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

Transcription

1 Original Article Solar radiation reflective coating material on building envelopes: Heat transfer analysis and cooling energy saving Energy Exploration & Exploitation 17, Vol. 35(6) ! The Author(s) 17 DOI: / journals.sagepub.com/home/eea Yin Zhang, Enshen Long, Yanru Li and Pan Li 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 ¼ 0.59), the average indoor air temperature is about 2.4 C 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/m 2, 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 College of Architecture and Environment, Sichuan University, Chengdu, China Corresponding author: Yin Zhang, College of Architecture and Environment, Sichuan University, Chengdu , China. cdzhangyin@163.com Creative Commons CC-BY: This article is distributed under the terms of the Creative Commons Attribution 4.0 License ( which permits any use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (

2 Zhang et al. 749 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, 02; Zhang et al., 15). Therein, buildings account for about 30% of total energy consumption (Luis et al., 08). 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., 11). 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., 15). 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, 08; Yang et al., 07). 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., 08). Gregory et al. (08) 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. (14) 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 (14) 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., 16; Rajendran and Smith, 15). Bakirci (08) 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. (13) investigated the temperature decline effect of window sunshade to decrease indoor cooling load caused by solar radiation. Santamouris et al. (08) 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. (08) tested the thermal physical properties of retroreflective surface material. Based on that, Yuan et al. (15) 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. (14) found that using retro-reflective materials as building envelope coating materials could mitigate the urban heat island. Meng et al. (16, 15) 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.

3 750 Energy Exploration & Exploitation 35(6) 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., 12; Wang et al., 14). Figure 1 gives the heat transfer process of building external walls. The transient heat transfer equations for the external wall are as w w c ¼ 2 T w 2 ð1þ Boundary conditions h out ðt out T w,out Þþq r,out ¼ k j x¼0 ð2þ h in ðt in T w,in Þþq r,in ¼ k j x¼l ð3þ where w, c p,w and k w are the wall density (kg/m 3 ); specific heat (J/(kg C)) and thermal conductivity (W/(m C)), respectively; q r,in means the indoor heat gains of people, equipment and solar radiation penetrated from windows (W/m 2 ), while q r,out contains outdoor heat gains of solar radiation, long wave radiation from the ground and sky (W/m 2 ). According to available research (Fang and Li, 00), the equivalent temperature of solar radiation is related to the thermal physical properties of the external surface Wall Solar radiation Long wave radiation q sol q lw q in Indoor radiation T out h out h in T in Outdoor heat convection Indoor heat convection 0 L x Figure 1. Heat transfer processes of building external walls.

4 Zhang et al. 751 material of building envelopes, which determines the ratio of solar absorption (a sol ) and reflection (r sol ) for the wall surface T sol,eq ¼ a solq sol h out ¼ ð1 r solþq sol h out The long wave radiation contains two parts: sky radiation and ground radiation. According to the heat radiation principle, there is ð4þ q lw,out ¼ q sky þ q g ¼ " w,sky w,sky ½T 4 w,out T4 sky Š þ " w,g w,g ½T 4 w,out T4 g Š Then the equivalent temperatures of long wave radiations can be obtained 8 h i " w,sky w,sky T 4 w,out T4 sky >< T sky,eq ¼ h h out i " w,g w,g T 4 w,out T4 g >: T g,eq ¼ h out ð5þ ð6þ Based on the equivalent radiation temperatures, equation (2) can be changed into the following w h out T out T w,out þðt sol,eq T sky,eq T g,eq Þ ¼ j x¼0 ð7þ The convective heat transfer coefficient of wall external surface can be regarded as a constant value, h out ¼ 23.3 W/(m 2 C) (Zhang et al., 13). For the internal surface, convective heat transfer coefficient can be obtained by the recommended equation from ASHRAE Handbook (05) h in ¼ 1:31ðTÞ 1 3 ¼1:31ðTw,in T in Þ 1 3 ð8þ 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 q r,out q r,out t a,out h w,out t a,out h w,in t a,in ACH q in t a,in h w,in ACH hw,out q in Figure 2. Schematic diagram of simplified two-plate room model.

