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Abstract

Heat transfer through building envelopes constitutes the dominant part of indoor cooling load in summer. Coating building external walls with high reflectivity materials proves to be an effective way to reduce heat gains from solar radiation and save cooling energy consumption accordingly. In this paper, the transient heat transfer model of building external envelopes is established and validated through experiment, to investigate the thermal performance of building walls coated with retro-reflective materials. Moreover, taking an office building in Chengdu as an illustrative example, the cooling energy saving potential of such retro-reflective material coated building is evaluated in summer. The experiment results show that for the building box with retro-reflective coating materials (r = 0.59), the average indoor air temperature is about 2.4℃ lower than the reference box without coating materials, resulted from decreasing heat absorption of solar radiation for external walls. Furthermore, the illustrative example in Chengdu shows the cooling load can be reduced by about 9.1 W/m², with such retro-reflective coating materials for building external walls, saving 15.2% electricity consumption in a whole summer. The incremental investment for coating can be paid back by 9.1 years for the studied case. Moreover, economic analysis and comparison indicate that such coating material is more applicable to southern cities in China, since the payback period is shorter due to more cooling energy saving for those with hot summer. This work can provide guidance for practical building envelope thermal design.

Keywords

Solar radiation heat transfer building wall coating material cooling load

Introduction

With the rapid modernization and urbanization over the recent two decades, global energy usage keeps growing dramatically with an average annual rate of over 10% (Douthwaite, 2002; Zhang et al., 2015). Therein, buildings account for about 30% of total energy consumption (Luis et al., 2008). Hence, the increasing demand for building energy supplies, coupled with the severe global environment problem, stimulates the search for more energy efficient and low emission buildings and systems (Deng et al., 2011). According to the latest statistic data, heating ventilation and air conditioning (HVAC) system constitutes the main part of building energy consumption, to meet indoor thermal comfort requirement (Zhang et al., 2015).

Optimization of building thermal design, especially for the building envelopes, plays an important role in improving indoor thermal comfort and saving energy consumption for space cooling and heating (Lee and Braun, 2008; Yang et al., 2007). It is reported that the main indoor cooling loads derive from the heat gain through building external envelopes including walls, windows and ceiling, caused by indoor and outdoor temperature difference (Asan and Sancaktar, 1998). Many researchers are dedicated to building passive system optimization to reduce heat transfer through envelopes (Luis et al., 2008). Gregory et al. (2008) investigated the effect of thermal mass on thermal performance of residential buildings and defined the decrement factor for building walls based on the indoor and outdoor temperature difference. Cheng et al. (2014) simulated the transient heat transfer process of building envelopes and obtained the time lag and decrement factors for typical building envelope materials. Talyor and Miner (2014) studied the influence of thermal–physical properties of building envelopes on temperature distribution, in order to evaluate the indoor thermal uncomfortable degree.

Except for the influence of ambient temperature, solar radiation also plays an indispensable role in determining indoor thermal comfort level (Gul et al., 2016; Rajendran and Smith, 2015). Bakirci (2008) put forward the theoretical model to evaluate the solar energy conversion and utilization system. He studied the capability of solar energy absorption for passive building envelopes and maintained that solar radiation highly impacts building load demands during the daytime. Berry et al. (2013) investigated the temperature decline effect of window sunshade to decrease indoor cooling load caused by solar radiation. Santamouris et al. (2008) utilized innovative materials of high reflectivity on building external walls to reduce heat gains and found that although the direct reflectivity of such material could reach as high as 0.8, the overall cooling effect was not that desirable because of the diffuse reflection. Then Nishioka et al. (2008) tested the thermal–physical properties of retro-reflective surface material. Based on that, Yuan et al. (2015) coated building external walls with retro-reflective materials and simulated the influence of building surfaces with different reflective characteristics on the albedo of urban canyons. Rossi et al. (2014) found that using retro-reflective materials as building envelope coating materials could mitigate the urban heat island. Meng et al. (2016, 2015) measured the indoor air temperature variations in the building coated with retro-reflective materials and explained the heat transfer mechanism for the walls.

Available research on the retro-reflective materials mainly focused on material preparation or its mechanical and thermal–physical property study, whereas seldom studied the cooling energy saving effect of buildings coated with such materials. During recent years, applications of high reflectivity materials in buildings have aroused more and more concerns. How to evaluate the cooling energy saving potential of such building envelope coating material is a new and important research topic. In this paper, in order to investigate the thermal performance of building external walls with retro-reflective materials, the transient heat transfer model is built. Moreover, the electricity consumption (EC) of such retro-reflective material coated office building in Chengdu is evaluated in a whole summer season based on the established model. In addition, economic analysis is conducted to compare the payback periods of coating for five typical cities located in different climate zones in China. This work can provide guidance for practical building envelope thermal design.

