Mitigating solar heat gain through windows of buildings by reflective films A case study in Iraq

Abstract

Keywords: Cooling; Radiation; Retro- reflective; Shading; Window

Introduction

The past few decades have seen an increase in energy consumption and related greenhouse gas emissions. Buildings are one of the main sectors with energy demand, especially air-conditioning. As an example of a very hot and dry climate, Iraq experiences harsh summers that increase

the cooling energy consumption by up to 70% [1]. In order to reduce the thermal impact on buildings, effective ways, such as thermal insulation materials and adequate windows should be implemented to satisfy energy-efficient buildings. Window is a necessary element for daylight and vision, but it allows undesirable heat to let in in summer

*Corresponding author. *E-mail address: tawfeeqwasmi@uomustansiriyah.edu.iq This paper was recommended for publication in revised form by Editor-in-Chief Ahmet Selim Dalkılıç Published by Yıldız Technical University Press, İstanbul, Turkey Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

or wanted heat to escape in winter through its thin pane of glass or vents. Thus, it reduces the building thermal performance in comparison with the else parts of the building envelope, like the roof and the walls. Currently, many procedures are used as effective ways to reduce the solar radiation in summer, such as using windows of high thermal performance (double- or triple-glazing), overhangs, louvers and shadings [2]. Furthermore, glazing can be integrated with photovoltaic layers to compose what so-called solar windows. These windows convert incident solar radiation into electrical power, and allow for space heating or lighting. Recently, many studies have been conducted to develop building-integrated PV panels [3]. Shading is the most effective passive approach that reduces the heat gain through the window, and improves the indoor thermal comfort. It may come as reflective layers and nets or as tinted films. The basic idea of window shading is to transmit visible light as much as possible, while reflecting the heat component back to the outside [4]. On this aspect, a Low-E glass, which is layered by metallic oxides, is commonly implemented for this purpose [5]. This type of glass can reduce the emission of IR and UV rays to the inside, and keep a desired intensity of light passing through. Low-E glass is coated by a thin and transparent layer helps reflect a band of thermal rays. In the winter, low-E glass avoids the heat escaping to the outdoor, thus keep the inside warm with almost stable indoor temperature [5, 6]. The shaded low-E double-glazing window is very effective to allow the light inside while keeping the heat out. It’s vital to utilize such kind of glass in combination with suitably controlled blinds, specifically blinds that allow the light to enter at a slat angle and sufficiently block the direct solar radiation component above a threshold value [7]. Glass absorbs some colors or wavelengths of light and reflects others. Even a black surface that is glossy reflects. Lighter colored objects reflect the majority of light that hits them, and darker objects absorb most of the light. There are three basic types of reflectivity: Scattered, specular, and retro [8]. Scattered or diffused reflectivity is the common way of reflectivity. In this type, when light hits the surface, it bounces off and goes in multiple directions. Specular or mirror reflectivity is when most of a beam of light reflects in another direction depending on the angle of

the surface. Finally, the retro reflectivity is where the light reflects only back to the source direction. Man-made retro reflectivity is created in two ways. One is through the use of perfectly round glass spheres that take in light, bend it, and then return it. The other method is through the use of prisms that also bend light and return it. Prismatic arrays have more efficiency than glass beads. Metallizing beads or prisms enhance reflectivity by giving them a mirror finish [8, 9]. Reflective coatings can be classified as high-reflective coatings (HR-coatings) and retro-reflective coatings (RR-coatings) [10]. For HR-coatings, reflective radiation would be intercepted via adjacent buildings; therefore, HR-coatings have some drawbacks. The drawbacks appear clearly for the case of multiple buildings, where there is an accumulated heat outdoor [10]. The RR-coatings have attracted further attention owing to its capability of reflecting solar radiation back along the incident direction, which reduces the absorbed radiation amount by the closest constructions [10]. RR-coating may decrease the cooling load in summer, but it increases the heat demand during the winter months. To improve seasonal variation and achieve minimum annual energy, the best retro-reflectivity should be selected according to the specific weather characteristics of the region [11]. Therefore, a retro-reflective film has been proposed as an active method for blocking the solar radiation in summer and avoiding the overheating of indoor space. Table 1 shows some aspects of using reflective coatings [12-17]. Shading minimizes the incident solar radiation upon surfaces, especially through windows, and hence dramatically affects the building energy performance. In large buildings, it is found that the films that have suitable reflection properties are able to achieve energy savings up to 30% [18]. Technologies, such as layering low-E shading films are common recently in energy-efficient glazing units. The films with retro-reflective properties have been installed in high-rise buildings in many locations worldwide. In traditional reflective films (HR-coatings), the reflected rays from the external face of the glass surface lies toward neighboring objects and buildings, thus rises the average surrounding temperature outdoor, and helps in increasing the possibility of thermal bridging to indoor [19]. On the architectural

