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The effect of reflective coatings on building surface temperatures, indoor environment and energy consumption—An experimental study
Energy and Buildings 43 (2011) 573–580
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
The effect of reflective coatings on building surface temperatures, indoor environment and energy consumption—An experimental study Hui Shen a , Hongwei Tan b , Athanasios Tzempelikos a,∗ a b
School of Civil Engineering, Purdue University, 550 Stadium Mall Dr., West Lafayette, IN 47907 USA College of Mechanical Engineering, Tongji University, 1239 Siping Rd., Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 24 June 2010 Received in revised form 8 October 2010 Accepted 18 October 2010 Keywords: Reflective coatings Electricity consumption Envelope materials Indoor environment
a b s t r a c t This paper presents an experimental study on the impact of reflective coatings on indoor environment and building energy consumption. Three types of coatings were applied on identical buildings and their performance was compared with three sets of experiments in both summer and winter. The first experiment considers the impact of coatings on exterior and interior surface temperatures, indoor air temperatures, globe temperature, thermal stratification and mean radiant temperatures for non-conditioned buildings (free-floating case); the second one focused on the impact of coatings on reduction of electricity consumption in conditioned spaces; in the third experiment, the impact of different envelope material properties equipped with different coatings was investigated. The results showed that, depending on location, season and orientation, exterior and interior surface temperatures can be reduced by up to 20 ◦ C and 4.7 ◦ C respectively using different coatings. The maximum reduction in globe temperature and mean radiant temperature was 2.3 ◦ C and 3.7 ◦ C in that order. For the conditioned case, the annual reduction in electricity consumption for electricity reached 116 kWh. Nevertheless, the penalty in increased heating demand can result in a negative all-year effect in Shanghai, which is characterized by hot summers and cold winters. © 2010 Elsevier B.V. All rights reserved.
1. Introduction and background Solar radiation incident on building envelope can be absorbed, reflected and transmitted and it influences interior and exterior surface temperatures, heat flow entering the building, and hence indoor thermal environment. Increasing the envelope reflection coefficient (exterior surface reflectivity weighted by the spectral energy distribution and integrated over the solar spectrum) results in reduced absorbed solar radiation and reduced surface temperatures, which allow reduction of conduction heat transfer to the building interior. Utilization of solar reflective coatings has shown great potential to reduce solar heat gain, cooling loads and peak power loads while improving indoor thermal conditions. The following sections present a short literature review on previous studies with reflective coatings. 1.1. Previous experimental studies—the impact of reflective coatings on surface and indoor air temperature A number of studies on reflective coatings have been carried out, commonly focused on warm climates and summer conditions. Most
∗ Corresponding author. Tel.: +1 765 496 7586; fax: +1 765 494 0395. E-mail address: [email protected] (A. Tzempelikos). 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.10.024
studies on this topic are experimental and contain measurements of reduction in interior and exterior surface temperatures and their impact on room air temperature. Givoni and Hoffman [1] compared the resulting indoor temperature for unventilated buildings in Israel and they reported that it could be 3 ◦ C cooler in the summer when the buildings were painted white compared with when painted grey. Synnefa et al. [2] reported “cool materials” for buildings and other urban surfaces application, and measured a surface temperature reduction of 4 ◦ C for white concrete tile when painted with reflective coatings. Cheng et al. [3] performed experiments with test cells to investigate the influence of envelope color on indoor temperatures under hot and humid weather condition. They showed that the maximum difference of inside air temperature between a black and a white cell was about 12 ◦ C for lightweight construction. Taha et al. [4] measured the reflectivity and surface temperatures of various materials used in urban surfaces and found that white electrometric coatings with a reflectivity of over 0.72 could be as high as 45 ◦ C cooler than black coatings with a reflectivity of 0.08. Bansal et al. [5] performed experiments under different conditions and found that black painted envelopes resulted in up to 7 ◦ C (per unit volume of interior space) higher temperature than the corresponding white painted envelopes. Uemoto et al. [6] showed that cool colored paint formulations produced significantly higher near infrared radiation reflectance than conventional paints of similar colors, and measured a more than 10 ◦ C reduction in surface
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temperatures when exposed to infrared radiation. Reagan and Acklam [7] calculated the total building heat gain reduction when roof surface reflectivity increased from 0.35 to 0.75. In July, the reductions were 6.4% and 4.8% in Tucson, Arizona with ceiling thermal resistances of 2.5 and 5.88 m2 K/W respectively. Griggs [8] found up to 65% reduction in heat flux through a white roof compared to black roof with the same thermal resistances of 1.32 m2 K/W. 1.2. Previous experimental studies—the impact of reflective coatings on energy consumption and peak demand Several published articles focus on the potential reduction in energy consumption for air conditioning and peak demand, which are important parts of energy costs. Energy consumption and peak demand savings with reflective coatings strongly depend on climatic conditions (except for the envelope and coating material properties and space type/configuration). Most studies were conducted in California: Akbari et al. [9] compared cooling energy and peak power consumption of two identical school bungalows with different roof reflectivities and found a 3.1 kWh (35%) savings in cooling energy as well as 0.6 kW peak demand savings with reflective roofs. They also concluded [10] that increasing the roof reflectance of commercial buildings in California from about 20% to 60% decreased the roof temperature on hot summer afternoons by 7.2 ◦ C. In another study [11], they reported electricity savings of 0.5 kWh/day (or 33 Wh/m2 /day) by increasing roof reflectivity from 26% to 72%, which translates into annual energy savings of about 125 kWh. For the same location, Hildebrandt et al. [12] measured daily air-conditioning savings of 17%, 26% and 39% in an office, a museum and a hospital with high reflectivity roofs. Parker et al. [13] monitored six homes in Florida before and after application of high-albedo coatings on their roofs. Reduction in air-conditioning electricity consumption was measured between 11% and 43% with an average saving of 9.2 kWh/day, and reduction in peak power demand (occurs between 5 and 6 pm) was 0.4–1.0 kW with an average reduction of 0.7 kW. They also monitored seven retail stores within a strip mall in Florida. After applying a reflective roof coating, a 7.5 Wh/m2 (25%) drop in daily summertime cooling-energy use and a 0.65 W/m2 (29%) decrease in demand were realized [14]. Akridge [15] reported daily savings of 75 Wh/m2 (28%) for an education building in Georgia by painting the galvanized roof with white acrylic coating. The same researcher [16] also measured a reduction of 33 ◦ C in peak roof temperature of a single storey building after application of a thermal control coating. In Nevada, Akbari and Rainer [17] measured daily air-conditioning energy savings of 33 Wh/m2 (only 1%) in two telecommunication regeneration buildings. In Texas, Konopacki and Akbari [18] measured daily energy savings of 39 Wh/m2 (11%) and peak-power reduction of 3.8 W/m2 (14%) in a large retail store when a reflective membrane was used. Energy savings in an office building in Mississippi reached 22% after application of a reflective roof coating [19]. In Hong Kong, Cheung et al. [20] showed that 30% reduction in solar absorptance can achieve 12% saving in annual required cooling energy. 1.3. Previous computational studies on reflective coatings Other studies employed modeling to calculate the potential benefits of reflective materials. Taha et al. [21] performed simulations and predicted a cooling load reduction of 18.9% for summer days in California when the roof and walls reflectivity was increased from 0.30 to 0.90. The simulated ceiling and wall thermal resistances were 5.28 and 3.35 m2 K/W respectively. Anderson [22] found similar reductions for a flat concrete roof of a simple single room. Konopacki et al. [23] simulated the direct energy savings from high reflective roofs in 11 US metropolitan areas, and computed
Fig. 1. Spectral reflectivity of coatings over the solar spectrum.
