RADIANT COOLING EFFECT OF CEILING SLIT CAPABLE OF BOTH DAYLIGHT HARVESTING AND SOLAR HEAT REMOVAL

With the increasing interest in the effects of solar radiation on indoor environmental quality and occupants’ well-being, a balance should be established between daylight harvesting and reducing solar load. Solar heat removal in space can be realized with the help of radiant cooling systems. Previous studies have reported on the enhancement of the cooling capacity of radiant cooling systems owing to the absorption of solar radiation at cooled surfaces; this characteristic is expected to increase the flexibility of the design of windows and facades in daylight harvesting spaces. The present study aims to verify the radiant cooling effect of a ceiling slit comprising concrete and embedded water circulation pipes. Field measurements were obtained during spring and summer at a research institute located in Fukui prefecture. A groundwater cooling system was used to provide chilled water to the ceiling slit, enabling both daylight harvesting and solar heat removal. In this study, absorption and removal of solar heat gain at the cooled surface and thermal comfort in the office space were evaluated. The office space was verified to be comfortable, and the usefulness of radiant cooling system under high intensity solar radiation was demonstrated. To verify the solar heat removal performance of the ceiling slit, the heat flux measured at the cooled surface (the amount of heat absorbed and dissipated by the cooled surface) was compared with the system-side heat extraction rate (the amount of heat removed by circulating chilled water), which was calculated using the measurement results. The radiant surface heat flux reached a maximum of 190 W/m2, and the heat extraction rate reached a maximum of 126 W/m2 under solar radiation. Even when no water supply to the ceiling slit was present, the surface heat flux (heat absorption) reached a maximum of 138 W/m2 during the daytime (August 29). Therefore, the measurement results evidently demonstrate that the radiant cooling capacity should be evaluated by the system-side heat extraction rate rather than the radiant surface heat flux. The fluctuation range of the system-side heat extraction rate was smaller than that of the radiant surface heat flux due to the thermal capacity of the ceiling slit, similar to the self-regulating effect of thermo-active building systems. The field measurement results demonstrated that a comfortable office space with daylighting was realized by directly extracting solar heat and accumulated hea