Model specification to determine thermal conductivity of soils

Abstract

Developments in technology have led to an increasing interest in alternative sources of energy, which includes the ground and, in particular, the zone of influence of building foundations. The temperature in that zone in northern Europe is about 10–13°C, which is about the mean air temperature. It is possible to take advantage of this stable temperature to cool/heat buildings by circulating liquid through a closed loop within this zone and passing it through a heat exchanger or heat pump connected to a circulation system within the building. The number of loops required depends on the difference in temperature between the building and the ground, and on the thermal properties of the ground. This paper describes a model specification for determining the thermal characteristics of soils and rocks obtained from routine site investigations. Tests on cylindrical specimens have been used to develop the equipment, test procedure and analysis. The results indicate that the technique is simple, reliable, and produces typical results for sand and clays.

Influences on the thermal efficiency of energy piles

“…The ground thermal conductivity is set to 2 W/mK, which could be considered a mid range value for geomaterials (Banks, 2012). The ground specific heat is set to 1600 J/kgK, which could be considered more typical of soils than the values used for the case study, as it implies a volumetric heat capacity of 3 MJ/m 3 K (Clarke et al 2008). As the ground properties cannot be controlled at any given site this parameter was not part of the sensitivity analysis.…”

A comparison of laboratory and in situ methods to determine soil thermal conductivity for energy foundations and other ground heat exchanger applications

“…The thermal cell is based on a design by Clarke et al [11], the recommended method for laboratory soil thermal conductivity testing according to the Ground Source Heat Pump Title Suppressed Due to Excessive Length 3 Association (GSHPA) [18]. A diagram of the apparatus is shown in Figure 2.…”

“…If Q cannot be measured directly, measurement of the temperatures in the specimen as it cools after the power is switched off (the recovery phase) can be used to determine the heat transfer coefficient between the top of the soil and the air, and hence the power. This approach, proposed by Clarke et al [11], uses the lumped capacitance method, which is only valid when the temperature difference across the soil is small compared with the temperature difference between the soil surface and the ambient temperature [21]:…”

“…Bi is the Biot number, a dimensionless group which is the ratio of resistances to heat transfer by conduction and convection. Where this is satisfied, the temperature of the soil at time t is [11]:…”

Energy piles: site investigation and analysis

“…Single-u and double-u shaped piping configurations were installed to a depth of 14.2m into concrete piles of diameter 250mm and 350mm respectively. The dominant geological formation along the length of the installed energy piles is saturated sand with gravel which has an expected thermal conductivity in the range 1.5W/mK to 5.02W/mK (Bristow et al, 2001, Clarke et al, 2008, EED, 2010, Goodrich, 1986, Midttomme and Rolandset, 1998 Polyethylene piping with an outer diameter of 40mm and wall thickness of 3.7mm was used for the installations.…”