US20160273805A1

US20160273805A1 - Reflective mirror for light concentrating system, and light concentrating system

Info

Publication number: US20160273805A1
Application number: US15/037,184
Authority: US
Inventors: Clement PILLETTE
Original assignee: Tokyo Ohka Kogyo Co Ltd
Current assignee: Tokyo Ohka Kogyo Co Ltd
Priority date: 2013-11-19
Filing date: 2013-11-19
Publication date: 2016-09-22
Also published as: EP3071899A1; JP2016538520A; WO2015075759A1; EP3071899A4

Abstract

A reflective mirror for a light concentrating system, having high reflectance and superior durability; and a light concentrating system including this reflective mirror for a light concentrating system. The reflective mirror includes a substrate and a reflective layer provided on the substrate, in which the reflective layer includes a material having a refractive index at a wavelength of 2500 nm of no less than 2.5 and having a Mohs hardness of 12 to 15. The light concentrating system includes the reflective mirror and a light receiving part that receives light reflected by the reflective mirror.

Description

TECHNICAL FIELD

The present invention relates to a reflective mirror for a light concentrating system, and to a light concentrating system including this reflective mirror for a light concentrating system.

BACKGROUND ART

In recent years, in terms of global environmental problems, a sunlight concentrating system has been attracting attention as one of the systems for obtaining eco-friendly renewable energy. A sunlight concentrating system concentrates sunlight on a light receiving part by way of a reflective mirror; heats a heating medium such as water, oil or molten salt by utilizing obtained solar heat; generates high-temperature and high-pressure vapor by vaporizing water by way of the heated heating medium; and generates electricity by driving a steam turbine by way of the vapor. A sunlight concentrating system provides advantages, for example, that the environmental load is low since a greenhouse gas such as carbon dioxide is not generated while generating electricity; electricity can be generated regardless of weather and time of day or night since heat can be stored; relatively high energy conversion efficiency can be achieved; etc. Mainly known types of sunlight concentrating systems include a parabolic trough type, a linear Fresnel type, a tower type, and a parabolic dish type.
Conventionally, a reflective mirror including a reflective layer composed of silver, aluminum or the like is generally used as a reflective mirror that is used for a sunlight concentrating system. In general, a transparent protective layer composed of glass or the like is formed on such a reflective layer. More specifically, for example, Patent Document 1 discloses a reflective mirror for a sunlight concentrating system, in which the reflective mirror includes: a substrate; a metal reflective film formed of aluminum, silver or the like adhered onto the substrate; and a transparent inorganic protective film formed of a glassy film adhered onto a surface of the metal reflective film (Patent Document 1).

CITATION LIST

Patent Literature

PTL 1: Patent Document 1: Japanese Unexamined Patent Application, Publication No. S57-4003

SUMMARY OF INVENTION

Technical Problem

A light concentrating system requires durability against an operating environment. For example, a sunlight concentrating system is often used under a harsh environment such as a desert area where a long sunshine duration and a vast land are available, and is therefore required to be able to endure being used under such a harsh environment. Above all, a reflective mirror used for a light concentrating system is required to suppress aged deterioration of reflectance. For example, in a sunlight concentrating system, a reflective mirror is strongly desired to be unlikely to be damaged even if dusts or the like collide against the reflective mirror at a high speed. In a conventional reflective mirror, if a reflective layer is not protected by a transparent protective layer composed of glass or the like, and is exposed directly to an external environment, the reflective layer composed of silver, aluminum or the like is likely to be damaged due to dusts or the like, and is easily oxidized and corroded; therefore, the reflectance of the reflective mirror will deteriorate. Even if the reflective layer is protected by a transparent protective layer as disclosed in Patent Document 1, the transparent protective layer is not sufficiently strong and is therefore likely to crack due to collisions with dusts or the like, resulting in invasion of water and oxygen from the crack, oxidizing and corroding the reflective layer; therefore, the reflectance of the reflective mirror is likely to deteriorate.
A reflective mirror used for a light concentrating system is required to have higher reflectance such that as much light as possible can be concentrated on a light receiving part. For example, when intended to concentrate heat on a light receiving part, the reflectance in the far-infrared range is required to be high. However, as described above, in the conventional reflective mirror as disclosed in Patent Document 1, at least a transparent protective layer is generally formed on a reflective layer in terms of protecting the reflective layer, and the transparent protective layer itself absorbs the light; therefore, deterioration of reflectance is unavoidable.
The present invention has been made in view of such conventional circumstances, and an object of the present invention is to provide: a reflective mirror for a light concentrating system, having high reflectance and superior durability; and a light concentrating system including this reflective mirror for a light concentrating system.

