Outer wall film capable of reflecting sunlight back to sky

Abstract

The invention discloses an outer wall film capable of reflecting sunlight back to the sky, and relates to the technical field of building outer wall energy conservation. The outer wall film is composed of a plurality of arranged reflection units, each reflection unit is provided with a first reflection face and a second reflection face which are perpendicular to each other, and the section defined by the first reflection face, the second reflection face and the building outer wall interface is a right triangle. And adjusting an included angle formed between the first reflecting surface and the building outer wall interface or an included angle formed between the second reflecting surface and the building outer wall interface so as to reflect sunlight with the incident angle larger than alpha back to the sky. According to the outer wall film, the two orthogonal reflecting surfaces of the reflecting unit form a right-angled triangle geometry, and the right-angled triangle geometry and the outer wall surface of a building form a reflecting interface system with a controllable included angle. The energy transfer limitation of traditional single reflection is broken through through the geometrical optics principle, secondary heating of the ground is avoided while heat absorption of the building surface is blocked, and space transfer of solar radiation energy is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of building exterior walls, in particular to an exterior wall film that reflects sunlight back to the sky. Background Art

[0002] In recent years, the accelerated pace of global urbanization has driven a continuous increase in building density. The resulting heat island effect has become a major challenge hindering sustainable urban development. Heat accumulation caused by urban surface hardening, combined with artificial heat sources, creates a significant temperature gradient. According to meteorological monitoring data from multiple countries, summer temperatures in the central areas of megacities can be 5-8°C higher than in suburban areas. This thermally altered environment not only exacerbates the surge in energy consumption caused by air conditioning system loads but also alters local microclimate cycles, leading to the retention of pollutants such as PM2.5. As the primary interface for energy exchange, the conflict between solar radiation absorption and reflection in building envelope systems is particularly prominent. While existing high-reflectivity coatings can reduce heat gain within the building itself, specular reflection multiplies the near-Earth solar radiation flux. Multiple reflections from the geometry of urban canyons create heat traps, leading to abnormally high surface temperatures. This energy transfer mechanism essentially transforms the thermal management dilemma at the building level into a deteriorating urban thermal environment at a larger scale, exposing the inherent shortcomings of traditional technical approaches in regulating systemic thermal balance.

[0003] An International Energy Agency study indicates that building cooling energy consumption in tropical cities now accounts for over 40% of total electricity consumption, with an average annual growth rate of 5.2%. Against this backdrop, optimizing the thermal performance of building facades has become a crucial breakthrough in alleviating the energy crisis. Existing technologies often focus on improving material reflectivity, but overlook the holistic nature of urban spaces as complex thermodynamic systems. Solar radiation, after being reflected by building facades, undergoes secondary absorption by low-reflectivity surface media such as asphalt pavement. This energy redistribution process raises urban canopy air temperatures by 2-3°C compared to conventional operating conditions, directly undermining the expected benefits of building energy-saving retrofits. More critically, the continued increase in surface longwave radiation and the turbulence suppression effect of the urban canopy create a positive feedback loop, resulting in diminishing returns in the long term for traditional cooling methods, revealing systemic flaws in existing solutions in energy routing. Summary of the Invention

[0004] The technical problem to be solved by the present invention is that traditional building exterior walls reflect solar radiation to the ground.

[0005] In order to solve the above problems, the present invention proposes the following technical solutions:

[0006] Disclosed is an exterior wall film that reflects sunlight back toward the sky. The exterior wall film is composed of a plurality of arranged reflective units, each of which has a first reflective surface and a second reflective surface perpendicular to each other, wherein the cross-section enclosed by the first reflective surface, the second reflective surface, and the interface of the building's exterior wall forms a right triangle. The angle formed between the first reflective surface and the interface of the building's exterior wall, or the angle formed between the second reflective surface and the interface of the building's exterior wall, is adjusted to reflect sunlight with an incident angle greater than α back toward the sky, where α is 0-90°.

[0007] A further technical solution is that the angle formed between the first reflecting surface and the interface of the building exterior wall is ∠A=45°-α / 2, and the angle formed between the second reflecting surface and the interface of the building exterior wall is ∠B=45°+α / 2.

[0008] A further technical solution is that α is determined by the following formula:

[0009]

[0010] H is the solar altitude angle, and A is the solar azimuth angle.

[0011] A further technical solution is that the reflectivity of the first reflecting surface is above 95%, and the mid-infrared emissivity is less than 5%.

