Exercise on designing a collective borefield for an apartment building

When designing collective borefields for apartment buildings, the process is more complicated than simply multiplying the design for a single apartment by the number of units. In this exercise, we will explain how to approach the design of collective borefields and the role that the simultaneity factor plays.

The exercise

In our last exercise, we looked at how a borefield could be designed for a single building. One might be tempted, when it comes to collective systems, to design for one apartment and then multiply it by the total number of units to obtain a final design. However, there are some problems with this approach:

• The different boreholes interfere with each other, leading to different long term behaviour (read our article on the problem of thermal interference here).
• For collective systems, the resulting peak power on the borefield is not the same as the sum of all the peak demands, due to something called simultaneity (which you can read more about here).

For these reasons, the design of collective borefields deserves an exercise of its own, where we can dive into the intricacies of the process.

!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: 0°C
• Maximum average fluid temperature threshold: 17°C (passive cooling)
• Simulation period: 25 years
• First month of the simulation: January

Ground input parameters

• 2 W/(mK) from 50 – 200 m
• Volumetric heat capacity: 2.4 MJ/(m³K)
• Surface temperature: 9.6°C
• Geothermal heat flux: 0.07 W/m²

Borehole resistance input parameters

The parameters for the pipe are:

• Double 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: 35 mm
• Grout: 1.5 W/(mK)

The fluid is 25 v/v% MPG with a 0.21 kg/s flow rate for every apartment.

Thermal load input parameters (per apartment)

• Peak heating demand: 3.4 kW
• Peak cooling demand: 2 kW
• Yearly heating demand: 5.1 MWh
• Yearly cooling demand: 1.4 MWh
• Yearly domestic hot water demand: 1.6 MWh
• SCOP: 5 (heating)
• SCOP: 3 (DHW)
• SEER: 20 (passive cooling)

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.

1. Calculate the required borehole depth for the case with only a single apartment.
2. Calculate the temperature profile for this borehole depth when placed in a 20 by 4 grid (to obtain 80 boreholes in total).
3. Correct the borehole depth for these 80 boreholes to stay within the temperature limits.
4. Calculate the temperature profile taking into account simultaneity.
5. Calculate the temperature profile taking into account simultaneity and the change in peak duration.
6. Calculate the temperature profile using the upscaling method for collective systems.
7. Calculate the final required depth for 80 boreholes using the load profile from the previous question.

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

When the required depth is calculated for a single apartment, a borehole depth of 96 m is obtained. As shown in the temperature profile below, this depth is determined by the maximum cooling peak (2 kW) rather than the maximum heating load, since the minimum average fluid temperature of 1.45°C is well above the 0°C threshold. The slight imbalance of around 3.68 MWh in extraction is hence not a determining design factor in this case.

Question 2

For the design of the 80 apartments, an initial estimate is to create a grid of 20 by 4 boreholes, each with the same 96 m depth found earlier, and then simulate the temperature profile. The result is shown in the graph below, where the thermal demand is simply taken as 80 times that of a single apartment.

As you can see, the imbalance (now 80 times greater) is clearly visible in this case. The reason is that, with a full borefield instead of a single borehole, the boreholes interact with each other, leading to a more pronounced long term temperature effect. It is clear that the borehole depth needs to be increased to meet the requirements.

Question 3

When the borehole depth is increased to 150 m, the average fluid temperatures remain within the limits. It is also clear that the design is now determined by the heating demand rather than the cooling demand, due to the more pronounced role of the imbalance in the design.

The design above, which results in a total borehole length of 11,920 m, is a very conservative approach. It assumes that 100 percent of the peak heating demand (272 kW in total, being 80 × 3.4 kW) is placed on the borefield, and the same applies for cooling. This assumption is challenged in the follow up simulation.

Question 4

Simultaneity is the concept whereby different users connected to the same collective system do not experience their peak demand at the same time. For 80 users of a collective system, the total interference is around 63 percent (based on the research of Winter et al., 2001). This means that, of the total peak heating demand of 272 kW (and likewise for cooling), only 172 kW is expected to be the maximum power required from the geothermal system. More information on the simultaneity factor can be found in our article on this subject.

When simultaneity is taken into account, the same borefield design with a depth of 150 m now gives a minimum average fluid temperature of 1.74 °C, as shown below. This means that the borefield would be greatly oversized, since we are allowed to go as low as 0 °C. However, this solution may be too optimistic.

Question 5

When it comes to simultaneity, not only the resulting peak power is important but also the peak duration. As discussed earlier, this second aspect of simultaneity has not been researched as extensively as the reduction in collective peak power. The cost of a lower resulting peak power is an increased peak duration.

Previously, we proposed the hypothesis that the peak duration of a collective system scales proportionally to the square root of the number of connected users. In our case, this would mean that the peak duration of the collective system is about nine times longer than that of an individual apartment, giving a peak duration of 72 hours instead of 8. This results in the temperature profile shown below.

!Caution
Please note that this heuristic is not backed up by literature. It is a first estimate based on the Central Limit Theorem in statistics. The approach in the next question provides a more robust way of addressing the peak duration.

In the temperature profile above, the minimum average fluid temperature is 1.39 °C, which is already lower than in the previous simulation but still indicates some oversizing. However, working with the heuristic scaling of the peak duration is somewhat arbitrary. For this reason, in the next simulation we will use the newest method in GHEtool to generate an hourly profile for collective systems.

Question 6

In one of our latest updates, it became possible to generate hourly load profiles in GHEtool Cloud (more information in this article). Hourly profiles have the advantage that there is no need to specify a peak duration, since at this time resolution the peak duration is implicit in the load profile.

Using this method to simulate the collective borefield, we obtain the temperature profile shown below, with a minimum average fluid temperature of 0.82 °C.

The minimum average fluid temperature when using this method differs from that in Question 6. This can be explained by examining the hourly load profile generated in GHEtool, shown below.

As can be seen, the peak power is limited to 172 kW (due to the simultaneity of 63 percent), which results in a longer peak duration. The peak in January lasts for almost 10 days, or about 240 hours. This is far longer than the 72 hours assumed earlier and explains why the fluid temperatures are lower with this approach than with the heuristic scaling of the peak duration.

!Note
It is difficult to determine which result or methodology is the most accurate, but creating an hourly load profile for collective systems offers a clear workflow and provides insights into the expected load profile that cannot be obtained when working with monthly loads and heuristic scaling of certain parameters. For this reason, we recommend using this methodology when working with collective systems.

Question 7

As a final exercise, the required borehole depth is calculated again, this time using the hourly load profile from the previous exercise. The result shows that the required depth is just under 140 m, saving about 800 m in total borehole length compared with the conservative approach in Question 3.

Conclusion

In this exercise, we explored the design of a borefield for an apartment building with 80 units. We learned that designing collective systems based solely on the design of a single unit leads to a significant underestimation of the required borehole length. On the other hand, assuming a resulting peak load equal to the sum of all demands results in an overestimation.

By applying the concept of simultaneity and using the new method to generate hourly load profiles in GHEtool Cloud, you can design more accurate, cost effective, and reliable collective borefields with confidence.

References

• Watch our video explanation over on our YouTube page by clicking here.