Page 105 - Analysis and Design of Energy Geostructures

Heat and mass transfers in the context of energy geostructures 77

Table 3.3 Typical values of soil porosity.
Soil type Porosity range, n [ ]
Uniform materials Equal spheres (theoretical values) 0.26 , n , 0.48
Standard Ottawa sand 0.33 , n , 0.44
Clean, uniform sand (fine or medium) 0.29 , n , 0.50
Uniform, inorganic silt 0.29 , n , 0.52
Well-graded Silty sand 0.23 , n , 0.47
materials Clean, fine to coarse sand 0.17 , n , 0.49
Micaceous sand 0.29 , n , 0.55
Silty sand and gravel 0.12 , n , 0.46
Mixed soils Sandy or silty clay 0.20 , n , 0.64
Skip-graded silty clay with stones or rock 0.17 , n , 0.50
fragments
Well-graded gravel, sand, silt and clay mixture 0.11 , n , 0.41
Clay soils Clay (30% 50% clay sizes) 0.33 , n , 0.71
Colloidal clay (0.002 mm . 50%) 0.37 , n , 0.92
Organic soils Organic silt 0.35 , n , 0.75
Organic clay (30% 50% clay sizes) 0.41 , n , 0.81
Source: Modified after Rees, S., Adjali, M., Zhou, Z., Davies, M. & Thomas, H., 2000. Ground heat transfer effects on
the thermal performance of earth-contact structures. Renew. Sustain. Energy Rev. 4 (3), 213 265.

2. In general, the denser the soil, rock, or concrete is, the smaller the pores and the
greater the contact points between solid grains are [typical values of porosity, n,
which are intrinsically related to dry density, ρ , are reported for soils in Table 3.3
d
with reference to the work of Rees et al. (2000)]. The thermal conductivity of
minerals is greater than that of water and air. For this reason, the greater the dry
density of geomaterials is for the same fluid filling the pores, the higher the overall
effective thermal conductivity of the medium will be.
3. Since water has a higher thermal conductivity than air, a geomaterial with pores
filled with water is characterised by a higher thermal conductivity than the same
material with the pores filled with air. The different magnitude between the ther-
mal conductivity of minerals, water and air also indicates that conduction heat
transfer primarily occurs between the solid particles rather than through the fluid
phase(s) (cf. Fig. 3.4). This phenomenon is particularly pronounced when a porous
material is completely dry and heat flows mainly through the contact points
between the grains instead of through the air-filled pores because of the greater
thermal conductivity of the grains compared to that of the air. This phenomenon
still characterises porous materials with increasing water content, although the pres-
ence of water layers of increasing thickness that cover the solid particles until the
pores are completely filled with water indicates a more significant proportion of
heat flowing in the water-filled pores compared to a dry case. A typical relationship