Table 1. Usages of some reflective coating with corresponding features Item

A roof in the building with a HR-reflective layer has 10% less When RR-coating is used for the building envelope, up to heat gain comparing to the conventional one [12]. 6% heat reduction can be seen comparing to HR-coating one [15].

HR-coating for south wall can decrease the indoor air temperature up to 5oC comparing to that without coating [13].

The air temperature inside a tent can be decreased by 7oC as maximum using RR-coating [16].

A building’s roof with HR-coating consumes cooling load 5-35% less than the ordinary one [14].

A model showed that the cooling demand can be decreased up to 20% using RR-coating on a large window [17].

aspects, reflective films have many advantages related to the aesthetic values. Since modern architecture utilizes curved forms, hence it is important to develop flexible films [20, 21]. Within the subject, several studies have presented some ideas upon the importance of shading the windows in controlling the indoor overheating in hot climates (or for cooling purposes) with some examples of the energy saving applications or available data. The studies focus upon various configurations for reflective shadings as films, coatings or layers. The ideas in these studies may have some developed suggestions or ability to satisfy a range of applications. They show reliable research works focusing upon the contribution of shading films in mitigating the solar rays in and corresponding overheating, as well as improving the characteristics and performance of the reflective shadings. The studies looked into the selecting of proper window films depending on the heat insulation features [22-25], or on the optical properties of the films [26, 27]. Some of the studies investigated the effect of the film on the energy saving for air-conditioning [24, 25, 28-30], or on the internal glass temperature [27, 31, 32]. However, numerous studies practiced the window films in testing rooms [29, 30, 33], in rig models [22, 31, 32], or just used computational methods [23, 34]. The retro-reflective films were the interest of some studies to enhance the indoor and outdoor thermal conditions in summer [28, 35, 36]. The current study aims to study the role of different reflective films in mitigating the solar radiation passing through regular glazed window and corresponding heat gain for a building in Iraq. The research presents the results of a realistic and actual investigation for the effect of using shading films on the heat transmitted through the glass of a building in an extreme hot area, with a thermal evaluation of multiple patterns of these films. The originality is implementing retro-reflective films beside common shading films with different shading rates. It needs to concentrate on the passive ways to reduce heat gain and reduce energy cooling for such conditions.

Materials And Methods

In this study, a set of thin colored films have been used with high reflectivity. The films have different colors, as black, red and gray, as shown in Figure 1. The films have a range of shading rates from 30 to 80% [37]. The specifications of these films are mentioned in Table 2. Furthermore, a retro-reflective film (with red color and 50% shading rate) has been served as an advanced case. The structure of the retro-reflective film surface has concentric circles. The whole films have been purchased from the local market as Chinese products. Window under test, shown in Figure 2, was chosen to be in a room in the Lab of Materials-Mustansiriyah University, Baghdad. The dimensions of the room are 6 x 5 x 4 m3. The window has dimensions of 1.7 m by 1.4 m of 6 mm thick, and it faced the south orientation. A glass area of 1 m2 was confined, and the remainder was insulated using Styrofoam

Results And Discussion

Ambient Conditions in Summer Data were recorded during the summer season (August and September, 2023), where the maximum ambient temperature could reach up to 50oC, as shown in Figure 4. The direct solar radiation during midday could reach up to 1150

W/m2, as depicted in Figure 5. The maximum intensity of the light outdoor ranged between 120,000-160,000 Lux, as displayed in Figure 6. Note that the readings in days of dust were displaced to show reliable analysis. Data obtained reveal that the highest indoor temperature took place about 2-3 PM owing to overheating as a result of accumulation inside. The outdoor solar radiation intensity ranged between (250 W/m2) in the early morning to (1100 W/m2) at 11:45 AM. Also, note that the solar intensity after noon is comparatively lower than that before it owing to the usual dusty weather in evening as a result of wind movement passing through the western desert. Collected Data The study evaluates the contribution of reflective films in reducing the overheating indoor and corresponding effects. The study is seeking into stating the effect of film

Figure 4. Variation of ambient temperature during the summer season (August and September).

