22 May 2008

EXPERIMENTAL INVESTIGATION ON A SMALL CAPACITY HEAT PUMP PROTOTYPE WITH CO2 AS WORKING FLUID


The recent European regulation on Fluor gases, pointing out the greenhouse
effect of present refrigerants, has put in relief the need of research on new fluids as
substitution. For systems based on the mechanical compression cycle, some solutions has
just begun to rise, by using fluids so called natural refrigerants such as carbon dioxide. The
use of CO2 in heat pump leads to a transcritical cycle very suitable to achieve high
temperature for hot tap water. On the other hand, the lower temperature level in the
gascooler can be valuable for space or floor heating. A small unit prototype with a heating
capacity up to 2.5 kW is tested in different conditions. Among the inlet parameters, the main
performances are estimated versus the evaporation temperature from 0°C to 10°C, the high
pressure from 75 bar to 110 bar and the CO2 charge from 400 g to 600 g. The critical
relevant points of the system are the gascooler, the compressor and the oil circulation. Thus,
two different technologies of gascooler have been installed and tested: shell and tubes with
inner diameter of 2 mm and plates heat exchanger granted to work up to 150 bar. The
amount of oil in the compressor has been also investigated, leading to an improvement of the
performances by reducing it.