5 752 Energy Exploration & Exploitation 35(6) (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., 13, 06). For the energy conversation of the indoor air, there is V room c ¼ XN j¼1 Q w,j þ Q win þ Q in þ Q ACH þ Q C ð9þ 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 ¼ h w,in ðt w,in,j T in ÞA w,j ð10þ Q win ¼ U win ðt out T in ÞA win Q ACH ¼ Vc p ACHðT out T in Þ=3600 ð11þ ð12þ 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 ¼ 0, T in 5 T H ð13þ ðt in T H ÞV room c p =, T in 4 T H 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 ¼ X Q C ðþ COPðÞ ¼ X Q C ðþ f ½RðÞ, T a ðþš 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, 17; Zhang et al., 15). 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) n o d exp 2½ RðÞ b c Š 2 e a COPðÞ ¼a þ p c ffiffi 2 þ 1 þ exp T aðþ f g ð14þ h i ð15þ

6 Zhang et al. 753 Figure 3. Electrical chillers COP variation with changing load ratios and ambient temperatures. where a ¼ , b ¼ 77.10, c ¼ , d ¼ 764,654.7, e ¼ , f ¼ 24.57, g ¼ 1.08, correlation coefficient ¼ 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). 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., 15, 13). 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 C, 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)). 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, 10). To reduce the

7 754 Energy Exploration & Exploitation 35(6) Climatic parameters Start Building parameters Heat transfer model Thermal performance model of chiller Overall heat gains Cooling load Cooling power provided by chiller Indoor air energy conservation Indoor air temperature Tin No Iteration Tin TH? Yes Total electricity consumption by chillers Output Figure 4. Calculation flow chart. 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 ), a spectrophotometer (type: CARY 5000, wavelength range: nm, resolution0.048 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). As Figure 6 shows, the chosen retro-reflective materials have high solar reflectivity for visible and ultraviolet sunlight ( mm), but relatively low reflectivity near infrared area. According to the standard ASTME , the average solar reflectivity can be expressed by R R 1800 Z S rðþd r ¼ R S d 350 S rðþd 1800 R 1800 S d ¼ 0:9756 ð16þ 350 S d 350 where r() is the wavelength-based reflectivity derived from the spectrophotometer and S represents the solar energy ratio for different wavelength (MIL DTL 64159(MR)-07). The average solar reflectivity of the chosen retro-reflective coating materials can be calculated out that r 1 ¼ 0.590, r 2 ¼ 0.543, r 3 ¼ Hence, these retro-reflective materials show higher solar reflectivity than ordinary building envelope materials (i.e. r ¼ ). In the following section, the experiment is conducted to investigate the temperature reduction and

8 Zhang et al. 755 Figure 5. Reflectivity measurement of three retro-reflective coating materials through spectrophotometer r(%) Material 1 Material 2 Material l (nm) Figure 6. Solar reflectivity of three samples of chosen retro-reflective coating materials for different wavelength sunlight.

9 756 Energy Exploration & Exploitation 35(6) Building box model Box 1: no coating material Box 2: with coating material Polystyrene foam (40 mm) retro-reflective material (1 mm) Thermocouple Stainless steel (0.5 mm) Data collector Figure 7. Schematic diagram of experiment on building envelope thermal performance with retro-reflective coating material. cooling energy saving effect for building envelopes with such retro-reflective coating material (r ¼ 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 ¼ 800 mm 1000 mm 1300 mm) are constructed to compare the building envelope thermal performance with and without retroreflective coating materials (1 mm), respectively. The main structure envelope materials of these two building boxes are the same (40 mm polystyrene foam 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). 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 16). Figure 9 gives the hourly measured ambient temperature and solar radiation intensity in Chengdu during the experiment period. 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 retroreflective coating material are supposed to be 0.041, 100 and.6 W/m K, respectively (Li, 12). For long wave radiations, the equivalent radiation temperatures can be regarded as constants (e.g. T sky,eq ¼ 3.0 C, T g,eq ¼ 0.6 C) (Xiao, 10). Box 2 is covered by the

10 Zhang et al. 757 Figure 8. Pictures of experiment system (on the roof of department building of College of Architecture and Environment, Sichuan University, Chengdu, China, 4 August 16) Solar radiation Temperature q sol (W/m 2 ) T out ( o C) Time (h) Figure 9. Climatic parameters in Chengdu from 4 to 9 August 16.

11 758 Energy Exploration & Exploitation 35(6) Box 2_test Box 1_test Box 2_calculate Box 1_calculate T ( o C) Time (h) Figure 10. External surface temperature variations of the two experimental building boxes in one summer day (8 August 16). aforementioned retro-reflective coating material (r ¼ 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. 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 C, 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 C. For the indoor environment, the air temperature reaches its maximal value at around 2 p.m. and that of box 2 is 6 C 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 C while that of box 2 is 28.3 C. 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. 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 southfacing 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/m 2 (Zhang et al., 13) and the triggering temperature of opening air conditioner for space cooling is 28 C (Standard of

