosed subject matter can exhibit one or more of the following features:

• when applied to a metallic substrate (e.g., carbon steel, alloy steel, galvanized steel, stainless steel, copper, aluminum, and nickel based superalloys), the coating has sufficient thermal durability (i.e., does not ablate over time) to withstand high temperatures (e.g., at least 450°C, 500°C, 550°C, 600°C, 650°C, 750°C, or higher) over a sustained period of time (i.e., hundreds or thousands of consecutive hours or in accelerated tests of exposure conditions of, for example, at least 2000 hours);

• when applied to a metallic substrate with a coating thickness after curing less than or equal to ΙΟΟμιη, whether as a single layer or as a primer layer with a top coating layer, the coating does not peel from the article or exhibit cracking after curing;

• when applied to a metallic substrate with a coating thickness after curing greater than or equal to 2μιη, the coating remains subjectively or objectively black and/or undergoes very slow optical degradation (e.g., fading of the coating which reduces absorptivity in the solar spectrum or in the visible portion of the solar spectrum);

• when applied to a metallic substrate with a coating thickness after curing less than or equal to ΙΟΟμιη, the coating adequately protects the metallic substrate from environmental degradation, i.e., from exposure to the elements (for example, the coating can be shown to protect the metallic substrate during an accelerated test in an atmosphere of 85% relative humidity for a period of at least 200 hours, 250 hours, 300 hours, 1000 hours or more, and/or an atmosphere of salt fog for a period of 8 hours or more, 24 hours or more, 48 hours or more, or after 2000 hours or more in dry air at a temperature used for accelerated testing;

• when applied to a metallic substrate with a coating thickness after curing less than or equal to ΙΟΟμιτι, the coating has sufficient mechanical durability to withstand one or more abrasion tests, for example a falling sand test such as ASTM D9868; and

• when applied to a metallic substrate the coating and/or a pigment component thereof has an absorptivity with respect to solar radiation in the wavelength range from 250nm to 3000nm (known as the AM 1.5 spectrum) of greater than 80% or greater than 90% or greater than 95%.

The coating, including the layer closest to a metallic substrate in the case that more than one layer is applied, can be formulated to protect the metal surface (substrate) from high temperature oxidation.

In an embodiment, a high-temperature coating for use in a solar thermal system can include (i) a binder such as a metal alkoxide binder or a high-temperature inorganic binder which irreversibly converts to an inorganic binder (such as for example silica or glass) after heating at a high temperature (e.g., at or above 200°C or above 350°C), (ii) an organic solvent system which may include a carrier liquid solvent and a co-solvent and (iii) and an inorganic filler or a metallic filler. If an inorganic filler is selected it can include a ceramic material, e.g., talc, that is coated or otherwise treated with a thermally conductive metal such as, for example, gold, where the coating or treating can provide additional thermal conductivity through or around the crystalline structure of the ceramic material. If the coating is to be applied as the only layer then it can also include an inorganic black pigment, and if the coating is to be applied as a top layer of a multi-layer system of coatings then it can also include an inorganic black pigment.

A suitable binder can be a heat resistant polymeric binder. A suitable binder can include at least one of silicone resins, silicone resin copolymers, silicone-polyester resin, and silicone- epoxy resins. For example, a binder can include a silicone resin selected from a methyl polysiloxane, a phenyl polysiloxane, a medium-hard phenylmethyl silicone resin, a medium- hard high solid phenylmethyl silicone resin, a soft phenylmethyl silicone resin, a dimethyl polysiloxane, a phenyl-methyl polysiloxane, a propyl-phenyl polysiloxane silicone resin or any combinations thereof, or a polydimethylsilazane. In an example, 30-80% (wt/wt) phenylmethyl polysiloxane resin in xylene can be used. In another example, 60-70% (wt/wt) phenylmethyl polysiloxane resin in xylene can be used. A suitable binder can include a frit-based binder, an alumina based binder, a phosphate-based binder, a zirconia-based binder, or a precursor or combination of any of these. In some embodiments, a suitable binder can include at least one of the following exemplary binders: in silanes, Poly(phenyl-methylsilane), (commercially available from Gelest, USA); in borosiloxanes, Poly (boro-diphenylsiloxane), PBDS, (commercially available as SSP- 040 from Gelest, USA); in polysilazanes, Poly 1,1 dimethyl silazane telomer (commercially available as SN-2M01-1 from Gelest USA), or Poly 1,1-dimethylsilazane cross linked

(commercially available as PSN-2M02 from Gelest, USA), or Ceraset PSZ-20 (commercially available from AZ Electronic Materials, Germany), or Ceraset PURS 20 (commercially available from AZ Electronic Materials, Germany), or KiON HTT 1800 (commercially available from AZ Electronic Materials, Germany), or KiON HTA 1500 rapid cure (commercially available from AZ Electronic Materials, Germany); in siloxanes, a methyl-phenyl polysiloxane in xylene (commercially available as SILRES® REN 60 or SILRES® REN 80 from Wacker Chemie AG), or a propyl -phenyl polysiloxane (commercially available as SILRES® REN 100 from Wacker Chemie AG); in titania (Sol gel), Titania Tyzor TE precursor (triethanolamine titanium complex, commercially available from DuPont, USA); in inorganic binders, a Sol gel such as Alumina sol-gel for example Bohamit or Disperal or Disperal P3, AIO(OH)

(commercially available from Sasol, Germany).