Solution to Problem

The present inventor have earnestly studied and completed the present invention by finding that the above problems can be solved by using a reflective layer composed of material having a refractive index at a wavelength of 2500 nm of no less than 2.5 and having a Mohs hardness of 12 to 15. More specifically, the present invention provides the following.
A first aspect of the present invention is a reflective mirror for a light concentrating system, including a substrate and a reflective layer provided on the substrate, and the reflective layer is composed of material having a refractive index at a wavelength of 2500 nm of no less than 2.5 and having a Mohs hardness of 12 to 15.
A second aspect of the present invention is a light concentrating system including: the reflective mirror for a light concentrating system; and a light receiving part that receives light reflected by the reflective mirror for a light concentrating system.

Advantageous Effects of Invention

According to the present invention, it is possible to provide: a reflective mirror for a light concentrating system, having high reflectance and superior durability; and a light concentrating system including this reflective mirror for a light concentrating system.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view showing a part of a light concentrating system 1 according to an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention is hereinafter described in detail with reference to the drawing.
FIG. 1 is a side view showing a part of a light concentrating system 1 according to the embodiment of the present invention. The light concentrating system 1 is a sunlight concentrating system for concentrating sunlight Ito obtain heat, and generating electricity by the obtained heat. FIG. 1 illustrates a sunlight concentrating system of a parabolic trough type as an example. As shown in FIG. 1, the light concentrating system 1 according to the embodiment of the present invention includes at least: a reflective mirror 2 for a light concentrating system; a frame 3; an arm 4; a light receiving part 5; a driving part 6; and a support 7.
Each part of the light concentrating system 1 is hereinafter described in detail.
The reflective mirror 2 for a light concentrating system includes a substrate 21 and a reflective layer 22 provided on the substrate 21. The mirror 2 is provided on the frame 3. The reflective mirror 2 for a light concentrating system functions as a total reflection mirror having high reflectance. The reflective mirror 2 for a light concentrating system is a reflective curved mirror (reflective concave mirror), which is arcuate in a lateral view, and which is shaped like an elongated trough extending in a direction perpendicular to the plane of the page. The reflective mirror 2 for a light concentrating system is a surface reflective mirror, in which other layers such as a transparent protective layer are not provided on the reflective layer 22. An outermost surface of the reflective mirror 2 for a light concentrating system (i.e. a surface of the reflective layer 22) forms a part of a parabola in the lateral view, and sunlight R reflected by the reflective mirror 2 for a light concentrating system can be linearly concentrated on a focus of the parabola (rectilinearly extending in the direction perpendicular to the plane of the page) by adjusting an orientation of the reflective mirror 2 for a light concentrating system. As will be described later, the light receiving part 5 is located at the focus, and the sunlight R reflected by the reflective mirror 2 for a light concentrating system is concentrated on the light receiving part 5.
The substrate 21 is not limited in particular as long as the substrate can support the reflective layer 22. Examples of material for the substrate 21 include a resin such as polyethylene terephthalate, inorganic material such as glass and metal, etc. The resin may include inorganic fiber such as carbon fiber and glass fiber. Above all, resin including inorganic fiber is preferable, and polyethylene terephthalate including carbon fiber is more preferable, since a light-weighted and highly strong substrate 21 can be easily obtained with such material. Examples of a thickness of the substrate 21 include a range of 4 to 8 mm. Furthermore, the back side of the substrate 21 is preferably coated with a material which is highly resistant to abrasion (Mohs hardness above 8, for example)
The reflective layer 22 is composed of material having a refractive index at a wavelength of 2500 nm of no less than 2.5 and having a Mohs hardness of 12 to 15. The material may be composed of a single type of substance, or may be composed of at least two types of substances. If the refractive index is less than 2.5, the reflectance of the reflective layer 22 is unlikely to be improved, and the sunlight R concentrated on the light receiving part 5 is likely to be decreased. The upper limit of the refractive index is not limited in particular, but is about 4.5 in practice. If the Mohs hardness is less than 12, the durability of the reflective layer 22 is likely to be insufficient; therefore, the reflective layer 22 is likely to be damaged due to dusts or the like, and the reflectance of the reflective layer 22 is likely to deteriorate.
Specific examples of the material composing the reflective layer 22 include silicon carbide, tungsten carbide, lonsdaleite, etc. Above all, silicon carbide is preferable, because both the refractive index at a wavelength of 2500 nm and the Mohs hardness satisfy the above range, and further because the hydrophobic property is as high as comparable to that of titanium dioxide, and the antifouling property is superior. Examples of a thickness of the reflective layer 22 include a range of 10 to 100 micrometers.
With a well-known method, the reflective mirror 2 for a light concentrating system can be manufactured by forming the reflective layer 22 by laminating, on the substrate 21, a layer composed of material having a refractive index at a wavelength of 2500 nm of at least 2.5 and having a Mohs hardness of 12 to 15.
The frame 3 supports the reflective mirror 2 for a light concentrating system as well as the arm 4. The driving part 6 is fixed to the frame 3. The frame 3 is provided in the upper portion of the support 7 at a position where the driving part 6 is fixed.
The arm 4 supports the light receiving part 5. One end of the arm 4 is supported by the frame 3, and the light receiving part 5 is disposed at another end of the arm 4. The length, angle, etc. of the arm 4 are appropriately adjusted such that the light receiving part 5 receives the sunlight R linearly concentrated by the reflective mirror 2 for a light concentrating system.
