HIGH PERFORMANCE THERMALLY-SPRAYED ABSORBER COATING

Abstract

A method for coating by thermal spraying a substrate for solar applications with a temperature-resistant and high-absorbance ceramic micro-structured coating includes the following steps: preparing a powder mixture including ceramic microparticles powder and polyester microballs powder, a percentage of the polyester microballs in the powder mixture being between 10 and 30% w/w; spraying the powder mixture onto the substrate by a thermal spray process in order to apply a coating layer on the substrate; and heating the substrate having the coating layer to a temperature of at least 400 C. so as to evaporate the microballs of polyester from the coating layer, leaving porosities at a place of the polyester microballs. Parameters of the spraying step and particle size are chosen so that the coating layer is applied in a thickness of between 50 and 150 microns.

Claims

1. A method for coating by thermal spraying a substrate for solar applications with a temperature-resistant and high-absorbance ceramic micro-structured coating, comprising the following steps: preparing a powder mixture comprising ceramic microparticles powder and polyester microballs powder, a percentage of the polyester microballs in the powder mixture being between 10 and 30% w/w; spraying the powder mixture onto the substrate by a thermal spray process in order to apply a coating layer on the substrate; and heating the substrate having the coating layer to a temperature of at least 400 C. so as to evaporate the microballs of polyester from the coating layer, leaving porosities at a place of the polyester microballs, wherein parameters of the spraying step and particle size are chosen so that the coating layer is applied in a thickness of between 50 and 150 microns.

2. The method according to claim 1, wherein the thermal spray process comprises a plasma spray process.

3. The method according to claim 1, wherein the ceramic microparticles include spinel structure particles and/or perovskite particles.

4. The method according to claim 3, wherein the spinel structure particles comprise manganese-cobalt oxide (MCO) particles.

5. The method according to claim 3, wherein the perovskite particles comprise lanthanum-manganese or lanthanum-cobalt/chromium oxide particles.

6. The method according to claim 5, wherein the perovskite particles comprise lanthanum-strontium-cobalt-ferrite (LSCF) particles or lanthanum strontium manganite particles (LSM).

7. The method according to claim 1, wherein a size of the ceramic microparticles is between 5 and 50 microns.

8. The method according to claim 1, wherein a size of the polyester microballs is between 40 and 150 microns.

9. The method according to claim 1, wherein the substrate is maintained under 100 C. before and during spraying the powder mixture.

10. The method according to claim 1, wherein the substrate is comprises a solar receiver having heat exchange tubes comprising steel or Ni-based alloy.

11. The method according to claim 1, wherein the coating is applied as one single layer or as one layer on a sub-layer.

12. A coated substrate for solar applications having a temperature-resistant and high-absorbance ceramic micro-structured coating, obtained by the method according to claim 1.

13. The coated substrate according to claim 12, wherein the coating porosities have an average diameter of 20 to 50 microns.

14. A solar receiver, comprising: heat exchange tubes comprising the coated substrate according to claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. Other features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:

[0066] FIG. 1 schematically represents the key parameters of the coating development strategy according to the present invention.

[0067] FIG. 2 schematically represents the method of performing a solar coating according to the present invention.

[0068] FIG. 3A shows an example of electron micrograph of a coated sample after plasma spraying of the powder mixture comprising ceramic microparticles powder and polyester microballs powder, according to the present invention.

[0069] FIG. 3B shows an electron micrograph of the coated sample of FIG. 3A after further heat treatment, according to the present invention.

DETAILED DESCRIPTION

[0070] In an embodiment, the present invention provides a method for supplying an absorber coating with high performances at high temperature, especially a method for providing an absorber coating with higher performances intended for solar receivers operated at temperatures higher than 850 C.

[0071] In an embodiment, the present invention provides a coating with an increased lifetime and a lifetime of minimum 5 years without any optical and mechanical performance degradations, leading to a reduced on-site maintenance.

