Modelica Based Dynamic Modeling of an Air-Side Economizer

For the heating, ventilating, and air conditioning (HVAC) systems for commercial buildings, the cooling coils in air handling units (AHU) account for a significant fraction of total building energy consumption and have a major impact on comfort conditions and maintenance costs. Development of cost-effective advanced control strategies will enhance the performance and efficiency of AHU. The control design process can be greatly facilitated with simulation on high-fidelity dynamic model prior to experimental validation and implementation. This paper presents a dynamic model for an air-side economizer. The model development was based on Dymola and AirConditioning Library with some revision on heat exchanger modeling. For chilled-water cooling coil modeling, the major challenges include the variation of coil surface conditions under flow rate changes and partially-dry-partially-wet operations. This study proposes a dynamic coil model that is capable of predicting cooling performances under fully dry, partially-dry-partially-wet, and fully wet conditions. Validation with experimental data from a benchmark study was conducted under both dry and wet surface conditions. The model predicted the experimental results quite well for both transient and steady-state behaviors. In addition, other moist air components, such as dampers, fans, ducts and room, were developed. Such transient model will lay a more quality foundation for controller validation at the simulation phase.