Radiative heat exchanger with diffuse reflection

Abstract

The present invention relates to a radiative heat exchanger comprising a radiative surface facing the sky. The majority of the radiative surface has been roughened to provide a roughened zone having a Mean Width of the Profile Elements, Rsm, equal to or greater than 15µm (Rsm ≥ 15µm); and a Mean Height of the Profile Elements, Rc, equal to or greater than 0.25µm (Rc ≥ 0.25µm). Rsm and Rc are measured on an evaluation length of 4.0 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm.

Description

RADIATIVE HEAT EXCHANGER WITH DIFFUSE REFLECTIONTechnical Field

[0001] The present invention relates to the field of passive radiative cooling systems. In particular it relates to radiative heat exchangers with diffuse reflection.Prior Art.

[0002] With climate change, the demand for cooling increases and represents an important energy consumption. Therefore, less energy consuming systems such as passive radiative cooling are currently developed without external energy sources.

[0003] Passive radiative cooling is based on the principle of the 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.

[0004] Currently main technologies for radiative heat exchanger involves typically either an emissive paint with micro particles that both reflect the sun light and emit infrared, or a stack with a transparent but highly emissive layer on top of reflective surface, as for example a metallic layer.

[0005] The more reflective is the radiative heat exchanger, the better it is for the passive radiative cooling performance during day light. However, one drawback of the excellent sunlight reflection is that it can cause glare. Texturing could reduce glare. However, it has been found that texturing the radiative surface of the radiative heat exchanger is not an adequate solution since it results in an increase of the optical path of sunlight inside the exchanger. Texturing therefore increases the absorption of sunlight and so reduce the radiative cooling performance of the radiative heat exchanger.

[0006] Therefore, there is still a need in the art to provide a radiative heat exchanger that not only provides highly performant passive radiative cooling performance, but as well avoid light pollution while remaining cost effective and easy to produce.Summary of invention

[0007] The present invention relates to a radiative heat exchanger comprising a radiative surface facing the sky, wherein the majority of the radiative surface has been roughened to provide a roughened zone having: a) a Mean Width of the Profile Elements, Rsm, equal to or greater than 15pm (Rsm > 15pm), preferably equal to or greater than 35pm (Rsm > 35pm), preferably equal to or greater than 42pm (Rsm > 42pm), preferably equal to or greater than 50pm (Rsm > 50pm), more preferably equal to or greater than 60pm (Rsm > 60pm). Preferably, the roughened zone has a Mean Width of the Profile Elements, Rsm equal to or lower than 150pm (Rsm < 150pm), equal to or lower than 120pm (Rsm < 120pm), preferably equal to or lower than 100pm (Rsm < 100pm), preferably equal to or lower than 90pm (Rsm < 90pm); and b) a Mean Height of the Profile Elements, Rc, equal to or greater than 0.25pm (Rc > 0.25pm), preferably equal to or greater than 0.5pm (Rc > 0.50pm), more preferably equal to or greater than 1.00pm (Rc > 1.00pm). Preferably, the roughened zone has a Mean Height of the profile elements, Rc, equal to or lower than 8.00pm (Rc < 8.00pm), preferably equal to or lower than 5.50pm (Rc < 5.50pm), preferably equal to or lower than 3.00pm (Rc < 3.00pm), more preferably equal to or greater than 2.00pm (Rc < 2.00pm).Rsm and Rc are measured on an evaluation length of 4.0 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm.

[0008] The radiative heat exchanger of the present invention is preferably a 'mirror' and therefore comprises a glass substrate having a first surface being the radiative surface and a second opposite surface. The second surface is a coated surface, being provided with at least one metallic reflective layer, preferably aluminium and / or silver. Preferably, the metallic reflective layer is a silver reflective layer comprising silver in an amount equal to or greater than 700mg / m2of silver; preferably equal to or greater than 800mg / m2, preferably equal toor greater than 1000mg / m2, preferably equal to or greater than 1200mg / m2, preferably equal to or greater than 1400mg / m2, more preferably equal to or greater than 1500mg / m2. It preferably 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 metallic reflective layer and the at least one paint layer is deposited on the at least one metallic reflective layer or preferably on the at least one protective layer, if present. Typically a protective layer is present and preferably the protective layer is a copper layer, and more preferably copper is deposited in an amount equal to or greater than 250mg / m2, preferably equal to or greater than 400mg / m2. Typically, at least a paint layer, preferably at least two paint layers are present; more preferably in addition to a protective layer.

[0009] Improved radiative heat exchanger can comprise the glass substrate at 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 4.0 mm, preferably equal to or lower than 3.0 mm and more preferably equal to or lower than 1.5 mm.