Alternatively or additionally, one or more of the following binders can be used: a phenyl- methyl silicone resin in xylene (commercially available as SILIKOPHEN® P 80/X from Evonik Tego Chemie GmbH), a phenyl-methyl silicone resin having >95% solids, 2-propanol, 1- methoxy, acetate (commercially available as SILIKOPHEN® P 80/MPA from Evonik Tego Chemie GmbH), a phenyl-methyl silicone resin (commercially available as SILIKOPHEN® P 40/W or SILIKOPHEN® P 50/X from Evonik Tego Chemie GmbH), a methyl polysiloxane (commercially available as SILRES® KX from Wacker Chemie AG), a phenyl polysiloxane (commercially available as SILRES® 601 from Wacker Chemie AG), a silicone resin containing phenyl groups (commercially available as SILRES 602® from Wacker Chemie AG); a phenylmethyl silicone resin (commercially available as SRP150 from GE Bayer Silicones); a medium-hard phenylmethyl silicone resin(commercially available as SRP501 from GE Bayer Silicones); a medium-hard high solid phenylmethyl silicone resin (commercially available as SRP 576 from GE Bayer Silicones); soft phenylmethyl silicone resin (commercially available as SRP 851 from GE Bayer Silicones). A phenyl-methyl silicone resin in xylene or other siloxanes binder composition from other suppliers can also be suitable.

In embodiments, the coating includes a polymer-based binder, and the type of polymer as well as the ratio of polymer to other components can affect final coating properties, such as, for example, adhesion, optical properties (e.g., light absorption and reflection), corrosion resistance, and long-term high-temperature durability and thermal shock resistance. Binder concentration within the coating can be within a range of 5% to 80% (wt/wt), or in other embodiments can be within a range between 20% and 70% (wt/wt). The ratio of binder to solids (for example, filler and pigment) can be between 1: 1 and 3: 1, and in other embodiments between 1: 1 and 2: 1, where all ratio on a wt:wt basis.

In embodiments, the addition of substantially plate-like (or platelet-like or planar) particles of a filler material resistant to high temperatures can improve the resistance of the coating to corrosion and/or to detrimental effects of high temperatures. This can be visibly noticeable after 500 or 1000 or 2000 hours at a temperature above when a coating with planar particles of a high-temperature filler material remains substantially black, while a coating without such particles would be less 'black' and more 'gray' .

In embodiments, the addition of planar particles (which can be substantially planar) of filler material can enhance the thermal conductivity of the coating, and especially if the filler material comprises a metal or a metal alloy or a ceramic material with a thermal conductivity of at least 3 Watts per meter per degree Kelvin. Alternatively, the thermal conductivity of the coating after application and curing on a metallic substrate can be at least 0.5 Watts per meter per degree Kelvin or at least 1.0 Watts per meter per degree Kelvin.

In embodiments, the addition of planar particles of filler material can reduce the emissivity of the coating after the coating is applied and cured on a metallic substrate that is a part of an insolation-receiving surface. The emissivity of a coating without such particles would be higher than 80% or higher than 90% while the emissivity of a coating with such particles can be less than 80%, or less than 75%, or less than 70%.

In embodiments, enhancements to corrosion, abrasion, and hot oxidation resistance, thermal conductivity and high-temperature stability, and reduction of emissivity, can be improved by incorporating a filler of planar particles that form overlapping strata of the particles when applied to a metallic substrate. Enough of the material should be provided to ensure that the overlapping strata cover most of the surface of the metallic substrate coated. If the particles are larger than the thickness of a layer of the coating after application and curing, or close to the thickness thereof, then it is more likely that the particles will settle in strata that are roughly parallel to the substrate and not settle on their edges.

In embodiments, a metallic or ceramic-containing filler material can strengthen one or more layers of coating applied to a metallic substrate. In particular, as the planar particles (or 'platelets') form overlapping strata, this alignment may serve to improve adhesion strength of the coating as a standalone coating or as a primer. The overlap of the platelets can reinforce the coating during drying and/or curing. The platelets may also reduce internal stress due to thermal expansion/contraction and increase flexibility and crack-resistance of the dried and/or cured coating. The planar particles can provide a measure of barrier protection, since the platelets align parallel to the article surface and provide low moisture and gas permeability through the strata of platelets. The relatively high aspect ratio of the individual platelets may provide beneficial rheological properties and improve sag resistance. The overlapping strata of planar particles can prevent or slow oxidation of the metallic substrate after coating, drying and/or curing.

