CO₂ heat pumps: 20% better than state-of-the-art systems

Hot water heat pumps using the natural refrigerant CO₂ (R744) are 20% more efficient than the most energy-efficient HFC or propane systems. In terms of energy efficiency and use of renewable heat they even outperform state-of-the-art solar heating systems, according to a new report by SINTEF.

CO₂ heat pumps for hot water production can save up to 75% of primary energy needed to operate a conventional domestic hot water (DHW) system with electric immersion heaters. Moreover, they can save 20-35% of annual primary energy even compared to systems based on solar collectors, while outperforming these cutting-edge solar heaters also regarding the use of renewable heat:

“We calculated that a 25 kW groundwater CO₂ heat pump water heater in Norway will achieve annual energy savings of about 70-75% compared to a conventional domestic hot water heating system with electric immersion heaters. Case studies have also clearly demonstrated that CO₂ heat pumps are one of the most promising technologies for centralized hot water heating in hotels, hospitals, or apartments,” says Jørn Stene, research scientist at SINTEF Energy Research.

These key findings are summarized in a study presented by Stene at the IEA International Heat Pump Conference in May. The paper confirms that the use of CO₂ heat pumps for residential and non-residential is more energy-efficient than all HFC-based systems, propane or solar heaters currently available on the market.

The comparison and its results

To compare the performance of an R744 unit with different optimized heat pump systems using fluorinated gases or propane, four different heat pump water heaters were tested at varying evaporation (-10°C to +10°C), inlet city water (5°C to 30°C) and outlet water temperature (60°C to 85°C). Two systems working with R-134a and R-290 and equipped with either a subcooler, condenser and desuperheater or a suction gas heat exchanger, condenser and desuperheater achieved a similar Coefficient of Performance (COP). A third heat pump using the same refrigerants but only a condenser and a desuperheater achieved a 15% lower COP than the other systems.

The best performing system, however, was the CO₂ heat pump, which on average led to a 20% higher COP than the most efficient R134a or R290 systems. Depending on the evaporation temperature, the R744 unit thus achieved a COP of about 3.7 to 5.4. This was mainly due to a higher compressor efficiency and the excellent temperature fit in the gas cooler between the CO₂ and the water, minimizing the average CO₂ temperature during heat rejection.

Case Study – Design & Evaluation of CO₂ HPWH

When using groundwater at 7°C as the heat source, the seasonal performance factor (SPF) for the CO₂ hot water heater is around 3.8, corresponding to a 70-75% energy savings compared to conventional systems. This was confirmed in an in-depth analysis of CO2 hot water heaters by the Norwegian University of Science and Technology. The study was conducted to estimate the potential of R744 heat pumps for a new housing initiative in Bergen, Norway, involving several apartment blocks built with passive house standard. The analysis showed that the lower the inlet water temperature in the gas cooler the higher the COP.