[0012] A further technical solution is that the reflectivity of the second reflecting surface is above 95%, and the mid-infrared emissivity is less than 5%.

[0013] A further technical solution is that the exterior wall film includes a base layer, a reflective layer and an aluminum-plated layer located between the base layer and the reflective layer, and a side of the reflective layer away from the aluminum-plated layer is a first reflective surface or a second reflective surface.

[0014] A further technical solution is that the base layer is made of acrylic resin and has a thickness of more than 5 μm.

[0015] A further technical solution is that the material of the aluminum plating layer is aluminum, the thickness is 80-200nm, and it has a full spectrum reflectivity of 90% or above.

[0016] A further technical solution is that the reflective layer is made of aluminum oxide and has a thickness of 10-50 nm.

[0017] In a second aspect, the present invention provides a method for preparing the above-mentioned exterior wall film that reflects sunlight back to the sky, comprising the following steps:

[0018] Process the exterior wall film master template with preset angles and sizes;

[0019] A base layer is prepared based on the exterior wall film master through a mold forming process;

[0020] An aluminum film is deposited on the surface of the substrate layer by a thermal evaporation process to obtain an aluminum coating layer;

[0021] A reflective layer is plated on the surface of the aluminum plated layer by adopting a magnetron sputtering process.

[0022] The present invention also provides a building exterior wall, comprising the exterior wall film for reflecting sunlight back to the sky.

[0023] Compared with the prior art, the present invention can achieve the following technical effects:

[0024] The present invention provides an exterior wall film that reflects sunlight back toward the sky. The film is composed of a plurality of arrayed reflective units, each of which has a first and a second reflective surface perpendicular to each other. The cross-section enclosed by the first and second reflective surfaces and the building's exterior wall forms a right triangle. The angle formed between the first and second reflective surfaces and the building's exterior wall is adjusted to reflect sunlight with an incident angle greater than α back toward the sky, where α ranges from 0 to 90°. The exterior wall film of the present invention achieves directional control of solar radiation through the array of reflective units. Each reflective unit is composed of two orthogonal reflective surfaces forming a right-angled triangle geometry, which form a reflective interface system with a controllable angle with the building's exterior wall. When the solar incident angle exceeds a preset threshold α, the two reflective surfaces work together to direct the incident light toward the atmosphere through one or more reflections, forming a sky-reflection path. This structure overcomes the energy transfer limitations of traditional single reflections by leveraging the principles of geometric optics. While blocking heat absorption by the building surface, it also prevents secondary heating of the ground, achieving spatial transfer of solar radiation energy.

[0025] Furthermore, by precisely setting the angle between the first reflective surface and the wall (45°-α / 2) and the complementary angle between the second reflective surface and the wall (45°+α / 2), a light path control mechanism is constructed that can dynamically match the solar altitude angle according to the latitude and wall orientation of different regions, thereby effectively alleviating the urban heat island effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 Schematic diagram of the structure of an exterior wall film for reflecting sunlight back to the sky, provided in an embodiment of the present invention; in the figure, (b) is a cross-sectional schematic diagram of the reflection unit and the building wall; (c) is a schematic diagram of the microstructure of the exterior wall film.

[0028] Figure 2Schematic diagram of the relationship between the solar altitude angle, azimuth angle and incident angle α.

[0029] Figure 3 The reflection of sunlight with different incident angles α by exterior wall films with different angle structures.

[0030] Figure 4 This is a scatter plot of the sunlight incident angle α and temperature on the east wall of Dongguan.

[0031] Figure 5 Optical properties of three different walls.

[0032] Figure 6 Thermal properties of three different walls.

[0033] Reference numerals

[0034] Exterior wall film 10, building exterior wall interface 20, first reflective surface 11, second reflective surface 12, base layer 1, aluminum coating layer 2, reflective layer 3. DETAILED DESCRIPTION

[0035] The following will be combined with the accompanying drawings of the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments. Similar component numbers in the drawings represent similar components. Obviously, the embodiments described below are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0036] It will be understood that when used in this specification and the appended claims, the terms “comprises” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0037] The "triangular structure" of the present invention refers to a structure in which the cross-section of the first reflecting surface 11, the second reflecting surface 12 and the building exterior wall interface 20 of the reflecting unit is a right-angled triangle. The "triangular structure" is also used in the specification of the present invention to refer to the exterior wall film of the present invention.

