Free Piston Compressor with a Variable Volume Ratio: Insights on Modeling Approach and Control Strategy

摘要

Increasing requirements to heat pump performance ratings as well as pressing environmental concerns force manufacturers to evaluate the potential of natural refrigerants such as ammonia, propane or carbon dioxide, and implement advanced cycle architectures. Beside the use of the appropriate working fluid, the compressor should have ideally an optimal built-in volume ratio (BVR) for any pressure ratio during its operation. Practically, such mode of operation is only possible if the BVR can be made adjustable in real time in order to follow pressure fluctuations due to possible variable heat pump loads. The concept of such a machine with an embedded linear alternator has been developed and patented. The rotation of the piston is used to adjust the timing of the ports so that, after the compression, the pressure in the cylinder is equal to the required one in the discharge line. The intake port has been designed in such a way, that it remains open during the suction phase. Therefore, no pressure difference is needed to actuate the ports and throttling losses can be nearly avoided. The reciprocating movement of the piston used to compress the working fluid is fully controlled which eliminates the need of springs typically used in similar machines. The dead volume of this machine is only defined by the machining tolerances, so it can be made extremely small allowing high pressure ratios. A test setup is used to validate the theoretical model and investigate the influence of the key parameters of the machine.