Help with geothermal solution

Dear forum participants, Despite various research, I am still missing valid information. Therefore, I am posting my question here in the forum in the hope of receiving feedback. We are planning a KFW 55 house in Oranienburg with 150 sqm of living space and a general contractor. Part of the contract was actually a geothermal solution with deep drilling and a 6kW Junkers solution (STM 60). The solution serves both heating and hot water preparation. Now the local water authority has thrown a wrench in the works and does not allow deep drilling. We are allowed to go up to a maximum depth of 3.5m without a permit. According to the soil survey, there are about 30 cm of topsoil, followed by non-cohesive sands. At 2.6m, flowing groundwater is encountered. In the neighborhood, there are apparently also private wells for drinking water extraction. My question at this point is: which solution (ground, basket, trench collector) is sensible given the outlined conditions and what building area must be expected? Thanks in advance for your support and best regards!

Hello,

...We are planning a KfW 55 house in Oranienburg with 150 sqm living space and a general contractor.

That is actually near me.

...Part of the contract was originally a geothermal solution with deep drilling and a 6kW Junkers solution (STM 60). ....Now the local water authority has put a stop to it and does not allow deep drilling.

The GC/contractor should have known that!

...My question at this point: Which solution (ground, basket, trench collector) makes sense under the given conditions and what building area must be expected?

There are various alternatives here, but they require appropriate areas, especially surface collectors, as the name already suggests. It should also be noted that the course of the supply temperature to the heat pump follows the ambient temperature more closely due to the near-surface depth. This results in greater fluctuations compared to vertical drilling. In addition, especially on moist sites, longer icing effects on the affected areas can occur in spring.
Trench collectors can be a cost-effective alternative, provided one can contribute significant own work. For professional work, it hardly differs from the vertical probe. In addition, companies usually have little or no experience with this. However, the garden looks like a "battlefield" afterwards.
The third alternative is geothermal baskets (spiral collectors), which extract heat very concentrated (lowest area requirement). Unfortunately, quite expensive!
For all collectors, the local groundwater level has an influence on installation costs (possibly water management required).
The respective area requirement must be calculated exactly based on the cooling capacity of the heat pump. There are no general assumptions here.
6 kW for a KfW EH55 seems a bit high to me. Does a heating load calculation exist?
As a last alternative, an air heat pump could be considered. You save the costs of an expensive brine installation but have to accept a somewhat lower annual performance factor. For an EH 55 house, that should not really be a problem. The difference can be calculated.
Important for all heat pumps: precise calculation of heating load, room heating loads, air volumes (if controlled residential ventilation is planned), heating surfaces and pipe hydraulics.
Heat pumps only achieve a high annual performance factor if the system is optimized for this; otherwise, there is a risk of a money pit.

Best regards.

Hello €uro

Thank you very much for the quick feedback. I have requested the documents for the thermal protection certificate from the general contractor. I currently cannot say to what extent a heating load calculation is available here. However, I will follow up on that. What interests me significantly is the fact of the low groundwater level. Could this possibly be used positively? For example, if the circulation pipes (e.g., as a coil) are installed in the flowing groundwater and this results in better heat exchange. Or am I wrong with this approach now?

Best regards!

Hello,

I requested the documents for the thermal insulation verification from the general contractor.

The thermal insulation verification (Energy Saving Ordinance/KfW verification) does not replace a heat load calculation according to DIN 12831! Also, the dimensioning of a heating system is not possible based on this and is not intended by the legislator!

...What interests me significantly is the fact of the low groundwater level. Can this possibly be used positively?

Yes, because water is an excellent energy reservoir and transporter! It is no coincidence that H2O is used on the secondary side for heat distribution and transfer in most heating systems! Only in PH can this be dispensed with.

...So if the circulation pipes (e.g. as a spiral) are installed in flowing groundwater and this results in better heat exchange. Or am I wrong with this approach?

Completely correct! Interesting here is the flow direction of the groundwater, if known. However, it is not necessarily required to "penetrate" deep into the groundwater, because the adjacent soil layers benefit from the heat transport and exchange caused by the groundwater.
Dry soils (e.g. sand) provide significantly less annual extraction capacity compared to "wet" substrates. Sometimes having "wet" feet can also be an advantage for energy supply, which is rather disadvantageous in basement construction!

Best regards

Dry soils (e.g. sand) provide far less annual withdrawal capacity compared to "wet" substrates

.

if we now break down this information to my indicated conditions... So sandy soil permeated by groundwater at a depth of 2.50m.... Are these now sensible and usable conditions for a geothermal solution and if so, for which one (from the perspective of minimal space requirements)

....Are these now reasonable and usable conditions for a geothermal solution and if so for which ones (from the perspective of minimal space requirement)

Good conditions, geothermal baskets. Best regards.