WO2015093403A1 - Sunlight-reflecting mirror unit, solar thermal power generation device, and method for cleaning sunlight-reflecting mirror - Google Patents

Abstract

The objective of the present invention is to provide: a sunlight-reflecting mirror unit which is capable of maintaining high cleanliness and reflectance of the surface; a solar thermal power generation device; and a method for cleaning a sunlight-reflecting mirror. The present invention is a sunlight-reflecting mirror unit which has a function of cleaning a sunlight-reflecting mirror. This sunlight-reflecting mirror unit is characterized in that: the surface of the sunlight-reflecting mirror has a contact angle with water of 30° or less; and this sunlight-reflecting mirror unit is provided with a cleaning means for forming a liquid film, which flows on the surface of the sunlight-reflecting mirror, by continuously or intermittently supplying a liquid that contains water to the surface.

Description

Sunlight reflecting mirror unit, solar power generation device, and method for cleaning sunlight reflecting mirror

The present invention relates to a solar reflection mirror unit, a solar thermal power generation apparatus, and a method for cleaning a solar reflection mirror capable of maintaining high cleanliness and reflectance of the surface of the solar mirror.

Known solar power generation devices include solar cells that directly convert sunlight into electric power, solar thermal power generation devices that use sunlight reflecting mirrors to collect sunlight and generate the resulting heat as a medium. It has been.
According to the solar thermal power generation apparatus, power can be generated regardless of day or night by storing the obtained heat. In the long term, the power generation efficiency of the solar thermal power generation device is higher than that of the solar cell, and sunlight can be used effectively.

Solar power generators are often used in desert areas. Dirt such as dust is likely to adhere to the sunlight reflecting mirror installed outdoors, and this dirt has been a cause of lowering the reflectivity and thus power generation efficiency.
In particular, dirt derived from dust in a desert area is different from normal dirt, and forms a strong sand film and adheres to the surface of the sunlight reflecting mirror. This is considered to be one of the causes that condensation occurs on the surface of the solar reflective mirror in a desert area where the temperature difference between day and night is large. When condensation occurs on the surface of the sunlight reflecting mirror, dust-derived substances (for example, NaCl, CaCO ₃ , SiO ₂ etc.) deposited on the surface, pollutants in the atmosphere (eg, SiO _x etc.), etc. It dissolves and reacts to form an insoluble salt. Thereafter, the water evaporates and insoluble salts and dust particles aggregate to form a strong sand film.

¡To suppress the decrease in reflectivity due to dirt, it is necessary to periodically clean the surface of the solar reflective mirror. However, since a large number of thousands of solar reflective mirrors are usually installed on a vast site, the cleaning work is a laborious work requiring a lot of labor and time.

Therefore, a sunlight reflecting mirror excellent in surface antifouling properties has been studied.
For example, there has been proposed a sunlight reflecting mirror that includes a layer containing a photocatalyst on the outermost surface of a sunlight reflecting mirror and decomposes the attached organic matter (see, for example, Patent Document 1).
There has also been proposed a solar reflective mirror that is provided with a hydrophilic layer containing a hydrophilic polymer, a metal alkoxide compound, and colloidal silica on the surface of the solar reflective mirror so that dirt is less likely to adhere and can be easily cleaned (for example, patents). Reference 2).
In addition, a solar reflective mirror with excellent antifouling property against oil dust, acid rain, dirt, etc. has been proposed by forming a fluororesin thin film with excellent durability on the surface of the solar reflective mirror. (For example, refer to Patent Document 3).

In addition to the antifouling property of the surface, a cleaning method for increasing the cleanliness of the surface has been studied.
For example, a method has been proposed in which nano-bubbles are used to easily clean dirt adhered without damaging the surface (see, for example, Patent Document 4).

However, since the degradation rate of dirt by the photocatalyst is slower than the speed at which a sand film is formed, dirt is still deposited on the surface in a harsh environment such as outdoors.
In addition, according to the hydrophilic layer, when there is rain, the water gets wet and the dirt is washed away from the surface, but in the desert area where there is little rain, such self-cleaning action cannot be expected, and regular cleaning work is still necessary It is.
On the other hand, a sunlight reflecting mirror having a water-repellent and oil-repellent surface made of fluororesin tends to cause water droplets to adhere to the surface due to rainfall and condensation, and the reflectance is significantly reduced. Even after the water droplets evaporate, it is difficult to maintain a high reflectance because dust substances and the like dissolved in the water droplets remain.

Thus, in a harsh outdoor environment such as a desert, it was not possible to sufficiently prevent the accumulation of dirt simply by providing antifouling properties. Even when a cleaning method having a high cleaning effect such as nanobubbles is used, in order to maintain a high reflectance, it is necessary to frequently perform a cleaning operation, and the cleaning load does not change much.

International Publication No. 2011/0778024 JP 2012-8166 A Utility Model Registration No. 3149312 JP 2013-139958 A

The present invention has been made in view of the above problems and circumstances, and the problem to be solved is a solar reflective mirror unit, a solar thermal power generation apparatus, and a solar reflective mirror cleaning method capable of maintaining high surface cleanliness and reflectivity. Is to provide.

In order to solve the above problems, the present inventors maintain high cleanliness and reflectivity of the surface of the solar reflective mirror installed in a harsh outdoor environment in the process of examining the cause of the above problems and the like. Therefore, it was thought that it was necessary to remove the dirt as soon as possible without attaching dirt to the surface. The present inventors have found that this can be realized by coating the surface with a liquid film and further flowing the liquid film, and have reached the present invention.

That is, the subject concerning this invention is solved by the following means.
1. A solar reflective mirror unit having a solar reflective mirror cleaning function,
The contact angle with water on the surface of the solar reflective mirror is 30 ° or less,
A solar reflective mirror comprising: cleaning means for continuously or intermittently supplying a liquid containing water to the surface of the solar reflective mirror to form a liquid film flowing on the surface. unit.

2. The sunlight reflecting mirror unit according to item 1, wherein a contact angle of the surface of the sunlight reflecting mirror with water is 20 ° or less.

3. 3. The solar light reflecting mirror unit according to claim 1, wherein a hydrophilic layer containing a photocatalyst that is made hydrophilic by sunlight is provided on the surface of the solar light reflecting mirror.

4). 4. The solar light reflecting mirror unit according to item 3, wherein a band gap of the photocatalyst is in a range of 2.4 to 5.2 eV.

5. The solar light according to any one of claims 1 to 4, wherein a covering area ratio of the liquid film with respect to the entire surface of the solar light reflecting mirror is in a range of 50 to 100%. Reflective mirror unit.

6). The solar reflective mirror unit according to any one of claims 1 to 5, further comprising a means for adjusting a flow rate of the liquid film.

7). The liquid film recovery means that has finished flowing on the surface of the sunlight reflecting mirror,
The first to sixth aspects, wherein the cleaning means re-supplyes the liquid film liquid collected by the collecting means to the surface of the sunlight reflecting mirror to form the liquid film. The solar reflective mirror unit as described in any one of to.

8). The sunlight reflecting mirror according to any one of claims 1 to 7, wherein the sunlight reflecting mirror is a film-like mirror having at least a sunlight reflecting layer on a resin film. unit.

9. A solar thermal power generation apparatus comprising the sunlight reflecting mirror unit according to any one of items 1 to 8.

10. A method of cleaning a solar reflective mirror,
The contact angle with water on the surface of the solar reflective mirror is 30 ° or less,
A method for cleaning a solar reflective mirror, comprising: supplying a liquid containing water continuously or intermittently to the surface of the solar reflective mirror to form a liquid film flowing on the surface.

The above-mentioned means of the present invention can provide a solar reflection mirror unit, a solar thermal power generation apparatus, and a solar reflection mirror cleaning method capable of maintaining high surface cleanliness and reflectivity.

The expression mechanism or action mechanism of the effect of the present invention is not clear, but is presumed as follows.
In the present invention, since the contact angle of the surface of the solar reflective mirror with water is 30 ° or less, when a liquid containing water is supplied onto the surface, the liquid is wetted and spread on the surface. Can be formed. Since the liquid film covers the surface continuously or intermittently, it is possible to prevent dirt from adhering to the surface.
Further, since the liquid film flows on the surface, the dirt attached to the liquid film can be quickly removed from the surface. Even when the coating with the liquid film is partial, the liquid film flowing through the dirt adhered on the surface is washed away, so that the cleanliness of the surface can be maintained high.
Since the cleanliness of the surface is kept high, the reflectance of the sunlight reflecting mirror is not lowered by dirt, and the reflectance can be kept high.

The front view which shows schematic structure of the sunlight reflective mirror unit which concerns on this Embodiment Sectional drawing which shows the structure of the sunlight reflective mirror which concerns on this Embodiment The figure which shows the structure of the solar thermal power generation apparatus which concerns on this Embodiment.

The sunlight reflecting mirror unit of the present invention comprises a sunlight reflecting mirror having a surface with a contact angle with water of 30 ° or less, and a cleaning means for forming a liquid film on the surface of the sunlight reflecting mirror. And This feature is a technical feature common to the inventions according to claims 1 to 10.

As an embodiment of the present invention, it is preferable that the contact angle with water on the surface of the solar reflective mirror is 20 ° or less from the viewpoint of preventing the adhesion of dirt to the surface by increasing the coating area ratio of the liquid film. .
Further, when the liquid film coverage is in the range of 50 to 100%, it is possible to sufficiently prevent the adhesion of dirt.

