A CFD study of energy saving potential in occupied zone in mixing and displacement ventilations

Heating, Ventilation and Air Conditioning (HVAC) system accounts for 40%-50% of total building energy usage, and thus it is significant to optimize HVAC systems to reduce energy consumption in buildings. In terms of control strategy, the conventional temperature control (i.e. temperature sensor is installed inside a return air duct) is to use a single thermostat to control variable-air-volume (VAV) boxes based on the temperature difference between the air return temperature and the room temperature set point. However, the return air temperature measured at the return air duct (usually installed at ceiling level) cannot reflect the real temperature in the occupied level or breathing zone. In order to improve the indoor temperature control, it is suggested the temperature sensors should be installed in the occupied level to capture the temperature that occupants really need. A typical office room is selected and both mixing and displacement ventilation are numerically studied to explore the energy saving potentials in this paper. By comparing with the traditional temperature control, 20% (203m³/h) and 23% (81m³/h) of airflow rate can be saved in mixing ventilation in fully and half occupied condition; in displacement ventilation, at least 25% (360m³/h) and 44% (504m³/h) of airflow rate reduction can be achieved without scarifying thermal comfort. Therefore, the proposed configuration for a room temperature control may lead to a significant energy saving of air-conditioning systems.