CO2 + propane mixture as working fluid for refrigeration in hot climates. Experimental results of energy efficiency tests.

Adding another compound to CO2 results in an increase of the critical temperature. Hence, it is possible to operate subcritical cycles at higher external temperatures. This has been recently experimented for heat pumps (Bouteiller 2017, Bouteiller et al. 2016) and has shown increased performance in certain conditions as well as lower working pressures. The present study investigates CO2 mixtures for refrigeration or air conditioning applications in hot climates. As an example of such applications, we examine here 4 different liquid cooling scenarios: A brine is cooled from 12 to 5 °C in order to test an air conditioning situation. In the 3 other scenarios, temperatures of the brine are fixed respectively at: 5 / 0°C, 0 / -5°C and -5°C / -10°C. A simple stage pure CO2 cycle is used as reference, with fixed external conditions. Heat sink temperatures are set at 35 °C and 40 °C to account for hot external conditions. The optimal efficiency and optimal high pressure for pure CO2 were first measured in those conditions and taken as reference. Then, performances obtained with 3 different mixture compositions (5%, 10% and 15% molar fraction of propane) in the same conditions are compared to this reference. Improvements in optimal performances and high pressure level as well as the occurrence of condensation is discussed. The results show that the operating pressure is always lowered and that in some conditions, the COP can be increased by 20%