From the viewpoint of obtaining the decomposition action of dirt, it is preferable that the surface of the sunlight reflecting mirror contains a photocatalyst that becomes hydrophilic by sunlight.
From the viewpoint of effectively using ultraviolet light on the short wavelength side in the ultraviolet region of sunlight, the band gap of the photocatalyst is preferably in the range of 2.4 to 5.2 eV.

As an embodiment of the present invention, in order to reduce the cost, it is preferable to include means for adjusting the flow rate of the liquid film. From the same point of view, it is also possible to provide liquid film recovery means and re-supply the liquid film liquid recovered by the cleaning means.
Further, it is preferable that the sunlight reflecting mirror is a film-like mirror because it can be curved and used.

The solar reflective mirror unit of the present invention can be suitably provided in a solar thermal power generation apparatus. Thereby, power generation efficiency can be improved.

Hereinafter, the present invention, its components, and modes for carrying out the present invention will be described in detail.
In the present application, “˜” is used to mean that the numerical values described before and after it are included as a lower limit value and an upper limit value.

[Sunlight reflection mirror unit]
FIG. 1 shows a schematic configuration of a sunlight reflecting mirror unit 1 according to an embodiment.
As shown in FIG. 1, the sunlight reflecting mirror unit 1 includes a sunlight reflecting mirror 10 and a cleaning unit 20 as a means for cleaning the surface of the sunlight reflecting mirror 10.

The sunlight reflecting mirror 10 has at least a sunlight reflecting layer on a substrate.
The surface of the sunlight reflecting mirror 10 exhibits hydrophilicity with a contact angle with water of 30 ° or less.
When the surface is hydrophilic, a liquid containing water supplied by the cleaning unit 20 can be wetted and spread on the surface, and a liquid film can be formed on the surface.

The contact angle between the surface of the sunlight reflecting mirror 10 and water is more preferably 20 ° or less.
If it is 20 ° or less, the wettability of the surface can be further increased, and the area that can be covered with the liquid film formed on the surface is expanded, so that the adhesion of dirt is prevented over a wider area and the dirt is removed. can do.

[Cleaning means]
The cleaning unit 20 continuously or intermittently supplies a liquid containing water to the surface of the sunlight reflecting mirror 10 to form a liquid film that flows on the surface.
A flowing liquid film refers to an aggregate of liquid molecules in which a plurality of liquid molecules flowing on the surface are continuously arranged on the surface to form a thin film. On the surface, the liquid molecules may be in a two-dimensional array, or the liquid molecules may be in a three-dimensional array by overlapping, and the thickness of the liquid film is not particularly limited.

The adhesion of dirt to the surface of the sunlight reflecting mirror 10 can be prevented by the flowing liquid film. Moreover, the dirt adhering to the liquid film can be quickly removed from the surface, and the dirt adhering to the surface of the solar reflective mirror 10 can be washed away. Since the liquid film is formed continuously or intermittently, the dirt can be removed constantly or intermittently, and high cleanliness of the surface of the solar reflective mirror 10 can be maintained. Because of the high cleanliness, cleaning is unnecessary or less frequent cleaning is sufficient, and not only the amount of cleaning work, but also the amount of water required for cleaning, the amount of additives, and hot water cleaning can reduce heating energy and reduce the cleaning load. It can be reduced overall. In addition, the amount and frequency of use of various cleaning tools such as brushes, sponges, cloths, felts, and cleaning machines necessary for cleaning can be reduced.

For example, as shown in FIG. 1, the cleaning unit 20 includes a tank 21, a pump P, a transfer pipe 22, and a supply pipe 23.
The supply pipe 23 has a plurality of apertures provided at equal intervals, and is arranged along one end of the sunlight reflecting mirror 10 on the upper side in the gravity direction. The cleaning unit 20 stores a liquid containing water in the tank 21, and continuously or intermittently sends the liquid from the tank 21 through the transfer pipe 22 to the supply pipe 23 by the pump P. A liquid is supplied onto the surface of the sunlight reflecting mirror 10 via the surface. When the surface of the sunlight reflecting mirror 10 is inclined with respect to the horizontal plane, the liquid supplied on the surface flows in the gravity direction B by its own weight. When the surface of the sunlight reflecting mirror 10 is installed in parallel with the horizontal plane and it is difficult to flow due to its own weight, the cleaning unit 20 blows air from the supply pipe 23 side using a blower, increases the liquid supply pressure, etc. The liquid may be flowed.

Note that a syringe, a piston, a rotary, a diaphragm, or the like can be used in place of the pump P as long as the liquid can be sent out continuously or intermittently.
Further, the diameter and the arrangement interval of the openings of the supply pipe 23 can be adjusted so that the liquid is evenly supplied to the entire width of one end of the mirror surface on which the supply pipe 23 is arranged. For example, when the amount of liquid supplied per unit time is not large, openings having the same diameter are arranged uniformly. On the other hand, when the amount of liquid supplied per unit time is large, the diameter of the opening is increased as it is closer to the outlet than the inlet in the flow direction of the liquid in the supply pipe 23, and the interval between the holes is decreased as it is closer to the outlet. To do.
The direction of the opening is preferably parallel to the mirror surface or upward with respect to the mirror surface from the viewpoint of supplying the liquid uniformly over the entire width of the mirror surface.
Instead of an opening, a supply pipe in which slits are provided at equal intervals over the entire width, a valve such as a valve or a faucet, or a supply pipe in which valves are provided at equal intervals can also be used.

The liquid supply means is not limited to the supply pipe 23 as long as the liquid can be supplied over the entire width of the mirror surface.
For example, as shown in FIG. 1, an absorber 24 may be disposed near the liquid outlet from the supply pipe 23, and the liquid may be supplied to the surface of the sunlight reflecting mirror 10 through the absorber 24. . Since the liquid supplied from the supply pipe 23 is absorbed and diffused by the absorber 24, the liquid can be supplied uniformly over the entire width of the mirror surface without concentration of the liquid supply position.
Such an absorber 24 may be provided in four directions of the sunlight reflecting mirror 10 or may be disposed in the supply pipe 23.
The material of the absorber 24 is not specifically limited, For example, it can be resin, a fiber, a pulp, etc.

The liquid containing water used for forming the liquid film is a liquid containing water as a main component. “Water as a main component” means that the content of water in the liquid component excluding the solid component is in the range of 30 to 100% by mass.
Although water is a valuable resource in desert areas, steam generated during power generation can be used in a solar thermal power generation device. Further, the water component itself contaminates the mirror surface, and there is almost no adverse effect such as a decrease in reflectivity or a decrease in power generation efficiency. Therefore, it is convenient as a liquid for cleaning the mirror surface. Furthermore, since the boiling point under atmospheric pressure is high, evaporation of the liquid film in a high temperature environment such as a desert can be suppressed.

If the liquid containing water has water as a main component, from the viewpoint of preventing the adhesion of dirt and facilitating the removal of dirt, organic solvents, surfactants, salts, acids / bases, resins, fibers, It can also contain additives such as particulate matter. Although there is no restriction | limiting in particular in an additive, The additive compatible with water is preferable.

Examples of the organic solvent that can contain a liquid containing water include alcohols such as methanol, ethanol, propanol, butanol, isopropyl alcohol, ethylene glycol, and propylene glycol, and hydrocarbons such as acetone and methylene chloride.
Examples of the surfactant include anionic, cationic, amphoteric, and nonionic, specifically, anionic surfactants such as polyoxyethylene alkyl ether acetate, dodecylbenzene sulfonate, and laurate, Nonionic surfactants such as polyoxyethylene alkyl ethers are listed.
Examples of the salts include sodium chloride and sodium hydrogen carbonate, and examples of the acids include acetic acid and phthalic acid.
By these additives, it is possible to easily remove the dirt by the liquid film and improve the cleaning power at the time of cleaning.

Examples of the resin include polyethylene glycol, polypropylene glycol, polycarboxylic acid, polyvinyl alcohol, hydroxyethyl cellulose, agar, and gelatin. With the resin, the viscosity of the liquid film can be increased, and adhesion of dirt to the surface can be prevented.
Examples of the fibers include nylon, polyester, acrylic, vinyl chloride, cotton, hemp, silk and the like.
Examples of the particulate material include inorganic particles such as silica, alumina, zirconia, and ceria, and organic particles such as crosslinked polystyrene and crosslinked polymethyl methacrylate.
According to the fibers and the particulate matter, dirt can be adsorbed on the fibers or the particulate matter in the liquid film.

The ratio of the coating area of the liquid film to the entire surface of the sunlight reflecting mirror 10 is preferably as high as possible from the viewpoint of reducing the probability of contact between the surface and the dirt, preferably in the range of 40 to 100%, more preferably in the range of 50 to 100%. It is.
In particular, if it is in the range of 50 to 100%, the surface of the solar reflective mirror 10 can be covered sufficiently with the liquid film to remove dirt, and the reflectance can be maintained higher.
The coating area ratio (%) of the liquid film is the product of the overall length in the length direction of the surface of the solar reflective mirror 10 and the overall length in the direction perpendicular to the length direction. And the percentage of the total area of the liquid film on the mirror surface with respect to the area of the entire mirror surface.
The covering area ratio of the liquid film can be adjusted by reducing the contact angle with the water on the surface of the solar light reflecting mirror 10 or changing the supply amount of liquid containing water per unit time.