the annual electricity savings in old residences, new residences and old/new office buildings to be 55%, 15% and 25% respectively. A simulation study by Shariah et al. [24] for two mild and hot climates, showed that, as the reflectance increases from 0 to 1, the total energy load decreases by 32% and 47% for noninsulated buildings and by 26% and 32% for insulated buildings. Wang et al. [25] developed a dynamic model and compared the annual cooling load, heating load and electricity consumption of a retail shed coated with solar reflective materials for six locations around the world. Other studies also used computer simulations to estimate the effect of reflective roofs [26–28]. Finally, Tang and Zhou [29] analyzed the relationship between outdoor sol-air temperature and solar radiation absorptance and investigated the influence of wall reflectance on annual building energy consumption for three Chinese cities representing hot summer and cold winter zone. Akbari and Levinson [30] reviewed and compared the technical development of cool-roof provisions in the ASHRAE Standards [31,32], and California Title 24 Standards and discussed the treatment of cool roofs in other standards and energy efficiency programs. This paper presents an experimental study about the impact of reflective coatings on building surface temperatures, air temperature, globe temperature, energy consumption and thermal comfort for buildings located in Shanghai, China. This location is characterized by hot summers and cold winters, and the overall effects of reflective coatings are complex considering the potential benefits in the summer and the potential penalties during winter. In parallel, another experiment with four smaller test cells was carried out to investigate the impact of envelope material thermal properties combined with reflective coatings.
2. Properties of selected coatings Three types of coatings were included in this study. Their spectral reflectivity was measured with a spectrophotometer over the main solar range and it is shown in Fig. 1. Coating A is general coating and it is used as the base case; coatings B and C are reflective coatings that have similar reflectivity over the visible spectrum but significant differences in the infrared spectrum. Both of these coatings reflect more at the infrared which accounts for a large portion of solar radiation and therefore they have the potential of reducing solar heat gain and, consequently, cooling load. The integrated average reflectivities for coatings A, B and C are 32%, 42% and 61% respectively. The main components of the coatings are summarized in Table 1.
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Fig. 2. (a) Building surface temperature measuring points. (b) Room air temperature measurement points at different heights.
Table 1 Type and main components of selected coatings. Coating
Type/main agents
Color
Paint thickness
A B C
Ordinary coating Reflective coating: acrylic, pigment, ceramic particle High reflective coating: ethylene tetrafluoride, pigment, butyl acetate
Light blue Light blue Light blue
30–40 m 30–40 m 30–40 m
Table 2 Properties of measured envelope material samples. Sample
Dimensions (m)
Conductance (W/m-K)
Resistance (m2 -K/W)
Galvanized steel sheet Polystyrene plate PE plastic board Concrete slab
0.5 × 0.5 × 0.006 0.5 × 0.5 × 0.07 0.5 × 0.5 × 0.004 0.5 × 0.5 × 0.03
44.2 0.041 0.48 0.79
1.36 × 10−5 1.71 7.1 × 10−3 3.8 × 10−2
3. Experimental setup and data collection 3.1. Surface temperatures and indoor environment In order to measure the effect of reflective coatings on envelope temperatures, indoor thermal conditions and energy demand, two identical buildings were built and placed near each other. They have the same dimensions of 7 m × 7 m × 4 m, the same envelope components and are equipped with the same HVAC system. One building was painted with coating A (Building A) and the other was painted with coating C (Building C). As this study is focused on envelope and indoor temperature improvement and envelope heat gain reduction with reflective coatings, the buildings have no windows. Building surface temperatures, indoor air tempera-
ture at different heights, indoor globe temperature and electricity consumption were measured and recorded every 10 min. The meteorological conditions, including environmental air temperature, global solar radiation and direct normal radiation, were recorded with the same frequency using a weather station installed near the buildings. The measuring points are shown in Fig. 2(a) and (b). Ten thermo recorder TR-71W sensors (one recorder has two channels) were used to measure and monitor temperatures. Each of them was assigned a number as shown in Fig. 2. Recorders numbered 1–5 measured surface temperatures of the roof and the vertical walls (channel 1 for interior surface temperatures; channel 2 for exterior surface temperatures). Recorders numbered 6–9 measured indoor air temperatures from floor to roof height at 0.5 m intervals.
Fig. 3. Schematic and pictures of test cells for evaluation of effect of envelope properties with reflective coatings.
