Reliable and Durable Grouting of Borehole Heat Exchangers

Abstract

Borehole heat exchangers are installed for the use of sustainable geothermal energy for domestic heating and cooling supply. Vertical heat exchangers are a reliable and common technique to make geothermal heat accessible. A well-engineered heat pump technology already exists, whereas the quality of the borehole heat exchanger constructions could be improved. The grouting of the borehole, shortly after the heat transfer pipes have been installed, plays a prominent role. The cementitious grouts are supposed to ensure the performance of these systems for a very long time. According to the applicable regulations the grouting of the borehole has to secure the heat transport from the rock to the heat carrier fluid, to seal the borehole to the surface to prevent contamination and to seal penetrated aquifers. So far most of the boreholes in Germany are grouted with clay/cement or thermally enhanced suspensions that were designed especially for this use. In practice sometimes quality problems appear due to the lack of suitable equipment, construction and documentation. At the moment there are few quality checks after the installation of the borehole heat exchangers in the ground. Laboratory methods and grouts have been tested and adapted to the requirements of the system. Practically used grouts are not frost proof and show structural damages by freezing. Nevertheless it is possible to mix freeze-thaw consistant grouts for borehole heat exchangers.