Besides, the selection of appropriate refrigerant is also vital considering environment issues. Since the last
century, R22 was as most common refrigerant in heat pump water heater because of its good thermal properties.
M.V. Venkataramana and Padmanabhan et al. had proven that R22 was superior to R134a in terms of coefficient
of performance and exergy efficiency in the system with separate refrigerant of R22 and R134a (Venkataramana et
al., 2013). For a long time, R134a was considered as a good substitute of CFC and widely used in HVAC&R
systems. According to simulation research, J.B. Chen et al. proposed that R134a was superior to R22 with respect
to the system performance (Chen et al., 2010). But it still should be verified by experiment. However, the harm of
R22 and R134a is obvious to all. They all had an unfriendly environment. So, it is necessary to choose right