Passive radiative cooling

Abstract

The present invention relates to a passive cooling system comprising a radiative heat exchanger and a convective heat exchanger comprising a heat transfer fluid. The radiative heat exchanger comprises a glass substrate having at least a silver reflective layer comprising at least ≥700mg / m2 of silver.

Description

Passive Radiative CoolingTechnical Field

[0001] The present invention relates to the field of passive radiative cooling systems and in particular, to sustainable passive radiative cooling systems.Prior Art

[0002] Decreasing energy consumption is not only economically advantageous but has also a positive impact on the environment. Decreasing energy also applies in the field of heating and cooling. With climate change, the demand for cooling increases and represents an increasingly important energy consumption.

[0003] Passive radiative cooling can be achieved, without external energy sources, through emission of infrared black body radiations to space beyond the atmosphere, that function at a heat sink and trap heat in the form of electromagnetic radiation. In particular, radiative cooling is achieved in the wavelengths of infrared radiations comprised between 8pm to 13pm corresponding to the earth's atmosphere transparency window in mid infrared. The cooling effect takes advantage of the very low space temperature. Therefore, an outdoor object emitting radiation in these wavelengths will be subjected to a cooling, leading to a temperature below the temperature of ambient air thanks to energy transfer to the cold space through this atmospheric window. There are currently two main technologies to do so, one involves an emissive paint with micro particles that reflect the sun light and emit infrared, and the second way is a reflective coated stack on a substrate, as for example via a silver layer.

[0004] This passive cooling effect is most pronounced at night, when, by emitting radiation to the very cold space, a black body is cooling through a very well-known process called "night cooling". This cooling increases with the emissivity of the black body. Nevertheless passive cooling during day time is also of increasing interest, even ifmore difficult to accomplish. During the day, the cooling effect is affected by the sun energy of the incoming solar radiation that overpowers outgoing radiation to space at -273°C. For this reason, cooling below ambient air temperature during the daytime through passive cooling, is a technical challenge. The heating of the system by solar radiation can be avoided though efficient reflection.

[0005] Hence, the radiative surface of the passive radiative cooling system, i.e. the surface facing the sky, should have low absorption of sunlight and a high reflectance of the solar energy at wavelengths of 0.3pm to 2.5pm. It should be further characterised by a high emittance over higher infrared wavelength, more particularly at wavelengths of 8pm to 13pm.

[0006] WO2020214989A1 discloses radiative cooling systems comprising a material with an emittance of at least 80% in the spectral range of 5pm to 15pm together with an absorption or reflection in the spectral range of 275nm to 375nm (UV) capable of achieving a cooling rate of at least 10 W / m2at 300°K ambient air temperature. The material comprises at least one of ZnO, Si, HfCh or ZnCh.

[0007] To improve day cooling, W02021 / 137080A1 discloses passive cooling article that includes a plurality of first elements defining a high absorbance in the atmospheric window wavelength range and defining high average reflectance in the solar wavelength range and a plurality of second elements defining a low absorbance in the atmospheric window wavelength range and defining high average reflectance in the solar wavelength range. The high emissivity elements facing the sky and the low emissivity elements are shaded.

[0008] Some passive radiative cooling solutions have already been proposed in the art. Most solutions are based on polymeric material that cannot provide long term effectiveness due to mechanical and chemical resistance under sunlight. Therefore, there is still a need to find sustainable technical solutions that not only have high cooling capacity but as well are made of sustainable materials and last longer in outdoor conditions.Summary of invention

[0009] The present invention relates to a passive radiative cooling system comprising a radiative heat exchanger and a convective heat exchanger. The radiative heat exchanger comprises a glass substrate having a first surface and a second opposite surface. The first surface is a radiative surface, and the second surface is a coated surface, being provided with at least one silver reflective layer comprising at least >700mg / m2of silver. The convective heat exchanger comprises a heat transfer fluid, and faces the coated surface of the radiative heat exchanger.

[0010] In a preferred embodiment, the radiative heat exchanger provides a regular luminous coefficient equal to orgreater than 86%, for a thickness between 2mm and 6mm. Preferably, the amount of silver comprised in the silver reflective layer is equal to or greater than 800 mg / m2, equal to or greater than 1000 mg / m2, equal to or greater than 1200 mg / m2, equal to or greater than 1400 mg / m2, equal to or greater than 1500 mg / m2.

[0011] The radiative heat exchanger typically further comprises at least a protective layer and / or at least a paint layer. The at least one protective layer is deposited on the at least one silver layer. The at least one paint layer is deposited on the at least one silver layer or preferably on the at least one protective layer, if present. Preferably, the radiative heat exchanger does comprise at least a protective layer, preferably a copper protective layer. More preferably, copper is deposited in an amount equal to or greater than >250 mg / m2, preferably equal to or greater than >400 mg / m2. Preferably, the radiative heat exchanger further comprises at least a paint layer, preferably at least two paint layers; more preferably in addition to the protective layer.

[0012] In particular, the glass substrate of the radiative heat exchanger has a thickness equal to or greater than 0.5 mm, preferably equal to or greater than 0.8 mm and more preferably equal to or greater than 1.0 mm. Preferably, its thickness is equal to orlowerthan 4 mm, preferably equal to or lower than 3 mm and more preferably equal to or lower than 1.5 mm.

[0013] In a preferred embodiment, the glass substrate of the radiative heat exchanger is a soda lime silicate glass. In particular, the glass substrate comprises a total iron content expressed as Fe20s of 20ppm to 2000ppm (20ppm < Fe20s < 2000ppm), based on the total weight of the glass composition.

[0014] In a preferred embodiment, the content of total iron expressed in total Fe20s in the glass composition, is comprised between 300ppm and lOOOppm (300ppm < Fe20s < lOOOppm). Preferably, it is comprised at a level equal to or greater than 400ppm, 450ppm, 500ppm, 550ppm, 600ppm and even, 650ppm by weight of total glass composition and / or preferably, the total iron expressed in total Fe20s, is comprised at a level equal to or lower than 900ppm, equal to or lower than 850ppm, equal to or lower than 800ppm, and even, equal to or lower than 750ppm by weight of total glass composition.

[0015] In a more preferred embodiment, the content of total iron expressed in total Fe20s in the glass composition, is comprised between 20ppm and less than 300ppm (20ppm < Fe2C>3 < 300ppm). Preferably it is comprised at a level equal to or greater than 40ppm, preferably equal to or greater than 50ppm, preferably equal to or greater than 60ppm, preferably equal to or greater than 70ppm, preferably equal to or greater than 80ppm, preferably equal to or greater than 90ppm, even, equal to or greater than lOOppm by weight of total glass composition and / or is comprised at a level equal to or lower than 250ppm, preferably equal to or lower than 200ppm, preferably equal to or lower than 175ppm and even, more preferably equal to or lower than 150ppm by weight of total glass composition.