[0010] It is preferred that glass substrate is a soda lime silicate glass, preferably having a total iron content expressed as FejOa of 20ppm to 2000ppm (20ppm < FejOa < 2000ppm). In one embodiment, the glass composition of the glass substrate has a content of total iron expressed in total FejOa, comprised between 300ppm and lOOOppm (300ppm < FejOa < 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 FejOa, 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. In another embodiment, the glass composition comprises a content of total iron expressed in total FejOa, comprised between 20ppm and less than 300ppm (20ppm < FejOa < 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, morepreferably equal to or lower than 150ppm by weight of total glass composition. Typically, the redox of the glass composition, expressed in FeO / FejOa, is equal to or lower than 30% (FeO / FejOa < 30%), preferably equal to or lower than 28% (FeO / FejOa < 28%), preferably equal to or lower than 25% (FeO / FejOa < 25%), more preferably equal to or lower than 23% (FeO / FejOaS < 23%) and even, more preferably equal to or lower than 20%.

[0011] It is preferred that the radiative heat exchanger provides a regular luminous coefficient equal to or greater than 86%, for a thickness between 2mm and 6mm. For radiative heat exchanger based on glass substrate and metallic reflective layer, it is preferred that the radiative heat exchanger has a visible light transmission, LTD4, for the glass substrate at a thickness of 4mm, equal to or greater than 88.5%, preferably equal to or greater than 89.0% preferably equal to or greater than 90.5%. At a thickness of 1mm, it has preferably 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%.

[0012] The radiative heat exchanger aims at providing 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 / or 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).

[0013] The radiative heat exchanger comprises a radiative surface facing the sky and an opposite surface, referred to as the 'contact surface'. The present invention further relates to a passive radiative cooling system comprising a radiative heat exchanger as described above and a convective heat exchanger comprising a heat transfer fluid. The convective heat exchanger is in contact with the contact surface of the radiative heat exchanger.Brief description of drawings

[0014] 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 of the 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.

[0015] Figure 1 shows a cross sectional schematic view of a radiative heat exchanger according to one embodiment of the present invention.Detailed description of the invention

[0016] The present invention relates to the field of passive radiative cooling systems. Passive radiative cooling technology can also be herein after referred to by its acronym 'PRC'. 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 function of the convective heat exchanger is to cool down a heat transfer fluid. It comprises 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.

[0017] The radiative 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 radiative 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 - faces the sky, is herein after referred to as the 'radiative surface'. The opposite surface, i.e. the surface of the radiative heat exchanger facing with the convective heat exchanger is herein referred to as the 'contact surface'.

[0018] 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.

[0019] 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 transmitted through the radiative layer back to the exterior environment, preventing the sunlight to heat the convective heat exchanger.

[0020] The objective of the present invention is to design a radiative heat exchanger that provides improved passive radiative performance while avoiding glare. Indeed, the objective of the present invention is to provide a highly efficient passive radiative cooling system that can be used in any part of the world without restrictions, whatever the sunlight profile, type of habitat and population density, whether close or not to cities and airport and therefore that does not create any light pollution.

[0021] The glare of a light source, like the sun, on a reflective surface is representing the amount of light reflected by this surface in the direction of the observer and that can indeed cause visual discomfort if this amount of light is too important. On flat reflective surfaces, light reflection occurs in the specular direction i.e. the angle between the reflected light and the surface is the same than the angle between the incident light and the surface. Specular reflections of the sun on passive radiative cooling installations can cause visual disturbance and is herein referred to as glare. This phenomenon can be problematic for some human activities, in particular transport such as automotive mobility, aviation (pilot and air controllers), ... This phenomenon is even more pronounced for passive radiative cooling installations as they need to achieve very high sunlight reflection, and thus are typically much more reflective than other surfaces providing glare.

[0022] High luminosity can reduce visual performance by reducing the perception of contrast and / or blindness. Glare, can cause difficulties for drivers, pilots or controllers to perceive their environment. It depends on the position (distance and angular position) of the light source in relation to the eye, its apparent surface and its luminance. Also considered are the dangers induced by a surprise effect caused by the appearance of a light source in the field of vision. This 'surprise effect' is all the more pronounced when the glare is lateral in relation to axis, because the brain perceives the change in state (the glare) without immediately identify the cause. For example, visual disturbance located near aerodromes may disturb pilots during phases of flight close to the ground or hinder the smooth operation of the control tower, in particular within a 3 km radius.

[0023] It has been found that texturing the radiative surface can be used to reduce specular reflection and therefore light pollution. Even if texturing the radiative surface should not be an adequate solution since it increases of the optical path for the sunlight through the emissive part of the radiative exchanger to the reflective part and back to through theemissive part to the sky. Texturing therefore increase the absorption of sunlight and so reduce the radiative cooling performance of the radiative heat exchanger.

[0024] However, it has been surprisingly found by texturing the radiative surface of the radiative heat exchanger to provide specific roughening technical features, the unwanted increase in light absorption can be outperformed by an increased emissivity of the radiative surface and therefore, can increase the overall performance of the radiative heat exchanger.Radiative heat exchanger Performance

[0025] The radiative heat exchanger performance can be assessed by the Free Cooling Power (FCP) and by the Yearly Cooling Energy Production (YCEP). 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.