[0072] In an embodiment, the present invention provides a coating thickness which is minimal while providing the best compromise between performances (such as adherence, thermal properties, conductivity) and cost.

[0073] In an embodiment the present invention relates to a method for coating by thermal spraying a substrate for solar applications with a temperature-resistant and high-absorbance ceramic micro-structured coating, comprising the following steps: [0074] preparing a powder mixture comprising ceramic microparticles powder and polyester microballs powder, the percentage of the polyester microballs in the powder mixture being comprised between 10 and 30% w/w; [0075] spraying the powder mixture onto the substrate by a thermal spray process in order to apply a coating layer on the substrate; [0076] heating the substrate having the coating layer to a temperature of at least 400 C. so as to evaporate the microballs of polyester from the coating layer, leaving porosities at the place of the polyester microballs; [0077] wherein spraying step parameters and particle size are chosen so that the coating layer (1) is applied in a thickness comprised between 50 and 150 microns.

[0078] According to preferred embodiments of the invention, the method is further limited by one of the following features or by a suitable combination thereof: [0079] the thermal spray process is a plasma spray process; [0080] the ceramic microparticles are selected from the group of spinel structure particles and perovskite particles; [0081] the spinel structure particles are manganese-cobalt oxide (MCO) particles; [0082] the perovskite particles are lanthanum-manganese or lanthanum-cobalt/chromium oxide particles; [0083] the perovskite particles are lanthanum-strontium-cobalt-ferrite (LSCF) particles or lanthanum strontium manganite particles (LSM); [0084] the size of the ceramic microparticles is comprised between 5 and 50 microns; [0085] the size of the polyester microballs is comprised between 40 and 150 microns; [0086] the substrate is maintained under 100 C. before and during spraying the powder mixture; [0087] the substrate is a solar receiver composed of heat exchange tubes made of steel or Ni-based alloy; [0088] the coating is applied according one single layer or according one layer on a sub-layer.

[0089] The present invention also relates to a coating manufactured with the method described above, and to a coated substrate suitable for solar applications, having a temperature-resistant and high-absorbance ceramic micro-structured coating such as described above.

[0090] Preferably, the coating porosities have an average diameter of 20 to 50 microns.

[0091] Another aspect of the invention relates to a solar receiver comprising heat exchange tubes made of the coated substrate as described above.

[0092] The present invention relates to a new thermal spray method for applying on a substrate 3, generally being metallic (e.g. steel), a simple (single) layer solar selective coating 1. This type of coating can be applied on substrate 3 by different thermal spray applications such as power flame spray or high velocity oxyfuel spray (HVOF) but the selected method is preferably plasma spray method. In plasma spray, a high frequency arc is ignited between an anode and a tungsten cathode. A gas flowing between the electrodes is ionized such that a plasma plume having a length of several centimeters develops. The temperature within the plume can be as high as 16000K. The particles velocity is 100-300 m/s. The spray material is injected as a powder outside of the gun nozzle into the plasma plume, where it is melted and projected onto the substrate surface.

[0093] According to the invention, a mixture of ceramic powders and microballs of polyester 2 is deposited onto substrate 3 by a thermal spray process, and preferably by air-plasma spray (APS) process using a plasma torch 5. In the plasma process, the mixture 2 is melted and projected on substrate 3, adhering and solidifying on the surface thereof to form the coating layer 1 (see FIG. 3A). Thereafter the projected microballs of polyester present in the mixture of powders are going to divide up in coating layer 1. Further, the substrate comprising coating layer 1 is heated to a high temperature (>400 C.), which leads to the evaporation of the microballs of polyester, leaving local porosities 4 instead (see FIG. 3B).

[0094] Further, these porosities 4 are going to act as a light trap 6 and so to allow increasing the absorbance of the coating 1. Once applied to the surface of the solar receiver, this coating 1 will thus allow to absorb a maximum of solar energy (94.5-95.5% of absorptivity in the solar spectrum 400-2500 nm) and reemit a minimum thereof (75-80% of emissivity in the infrared spectrum 1-20 m), thereby increasing the efficiency of the solar receiver panel from 90.5% to 91.35% (+0.85% efficiency with respect to prior art paint such as Pyromark paint). The lifetime is estimated to increase from 1 year to 5 years as 1000 additional cycles can be performed at 750 C., as inferred from bending tests (not shown).

