Dynamic Modeling and Self-Optimizing Control of Air-Side Economizers

For the heating, ventilating, and air conditioning (HVAC) systems for commercial buildings, there has been a greater demand for reducing energy consumption. The economizers have been developed as a class of energy saving devices that may increase the energy efficiency by taking advantage of outdoor air during cool or cold weather. However, in practice, many economizers do not operate in the expected manner and waste even more energy than before installation. Better control strategy is needed for optimal and robust operation. This paper presents two related aspects of research on dynamic modeling and control for economizers. First, a Modelica based dynamic model is developed for a single-duct air-side economizer. The model development was based on Dymola and AirConditioning Library with some revision on water medium and heat exchanger modeling. Such transient model will lay a more quality foundation for control design. Second, for a threestate operation for air-side economizers, a selfoptimizing control strategy is developed based on the extremum seeking control (ESC). The mechanical cooling can be minimized by optimizing the outdoor air damper opening via extremum seeking. Such has much less dependency on the knowledge of economizer model, and thus has more promise for practical operation. In addition, an anti-windup ESC scheme is proposed as an enhancement for the existing ESC techniques. The simulation results validated the effectiveness of the dynamic model of the economizer, demonstrated the potential of using ESC to achieve the minimal mechanical cooling load in a selfoptimizing manner, and illustrated the possibility of ESC malfunctioning under actuator (damper) saturation and the capability of anti-windup ESC in preventing such undesirable behavior.

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