Review on recent developments of variable refrigerant flow systems since 2015

Variable refrigerant flow systems (VRFS) are gradually gaining popularity in commercial and residential buildings owing to their satisfactory part-load performance, flexible control, and ease of installation and maintenance. However, the characteristic complex system structure of VRFS makes it difficult to understand their performance. This study presents a comprehensive review of VRFS concerning their design and optimisation, control strategies, modelling and simulation, and fault detection and diagnosis (FDD). This review begins with the developments in design and optimisation of VRFS in terms of alternative refrigerants, crucial components, and innovative systems. Subsequently, the control strategies of VRFS including energy efficiency, improved humidity, and defrosting operation are reviewed. Meanwhile, the research objectives on modelling and simulation of VRFS could be divided into three aspects: investigation of the inherent energy performance, prediction of energy consumption in buildings, and comparisons of energy saving potential. In addition, the advances in FDD for VRFS, based on the rapid development of data mining technologies, are presented. The main conclusions drawn from this detailed review provide a comprehensive overview of VRFS and inspire new insights and feasible solutions for relevant researchers, product developers, practitioners, and policymakers in this field.