What is the COP?
The Coefficient of Performance (COP) demonstrates the efficiency of a technology. A ground source heat pump’s Coefficient of Performance (COP) is high because it uses free heat energy from the ground, making it a highly efficient technology. The higher the COP, the more efficient the technology and less electrical power required.
For each kilowatt of electric power supplied to run the ground source heat pump unit, around four kilowatts of heat energy are produced. This means the heat pump has a COP of 4; it is safe to say on average this is a typical ground source heat pump COP. Compare this to boilers, which typically have a COP of 1. In practice then, a high COP of 4 means the end user benefits from significantly reduced carbon emissions and lower running costs, compared to traditional heating systems.
The COP varies depending on the temperature difference between the ground temperature (inlet temperature) and the required heat output temperature (outlet temperature, i.e. your heating distribution system).
See the below sections for how COP varies with inlet and outlet temperatures.
As the inlet temperature increases from the ground, the COP will also increase. This is simply as the compressor does not have to work as hard, as the inlet temperature increases, to reach the required outlet temperature.
The inlet temperature test condition that all heat pumps should be tested to determine the COP is 0oC as per EN14511.
The graph below indicates how COP changes for different kW ratings with a change in ground temperature. The graph above has been generated using the methodology laid down in EN14511 and is as a guide only.
The graph shows an increase in ground temperature is beneficial to the efficiency of the heat pump. In reality although all heat pumps should be tested at 0oC inlet, the ground temperature rarely falls this low and source temperatures of around 2oC to 4oC can be expected depending on the time of year.
COP inlet temperature graph
The higher the outlet temperature of the heat pump, the more work the compressor has to do to achieve this temperature. Therefore the more power the heat pump requires and the lower COP.
The temperature leaving the heat pump will actually be less than this ‘compressor off’ temperature due to losses across the heat exchanger within the heat pump.
The graph below indicates how COP changes for different kW ratings with a change in the temperature of the refrigerant coming off the compressor. The graph has been generated using the methodology laid down in EN14511 and is as a guide only.
The main outlet temperature test condition that all ground source heat pumps should be tested to determine the COP is 35oC as per EN14511. This (with a set inlet of 0oC) is to try and provide a common point so one heat pump's performance can be compared to another.
It can be seen that the outlet temperature from the heat pump can have a significant effect on the efficiency and hence COP of the heat pump. This is why heat pumps are ideally suited for underfloor mounted in screed which only requires a low inlet temperature of 35oC and how using radiators may result in a lower COP due to the higher flow temperatures they require.
COP outlet temperature graph