Geothermal ground heat exchangers in Malta : thermal performance assessment of infill grouts

Abstract

The aim of this paper was to assess the thermal conductivity of proprietary grout mixes versus local ones in Malta. Ground source heat pump (GSHP) systems consume 75% less electricity than conventional environmental control systems and have lower maintenance costs. On the other hand the installation costs of GSHPs are comparatively higher. Attention has therefore been focused to try to lower the costs of installing the borehole heat exchange system. The grout is one of the key factors that influence the efficiency of ground source heat pumps, being the principal conductive medium to the natural terrain. This paper explores the variations in the thermal conductivity of different sand-cement grouts. It compares and evaluates options for the suitability of these materials for backfilling vertical boreholes of ground heat exchangers. In this study sixteen different sand-cement grouts have been designed, evaluated and compared with materials, which have been conventionally used to fulfil this purpose. Imported silica sand and local sand mixes were formulated, tested and compared so that the potential use of local sands could be closely examined. Output results prove that local sand-cement grouts improve thermal conductivity values by 27% and are also 17% less expensive when compared to silica sand-cement and bentonite grouts. This win-win scenario points towards improvements in both in thermal conductivity and cost effectiveness when using local resources.