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4.1. Results and discussion for free floating case The measurement period included summer, winter and shoulder season conditions to evaluate the effect of reflective coatings on indoor thermal environment for buildings A and C. During the measurement period, data was collected from 9:00 am to 6:00 pm. Representative weather data for a 3-day period during winter and summer is shown in Fig. 4. Average daily total solar radiation is 13.6 MJ/m2 in summer and 10.7 MJ/m2 in winter. Average daily
800
25
700
20
600 500
15
400
10
300 200
5
100 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 9:50 10:50 11:50 12:50 13:50 14:50 15:50 16:50 17:50 9:40 10:40 11:40 12:40 13:40 14:40 15:40 16:40 17:40
0
0
700
20
600
Temperature (ºC)
2
25
Solar radiation (W/m )
Time
4. Experimental results and discussion This study includes three separate experiments. The two main experiments were carried out from August 2008 to March 2009, to compare indoor thermal conditions in building A and building C under real weather conditions. The first case considered free floating conditions in the rooms, while in the second case the spaces were conditioned and electricity consumption for air conditioning was measured. The third experiment took place in September 2008, comparing coating effects applied on different envelope surface materials. In the following sections, representative experimental results are presented for all cases.
1000 900
30
2
35
Temperature (ºC)
In parallel with the main experimental measurements, another secondary study was carried out in order to investigate the impact of envelope material thermal properties combined with reflective coatings. Four test cells with roof dimensions 0.5 m × 0.5 m were built and used for measuring the resulting surface temperatures and heat flow through the roof section using the four different materials presented in Table 2. For each material, the three selected coatings were tested. Surface temperatures were measured with thermo Recorder TR-52, and heat flux through samples was measured with Heat Flow Meter HFM-215. Fig. 3 shows a schematic of the testing configuration and sample pictures.
Outdoor air temperature Global solar radiaon Direct normal solar radiaon
Solar radiation (W/m )
3.2. Effect of envelope properties and reflective coatings
500
15
400 10 300 5
200
0 -5
100 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 9:50 10:50 11:50 12:50 13:50 14:50 15:50 16:50 17:50 9:40 10:40 11:40 12:40 13:40 14:40 15:40 16:40 17:40
576
0
Time Fig. 4. Representative weather data for (a) summer and (b) winter.
mean ambient temperature is 28.6 ◦ C in summer and 11.3 ◦ C in winter. The results show that reflective coatings have a significant effect on surface temperatures. Since the roof has a different construction from walls, and buildings are usually multi storied, only wall surface temperatures were compared to identify the most applicable
Fig. 5. Maximum and average surface temperature reductions for exterior surfaces in (a) summer (b) winter and for interior surfaces in (c) summer (d) winter.
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Fig. 7. Measured indoor (a) air temperature and (b) globe temperature at 1.5 m from floor.
difference of surface heat gain. Therefore, a west wall in Shanghai is the best location to use reflective coatings because it has the greatest heat gain reduction in summer and less heat loss increase in winter when compared to other orientations. Another factor that has to be considered is that west walls usually have smaller window ratio than south walls. Measured indoor air temperature and globe temperature within work area (at 1.5 m from floor) are shown in Fig. 7. The results indicate that application of reflective coatings reduces indoor air temperature and globe temperature in both summer and winter. The average reduction in air temperature is 1.8 ◦ C in the summer and 1.6 ◦ C in winter; globe temperature was reduced by 2.3 ◦ C in the summer and by 2.1 ◦ C in winter respectively. The mean radiant temperature is a key factor influencing occupants’ thermal comfort; it can be calculated from the measured wall surface temperatures (Eq. (1)). 4
Tr = T1 4 Fp−1 + T2 4 Fp−2 + · · · + TN 4 Fp−N Fig. 6. Measured (a) exterior and (b) interior surface temperatures for west wall and (c) exterior and (d) interior surface temperatures for south wall.
orientation. Maximum and average surface temperature reductions for every wall are shown in Fig. 5. In the summer, the greatest exterior surface temperature reduction occurs on the west wall (maximum 19.9 ◦ C, average 6 ◦ C), with the south wall following. In winter, south wall has the greatest exterior temperature reduction (maximum 17.2 ◦ C, average 8.3 ◦ C), with the west wall following. Interior surface temperature reduction shows a similar trend. Detailed measurements of surface temperatures for the west and south wall with two coatings are presented in Fig. 6. It should be noticed that the difference between exterior and interior surface temperature reduction indicates the
(1)
where Tr is mean radiant temperature, K; TN is the surface temperature of surface N, K; Fp−N is the angle factor between a person and surface N. Mean radiant temperatures were calculated for a standing person (height: 1.7 m) in the middle of the room using the measured surface temperatures and the respective angle factors [33], as shown in Fig. 8. Average reductions in calculated mean radiant temperature are 1.82 ◦ C and 1.87 ◦ C in summer and winter. The maximum reductions are 3.7 ◦ C and 3.3 ◦ C respectively, appearing at 12:30 in summer and 11:50 in winter. Fig. 9 illustrates the vertical air temperature distribution at these two peak times. Thermal stratification naturally occurs in both Buildings A and C, and the difference in temperatures along the height of the (identical) rooms is significant. At 1.7 m from the floor (occupant height), the temperature in Building A is 3 ◦ C higher than the temperature in Building C in the summer (similarly in winter).
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Fig. 8. Mean radiant temperature calculated from measured surface temperatures in (a) summer days and (b) winter days.
4.2. Results and discussion for conditioned case In the second set of experiments, both Buildings A and C were conditioned (indoor temperature set point was 24 ◦ C in all seasons).
Fig. 9. Measured indoor air temperature at different heights for (a) summer and (b) winter peak times with different coatings.
Fig. 10. Average surface temperature reductions (A/C on) in (a) summer and (b) winter.
