We are often asked: "Isn't the influence of the center bar on the performance of the separatus probe too great? Will it cause a thermal short circuit?" In Part 2 of this double article, we will discuss in detail research results and evaluations from real-life operation of the separatus geothermal probe. We will show how to deal with this.
Thermal short circuit – briefly explained
In Part 1 of this series, we explained what a "thermal short circuit" is and its significance in the entire context of a fully installed geothermal probe. If you haven't read this article yet, we highly recommend it!
Briefly explained: There is a desired temperature difference between the supply and return lines in the geothermal probe. A certain thermal short-circuit effect always occurs with temperature differences. It's obvious that you want to minimize this effect as much as possible. But it's by far not the most important factor when it comes to the performance of a geothermal probe. This is where the cost-benefit ratio comes into play: What adjustments can be made to maximize the cost of a geothermal probe without overly compromising efficiency?
In general, a narrow bore diameter is beneficial for both the cost and efficiency of geothermal probes. Therefore, this must be the number one adjustment factor, making separatus the preferred system.
First things first: what can be said about the thermal short circuit in the separatus system from a research perspective?
Step 1: CFD simulations
The warm and cold streams are separated by a nearly 3 mm thick ridge, and heat is exchanged in both directions. Whether the ridge has any influence is beyond question. What matters is how much influence the central ridge in the pipe has on the overall performance of a geothermal probe. This question has been central to the development of splitpipe technology since the initial design phases.
We wanted to know how significant the differences between thermal short circuits and conventional probe designs are. Early on, it became apparent that additional insulation of the bridge had little impact on the effect and made production more challenging. The central bridge, made of PE material, thus forms the heat exchange surface.
CFD simulations were conducted by the Eastern Switzerland University of Applied Sciences (OST). The question was as follows: Assuming a probe consisting of two separate pipes with the same overall cross-section as the separatus split pipe, how much better would its heat transfer performance be? The answer: The difference between the split pipe and the separate pipes is only a few tenths of a degree. Scenarios were calculated with different flow velocities and viscosities. On average, the difference in heat transfer performance in these theoretical considerations was just approximately 7%.
This naturally raises the question: How much additional borehole length, if any, would be required to achieve this? This depends on many other boundary conditions, but in this case, it can be narrowed down to a few percent. Simulation programs such as EED and GHEtool can clearly demonstrate the significant impact of a change in borehole resistance. In conventional cases, a 7% difference in heat transfer performance results in a mere 2 to 3% difference in drilling meters.
Conclusion: Thermal short-circuiting is a factor discussed in theory, but in reality, it plays a practically negligible role in this application. Other factors have a much greater influence: geology, borehole construction, surface area, and flow regime. Heat absorption must therefore be considered as a whole when comparing different geothermal probe designs.
Overview of influences and their effects
| Positively influencing factors on the heat transfer of a separatus probe | Positively influencing factors on the heat transfer of a U-probe |
|---|---|
| Small drilling diameter, good connection to the ground | Reduced thermal short circuit |
| Less insulating backfill material | Larger pipe surface |
| Higher thermal conductivity of backfill material is possible | Larger brine volume |
| Series connection, thus turbulent flow | |
| Even after a long running time, no cold “dead space” inside between the pipes |
Experts agree that each of these factors influences the overall performance of a geothermal probe. It becomes difficult to assess the true magnitude of each individual influence. As the CFD simulations mentioned above show, the differences are sometimes so marginal that they are difficult to measure or confirm in practice. The short-term measurement methods available today were not developed for comparing probe designs and are therefore unsuitable because they do not reflect real-life operation with a heat pump.
Therefore, the crucial question is: What does it look like in practice?
Step 2: Practical test
We have subjected separatus to practical testing using several test systems. The longest-operating system is the "Pernter" reference system.
With this test system operating in real-world operation, we can test different probe designs under different flow rates and extraction rates. If the claim that a significant thermal short circuit occurs in the separatus split pipe were true, then this should also be visible in measurements at different temperature ranges.
The fact is that short-term fluctuations in the temperature spread across the probes don't really have a noticeable impact on the performance ratio. There are even documented measurement periods in which the ratio favored the separatus split-pipe probe despite a larger temperature difference. Therefore, other factors must be responsible for the performance difference.
Click here for the reference report.
That means?
So how should we deal with these findings? Should thermal short circuits be considered in the planning process? If so, how?
As already explained, other factors have a much greater influence on the design of a geothermal probe: borehole structure, surface, turbulence... The so-called "thermal borehole resistance" must be considered as a whole.
All of these factors are taken into account when professionally planning and designing a geothermal probe system. Naturally, thermal short-circuiting is also factored in. Our planning recommendation for "EED" provides practical guidance on this. The separatus geothermal system can be directly selected in the geothermal calculation program "GHEtool," which already takes all parameters into account.
Our recommendations are: First and foremost, the smallest possible bore diameter should be chosen. Slim boreholes reduce construction costs and simultaneously have a positive impact on thermal efficiency. At the same time, less backfill material is required, making thermally enhanced backfill affordable and practical. Furthermore, separatus should always be designed with turbulent flow conditions in mind, because thanks to the simple series connection, sufficient flow velocity can be achieved even with very short boreholes and very low flow rates.
If these basic principles are observed, the usual tables from design guidelines such as VDI 4640 or SIA 384-6 can also be used for simple, smaller systems. This is because efficiency differences between separatus and double-U probes are then almost nonexistent. Differences in one direction or the other are within the normal range of fluctuation for any system. However, significant differences in construction costs are certainly noticeable...