Fundamental study on vane type compressor/expander combination for carbon dioxide

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The objectives of this study are development of recovery techniques for a throttling loss in a refrigeration system working with carbon dioxide and improvement of the cycle performance. In this study, a prototype of a vane type expander was developed and its performance was evaluated experimentally and theoretically. The theoretical study shows that the cycle performance is greatly improved by using the expander and the losses caused by bypass and pre-expansion at an off-design point are clarified. The total efficiency of the prototype expander developed in this study was 0.43 at 2000 rpm, the power of 220 W was recovered and the trans-critical expansion process was successfully measured. With using a mathematical model developed to analyze the trans-critical expansion process, the performance of the prototype machine was examined carefully. Each efficiency and loss analysis show that the leakage through a rotor face and the pressure drop at a inlet port are major factors decreasing the performance. It is convinced, therefore, that the required performance is achievable by optimization of the specifications and applying a seal device. The improvement of the performance is expected to be 24 % by applying the expander developed in this study. In addition, a single rotor type compressor/expander unit, which has many problems to solve but is expected to have high potential, was developed and tested. Since manufacturing accuracy was not enough against radial load acting on the rotor, the compressor side did not work well although the unit rotated. The single rotor type unit with the accurate machining will be developed and the operation of a compressor/expander unit of dual rotor type developed already will be done in future. With clarifying the performance improvement and characteristics of the cycle by using these compressor/expander units, we will achieve a practical use of CO_2 refrigeration unit with the compressor/expander units.