30 May 2005
Natural refrigerants used in the refrigeration and air-conditioning systems are gathering attention as
public interest in environmental issues increase. Amongst these, CO2 is a non-combustible, non-toxic,
safe refrigerant. Development of applications for using its beneficial properties for heat generation with a
heat pump is currently being pursued. However, CO2 is not used for low temperature applications as it
produces poor cycle efficiency at low temperatures.
We considered a two-stage compression, two-stage expansion cycle as a way of improving cycle
efficiency when using CO2 refrigerant. The two-stage compression expansion cycle has a gas-liquid
separator installed at the intermediate pressure point in the middle of the pressure-reducing circuit of a
common refrigeration cycle using a two-stage compressor. The refrigerant is separated into gas and
liquid, with the gas returning to the compressor’s second stage inlet, and the liquid being further reduced
in pressure and fed to the evaporator.
Presuming that an identical cycle was used at low temperatures, the two-stage expansion cycle is
effective because the CO2 refrigerant is drier at the entrance of the evaporator than an HFC refrigerant.
The main paper reports the characteristics we investigated using a test version of the two-stage
compression two-stage expansion cycle that we built.