Two-stage transcritical carbon dioxide cycle optimisation: A theoretical and experimental analysis

The aim of this paper is to investigate, both experimentally and theoretically, the potential of improving the cycle efficiency through two stage compression with intermediate cooling, at operating conditions typical of air conditioning. The experimental set-up consists of two closed loop air circuits acting as heat sink and heat source for gas-cooler and evaporator respectively. The tested refrigerating circuit includes two tube-and-fin heat exchangers as gas-cooler and evaporator, a back-pressure valve as throttling device and a double-stage semi-hermetic compound, two-piston, reciprocating compressor equipped with oil separator and intercooler. A full set of thermocouples, pressure transducers and flow-meters allows measurement of all the main parameters of the CO2 cycle, enabling to perform heat balance both air and refrigerant side. Tests were run at fixed evaporation pressure, evaporator outlet superheating and gas-cooler outlet temperature, varying the gas-cooler outlet pressure in the range 8-11 MPa. The optimal gas-cooler pressure for this application as well as the effect of the intercooler efficiency on the cycle performance were investigated. A FORTRAN code for the simulation of an improved two-stage cycle was validated against the experimental results; a theoretical analysis performed with this code is proposed for optimisation and energy performance evaluation of such a cycle.