CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the United States national phase of International Application No. PCT/JP2019/028145 filed Jul. 17, 2019, and claims priority to Japanese Patent Application No. 2018-137746 filed Jul. 23, 2018, the disclosures of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a radiative cooling device configured such that an infrared radiative layer for radiating infrared light from a radiative surface and a light reflective layer disposed on the side opposite to the presence side of the radiative surface of the infrared radiative layer are provided in a mutually stacked state.

BACKGROUND ART

Such radiative cooling device is provided for use in cooling various kinds of cooling targets or objects, with an arrangement e.g. that infrared light (radiation) radiated from the radiative surface of the infrared radiative layer is transmitted through the atmospheric window (e.g. a range of wavelength that allows good transmission therethrough of infrared light having wavelengths ranging from 8 μm to 14 μm), thereby to cool a cooling target (a target or an object to be cooled) located on the side of the light reflective layer opposite to the presence side of the infrared radiative layer.

By the way, the light reflective layer reflects the light (visible light, ultraviolet light, infrared light) transmitted through the infrared radiative layer from the radiative surface, thus preventing such light (visible light, ultraviolet light, infrared light) transmitted through the infrared radiative layer from being projected onto the cooling target to heat this cooling target.

Incidentally, the light reflective layer has a further function of reflecting not only the light transmitted through the infrared radiative layer, but also the infrared light radiated from the infrared radiative layer to the presence side of the light reflective layer back toward the infrared radiative layer. However, the following explanation will be made on a premise that the light reflective layer is provided for the purpose of reflecting the light (visible light, ultraviolet light, infrared light) transmitted through the infrared radiative layer.

As a first conventional example of such radiative cooling device, there are known one configured such that the infrared radiative layer is comprises a laminar body consisting of a layer of SiO2, a layer of MgO or glass (optical glass) and the light reflective layer comprises a diffusive reflective body or a multi-layered body comprised of a multiple layers of a metal layer formed of silver, a layer of TiO2and a layer of SiO2disposed in alternation (see e.g. Patent Document 1).

BACKGROUND ART DOCUMENT

Patent Document

Patent Document 1: detailed disclosure of U.S. Patent Application Publication No. 2015/0338175

SUMMARY OF THE INVENTION

Problem to be Solved by Invention

The radiative cooling device is to be configured such that its reflectance under the condition of the infrared radiative layer and the light reflective layer being stacked provides a high reflectance for the wavelength range (e.g. wavelength equal to or higher than 400 nm and equal to or less than 1800 nm, seeFIG. 26) where the intensity of the solar light energy is high.

Namely, while the infrared radiative layer has a high transmittance for the wavelength range where the solar light energy is high, the light reflective layer is provided with a high reflectance to reflect sufficiently the light transmitted through the infrared radiative layer.

The wavelength range where the solar light energy is high includes the range of visible light (equal to or greater than 400 nm and equal to or less than 1800 nm). However as a result of the arrangement of the light reflective layer reflecting the visible light range with a high reflectance, when the radiative cooling device is seen from the presence side of the radiative surface of the infrared radiative layer, coloring condition is not felt as e.g. a condition similar to that of a mirror surface is felt instead.

Namely, the present inventors previously conducted research for and developed a radiative cooling device configured such that the infrared radiative layer comprises a glass (white plate glass) selected from the group consisting of alkali free glass, crown glass and borosilicate glass and the light reflective layer comprises a metal layer formed of silver having a thickness equal to or greater than 300 nm, so that as the light transmitted through the infrared radiative layer is reflected appropriately by the light reflective layer, thus cooling a cooling target appropriately. However, with this radiative cooling device, when the radiative cooling device is viewed from the presence side of the radiative surface of the infrared radiative layer, its back face is felt like a condition similar to that of a silver-colored mirror face, no coloring effect being felt.

However, for the sake of improvement of the aesthetic effect, it was desired that coloring of blue, pink, etc. should be felt or present when the radiative cooling device is viewed from the presence side of the radiative surface of the infrared radiative layer.

More particularly, the radiative cooling device is intended or assumed to be used for installation in a roof of a house, a roof of an automobile, etc. In such case, in order to allow the radiative cooling device as viewed from the presence side of the radiative surface of the infrared radiative layer to exhibit a color which harmonizes with the color of the surrounding, it may be desired that coloration effect can be felt appropriately when the radiative cooling device as viewed from the presence side of the radiative surface of the infrared radiative layer.

Incidentally, in the following description, a condition of coloration effect felt when the radiative cooling device is viewed from the presence side of the radiative surface of the infrared radiative layer will be referred in short as a “condition of coloration of the radiative surface”.

The present invention has been made in view of the above-described state of the art and its object is provide a radiative cooling device that provides coloration of the radiative surface while maximally avoiding reduction in its radiative cooling performance due to absorption of solar light.

