Improvements in a CO2 transcritical plant working with two different subcooling systems.

Natural refrigerant CO2 is an “old-new refrigerant” that is being implanted as a green solution against high-GWP greenhouse gases. Its particular low critical temperature makes carbon dioxide difficult to be implemented in warm countries where transcritical operating conditions are commonly achieved. In order to be competitive over artificial refrigerants, transcritical cycles must improve its levels of COP and cooling capacity by using different cycle configurations mainly focused in reducing the temperature at the gas-cooler exit. In this work, two different subcooling systems based on the use of an IHX and a dedicated mechanical subcooling, are presented and experimentally tested at a -10ºC evaporating level under two different heat rejection temperatures at the gas-cooler (30 and 35ºC). Working with IHX, the results show a maximum improvement in cooling capacity and COP ranged between [1.9 ÷ 11.7]% and [0.8 ÷ 12.2]%, respectively, with an average increment in the discharge temperature of 11.6 K. The same tests performed with a dedicated mechanical subcooling using R600a as working fluid, brings an improvement in cooling capacity and COP ranged between [27.2 ÷ 42.8]% and [5.1 ÷ 19.3]%, respectively, with a maximum increment in the power consumption of the refrigerating plant of 20.9%.