Soil Thermal Conductivity Testing

Carbon Zero Consulting provide a unique service to measure soil thermal conductivity in situ at the intended site of installation of pipework.

Procedures for thermal response testing (TRT) of vertical closed loop boreholes are well known and documented. Design simulation software or MCS ‘lookup’ tables for horizontal closed loop schemes also requires a representative value of soil thermal conductivity (λ) as input data. Databases of soil thermal conductivity data are not readily available or attributable to a particular soil type – especially as the installer is unlikely to have expertise in soil type recognition. As a result, many installers have little or no information with which to base their calculation of heat exchange pipe length to meet building heating loads.

The Soil Survey

We utilise a ‘Hukseflux FTN01′ Field Thermal Needle Probe System (see image below). The probe itself comprises a thermal needle of diameter 6.3 mm and 170 mm length, mounted on a 1.5 m long handle (Figure 1). The probe is pushed into the soil at the target depth, for example, at the base of a trial pit or at the base of a hand-drilled narrow auger hole. The probe is left for 5 minutes to equilibrate with the ambient soil temperature. The probe’s readout unit has a facility to detect whether the temperature has adequately stabilised prior to commencement of the heating test.

Soil Thermal Conductivity

Soil thermal tool

During the test itself, a small electrical voltage is applied to a resistance element in the probe. This results in a constant heat power being generated, which propagates radially into the surrounding soil. The temperature of the probe increases with time and this increase is monitored over a heating cycle of around 5 minutes. If radial heat conduction is assumed, the temperature should increase in proportion to the logarithm of time, according to a standard “line source” approximation. Analysis of the rate of temperature increase provides a value of the surrounding soil thermal conductivity.

The determined thermal conductivity is representative of a cylinder of soil around 100 – 300 mm diameter around the probe. A representative assessment of a site’s “bulk” soil conductivity (i.e. a single value that can be used in design software) requires a number of individual determinations distributed across a site; normally 12 – 15 measurements for an ‘average’ domestic ground loop area.

A survey to obtain sufficient samples for a domestic application requires 1 day surveying in the field plus time for data analysis and reporting.

Call or email us to find out more. You can also download further information on the needle probe and our ‘Soilheat’ service.