[0038] The "triangular structure wall" of the present invention refers to a wall covered with the exterior wall film of the present invention that reflects sunlight back to the sky.

[0039] See also Figure 1An embodiment of the present invention provides an exterior wall film that reflects sunlight back toward the sky. As shown in the figure, the exterior wall film 10 is composed of a plurality of arranged reflective units. Each reflective unit has a first reflective surface 11 and a second reflective surface 12 that are perpendicular to each other. The cross-section enclosed by the first reflective surface 11, the second reflective surface 12, and the building exterior wall interface 20 forms a right triangle. The angle formed by the first reflective surface 11 and the building exterior wall interface 20, or the angle formed by the second reflective surface 12 and the building exterior wall interface 20, is adjusted to reflect sunlight with an incident angle greater than α back toward the sky, where α is 0-90°.

[0040] In a specific embodiment, the reflectivity of the first reflective surface is greater than 95%, and the mid-infrared emissivity is less than 5%.

[0041] In a specific embodiment, the reflectivity of the second reflective surface is greater than 95%, and the mid-infrared emissivity is less than 5%.

[0042] See further Figure 1 (c) shows that the exterior wall film of this embodiment includes a base layer 1, a reflective layer 3 and an aluminum-plated layer 2 located between the base layer 1 and the reflective layer 3, and the side of the reflective layer 3 away from the aluminum-plated layer 2 is a first reflective surface 11 or a second reflective surface 12.

[0043] In this embodiment, the base layer 1 is made of acrylic resin (C3H4O2). n , with a thickness of 5 μm or more, such as 5 μm, 10 μm, 30 μm, etc. (C3H4O2) n , a material commonly used in micro- and nano-fabrication, not only possesses excellent formability and uniformity, but also achieves nanometer-level precision and possesses excellent physical and chemical properties. Furthermore, this material is low-cost and has mature manufacturing processes, offering strong commercial advantages.

[0044] In this embodiment, the aluminum-plated layer 2 is made of aluminum and has a thickness of 80-200 nm, for example, 100 nm or 150 nm. Aluminum film has a full-spectrum reflectivity of 90% or higher, exhibiting excellent reflectivity from the visible to infrared range, with a reflectivity of 90-98%. The average reflectivity across the full spectrum is close to 90%, demonstrating high reflectivity and low transmittance.

[0045] In this embodiment, the reflective layer 3 is made of aluminum oxide and has a thickness of 10-50 nm, for example, 30 nm.

[0046] In this embodiment, the first reflective surface and the second reflective surface are made of the same material and have the same reflectivity and mid-infrared emissivity.

[0047] The embodiment of the present invention also provides a method for preparing the above-mentioned exterior wall film that reflects sunlight back to the sky, comprising the following steps:

[0048] First, laser direct writing and etching technology is used to process a preset angle exterior wall film master. Based on the master, a base layer with the same geometric features is prepared through a mold forming process. Then, an aluminum coating layer 2 is deposited on the surface of the base layer through a thermal evaporation process. Finally, a magnetron sputtering process is used to cover the aluminum film surface of the aluminum coating layer 2 with an aluminum oxide reflective layer 3. The high density of aluminum oxide achieves antioxidant and weather resistance enhancement, ensuring the long-term stability of the exterior wall film in outdoor environments.

[0049] The exterior wall film provided by the embodiment of the present invention that reflects sunlight back to the sky is an aluminum-based composite film layer. The aluminum-plated layer has a reflectivity of more than 95% and a bright Al2O3 reflective layer to achieve two-way control during the day and night:

[0050] During the day: it can efficiently reflect sunlight, significantly reducing the temperature of building walls, sidewalks and urban canyons; at night: the 5% mid-infrared emissivity makes the energy lost by thermal radiation very small, thereby playing a certain role in thermal insulation of the wall.

[0051] In summary, the exterior wall film provided by the present invention that reflects sunlight back to the sky achieves all-weather thermal management through unique geometric structures and material properties. The film is attached to the exterior surface of the building and, during the day, reflects solar radiation directionally to the sky through a double-reflecting surface system, effectively reducing the building surface's absorption of solar energy, thereby reducing the building's cooling energy consumption and suppressing wall temperature rise; at the same time, it avoids the problem of secondary heat absorption of the ground caused by traditional highly reflective materials, significantly alleviating the urban heat island effect. At night, the low infrared emissivity characteristic is used to reduce wall heat radiation loss and maintain thermal stability inside the building. This day and night coordinated control mechanism not only improves the building's energy efficiency, but also provides an innovative solution for improving microclimate and promoting green city construction by blocking the vicious cycle of the urban thermal environment.