The average daily total solar radiation is 15.5 MJ/m2 in summer and 10.9 MJ/m2 in winter, while the average daily mean ambient temperature is 28.6 ◦ C in summer and 11.3 ◦ C in winter. Fig. 10 presents the measured average temperature differences of exterior and interior surfaces for each wall. Air conditioning operation results in smaller temperature decrease. Since the indoor air temperature are controlled at 24 ◦ C in both summer and winter, all walls have similar temperatures in Buildings A and C. Heat gains through walls depend on exterior surface temperatures. As shown in Fig. 10, exterior temperature reduction in summer is smaller than in winter, so it is reasonable to expect a smaller heat gain reduction in summer than heat loss increase in winter. The measured electricity consumption for air conditioning also indicates such a trend (Fig. 11). Note that the average air temperature of working hours in cooling period for Shanghai is 29.3 ◦ C, and the average daily total radiation is 18.2 MJ/m2 ; respectively, the average temperature of working hours in heating period is 10.4 ◦ C, and the average daily total radiation is 10.1 MJ/m2 . The reduction in daily electricity consumption for September is 2.62 kWh between Building A and C, while the increase in daily electricity consumption for December is 2.55 kWh. Annual electricity consumption changes can be estimated as follows: assuming that the electricity consumption has a linear relationship with indoor-outdoor temperature difference, and considering that the recommended cooling and heating set points in Shanghai are 26 ◦ C and 18 ◦ C respectively. The cooling degree hour within working time is 2443.1 h and the heating degree hour within working time is 11471.6 h in Shanghai. Then the total reduction of cooling electricity consumption is estimated to be 116.4 kWh and increase of heating electricity consumption is equal to 327 kWh. In this estimation, only envelope heat gain is considered, neglecting all other heat gains, for example occupancy and equipment heat gains which
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Fig. 11. Measured electricity consumption with air conditioning for representative days in (a) September and (b) December with two different coatings.
increase cooling loads but reduce heating loads, so a better performance can be expected. The fact that summer electricity shortage is a long municipal problem adds merits to application of reflective coatings. Furthermore, it should be noted that, although air conditioning is on in both Buildings A and C (and hence the two buildings have very similar air temperatures), the application of reflective coatings still influences indoor thermal comfort by changing the temperature difference between interior surfaces and air temperature.
Fig. 12. Measured (a) exterior surface temperatures, (b) interior surface temperatures and (c) heat flux for concrete slab sample.
4.3. Comparison of coating effects applied on different surface materials
Table 2 were painted with coatings A, B and C. Their reflectances were measured with a spectrophotometer (wavelength range: 300–2100 nm). It was found that the coating reflectance is not affected by the material. Every set of samples was then simultaneously exposed to same environmental conditions. Surface
In order to investigate the possible influence of different surface materials on coating performance, the four materials presented in
Table 3 Average measured temperatures, heat fluxes and weather data. Sample
Galvanized steel sheet Polystyrene plate PE plastic board Concrete slab
Tex (◦ C)
Tin (◦ C)
q (W/m2 )
A
B
C
A
B
C
A
B
C
44.2 48.8 44 42.3
42.6 47.3 42.3 40.6
39.5 42.6 39.7 37.8
43.5 39.3 43.9 41.2
42.2 38.5 42.2 39.5
39.4 37.7 39.4 37.4
55 17.5 44.6 39.1
43.5 13.9 30.7 26.2
23.4 8.3 12.9 11.6
To (◦ C)
I (W/m2 )
30.3 32.6 31.3 31.4
434 548 435 388
Tex : average exterior surface temperature; Tin : average interior surface temperature; q: average heat flux; To : average outdoor air temperature; I: average solar radiation.
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temperatures and heat fluxes were measured. Fig. 12 presents the measured exterior surface temperatures, interior surface temperatures and heat fluxes in the concrete slab. Average measured temperatures and heat fluxes for all four types of tested materials are listed in Table 3. Comparing reductions in surface temperatures and heat flux as function of reduction in absorbed solar radiation, the experimental results showed that material thermal resistance has a small effect on exterior surface temperatures; however, it has a more profound impact on interior surface temperatures and of course on heat flux. 5. Conclusion This paper presents experimental results for the impact of solar reflective coatings on building surface temperatures, indoor environment, heat gains and electricity consumption under real weather conditions in summer and winter. Three types of coatings were applied on identical buildings and their performance was compared with a set of three separate experiments: free-floating case, conditioned spaces and different envelope materials. For the non-conditioned case, the results indicate the significant effect of the coatings on lowering building surface temperatures. In the summer, an increase of surface reflectance from 32% to 61% resulted in average reduction of exterior surface temperature reduction of 6 ◦ C on the west wall, and in the winter a respective reduction of 8.3 ◦ C on the south wall. The maximum reduction in exterior surface temperature of the west wall reached 19.9 ◦ C in the summer. Reductions in interior surface temperatures showed similar trends. The average reductions in indoor air temperature and globe temperature at 1.5 m above ground were in the order of 2 ◦ C during both seasons. Mean radiant temperatures were calculated from the measured surface temperatures and respective angle factors. For a standing person in the center of the room, the average reductions reached 1.8 ◦ C in both summer and winter. For the conditioned case, the indoor air temperature was controlled at 24 ◦ C. A reduction of 2.62 kWh in electricity consumption was measured during a representative summer day, while an increase of 2.55 kWh was observed during a typical winter day. Considering the recommended set points for cooling (26 ◦ C) and heating (18 ◦ C) in Shanghai, the estimated net overall effect of increasing envelope reflectance from 32% to 61% is negative. Finally, four different envelope materials were painted with three different coatings (reflectivities: 32%, 42% and 61%) and were exposed to the same environment conditions. Surface temperatures and heat fluxes were measured and compared. The thermal resistance of materials has a significant impact on the coatings effect—the coatings perform better when applied on materials with smaller thermal resistance. Acknowledgements The authors would like to thank Daikin Industries Ltd. for the financial and operational support. References [1] B. Givoni, M.E. Hoffiman, Effect of building materials on internal temperatures, Research Report, Building Research Station, Technion Haifa, 1968. [2] A. Synnefa, M. Santamouris, I. Livada, A study of the thermal performance of reflective coatings for the urban environment, Solar Energy 80 (2006) 968– 981. [3] V. Cheng, E. Ng, B. Givoni, Effect of envelope colour and thermal mass on indoor temperatures in hot humid climate, Solar Energy 78 (2005) 528–534.