Figure 5. Variation of direct solar radiation during the summer season (August and September).

Figure 6. Variation of outdoor light intensity during the summer season (August and September). nature, color and shading rate on the thermal conditions,

the reflective film is suitable in such hot region and climatic

to reconcile between the allowing of suitable amount of light

to in and avoid the heat as much as possible. Therefore, the

17. Sep – 19 Sep

The case studies include: Without film, with film (different colors and shading rates), and with retro-reflective film. These case studies covered the suggested duration (6 Aug-19 Sep), as evinced in Table 5. The obtained data for the surface temperatures of the glass are illustrated as following: Figure 7 in case of no film is used, Figure 8 in case of using black film, Figure 9 in case of using red film, Figure 10 in case of using gray film, and Figure 11 in case of using retro-reflective red film.

The obtained data for the solar radiation through the glass are shown as following: Figure 12 in case of no film is used, Figure 13 in case of using black film, Figure 14 in

Figure 7. Surface temperature of the glass in case of no film.

Figure 8. Surface temperature of the glass in case of using black film.

Figure 9. Surface temperature of the glass in case of using red film.

Figure 10. Surface temperature of the glass in case of using gray film.

Figure 11. Surface temperature of the glass in case of using retro-reflective red film. case of using red film, Figure 15 in case of using gray film, and Figure 16 in case of using retro-reflective red film. The results manifested the potential of black sheets to eliminate the solar radiation by more than 70%. Note that the orientation of the window is toward the south, where the sun has

a high angle at the noon time. This means that the solar rays are incident on the roof. Therefore, the readings of indoor solar intensity at noon time were lower comparing to that recorded at morning or evening. Also, note that the recorded radiation is the instantaneous value.

Figure 12. Solar radiation across the glass in case of no film.

Figure 13. Solar radiation across the glass in case of using black film.

Figure 14. Solar radiation across the glass in case of using red film. The obtained data for the light intensity through the glass are shown in the following figures: Figure 17 for the case of no film is used, Figure 18 for the case of using black film, Figure 19 for the case of using red film,

Figure 20 for the case of using gray film, and Figure 21 for the case of using retro-reflective red film. Note that the external readings for the light intensity have recorded low values due to the sun shade provided by

Figure 15. Solar radiation across the glass in case of using gray film.

Figure 16. Solar radiation across the glass in case of using retro-reflective red film.

Figure 17. Light intensity across the glass in case of no film. the overhang. However, the light intensity still higher comparing to other places or conditions. Where, the summer in Baghdad is usually clear with the absence of

clouds mostly. Also, the location of the selected building and the orientation toward the sun offer high light intensity.

Figure 18. Light intensity across the glass in case of using black film.

Figure 19. Light intensity across the glass in case of using red film.

Figure 20. Light intensity across the glass in case of using gray film.

Figure 21. Light intensity across the glass in case of using retro-reflective red film. Behavior of the Films By looking deeply into the films, it can be recognized that both black film and retro-reflective film have shown the best reduction in the intensity of solar radiation that measured indoor (about 55% reduction), then red, and then gray. The gained radiation is the main reason for the overheating. However, the using of black film leads to undesired rise in the internal surface temperature due to the capability of black surfaces (or dark colors) in absorbing more heat [39]. Red and gray films elucidated less temperature rise. On the other hand, the retro-film didn’t show that the rise in the temperature owing to the capability of retro-film in reflecting the incident rays back to the original direction they come from. Therefore, retro-films have the minimum possibility for diffusing or absorbing the radiation [8]. Regarding the light, the black film with a high shading rate reduces the intensity below 7000 Lux through the daytime, while the retro-film keeps it higher (between 7000-10000 Lux). Therefore, a (80%) shading rate is not recommended. Heat Gain The capacity of heat gain via a window is influenced by several factors, including types of glass and frame, orientation, shading, air leakage and heat bridge (a region in the frame conducts heat faster than nearby regions). Heat transfer across this element can be determined by [40]: (1) Where: U: The overall heat transfer coefficient (W/m2.K) of window A: The area of window (m2) to: the temperature of outdoor (oC) ti: The indoor temperature (oC) SHGC: Solar heat gain coefficient I: Incident radiation (W/m2) The overall coefficient of heat transfer through the window can be calculated as [40]:

