consumption. As the first step of calculating indoor energy consumption, this study investigated the net radiation heat gain of building exterior surf

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1 Analysis of the Net Radiation Heat Gain of Buildings by Changing Building Distance in Guangzhou and Sendai J. HU 1,*, Y. XUAN 2, and A. MOCHIDA 1 1 Graduate School of Engineering, Tohoku University, Sendai, Japan 2 Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan ABSTRACT The present authors have previously conducted the optimization study of building arrangement from the viewpoint of outdoor thermal acceptability. However, on the other hand, it is also necessary to examine building arrangement in terms of indoor energy consumption. This study investigated the net radiation heat gain of building exterior surface by changing building distance, and clarified the reduction potential of cooling load of a single building and a cluster of buildings with different building arrangements. Guangzhou, China and Sendai, Japan were selected to investigate the influence of different solar altitudes. The simulation results showed that the exposure level and orientation of the surfaces were closely related to heat gain of the surfaces. Cooling load per square meter can be reduced by decreasing building distance, and the potential reduction of cooling load can also be expected by changing the thermal properties of the surfaces. With regard to energy consumption in a certain area, controlling the heat gain of the whole buildings is more effective compared to that of a single building. KEYWORDS Building distance, Building shadow, Heat gain, Shortwave radiation, Longwave radiation INTRODUCTION With global warming and UHI (urban heat island) effects, the highest air temperature in summer season sets another new record year by year and extreme hot days last longer and longer. For that reason, in order to create more acceptable area and reduce heatstroke patients, the previous study (Xuan et al. 213, Yang et al. 213) attempted to optimize building arrangement from the viewpoint of outdoor thermal acceptability. On the other hand, recently, more energy is required for air conditioning and it causes power shortage especially during peak demand periods. Therefore, it is also necessary to examine building arrangement in terms of indoor energy * Corresponding author xuan@nagoya-u.jp, mochida@sabine.pln.archi.tohoku.ac.jp 243

2 consumption. As the first step of calculating indoor energy consumption, this study investigated the net radiation heat gain of building exterior surface by changing building distance (shown in Fig. 1), and consequently, clarified the reduction potential of cooling load of buildings. Since the amount of incoming solar radiation varies with latitude, Guangzhou (113 33'E, 23 17'N), China and Sendai (14 52'E, 38 16'N), Japan were selected as examples. Estimated area Building area NORTH WEST ROOF D/H=.24 (D=1m) D/H=.48 (D=2m) D/H=.71 (D=3m) SOUTH ROAD EAST D/H=.95 (D=4m) D/H=1.19 (D=5m) D/H=1.43 (D=6m) (1) Analysis model (2) Building arrangements in the target area H: building height, 42m D: building distance in the north-south direction Fig. 1 Analysis model and building arrangements in the target area Table. 1 Case name Analysis cases Analysis date Latitude ( º) Building distance (D) (m) Building height (H) (m) ANALYSIS OUTLINE Fig. 1 shows the ideal model of the typical residential area in Guangzhou, China. As seen from it, the buildings (6m 15m 42m) are distributed in parallel with each D/H (-) 1.24 GZ GZ-3 Guangzhou /21 GZ-4 (23º17'N) 4 (14 stories).95 GZ GZ SD SD-3 Sendai /21 SD-4 (38 16'N) 4 (14 stories).95 SD SD GZ: Situation in Guangzhou using air temperature and latitude in Guangzhou SD: Situation in Sendai using air temperature and latitude in Sendai 244

3 other and face to the south. The focus area is the target area (3m 3m), and a total of six building distances from 1m to 6m in the north-south direction in the area were studied. Table 1 lists analysis cases in this study. In order to clarify the characteristics of heat gain of the surfaces with different orientations, the building surface was divided into five parts: east, west, north, south and roof (shown in Fig. 1 (2)). Heat gain (or the absorbed heat by walls) here means the radiation energy arrived and absorbed by the surfaces of buildings. Therefore, in the shortwave case, the heat gain by shortwave radiation refers to the difference of components (1) and (2) in Fig. 2, while in the longwave case, the heat gain by longwave radiation means the difference of components (3) and (4). In this paper, heat gain with positive value indicates absorption while negative value indicates emission. Fig. 2 Components of radiant heat transport at building surface Exterior Mortar (25) Aerated Concrete (2) Interior Mortar (25) Exterior Mortar (25) Fine-Stone Concrete (2) Cement Mortar (2) Insulation Layer (3) Reinforced Concrete (1) Interior Mortar (25) Concrete (1) Gravel (1) Soil (3) (1) Building wall (2) Roof (3) Ground Fig. 3 Layers of building surface and ground in radiation simulation (the unit is mm.) The net radiation heat gain of building exterior surfaces was obtained by radiation simulation. To ensure high prediction accuracy, three-dimensional multireflections of shortwave and longwave radiations were taken into consideration. The shape factor (i.e. configuration factor or view factor) was calculated using the Monte-Carlo method and the radiative heat transfer was calculated using Gebhart s absorption factor. The simulation was fulfilled by commercial software, STAR-RADX V3.2 with additional codes. The radiation simulation was executed every ten minutes and the output data were stored every one hour. To make it possible to compare the net radiation heat gain of building exterior 245