Selection of the filler material (i.e., the planar particles) and its concentration in the coating can affect the resulting properties of the coating, such as, but not limited to: emissivity, optical properties, thermal resistance, adhesion, corrosion resistance, abrasion resistance and hot oxidation resistance. For example, the filler material can be at a concentration between about 1% (wt/wt) and about 60% (wt/wt).

According to embodiments, a coating can be in the form of a liquid composition like a paint, and can include a solution and/or a colloid and/or a suspension. The coating can include a carrier liquid, such as an aqueous or organic solvent for ease of application to a surface of an article, for example an insolation-receiving surface of a heat transfer member in a solar receiver. A solvent can serve as a carrier for the various components of the liquid coating. In addition, a solvent can dissolve or facilitate the dissolution of a binder in the coating, thereby reducing the viscosity thereof to a suitable level for application. Modes of application can include, but are not limited to, brush, roller, pressure spray, ultrasonic spray, electrostatic spray, and airless spray. After application of the coating, the solvent can evaporate, thus leaving behind the other components of the paint formulation to form the coating on the desired article.

A solvent can include, for example, at least one of glycol ethers, aromatic naphtha solvents, and members of the xylene family (e.g., m-xylene, p-xylene, o-xylene, and/or mixtures thereof), butyl acetate, toluene, and combinations thereof. For example, the organic solvent can be at least one of 4-chlorobenzotrifluoride(4-CBTF) propylene glycol mono methyl ether (commercially available as DOWANOL™ PM from Dow Chemical Company), dipropylene glycol mono methyl ether (commercially available as DOWANOL™ DPM from Dow Chemical Company), dipropylene glycol (mono methyl ether acetate) (commercially available as

DOWANOL™ DPM A from Dow Chemical Company), tripropylene glycol mono methyl ether (commercially available as DOWANOL™ TPM from Dow Chemical Company), propylene glycol mono n-butyl ether (commercially available as DOWANOL™ PnB from Dow Chemical Company), dipropylene glycol mono butyl ether (commercially available as DOWANOL™ DPnB from Dow Chemical Company), tripropylene glycol mono n-butyl ether (commercially available as DOWANOL TPnB from Dow Chemical Company), propylene glycol mono propyl ether (commercially available as DOWANOL™ PnP from Dow Chemical Company), dipropylene glycol mono propyl ether (commercially available as DOWANOL™ DPnP from Dow Chemical Company), propylene glycol butyl ether (commercially available as

DOWANOL™ TPnB-H from Dow Chemical Company), propylene glycol mono methyl ether acetate (commercially available as DOWANOL™ PMA from Dow Chemical Company), diethylene glycol mono butyl ether (commercially available as DOWANOL™ DB from Dow Chemical Company), other ethylene or propylene glycol ethers, xylenes (m-xylene, p-xylene, o- xylene or any mixture thereof), t-butyl acetate, n-butyl acetate, and toluene. Other solvents can also be used according to one or more contemplated embodiments in order to comply with environmental requirements related to volatile organic compounds (VOC).

In some embodiments, it is possible to employ a solvent system which includes a solvent and a co-solvent. In some embodiments, a co-solvent may be used to disperse an inorganic or metallic filler. The co-solvent may have an undesirably high evaporation rate. In order to reduce the rate of evaporation, a solvent to lower the evaporation rate may be introduced. In some examples, the solvent may be 4-chlorobenzotrifluoride (4-CBTF) and the co-solvent may be dipropylene glycol methyl ether (DPM) and/or glycol methyl ether acetate (DPMA). The total solvent/co- solvent concentration can be in the range from 0% (wt/wt) to 80% (wt/wt), for example, between 10% (wt/wt) and 45% (wt/wt).

In embodiments, a coating can include at least one of a wetting agent and a dispersing agent. Additionally or alternatively, the coating can include a thickening agent, a de-foaming agent, an anti-foaming agent, an electrostatic spray agent, a spray enhancing agent, an anti- sedimentation agent, a rheological agent, an adhesion promotion agent, and an anti-corrosive agent. The dispersing agents can de- agglomerate the particles in the paint formulation and reduce solid precipitation in the paint formulation. Such dispersing agents can include at least one of, for example, an alkylolammonium salt of a block copolymer with acidic groups

(commercially available as DISPERBYK®-180 from BYK Additives), a solution of a carboxylic acid salt of polyamine amides (commercially available as ANTI-TERRA®-204 from BYK Additives), a solution of a copolymer with acidic groups (commercially available as DISPERBYK®-110 from BYK Additives), and a copolymer with acidic groups (commercially available as DISPERBYK®-111 from BYK Additives).

The wetting agent can reduce surface tension of the paint formulation and thereby improve paint film properties and adhesion to the surface of the article. Such wetting agents can include a polyether modified poly-dimethyl- siloxane (commercially available as BYK®-333 from BYK Additives). De-foaming agents can include a silicone-free solution of foam destroying polymers (commercially available as BYK®-052, BYK®-054, or BYK®-057 from BYK Additives), a polyacrylate-based surface additive (commercially available as BYK®-392 from BYK Additives), or a silicone-free air release additive (commercially available as BYK®- A 535 from BYK Additives).

