Optimization and sequencing of chilled-water plant based on extremum seeking control

Abstract

Chilled-water plants with multiple chillers are the backbone of ventilation and air conditioning (VAC) systems for commercial buildings. Optimal operation and sequencing of such plant has great impact on building energy efficiency. Large variations in equipment characteristics and operating conditions make it difficult and expensive to obtain plant models required by model-based control/optimization approaches. For a variable primary flow (VPF) chilled-water plant with parallel chillers, a model-free optimization and sequencing strategy is proposed to maximize energy efficiency in real time, based on a penalty-function based multivariate extremum seeking control (ESC). The ESC takes the total power consumption (chiller compressors + cooling tower fan + condenser water pumps + input-saturation penalty) as feedback, while tower fan airflow, condenser water flows and evaporator leaving chilled-water temperature setpoint as manipulative inputs. A band-pass filter array replaces the high-pass filter in the standard ESC to reduce the cross-channel interference. The chiller sequencing is enabled with input saturation related signals. Simulation study is performed under several testing conditions using a Modelica based dynamic simulation model of a chilled-water plant with two parallel chillers, one cooling tower, one air-handling unit and one zone. Simulation results validate the effectiveness of the proposed strategy with significant energy saving demonstrated.