JPS58160765A - Selective absorber for solar heat collector - Google Patents

Abstract

PURPOSE:To increase the ratio of sunlight absorbance modulus to radiation heat, by arranging an electoconductive sheet having perforated holes arranged regularly in front of an absorber having a high sunlight absorbance modulus. CONSTITUTION:The electroconductive sheet 2 such as a metal mesh sheet in which the perforated holes 2a are arranged regularly is arranged just below a transparent cover 1, and the absorber 3 having a high sunlight absorbance modulus is placed just below the electroconductive sheet 2 so that a selective absorber is formed. When the size of the preforated holes 2a is approximately in the range of the wave length of sunlight, most of the sunlight arriving at the mesh sheet 2 passes through the mesh sheet 2, and most of heat and light having wave lengths longer than that of the sunlight is reflected thereby. Thus, by optimizing the size of the perforated holes 2a, a selective absorber can be obtained with the sunlight absorbance modulus being increased and the heat emissivity being decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a selective absorber for a solar collector. especially,
By placing an electrically conductive thin plate with regularly arranged through holes in front of an absorber with a high solar absorption rate, it absorbs electromagnetic waves in the wavelength range of the sun and absorbs electromagnetic waves in a longer wavelength range. This invention relates to a selective absorber that reflects electromagnetic waves.

As an absorber for a solar heat collector, one that has a large absorption rate for the sun jtK and a small thermal emissivity at the operating temperature of the collector, in other words, a material that has a large absorption rate for electromagnetic waves in the wavelength range of sunlight, An absorber with a high reflectance for electromagnetic waves in a longer wavelength range is required to improve the heat collection efficiency of the collector. Absorbers with these characteristics are called selective absorbers.

Conventionally, (1) the absorption edge on a metal surface with high reflectance in the infrared region is 1000 to 3000n! (2) A dielectric film is placed on a metal surface that has a high reflectance in the infrared region. (3) The thickness of the dielectric film is adjusted appropriately so that the absorption due to the optical interference effect is included in the sunlight absorption rate. It also absorbs short wavelength light (sunlight) through the cavity effect, while absorbing long wavelength light (heat) through a smooth surface.
In other words, devices have been devised that operate with a low thermal emissivity.

This invention proposes a selective absorber based on a structure and principle different from the conventional ones as described above. First, the structure and principle of the selective absorber according to the present invention will be explained using one embodiment of the present invention.

FIG. 1 is an enlarged perspective view of an embodiment according to the present invention.
1 is a transparent cover, (2) is an electrically conductive thin plate provided directly under the transparent cover (!), and has regularly arranged through holes (2a), such as a metal mesh plate (hereinafter referred to as , this thin plate or its equivalent is called a mesh plate), (a+ indicates an absorber with a high sunlight absorption rate, which is provided facing the mesh plate (2).

The sunlight that passes through the transparent cover (11) is exposed to the mesh plate (2).
incident on . At this time, the mesh plate (2) is made of an electrical conductor, that is, one that has a high reflectance in the infrared region, and the dimensions of the first through hole (21L) are made to be about the same as the wavelength range of sunlight, and the through hole ( 2a) are arranged regularly, most of the sunlight passes through the mesh plate (2), and most of the light (heat) in the wavelength range H, which is longer than that of sunlight, passes through the mesh plate (2).
2) It is reflected by K. That is, in the present invention, a metal mesh plate (2) and an absorber (3) are combined.

By optimizing the dimensions of the mesh plate (2).

The objective is to obtain a selective absorber with high solar absorption and low thermal emissivity.

The transmission phenomenon of microwave radio waves or far-infrared light through metal mesh is clear from waveguide management theory and various experiments. According to Mitsuishi et al.
Journal of AppliedPhysi9s
, Volume 2, Pages 574-577, 1! US year) 8
It is shown that the transmittance of microwave radio waves reaches its maximum near the wavelength of 250 μm in a metal mesh with a 2 μm wire and a 212 μm return period.

Regarding fine metal mesh, see Mr. Rank (pplie40ptics, Volume 1, Page 64).
3-64, 1162) has a width of 12 μm and 2 periods of 50 μ
The hazy metal mesh exhibits a peak transmission wavelength of approximately 60 μm. Furthermore, Mr. Sakai et al.
sics.

Volume 1s, pages 131-140. 19711) is a 1500 metal mesh with a period of 1t1μm and 115μm.
The peak transmission wavelength is shown.

However, all of these documents aim at improving the transmittance of electromagnetic waves that pass through a metal mesh and narrowing the band of the transmitted electromagnetic waves, targeting electromagnetic waves with long wavelengths. On the other hand, in this invention, as much sunlight as possible is allowed to pass through.

It is aimed at a metal mesh that reflects electromagnetic waves with longer wavelengths (as much as possible), and the wavelength of the electromagnetic waves that are transmitted through the metal mesh is much shorter than that in the literature. The range is the wavelength range of sunlight from 0.3 to
2. ．． It has a wide area of 5 μm.