12 Zhang et al. 759 (a) (b) (c) T ( o C) T ( o C) T ( o C) Box 1 Box Time (h) Box 1 Box Time (h) Box 1 Box Time (h) Figure 11. Temperature variations of the two experimental building boxes in six consecutive summer days (4 9 August 16). (a) External surface, (b) internal surface and (c) indoor air. PR China (GBJ 19-87), 01) (Zhang et al., 15). 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, 05). 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 ¼ 0.30), the average cooling load is 60.6 W/m 2. While after coating with the retro-reflective materials (e.g. r ¼ 0.59), average cooling load decreases to 51.5 W/m 2 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 ¼ 5.8. Thus, for the whole office building in Chengdu (Figure 12), the total EC for space cooling in summer can be reduced from to kwh (by 15.2%), after coating external walls with such retro-reflective materials.

13 760 Energy Exploration & Exploitation 35(6) Figure 12. An office building in Chengdu (area 6900 m 2, external surface area 5865 m 2, 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 (c p ¼ 1.5 MJ/m 3 C, k ¼ 1 W/m C) External wall (double layers) 0.25 m reinforced concrete (c p ¼ 2.3 MJ/m 3 C, k ¼ 1.74 W/m C) m polystyrene board (c p ¼ MJ/m 3 C, k ¼ W/m C) Double-glazing window U ¼ 3.1 W/m 2 C, SC ¼ 0.44 Internal thermal disturbance Dynamic change with the work scheduling of typical office buildings, the peak value is 35.4 W/m 2 28 C T H 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

14 Zhang et al Ambient temperature Solar radiation T out ( o C) Q sol (W/m 2 ) Time (h) Figure 13. Ambient temperature and solar radiation in Chengdu from 31 May to 30 September in one typical year. (a) Without coating material With retro-reflective materials 1 Q c (W/m 2 ) (b) Q c (W/m 2 ) Time (h) Without coating material With retro-reflective materials /4 8/5 8/5 8/7 8/8 8/9 Time (h) 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.

15 762 Energy Exploration & Exploitation 35(6) Figure 15. Five typical cities in different climate zones in China. 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 kwh in summer, leading to CNY operation cost saving each year (unit electricity price is 0.56 CNY/kW h for domestic usage (Li, 12)). On the other hand, the unit price of retro-reflective coating materials is about 40 CNY/m 2 in China (Li, 12). Thus, the incremental capital investment for building coating is assessed to be CNY (4692 m 2 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, 16)). 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). 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 kwh after coating, resulting in only 6.8 years payback period there. By contrast, the payback

16 Zhang et al. 763 Electricity Consumption ( 10 5 kwh) EC_no coating material EC_with coating material Payback period Harbin Beijing Chengdu Kunming Guangzhou Payback period (year) Figure 16. Energy saving potential and economic analysis comparison among five typical cities in China. 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 retroreflective 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 (1) 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 ¼ 0.59) is about 2.4 C lower than that of the reference box. (2) The average cooling load can be reduced by about 9.1 W/m 2, with such retro-reflective coating materials for the office building in Chengdu. Moreover, EC for space cooling decreases from to kwh, leading to 15.2% energy saving in a whole summer season. (3) For the studied case, operation cost for space cooling in summer can be saved by CNY. However, the capital investment for coating increases by 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.

17 764 Energy Exploration & Exploitation 35(6) 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. 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 (16YFC ), National Natural Science Foundation of China ( ) and China Scholarship Council ( ). References American Society of Heating Refrigerating and Air-Conditioning Engineers (ASHRAE). ASHRAE Handbook Fundamentals, Chapter 3. Heat Transfer. Atlanta: ASHRAE, 05. Asan H and Sancaktar YS (1998) Effects of wall s thermophysical properties on time lag and decrement factor. Energy and Buildings 28: ASTME Standard test method for solar energy transmittance and reflectance (terrestrial) of sheet materials, 07. Bakirci K (08) Models for determination of solar energy potential. Energy Exploration & Exploitation 26(5): Berry R, Livesley SJ and Aye L (13) Tree canopy shade impacts on solar irradiance received by building walls and their surface temperature. Building and Environment 69: Cheng R, Wang X and Zhang YP (14) Analytical optimization of the transient thermal performance of building wall by using thermal impedance based on thermal-electric analogy. Energy and Buildings 80: China Meteorological Administration. China Architecture-Specific Meteorological Data Sets for Thermal Environment Analysis. Beijing: China Architecture & Building Press, 05 (in Chinese). Deng J, Wang RZ and Han GY (11) A review of thermally activated cooling technologies for combined cooling, heating and power systems. Progress in Energy and Combustion Science 37: Douthwaite R (02) Building limited fossil energy supplies into the world monetary system. Energy Exploration & Exploitation (6): Fang XD and Li YZ (00) Numerical simulation and sensitivity analysis of lattice passive solar heating walls. Solar Energy 69(1): Gregory K, Mohtaderi B, Sugo H, et al. (08) Effect of thermal mass on the thermal performance of various Australian residential construction systems. Energy and Buildings 40:

