Over the last chapters, we have talked about short- and long-term temperature effects and what influences them. In this chapter, we will use GHEtool Cloud to bring this theory into practice and learn about the effects of the temperature gradient in the ground, the type of antifreeze, the borefield configuration and so forth.
The exercise
The case for this exercise is based on a real-life office building located in the city of Ghent (Belgium). Throughout this exercise, you’ll explore the influence of the geothermal temperature gradient on the design, compare the impact of using MPG versus water as a heat transfer fluid, assess the choice between single and double U-tube configurations, and gain general insights into designing borefields for buildings with a high cooling demand.
Since this will be the first time in this course we’ll make a simulation, we will only use the traditional assumptions when designing borefields. This means that we are going to, purposely for now, disable all more advanced and accurate models in GHEtool. Later in this course, we will toggle them back on to clearly illustrate their added benefit.
Input parameters
Below, the different required input parameters are given to follow along with this simulation.
General input parameters
The simulation will be done with a minimum average fluid threshold of 2°C (so we have regime 0-4°C across the borefield and we can prevent local freezing) and a maximum average fluid temperature of 17°C (for passive cooling). The simulation will start in January and the simulation period will be 40 years.
Ground properties
We are going to work with a homogeneous ground layer, however, since in reality there are different layers at this location, we will work with a ground thermal conductivity of 1.6 W/(mK) when our borehole is 150 m deep and 1.7 W/(mK) when it is 100 m deep. The ground volumetric heat capacity is 2.4 MJ/(m³K) and for the temperature the database entry of ‘BEL-Gent’ can be used.
Borefield parameters
We will work with rectangular borefields in this exercise (although, feel free to make alternative configurations) with an equal borehole spacing in length and width direction of 6 m. The buried depth is 0.7 m and the initial, starting configuration will be 15 by 14 boreholes of 150 m borehole depth.
Borehole resistance parameters
For our initial simulation, a double DN32 U tube will be installed in the borehole, with a ground of 1.5 W/(mK), a distance of the pipe to the borehole centre of 35 mm and a borehole diameter of 140 mm. The heat transfer fluid is 25 v/v% of MPG and the total, constant, flow rate through the borefield is 35 l/s.
Thermal demand input
For the thermal demand, we will work with a monthly load profile as this will be a feasibility study with rough estimates. The peak heating and cooling demands are respectively 306 kW and 336 kW with a yearly energy demand of 398 MWh and 269 MWh. The seasonal efficiency of the heat pump is 4.5 for heating and our seasonal energy efficiency ratio for passive cooling is 20.
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.
- Given the original borefield design of 15×14 boreholes @150 m, is this a good design?
- Imagine there is a difficult ground layer at 110 m below surface. How many boreholes should we drill extra if we decrease the borehole depth to 100 m? Try to think about this first before you start simulating.
- What happens if we update the thermal conductivity of the ground to the correct value? Can we change the design?
- What happens to the temperature profile if we change the fluid to water?
- How can we redesign our borefield to be more cost-efficient?
- What happens to our design if we switch from a double U tube to a single U tube?
- What is the difference when we work with a single DN40 when compared to a single DN32?
The solution in GHEtool Cloud
Question 1
The temperature profile with the initial conditions, as simulated with GHEtool, can be seen in the figure below. There is absolutely no problem with the minimum average fluid temperature, since it stays 40 years above the 10°C. The maximum average fluid temperature is 17.19°C which is slightly above our threshold of 17°C.
The almost philosophical question is now: is this borefield designed correctly. Well, it depends. If you are quite sure about your peak power or it is perhaps somewhat on the smaller side, it is more important to stick to your temperature limits. However, in this case, we are doing a feasibility study of a big project with just first estimates. Probably, there is quite a lot of safety already in them, so crossing the temperature limit with just 0.19°C, is not that big of an issue.
In the end, engineering is all about making sure the system works by working with safety factors and depending on where you take some extra margin, the temperature limits can be more or less strict.
Question 2
When the maximum borehole depth is reduced to 100 m, your first instinct could be to increase the total number of boreholes by 30% in order to keep the same total borehole length. However, the temperature profile below only required us to add 15 boreholes, for a total of 225. The maximum average fluid temperature is here 17.18°C.
The main reason for this is that, due to our temperature gradient, the average undisturbed ground temperature is now 12.02°C, which is significantly lower than the 13.27°C in the previous question, shifting all the lines down.
Perhaps you noticed that the amplitude of the sinusoidal profile is significantly bigger in this case than in the previous simulation. If you recall our previous chapter on the effective borehole thermal resistance, we said that (besides the resistance), also the total borehole length is important.
In this case, we have significantly less borehole length, meaning that our specific heat injection is almost 30% higher. This increases the temperature difference between the borehole wall and the fluid temperatures. However, this effect is compensated with a better borehole resistance and a lower ground temperature.
Question 3
In the previous simulation, chances are that you forgot to change your ground thermal conductivity to 1.7 W/(mK), since now our depth has changed. (Congratulations if you did not forget this!)
This is also beneficial, so we again take away 15 boreholes and end up with the original 15 x 14 configuration but now with only 100 m of borehole depth instead of 150 m. The maximum fluid temperature during peak is 17.22°C.
Question 4
Up until now, all the simulations were carried out with 25 v/v% of MPG, which give us a freeze protection down to -11°C. However, looking at these profiles, this is absolutely not needed. If we change our fluid in GHEtool to water, we end up with a transient flow rate with a Reynolds number of 2442, which already has some turbulence. This decreases our effective borehole thermal resistance from 0.1547 mK/W down to 0.1273 mK/W. The maximum average fluid temperature is now 0.5°C lower than before: 16.76°C.
Question 5
When we are working with water, our minimum average fluid temperature is no longer 2°C but closer to 6°C (although it can differ depending on the requirements of your heat pump manufacturer). The borefield size could be reduced to just 15 x 13 boreholes due to the higher Reynolds number and hence lower borehole resistance.
Question 6
When we switch from a double DN32 U tube to a single DN32, the Reynolds number jumps to 5971, making the flow 100% turbulent and reducing the convective heat resistance. However, the borehole resistance increased from 0.1142 mK/W to 0.1358 mK/W. Since both our single and double U tubes were already rather turbulent (the double U was already far in the transient regime), there was not so much to be gained here. Since the single U has less contact area than the double U, overall our borehole resistance will be slightly worse.
The maximum average fluid temperature is now 17.29°C.
Question 7
As a last variation, the single DN32 was replaced by a single DN40. Here the flow is still turbulent (Re=4762), but the borehole resistance is again slightly better with 0.1257 mK/W, giving us a maximum average fluid temperature of 17.11°C. This is because the DN40 has a higher surface area, making it easier to transfer heat and since the flow remains turbulent, this is the most important parameter.
Conclusion
In this chapter, we made our first simulations in GHEtool Cloud for an office building. By making intelligent design choices, we were able to reduce the total borehole length with more than 30%. It is clear that all design choices (the borehole depth, the type of antifreeze, the number of U tubes …) all influence the borefield design.
In the next part, we will dive further into the borefield physics, taking a look at newer models for more accurate simulations.
Questions
Downloads
- Download GHEtool simulation from this chapter here.