Utilizing multi-connected three-fluid heat exchangers in indoor units to improve the low load-ratio performance of variable refrigerant flow systems
Fuhai Zha, Yuan Wang, Chenjiyu Liang, Wenxing Shi, Xianting Li, Defang Guo, Shoubo Mao
Abstract
Variable refrigerant flow (VRF) systems can easily fall into low part load ratio (PLR) conditions during operation in commercial buildings throughout the year, which may result in poor energy efficiency of VRFs. This study proposes a system to improve the low PLR performance of VRFs by multi-connected indoor units, in which three-fluid heat exchangers are used in the indoor units and all indoor units are connected by a water loop. The ε−NTU, efficiency-frequency correction, and manufacturer-fitted models are used for heat exchangers, compressors, and fans/pumps respectively. Based on the component models and refrigeration/heat pump cycle, the numerical models for the reference system (conventional VRFs) and the proposed system are established. Taking a typical office building in Nanjing, China as an example, the energy performance and economy of the proposed systems with the same capacity of each outdoor unit as reference system (Prop. S1) and smaller capacity of each outdoor unit than reference system (Prop. S2) are analyzed and compared with those of the reference system. The results show that: (1) When the PLR is low, the compressors of some VRFs can stop, and the required energy can be supplied by other VRFs through the water loop, which results in the performance improvements of 30.2 % and 32.3 % for the Prop. S1, and 36.4 % and 57.4 % for the Prop. S2 when the PLR are 18 % and 9 %, respectively, in cooling conditions; (2) During typical weeks in summer and winter, the Prop. S1 saves 6.0 % and 14.2 % energy, and the Prop. S2 saves 8.0 % and 20.4 % energy, respectively; (3) The Prop. S1 and Prop. S2 save 9.5 % and 13.5 % energy throughout the year, respectively, and the static payback periods for Prop. S1 and Prop. S2 are 1.65 and 1.89 years, respectively. The results show that the proposed systems are promising for improving the energy efficiency of air-conditioning systems in an economical way.