Energetic and Exergetic Analysis of a Transcritical NO Refrigeration Cycle with an Expander.

Comparative energy and exergy investigations are reported for a transcritical NO refrigeration cycle with a throttling valve or with an expander when the gas cooler exit temperature varies from 30 to 55 °C and the evaporating temperature varies from -40 to 10 °C. The system performance is also compared with that of similar cycles using CO. Results show that the NO expander cycle exhibits a larger maximum cooling coefficient of performance (COP) and lower optimum discharge pressure than that of the CO expander cycle and NO throttling valve cycle. It is found that in the NO throttling valve cycle, the irreversibility of the throttling valve is maximum and the exergy losses of the gas cooler and compressor are ordered second and third, respectively. In the NO expander cycle, the largest exergy loss occurs in the gas cooler, followed by the compressor and the expander. Compared with the CO expander cycle and NO throttling valve cycle, the NO expander cycle has the smallest component-specific exergy loss and the highest exergy efficiency at the same operating conditions and at the optimum discharge pressure. It is also proven that the maximum COP and the maximum exergy efficiency cannot be obtained at the same time for the investigated cycles.