Heat source temperature is a decisive factor for the output and efficiency of a heat pump. The advantage of geothermal is that the temperature of the soil or groundwater, unlike the outside air temperature, changes very little throughout the year. The efficiency of the heat pump remains high when the weather is coldest and the heat requirements of your home are greatest.
 

Seasonal Variation in Ground Temperature

Closed loop collectors

The purpose of the geothermal collector is to absorb stored energy from the ground and transmit this energy to the refrigerant at the evaporation stage, through a heat exchanger. This can be achieved by circulating brine, a mixture of anti-freeze and water, through coils of absorber pipe laid in the ground. The pipes can be laid in horizontal coils or in vertical loops contained in boreholes. Geothermal loops can be used for both removal of energy for heating and discharge of energy for cooling. The packing around the loop acts as a thermal connection to the soil and helps to stabilise it in the soil structure. It is therefore an important quality factor with respect to output and durability. Durability is an important factor, as a geothermal loop is a stationary object and, under normal circumstances, should last as long as the building itself.
 

The horizontal collector

    

Loops of HDPE absorber pipe, 100m long, are laid horizontally in the soil. The pipes are laid at around 1.2 to 1.5 m depth, (0.3m below the regional freezing depth) where the soil temperature remains relatively constant throughout the year. For the smallest layout grid of 0.25m pipe spacings, each 100m loop of absorber pipe can collect around 1kW of heat from the ground, i.e. one 100m loop is required for every 1.333kW heating capacity.

Geothermal probes

Hair-pin shaped loops of high quality HDPE plastic tubing are encased in a vertical borehole, around 12cm in diameter. The tubing is hard wearing and a good conductor. The specific removal capacity (W/m) is only slightly influenced by the diameter of the hole, arrangement of pipes and pipe fittings. Far more important is the length of the hole, as capacity increases in proportion to the length. Boreholes are drilled to a maximum depth of 130m. The boreholes must be laid far enough apart so as not to affect each other.
 

Ground-water source – open loop

It is possible in some cases to take groundwater directly through the evaporator, or through an intermediate heat exchanger, to transmit heat to the refrigerant. The groundwater is extracted from one borehole and re-injected into a second borehole after the heat pump. The advantage of this heat source is that the water comes into the heat pump at the ground temperature, around 10ºC. The higher temperature of the source in the evaporation stage of the refrigeration cycle benefits the heat pump efficiency.

As long as the groundwater is available in sufficient quantities, of sufficient quality and at a reasonable depth, this heat source can produce very high efficiencies. General requirements for the suitability of your site for groundwater source are:
 

Temperature:
The temperature of the groundwater at the inlet should not be below around 8ºC and should not be cooled below 4ºC (consult local regulations). The lower the groundwater temperature, the higher the necessary extraction rate. There are some places in Northern Europe where the groundwater is too cold to use as a geothermal heat source, since enormous extraction rates are required. The European average temperature for groundwater is around 10 ºC. The temperature of the groundwater in any region will not vary by more than a few degrees throughout the year.
 

Quantity:
The system must be able to provide the maximum extraction rate for the heat pump size at the minimum flow temperature. The groundwater must be available in large quantities. Local regulations for the extraction of groundwater will apply.
 

Quality:
To protect the heat exchanger from corrosion, the chemical composition of the groundwater must be suitable. The water must be sufficiently free of deposits and coarse dirt.
 

Depth:
The groundwater must be available at a depth of no more than 15m, or the energy requirements of the pump will be large.
 

A test borehole should be drilled before work begins to investigate the depth and thickness of the water bearing layers in the ground and the chemical composition of the groundwater. Long term faultless operation is only possible where adequate and consistent quality is present in the groundwater. A well for a heat pump may be pumped continuously for 20 hours, unlike a household water well, so it is important to bear this in mind.