Thermodynamic analysis of a CO2 air-conditioning concept for multi-family houses in temperate and subtropical climates based on experimental data

Abstract

The use of synthetic refrigerants is strongly restricted worldwide to combat the double threat of ozone layer depletion (ODP) and global warming (GWP). Carbon dioxide (CO2), as a natural refrigerant, has been attracting more and more attention in applications involving refrigeration and air-conditioning systems. With reference to a transcritical CO2 cycle, this study evaluates the energetic and exergetic efficiency of an air-conditioning system for multi-family houses in temperate and subtropical climates. For this purpose, multi-family houses in three European countries (Germany, Italy, and Spain) with an identical cooling capacity of 19.4 kW at the design point, but different cooling areas and annual cooling demands, are considered. The air-conditioning system has been investigated by simulation over the period of one year, considering the corresponding cooling profiles depending on time and location (Munich, Florence, and Malaga). The applied off-design model of the CO2 plant is validated by experimental data. The relative deviation between the simulation and experimental results concerning the cooling capacity is below 5.5%. For the Munich case, the results show, for the cooling area of 375 m2, a yearly averaged energy efficiency ratio (EER) of6.82. Based on the identical system capacity, a living area of 322 m2 and 283 m2 can be cooled for the Florence and Malaga cases, respectively. Due to the different climatic boundary conditions, a lower yearly performance with an EER of6.07 for the Florence case and 5.87 for the Malaga case are obtained. In terms of the exergy loss for each component, the results show that the highest losses are obtained for compressor and gas cooler, respectively. Together they account for more than 50% percent of the total exergy loss of the system. The lowest losses are related to the recuperator with about 6.6% of the total exergy loss. Furthermore, the results indicate that the studied CO2 system is more appropriate for countries in the temperate zone than countries in the subtropical zone. Moreover, the considered CO2 air-conditioning concept shows an acceptable EER compared to the typical EER values of the European market, which is based on conventional air-conditioning systems, coupled with a significant reduction in the global warming. Therefore, the findings and implications of this work address the gap and remove doubts on using carbon dioxide as a working fluid for the air-conditioning of multi-family houses in temperate and subtropical regions.