Geothermal borehole fields
Shallow geothermal energy offers a promising low-carbon solution for heating and cooling buildings. The most widely used shallow geothermal systems are vertical ground-source heat pumps (GSHPs). These systems exchange heat with the ground through borehole heat exchangers (BHEs), typically installed at depths of up to 400 m.
Dimensioning borehole fields is complicated because it involves multi-physics modeling, long-term performance, and site-specific constraints. Because boreholes are thermally coupled, heat extracted or injected into one borehole affects nearby ones (known as thermal interference). Therefore, effective design must consider cumulative thermal loads and peak load effects over decades of operation (typically 20–50 years). It must ensure that fluid temperatures remain within acceptable bounds throughout the system's lifetime to prevent ground freezing or overheating.
How it works
TESSA employs the temporal superposition method and g-function modeling to predict borehole fluid temperatures over decades of operation. This approach is built on validated methods from academic literature and our own published research.
The extreme fluid temperatures are estimated by superposing three load components:
- Long-term component — represents the annual average load.
- Seasonal (periodic) component — captures seasonal variations.
- Short-term (pulse) component — accounts for peak loads lasting hours to days.
Each load component is associated with the respective ground thermal resistances, plus the borehole thermal resistance.
To calculate these thermal resistances, TESSA uses the g-function method developed by Dr. Per Eskilson. The g-function is a dimensionless temperature response function that describes how ground temperature evolves in response to a unit heat load over time in a borehole field. This method is also used in industry-standard tools such as Earth Energy Designer (EED).
TESSA integrates Pygfunction, developed by Prof. Massimo Cimmino, which uses the analytical finite line source solution to compute g-functions. Unlike tools limited to simple geometries (rectangular or U-shaped fields), Pygfunction enables the design of arbitrary borehole field configurations.
Required inputs
To run a dimensioning analysis, TESSA requires the following inputs:
- Borehole field heat exchange profile.
- Borehole specifications (dimensions and thermal properties).
- Borehole field layout (coordinates of each BHE).
- Ground thermal properties.
- Constraints on minimum and maximum fluid temperatures.
Validation with Energy Earth Designer (EED)
To validate the borehole field dimensioning implemented in TESSA, the results were compared with those obtained from Energy Earth Designer (EED), a widely used software for geothermal system design. Two representative calculation cases were defined and simulated in both tools under identical boundary conditions.
Definition of validation scenarios
Borefield configuration
- 2 × 3 rectangular layout (6 boreholes in total)
- Borehole spacing: 10 m
Borehole specification
- Double-U pipe configuration
- Borehole diameter: 110 mm
Ground properties
- Surface temperature: 8 °C
- Ground thermal conductivity: 3.5 W/(m·K)
- Volumetric heat capacity: 2.16 MJ/(m³·K)
- Geothermal heat flux: 0.06 W/m²
Heat carrier fluid
- 25% ethylene glycol
System configuration
- Heating provided by heat pump (COP = 3)
- Cooling provided via direct cooling
Sizing parameters
- Simulation period: 50 years
- Maximum mean fluid temperature: 15 °C
- Minimum mean fluid temperature: −1.5 °C
Load cases
- Case 1: heating-only operation.
- Case 2: heating with seasonal cooling (providing partial thermal regeneration).
| Case | Annual Heat (MWh) | Max Monthly Heat (MWh) | Peak Heat (kW) | Annual Cooling (MWh) | Max Monthly Cooling (MWh) | Peak Cooling (kW) |
|---|---|---|---|---|---|---|
| 1 | 182 | 26.8 | 91 | 0 | 0 | 0 |
| 2 | 182 | 26.8 | 91 | 50 | 20 | 42 |
Results
| Case | Borehole Length (EED) (m) | Borehole Length (TESSA) (m) | Deviation |
|---|---|---|---|
| 1 | 262 | 250 | −4.7% |
| 2 | 229 | 220 | −4.1% |
The deviation between TESSA and EED remains below 5% in both cases. In Case 1 (heating-only), TESSA slightly underestimates the required borehole length compared to EED. In Case 2, the introduction of cooling loads reduces the required borehole length due to thermal regeneration of the ground. Both tools consistently capture this regeneration effect, and the relative deviation remains similarly small. Overall, the comparison demonstrates the reliability of TESSA's implementation for long-term geothermal sizing calculations.