عنوان مقاله [English]
A significant part of the total annual energy consumption is used in buildings. The most part of the energy consumption in buildings is due to the HVAC systems. As a result, the efficiency of air conditioning systems plays an important role in energy saving. In the recent years, variable refrigerant flow (VRF) systems have become popular, especially for commercial buildings, due to their high efficiency and their ability in simultaneous heating and cooling. In this research, VRF systems are investigated from the second law of thermodynamic point of view; the effect of design parameters and the refrigerant type on the system performance is studied. At first, the thermodynamic cycle of VRF systems is modeled. Comparison of the numerical results with the available experimental data shows that the developed model can predict the system behavior with reasonable good accuracy. Based on the results, the coefficient of performance (COP), the exergy destruction, and the exergy efficiency of the VRF system are evaluated to investigate the system effectiveness. The results indicate that the compressor and the condenser are responsible for the most exergy destruction in the system, while the exergy destruction in the sub-cooling heat exchanger and the expansion valve is the least. In addition, the refrigerant type is an important parameter which may affect the system performance. Considering the importance of the sub-cooling heat exchanger, the effect of the heat exchanger on system performance has also been investigated. It has been found that the effect of the sub-cooling heat exchanger depends on the refrigerant type. In the systems using R-134a and R-407c, the heat exchanger improves the system effectiveness; if R-22 and R-410a are used as a refrigerant, the system performance decreases. The results of this paper are useful in getting a better understanding of the VRF systems and in designing more efficient ones.