International Journal of Thermal Sciences 46 (2007) 180–187 www.elsevier.com/locate/ijts Optimization of two-stage transcritical carbon dioxide heat pump cycles Neeraj Agrawal, Souvik Bhattacharyya ∗ , J. Sarkar Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India Received 7 October 2005; received in revised form 3 April 2006; accepted 3 April 2006 Available online 8 June 2006 Abstract Optimization studies of two-stage transcritical carbon dioxide heat pump cycles, incorporating options such as flash gas bypass, flash intercool- ing and compressor intercooling, are presented based on cycle simulation. Sub-critical and super-critical thermodynamic and transport properties of carbon dioxide coded and then integrated with the simulation code for further analyses. Results exhibit improvement in performance by adopt- ing optimal operating conditions. The optimum interstage pressure, thus obtained, deviate from the classical estimate of geometric mean of gas cooler and evaporator pressure. It is observed that the flash gas bypass system yields the best performance among the three two stage cycles ana- lyzed. Internal heat exchanger effectiveness and compressor isentropic efficiency shows marginal influence on the system performance. Internal heat exchanger effectiveness shows marginal influence on the system performance while compressor isentropic efficiency shows an about 10% variation in COP. However, optimum gas cooler pressure and optimum intermediate pressure are only marginally affected. Based on the cycle simulations, correlations of optimum gas cooler pressure and inter-stage pressure in terms of gas cooler temperature and evaporator temperature are obtained. This would be useful as a guideline in design of such systems.  2006 Elsevier Masson SAS. All rights reserved. Keywords: CO 2 heat pump; Multistage; Inter-stage pressure; Transcritical cycle; Optimization 1. Introduction Natural refrigerants, in general, and particularly CO 2 is in- creasingly becoming the refrigerant of choice to replace the environmentally harmful CFCs and HCFCs. CO 2 exhibits ex- cellent heat transfer properties and is non-flammable and non- toxic. It has relatively lower specific volume, resulting in com- ponent size reduction for the same operating conditions. Lorentzen and Pettersen [1–3] have shown in their seminal studies that difficulties connected with the low critical tem- perature of CO 2 (31.1 ◦ C) can be successfully overcome by operating the system in the transcritical mode, where single- phase heat rejection occurs above the critical temperature in the gas cooler instead of condenser as in conventional systems, and where pressure and temperature can be controlled indepen- dently to obtain optimum performance. The gliding temperature in the gas cooler makes the CO 2 systems more economical for simultaneous cooling and heating applications. One of the ma- * Corresponding author. Tel.: +91 3222 282904; fax: +91 3222 255303. E-mail address: souvik@mech.iitkgp.ernet.in (S. Bhattacharyya). jor advantages of transcritical CO 2 heat pump systems is the high temperature lift compared to others. The performance decline of CO 2 single stage systems at high heat rejection temperature can be effectively overcome by em- ploying a two-stage or multistage system with an intercooler in between compression stages in parallel with the gas cooler. The selection of the intermediate pressure is an important parameter for a multistage system. Several researchers [4–6] have investi- gated the optimum inter-stage pressure on the basis of minimum work requirement. There is a fair agreement on the fact that the optimum intermediate pressure in a two-stage refrigeration system is quite close to the classical estimate, given by the geo- metric mean of gas cooler and evaporator pressure. Gupta and Prasad [7] optimized three stage refrigeration systems graphi- cally for refrigerants R12, R22, and R714. They also developed the correlations to account for the effects of subcooling of con- densate and super-heating of vapour in the evaporator each up to 15 K. It was concluded that staging is most beneficial for R714. Gupta [8] investigated a cascade refrigeration-heat pump system using R-22 in heat pump side and R-13 in the refrigera- 1290-0729/$ – see front matter  2006 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2006.04.011