[0016] It is preferred that the glass composition of the glass substrate of the radiative heat exchanger has a redox, expressed in FeO / Fe2O3, equal to or lower than 30% (FeO / Fe2O3 < 30%), preferably equal to or lower than 28% (FeO / Fe2O3 < 28%), preferably equal to or lower than 25% (FeO / Fe2O3 < 25%), more preferably equal toor lower than 23% (FeO / Fe20s3 < 23%) and even, more preferably equal to or lower than 20%.

[0017] For the purpose of the present invention, it is preferred that the glass substrate at a thickness of 4mm, has a visible light transmission, LTD4, equal to or greater than 88.5%, preferably equal to or greater than 89.0% preferably equal to or greater than 90.5%. It is further preferred that the glass substrate at a thickness of 1mm, has a visible light transmission, LTD1, equal to or greater than 90.0%, preferably equal to or greater than 91.0% preferably equal to or greater than 91.5%.

[0018] For the purpose of the present invention, it is preferred that passive radiative cooling system has a free cooling power equal to or greater than 10 W / m2(FCP > 10W / m2), preferably equal to or greater than 25 W / m2(FCP > 25W / m2) and preferably a yearly cooling energy production is equal to or greater than 300kWh / m2(YCEP >300kWh / m2), preferably equal to or greater than 1000kWh / m2(YCEP > 1000kWh / m2), more preferably equal to or greater than 2500kWh / m2(YCEP > 2500kWh / m2), and still more preferably equal to or greater than 5000kWh / m2(YCEP > 5000kWh / m2).

[0019] The present invention further relates to the use of a glass substrate having at least a silver reflective layer comprising at least >700mg / m2of silver, preferably providing a regular luminous coefficient equal to or greater than 86% for a coated glass substrate having a thickness between 2mm and 6mm, as a heat radiative exchanger within a passive radiative colling system. All preferred embodiments described above apply to heat radiative exchanger.Brief description of drawings

[0020] This and other aspects of the present invention will now be described with reference to details discussed below. The following description and drawings are illustrative ofthe disclosure and are not to be construed as limiting the disclosure. The drawings are not to scale and should not be considered as a limitation of the invention.

[0021] Figure 1 shows a temperature profile comparing one embodiment of a radiative heat exchanger of the prior art with two embodiments of radiative heat exchangers of the present invention.

[0022] Figure 2 shows a cross sectional schematic view of a radiative heat exchanger according to one embodiment of the present invention.

[0023] Figure 3 shows a schematic view of a passive radiative cooling system according to one embodiment of the present invention wherein heat exchange is achieved through direct contact between the fluid and the radiative heat exchanger.

[0024] Figure 4 shows a schematic view of a passive radiative cooling system according to another embodiment of the present invention wherein heat exchange is achieved through a pipe. Figure 4b shows a top view of the convective heat exchanger.

[0025] Figure 5 shows a schematic view of a passive radiative cooling system according to another embodiment of the present invention wherein heat exchange is achieved through a roll bond panel.Detailed description of the invention

[0026] The objective of the present invention is to design a day and night passive radiative cooling system. In particular, the objective of the present invention is to design a sustainable passive radiative cooling system that provides not only superior cooling performance but as well remains performant over a long period of time, in diverse exterior conditions. Passive radiative cooling technology can also be herein after referred to by its acronym 'PRC'.

[0027] The present invention relates to an efficient passive radiative cooling system that minimizes the heating by solar radiation and maximizes the cooling performance, while remaining durable in outdoor environment for a long period of time.

[0028] The present invention relates to an efficient PRC system that not only is very efficient at night but as well during day light and even in high sunshine conditions. Hence, another objective of the PRC of the present invention is to optimize the performance of the reflective layer within the radiative heat exchanger.

[0029] By 'a long period of time', it is understood at least 15 years, preferably at least 20 years and even more at least 25 years.

[0030] PRC systems typically comprise a radiative heat exchanger and a convective heat exchanger. The function of the radiative heat exchanger is to achieve efficient heat dissipation via radiation and convection. The convective heat exchanger comprising an inlet port, an outlet port and a heat transfer fluid coupled to the radiative heat exchanger, so that the heat transfer fluid enters the inlet port at a first temperature and exits at a second temperature that is lower than the first temperature.

[0031] The radiation heat exchanger typically comprises a reflective layer that reflects sunlight, from UV to near infrared at the wavelengths of about 300nm to 2500nm. It further comprises a radiative layer (also referred to as IR radiative layer and a mid-IR radiative layer) with good emissivity at the wavelengths above 5pm. This radiative layer is typically positioned above the reflective layer in the direction of the sky. The convective heat exchanger is positioned below the radiation heat exchanger, in the direction of the sky. The surface of the PRC system being the surface of the radiative heat exchanger, being the surface of the radiative layer, that faces the sky, is herein after referred to as the 'radiative surface'.

[0032] The radiative layer acts as a grey body and so emits mid-IR energy - typically above 5pm depending on its temperature. In particular, the radiative layer emits in the atmosphere transparent window i.e. between about 8pm and 13pm providing the cooling performance.

[0033] Since the sunlight will pass through the radiative layer twice, it is critical to minimise its sunlight absorption. The reflective layer reflects indeed the sunlight transmittedthrough the radiative layer back to the exterior environment, preventing the sunlight to heat the convective heat exchanger.

[0034] The present invention relates to a PRC system comprising a radiative heat exchanger combined with a convective heat exchanger. The convective heat exchanger allows the transfer of heat from a heat transfer fluid to be cooled by the radiative heat exchanger.

[0035] The present invention relates to a passive radiative cooling system comprising : a. A radiative heat exchanger comprising a glass substrate having a first surface and a second opposite surface; wherein i. the first surface is a radiative surface, ii. the second surface is a coated surface, being provided with at least one silver reflective layer comprising at least >700mg / m2of silver; b. a convective heat exchanger comprising a heat transfer fluid, said convective heat exchanger faces the coated surface.

[0036] The present invention relates to a passive radiative cooling system wherein the radiative heat exchanger comprises a mirror. The mirror is defined as a glass substrate having a first surface and a second opposite surface. The second surface is a coated surface, being provided with at least one silver reflective layer comprising at least >700mg / m2of silver.

[0037] It has been found surprisingly that mirrors are indeed very effective and sustainable elements that can be used as the radiative heat exchanger in passive radiativecooling systems. The mirror provides the radiative cooling function and its contact with the heat transfer fluid of the convective heat exchanger provides the required heat exchange. Mirrors comprise both elements of light reflective layer being a metallic layer being a silver layer and infrared radiative layer being the glass substrate.

