CO2 Heat Pumps in Practice

CO2 heat pumps are rapidly becoming available in a wide variety of configurations and from a wide range of suppliers. In this final article, we use the Revere™ CO2 heat pumps available from Automatic Heating to demonstrate the key performance features of CO2 heat pumps.

CO2 Heat Pump Efficiency

The COP of CO2 heat pumps is higher than for HFC heat pumps in applications where a high temperature difference is required between the inlet and outlet of the water being heated. This is illustrated in Figure 4.

Figure 4. Efficiency of CO2 heat pump as a function of ambient temperature (domestic hot water application).

The figure demonstrates a number of generic features of CO2 heat pumps:

The efficiency is routinely higher than equivalent HFC heat pumps in domestic hot water application.
The increase in COP is roughly fixed across non-freezing ambient temperatures. This means that the relative efficiency benefit increases as the ambient temperature drops towards zero.
The maximum hot water temperature achieved by the CO2 system is higher than that for the HFC system.

This article series is kindly contributed by Dr Paul Bannister, a thought leader and public speaker on energy and energy efficiency issues in Australia. For other articles by Dr Paul Bannister, please refer to our news section.

CO2 Heat Pump Capacity

Similar effects are visible in the capacity curve of CO2 heat pumps as a function of ambient temperature, as illustrated in Figure 5.

Figure 5. Heating capacity of CO2 heat pump as a function of ambient temperature (domestic hot water application)

For Australian conditions, one is typically designing for minimum ambient temperatures between minus 2°C and 10°C. Looking at the figures, it can be seen that the oversizing required for the CO2 heat pump to compensate for the low temperature performance is far smaller than it is for the HFC heat pump.

Outlet temperatures

HFC heat pump technologies are generally limited to maximum temperatures in the region of 45-55°C. CO2 heat pumps can work at temperatures of up to 90°C, while maintaining a high COP, as shown in Figure 6.

Figure 6. Effect of outlet temperature on CO2 heat pump performance (domestic hot water application)

Greenhouse Gas Benefits

Comparing a heat pump to a condensing boiler, the degree of greenhouse gas benefits depends on the ratio of the greenhouse gas coefficients of electricity and gas. Using the current greenhouse gas coefficients for Australia, the threshold COPs for a greenhouse benefit are as shown in Figure 7 . It can be seen that in domestic hot water applications, a CO2 heat pump at a notional COP of 4 is beneficial in all states other than Victoria; as the grid decarbonises, this situation will improve further.