Exercise on designing a borefield for a single building

In this exercise, we will investigate how you can design a borefield for a single family building. The goal is to learn the difference between designing for the heating or cooling demand and to see the difference in required borehole length when working with either active or passive cooling. Next to that, we will stress the difference between designing for a modulating or an on/off heat pump and the importance of the flow rate.

The exercise

The case for this exercise is based on a real-life single-family building located in the city of Antwerp (Belgium). To design a suitable borefield for this building, you’ll need to draw on knowledge from a range of previous articles (which we’ll reference where necessary). Throughout this exercise, you’ll explore the different questions that arise when designing a borefield for a smaller project. Typically, geothermal borefields were designed to cope with the heating demand of the building, where cooling was seen as ‘a nice to have’. However, nowadays, with the increasing demand for summer comfort, this is becoming more and more important. In this exercise, you’ll familiarise yourself with these design questions for a single-family building.

!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 (passive cooling) | 25°C (active cooling)

!Note
These temperatures are selected to ensure the cooling can operate in a passive or free mode (hence the upper limit of 17°C). The lower limit of 2°C is set to avoid negative temperatures within the borehole. An average fluid temperature of 2°C typically corresponds to a supply temperature of 0°C and a return temperature of 4°C, assuming a temperature difference ($\Delta T$) of 4°C.

• Simulation period: 40 years
• First month of the simulation: January

Ground input parameters

• 1.9 W/(mK) from 50 – 200 m
• Volumetric heat capacity: 2.4 MJ/(m³K)
• Location: ‘Bel-Antwerpen’

Borefield input parameters

All the borefield in this exercise are on a line, with an equal borehole spacing in length and width of 6 m. The buried depth is 1 m and the initial, starting configuration is 1 x 3 boreholes with a borehole depth of 90 m.

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: 1.8 W/(mK)

The fluid is 25 v/v% MPG with a 0.7 l/s flow rate for the entire borefield.

!Note
In a future article, we will discuss in detail how you can calculate the flow rate through your system. One way is to look at the technical documentation of your heat pump and check what the rated flow rate is.

Thermal load input parameters

• Building peak heating demand: 9.2 kW
• Modulating heat pump of 12 kW
• Yearly heating demand: 12 MWh
• Building cooling demand: 8.7 kW
• Yearly cooling demand: 6.1 MWh
• Yearly domestic hot water demand: 2.1 MWh
• SCOP: 4.87 (heating)
• SCOP: 3.13 (DHW)
• SEER: 20 (passive cooling)
• SEER: 6 (active 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. Given the original borefield design of 1 × 3 boreholes @ 90 m, can the heating and cooling demand be met?
2. What is the required total borehole length if we design the borefield such that the cooling demand can be met with passive cooling?
3. How can we reduce the total borehole length?
4. How does the design change if, instead of passive cooling, active geothermal cooling is used?
5. If we use an on/off heat pump instead of a modulating one, would that change our design?

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

With a total borehole length of 267 m, this first design is a traditional design for shallow geothermal borefields for a residential building. It is based on the heating demand and will always stay above the minimum threshold of 2°C (the actual minimum appears to be 2.22°C).

!Note
One might expect that 3 boreholes of 90 m would yield a total borehole length of 270 m, rather than 267 m. However, there is a difference between borehole length and borehole depth. Since there is a buried depth of 1 m, the active borehole length is actually 89 m instead of 90 m, which explains the difference.

Using the input parameters we had, we see that the borehole has a laminar flow regime during both heating and cooling, so the effective borehole thermal resistance is more or less equal (more information about this can be found here). With this design, we actually see that we are not able to provide the required summer comfort, as the maximum average fluid temperature of 20.42°C significantly exceeds our limit of 17°C for passive cooling.

