Experimental investigation of thermal conductivity of soils and borehole grouting materials

Abstract

Exploitation of thermogeology energy in heating and cooling of buildings starts to spread worldwide as an alternative renewable source of heat energy. The thermal conductivity of soils is among the critical parameters required to achieve a proper design of ground heat exchangers or any underground systems that involve thermo-active processes. This research is a part of study related to the laboratory measurements of thermal conductivity of soils and thermal grouts used for borehole heat exchangers. The first part of this project involves a design of a new thermal cell that can be used to measure the thermal conductivity of soils. The design of the apparatus is based on the application of Fourier’s law at steady state condition where unidirectional heat flux is generated through two identical specimens. A new concept of minimizing the radial heat losses that occur due to the ambient temperature interface (ATI) using a thermal jacket as a heat insulation barrier has been introduced in the design and experimentally performed. The obtained results and the analysis of the heat flow reveal that the longitudinal heat flow can be maximized and the radial heat flow can be minimized when the thermal jacket is used with proper temperature control. Also, it has been revealed that the measured thermal conductivity of soils is sensitive to further boundary conditions such as thermocouples and temperature of sink disks. In addition to its simplicity, the new cell can be used for undisturbed field samples (U100 samples) as well as laboratory-prepared specimens. The sample preparation and the test procedure for the two different soil conditions highlighted the simplicity of using the new apparatus in measurement of the thermal conductivity of soils. The second part of this research concerns a production of new thermal grout for borehole heat exchangers using unwanted industrial and domestic materials (PFA and ground glass-low cost) and the commodity fluorspar, all of which have relatively high thermal conductivity. The thermal conductivity of different PFA based grouts that comprise different enhancing materials at different mix proportions has been measured dry and at saturation using the new thermal call. The results highlighted the effect of mineralogy and the particle size distribution of the mix constituents on the thermal conductivity of the grout. The results showed that a combination of fluorspar with coarse ground glass can provide good thermal enhancement in both dry and saturated conditions. The grout that consist of 20% cement, 30% PFA, 15% coarse ground glass and 35% fluorspar by weight with dry and saturated thermal conductivity of 1.283 and 1.985 𝑊/𝑚.𝐾 respectively can be considered as a suitable grout that can be used successfully in UK. Comparing with thermally enhanced bentonite (1.46 𝑊/𝑚.𝐾), it is expected that with London Clay Formation optimal performance of borehole heat exchangers and cost savings would be achieved using the selected grout. The work done in the final part can be considered as an application of the new steady state thermal cell in the estimation of the thermal conductivity of sandy soils. Also, it can be considered as a case study where the thermal conductivity was measured for soils that have not been previously thermally tested (Tripoli sand). The effects of the porosity and degree of saturation on the thermal conductivity of Tripoli sand were investigated. The results of twenty experimental tests showed that the effect of the saturation degree is significant compared with the effect of dry density especially at saturation degree less that 10%. Also, the results revealed that the thermal conductivity is approximately linearly proportional to the dry density at all levels of saturation. The validation of some existing selected prediction models showed that none of the selected models is able to correctly match the thermal conductivity of Tripoli sand at all conditions. However, some models were more accurate than others in certain conditions. It is also concluded that all presented models failed to estimate the thermal conductivity of such soil in low or partially saturated conditions where convection started to play a role in the heat transfer mode. On the other hand, the variation of thermal conductivity of Tripoli sand can be fittingly described as logarithmic function of the water content at all levels of porosity with R2 value ranges between 0.9694 and 0.9732. As a result, an empirical model based on the experimental results expressing the thermal conductivity in terms of water content and porosity has been obtained and validated.