(2) Where, (g) refers to the glass, and (f) refers to the frame. By simplification this relation through neglecting the frame and conduction via glass, it yields; (3) Where, ho and hi are the natural convection heat transfer coefficients for external and internal surfaces of the glass, respectively. These coefficients can be found based on Nusselt number, as following [41]; (4) Where, k is the thermal conductivity of air, while l is the height of glass. Nusselt number can be calculated based on outdoor and indoor air conditions as following [41]; (5) Where, Gr is Grashof number, while Pr is Prandtl number. And, they can be determined depending on air properties at film temperature (average temperature between ambient and surface of glass). Solar heat gain coefficient (SHGC) of the window is the ratio of the incident solar radiation that can pass through the window to indoor [42]. Thus, it can been calculated by; SHGC = Iin / Iout

Now, the average values of U-factor and SHGC for the single glazed window of various films, depending on the behavior of the films under the current study, are listed in Table 6 [See the appendix for more information].

Table 6. Thermal parameters of the glasses determined under the current study Type of glass

shading film can reduce the transmitted radiation by more than 50%, and makes a reduction in the heat gain up to 400 W/m2, which is too close to that determined in the current study. Cooling Load In order to determine the air-conditioning load for a 20 m2 room as a standard area for a room with various cases, a local application called “Iraq Passive Planning Package (IPPP)” [43] was employed. And, this program includes equations and relations mentioned in ASHRAE handbook [44]. The cooling load was estimated based on the transmitted heat from outside without the existence of any indoor heat. Note that the load represents a magnitude calculated for the entire day and for the whole month. Materials, elements and limits assumed during the calculation are listed in Table 7. The total cooling load can be determined by:

Figure 22. Average heat gain through the window for various cases at daytime.

Figure 22 shows the average heat gain through the window for different cases relying upon the resulted data. Here, the outdoor temperature is fixed at 50oC, as the maximum ambient temperature in summer. Also, consider the indoor temperature is fixed at 24oC, as the design value. It can be observed that the conventional heat gain through the regular window without any films is about 660 W/m2, which is relatively high in contrast with the cases of using the film. Where, the least heat gain is 315 W/m2 (about 52% decrement) achieved by using black film of 80% shading rate, but with undesired intensity of light indoor. The case of using retro-reflective film also portrayed a low heat gain, which is 330 W/m2 (about 50% decrement) with an acceptable light intensity indoor. Note that Yousif et al. [31] have mentioned in their study that a

(7) Where,; Q represents the heat transfer through a certain element (wall, roof, floor, window and door), and it can be found by: (8) Where, U is the overall heat transfer coefficient though the element, and A is the exposed surface area, while ΔT is the temperature discrepancy across the element. The ventilation loses is given by: (9)

Table 7. Features of the typical room assumed to determine the cooling load Part

1 cm gypsum, 25 cm Brick and 1 cm cement plaster. There’re (3) internal walls and merely one external wall.

Conclusion

The role of certain reflective films in mitigating the solar heat gain through regular glazed window and corresponding cooling load was investigated for a building in Iraq as an example of a hot arid climate. The study measured three parameters: Internal surface temperature of the glass, internal intensity or radiation, and internal intensity of light. The main findings of the results can be concluded as:

1. The use of reflective films (50% shading) reduces the

heat gain through the window and decreases the corresponding energy consumption by 290, 400, 480 and 540 W, for the cases of using gray, red, black and retro-reflective films, respectively.

2. The corresponding energy savings comparing to the

regular clear glass are 3.7, 5.1, 6.2, and 6.9%, respectively

4. Films with high shading rates can reduce the radiation

indoor, but they have a drawback due to the weak light indoor. Therefore, they are not recommended.

5. The use of retro-films elucidates a suitable light indoor

and doesn’t manifest the rise in the surface temperature owing to the capability of retro-film in reflecting the incident rays right back without diffusion.

Acknowledgment

The authors would like to express their gratitude to the College of Engineering in Mustansiriyah University, Iraq.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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