The Effects on an Indoor Thermal Environment with Simple Hydroponic Cultivation on Rooftops

Abstract

We developed a simple and light weight hydroponic system using rockwool substrate for growing plants as a rooftop vegetation. The system was set with the vegetation section on the rooftop of the university building at the height of 16m above the ground level, where tomato plants were cultivated. A control section with no plants was also set on the same floor.
Humidity in the air surrounding the vegetation section increased by approximately 15% due to the tomato plants transpiration. Air temperature in the vegetation section decreased by approximately 1°C compared to that of the control section on cloudy days, while it seldom decreased on clear days. The rooftop surface temperature of the control section rose to over 50°C at clear midday, while at the vegetation section it infrequently rose to about 40°C, which made the difference in the total heat transfer into the room below by 2MJm^-2 in the value integrated over clear daytime. As a result, the air temperature in the room beneath the vegetation section was kept approximately 2°C lower than that beneath the control section after July. Heat flux through the multi-layer slab of the rooftop was obtained by means of the response factor method, and the latent heat transfer was estimated by the amount of supplied tap water for cultivation and then the heat balance on rooftop floor was examined. Sensible heat flux of the vegetation section was the same as that of the control, which corresponds to the fact that the air temperature of the vegetated area did not decrease. At the vegetation section the latent heat flux shared 50-60% of the net radiation flux, and the heat flux to the concrete floor decreased to approximately 25% of that at the control section. The results indicate that the vegetation system can be successfully applied to reduce the thermal load on buildings and to moderate dry climates in urbanized areas.