Multi-family house in challenging geological conditions

Project comparison between DU-32 and separatus

When planning a geothermal system, thermal output is not the only deciding factor. The maximum drilling depth, geological conditions, borehole diameter, hydraulics, and execution risks must also be considered.

For a multi-family building with heating, hot water, and cooling requirements, a comparative analysis of different options was conducted over a period of 50 years. The comparison included a conventional ground source heat pump field with double U 32 probes and a field with separate geothermal probes.

The results show that more drilling does not automatically mean more effort. With separatus, the maximum drilling depth can be reduced, the overall drilling length shortened, and the borehole diameter significantly decreased.

High energy demand and combined use

The building has an annual heating demand of 80 MWh and a hot water demand of 85 MWh. An additional 28 MWh of cooling energy is required annually.

The maximum heating output is 60 kW. A peak cooling output of 35 kW was factored in. Therefore, the system must not only reliably provide heat in winter, but also cool during the summer.

This combined use places increased demands on the thermal and hydraulic design of the ground source heat pump field.

Challenging geological conditions

The geological conditions at the site increase the risk with increasing drilling depth. Horizontal directional drilling is mandatory at depths of 100 meters and above.

At the same time, the fine-grained, unconsolidated rocks pose a risk of instability. Deeper drilling can necessitate additional technical assessments, place higher demands on drilling technology, and lead to greater uncertainties regarding costs and schedules.

From a risk minimization perspective, it made sense to examine a variant with a lower maximum drilling depth.

Neighbor probes taken into account in the simulation

Ground source heat pump fields do not operate in isolation. Existing and future installations in the surrounding area can influence the ground temperature in the long term.

The simulation took into account existing neighboring probes and potential future installations. The initial subsurface temperature was reduced accordingly.

In addition, combined heating and cooling operation was incorporated. The heat introduced in summer contributes to the regeneration of the subsoil. The flowing groundwater present at the site supports natural thermal regeneration.

The simulation thus not only depicts the current state, but also takes into account the long-term development of the system over 50 years.

Scenario with double U 32 probes

The conventional version consists of 13 double U 32 probes, each with a depth of 150 meters.

This results in a total drilling length of 1,950 meters. The borehole diameter is 132 millimeters.

With a total volume flow of 12.2 m³/h and a heat transfer medium containing 20 percent monoethylene glycol, a laminar flow is established during heating operation.

The Reynolds number during heat extraction is 1,851. The effective thermal borehole resistance is 0.18 mK/W.

The minimum average fluid temperature after 50 years is minus 1.3 °C. During cooling operation, a maximum average fluid temperature of 15.1 °C is reached.

This variant therefore meets the thermal requirements. However, the laminar flow limits the heat transfer between the brine and the pipe wall.

Scenario with separate geothermal probes

The second variant consists of 15 separate geothermal probes, each with a depth of 120 meters.

The total drilling length is 1,800 meters. Despite two additional boreholes, the total drilling length is thus reduced by 150 meters.

At the same time, the maximum drilling depth per probe decreases by 30 meters. The borehole diameter can be reduced from 132 to 100 millimeters.

During heating operation, the Reynolds number is 2,384, and during cooling, it is 4,076. In both cases, turbulent flow is achieved.

The effective thermal borehole resistance is 0.12 mK/W during heat extraction and 0.11 mK/W during heat injection.

The minimum average fluid temperature after 50 years is -1.47 °C. The maximum fluid temperature during cooling operation is 14.6 °C.

Both variants therefore meet the requirements for long-term fluid temperatures.

Reduced overall drilling length despite additional drilling.

At first glance, a higher number of boreholes might seem like a disadvantage. However, what matters is not just the number, but the total drilling output and the effort required per borehole.

The following differences emerge in comparison:

• Double U 32 with 13 boreholes of 150 meters each
• separatus with 15 boreholes of 120 meters each
• 1,950 meters total drilling length at twin U 32
• 1'800 Meter total drilling length at separate
• 132 millimeter borehole diameter for double U 32
• 100 millimeter borehole diameter at separatus

The separatus variant therefore requires 150 meters less total drilling length and reduces the borehole diameter by 42 millimeters.

