Investigation of energy efficiency of a three-pipe “VRF” system with and without waste energy recovery in transitional seasons

Abstract

Building sector has become the largest consumer of primary energy in the world. This leads to a huge consumption of fossil fuels and a large impact on the environment. It is recognized that improving heating, ventilation and air conditioning (HVAC) systems, which consume almost half of all commercial building energy, is critical. It is also estimated that there will be a significant increase in HVAC installations around the world, as the demands for indoor comfort in buildings increase. The variable refrigerant flow (VRF) system, as one of the emerging HVAC technologies, has been and is still widely used in Asia, Europe and has a rapidly growing market share in North America for decades. The article analyses a multi-unit air conditioning system that can independently change the refrigerant flow depending on the cooling or heating load. The system configuration with heat recovery (VRF-HR) can recover heat from the indoor units in the cooling zones and transfer it to the heating zones, simultaneously performing heating and cooling functions. Since such a system can use waste heat, the required power of the outdoor unit of VRF-HR system is reduced compared to conventional heat pump systems. The work investigates a three-pipe variable refrigerant flow (VRF) system with hot water preparation in the transition season, analyzes the efficiency of this system in simultaneously preparing hot water, and using waste energy for cooling. The obtained data are compared with the traditional VRF system for separate preparation of hot water and cooling. The results showed that a variable refrigerant system with a heat recovery configuration can be about 38% more efficient in hot water preparation compared to traditional systems without heat recovery. And when evaluating the amount of coolness transferred by internal evaporators, it can reach up to 80%. Also, a heat recovery system can delay freezing of the outdoor unit’s evaporator, so defrost cycles can occur less often, which not only increases system efficiency, but also better meets heat or cooling needs and extends the life of system components.