Optimization of Supply Air Temperature Reset Schedule for a Single Duct VAV Systems

In a single duct variable air volume (SDVAV) system, the supply air temperature is usually set as a constant value. Since this constant setpoint is selected to satisfy the maximum cooling load conditions, significant reheat will occur once the airflow reaches the minimum and the heating load increases. Resetting the supply air temperature (SAT) higher during the heating season can reduce the reheat. However, air flow will increase when the SAT is higher which consume extra fan power. Therefore, to minimize the total operating cost of a SDVAV system, the supply air temperature is typically reset based on outside air temperature (OAT) with a linear reset schedule. However, the linear reset schedule is often determined based on the engineer’s experience and it may not represent the optimal reset schedule for each building. This thesis documents a study to determine the optimized supply air temperature reset schedule for SDVAV systems and analyzes the influencing factors under different operation scenarios. The study was divided into five main sections. The first section introduces the research background and objective. Literature review is documented after the introduction. The third section describes the methodology used in this study and the fourth section develops an in-depth discussion and analysis of the impact of the key influencing factors: minimum air flow ratio; ratio of exterior zone area to total floor area (i.e., exterior area ratio); internal load and the prices of the electricity; the cooling and the heating energy. The simulation results using EnergyPlus Version 2.1.0 for various operation scenarios are investigated in this section. The last section is a conclusion of the whole study. The optimized supply air temperature can be set with respect to the OAT. The study found that instead of a simple linear relationship, the optimal reset schedule has several distinctive segments. Moreover, it is found that the optimal supply air temperature reset schedule should be modified with the change of operation conditions (e.g., different minimum flow ratios and internal loads). Minimum air flow ratio has a significant impact on energy consumption in a SDVAV system. Exterior area ratio determines zone load distribution and will change system load indirectly. For buildings with small internal load, a more aggressive supply air temperature reset tactic can be implemented. In addition, the cost of electricity, cooling and heating energy can determine which end use energy (i.e., reheat energy and fan power) should take the priority