Managing heat rejected from air conditioning systems to save energy and improve the microclimates of residential buildings

Simulations are conducted to investigate the influence and improvement potential of air conditioning heat rejection management of residential buildings on microclimate and energy use. The microclimate and building energy use are simulated on a typical high-rise building in Taipei, Taiwan, on a summer night. Heat rejection from the air conditioners is estimated with a building energy program, EnergyPlus, and a computational fluid dynamics (CFD) program, Windperfect, is used to analyze how heat rejection affects the outside thermal environment. Results show that heat rejection from air conditioners worsens the thermal environment below the urban canopy, thus increasing building energy use. Three countermeasure cases of heat rejection management, which consider the type of cooling system and its installation position, are proposed in this study. The average air temperature increase around the buildings caused by heat rejection was analyzed by transferring the simulation results of the building energy program to the CFD model on an hourly basis. Results show that the air temperature next to the building envelope and the air temperature around air conditioners decrease and that there is a reduction in electricity consumption by the air conditioners when a split-type air conditioner is installed on each floor or on every third floor. A reduction in the ambient air temperature below the urban canopy can be obtained by placing a cooling tower on the roof of the building.