Thermal performance of concrete wall panel with advanced coating materials: a sustainable design

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1 Topic: T2.1 Design and Innovation Reference number: 2046 Thermal performance of concrete wall panel with advanced coating materials: a sustainable design Ao Zhou 1, Denvid Lau 1, * 1. Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China Abstract: Buildings account for a large portion of world s energy consumption and greenhouse gases emissions nowadays, and it is known that air-conditioning system is one of major causes for energy consumption in building sector to provide thermally comfortable indoor environment for occupants. In order to decrease the huge building energy consumption and greenhouse gases emissions, the building envelope ought to be well designed so that the thermal performance of buildings is improved and sustainability of built environment can be achieved. In this paper, a comparative study of various advanced coating materials which can be applied to concrete surface is conducted to determine the effectiveness and efficiency of our proposed concrete wall panel design. The temperature variations of concrete panels with different coating materials under radiation source are measured to evaluate the thermal performance and effect on lowering concrete surface temperature facing the heat source. Both the radiation and ambient air temperature are applied to the tested specimens. The experimental work has demonstrated that the application of coating materials can effectively decrease the surface temperature under hot climate and improve the energy efficiency of buildings, eventually contributing to a drop in cooling loads and energy consumption of concrete buildings. Key words: thermal performance, sustainable design, coating materials 1 Introduction Nowadays, buildings account for a large portion of world s total energy consumption and greenhouse gases emissions, and the proportion is expected to further increase gradually. In United States of America, more than 40% of building energy was used for space heating or cooling in 2010, occupying the largest portion of building energy [1]. It is known that air-conditioning system is one of major causes for energy consumption in building sector to provide indoor thermal comfort for occupants. The building envelope absorbs solar radiation, and it induces the increase of exterior surface temperature, including roof and exterior side wall surface, and interior surface temperatures. Then, indoor temperature rises accompanying a negative influence on thermally comfortable indoor environment. Consequently, excess electricity and energy is required for air-conditioning system to maintain the desired indoor temperature, pulling up the electricity and energy consumption of building. In order to decrease huge building energy consumption and decrease greenhouse gases emissions, the building envelope ought to be well designed to minimize the unwanted heat gain from solar radiation so that the thermal performance of buildings is improved and * Corresponding author at: Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China. Tel.: ; fax: address: denvid.lau@cityu.edu.hk.

2 sustainability of built environment can be achieved [2]. A lot of studies were conducted to improve the thermal performance of building envelope from the perspective of building type, orientation and construction materials, etc. Previous work has demonstrated that appropriate design of building envelope can reduce the consumed energy and caused greenhouse gases emissions significantly, which is consistent with the concept of sustainable building. A sandwich concrete/gypsum wall panel design was proposed to protect building from unwanted heat gain and loss with environment, which has shown good thermal insulating performance [3]. Heat reflective coating is a newly developed material in recent years that shows good thermal insulation performance. Most of the studies on reflective coating are focused on hot climate or summer condition. The reflective coating materials can reduce absorbed solar radiation, resulting in reduction of exterior surface temperature and heat transfer from exterior to interior. Application of reflective coating materials possesses great potential to decrease solar heat gain, exterior surface temperature and cooling loads for air-conditioning system when maintaining thermally comfortable indoor environment, and finally reduces building energy cost [4,5,6]. It is expected that a lower indoor temperature (without air-conditioning system) can be obtained using proposed coating materials, which is shown in Figure 1(a). Fig.1 (a) The left building envelope is the conventional precast concrete wall panel, while the right one applies different coating materials. If there is no air-conditioning system, the building whose exterior surface is covered with coating materials exhibits a lower indoor temperature than that of conventional concrete building without coating; (b) a schematic diagram of applying coating material to illuminated surface of concrete In this research work, the thermal performance of commercially available reflective coating materials, including ordinary exterior wall white painting and hollow glass beads coating, which can be used in building envelopes, is investigated through experimental observation. All coating materials are applied to precast

