Measurements and modelling of a novel oil-free refrigeration system

Vapour compression refrigeration (VCR) system is one of the most commonly used refrigeration methods for domestic refrigeration, heat pump, and automobile air conditioning. The ever rapid rise in the number of VCR units causes a dramatic increase in energy demand and greenhouse emissions, which results in a significant contribution to the global warming. To mitigate the greenhouse gas emission and improve energy efficiency, high efficiency and low carbon oil-free refrigeration system with capacity modulation can be considered as a potential solution for VCR units. A novel oil-free refrigeration system (ORS) is introduced in this thesis. The performance of the ORS has been experimentally and numerically investigated. The ORS consisting of two balanced novel oil-free linear compressors, an off-the-shelf water-cooled coaxial condenser and an evaporator with an electric heater has been instrumented. A series of experiments have been carried out over a wide range of test conditions to investigate the performance of the linear compressor using R134a. At a compressor stroke of 11 mm and a pressure ratio of 2.0, the oil-free linear compressor can achieve a volumetric efficiency of 71%. The oil-free linear compressor is capable of capacity modulation in response to heat load. Part load efficiency is even higher. An analytical model of the linear compressor has been developed. The drop-in performance of R1234yf for R134a system also has been experimentally investigated using the ORS test rig. At a condenser temperature of 40 °C, the coefficient of performance (CoP) of R1234yf is 5-20% lower than R134a. The effect of the refrigerant charge and refrigerant distribution for the ORS using R134a has been experimentally evaluated. The experimental results show that an optimal refrigerant charge, which allows the system to achieve the highest efficiency, varies with operating conditions. A higher compressor stroke and a lower pressure ratio tend to have a higher optimal refrigerant charge. To provide the numerical ORS model with evaporation heat transfer and pressure drop of refrigerants, two existing correlations for heat transfer and pressure drop for R1234yf, R152a and R134a have been improved based on the experimental data. Overall, the modelling results agree well with the measurements. A comprehensive numerical system model for the ORS consisting of linear compressor model, a heat exchanger model, and a refrigerant distribution model has been proposed using MATLAB and Simulink. The model is able to predict mass flow rate, power input, pressure drop and heat transfer in heat exchangers, and CoP of the refrigeration system. Overall, the modelling results agree well with measurements. The numerical model can be used for future work on low charge ORS design using oil-free linear compressor and microchannel heat exchangers for various applications.