Eden Geothermal drilling rig (supply: Exeter Science Centre, YouTube screenshot)
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 printed 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 several types 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 printed within the Geothermics journal (https://doi.org/10.1016/j.geothermics.2023.102888)
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 completely different components of Europe and the US. The research states that the standard depth of closed-loop borehole warmth exchangers (BHE) in lots of European nations 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 house. The warmth is then extracted by pumping the fluid again to the floor by way of the central pipe, usually 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, usually produced from excessive density or cross-linked polyethylene. U-tubes can both be suspended inside 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 software.
Technique and comparability to thermal response checks
Simulations had been executed 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 executed 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 completely different system design parameters to judge warmth extraction, strain drop, and parasitic losses.
The coaxial system confirmed higher warmth extraction charges, doubtless resulting from elevated space of contract between the annual house and the encircling stable rock/grout. By way of thermal effectivity, the coaxial system was solely marginally higher than the double U-tube due to different issues like price, engineering practicality, and potential strain losses.
The strain drop within the system is way decrease in coaxial, at 85 kPa, in comparison with the only 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 comparable 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 improve in strain drop with depth, thus leading to an rising pumping energy.
Circulation fee had a major impact on each thermal energy and strain drop. Better move charges (> 3 L/s) result in a rise in warmth extraction charges for all configurations, with coaxial offering the very best achievable thermal energy. Rising move charges correlate to a disproportionate improve in strain drop for all MDBHE configuration kind, indicating that strain loss is proportional to the sq. of the move fee.
Curiously, the best thermal energy recorded was for the U-tube configuration mannequin on the lowest move fee (1 L/s). This was resulting from better fluid velocities within the slim pipe, ensuing to better warmth extraction.
- Rock thermal conductivity
Each outlet temperature and thermal energy have optimistic near-linear correlations with rock thermal conductivity, with the very best fee of improve noticed in coaxial methods.
Conclusions
The research concludes that coaxial borehole warmth change carry out greatest when it comes to optimizing geothermal warmth extraction and minimizing hydraulic strain losses in mid-to-deep BHEs. The advice is then to undertake such design when setting up boreholes at depths better than 500 meters.
Single U-tubes are viable in eventualities with move charges of 1 L/s. They, nonetheless, present low thermal output at depth and should have elevated parasitic pumping losses at depths past 500 meters.
Supply: Brown et al., 2024