[0095] The process parameters affect the microstructure and properties of the coating layer. Appropriate selection of the material to be applied is essential (type, characteristics, geometry, dimensions). Finer particles are susceptible to be vaporized, coarser particles lead to a lack of fusion which is not suitable for the formation of a dense coating layer with good adhesion to the substrate. The inventors discovered that the thickness of the coating layer is influenced by the mixture projection parameters and the size of the projected particles. Both can be chosen to obtain a layer thickness comprised between 50 and 150 microns. Thin coating is obtained with the smallest particle sizes.

[0096] According to one embodiment, ceramic powder is preferably spinel structure particles (with chemical structure (AB).sub.2O.sub.3, where A and B are metallic cations). More preferably the spinel-structured material is manganese-cobalt oxide (MCO) under the form of Mn.sub.1.5Co.sub.1.5O.sub.4.

[0097] Still according to another embodiment, ceramic powder can also be perovskite particles (with chemical structure (AB).sub.3O.sub.4, where A and B are metallic cations), such as lanthanum-manganese and lanthanum-cobalt/chromium oxides, and preferably lanthanum-strontium-cobalt-ferrite (Sr-doped LaCo.sub.1-xFe.sub.xO.sub.3 or LSCF) or lanthanum strontium manganite (LSM).

[0098] The size of the ceramic powder particles is preferably comprised between 5 and 50 microns.

[0099] The size of the polyester balls is preferably comprised between 40 to 150 microns, and still preferably with a mean size about 60 microns.

[0100] Particle size analysis or determination is obtained by methods known of the one skilled in the art, such as laser diffraction, sieve analysis (e.g. according to ASTM B214), etc.

[0101] According to one embodiment, the percentage of polyester balls in the mixture 2 is comprised between 10 and 30% (w/w), and preferably 20% (w/w).

[0102] The proposed solution is to apply by plasma spray process a specific mixture of high-temperature stable powders in order to form the coating. This technology insures a very good adhesion of the coating on the substrate by mechanical cohesion even at very high-temperature (see electron micrographs, FIGS. 3A and 3B).

[0103] According to one embodiment, texturing the surface by ageing the coating is a proposed approach to increase the solar absorptivity by generating multiple internal reflections.

[0104] One advantage of the present invention is that the coating achieved by plasma spray in the present invention exhibits better optical properties, what allows to improve the efficiency of the solar receiver, and a longer lifetime at high-temperature, which allows to reduce the on-site maintenance operations.

[0105] Another advantage is the formation of relatively thin coatings thanks to using smaller polyester and ceramic particles. This reduces the cost of the coating as the cost of the particles can vary by a factor of 10 with increasing particle size.

[0106] In conclusion, thermally-sprayed absorber coating obtained by the plasma spray method of the present invention exhibits an improved behaviour in surface degradation, an improved life-time, and reduced costs of maintenance, while improving the absorbance properties. This new solution will give the opportunity to the CSP customers to economize money by reducing the on-site maintenance operation number and shutdown periods of the power plant, which is a commercial advantage for the solar receiver supplier.

[0107] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below. Additionally, statements made herein characterizing the invention refer to an embodiment of the invention and not necessarily all embodiments.

[0108] The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article a or the in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of or should be interpreted as being inclusive, such that the recitation of A or B is not exclusive of A and B, unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of at least one of A, B and C should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of A, B and/or C or at least one of A, B or C should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.

REFERENCE SYMBOLS

[0109] 1 Coating [0110] 2 Mixture of ceramic powders and polyester microballs (plasma spray) [0111] 3 Substrate [0112] 4 Porosities [0113] 5 Plasma torch [0114] 6 Light traps