[0038] In the present text, the terms "mirror", 'coated glass substrate" and "radiative heat exchanger" can be used interchangeably.

[0039] In the passive radiative cooling system of the present invention, the first surface of the glass substrate is facing the sky and is herein referred to as 'the radiative surface' and the surface facing with the convective heat exchanger is herein referred to as the 'contact surface'.

[0040] Indeed, it has been found that the silver layer comprising at least >700mg / m2of silver, is a very effective light reflective layer in reflecting most of the solar radiation. The glass substrate functions as the radiative layer and is positioned above the silver reflective coating in the direction of the sky. The glass substrate, has a high emissivity in the infrared domain and therefore is able to transfer heat through IR radiation to the atmosphere and deep space.

[0041] Preferably, the mirror provides a regular luminous coefficient equal to or greater than 86% for a mirror having a thickness between 2mm and 6mm, as defined in the European Norm EN 1036-1 / 2007 (E). As further described therein, the luminous coefficient is measured according to the European Norm EN410. For thicker mirrors having a thickness between 8mm and 10mm, the luminous coefficient is equal to or greaterthan 83%. Forthinner mirror, the luminous coefficient threshold will increase accordingly.

[0042] Preferred mirrors for use as the radiative heat exchanger in the PRC systems of the present invention comprise a silver reflective layer comprising silver in an amount equal to or greater than 800 mg / m2, equal to or greater than 1000 mg / m2, equal to or greater than 1200 mg / m2, equal to or greater than 1400 mg / m2, equal to orgreater than 1500 mg / m2. Typically it is equal to or lower than 2000 mg / m2, equal to or lower than 1800 mg / m2. The more silver is deposited on the glass substrate, the more performant is the reflective layer up to a reflection optimum. These values offer a good compromise between good reflection values and an acceptable production cost. The thickness of the silver layer may be greater than or equal to 65nm, 70nm, 80nm, 90nm, lOOnm, llOnm, 120nm, 130nm or 140nm. It may be typically equal to or lower than 200nm, 180nm, 160nm or 150nm.

[0043] Preferred mirrors - at their specific thickness, have a light reflection measured according to ISO 9050:2003 (measured through the surface of the glass, at normal incidence, under illuminant D65, 2°) greater than or equal to >85%, preferably >90%, >91%, >92%, >93%, >94% and preferably >95%.

[0044] Preferred mirrors - at their specific thickness, have an energy reflection measured according to the norm ISO 9050:2003 standard point 3.5.4. (measured at the surface of the glass, with an angle of incidence of 8° with respect to the normal, air mass of 1.5, solar irradiance spectrum given in ASTM G173 'direct and circumsolar') greater than or equal to >82%, >84%, >85% and preferably > 86% for glass substrate having a clear glass compositions having a content of total iron expressed in total Fe20s, comprised between 300ppm and lOOOppm (300ppm < Fe20s < lOOOppm) by weight of total glass composition. Preferably the mirrors have an energy reflection greater than or equal to >90%, >92%, >93% or >94% for glass substrate having an extra-clear glass composition having a content of total iron expressed in total Fe20s, is comprised between 20ppm and lower than 300ppm (20ppm < Fe20s < 300ppm) by weight of total glass composition.

[0045] According to a preferred embodiment of the invention, the silver layer has silver grains with an average size of between 10 nm and 200 nm, preferably between 20 nm and 120 nm. This average grain size can be determined by observing the surface of the silver layer using SEM-FEG (scanning electron microscope with field emission guns).

[0046] As described in patent application WO2013 / 057256 published on April 25, 2013 by AGC Glass Europe, on page 3 line 14 to page 5, line 7, incorporated herein by reference: a preferred mirror for the PRC system of the present invention, is a mirror comprising a glass substrate covered with a layer of silver, itself covered with at least one layer of paint, in which the intensity ratio of the crystallographic orientations within the silver layer is less than 5.0, wherein the silver layer has a correlation length (CLz), measured by X-ray diffraction using the Scherrer method, greater than 27.0nm, preferably greater than 28.0nm, more preferably greater than 30.0nm.

[0047] Mirrors can further comprise additional layers such as sensibilisation layer(s), activation layers), passivation layer(s), protective layer(s), silanisation layer(s) and / or paints layer(s).

[0048] Activating the glass surface on which the silver layer is to be deposited typically contributes to the ageing and / or corrosion resistance of the mirrors and / or to their durability. Such material(s) may be selected from the group of elements consisting of bismuth, chromium, gold, indium, nickel, palladium, platinum, rhodium, ruthenium, titanium, vanadium and zinc. Palladium and / or ruthenium is generally preferred.

[0049] In some embodiments, the silver layer can be treated with a silane before the paint is applied. The presence of traces of silane on the surface of the silver layer on the side of the paint layer(s) can contribute to the mirror's resistance to mechanical stress and / or corrosion. However, in preferred embodiments of mirrors of the present invention, no activation layer is used and even less palladium is used since it has been found that palladium decreases significantly the energetic reflection. It has been further found that passivation and silane layers are in general not required in preferred embodiments wherein protection is provided by a protective layer, preferably by a copper layer.

[0050] In a preferred embodiment, the mirror to be used on the PRC system of the present invention can comprise a sensibilisation layer, a protective layer and / or paintlayer(s). When present, the sensibilisation layer is comprised between the glass substrate and the silver layer. When present, the at least one protective layer is deposited on the at least one silver layer. When present, the at least one paint layer is deposited on the at least one silver layer or preferably on the at least one protective layer, if present.

[0051] Preferably, the mirror of the present invention comprises a sensibilisation layer. Preferably, the sensibilisation layer is a Tin layer that can be further deposited during a step of sensitising the surface of the glass substrate on which the silver layer is to be deposited. Sensibilisation can contribute to the good adhesion of the silver layer to the glass substrate.

[0052] Preferably, the mirror to be used in the PRC system of the present invention comprises a protective layer, more preferably a copper layer. Copper is preferably deposited in an amount equal to greater than >200 mg / m2, equal to greater than >250 mg / m2, more preferably equal to greater than >400 mg / m2and preferably equal to lower than <750mg / m2, more preferably equal to lower than <600mg / m2. This represents thickness between 130 and 700 nm. The protective layer is found to protect against UV and therefore enhance the durability of the mirror.

