Cooling and Energy Saving System Utilizing Photo-induced Hydrophilicity on TiO_2 Film

城市变暖（热岛现象）是环境问题之一。防止这种现象的有效方法是增加绿地面积或水面面积。因此，我们设计了一个新颖的系统，通过在夏季将储存的雨水连续喷洒到建筑物的TiO_2涂层表面来增加水面。由于已知TiO_2表面在紫外光照射下表现出光致亲水性，太阳能光将建筑表面转化为高度亲水性。高亲水性在最大限度地减少形成水膜所需的水量方面起着重要作用。高亲水性形成的薄水膜也表现出从水面有效蒸发的特性。蒸发产生有效的潜热通量，冷却建筑表面及其周围的大气。冷却效果也减少了空调的使用量，从而减少了能源消耗和人工热排放。因此，我们的系统既节约了能源，又减少了城市变暖。该体系的关键在于TiO_2材料表面的高亲水性。研究了TiO_2涂层材料表面形成的水膜厚度与表面接触角（亲水性指标）与冷却效率之间的关系。结果表明，随着TiO_2涂层材料表面光致亲水性的增加，冷却效率也随之提高。随着TiO_2涂层表面接触角的减小，水膜厚度逐渐变薄。这些结果表明，冷却效率的提高与材料上形成的水膜的厚度有关，因为薄的水膜导致有效的蒸发。通过实际建筑和住宅，验证了该系统的制冷效果。

Urban warming (heat island phenomenon) is one of the environmental problems. An effective method to prevent the phenomenon is to increase the amount of green tract land or surface area of water. Therefore, we have designed an original system to increase water surfaces by continuously sprinkling stored rainwater onto the TiO_2 coated surfaces of buildings in summer. Since TiO_2 surface is known to show photo-induced hydrophilicity under UV light irradiation, the solar light converts the building surfaces to a highly hydrophilic one. The high hydrophilicity plays an important role in minimizing the amount of water needed to form a water film. And also, the thin water film formed by high hydrophilicity shows an effective evaporation from the water surfaces. The evaporation generates the effective latent heat flux, cooling down the building surfaces and its surrounding atmosphere. The cooling effect also reduces the amount of air conditioning used, which reduces energy consumption and artificial heat emission. Thus, our system saves energy as well as reduces the urban warming.A key to this system is the high hydrophilicity of TiO_2 material surfaces. We investigated the relationship between the contact angle, an indicator of the hydrophilicity, and the cooling efficiency by the evaporation linking the thickness of water film formed on the TiO_2 coated material surfaces. The results demonstrate that the cooling efficiency increases with increasing the photo-induced hydrophilicity of the TiO_2 coated material surfaces. And water film thickness became thiner with decreasing the contact angle on the TiO_2 coated material. These results suggested that the increase in cooling efficiency relates to the thickness of water film formed on the material, because the thin water film leads to the efficient evaporation.The cooling effect of the system was also confirmed using the actual building and house.