Optimal chiller sequencing control in an office building considering the variation of chiller maximum cooling capacity

Abstract

For air conditioning system with multiple chillers, sequencing control is crucial in achieving operating energy efficiency while maintaining indoor thermal comfort in commercial buildings. In this study, an optimal strategy was proposed to improve the chiller sequencing control for energy efficient and reliable operation. The chiller model and the effectiveness-NTU (number of transfer unit) cooling tower model were developed for calculating the maximum cooling capacity of the individual chiller. Then, a simple correlation of maximum cooling capacity of the chiller was established in order to simplify the calculation process. For predicting the chiller partial load ratio (PLR) accurately, more accurate part-load performance curves were presented, considering the effect of maximum cooling capacity of chiller. The cooling load characteristic of typical working days in an office building in Zhuhai was also analyzed. Based on the cooling load profiles, the optimal chiller sequencing control strategy was formulated by using chiller model and branch and bound algorithm. In addition, three different chiller sequencing control strategies (strategies A–C) were tested and compared by implementing them in different chilled system configurations in the office building. The results suggest that the performances of strategies are varied for different chilled system configurations. The performance of the optimal strategy C, considering the variation of maximum cooling capacity of chiller, achieves the highest energy efficiency and reliability for different systems, especially for the system configuration in case 1. Strategy B also considers the effect of maximum cooling capacity of chiller and is appropriate for the system of constant speed chiller with dedicated pumps (i.e. case 2). Strategy A is a conventional strategy, which can neither guarantee the energy saving operation of chiller plant nor the sufficient supply of cooling capacity. To conclude, the proposed optimal chiller sequencing control strategy C can be directly applied in the real chiller plant.