(522e) Theoretical and Experimental Investigation of An Absorption Refrigeration System Using R134/[Bmim][PF6] Working Fluid

Energy recycling is an overarching goal in many areas including high performance computing and data storage systems where all the electrical energy is converted into low-quality waste heat. To this end, the first experimental demonstration of an absorption refrigeration system operating on recycled energy using a safe refrigerant pair has been achieved. The refrigerant was 1,1,2,2-tetrafluoroethane (R134) and the absorption media was an ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]). ILs are liquid salts at ambient temperature and are a promising class of absorbents in an absorption refrigeration system because they have tunable properties, high molar uptake for specific gases, zero vapor pressure, high thermal stability, and environmental safety.

There is limited information on the thermodynamic and transport properties of IL/refrigerant mixtures at the temperatures and pressures that are relevant to operation of the refrigeration systems. A Redlich-Kwong type equation of state model was employed to analyze potential working fluids and overall system performance. The two phase pressure drop equation and binary interaction parameters were introduced to improve the computational model. Using the model, the R134/[bmim][PF₆] fluid pair was compared to previous studies using 1,1,1,2-tetrafluoroethane (R134a) and the same IL. The R134/[bmim][PF₆] fluid pair had up to 92% greater cooling-to-total-energy efficiency than the R134a/[bmim][PF₆] fluid pair even though R134 and R134a are isomers and have similar physical properties. The coefficient of performance (cooling capacity/pumping work) of the R134/[bmim][PF₆] fluid pair was up to three times larger than that of R134a/[bmim][PF₆] fluid pair when only waste heat was used at the desorber. The maximum projected coefficient of performance was 161 for the R134/IL system.

A working refrigeration system with R134/[bmim][PF₆] was constructed. For enhanced performance and miniaturization, microchannel heat exchangers were fabricated on copper plates for evaporator, condenser, absorber, and desorber. The system performance was experimentally evaluated using the measured pressure, temperature and heater power values. The measurement of the system performance showed that R134/[bmim][PF₆] had 1.9 times larger cooling capability compared to R134a/[bmim][PF₆] at a desorber temperature as low as 63°C.