In this article, we will give the final perspective on the single or double U-tube debate, namely the practical aspects. Besides that, we will revisit the thermal and hydraulic aspects, but now look at some innovative probes such as the Separatus, TurboCollector, and GEROtherm VARIO and FLUX.
!Note
If you have not read the previous two articles in this series, you can find the one on the thermal aspects here and the one on hydraulic aspects here.
Single or double? That is the question
In the world of geothermal design, few topics seem as sensitive or as likely to spark debate as the question of using a single or a double U-tube. In the last two weeks, we already explored this question from the thermal perspective, looking at the effective borehole thermal resistance. Here, we found that we needed to simulate our specific situation in order to know which one was actually performing better.
Last week, we talked about the hydraulic aspects and what the pressure drop of both a single and a double U is. Here, we learned that when working with the same diameters, a single U-tube always performs worse compared to the double U. When we compared single and double U-tubes with different diameters, the situation became more complicated, and case specific simulations were required.
Today, we take a look at the final piece of the puzzle: the practical aspects as well as some innovative probe designs that could add some extra nuance to the debate.
Practical aspects
Many drillers prefer to work with a single U-tube over a double one, due to a variety of reasons, which we have classified into two categories: single U-tubes are cheaper and single U-probes are easier to install. Both are explained below.
Single U-tubes are cheaper
When comparing single to double U-tubes, the single ones are typically cheaper due to several reasons:
- Since we only have half the number of probes, the material cost for the probes is lower.
- Since single U-tubes have less internal volume than double U-tubes, we can also save on the amount of antifreeze needed for the system.
- The counterbalance weight that is used to pull the probes down into the borehole can be smaller for a single U than for a double, since there is less PE material to pull down.
- The number of welds is also lower when using a single U-tube instead of a double one, leading to time savings and a reduction in material cost as well. Besides that, since there are fewer welds, there is also a reduced risk of leakages.
Single U-tubes are easier to install
When you drill a borehole, there is only limited space to install a geothermal probe. It is not hard to imagine that given the available space, it is easier to let two pipes (that is, a single U-tube) down the borehole instead of four (for a double U-tube). In the first case, there is more room left for the pipes, meaning they go down faster, leading to shorter installation time and therefore lower installation cost. Besides that, there is less risk of a collapse of the borehole due to the faster installation, increasing the chance of a good installation.
The last aspect is that since there is more space available in the borehole, it is also easier to backfill it with grout, contributing to a well-functioning system.
Conclusion practical aspects
For the first time in this series, there seems to be a clear winner. When looking at the practical reasons for installing a single U-tube or a double U-tube, the single one comes out as a clear winner.
Special probes
To end this series, let us take a closer look at some innovative or special probes and at the (other) insights they might bring to the debate. In the following subsections, we will discuss the separatus, the TurboCollector, and the conical GEROtherm VARIO and FLUX probes.
!Note
Unless stated otherwise, a DN32 pipe, a borehole diameter of 140 mm with a length of 100 m and a grout with a thermal conductivity of 1.5 W/(mK) are assumed in this article.
Separatus
The separatus heat exchanger is a single pipe (DN50) in which a membrane (the separator) is installed. Due to this membrane, both the entering and returning flow can take place in the same pipe, making it even easier to install than a single U-tube, since there is now only one pipe going down instead of two.
!Note
If you have not read our article on the model development of the separatus probe, you can find it here.
The separatus probe (as discussed in an earlier article) can be compared with a single U-tube in terms of both thermal and hydraulic behaviour. In the figure below, a comparison is shown for the effective borehole thermal resistance of both a single and a double DN32, as well as a separatus probe.
In the graph above, it is clear that the separatus transitions, just like the single DN32 (albeit somewhat later), towards a turbulent state sooner than the double DN32 counterpart. This significantly reduces the borehole resistance, but not enough to come close to the performance of the single U-tube, let alone the double one. The reason is that the separatus, since both the entering and returning fluid are close together, has quite a lot of resistance in the grout (as can be seen in the illustration below).
When the separatus is installed in a smaller borehole of, let us say, 90 mm, the grout resistance is significantly lower, leading to the revised graph below. Here, one can see that the entire curve has moved downward when going from a 140 mm diameter to a 90 mm diameter, even outperforming the double U-tube in a flow range between 0.27 and 0.48 l/s.
When we look at the pressure drop of these different probes, we see that the separatus is somewhere in the middle between a traditional single DN32 and a double one.
TurboCollector
Our next innovative probe is the TurboCollector from Muovitech. This is a traditional smooth pipe in which clockwise and counter clockwise rotated fins are installed to enhance turbulence at a lower flow rate.
