HIGH TEMPERATURE SOLAR SELECTIVE COATINGS

• US 20100313875A1
• Filed: 10/18/2007
• Published: 12/16/2010
• Est. Priority Date: 10/18/2007
• Status: Active Grant

• Current Assignee: Alliance For Sustainable Energy LLC
• Original Assignee: Alliance For Sustainable Energy LLC
• Inventors: Kennedy, Cheryl E.
• Granted Patent: US 8,893,711 B2
• Time in Patent Office: Days
• US Class Current: 126/652
• CPC Class Codes: F24S 10/45 the enclosure being cylindrical
• CPC Class Codes: F24S 70/225 for spectrally selective absorption
• CPC Class Codes: F24S 70/30 Auxiliary coatings, e.g. anti-reflective coatings
• CPC Class Codes: G02B 5/281 designed for the infrared light
• CPC Class Codes: Y02E 10/44 Heat exchange systems

Abstract

Improved solar collectors (40) comprising glass tubing (42) attached to bellows (44) by airtight seals (56) enclose solar absorber tubes (50) inside an annular evacuated space (54. The exterior surfaces of the solar absorber tubes (50) are coated with improved solar selective coatings (48) which provide higher absorbance, lower emittance and resistance to atmospheric oxidation at elevated temperatures. The coatings are multilayered structures comprising solar absorbent layers (26) applied to the meta surface of the absorber tubes (50), typically stainless steel, topped with antireflective Savers (28) comprising at least two layers 30, 32) of refractory metal or metalloid oxides (such as titania and silica) with substantially differing indices of refraction in adjacent layers. Optionally, at least one layer of a noble metal such as platinum can be included between some of the layers. The absorbent layers cars include cermet materials comprising particles of metal compounds is a matrix, which can contain oxides of refractory metals or metalloids such as silicon. Reflective layers within the coating layers can comprise refractory metal silicides and related compounds characterized by the formulas TiSi. Ti₃SiC₂, TiAlSi, TiAN and similar compounds for Zr and Hf. The titania can be characterized by the formulas TiO₂, Ti₃O₅. TiOx or TiO_xN_1−x with x 0 to 1. The silica can be at least one of SiO_2x, SiO_2x or SiO_2xN_1−x with x=0 to 1.

Claims

1. A method for manufacturing a solar selective absorption material, characterized essentially by the following steps:

a) providing a substrate comprising an infrared reflecting metal;

b) applying thereto a layer of an IR reflective material;

c) applying to said layer of IR reflective material at least one layer of a solar absorbent material;

d) optionally, applying to said layer of solar absorbent material another layer of IR reflective material, followed by another layer of solar absorbent material, which steps can be repeated; and

e) applying at least one surface layer of solar antireflective material, to produce a coated substrate having overall high Solar absorbance and low IR emittance at elevated temperatures.

2. The method of claim 1 wherein said layers of solar selective absorbtion material are characterized by being applied by at least one physical or chemical vapor deposition method. Wherein the said at least one physical vapor deposition method is selected from the group consisting of electron beam evaporation, ion beam assisted deposition or sputtering.

3. The method of claim 1 wherein said IR reflective layers are characterized by comprising at least one refractory metal silicide, boride or carbide. Wherein the said refectory metal silicide is at least one titanium silicide characterized by the formula TixSiy, where x=1, 3 or 5 and y=1, 2 or 4.

4. The method of claim 1 wherein said solar absorbent material is characterized by comprising a refractory metal or metalloid oxide, nitride or oxynitride.

5. The method of claim 1 wherein said surface layer of antireflective material is characterized by comprising at least two adjacent thin layers of at least one refractory metal oxide, nitride, or oxynitride and at least one metalloid oxide, nitride, oxynitride, or fluoride with said adjacent layers having substantially differing indices of refraction, which layers can be repeated. Wherein the said antireflective layers are characterized by comprising at least one metalloid oxide, each having a different index of refraction, wherein said metalloid is selected from the group consisting of silicon and germanium.

6. The method of claim 3 wherein said refractory metal is selected from the group consisting of Group IVA metals (titanium, zirconium and hafnium), Group VA metals (vanadium, niobium and tantalum) and Group VI metals (chromium, molybdenum and tungsten).

7. The method of claim 1 wherein said substrate is characterized by comprising stainless steel that can be curved or tubular.

8. The method of claim 1 wherein at least one layer of said IR reflective material is characterized by comprising a noble metal selected from the group consisting of platinum, palladium, rhodium, ruthenium, indium, gold and osmium.

9. The method of claim 1, further characterized by a step of texturizing the final layer of material, comprising antireflective material, to increase solar absorption and minimize surface reflection. Wherein, the said final layer is texturized by a process of depositing said antireflective material at an acute angle to the surface of said final layer (or at least one step of etching).

10. The method of claim 1, further characterized by materials being deposited to form at least one of said solar absorbent and IR reflective layers as cermets.

