An assessment of the thermal performance of infill grout mixes for use within geothermal heat exchangers in Malta

Abstract

Ground source heat pump systems consume 75% less electricity than conventional electricity powered heating and cooling systems and have lower maintenance costs. On the other hand the installation costs of the said systems are comparatively much higher, therefore attention has been focused worldwide to try to lower the costs of installing the borehole heat exchanging system required. The grout is one of the key factors which influence the efficiency of ground source heat pump systems. This dissertation explores the variations in the thermal conductivities 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. Silica sand and local sand mixes were formulated, tested and compared so that the potential use of local sands within such formulations can be examined. Thermal conductivity tests on the grout samples formulated show that silica sand-cement grouts exhibit very good thermal conductivity values which could result in a reduction of the required borehole depth to provide an equivalent heat transfer capacity. The thermal conductivity of grouts containing local sands decreases slightly when compared to that of silica but it is still higher than the thermal conductivity of bentonite grouts which have been used conventionally. In addition, thermal conductivity tests on samples which have been oven dried show that all the sand-cement grouts examined are capable of retaining their heat transfer characteristics under drying conditions. Apart from exhibiting better thermal conductivity values, local sand-cement grouts are 27% and 17% more cost effective when compared to silica sand-cement and bentonite grouts respectively. This points towards a further potential for improvement both in thermal conductivity and cost effectiveness.