Solution

A radiative cooling device according to the present invention configured such that an infrared radiative layer for radiating infrared light from a radiative surface and a light reflective layer disposed on the side opposite to the presence side of the radiative surface of the infrared radiative layer are provided in a mutually stacked state, the radiative cooling device being characterized in that:

the light reflective layer is arranged such that a first metal layer made of silver or silver alloy and having a thickness equal to or greater than 10 nm and equal to or less than 100 nm, a transparent dielectric layer and a second metal layer for reflecting light transmitted through the first metal layer and the transparent dielectric layer are stacked in this order on the side closer to the infrared radiative layer; and

the transparent dielectric layer has a thickness that causes a resonance wavelength of the light reflective layer to be a wavelength included in wavelengths equal to or greater than 400 nm and equal to or less than 800 nm.

Namely, as the thickness of the first metal layer formed of silver or silver alloy is set to a range equal to or greater than 10 nm and equal to or less than 100 nm, it is possible to allow the visible light to be transmitted appropriately while implementing optical control (resonance of the resonance wavelength) appropriately.

That is, with silver or silver alloy, the thinner, the higher the transmittance for solar light transmission. In contrast, the reflectance thereof decreases. Therefore, if the thickness of the first metal layer is set to be greater than 100 nm, it will become unable to appropriately transmit the visible light transmitted through the infrared radiant layer, so that coloration of the radiative surface through absorption of light utilizing resonance of the resonance wavelength becomes impossible. On the other hand, if the thickness of the first metal layer is set to be smaller than 10 nm, appropriate reflection of light becomes impossible, so that appropriate optical control (resonance of the resonance wavelength) will become impossible. As a result, coloration of the radiative surface by light absorption utilizing resonance of the resonance wavelength becomes impossible.

And, the visible light transmitted through the first metal layer will basically be transmitted through the transparent dielectric layer and then reflected by the second metal layer to be emitted again from the radiant surface of the infrared radiation layer into the atmosphere. In this, since the thickness of the transparent dielectric layer is configured as a thickness that causes a resonance wavelength of the light reflective layer to be a wavelength included in the wavelengths equal to or greater than 400 nm and equal to or less than 800 nm, the light of the narrow band range centering about whichever resonance wavelength of the visible light transmitted through the first metal layer (i.e. the light having a wavelength equal to or greater than 400 nm and equal to or less than 800 nm) will be absorbed by the light reflective layer by the optical control arrangement (resonance of the resonance wavelength).

That is, the light of the narrow band range centering about the resonance wavelength included in the visible light transmitted through the first metal layer to reach the transparent dielectric layer will be reflected back and forth in repetition between the first metal layer and the second metal layer and will be absorbed by the first metal layer and/or the second metal layer in the course of this process.

As a result, the visible light reflected by the light reflective layer and emitted from the radiative surface of the infrared radiant layer into the atmosphere will not include the light of the narrow band range absorbed by the light reflective layer. Thus, when the radiative cooling device is viewed from the presence side of the radiative surface of the infrared radiative layer, a coloration condition (effect) will be felt and present.

And, by changing the thickness of the transparent dielectric layer thereby to change the wavelength used as the resonance wavelength included in the wavelengths equal to or greater than 400 nm and equal to or less than 800 nm, the color exhibited by the radiative cooling device when viewed from the presence side of the radiative surface of the infrared radiative layer can be changed (seeFIG. 27).

Moreover, since the light of the narrow band range centering about the resonance wavelength of the visible light (namely, the light having wavelengths equal to or greater than 400 nm and equal to or less than 800 nm) is absorbed by the light reflective layer, even though there occurs certain reduction in the radiative cooling performance due to rise of temperature resulting from solar light absorption, since this is absorption of light of narrow band range, the resultant reduction in the radiative cooling performance will be small.

In short, according to the characterizing feature of the present invention, it is possible to provide a radiative cooling device that provides coloration (effect) of the radiative surface while maximally avoiding reduction in its radiative cooling performance due to absorption of solar light.

According to a further characterizing feature of the radiative cooling device of the present invention, the second metal layer is formed of silver or silver alloy having a thickness equal to or greater than 100 nm.

Namely, with silver or silver alloy, the greater its thickness (membrane thickness), the greater its reflectance. In particular, if the thickness exceeds 100 nm, the reflectance for the infrared light or visible light (wavelengths equal to or greater than 400 nm and equal to or less than 800 nm) increases to such a degree to exceed 90%.

Therefore, since the thickness of the second metal layer formed of silver or silver alloy is set equal to or greater than 100 nm, by appropriately reflecting the infrared light and/or visible light transmitted through the first metal layer and the transparent dielectric layer, it is possible to prevent the infrared light and/or visible light from reaching a cooling target positioned on the side of the light reflective layer opposite to the presence side of the infrared radiative layer, whereby cooling of the cooling target may be carried out appropriately.

