Page 108 - Analysis and Design of Energy Geostructures

80 Analysis and Design of Energy Geostructures

Table 3.4 Thermal conductivity of materials characterising energy geostructures.
Material Thermal conductivity, λ [W/(m C)]

Dry Saturated
Clay 0.4 1.0 0.9 2.3
Silt 0.4 1.0 0.9 2.3
Sand 0.3 0.8 1.7 5.0
Gravel 0.4 0.5 1.8
Peat 0.2 0.7
Claystone/siltstone 1.1 3.5
Sandstone 1.3 5.1
Quartzite 3.6 6.6
Marl 1.5 3.5
Limestone 2.5 4.0
Argillaceous schists 1.5 2.1
Micaschists 1.5 3.1
Metaquartzite 5.8
Marble 1.3 3.1
Gneiss 1.9 4.0
Rhyolithe 3.1 3.4
Peridotite 3.8 5.3
Granite 2.1 4.1
Gabbro 1.7 2.5
Diorite 2.0 2.9
Basalt 1.3 2.3
Concrete 0.9 2.0
Steel 14 60
Polyethylene (low-density) 0.39
Source: Data from Pahud, D., 2002. Geothermal Energy and Heat Storage. Laboratorio di Energia, Ecologia ed
Economia, Canobbio.

3.4.3 Remarks about conduction
Conduction dominates heat transfer in materials that are characterised by potential
pore sizes negligibly small in relation to the considered solid volume. Based on this
consideration:
• Heat transfer in reinforced concrete is generally dominated by conduction for the
features of the concrete mix design and the steel that are likely to characterise
energy geostructures. Conduction also governs the heat exchange across the thick-
ness of the pipes embedded in energy geostructures.
• Heat transfer in soils is often dominated by conduction. However, considering the
specific features of any considered site is essential to verify the validity of the previ-
ous statement. For example these features include the size of the solid particles and