Mathematical model

Heat transfer of external wall

In order to simplify analysis, the two-plate room model is used and this model has been validated before for building indoor environment simulations (Jiang et al., 2012; Wang et al., 2014). Figure 1 gives the heat transfer process of building external walls. The transient heat transfer equations for the external wall are as follows $ρ_{w} c_{p, w} \frac{\partial T_{w}}{\partial τ} = k_{w} \frac{\partial^{2} T_{w}}{\partial x^{2}}$ (1)

Figure 1.

Heat transfer processes of building external walls.

Boundary conditions $h_{out} (T_{out} - T_{w, out}) + q_{r, out} = - k_{w} \frac{\partial T_{w}}{\partial x} |_{x = 0}$ (2) $h_{in} (T_{in} - T_{w, in}) + q_{r, in} = k_{w} \frac{\partial T_{w}}{\partial x} |_{x = L}$ (3) where ρ_w, c_p,w and k_w are the wall density (kg/m³); specific heat (J/(kg℃)) and thermal conductivity (W/(m℃)), respectively; q_r,in means the indoor heat gains of people, equipment and solar radiation penetrated from windows (W/m²), while q_r,out contains outdoor heat gains of solar radiation, long wave radiation from the ground and sky (W/m²).

According to available research (Fang and Li, 2000), the equivalent temperature of solar radiation is related to the thermal–physical properties of the external surface material of building envelopes, which determines the ratio of solar absorption (a_sol) and reflection (r_sol) for the wall surface $T_{sol, eq} = \frac{a_{sol} q_{sol}}{h_{out}} = \frac{(1 - r_{sol}) q_{sol}}{h_{out}}$ (4)

The long wave radiation contains two parts: sky radiation and ground radiation. According to the heat radiation principle, there is $q_{lw, out} = q_{sky} + q_{g} = σ ɛ_{w, sky} ϕ_{w, sky} [T_{w, out}^{4} - T_{sky}^{4}] + σ ɛ_{w, g} ϕ_{w, g} [T_{w, out}^{4} - T_{g}^{4}]$ (5)

Then the equivalent temperatures of long wave radiations can be obtained ${T_{sky, eq} = \frac{σ ɛ_{w, sky} ϕ_{w, sky} [T_{w, out}^{4} - T_{sky}^{4}]}{h_{out}} T_{g, eq} = \frac{σ ɛ_{w, g} ϕ_{w, g} [T_{w, out}^{4} - T_{g}^{4}]}{h_{out}}$ (6)

Based on the equivalent radiation temperatures, equation (2) can be changed into the following form $h_{out} [T_{out} - T_{w, out} + (T_{sol, eq} - T_{sky, eq} - T_{g, eq})] = - k_{w} \frac{\partial T_{w}}{\partial x} |_{x = 0}$ (7)

The convective heat transfer coefficient of wall external surface can be regarded as a constant value, h_out = 23.3 W/(m²℃) (Zhang et al., 2013). For the internal surface, convective heat transfer coefficient can be obtained by the recommended equation from ASHRAE Handbook (2005) $h_{in} = 1.31 (Δ T)^{\frac{1}{3}} = 1.31 (T_{w, in} - T_{in})^{\frac{1}{3}}$ (8)

Based on the established model, the heat transfer amount through building external walls can be obtained. Furthermore, it can be seen that high solar reflectivity (r_sol) is preferable for low heat gain from solar radiation (q_sol), which can decrease the cooling load demand and save air conditioning energy consumption accordingly in summer.

Simplified room model

In real applications, except for heat transfer through building walls, cooling load is also caused by heat gains from windows and indoor heat source. As Figure 2 shows (two-plate room model), the external wall is viewed as one plate and all internal envelopes (e.g. wall, ceiling and floor) are lumped into another plate, regardless of the long wave radiations between them. Such a simplified room model has been validated before and the calculation error is within 10%, which is quite acceptable for engineering applications (Zhang et al., 2013, 2006).

Figure 2.

Schematic diagram of simplified two-plate room model.

