For a multi-family building with heating, hot water and cooling requirements, a separatus simulation compares two borehole heat exchanger fields over 50 years: 13 double U-32 probes à 150 metres and 15 separatus probes à 120 metres. In comparison, the separatus variant requires 150 metres less total drilling length and 30 metres less depth per probe. According to the project report, both variants meet the targeted requirements for long-term fluid temperatures.
For the decision about geothermal drilling, total drilling length, depth, drilling diameter and the simulated temperatures are jointly relevant here. The results apply to the documented project conditions.
What needs does the Earth probe field have to cover?
- Annual heating demand · Project value: 80 MWh
- Annual hot water demand · Project value: 85 MWh
- Annual cooling demand · Project value: 28 MWh
- Maximum heating capacity · Project value: 60 kW
- Cooling Peak Performance Considered · Project Value: 35 kW
- Simulation period · Project value: 50 years
The simulation takes into account heating and cooling mode, existing neighboring probes and possible future plants. According to the separatus project report, the initial temperature of the substrate was reduced. The report also cites running groundwater and summer heat input as contributing to the thermal regeneration of the subsurface.
Why the drilling depth plays a role
For the location described, the project report specifies a specification for fluid-assisted rotary drilling from 100 metres depth and instability risks due to fine-grained loose rocks. A location and associated source of authorisation shall not be mentioned; the 100-Metre indication is therefore exclusively a documented project condition.
The separatus variant reduces the maximum depth from 150 to 120 metres. 120 metres are also above the mentioned 100 metre threshold. The shorter drilling therefore does not result in any exemption from the irrigation drilling specification described in the project. A measured risk reduction or concrete cost savings does not show the comparison.
Two probe fields in direct comparison
All of the following information comes from the separatus project report "Geothermal correctly dimensioned". The temperature and resistance values are simulation results.
- Number of probes · Double-U 32: 13 · separatus: 15
- Depth per probe · Double U 32: 150 m · separatus: 120 m
- Total drilling length · Double U 32: 1.950 m · separatus: 1.800 m
- Hole diameter · Double U 32: 132 mm · separatus: 100 mm
- Reynolds number for heat extraction · Double U 32: 1.851 · separatus: 2.384
- Reynolds number for cooling · Double U 32: not specified · separatus: 4.076
- Effective borehole thermal resistance on heat extraction · Double U 32: 0,18 mK/W · separatus: 0,12 mK/W
- Effective borehole thermal resistance at heat input · Double U 32: not specified · separatus: 0,11 mK/W
- Minimum average fluid temperature after 50 years · Double U 32: −1,3 °C · separatus: −1,47 °C
- Maximum fluid temperature in the cooling mode according to report* · Double U 32: 15,1 °C · separatus: 14,6 °C
*The original report expressly refers to the DU-32 value as the average fluid temperature; the separatus cooling value does not have this specific designation. These differences in designation limit interpretation.
For the DU-32 variant, the report mentions a total volume flow of 12,2 m3/h and a heat transfer medium with 20 percent monoethylene glycol. Such information is not explicitly repeated for the separatus variant.
Computationally, there are two additional separatus drillings, 150 metres less total drilling length, 30 metres less depth per probe and a drill hole diameter smaller by 32 millimeters.
What can be derived from the hydraulic results
The project report describes the DU-32-heating mode as laminar and identifies lower effective thermal well resistances for separatus. For the separatus-heating mode it specifies a Reynolds number of 2.384. The blanket classification of both separatus operating cases as turbulent is not further justified in the report. From this information alone, no reliable turbulent heating mode is derived here.
The simulated minimum average fluid temperature at separatus with −1,47 °C is lower than at DU 32 with −1,3 °C. In cooling mode, the report mentions 14,6 °C for separatus and 15,1 °C for DU 32. Thus, the comparison does not demonstrate a general temperature advantage in both operating modes. It does not contain measured efficiency values or an annual working number.
Which decision supports the comparison
For this project, a higher number of drillings is offset by a smaller total drilling length at a lower individual depth. The project report considers both variants to be sufficient in terms of long-term fluid temperatures. The decision can thus be made on the basis of concrete variants, instead of solely on the basis of the number of probes.
The product and planning specifications of separatus tie the statement of a quasi equivalent thermal performance to small drilling diameters, thermally improved borehole grouting and planned turbulent flow. This general statement does not replace proof of the respective conditions in the project. This report does not mention thermal conductivity of the borehole grouting used.
Support for design / system sizing and simulation
According to the project report, separatus offers long-term thermal simulation, hydraulic design, pressure-drop calculation and the comparison of different probe systems. This includes considering neighboring probes, evaluating heating and cooling operations, and optimising drilling depth and drilling diameter.
Are you planning an Earth probe field with difficult geology or combined heating and cooling needs? Send separatus load profile, land situation and geological boundary conditions for a variant comparison.
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