The light receiving part 5 is formed to be hollow such that a heating medium such as water, oil or molten salt can flow through the inside of the light receiving part 5. The light receiving part 5 is disposed at the focus of the parabola formed by the surface of the reflective layer 22. The light receiving part 5 is shaped like a pipe extending in the direction perpendicular to the plane of the page. The heating medium inside the light receiving part 5 is heated by the sunlight R linearly concentrated on the light receiving part 5. The light receiving part 5 is connected to a heat exchanger (not shown), and the heating medium circulates inside the light receiving part 5 via the heat exchanger. Since the reflective layer 22 is composed of material with high reflectance, even if the shape of the surface of the reflective layer 22 forms only a part of the parabola in the lateral view, a sufficiently large amount of light can be concentrated on the focus of the parabola. Therefore, by appropriately selecting the position of the part, the light receiving part 5 can be provided at a position deviated from the path of the sunlight I incident on the reflective mirror 2 for a light concentrating system. This makes it possible to prevent the light receiving part 5 from interrupting the sunlight I incident on the reflective mirror 2 for a light concentrating system. As a result, a larger amount of sunlight I is incident on the reflective mirror 2 for a light concentrating system, is reflected by the reflective layer 22, and is concentrated on the light receiving part 5.
The driving part 6 changes the orientation of the reflective mirror 2 for a light concentrating system by following the solar motion. The driving part 6 is configured by a motor and the like, and is fixed to the frame 3. When the driving part 6 is driven, the frame 3 is rotated around an axis extending through the driving part 6 in the direction perpendicular to the plane of the page. This is accompanied by rotation of the reflective mirror 2 for a light concentrating system, the arm 4, and the light receiving part 5 as well, around the axis. The driving part 6 rotates the frame 3 such that the sunlight R reflected by the reflective mirror 2 for a light concentrating system is constantly concentrated on the light receiving part 5.
The support 7 supports the frame 3 and, through the frame 3, the reflective mirror 2 for a light concentrating system, the arm 4, the light receiving part 5, and the driving part 6. The frame 3 is provided in the uppermost portion of the support 7 at the position of the driving part 6.
Next, operations of the light concentrating system 1 according to the embodiment of the present invention are described with reference to FIG. 1.
The sunlight I is incident on the reflective mirror 2 for a light concentrating system, and is then reflected by the reflective layer 22. The reflected sunlight R is linearly concentrated on the light receiving part 5, and heats the heating medium inside the light receiving part 5. The heated heating medium flows through the inside of the light receiving part 5, vaporizes the water in the heat exchanger (not shown) connected to the light receiving part 5, and generates high-temperature and high-pressure vapor. This vapor drives a steam turbine (not shown) to generate electricity. The heating medium having passed through the heat exchanger further flows through the inside of the light receiving part 5, and is heated again by the sunlight R reflected and concentrated by the reflective layer 22. Since the reflective mirror 2 for a light concentrating system follows the solar motion by way of the driving part 6, the sunlight R reflected by the reflective mirror 2 for a light concentrating system is constantly concentrated on the light receiving part 5.
The light concentrating system 1 according to the embodiment of the present invention achieves the following effects. In the reflective mirror 2 for a light concentrating system used in the light concentrating system 1, the material composing the reflective mirror 22 has a high refractive index of no less than 2.5 at a wavelength of 2500 nm. Therefore, the reflectance of the reflective layer 22 is likely to be high, and a larger amount of sunlight R can be concentrated on the light receiving part 5.
Moreover, the reflective layer 22 is composed of material having sufficient durability with Mohs hardness in a range of 12 to 15. Accordingly, the reflective layer 22 is unlikely to be damaged by dusts or the like, and the reflectance is likely to be maintained over time. Therefore, the reflective layer 22 can be used as a single layer without the need to be protected by a transparent protective layer. In other words, the reflective mirror 2 for a light concentrating system can be configured as a surface reflective mirror, and the sunlight can be prevented from being absorbed by a transparent protective layer. Consequently, the reflectance of the reflective mirror 2 for a light concentrating system can be effectively improved. Moreover, the process of manufacturing the reflective mirror 2 for a light concentrating system can be simplified, since a transparent protective layer does not need to be formed.
Table 1 shows reflectance and Mohs hardness of various reflective mirrors. “Si—C” in Table 1 represents the reflective mirror 2 for a light concentrating system including the reflective layer 22 composed of silicon carbide, in which the Mohs hardness of “Si—C” indicates Mohs hardness of silicon carbide forming the reflective layer 22. “Thick glass” in Table 1 represents a reflective mirror including: a reflective layer including silver; and a surface layer including silica glass, in which the Mohs hardness of the “Thick glass” indicates Mohs hardness of the silica glass forming the surface layer. “Aluminum” in Table 1 represents a reflective mirror including a reflective layer including aluminum. “Silvered Polymer” in Table 1 represents a reflective mirror including: a reflective layer including silver; and a polymer layer including polymethyl methacrylate (PMMA) resin.
As shown in Table 1, the “Si—C” that is the reflective mirror for a light concentrating system according to the present invention shows extremely high reflectance of 100%, and has superior durability with Mohs hardness of no less than 12. In contrast, “Thick glass”, “Aluminum” and “Silvered Polymer” that are conventionally used reflective mirrors show insufficient reflectance of 93.50% at best, and besides have inferior durability with low Mohs hardness.

