This is a U.S. National Phase Application under 35 U.S.C. 371 of International Application PCT/JP2008/069448, filed on Oct. 27, 2008.
This application claims the priority of Japanese Application No. 2007-286675 filed on Nov. 2, 2007, the entire content of which is hereby incorporated by reference.
The present invention relates to an optical element, in particular, to an optical element suitably used to reflect sunlight in a sunlight condensing system to utilize sunlight as energy.
Thermal power generation which burns fossil fuel and generates electric power has comparatively low facility cost and the restriction for the installation of an electric power plant is not sever, thermal power generation has been widely used all over the world. Incidentally, the amount of emission of CO2 (carbon dioxide) to cause global warming is increasing continuously every year, therefore, there is the actual circumstances that the reduction of CO2 becomes urgent requirement from a viewpoint of global environment protection. Moreover, since an amount of fossil fuel is limited, fossil fuel should be utilized with saving so as not to be depleted before an energy production technique taking a position of thermal power generation is established. For this reason, although other electric power generating techniques to supplement thermal power generation have been sought, there is a problem that nuclear power generation and hydraulic power generation are hard to be utilized, because the installation of their electric power plants is restricted.
On the other hand, as clean energy which does not give a load to the environment, sunlight energy attracts attention. Generally as a method of changing sunlight into energy, a solar battery has been well known. However, with a current technique, there is the actual circumstance that the electric power generating cost of a solar battery is relatively high in comparison with other plants.
In contrast to this, it has been also considered that sunlight is used directly as energy at relatively low cost Patent Document 1 discloses a technique that sunlight is condensed and changed into heat energy and the heat energy is converted into electrical power. More concretely, in the technique, sunlight is reflected by a large number of reflective mirrors (heliostat) arranged a tower and is condensed into a heat exchanging unit by a condensing mirror mounted on the tower so as to heat the heat exchanging unit, and then the heat energy obtained by the heat exchanging unit is sent to an electric power generating unit, whereby electric power is generated.
Incidentally, in order to convert sunlight into energy efficiently, there is a problem that how to set up the optical characteristic of a reflective mirror.
Here, in a common reflective mirror, an Ag vapor-deposited film is used in many cases. As shown in
On the other hand, in precision optical devices etc., a technique to reflect light with a dielectric multilayer is known. By the use of such a dielectric multilayer, it becomes possible to obtain a high reflectance ratio even in a wide wavelength band. However, in order to correspond to the wide wavelength band, it may be necessary to increase the number of layers. As a result, the thickness of a film tends to become ticker. However, in the case of reflecting light with such a dielectric multilayer with a large thickness, if light enters its reflective surface from a direction vertical to the reflective surface, there is no problem. However, as an incidence angle of light to the reflective surface become smaller, the optical path length of light passing through the dielectric multilayer becomes longer. As a result, there is problem that it becomes difficult to obtain an expected reflective characteristic. This means that when sunlight enters with a shallow incidence angle in morning or evening, the utilization efficiency of light decreases.
The present invention has been achieved in view of these problems, and an object of the present invention is to provide an optical element for reflecting sunlight, which is excellent in resistance to weather and has a high reflectance ratio in a wide wavelength band.
In a plate-shaped optical element for reflecting sunlight, the optical element of the present invention is characterized in that a dielectric multilayer and a metal vapor-deposited film are formed on at least one of a surface where sunlight enters and a surface opposite to the surface of a base plate and the above-mentioned dielectric multilayer is formed on a surface where sunlight enters than the above-mentioned metal vapor-deposited film.
Further, in a plate-shaped optical element for reflecting sunlight, the optical element of the present invention is characterized in that a first reflective film is formed on a surface of a base plate where sunlight enters and a second reflective film to reflect light with a wavelength band having passed through the first reflective film is formed on a surface opposite to the surface of the base plate where sunlight enters.
With reference to
Here, an important point is that since a dielectric multilayer is made such that a low refractive layer and a high refractive layer are laminated, the dielectric multilayer has a characteristic to allow light in a wavelength band other than its reflecting wavelength band to pass through itself different from a metal vapor-deposited film. Therefore, if a dielectric multilayer is formed at a light-entering side than the metal vapor-deposited film, light in a wavelength band where the dielectric multilayer has a high reflectance ratio, is reflected by the dielectric multilayer with the high reflectance ratio. On the other hand, light in a wavelength band where the dielectric multilayer has a low reflectance ratio passes through the dielectric multilayer, and thereafter, is reflected by the metal vapor-deposited film with a high reflectance ratio. Therefore, even if a metal vapor-deposited film and a dielectric multilayer are laminated, these films are refrained from restricting reflection with each other, whereby it can be possible to realize reflection with high efficiency by the total.
There is no restriction in a kind of metals to be vapor-deposited. However, it is desirable to use aluminum from viewpoints of cost, resistance to weather, etc. The dielectric multilayer is made such that a high refractive index layer and a low refractive index layer are laminated. For example, it is disclosed in Japanese Unexamined Patent Publication No. 2005-292462. The base plate is desirably glass or plastic.
