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Geothermal depth is a critical factor that determines the viability and efficiency of air conditioning systems based on ground thermal energy. This article analyzes the technical standards, practical cases and technological advances to optimize its installation, surpassing the information currently available.
Why Does Depth Define Geothermal Efficiency?
The subsoil acts as a natural thermos:
- Up to 15 m: Stable temperature (15-18°C in Spain) independent of the season.
- >20 m: Increases 3°C every 100 m due to geothermal gradient.
This stability allows heat pumps to operate with COPs of 4-5, reducing energy consumption by up to 70% vs. conventional systems.
Capture Systems: Depths and Applications
| Type | Depth | Requirements | Average Cost |
|---|---|---|---|
| Horizontal | 1.2 – 2 m | 2x usable living area | 8.000-12.000€ |
| Vertical | 50 – 150 m | Reduced land (≥10 m²/well) | 15.000-25.000€ |
| Deep | 300 – 3.000 m | Industrial/urban projects | 50.000€+ |
1. Horizontal systems (1.5-2 m)
- Materials: 32-40 mm pipes in trenches filled with bentonite.
- Efficiency: 18 W/m linear (requires 35-55 m per kW).
- Practical example: For a 150 m² house in Madrid, 450 m of pipe in 1.5 m deep trenches is required.
2. Vertical Systems (50-150 m)
- Standard wells: Diameter 150 mm, with 4 pipes PE100 PN16 and bentonite-cement filling.
- Power: 50-70 W/m in granitic soils, 30-40 W/m in clay soils.
- Case study: Single-family house in Barcelona with 2 wells of 100 m (16 kW thermal).
3. Deep geothermal (>300 m)
- Technology: 300 m probes with rough outer layer for maximum thermal contact.
- Applications: District heating (e.g., neighborhood of 500 houses in Malaga with 10 wells of 1,500 m).
4 Factors that Determine Optimal Depth
- Thermal Conductivity of Soil
- Wet clays: 2.5 W/mK → 20% shorter wells.
- Fractured granites: 3.5 W/mK → Higher efficiency.
- Building Energy Demand
- Calculation: 50 W/m² for heating (CTE DB-HE 2024).
- Example: 500 m² office in Seville → 25 kW → 3 wells of 85 m.
- Local Regulations
- Minimum distance: 2 m from trees, 1.5 m from foundations.
- Permits: Geotechnical report required for wells >30 m (Law 22/1973).
- Probe Technology
- Coaxial probes: 15% more efficient than U-tubes at >100 m.
Updated Costs (2025)
| Concept | Horizontal | Vertical (100 m) |
|---|---|---|
| Installation | 85-120 €/m linear | 180-250 €/m well |
| Maintenance | 150 €/year | 300 €/year |
| IDAE Grants | Up to €4,000 | Up to €7,000 |
Example of amortization: Villa in Bilbao with vertical system (18.000€) saves 1.200€/year → ROI in 12 years.
Myths vs. Realities
❌ Myth: “The deeper the well, the better the yield”.
✅ Reality: Beyond 150 m, the temperature increase (3°C/100m) does not compensate the cost (+40% per well).
❌ Myth: “Horizontal systems are less efficient”.
✅ Reality: COP 4.2 is achieved using only 1.2 m depth.
Technical Frequently Asked Questions
What is the temperature at X meters?
- 10 m: 15°C (stable all year round).
- 100 m: 25-28°C (Mediterranean zone).
- 500 m: 40-45°C (ideal for district heating).
How to choose between horizontal and vertical?
- Horizontal: Land >500 m², renovations with garden.
- Vertical: Urban spaces, rocky soils.
Conclusion: Keys to Maximize Investment
- Always perform a thermal response test (TRT) to adjust the depth.
- Opt for coaxial probes in wells >80 m to gain 15% efficiency.
- Take advantage of subsidies such as the REPower EU Plan, which covers up to 40% of the installation.
Geothermal is not just digging: it is precision engineering where every meter counts. With advances in materials and 2025 regulations, achieving systems with returns in <10 years is now a technical and economic reality.