11. The method of claim 10, further characterized in that a deposited solar absorbent cermet comprises particles of at feast one refractory metal silicide in a matrix of at least one refectory metal oxide or silica. Wherein the said refractory metal is selected from the group consisting of Group IV A metals (titanium, zirconium and hafnium), Group VA metals (vanadium, niobium and tantalum) and Group VI metals (chromium, molybdenum and tungsten). Wherein said refractory metal, silicide is at least one titanium silicide characterized by the formula TixSiy, where x=1, 3 or 5 and y=1, 2 or 4.

12. The method of claim 10, characterized in that a deposited IR reflective cermet comprises particles of titanium silicide in a matrix of at least one refractory metal oxide or silica. Wherein at least two layers of said cermets are applied, each having a different index of refraction and volume traction of metal to refractory metal oxide or silica matrix material. Wherein the layer of said cermet closest to said substrate has a relatively low index of refraction and a relatively high metal volume time fraction, with at least one layer applied above that layer having a relatively high index of refraction and relatively lower metal volume fraction.

13. The method of claim 10 wherein at least one layer of said IR reflective material is characterized by comprising a noble metal selected from the group consisting of platinum, palladium, rhodium, ruthenium, indium, gold and osmium.

14. A solar selective coated substrate prepared in accordance with the method of claim 1, 3, 4, 5, 7, 8.

15. A solar selective coaled substrate comprising at least one cermet, prepared in accordance with the method of claim 10, 11, 12, 13.

21. A collector receiver tube for a solar parabolic trough system comprising a tubular glass envelope having connecting bellows at each end thereof secured by airtight seals and means for evacuating said envelope, concentrically enclosing a solar absorber tube therein, characterized by said absorber tube being a coated stainless steel substrate having a solar selective coating in accordance with claim 15, 16, 17, 18, 19, 20.

22. A collector receiver tube (40) for a solar parabolic trough system comprising a tubular glass envelope (42) having connecting bellows (44) at each end thereof secured by airtight seals (56) and means for evacuating said envelope (46), concentrically enclosing a solar absorber tube (50) therein, characterized by said absorber tube (50) being a coaled stainless steel substrate (20) having a solar selective coating (48) in accordance with claim 21.

23. A process of absorbing solar energy and heating a heat exchange fluid, characterized by passing a heat exchange fluid through at least one absorber tube of claim 22 while said at least one absorber tube is exposed to sunlight.

16. A solar selective coated substrate comprising an infrared reflecting substrate, at least one layer of solar absorbent material and at least one layer of antireflective material, characterized by:

a substrate comprising stainless steel, a plurality of layers of solar absorptive material comprising at least one refractory metal oxide, nitride or oxynitride and a plurality of layers of IR reflective material comprising at least one refractory metal silicide, boride or carbide, topped with at least one layer of solar antireflective material comprising at least one layer of a refractory metal oxide and at least one layer of a silica, producing a material having high solar absorptance and low IR emittance at elevated temperatures.

17. The coated substrate of claim 16, further characterized by resistance to oxidation at temperatures above about 450 deg. C.

18. The coated substrate of claim 16, characterized by having at least two layers of solar absorptive material having low indices of refraction, at least one layer of solar absorptive material having a high index of refraction, adjacent to at least one low index layer and at least two layers of IR reflective material adjacent to said layers of absorptive material. The coated substrate is further characterized by at least two layers of solar absorbent material comprising at least one titania or silica, at least three IR reflective layers adjacent to said absorbent layers and comprising a titanium, zirconium, or hafnium silicide, boride, carbide or ternary compound comprising titanium or silicon, oxygen and nitrogen, and a solar antireflective layer as a top layer, comprising at least one thin layer each of at least one titania (30) and silica. Wherein said titania comprises at least one of TiO₂, Ti₃O₅, TiO_2x or Ti_2xO_xN₁₋

x, with x=0 to 1, said silica comprises at least one of SiO₂2, SiO_2x or SiO_2xN<sub>1−

x</sub>, with x=0 to 1 and the titanium silicide is at least one of Ti₃Si, Ti₅Si₃, Ti₅Si₄, TiSi or TiSi₂.

19. The coated substrate is characterized by a top layer of antireflective material which is texturized to increase solar absorption and minimize surface reflection. Wherein said top layer of antireflective material is characterized by comprising at least one silica having a low index of refraction.

20. A solar absorbent coaled substrate comprising an infrared reflecting substrate, at least one layer of solar absorbent material and at least one layer of antireflective material, characterized by a reflective substrate comprising stainless steel, at least two solar absorbent layers of cermet materials comprising particles of titanium silicide in a matrix comprising at least one refractory metal or metalloid oxide, each layer of said cermet material having either a high or low index of refraction and a differing volume fraction of said titanium silicide expressed as metal to the metal oxide matrix and an outer antireflective layer comprising at least one layer of titanium dioxide having a high index of refraction adjacent to at least one layer of silica having a low index of refraction.