Mitigation of heating of an urban building rooftop during hot summer by a hydroponic rice system

• A rice hydroponic system mitigated rooftop heating in summer.
• Mitigation effects are explained by ambient air temperature and solar radiation.
• Solar radiation was the primary factor affecting thermal mitigation effects.
• A normalized mitigation index represents the cooling effect of the hydroponic system.

The use of green roofs is an important method for mitigating heating of urban rooftop environments. Our study aimed to demonstrate the mitigation of thermal effects on a hot rooftop during summer by a hydroponic system in which rice was grown. The system was installed on the top of a commercial building in the large city of Osaka, Japan; the roof was divided into two areas, one bare, the other covered by the hydroponic system. In both areas, we measured thermal factors, such as air temperature, rooftop surface temperature, and conductive heat flux; from the data we calculated three thermal mitigation indices. We also propose normalized types of mitigation indices. Mitigation effects on the thermal environment by the hydroponic system could be well estimated from ambient air temperature and solar radiation; the effects were better explained by solar radiation than by ambient air temperature. The results indicate that during the hot season, the system's mitigation effects on the thermal environment can be predicted from solar radiation level, and the normalized mitigation index is an appropriate index for estimating the cooling effect. Our results suggest that the hydroponic system might affect the energy flow in two ways: the proportions of sensible heat flux and latent heat flux are mainly affected by evaporative cooling and the portion of conductive heat flux is mainly affected by radiation shielding.

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