Geothermal collector
Geothermal collectors are geothermal heat exchangers that mainly consist of plastic pipes laid horizontally in the ground (as opposed to geothermal probes that are vertically inserted into a borehole ). They are primarily used as a heat source for heat pump heating . The heat yield of the surface collectors installed at a shallow depth is heavily dependent on the seasonal temperature profile in the soil layers close to the surface. The manufacture of a surface collector is therefore particularly worthwhile in lower-lying regions with mild weather. Since the ground temperature increases as a result of the ingress of rainwater as well as solar radiation, unsealed and unshaded open spaces such as lawns are ideal for installing geothermal collectors.
Also air fountain deprive the ground heat at a shallow depth. They are operated together with ventilation systems or air / water heat pumps .
laying
The collector pipes are laid at least 20 cm below the local frost line, typically at a depth of 1 to 1.5 meters.
Depending on the laying scheme, one speaks of trench or surface collectors .
In the case of surface collectors, the collector pipes are often laid in a meandering shape with a pipe spacing of 1.2 to 1.5 m, as is the case with underfloor heating . In order to enable a high level of heat extraction in poorly insulated buildings in the short term, it can make sense to choose a closer pipe spacing or to use prefabricated registers made of so-called capillary pipes .
A space-saving form of a surface collector is the so-called ring trench collector . The pipes are laid in loops in an annular trench up to 2 m deep. This allows more soil to be developed with less excavation volume, which lowers the costs compared to the other types of surface collector. This design is propagated by various manufacturers of modulating geothermal heat pumps as an optimal cost-effective alternative to a geothermal probe.
So-called spiral collectors or geothermal heat baskets have a significantly lower space requirement than surface collectors, but have to be installed deeper. They can also be used on smaller plots if there is neither space for surface collectors nor access to deep drilling equipment for geothermal probes.
While geothermal probes with a generous design pump brine at a temperature of 10 ° C all year round, the average soil temperature at the beginning of February at a depth of one meter is 2 ° C. At a depth of two meters it is 4.5 ° C. In contrast, the soil temperature at this depth can reach 20 ° C in August.
Depending on the nature of the soil and in particular on the water content, the time lag between air and soil temperatures at a depth of 6 to 12 m reaches six months, so that here the qualitative temperature profile is exactly the opposite of that on the earth's surface. However, the absolute fluctuation amplitudes at this depth are only very small anyway.
In order to use the storage capacity of the soil, collectors would have to be laid at this depth. The energy stored in the deeper soil layers is not completely regenerated by the heat supplied from the earth's surface in summer, while the heat stored in higher layers is lost again early on due to the natural cooling.
A conceivable compromise would be to fill the top of the geothermal collector with heat-insulating material such as light slag , glass foam gravel , perlite , expanded slate or expanded clay . This would increase the phase shift between the surface of the earth and the ground at the level of the collector, so that in winter a greater amount of energy would be available in the layers of the earth below the collector.
Heat transfer medium
A water-glycol mixture circulates through the closed pipe system as a heat transfer medium . The heat brought into the ground by rain and sun is extracted via the carrier medium and fed to the heat pump .
power
The heat output that can be achieved depends on the nature of the soil . Dry, coarse-grained soils are less suitable for heat transport than moist groundwater soils. That is why the performance of classic surface collectors varies from 10 W / m² to 35 W / m² for groundwater floors. With a heat input of 20 W / m², a floor area of 300 m² is required to supply a heat pump heating system with 6 kW output. This roughly corresponds to twice the living space to be heated . A ring trench collector designed for the same output requires less space compared to the classic design.
Both the use of a heat pump itself and the installation of geothermal collectors at a shallow depth is particularly useful if the temperature difference between the heat source and the heat sink is sufficiently small. The year of construction and the thermal insulation properties of the building are only indirectly relevant. Due to the higher flow temperatures required, the operation of a heat pump for heating old buildings is not profitable in some cases. Information about this is provided by performing a thermal calibration. If the heat sink is correctly calibrated, flow temperatures that are suitable for heat pumps often result, even in old buildings. However, such a comparison is seldom carried out and is often run with excessively high heating curves and curtailment of the heating surfaces through individual room controls, which then leads to excessive power consumption by the heat pump. It should be noted that a high heat demand requires the development of a possibly very large collector surface. If the collector field is too small, the ground freezes and efficiency drops from December to February. However, due to the large amount of latent heat at 0 ° C, the brine temperature is stabilized so that high heat requirements can also be met. However, depending on the type of soil and the amount of ice that has formed, undesirable uplifts can occur.
advantages
- closed system with harmless brine medium
- very favorable manufacturing costs due to:
- easy development of the heat source with the help of the construction machines that are already available for major construction projects
- Use of simple PE pipe to manufacture the collector
- unproblematic laying in own contribution
disadvantage
Geothermal collectors require a large area compared to geothermal probes that are installed in deep boreholes.
In cold winters, the natural soil temperature can drop to 0 ° C at a depth of one meter. A relatively uniform level of 10 ° C is only reached at depths of 6 m.
The cooling of the soil can delay the beginning of the growth period of plants that grow above the collector by up to two weeks. Deep-rooted plants that are not adapted to cold climates develop poorly.
Dimensioning
The simulation model, which can be downloaded free of charge from the Internet, can be used to dimension or calculate the performance. This model makes it easy to compare the geothermal probe. A free online tool is available for designing and dimensioning a ring trench collector.
swell
- ↑ Climate time series - soil temperature , Potsdam Institute for Climate Impact Assessment
- ↑ Soil temperature ( Memento from February 16, 2010 in the Internet Archive )
- ↑ Bernd Glück: Simulation model for geothermal collectors, 2008
- ↑ Bernd Glück: Simulation model for geothermal probes , 2008
- ↑ Trenchplanner: Trenchplanner