[0026] The free cooling power, FCP, is a simulated value, calculated thanks to the well-known MatLab software (method A), using the formula below:25nm 2500nmFCP [W / m2] = C — H = B o TatmdA — S a dA = 300nm = 300nm

[0027] 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.

[0028] 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 expressed in W / m2 / nm with a the absorption of the radiative heat exchanger.

[0029] Every value is normalized per unit of surface.

[0030] 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 sunlightexposure at ambient temperature. If the value of the radiated power toward deep space (C) is higher than the value of heating power through 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 provides cooling.

[0031] It has been found that texturing induces an unwanted increase in light absorption due to the increased optical path. It corresponds to the heating power H in the FPC equation above. Indeed, the sunlight power clearly outperforms the radiative power C. It means that a small increase of the absorption will hinder the radiative cooling performance of the whole system.

[0032] However, it has been surprisingly found that texturing the radiative surface to provide specific morphological properties characterised by (a) a Mean Width Of The Profile Elements, Rsm equal to or greater than 15pm and (b) a mean height of profile elements, Rc equal to or greater than 0.25pm; provides a positive increase in emissivity. It corresponds to the radiative power C in the FCP equation above. The increase in emissivity outperforms the increase in absorption so that the free cooling power of the radiative heat exchanger is increased.

[0033] 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 at a fixed temperature (30°C) over the whole year and therefore takes into account the real averaged climatic conditions (sun, temperature, clouds,...).

[0034] In a preferred embodiment of the present invention, radiative heat exchanger has a free cooling power of at least 10W / m2(FCP > 10W / m2), preferably at least 25 W / m2(FCP > 25W / m2).

[0035] In a preferred embodiment of the present invention, the radiative heat exchanger 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).Surface Roughness parameters

[0036] The present invention therefore relates to a radiative heat exchanger wherein its radiative surface is characterised by specific surface roughness parameters (Rsm and Rc) to provide both technical advantages of antiglare benefit and an improved radiative cooling performance. The terms "Roughening", "Texturing" and "etching" can be used herein interchangeably.

[0037] Roughness values are quantitative measures that characterize the surface texture of materials. These values are defined through several parameters that describe the microlevel variations on a surface's profile, impacting its functionality and interaction with other materials. The roughened surface can be characterized by different roughness values defined in the standard ISO 4287-1997. The texture is a consequence of the existence of surface irregularities / patterns. These irregularities consist of bumps called "peaks" and cavities called "valleys". On a section perpendicular to the textured surface, the peaks and valleys are distributed on either side of a "center line" (algebraic average) also called "mean line". In a profile and for a measurement along a fixed length (called "evaluation length").

[0038] Hence, the present invention relates to a radiative heat exchanger comprising a radiative surface facing the sky and an opposite contact surface, wherein at least part of the radiative surface has been roughened to provide a roughened zone having: a) Mean Width of the Profile Elements, Rsm, equal to or greater than 15pm (Rsm > 15pm), preferably equal to or greater than 35pm (Rsm > 35pm), preferably equal to or greater than 42pm (Rsm > 42pm), preferably equal to or greater than 50pm (Rsm > 50pm), more preferably equal to or greater than 60pm (Rsm > 60pm); wherein Rsm is measured on an evaluation length of 4.0 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm; and b) a Mean Height of the Profile Elements, Rc, equal to or greater than 0.25pm (Rc > 0.25pm), preferably equal to or greater than 0.50pm (Rc > 0.50pm), more preferably equal to or greater than 1.0pm (Rc > 1.00pm); wherein Rc is measured on an evaluation length of 4.0 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm.In the context of the present invention, the expression 'facing the sky' is to be understood as defining the intended operational orientation of the radiative surface. It specifies that the surface is arranged to have a clear line of sight to the sky, thereby enabling it to radiate thermal energy outwards, using the cold expanse of the upper atmosphere and outer space as a passive cold sink. This terminology defines a functional and orientational characteristic of the heat exchanger per se and does not imply that the sky itself is a component of the claimed apparatus. A person skilled in the art of thermal engineering would readily understand this term as referring to the device's configuration for effective radiative cooling, in a manner analogous to how other technical components are described by their orientation relative to their operational environment.

[0039] Preferably, the majority of the radiative surface is roughened. Preferably, at least 75%, at least 90% and even more preferably 100% of the radiative surface of the heat exchanger is roughened.

[0040] In a preferred embodiment for improved radiative cooling performance, the textured zone has a mean height of profile elements, Rc, equal to or lower than 8.00pm (Rc < 8.00pm), preferably equal to or lower than 5.50pm (Rc < 5.50pm), preferably equal to or lower than 3.00pm (Rc < 3.00pm), more preferably equal to or lower than 2.00pm (Rc < 2.00pm).

[0041] Preferably, the textured zone has a Mean Width of the Profile Elements, Rsm equal to or lower than 150pm (Rsm < 150pm), equal to or lower than 120pm (Rsm < 120pm), preferably equal to or lower than 100pm (Rsm < 100pm), preferably equal to or lower than 90pm (Rsm < 90pm).