[0052] It can be understood that the core innovation of the exterior wall film that reflects sunlight back to the sky provided by the embodiment of the present invention lies in constructing a right-angle reflecting geometry composed of a first reflecting surface 11, a second reflecting surface 12 and an interface 20 of the building's exterior wall. Selective reflection of solar radiation is achieved by precisely controlling the spatial angle of the reflecting surface (rather than limiting the specific size): when the incident angle exceeds the set threshold α, the incident light is precisely directed to the atmosphere through the orthogonal optical path control of the double reflecting surfaces. This parametric design gives the system the advantage of geometric freedom, so that the reflecting unit can achieve the expected optical performance without being restricted to a specific size ratio, which not only ensures the heat reflection efficiency, but also expands the adaptability of the building facade. Therefore, the embodiment of the present invention does not limit the size and size of the triangular structure.

[0053] In some embodiments, the angle formed between the first reflecting surface and the interface of the building exterior wall is ∠A=45°-α / 2, and the angle formed between the second reflecting surface and the interface of the building exterior wall is ∠B=45°+α / 2.

[0054] In some embodiments, the α is determined by the following formula:

[0055]

[0056] H is the solar altitude angle, and A is the solar azimuth angle.

[0057] The determination of the α formula is described as follows:

[0058] See also Figure 2 In this embodiment, taking the east wall as an example, the relationship between the sun's altitude angle and azimuth angle and the incident angle α of the triangular structure of the present invention is established: the light (X, Y, Z) is projected onto the vertical surface of the exterior wall film, and the angle between the light and the horizontal plane on this plane is recorded as α. When the exterior wall film is on the east wall, the sun's movement trajectory is determined by the two variables of altitude angle (H) and azimuth angle (A). In order to accurately express the relationship between the two, we introduce The vector represents the component of the sun's rays in the y direction and is expressed as:

[0059]

[0060] r represents the distance of the light from the origin O, H∈[0,π / 2], A∈(0,π); The vector represents the component of the sun's rays projected onto the plane yoz, expressed as:

[0061]

[0062] α(H, A) (abbreviated as α) is the angle between the light projected onto the yoz plane and the horizontal plane xoy. Therefore, the incident angle α can be deduced from (1) and (2) as α(H, A), which is expressed as:

[0063]

[0064] Therefore, according to formula (3), the incident angle α is related to the solar altitude angle H and the solar azimuth angle A. α is used to describe the incident angle of the sun on the east wall to study the incidence and reflection of sunlight on the exterior wall film. Similarly, when the exterior wall film is installed on the south, west, and north walls, the design can also be carried out on these three walls based on this design concept.

[0065] Specifically, based on the angle between the first reflective surface on the exterior wall film and the wall (45°-α / 2) and the complementary angle between the second reflective surface and the wall (45°+α / 2), the relationship between the internal angles of the right triangle of the reflective unit can be expressed as [(45-α / 2)°, (45+α / 2)°, 90°].

[0066] To verify that the reflective unit of the above-mentioned inner angle structure [(45-α / 2)°, (45+α / 2)°, 90°] can reflect sunlight with an incident angle greater than α back to the sky, this embodiment constructs a triangular structure of exterior wall films with different α values:

[0067] α=10°, [40°, 50°, 90°];

[0068] α=30°, [30°, 60°, 90°];

[0069] α=50°, [20°,70°,90°];

[0070] α=70°, [10°, 80°, 90°].

[0071] By performing ray tracing simulation, the angle between the reflected light and the horizontal plane is defined as β. When β ≥ 0, if the existence of surrounding obstructions is not considered, the light can be considered to be effectively reflected back to the sky, and the corresponding reflection ratio is defined as the sky reflectivity. The reflection of the above four types of exterior wall films to sunlight with different incident angles α is shown as follows: Figure 3 shown.

[0072] The results show that the sky reflectivity of exterior wall films with different structures gradually increases with the increase of the incident angle α of sunlight, and can theoretically reach 100% at the corresponding critical α. This result shows that by constructing a suitable right-angled triangle structure of reflective units, it is possible to reflect sunlight with an incident angle greater than α back to the sky.