3 concrete slab, which is one of most common used materials in buildings industry. The impact of coating materials on exterior, interior surfaces temperature and indoor air temperature is studied. The thermal performance of concrete slab with different coatings is monitored. The surfaces and indoor air temperature variation under radiation as well as cooling is measured and compared to that of uncoated concrete slab. is envisioned that proposed coating material can effectively reduce heat absorption and cooling load for buildings, and thus save a great amount of building energy consumed by air-conditioning system. It 2 Experimental program 2.1 Materials Precast concrete, which is one of most extensively used construction materials, is adopted here as substrate material in this experimental study because of following benefits. First, each precast concrete unit can be standardized in shape and dimensions, eliminating the need of supporting formwork in situ. Second, the quality of precast concrete is more consistent and reliable compared to cast in situ concrete. Two types of coating materials are included in this experimental study. Coating A is a general white exterior wall painting; coating B is a reflective coating. Both two materials possess the potential to reduce heat gain, and thus cooling load. The main components and property of these coating materials are shown in Table 1. Table1 Description of coating materials Coating Main component Color Painting thickness A Ordinary exterior acrylic insulating painting White mm B Titanium oxide, hollow glass beads styrene-acrylic emulsion White mm 2.2 Specimens The coating materials are applied to the exterior surfaces of conventional precast concrete panel, which is shown in Figure 1(b). Three concrete panels are adopted in this experiment, namely uncoated concrete panel (reference), covered with coating A and covered with coating B. The photos of two specimens covered with coatings are shown in Figure 2. It should be mentioned that three concrete panels are identical and the dimensions of concrete panel is 450mm 200mm 60mm (Length Width Height). The coated surface of concrete specimens are grinded and cleaned so that there is no grease and ash before applying the coatings. The application of coating materials is based on the instruction of products.

4 Fig.2 The photos of two specimens: (a) coated with coating A; (b) coated with coating B 2.3 Instrumentation and data collection When the earth and sun is spaced at mean earth/sun distance, the total solar irradiance on a normal surface at earth is accepted as W m -2 [7]. The total solar irradiance changes by around 6.6% because of the variation of earth/sun distance. In order to simulate the solar irradiance, the halogen lamp is adopted as the heat radiation source irradiating the specimens consistently throughout the experiment. The halogen lamp can generate high intensity radiation which matches solar spectra closely. Furthermore, by using halogen lamp as the heat radiation source, the weather conditions can be ignored and sunlight duration can be easily controlled so that the test condition for all samples is consistent and experimental errors can be minimized. The power of halogen lamp is 1300 W and the halogen lamp is placed at 500 mm away from the illuminated surface of specimens, which is shown in Figure 3. The illuminated surface of specimen in the experiment refers to exterior surface of building (outdoor) and unilluminated surface refers to interior surface of building (indoor). During experiment, the halogen lamp was kept on for 12 hours continuously. Both the illuminated surface temperature, unilluminated surface temperature and unilluminated air temperature were measured in one minute interval through thermocouples with TDS-303 Data Logger system. The measurement of equipment ranges from -10 o C to 200 o C and the accuracy is ±0.5% or ±0.5 o C (whichever is greater). After 12 hours radiation, the halogen lamp was turned off and the concrete panel was cooled down until ambient air temperature. Then the specimen was replaced by another one before next test. By monitoring the variation of illuminated surface temperature, unilluminated surface temperature and unilluminated air temperature, the thermal performance of different specimens can be evaluated.

5 Fig.3 Experimental setup 3 Results and discussion 3.1 Surface temperature An equation which can describe the thermal balance of insulated underneath surface under the sun, as is similar to the case in this experiment, is followed: (1 a)i = εσ(t 4 s T 4 sky ) + h(t s T a ) (1) where a is reflectivity of surface; I is total radiation on the surface; ε is emissivity; σ is Stefan-Boltzmann constant; T s is equilibrium surface temperature; T sky is radiant sky temperature; h is convective coefficient; T a is the ambient air temperature [8]. When the lamp is on, the surface reflectivity is the most important parameter affecting the thermal performance of specimens. The ambient air temperature during testing is 23.6 o C. The measured illuminated surface, unilluminated surface and unilluminated air temperature is summarized in Table 2. The illuminated surface temperature after 12 hours radiation could reach up to 47.9 o C for uncoated concrete panel, and 46.8 o C and 40.1 o C for coating A panel and coating B panel, respectively. Through observing these temperatures, the thermal insulation performance of different specimens can be evaluated. The illuminated surface temperature of coating A panel is 1.1 o C lower than that of uncoated panel, meaning coating A material can reduce the heat gain slightly. Furthermore, the illuminated surface temperature of coating B panel is 7.8 o C lower than that of reference, implying that coating B can reduce heat gain and improve the thermal insulation capability effectively and efficiently. Table2 The temperature of specimens after 12 hours radiation Specimens Illuminated surface temperature ( o C) Unilluminated surface temperature ( o C) Unilluminated air temperature ( o C)

