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Why is COP crucial to borehole dimensioning?

COP (Coefficient Of Performance) is the figure expressing the efficiency of the heat pump

It has many similar characteristics to the MPG (Miles Per Gallon) figure given for cars, in that

  • Manufacturers claims are to be treated with care
  • Claimed figures are often optimistic and can only be achieved under optimum conditions
  • COP and MPG both depend upon driving syle, i.e. how the unit is operated
  • Using a big saloon for urban shopping trips is not usually the most efficient thing to do, neither is oversizing a GSHP/borehole array.

COP is calculated simply by dividing the energy output by the heat pump measured in kilowatts by the energy fed in to the heat pump, (electricity from the mains supply). Therefore the higher the COP, the more efficient the heat pump is and the more money can be saved.

Example:  Energy consumption of heat pump 3 kW

                Energy output of heat pump 12 kW

                COP = 12 / 3 = 4

However  The COP is strongly dependent upon the input and output temperature of the medium (for GSHP's, Glycol)

The higher the temperature of the geothermal boreholes (i.e. ground temperature), the higher the COP

The lower the temperature of the output (e.g. underfoor heating), the higher the COP

The higher the difference between input and output temperatures, the more energy is expended, the harder the heat pump has to work to "pump up" the value from one temperature to the other and therefore, the lower the COP

Heat pump manufacturers are therefore adept at choosing favourable input/output temperature values when stating COP claims.

One might think that the higher the COP, the more efficient the unit is and therefore the fewer boreholes are needed. In fact, exactly the opposite is true.

A higher COP means that a greater proportion of the delivered heat to the building must be extracted from the ground and therefore the more borehole metres needed. More heat energy from the ground, less from electricity

A more efficient heat pump will therefore need a more costly array than a low efficiency heat pump. The benefit comes from increased energy saving.