Experimental study on the thermal performance of building external window greenery in a subtropical climate

Previous research has found that building greenery systems can cool both buildings and their surrounding environments during the summer. However, the window greenery system, which is a widely adopted form of building greenery, has received limited attention. This study analyzed the thermal performance of window greenery systems in general and the building-integrated window gardens in specific, based on in-situ measurements in a subtropical city. The results show that the cooling effects of the window garden on the envelope were significant near the bottom region. During the air-conditioning period, the heat flux through the bottom wall and windowpane interior surfaces was decreased by 29.0 % and 25.8 %, respectively. A regression model for heat flux through the bottom windowpane was proposed, which estimated that the reduction in radiative heat transfer by plants was 88.0 %. The equivalent heat transfer coefficient for the walls adjacent to the window garden was calculated. Window gardens also improved the outdoor microclimate, reduced the average air temperature over planting troughs by 1.0 C-degrees, and increased the average relative air humidity by around 10.0 % throughout the day. Given that window gardens' cooling effects strongly depend on water evaporation, a win-win strategy for energy and water saving by harvesting the air conditioning condensate for irrigation is proposed. Based on an estimation, the condensate could fully meet the daily requirements of window greenery, confirming the feasibility of the win-win strategy.