The refractive index of the liquid film formed by the cleaning unit 20 is lower than the refractive index of the surface of the solar reflective mirror 10 from the viewpoint of concentrating solar energy and efficiently converting it into thermal energy. It is preferably higher than the refractive index of air.
Thereby, the reflection by the interface of air and a liquid film can be suppressed, and the transmitted light amount to the reflection layer of the sunlight reflective mirror 10 can be increased.

In addition, the inclination angle of the surface of the sunlight reflecting mirror 10 with respect to the horizontal plane may be changed according to the incident direction of sunlight. In this case, the same transfer pipe 22 and supply pipe 23 may be disposed at both ends of the sunlight reflecting mirror 10 in the gravity direction, and the liquid may be supplied from the transfer pipe 22 and the supply pipe 23 located on the upper side in the gravity direction.

[Recovery means]
As shown in FIG. 1, the sunlight reflecting mirror unit 1 includes a collection container 30 as a means for collecting the liquid film that has finished flowing on the surface of the sunlight reflecting mirror 10, and cleans the liquid in the collected liquid film. The liquid film can be formed again on the surface of the sunlight reflecting mirror 10 by being re-supplied by 20.
Thereby, the collected liquid can be reused, and the cost can be reduced. In the case where the sunlight reflecting mirror 10 has a small amount of rainfall in a desert or the like and water is disposed in a precious environment, the cost can be further reduced.

As the collection container 30, for example, a semi-cylindrical collection container disposed along the other end facing the supply pipe 23 of the sunlight reflecting mirror 10 can be used. The liquid film that has finished flowing on the surface of the sunlight reflecting mirror 10 is recovered and stored in the tank 21 by the recovery container 30. In order to remove dirt components contained in the collected liquid, it is preferable to provide a filter or the like at the liquid inlet to the tank 21.

[Adjustment means]
The sunlight reflecting mirror unit 1 can also include means for adjusting the flow rate of the liquid film formed by the cleaning unit 20. The surface cleanliness can be adjusted by adjusting the flow rate.
For example, in an environment where much dirt is attached, the flow rate of the liquid film is increased to increase the cleanliness. In an environment where there is little adhesion of dirt, the cost required for cleaning can be reduced by reducing the flow rate of the liquid film to reduce the degree of cleaning.

FIG. 1 shows an example in which a control device 40 for changing the rotational speed of the pump P is provided as adjusting means, and the supply rate per unit time of the pump P is adjusted by the control device 40 to adjust the flow rate of the liquid film. Show.
If the flow rate of the liquid film can be adjusted, not only the control device 40 but also a tank and a valve are provided as adjusting means between the pump P and the supply pipe 23, and the flow rate of the liquid film is determined by the valve opening degree of the valve. Can also be adjusted. Even when there is no pump P, liquid can be poured into the tank and the flow rate of the liquid film can be adjusted by the valve opening.
In the case where a plug or valve is provided at the liquid supply port in the supply pipe 23, the flow rate of the liquid film is similarly adjusted by adjusting the valve opening degree using the plug or valve as an adjusting means. Can also be adjusted.
The flow rate of the liquid film may be adjusted by using a blower as the adjusting means and blowing air from the flow direction of the liquid film or the direction opposite to the flow direction to promote or suppress the flow of the liquid film.

Next, details of the sunlight reflecting mirror will be described.
[Sunlight reflection mirror]
The sunlight reflecting mirror 10 can be a plate-like mirror using a glass plate, a metal plate or the like as a substrate, but is preferably a film-like mirror using a resin film as a substrate. Since the film-like mirror is highly flexible, it can be deformed into a curved surface or the like in accordance with the position where sunlight is condensed when used in a solar power generation apparatus.

FIG. 2 is a cross-sectional view showing a configuration example when the sunlight reflecting mirror 10 is a film-like mirror. In FIG. 2, the arrow represents the incident direction A of sunlight.
As shown in FIG. 2, the sunlight reflecting mirror 10 includes an anchor layer 12, a reflecting layer 13, a corrosion preventing layer 14, an adhesive layer 15, an ultraviolet absorbing layer 16 and a hydrophilic layer 17 in this order on a resin film 11. I have.
Details of each layer will be described below.

[Resin film]
As the resin film 11, various conventionally known resin films can be used as long as the sunlight reflecting mirror 10 can be formed into a film shape.
Examples of the resin film 11 include cellulose ester films, polyester films, polycarbonate films, polyarylate films, polysulfone (including polyethersulfone) films, polyethylene terephthalate, polyester films such as polyethylene naphthalate, polyethylene films, Polypropylene film, cellophane, cellulose diacetate film, cellulose triacetate film, cellulose acetate propionate film, cellulose acetate butyrate film, polyvinylidene chloride film, polyvinyl alcohol film, ethylene vinyl alcohol film, syndiotactic polystyrene film, polycarbonate film , Norbornene resin fill , Polymethyl pentene film, polyether ketone film, polyether ketone imide film, a polyamide film, a fluororesin film, a nylon film, polymethyl methacrylate film, and acrylic films.
Among these, a polycarbonate film, a polyester film, a norbornene resin film, or a cellulose ester film is preferable, and a polyester film or a cellulose ester film is more preferable.

The resin film 11 may be a film manufactured by a melt-flow method or a film manufactured by a solution-flow method.
The thickness of the resin film 11 can be set to a thickness corresponding to the type of resin. In general, it is preferably within the range of 10 to 400 μm, more preferably within the range of 20 to 300 μm, and even more preferably within the range of 30 to 200 μm.

[Anchor layer]
The anchor layer 12 is provided between the resin film 11 and the reflective layer 13 in order to improve the adhesion of the reflective layer 13 to the resin film 11. The anchor layer 12 can increase heat resistance and prevent the resin film 11 from being deteriorated due to heat generated when the reflective layer 13 is formed. Moreover, the surface of the resin film 11 can be smoothed, and it is also possible to prevent the reflectance of the reflective layer 13 from decreasing.

The anchor layer 12 is preferably highly transparent in order to increase the amount of light transmitted to the reflectance of the reflective layer 13.
The material of the anchor layer 12 is not particularly limited as long as high adhesion, heat resistance, reflectance, and transparency can be obtained. For example, a resin can be used. Examples of preferable resins include polyester resins, acrylic resins, melamine resins, epoxy resins, polyamide resins, vinyl chloride resins, vinyl chloride vinyl acetate copolymer resins, and the like. These resins can be used alone or in combination of two or more. Among these, from the viewpoint of obtaining weather resistance, a mixed resin of a polyester resin and a melamine resin is preferable, and a thermosetting resin in which a curing agent such as isocyanate is further mixed is more preferable.

The thickness of the anchor layer 12 is preferably within a range of 0.01 to 3.00 μm, and preferably within a range of 0.1 to 1.0 μm, from the viewpoint of improving adhesion, smoothness, and reflectance. Is more preferable.
As a method for forming the anchor layer 12, conventionally known coating methods such as a gravure coating method, a reverse coating method, and a die coating method can be used.

(Reflective layer)
The reflection layer 13 is provided to reflect the sunlight incident on the sunlight reflecting mirror 10.
In order to prevent the resin film 11 from being deteriorated by sunlight, the reflective layer 13 is preferably disposed closer to the sunlight incident side than the resin film 11.
The reflectance of the reflective layer 13 is preferably 80% or more, and more preferably 90% or more. The reflectance of the reflective layer 13 refers to regular reflectance.

As the material of the reflective layer 13, metals such as aluminum, silver, chromium, nickel, titanium, magnesium, rhodium, platinum, palladium, tin, gallium, indium, bismuth, gold and the like can be used. Among these, aluminum or silver is preferable and silver is more preferable from the viewpoint of obtaining high reflectance and corrosion resistance. Two or more layers each composed mainly of aluminum and silver can be laminated to form the reflective layer 13. Thereby, the reflectance with respect to light having a wavelength in the range of about 400 to 2500 nm from the visible light region to the infrared region can be increased, and the dependency of the reflectance on the incident angle can be reduced.

Moreover, as a material of the reflective layer 13, from the viewpoint of improving the durability of the reflective layer 13, an alloy of two or more of the above metals may be used. Examples of such an alloy include silver alloys of silver and other metals. As another metal used for the silver alloy, gold is preferable from the viewpoint of improving moisture resistance and reflectance.
When a silver alloy is used, the ratio of the number of silver atoms to the total number of atoms of silver and other metals in the reflective layer 13 is preferably in the range of 90.0 to 99.8%. The ratio of the other metal to the total number of atoms of silver and the other metal in the reflective layer 13 is preferably in the range of 0.2 to 10.0% from the viewpoint of obtaining durability.

As a method for forming the reflective layer 13, either a wet method or a dry method can be used.
The wet method is a general term for a plating method, and is a method of forming a metal film by depositing a metal from a solution. As a specific example of the wet method, there is a silver plating formation method using a silver mirror reaction.
The dry method is a general term for a vacuum film-forming method, and examples thereof include a resistance heating vacuum deposition method, an electron beam heating vacuum deposition method, an ion beam assisted vacuum deposition method, an ion plating method, and a sputtering method. Since the elongate resin film 11 can be used for manufacture of the sunlight reflective mirror 10, the vapor deposition method which can form the reflection layer 13 continuously by a roll-to-roll system is preferable.

In addition, when forming the reflective layer 13 using silver, in addition to the wet method and the dry method, a coating film containing a silver complex compound capable of vaporizing and leaving the ligand is formed, and the coating film is formed. A method of firing can also be adopted.