!Note
If you have not read our article on the model development of the TurboCollector, you can find it here.
The graph below shows the effective borehole thermal resistance for the smooth single and double DN32 U-tube, as well as the single DN32 TurboCollector. As can be seen, the single smooth and TurboCollector pipes follow similar trajectories, with the TurboCollector transitioning sooner into a turbulent state (Reynolds number of 1800 instead of 2300).
In the graph below, the pressure drop is also shown. Here, it is clear that the TurboCollector has very similar hydraulic behaviour to the smooth pipe, except in the region where the smooth pipe is still laminar and the TurboCollector is already in the transient regime.
A final comparison we can make is between the double DN32 and the single DN40 TurboCollector. In the graph below, this line is added from the thermal perspective.
One can see that this DN40 TurboCollector outperforms the smooth double DN32 option even more than the regular DN32 probe. This is because a DN40 probe has more surface area to transfer heat. The downside is that since we now have a larger diameter, the transition to turbulence starts at a slightly higher flow rate (about 0.2 l/s instead of 0.17 l/s), so we lose a small range in which the single (TurboCollector DN40) pipe performs better than the double DN32.
The other main advantage of switching from a DN32 to a DN40 is clearly visible in the hydraulic graph. As discussed in our previous article, the diameter has a very important effect on the resulting pressure drop, causing the single DN40 TurboCollector to have almost the same pressure drop as the smooth DN32 alternative.
GEROtherm VARIO and FLUX
The last innovative probes we want to discuss in this article are the conical GEROtherm VARIO and FLUX probes of HakaGerodur. The idea behind this pipe is that the wall thickness, which is required for a certain pressure rating, is only needed at the bottom of the borehole, not at the top. Therefore, they developed a conical design that has, on average, a smaller wall thickness, leading to a reduced pressure drop.
!Note
If you have not read our article on the model development of the GEROtherm VARIO and FLUX probes, you can find it here.!Note
Since these conical probes were developed for deeper boreholes, the assumptions in the following graphs were changed to a borehole length of 350 m, a diameter of 170 mm, and a pressure class of PN32 (unless stated otherwise).
The graph below shows the borehole thermal resistance of a single and a double DN40 U-tube compared to a single FLUX DN43. As can be seen, due to its slightly larger diameter, the FLUX probe begins the transition to a turbulent regime somewhat later, indicated by the first bending point. The second occurs when the top part (which, due to the conical design, has a larger inner diameter) also enters the transient flow range.
It can be seen that the FLUX probe has better thermal performance than the traditional alternative, since the conical design results in an average wall thickness that is slightly lower, reducing the conductive resistance through the pipe wall. Between 0.3 and 0.47 l/s (and also at very low flow rates), it has the best thermal performance.
!Note
Since this borehole is way longer, the general shape of the graph is also different and way steeper in the laminar regime.
In the hydraulic graph below, it is clear that due to its conical design, the pressure drop of the single FLUX DN43 probe already comes close to that of the traditional double DN40 PN32 option.
When we do the same approach as before and switch to an even larger diameter, now a DN53 PN38 FLUX probe, we obtain a thermal behaviour that is again slightly different. Due to the larger diameter, the transition to turbulence starts later, causing the range in which the single FLUX performs better than the double DN40 to shrink when switching from a FLUX DN43 to a DN53. However, the performance of this larger pipe is very close to that of a double U-tube at higher flow rates.
As a final graph, we have the hydraulic performance of the FLUX DN53 probe. From this, we see that this solution is, for all flow rates, the alternative with the lowest pressure drop. This means that this solution also has a range of flow rates for which it is both hydraulically and thermally beneficial when compared to the traditional double DN40 U-tubes.
Conclusion
This article provided the final perspective on the question: which one is better, the single or double U-tube? When looking at this question from a practical point of view, the single U-tube was always the preferred option, due to its easier and probably cheaper installation.
As a final nuance, we also considered innovative probe designs such as the separatus, TurboCollector, and GEROtherm VARIO and FLUX probes. Here, it was once again clear that the pipe diameter, flow rate, and in the case of the separatus, the borehole diameter, all play an important role in determining which solution performs better thermally and hydraulically.
After three weeks of discussing this topic, it should now be clear that there is no silver bullet. Every project is unique and therefore requires its own simulations and considerations: practical, thermal, and hydraulic alike. We hope that this series has given you some insight into the different viewpoints and that you now have a few starting points to carry out your own simulations and compare different design options using GHEtool Cloud.
References
- Watch our video explanation over on our YouTube page by clicking here.
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