Air-Handling Unit Supply Air Temperature Optimal Economizer Control Experiment

Abstract

Most air handling units (AHUs) in commercial buildings have an air economizer cycle for free cooling under certain outside air conditions. During the economizer cycle, the outside air and return air dampers are modulated to seek supply air temperature at its setpoint. The supply air temperature is typically set at 13 °C (55 °F) to control humidity in the space. However, dehumidification is not necessary when the outside air is dry. Meanwhile, the space may have less cooling load due to envelope heat loss and/or occupant schedule changes. These facts provide an opportunity to use higher supply air temperature to reduce or eliminate mechanical cooling and terminal box reheat. On the other hand, a higher supply air temperature requires increased air flow as well as fan power. Therefore, an optimization question was formed, through which an optimal supply air temperature is identified to minimize total energy consumption. In our previous studies, through simulation, 90% of energy savings were concluded and a universal control sequence was also proposed for implementing the optimal control strategy. In this paper, experiments were conducted to validate the previously documented theory concerning the optimal supply air temperature reset. The previously recommended universal control sequence is implemented into the building automation system for an air-handling unit control to make the program ready for the next step of verifying energy savings previously simulated. This paper presents optimization control system setup and experimental results showing the program tuning procedures, through which the program is ready for the next step.