(Corrosion prevention layer)
The corrosion prevention layer 14 is provided to prevent the reflection layer 13 from being corroded.
The corrosion prevention layer 14 is preferably provided adjacent to the reflective layer 13. As shown in FIG. 2, one corrosion prevention layer 14 may be adjacent to one side of the reflective layer 13, and two corrosion prevention layers 14 are provided adjacent to both sides of the reflective layer 13. May be.

The corrosion prevention layer 14 is a layer containing, for example, a corrosion inhibitor.
When silver is used for the reflection layer 13, the corrosion inhibitor preferably has an adsorptive group for silver.
Corrosion inhibitors having an adsorptive group for silver include amines and derivatives thereof, compounds having a pyrrole ring, compounds having a triazole ring, compounds having a pyrazole ring, compounds having a thiazole ring, compounds having an imidazole ring, indazole Examples thereof include compounds having a ring, copper chelate compounds, thioureas, compounds having a mercapto group, and naphthalene compounds. These corrosion inhibitors can be used alone or in combination of two or more.

Examples of amines and derivatives thereof include ethylamine, laurylamine, tri-n-butylamine, O-toluidine, diphenylamine, ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, monoethanolamine, diethanolamine, triethanolamine, 2N- Dimethylethanolamine, 2-amino-2-methyl-1,3-propanediol, acetamide, acrylamide, benzamide, p-ethoxychrysoidine, dicyclohexylammonium nitrite, dicyclohexylammonium salicylate, monoethanolamine benzoate, dicyclohexylammonium benzoate, diisopropyl Ammonium benzoate, diisopropylammonium nitrite Cyclohexylamine carbamate, nitronaphthalene nitrite, cyclohexylamine benzoate, dicyclohexylammonium cyclohexanecarboxylate, cyclohexylamine cyclohexane carboxylate, dicyclohexylammonium acrylate, cyclohexylamine acrylate or mixtures thereof.

Examples of compounds having a pyrrole ring include N-butyl-2,5-dimethylpyrrole, N-phenyl-2,5-dimethylpyrrole, N-phenyl-3-formyl-2,5-dimethylpyrrole, and N-phenyl-3. , 4-diformyl-2,5-dimethylpyrrole or a mixture thereof.

Examples of the compound having a triazole ring include 1,2,3-triazole, 1,2,4-triazole, 3-mercapto-1,2,4-triazole, 3-hydroxy-1,2,4-triazole, 3- Methyl-1,2,4-triazole, 1-methyl-1,2,4-triazole, 1-methyl-3-mercapto-1,2,4-triazole, 4-methyl-1,2,3-triazole, Benzotriazole, tolyltriazole, 1-hydroxybenzotriazole, 4,5,6,7-tetrahydrotriazole, 3-amino-1,2,4-triazole, 3-amino-5-methyl-1,2,4- Triazole, carboxybenzotriazole, 2- (2'-hydroxy-5'-methylphenyl) benzotriazole, 2- (2'-hydroxy) -5'-tert-butylphenyl) benzotriazole, 2- (2'-hydroxy-3 ', 5'-di-tert-butylphenyl) benzotriazole, 2- (2'-hydroxy-4-octoxyphenyl) Examples thereof include benzotriazole and a mixture thereof.

Examples of the compound having a pyrazole ring include pyrazole, pyrazoline, pyrazolone, pyrazolidine, pyrazolidone, 3,5-dimethylpyrazole, 3-methyl-5-hydroxypyrazole, 4-aminopyrazole, and mixtures thereof.
Examples of compounds having a thiazole ring include thiazole, thiazoline, thiazolone, thiazolidine, thiazolidone, isothiazole, benzothiazole, 2-N, N-diethylthiobenzothiazole, P-dimethylaminobenzallodanine, 2-mercaptobenzothiazole, etc. Or a mixture thereof or the like can be mentioned.

Examples of the compound having an imidazole ring include imidazole, histidine, 2-heptadecylimidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, 2-undecylimidazole, 1-benzyl-2-methyl Imidazole, 2-phenyl-4-methylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2-undecyl Imidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, 4-formylimidazole, 2-methyl-4-formylimidazole, 2-phenyl 4-formylimidazole, 4-methyl-5-formylimidazole, 2-ethyl-4-methyl-5-formylimidazole, 2-phenyl-4-methyl-4-formylimidazole, 2-mercaptobenzimidazole, etc. Or a mixture thereof or the like can be mentioned.

Examples of the compound having an indazole ring include 4-chloroindazole, 4-nitroindazole, 5-nitroindazole, 4-chloro-5-nitroindazole, and a mixture thereof.

Examples of copper chelate compounds include acetylacetone copper, ethylenediamine copper, phthalocyanine copper, ethylenediaminetetraacetate copper, hydroxyquinoline copper, and mixtures thereof.
Examples of thioureas include thiourea, guanylthiourea, and mixtures thereof.

As a compound having a mercapto group, mercaptoacetic acid, thiophenol, 1,2-ethanediol, 3-mercapto-1,2,4-triazole, 1-methyl-3-mercapto can be used by adding the above-described materials. -1,2,4-triazole, 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, glycol dimercaptoacetate, 3-mercaptopropyltrimethoxysilane, or a mixture thereof.
Examples of naphthalene compounds include thionalide.

Moreover, the corrosion prevention layer 14 can also contain antioxidant as a corrosion inhibitor.
As the antioxidant, it is preferable to use a phenol-based antioxidant, a thiol-based antioxidant, or a phosphite-based antioxidant.

Examples of phenolic antioxidants include 1,1,3-tris (2-methyl-4-hydroxy-5-tert-butylphenyl) butane and 2,2′-methylenebis (4-ethyl-6-tert-butylphenol). ), Tetrakis- [methylene-3- (3 ′, 5′-di-t-butyl-4′-hydroxyphenyl) propionate] methane, 2,6-di-t-butyl-p-cresol, 4,4 ′ -Thiobis (3-methyl-6-t-butylphenol), 4,4'-butylidenebis (3-methyl-6-t-butylphenol), 1,3,5-tris (3 ', 5'-di-t- Butyl-4'-hydroxybenzyl) -S-triazine-2,4,6- (1H, 3H, 5H) trione, stearyl-β- (3,5-di-t-butyl-4-hydroxyphenyl) propio Triethyl glycol bis [3- (3-tert-butyl-5-methyl-4-hydroxyphenyl) propionate], 3,9-bis [1,1-dimethyl-2- [β- (3 -T-butyl-4-hydroxy-5-methylphenyl) propionyloxy] ethyl] -2,4,8,10-tetraoxoxaspiro [5,5] undecane, 1,3,5-trimethyl-2,4 , 6-tris (3,5-di-t-butyl-4-hydroxybenzyl) benzene and the like. In particular, the phenolic antioxidant preferably has a molecular weight of 550 or more.

Examples of the thiol antioxidant include distearyl-3,3′-thiodipropionate, pentaerythritol-tetrakis- (β-lauryl-thiopropionate), and the like.

Examples of phosphite antioxidants include tris (2,4-di-t-butylphenyl) phosphite, distearyl pentaerythritol diphosphite, di (2,6-di-t-butylphenyl) pentaerythritol diphosphite. Phosphite, bis- (2,6-di-t-butyl-4-methylphenyl) -pentaerythritol diphosphite, tetrakis (2,4-di-t-butylphenyl) 4,4′-biphenylene-diphosphonite, Examples include 2,2'-methylenebis (4,6-di-t-butylphenyl) octyl phosphite.

In addition, the corrosion prevention layer 14 can also use the said antioxidant and light stabilizer together. Examples of light stabilizers that can be used in combination include hindered amine light stabilizers and nickel ultraviolet light stabilizers.
Examples of hindered amine light stabilizers include bis (2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis (1,2,2,6,6-pentamethyl-4-piperidyl) sebacate, Bis (1,2,2,6,6-pentamethyl-4-piperidyl) -2- (3,5-di-t-butyl-4-hydroxybenzyl) -2-n-butylmalonate, 1-methyl- 8- (1,2,2,6,6-pentamethyl-4-piperidyl) -sebacate, 1- [2- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionyloxy] ethyl ] -4- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionyloxy] -2,2,6,6-tetramethylpiperidine, 4-benzoyloxy-2,2,6, 6-tetramethyl Piperidine, tetrakis (2,2,6,6-tetramethyl-4-piperidyl) -1,2,3,4-butane-tetracarboxylate, triethylenediamine, 8-acetyl-3-dodecyl-7,7,9 , 9-tetramethyl-1,3,8-triazaspiro [4,5] decane-2,4-dione.

In particular, as the hindered amine light stabilizer, a hindered amine light stabilizer containing only a tertiary amine is preferable. Specifically, bis (1,2,2,6,6-pentamethyl-4-piperidyl) is preferable. Sebacate, bis (1,2,2,6,6-pentamethyl-4-piperidyl) -2- (3,5-di-t-butyl-4-hydroxybenzyl) -2-n-butyl malonate, 1 2,2,6,6-pentamethyl-4-piperidinol / tridecyl alcohol and 1,2,3,4-butanetetracarboxylic acid are preferred.

Examples of the nickel-based ultraviolet stabilizer include [2,2′-thiobis (4-t-octylphenolate)]-2-ethylhexylamine nickel (II), nickel complex-3,5-di-t-butyl-4 -Hydroxybenzyl monophosphate, nickel dibutyl dithiocarbamate, etc.

