C22 -- Reducing HVAC Energy Consumption Through Optimal Sub-Zoning Considering Occupant-Centric Control (OCC)

Improving thermal zoning is a significant means to reduce HVAC energy consumption and improve thermal comfort. Virtual division of super-zones (space bound by physical partitions) into zones (area catered by each air handling unit (AHU)) and sub-zones (regions catered by each diffuser) are gaining popularity for commercial buildings such as open-plan offices with occupant-centric control (OCC). Since thermal coupling from adjacent sub-zones are prominent in such virtual divisions, unplanned sub-zones can lead to energy wastage. Architects and HVAC designers sometimes use their domain knowledge to delineate these zones based on their thermal and functional requirements, occupancy schedule, and system capabilities. Current literature, however, does not address a systematic method of sub-zoning or its optimal size. Our work addresses this gap by exploring a method of parameterized sub-zoning. The effect of spatial parameters such as (1) spatial design (2) location of diffusers and (3) thermostats; dynamic parameters such as (4) ambient conditions, (5) occupancy; and HVAC parameters such as (6) type of diffusers and (7) air supply conditions are analyzed using layouts of existing open-plan offices in India. Transient state parametric analysis of sub-zonal conditioning is performed on a typical open-plan office through CFD simulations employing ANSYS fluent after validating the numerical model using experimental results.

Results indicate that (1) planned sub-zonal conditioning can reduce total energy consumption up to 40% despite an increase in return air temperature; (2) Use of setback conditions in unoccupied sub-zones improve cooling time and helps in reducing sharp vertical thermal gradients; (3) Relative location of diffusers and air supply velocity are two parameters that influence the spread of jets and tuning them is critical in sub-zonal OCC. Our findings enhance literature on thermal zoning highlighting the possibilities and limitations of sub-zonal conditioning and highlights the need for integrated architectural and HVAC design.