17 October 2006

Development of reversible residential air conditioners and heat pumps using CO2 as working fluid

Since 1997, SINTEF Energy Research and The Norwegian University of Science and Technology
(NTNU) have been investigating and developing reversible residential air conditioners and heat pumps
(RAC split-type units) using carbon dioxide (CO2) as their working fl uid. A third-generation prototype
CO2 RAC split-type unit has recently been constructed and extensively tested in heating and cooling
modes. The test results have been used for calculating the seasonal heating and cooling performance (SPF)
for two different climates; Greece (Athens) and Norway (Oslo). The results have been compared with
manufacturer’s data with verifi ed rating points for the most energy-effi cient Japanese R410A split-type
unit available on the market.
In both the heating mode and the cooling mode, the calculated SPF for the CO2 and R410A units in the
Oslo climate were more or less identical. However, in the cooling mode in the Athens climate, the SPF of
the CO2 unit was about 17 % lower than that of the R410A unit. Further development and optimization
of the CO2 unit, e.g. by utilizing microchannel heat exchanger technology, increasing the isentropic effi
ciency of the compressor and/or using an ejector or expander for expansion work recovery, will be necessary
before the CO2 unit will be able to match or outperform the market-leading R410A unit in terms
of energy effi ciency. However, since the CO2 unit already matches many of the better R410A
units on the market, CO2 must be regarded as a promising working fl uid in reversible air-conditioning
and heat pump units for residential use.