The thickening and/or anti- sedimentation agent can provide the desired viscosity of the primer paint formulation, for example, based on the method of coating and/or to reduce particle sedimentation. Such thickening and/or anti- sedimentation agents can include a solution of a modified urea (commercially available as BYK®-410 from BYK Additives), BYK®-430 or BYK®-431 from BYK additives, bentonites, and hydrophobic pyrogen silica (commercially available as AEROSIL® R 972 from Evonik Industries). Electrostatic spray agents may increase the conductivity of the paint formulation to assist in spraying. Such spray agents can include a cationic compound additive (commercially available as EFKA® 6780 from BASF Corporation) or a conductivity promoter for coatings (commercially available as LANCO™ STAT L 80 from Lubrizol Deutschland GmbH).

The coating according to some embodiments of the disclosure may be applied by itself or in combination with one or more surface treatments or other layers. For example, a metal article may be provided with one or more of a substrate surface treatment (e.g., grit blasting, shot blasting or ball blasting) and a high-temperature heat-resistant solar- absorbing coating (e.g., the instant coating formulation) as a top coating layer (absorbing layer).

In embodiments, a coating as disclosed herein can be applied to the external surface (or at least a portion thereof) of a heat transfer member or an assembly of heat transfer members such as a pipe assembly comprising one or more pipes. If used as a primer layer the coating can be provided at a thickness of between Ιμιη and ΙΟΟμιη or 200μιη after application and curing. Alternatively or additionally, each layer of the coating can have a dry thickness less than ΙΟΟμιη. Application of a coating can include (a) applying a layer of the coating over a metallic surface, for example of a heat transfer member, at a time when a binder dispersed in the coating is a metal alkoxide binder or high temperature inorganic binder, and (b) subsequently heating the layer (e.g., at a temperature greater than 200 degrees Celsius or greater than 350 degrees Celsius) to cure the coating layer or layers on the metallic surface, thereby irreversibly converting the metal alkoxide binder or high temperature inorganic binder into an inorganic and/or ceramic binder.

Referring again to the figures, the metal article can be a pipe 202 of a receiver 200 in a solar thermal system. For example, one or more of the coatings/treatments described herein may be applied to at least a portion of the exterior surface of pipe 202, as shown in FIG. 3. FIG. 3 shows an isometric cutaway view of pipe 202 including two illustrative coating layers 308 and 312 that have not been drawn to scale. Pipe 202 has a metal wall 304 separating an interior volume 301 of pipe 202 from the external environment. Water and/or steam or other heat transfer fluid or working fluid, which may be preheated and/or pressurized, flows through the pipe interior volume. An exterior surface side 306 of the metal wall 304 can receive reflected insolation from the field of heliostats, so as to heat the metal wall 304 and thereby the fluid conveyed therethrough. The one or more levels of coating applied to the exterior surface 306 can improve absorption of solar insolation and/or protect the metal surface and/or reduce emissivity.

The exterior surface side 306 of the metal wall 304 can optionally be pre-treated prior to application of any other layers. For example, the surface 306 can be subjected to at least one of grit-blasting, shot blasting and ball blasting. Alternatively or additionally, one or more layers of paint or other formulations can optionally be provided between a first coating layer 308 and the pipe surface 306.

In the example illustrated in FIG. 3, a first layer 308 of a coating according to any of the embodiments in this disclosure is provided on the pipe surface 306 and a second layer 312 of coating is provided atop that. First layer 308 may include a solar- absorptive black pigment, but this may be omitted because of the presence of a solar-absorptive black pigment in the second (outer) layer 312. First layer 308 includes overlapping strata of planar particles of a material that enhances the thermal conductivity and reduces the emissivity of the coating, the material typically being a metal or metal alloy or a ceramic with high thermal conductivity. The second, outer layer 312 may optionally include similar arrangements of planar particles. In another example that is not illustrated but which can be understood from FIG. 3, only a single coating layer 308 is applied to the pipe surface 306, and the single layer includes both the overlapping strata of planar particles and a solar-absorptive black pigment. In still other examples, additional coating layers may be provided, where, in general, at least the first layer includes the

overlapping layers of planar particles and at least the outermost layer includes a solar- absorptive black pigment.

The thickness of each coating layer can be 5μιη or less, or less than 20μιη, 50μιη, or ΙΟΟμιη. After curing, each layer can have a thickness in the range from 5μιη to ΙΟΟμιη, for example, between 5μιη and 50μιη. Prior to curing, each layer can have a wet film thickness in the range from 5μιη to ΙΟΟμιη. Alternatively or additionally, each layer of the applied paint formulation can have a wet film thickness of about 20-150μιη.

