Transcritical CO2 Heat Pump Simulation Model and Validation for Simultaneous Cooling and Heating

Authors: Jahar Sarkar

Abstract:

In the present study, a steady-state simulation model has been developed to evaluate the system performance of a transcritical carbon dioxide heat pump system for simultaneous water cooling and heating. Both the evaporator (including both two-phase and superheated zone) and gas cooler models consider the highly variable heat transfer characteristics of CO2 and pressure drop. The numerical simulation model of transcritical CO2 heat pump has been validated by test data obtained from experiments on the heat pump prototype. Comparison between the test results and the model prediction for system COP variation with compressor discharge pressure shows a modest agreement with a maximum deviation of 15% and the trends are fairly similar. Comparison for other operating parameters also shows fairly similar deviation between the test results and the model prediction. Finally, the simulation results are presented to study the effects of operating parameters such as, temperature of heat exchanger fluid at the inlet, discharge pressure, compressor speed on system performance of CO2 heat pump, suitable in a dairy plant where simultaneous cooling at 4oC and heating at 73oC are required. Results show that good heat transfer properties of CO2 for both two-phase and supercritical region and efficient compression process contribute a lot for high system COPs.

Keywords: CO2 heat pump, dairy system, experiment, simulation model, validation.

Digital Object Identifier (DOI): doi.org/10.5281/zenodo.1330887

References:

[1] P. Neksa, "CO2 heat pump systems," Int. Journal of Refrigeration, vol. 25, pp. 421-427, 2002.
[2] L. Cecchinato, M. Corradi, E. Fornasieri and L. Zamboni, "Carbon dioxide as refrigerant for tap water heat pumps: A comparison with the traditional solution," Int. Journal of Refrigeration, vol. 28, pp. 1250- 1258, 2005.
[3] R. Yokoyama, T. Shimizu, K. Ito and K. Takemura, "Influence of ambient temperatures on performance of a CO2 heat pump water heating system," Energy, vol. 32, pp. 388-398, 2007.
[4] S. D White, M. G. Yarrall, D. J. Cleland and R. A. Hedley, "Modelling the performance of a transcritical CO2 heat pump for high temperature heating," Int. Journal of Refrigeration, vol. 25, pp. 479-486, 2002.
[5] S. G. Kim, Y. J. Kim, G. Lee and M. S. Kim, "The performance of a transcritical CO2 cycle with an internal heat exchanger for hot water heating," Int. Journal of Refrigeration, vol. 28, pp. 1064-1072, 2005.
[6] J. Sarkar, S. Bhattacharyya and M. Ramgopal, "Simulation of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applications," Int. Journal of Refrigeration, vol. 29, pp. 735-743, 2006.
[7] N. Agrawal and S. Bhattacharyya, "Optimized transcritical CO2 heat pumps: Performance comparison of capillary tubes against expansion valves," Int. Journal of Refrigeration, vol. 31, pp. 388-395, 2008.
[8] T. M. Ortiz, D. Li and E. A. Groll, "Evaluation of the performance potential of CO2 as a refrigerant in air-to-air air conditioners and heat pumps: system modelling and analysis," ARTI final report, no. 21CR/610-10030, 2003.
[9] V. Gnielinski, "New equations for heat and mass transfer in turbulent pipe and channel flow," International Chemical Engineering, vol. 16, pp. 359-366, 1976.
[10] S. S. Pitla, E. A. Groll and S. Ramadhyani, "New correlation to predict the heat transfer coefficient during in-tube cooling of turbulent supercritical CO2," Int. Journal of Refrigeration, vol. 25, pp. 887-895, 2002.
[11] X. Fang, C. W. Bullard and P. S. Hrnjak, "Heat transfer and pressure drop of gas coolers," ASHRAE Transactions, vol. 107, pp. 255-266, 2001.
[12] S. H. Yoon, E. S. Cho, Y. W. Hwang, M. S. Kim, K Min and Y. Kim, "Characteristics of evaporative heat transfer and pressure drop of carbon dioxide and correlation development," Int. Journal of Refrigeration, vol. 27, pp. 111-119, 2004.
[13] J. Sarkar, S. Bhattacharyya and M. Ramgopal, "Transcritical CO2 heat pump prototype development for simultaneous water cooling and heating," International Congress of Refrigeration, Beijing, 2007.