Experimental assessment on operation efficiency and output characteristics of an oil-free linear compressor in a refrigeration system under various parameters

Improving compressor performance in domestic refrigerators is a significant contribution to saving energy and reducing carbon emissions. Compared with conventional reciprocating compressors, linear compressors have many advantages, including high efficiency, simple control and oil-free operation, and thus can be an alternative choice. Understanding the operating mechanism of linear compressors is important to improve compressor efficiency. The real-time experimental investigation to understand the operation mechanism of linear compressor is still relatively scarce. Based on it, in this work, the output characteristics of the linear compressor need to be further investigated real-timely. A linear compressor prototype is developed and a real-time measurement system with the prototype is established. The effect of various parameters (refrigerant charge amount, operation frequency, piston stroke, discharge pressure and suction pressure) on the operation efficiency and output characteristics is analyzed in detail and the comparison between two refrigerants (R134a and R600a) is also studied. The results suggest that the refrigerant charge amount has little effect on the motor efficiency under fixed working conditions and stroke. At the resonant frequency (here 62.5 Hz), the motor efficiency, overall isentropic efficiency and COP are maximum while the volume efficiency increases only slightly at 35-75 Hz. With the increase of stroke, the volumetric efficiency and overall isentropic efficiency gradually increase while the motor efficiency decreases slightly due to the reduction of magnetic field intensity. As the pressure difference between the suction and discharge pressure increases, the increase of piston offset leads to the reduction of magnetic field intensity and thus the motor efficiency gradually decreases. The motor efficiency for R600a (89 %) is higher than that for R134a (84 %) at the same stroke while the cooling capacity for R600a (116.2 W) is lower than that for R134a (223.7 W). Finally, it is suggested that the key factors affecting the compressor performance are the frequency and piston offset characteristics. The main reason for low efficiency in resonance is mainly due to the presence of piston offset.