Development of a dry mortar with nanosilica and different types of industrial waste for the application in borehole heat exchangers

Abstract

A geothermal borehole is a heat exchanger between the soil and a heat transfer fluid. This fluid flows throw in the geothermal pipes, which has been inserted into borehole. Using a Ground Source Heat Pump (GSHP) systems, the fluid provides heating and cooling buildings.The space between the geothermal pipes and the ground is filled with a geothermal backfill. This geothermal fill must be having high thermal conductivity for facilitate the heat flow. In this way this fill is an important element in a Borehole Heat Exchanger as its choice can result in significant economic and energy savings during the geothermal installation’s lifetime. However, in general it has not received enough attention.The geothermal backfill materials formed by a mixture of cement, fine aggregate, sand and/or additions are known as geothermal grouts. The aggregated additives and the other materials aggregated confers to the geothermal backfill high thermal conductivity. This property is rejected in the conventional grouts used in buildings.On the other hand, the use of industrial waste or by-products in geothermal grouts is considered more sustainable, for it reduces landfill volume and the need of exploiting new mineral resources.This paper describes the development of a geothermal grout, named MG 7. MG-7 has improved thermal properties compared with conventional grouts. In this case, mining and by-products from surrounding companies have been used, following the principles of the circular economy.To determinate the geothermal grout properties different techniques and conventional equipment has been used. However, to calculate the thermal conductivity a specific device developed by the authors. Finally, it has been obtained a pre-dosed geothermal grout with a thermal conductivity of 2.01 ± 0.08 W/m∙K (K = 2), in which 30 % of the aggregates come from industrial waste, such as mine tailings, ladle furnace slag, fly ash and silica fume. It also contains 2 % of silica nanoparticles.