4 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: surfaces at different latitudes, the same analysis date (21 st June) and each local solar time were adopted. Local indoor temperature was set to 26ºC. The ground and building surfaces were assumed to be covered with concrete. The structures of building envelop (wall and roof) and ground are shown in Fig. 3. RESULTS Heat Gain by Shortwave Radiation GZ-3 GZ-6 SD-3 SD GZ-3 GZ-6 SD-3 SD-6 (1) Roof surfaces (2) Road surfaces GZ-3 GZ-6 SD-3 SD GZ-3 GZ-6 SD-3 SD-6 (3) East surfaces (4) West surfaces GZ-3 GZ-6 SD-3 SD GZ-3 GZ-6 SD-3 SD-6 (5) North surfaces (6) South surfaces Fig. 4 Heat gain by shortwave radiation As seen from Fig. 4 (1), for the roof surface in Guangzhou, there is no change in the shortwave radiation heat gain as building distance decreases, while in Fig. 4 (2), for the road surface, a slight decrease is observed. This means that the obstruction of the sunshine by reducing building distance cannot be expected too much due to the high solar altitude in Guangzhou. Being affected by the reflections of other surfaces, the peak value of the shortwave radiation heat gain for the road surface is greater than that of the roof surface (at 12:). In Sendai, the shortwave radiation heat gains for the road surfaces decrease apparently as building distance decreases. This is due to the 246

5 creation of building shadow by decreasing building distance in Sendai. Fig. 5 gives the distributions of surface temperatures of ground and building in Guangzhou and Sendai. As shown in Fig. 5, due to the obstruction of the sunshine by buildings, a significant building shadow is observed behind buildings in Sendai. N Guangzhou (Solar altitude: 89.6º) D/H=.24 (D=1m) D/H=.24 (D=1m) [ºC ] 56. E D/H=.71 (D=3m) D/H=.71 (D=3m) 3. Sendai (Solar altitude: 75.3º) D/H=1.43 (D=6m) D/H=1.43 (D=6m) (1) Guangzhou (2) Sendai Fig. 5 Distribution of surface temperature of the vertical surfaces According to Figs. 4 (3) ~ (6), the shortwave radiation heat gains on the vertical surfaces are apparently less than that on the horizontal surfaces. The heat gains by shortwave radiation per square meter on the east surfaces reach the peak values in the morning and those on the west surfaces reach peak values in the afternoon in the two cities. The reason is that the surface which is under the direct sunshine receives the larger shortwave radiation. When focusing on Figs. 4 (5) and (6), with a decrease in building distance, the heat gains on the north and south surfaces decrease. The heat gain on the south surfaces in Sendai is apparently greater than that in Guangzhou (Fig. 4 (6)). This is due to the lower solar altitude leading more direct sunshine on the south surface during the whole day. Heat Gain By Longwave Radiation As shown in Figs. 6 (1) and (2), the longwave radiation heat gains on the horizontal surfaces (roof and road) have negative values during the daytime. This means that these surfaces emit heat in terms of the longwave radiation. The horizontal surfaces being highly exposed to the sunshine due to the high solar altitude strongly absorb the 247