FIGS. 4A, 4B, 4C, and 4D are illustrative examples of planar particles 350 of a metallic or ceramic-containing material with thermal conductivity and emissivity properties as disclosed in the various embodiments. Each of the various examples is shown in both plan view and elevation view. As can be seen in the elevation views, planar particles can be substantially planar with some variation, with varying thicknesses and irregular surfaces. As can be seen in FIG. 4D, any particle can have unformed or open spaces 351. As can be seen in FIG. 4C, a particle need not have a convex shape, and in fact the illustrated shapes are merely exemplary and the particles can have any shape. Each particle has a first maximum dimension Dl, a second maximum dimension D2, and a third maximum dimension D3. The plan view is a projection of the largest 'face' or facet of each respective illustrated particle with maximum dimensions Dl and D2, and the elevation view is a corresponding projection of a smaller 'face' with maximum dimensions Dl and D3. Any of Dl, D2 and D3 can differ from particle to particle. D3 is defined herein as the smallest of Dl, D2 and D3 for each particle. In some embodiments, D3 is between 1 micron and 10 microns, inclusive, for at least 50% of the planar particles. In some

embodiments, D3 is between 0.5 micron and 20 microns, inclusive, for at least 95% of the planar particles. The D3 dimension, being thin relative to the other two dimensions, allows the planar particles to 'stack up' when the coating is applied to a substrate and 'lie down' parallel to the substrate.

FIG. 5 illustrates a cross section along a longitudinal section of a coated pipe wall 304, where planar particles 350 in coating layer 308 are shown settled in overlapping strata that are substantially parallel to pipe wall 304 and its outer surface 306. Coating layer 308 has a thickness D4 and contains planar particles 350 as filler as well as other components according to any of the embodiments described herein. According to some embodiments a second coating layer 312 can be applied, and according to other embodiments additional coating layers can be applied; in any case coating layer 308 will have a thickness D4 and other coating layers such as a second coating layer 312 will have another thickness which can be the same as that of coating layer 308 or different.

FIG. 6 illustrates, in plan view, a plurality of substantially planar particles 350 that have settled into overlapping strata. The shaded areas of particles 350 are projections of the apparent area in plan view, depending on the angular orientation of the respective particles after application (or after application and curing) of the coating on a metallic substrate, and because of variations in angular orientation the shaded areas do not necessarily correspond one-to-one with respective maximum dimensions Dl and D2. In embodiments, there can be residual areas with no 'coverage' of the particles 350, as indicated in FIG. 6, even if the substrate is evenly covered with a coating layer 308, i.e., the distribution of particles 350 within coating layer 308 is not entirely uniform and the ultimate angular orientations of the respective particles are not entirely parallel. In some embodiments, the overlapping strata of particles 350 provide coverage of at least 90% or at least 95% or at least 99% of the surface area of metallic substrate 306 that is coated with the coating 308.

In order to ensure that the particles 350 settle into overlapping strata, it may be desirable that at least one of the two larger dimensions Dl or D2 is thicker than coating layer thickness D4. In embodiments, at least one of Dl or D2 is larger than D4 for at least 50% of planar particles 350. In some embodiments, at least one of Dl or D2 is larger than D4 for at least 75% of planar particles 350. In some embodiments, both Dl are D2 are larger than D4 for at least 50% of planar particles 350. In some embodiments, both Dl and D2 are larger than D4 for at least 75% of planar particles 350.

In an example, the thickness D4 of a first coating layer 308 or second coating layer 312 is less than or equal to 200 microns or less than or equal to 150 microns, and for at least 50% of the particles 350 at least one of Dl and D2 is greater than 150 microns or greater than 200 microns. In another example, the thickness D4 of a first coating layer 308 or second coating layer 312 is less than or equal to 200 microns or less than or equal to 150 microns, and for at least 50% of the particles 350 both Dl and D2 are greater than 150 microns or greater than 200 microns. In another example, the thickness D4 of a first coating layer 308 or second coating layer 312 is less than or equal to 200 microns or less than or equal to 150 microns, and for at least 50% of the particles 350 both Dl and D2 are greater than 150 microns or greater 200 microns. In yet another example, for at least 95% of the particles 350 at least one of Dl and D2 is great than 30 microns. In still another example, for at least 95% of the particles 350 both Dl and D2 are greater than 30 microns.

A projection of the largest 'face' of a substantially planar particle 350 in the x-y plane (the plane parallel to external surface 306 of a substrate 304), in other words the apparent surface area of the plan view of a particle 350 as illustrated in FIG. 7, has an area Al. In the simplified illustration of FIG. 7, where the angular orientation of particle 350 is— on one axis— at an angle a parallel to surface 306, and on another axis (unseen, the axis perpendicular to the drawing) parallel to surface 306, it can be seen that the projected area Al is substantially equal to the actual area of the largest face of the particle 350 multiplied by the cosine of angle a.