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
The effect of reflective coatings on building surface temperatures, indoor environment and energy consumption—An experimental study Hui Shen a , Hongwei Tan b , Athanasios Tzempelikos a,∗ a b
School of Civil Engineering, Purdue University, 550 Stadium Mall Dr., West Lafayette, IN 47907 USA College of Mechanical Engineering, Tongji University, 1239 Siping Rd., Shanghai 200092, China
a r t i c l e
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Article history: Received 24 June 2010 Received in revised form 8 October 2010 Accepted 18 October 2010 Keywords: Reflective coatings Electricity consumption Envelope materials Indoor environment
a b s t r a c t This paper presents an experimental study on the impact of reflective coatings on indoor environment and building energy consumption. Three types of coatings were applied on identical buildings and their performance was compared with three sets of experiments in both summer and winter. The first experiment considers the impact of coatings on exterior and interior surface temperatures, indoor air temperatures, globe temperature, thermal stratification and mean radiant temperatures for non-conditioned buildings (free-floating case); the second one focused on the impact of coatings on reduction of electricity consumption in conditioned spaces; in the third experiment, the impact of different envelope material properties equipped with different coatings was investigated. The results showed that, depending on location, season and orientation, exterior and interior surface temperatures can be reduced by up to 20 ◦ C and 4.7 ◦ C respectively using different coatings. The maximum reduction in globe temperature and mean radiant temperature was 2.3 ◦ C and 3.7 ◦ C in that order. For the conditioned case, the annual reduction in electricity consumption for electricity reached 116 kWh. Nevertheless, the penalty in increased heating demand can result in a negative all-year effect in Shanghai, which is characterized by hot summers and cold winters. © 2010 Elsevier B.V. All rights reserved.
1. Introduction and background Solar radiation incident on building envelope can be absorbed, reflected and transmitted and it influences interior and exterior surface temperatures, heat flow entering the building, and hence indoor thermal environment. Increasing the envelope reflection coefficient (exterior surface reflectivity weighted by the spectral energy distribution and integrated over the solar spectrum) results in reduced absorbed solar radiation and reduced surface temperatures, which allow reduction of conduction heat transfer to the building interior. Utilization of solar reflective coatings has shown great potential to reduce solar heat gain, cooling loads and peak power loads while improving indoor thermal conditions. The following sections present a short literature review on previous studies with reflective coatings. 1.1. Previous experimental studies—the impact of reflective coatings on surface and indoor air temperature A number of studies on reflective coatings have been carried out, commonly focused on warm climates and summer conditions. Most
∗ Corresponding author. Tel.: +1 765 496 7586; fax: +1 765 494 0395. E-mail address: [email protected] (A. Tzempelikos). 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.10.024
studies on this topic are experimental and contain measurements of reduction in interior and exterior surface temperatures and their impact on room air temperature. Givoni and Hoffman [1] compared the resulting indoor temperature for unventilated buildings in Israel and they reported that it could be 3 ◦ C cooler in the summer when the buildings were painted white compared with when painted grey. Synnefa et al. [2] reported “cool materials” for buildings and other urban surfaces application, and measured a surface temperature reduction of 4 ◦ C for white concrete tile when painted with reflective coatings. Cheng et al. [3] performed experiments with test cells to investigate the influence of envelope color on indoor temperatures under hot and humid weather condition. They showed that the maximum difference of inside air temperature between a black and a white cell was about 12 ◦ C for lightweight construction. Taha et al. [4] measured the reflectivity and surface temperatures of various materials used in urban surfaces and found that white electrometric coatings with a reflectivity of over 0.72 could be as high as 45 ◦ C cooler than black coatings with a reflectivity of 0.08. Bansal et al. [5] performed experiments under different conditions and found that black painted envelopes resulted in up to 7 ◦ C (per unit volume of interior space) higher temperature than the corresponding white painted envelopes. Uemoto et al. [6] showed that cool colored paint formulations produced significantly higher near infrared radiation reflectance than conventional paints of similar colors, and measured a more than 10 ◦ C reduction in surface
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temperatures when exposed to infrared radiation. Reagan and Acklam [7] calculated the total building heat gain reduction when roof surface reflectivity increased from 0.35 to 0.75. In July, the reductions were 6.4% and 4.8% in Tucson, Arizona with ceiling thermal resistances of 2.5 and 5.88 m2 K/W respectively. Griggs [8] found up to 65% reduction in heat flux through a white roof compared to black roof with the same thermal resistances of 1.32 m2 K/W. 1.2. Previous experimental studies—the impact of reflective coatings on energy consumption and peak demand Several published articles focus on the potential reduction in energy consumption for air conditioning and peak demand, which are important parts of energy costs. Energy consumption and peak demand savings with reflective coatings strongly depend on climatic conditions (except for the envelope and coating material properties and space type/configuration). Most studies were conducted in California: Akbari et al. [9] compared cooling energy and peak power consumption of two identical school bungalows with different roof reflectivities and found a 3.1 kWh (35%) savings in cooling energy as well as 0.6 kW peak demand savings with reflective roofs. They also concluded [10] that increasing the roof reflectance of commercial buildings in California from about 20% to 60% decreased the roof temperature on hot summer afternoons by 7.2 ◦ C. In another study [11], they reported electricity savings of 0.5 kWh/day (or 33 Wh/m2 /day) by increasing roof reflectivity from 26% to 72%, which translates into annual energy savings of about 125 kWh. For the same location, Hildebrandt et al. [12] measured daily air-conditioning savings of 17%, 26% and 39% in an office, a museum and a hospital with high reflectivity roofs. Parker et al. [13] monitored six homes in Florida before and after application of high-albedo coatings on their roofs. Reduction in air-conditioning electricity consumption was measured between 11% and 43% with an average saving of 9.2 kWh/day, and reduction in peak power demand (occurs between 5 and 6 pm) was 0.4–1.0 kW with an average reduction of 0.7 kW. They also monitored seven retail stores within a strip mall in Florida. After applying a reflective roof coating, a 7.5 Wh/m2 (25%) drop in daily summertime cooling-energy use and a 0.65 W/m2 (29%) decrease in demand were realized [14]. Akridge [15] reported daily savings of 75 Wh/m2 (28%) for an education building in Georgia by painting the galvanized roof with white acrylic coating. The same researcher [16] also measured a reduction of 33 ◦ C in peak roof temperature of a single storey building after application of a thermal control coating. In Nevada, Akbari and Rainer [17] measured daily air-conditioning energy savings of 33 Wh/m2 (only 1%) in two telecommunication regeneration buildings. In Texas, Konopacki and Akbari [18] measured daily energy savings of 39 Wh/m2 (11%) and peak-power reduction of 3.8 W/m2 (14%) in a large retail store when a reflective membrane was used. Energy savings in an office building in Mississippi reached 22% after application of a reflective roof coating [19]. In Hong Kong, Cheung et al. [20] showed that 30% reduction in solar absorptance can achieve 12% saving in annual required cooling energy. 1.3. Previous computational studies on reflective coatings Other studies employed modeling to calculate the potential benefits of reflective materials. Taha et al. [21] performed simulations and predicted a cooling load reduction of 18.9% for summer days in California when the roof and walls reflectivity was increased from 0.30 to 0.90. The simulated ceiling and wall thermal resistances were 5.28 and 3.35 m2 K/W respectively. Anderson [22] found similar reductions for a flat concrete roof of a simple single room. Konopacki et al. [23] simulated the direct energy savings from high reflective roofs in 11 US metropolitan areas, and computed
Fig. 1. Spectral reflectivity of coatings over the solar spectrum.
