This paper describes the design, construction, validation, and calibration of a thermal conductivity measuring apparatus for geothermal backfill materials in the range from 0.13–2.80 W/m·K. The developed apparatus is based on the Transient Hot Wire (THW) method whose mathematical basis is the Infinite Linear Source (ILS) model. The apparatus consists of a nichrome hot wire, an adjustable direct current power supply, a temperature sensor (K-type thermocouple), and a datalogger. For the validation and calibration of the developed apparatus, four standard samples have been used with a known thermal conductivity, to 3.0 W/m·K. Furthermore, the thermal conductivity of four geothermal backfill materials of common use (bentonite, neat cement, cement–sand mortar, and cement–bentonite mortar) has been measured using both the developed apparatus and a commercial meter.

The aim of the present study is to develop an apparatus for the measurement of the thermal conductivity of geothermal grouting materials. The apparatus, named MCT, is designed and constructed as a direct application of a mathematical model of heat transference for conduction in an infinite homogeneous isotropic medium using a linear heat source of infinite length, infinitesimal radius and radial heat flow. This application is known as the transient hot wire method. The apparatus mainly consists of a hot wire, a power supply, a temperature sensor and a datalogger. The commissioning of the developed apparatus is carried out by means of the calibration of the temperature sensor, as well as measurements of thermal conductivity using four reference samples whose thermal conductivity is known. Each of the reference samples is formed of two solid rectangular prisms of the same material and of the same dimensions. MCT is precise and accurate. In good experimental conditions the uncertainty of the measurements is within 10%. In addition, the MCT apparatus is light and with reduced dimensions.

No abstract

Thermal conductivity is an essential property of the geothermal grout, which in turn is considered the most important element in a vertical closed-loop ground heat exchanger. The main aim of this work is to determine the thermal conductivity of four of the most commonly used geothermal grouting materials using a homemade apparatus, developed by the research team. The tests were also performed with a commercial thermal conductivity meter, the Shotherm QTM-F1. Both the homemade apparatus, called MCT, and the commercial thermal conductivity meter are based on the transient hot wire method. The components used to make the specimens are cement, silica sand, bentonite, water and superplasticizer. A laboratory mortar mixer was used to prepare all mixtures. Later, the samples were cured in a water bath for 7 days and finally the samples were dried at 60 °C for 24 h before testing. Each of the samples tested is formed of two solid rectangular prisms of the same material and of the same dimensions (180 mm × 70 mm × 50 mm). The obtained results with the MCT apparatus are similar to the data reported in the literature and to those obtained using the Shotherm QTM-F1thermal conductivity meter. The deviations of the values measured with the MCT apparatus are, in all cases, less than 10%.

No abstract