Does the thermal short circuit have to be taken into account for a borehole heat exchanger (BHE)?
Yes. Anyone planning geothermal heating should consider the heat exchange between the pre- and return together with the entire well construction. The thermal short-circuit is part of the design / system sizing; from its presence alone, no reliable statement about the performance of an borehole heat exchanger system can be derived. So the effect can be classified technically.
In the Splitpipe probe, a central web separates the preliminary and return within a tube. Due to the temperature difference, heat is exchanged via this web. The mentioned article describes this exchange as a thermal short-circuit effect.
What do the described simulations say?
The separatus specialist contribution reports on CFD simulations of the Eastern Swiss University of Applied Sciences (OST). The Splitpipe and two separate pipes with the same cross section were compared. This is a theoretical comparison, not a field proof for a particular heating plant.
According to the expert article, the heat transfer capacity of the Splitpipe was on average about 7 % lower. For the transfer to the design / system sizing under consideration therein, the contribution 2–3 gives % more drilling metres. Both figures come from the original separatus specialist contribution. They are not a general rule of interpretation. A stand-alone simulation report with complete boundary conditions is not available for this classification.
What can be derived from the reported practical operation?
The article describes the reference plant “Pernter” as a test system for different types of probes, volume flows and extraction capacities. separatus reports on continuous temperature and performance records.
The underlying measurement series are not included in the article. Therefore, neither a quantified power advantage nor the proof that the thermal short-circuit in operation would in principle be negligible can be derived from this.
Which conditions are crucial for the probe comparison?
The product and design specifications of separatus describe the thermal performance compared to a double U-tube BHE as quasi equivalent under the following conditions: small drilling diameter, thermally improved borehole grouting and planned turbulent flow. The associated diameter comparison is 100 mm versus 132 mm. These conditions are part of the statement and must be mentioned in a comparison.
For the borehole grouting, separatus recommends a thermal conductivity over 2 W/mK.. Thermal well resistance bundles influences of well construction and heat transfer after the mentioned separatus technical contribution. For buyers, this means that the choice of the probe should be based on a design / system sizing of the entire borehole heat exchanger system.
What information helps with planning?
For a variant comparison, load profile, land situation and maximum drilling depth are suitable initial data. separatus also mentions this information in the article “Short borehole heat exchangers efficient planning” for a project enquiry. Have your project check which well structure and which hydraulic design fit.
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