The smaller diameter reduces the amount of material excavated, the need for backfill material, energy consumption during drilling, and increases drilling speed. At the same time, smaller drilling rigs can be used.

A shallower drilling depth reduces the execution risk.

In challenging geological conditions, the maximum drilling depth is a significant risk factor. The deeper a borehole is drilled, the longer it remains open, and the greater the risk of instability, material intrusion, or problems with probe installation.

Reducing the probe depth from 150 to 120 meters limits work in the particularly challenging depth range. This reduces technical uncertainty and can lower costs and schedule risks.

This advantage cannot be assessed solely based on the drilling depth. Especially in difficult soil conditions, a shorter and more controllable borehole can be crucial for project success.

Hydraulics as a key planning factor

The thermal performance of a ground source heat pump depends not only on the subsurface and the length of the probe, but also on the flow characteristics of the heat transfer medium.

In laminar flow, the fluid moves largely in ordered layers. The heat transfer between the fluid and the pipe wall is lower.

In turbulent flow, the fluid is mixed more thoroughly. This improves heat transfer and reduces thermal borehole resistance.

The project example clearly demonstrates this difference. The double U 32 variant remains laminar during heat extraction. The separatus variant achieves more favorable flow conditions and a fully turbulent flow during cooling.

Therefore, a hydraulic design is an essential component of any reliable dimensioning.

Shorter probes can be more efficient at cooling.

Shorter geothermal probes offer a further advantage when planning systems with cooling operation.

The ground temperature increases with depth due to the geothermal temperature gradient. Shorter probes are, on average, located in cooler soil layers than deeper probes.

This is advantageous for cooling. The lower the temperature of the ground, the greater the temperature difference between the building and the heat source. This allows heat to be transferred to the ground more efficiently.

In a project comparison, this is evident in the maximum fluid temperatures during cooling operation:

• Double U 32 with a maximum temperature of 15.1 °C
• separate with a maximum of 14.6 °C

The shorter separatus variant therefore has a lower maximum fluid temperature. This improves cooling conditions and can increase system efficiency.

This advantage is particularly relevant for buildings with high cooling requirements or for systems that use passive cooling.

Cooling supports the regeneration of the subsoil.

Cooling not only improves comfort in summer, but also has a positive impact on the long-term energy balance of the geothermal probe field. During the heating season, heat is extracted from the ground. In cooling mode, some of this energy is returned to the ground.

This regeneration stabilizes ground temperatures and can improve the long-term performance of the system. Short-term, regenerative ground source heat pump systems are therefore particularly interesting for buildings with balanced heating and cooling requirements.

Technical and economic comparison

Both variants meet the thermal requirements for over 50 years.

However, the separatus system offers several advantages in direct comparison:

• 150 meters less total drilling length
• 30 meters lower maximum drilling depth
• 42 millimeters smaller borehole diameter
• turbulent flow conditions
• lower thermal borehole resistance
• lower execution risk
• better conditions for cooling operation
• reduced need for drilling and backfilling material

These factors not only affect technical quality. They primarily influence investment costs, the construction process, and long-term operational efficiency.

The best solution arises from comparing different options.

This project demonstrates that the best design is not necessarily the one with the fewest boreholes. The crucial factor is the interplay of total borehole length, probe depth, borehole diameter, hydraulics, geology, cooling operation, and execution risk.

A reliable simulation makes these relationships visible and provides an objective basis for decision-making.

In this comparison, separatus proves to be the more suitable solution. This variant reduces the overall drilling length and maximum depth, improves hydraulic conditions, and lowers the risk during execution.

Support with dimensioning and simulation

We support planning offices, drilling companies, installers and building owners in the development of economical geothermal systems.

Our services include:

• Long-term thermal simulation according to SIA 384/6 & VDI 4640/2
• hydraulic design
• Pressure loss calculation
• Comparison of different probe systems
• Consideration of existing and future neighboring probes
• Optimization of drilling depth and borehole diameter
• Evaluation of heating and cooling operation
• Support in complex geological boundary conditions

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

Are you planning a ground source heat pump field with challenging geology, limited drilling depth, or combined heating and cooling requirements?

Send us the load profile, site conditions, and geological boundary conditions. We will examine the possible options.