[0073] Furthermore, the inventors used theoretical calculations to obtain the temperature data of the east-facing wall in Dongguan throughout the year, and combined these data with the incident angle α to draw a scatter plot between the incident angle α and the temperature of the east-facing wall, as shown in Figure 2. Figure 4 As shown in the figure, from the perspective of living comfort, it is not necessary to reflect all the sunlight hitting the wall back to the sky. For example, the incident angle of sunlight in the morning is low, and the corresponding temperature is also low. Therefore, it is necessary to selectively reflect the higher temperature (high radiation intensity sunlight) back to the sky and retain the lower temperature (low radiation intensity sunlight). Figure 4 The results show that when the incident angle α = 10°, the temperature begins to rise significantly. At this angle, the high-intensity sunlight has a greater impact on the indoor temperature. Therefore, this embodiment selects the incident angle α = 10° as the reflection threshold to construct the exterior wall film.

[0074] Combine Figure 3 As a result, in the reflective structure with an internal angle of [40°, 50°, 90°], the first reflective surface 11 and the second reflective surface 12 can ensure that sunlight with an incident angle greater than 10° is reflected, thereby significantly reducing the wall temperature.

[0075] The following uses the exterior wall film constructed with an internal angle of [40°, 50°, 90°] as an example to compare the energy transfer differences between a highly reflective wall, a highly absorptive wall, and a triangular structure wall covered with the exterior wall film of this embodiment. The results are as follows: Figure 5 shown.

[0076] Highly reflective walls, Figure 5 (a)-(b): Although the mirror reflection mechanism reduces the heat absorption of the wall itself, it will cause the ground to absorb the reflected energy again, thereby increasing the temperature, which in turn causes the temperature of the urban canyon to rise and produce the urban heat island effect.

[0077] Highly absorbent walls, Figure 5 (c)-(d): The walls will absorb most of the solar radiation, causing the building temperature to rise, resulting in energy loss, increasing the city’s electricity burden and CO2 emissions.

[0078] Triangular structure wall, Figure 5 (e)-(f): Within an incident angle range of 10°-80°, the first and second reflective surfaces control the optical path, directing over 99% of the reflected light toward the sky. Due to the highly reflective aluminum deposited on the surface, the wall absorbs very little heat and reflects excess solar radiation back toward the sky, preventing it from being reabsorbed by the ground. Therefore, the triangular film structure not only keeps the wall cooler but also reduces the amount of energy received by the ground, achieving energy savings for the building itself and mitigating urban heat islands.

[0079] Taking the exterior wall film constructed with internal angles of [40°, 50°, 90°] as an example, the paths of reflected light with incident angles α ranging from 10° to 90° were explored. The experiment shows that:

[0080] 10°-50° range: The reflected light will split into two beams, one beam returns along the original path, and the other beam is reflected back to the sky at a higher angle.

[0081] 50° critical point: single reflection mode, the reflected light will return along the original path.

[0082] 50°-90° range: The reflected light is a single beam, and the reflection angle is stable at more than 10°, ensuring that the light can return to the sky.

[0083] The above-mentioned results of the paths of the reflected light further confirm that the exterior wall film for reflecting sunlight back to the sky constructed by the present invention can reflect sunlight back to the sky.

[0084] Construct the following experimental samples and conduct thermal experiments outdoors:

[0085] High-reflectivity wall samples: A 50nm thick layer of aluminum is deposited on a smooth metal plate to create an aluminum coating. A 30nm thick Al2O3 reflective layer is then applied to the aluminum coating to ensure high reflectivity. The reflectivity is 0.9-0.95, and the absorptivity is 0.05-0.1.

[0086] High-absorption wall sample: A layer of high-absorption black paint with a thickness of 80nm was sprayed on a smooth metal plate. The absorption rate was 0.93-0.95, and the reflectivity was 0.05-0.07.

[0087] The triangular structure exterior wall film sample of the present invention has an internal angle of [40°, 50°, 90°]: the base layer is acrylic resin (C3H4O2)n with a thickness of 5μm; the aluminum plating layer is aluminum with a thickness of 50nm; and the reflective layer is bright aluminum oxide with a thickness of 30nm.

[0088] High-absorption ground sample: Spray a layer of high-absorption black paint on a smooth metal plate with a thickness of 80nm. Absorption rate: 0.93-0.95, reflectivity: 0.05-0.07.

[0089] The three samples were attached to the east-facing wall respectively, and the temperature of the three walls and the temperature radiated to the ground were measured within a specific time. The experimental results are as follows: Figure 6 shown.

