Simulation Results for an Air-Conditioning System With Secondary Refrigerant Loops

There are some highly efficient and environmentally safe hydrocarbons and HFCs proposed as R22 substitutes, but they are flammable. The use of these refrigerants in an air-conditioning/heat pump system may not be acceptable. The present research looks into the prospects of using an additional loop with a secondary refrigerant as a thermal coupling between the primary refrigerant in an outdoor compact unit and the space air to be conditioned. The secondary refrigerant is a liquid and, hereinafter, will be referred as “heat transfer fluid.” This way the refrigerant used in the vapor compression cycle does not come into the space, which assures a safer operation. Simulation of the system performance was conducted with R22, R134a, R290 (propane), and an R32/R152a (35/65) blend. The R32/R152a blend proved to be the most energy efficient. For the refrigerants studied, the use of one loop with water as the heat transfer fluid decreases the coefficient of performance (COP) by 8.5–15.3% compared to the R22 baseline, without the secondary loop. The use of anti-freeze as heat transfer fluid instead of water is even less efficient since the heat transfer characteristics of anti-freeze are poorer. Using a water-glycol mixture as an anti-freeze heat transfer fluid, instead of water, showed an additional 8.2–9.3% COP loss. The system with two secondary loops with water shows an even higher decrease in COP by 23.1–31.1% compared to the baseline.