Study on the Optimal Control Strategy for Condensing Side of Chiller Systems

Abstract

The performance curves of typical chillers available from the present market are investigated. These performance curves are fitted to develop a chiller model, which together with pump and cooling tower models are used to study the optimal control strategy for condensing side of chiller systems. The optimization objective is to minimize the total energy use of chillers, condensing pumps, and cooling tower fans. The optimization problem is solved by traversing all possible combinations of chiller water flow rates, cooling tower air flow rates, and ambient air wet-bulb temperatures. Then the optimal solutions are analyzed to find out applicable rule-based control strategies that are not far from optimal solutions and reliable as well. The rule-based control strategy is to keep one or two constant cooling water flow rates during whole operational season according to the number of running chillers and to keep cooling tower air flow rate at a constant ratio to cooling water flow rate. Because this rule-based control strategy does not use sensor measured data, such as wet-bulb air temperature, it is free of sensor faults and can be more robust and reliable than the currently used control strategies, most of which use cooling water temperature or ambient air wet-bulb temperature to decide the cooling water flow rate or air flow rate of cooling tower fans.