Design method for multi-stage fresh air treatment system with high seasonal energy efficiency

Compared to traditional air conditioning systems that only use high-grade chilled/hot water, multi-stage air treatment (MAT) systems use lower grade chilled/hot water and achieve higher energy efficiency. However, the area of heat exchangers in the existing MAT system is designed based on the maximum value of all working conditions, which is large and not appropriate for partial load conditions. Therefore, this study proposes a method for MAT system design wherein the area of heat exchangers can be appropriate for different working conditions. In this study, optimal area ranges for various working conditions in a fresh air system are calculated, and the overlapping range of these optimal area ranges for all working conditions is determined. Then, the performance of the proposed MAT system is numerically calculated, and six different climate zones are selected to evaluate the applicability of the proposed design methods. The results show that: (1) when the system electric power fluctuates within a small range of 3%, the designed area of heat exchangers can vary over a large range of approximately 50% to 150%; (2) compared to the existing MAT system, the designed area of heat exchangers in the proposed MAT system is reduced by 50.0%, and the maximum energy saving rates in the cooling and heating seasons are 10.9% and 9.8%, respectively; (3) in the selected six climate zones, the duration ratio of working conditions where the designed area is not within the optimal range is less than 5%, indicating good applicability of the proposed design method.