Determination of Thermal Properties for Horizontal Ground Collector Loops

Abstract

Horizontal closed loop ground collectors for ground source heat pumps are located within the soil and the top of the underlying, unconsolidated geology. Estimating thermal properties for this zone is difficult as it is heterogeneous and is subject to seasonal water content variations. Field measurements taken with needle probe instruments only provide data for the small annulus around the needle probe and are a snapshot in time highly dependent on the state of saturation. Apparent thermal diffusivity can be determined from soil temperature measurements. The technique utilises the decrease in amplitude and increase in phase shift with depth of a transmitted heat pulse in the ground, the magnitudes of which are determined by thermal diffusivity. Soil temperature data from 65 United Kingdom Meteorological Office weather stations have been used to calculate soil thermal diffusivity values. These are located throughout the UK, including different soil types and occupying the depth range of a horizontal loop ground collector. Using determinations from seasonal temperature cycles spanning several years mean that the resulting thermal diffusivities are seasonally averaged, site specific estimates that are more representative of the ground conditions than diffusivity values determined in the laboratory or obtained by point measurements using field needle probes. Associated thermal conductivities have been estimated from the thermal diffusivities from knowledge of soil texture. These determinations have been compared against other thermal property estimation schemes and provide a data set that can be used for assessing and calibrating modelled data sets.