In the selective absorber configured as shown in FIG. 1 below,
The relationship between the dimensions and shape of the through holes of the mesh plate (2), solar absorption rate, and thermal emissivity will be explained.

The gz diagram is a diagram showing the dimensions of the mesh plate, where D is the period in two orthogonal directions of the lattice, and a is the wire @ or wire diameter.

T indicates thickness. In this case, the S width (dl and thickness) (which is the penetration depth of electromagnetic wave energy) must also be sufficiently large, and the mesh plate (2) must be made of a material with high reflectivity in the infrared region, that is, electrical It must be made from a suitable conductor. As the absorber (3), a substrate made of metal, plastic, etc. coated with black paint or the like to increase the sunlight absorption rate may be used.

The taS diagram shows the spectral absorption rate of the selective absorber configured as described above. In this case, the 9-spectral absorption rate is determined by the wavelength (λ), the period Φ of the mesh plate (2), the line width ((1), etc., and is the value (d/D ) divided by the line width (d) by the period p). The spectral absorption rate is expressed as a function of the wavelength of light divided by the period (λ/D) using as a parameter.
Te, horizontal axis tλ/D m height), (1/D=120, a, spectral absorption rate for 25, respectively, WXs diagram (a)
and (b). Here, the mesh plate (21) had a size with a period in the range of 300 nn+ to 11,000 nm, and the absorber (3) was a stainless steel plate with black paint painted on it and had a sunlight absorption rate of L95 or higher.

As is clear from Figure 3, the spectral absorption rate is at its maximum near λ/D = 1.2, and the width of the 0-spectral absorption curve increases as the d/D value decreases. . In order to increase the sunlight absorption rate for something with such spectral absorption characteristics, it is sufficient to select a mesh plate (210 cycles) so that the spectral energy distribution of sunlight matches the vicinity of this maximum absorption value. Although decreasing the d/D value is advantageous in increasing the sunlight absorption rate, decreasing the d/D value results in large absorption on the long wavelength side.

Therefore, the thermal emissivity also increases, so the d/D value cannot be reduced indefinitely. Therefore, there is a range of D and (1/D values) that optimizes the solar absorption and thermal emissivity of the selective absorber of this invention.

Table 1 shows the solar absorption and thermal emissivity of the selective absorber of this invention for D and d/D ii.

These values were calculated using the curve in FIG. 3, the sunlight spectral energy distribution at Air Mass 2, and the blackbody radiation spectral energy distribution at 100'' IC using a conventional method.

Table 1 As is clear from Table 1, the mesh plate (2) with a period of 5Q (inlk) exhibits a large solar absorption rate and a small thermal emissivity, and is suitable as an absorber for solar heat collectors, especially in sunlight. The ratio of absorption rate to thermal emissivity (α/g) is large, making it suitable for high-temperature heat collecting absorbers.
Also, the mesh board from soon to soon has a large sunlight absorption rate and α/1 value, and is suitable as an absorber for heat collection. However, from m-phase SOOnm to goonno range 0
40, the sunlight absorption rate decreases significantly, making it unsuitable for use as a heat collecting absorber. On the other hand, regarding the d/D value, the range from 0.20 to (L25) gives a large solar absorption rate and small thermal emissivity, and is suitable as an absorber for heat collection. For example, when (1/D = 0.30, the period a
Solar absorption rate is 1 for 4C from oonm to 800nm
4 or less. In addition, (when the L/D value is rL20 or less, the solar absorption rate decreases and the thermal emissivity increases. For example, d/D: (at L15, the period is 3 G On1 m1
For @00 nm, the solar absorption rate is less than α5 and the thermal emissivity is more than 00%, making it unsuitable as an absorber for heat collection. In other words, the mesh plate (2) has a circumference of '1j
4(D) is in the range from soonm to 800nm, and the line width (
A material having a ratio of a) to period (a/p) in the range of 0.20 to 0.25 is suitable as the heat collecting absorber of the present invention.

In the above embodiment, a metal plate or a plastic plate coated with black paint was used as the absorber +3 placed directly under the mesh plate (2).
) In addition to these, a black fluid can also be used. Next, an example thereof will be explained.

FIG. 4 is an enlarged perspective view of an embodiment according to the present invention.
) is a transparent cover made of a glass plate, plastic plate, etc., C2) is a mesh plate provided facing the transparent cover (11), (311I'i sunlight absorber).

It consists of a container (4) and a black fluid (5) flowing through it.

In the container (4), the surface facing the mesh plate (2) is made of a material with low solar reflectance, such as glass, acrylic resin, plastics such as fluorine resin, etc. The black fluid (5) may be one that has a high solar absorption rate; for example, a fine powder of carbon black, silicon, germanium, etc. dispersed in water or an organic solvent, or one colored with a black dye. Water or organic solvents are suitable.

The sunlight that passes through the transparent cover (2) and enters the mesh plate (2) is exposed to the mesh plate (2) as explained in the above embodiment.
) passes through the mesh plate +21 and enters the surface of the container (4).

