COATED GLASS FOR SOLAR REFLECTORS

Abstract

The present invention relates to a heat treated coated glass for a solar reflector comprising: a heat treated glass, and an anti-soiling coating of TiO.sub.2 over one side of the heat treated glass, said heat treated coated glass obtainable by a process comprising: applying over one side of a glass substrate a sol-gel solution in liquid form obtained from the hydrolysis and condensation reactions between a precursor of TiO.sub.2 and water, and subjecting the glass thus coated to a heat treatment process by which the glass substrate is converted into a heat treated glass and concurrently the coating densities and forms a solid anti-soiling coating of TiO.sub.2 over one side of the glass.

Claims

1. A heat treated coated glass for a solar reflector comprising: a heat treated glass, and an anti-soiling coating of TiO.sub.2 over one side of the heat treated glass, said heat treated coated glass obtainable by a process comprising: applying over one side of a glass substrate a sol-gel solution in liquid form obtained from the hydrolysis and condensation reactions between a precursor of TiO.sub.2 and water, and subjecting the glass thus coated to a heat treatment process by which the glass substrate is converted into a heat treated glass and concurrently the coating densifies and forms a solid anti-soiling coating of TiO.sub.2 over one side of the glass.

2. The glass according to claim 1 wherein the sol-gel process uses a sol-gel solution comprising: at least one inorganic precursor agent of formula
Ti(X).sub.4 (I) wherein the X groups, being the same or different, are hydrolyzable groups preferably selected from OR alkoxy, OC(O)R acyloxy and halogens; or a hydrolyzate of this precursor agent; at least one organic solvent; water; and optionally a hydrolysis and condensation catalyst.

3. The glass according to claim 2, wherein R is a C.sub.1-C.sub.6 alkyl radical.

4. The glass according to any one of claims 2 to 3, wherein the precursor agent of formula (I) is tetrabutyl titanate (TNBT).

5. The glass according to any one of claims 2 to 4, wherein the organic solvent is selected from an alkanol and preferably is ethanol.

6. The glass according to any one of claims 2 to 5, wherein the hydrolysis and condensation catalyst is selected from an inorganic acid and preferably is HCl.

7. The glass according to any one of claims 2 to 6, wherein the sol-gel solution comprises TNBT, ethanol, water and HCl.

8. The glass according to claim 7, wherein the sol-gel solution has the following molar ratios: TNBT/Ethanol/Water=0.5-1.5:20-60:1-4.

9. The glass according to claim 8, wherein the sol-gel solution has the following molar ratios: TNBT/Ethanol/Water=TNBT/Ethanol/Water=1:20:4.

10. The glass according to any one of the preceding claims, wherein the heat treatment process consists of thermal heat strengthening.

11. The glass according to any one of the preceding claims, wherein the heat treatment process consists of thermal tempering.

12. The glass according to any one of the preceding claims, wherein the anti-soiling coating has a thickness from about 50 nm to about 1 micron, preferably about 150 nm thick.

13. A solar reflector comprising a heat treated coated glass as defined in any one of claims 1 to 12 and a mirror coating, wherein the anti-soiling coating and the mirror coating are located on opposite sides of the glass substrate.

14. A solar installation comprising the solar reflector as defined in claim 13.

15. A method of manufacturing a heat treated coated glass for a solar reflector, said method comprising: applying over one side of a glass substrate a sol-gel solution in liquid form obtained from the hydrolysis and condensation reactions between a precursor of TiO.sub.2 and water, and subjecting the glass thus coated to a heat-treating process by which the glass substrate is converted into a heat treated glass and concurrently the coating densifies and forms a solid anti-soiling coating of TiO.sub.2 over one side of the glass substrate.

16. The method according to claim 15, wherein the solution is applied over the glass substrate by dip-coating, spray-coating, flow-coating, roll-coating, or the like.

17. Use of a heat treated coated glass as defined in any one of claims 1 to 12 as an element of solar reflectors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] To better understand the invention, its objects and advantages, the following figures are attached to the specification in which the following is depicted:

[0038] FIG. 1 is a schematic diagram of a solar reflector according to an embodiment of the present invention. In particular, the reflector is made of a heat treated glass pane having an anti-soiling coating of TiO.sub.2 and a reflective coating (e.g. Ag) and its protective layers located on the opposite (rear) side from the anti-soiling TiO.sub.2 coating.

