GREEN LOCAL HEATING WITH GEOTHERMAL HEATING
QUESTIONS & ANSWERS
What is soil regeneration and why is it important (so-called freezing)?
When a building is heated with a geothermal probe array, the probe extracts heat from the ground. The problem is that, depending on the soil layers drilled, this heat only flows very slowly after the probes in a defined amount. Mean values can be taken from tables in VDI Guideline 4640 or, in the case of larger probe fields, are determined on the basis of a pilot bore using a Geothermal Response Test GRT in situ. The influence of the sun is limited to depths of up to 15 m below ground. Greater horizontal heat and cold transport takes place when groundwater is present with a significant flow rate. In areas with many borehole heat exchangers, this can lead to major problems in the long term and must be taken into account in the simulation calculations.
Heating a building in winter can extract more heat from the ground over the long term (usual observation period 20 years) than can flow naturally. This leads to the ground cooling down in a certain radius around the probe, which reduces the efficiency of the system. If the geothermal probes are kept at a distance of 6m from each other, this does not pose a major problem for smaller systems, since the temperature levels off at a certain level and the function of the system is not disturbed, even in the long term. Nevertheless, regeneration can be worthwhile because the annual performance factor is increased. It is not absolutely necessary. However, this looks completely different when larger probe fields are sunk close together, as is increasingly the case in residential areas. Since in this case the horizontal heat transport is no longer given or even a cold plume reaches into the probe field, the ground could continue to cool down over the years to the point of freezing, so that the function of the system can no longer be guaranteed. Simulation calculations show that the ground temperature in densely built-up areas has fallen so low after a few decades that a geothermal system is no longer economical. This is particularly problematic with probes longer than 200 m. The ground cools down more slowly than with short probes, but since the solar radiation no longer has any influence at great depths, the long-term end temperature is significantly lower.
This problem can be solved simply by heating the ground in the summer, which regenerates the geothermal probes. The effort required is low and the benefits are considerable, which is why regeneration for the DGC local heating networks is mandatory and is taken into account in the planning.
How do I regenerate the geothermal probe sustainably?
The field of geothermal probes is used to cool the building in summer. The heat pump does not have to be switched on for this. The water in the heating pipes is simply cooled directly via a heat exchanger using the heat transfer medium circulating in the probes. The advantage here is that an air conditioning system does not have to be installed separately, which also saves money and electricity. The energy requirement is determined solely by the power consumption of the circulating pumps in the probe and heating circuit. If efficient pumps have been installed, this is very low. The plant will be designed and operated in such a way that the mean temperature of the soil does not change over the years. In buildings with surface heating and cooling, passive cooling is used in summer.
When an existing heat pump is replaced with a new, more efficient heat pump with a higher COP value, the borehole heat exchanger field may be too small for the new system, as more energy is then required from the ground and less electricity is required to drive the heat pump. This can lead to icing of the probe field. Changing from a heat pump that can only heat to a heat pump that can heat and cool can solve these problems and keep the probe array fully functional for an indefinite period of time. The retrofitting of a regeneration will in any case be considered in the planning in order to be able to prevent the temperature in the probe field from dropping too much if necessary. The DGC local heating networks take into account the technical development of heat pumps so that a later replacement does not affect the efficiency of the entire system.
groundwater limitations
In near-surface geothermal energy, brine heat pumps are used to extract thermal energy from the shallow subsoil. In contrast to deep geothermal energy, the boreholes in shallow geothermal energy are usually shallower, which results in some specific groundwater restrictions. Some of the most common groundwater limitations in shallow geothermal energy are:
availability of groundwater
The availability of sufficiently high groundwater levels with significant horizontal flow velocities in the vicinity of the heat pump wells is an important factor for the efficiency of near-surface geothermal energy.
soil condition
The soil conditions define the efficiency of the heat transfer.
environmental impact
As with any type of geothermal energy, shallow geothermal energy can also have an impact on the environment. These include, for example, the possible impairment of groundwater flows and effects on the water chemistry through the heat transfer medium in the event of leaks.
Permissions and Legal Restrictions
In the case of near-surface geothermal energy, water law permits and the regulations formulated there must be observed. It is important to carefully consider these groundwater limitations when planning and implementing shallow geothermal projects to ensure they are sustainable and environmentally friendly.
The DGC conservatively determines the required extraction values solely on the basis of the soil composition and structure. Thus, sustained changes in the groundwater level have no impact on the efficiency of the entire local heating network.
Restrictions on water rights permits
Water law permits can limit near-surface geothermal energy, as they regulate the use of water and groundwater. In many cases, shallow geothermal energy requires the use of drilling through aquifers and the handling of large volumes of water. For the use of near-surface geothermal energy, water law permits must therefore be obtained, which regulate the way water and groundwater are used. These permits may contain restrictions on the depth and diameter of the wells, the choice of heat transfer fluid and the material used to seal the wells.
It is important to note that the limitations and requirements of water law permits may vary by location and local conditions. It is therefore advisable to commission a DGC potential study before starting a geothermal project. This study provides comprehensive information about the applicable approval procedures and requirements.
Expansion of water protection areas
The expansion of water protection areas can have an impact on near-surface geothermal energy. Shallow geothermal energy refers to the use of geothermal energy at depths of up to 400 meters to heat or cool buildings. If an area is designated as a water protection area, this means that the groundwater resources in that area must be specially protected to prevent contamination. This may result in the use of shallow geothermal energy being restricted or even banned in this area to minimize the risk of groundwater contamination. In some cases, however, it may be possible to use shallow geothermal energy in water protection areas if certain conditions are met. For example, geological and hydrological studies can be carried out to ensure that the geothermal system does not contaminate groundwater, or biodegradable heat transfer fluids can be prescribed. The DGC potential study provides comprehensive information about the local requirements and necessary studies. The local regulations and guidelines are determined whether and how the use of near-surface geothermal energy is possible in a specific water protection area.
Expansion of fauna-flora habitat areas (FFH area)
In the Habitats Directive, a total of 92 habitat types and around 138 species and subspecies of common interest are listed for Germany, for which a system of networked protected areas must be set up. According to the Bird Protection Directive, special protection areas are to be designated for 110 species in Germany as well as for other regularly occurring migratory bird species. In Germany there are over 4,500 FFH areas and over 740 bird sanctuaries, some of which overlap. A total of 15.5 percent of the German land area is covered by so-called "Natura 2000 protected areas".
Nature protection areas and the expansion of renewable energy systems (RE systems) should not be mutually exclusive, but should be brought into harmony. RE systems serve climate protection and thus global environmental protection, without which local nature conservation is not possible. Bird sanctuaries have no prospect of survival without the expansion of renewable energy systems, which are climate protection measures. Ultimately, species extinction is caused by climate change and not by renewable energy systems. Threatened species are protected by the massive expansion of green electricity and green heat, because the extent of climate-damaging pollutant emissions is reduced and the basis of life for all species is secured.
DGC local heating networks in existing buildings or in new construction projects are renewable energy systems and do not restrict FFH or bird protection areas. Built-up and planned residential or commercial areas with a DGC local heating network based on near-surface geothermal energy also make this possible
Protection zones since the probe field is underground. Near-surface geothermal energy (ONG) has no negative impact on FFH areas. The expansion of such areas does not affect the approval of ONG district heating networks.