Water-side reversible CO2 heat pump for residential application

•Water/water R744 for space heating, cooling and hot water production.•Seasonal performances evaluated with original validated model.•Benchmark with R410A unit shows 22% higher energy consumption.•Two-phase ejector guarantees same seasonal performance as R410A unit.•Development of air/water ejector unit next step to overcome R410A efficiency.

This paper presents the energy assessment of a water/water R744 chiller/heat pump, working according to a transcritical cycle, used for winter heating, summer cooling and tap water production. The different functions (heating, cooling, hot water) are managed water side. The analysis of the R744 chiller/heat pump is based on an original simplified method, which is able to predict the energy performance of the unit based only on its performance data at the nominal rating conditions. The method was validated against experimental data. A comparison with a state-of-the-art R410A unit is presented. The monthly analysis shows that the CO2 unit is very efficient in hot water production, but penalised in heating and cooling service. The adoption of an ejector in place of the expansion valve makes the CO2 system reach the same energy consumption as the R410A unit, despite the presence of the water loop only in the R744 lay-out.