30 May 2005
Carbon dioxide (CO2) represents an environmentally benign and safe alternative to conventional
working fluids in residential brine-to-water heat pumps. A 6.5 kW prototype CO2 heat pump for
combined space heating and hot water heating has been extensively tested and analysed. The heat
pump was equipped with a unique counter-flow tripartite gas cooler for preheating of domestic hot
water (DHW), low-temperature space heating and reheating of DHW.
The CO2 heat pump was tested for simultaneous space heating and DHW heating, DHW heating
only and space heating only. The heat pump rejected heat to a floor heating system at supply/return
temperatures 33/28, 35/30 or 40/35ºC, and the set-point temperature for the DHW was 60, 70 or 80ºC.
The experimental results proved that a brine-to-water CO2 heat pump system may achieve higher seasonal
performance factor (SPF) than the most energy efficient brine-to-water heat pump systems on
the market, provided that the heating demand for hot water production constitutes at least 25% of the
total annual heating demand for the residence, the return temperature in the space heating system is
relatively low (<30ºC) and the thermodynamic losses in the DHW tank can be reduced to a low level.