Optimal Outside Air Control for Air Handling Units with Humidity Control

Abstract

Most air handling units (AHUs) in commercial buildings have the (air) economizer cycle to use outside air for free cooling under certain outside air conditions. Ideally the economizer cycle is enabled if outside air enthalpy is less than return air enthalpy. During the economizer cycle, outside air flow is modulated to seek mixed air temperature at a supply air temperature set point. Since the outside air may be dry during the economizer cycle, humidification is required for AHUs with humidity control. As a result, the economizer cycle saves cooling energy but requires excessive steam for humidification. Therefore the economizer cycle may not be economical. An optimal outside air control method is developed to minimize the total cost of mechanical cooling and steam humidification. The impacts of chilled water price, steam price, and space minimum humidity set point are analyzed. Finally the optimal outside air control zones are presented on a psychrometric chart under differential energy price ratios and minimum indoor humidity set points.