Obviously the projected area can be calculated for any angular orientation and shape of particle. In embodiments, the sum of Al (the combined projected surface areas of each of the particles particle) for the planar particles 350 in a layer of coating 308 or 312 covering all or part of a coated can be larger than the coated area. In embodiments, a fluid can be heated using insolation. A method for solar heating of a fluid can include directing insolation at high concentration, for example more than 100 suns (e.g., > 100 kilowatts per square meter) or more than 200 suns or more than 600 suns, onto the surface of a heat transfer member 202 in a solar receiver 200. The heat transfer member can comprise a metallic substrate 304 and a coating layer 308 applied to an external surface 306 of the heat transfer member. Coating layer 308 can provide high solar absorptivity, for example more than 90% with respect to the AM 1.5 spectrum, and can have emissivity at 20 degrees Celsius, or at a higher temperature, of less than 80% or less than 70%. At least the coating layer 308, and optionally an optional second coating layer 312, and optionally any optional additional coating layers, can comprise overlapping strata of planar particles that increase the thermal conductivity and reduce the emissivity of the coating. However many coating layers there are, at least the outermost one includes an oxide-based pigment that increases solar absorptivity. The method can additionally comprise conveying a fluid through an inner volume 301 of the heat transfer member 202, enabling the transfer of heating of the fluid from enthalpy in the metallic substrate 304 of the heat transfer member 202, the enthalpy having been converted from photonic energy absorbed by at least one of the coating layers.

In one or more first embodiments, a coating for an insolation-receiving metallic surface comprises planar particles of a material having thermal conductivity of at least 3 Watts per meter per degree Kelvin. The emissivity of the coating after application and curing on the metallic surface is less than 80%.

In the first embodiments or any other embodiment, the emissivity of the coating after application and curing on the metallic surface is less than 70%.

In the first embodiments or any other embodiment, the planar particles include a metal or metal alloy and have a melting point greater than 650 degrees Celsius or greater than 750 degrees Celsius.

In the first embodiments or any other embodiment, at least 95% of the planar particles are larger than 30 microns in at least one dimension.

In the first embodiments or any other embodiment, at least 50% of the planar particles are larger than 150 microns in at least one dimension.

In the first embodiments or any other embodiment, at least 95% of the planar particles are larger than 30 microns in each of two dimensions.

In the first embodiments or any other embodiment, at least 50% of the planar particles are larger than 150 microns in each of two dimensions.

In the first embodiments or any other embodiment, the smallest dimension of each of at least 95% of the planar particles is between 0.5 micron and 20 microns, inclusive.

In the first embodiments or any other embodiment, the smallest dimension of each of at least 50% of the planar particles is between 1 micron and 10 microns, inclusive.

In the first embodiments or any other embodiment, the planar particles form overlapping strata when the coating is applied to the metallic surface.

In the first embodiments or any other embodiment, the planar particles are at a concentration between 1% (wt/wt) and 60% (wt/wt).

In the first embodiments or any other embodiment, the planar particles comprise a metal or a metal alloy.

In the first embodiments or any other embodiment, the planar particles comprise a ceramic material with a metallic additive or coating.

In the first embodiments or any other embodiment, the planar particles comprise talc with a metallic coating.

In the first embodiments or any other embodiment, the planar particles comprise an alloy or superalloy of a metal selected from the group consisting of iron, nickel, cobalt, chromium, silver, and gold.

In one or more second embodiments, a heat transfer member with an insolation-receiving surface comprises a metallic substrate and a coating. The coating covers at least a portion of the insolation-receiving surface and includes at least one layer. The coating also comprises overlapping strata of planar particles that increase the thermal conductivity and reduce the emissivity of the coating.

In the second embodiments or any other embodiment, the coating consists of a single layer, and the coating additionally comprises an oxide-based pigment.

In the second embodiments or any other embodiment, the coating consists of at least two layers. At least the layer furthest from the metallic substrate comprises an oxide-based pigment, and at least the layer closest to the metallic substrate comprises overlapping strata of planar particles that increase the thermal conductivity and lower the emissivity of the coating.

In the second embodiments or any other embodiment, the planar particles are selected from the group consisting of ceramic-containing particles with thermal conductivity of at least 3 Watts per meter per degree Kelvin, and metallic particles.

In the second embodiments or any other embodiment, the planar particles in the coating are at a concentration between 1% (wt/wt) and 60% (wt/wt).

In the second embodiments or any other embodiment, the planar particles comprise a metal or a metal alloy. In the second embodiments or any other embodiment, the planar particles comprise a ceramic material with a metallic additive or coating.

In the second embodiments or any other embodiment, the planar particles comprise talc with a metallic coating.

In the second embodiments or any other embodiment, the planar particles comprise an alloy or superalloy of a metal selected from the group consisting of iron, nickel, cobalt, chromium, silver, and gold.

In the second embodiments or any other embodiment, the thermal conductivity of the coating is at least 0.5 Watt per meter per degree Kelvin.

In the second embodiments or any other embodiment, the overlapping strata cover at least 90% or at least 95% or at least 99% of the surface area of metallic substrate that is coated with the coating.

In the second embodiments or any other embodiment, at least 50% or at least 75% or at least 95% of the planar particles are larger in at least one dimension than the thickness of a coating layer comprising the particles.

In the second embodiments or any other embodiment, at least 50% or at least 75% or at least 95% of the planar particles are larger in two dimensions than the thickness of a coating layer comprising the particles.