[0053] Preferably, the mirror to be used in the PRC system of the present invention comprises at least one paint layer, preferably at least 2 or 3 paint layers. Typical paints may be of the acrylic, epoxy, alkyl or polyurethane type. They can be applied, for example, by roller or curtain. The paint covering the silver layer can be deposited in a single step, resulting in a single layer of paint, or in several steps, resulting in two or three layers of paint. When several layers of paint cover the silver, they may be of identical or different compositions. Typically, a base coat and a top coat paints are used. The paint layer has generally a thickness equal to or greater than >20pm, equal to or greater than >22pm, preferably equal to or greater than >25pm and generally a thickness equal to or lower than <40pm, equal to or lower than <38pm, preferably equal to or lower than <35pm. The paint covering the silver layer is preferably lead- free or substantially lead-free. This can be beneficial to the environment.'Substantially lead-free' means that the lead content of the paint is significantly lower than the lead content of lead-containing paints commonly used in the manufacture of mirrors. The lead content of a substantially lead-free paint as defined herein is less than 500 mg / m2, preferably less than 400 mg / m2 or even more preferably less than 300 mg / m2. The lead content of a lead-free paint as defined herein is less than 100 mg / m2, preferably less than 80 mg / m2 or even more preferably less than 60 mg / m2.

[0054] The combination of the protective copper layer and of the paint layers is preferred to provide the mirror with acceptable aging characteristics and sufficient corrosion resistance.

[0055] The sensibilisation tin layer deposited on the surface of the glass, are preferably deposited as 'islands'. 'Islands' means that the materials deposited, do not form a distinct and continuous layer, but are found discontinuously on the surface they treat.

[0056] Figure 2 represents one preferred embodiment of the present invention wherein the radiative heat exchanger (11) has the following structure to increase its reflective properties and its durability. From the radiative surface (A), it comprises in the following order: a glass substrate (1), Tin as sensibilization layer (2), a silver reflective layer (3), a copper protective layer (4) and 2 layers of paints (5, 5b); the last paint layer (5b) representing the contact surface (A'). Preferably the glass substrate has a clear glass composition and more preferably an extra clear glass composition and therefore a total iron content, expressed as Fe20s, comprised between 20ppm and <300ppm; thereby reducing the absorbing effect of the glass with respect to solar radiation. Preferably the thickness of the glass substrate is comprised between 0.5mm and 4mm, preferably is 1mm. One embodiment of a preferred mirror is the following : a glass substrate having an extra clear composition / a sensibilisation Sn layer / a silver layer Ag of 1350mg / m2 / a protective layer Cu of 450mg / m2 / first base coat layer of 30pm of acrylic paint / second top coat layer of 30pm of polyurethane paint.Maximisation of reflectance properties

[0057] In a preferred embodiment, to maximise reflectance of mirrors, and therefore to minimise the sunlight absorption by the glass substrate, one can work on the thickness of the glass substrate and / or on the composition of the glass substrate.

[0058] Conventional thicknesses for mirrors are comprised between >0.8mm and <6mm. For applications requiring curved reflectors; mirrors have typically a thickness equal to or greater than > 0.8mm, preferably > 0.9mm or more preferably > 1.1 mm and / or a thickness equal to or lower than < 1.5 mm; providing a typical thickness of about 0.95mm or 1.25mm. For applications requiring flat reflectors, mirrors have a thickness equal to or greater than > 2.0mm or preferably > 2.5mm and / or a thickness equal to or lower than < 6.0mm or preferably < 5.0mm. Indeed, decreasing the thickness of the glass substrates reduce light absorption since it minimises the optical path and thus interactions between the light and the glass material.

[0059] Hence, in a preferred embodiment of the present invention, the thickness of the glass substrate is equal to or lowerthan 4.0mm, preferably equal to or lower 3.0mm, preferably equal to or lower 2.0mm, and more preferably equal to or lower 1.5mm. In a preferred embodiment of the present invention, the glass substrate has a thickness equal to or greater than 0.5 mm, preferably equal to or greater than 0.8 mm and more preferably equal to or greater than 1.0 mm. Such a thin glass allows to decrease the optical path of solar radiation but remains sufficiently thick for robustness.

[0060] The g[a_s_s_com ositio_n is chosen to have a lowest possible sunlight absorbance. Absorbance is conventionally measured by optical method on devices such as Perkin- Elmer instruments, as commonly practiced by persons skilled in that art. It is preferred that the absorbance of the glass is at most <4%, preferably at most <3% in selecting glass raw material of low absorbance such as Iron. Indeed, not all the light wavelengths are absorbed in a same way. As commonly understood by person in the art, glass with conventional composition are transparent to visible light but not toUV light under 300nm and infrared above 3500nm. The components of the glass composition can therefore be carefully selected to provide a glass composition demonstrating selective sunlight absorbance. Such glass composition can therefore be formulated in a way to avoid heating of the material and help in solving the problem of heating through day light. Advantageously, the composition is tailored to limit the absorption of solar radiation comprised between 300 and 2500 nm. Therefore, attention should be paid to minimise the amount of iron and especially the amount of the FeO specie, as well as the amount of chrome, cobalt, nickel, selenium, copper, vanadium and / or magnesium.

[0061] Light absorption is minimized for glass composition having a low content of iron. Such glass composition are typically referred to as 'clear glass' or even 'extra clear glass', as defined herein under.

[0062] Typically, the glass composition is a soda-lime-silicate glass (SLS). SLS in the present invention are referred to in a broad sense and relate to any silicate glass which comprises the following components in weight percentage, expressed with respect to the total weight of glass (Comp. A). Preferably, the silicate glass composition (Comp. B) is a soda-lime-silicate-type glass with a base glass matrix of the composition comprising the following components in weight percentage, expressed with respect to the total weight of glass.

[0063] The glass composition comprises typically iron expressed as total Fe20s, at a level of 20ppm to 2000ppm based on the total weight of the glass composition.

[0064] For clear glass compositions, it is preferred that the content of total iron expressed in total Fe2C>3, is comprised between 300ppm and lOOOppm (300ppm < Fe20s < lOOOppm). Preferably, it is comprised at a level equal to or greater than 400ppm, 450ppm, 500ppm, 550ppm, 600ppm and even, 650ppm by weight of total glass composition. Preferably, total iron expressed in total Fe20s, is comprised at a level equal to or lower than 900ppm, 850ppm, 800ppm, and even, 750ppm by weight of total glass composition. More preferably, the glass composition is an extra clear glass composition since they favor good reflection values. For extra clear glass compositions, it is preferred that the content of total iron expressed in total Fe20s, is comprised between 20ppm and less than 300ppm (20ppm < Fe20s < 300ppm). Preferably, it is comprised at a level equal to or greater than 40ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm, and even, lOOppm by weight of total glass composition. Preferably, total iron expressed in total Fe20s3, is comprised at a level equal to or lower than 250ppm, 200ppm, 175ppm and even, 150ppm by weight of total glass composition.

