Review of the advances and applications of variable refrigerant flow heating, ventilating, and air-conditioning systems for improving indoor thermal comfort and air quality

The maintenance of a healthy and comfortable indoor environment consumes a significant amount of energy in the built environment. Heating, ventilating, and air-conditioning (HVAC) systems can provide a healthy indoor thermal environment and air quality. Variable refrigerant flow (VRF) HVAC systems utilize a refrigerant to transfer heat from a heat source to a heat sink by changing its phase from liquid to gas and from gas to liquid. VRF HVAC systems are becoming popular due to their flexible operation, particularly under dynamic thermal loading and weather conditions. The advances of the VRF HVAC system include the utilization of new materials and concepts that make the system robust and dynamic, give it high heat transfer capabilities, allow it to have a compact design, and make it energy efficient. Due to its energy efficiency, indoor thermal comfort and quality, and versatile applications, the VRF HVAC system is one of the most viable alternatives to conventional HVAC systems. As such, extensive efforts in the research, development, testing, and application of these systems have been made. Despite these advancements and the demand for high-quality, energy-efficient, and comfortable indoor thermal environments, VRF HVAC systems still require further development, which is the topic of this review paper.