How can an Earth probe field be planned at limited drilling depth?
A schoolhouse plan comparison shows a possible solution: The simulated separatus variant requires 16 instead of 17 drilling with 100 m depth. The calculated minimum fluid temperatures after 50 years are close to each other. The result shall apply to the project assumptions described; it is not a general commitment to save a well with separatus.
For clients and specialist planners, the example combines the question of the cost of geothermal heating with the thermal and hydraulic design / system sizing of the borehole heat exchanger field. It is a planning simulation, not measured operating results.
What requirements did the school house have?
- Maximum heating capacity · Approach: 71 kW
- Annual heat demand · Approach: 120,5 MWh
- Maximum cooling capacity · Approach: 27,3 kW
- Annual cooling demand · Approach: 6,5 MWh
- Maximum Drill Depth · Approach: 105 m
- Simulated depth per probe · batch: 100 m
- thermal conductivity Subsurface · Approach: 2,4 W/mK
- thermal conductivity injection material · batch: 2,0 W/mK
- Simulation duration · Approach: 50 years
For both variants, identical soil parameters were set. Following the original project report, the influence of neighbouring Earth probes over the simulation period was also considered. A location or name of the schoolhouse is not specified in the report.
What distinguishes the two planning variants?
- Drilling · separatus: 16 × 100 m · Double U 32: 17 × 100 m
- Total Drill Length · separatus: 1.600 m · Double U 32: 1.700 m
- Hole diameter · separatus: 100 mm · Double U 32: 135 mm
- Minimum fluid temperature according to 50 years · separatus: −1,47 °C · Double-U 32: −1,44 °C
- Reynolds number in plan comparison · separatus: 2.987 · Double U 32: 1.905
- Concentrate for mixing with 20 % monoethylene glycol · separatus: around 448 l · Double U 32: around 734 l
The total volume flow was set at 16,5 m3/h. The Reynolds numbers are reproduced here as values of the plan comparison; A safe flow rating or an isolated proof of performance does not follow in this diagram.
The separatus variant in the example 100 m has less total drilling length and a drill hole diameter smaller by 35 mm. The project comparison uses 135 mm for double U 32. It can thus be distinguished from the general separatus product comparison with 132 mm. The litres are the concentrate for the mixture, not the entire brine volume.
What does this mean for the decision on geothermal heating?
The original project report assesses both variants as thermally sufficient. The almost identical calculated minimum temperatures document the result of the respective overall design. They do not demonstrate the sole influence of probe design or flow.
The project report does not contain a quantified cost accounting. The impact of different drill lengths, diameters and material quantities on the investment must therefore be determined for the specific project.
The product and planning specifications of separatus link the statement of a quasi equivalent thermal performance to a small drilling diameter, thermally improved borehole grouting and planned turbulent flow. For borehole grout / grouting material he recommends 2 W/mK. The schoolhouse plan comparison, on the other hand, expects 2,0 W/mK; this documented project approach is not subsequently changed.
What data does a variant comparison need?
Send separatus the load profile, the land situation and the maximum drill depth. The testing of possible variants can build on this. The design services mentioned in the project report include long-term thermal simulation, hydraulic design, pressure-drop calculation and the comparison of probe systems and probe arrays.
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