[0065] In a preferred embodiment, the redox of the glass composition, expressed in FeO / Fe2C>3, is equal to or lower than 30% (FeO / Fe20s < 30%), preferably equal to or lower than 28% (FeO / Fe20s < 28%), preferably equal to or lower than 25% (FeO / Fe20s < 25%), preferably equal to or lower than 23% (FeO / Fe20s < 25%), more preferably equal to or lower than 20% (FeO / Fe20s3 < 20%). Preferably, the iron redox expressed in FeO / Fe20s, is equal to or greater than 15% (FeO / Fe20s > 15%), is equal to or greater than 15%. Indeed, the Fe2+iron specie absorbs more than the Fe3+iron specie.

[0066] Preferably, the soda-lime glass substrate has a visible light transmission, LTD1, equal to or greater than 90.0%, preferably equal to or greater than 91.0% at a glass and more preferably equal to or greater than 91.5% at a thickness of 1 mm. Preferably, the soda-lime glass substrate has a visible light transmission, LTD4, equal to orgreater than 88.5%, preferably equal to or greater than 89% at a glass and more preferably equal to or greater than 90.0% at a thickness of 4 mm. Indeed it has been found that higher is the light transmission, lesser is the sun energy absorptance by the glass substrate (at equivalent reflection). The glass substrate heats less, resulting is more cooling performance.

[0067] In present description and claims, to quantify the visible transmission (also called luminous transmission / transmittance orTL) of a glass sheet, one considers the visible transmission with illuminant D65 for a sheet thickness of 4 mm (LTD4) / 1 mm (LTD1) at a solid angle of observation of 2° (according to standard IS09050). The visible light transmission (TL LT) represents the percentage of radiation flux emitted between wavelengths 380 nm and 780 nm which is transmitted through the glass substrate.Radiative heat exchange Performance

[0068] In a preferred embodiment of the present invention, the PRC system has a free cooling power of at least 10W / m2(FCP > 10W / m2), preferably at least 25 W / m2(FCP > 25W / m2).

[0069] In a preferred embodiment of the present invention, the PRC system has a yearly cooling energy production of at least 300kWh / m2(YCEP > 300kWh / m2), preferably at least 1000kWh / m2(YCEP > 1000kWh / m2), more preferably at least 2500kWh / m2(YCEP > 2500kWh / m2), and still more preferably at least 5000kWh / m2(YCEP > 5000kWh / m2).

[0070] Free cooling power and yearly cooling energy production are calculated with the weather conditions corresponding to Belgium. FCP is measured under direct sunlight in summer, therefore with maximum sunlight irradiance around 900W / m2and clear sky conditions, it should correspond to the low performance side. Ambient temperature is considered to be 25°C. The YCEP is measured throughout the year and hence throughout the different weather conditions with a defined temperature of the liquid circulating in the convective heat exchanger.

[0071] The free cooling power, FCP, is a simulated value, calculated thanks to the well- known MatLab software (method A), using the formula below:

[0072] Wherein C is the radiated power toward deep space. C is calculated as the integral over the wavelength of 300nm to 25pm of B = emissivity of the tested product with o = the black body radiation (expressed in W / m2 / nm) and Tatm= the atmosphere transparency.

[0073] Wherein H is the heating power through solar irradiance and is calculated as the integral over the wavelength of 300nm to 2500nm of S = the solar irradiance of the tested product expressed in W / m2 / nm with a the absorption of the radiative heat exchanger.

[0074] Every value is normalized per unit of surface.

[0075] The free cooling power FCP represents the ability of an outdoor horizontal surface to evacuate heat (positive FCP) or to generate heat (negative FCP) under direct sunlight exposure at ambient temperature. If the value of the radiated power toward deep space (C) is higherthan the value of heating powerthrough solar irradiance (H), the radiative surface of the radiative heat exchanger emits more energy toward deep space than it receives from the sun and thus provide cooling.

[0076] The yearly cooling energy is another simulated value, calculated thanks to the well- known EnergyPlus software (method B). The yearly cooling energy represents the average value integrating all (Belgian) weather conditions during a full year. The simulation calculates the energy lost by an outdoor horizontal surface fixed temperature (30°C) over the whole year and therefore takes into account the real averaged climatic conditions (sun, temperature, clouds,...).Making Process

[0077] The radiative heat exchanger is typically a reflective stack deposited on the glass substrate, that comprises at least a silver layer. The silver layer may be deposited by any possible means, as wet coating or magnetron sputtering, for example. Preferably, the silver layer is made through a silvering method involving a very well- known wet chemical processes.

[0078] Mirrors can indeed be manufactured by physical vapor deposition ("PVD"). However, PVD mirrors have the disadvantages of a more complex and expensive process and generally do not show sufficient durability. Therefore, most mirrors are produced by wet-chemistry processes.

[0079] Mirrors can be manufactured by wet chemistry processes. One embodiment is herein described: For example, on a mirror production line, the glass sheets are usually transported along the line by roller conveyors. They are first polished and rinsed before being sensitized, for example by means of a solution of tin chloride sprayed on the glass; they are then rinsed again. An activating solution is then sprayed onto the glass sheets; this solution can be, for example, an acidic aqueous solution of PdCl2. The glass sheets then pass through a rinsing station where demineralized water is sprayed and then through the silvering station where a traditional silvering solution is sprayed, this solution being the result on the surface of the glass of a combination of two separately sprayed solutions, one comprising a silver salt and either a reducing agent or a base, the other comprising either the reducing agent or the base which is absent from the solution comprising the silver salt. The flow rate and concentration of the silvering solution sprayed onto the glass are controlled to form a silver layer of the desired thickness. The glass is then rinsed and immediately afterwards an aqueous solution of, for example, SnCl2 is sprayed onto the glass sheets as they move along the conveyor. After another rinse, the mirrors can be treated by spraying with a solution containing a silane. After a final rinse, the silvered glass sheets enter a drying station. The mirrors are then covered with one or more layers of paint. Each layer of paint is baked or dried before anyother layer of paint is applied, for example in a tunnel oven. Preferably, the paint is applied to the silver substrates as a continuous curtain of liquid paint falling onto the glass sheets.

[0080] In another embodiment, after the deposition of the silver layer, the glass is rinsed and immediately afterwards, a copper salt and a reducing agent are sprayed to form a copper layer on the surface of the glass. After another rinse, the silver and copper glass sheets enter a drying station, and the process continues with the deposition of one or more layers of paint.

