Geothermal collectors – function and benefits briefly explained

Geothermal collectors for brine/water heat pumps are shallow geothermal energy systems installed close to the surface, which extract thermal energy from the ground at a depth of approx. one to two metres. They are suitable for residential areas where deep drilling is not possible for legal reasons.

Geothermal collectors extract geothermal heat from close to the surface

For heat recovery, geothermal collectors are installed close to the surface and below the frost line, similar to an underfloor heating system. Depending on the region, this limit will vary and will significantly influence the soil depth. In practice, most geothermal collectors are located horizontally at 1.0 to 1.5 metres below the surface of the ground. The most common types include surface collectors, trench collectors, spiral collectors and geothermal basket collectors. Depending on the type of collector and installation, excavation or drilling can reach up to five metres in depth. When laying the pipes, it is important to ensure that they are spaced a certain distance apart. If they aren't, the collectors will extract too much heat from the ground in some locations, which may cause the area around them to freeze.

How geothermal collectors work

Geothermal collectors usually consist of plastic pipes with a diameter of two to four centimetres. The diameter of the pipes depends on both the conditions of the soil and the installation depth, and should match heat demand as closely as possible. An incorrect design will have a negative impact on economic efficiency and should be avoided at all costs. Irrespective of the type, a brine medium circulates inside the collectors, which absorbs the thermal energy from the ground and transfers it to the ground source heat pump.

Within the ground source heat pump, the extracted heat is transferred to a refrigerant, which then evaporates. In order to raise the resulting refrigerant vapour to the required temperature and use it for central heating backup or domestic hot water heating, it is compressed with the aid of a compressor. You can learn in detail about how this process works in the article  How the brine/water heat pump works.

Compared to geothermal probes, no permit is required for installation of geothermal collectors, just a notification to the local authority. In order to find the right collectors, several factors need to be taken into account beforehand. These include the installation technique and the conditions of the ground.

In practice, geothermal collectors are often installed horizontally and in a similar way to underfloor heating. The technology does, however, require a large area. The space that needs to be available very much depends on the heating requirements. As a rule of thumb, however, the collector area should be about one and a half to two times as large as the area to be heated. If space is limited, vertical installation is possible, e.g. as a trench collector or as a geothermal basket collector. It is important to take the maximum depth into account, as this can lead to an encroachment into the groundwater if the groundwater table distances are low.

When choosing the right geothermal collectors, the type of soil also plays a decisive role. A distinction is made between moist and dry soils, with the latter providing significantly more thermal energy. In all cases, care must be taken to ensure that the collector area is never covered or even built over. If it is, the soil will no longer be able to regenerate itself through sunlight and warm rain. The result: it will provide less heat.