the annual electricity savings in old residences, new residences and old/new office buildings to be 55%, 15% and 25% respectively. A simulation study by Shariah et al. [24] for two mild and hot climates, showed that, as the reflectance increases from 0 to 1, the total energy load decreases by 32% and 47% for noninsulated buildings and by 26% and 32% for insulated buildings. Wang et al. [25] developed a dynamic model and compared the annual cooling load, heating load and electricity consumption of a retail shed coated with solar reflective materials for six locations around the world. Other studies also used computer simulations to estimate the effect of reflective roofs [26–28]. Finally, Tang and Zhou [29] analyzed the relationship between outdoor sol-air temperature and solar radiation absorptance and investigated the influence of wall reflectance on annual building energy consumption for three Chinese cities representing hot summer and cold winter zone. Akbari and Levinson [30] reviewed and compared the technical development of cool-roof provisions in the ASHRAE Standards [31,32], and California Title 24 Standards and discussed the treatment of cool roofs in other standards and energy efficiency programs. This paper presents an experimental study about the impact of reflective coatings on building surface temperatures, air temperature, globe temperature, energy consumption and thermal comfort for buildings located in Shanghai, China. This location is characterized by hot summers and cold winters, and the overall effects of reflective coatings are complex considering the potential benefits in the summer and the potential penalties during winter. In parallel, another experiment with four smaller test cells was carried out to investigate the impact of envelope material thermal properties combined with reflective coatings.
2. Properties of selected coatings Three types of coatings were included in this study. Their spectral reflectivity was measured with a spectrophotometer over the main solar range and it is shown in Fig. 1. Coating A is general coating and it is used as the base case; coatings B and C are reflective coatings that have similar reflectivity over the visible spectrum but significant differences in the infrared spectrum. Both of these coatings reflect more at the infrared which accounts for a large portion of solar radiation and therefore they have the potential of reducing solar heat gain and, consequently, cooling load. The integrated average reflectivities for coatings A, B and C are 32%, 42% and 61% respectively. The main components of the coatings are summarized in Table 1.
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Fig. 2. (a) Building surface temperature measuring points. (b) Room air temperature measurement points at different heights.
Table 1 Type and main components of selected coatings. Coating
Type/main agents
Color
Paint thickness
A B C
Ordinary coating Reflective coating: acrylic, pigment, ceramic particle High reflective coating: ethylene tetrafluoride, pigment, butyl acetate
Light blue Light blue Light blue
30–40 m 30–40 m 30–40 m
Table 2 Properties of measured envelope material samples. Sample
Dimensions (m)
Conductance (W/m-K)
Resistance (m2 -K/W)
Galvanized steel sheet Polystyrene plate PE plastic board Concrete slab
0.5 × 0.5 × 0.006 0.5 × 0.5 × 0.07 0.5 × 0.5 × 0.004 0.5 × 0.5 × 0.03
44.2 0.041 0.48 0.79
1.36 × 10−5 1.71 7.1 × 10−3 3.8 × 10−2
3. Experimental setup and data collection 3.1. Surface temperatures and indoor environment In order to measure the effect of reflective coatings on envelope temperatures, indoor thermal conditions and energy demand, two identical buildings were built and placed near each other. They have the same dimensions of 7 m × 7 m × 4 m, the same envelope components and are equipped with the same HVAC system. One building was painted with coating A (Building A) and the other was painted with coating C (Building C). As this study is focused on envelope and indoor temperature improvement and envelope heat gain reduction with reflective coatings, the buildings have no windows. Building surface temperatures, indoor air tempera-
ture at different heights, indoor globe temperature and electricity consumption were measured and recorded every 10 min. The meteorological conditions, including environmental air temperature, global solar radiation and direct normal radiation, were recorded with the same frequency using a weather station installed near the buildings. The measuring points are shown in Fig. 2(a) and (b). Ten thermo recorder TR-71W sensors (one recorder has two channels) were used to measure and monitor temperatures. Each of them was assigned a number as shown in Fig. 2. Recorders numbered 1–5 measured surface temperatures of the roof and the vertical walls (channel 1 for interior surface temperatures; channel 2 for exterior surface temperatures). Recorders numbered 6–9 measured indoor air temperatures from floor to roof height at 0.5 m intervals.
Fig. 3. Schematic and pictures of test cells for evaluation of effect of envelope properties with reflective coatings.
