In this chapter, we will provide you with the answers to the question at the end of each chapter of the eight part or the course.
Question 1.1
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A residential building is being constructed next to an office building. Which of the three options in this chapter would you suggest at first glance?
A residential building and an office building typically have completely different load profiles: offices are used during the workdays, and residential buildings require their heating and cooling mostly in the morning, evening and weekends. But besides that, an office typically requires much more cooling than heating, so the imbalance is lower or even injection-dominated, whereas residential buildings (of course depending on your area) are typically extraction-dominated due to the higher heating and domestic hot water demand.
Therefore, connecting both to the same geothermal borefield would lower the overall imbalance and hence the required borefield size. It is probably easier to still work with individual heat pumps (at least one for the residential part and one for the office building), since both will probably have a completely different ownership structure and keeping the heat pumps separate can be the easiest way.
Question 1.2
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A new residential area is being constructed with 10 smaller two-storey buildings, each with 4 apartments, and 1 bigger six-storey building with 12 units. Would you advise an individual or collective system?
In this case, it is probably best to go for a semi-collective solution where each building has its own collective borefield. In the case of the 4 apartments, it could make sense to work with individual heat pumps, since there is not much to be saved when working with a centralised heat pump, due to the low degree of simultaneity. Working with small collective systems also has the advantage that, if one apartment has a significantly different use profile (for some reason), the load is spread out and averaged across the different boreholes.
In the building with 12 apartments, opting for a centralised heat pump can already be useful, but probably not yet, due to the smaller scale of the collectiveness.
Question 1.3
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Imagine that an energy service company (ESCO) will invest in, maintain and operate all the ground-source heat pumps together with the borefields. Given this central owner of all the systems, would your answer to the previous question change?
When there is a central owner of the system, the maintenance cost and complexity also play a role. Here, in the case of 12 apartments, it could be more beneficial to have one centralised heat pump to maintain instead of 12 individual ones. Also from a cost and investment perspective, having a centralised production unit in this building can be beneficial. For 4 units, it will depend on whether there is a central room to install the centralised heat pump.
Since all buildings have the same load profile (all of them are residential), there is not much to be gained by connecting them to a single geothermal borefield. Therefore, the ESCO will probably opt for collective solutions per building, since the cost of a horizontal distribution grid will probably not be economically interesting at this scale. This would, for example, be different when there is also a different type of building (for example, an office) nearby that can change the imbalance for the better.
Question 2.1
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After the increase of the borefield sizes in the first case, why is it that the interference becomes less?
There are two aspects that play a role here:
- Since the borefields are now larger, the specific heat extraction rate per borehole decreases, since the imbalance is now divided across more boreholes, making the contribution of each borehole to the overall interference slightly less.
- Since the extra boreholes are slightly further away from the other borefields, they experience a slightly lower interference from these fields, lowering the overall interference of that borefield.
Question 2.2
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What would be the effect on the design if the three hydraulically separated boreholes in the first case were combined into one borefield? You can assume the boreholes stay in their place, but they will now be hydraulically connected as in the case of a 5th-generation district heating and cooling grid.
When all the boreholes are connected into a single geothermal system, the initial design of 76 boreholes in total gives a minimum average fluid temperature of 1.18°C. This implies that the original number of boreholes is even too large for the entire project, whereas it was too small when the interference was accounted for. How is this possible?
The reason is that when all the boreholes are connected in a single geothermal borefield, the contribution of each borehole to the entire heat transfer changes. When interference is considered, each borefield is still calculated as a stand-alone system, with the boreholes that contribute the most located at the boundaries of the borefield. When all boreholes are hydraulically connected, the boreholes that contribute the most are still the ones on the boundary, but now on the boundary of the entire system. This significantly changes the way the energy is exchanged with the ground.
Question 2.3
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Run the interference calculation for the neighbourhood again, but now with only one borehole per system located at the geometric centre of the field. You can download the CSV file for this below.
The interference is in this case slightly higher than when the two boreholes are considered individually. For example, system 1 now has a total interference of -0.32°C instead of -0.31°C. This shows that the assumption of the geometric centre for small geothermal systems is rather accurate.
Question 2.4
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What would change if, instead of using the finite line source, the infinite line source was used to model the interference? Do this for the first and the second case.
In both cases, when the infinite line source is used, the total interference increases significantly. In the case of the three apartment buildings, the interference goes to -1.61°C (instead of -0.74°C). For the 20 houses, the difference is smaller (-0.68°C instead of -0.48°C for house 6), since here the boreholes are separated further from each other.
Question 3.1
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How does the required borefield size change when considering this simultaneity with the heuristic?
When the required borefield size and depth are recalculated, 56 boreholes (in a 14×4 grid, with equal spacings of 7 m) with a borehole depth of 184.41 m are found. This reduces the total required borehole length from 13112 m to 10271 m.
Question 3.2
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How does the required borefield size change when considering this simultaneity with the load shift?
When the same is done for the borefield with simultaneity and load shifting, the same 14×4 borefield with spacings of 7 m is found, but now with a slightly deeper borehole depth of 188.71 m. This brings the total borehole length to 10512 m, which is more than in the case of the heuristic.
This is in line with the temperature simulation of the hourly profile with simultaneity when compared to the monthly simulation. Here, the hourly simulation also gave slightly lower temperatures, indicating that a larger borefield is required.
Question 3.3
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How would the temperature profile change if a maximum simultaneity of 70% is assumed? Would this change the required borefield size?
When the simultaneity is reduced to 70%, the resulting heating power is 190.04 kW and the resulting cooling power is 112 kW. This leads to a lower minimum average fluid temperature of 0.47°C when compared to the one with the higher simultaneity of 0.67°C. This is because the peak power on the borefield is now (slightly) larger.
This also results in a different required sizing, with again 14×4 boreholes, spaced 7 m apart, but now with a depth of 192.85 m, resulting in a total borehole length of 10743 m, which is indeed higher than the 10512 m from before.