A new study from the Arctic University in Norway, published in the International Journal of Refrigeration, looked at the efficiency of integrated CO2 heat pumps that combine space and water heating and found them to be good performers under certain conditions.
“A CO2-based integrated heat pump system with an ejector is a recommended heating system for modern, well-insulated, low-energy buildings,” the study said.
Performance of these systems depends on several factors including inlet and outlet water temperatures; the size of gas coolers, evaporators and heat exchangers; compressor efficiency; and the profile of the heating demand.
The researchers found that integrated CO2 heat pumps can outperform traditional R410A systems when the following conditions are met:
- The ratio of space-to-water heating is less than 0.6 – 1.0; meaning that the space heating demand is less than the hot water requirements.
- The inlet water temperature is below 20°C (68°F).
- The heat exchangers are designed with tight temperature pinches below 10°K (10°C/18°F).
Buildings where the space-to-water heating ratio is less than 1.0 include modern low-energy buildings and buildings with large hot water requirements, such as hotels and sports complexes. The required inlet water temperature of less than 20°C (68°F) makes it suitable for moderate climates such as in Northern Europe.
A CO2-based integrated heat pump system with an ejector is a recommended heating system for modern, well-insulated, low-energy buildings”Eivind Brodal & Steve Jackson, Arctic University of Norway
In Norway, where the study was done, low-energy housing has annual space-to-water ratios of 0.40 – 0.85. In 2014, only 3% of Norway’s housing was low-energy, but this percentage is expected to increase to 40% in 2050.
The CO2-based integrated heat pump systems can be optimized by introducing an ejector, the researchers found. Ejectors improved the Coefficient of Performance (COP) of a CO2 system up to 11%, whereas an ejector in a R410A system only increased the COP by less than 3%.