In short, according to the further characterizing feature of the present invention, cooling of the cooling target can be carried out appropriately by appropriately reflecting the infrared light and/or visible light by the second metal layer.

According to a still further characterizing feature of the radiative cooling device of the present invention, the second metal layer is formed of aluminum or aluminum alloy having a thickness equal to or greater than 30 nm.

Namely, aluminum or aluminum alloy has a higher absorbance for the visible light than silver or silver alloy (seeFIG. 31).

Therefore, by forming the second metal layer of aluminum or aluminum alloy having a thickness equal to or greater than 30 nm, the light having the resonance wavelength and included in the visible light transmitted through the first metal layer to eventually reach the transparent dielectric layer can be absorbed by the second metal layer appropriately. Thus, in comparison with an arrangement of forming the second metal layer of silver or silver alloy, more distinct coloration effect can be obtained.

Incidentally, aluminum or aluminum alloy has a higher reflectance for the ultraviolet light than silver or silver alloy.

Thus, it can appropriately reflect ultraviolet light transmitted through the first metal layer, so that the ultraviolet light can be reliably prevented from reaching the cooling target located on the side of the light reflective layer opposite to the presence side of the infrared radiative layer, whereby the cooling of the cooling target can be carried out appropriately.

In short, according to the still further characterizing feature of the present invention, more distinct coloration effect can be obtained.

According to a still further characterizing feature of the radiative cooling device of the present invention, the second metal layer comprises the first layer formed of silver or silver alloy and the second layer formed of aluminum or aluminum alloy stacked and disposed in this order to the side closer to the transparent dielectric layer.

Namely, in case the first layer formed of silver or silver alloy and the second layer formed of aluminum or aluminum alloy are stacked with the first layer being located on the side close to the transparent dielectric layer, while the thickness (membrane thickness) of the first layer is reduced than the case of forming the second metal layer of silver or silver alloy, it is still possible to obtain an optical characteristics almost same as the case of forming the second metal layer of silver or silver alloy. Thus, it becomes possible to reduce the amount of expensive silver or silver alloy to be used.

Moreover, by decreasing the general thickness to 60 nm approximately while making the thickness of the first layer greater than 2 nm and making the thickness of the second layer equal to or greater than 30 nm, it is possible e.g. to form a second metal layer having almost same optical characteristics as the case of forming the second metal layer entirely of silver or silver alloy. Thus, the membrane thickness of the second metal layer can be smaller than the thickness (e.g. 100 nm) in the case of forming the second metal layer entirely of silver or silver alloy. Thus, manufacture cost reduction of the second metal layer is made possible.

In short, according to the still further characterizing feature of the radiative cooling device of the present invention, the cost reduction of the second metal layer is made possible through manufacture cost reduction through reduction in the amount of use of the expensive silver or silver alloy.

According to a still further characterizing feature of the radiative cooling device of the present invention, the transparent dielectric layer comprises a transparent nitride membrane.

Namely, by providing a transparent nitride membrane as the transparent dielectric layer and adjusting its thickness, adjusting of the color under the colored condition of the radiative surface can be carried out favorably.

As some specific examples of such transparent nitride membrane, Si3N4and AlN can be cited. The transparent nitride membrane, when formed into a membrane by such technique as sputtering, vapor deposition, etc., does not cause discoloration of silver or silver alloy in case the first metal layer is formed of silver or silver alloy, thus being advantageous in facilitating productivity improvement.

Incidentally, the transparent nitride layer, in case the first metal layer is formed of silver or silver alloy and the second metal layer is formed of aluminum or aluminum alloy, acts as a transparent anti-alloying layer for preventing alloying between silver or silver alloy of the first meal layer and aluminum or aluminum alloy of the second metal layer.

Namely, with suppression of alloying between silver and aluminum, with effective avoidance of light absorbance by the light reflective layer, the condition of appropriately reflecting light by the light reflective layer can be maintained for a long period of time.

In short, according to the still further characterizing feature of the radiant cooling device of the present invention, color adjustment under the colored condition of the radiative surface can be carried out favorably.

According to a still further characterizing feature of the radiative cooling device of the present invention, the transparent dielectric layer comprises a transparent oxide layer.

Namely, by providing a transparent oxide membrane as the transparent dielectric layer and adjusting its thickness, adjusting of the color under the colored condition of the radiative surface can be carried out favorably.

As some specific examples of such transparent oxide membrane, many kinds can be used, but, Al2O3, SiO2, TiO2, ZrO2, HfO2, Nb2O5, Ta2O5, which can be readily formed into membrane by the vapor deposition, the sputtering technique etc., can be cited.

Incidentally, the transparent oxide layer, in case the first metal layer is formed of silver or silver alloy and the second metal layer is formed of aluminum or aluminum alloy, acts as a transparent anti-alloying layer for preventing alloying between silver or silver alloy of the first metal layer and aluminum or aluminum alloy of the second metal layer.

