With buildings becoming increasingly complex and versatile, our borefield designs are also moving away from the traditional rectangular configuration. In this article, we explore the importance of using the actual coordinates of the boreholes when dealing with irregular borehole configurations, as well as the significance of the buried depth.
The exercise
Increasingly, borefield configurations are irregular, with some boreholes positioned on other sides of the project or even beneath the building. However, most designers still use the traditional rectangular layout to determine the total number of boreholes, which often results in oversized systems.
In this exercise, we explore the difference in temperature between a rectangular configuration with 90 boreholes and the actual irregular configuration with the same number of boreholes. In addition, we discuss the importance of buried depth and revisit the distinction between borehole length and depth.
The case used to illustrate these aspects is a multi-utility building comprising shops, residential units, and offices.
!Hint
To get the most out of this exercise, we strongly recommend attempting the design questions below before reading the provided solution. Borefield design is far from straightforward, and the best way to master its complexities is through hands-on experience.
Input parameters
General input parameters
- Minimum average fluid temperature threshold: -2°C
- Maximum average fluid temperature threshold: 17°C (active cooling)
- Simulation period: 50 years
- First month of the simulation: January
Ground input parameters
- Ground thermal conductivity: 1.6 W/(mK)
- Volumetric heat capacity: 2.4 MJ/(m³K)
- Surface temperature: 10°C
- Geothermal heat flux: 0.8 W/m²
Borehole resistance input parameters
The parameters for the pipe are:
- Single DN32 PN16 pipe (i.e. a wall thickness of 3mm and an outer diameter of 32mm)
- Borehole diameter: 140 mm
- Distance from pipe to borehole centre: 39 mm
- Grout: 1.8 W/(mK)
The fluid is 25 v/v% MPG with a 40 l/s flow rate for the entire borefield.
Thermal load input parameters
As this is a multi-utility building with different users and load profiles, some preprocessing of the data was carried out. Therefore, instead of working with the building load, we now work directly with the ground load (that is, the heat injection and extraction), and the SEER and SCOP values are no longer required.
- Peak extraction demand: 475 kW
- Peak injection demand: 400 kW
- Yearly extraction demand: 477 MWh
- Yearly injection demand: 380 MWh
- Peak duration extraction: 20 hours
- Peak duration injection: 8 hours
Borefield configuration
The project discussed here has 90 boreholes arranged in an irregular grid, as shown in the figure below. These coordinates were measured during the actual construction of the project, but they can also, for example, be exported from an AutoCAD file (as we discussed in this article).
!Hinweis
If you want to follow along and do the exercise yourself, you can download the coordinates hier.
The configuration shown above is, of course, not one that can be used for design in an early feasibility phase. Therefore, we start with the assumption of a rectangular configuration of 5 by 18 boreholes, with a borehole-to-borehole spacing of 5.5 m.
The vertical structure of the borehole is illustrated in the figure below. As shown, the boreholes are installed beneath the building, where the ground floor is located at 5 m above sea level (a.s.l.). During construction, an initial excavation pit is made to 0 m a.s.l., where drilling takes place. The boreholes are drilled to a depth of 140 m, and probes with a counterweight balance of 1.5 m in length are installed.
After drilling and pipe installation, the excavation pit is further deepened to -3 m a.s.l., where the cellar floor will be constructed. Half a metre lower, the horizontal connections of the boreholes will be installed.
Design questions
For this exercise, you are invited to answer the following design questions while tracking the total borehole length for each step. This will help you assess the cost and performance implications of various design changes.
!Hint
To keep your work well-organised, it is recommended to use a separate scenario for each design question.
- Calculate the temperature profile using the rectangular grid (where you enter the borefield relative to the ground surface or the buried depth).
- Calculate the temperature profile with a buried depth of 0.7m.
- Calculate the temperature profile using the real borefield coordinates.
Solution
Below you’ll find the answers to the design questions outlined earlier. It is important to emphasise that there is no single correct answer. The value of this exercise lies in understanding the reasoning behind each decision rather than strictly agreeing with every assumption.
Each geothermal project is unique, and the choices you make—regarding parameters, configurations, and thresholds—depend heavily on project-specific constraints, design priorities, and practical considerations. Use these answers as a guide, but don’t hesitate to challenge the assumptions and explore alternatives.
Question 1
The first challenge is to determine the borehole length, borehole depth, and buried depth in our specific project. The buried depth is defined as the level of the horizontal pipe connections relative to the ground surface. The active borehole length is the difference between the lowest point of the borehole and the buried depth. The borehole depth, on the other hand, is the difference between the lowest point of the borehole and the ground surface.
!Hinweis
If you have not read our article on this topic, you can find it hier.
In our case, the boreholes are constructed at a level of 0 m a.s.l. After drilling, the pipes are connected 3.5 m lower, as the pit is further excavated by 3 m and the pipes are installed an additional 0.5 m below that. Although one might be tempted to assume the buried depth is therefore 3.5 m, it is important to remember that this value must be defined relative to the ground surface, which is at 5 m a.s.l. The total buried depth is thus 8.5 m.
Next, if we want to define the borehole relative to the ground surface, we need the borehole depth. Although the drilling depth is 140 m, this is not the actual borehole depth, as it must be measured relative to the ground surface, which is 5 m higher. This brings the actual borehole depth to 145 m.
!Hinweis
For completeness, the borefield can also be defined relative to the buried depth. In this case, the borehole length is required. This parameter is not equal to 140 m, since 3.5 m of the borehole will be excavated away. The actual borehole length is therefore 136.5 m, which is exactly equal to the borehole depth minus the buried depth.
The temperature profile for this initial design is shown above. After 50 years, the temperature drops to -2.7°C, which is below the allowed minimum threshold of -2°C for this project.
Question 2
One question that may arise is what effect the buried depth has on this result, since other software tools, such as Earth Energy Designer (EED), do not require this parameter (as we discussed earlier in this article). Therefore, in this design variation, we set the buried depth to 0.7 m, which could, for example, represent a situation where the borefield is installed directly beneath the ground surface instead of under a building. The resulting temperature profile is shown below.
Although it is barely visible on the temperature plot above, the minimum average fluid temperature is now slightly higher (-2.35°C instead of -2.7°C). This is because, being closer to the ground surface, there is more regeneration from the environmental air temperature to the ground, resulting in a lower temperature drift.
On the other hand, the deepest point of the borefield (that is, the borehole depth) is now also closer to the surface, so the average ground temperature is 13.45°C in this case, compared with 13.84°C previously. The fact that the ground temperature is lower while the fluid temperature is higher (which might seem unexpected) highlights the importance of considering buried depth in actual designs.
Question 3
As a last design variation, we will simulate the actual irregular borefield configuration. This you can easily do by importing the coordinates as we showed in this tutorial. This gives the temperature profile below.
!Hinweis
When working with custom or irregular configurations in GHEtool, your exact input is used for the design. In contrast to other design tools, GHEtool does not convert the irregular configuration into a traditional rectangular design that only resembles your layout, but instead designs directly with your actual project configuration.
We now see that the minimum average fluid temperature is -1.69°C, which is above the temperature threshold of -2°C and significantly higher than the -2.7°C obtained with the traditional rectangular configuration. When using the actual configuration of the boreholes, the borefield is correctly sized (and perhaps even slightly oversized), whereas the initial design would have been undersized.
Fazit
In this exercise, a borefield with an irregular borehole configuration was examined. It was found that, compared with a traditional rectangular field, the actual irregular configuration resulted in an average fluid temperature that was 1°C higher, which could lead to significant savings in investment cost. In addition, the influence of the buried depth was demonstrated.
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