[0042] The Mean Width of the Profile Elements, Rsm indicates the average value of the length of the profile element along the sampling length; Xsi is the length of a single profile element. The peaks (valley) that constitute elements have minimum height and length standards such that they will be treated as noise and considered a part of the preceding valley (peak) if the height (depth) is less than 10% of the maximum height or the length is less than 1% of the segment length.tSm.PSm.WSm nSXsiMean width of the profile elements (for a roughness profile)

[0043] The Mean Height Of Profile Elements, Rc, indicates the average value of the height of the curve element along the sampling length. Profile elements consist of a peak and a neighboring valley. The peaks (or valleys) that constitutes as an element have minimum height and length standards such that they will be treated as noise and considered a part of the preceding valley (or peak) if the height (or depth) is less than 10% of the maximum height or the length is less than 1% of the segment length.

[0044] The roughness values according to the invention may be measured with a profilometer using 2D profiles (according to ISO4287 standard). Alternatively, one can use the technique of 3D profilometry (according to ISO 25178 standard) but isolating a 2D profile which then gives access to the parameters defined in the ISO4287 standard.

[0045] According to the invention, the roughness values are measured with a Gaussian filter, which is a filter of long wavelengths, also called profile filter Ac. It is used for separating the components of roughness / texture from components of undulation of the profile. The evaluation length, L, according to the invention is the length of the profile used to evaluate the roughness. Base length, I, is the part of the evaluation length used to identify irregularities characterizing the profile to assess. The evaluation length, L, is divided / cut into n base lengths, I, which depend on the profile irregularities. The base length, I, corresponds to the "cut-off" wavelength (or limit wavelength) of the Gaussian filter (I = Ac). Typically, the evaluation length is of at least five times the base length. In roughness measurements, ashort wavelength filter (profile filter As) is also commonly used to eliminate the effects of very short wavelengths which are background noise.

[0046] Roughening can be achieved by any suitable method such as etching (acid, alkaline, liquid or vapor), laser texturing, sand blasting, embossing, rolling, mechanical polishing, engraving, and / or vapor deposition (e.g., chemical or physical vapor deposition), preferably by etching. Acid etching is preferred since it provides process control, short process time and flexibility. Acid etching can comprise a pre-etching step and / or a post etching step in addition to the main etching step. The other 'non chemical' techniques provide on the other hand a more sustainable option by indeed avoiding the use of chemicals.

[0047] For acid etching method, the etching solution used may typically include hydrofluoric acid, sulfuric acid, ammonium bifluoride, potassium bifluoride, tin chloride, sodium carbonate, and a water miscible organic solvent. In some embodiments, the etching solution may include 30-60 mol% (molar percentage) hydrofluoric acid, 1-18 mol% sulfuric acid, 5-25 mol% ammonium bifluoride, 0-7 mol% potassium bifluoride, 0-25 mol% water miscible organic solvent, and water. In other embodiments, the etching solution may comprise 4-15 mol% hydrofluoric acid, 0-30 mol% ammonium bifluoride, 0-7 mol% potassium fluoride, 0-15 mol% sodium carbonate and 0-15 mol% water miscible organic solvent, and water. In yet other embodiments, the water miscible organic solvent in the etching solution may be an alcohol (such as ethanol and iso-propanol), ethylene glycol, propylene glycol, glycerol, or combinations thereof. Other etching solutions may include 0-15 mol% potassium bifluoride, 0-15 mol% ammonium bifluoride, 0-15 mol% sulfuric acid, 0-5 mol% tin chloride and 0-15 mol% water miscible organic solvent.

[0048] Sandblasting is another suitable method and consists of compressed air, water and solid particles such as sand, SiC and / or alumina projected on the layer. Depending on the solid particles composition, shape and size and depending on the process parameters, the roughness technical parameters of the present invention can be easily achieved by persons skilled in that art.Radiative heat exchanger technologies

[0049] Several technologies are available for the radiative heat exchanger as long as the radiative surface is textured in such a way to provide the required texture parameters of Rsm equalto or greater than 15pm and Rc equal to or greater than 0.25pm. According to the invention, the radiative heat exchanger can be a polymer layer with dispersed particles and / or a metallic reflective layer.

[0050] For example, in one embodiment, the radiative heat exchanger includes a polymer layer having uniformly dispersed inorganic particles, used to radiate heat outward in the form of electromagnetic waves. The polymer layer forms the radiative surface of the heat exchanger and is disposed on the surface of the radiative cooling layer via adhesives and / or composite adhesive layers. The adhesive can be one or more of polyacrylic adhesive, polyurethane adhesive, and epoxy resin adhesive, with a conventional thickness of 3pm to 15 pm, preferably 10pm.