6 Uncoated panel Coating A panel Coating B panel The recorded temperature of illuminated surface and unilluminated surface is shown in Figure 4 and Figure 5, respectively. In first 200 minutes, both the illuminated and unilluminated surface temperature rose quickly. After that, the increasing rate slowed down, implying the thermal equilibrium between specimen and environment was approached gradually. Meanwhile, the illuminated surface temperature of uncoated and coating A specimen was close to each other, which is far higher than that of coating B specimen. These results illustrated that the ordinary white painting has little effect on reducing exterior surface temperature, while the reflective coating B containing titanium oxide and hollow glass beads can reduce surface temperature and energy absorption of building exterior wall significantly, and thus protect buildings and save energy. Fig.4 The temperature variation against time (T-t curve) of illuminated surface in different samples

7 Fig.5 The unilluminated surface temperature variation against time (T-t curve) of different specimens 3.2 Air temperature The unilluminated air temperature variation against time (T-t curves) of three specimens is indicated in Figure 6. The unilluminated air temperature is used to represent the indoor temperature in this study when evaluating the thermal impact of coating material on indoor environment. The results demonstrate that application of reflective coating can decrease the indoor air temperature and indoor temperature fluctuation. The average unilluminated air temperature of uncoated panel is higher than that of coating A panel 0.59 o C, while higher than that of coating B 2.2 o C. Based on experimental results, the energy saving effect through two coating materials can be roughly estimated. It is reported that the general set indoor temperature for air-conditioning system is 20 o C. By using coating material A and B, the percentage of energy saving can be calculated as 0.5 / ( ) 100% = 7.2% and 2.2 / ( ) 100% = 31.8%. Taking the huge amount of energy consumed by air-conditioning system into consideration, the application of coating material leads to a great amount of energy saving, further eliminating a big drop in greenhouse gases emissions. Fig.6 The unilluminated air temperature variation against time (T-t curve) of different specimens 4 Conclusions This paper presents an experimental study on advanced coating materials which can be applied to concrete surface. The surface temperatures and air temperature were measured to determine the effectiveness and efficiency of concrete wall panel covered with proposed coatings. Two coatings were applied to identical concrete panels and their thermal performance was evaluated. The experiments have demonstrated the significant effect of reflective coating material B on lowering the building surface temperature and indoor temperature. The application of reflective coating B resulted in a drop on exterior surface temperature, up to 7.8 o C. The interior surface temperature indicated the similar reduction, up to 6.3 o C. The average decrease of indoor air temperature was 2.2 o C, implying that the

8 energy consumed by air-conditioning system can be saved significantly when the proposed reflective coating material is applied to building envelope. Eventually, a remarkable drop in cooling loads, energy consumption and greenhouse gases emissions is obtained. Acknowledgements The authors are grateful to the support from Croucher Foundation through the Start-up Allowance for Croucher Scholars with the Grant No , and the support from the Research Grants Council (RGC) in Hong Kong through the Early Career Scheme (ECS) with the Grant No References [1] L. D&R International Buildings Energy Data Book [J]. US Department of Energy [2] L. Bernstein, P. Bosch, O. Canziani et al. Climate Change 2007: Synthesis Report, Intergovernmental Panel on Climate Change [J]. Valencia, Spain, [3] A. Zhou, K.W. Wong, D. Lau. Thermal insulating concrete wall panel design for sustainable built environment [J]. The Scientific World Journal, [4] A. Synnefa, M. Santamouris, I. Livada. A study of the thermal performance of reflective coatings for the urban environment [J]. Solar Energy, 2006, 80(8): [5] H. Shen, H. Tan, A. Tzempelikos. The effect of reflective coatings on building surface temperatures, indoor environment and energy consumption an experimental study [J]. Energy and Buildings, 2011, 43(2): [6] R. Yao. Design and Management of Sustainable Built Environments [M]. Springer [7] Standard A. E490-00, 2006a, Standard solar constant and zero air mass solar spectral irradiance tables [J]. ASTM International, West Conshohocken, PA, [8] American American Society of Heating, Refrigeration, and Air-Conditioning Engineers. ASHRAE Handbook Fundamentals. ASHRAE. 1989