Our experts are often asked: "Does it matter if the supply and return lines of a geothermal probe are close together? Don't the lines influence each other?" This often leads to questions like: "Isn't the bridge of the separatus split-pipe geothermal probe a disadvantage?" In Part 1 of this double article, we try to answer these questions as simply and clearly as possible.
What exactly is meant by “thermal short circuit”?
To answer this question, we cannot avoid a little theoretical study – we need to briefly explain the general functioning of a heat pump: On the one hand, it extracts energy from a source in order to increase the temperature in the house's heat distribution system on the other. To do this, a heat transfer medium (brine, water, air) must constantly circulate on the source side (the left side in the image below) to provide the required heat energy. On the source side, a distinction is made between open and closed systems. Open systems are heat pumps that openly draw in the ambient air or groundwater and release it back into it. With geothermal probes, on the other hand, the heat transfer medium circulates in a closed pipe system that absorbs the ambient heat via heat conduction.
The following problem arises, particularly in open systems: Too little distance between the intake and return ports leads to a short circuit. For example, this phenomenon occurs in air-source heat pumps when the cooled air is sucked back in immediately or with only a small detour. The medium never has the opportunity to warm itself sufficiently from the environment. The efficiency of such a heat pump collapses.
Does such a short circuit also occur in closed systems?
In the closed pipe system of the geothermal probe system, the medium cannot be directly sucked back in through a short circuit – it can only circulate the entire way through the geothermal probe until it returns to the heat pump in a heated state. However, the question always arises as to whether heat transfer via conduction between the cold and warm sides of the ground might still occur. This would also be tantamount to a thermal short circuit. Does this effect exist? What does science say?
For theoretical analysis, CFD simulations are now performed. A CFD simulation (Computational Fluid Dynamics) is a numerical flow simulation performed using a computer model that calculates the movement of the fluid in the geothermal probes and represents the heat transfer while taking many complex phenomena into account. CFD simulations show that there are many factors that influence heat conduction within a geothermal probe borehole.
Some of these factors have a strong impact, others a weaker one. The thermal conductivities of the rock and the backfill material play a particularly important role. The flow conditions of the heat transfer medium (laminar or turbulent) and the heat transfer at the interfaces are also important. And everyone agrees: The position of the probe tubes within the borehole also makes a difference, because heat always flows from warm to cold – this physical law cannot be completely stopped. But how much of an impact can this have on overall performance?
Thermal short circuit – what is its significance?
One of the largest studies* ever conducted shows that, among all these factors, the choice of backfill material has one of the greatest impacts. It cites the quality of the backfill next – the more meticulous the work, the higher the efficiency of the geothermal probe. It also shows that the so-called "thermal borehole resistance" tends to increase with larger diameters, meaning the heat transfer performance of the geothermal probe decreases. And the larger the selected borehole diameter, the greater the impact of material selection and poor workmanship.
Interestingly, the study also examined the use of borehole spacers. Spacers were originally developed with the intention of separating the boreholes from one another in order to minimize thermal interference ("short circuits") between them. But what conclusion does the practical study reach? The use of spacers can, paradoxically, lead to increased borehole resistance due to an increased risk of water or air inclusions. Spacers must have been invented at a desk—people thought they could squeeze every last percent out of a borehole—but in practice, they are counterproductive. Conclusion: The quality of the borehole for the geothermal probe and the selection of thermally high-quality materials are far more important than any minimal thermal short circuit that might occur.
Here's a comparison: Imagine you want to buy a thermos flask. A good thermos flask keeps coffee or tea hot for a long time. There are two models in the store. The expensive one boasts very thick insulation, but it doesn't have a lid. The other one is cheaper, has slightly thinner insulation, but comes with a lid that closes. What do you think – which one keeps your coffee hot longer? So, what's the key? The insulation value of the flask wall, or the overall design?
It's exactly the same with a geothermal probe: the overall product counts. Minimal heat conduction between the supply and return lines isn't crucial. While a thermal short circuit is theoretically possible in a geothermal probe, it's so small that it's barely measurable. What matters is that the entire system is well-planned: a narrow borehole, good probe embedding, and suitable hydraulics. Overall, this counts much more than a small thermal short circuit in the ground, mathematically to the third decimal place.
At separatus, we place great value on holistic planning and execution. Our separatus partners stand for expertise and quality. Planning recommendations for the separatus system include the smallest possible drill diameter, thermally enhanced backfill, and turbulent flow.
In the next part of this double article, we will detail the investigations conducted on the separatus splitpipe technology. We will present the results of CFD simulations conducted by the OST University of Applied Sciences, as well as analyses of geothermal probes conducted in real operation.
* https://www.geoenergie-konzept.de/thermischer-bohrlochwiderstand