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NUMERICAL STUDY on SPECULAR SOLAR REFLECTORS AIMED at INCREASING SOLAR REFLECTIVITY of BUILDING ENVELOPE Masatoshi Nishioka1, C

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NUMERICAL STUDY on SPECULAR SOLAR REFLECTORS AIMED at INCREASING SOLAR REFLECTIVITY of BUILDING ENVELOPE Masatoshi Nishioka1, C Proceedings of BS2013: 13th Conference of International Building Performance Simulation Association, Chambéry, France, August 26-28 NUMERICAL STUDY ON SPECULAR SOLAR REFLECTORS AIMED AT INCREASING SOLAR REFLECTIVITY OF BUILDING ENVELOPE Masatoshi Nishioka1, Craig Farnham1, Minako Nabeshima1, Masaki Nakao1 1Osaka City University, Osaka, Japan Further, in terms of heat island mitigation, absorption ABSTRACT of solar radiation leads to an increase of convective Techniques that improve solar reflectivity of urban heat to the urban atmosphere, so it is desirable to surfaces to mitigate urban heat island phenomenon reflect away as much of the solar radiation as have become widespread with the use of highly- possible. For these purposes, cool-roof technology reflective paints (Kondo, 2008). Since these has become popular in recent years, and the typical reflective paints have a higher proportion of diffuse application is highly reflective paint. reflection, there is an anxiety that if used on the sides In order to mitigate the heat island phenomenon, the of buildings, much of the reflected solar radiation is amount of solar absorption on urban surfaces should scattered toward the ground and surrounding be reduced. For example, in the case of a roof surface buildings. This becomes even more of a problem in Fig. 1, reflection in any direction will cause no when used in narrow street canyons. Therefore, we trouble. However, in the case of an urban canyon, aim to develop reflectors with the ability to direct the more than half of the reflected radiation is downward reflected solar radiation back to the sky. and will ultimately be absorbed by the ground or Retroreflective surfaces are widely used in order to lower buildings. Consequently, for the cool-roof increase visibility of road signs at night (Woltman, concept to be effectively applied to walls as a heat 1982), because they reflect car headlights back in the island countermeasure, solar reflectors should be direction from which they came. However, if the aim designed so that the reflected radiation is directed is to use retroreflectors on building envelopes for back towards the sky. solar radiation reflection, the diurnal apparent motion of the sun makes it difficult to reflect efficiently. SURFACE TEMPERATURE There was little knowledge or past research about DISTRIBUTION AND SOLAR such reflective performance, therefore we examine it RADIATION in this paper. The typical daily heat balance of an urban surface In this research, we use corner cube retroreflectors that is open to the sky is shown in Fig. 2. The latent consisting of four pieces of aluminum reflector panel heat component caused by moisture evaporation is with a glossy surface. At first, the reflective disregarded for this simple expression. The heat performance of the retroreflector consisting of ideal source is solar radiation absorbed by the urban mirrors are evaluated by numerical simulation. Then, surface. The heat is dispersion has two components, more realistic reflectors consists of aluminum mirrors heat convection to the urban atmosphere and are evaluated. The reflectivity of the mirror plate is radiation to the sky. Because the cooling and the measured by spectrophotometer and the bidirectional heating on the urban surface for one day are roughly reflectance distribution is measured by in balance, the thermal storage to the ground can be goniophotometer. These radiant properties were used in the numerical simulation using a Monte Carlo path tracing method. Roof (reflect to sky) INTRODUCTION When considering the thermal function of building exterior walls, protection of the interior space from the changing exterior climate is not their only role. They are also one of the components of the urban surface, influencing the urban heat island phenomenon. The major role of exterior walls is to thermally Urban canyon isolate the interior space from the outside climate. (reflect among canyon) Enhanced solar reflectance of the surface of building roof materials, called “cool-roof”, can reduce the heat load from solar radiation in the summer season. Figure 1 Overview of reflection on urban surfaces - 3041 - Proceedings of BS2013: 13th Conference of International Building Performance Simulation Association, Chambéry, France, August 26-28 C) 250 Solar radiation Long wave radiation deg 200 (Direct) (Sky) balance 150 Kd,0 Ks,0 L*0 地面Ground 100 南向き壁面South face Convective heat 北向き壁面North face H 日平均表面温度(℃) 50 Surface temperature ( temperature Surface 0 0246810 12 φ di/φ si Figure 2 Daily heat balance on urban surface open Figure 4 �,,�, and daily mean surface temperature to the sky No convective heat transfer. Sky plane 34 地面Ground C) 南向き壁面South face 32 deg 北向き壁面North face =50 h 30 North face wall 28 South face wall 26 24 日平均表面温度(℃) 22 Ground ( temperature