[0051] The polymer layer includes in general a base layer, that is typically a film layer such as at least one of a PEN (polyethylene naphthalate) film layer, a PET (polyethylene terephthalate) film layer, a PI (polyimide) film layer, a PE (polyethylene) film layer, a PVDF (polyvinylidene fluoride) film layer, a PP (polypropylene) film layer, and a PVC (polyvinyl chloride) film layer.

[0052] The polymer layer can be textured or an additional layer can be added on the base layer to provide the embossed structure with the Rc and Rsm required parameters. The Roughened polymeric layer can be produced via conventional film making processes wherein a mirror roller is replaced with a frosted roller at the position of the casting film head, to form an embossed layer after cooling. In another embodiment, sanding roller may be provided so that one surface of the substrate is provided with an embossed structure to form a singlesided embossed protective layer. Two grinding rollers may also be provided, so that both surfaces of the protective layers are provided with embossed structures to form a doublesided embossed protective layer.

[0053] Another embodiment can encompass a flexible, transparent passive radiative cooling structure in the form of a flexible film with a first surface and a second surface, and an inner region between the two surfaces. The cooling structure is transparent to visible light, and configured to absorb and emit infrared radiations. Embedded particles of a first material are distributed within a flexible film formed of a second material. The first material can be silica glass. Preferably, the particles have an average per particle volume size of greater than 14,200 pm3, and volume percentages above 25%, preferably between 25% and 73%. The embedded particles can be distributed in a disorderly manner or in a uniform manner withinthe flexible film formed of the second material being typically a thermoplastic polymer. The particles can have ellipsoidal shape, and preferably, the particles are spheroidal in shape, with average diameters of greater than 30 pm.Preferred radiative heat exchanger

[0054] In a preferred embodiment of the present invention, the radiative heat exchanger is a 'mirror'. Indeed, mirrors are very effective radiative heat exchangers. Mirrors can not only provides superior cooling performance but are as well sustainable since they remain performant over a long period of time, in diverse exterior conditions. 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. In addition, mirrors as the radiative heat exchanger in PRC system, provide efficient radiative cooling not only is at night but as well during day light and even in high sunshine conditions.

[0055] Indeed, it has been found that the silver layer of a mirror is a very effective light reflective layer in reflecting most of the solar radiation. The glass substrate functions as a the radiative layer and is positioned above the silver reflective coating in the direction of the sun. 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.

[0056] Hence, in a preferred embodiment, the radiative heat exchanger comprises a glass substrate having a first surface and a second opposite surface. The first surface is the radiative surface. The second surface is a coated surface, being provided with at least one reflective metallic layer, preferably an aluminium and / or silver layer, more preferably a silver layer comprising at least >700mg / m2of silver.

[0057] 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.

[0058] 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 NormEN 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 greater than 83% as defined in the European Norm EN 1036-1 / 2007 (E). For thinner mirror, the luminous coefficient threshold will increase accordingly.

[0059] Preferred mirrors for use as the radiative heat exchanger 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 or greater 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.

[0060] 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%.

[0061] 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 FejOa, comprised between 300ppm and lOOOppm (300ppm < FejOa < 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 FejOa, is comprised between 20ppm and lower than 300ppm (20ppm < FejOa < 300ppm) by weight of total glass composition.

[0062] 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).

[0063] 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.

[0064] 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).

[0065] 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.

[0066] 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.

[0067] In a preferred embodiment, the mirror of the present invention can comprise a sensibilisation layer, a protective layer and / or paint layer(s). When present, the sensibilisation layer is comprised between the glass substrate and the silver layer. Whenpresent, 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.

[0068] 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.

[0069] Preferably, the mirror 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.

[0070] Preferably, the mirror 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.

[0071] 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.

[0072] 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.

[0073] Figure 1 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) having a roughened zone (la), 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 FejOa, 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 wherein its entire radiative surface has been etched to provide a Rc comprised between 1.0pm and 2.0pm and a Rsm of at least 60pm. The glass substrate has 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.

[0074] In the preferred embodiment wherein the radiative heat exchanger is a glass substrate with a metallic reflective layer, the roughening step can occur on the glass substrate before or after metallic reflective layer is deposited. In the embodiment wherein the roughening step occurs on the glass substrate after metallic reflective layer is deposited and then the metallic reflective layer needs to be protected. For example, in the case of roughening by acid etching, then then the metallic reflective layer can be protected by an acid etching resistantmask or -tape. Then, the glass is dipped in acid-etching solution at a certain temperature during a time, t. The glass sheet is then removed from the etching bath and immediately washed with an aqueous detergent. The mask is removed by hot water and mechanical action (sponge or soft brush) or by mechanical peeling, directly after the rinsing.Maximisation of reflectance properties

[0075] 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.

[0076] 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.

[0077] Hence, in a preferred embodiment of the present invention, the thickness of the glass substrate is equal to or lower than 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.

[0078] The glass composition 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 to UV 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 limitthe 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.

[0079] 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.

[0080] 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.

[0081] The glass composition comprises typically iron expressed as total Fe2O3, at a level of 20ppm to 2000ppm based on the total weight of the glass composition.