6 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: : 2: 4: 6: 8: 1: 12: 14: 16: 18: 2: 22: : 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 21: 23: 1 GZ-3 1 GZ GZ-6 SD-3 SD GZ-6 SD-3 SD (1) Roof surfaces (2) Road surfaces GZ-3 GZ-6 SD-3 SD GZ-3 GZ-6 SD-3 SD (3) East surfaces (4) West surfaces GZ-3 GZ-6 SD-3 SD GZ-3 GZ-6 SD-3 SD-6 (5) North surfaces (6) South surfaces Fig. 6. Heat gain by Longwave radiation shortwave radiation as shown in Fig. 4 and their surface temperatures increase rapidly, and therefore, more longwave radiations are emitted from these surfaces. As shown in Fig. 6 (2), the longwave radiation emission of the road surface in Sendai is smaller than those in Guangzhou. With an increase in building distance, the longwave radiation emission becomes more. In, the shortwave radiation is difficult to reach the area of building shadow (shown in Fig. 5) and therefore the increase of the surface temperature under building shadow is smaller compared with that in the exposed area. This leads the smaller longwave radiation emission from the road surface. Figs. 6 (3) ~ (6) show that the values of the heat gain by the longwave radiation on the vertical surfaces are almost plus in both Guangzhou and Sendai. Due to less exposure level of the vertical surfaces compared with that of the horizontal surfaces, the shortwave radiation heat gain on the vertical surfaces is less than that on the horizontal surfaces, and this will lead to a relatively lower surface temperature on the vertical surfaces. This indicates that the vertical surfaces will absorb the longwave radiation. Under strong solar radiation, with the surface temperature growing up, the 248

7 Dailyradiation gain (MJm -2 day -1 ) Daily radiation gain (MJm -2 day -1 ) east and west surfaces even begin to emit longwave radiation. Daily Net Radiation Heat Gain EAST WEST 6 EAST WEST 4 NORTH SOUTH 4 NORTH SOUTH Building distance (m) (1) Guangzhou (2) Sendai Fig. 7. Daily net radiation gain on the vertical surfaces in Guangzhou and Sendai Building distance (m) Fig. 8 Total daily net radiation heat gain of one building in different building arrangements Fig. 9 Daily total net radiation gain of all buildings in the target area in different building arrangements Fig. 7 shows the daily net radiation heat gains on the vertical surfaces of one building in different building arrangements in Guangzhou and Sendai. The heat gains on the vertical surfaces are increasing as building distance increases. In the short building distance (1m to 3m), the heat gains on the east and west surfaces far more exceed those on the north and south surfaces. As the building distance gets greater, with the effects of building shadow becoming less, the heat gain on each vertical surface is becoming more, and the differences among the vertical surfaces are becoming less. Fig. 8 shows the radiation heat gain of one building in different building arrangements in Guangzhou and Sendai. As seen from it, the heat gain of the building increases as building distance increases. In the 1m cases, the radiation heat gains in Guangzhou and Sendai are at the same level, but in the 6m cases, the heat gain of the building in Guangzhou is more than that in Sendai. Fig. 9 shows the radiation heat gain of all the buildings in the target area in different arrangements. As seen from it, the total radiation heat gains of the buildings are apparently decreases as the building distance gets larger. With the building distance changing from 1m to 6m, the amount of the buildings in the target area decreases. This makes the radiation heat gain of the whole target area become less as building distance increases, though the heat gain of one building grows with an 249

8 increase in building distance. The total radiation heat gain of all the buildings in the target area in Guangzhou is a little more than that in Sendai. CONCLUSIONS (1) The shortwave radiation heat gain mainly depends on the exposure level of the surfaces to the sunshine. The longwave radiation heat emission mainly depends on the amount of the shortwave radiation absorbed by the surfaces. (2) The ratio of the roof surface area to the whole building surface area is relatively small and therefore it is necessary to assess the reduction potential of cooling load by considering both the intensity of the net radiation heat gain and the area of the corresponding surface. (3) The net radiation heat gains of the vertical surfaces for one building decrease as building distance decreases, however, the total net radiation heat gains of all buildings in the target area increase. With regard to energy consumption in a certain area, controlling the heat gain of the whole buildings is more important compared with that of a single building. ACKNOWLEDGEMENT This study is supported by the strategic Japanese-Chinese Cooperative Program of JST and MOST (Grant No. 211DFA9121) REFERENCE Y. Xuan, G. Yang, Q. Li and A. Mochida: Fundamental study on building arrangement to maximize thermal acceptability of outdoors at different latitudes (Part 1), Summaries of Technical Papers of Annual Meeting Architectural Institute of Japan, 213 G. Yang, Y. Xuan, Q. Li, A. Mochida: Fundamental study on building arrangement to maximize thermal acceptability of outdoors at different latitudes (Part 2), Summaries of Technical Papers of Annual Meeting Architectural Institute of Japan, 213 S. Yoshida, S. Murakami, R. Ooka, A. Mochida, Y. Tominaga: CFD Prediction of Thermal Comfort in Microscale Wind Climate, Proceedings of the 3rd International Symposium on Computational Wind Engineering 2; H. Takebayashi, M. Moriyama: Analysis of the relationships between the properties of an urban street canyon and its radiant environment from the viewpoint of the introduction of appropriate urban heat island mitigation technologies, 6th Japanese-German Meeting on Urban Climatology. 25