TABLE 1

Reflective Mirror	Si—C	Thick glass	Aluminum	Silvered Polymer

Reflectance (%)	100	93.50	91	90
Mohs Hardness	13	7.0	3	4-5

In the above embodiment, the light receiving part 5 is provided at the position deviated from the path of the sunlight I incident on the reflective mirror 2 for a light concentrating system. As a result, the light receiving part 5 does not interrupt the sunlight I that is incident on the reflective mirror 2 for a light concentrating system; therefore, a larger amount of sunlight I is incident on the reflective mirror 2 for a light concentrating system, is reflected by the reflective layer 22, and is concentrated on the light receiving part 5. In contrast, in conventional sunlight concentrating systems of a parabolic trough type, a light receiving part is provided in a path of sunlight incident on a reflective mirror, and the light receiving part therefore interrupts a part of the sunlight incident on the reflective mirror 2. Therefore, in the above embodiment, a larger amount of sunlight R can be concentrated on the light receiving part 5 as compared to the conventional cases.
By using silicon carbide as the material for the reflective layer 22, stains are unlikely to adhere to the surface of the reflective layer 22 because of the superior antifouling property of the silicon carbide, and the labor and cost of maintenance for removing stains can therefore be considerably reduced.
The preferred embodiment of the present invention has been described above. However, the present invention can be carried out in various aspects without being limited to the embodiment described above.
For example, although a sunlight concentrating system of the parabolic trough type has been described in the above embodiment, the reflective mirror 2 for a light concentrating system can be applied to a sunlight concentrating system of any of a linear Fresnel type, a tower type, or a parabolic dish type.
The light concentrating system 1 has been described by exemplifying a sunlight concentrating system in the above embodiment; however, the light concentrating system 1 and the reflective mirror 2 for a light concentrating system can be applied to general light concentrating systems other than the sunlight concentrating system.
In the above embodiment, the light receiving part 5 may be replaced with a solar cell. This makes it possible to generate electricity by irradiating the solar cell with a larger amount of sunlight R concentrated by the reflective mirror 2 for a light concentrating system.

Reference Signs List

1 light concentrating system
2 reflective mirror for a light concentrating system
21 substrate
22 reflective layer
3 frame
4 arm
5 light receiving part
6 driving part
7 support
I incident light
R reflected light

Claims

What is claimed is:

1. A reflective mirror, comprising a substrate and a reflective layer provided on the substrate, wherein the reflective layer is composed of material having a refractive index at a wavelength of 2500 nm of no less than 2.5 and having a Mohs hardness of 12 to 15.

2. The reflective mirror according to claim 1, wherein the reflective mirror is a surface reflective mirror.

3. The reflective mirror according to claim 1, wherein the reflective mirror is used for a sunlight concentrating system.

4. A light concentrating system comprising: the reflective mirror according to claim 1; and a light receiving part that receives light reflected by the reflective mirror.

5. The light concentrating system according to claim 4, further comprising a driving part that changes an orientation of the reflective mirror by following the solar motion.

6. The reflective mirror according to claim 1, wherein the reflective mirror is used for a light concentrating system.