Most of that sunlight is absorbed by or transmitted through the surface of the container (4). The sunlight absorbed by the surface of the container (4) is converted into heat and transferred to the black fluid (5) inside the container (4), and the sunlight that has passed through the container (4) is transferred to the black fluid (51).
Both can be effectively taken out as heat to the external system. In this case, the transparent cover (1) is 3M thick soda glass with low iron content, the mesh plate is made of nickel with a width of 0.2 μm and a repeating period of 0.8 μm), and the container (
4) was made from 31EJE soda glass, and 5oz of ethylene glycol was used as the black fluid (5).
Carbon black triple tS is separated in a 1% aqueous solution.

In the absorber for a solar heat collector constructed from the above, a solar absorption rate of 1i or more and a thermal emissivity of αo1 or less were obtained for an absorber temperature of 60°C.

Further, as the mesh plate (2), it is also possible to use a dielectric plate having a high solar transmittance and provided with a thin layer made of an electrical conductor and having through holes.

Next, i1i! I will clarify.

FIG. 5 is an enlarged perspective view of an embodiment according to the invention.
) is a transparent cover made of glass plate, plastic plate, etc., (2) is a transparent cover (1) is a mesh plate provided facing K, (6Hi mesh plate (dielectric plate that serves as a substrate for holding lead) The body plate (3) shows a sunlight absorber made of an iron plate coated with black paint, etc. In order to form the mesh plate (21) on the dielectric plate (6), KFi, for example, a glass plate is used.

After forming a thin film of metal such as chromium on a plastic plate such as polyethylene using a normal vapor deposition method, the mesh plate (through holes such as 21 It is good to have 2a).

Note that the long surface of the dielectric plate (6) facing the absorber (3) K is required to have a large reflectance for heat dissipation (radiated infrared rays) from the absorber (3).

As an example, a mesh plate (2) of a chromium vapor-deposited film with a line width of 12 μm and a return period of 8 μm formed on a dielectric plate (6) made of polyethylene, and an absorber made of an iron plate coated with black paint (
When 3) is combined, a solar absorption coefficient of 0.50 or more and a thermal emissivity of &1 or less were obtained in experiments at a temperature of 60° C. for the a-collection body (3).

In addition, in the above embodiment, the through holes (2a
) as a thin plate or layer with a square through-hole R(
2a) are shown arranged two-dimensionally at the same period, but the shape of the 0 through holes (2&) may be a square, a polygon such as a hexagon, a circle, etc., and have the same effect. The array may be two-dimensionally arranged according to a certain rule. Also, as a thin plate or thin layer, it is plate-shaped! Although it is made of wire, it may also be made of wire mesh made of thin wires knitted together.

As described above, the selective absorber according to the present invention has a large solar absorption rate and a small thermal emissivity.

In particular, the ratio of sunlight $L morbidity and thermal emissivity (α/ε) 115
0 or higher, compared to about 10 fifK of conventional selective absorbers made of copper plated with black chrome, and can be used in extremely large, high-temperature heat collectors.

Extremely effective.

[Brief explanation of the drawing]

View, i! FIG. 3 is a spectral absorption characteristic diagram of a selective absorber according to one embodiment of the present invention, and FIG. 4 is an enlarged perspective view showing a selective absorber according to another embodiment of the present invention. FIG. 5 is an enlarged perspective view showing a selective absorbent body according to another embodiment of the present invention. (11...Transparent cover, C21...Mesh plate (electrically conductive thin plate), +31...Solar light absorber, (4)
... Container, (5) ... Black fluid, (6) ... Dielectric plate. In Figure 1, the same reference numerals indicate the same or corresponding parts. Agent Kuzuno Nobu - Ki (instead?chi L half 2 evil*J @ (toge enters 1・νku income rate 4- to ¥-5a shaku P%L

Claims (1)

[Scope of Claims] (11) A selective absorber for a solar collector, comprising an electrically conductive thin plate having regularly arranged through holes arranged in front of an absorber with a high solar absorption rate. (2) Electricity Period p) of 0 through holes in a conductive thin plate
30On! The range is from El to 80 Onll, and the value (d/D) obtained by dividing the line width (d) by the period is rL2
A selective absorber for a patented rector, characterized in that the absorption range is from 0 to 0.25. (3) An absorber consisting of an electrically conductive thin plate with regularly arranged through holes, a surface with low solar reflectance, and a black fluid with high solar absorption filled under the surface. Selective absorber for solar heat collector placed on the front. (4) In the electrically conductive thin plate κ, the period of the through holes (1
)> in the range from 300nm to IOonll. Also, the line width ((1) divided by the period (a/p) is 0.2
0 to a. The selective absorber for a solar collector according to claim 3, characterized in that ζ is in the range of 25. (5) An electrically conductive thin layer with regularly arranged through holes formed on a dielectric thin plate with high solar transmittance is placed in front of an absorber with high solar absorption. Selective absorber for solar collectors. (6) In an electrically conductive thin plate or layer. The period p of the through holes was set in the range of 300nll to IOOnm. Also, seven IIm ((1) divided by the period ((1/
D) The selective absorber for a solar collector as set forth in claim aI5, characterized in that it has a (L20 to 125) range.