Freeze-Thaw-Cycles on Borehole Heat Exchanger Grouts: Impact on the Hydraulic Properties

Abstract

In this paper a testing device is described that measures the hydraulic conductivity of grout specimens for Borehole Heat Exchangers (BHE). During the operation of closed-loop ground source heat pumps running with antifreeze, freezing of the backfill can occur due to extensive heat extraction. This laboratory device can assess the influence of frost on the hydraulic seals of BHEs. The device is based on a triaxial flexible wall permeameter. A freely selectable number of cyclic freeze-thaw-stresses as well as a confining pressure simulating radial earth pressure (r 2 ¼ r 3) can be applied. Specimens are composed of an annular grout body and a polyethylene pipe simulating the BHE system. The freezing direction is perpendicular to the vertical axis of the BHE from the inside to the outside. Numerical coupled modeling was applied to verify the results of the temperature distribution inside the specimens. It was observed and modeled that the propagation of the frost front and the fabric disintegration processes are correlated. Results of three different grouting materials will be presented. With its relative small dimensions the device can be easily implemented into soil mechanical laboratories and thus can contribute to quality control of grouts.

Impact of operational temperature changes and freeze–thaw cycles on the hydraulic conductivity of borehole heat exchangers

“…The annulus is usually grouted from the bottom to the top with a cement-based suspension to seal-off the BHE. Both the vertical and horizontal hydraulic conductivity of a BHE system must be minimized to prevent vertical groundwater flow and thereby avoid aquifer interconnection and possible contaminant transport from the surface (Anbergen et al 2014 ). The integrity of a BHE is of utmost importance for a sustainable system operation.…”

Thermal properties variations in unconsolidated material for very shallow geothermal application (ITER project)

“…For example, dry soils, unlike those saturated or half-saturated, do not show thermal conductivity variations with temperature (Leong et al, 1998). Moreover, repeated thermal anomalies, as freezethaw cycles induced by borehole heat exchangers (BHE), affect the hydraulic and geotechnical behaviour of sediments and grouting material near the exchangers (Anbergen et al, 2014(Anbergen et al, , 2015Dalla Santa et al, 2016).…”

Experimental analysis of enhanced cement-sand-based geothermal grouting materials

“…Although the research done in the last few years is not extensive, some investigations can be highlighted, such as those where the thermal conductivity of different bentonite-based grouts with different types and quantities of sands and graphites is evaluated [6,7]. A more thorough characterization was carried out in [8][9][10][11][12][13], where mechanical strength, thermal performance and permeability of bentonite-based grouts were analysed before and after they were subjected to freezing damage and heating-cooling cycles. The favourable influence of the graphite on the thermal performance of the grouting materials, its adverse effect on the mechanical behaviour and the negative impact of the high w/s (water/solid) ratios of the very workable admixtures, are some of the important conclusions of these investigations.…”