A study of high efficiency CO2 refrigerant VRF air conditioning system adopting multi-stage compression cycle

CO2 refrigerant is expected to be use for air conditioners due to its environmental property of GWP = 1, non-flammability and low toxicity and has already been applied to hot water supply system, refrigerators and freezers. However, since cycle COP of CO2 is poorer than that of HFC in applying to air conditioner, it cannot be expected to improve the performance significantly by simply improving the efficiency of a conventional single-stage cycle and components consisting of the cycle.  Therefore, it is important to improve the cycle efficiency, especially in the performance of cooling which is a weak point of CO2. Challenges also lie in reducing weight of components such as compressor and heat exchangers resulted in weight gain to resist high pressure and to achieve high efficiency in the system. In this study, a prototype of CO2 refrigerant VRF air conditioning system is constructed and evaluates the performance. The system is adopting the four-stage compression cycle applying a new type compressor and heat exchangers and so on aimed for high efficiency cycle and reducing weight As a result of the evaluation, it is confirmed that CO2 cycle performance can be improved in similar extent to HFC refrigerant cycle. On the other hand, this high efficiency cycle consists of a lot of components and becomes more complex than HFC refrigerant cycle; therefore the size and weight of outdoor unit increases to house these components. Further technical development will be necessary to overcome these weaknesses

Evaluation and Optimization of System Performance using HFO-mix Refrigerants for VRF and Mini-split Air-Conditioner

Global demand for air conditioning is expected to increase continuously. In order to minimize the impact on the climate as a total amount of CO2 equivalent, it is essential to continue to search for better refrigerants. On the other hand, product size and cost are important factors in providing value to users. This time, while using the experiment result of the mini-split type air-conditioner reported in the past, in addition, we conducted a new experiment on multi-type VRF air-conditioner. Then, the difference in COP performance between R-410A and alternative candidate refrigerants was evaluated. In addition, we studied the product design using R-452 B, R-454 B by simulation and compare the size when the performance is made identical by changing the system specifications with alternative refrigerant candidates. As a result, common results were obtained in both systems. It was found that R-452B and R-454B have better performance than R-410A. On the other hand, it was found that these refrigerants need to increase both the indoor heat exchanger and the outdoor heat exchanger to 120% in order to obtain the same performance as R-32