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4.1. Results and discussion for free floating case The measurement period included summer, winter and shoulder season conditions to evaluate the effect of reflective coatings on indoor thermal environment for buildings A and C. During the measurement period, data was collected from 9:00 am to 6:00 pm. Representative weather data for a 3-day period during winter and summer is shown in Fig. 4. Average daily total solar radiation is 13.6 MJ/m2 in summer and 10.7 MJ/m2 in winter. Average daily
800
25
700
20
600 500
15
400
10
300 200
5
100 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 9:50 10:50 11:50 12:50 13:50 14:50 15:50 16:50 17:50 9:40 10:40 11:40 12:40 13:40 14:40 15:40 16:40 17:40
0
0
700
20
600
Temperature (ºC)
2
25
Solar radiation (W/m )
Time
4. Experimental results and discussion This study includes three separate experiments. The two main experiments were carried out from August 2008 to March 2009, to compare indoor thermal conditions in building A and building C under real weather conditions. The first case considered free floating conditions in the rooms, while in the second case the spaces were conditioned and electricity consumption for air conditioning was measured. The third experiment took place in September 2008, comparing coating effects applied on different envelope surface materials. In the following sections, representative experimental results are presented for all cases.
1000 900
30
2
35
Temperature (ºC)
In parallel with the main experimental measurements, another secondary study was carried out in order to investigate the impact of envelope material thermal properties combined with reflective coatings. Four test cells with roof dimensions 0.5 m × 0.5 m were built and used for measuring the resulting surface temperatures and heat flow through the roof section using the four different materials presented in Table 2. For each material, the three selected coatings were tested. Surface temperatures were measured with thermo Recorder TR-52, and heat flux through samples was measured with Heat Flow Meter HFM-215. Fig. 3 shows a schematic of the testing configuration and sample pictures.
Outdoor air temperature Global solar radiaon Direct normal solar radiaon
Solar radiation (W/m )
3.2. Effect of envelope properties and reflective coatings
500
15
400 10 300 5
200
0 -5
100 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 9:50 10:50 11:50 12:50 13:50 14:50 15:50 16:50 17:50 9:40 10:40 11:40 12:40 13:40 14:40 15:40 16:40 17:40
576
0
Time Fig. 4. Representative weather data for (a) summer and (b) winter.
mean ambient temperature is 28.6 ◦ C in summer and 11.3 ◦ C in winter. The results show that reflective coatings have a significant effect on surface temperatures. Since the roof has a different construction from walls, and buildings are usually multi storied, only wall surface temperatures were compared to identify the most applicable
Fig. 5. Maximum and average surface temperature reductions for exterior surfaces in (a) summer (b) winter and for interior surfaces in (c) summer (d) winter.
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Fig. 7. Measured indoor (a) air temperature and (b) globe temperature at 1.5 m from floor.
difference of surface heat gain. Therefore, a west wall in Shanghai is the best location to use reflective coatings because it has the greatest heat gain reduction in summer and less heat loss increase in winter when compared to other orientations. Another factor that has to be considered is that west walls usually have smaller window ratio than south walls. Measured indoor air temperature and globe temperature within work area (at 1.5 m from floor) are shown in Fig. 7. The results indicate that application of reflective coatings reduces indoor air temperature and globe temperature in both summer and winter. The average reduction in air temperature is 1.8 ◦ C in the summer and 1.6 ◦ C in winter; globe temperature was reduced by 2.3 ◦ C in the summer and by 2.1 ◦ C in winter respectively. The mean radiant temperature is a key factor influencing occupants’ thermal comfort; it can be calculated from the measured wall surface temperatures (Eq. (1)). 4
Tr = T1 4 Fp−1 + T2 4 Fp−2 + · · · + TN 4 Fp−N Fig. 6. Measured (a) exterior and (b) interior surface temperatures for west wall and (c) exterior and (d) interior surface temperatures for south wall.
orientation. Maximum and average surface temperature reductions for every wall are shown in Fig. 5. In the summer, the greatest exterior surface temperature reduction occurs on the west wall (maximum 19.9 ◦ C, average 6 ◦ C), with the south wall following. In winter, south wall has the greatest exterior temperature reduction (maximum 17.2 ◦ C, average 8.3 ◦ C), with the west wall following. Interior surface temperature reduction shows a similar trend. Detailed measurements of surface temperatures for the west and south wall with two coatings are presented in Fig. 6. It should be noticed that the difference between exterior and interior surface temperature reduction indicates the
(1)
where Tr is mean radiant temperature, K; TN is the surface temperature of surface N, K; Fp−N is the angle factor between a person and surface N. Mean radiant temperatures were calculated for a standing person (height: 1.7 m) in the middle of the room using the measured surface temperatures and the respective angle factors [33], as shown in Fig. 8. Average reductions in calculated mean radiant temperature are 1.82 ◦ C and 1.87 ◦ C in summer and winter. The maximum reductions are 3.7 ◦ C and 3.3 ◦ C respectively, appearing at 12:30 in summer and 11:50 in winter. Fig. 9 illustrates the vertical air temperature distribution at these two peak times. Thermal stratification naturally occurs in both Buildings A and C, and the difference in temperatures along the height of the (identical) rooms is significant. At 1.7 m from the floor (occupant height), the temperature in Building A is 3 ◦ C higher than the temperature in Building C in the summer (similarly in winter).
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Fig. 8. Mean radiant temperature calculated from measured surface temperatures in (a) summer days and (b) winter days.
4.2. Results and discussion for conditioned case In the second set of experiments, both Buildings A and C were conditioned (indoor temperature set point was 24 ◦ C in all seasons).
Fig. 9. Measured indoor air temperature at different heights for (a) summer and (b) winter peak times with different coatings.
Fig. 10. Average surface temperature reductions (A/C on) in (a) summer and (b) winter.
