A series of simplified model calculations was used to investigate the thermal behavior of a solid before applying the more complex Tromm Site model.

In the first test, a square block (100 m × 100 m) was subjected to atmospheric pressure and a surface temperature of 10 °C; a constant heat flux of 0.065 W m^⁻² was applied to the base. The matrix permeability was extremely low (1 × 10^-20 m²), and the thermal conductivity was 2.5 W m^-1 K^-1. The result: The temperature field is nearly purely conductive, convective flows are negligible, and the ground temperature rises to only about 12.5 °C.

A sensitivity analysis examined three permeability values (1 × 10^-20, 1 × 10^-15, 1 × 10^-12 m²). Here, too, heat transfer remains predominantly conductive; higher permeability results only in slightly higher temperatures throughout the volume.

A third, geologically more realistic model featured an inclined cross-section with a 50 m fracture located in the center. The boundary conditions remained unchanged. The simulation demonstrated that groundwater flows from higher to lower elevations and that the fracture significantly distorts the flow field: local circulation patterns develop within it, and the Darcy velocity is significantly higher than in the surrounding rock.

The results underscore that when permeability is very low, heat conduction is the dominant mode of transport, whereas structural features such as fractures significantly influence hydraulic behavior, providing important insights for further modeling of complex geothermal systems.