Surface 20 0 2 4 6 8 10 12 d=10 φ di/φ si Figure 3 2D urban canyon shape with East-West Figure 5 �,,�, and daily mean surface temperature axis Convective heat transfer coefficient as 18 W/m2K ignored here. The formula represented by the above heat transfer is as follows. �, = �,�, (3) ∗ �, + �, − � − � = 0 (1) �, = �,�, (4) ∗ The irradiation of a plane depends on the incoming � = �� − � (2) angle, so the factor �, is determined by the incoming angle and the presence or absence of The direct solar radiation to an unshielded, open sunshine. The factor �, for sky solar radiation is the surface is �, , the sky radiation is �, , the balance sky view factor of the location. The heat balance ∗ of long-wave radiation is � , and the heat convection equation at surface � is expressed as Eq.5. is �.The Stefan-Boltzmann constant is � , the surface temperature is � , and the atmospheric radiation is ∗ �,�, + �,�, − �,� = 0 � . (5) However, actual urban surfaces are usually not The surface temperature is expressed as Eq.6 , simple horizontal planes as in Fig. 2. They are derived from Eq.2 and Eq.5. uneven and complex shapes as in Fig. 1. A more / complex phenomenon is caused by complex shapes. �, + � �, � = + � (6) For the simple expression, consider the case where � , � the heat balance is determined only by the radiation , balance with the convection component is assumed to In the case of no direct solar radiation, the right- be 0 in Eq.1. The direct solar radiation incoming to hand side of Eq.6 is free from the factors �,,�,, the surface free of solar shielding is �,, and sky therefore the temperature at shielded surfaces and the solar radiation is �,. However, the typical surfaces temperature at unshielded surfaces will be equal. in an urban canyon are not free from solar shielding, Therefore, it is the second term in the right-hand side of Eq.6 which determines the temperature so the two factors �,,�, are introduced in Eq.3 distribution on the urban canyon surface. and Eq.4. As the factors �,,�, depend on the position in the urban canyon, those factors have A numerical simulation performed for the case of subscript � indicating a position. clear model solar conditions on 21,Jun in Osaka - 3042 - Proceedings of BS2013: 13th Conference of International Building Performance Simulation Association, Chambéry, France, August 26-28 (latitude: 34.7N, longitude: 135.5E) was performed for the urban canyon with the shape are shown in Fig. D F 3. Fig. 4 shows the surface temperature distribution Mirror鏡面Ⅳ-IV Mirror鏡面Ⅰ-I A calculated by Eq.6 under the condition of no C convection. Solar radiation is set for June 21 with Mirror鏡面Ⅱ-II Mirror 鏡面Ⅲ-III E clear sky. Surface material is 150mm thick concrete with fully-insulated back side. Solar absorptivity and B emissivity are both set to 1. The results are (a) Reflector unit (b) Reflector array reasonable considering that heat dissipation through Figure 6 Overview of retroreflector convection is not included. Though Fig. 4 is an unrealistic thought experiment, Fig. 5 shows more realistic condition, with a convection heat transfer Mirror鏡面Ⅳ-IV component included. The convective heat transfer Mirror鏡面-ⅣIV Mirror鏡面Ⅰ-I Mirror鏡面Ⅰ-I coefficient is set to 18 W/m2K , with an atmosphere Mirror鏡面Ⅲ-III Mirror鏡面Ⅲ-III temperature varying over the range 23.5 +/- 4 deg C. Mirror鏡面-IIⅡ Mirror鏡面-IIⅡ In the ground temperature distribution, the same trend as Fig. 4 is shown, although the effect on temperature of North-face wall and South-face wall (a) Reflection 3 times (b) Reflection 5 times is slight, because the solar radiation is very little. As described above, the surface temperature 鏡面Ⅳ 鏡面Ⅳ distribution is governed by the second term of right- Mirror-IV Mirror鏡面Ⅰ-I Mirror-IV 鏡面MirrorⅠ -I hand side of Eq.6 Therefore, if the direct solar Mirror鏡面Ⅲ-III Mirror鏡面Ⅲ-III radiation can be reflected to the sky, rather than to Mirror鏡面-ⅡII Mirror鏡面-IIⅡ other canyon surfaces, the surface temperature in urban canyon will be decreased. (c) Reflection 1 time (d) Reflection 2 times Figure 7 Typical 4 paths of reflection in RETROREFLECTOR retroreflector unit A directive reflector is made with a specularly reflective material. The direction of reflection can be North北 controlled to account for the sun position depending on the time and season. By automatically controlling a motorized mount, the mirror facing can direct solar radiation as desired. However, a means to control the Mirror-II direction of reflection without using a movable 鏡面Ⅱ 鏡面Ⅰ 東East Mirror-I mechanism is desirable from the viewpoint of cost. γ Overview of retroreflector Given the issues mentioned above, the authors have Line谷線：鏡面Ⅰと鏡面Ⅱの of intersection focused on a retroreflector. Retroreflection is the with交線 mirror-I & II phenomenon of light rays striking a surface and reflecting back along the same path to the source of (a) Azimuth angle � the light. A retroreflector can do this without using a movable mechanism, just by the nature of its Zenith天頂方向 structure. It is an ideal reflector for the sun for our purpose, sending solar radiation back to the sky P regardless of the solar position.
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