Thermal Effect of Evaporative Cooling by Spraying Water on Membrane-structured Roof

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  • ISHIKAWA Yukio
    Environmental and Mechanical Engineering Section, Takenaka Technical Research Laboratory

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  • 膜構造屋根散水システムの熱的効果に関する研究
  • マク コウゾウ ヤネ サンスイ システム ノ ネツテキ コウカ ニ カンスル

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With the aim of embodying its educational policy whereby emphasis is placed on a favorable relationship with nature, H High School, in Hiroshima City (34°22'N and 132°26'E), has adopted a tension membrane-structured roof for its gymnasium, to create a bright space integrated with the campus which is surrounded by greenery. This roof is framed with pieces of glue-laminated lumber, on which a 0.8mm thick single-layered membrane composed of glass fiber base sheet coated with teflon is laid. While the membrane-structured roof has the major advantage of being translucent, it also admits the penetration of solar radiation, so a thorough study of the indoor thermal environment in the summer period would be required. Since this facility is a gymnasium for the junior and senior high school students, the plan excluded the use of active cooling, but aimed at planning and implementing the proposed roof spraying system as a means of passive cooling utilizing natural energy. The principles are that water is sprayed onto the membrane roof surface to reduce its temperature through the cooling effect of evaporation into the open air, thereby facilitating the passive cooling of the gymnasium inner space. This paper begins firstly with outlining the roof spraying system by means of evaporative cooling, and discusses the field measurement results of the thermal effects, which were carried out on typical summer days. Through field measurements, the author has proved that roof spraying reduces the indoor PMV, which further leads to a decrease in PPD, and SET^* is also reduced, which enhances the indoor thermal environment, producing a noticeable passive cooling effect. Furthermore, this study has provided field measurement data representing the lighting and thermal characteristics of the single-layered, membrane-structured gymnasium. Secondly, a theoretical analysis of the thermal effect is made, followed by verification of results between analysis and field measurements. As both results concur, theoretical simulation is carried out by using the meteorological data of Hiroshima City then available. Here, the results of the predicted effect of passive cooling during the summer period are discussed.

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