The corrosion inhibitor preferably has a molecular weight of 800 or less. By containing such a low molecular weight corrosion inhibitor, the corrosion inhibitor can easily move to the interface with the reflective layer 13, and the corrosion prevention action is improved.
The content of the corrosion inhibitor in the corrosion prevention layer 14 varies depending on the corrosion inhibitor used, but is preferably in the range of 0.01 to 1.00 g / cm ³ .

(Adhesive layer)
The adhesive layer 15 is provided to improve the adhesion between the corrosion prevention layer 14 and the ultraviolet absorption layer 16.
The adhesive layer 15 can be formed in the same manner as the anchor layer 12 as long as high adhesiveness is obtained.

[UV absorbing layer]
The ultraviolet absorption layer 16 is provided in order to prevent deterioration of each layer due to ultraviolet rays of sunlight transmitted through the hydrophilic layer 17.
The ultraviolet absorbing layer 16 is preferably an acrylic resin layer having an ultraviolet absorbing group or containing an ultraviolet absorber from the viewpoint of increasing the flexibility and weather resistance of the sunlight reflecting mirror 10 and reducing the weight.

Examples of the ultraviolet absorber that can be contained in the ultraviolet absorbing layer 16 include organic compounds such as benzophenone, benzotriazole, phenyl salicylate, triazine, and benzoate, and inorganic compounds such as titanium oxide, zinc oxide, cerium oxide, and iron oxide. Is mentioned.
In order to reduce the bleeding out of the ultraviolet absorber, it is preferable to use a high molecular weight ultraviolet absorber having a molecular weight of 1000 or more. Preferably, the molecular weight is in the range of 1000 to 3000.

Examples of the benzophenone ultraviolet absorber include 2,4-dihydroxy-benzophenone, 2-hydroxy-4-methoxy-benzophenone, 2-hydroxy-4-n-octoxy-benzophenone, 2-hydroxy-4-dodecyloxy-benzophenone, 2- Hydroxy-4-octadecyloxy-benzophenone, 2,2'-dihydroxy-4-methoxy-benzophenone, 2,2'-dihydroxy-4,4'-dimethoxy-benzophenone, 2,2 ', 4,4'-tetra And hydroxy-benzophenone.

Examples of benzotriazole ultraviolet absorbers include 2- (2'-hydroxy-5-methylphenyl) benzotriazole, 2- (2'-hydroxy-3 ', 5'-di-t-butylphenyl) benzotriazole 2- (2'-hydroxy-3'-t-butyl-5'-methylphenyl) benzotriazole, 2,2'-methylenebis [6- (2H-benzotriazol-2-yl) -4- (1, 1,3,3-tetramethylbutyl) phenol] (molecular weight 659; examples of commercially available products are LA31 manufactured by ADEKA), 2- (2H-benzotriazol-2-yl) -4,6-bis (1 -Methyl-1-phenylethyl) phenol (molecular weight 447.6; an example of a commercially available product is Tinuvin 234 manufactured by BASF Japan Ltd.).

Examples of the phenyl salicylate ultraviolet absorber include phenylsalicylate, 2-4-di-t-butylphenyl-3,5-di-t-butyl-4-hydroxybenzoate, and the like. Examples of the hindered amine ultraviolet absorber include bis (2,2,6,6-tetramethylpiperidin-4-yl) sebacate.

Examples of triazine ultraviolet absorbers include 2,4-diphenyl-6- (2-hydroxy-4-methoxyphenyl) -1,3,5-triazine, 2,4-diphenyl-6- (2-hydroxy-4). -Ethoxyphenyl) -1,3,5-triazine, 2,4-diphenyl- (2-hydroxy-4-propoxyphenyl) -1,3,5-triazine, 2,4-diphenyl- (2-hydroxy-4) -Butoxyphenyl) -1,3,5-triazine, 2,4-diphenyl-6- (2-hydroxy-4-butoxyphenyl) -1,3,5-triazine, 2,4-diphenyl-6- (2 -Hydroxy-4-hexyloxyphenyl) -1,3,5-triazine, 2,4-diphenyl-6- (2-hydroxy-4-octyloxyphenyl) -1,3,5- Liazine, 2,4-diphenyl-6- (2-hydroxy-4-dodecyloxyphenyl) -1,3,5-triazine, 2,4-diphenyl-6- (2-hydroxy-4-benzyloxyphenyl)- 1,3,5-triazine, [2- (4,6-diphenyl-1,3,5-triazin-2-yl) -5- (hexyl) oxyphenol] (trade name Tinuvin 1577FF, manufactured by BASF Japan Ltd.) , [2- [4,6-bis (2,4dimethylphenyl) -1,3,5-triazin-2-yl] -5- (octyloxy) phenol] (trade name CYASORB UV-1164, Cytec Industries, Inc. Manufactured) and the like.

Examples of the benzoate-based ultraviolet absorber include 2,4-di-tert-butylphenyl-3,5-di-tert-butyl-4-hydroxybenzoate (molecular weight 438.7; examples of commercially available products) Sumisorb 400) manufactured by Sumitomo Chemical Co., Ltd.

In addition, each of the above ultraviolet absorbers may be used in combination of two or more thereof as necessary. Further, if necessary, an ultraviolet absorber other than the above-described ultraviolet absorber, for example, a salicylic acid derivative, a substituted acrylonitrile, a nickel complex, a benzophenone-based ultraviolet absorber, a triazine-based ultraviolet absorber, or the like can be contained.

The content of the ultraviolet absorber in the ultraviolet absorbing layer 16 is preferably in the range of 0.1 to 20.0 mass%. More preferably, it is in the range of 0.25 to 15.00% by mass, and still more preferably in the range of 0.5 to 10.0% by mass.
If it is 0.1 mass% or more, adhesiveness is favorable, and if it is 20 mass% or less, weather resistance is favorable.

The thickness of the ultraviolet absorbing layer 16 is preferably in the range of 20 to 150 μm, more preferably in the range of 40 to 100 μm. Within this range, the transmittance of incident light can be improved, and an appropriate surface roughness can be imparted to the surface of the solar reflective mirror 10.

As the acrylic resin that can be used for the ultraviolet absorbing layer 16, a methacrylic resin is preferable. The methacrylic resin is a polymer mainly composed of methacrylic acid ester, and may be a homopolymer of methacrylic acid ester, 50% by mass or more of methacrylic acid ester, and other monomers less than 50% by mass. And a copolymer thereof. Here, as the methacrylic acid ester, an alkyl ester of methacrylic acid is usually used. A particularly preferred methacrylic resin is polymethyl methacrylate resin (PMMA).

From the viewpoint of obtaining heat resistance of the solar reflective mirror 10, the methacrylic resin preferably has a glass transition temperature of 40 ° C or higher, more preferably 60 ° C or higher. This glass transition temperature can be appropriately set by adjusting the type of monomer and the ratio thereof.

The ultraviolet absorbing layer 16 can also contain the same antioxidant as the corrosion preventing layer 14. Further, a light stabilizer may be used in combination with the antioxidant. The antioxidant can prevent deterioration of the acrylic resin during melt film formation. Since the antioxidant captures radicals even after film formation, deterioration of the acrylic resin layer can be prevented.

[Hydrophilic layer]
The hydrophilic layer 17 is provided on the outermost surface of the sunlight reflecting mirror 10. Therefore, as described above, the contact angle with water on the surface of the hydrophilic layer 17 is 30 ° or less, and preferably 20 ° or less.
The contact angle with water (°) was 30 seconds after 3 μL of water was dropped on the hydrophilic layer 17 in an environment of a temperature of 23 ° C. and a relative humidity of 55% RH in accordance with JIS-R3257. It can be measured using a total of DM300 (manufactured by Kyowa Interface Chemical Co., Ltd.). It shows that hydrophilic property is so high that the measured contact angle is small.

The hydrophilic layer 17 can contain a hydrophilizing agent in order to make the contact angle with water on the surface 30 ° or less.
Examples of the hydrophilizing agent that can be used include compounds containing metal elements, such as oxides, nitrides and carbides containing metal elements such as Si, Ti, Al, Sn, Fe, Zn, Sb, and Zr. .
In order to enhance the hydrophilicity, the hydrophilic layer 17 can also contain metal particles such as silica particles, alumina particles, titania particles, zirconia particles, etc., in addition to the above compounds. By using metal particles, the surface roughness is increased, the hydrophilicity is improved, and the hydrophilic layer 17 having a contact angle of 30 ° or less can be formed. Further, if the surface roughness is large, it becomes difficult for dirt to come into contact with the surface of the hydrophilic layer 17, and the liquid film penetrates into the interface between the hydrophilic layer 17 and the surface, and the dirt is easily removed.
The hydrophilic layer 17 may contain a silicate compound, polysilazane having a Si—N bond as a basic skeleton, and the like. Examples of the silicate compound that can be used in combination include tetrahydroxysilane, tetramethoxysilane, tetraethoxysilane, and tetraethoxyoxysilane.

The hydrophilic layer 17 can also be a layer containing a photocatalyst that is rendered hydrophilic by sunlight as a hydrophilizing agent. A photocatalyst is a substance that, when irradiated with light having energy larger than the band gap between the conduction band and the valence band, the electrons in the valence band are excited to generate conduction electrons and holes.
By containing the photocatalyst, it is possible to obtain a decomposition action of dirt attached to the surface of the sunlight reflecting mirror 10, and it is possible to obtain higher hydrophilicity.

