Optimization of return vent height for stratified air distribution system with impinging jet supply satisfying threshold of |PMV| < 0.5

The stratified air distribution (STRAD) ventilation system addresses much attention due to the less cooling energy consumption than conventional mixing ventilation systems in maintaining a proper indoor environment. In recent literature, the vent height of STRAD systems has been studied. However, in previous studies, the impacts of over-cooling and/or over-warming on cooling load as well as, consequently, on the optimal return vent height were not systematically considered. In practice, the indoor environment (e.g., predictive mean vote PMV, indoor air quality IAQ) is sensitive to the supply condition (e.g., supply temperature) and the return vent height. This study aims to optimize the return vent height of a STRAD ventilation system with impinging jet supply under a general thermal comfort condition, i.e., threshold of |PMV| < 0.5. The evaluated indices include the cooling coil load, the temperature difference between the head and ankles, the average CO₂ concentration, the average mean age of air, and the predicted percentage of dissatisfied. The research outcomes indicate that: a) the increase of supply temperature generally avoids overcooling and reduces cooling coil load; b) the cooling coil load increases and c) the IAQ generally upgrades as the return vent height rises; d) the optimal return vent height with the impinging jet ventilation is recommended as 2.625 m (i.e., near the ceiling), which is different from previously suggested mid-level return vent. Under this optimal return vent height, the supply temperature can be increased to 22 °C with resultant |PMV| < 0.5, lower cooling coil load, and better IAQ comparing to the recommended case of mid-level return vent with a supply temperature of 18 °C.