In the second embodiments or any other embodiment, the metallic substrate comprises a metal alloy selected from the group consisting of steels, alloy steels and nickel superalloys.

In the second embodiments or any other embodiment, an inner volume thereof comprises a conduit for a fluid.

In the second embodiments or any other embodiment, the coated substrate has absorptivity of at least 90% with respect to the AM 1.5 spectrum, and emissivity of less than 80%.

In the second embodiments or any other embodiment, the coated substrate has at least one of: (i) absorptivity of at least 95% with respect to the AM 1.5 spectrum, and (ii) emissivity of less than 70%.

In the second embodiments or any other embodiment, the projected surface area of each of the planar particles in a coating layer, as projected on an x-y plane parallel to the metallic substrate, is greater than the surface area of the coated metallic surface.

In one or more third embodiments, a method for solar heating of a fluid comprises directing insolation at a concentration of at least 100 suns onto a surface of a heat transfer member. The heat transfer member comprises a metallic substrate and a coating with an absorptivity of at least 90% with respect to the AM 1.5 spectrum and emissivity of less than 80%. The method can further comprise conveying the fluid through an inner volume of the heat transfer member. The coating includes at least one layer, and at least a layer of the coating closest to the metallic substrate comprises overlapping strata of planar particles that increase the thermal conductivity and reduce the emissivity of the coating.

In the third embodiments or any other embodiment, the coating consists of at least two layers, and at least a layer of the coating furthest from the metallic substrate includes an oxide- based pigment.

In the third embodiments or any other embodiment, the coated substrate has emissivity of less than 70%.

In the third embodiments or any other embodiment, the planar particles are selected from the group consisting of ceramic-containing particles with thermal conductivity of at least 3 Watts per meter per degree Kelvin, and metallic particles.

In the third embodiments or any other embodiment, the planar particles in the coating are at a concentration between 1% (wt/wt) and 60% (wt/wt).

In the third embodiments or any other embodiment, the planar particles comprise a metal or a metal alloy.

In the third embodiments or any other embodiment, the planar particles comprise a ceramic material with a metallic additive or coating.

In the third embodiments or any other embodiment, the planar particles comprise talc with a metallic coating.

In the third embodiments or any other embodiment, the planar particles comprise an alloy or superalloy of a metal selected from the group consisting of iron, nickel, cobalt, chromium, silver, and gold.

The foregoing descriptions apply, in some cases, to illustrative examples, but these examples can be extended to production techniques. For example, where quantities and techniques apply to the illustrative examples, they should not be understood as limiting. In addition, although specific chemicals and materials have been disclosed herein, other chemicals and materials may also be employed according to one or more contemplated embodiments.

Features of the disclosed embodiments may be combined, rearranged, omitted, etc., within the scope of the invention to produce additional embodiments. Furthermore, certain features may sometimes be used to advantage without a corresponding use of other features.

It is thus apparent that there is provided in accordance with the present disclosure, high- temperature solar- absorptive coatings with high thermal conductivity and low emissivity, and methods for use thereof. Many alternatives, modifications, and variations are enabled by the present disclosure. While specific embodiments have been shown and described in detail to illustrate the application of the principles of the present invention, it will be understood that the invention may be embodied otherwise without departing from such principles. Accordingly, Applicant intends to embrace all such alternatives, modifications, equivalents, and variations that are within the spirit and scope of the present invention.

Claims

1. A coating for an insolation-receiving metallic surface, the coating comprising: planar particles of a material having thermal conductivity of at least 3 Watts per meter per degree Kelvin,

wherein the emissivity of the coating after application and curing on the metallic surface is less than 80%.

2. The coating of claim 1, wherein the emissivity of the coating after application and curing on the metallic surface is less than 70%.

3. The coating of claim 1, wherein the planar particles include a metal or metal alloy and have a melting point greater than 650 degrees Celsius or greater than 750 degrees Celsius.

4. The coating of claim 1, wherein at least 95% of the planar particles are larger than 30 microns in at least one dimension.

5. The coating of claim 1, wherein at least 50% of the planar particles are larger than 150 microns in at least one dimension.

6. The coating of claim 1, wherein at least 95% of the planar particles are larger than 30 microns in each of two dimensions.

7. The coating of claim 1, wherein at least 50% of the planar particles are larger than 150 microns in each of two dimensions.

8. The coating of claim 1, wherein the smallest dimension of each of at least 95% of the planar particles is between 0.5 micron and 20 microns, inclusive.

9. The coating of claim 1, wherein the smallest dimension of each of at least 50% of the planar particles is between 1 micron and 10 microns, inclusive.

10. The coating of claim 1, wherein the planar particles form overlapping strata when the coating is applied to the metallic surface.

11. The coating of claim 1, wherein the planar particles are at a concentration between 1% (wt/wt) and 60% (wt/wt).

12. The coating of claim 1, wherein the planar particles comprise a metal or a metal alloy.

13. The coating of claim 1, wherein the planar particles comprise a ceramic material with a metallic additive or coating.