Examples of the photocatalyst that can be contained in the hydrophilic layer 17 include anatase-type titanium oxide (band gap; 3.2 eV), rutile-type titanium oxide (band gap; 3.0 eV), and zinc oxide (band gap; 3.2 eV). , Tin oxide (band gap; 3.5 eV), tungsten oxide (band gap; 2.5 eV), potassium tantalate (band gap; 3.4 eV), strontium titanate (band gap; 3.2 eV), zirconium oxide ( Band gap; 5.0 eV), niobium oxide (band gap; 3.4 eV), and the like. Note that since the band gap of the photocatalyst has a distribution depending on the crystal structure of the photocatalyst, the degree of purification, and the like, the actually measured bad gap may have a difference of about ± 0.2 eV from the band gap described above.

The band gap of the photocatalyst is preferably in the range of 2.4 to 5.2 eV.
By using a photocatalyst having a band gap of 5.2 eV or less, the photocatalyst can be excited by the energy of sunlight reaching the earth without being absorbed, scattered or attenuated in the atmosphere. In addition, by using a photocatalyst of 2.4 eV or more, it is possible to suppress a decrease in solar reflectance due to absorption of visible light and to suppress a decrease in power generation efficiency. By using a photocatalyst that absorbs and excites light on the short wavelength side as much as possible, it becomes possible to use not only the wavelength range of visible light mainly used for power generation, but also the optical energy in the wavelength range of ultraviolet light, Power generation efficiency is improved.

The hydrophilic layer 17 can contain a small amount of a platinum group metal such as Pt, Pd, Ru, Rh, Ir, Os, etc. in order to enhance the photocatalytic activity.
Moreover, the hydrophilic layer 17 can contain metal particles, such as the silica particle mentioned above, an alumina particle, a titania particle, a zirconia particle, with a photocatalyst. A silicate compound, polysilazane having a Si—N bond as a basic skeleton, or the like may be used in combination.

The hydrophilic layer 17 containing a photocatalyst can be formed by applying a dispersion of photocatalyst particles by a conventionally known coating method.
When the heat resistance of the resin film 11 is high, the hydrophilic layer 17 containing a photocatalyst can be formed by a sol coating baking method, an organic titanate method, a vacuum film forming method, or the like.
The sol coating and baking method is a method in which anatase-type titanium oxide sol is applied by a coating method such as a gravure coating method, a reverse coating method, or a die coating method, followed by baking.
The organic titanate method is a method in which a coating solution obtained by partially or completely hydrolyzing an organic titanate is applied by a conventionally known coating method such as a gravure coating method and dried. By drying, hydrolysis of the organic titanate is completed to produce titanium hydroxide, and an amorphous titanium oxide layer is formed by dehydration condensation polymerization of titanium hydroxide. Thereafter, firing is performed at a temperature equal to or higher than the crystallization temperature of anatase, and amorphous titanium oxide is phase-transformed into anatase-type titanium oxide.
The vacuum film formation method is a method of forming an amorphous titanium oxide layer by a vacuum deposition method, a sputtering method, or the like. Thereafter, phase transition is made to anatase-type titanium oxide by firing.

The hydrophilic layer 17 may be a layer in which a surface treatment such as a plasma treatment or an etching treatment is performed on the surface of the substrate as long as the hydrophilic layer 17 exhibits hydrophilicity such that the contact angle with water is 30 ° or less.

An inorganic coat layer may be provided between the hydrophilic layer 17 and the ultraviolet absorption layer 16. When the hydrophilic layer 17 contains a photocatalyst described later, the inorganic coating layer can prevent an organic compound such as an acrylic resin contained in the ultraviolet absorbing layer 16 from being decomposed by the photocatalyst.
Examples of the material for the inorganic coating layer include a silicate compound such as tetraethoxysilane and a layer containing an alcohol such as methanol. Moreover, in order to improve the adhesiveness of the hydrophilic layer 17 and the ultraviolet absorption layer 16, an inorganic coat layer can contain the structural component of a hydrophilic layer, or the structural component of an ultraviolet absorption layer. The inorganic coat layer may be a single layer or a plurality of layers.

The layer thickness of the hydrophilic layer 17 is a suitable layer thickness depending on the refractive index of the hydrophilic layer 17, the contained components, the wavelength range of light used for power generation among the sunlight incident on the sunlight reflecting mirror 10, and the like. Should be selected. Since use of a wavelength range as wide as possible leads to improvement of power generation efficiency, it is preferable that the layer thickness is thin in consideration of light absorption by the hydrophilic layer 17. Since it is the extreme surface layer that expresses hydrophilicity, if it has a layer thickness of about several nanometers, hydrophilicity necessary for forming a liquid film can be expressed.
In the case of forming the hydrophilic layer 17 using photocatalyst particles, a layer thickness of at least about 1/2 of the particle diameter of the photocatalyst particles is necessary from the viewpoint of retention of the photocatalyst particles and prevention of falling off. If the thickness is 10 nm, a layer thickness of 5 nm or more is necessary. When the hydrophilic layer 17 containing the photocatalyst is formed by vapor deposition, sputtering, or the like, a layer thickness for expressing the photocatalytic function, that is, a layer thickness that establishes a crystal structure is required, and the photocatalyst particles are contained. As in the case, a layer thickness of 5 nm or more is necessary.
From the above viewpoint, generally, the thickness of the hydrophilic layer 17 can be selected within a range of 5 to 300 nm.

The hydrophilic layer 17 can also contain a surfactant, a leveling agent, an antistatic agent and the like.
The surfactant is effective for smoothing the surface of the hydrophilic layer 17. Specific examples of the surfactant that can be used include the same examples as the surfactant that can contain the above-described liquid containing water.
Leveling agents are effective in reducing small irregularities on the surface. As the leveling agent, for example, a dimethylpolysiloxane-polyoxyalkylene copolymer (for example, SH190 manufactured by Toray Dow Corning Co., Ltd.) is suitable as the silicone leveling agent.
The antistatic agent is effective in improving the antifouling property of the film mirror. When the hydrophilic layer 17 has conductivity due to the antistatic agent, the electric resistance value on the surface of the film mirror unit can be reduced. Further, by forming an antistatic layer through a very thin layer between the layer adjacent to the hydrophilic layer 17 or the hydrophilic layer 17, the electrical resistance value on the surface of the film mirror unit can be reduced, and the antifouling property can be obtained. It is possible to improve.

[Other layers]
The sunlight reflecting mirror 10 is not limited to the above-described layers, and may include other layers.
For example, the solar reflective mirror 10 can include a hard coat layer between the ultraviolet absorbing layer 16 and the hydrophilic layer 17 in order to prevent damage to each layer.
As a material for the hard coat layer, acrylic resin, urethane resin, melamine resin, epoxy resin, organic silicate compound, silicone resin, and the like can be used. From the viewpoint of hardness and durability, a silicone resin or an acrylic resin is preferable.
The hard coat layer may contain additives such as the above-described corrosion inhibitors, antioxidants, light stabilizers, surfactants, leveling agents, and antistatic agents.

[Solar thermal power generator]
The solar thermal power generation device of the present invention may be any type of solar thermal power generation device such as a trough type, a Fresnel type, or a tower type as long as it includes the solar light reflecting mirror unit of the present invention.
Hereinafter, as an embodiment of the solar thermal power generation apparatus of the present invention, a trough type solar thermal power generation apparatus including the solar reflective mirror unit 1 will be described.

FIG. 3 shows a schematic configuration of the trough type solar thermal power generation apparatus 100.
As shown in FIG. 3, the solar thermal power generation apparatus 100 includes a sunlight reflecting mirror unit 1 (sunlight reflecting mirror 10 and cleaning unit 20), a support member 50, an angle adjusting unit 60, a heat storage tank 71, a heat exchanger 72, A generator 73, a heat collecting tube 80, and the like are provided.

The solar power generation apparatus 100 reflects sunlight by the curved sunlight reflecting mirror 10 and condenses it on the heat collecting tube 80, heats the heat medium in the heat collecting tube 80, generates steam by a heat exchanger, and generates power. I do.
In FIG. 3, only one solar reflective mirror 10 is shown, but usually, several thousand units of solar reflective mirrors 10 are installed, and heat collecting tubes provided corresponding to the respective solar reflective mirrors 10. 80 is connected to the heat storage tank 71.

In the arcuate support member 50, the support member 51 in the arc portion supports the solar light reflecting mirror 10 in a curved shape, and the support member 52 in the string portion supports the heat collecting tube 80.
The arrangement positions of the sunlight reflecting mirror 10 and the heat collecting tube 80 are adjusted so that the sunlight reflected by the sunlight reflecting mirror 10 is condensed on the heat collecting tube 80.

In the solar thermal power generation apparatus 100, a supply pipe 23 and an absorber 24 are attached to the upper end and the lower end of the sunlight reflecting mirror 10, respectively. The cleaning unit 20 transfers the liquid containing water in the tank 21 to the two supply pipes 23 by the pump P, and supplies the liquid to the surface of the sunlight reflecting mirror 10 by the supply pipe 23 and the absorber 24. it can. The cleaning unit 20 can also supply the liquid by only one of the supply pipe 23 and the transfer pipe 22 by providing an open valve on or both of them to open and close. Since the inclination angle of the mirror surface of the sunlight reflecting mirror 10 is adjusted according to the incident angle of sunlight, the cleaning unit 20 supplies water to the supply pipe 23 located on the upper side in the gravity direction of the two supply pipes 23. Transport liquid containing.