Namely, with suppression of alloying between silver and aluminum, with effective avoidance of light absorbance by the light reflective layer, the condition of appropriately reflecting light by the light reflective layer can be maintained for a long period of time.

In short, according to the still further characterizing feature of the radiant cooling device of the present invention, color adjustment under the colored condition of the radiative surface can be carried out favorably.

According to a still further characterizing feature of the radiative cooling device of the present invention, the infrared radiative layer comprises glass selected from the group consisting of alkali free glass, crown glass and borosilicate glass.

Namely, alkali free glass, crown glass and borosilicate glass are relatively inexpensive, yet having high transmittance for the wavelength equal to or greater than 400 nm and equal to or less than 1800 nm where the intensity of the solar light energy is high (e.g. equal to or greater than 95%) and have a characteristics of high emission intensity for radiating infrared light having wavelengths corresponding to the atmospheric window (e.g. window that transmits infrared light having wavelength equal to or greater than 8 μm and equal to or less than 14 μm).

Therefore, by constituting the infrared radiative layer of any glass selected from the group consisting of alkali free glass, crown glass and borosilicate glass, it is possible to obtain a radiative cooling device having high cooling capacity while achieving cost reduction in the general arrangement.

In short, according to the still further characterizing feature of the radiative cooling device of the present invention, it is possible to obtain improvement in the cooling capacity while achieving cost reduction in the general arrangement.

According to a still further characterizing feature of the radiative cooling device of the present invention, the infrared radiative layer is used as a substrate, on which the first metal layer, the transparent dielectric layer and the second metal layer are stacked.

Namely, since the infrared radiative layer is used as a substrate and the first metal layer, the transparent dielectric layer and the second metal layer are stacked on this substrate, it is possible to achieve thickness reduction of the general arrangement while achieving simplification of the general arrangement.

Incidentally, in case the first metal layer, the transparent dielectric layer and the second metal layer are to be stacked on the infrared radiative layer as a substrate, if the first metal layer, the transparent dielectric layer and the second metal layer are thin, the first metal layer, the transparent dielectric layer and the second metal layer will be stacked one after another by the sputtering technique or the like.

Namely, in comparison with an arrangement in which a substrate for stacking is provided separately and on this stacking substrate, the second metal layer, the transparent dielectric layer and the first metal layer are stacked one after another by e.g. the sputtering technique or the like, after which at the portion of the first metal layer on the side opposite to the presence side of the second metal layer, an infrared radiative layer separately fabricated will be placed to be stacked thereon or in which the infrared radiative layer is stacked by the sputtering technique or the like at the portion of the first metal layer on the side opposite to the presence side of the transparent dielectric layer, there is no need to provide a stacking substrate. Consequently, it is possible to achieve simplification of the general arrangement and also to achieve membrane thickness reduction of the general arrangement.

In short, according to the still further characterizing feature of the radiative cooling device of the present invention, simplification of the general arrangement can be achieved and moreover reduction of the membrane thickness of the general arrangement can be achieved also.

According to a still further characterizing feature of the radiative cooling device of the present invention, a gapless contact layer is stacked between the infrared radiative layer and the first metal layer.

Namely, since a gapless contact layer is stacked between the infrared radiative layer and the first metal layer, it is possible to suppress occurrence of damage such as peeling-off of the first metal layer of the light reflective layer from the infrared radiative layer formed of glass, etc. due to e.g. temperature change, etc. Thus, durability can be improved.

In short, according to the still further characterizing feature of the radiative cooling device of the present invention, durability can be improved.

According to a still further characterizing feature of the radiative cooling device of the present invention, on the side of the second metal layer opposite to the presence side of the transparent dielectric layer, an anti-oxidization layer is stacked.

Namely, on the side of the second metal opposite to the presence side of the transparent dielectric layer, an anti-oxidization layer is stacked. Therefore, oxidization deterioration of the second metal layer can be suppressed, so that the durability can be improved.

In short, according to the still further characterizing feature of the radiative cooling device of the present invention, deterioration of the second metal layer can be suppressed, whereby the durability can be improved.

According to a still further characterizing feature of the radiative cooling device of the present invention, the radiative surface of the infrared radiative layer has unevenness for light diffusion.

Namely, since the radiative surface of the infrared radiative layer has unevenness for light diffusion, the colored condition of the radiative surface of the infrared radiative layer, namely, the colored condition of the radiative cooling device as viewed from the presence side of the radiative surface of the infrared radiative layer, can be appropriately obtained even when the radiative surface is viewed from various directions.

In short, according to the still further characterizing feature of the radiative cooling device of the present invention, the colored condition of the radiative surface can be appropriately obtained even when the radiative surface is viewed from various directions.

