Effects of refrigerant charge amount on the performance of a transcritical CO 2 heat pump Honghyun Cho, Changgi Ryu, Yongchan Kim * , Ho Young Kim Department of Mechanical Engineering, Korea University, Anam-Dong, Sungbuk-Ku, Seoul 136-701, South Korea Received 28 April 2005; received in revised form 8 September 2005; accepted 13 September 2005 Available online 14 November 2005 Abstract A typical transcritical CO 2 system shows lower performance than conventional air conditioners in cooling mode operation. In addition, the CO 2 system shows a large variation of the performance according to refrigerant charge whereas the conventional systems do not show large variation. In this study, the performance of the CO 2 heat pump was measured and analyzed by varying the refrigerant charge amount at standard cooling condition. In addition, the performance sensitivity of the CO 2 system as a function of refrigerant charge was compared to those for the R22, R410A, and R407C systems. The cooling COP of the CO 2 system was reduced more significantly at undercharged conditions than at overcharged conditions as the deviation from the optimal charge increased. The expansion loss was the dominant factor affecting system performance at undercharged conditions, while the gascooler loss became the major parameter at overcharged conditions. Among the systems investigated and compared in this study, the CO 2 system showed the most reduction in performance at undercharged conditions. q 2005 Elsevier Ltd and IIR. All rights reserved. Keywords: Heat pump; CO 2 ; Transcritical cycle; Experiment; Performance; Parameter; Refrigerant charge Impact de la charge en frigorige `ne sur la performance d’une pompe a ` chaleur au CO 2 transcritique Mots cle ´s : Pompe a ` chaleur ; CO 2 ; Cycle transcritique ; Expe ´rimentation ; Performance ; Parame `tre ; Charge en frigorige `ne 1. Introduction Due to environmental concerns over global warming from hydrofluorocarbon (HFC) refrigerants, the application of carbon dioxide (CO 2 , R744) in mobile air-conditioners, water heaters, and heat pumps [1–6] has been studied extensively. CO 2 is environmentally safe and has favorable thermodynamic properties as a refrigerant. Domestic water heating systems operating under a transcritical CO 2 cycle show higher performance than conventional water heaters [1,2]. However, comfort cooling applications of the transcritical CO 2 cycle yields lower performance than the typical unitary equipment using conventional refrigerants such as R22, R134a, and R410A [3,4] because of the large expansion losses and higher irreversibility during the gas cooling process. In addition, the performance of the transcritical CO 2 system is more sensitive to refrigerant International Journal of Refrigeration 28 (2005) 1266–1273 www.elsevier.com/locate/ijrefrig 0140-7007/$35.00 q 2005 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2005.09.011 * Corresponding author. Tel.: C82 2 3290 3364; fax: C82 2 921 5439. E-mail address: yongckim@korea.ac.kr (Y. Kim).