DETAILED DESCRIPTION OF THE INVENTION

[0039] As used herein, the glass substrate refers to a glass pane, typically a flat annealed glass pane. The glass substrate may be from about 1-10 mm thick in different embodiments of this invention, and maybe of any suitable color. In certain instances, glass (e.g., soda lime silica type glass) substrate is from about 3-10 mm thick, most preferably about 3-6 mm thick. Preferably, the glass substrate is a low-iron high energy transmission silicate glass.

[0040] It is an object of the present invention to provide a heat treated coated glass useful for solar reflectors. Such a glass comprises: [0041] a heat treated glass, and [0042] an anti-soiling coating of TiO.sub.2 over one side of the heat treated glass, said coating being prepared by means of a sol-gel process which comprises a thermal treatment taking place under the same process conditions for heat treatment of the glass.

[0043] The sol-gel process is a widely known method for producing solid materials from small molecules, which involves conversion of monomers into a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. In a typical sol-gel process, a colloidal suspension, or a sol, is formed from the hydrolysis and condensation reactions of the precursors, which are usually inorganic metal salts or metal organic compounds such as metal alkoxides. Complete polymerization and loss of solvent leads to the transition from the liquid sol into a solid gel phase.

[0044] A suitable sol-gel solution to be used in the present invention to form the TiO.sub.2-based anti-soiling coating comprises: [0045] at least one inorganic precursor agent (i.e. TiO.sub.2-precursor) of formula

Ti(X).sub.4 (I) [0046] wherein the X groups, being the same or different, are hydrolyzable groups preferably selected from OR alkoxy, OC(O)R acyloxy and halogens; or a hydrolyzate of this precursor agent; [0047] at least one organic solvent; [0048] water; and [0049] optionally a hydrolysis and condensation catalyst.

[0050] As used herein, sol-gel solution refers to the mixture of substances which has to be combined to produce the initial colloidal solution (sol) and the subsequent integrated network (gel) which finally results in a dense coating based on TiO.sub.2.

[0051] Compound (I) is the precursor for TiO.sub.2-based matrix. In certain embodiments, X is selected from OR alkoxy and OC(O)R acyloxy groups, wherein R is an alkyl radical, preferably a C.sub.1-C.sub.6 alkyl radical. In other embodiments, X is an halogen such as Cl, Br or I.

[0052] Preferably, the X groups are alkoxy groups, and in particular methoxy, ethoxy, propoxy (such as iso-propoxy) and butoxy (such as n-butoxy) groups.

[0053] Preferred compounds of formula (I) are tetraalkyl titanates. Amongst them, titanium isopropoxide (also commonly referred to as tetraisopropyl titanate, titanium tetraisopropoxide or TTIP) or titanium n-butoxide (also commonly referred to as tetrabutyl titanate or TNBT) will be advantageously used. More preferably, the precursor agent of formula (I) is tetrabutyl titanate (TNBT).

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[0054] As used herein, the term about means a slight variation of the value specified, preferably within 10 percent of the value specified. Nevertheless, the term about can mean a higher tolerance of variation depending on for instance the experimental technique used. Said variations of a specified value are understood by the skilled person and are within the context of the present invention. Further, to provide a more concise description, some of the quantitative expressions given herein are not qualified with the term about. It is understood that, whether the term about is used explicitly or not, every quantity given herein is meant to refer to the actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including equivalents and approximations due to the experimental and/or measurement conditions for such given value.

[0055] Inorganic precursor agents of formula (I) that are present in the sol generally account for about 5 to about 40% by weight of the total weight (including all other compounds that are present in the sol-gel solution, in particular the solvent) of the sol-gel solution. In particular embodiments, the weight percentage of the inorganic precursor ranges from about 10 to about 35 wt % or from about 12 to about 30 wt %.