The average daily total solar radiation is 15.5 MJ/m2 in summer and 10.9 MJ/m2 in winter, while the average daily mean ambient temperature is 28.6 ◦ C in summer and 11.3 ◦ C in winter. Fig. 10 presents the measured average temperature differences of exterior and interior surfaces for each wall. Air conditioning operation results in smaller temperature decrease. Since the indoor air temperature are controlled at 24 ◦ C in both summer and winter, all walls have similar temperatures in Buildings A and C. Heat gains through walls depend on exterior surface temperatures. As shown in Fig. 10, exterior temperature reduction in summer is smaller than in winter, so it is reasonable to expect a smaller heat gain reduction in summer than heat loss increase in winter. The measured electricity consumption for air conditioning also indicates such a trend (Fig. 11). Note that the average air temperature of working hours in cooling period for Shanghai is 29.3 ◦ C, and the average daily total radiation is 18.2 MJ/m2 ; respectively, the average temperature of working hours in heating period is 10.4 ◦ C, and the average daily total radiation is 10.1 MJ/m2 . The reduction in daily electricity consumption for September is 2.62 kWh between Building A and C, while the increase in daily electricity consumption for December is 2.55 kWh. Annual electricity consumption changes can be estimated as follows: assuming that the electricity consumption has a linear relationship with indoor-outdoor temperature difference, and considering that the recommended cooling and heating set points in Shanghai are 26 ◦ C and 18 ◦ C respectively. The cooling degree hour within working time is 2443.1 h and the heating degree hour within working time is 11471.6 h in Shanghai. Then the total reduction of cooling electricity consumption is estimated to be 116.4 kWh and increase of heating electricity consumption is equal to 327 kWh. In this estimation, only envelope heat gain is considered, neglecting all other heat gains, for example occupancy and equipment heat gains which
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Fig. 11. Measured electricity consumption with air conditioning for representative days in (a) September and (b) December with two different coatings.
increase cooling loads but reduce heating loads, so a better performance can be expected. The fact that summer electricity shortage is a long municipal problem adds merits to application of reflective coatings. Furthermore, it should be noted that, although air conditioning is on in both Buildings A and C (and hence the two buildings have very similar air temperatures), the application of reflective coatings still influences indoor thermal comfort by changing the temperature difference between interior surfaces and air temperature.
Fig. 12. Measured (a) exterior surface temperatures, (b) interior surface temperatures and (c) heat flux for concrete slab sample.
4.3. Comparison of coating effects applied on different surface materials
Table 2 were painted with coatings A, B and C. Their reflectances were measured with a spectrophotometer (wavelength range: 300–2100 nm). It was found that the coating reflectance is not affected by the material. Every set of samples was then simultaneously exposed to same environmental conditions. Surface
In order to investigate the possible influence of different surface materials on coating performance, the four materials presented in
Table 3 Average measured temperatures, heat fluxes and weather data. Sample
Galvanized steel sheet Polystyrene plate PE plastic board Concrete slab
Tex (◦ C)
Tin (◦ C)
q (W/m2 )
A
B
C
A
B
C
A
B
C
44.2 48.8 44 42.3
42.6 47.3 42.3 40.6
39.5 42.6 39.7 37.8
43.5 39.3 43.9 41.2
42.2 38.5 42.2 39.5
39.4 37.7 39.4 37.4
55 17.5 44.6 39.1
43.5 13.9 30.7 26.2
23.4 8.3 12.9 11.6
To (◦ C)
I (W/m2 )
30.3 32.6 31.3 31.4
434 548 435 388
Tex : average exterior surface temperature; Tin : average interior surface temperature; q: average heat flux; To : average outdoor air temperature; I: average solar radiation.
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temperatures and heat fluxes were measured. Fig. 12 presents the measured exterior surface temperatures, interior surface temperatures and heat fluxes in the concrete slab. Average measured temperatures and heat fluxes for all four types of tested materials are listed in Table 3. Comparing reductions in surface temperatures and heat flux as function of reduction in absorbed solar radiation, the experimental results showed that material thermal resistance has a small effect on exterior surface temperatures; however, it has a more profound impact on interior surface temperatures and of course on heat flux. 5. Conclusion This paper presents experimental results for the impact of solar reflective coatings on building surface temperatures, indoor environment, heat gains and electricity consumption under real weather conditions in summer and winter. Three types of coatings were applied on identical buildings and their performance was compared with a set of three separate experiments: free-floating case, conditioned spaces and different envelope materials. For the non-conditioned case, the results indicate the significant effect of the coatings on lowering building surface temperatures. In the summer, an increase of surface reflectance from 32% to 61% resulted in average reduction of exterior surface temperature reduction of 6 ◦ C on the west wall, and in the winter a respective reduction of 8.3 ◦ C on the south wall. The maximum reduction in exterior surface temperature of the west wall reached 19.9 ◦ C in the summer. Reductions in interior surface temperatures showed similar trends. The average reductions in indoor air temperature and globe temperature at 1.5 m above ground were in the order of 2 ◦ C during both seasons. Mean radiant temperatures were calculated from the measured surface temperatures and respective angle factors. For a standing person in the center of the room, the average reductions reached 1.8 ◦ C in both summer and winter. For the conditioned case, the indoor air temperature was controlled at 24 ◦ C. A reduction of 2.62 kWh in electricity consumption was measured during a representative summer day, while an increase of 2.55 kWh was observed during a typical winter day. Considering the recommended set points for cooling (26 ◦ C) and heating (18 ◦ C) in Shanghai, the estimated net overall effect of increasing envelope reflectance from 32% to 61% is negative. Finally, four different envelope materials were painted with three different coatings (reflectivities: 32%, 42% and 61%) and were exposed to the same environment conditions. Surface temperatures and heat fluxes were measured and compared. The thermal resistance of materials has a significant impact on the coatings effect—the coatings perform better when applied on materials with smaller thermal resistance. Acknowledgements The authors would like to thank Daikin Industries Ltd. for the financial and operational support. References [1] B. Givoni, M.E. Hoffiman, Effect of building materials on internal temperatures, Research Report, Building Research Station, Technion Haifa, 1968. [2] A. Synnefa, M. Santamouris, I. Livada, A study of the thermal performance of reflective coatings for the urban environment, Solar Energy 80 (2006) 968– 981. [3] V. Cheng, E. Ng, B. Givoni, Effect of envelope colour and thermal mass on indoor temperatures in hot humid climate, Solar Energy 78 (2005) 528–534.
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