Individual and collective borefields

Geothermal borefields can be used in many different scenarios, but one general distinction is between individual and collective geothermal systems. In this chapter, three different types of geothermal systems, from fully individual to fully collective are discussed together with when one could be a better choice than the other. The distinctions in this chapter will help to understand the concept of simultaneity and interference next.

Three types of geothermal borefield systems

Whenever you have a project with different units or buildings, such as a newly built neighbourhood or an apartment building that you want to provide with geothermal heating and cooling, you are faced with a question: do we install a collective or an individual system? In the image below, three general types of solutions are shown.

On the one extreme end, there is the fully individual solution (left). Here, each apartment or building unit has its own production of heating and cooling (i.e. its own ground-source heat pump) which is connected to its own separate borehole or borefield.

On the other extreme, there is the fully collective solution (right). Here, the heating and cooling production are centralised and distributed to the different building units. When the units are located in separate buildings (for example a whole neighbourhood), this solution becomes the traditional heating and cooling network.

The last option is to work with individual production units for heating and cooling that are connected to a centralised borefield. This solution, which gained a lot of interest lately in regions like the Netherlands, is similar to what is called fifth-generation district heating and cooling (5DHC), where the geothermal temperatures are shared among different users who can decide themselves if they want to use it for heating or cooling.

All three of the options above have their own use cases, advantages and disadvantages. In the following sections each of these options will be discussed in more detail, focusing on the geothermal design aspects (peak power and imbalance) as well as general concerns (cost, maintenance and societal effects).

Individual production, individual borefield

When all your buildings or building units (like apartments) have their own individual ground-source heat pump system, the design is identical to what we have been doing before in this course. Conceptually, you need to make sure that each geothermal borehole (or borefield) can deliver 100% of the peak loads of the building as well as the yearly energy demand. The combination of these effects can put quite a lot of pressure on the design of the individual borefields.

In contrast to collective geothermal systems, individual boreholes or borefields are often placed further away from each other than in the case of a traditional borefield with a fixed borehole-to-borehole spacing. Imagine, for example, a neighbourhood where each individual house has its own geothermal borehole or boreholes. Typically, these boreholes would be further apart than if they were centralised in a single field. This larger spacing can help to cope with the imbalance, as discussed in Part 5.5.

Nevertheless, the fact that your borefields are connected to individual installations does not mean that they are completely separate from each other. Just like the different boreholes in a single field influence each other, so does any pair of geothermal borefields have a certain degree of thermal interference. This will be discussed in Part 8.2.

Conclusion

In summary, one can say that the total borehole length required for a fully individual system is governed by three factors:

• A higher resulting peak power (100% of the building load).
• A higher geothermal imbalance.
• A potentially lower influence of the imbalance, due to the borehole spacing.

Although the relative importance of these three factors can vary from project to project, one can say that in the case of apartment buildings, where the boreholes are relatively close together due to the limited space, an individual solution will lead to more borehole metres. In the case of a neighbourhood, where the boreholes are spaced further apart, the individual solution can be slightly smaller.

With regard to maintenance costs, these are relatively high for this type of solution, since servicing multiple heat pumps leads to higher cumulative expenses. In addition, as every building has its own heating and cooling unit, this will result in higher electrical peak loads on the grid, requiring greater capacity (as discussed in Part 1.1).

Individual production, collective borefield

A second solution which is increasingly popular is to decentralise the ground-source heat pumps but to connect them to a single borefield. This could be especially interesting in cases where, due to, for example, space restrictions, the boreholes would already be rather close, or in the context of a fifth-generation district heating network with both heating- and cooling-dominated buildings. There are two major benefits when working with a centralised borefield:

1. Since all the units are now coupled with the same geothermal source, only the cumulative imbalance needs to be considered for the borefield design. This imbalance will always be equal to or smaller than the sum of the individual imbalances of each unit, for example in the case of a multi-level building with a shop downstairs with a high cooling demand and residential apartments on top. The resulting borefield will therefore be more balanced than in the individual case where each borehole should be able to cope with the individual imbalance.
2. Since not all heat pumps work at the same time, the resulting peak power on the ground can also be smaller. This effect is called simultaneity and will be discussed later in Part 8.3.

Conclusion

When working with a collective borefield, a lower peak power and imbalance can potentially be considered, leading to a more economical design. However, if the boreholes are now closer to one another than in the individual case, the influence of the imbalance might be greater. The final decision on whether this is beneficial or not ultimately depends on the specific project.

Generally speaking, for apartments where the boreholes are already quite close together, a collective borefield is likely to be cost-efficient. For neighbourhoods, this is usually not the case, due to the high investment cost of the distribution pipes for the 5th-generation district heating and cooling network, unless the imbalance of the entire system is greatly reduced due to the collective nature.

Since there are still multiple heat pumps installed, the cumulative maintenance cost of this solution remains relatively high, as does the total required peak electricity power on the electricity grid. Because simultaneity cannot be guaranteed and regeneration is somewhat more complex in this case, the borefield is generally larger than in the following option.

Collective production, collective borefield

The final option is to opt for a centralised heating and cooling production coupled to a centralised borefield. This is basically the traditional concept of a district heating and cooling network with a centralised production system. Due to the centralised nature of the source, the design can benefit from an overall typically smaller imbalance compared to the individual designs.

However, in contrast to the collective borefield with individual heat pumps, having a centralised production system means that we can now be sure of our lower peak power, both thermally and hydraulically, since this centralised heat pump will typically be smaller than the sum of all the individual installations.

Due to the centralised design, the boreholes are likely to be closer together, which can increase the effect of imbalance. Nevertheless, since we also have a centralised production unit, it becomes relatively easy to mitigate this by creating a hybrid system or incorporating regeneration, as will be discussed in Parts 9 and 10.

Conclusion

Having a fully collective system will result in lower peak power demands on the borefield and a typically lower imbalance. Therefore, when compared to the case with individual production units and a collective borefield, this solution will generally lead to smaller geothermal systems, although it still depends on the specific project and borefield geometry. In general, this solution tends to be more cost-efficient for apartments, while it may be more expensive for neighbourhoods due to the need for insulated distribution pipes for heating and cooling in the 4th-generation district heating and cooling.

Since we now have only a single system to maintain, the cumulative maintenance cost is lower and the required electrical capacity from the grid is also reduced. Collective systems are easier to optimise and are therefore sometimes the preferred option of an Energy Service Company (ESCO).

Conclusion

In this chapter, three different geothermal solutions were discussed: the fully individual system (where both the heat pump and boreholes are individual), the fully collective system (with collective heat pump and a shared borefield), and a combined solution, where separate heat pumps per unit are connected to a central geothermal source.

Although the specific cost savings of moving from individual to collective systems may depend on the characteristics of each project, moving towards a collective borefield offers the advantage of working with a combined imbalance rather than addressing it on a building-by-building basis. In addition, centralising production can help reduce maintenance costs, limit the peak power on the borefield, and lower the required electrical capacity.

In the next chapter, the concept of interference between different, hydraulically separated geothermal systems will be discussed.

Questions

References

• Lund, H, Werner, S., Wiltshire, R., Svendsen, S., Thorsen, J.E., Hvelplund, F.,Mathiesen, B.V. (2014). 4th Generation District Heating (4GDH): Integrating smart thermal grids into future sustainable energy systems, Energy, Volume 68, Pages 1-11, 0360-5442, https://doi.org/10.1016/j.energy.2014.02.089.