!Note
Nowadays, more and more borefields are designed with a minimum temperature of 0°C. Although such a system might function adequately, it does not account for thermal interference between different boreholes. Moreover, with fewer borehole metres, the peak temperature during cooling would be even higher (more information here). For a more robust, safe, and highly efficient design, we therefore recommend designing with a minimum temperature of 2°C.

Question 2

To find the required borefield size that is also able to cool our building, one can use the ‘calculate required depth’ aim in GHEtool for the same 3 boreholes. When we do so, you’ll encounter a gradient error (more information about this can be found in our separate article). Since 90 m was not sufficient to meet the cooling demand, deeper drilling is required. However, as this also increases the ground temperature, it may occur that no feasible solution exists with just 3 boreholes.

When we use 4 boreholes instead of 3, the required borehole depth can be calculated and is just under 150 m. Now, the fluid temperature stays below the maximum limit of 17°C, but we need a total borehole length of 590 m instead of the 267 m from before.

There are two major reasons why the difference between our first design and this passive one is so significant. Firstly, since we are now drilling deeper, the ground temperature is approximately 1°C higher, making passive cooling more difficult (and requiring even more borehole length). Secondly, as the flow rate is now divided across four instead of three boreholes, the effective borehole resistance increased from 0.1308 mK/W to 0.1630 mK/W, making it harder for the borefield to exchange energy with the ground.

!Caution
If you work with a flow rate per borehole, switching from three to four boreholes would imply a 33% increase in the total flow rate through the system. However, this does not happen in practice, as the flow rate is determined by the building’s demand. Therefore, when changing the number of boreholes (and in general), it is better to work with a flow rate for the entire borefield. In a future article, we will cover the calculation of the flow rate in more detail.

Question 3

One way to try to reduce the total borehole length is to adjust the flow rate. However, since the total flow rate is fixed, the only option is to connect the boreholes in pairs of two in series. In this configuration, the total flow rate is divided by two instead of by four, as is the case when all boreholes are connected in parallel (read this article for more information).

When we do this, our Reynolds number is now 3016 during cooling (i.e. injection), giving us a significantly better effective borehole thermal resistance of 0.0846 mK/W, reducing the required borehole depth to just below 90 m. This brings the total borehole length to 354 m, which is more than in our first scenario, but significantly less than in the previous one.

Question 4

Another way to cope with the high cooling demand is to use active cooling. If we do this, our design is very similar to the first scenario, due to the fact that both borefields are effectively designed for the heating demand (since the temperature limit of 25°C is not reached).

!Note
As we have discussed several times already (see for example this article), it is also possible to combine active and passive cooling. However, for residential projects, this is typically not done due to the higher investment cost.

!Note
Switching from active to passive cooling is not straightforward, as it may require different emission systems. For example, it is not advisable to circulate water at 12°C through a floor cooling system. Please discuss the capabilities of your emission system with your installer and work from there.

Question 5

Up until now, we have worked with a modulating heat pump of 12 kW, which had the advantage that we could design for a peak heating demand of 9.2 kW — which is what the building actually requires. However, if you use an on/off heat pump of 12 kW, the heat pump can only deliver 12 kW, so your geothermal borefield must be designed to cope with that full capacity.

When we take our original design and set the maximum power to 12 kW, we see that the temperature during heating now drops to 0.29°C — well below our 2°C threshold. This highlights the importance of knowing which type of heat pump will be installed and, if it is a modulating heat pump, ensuring the installer limits the rated power to match the actual building demand.

In general, it is true that oversizing the ground source heat pump will lead to an oversized borefield — and thus a significantly higher investment cost.

Conclusion

This exercise discussed the different design questions that arise when designing a borefield for a single-family building. Whereas borefields were previously sized solely to cope with the heating demand, taking into account the ever-increasing cooling demand can lead to significantly more borehole metres (and hence a higher investment cost). However, when clients expect a high level of summer comfort, this might be the preferred approach. For lower budgets, a possible solution is to opt for active geothermal cooling — provided the emission system allows for it.

References

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