[0082] For clear glass compositions, it is preferred that the content of total iron expressed in total Fe2O3, is comprised between 300ppm and lOOOppm (300ppm < Fe2O3< 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 FejOa, 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 FejOa, is comprised between 20ppm and less than 300ppm (20ppm < FejOa < 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 FejOaS, is comprised at a level equal to or lower than 250ppm, 200ppm, 175ppm and even, 150ppm by weight of total glass composition.

[0083] In a preferred embodiment, the redox of the glass composition, expressed in FeO / FejOa, is equal to or lower than 30% (FeO / FejOa < 30%), preferably equal to or lower than 28% (FeO / FejOa < 28%), preferably equal to or lower than 25% (FeO / FejOa < 25%), preferably equal to or lower than 23% (FeO / FejOa < 25%), more preferably equal to or lower than 20% (FeO / FejOaS < 20%). Preferably, the iron redox expressed in FeO / FejOa, is equal to or greater than 15% (FeO / FejOa > 15%), is equal to or greater than 15%. Indeed, the Fe2+iron specie absorbs more than the Fe3+iron specie.

[0084] 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 or greater 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.

[0085] In present description and claims, to quantify the visible transmission (also called luminous transmission / transmittance or TL) 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.Mirror Making Process

[0086] The radiative heat exchanger is preferably a reflective stack deposited on the glass substrate, that comprises at least a metallic reflective layer, preferably an aluminium and / or silver layer, more preferably 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.

[0087] 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.

[0088] 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 PdCL. 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, SnCL 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 any other 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.

[0089] 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.

[0090] A suitable process to produce preferred mirrors is adapted from the above process 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.

[0091] Hence, a preferred mirror 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.Convective heat exchanger and PRC systems

[0092] The radiative heat exchanger comprises a radiative surface facing the sky and an opposite surface, referred to as the 'contact surface'. The present invention further 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. The heat exchange between the contact surface of the radiative heat exchanger and the heat transfer fluid of the convective heat exchanger may be achieved through different non limiting embodiments.

[0093] 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.

[0094] The objective of the convective heat exchanger is to maximise thermal conductivity between the radiative heat exchanger 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.

[0095] 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 north in 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.

[0096] 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.

[0097] According to a first embodiment of the invention, the heat transfer fluid is in direct contact to the contact surface of the radiative heat exchanger. The convective heat exchanger 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 being normal to the contact surface and a bottom horizontal wall parallel to the contact surface. The enclosure is closed by the contact surface of the radiative heat exchanger. 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.

[0098] 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 flow path 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.

[0099] 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.

[0100] According to a second embodiment , the liquid heat exchanger circulates within a pipe of an exchanger plate. The convective heat exchanger is a heat exchanger plate comprising a pipe. 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. More particularly, the thermal conductive plate is an aluminium or copper plate. The heat conductive plate is in contact to the contact surface of the radiative heat exchanger.

[0101] According to a third embodiment, the convective heat exchanger is a 'roll bond' panel wherein heat transfer fluid is heat transfer within a patterned. 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 can be made by an embossing press on a first aluminium plate which is then stacked to a second non-embossed planar aluminiumplate. The planar plate of the roll bond panel is in contact with the contact surface of the mirror.

[0102] 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 to provide liquid / waterproofness and air / tightness, mechanical stability, resist to the heat transfer fluid pressure.Application and Use

[0103] The radiative heat exchanger of the present invention can typically be 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, ...

[0104] The present invention further relates to the use of a heat radiative exchanger within a passive radiative cooling system. All preferred embodiments described above, apply to the radiative heat exchanger in its use.DefinitionsAs 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.For the 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 art.Examples

[0105] 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.

[0106] The Free Cooling Power (FCP) of radiative heat exchangers has been tested by measuring their temperature by thermos-couples when placed horizontally 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.

[0107] A sheet of soda-lime-silicate glass (SLS) sold under the tradename Sunmax Premium by AGC glass Europe having a thickness of 1.0 mm thickness (40cm x 40cm) is used as the glass substrate for the mirror. The following layers are deposited on the surface by a conventional method to create 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%.

[0108] The mirror is then washed with an aqueous detergent and dried. The surface wherein the different layers above have been deposited, is protected by an acid etching resistant mask. Then, the glass is dipped in 200 mL of an acid-etching solution at 20-25°C during a time, t. The glass sheet is then removed from the etching bath and immediately washed with an aqueous detergent. The mask is removed by hot water and mechanical action (sponge orsoft brush) directly after the rinsing. Different etching solutions during different periods of time were used.

[0109] The different etched radiative surface are analyzed in terms of texture / surface roughness properties. Surface roughness measurements were performed using a 3D optical profiler Sensofar Model S neox 090 with a Nikon 50x lens, using the "SensoSCAN S neox 7.7" software, on an evaluation length of 4 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm. Roughness values are calculated from the profile measured according to ISO4287 standard. The sample is first cleaned with detergent and dried. It is then placed under the microscope and after conventional settings, the profile of a 2D acquisition is then initiated.

