An absorption refrigeration system using ionic liquid and hydrofluorocarbon working fluids
Kim, Sarah Sungeun
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Efficient heat management in energy intensive applications such as server and data centers has become a national concern due to the magnitude of the energy consumed. In that matter, the absorption refrigeration system is an attractive solution because the abundant waste heat available in the data centers can be recycled to run the heat pump, which will bring about significant cooling cost savings. The use of absorption refrigeration has been limited due to the drawbacks related to the working fluids in commercially available equipment. Recently, ionic liquids (ILs) have been suggested as the absorbent in absorption heat pumps due to their tunable properties, negligible volatility and high thermal stability. The non-random-two-liquid-model was initially used to analyze the feasibility of the new IL based working fluid. Hydrofluorocarbons (HFCs) were paired with IL absorbents due to their good properties as refrigerants. The cooling-to-total-energy (CE) efficiency had a local maximum with respect to desorber temperature due to the solubility limit at lower temperatures and large heating requirements at higher temperatures. The waste heat recycling coefficient of performance (COP) continually increased with respect to desorber temperature and among the HFCs studied in this work, R134 gave the highest COP value, which is up to 40 times higher than that of typical vapor compression systems and 60 times higher than NH3/H2O and H2O/LiBr absorption refrigeration systems. A Redlich-Kwong equation of state (RK-EOS) was employed for accurate computation of mixture properties over a wide range of operating conditions. Analysis using the RK-EOS model showed that the CE trend in refrigerants followed the trend of solubility in the [bmim][PF6] IL. However, the trend in COP was different from that of CE as the operating pressure ranges became an important factor. Required pumping work of the working fluids has also been analyzed using a two phase pressure drop equation and the results show that the impact of viscous IL flow is insignificant compared to the total pumping work. The HFCs studied in this work have very similar structures. However, the extent of solubility and system efficiency in the same IL, [bmim][PF6], made a large difference. Most surprisingly, even when the refrigerant had the same chemical formula, the change in fluorine position in tetrafluoroethane showed significantly different system performance. The symmetrical tetrafluoroethane had superior CE and COP over the asymmetrical tetrafluoroethane most likely due to the higher probability to form hydrogen bonding with the absorbent. The computational results for various HFC/IL pairs show that in selecting the working fluid pairs, the refrigerant should have high overall solubility in the IL and a large gradient of solubility with respect to temperature. Also, refrigerants with small pressure ranges are preferred. In addition to the simulation study, a bench-top absorption refrigeration system was built and operated using IL based working fluids for the first time. The effect of cooling was observed by operating the test system. The experimental results were congruent with the predictions from the modeling work. In conclusion, an absorption refrigeration system based on the IL chemical compressor has been shown to be a promising solution in applications which need efficient cooling and generate abundant waste heat.