How to avoid imbalance?

Imbalance is a central and important aspect in the world of geothermal design. Last week, we discussed how you can cope with it when designing your borefield, but today we will take a step back and look at different ways to avoid it.

What is imbalance?

Imbalance is the yearly heating or cooling of the ground caused by a difference between the energy extracted from and injected into it on an annual basis. In that sense, it is entirely determined by your building’s energy demand and is something you simply have to learn to cope with. Below, you can see an example of an extraction dominated borefield.

The geothermal demand above translates into the extraction dominated borefield shown below. As you can see, the temperature drops year after year because more energy is extracted than injected. This places considerable stress on the minimum threshold at the end of the simulation period, as this becomes the critical design point. The greater the imbalance, the more borehole metres (and therefore investment) are required to cope with it.

!Note
Not all imbalance will by definition lead to more borehole metres and therefore higher investment costs. When there is a certain imbalance but your borefield is already limited in the first year due to a high peak power, it becomes less important. More information can be found in our article on the borefield quadrants.

Avoid or cope?

It is clear that imbalance can cause considerable challenges for your borefield design, and as with most challenges in life, you can either try to avoid it or learn to cope with it.

Last week, we looked at different ways how to cope with the imbalance once it was given. (You can find the article here.) This week, we will take a step back and look at the different design options available to avoid imbalance. This can be done in two ways: by considering the architectural decisions in the building or by using a more system-based approach. Both will be explained below.

Graphical representation of the origin of imbalance and where you can cope with it.

Architectural approach

As mentioned before, imbalance is a consequence of the building demand for heating, cooling, and domestic hot water, but those demands are not set in stone. Of course, aspects such as occupancy and comfort levels are rather strict and immutable, but at the architectural design stage of a project, there are several factors we can influence that will determine the final heating and cooling demand of the building. A few of these are discussed below.

Solar shades

One of the most common discussions in the architectural phase of a project is whether to install solar shades to prevent the building from overheating in summer.

Although this might be a good idea for comfort reasons, installing solar shading will decrease the cooling demand of the building, and when you have an extraction-dominated borefield (where the temperature will drop year after year), this is not ideal. In that situation, you actually want a higher cooling demand to help balance your borefield.

!Note
Instead of placing solar shades, it would be better to explore ways to keep the building cool without reducing the cooling demand itself, since that affects the imbalance. For example, you might use cool fans to help cope with the high cooling demand in summer. However, always make sure that your borefield can also handle the peak cooling demand.

Windows

Besides solar shades, the size and location of windows also have a significant influence on imbalance. First, the size of the windows has a double effect: it increases solar gains in summer (raising the cooling demand and thus the injection load on the borefield), but on the other hand, it also increases conductive heat losses in winter (raising the heating demand and thus the extraction load on the borefield).

The relative magnitude of these effects is determined by the positioning or location of the windows. A window on the northern façade of the building will increase the cooling load only slightly, whereas a window on the southern façade will cause a considerable increase in cooling demand. The heat losses in winter are less sensitive to orientation.

Besides the size and orientation of the windows, their U and g values also make a difference. The first parameter affects the insulation quality of the window (and therefore influences the heating or extraction demand), while the second affects solar gains (which influence the cooling or injection demand).

Floor material

A final topic that has a significant influence on the geothermal imbalance is the floor material. Both situations below use floor heating systems, but one is finished with ceramic tiles, while the other has a solid wooden floor.

Besides the difference in aesthetics, the thermal behaviour of both floors is extremely different. The ceramic floor is a very good conductor of heat, whereas the wooden floor acts more as an insulator. This means that if you want to use floor cooling, a ceramic floor will perform almost 50 percent better than a wooden one. This not only affects thermal comfort but also the imbalance, since a wooden floor will capture less of the cooling demand, resulting in a lower injection load on the borefield.

System approach

The options discussed above can be used to modify the building demand to some extent, but before we address imbalance in our geothermal design, there is another approach we can take to avoid it: the system approach. Below, both the options of hybridisation and regeneration are discussed.

Hybridisation

We design an HVAC system to provide the building with the required heating and cooling, but depending on the configuration, only a certain portion of this demand is met by the borefield. When we revisit the graph on imbalance, it becomes clear that the building has a higher heating demand than cooling demand (as seen by a higher extraction than injection of heat), which causes the imbalance.

One approach to balance the borefield is to take part of the building’s heating demand out of the geothermal system and assign it to another technology, such as an air to water heat pump, gas boiler, or electric heater. In this way, we do not alter the building demand itself, but we eliminate the geothermal imbalance by shifting part of the load elsewhere.

!Note
As mentioned in our article on the borefield quadrants, the injection and extraction loads are not identical to the building’s cooling and heating demands, since a heat pump operates between them. In addition, if domestic hot water is also produced using the ground source heat pump, part of the extraction load comes from that. For the purpose of this article, however, we abstract from this.

Example of a hybrid system

Although hybrid systems (as we discussed extensively in our articles on that topic) are typically used in larger installations, they can also appear in smaller residential projects. One example is a building with floor heating and cooling coupled to a geothermal heat pump, which becomes too hot in summer. To avoid overheating, the owners install a separate air conditioning split unit to meet the cooling demand. We then have a hybrid system where both a geothermal and an air source heat pump are present.

However, if the geothermal system is extraction dominated, the hybrid system described above will not be beneficial for the imbalance. The split unit can easily cool the room temperature to 21°C, leaving no cooling load for the floor cooling system. As a result, the amount of heat injected back into the borefield will be almost zero, potentially causing a significant increase in imbalance.

On the other hand, if the split unit is also used to assist with heating in winter, this can take part of the building’s heating demand away from the borefield, which would be beneficial for the borefield’s balance.

Regeneration

Another approach to reduce the imbalance on the borefield is regeneration. Here, instead of taking part of the borefield load away to compensate for the imbalance, we increase the opposing load. For example, in the case below, where extraction is higher than injection, we can increase the injection demand by installing dry coolers, solar absorbers, or similar systems. By doing so, the imbalance is again compensated.

!Caution
When regenerating your borefield, you are increasing the load on it. It is important to simulate whether your borefield can actually handle this additional load. For example, in the case above, if you were to install solar absorbers to inject extra heat into the ground, this would increase the injection power and therefore also the average fluid temperature inside the borehole. You need to ensure that you can still deliver the required cooling to your building — in other words, make sure that your regeneration does not interfere with actual building comfort.

!Coming soon
Next year, we will be releasing modules for regeneration (next to hybridisation) into GHEtool Cloud. Stay tuned for that!

Conclusion

Imbalance plays an important role in the work of a geothermal designer, and while you can try to cope with it during the borefield design stage, it is better to avoid imbalance altogether. In this article, we focused on two approaches to achieve this: the architectural approach and the system approach.

The first highlighted the importance of building design choices related to window size and placement, solar shading, and also the impact of floor type when working with floor heating and cooling. The second introduced the addition of another technology to the HVAC system to create either a hybrid system, which takes part of the load away from the borefield to keep it balanced, or a regenerated borefield, where we counteract the imbalance by injecting or extracting extra heat into or from it.

References

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