It is desirable that the above-mentioned dielectric multilayer has a reflectance ratio of 90% or more for light with a wavelength of 1.0 μm or less, and the above-mentioned metal vapor-deposited film has a reflectance ratio of 90% or more for light with a wavelength of 1.0 μm or more.
It is desirable that the above-mentioned dielectric multilayer is formed on a surface of the above-mentioned base plate where sunlight enters and the above-mentioned metal vapor-deposited film is formed on a surface of the above-mentioned base plate opposite to the surface where sunlight enters. However, a dielectric multilayer and a metal vapor-deposited film may be formed on a surface where sunlight enters, or a dielectric multilayer and a metal vapor-deposited film may be formed on a surface opposite to a surface where sunlight enters.
Hereafter, with reference to drawings, an embodiment of the present invention will be described more in detail.
On the upper end of the fork 7, a concave mirror 13 is held so as to rotate and shift freely in the direction of an elevation angle (B direction). The concave mirror 13 is shaped in the form of a rectangular plate and has a reflective surface being a curved surface (including an aspheric surface, a paraboloidal surface, etc.). However, this reflective surface may be a flat surface.
Circular pipes 14 are fixed to the reverse side of the concave mirror 13. As shown in
On the other hand, as shown in
The height of the concave mirror 13 of the heliostat 5 becomes gradually high as the position of the concave mirror 13 separates from the elliptic mirror 1 at the central section. This is because a concave mirror 13 is made to be prevented from becoming a shadow for another concave mirror 13 at the time of reflecting sunlight, whereby a shading loss can be prevented from taking place.
Moreover, in
With regard to reflection on the inside of this condensing mirror 4, it may be preferable in consideration of loss of light that, as shown in
Examples of an optical element capable of being applied with the present invention include the elliptic mirror 1, the concave mirror 13, and the condensing mirror 4.
The film thickness data of the dielectric multilayer used in Example suitably for an optical element shown in
In the example in which the dielectric multilayer shown in Table 1 was formed on a light entering surface of a glass-made base plate and a metal vapor-deposited film made from a material of Cu was formed on a surface of the base plate at the opposite side to the light entering surface,
The film thickness data of the dielectric multilayer used in Example suitably for an optical element shown in
In the example in which the dielectric multilayer shown in Table 2 and a metal vapor-deposited film made from a material of Al were formed on a light entering surface of a glass-made base plate in this order from the light-entering side,
In any embodiment, the dielectric multilayer had a reflectance ratio of 90% or more for light having a wavelength of 1.0 μm or less and the metal vapor-deposited film had a reflectance ratio of 90% or more for light having a wavelength of 1.0 μm or more. Therefore, in combination of the dielectric multilayer and the metal vapor-deposited film, it was able to provide a reflectance ratio of 90% or more for a wide wavelength band. Furthermore, in the case that an incidence angle is 20 degrees, the dielectric multilayer of this embodiment has a wavelength band having a reflectance ratio of 6% or less at three position in a range of a wavelength of 1.9 μm to 2.4 μm, and in the case that an incidence angle is 50 degrees, the dielectric multilayer of this embodiment has a wavelength band having a reflectance ratio of 8% or less at three position in a range of a wavelength of 1.7 μm to 2.2 μm. Accordingly, the number of layers can be suppressed and incidence angle characteristic can be made good.
As mentioned above, although the present invention has been explained with reference to embodiments, the present invention should not be interpreted as being limited to the above-mentioned embodiment, and of course, modification and improvement can be made suitably.
Number | Date | Country | Kind |
---|---|---|---|
2007 286675 | Nov 2007 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/JP2008/069448 | 10/27/2008 | WO | 00 | 4/29/2010 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2009/057551 | 5/7/2009 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
4209222 | Posnansky | Jun 1980 | A |
5177396 | Gielen et al. | Jan 1993 | A |
5563734 | Wolfe et al. | Oct 1996 | A |
6252155 | Ortabasi | Jun 2001 | B1 |
7385659 | Kotchick et al. | Jun 2008 | B2 |
7638708 | Fork et al. | Dec 2009 | B2 |
7790978 | Dziendziel et al. | Sep 2010 | B1 |
7851054 | Weber et al. | Dec 2010 | B2 |
20070137691 | Cobb et al. | Jun 2007 | A1 |
20070178316 | Mellott | Aug 2007 | A1 |
20110134515 | Banerjee et al. | Jun 2011 | A1 |
Number | Date | Country |
---|---|---|
03-293301 | Dec 1991 | JP |
09-15407 | Jan 1997 | JP |
11-119105 | Apr 1999 | JP |
11-508379 | Jul 1999 | JP |
2002-254550 | Sep 2002 | JP |
2006-227099 | Aug 2006 | JP |
Entry |
---|
Machine translation of JP 09-015407A from http://www4.ipdl.inpit.go.jp/. |
Number | Date | Country | |
---|---|---|---|
20100258187 A1 | Oct 2010 | US |