The heat collection tube 80 has a double structure of an outer tube 81 and an inner tube 82, and transfers the heat medium supplied to the inner tube 82. The heat medium is heated by the sunlight reflected by the sunlight reflecting mirror 10 in the heat collecting tube 80 and then transferred to the heat storage tank 71. During power generation, steam is generated by the heat medium transferred from the heat storage tank 71 to the heat exchanger 72, and the generator 73 rotates the turbine with the steam to generate electric energy.
In order to increase the absorption rate of sunlight, the outer tube 81 is preferably a transparent glass tube, and the outer surface of the inner tube 82 is preferably colored black. Further, if the space between the outer tube 81 and the inner tube 82 is a vacuum heat insulating space, loss of heat from the heat medium can be reduced.

The angle adjustment unit 60 adjusts the tilt angle of the mirror surface of the sunlight reflecting mirror 10 in accordance with the incident direction of sunlight so that as much sunlight as possible can be collected.
Specifically, the angle adjustment unit 60 rotates around the rotation shafts 61R and 61L attached to the support members 50 at both ends in the left-right direction of the sunlight reflecting mirror 10, respectively. A motor 62 and a speed reducer 63 are attached to the rotation shaft 61R, and the rotation shaft 61L is rotatably supported by a bearing 64. The angle adjusting unit 60 drives the motor 63 and the speed reducer 64 by the control device 65 to control the rotation angle of the sunlight reflecting mirror 10. The control device 65 acquires a sunlight detection signal by the sunlight sensor 66 attached to the support member 50, and determines the rotation angle so that the amount of sunlight is maximized.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto. In addition, although the display of "part" or "%" is used in an Example, unless there is particular notice, it represents "mass part" or "mass%".

Example I
[Sunlight reflection mirror 1]
An anchor layer having a thickness of 0.1 μm was formed on one side of a 100 μm-thick polyethylene terephthalate film (hereinafter referred to as PET film) obtained by biaxial stretching. The anchor layer is made of polyester resin Esper 9940A (Hitachi Chemical Industry Co., Ltd.), melamine resin, diisocyanate crosslinking agent tolylene diisocyanate and hexamethylene diisocyanate (Mitsui Chemicals Fine Co., Ltd.), respectively 20: 1: A resin mixed at a mass ratio of 1: 2 was applied and formed by a gravure coating method.
Next, a reflective layer having a thickness of 80 nm was formed by vacuum evaporation using silver.

On the reflective layer, a corrosion prevention layer coating solution was applied by a gravure coating method to form a corrosion prevention layer having a thickness of 0.1 μm. As a coating solution, 10% by mass of Tinuvin 234 (manufactured by Ciba Japan Co., Ltd.) in a resin in which Esper 9940A and tolylene diisocyanate are mixed at a resin solid content ratio (mass ratio) of 10: 2, respectively. Was used as a corrosion inhibitor.

Next, on the corrosion prevention layer, vinylol 92T (acrylic resin adhesive, Showa Polymer Co., Ltd.) was applied to a thickness of 0.1 μm to form an anchor layer. An acrylic resin film formed by a solution casting method was laminated on the anchor layer to form an ultraviolet absorbing layer. The arithmetic average roughness Ra of the surface of the ultraviolet absorbing layer was 0.1 μm, and the layer thickness was 50 μm.

Next, in a solvent in which ethanol, isopropyl alcohol, and methanol are mixed at a mass mixing ratio of 10:60:30, tetraethoxysilane is added at 1% by mass, and further mixed with 1% by mass of acetic acid aqueous solution. .3% by mass was added to prepare an inorganic coating layer coating solution. The prepared coating solution was applied on the ultraviolet absorbing layer by a die coating method so that the film thickness after drying was 100 nm, thereby forming an inorganic coating layer. Further, a dispersion in which 1% by mass of silica (SiO ₂ ) particles having an average particle diameter of 50 nm was dispersed in the same coating liquid as that of the inorganic coat layer was applied on the inorganic coat layer to form a hydrophilic layer. At the time of coating, the solar reflective mirror 1 was manufactured by coating by a die coating method so that the film thickness after drying was 80 nm.

[Sunlight reflecting mirrors 2-7]
In the production of the solar reflective mirror 1, the same manner as the solar reflective mirror 1 except that the silica particles used in the hydrophilic layer were replaced with photocatalyst particles shown in the column of hydrophilizing agent in Table 1 below. Thus, each of the sunlight reflecting mirrors 2 to 7 was manufactured. In Table 1 below, the a type of the titanium oxide particles (TiO ₂ ) indicates an anatase type, and the r type indicates a rutile type.

[Sunlight reflecting mirrors 8 and 9]
In the production of the solar reflective mirror 1, the same procedure as in the solar reflective mirror 1 except that a hydrophilic layer was formed using a dispersion in which silica particles were dispersed in an amount of 0.6 and 0.8% by mass, respectively. Thus, sunlight reflecting mirrors 8 and 9 were manufactured.

[Sunlight reflecting mirrors 10 and 11]
A glass plate was prepared as a base material, and a reflective layer having a thickness of 80 nm was formed on one side of the glass plate using silver by vacuum deposition. A corrosion prevention layer was formed on the reflective layer in the same manner as the solar light reflecting mirror 1. In order to bend the mirror surface, the flat glass plate was heated to be bent above the softening point and then cooled, and a reflection layer and a corrosion prevention layer were formed outside the circumferential surface of the curved glass plate.
Next, after rubbing the other surface of the glass plate with a cloth containing an aqueous dispersion of CeO ₂ particles (abrasive), the glass plate was washed with water, and CeO ₂ particles adhered to the surface of the glass plate. At the same time, the dirt was removed.
A hydrophilic layer was formed on the cleaned surface in the same manner as the sunlight reflecting mirror 1 to produce a sunlight reflecting mirror 10.

In the production of the solar reflective mirror 10, the solar reflective mirror is the same as the solar reflective mirror 10 except that the silica particles used in the hydrophilic layer are replaced with anatase-type titanium oxide particles as a photocatalyst. 11 was produced.

[Sunlight reflecting mirrors 21 and 22]
In the production of the solar reflective mirror 1, the solar reflective mirror 21 was produced in the same manner as the solar reflective mirror 1 except that the hydrophilic layer was formed without adding silica particles.
In the production of the solar reflective mirror 10, the solar reflective mirror 22 was produced in the same manner as the solar reflective mirror 10, except that the hydrophilic layer was not formed.

[Sunlight reflecting mirror units 1 to 9]
SE-6010 (acrylic resin adhesive, manufactured by Showa Polymer Co., Ltd.) was applied to the surface of the sunlight reflecting mirrors 1 to 9 opposite to the surface on which the anchor layer of the PET film was formed, and an adhesive having a thickness of 1 μm. A layer was formed. A metal support (aluminum plate manufactured by Sumitomo Light Metal Co., Ltd.) having a thickness of 0.5 mm was attached to the PET film via this adhesive layer. Thereafter, as shown in FIG. 3, the mirror surfaces of the sunlight reflecting mirrors 1 to 9 were curved and attached to the support member.

Next, a supply pipe provided with openings having a diameter of 2 mmφ at equal intervals of about 10 mm was attached to one end of each of the sunlight reflecting mirrors 1 to 9. Furthermore, after attaching a commercially available pump to the tank, a supply pipe was connected to the pump to obtain each of the sunlight reflecting mirror units 1-9. The tank was charged with 100% by mass of water without additives as a liquid to be supplied.

[Sunlight reflecting mirror units 10 and 11]
An adhesive layer was formed on the corrosion preventing layers of the solar reflective mirrors 10 and 11 in the same manner as the solar reflective mirror units 1 to 9, and a metal support was attached. Thereafter, in the same manner as the above-described sunlight reflecting mirror units 1 to 9, a supporting member is attached to the sunlight reflecting mirrors 10 and 11, and further, a cleaning means (supply pipe, pump, tank, etc.) is provided, and the sunlight reflecting mirror is provided. Units 10 and 11 were obtained. The tank was charged with 100% by mass of water without additives as a liquid to be supplied.

[Sunlight reflection mirror unit 12]
The solar reflective mirror unit 2 is the same as the solar reflective mirror unit 2 except that a liquid composed of 1% by mass of ethanol and 99% by mass of water is put into the tank instead of 100% by mass of water. Thus, a sunlight reflecting mirror unit 12 was obtained.

[Sunlight reflection mirror unit 13]
In the sunlight reflecting mirror unit 2, a pump was set so as to intermittently supply the liquid, and the sunlight reflecting mirror unit 13 was obtained. The liquid supply interval was 10 minutes every hour.

[Sunlight reflection mirror unit 14]
In the said sunlight reflective mirror unit 2, the sunlight reflective mirror unit 14 was obtained like the sunlight reflective mirror unit 2 except having arrange | positioned the absorber adjacent to the opening part of a supply pipe | tube. The absorber was a resin sponge, and the length of the sponge was set to a length corresponding to the entire width of one end of the mirror surface in the same manner as the supply pipe.

