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Refrigeration systems have to operate at both full- and part-load conditions. The condenser pressure is a function of the daily or seasonally ambient temperature variations. Whereas, the evaporation pressure is defined by the cooling load which has its own fluctuations. As a consequence, the compressor should dynamically adjust the built-in volume ratio to the compression ratio of the cycle. To this end, piston compressors can provide to some extent flexibility to the system designer. Generally, the discharge valve opens when the in-cylinder pressure becomes slightly higher than the condenser pressure. The drawback of this approach is an over-compression, of which magnitude is dependent on a specific design of the check valve. This over-compression leads to notable losses and decreasing the overall efficiency of the system.
In this work, a free-piston compressor with an integrated linear motor is proposed that utilizes the rotation of the piston to open and close the intake and discharge valves at any moment using the feedback from the pressure sensors. This design allows to open the discharge valve precisely when the pressures equalizes, eliminating over-compression losses. Due to the fact that the valve openings are located on the cylinder walls, the opening can be made much larger than a check valve located in the flange of the cylinder. Thus, the throttling losses can be significantly reduced. The clearance volume is very small which results in a higher volumetric efficiency. The proposed design provides a better flexibility to deliver the target cooling capacity: the stroke length, the frequency and the port timing can be widely adjusted. Furthermore, the unit can be reversed on-the-fly to act as an expander which makes this device attractive to use in reversible heat pump / organic Rankine cycle systems.