18 Zhang et al. 765 Gul M, Kotak Y and Muneer T (16) Review on recent trend of solar photovoltaic technology. Energy Exploration & Exploitation 34(4): Jiang F, Wang X and Zhang YP (12) Analytical optimization of specific heat of building internal envelope. Energy Conversion and Management 63: Lee KH and Braun JE (08) Model based demand limiting control of building thermal mass. Building and Environment 43: Li MF and Ju YL (17) The analysis of the operating performance of a chiller system based on hierarchal cluster method. Energy and Buildings 138: Li P (12) Research of the cooling effect and energy saving on retro-reflective material. Master Dissertation, Sichuan University, Chengdu, China (in Chinese). Liu CT and Hsieh BZ (16) Assessment model for estimating CO 2 commercial storage capacity in saline formations: A case study in Taiwan. International Journal of Greenhouse Gas Control 49: Luis PE, Jose O and Christine P (08) A review on buildings energy consumption information. Energy and Buildings 40(3): Meng X, Luo T, Wang ZY, et al. (16) Effect of retro-reflective materials on building indoor temperature conditions and heat flow analysis for walls. Energy and Buildings 127: Meng X, Wang CX, Liang WJ, et al. (15) Thermal performance improvement of prefab houses by covering retro-reflective materials. Procedia Engineering 121: MIL DTL 64159(MR)-07 (07) Coating, water dispersible aliphatic polyurethane, chemical agent resistant. Nishioka M, Inoue S and Sakai H (08) Retroreflective properties calculating method based on geometrical-optics analysis. Proceedings of Japanese Architectural Institute (AIJ) Environmental Engineering 633: Rajendran P and Smith H (15) Modelling of solar irradiance and daylight duration for solarpowered UAV sizing. Energy Exploration & Exploitation 34(2): Rossi F, Pisello AL, Nicolini A, et al. (14) Analysis of retro-reflective surfaces for urban heat island mitigation: a new analytical model. Applied Energy 114: Santamouris M, Synnefa A, Kolokotsa D, et al. (08) Passive cooling of the built environment-use of innovative reflective materials to fight heat islands and decrease cooling needs. International Journal of Low Carbon Technology 3(2): Talyor RA and Miner M (14) A metric for characterizing the effectiveness of thermal mass in building materials. Applied Energy 128: Wang X, Cheng R, Zeng RL, et al. (14) Ideal thermal physical properties of building wall in an active room. Indoor and Built Environment 23: Xiao W (10) Study of the direct-gain solar heating in remote southwest Tibet. PhD Dissertation, Tsinghua University, Beijing, China (in Chinese). Yang L, Lam JC and Liu JP (07) Analysis of typical meteorological years in different climates of China. Energy Conversion and Management 48(2): Yuan J, Farnham C and Emura K (15) Development of a retro-reflective material as building coating and evaluation on albedo of urban canyons and building heat loads. Energy and Buildings 103: Zhang Y, Zhang YP, Wang X, et al. (13) Ideal thermal conductivity of a passive building wall: Determination method and understanding. Applied Energy 112: Zhang YP, Lin KP, Zhang QL, et al. (06) Ideal thermophysical properties for free-cooling (or heating) buildings with constant thermal physical property material. Energy and Buildings 38(10): Zhang YP, Zhang Y, Shi WX, et al. (15) A new approach based on the inverse problem and variation method for solving building energy and environment problems: Preliminary study and illustrative examples. Building and Environment 91:

19 766 Energy Exploration & Exploitation 35(6) Appendix Notation a solar absorption ratio (%) A area (m 2 ) ACH air exchange rate (h 1 ) c p specific heat ((J/(kg C)) COP coefficient of performance EC electricity consumption (kw h) h convective heat transfer coefficient (W/(m 2 K)) k thermal conductivity (W/(m K)) L length/thickness (m) q heat flow density (W/m 2 ) Q heat capacity (W) r solar reflectivity (%) R load ratio T temperature (K/ C) U overall heat transfer coefficient (W/(m 2 K)) V volume (m 3 ) density (kg/m 3 ) time (s) view factor Subscripts a c g in lw out sky sol w win ambient cooling ground indoor long wave outdoor sky solar radiation wall window