For the energy conversation of the indoor air, there is $V_{room} ρ c_{p} \frac{\partial T_{in}}{\partial τ} = \sum_{j = 1}^{N} Q_{w, j} + Q_{win} + Q_{in} + Q_{ACH} + Q_{C}$ (9) where Q_in means the indoor heat source (W); Q_ACH, Q_w, Q_win represent the ventilation heat gains, indoor air heat convection with walls and window, respectively (W), which can be expressed by $Q_{w, j} = h_{w, in} (T_{w, in, j} - T_{in}) A_{w, j}$ (10) $Q_{win} = U_{win} (T_{out} - T_{in}) A_{win}$ (11) $Q_{ACH} = ρ {Vc}_{p} ACH (T_{out} - T_{in}) / 3600$ (12)

In equation (9), Q_C means the cooling power provided by the air conditioner (i.e. electrical chiller), when the indoor air temperature is higher than the upper value of thermal comfort zone $Q_{C} = {0, T_{in} < T_{H} (T_{in} - T_{H}) V_{room} ρ c_{p} / Δ τ, T_{in} > T_{H}$ (13)

Cooling energy consumption

As equations (9) and (13) show, the cooling energy usage mainly derives from the EC of air conditioners (e.g. electrical compression chiller) for space cooling in summer $EC = \sum_{τ} \frac{Q_{C} (τ)}{COP (τ)} = \sum_{τ} \frac{Q_{C} (τ)}{f [R (τ), T_{a} (τ)]}$ (14) where COP represents the coefficient of performance, which is the function of equipment load ratio under off-design working conditions (i.e. ratio of practical cooling power to the rated one, R) and ambient temperature (T_a) that impacting the condensing temperature of chillers (Li and Ju, 2017; Zhang et al., 2015). In practical applications, the ambient temperature and cooling load always fluctuate during cooling seasons, so COP of chillers vary timely. In this paper, a centrifugal chiller (a typical electrical chiller widely used in air conditioning system) is chosen as the energy supply equipment for space cooling in summer. Its thermal performance under part load conditions were tested (done by China Southwest Architectural Design and Research Institute Corporation). The fitting expression of COP is as follows (through 1stOpt software) $COP (τ) = a + \frac{d \times \exp {- 2 [\frac{R (τ) - b}{c}]^{2}}}{c \times \sqrt{\frac{π}{2}}} + \frac{e - a}{1 + \exp [\frac{T_{a} (τ) - f}{g}]}$ (15) where a = − 516.15, b = 77.10, c = −1165.11, d = −764,654.7, e = −517.08, f = 24.57, g = −1.08, correlation coefficient = 0.996. It can be seen from Figure 3 that COP decreases with increasing ambient temperature, while it declines considerably with declining load ratio (e.g. COP stands at about 4.0 when the chiller works under about 30% part load condition).

Figure 3.

Electrical chillers’ COP variation with changing load ratios and ambient temperatures.

Based on the established model, the hourly cooling load, indoor air temperature and EC can be calculated out. For such a non-linear programme, numerical iteration calculation method is utilized to solve equations (1) to (15) (Zhang et al., 2015, 2013). Figure 4 gives the flow chart of solving such a non-linear programme through MATLAB software. Building information (i.e. thermal–physical properties of external envelopes) and climatic parameters (i.e. ambient temperature and solar radiation intensity) are input to the calculation module to obtain the heat gains through envelopes (equations (1) to (7)) and thermal performance of the electrical chiller (equation (15)). In the starting calculation cycle, cooling power provided by chillers is determined initially. Then the energy balance relationship of indoor air (equation (9)) is used to determine hourly indoor air temperature. Only if the hourly indoor air temperature is lower than the setting upper value for thermal comfort (i.e. 28℃, according to China HVAC design standard, GBJ 19-87), can the iteration be finalized. Otherwise, the new calculation cycle starts by changing cooling power of electrical chiller. Finally, the total EC in the whole cooling season can be obtained through the integration of hourly energy usage (equation (14)).

Figure 4.

Calculation flow chart.

Test and experiment

Reflectivity measurement

Concrete and cement are the mainly used structure materials of building envelopes, and their solar radiation reflectivity (r_sol) are 0.27 and 0.30, respectively (Xiao, 2010). To reduce the solar radiation heat gain in summer, retro-reflective coating material can be painted on the external surface of building walls. According to the standard test method for solar energy reflection of sheet materials (ASTME 424-71-2007), a spectrophotometer (type: CARY 5000, wavelength range: 00–2500 nm, resolution≤0.048 nm) is used to measure the solar reflectivity for different wavelength sunlight. As Figure 5 shows, three samples (locally available and easily purchased) of chosen retro-reflective coating materials are as follows: (1) material 1 – prism type, (2) material 2 – glass ball (made in China) and (3) material 3 – glass ball (made in US).