[0081] A suitable process to produce preferred mirrors is adapted from the above proves and described in patent application WO2013 / 057256 published on April 25, 2013 by AGC Glass Europe, on page 8 line 14 to page 9, line 5; incorporated herein by reference.

[0082] Hence, a preferred mirror for the PRC system of the present invention is obtained as such: After sensibilisation of the glass surface, the silver is deposited by reduction of an ammoniacal silver nitrate solution and results in a silver reflecting layer comprising an amount of silver equal to or greater than 700 mg / m2, equal to or greater than 800 mg / m2, equal to or greater than 1000 mg / m2, equal to or greater than 1200 mg / m2, equal to or greater than 1400 mg / m2, equal to or greater than 1500 mg / m2. The silver layer is then covered with a protective copper layer and finally one or more paint layers are deposited for the protection and durability of the mirror. The copper layer is protecting the paints layers against UV and is deposited to obtain an amount of cupper equal to or greater than >250 mg / m2, preferably equal to or greater than >400 mg / m2. Finally a single or multiple paint coating is deposited above the silver and copper layers. The paint layer may be any kind of polymeric coverage capable of protecting the silver layer as for example an acrylic, epoxy, alkyd, polyethylene or polyurethane based polymer. Please refer to examples 1, 2 and 3 described in patent application WO2013 / 057256 published on April 25, 2013 by AGC Glass Europe, on page 9 line 14 to page 10, incorporated herein by reference.

[0083] According to the invention, the heat exchange between the contact surface (A') of the radiative heat exchanger (11) and the heat transfer fluid of the convective heat exchanger (12) may be achieved through different non limiting embodiments illustrated in Figures 3 to 5.

[0084] Through the passive radiation cooling process and through convection with the cooler ambient temperature, the contact surface of the radiative heat exchanger is colder than the heat transfer fluid and therefore allows to cool the heat transfer fluid within the convective heat exchanger. The passive radiative cooling system of the present invention, offers an effective, low energetic consumption and sustainable cooling technology, effective both during day and night.

[0085] The objective of the convective heat exchanger (12) is to maximise thermal conductivity between the radiative heat exchanger (11) and the heat transfer fluid. The convective heat exchanger typically comprises a inlet port, an outlet port and a fluid path arranged between the inlet port and the outlet port. The heat transfer fluid enters the inlet port at a first temperature and exits the outlet port at a second temperature lower than the first temperature.

[0086] In some embodiments, the PRC system can further include a control system configured to control at least one operating parameter such the flow rate of the heat transfer fluid in the fluid path, the temperature of the heat transfer fluid entering the inlet port, and a temperature of the fluid exiting the outlet port. The PRC system can further comprise control valves for fluid flows controls. In some embodiments, the PRC system includes a mechanism configured to change the angle of the PRC system. The PRC system can be oriented at an angle from the sun, such as between 5 and 20 degrees, inclusive; and preferably between 7 and 12 degrees, inclusive. Typically, the radiative heat exchanger could be titled with an angle facing the northin the northern hemisphere and with an angle to the south in the southern hemisphere, to minimise sunlight. The heat transfer fluid may flow passively or actively. In some embodiments, the PRC system can further comprise a pump for pumping the heat transfer fluid through each fluid path of the plurality of cooling panels, such as a thermosyphon.

[0087] In one embodiment, the present invention relates to a cooling system that comprises a plurality of PRC systems, wherein the fluid paths of the plurality of the multiple PRC systems are coupled. In some embodiments of the cooling systems, the plurality of PRC systems are arranged in an array. They can be arranged in parallel such that each inlet port of the plurality of PRC system is coupled together and each outlet port of the plurality of PRC system is coupled together. Each inlet port receives a fluid at an inlet temperature, and each outlet port outputs the fluid at an outlet temperature less than the inlet temperature.

[0088] According to a first embodiment of the invention, the heat transfer fluid is in direct contact to the contact surface of the mirror. As shown on the Figure 3, the convective heat exchanger (12) comprises a hollow enclosure designed to allow the circulation of the heat transfer fluid inside the enclosure. The hollow enclosure is limited by side vertical walls (C, D, E and F) being normal to the contact surface and a bottom horizontal wall (B) parallel to the contact surface. The enclosure is closed by the contact surface of the radiative heat exchanger (A'). In this embodiment, the heat transfer fluid flows in direct contact to the contact surface of the radiative heat exchanger. In such embodiment, the contact surface being typically a paint layer, should be designed to resist mechanically and chemically to the heat transfer fluid. In such case, the paint layer is preferably chosen from epoxy, polyurethane, acrylic or alkyl based polymer.

[0089] Figure 3 is very schematic and both entrance or exit could have any other position within the vertical walls, any size or any form. The hollow enclosure can comprises more than one entrance and / or more than one exit for the fluid circulation. The position, number, size and form of the entrance(s) and exit(s) will define the flowpath of the heat transfer fluid and can be designed to maximize the heat exchange. Preferably, the hollow enclosure further comprises baffles to force some fluid path and therefore increase the heat exchange.

[0090] In the above described embodiment, a heat conductive plate may be added to the contact surface of the radiative heat exchanger to improve the efficiency of the heat exchange.

[0091] According to a second embodiment , the liquid heat exchanger circulates within a pipe of an exchanger plate. As illustrated in Figure 4 and 4b, the convective heat exchanger (12) is a heat exchanger plate (7) comprising a pipe (8). Preferably, the pipe is made of a heat conducting material such as copper, more preferably the pipe is a copper serpentine welded to a thermal conductive plate as illustrated in Figure 4(b). More particularly, the thermal conductive plate is an aluminium or copper plate. The heat conductive plate (7) is in contact to the contact surface (A') of the radiative heat exchanger.

[0092] According to a third embodiment, the convective heat exchanger (12) is a 'roll bond' panel wherein heat transfer fluid is heat transfer within a patterned circuit as illustrated in Figure 5. Roll bonding is a process that uses hot and cold rolling to manufacture absorbers in which the heat transfer fluid circuit is directly integrated into the panel. When the absorber is made of aluminium, heat transfer is very important. In particularly, the "roll bond" panel (9) can be made by an embossing press on a first aluminium plate which is then stacked to a second non-embossed planar aluminium plate. The planar plate of the roll bond panel is in contact with the contact surface of the mirror.

[0093] According to any embodiment of the invention, the heat transfer fluid is not limited and can be any suitable heat transfer fluid. The heat transfer fluid is typically water that can be mixed with a corrosion inhibitor and / or a fluid to prevent freezing. Preferably, the heat transfer heat transfer fluid is chosen from water, water with glycol and / or or water with propanol. The convective heat exchanger is designed toprovide liquid / waterproofness and air / tightness, mechanical stability, resist to the heat transfer fluid pressure.

