Development and evaluation of a reversible CO2 residential air conditioning system compared to a state-of-the art R410a unit

This paper was presented at the IIR Gustav Lorentzen Conference in Glasgow (Sep. 2004). A reversible CO2 prototype RAC split unit system was designed, built and installed in two calorimetric test chambers. The unit is based on the Lorentzen Cycle with a receiver and a suction line heat exchanger. Both the indoor and the outdoor heat exchangers have tube-in-fin design. The hermetic compressor has two compression stages and is inverter driven. In heat pump mode the CO2 prototype unit can either operate as a conventional one-stage circuit or as a two-stage system. In air-conditioning mode, there is an option for cooling the compressor first stage discharge gas. Experimental tests are under preparation. Realistic simulations were carried out with the advanced in-house simulation program CSIM using input data for all components of the prototype. CSIM is a steady state circuit simulator (primarily) for trans-critical CO2 cycles, which uses well calibrated models for all components. A state of the art inverter driven R410A split unit was tested experimentally as a baseline unit. The results obtained were compared with simulation results for the CO2 prototype. Results indicate that the prototype CO2 unit should perform almost as well as the R-410A baseline unit in AC mode at 27.8°C and 35°C ambient temperature. In heat pump mode, the prototype CO2 unit is predicted to perform around 30 to 40% better at -5°C and 5°C ambient temperature. Future experimental work will reveal whether the potential for this new CO2 system can be achieved. Simulation of the CO2 prototype heat exchangers revealed that the air and refrigerant flows should be (cross) co-current in operating as evaporator and (cross) counter-current operating as gas cooler. Furthermore, increased indoor heat exchanger size will reduce the optimum high side pressure for CO2 systems in heat pump mode, leading to improved performance. Increased heat exchanger size in conventional HFC-systems cannot reduce the need for high condensing pressures if high outlet air temperature or capacity is required. The COP will suffer. Source: 6th IIR Gustav Lorentzen Natural Working Fluids Conference, Conference papers from Day 3 Sessions 5, 6&7, 5/A/10.10. Authors: A. Jakobsen, G. Skaugn, T.V. Skiple, P. Neksa, T. Andresen. You can order the proceeding CD-ROM of the Glasgow conference by clicking here.