Simulation of the optimal heat rejection pressure for transcritical CO₂ expander cycle

Abstract

In order to optimize and control transcritical CO₂ refrigeration cycle, a mathematical model was developed to simulate the system performance. The simulation results show that a maximum COP exists at the optimal heat rejection pressure not only for throttle valve cycle but also for expander cycle. Also, the optimal heat rejection pressures of the throttle valve cycle are greater than those of the expander cycle under the same condition. In order to further obtain correlation of the optimal heat rejection pressure for transcritical CO₂ expander cycle, it is necessary to analyze the impact degree of compressor efficiency, expander efficiency, gas cooler outlet temperature and evaporation temperature. Based on the simulation results, the values of the optimal heat rejection pressure for the expander cycle were regressed in terms of gas cooler outlet temperature and evaporation temperature at given compressor efficiency and expander efficiency. Finally, two types of polynomial correlations were obtained. One is cubic form, with an average deviation of less than 0.5% and the other is simplified form, with an average deviation of less than 1%. It is, therefore, convenient to use either correlation to simulate the performance of transcritical CO₂ expander cycle.

Key words： transcritical CO₂ cycle; optimal heat rejection pressure; simulation; expander

Acknowledgements

This research was supported by the National Natural Science Foundation of China (Grant No. 50506019) and the Development of Science and Technology Foundation of Colleges and Universities in Tianjin (No. 20071116).

References

1 Hwang Y. Comprehensive investigation of carbon dioxide refrigeration cycle. Dissertation for the Doctoral Degree. Department of Mechanical Engineering, University of Maryland College Park, USA, 1996

2 Kauf F. Determination of the optimum high pressure for transcritical CO₂-refrigeration cycles. International Journal of Thermal Sciences, 1999, 38(4): 325–330

3 Liao Shengming, Zhao Tianshou, Jakobsen A. A correlation of optimal heat rejection pressures in transcritical carbon dioxide cycles. Applied Thermal Engineering, 2000, 20(9): 831–841

4 Robinson D M, Groll E A. Efficiencies of transcritical CO₂ cycles with and without an expansion turbine. International Journal of Refrigeration, 1998, 21(7): 577–589

5 Zha Shitong. Study and development of Transcritical CO₂ expander. Dissertation for the Doctoral Degree. Tianjin: School of Mechanical Engineering, Tianjin University, 2002 (in Chinese)

6 Yang Junlan, Ma Yitai, Liu Shengchun. Performance investigation of transcritical carbon dioxide two-stage compression cycle with expander. Energy, 2007, 32(3): 237–245

7 Wang Jinggang, Ma Yitai, Wei Dong, Wang Kanhong. Study of transcritical CO₂ two stage compression with turbine cycle. Journal of Refrigeration, 2001, (2): 6–11 (in Chinese)

8 Ma Yitai, Yang Junlan, Guan Haiqing, Li Minxia. Configuration consideration for expander in transcritical carbon dioxide two-stage compression cycle. Transactions of Tianjin University, 2005, 11(1): 53–58

9 Yang Junlan, Ma Yitai, Li Minxia, Guan Haiqing. Exergy analysis of transcritical carbon dioxide refrigeration cycle with an expander. Energy, 2005, 30(7): 1162–1175