[0094] The embodiments illustrated in Figures 3 to 5, are by no way a limitation in the design of the hollow enclosure, heat exchanger plate or roll bond panel. For example, pipe and embossing may have any design to fulfil and even improve the heat exchange.Durability

[0095] The PRC system of the present invention is designed to be exposed to outdoor conditions and therefore should demonstrate enough durability. This is particularly critical for the radiative surface of the mirror that is indeed facing the sky.

[0096] Therefore, the mirrors of the PRC system of the present invention must pass long term evaluation and meet the chemical and mechanical specifications of the invention. For chemical resistance such as corrosion, the mirror must be conform to the tests described in the norm ISO 9227-2017, in the norm ISO 6270-1 (1998), in the norm ISO 6270-2 (2005) and in the norm ASTM D4587 (2011). The mechanical resistance is assessed by the tests described in the norm ISO 2409 (2013) and ISO 7784-2 (1997).

[0097] The PRC systems of the present invention are typically used to reject energy as heat supplied by a refrigeration cycle, a cooling jacket of equipment, an air conditioning system, a thermal reservoir, a coolant conditioning system (e.g., such as an intermediate cooling loop, a system for providing coolant to multiple systems), and / or any other suitable cooling load. For example, a coolant conditioning system may be configured to provide coolant at predetermined conditions (e.g., values of temperature, pressure, enthalpy, flow rate, density, phase(s), or other properties) for cooling. Most common applications cooling systems for data centres, refrigerating systems in the food industry, ...

[0098] The present invention further relates to the use of a heat exchanger comprising a glass substrate having at least a silver reflective layer comprising at least >700mg / m2of silver, preferably providing a regular luminous coefficient equal to or greater than 86% for a thickness between 2mm and 6mm, as a heat radiative exchanger within a passive radiative cooling system. All preferred embodiments described above, apply to the radiative heat exchanger in its use.Definitions- As used herein, the term comprising or comprise, are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in the specification and claims, the terms "comprises" and "comprising" and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.Forthe sake of clarity, when using terms like "under", "below", "above", "lower", "upper", "first" or "last" herein, it is always in the context of a sequence of layers starting from the glass below, going upward, further away from the glass. Such sequences may comprise additional intermediate layers, in between the defined layers, except when a direct contact is specified.Further, as used herein, the terms "deposited over", "provided over", "in contact" or "facing" mean deposited over, provided on, in contact or facing but not necessarily in surface contact with. For example, a coating "deposited over" a substrate does not preclude the presence of one or more other coating films of the same or different composition located between the deposited coating and the substrate.Unless otherwise defined, all technical and scientific terms used herein are intended to have the same meaning as commonly understood to one of ordinary skill in the artExamples

[0099] The invention will be illustrated by some examples but one must understand that those examples are by no way limiting the scope of the invention.

[0100] The performance of the PRC system has been tested by measuring the temperature of several radiative heat exchangers that have been placed under direct sunlight during summer in Belgium. The temperature is measured on the contact (bottom) surface of the radiative heat exchanger; i.e. the surface opposite to the sunlight. The following radiative heat exchangers have been tested :

[0101] Examples of the invention are :Example 1 : The radiative heat exchanger is a mirror having the following structure: Glass (1 mm) / Ag (125 nm) / Cu (520 nm) / acrylic paint (27 pm) / Polyurethane paint (27 pm). The silver layer comprises 1350mg / m2of silver and the luminous coefficient according to Norm EN410 is 95.2%Example 2 : The radiative heat exchanger is a mirror having the following composition: Glass (4 mm) / Ag (125 nm) / Cu (520 nm) / acrylic paint (27 pm) / Polyurethane paint (27 pm). The silver layer comprises 1350mg / m2of silver and the luminous coefficient according to Norm EN410 is 93.0%.Comparative examples are :- Example 3 : The radiative heat exchanger is a white paint on a glass substrate of 4mm.- Example 4 : The radiative heat exchanger is a grey paint on a glass substrate of4mm.- Example 5 : The radiative heat exchanger is a Lacobel white (White organic paint on a glass substrate of 4mm ) as commercially available from AGC Glass Europe.

[0102] In all examples of the invention and comparative examples, the glass substrate has the following extra clear composition by weight of the total composition: 72% SiO2, 7.9% CaO, 0.015% de K20, 13.9% de Na2O, 88ppm of Fe2O3, 1% AI2O3, 4.5% MgO and minors.Figure 1

[0103] Figure 1 shows the temperature profiles (Expressed in °C) versus time (8:24 to 22:48), of examples 1 and 2 of the present invention and of comparative example 5, versus ambient air temperature in Anderlues (Belgium) on 21 august 2023.

[0104] The experiment consisted of having 3 radiative heat exchangers placed horizontally under direct sunlight exposure. The temperature of each contact surface were monitored by thermos-couples. The objective is to measure the temperature since a lower than ambient temperature demonstrates a positive FCP and therefore effective radiative cooling whereas a temperature above the ambient temperature demonstrate a negative FCP and therefore no radiative cooling.

[0105] As seen on Figure 1, the temperature profiles of the mirror of the present invention are below the ambient air temperature, even during the sunny conditions of day time. This also shows that thinner glass substrate provides improved coolingpotential since thinner glass substrate absorbs less light. After about 6PM, when the sun has disappeared due to shading (night cooling), the performance of the comparative example becomes comparable to the mirrors of the present invention. In contrast during day time, the thermal absorbance of the glass substrate of the comparative radiative heat exchanger is too high as indicated by the glass substrate's temperature greatly superior to the ambient air temperature.Table 1

[0106] Table 1 summarizes the performance obtained by the radiative heat exchangers of the two examples 1 and 2 of the present invention and two comparatives examples 3 and 4 : reflectance, emissivity and cooling capability were measured and calculated. The free cooling power calculated as per method described above assesses the cooling capability of the radiative surfaces during a sunny day representing the least favourable conditions. The yearly cooling energy calculated as per method B described above, represents the mean value integrating all the yearly conditions.

[0107] The Optical characteristics of Reflectance (%) between 380 to 2500 nm are measured in accordance the norm IS09050 (2003) adapted with solar spectrum from norm ASTM G173 (2003). Emittance is calculated according to norm EN12898. Measures are realized on a Perkin-Elmer instrument.

[0108] Results in Table 1 below clearly shows that comparative examples wherein the white or grey paint is used as the light reflective layer, are not effective technologies to provide passive radiative cooling during the day because heat absorption by the glass substrate is too important. The grey paint is even less reflective and therefore the least performant. Indeed, the reflectance percentage is 30%-70% respectively versus the 93.6% - 96.5% for the examples of the present invention. This is translated into a negative free cooling power.