[0110] The reference is the mirror configuration described herein above wherein its radiative surface has not been roughened. Except from the roughening feature of the radiative surface, all tested mirrors are of the same composition and come from the same production.

[0111] The roughened radiative surface of comparative example 1 is obtained with an acid-etching aqueous solution comprising KHFj at 5.0 mol%, NH4HF2 at 0.5 mol%, SnCL at 0.25 mol%, H2SO4 at 1.0 mol% and glycerol at 0.5 mol%. The etching time is 20 seconds. The roughened surface is characterized by textures features : Rc of 0.18pm and Rsm of 8.3pm.

[0112] The roughened radiative surface of Example 2 of the present invention: is obtained with an acid-etching aqueous solution comprising HF at 50.0 mol%, H2SO4 at 10.0 mol%, NH4HF2 at 8.0mol%, KHF2 at 2.5mol% and TWEEN 60 at 0.035mol%. The etching time is 30 seconds. The roughened surface is characterized by textures features: Rc of 1.3pm and Rsm of 64.4pm.

[0113] The roughened radiative surface of Example 3 of the present invention is obtained with an acid-etching aqueous solution comprising NH4HF2 at 15.0 mol%, KHF2 at 2.0 mol%, Na2COs at 4.0 mol%, HF at 8.0 mol% and H2SO4 at 5.0 mol%. The etching time is 300 seconds. The roughened zone is characterized by textures features: Rc of 5.2pm and Rsm of 47.5pm.

[0114] The roughened radiative surface of comparative example 4: the sandblasting technique has been used with a Corindon 80 for a period of time of 20sec at a pressure of 4 to 7 bars max. The roughened zone is characterized by textures features: Rc of 11.0pm and Rsm of 9.4pm.

[0115] Absolute temperatures are not shown in Figure A below. The test below assesses the impact of roughnening, and hence it is the difference in temperature reached by the different tested radiative heat exchangers that were measured, versus the reference.

[0116] Figure A below shows temperature profiles (Expressed in °C) versus time (24 hours from midnight on midnight the following day) of examples 2 and 3 of the present invention and of comparative examples 1 and 4, versus the temperature of the reference, in Anderlues (Belgium) on 21-22 September 2024. Temperature is measured by thermocouples at the contact surface of the mirrors. Mirrors are positioned horizontally, facing the sky in an open field.

[0117] Figure A:Temperature of roughened mirrors versus referenceTimeE 2 — • • Comp. Ex.4 -- --- Comp. E l — - ~ Ex.3

[0118] A negative value between the temperature of the tested roughened mirror and the reference, indicates that the roughened mirror has a lower temperature, and hence is cooler than the reference temperature. This demonstrates a higher FCP than the reference and therefore an improved effective radiative cooling. A positive value between the temperature of the tested roughened mirror and the reference, indicates that the roughened mirror has a higher temperature and hence is warmer than the reference. This demonstrate a lower FCP than reference and therefore a decrease in radiative cooling performance.

[0119] Two main time periods can be distinguished : (1) after 6PM and before 6AM when there is no sunlight (referred herein after as 'night') and (2) between 10:30AM and 6PM where there is sunlight (referred herein after as 'day').

[0120] Comparative example 4 provides not only much higher temperature during the day but overall similar if not higher temperature at night. Therefore, the roughened surface feature of the radiative surface, even if it provides anti-glare benefit, adversely affects the cooling performance of the radiative heat exchanger. The roughening of the radiative surface causes increased absorption of the sunlight versus reference and such increase in absorption is overtaking the potential gain of increased emissivity, resulting in smaller Free Cooling Power than reference.

[0121] At night, comparative example 1 and examples 2 and 3 of the present invention exhibit a lower temperature than the reference by roughly -0.5°C. Roughening induces a higher emissivity and thus increases the value of the component C in the calculation of the free cooling power (FCP).

[0122] However, comparative example 1 provides higher temperature difference during the day even if in a much lesser extent than comparative example 4. The increase in temperature difference during the day cannot be nevertheless compensated by the decrease in temperature difference during the night. Therefore, the roughened surface feature of the radiative surface, even if it provides anti-glare benefit, still adversely affects the cooling performance of the radiative heat exchanger.

[0123] On the contrary, example 3 of the invention provides as well higher temperature difference during the day but in a lesser extent than comparative example 1. The increase in temperature difference during the day is compensated by the greater decrease in temperature during the night. There is a small increase of absorption (component H of Free Cooling Power) and therefore, the FCP (daytime) of example 3 is slightly smaller than Reference. However, the overall cooling power (day and night) is higher than reference (see Table 1 below ) thanks to an increase in emissivity. Therefore, the specific roughened surface features of the radiative surface, provide not only anti-glare benefit, but as well improved cooling performance of the radiative heat exchanger.

