A research from the College of Glasgow compares the thermal and hydraulic efficiency of various single-well borehole warmth exchanger methods.
A not too long ago revealed paper by Christopher S. Brown, Isa Kolo, David Banks, and Gioia Falcone from the James Watt College of Engineering on the College of Glasgow compares the thermal and hydraulic efficiency of the various kinds of single-well, medium-depth borehole warmth exchanger designs.
The complete paper, “Comparability of the thermal and hydraulic efficiency of single U-tube, double U-tube, and coaxial medium-to-deep borehole warmth exchangers,” has been revealed within the Geothermics journal (
Design comparability
The three designs being in contrast on this research are all single-well, closed-loop methods. These designs are very generally carried out in several components of Europe and the US. The research states that the everyday depth of closed-loop borehole warmth exchangers (BHE) in lots of European international locations has been steadily rising. For the aim of this research, the middle-deep vary is taken into account to be 500 meters to 1000 meters depth.
In a coaxial DBHEs, fluid is warmed by the pure geothermal gradient because it flows downward by way of the annual area. The warmth is then extracted by pumping the fluid again to the floor through the central pipe, sometimes at a quicker velocity to attenuate warmth loss.
U-tubes and double U-tubes flow into a warmth switch fluid down and up the outlet by way of absolutely enclosed pipes, sometimes constituted of excessive density or cross-linked polyethylene. U-tubes can both be suspended throughout the column of pure groundwater, or sealed in place by a grout with low permeability and excessive thermal conductivity. The usual outer diameter of U-tubes can differ for every utility.
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Technique and comparability to thermal response checks
Simulations had been performed utilizing the borehole warmth exchanger warmth transport module of the open-source OpenGeoSys software program. The fashions take into account warmth switch within the rock formation, the grout area, and the inlet and outlet pipes.
Validation of the fashions was performed by evaluating the output values to these obtained utilizing precise thermal response checks. The modelled information for U-tube methods have an error of solely 0.4% in comparison with real-world values, whereas that for coaxial methods had an identical error of ~0.6%
Mannequin response to design parameters
The person fashions had been then evaluated based mostly on totally different system design parameters to guage warmth extraction, stress drop, and parasitic losses.
The coaxial system confirmed higher warmth extraction charges, doubtless resulting from elevated space of contract between the annual area and the encompassing strong rock/grout. When it comes to thermal effectivity, the coaxial system was solely marginally higher than the double U-tube due to different concerns like value, engineering practicality, and potential stress losses.
The stress drop within the system is way decrease in coaxial, at 85 kPa, in comparison with the one U-tube and double U-tube configurations, at 1.46 MPa and 423 kPa, respectively. This then interprets to a decrease output wanted for the circulation pump to run the system for the coaxial configuration, given related flowrates.
Rising the depth of the borehole will increase the outlet temperature and thermal energy for all borehole configurations. There may be additionally a linear enhance in stress drop with depth, thus leading to an rising pumping energy.
Circulation charge had a big impact on each thermal energy and stress drop. Higher movement charges (> 3 L/s) result in a rise in warmth extraction charges for all configurations, with coaxial offering the best achievable thermal energy. Rising movement charges correlate to a disproportionate enhance in stress drop for all MDBHE configuration kind, indicating that stress loss is proportional to the sq. of the movement charge.
Apparently, the best thermal energy recorded was for the U-tube configuration mannequin on the lowest movement charge (1 L/s). This was resulting from higher fluid velocities within the slender pipe, ensuing to higher warmth extraction.
Rock thermal conductivity
Each outlet temperature and thermal energy have optimistic near-linear correlations with rock thermal conductivity, with the best charge of enhance noticed in coaxial methods.
Conclusions
The research concludes that coaxial borehole warmth alternate carry out finest when it comes to optimizing geothermal warmth extraction and minimizing hydraulic stress losses in mid-to-deep BHEs. The advice is then to undertake such design when establishing boreholes at depths higher than 500 meters.
Single U-tubes are viable in eventualities with movement charges of 1 L/s. They, nevertheless, present low thermal output at depth and will have elevated parasitic pumping losses at depths past 500 meters.
Supply: Brown et al., 2024