[0109] The results also show that by decreasing the thickness, the absorption decreases and less thermal energy is absorbed, resulting in a more efficient cooling performance.Please refer to the example 1 of the present invention wherein the thickness of the glass substrate has been decreased from 4mm to 1mm, providing a reflectance of 96.5% versus 93.6% and a free colling power of 25W / m2versus 10W / m2.

[0110] Table 1Table 2

[0111] The mirrors of both examples of the present invention have been tested for chemical and mechanical durability according to the mentioned norms. Both exemplified mirrors passed all tests, demonstrating excellent durability.

[0112] The chemical durability is evaluated with the norms ISO 9227-2017, ISO 6270-1 (1998), ISO 6270-2 (2005) and ASTM D4587 (2011). The mechanical durability is assessed with the norms ISO 2409 (2013) and ISO 7784-2 (1997)

[0113] Table 2

Claims

CLAIMSClaim 1. A passive radiative cooling system comprising : a. a radiative heat exchanger comprising a glass substrate having a first surface and a second opposite surface; wherein i. the first surface is a radiative surface, ii. the second surface is a coated surface, being provided with at least one silver reflective layer comprising at least >700mg / m2of silver; b. a convective heat exchanger comprising a heat transfer fluid, said convective heat exchanger faces the coated surface.Claim 2. The passive radiative cooling system according to claim 1 wherein the radiative heat exchanger provides a regular luminous coefficient equal to or greater than 86%, for a thickness between 2mm and 6mm.Claim 3. The passive radiative cooling system according to any one of the previous claims wherein the silver reflective layer comprises silver in an amount equal to or greater than 800 mg / m2, equal to or greater than 1000 mg / m2, equal to or greater than 1200 mg / m2, equal to or greater than 1400 mg / m2, equal to or greater than 1500 mg / m2.Claim 4. The passive radiative cooling system according to any one of the previous claims wherein the radiative heat exchanger further comprises at least a protective layer and / or at least a paint layer; wherein the at least one protective layer is deposited on the at least one silver layer and the at least one paint layer is deposited on the at least one silver layer or preferably on the at least one protective layer, if present.Claim 5. The passive radiative cooling system according to claim 4 wherein the radiative heat exchanger comprises at least a protective layer, preferably the protective layer is a copper layer, and more preferably copper is deposited in an amount equal to or greater than >250 mg / m2, preferably equal to or greater than >400 mg / m2.Claim 6. The passive radiative cooling systems of any one the claims 4-5 wherein the radiative heat exchanger further comprises at least a paint layer, preferably at least two paint layers; more preferably in addition to a protective layer.Claim 7. The passive radiative cooling system according to any one of the previous claims wherein the glass substrate has a thickness equal to or greater than 0.5 mm, preferably equal to or greater than 0.8 mm and more preferably equal to or greater than 1.0 mm; and / or a thickness of equal to or lower than 4 mm, preferably equal to or lower than 3 mm and more preferably equal to or lower than 1.5 mm.Claim 8. The passive radiative cooling system according to any one of the previous claims wherein the glass substrate is a soda lime silicate glass.Claim 9. The passive radiative cooling system according to any one of the previous claims wherein the glass substrate comprises a total iron content expressed as Fe20s of 20ppm to 2000ppm (20ppm < Fe20s < 2000ppm), based on the total weight of the glass composition.Claim 10. The passive radiative cooling system according to claim 9 wherein the content of total iron expressed in total Fe20s, is comprised between 300ppm and lOOOppm (300ppm < Fe2C>3 < lOOOppm); preferably, it is comprised at a level equal to orgreater than 400ppm, 450ppm, 500ppm, 550ppm, 600ppm and even, 650ppm by weight of total glass composition and / or preferably, the total iron expressed in total Fe20s, is comprised at a level equal to or lower than 900ppm, equal to or lower than 850ppm, equal to or lower than 800ppm, and even, equal to or lower than 750ppm by weight of total glass compositionClaim 11. The passive radiative cooling system according to any one of the previous claim 9 wherein the content of total iron expressed in total Fe20s, is comprised between 20ppm and less than 300ppm (20ppm < Fe20s < 300ppm); preferably it is comprised at a level equal to or greaterthan 40ppm, preferably equal to or greaterthan 50ppm, preferably equal to or greater than 60ppm, preferably equal to or greater than 70ppm, preferably equal to or greater than 80ppm, preferably equal to or greaterthan 90ppm, even, equal to or greater than lOOppm by weight of total glass composition and / or is comprised at a level equal to or lower than 250ppm, preferably equal to or lower than 200ppm, preferably equal to or lower than 1750ppm and even, more preferably equal to or lower than 150ppm by weight of total glass composition.Claim 12. The passive radiative cooling system according to any of the previous claims 8 to 11, wherein the redox of the glass composition, expressed in FeO / Fe2O3, is equal to or lower than 30% (FeO / Fe2O3 < 30%), preferably equal to or lower than 28% (FeO / Fe2O3 < 28%), preferably equal to or lower than 25% (FeO / Fe2O3 < 25%), more preferably equal to or lower than 23% (FeO / Fe20s3 < 23%) and even, more preferably equal to or lower than 20%.Claim 13. The passive radiative cooling system according to any of the previous claims wherein the glass substrate at a thickness of 4mm, has a visible light transmission, LTD4, equal to or greater than 88.5%, preferably equal to or greater than 89.0% preferably equal to or greater than 90.5%.Claim 14. The passive radiative cooling system according to any of the previous claims wherein the glass substrate at a thickness of 1mm, has a visible light transmission, LTD1, equal to or greater than 90.0%, preferably equal to or greater than 91.0% preferably equal to or greater than 91.5%.Claim 15. The passive radiative cooling system according to any of the previous claims, having a free cooling power equal to or greater than 10 W / m2(FCP > 10W / m2), preferably equal to or greater than 25 W / m2(FCP > 25W / m2).Claim 16. The passive radiative cooling system according to any of the previous claims having a yearly cooling energy production equal to or greater than 300kWh / m2(YCEP > 300kWh / m2), preferably equal to or greater than 1000kWh / m2(YCEP > 1000kWh / m2), more preferably equal to or greater than 2500kWh / m2(YCEP >2500kWh / m2), and still more preferably equal to or greater than 5000kWh / m2(YCEP > 5000kWh / m2).Claim 17. Use of a glass substrate having at least a silver reflective layer comprising at least >700mg / m2of silver, preferably providing a regular luminous coefficient equal to or greater than 86% for a mirror having a thickness between 2mm and 6mm, as a heat radiative exchanger within a passive radiative colling system.

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