[0090] The experimental temperatures of the three walls are as follows: Figure 6 As shown in (a), the triangular wall and the highly reflective wall have similar temperatures, both lower than the highly absorptive wall. At 10:01, the temperature difference between the triangular wall and the highly absorptive wall is the largest, with the triangular wall 14.7°C lower than the highly absorptive wall. Over the entire outdoor testing period, the average temperature of the triangular wall is 11.9°C lower than that of the highly absorptive wall. Figure 6 (b) shows the ground temperature in the areas where the three wall types are located. The ground temperature corresponding to the triangular film wall is the lowest, 19.3°C lower than the ground temperature corresponding to the high-reflectivity wall at 10:28, and 3°C lower than the ground temperature corresponding to the high-absorptivity wall at 10:27.

[0091] These results demonstrate the dual advantages of the exterior wall film constructed by this invention, which reflects sunlight back toward the sky, in terms of thermal management performance. In terms of wall temperature control, both the triangular structure film and the highly reflective wall significantly outperformed the highly absorptive wall, achieving a maximum instantaneous temperature difference of 14.7°C and a stable average temperature difference of 11.9°C, demonstrating its effective blocking of heat transfer from solar radiation to the building. Regarding ground temperature control, the surface temperature in the area corresponding to the triangular structure film was 19.3°C lower than that of the traditional highly reflective system and even 3°C lower than that of the highly absorptive system, demonstrating that its reflective mechanism successfully circumvents the secondary ground heat absorption problem caused by traditional highly reflective materials. This data demonstrates that this technology achieves a synergistic effect by simultaneously cooling the building and improving the urban thermal environment, resolving the conflicting relationship between "wall cooling" and "ground temperature rise" in traditional technologies. This provides a systematic solution for urban heat island mitigation that combines building energy conservation with environmental optimization.

[0092] In the above embodiments, the description of each embodiment has its own focus. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0093] The above description is a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions are intended to be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. An exterior wall film that reflects sunlight back to the sky, characterized in that: The exterior wall film is composed of a plurality of arranged reflective units, each of which has a first reflective surface and a second reflective surface perpendicular to each other, and the cross-section enclosed by the first reflective surface, the second reflective surface and the interface of the building exterior wall is a right triangle; the angle formed by the first reflective surface and the interface of the building exterior wall or the angle formed by the second reflective surface and the interface of the building exterior wall is adjusted to reflect sunlight with an incident angle greater than α back to the sky, where α is 0-90°.

2. The exterior wall film for reflecting sunlight back to the sky according to claim 1, characterized in that: The angle formed between the first reflecting surface and the interface of the building exterior wall is ∠A=45°-α / 2, and the angle formed between the second reflecting surface and the interface of the building exterior wall is ∠B=45°+α / 2.

3. The exterior wall film for reflecting sunlight back to the sky according to claim 1 or 2, characterized in that: The α is determined by the following formula: H is the solar altitude angle, and A is the solar azimuth angle.

4. The exterior wall film for reflecting sunlight back to the sky according to claim 1, characterized in that: The reflectivity of the first reflective surface is greater than 95%, and the mid-infrared emissivity is less than 5%.

5. The exterior wall film for reflecting sunlight back to the sky according to claim 1, characterized in that: The reflectivity of the second reflective surface is greater than 95%, and the mid-infrared emissivity is less than 5%.

6. The exterior wall film for reflecting sunlight back to the sky as claimed in claim 1, characterized in that: The exterior wall film comprises a base layer, a reflective layer and an aluminum-plated layer located between the base layer and the reflective layer. The side of the reflective layer away from the aluminum-plated layer is a first reflective surface or a second reflective surface.

7. The exterior wall film for reflecting sunlight back to the sky according to claim 6, characterized in that: The base layer is made of acrylic resin and has a thickness of more than 5 μm.

8. The exterior wall film for reflecting sunlight back to the sky according to claim 6, characterized in that: The aluminum plating layer is made of aluminum, has a thickness of 80-200 nm, and has a full spectrum reflectivity of 90% or more.

9. The exterior wall film for reflecting sunlight back to the sky according to claim 6, characterized in that: The reflective layer is made of aluminum oxide and has a thickness of 10-50 nm.

10. A building exterior wall, characterized in that: The invention comprises the exterior wall film for reflecting sunlight back to the sky as described in any one of claims 1 to 9.

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