[0124] Example 2 of the present inventionillustrates a preferred embodiment of the present invention providing a lower temperature difference, both during the night and the day periods. The temperature difference is lower by about -1°C even during the day. The small increase of absorption (component H of Free Cooling Power) is very marginal versus the increase of emissivity. The FCP (daytime) of example 2 is substantially higher than reference and overall cooling power (day and night) is higher than reference. Therefore, the specific roughened surface features of the radiative surface, provide not only anti-glare benefit, but as well improved cooling performance of the radiative heat exchanger.

[0125] Table 1 gives the averaged value of the data presented in Figure A above and illustrates the average temperature difference (°C) between the tested radiative heat exchangers and the reference from September 21, 2024 midnight to September 22, 2024 midnight. Table 1 gives the overall general improvement of the cooling performance.

[0126] Table 1 :

[0127] Over the 24hours period, example 3 of the present invention provides an average temperature difference versus reference of -0.23°C. Hence, the specific Rc and Rsm roughened surface features of the radiative surface of the radiative heat exchanger of the present invention provides not only anti-glare benefit but as well significantly improved cooling performance.

[0128] Over the 24hours period, example 2 of the present invention provides an average temperature difference versus reference of -0.75°C. Hence, the specific Rc and Rsm roughened surface features of the radiative surface of the radiative heat exchanger of the present invention provides not only anti-glare benefit but as well superior cooling performance.

Claims

CLAIMS1. A radiative heat exchanger comprising a radiative surface facing the sky, wherein the majority of the radiative surface has been roughened to provide a roughened zone having: a) a Mean Width of the Profile Elements, Rsm, equal to or greater than 15pm (Rsm > 15pm), preferably equal to or greater than 35pm (Rsm > 35pm), preferably equal to or greater than 42pm (Rsm > 42pm), preferably equal to or greater than 50pm (Rsm > 50pm), more preferably equal to or greater than 60pm (Rsm > 60pm) wherein Rsm is measured on an evaluation length of 4.0 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm; and b) a Mean Height of the Profile Elements, Rc, equal to or greater than 0.25pm (Rc > 0.25pm), preferably equal to or greater than 0.50pm (Rc > 0.50pm), more preferably equal to or greater than 1.00pm (Rc > 1.00pm); wherein Rc is measured on an evaluation length of 4.0 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm.

2. The radiative heat exchanger according to claim 1 wherein the roughened zone has a Mean Height of the Profile Elements, Rc, equal to or lower than 8.00pm (Rc < 8.00pm), preferably equal to or lower than 5.50pm (Rc < 5.50pm), preferably equal to or lower than 3.00pm (Rc < 3.00pm), more preferably equal to or lower than 2.00pm (Rc < 2.00pm).

3. The radiative heat exchanger according to any one of the preceding claims wherein the radiative heat exchanger comprises a glass substrate having a first surface being the radiative surface and a second opposite surface; wherein the second surface is a coated surface, being provided with at least one metallic reflective layer, preferably aluminium and / or silver.

4. The radiative heat exchanger according to claim 3 wherein the metallic reflective layer is a silver reflective layer comprising silver in an amount equal to or greater than >700mg / m2; preferably equal to or greater than 800mg / m2, preferably equal to or greater than 1000mg / m2, preferably equal to or greater than 1200mg / m2, preferably equal to or greater than 1400mg / m2, more preferably equal to or greater than 1500mg / m25. The radiative heat exchanger according to any one of the preceding claims 3 to 4 providing a regular luminous coefficient equal to or greater than 86%, for a thickness between 2mm and 6mm.

6. The radiative heat exchanger according to any one of the previous claims 3 to 5 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 metallic 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.

7. The radiative heat exchanger according to claim 6 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.

8. The radiative heat exchanger according of any one the claims 3 to 7 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.

9. The radiative heat exchanger according to any one of the previous claims 3 to 8 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 4.0 mm, preferably equal to or lower than 3.0 mm and more preferably equal to or lower than 1.5 mm.

10. The radiative heat exchanger according to any one of the previous claims 3 to 9 wherein the glass substrate is a soda lime silicate glass, preferably having a total iron content expressed as FejOa of 20ppm to 2000ppm (20ppm < FejOa < 2000ppm), preferably of 20ppm to less than 300ppm (20ppm < FejOa < 300ppm); based on the total weight of the glass composition.

11. The radiative heat exchanger according to any of the previous claims 3 to 10 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%.

12. The radiative heat exchanger according to any of the previous claims 3 to 10 wherein the glass substrate at a thickness of 1mm, has a visible light transmission, LTD1, equal to or greaterthan 90.0%, preferably equal to or greater than 91.0% preferably equal to or greater than 91.5%.13.The radiative heat exchanger 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).

14. The radiative heat exchanger 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).

15. A passive radiative cooling system comprising a radiative heat exchanger according to any of the previous claims and a convective heat exchanger comprising a heat transfer fluid; wherein the radiative heat exchanger comprises a contact surface opposite to the radiative surface and wherein the convective heat exchanger is in contact with the contact surface of the radiative heat exchanger.

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