[0056] Organic solvents or mixtures of organic solvents to be suitably used for preparing the sol-gel solution according to the invention are all classically employed solvents, and more particularly polar solvents, especially alkanols such as methanol, ethanol, isopropanol, isobutanol, n-butanol and mixtures thereof. Other solvents, preferably water-soluble solvents, may be used, such as 1,4-dioxane, tetrahydrofurane or acetonitrile. Ethanol is the most preferred organic solvent.

[0057] Generally, the organic solvent represents from about 40 to about 90% by weight of the sol-gel solution total weight (e.g. from about 50 to about 90 wt %, from about 60 to about 90 wt %, etc.). Water represents typically from about 0.5 to 10% by weight of the sol-gel solution total weight (e.g. from about 1 to about 9 wt %, from about 1 to about 6 wt %, etc.).

[0058] The medium in which the precursor agent of formula (I) is present is typically an acidic medium, the acidic character of the medium being obtained through addition, for example, of an inorganic acid, typically HCl, HNO.sub.3 or H.sub.2SO.sub.4 or of an organic acid such as acetic acid. This acid acts as a hydrolysis and condensation catalyst by catalyzing the hydrolysis of the X groups of the compound of formula (I). A particularly preferred hydrolysis and condensation catalyst is HCl. Nevertheless, base catalysts are also contemplated in the present invention for catalyzing the hydrolysis and condensation of the inorganic precursor agent.

[0059] According to preferred embodiments, the sol-gel solution comprises TNBT as inorganic precursor agent of formula (I), ethanol as organic solvent, water and HCl as catalyst. Typical molar ratios of these components are the following: TNBT/Ethanol/Water=0.5-1.5:20-60:1-4. HCl may be normally added until acid pH such as pH=2.

[0060] In a particular preferred embodiment, the sol-gel solution has the following composition TNBT/Ethanol/Water=1:20:4.

[0061] The above molar ratios are also contemplated for other sol-gel solutions i.e. with a different precursor agent of formula (I) and/or a different organic solvent.

[0062] The person skilled in the art will recognize that there are a number of technologies in the art suitable for applying the sol over the glass pane. These include, without limitation, dip-coating, spray-coating, flow-coating, roll-coating, and the like. Dip-coating and spray-coating are preferred coating processes.

[0063] In a dip-coating process, the glass substrate is covered/protected on one side and immersed in the sol, preferably at a constant speed. The substrate remains inside the sol for a while and is starting to be pulled up. Deposition of the coating layer on the substrate takes place while it is pulled up and the withdrawing is carried out preferably at a constant speed. The speed determines the thickness of the coating (faster withdrawal gives thicker coating material) and therefore its optical properties. According to a preferred embodiment, the withdrawing speed ranges from about 150 to about 250 mm/min, such as about 175 to about 225 mm/min, preferably about 200 mm/min.

[0064] Once the coating is deposited on the glass substrate, the glass is subjected to a heat-treatment processing (curing). In a conventional heat-treatment processing, the glass is properly positioned on the loading table of the heating oven and progressively heated to its bending temperature by continuous, or step-by-step, travelling through the heating tunnel. Radiation with electrical heat sources and / or convection by means of hot air heating can be used to heat up the glass. As the glass reaches the desired temperature, it is rapidly moved to the bending section, where the glass is bended to its desired curved shape and immediately heat strengthened or tempered (heat treatment) with rapid cooling by means of violent air blowing on both glass sides. After this heat treatment the glass is cooled down to a normal handling temperature (under 50 C.) by continuous travelling in a cooling tunnel where it is blown with atmospheric air coming from one or several fans.

[0065] In a particular embodiment the heat treatment is thermal heat strengthening or thermal tempering.

[0066] When thermally heat strengthened, the glass has compressive layers in both surfaces between 20 Mpa and 69 Mpa, resulting in improved mechanical properties with respect to typical annealed glass reflectors in use.

[0067] When thermally tempered, the glass has compressive layers in both surfaces in excess of 70 Mpa, resulting in improved mechanical properties with respect to typical annealed glass reflectors in use.

[0068] Advantageously, under the heat-treatment, the residual solvent and traces of organic compounds are driven off of the coating and TiO.sub.2 is covalently immobilized by TiOTi bonds on the glass so that the coating densifies and forms a durable coating with excellent anti-soiling properties.

