Thermal conductivity of soils for energy foundation applications

Abstract

Ground source heat pumps are a low-carbon method of providing space heating. Thermal energy is extracted by means of a heat transfer fluid pumped through a series of pipes buried in the ground. For new builds, construction costs can be minimised by installing the pipes within the building foundations, eliminating the need for further excavations. These are known as energy foundations. Designing such a system requires knowledge of the ground thermal properties, in particular the thermal conductivity. This can be determined by conducting a field thermal response test, or by laboratory tests on soil samples. In this thesis, the thermal response test was compared to the needle probe and thermal cell laboratory methods. For each method, the main sources of error were investigated. Previously, the needle probe transient temperature data was analysed by visual inspection or rules of thumb. A new analysis method was developed and trialled on agar-kaolin samples, which reduces errors associated with the previous methods. The greatest source of error in the thermal cell method was identified as heat losses. A finite element model of the thermal cell showed that it overestimates the thermal conductivity by at least 35% due to heat losses. The needle probe was found to be the more reliable method. Both laboratory methods gave significantly lower values of thermal conductivity than the thermal response test. Possible reasons for this include differences in scale and sampling disturbances. The final stage of this research considered the required accuracy in soil thermal conductivity measurement for a well-designed energy foundation system. A numerical model of an energy foundation system was used to simulate different thermal loading scenarios. Variations in thermal conductivity had little effect on balanced systems, but had a significant impact on heating only or cooling only systems.

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ORCID for William Powrie: orcid.org/0000-0002-2271-0826

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