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What is the potential for CO2 Heat Pump in the UK ?

The Climate in the UK is characterised by quite mild temperatures. The average winter temperatures are between 2C and 5C. However the humidity is general quite high which suggest that a significant amount of energy will be required for the defrost cycles.

What is interesting to know for heat pumps is their actual seasonal performance factor (mean annual COP) on a particular installation. It can be noticed that by knowing the average outside temperature and the return temperature to the heat pump over the week, the COP can be estimated quite accurately by adding the energy for the defrost, the controls and the back-up system (see table below). It is however important to keep in mind that it is just an approximation.

Rmk: The average of the COP over the week is not equal to the COP for the average temperature (dependence to the square of the outside temperature). But as the function is convex, it will lead us to underestimate the COP. Nevertheless, the mean COP over the period is not directly equal the average of the different COP since the power of the compressor is not always constant. It lead us to overestimate the COP since the power to the compressor tend to be less when the COP is higher...

If the mean average temperature is normally between 2 and 5 in winter in the UK, the heat pump will tend to operate more often at lower temperature. But the system also provides domestic hot water during summer where the performance might be very good (but the load will be limited). Considering the same energy demand for one week and a constant energy consumption for the controls, a best case and a worse case are defined to make a very rough estimation of what could be the range of the seasonal performance factor for a Sanyo Eco Cute system. (In this calculation, even if the water temperature to the heating system will increase at lower outside temperature, no increase of the average return temperature to the heat pump is considered.)

Worse Case

  • Energy for the defrost: 15kWh (50% higher than in Oban)
  • Energy for the electric: 20kWh (similar to Oban)
  • Outside temperature: -2C during the system operation
  • Return water temperature: 43C (like in Athelstaneford)

Best Case

  • Energy for the defrost: 6kWh (like for Athelstaneford)
  • Energy for the electric back-up: 1kWh (like for Athelstaneford)
  • Outside temperature: 4C during the system operation
  • Return water temperature: 36.2C (like in Oban)


It can be seen that the average COP are 1.6 for the worse case and 2.2 for the best case. Thus it can be roughly assumed that the seasonal performance factor of most of the Sanyo Eco Cute System will be between these 2 values.

But the design of the tank is not optimum and we suggest that the 70W of the stand-by control of the heat pump could be suppressed. In this case, 2 third of the energy for the heat pump controls could be saved. In addition, we suggest that the design of the tank could be improved by removing the sterilisation cycles. The return water temperature to the heat pump should then not be higher than the return water temperature from the heating system. Finally, a further cooling of 6C by the water coming from the city is considered as feasible. In this condition the average COP is also calculated in the worse and the best case.

In this new configuration, the seasonal performance factor of the Sanyo installation is assumed to be between 1.9 and 2.5 which would become really competitive with other heat pumps. In addition, the behaviour of the system in summer with only the domestic hot water demand is not known yet. It could be expected that the operation during the summer period increases a bit the performances...


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