Residential CO2 heat pump system for combined space heating and hot water heating

This paper was presented at the IIR Gustav Lorentzen Conference in Glasgow (Sep. 2004). A theoretical and experimental study was carried out for a residential brine-to-water CO2 heat pump system for combined space heating and hot water heating - a so-called integrated CO2 heat pump system. A 6.5 kW CO2 heat pump unit was constructed and extensively tested in order to document the performance and study the component and system behaviour over a wide range of operating conditions. The CO2 heat pump unit 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 two DHW gas cooler units were connected to a DHW system consisting of an unvented single-shell DHW tank, an inverter controlled pump and a water loop. The CO2 heat pump unit was tested in three different modes: simultaneous space heating and DHW heating, DHW heating only and space heating only. Most tests were carried out at an evaporation temperature of -5ºC. The heat pump unit gave off heat to a floor heating system at supply/return temperatures of 33/28, 35/30 or 40/35ºC. The set-point temperature for the DHW was 60, 70 or 80ºC. The test results proved that an integrated brine-to-water CO2 heat pump system may achieve the same or higher seasonal performance factor (SPF) than the most energy efficient state-of-the art brine-to-water heat pump systems as long as. 1) The ratio of the annual heat delivered for DHW production and the total annual heat delivered from the heat pump unit is minimum 25 to 30%. 2) The return temperature in the space heating system is about 30ºC or lower. 3) The city water temperature is about 10ºC or lower. 4) The thermodynamic losses in the DHW tank are small. The latter requires a special tank design in order to minimize mixing and conductive heat transfer between the hot and cold water during the tapping and charging periods. Source: 6th IIR Gustav Lorentzen Natural Working Fluids Conference, Conference papers from Day 3 Sessions 5, 6&7, 6/A/12.20. Author: J. Stene. You can order the proceeding CD-ROM of the Glasgow conference by clicking here.