Figure 5.

Reflectivity measurement of three retro-reflective coating materials through spectrophotometer.

As Figure 6 shows, the chosen retro-reflective materials have high solar reflectivity for visible and ultraviolet sunlight (0.3–2.3 µm), but relatively low reflectivity near infrared area. According to the standard ASTME 424-71-2007, the average solar reflectivity can be expressed by $r = \frac{\int S_{λ} r (λ) d λ}{\int S_{λ} d λ} \approx \frac{\int_{350}^{1800} S_{λ} r (λ) d λ}{\int_{350}^{1800} S_{λ} d λ} (\int_{350}^{1800} S_{λ} d λ = 0.9756)$ (16) where r(λ) is the wavelength-based reflectivity derived from the spectrophotometer and S_λ represents the solar energy ratio for different wavelength (MIL DTL 64159(MR)-2007).

Figure 6.

Solar reflectivity of three samples of chosen retro-reflective coating materials for different wavelength sunlight.

The average solar reflectivity of the chosen retro-reflective coating materials can be calculated out that r₁ = 0.590, r₂ = 0.543, r₃ = 0.487. Hence, these retro-reflective materials show higher solar reflectivity than ordinary building envelope materials (i.e. r = 0.2–0.3). In the following section, the experiment is conducted to investigate the temperature reduction and cooling energy saving effect for building envelopes with such retro-reflective coating material (r = 0.59).

Experiment system

Figures 7 and 8 give the schematic diagram and photos of the experiment system, respectively. Two building box models (L × W × H = 800 mm × 1000 mm × 1300 mm) are constructed to compare the building envelope thermal performance with and without retro-reflective coating materials (1 mm), respectively. The main structure envelope materials of these two building boxes are the same (40 mm polystyrene foam + 0.5 mm stainless steel × 2). Moreover, thermocouples are utilized to measure the temperatures of indoor air, envelope internal and external surfaces, respectively. Then the temperature data are collected by the automatic testing system (JTRG-II).

Figure 7.

Schematic diagram of experiment on building envelope thermal performance with retro-reflective coating material.

Figure 8.

Pictures of experiment system (on the roof of department building of College of Architecture and Environment, Sichuan University, Chengdu, China, 4 August 2016).

Except for the constructed building box models and temperature measurement system, the experiment equipment also contains climatic data collection system including outdoor air temperature, humidity and solar radiation intensity test. The experiment was conducted in six consecutive days in summer (from 4 to 9 August 2016). Figure 9 gives the hourly measured ambient temperature and solar radiation intensity in Chengdu during the experiment period.

Figure 9.

Climatic parameters in Chengdu from 4 to 9 August 2016.

Results and discussion

Temperature variation and comparison

The cooling load of constructed building boxes mainly comes from the heat gain through envelopes (four walls and ceiling with same materials). The aforementioned heat transfer model is used to calculate the temperature distribution, which is then compared to the measured one. The thermal conductivities of polystyrene foam, stainless steel and retro-reflective coating material are supposed to be 0.041, 100 and 20.6 W/m K, respectively (Li, 2012). For long wave radiations, the equivalent radiation temperatures can be regarded as constants (e.g. T_sky,eq = 3.0℃, T_g,eq = 0.6℃) (Xiao, 2010). Box 2 is covered by the aforementioned retro-reflective coating material (r = 0.59) and the two boxes are set under the same conditions. Figure 10 gives the external surface temperature variations for these two building boxes, respectively. It can be seen that the calculated surface temperature based on the built heat transfer model matches well with the tested one, and the average relative error only stands at 5%, which is acceptable for engineering applications.

Figure 10.

External surface temperature variations of the two experimental building boxes in one summer day (8 August 2016).