14. The coating of claim 1, wherein the planar particles comprise talc with a metallic coating.

15. The coating of claim 1, wherein the planar particles comprise an alloy or superalloy of a metal selected from the group consisting of iron, nickel, cobalt, chromium, silver, and gold.

16. A heat transfer member with an insolation-receiving surface, the heat transfer member comprising:

a metallic substrate; and

a coating that covers at least a portion of the insolation-receiving surface and includes at least one layer;

wherein the coating comprises overlapping strata of planar particles that increase the thermal conductivity and reduce the emissivity of the coating.

17. The heat transfer member of claim 16, wherein:

the coating consists of a single layer, and

the coating additionally comprises an oxide-based pigment.

18. The heat transfer member of claim 16, wherein:

the coating consists of at least two layers,

at least the layer furthest from the metallic substrate comprises an oxide-based pigment, and

at least the layer closest to the metallic substrate comprises overlapping strata of planar particles that increase the thermal conductivity and lower the emissivity of the coating.

19. The heat transfer member of any of claims 16-18, wherein the planar particles are selected from the group consisting of ceramic-containing particles with thermal conductivity of at least 3 Watts per meter per degree Kelvin, and metallic particles.

20. The heat transfer member of any of claims 16-18, wherein the planar particles in the coating are at a concentration between 1% (wt/wt) and 60% (wt/wt).

21. The heat transfer member of any of claims 16-18, wherein the planar particles comprise a metal or a metal alloy.

22. The heat transfer member of any of claims 16-18, wherein the planar particles comprise a ceramic material with a metallic additive or coating.

23. The heat transfer member of any of claims 16-18, wherein the planar particles comprise talc with a metallic coating.

24. The heat transfer member of any of claims 16-18, wherein the planar particles comprise an alloy or superalloy of a metal selected from the group consisting of iron, nickel, cobalt, chromium, silver, and gold.

25. The heat transfer member of any of claims 16-18, wherein the thermal conductivity of the coating is at least 0.5 Watt per meter per degree Kelvin.

26. The heat transfer member of any of claims 16-18, wherein the overlapping strata cover at least 90% or at least 95% or at least 99% of the surface area of metallic substrate that is coated with the coating.

27. The heat transfer member of any of claims 16-18, wherein at least 50% or at least 75% or at least 95% of the planar particles are larger in at least one dimension than the thickness of a coating layer comprising the particles.

28. The heat transfer member of any of claims 16-18, wherein at least 50% or at least 75% or at least 95% of the planar particles are larger in two dimensions than the thickness of a coating layer comprising the particles.

29. The heat transfer member of any of claims 16-18, wherein the metallic substrate comprises a metal alloy selected from the group consisting of steels, alloy steels and nickel superalloys.

30. The heat transfer member of any of claims 16-18, wherein an inner volume thereof comprises a conduit for a fluid.

31. The heat transfer member of any of claims 16-18, wherein the coated substrate has absorptivity of at least 90% with respect to the AM 1.5 spectrum, and emissivity of less than 80%.

32. The heat transfer member of any of claims 16-18, wherein the coated substrate has at least one of: (i) absorptivity of at least 95% with respect to the AM 1.5 spectrum, and (ii) emissivity of less than 70%.

33. The heat transfer member of any of claims 16-18, wherein the projected surface area of each of the planar particles in a coating layer, as projected on an x-y plane parallel to the metallic substrate, is greater than the surface area of the coated metallic surface.

34. A method for solar heating of a fluid, comprising:

directing insolation at a concentration of at least 100 suns onto a surface of a heat transfer member, the heat transfer member comprising a metallic substrate and a coating with an absorptivity of at least 90% with respect to the AM 1.5 spectrum and emissivity of less than 80%; and

conveying the fluid through an inner volume of the heat transfer member;

wherein:

the coating includes at least one layer, and

at least a layer of the coating closest to the metallic substrate comprises overlapping strata of planar particles that increase the thermal conductivity and reduce the emissivity of the coating.

35. The method of claim 34, wherein the coating consists of at least two layers, and at least a layer of the coating furthest from the metallic substrate includes an oxide-based pigment.

36. The method of any of claims 34-35, wherein the coated substrate has emissivity of less than 70%.

37. The method of any of claims 34-35, wherein the planar particles are selected from the group consisting of ceramic-containing particles with thermal conductivity of at least 3 Watts per meter per degree Kelvin, and metallic particles.

38. The method of any of claims 34-35, wherein the planar particles in the coating are at a concentration between 1% (wt/wt) and 60% (wt/wt).

39. The method of any of claims 34-35, wherein the planar particles comprise a metal or a metal alloy.

40. The method of any of claims 34-35, wherein the planar particles comprise a ceramic material with a metallic additive or coating.

41. The method of any of claims 34-35, wherein the planar particles comprise talc with a metallic coating.

42. The method of any of claims 34-35, wherein the planar particles comprise an alloy or superalloy of a metal selected from the group consisting of iron, nickel, cobalt, chromium, silver, and gold.