[Sunlight reflecting mirror units 21 to 23]
A sunlight reflecting mirror unit 21 was obtained in the same manner as the sunlight reflecting mirror unit 1 except that the sunlight reflecting mirror 1 of the sunlight reflecting mirror unit 1 was replaced with the sunlight reflecting mirror 21.
Moreover, the solar reflective mirror unit 22 was obtained in the same manner as the solar reflective mirror unit 10 except that the solar reflective mirror 10 of the solar reflective mirror unit 10 was replaced with the solar reflective mirror 22.
In addition, the solar light reflecting mirror unit 23 was obtained by removing cleaning means such as a supply pipe and a pump provided in the solar light reflecting mirror unit 2.

[Evaluation]
(Contact angle with water)
Each of the sunlight reflecting mirror units 1 to 14 and 21 to 23 was installed outdoors in a desert area. In each of the sunlight reflecting mirror units 1 to 11, 14 and 21 to 23, a liquid containing water is continuously supplied to the surfaces of the sunlight reflecting mirrors 1 to 11, 21 and 22 to form a liquid film which always flows. did. In the sunlight reflecting mirror unit 13, a liquid containing water is intermittently supplied to the surface of the sunlight reflecting mirror 2 to intermittently form a flowing liquid curtain. The surface of each of the sunlight reflecting mirrors 1 to 11 and 21 is a hydrophilic layer. The surface of the sunlight reflecting mirror 22 is a glass plate.

Before installation and one month and one year after installation, the contact angle (°) with the water on the surface of each of the solar reflective mirrors 1 to 11, 21 and 22 is determined according to JIS-R3257. It measured as follows.
3 μL of water was dropped on the surface of each of the sunlight reflecting mirrors 1 to 11, 21 and 22, and the contact angle (°) was measured after 30 seconds. The contact angle was measured using a contact angle meter DM300 (manufactured by Kyowa Interface Chemical Co., Ltd.) in an environment of a temperature of 23 ° C. and a relative humidity of 55% RH.

(Reflectance)
In parallel with the measurement of the contact angle, the solar reflective mirrors 1 to 11 and 11 to 23 of the solar reflective mirror units 1 to 14 and 21 to 23 are installed before being installed outdoors and after one month and one year after installation. The reflectance (%) of 21 and 22 was measured as follows.
Using a spectrophotometer U-4000 (manufactured by Hitachi, Ltd.) equipped with an integrating sphere reflection accessory, the incident angle of incident light is adjusted to 5 ° with respect to the normal of the reflecting surface, and the reflection angle is 5 °. The regular reflectance (%) of was measured. From the measured values obtained, the average value within the wavelength range of 400 to 700 nm was determined as the reflectance (%) of each of the solar reflective mirrors 1 to 11, 21 and 22.

(Liquid film coverage)
In parallel with the measurement of the contact angle, the ratio of the liquid film coverage (%) in each of the solar reflective mirror units 1 to 14 and 21 to 23 is measured before installation outdoors and after one month and one year after installation. ) Was measured as follows.
When the mirror surface is curved, the mirror surface was photographed by changing the angle in the photographing direction by 120 ° from the center (the position of the heat collecting tube 80 in FIG. 3) around the curved mirror surface. The obtained plurality of images were analyzed, and the area (m ² ) of the liquid film was obtained and added every 10 ° in the photographing direction. The area of the liquid film obtained by the addition was divided by the area (m ² ) of the mirror surface to obtain the coating area ratio (%) of the liquid film.
When the mirror surface was a flat surface, the entire mirror surface was photographed, and one image obtained was analyzed to determine the area (m ² ) of the liquid film. The area of the liquid film was divided by the area of the mirror surface (m ² ) to obtain the liquid film coverage area ratio (%).

Table 1 below shows the measurement results of the contact angle (°), the reflectance (%), and the coating area ratio (%) of the liquid film. In addition, the coating area ratio (%) of the liquid film of the following Table 1 has shown the range of the coating area ratio measured in one year from installation.

As shown in Table 1, in the sunlight reflecting mirror units 21 to 23 according to the comparative examples, dirt is adhered to the mirror surface, and the reflectance is reduced to about 80% in one month after installation. If it is continuously installed without washing, dirt is further adhered, conversion efficiency from solar energy to heat energy is reduced, and loss is increased, which is not preferable. On the other hand, according to the sunlight reflecting mirror units 1 to 14 according to the present invention, the reflectance is hardly lowered, the clean state before the installation is continued, and the high reflectance can be maintained. I understand. It can also be seen that when the area coverage of the liquid film is in the range of 50 to 100%, the reflectance can be maintained particularly high.

A solar thermal power generation apparatus using the solar reflective mirror units 1 to 14 and 21 to 23 was manufactured, and the contact angle and the reflectance were measured in the same manner. The same results were obtained. Further, when the contact angle and the reflectance were measured, the power generation efficiency of the solar thermal power generation apparatus was obtained. However, the solar reflective mirror units 1 to 14 showed almost no fluctuation in the power generation efficiency after one month and one year. It was. On the other hand, the solar reflective mirrors 21 to 23 showed a tendency that the power generation efficiency decreased in proportion to the passage of time.

Example II
Using the sunlight reflecting mirror unit 2 of Example I, supply of water per unit time by a pump so that the coverage ratio (%) of the liquid film is 100%, 50%, 40%, and 10%, respectively. The amount was adjusted, and in the same manner as in Example I, the contact angle (°), the reflectance (%), and the coating film area ratio (%) were measured.
Further, by using the sunlight reflecting mirror unit 14 of Example I, water per unit time by a pump so that the covering area ratio (%) of the liquid film becomes 100%, 50%, 40%, and 10%, respectively. In the same manner as in Example I, the contact angle (°), the reflectance (%), and the coating film area ratio (%) of the liquid film were measured.

Table 2 below shows the measurement results.

As shown in Table 2, it can be seen that when the covering area ratio is in the range of 50 to 100%, the decrease in contact angle and reflectance can be effectively suppressed.

Example III
A cylindrical liquid film recovery container was provided at the other end facing the supply pipe of the sunlight reflecting mirror unit 2 of Example I to obtain a sunlight reflecting mirror unit 2a. Further, a transfer pipe for connecting the recovery container to the tank is provided so that the liquid recovered by the recovery container can be stored in the tank. A filter was provided between the tank and the transfer pipe.
The sunlight reflecting mirror unit 2a was installed outdoors, and a liquid containing water was continuously supplied to the surface of the sunlight reflecting mirror 2 to form a constantly flowing liquid film. Further, the liquid film was recovered by the recovery means and returned to the tank, and the liquid returned to the tank was supplied again by the supply pipe 23.

1 month and 1 year after installation, when the contact angle and reflectance of the surface of the solar reflective mirror 2 were measured, the same measurement results as the solar reflective mirror unit 2 shown in Table 1 were obtained. It was confirmed that the cleanliness and reflectivity can be maintained high even when the liquid is reused.

The present invention can be used for the purpose of improving the power generation efficiency of solar thermal power generation.

DESCRIPTION OF SYMBOLS 1 Sunlight reflection mirror unit 10 Sunlight reflection mirror 11 Resin film 12 Anchor layer 13 Reflection layer 14 Corrosion prevention layer 15 Adhesion layer 16 Ultraviolet absorption layer 17 Hydrophilic layer 20 Cleaning means 21 Tank 22 Transfer pipe P Pump 23 Supply pipe 30 Recovery means 40 Control device 100 Solar power generation device 50 Support member 60 Angle adjustment unit 73 Generator 80 Heat collection tube

Claims (10)

1. A solar reflective mirror unit having a solar reflective mirror cleaning function,
   The contact angle with water on the surface of the solar reflective mirror is 30 ° or less,
   A solar reflective mirror comprising: cleaning means for continuously or intermittently supplying a liquid containing water to the surface of the solar reflective mirror to form a liquid film flowing on the surface. unit.
2. The solar reflective mirror unit according to claim 1, wherein a contact angle of the surface of the solar reflective mirror with water is 20 ° or less.
3. The solar light reflecting mirror unit according to claim 1 or 2, wherein a hydrophilic layer containing a photocatalyst that is made hydrophilic by sunlight is provided on a surface of the solar light reflecting mirror.
4. The solar reflective mirror unit according to claim 3, wherein a band gap of the photocatalyst is in a range of 2.4 to 5.2 eV.
5. The sunlight according to any one of claims 1 to 4, wherein a covering area ratio of the liquid film with respect to the entire surface of the sunlight reflecting mirror is in a range of 50 to 100%. Reflective mirror unit.
6. The solar reflective mirror unit according to any one of claims 1 to 5, further comprising means for adjusting a flow rate of the liquid film.
7. The liquid film recovery means that has finished flowing on the surface of the sunlight reflecting mirror,
   7. The liquid film is formed by the cleaning means resupplying the liquid film liquid collected by the collecting means to the surface of the sunlight reflecting mirror. 8. The solar reflective mirror unit as described in any one of to.
8. The solar light reflecting mirror according to any one of claims 1 to 7, wherein the solar light reflecting mirror is a film-like mirror having at least a sunlight reflecting layer on a resin film. unit.
9. A solar thermal power generation apparatus comprising the sunlight reflecting mirror unit according to any one of claims 1 to 8.
10. A method of cleaning a solar reflective mirror,
    The contact angle with water on the surface of the solar reflective mirror is 30 ° or less,
    A method for cleaning a solar reflective mirror, comprising: supplying a liquid containing water continuously or intermittently to the surface of the solar reflective mirror to form a liquid film flowing on the surface.

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