From household refrigerators and air conditioners to cooling chambers and heat pumps, refrigerants are widely used in both domestic and industrial equipments. Most of the vapor compression heat pump systems currently use halogenated refrigerants (R22 or R134a) because of their excellent thermodynamic properties and low price. However, the use of halogenated refrigerants imposes adverse environmental impacts such as ozone depletion and global warming. It is therefore necessary to find some alternative refrigerants which are environmentally friendly and of high-performance while retain cost-effectiveness. Recent studies have shown that ether compounds might be environmentally friendly refrigerants, but their flammable characteristics have restricted their applications. Incorporat...[ Read more ]

From household refrigerators and air conditioners to cooling chambers and heat pumps, refrigerants are widely used in both domestic and industrial equipments. Most of the vapor compression heat pump systems currently use halogenated refrigerants (R22 or R134a) because of their excellent thermodynamic properties and low price. However, the use of halogenated refrigerants imposes adverse environmental impacts such as ozone depletion and global warming. It is therefore necessary to find some alternative refrigerants which are environmentally friendly and of high-performance while retain cost-effectiveness. Recent studies have shown that ether compounds might be environmentally friendly refrigerants, but their flammable characteristics have restricted their applications. Incorporating ether compounds with fire retardants could be one effective way to solve this problem.

The research objectives for the present study were to formulate and evaluate environmentally friendly and high performance mixture refrigerants which could be applied for moderately high temperature (50 - 70 ℃) heat pumps. In particular, dimethyl ether (DME) was selected as the refrigerant and mixed with retardants, R134a and R227ea, respectively, to replace traditional refrigerants for R134a heat pumps because of its environmentally friendly properties and no requirements on the changes of the equipment structure in this study. Simulation and experimental results show that DME has higher coefficient of heating performance (COP_H) and more than twice unit mass heating capacity than R134a under moderately high temperature working conditions. The measured flammability range from lower flammable limit (LFL) to upper flammable limit (UFL) of DME was 3.30 - 22.79 % at room temperature (25 ℃). As the retardants, R134a and R227ea could decrease the flammable range significantly and the critical suppression mass ratios of DME-R134a and DME-R227ea mixtures were 5.87 and 3.27, respectively. Safe mixture refrigerants of DME-R134a (85.5 - 14.5 % in mass percentage) and DME-R227ea (23.3 – 76.7 % in mass percentage) were charged into heat pump system for their performance evaluation. Comparing with DME-R134a, DME-R227ea has better condensing pressure and discharge temperature but lower heating capacity. The optimal condensing temperatures for DME-R134a and DME-R227ea were found to be 52 and 56 ℃ with COP_H values being 4.30 and 4.22, respectively. Under the condition of evaporating temperature 30 ℃ and condensing temperature 60 ℃, COP_H values of DME-R134a and DME-R227ea are 16.2 % and 13.5 % higher than that of pure R134a, respectively.