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

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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.