[0069] All these operations, including bending and heat treatment, may be carried out in equipments which are conventional in the glass industry such those used in the manufacturing of heat-treated glass, well known to the skilled person, like for example in the automobile industry.

[0070] Furnace parameters (glass speed, temperatures, bender operation, air pressure, etc), furnace operations and their coordination are preferably fully automatic and controlled by means of a sophisticated computer control system.

[0071] The bent glass is then moved to a coating line to provide it with the necessary reflective capabilities and conducting a mirroring process.

[0072] According to the above procedure, anti-soiling coatings of TiO.sub.2 of very different thicknesses may be easily obtained in a cost-efficient manner. In certain embodiments, the anti-soiling coating of TiO.sub.2 ranges from about 50 nm to 1 about micron thick. In preferred embodiments, it is about 50-250 nm thick, more preferably about 100-200 nm thick and even more preferably about 150 nm thick.

[0073] The mirror coating may be of or include Al, Ag or any other suitable reflective material in certain embodiments of this invention.

[0074] Additionally, solar reflectors according to the invention may be provided with protective layers or coatings of the reflective coating on the rear side of the mirror. In a particular embodiment, the mirror coating of the reflector comprises a reflective coating (such as silver), a first protective layer comprising a metal (such as copper) and a second protective layer thereof of paint. In another particular embodiment, the mirror coating comprises a reflective coating comprising silver, a first protective layer comprising copper and three protective layers of paint.

[0075] In certain instances, the solar reflectors may be mounted on a steel or other metal based support system.

[0076] All the features described in this specification (including the claims, description and drawings) and/or all the steps of the described method can be combined in any combination, with the exception of combinations of such mutually exclusive features and/or steps.

[0077] The invention will be further illustrated by means of examples, which should not be interpreted as limiting the scope of the claims.

EXAMPLES

Example 1

Preparation of Heat Treated Coated Glass

[0078] A sol-gel solution was carried out mixing in a reactor 620 g of titanium n-butoxide as titanium precursor and 1520 g of ethanol as solvent. Then 65 g of water acidified with hydrochloric acid (catalyst of the reaction) until acid pH (pH=2) were added under vigorous stirring. The solution was then stirred during 24 hours at room temperature.

[0079] Glass samples of 270440 mm were cleaned with a cleaning agent, washed with distilled water and then dried in air. One of the sides of the sample was covered in order to deposit the coating only on the other side. This glass was dipped into the above-mentioned solution and withdrawn at a speed of 200 mm/min. After drying during 30 minutes at room temperature, the coated glass samples were thermally cured following a typical heat strength process for tempering glasses.

Example 2

Reflectance Study of Solar Reflector

[0080] The inventors have studied the reflectance of a solar reflector partially coated with the TiO.sub.2 coating of the present invention on of its surface. Several samples of 270440 mm were tested in outdoor conditions in the same environment of a CSP real plant. After 3 months in outdoor exposure with a cleaning frequency every two weeks, the reflectance measured before cleaning in the coated area was as average 2 ppt higher than in the uncoated one. These results are shown in Table 1.

TABLE-US-00001 TABLE 1 Sample Average .sub.coateduncated (ppt) Sample A 2 Sample B 2.2 Sample C 2.1 Sample D 2

[0081] Further, visually, the dust accumulation over the uncoated glass was clearly superior.

[0082] Among the advantageous properties of the coating, the inventors, have found out that surprisingly, after 15 months in outdoor exposure, the reflectance of the coated area has increased 0.2 ppt as average with regards to the initial values of the experiment, meanwhile, the reflectance of the uncoated area remains the same. These results are shown in Table 2.

TABLE-US-00002 TABLE 2 Average Average .sub.final coated initial coated .sub.final uncoated initial uncoated Sample (ppt) (ppt) Sample E 0.1 0.0 Sample F 0.3 0.2 Sample G 0.1 0.2 Sample H 0.2 0.1 Sample I 0.2 0.2

[0083] These studies prove that the TiO.sub.2 coating of the present invention has excellent anti-soiling properties.