Study on designmethodforcoolspotformationina semi-outdoor space by combining various passive cooling techniques

Abstract

In order to reduce heat in urban environments, the Japanese government has been enforcing mitigation and adaptation policies; however, due to insufficient design guidelines, it is still difficult to form a cool environment. The purpose of this study is to propose an evaluation index and design guideline for the formation of cool spots in semi-outdoor spaces using passive cooling methods, and to confirm the total cooling effect of the designed space by means of air temperature, MRT and sensible temperature. In order to form a cool spot in semi-outdoor space, it is important to utilize natural ventilation at windy condition, but also to decline radiative temperature and generate cool air at breeze condition. Therefore, we focused on evaporative cooling methods that are applied to vertical plane, and proposed a cooling evaluation index based on the ambient wet bulb temperature, with βTs, βTaJee and βTajor as the cooling efficiencies of the surface temperature, cool air on leeward and forward side of the passive cooling techniques, respectively. From the measurements of the individual methods, we quantified βTs, βTaJee and βTajor of trees, a Passive cooling louver, and lava walls under watering condition, and the values for βTs with these methods were found to be 0.12, 0.55, and 0.76, respectively, and βTaJee at a wind velocity >0.5 m/s were found to be 0.11, 0.17, and 0.36, respectively. A "Cool terrace" was designed for a semi-outdoor space with shade on the ceiling and water retentive blocks on the floor, on the basis of βTa (average βTaJee of all the vertical planes in the space) to achieve an air temperature decline by more than 1°C and MRT equivalent or lower than the ambient air temperature. The extent of evaporative cooling methods that needs to be applied was calculated based on the required βTa, and by taking into account the dominant wind direction of the area, the evaporative cooling methods were allocated as a "Cool terrace". The measurement result of the Cool terrace showed an air temperature decline of 1.5 to 2.7 °C, and an MRT equivalent to the representative air temperature at breeze. Moreover, the SET (standard new effective temperature) of the Cool terrace was at most 6 °C lower compared to the SET of the adjacent deck terrace, when the solar radiation was large, and was on an average 1 °C lower, when it was cloudy. The contribution of each environmental element to the reduction in SET was analyzed, and when the difference in SET 's was 6 °C, the contribution of MRT was-5.5 °C, air temperature was-0.8 °C, wind velocity was +0.1 °C, and humidity was +0.1 °C. From the results, the application of vertical passive cooling methods can provide cool environment in means of low air temperature and MRT at breeze, and since it is known that the ventilation (i.e. wind speed) can lower the sensible temperature as well34), these results imply that the combination of vertical passive cooling methods with different figures and cooling effects of can provide a cool environment continuously at various wind conditions.