In this chapter, we will provide you with the answers to the question at the end of each chapter of the second part or the course.
Question 1.1
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I want to make a monthly temperature simulation where there is 8000 kWh cooling in the summer, but this is purely baseload, without any peaks. What will the temperature profile look like?
When there are no peak powers, the energy for each month is delivered at baseload power, which is the smallest power that can deliver the demand energy during 730 hours (i.e. the number of hours in each month). In this case, the fluid temperature during cooling will equal the baseload temperature in the summer months. This is shown in the temperature profile below.
Question 1.2
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Imagine you have simulated your borefield with an hourly load with an initial undisturbed ground temperature of 11°C and you find that your minimum average fluid temperature is 0.2°C. You now do a TRT and the ground temperature turns out to be 11.5°C. How would this impact the results of your simulation?
All geothermal borefield simulations start with a certain undisturbed ground temperature. In this case, the real undisturbed ground temperature is 0.5°C higher than expected, meaning that our borehole wall temperature is also 0.5°C higher. Since the fluid temperature is connected to the borehole wall temperature, these temperatures will also rise by 0.5°C.
Question 2.1
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In order to save money, I want to work with a single U tube instead of a double one. Would this have an effect on the effective borehole thermal resistance and/or our total borehole length?
There is no definitive answer to this question, since it highly depends on the flow rate and whether the heat transfer is laminar or turbulent. If the double U probe is laminar and switching to a single U makes the fluid turbulent, chances are that your effective borehole resistance will be better than in the original situation and that you can get away with fewer borehole metres. If, by switching to a single U probe, your fluid remains laminar, your effective borehole thermal resistance will definitely be larger, requiring more borehole metres.
On the other hand, if you have fewer but deeper boreholes, your flow rate is typically higher (towards 1 l/s per borehole). In that case, your double U probe is probably turbulent. Switching to a single U probe makes the fluid even more turbulent, but this no longer has a significant impact on the heat transfer. Since the total heat transfer area is now smaller, the single U probe will perform worse in this case than the double U.
Question 2.2
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Imagine you are drilling you borehole in solid rock. Legal aspects aside, would you rather grout your borehole or just leave it open so it will be filled with groundwater?
There are both pros and cons when it comes to using groundwater filled boreholes. If you have a load profile that stays away from freezing temperatures, you could benefit from the buoyancy effect in the borehole itself. This enhances the heat transfer and will give you an effective borehole thermal resistance quite close to, or even better than, a grouted borehole, but at a lower investment cost.
In contrast, if your fluid temperatures become quite cold so that the borehole starts freezing, this buoyancy effect disappears and you will end up with a filling conductivity of 0.6 W/(mK), lowering your effective borehole thermal resistance. Chances are that you will have to drill deeper to compensate for that effect.
The question of whether the deeper, water filled borehole is more economical than the grouted shallower one highly depends on your economic context and the particular load profile of your project.
Question 3.1
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My borefield is perfectly balanced, but I have quite a high groundwater flow. Would this have a (positive/negative) effect on my design?
Groundwater flow is particularly useful when you have an imbalance, since it can take part of this imbalance away. However, in this case there is no imbalance, so there is no long-term temperature drift that could be mitigated by groundwater flow. But this does not mean that groundwater has no influence on the borefield.
As mentioned before, borefields are a way to store energy over the seasons, with the fluid temperatures in heating and cooling being influenced by the energy that was injected or extracted in the previous season. If groundwater flow displaces some of the cold stored in winter, this means that the borehole wall temperature, and hence the fluid temperatures, will be higher in summer.
The same holds for the stored heat from summer. If part of it is taken away by the groundwater, the borehole wall temperature will be lower, lowering the fluid temperatures.
This could not only cause an increase in the required borefield size, since now our temperatures are closer to both limits, but also decrease the overall efficiency of the system.
In short, having groundwater flow in a totally balanced system is rather disadvantageous. If there is an imbalance and the borefield is limited in the last year due to long-term temperature drift, groundwater flow would, in contrast, be beneficial.
Question 4.1
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Can you think of other ways to improve the design from the last question, still working with a single DN40 probe?
At the moment, all parameters related to the flow were varied, but we kept the grout thermal conductivity constant at 1.5 W/(mK) in all our simulations. If we increase this to a thermally enhanced grout of 2 W/(mK), our borehole resistance drops from 0.1257 mK/W to 0.1116 mK/W, lowering our maximum average fluid temperature to 16.86°C.
Other, perhaps more far stretched solutions might be to decrease the drilling diameter (effectively decreasing the grout resistance that way) or to increase the pipe diameter even further to a DN45 (or higher, as long as you stay turbulent) to increase the heat transfer surface area.
Question 4.2
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We discussed in Part 1.3 that the assumption of a linear geothermal gradient is not that accurate, especially in city centres. Since the project is located in the city of Ghent, chances are that there is indeed some urban heat island effect and the ground temperature is actually warmer in the first layers. How would you take this into account and what would be the effect on your borefield size?
At a borehole depth of 100 m, the average undisturbed ground temperature is 11.87°C. If we want to build in some safety in our design, we can set the average undisturbed ground temperature to, for example, 12.5°C in the ground data tab. This increases our maximum average fluid temperature by 0.63°C, which needs to be compensated for by increasing the number of boreholes again. In our situation, we needed 30 extra boreholes of 100 m, for a total of 225 boreholes, compared to the 195 boreholes in Question 7.
Question 4.3
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We now assumed that the simulation started in January. Would the result be different for a simulation starting in June? Why?
Borefields store energy over the seasons. By starting in January, the borefield is first cooled down before going into summer. If we were to start the simulation in June, the borefield temperature would be higher since it would not yet have been cooled down by the extraction load in winter. Since our borefield is limited by the cooling demand, this would increase the average fluid temperature for Scenario 7 from 17.11°C for a start in January to 17.42°C for a simulation started in June. The temperature profile is given below.
Downloads
- Download GHEtool simulation from this chapter here.