The measured external, internal and indoor air temperatures of two experimental building boxes are shown in Figure 11, respectively. It is clear that all temperatures fluctuate during this period and the variation trends keep consistent with that of the climatic parameters shown in Figure 9. Particularly, due to solar radiation in the daytime, the external surface of experimental building envelope can reach as high as 60℃, exceeding the ambient temperature. For the same building box, temperature declines monotonously from external surface to indoor air (T_w,out > T_w,in > T_in). Furthermore, the temperature of box 2 with retro-reflective coating materials is much lower than that of box 1 without coating materials (reference box), especially for the external surface, where such temperature difference can arrive at as high as about 20℃. For the indoor environment, the air temperature reaches its maximal value at around 2 p.m. and that of box 2 is 6℃ lower than box 1 during the daytime, resulting from more solar reflection and less heat gains. By contrast, there is almost no indoor air temperature difference between these two building boxes in the night. For the studied case, it is measured that the average indoor air temperature of box 1 reaches 30.7℃ while that of box 2 is 28.3℃. Therefore, coating building envelopes with high-reflectivity material is an effective way to improve indoor thermal comfort and decrease heat gains, which is favourable for saving cooling energy consumption in summer.

Figure 11.

Temperature variations of the two experimental building boxes in six consecutive summer days (4–9 August 2016). (a) External surface, (b) internal surface and (c) indoor air.

Cooling energy saving

Based on the room model, air conditioning energy consumption for space cooling can be calculated out for given known conditions. As Figure 12 shows, a typical room with a south-facing external wall in a multi-stories office building in Chengdu is chosen as an illustrative example (model built by PKPM software). The main geometrical and thermal–physical parameters of the studied room are listed in Table 1. It is assumed that the average value of indoor heat gains from people, lights and equipment is 10.8 W/m² (Zhang et al., 2013) and the triggering temperature of opening air conditioner for space cooling is 28℃ (Standard of PR China (GBJ 19-87), 2001) (Zhang et al., 2015). Figure 13 shows the ambient temperature and solar radiation intensity variations in Chengdu in the whole summer days (from 31 May to 30 September) in one typical year (China Meteorological Administration, 2005).

Figure 12.

An office building in Chengdu (area 6900 m², external surface area 5865 m², ratio of window to wall 0.2).

Table 1.

Room information of the office building in Chengdu.

Items	Values
Length × width × height	5.0 m × 4.0 m × 3.2 m
Ratio of window to wall	0.2
Internal wall	0.2 m concrete hollow block (ρc_p = 1.5 MJ/m³℃, k = 1 W/m℃)
External wall (double layers)	0.25 m reinforced concrete (ρc_p = 2.3 MJ/m³℃, k = 1.74 W/m℃) + 0.07 m polystyrene board (ρc_p = 0.048 MJ/m³℃, k = 0.046 W/m℃)
Double-glazing window	U = 3.1 W/m²℃, SC = 0.44
Internal thermal disturbance	Dynamic change with the work scheduling of typical office buildings, the peak value is 35.4 W/m²
T_H	28℃

Figure 13.

Ambient temperature and solar radiation in Chengdu from 31 May to 30 September in one typical year.

According to the established model in ‘Heat transfer of external wall’, ‘Simplified room model’ and ‘Cooling energy consumption’ sections, iteration calculation is activated (Figure 4). The results of hourly cooling loads in the studied office building in Chengdu are shown in Figure 14(a) and (b). It is clear that with retro-reflective coating materials, the hourly cooling load in the daytime declines significantly, due to the decreasing heat gains from solar radiation. For ordinary building envelope materials (e.g. r = 0.30), the average cooling load is 60.6 W/m². While after coating with the retro-reflective materials (e.g. r = 0.59), average cooling load decreases to 51.5 W/m² under the same working conditions. Then based on the climatic parameters (Figure 13) and equipment model (equation (15)), the average COP of electrical chiller can be obtained, that is COP = 5.8. Thus, for the whole office building in Chengdu (Figure 12), the total EC for space cooling in summer can be reduced from 2.11 × 10⁵ to 1.79 × 10⁵kWh (by 15.2%), after coating external walls with such retro-reflective materials.

Figure 14.

Hourly cooling load of the office room in Chengdu. (a) In whole summer (from 28 May to 30 September) and (b) from 4 to 9 August.

Economic analysis

Even though such coating materials are favourable for saving cooling energy consumption and operation cost of air conditioning systems in summer, the capital investment of building is about to rise since the price of retro-reflective materials is higher than that of ordinary ones. So economical factor plays an important role in feasibility analysis in practical applications. For the studied office building (Figure 12), the total EC for space cooling can be reduced by 3.2 × 10⁴kWh in summer, leading to 1.79 × 10⁴ CNY operation cost saving each year (unit electricity price is 0.56 CNY/kW h for domestic usage (Li, 2012)). On the other hand, the unit price of retro-reflective coating materials is about 40 CNY/m² in China (Li, 2012). Thus, the incremental capital investment for building coating is assessed to be 1.88 × 10⁵ CNY (4692 m² surface area of external walls). The payback period of incremental capital investment equals about 9.1 years (considering the discount rate, an interest rate of central bank in computations of present value, e.g. 10% (Liu and Hsieh, 2016)). So the payback period is acceptable, with respective to the much longer building lifetime.

