Temperature stratification and air change effectiveness in a high cooling load office with two heat source heights in a combined chilled ceiling and displacement ventilation system
Radiant chilled ceilings (CC) with displacement ventilation (DV) represent a promising integrated system design that combines the energy efficiency of both sub-systems with the opportunity for improved ventilation performance resulting from the thermally stratified environment of DV systems. Their combined cooling capacity is thought to be limited. The purpose of this study is to conduct laboratory experiments for a U.S. interior zone office with a very high cooling load (91.0 W/m2) and with two different heat source heights represented by computer CPUs (at floor level and at 1.52 m) to investigate their influence on room air stratification and air change effectiveness. The experiments were carried out in a climatic chamber equipped with 12 radiant panels, covering 73.5% of the ceiling, installed in the suspended ceiling. The cooling load removed by the panels varied between 0 and 92 W/m2 (based on radiant panel area) or between 0 and 68 W/m2 (based on room area). The average mean water temperature of the panels varied between 14.1-26.2°C. The displacement ventilation airflow rate varied between 4.0 and 9.9 l/(s m2), and the supply air temperature was kept constant at 18°C. The results showed that displacement ventilation and chilled ceiling are able to provide a stable thermal stratification and improved ventilation effectiveness compared to mixing ventilation for a wide range of configurations and system design even for extremely high cooling load (91 W/m2). Stratification and air change effectiveness decreases when a larger portion of the cooling load is removed by the chilled ceiling (surface temperature of the panel deceases). For every degree decrement of the panel the stratification decreases by 0.13°C and the ACE by 0.13. Moving the CPUs (representing 51% of the total room heat gain) from the floor level to 1.5 m height markedly increased the room median stratification (0.8°C) and the median air change effectiveness measured at 0.6 m (1.75). Therefore, increasing the height of heat sources reduced energy use and improved indoor air quality. When the CPUs where located in the higher location, the median stratification in the occupied zone was 2.95°C and the ACE at 0.6 m was 2.9. Moreover it was found that the higher the stratification the better the air change effectiveness.