Improvement of domestic CO_2 air conditioner by integrating compressor/expander combination

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As an attempt of increasing the energy efficiency of a refrigeration cycle working with carbon dioxide by recovering a throttling loss, basic performances of a vane type expander and a scroll type expander were clarified and trans-critical leakage flow occurring in the expander and performance of a cycle with an expander/compressor combination were examined in this study.The total efficiency of the vane expander is 59% by optimization of clearances and a reduction of a shaft seal. The trans-critical expansion process changes according to an inlet temperature of the expander, and although a delay of flash is observed when the inlet temperature is low, good performance is obtained over the range of tested temperature. The scroll expander which was made using a scroll compressor element without major modification shows the total efficiency of 56% and a potential of having enough efficiency with an optimized design. The trans-critical leakage flow was examined experimentally using a channel model and it can be analyzed by a homogenous flow model with taking account of inlet acceleration, frictional loss and acceleration loss in the leakage clearance although an improvement of the model is needed when the leakage becomes choked flow. In addition, a prototype of compressor/expander combined machine was developed and its operating characteristics were measured. Operating balance point and the performance of the cycle with the combined machine were examined with considering the performances of expander and compressor. An improvement of cycle performance is obtained by 21% compared to a cycle with an expansion valve. Although the cycle performance decreases under off-design conditions, the performance will be improved by a bypass or a pre-expansion. In this study, a superior compressor/expander combined machine was not developed yet. But achievements of this study show feasibility of a CO_2 air conditioning system.