The previous analysis in ‘Cooling energy saving’ section indicates that the cooling energy saving potential of coating materials highly depends on local climatic conditions, especially the ambient temperature and solar radiation intensity. Therefore, coating building external walls with retro-reflective materials may not be feasible for all situations or places if taking economic factors into consideration. Figure 15 gives five typical cities in different climate zones in China: Harbin, Beijing, Chengdu, Kunming and Guangzhou (China Building Thermal Design Standard, GBT 50176). By using the same method, the ECs before and after coating, as well as the payback periods, can be obtained for these cities, respectively (Figure 16).

Figure 15.

Five typical cities in different climate zones in China.

Figure 16.

Energy saving potential and economic analysis comparison among five typical cities in China.

The variation trend indicates that the cooling energy saving amount of building walls coated with retro-reflective materials declines approximately from south to north in China. For instance, Guangzhou, located in Hot Summer and Cold Winter zone, is of the highest EC for cooling in summer. And the electricity usage can be reduced by 4.4 × 10⁴kWh after coating, resulting in only 6.8 years payback period there. By contrast, the payback period reaches about 21 years in Harbin, since its cooling load is much lower than the other southern cities. Therefore, from the perspective of economic analysis, the studied coating material is more applicable to southern cities with hot summer in China, in order to make full use of solar radiation reflection effect of retro-reflective materials to decrease cooling load.

Conclusions

Heat transfer through building envelopes constitutes the dominant part of indoor cooling load in summer. Coating building external walls with high solar reflectivity materials is an effective way to reduce heat gains from solar radiation and save cooling energy consumption accordingly. In this paper, the transient heat transfer model of building external envelopes is established and validated through experiment, to investigate the thermal performance of building walls coated with retro-reflective materials. Moreover, taking an office building in Chengdu as an illustrative example, the cooling energy saving potential of such retro-reflective material coated building is evaluated in summer. In addition, economic analysis is conducted to compare the payback periods of such coating materials in different climate zones. It can be concluded that

Retro-reflective coating materials can decrease both the surface temperature of external walls and heat gains from solar radiation during the daytime. For the experiment case, average indoor temperature in the building box coated with retro-reflective materials (r = 0.59) is about 2.4℃ lower than that of the reference box.

The average cooling load can be reduced by about 9.1 W/m², with such retro-reflective coating materials for the office building in Chengdu. Moreover, EC for space cooling decreases from 2.11 × 10⁵ to 1.79 × 10⁵kWh, leading to 15.2% energy saving in a whole summer season.

For the studied case, operation cost for space cooling in summer can be saved by 1.79 × 10⁴ CNY. However, the capital investment for coating increases by 1.88 × 10⁵ CNY. The incremental capital investment can be paid back by 9.1 years in Chengdu. Through economic comparison, the payback period increases nearly from south to north in China. So such retro-reflective coating material is more applicable to southern cities with hot summer.

In practical engineering fields, building cooling load and energy consumption are influenced by various factors, such as building types, climatic conditions and indoor thermal comfort requirements, etc. Besides, reducing heat gains from solar radiation through coating envelopes with retro-reflective materials can decrease cooling load in summer, but it also inevitably increase heating load in winter, when solar radiation is much more preferable. The present work only discusses a simple case to show the cooling energy saving potential and preliminary application of retro-reflective coating materials used in building external envelopes. Although the specific results obtained from the studied case may not be applicable to all situations, the analysis approach used here is general. This work can provide guidance for practical building thermal design.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This research is financed by National Key Research and Development Program of China (2016YFC0700406),National Natural Science Foundation of China (51478280) and China Scholarship Council (201706245001).

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Solar radiation reflective coating material on building envelopes: Heat transfer analysis and cooling energy saving

Abstract

Keywords

Introduction

Mathematical model

Heat transfer of external wall

Simplified room model

Cooling energy consumption

Test and experiment

Reflectivity measurement

Experiment system

Results and discussion

Temperature variation and comparison

Cooling energy saving

Economic analysis

Conclusions

Footnotes

Declaration of conflicting interests

Funding

References