Project example planning:

School building with limited drilling depth

In geothermal systems with shallow borehole depths, the available drilling depth is not the only factor determining economic efficiency and operational reliability. Equally important are the hydraulic properties of the geothermal probes used, the borehole diameter, and the thermal design of the entire probe field.

We were tasked with designing a geothermal system for a school building with a maximum permissible drilling depth of 105 meters. The comparison included conventional double U 32 probes and separate geothermal probes.

The result demonstrates the optimization potential of a project-specific design. With separatus, the required design reliability could be achieved with one less borehole.

School building requirements

The planned geothermal system must cover a high heating demand and a seasonal cooling demand at the same time.

Key building data:

• maximum heating output of 71 kW
• annual heat demand of 120.5 MWh
• maximum cooling capacity of 27.3 kW
• annual cooling requirement of 6.5 MWh

The maximum drilling depth was limited to 105 meters. Therefore, the ground probes were simulated at a depth of 100 meters each.

Identical soil parameters were used for both variants. The thermal conductivity of the subsoil was set at 2.4 W/mK. A thermal conductivity of 2.0 W/mK was assumed for the injection material.

The influence of neighboring geothermal probes was also included over the entire simulation period of 50 years.

Comparison of the two probe systems

The variant with conventional double U 32 probes required 17 boreholes of 100 meters each. With separatus, 16 boreholes of 100 meters each were sufficient.

The minimum fluid temperature after 50 years was almost identical for both variants:

• separated -1.47 °C
• Double U 32 -1.44 °C

Both systems meet the thermal requirements. However, separatus achieves this result with one less borehole.

Additionally, the required borehole diameter is reduced from 135 millimeters for the double U 32 probe to 100 millimeters with separatus.

That means:

• 100 meters less total drilling length
• 35 millimeters smaller borehole diameter
• less drilling material and injection volume
• less effort required for connection and distribution
• comparable long-term design reliability

Why hydraulics are important for short ground probes

With deep ground source heat pumps, the available pipe length is often sufficient to achieve favorable flow conditions even with conventional systems. With short ground source heat pump fields, however, hydraulics can become the limiting factor.

A total flow rate of 16.5 cubic meters per hour was specified for the school building. The heat transfer medium consists of a mixture containing 20 percent monoethylene glycol.

Under these conditions, the double U 32 probe achieves a Reynolds number of 1905. The flow thus remains laminar. With laminar flow, the heat transfer between the brine and the pipe wall is lower.

Separatus achieves a Reynolds number of 2987 at the same total volume flow rate, resulting in turbulent flow conditions. The mixing of the fluid improves heat transfer and increases the thermal efficiency of the probe.

This hydraulic advantage makes it possible to achieve the same long-term fluid temperature with fewer total drilling meters.

Less brine volume and easier distribution

In addition to the borehole length and diameter, the required fluid volume also differs. For the mixture with 20 percent monoethylene glycol, approximately 448 liters of concentrate are needed for the separatus variant. For the double U 32 variant, it is approximately 734 liters.

The smaller system volume reduces material requirements and simplifies filling the system. At the same time, the reduced number of probes lowers the complexity of the connecting lines and the distribution system.

These factors directly affect investment costs and practical implementation.

Same design reliability with one less borehole

This project example shows that the number of boreholes alone does not determine the performance of a geothermal system.

An optimized combination of probe, hydraulics, borehole diameter, injection material and probe arrangement can significantly improve cost-effectiveness.

Especially in the case of schools, apartment buildings and larger geothermal probe fields, these differences have a significant impact on the overall costs.

Support with dimensioning and variant comparison

Short ground source heat pump fields must be designed specifically for each project. Standard assumptions are insufficient to fully exploit the available optimization potential.

We support planning offices, installers, drilling companies and building owners with:

• Dimensioning of ground source heat pump fields according to SIA 384/6 & VDI 4640/2
• long-term thermal simulation
• hydraulic design
• Pressure loss calculation
• Selection of suitable probe systems
• Comparison of separatus and standard systems
• Optimization of drilling depth, drill diameter and probe arrangement

Whether separatus or standard probe, the decisive factor is the technically and economically best solution for the respective project.

Are you planning a ground source heat pump field with limited drilling depth, a narrow plot of land, or challenging hydraulic boundary conditions?

Send us the load profile, the site conditions, and the maximum drilling depth. We will examine the possible options.