Environmental hazards due to extreme thermal stress induced by borehole heat Exchangers

In Ground Source Heat Pump systems (GSHP), a continuous circulation of a fluid inside the Borehole Heat Exchangers (BHE) transfers heat between the ground and the building's conditioning system. Therefore, the natural thermal status of the subsoil in the boreholes' surrounding is altered. The international community has already pointed out the importance of restricting the thermal anomalies induced in the soil, through the establishment of a minimum temperature threshold for the brines inside the probes. As a matter of fact, the heat carrier fluids often consist of mixtures of water and anti-freezing additives, which permit to decrease the temperature of the exchanger also below the water freezing point. This paper presents a first summary of how the cyclic thermal stress induced by a heat exchanger could change some physical and geotechnical properties of soils, in case the fluid is carried at sub-zero temperatures, therefore inducing cyclic freezingthawing and heating processes in the ground. The effects of these processes are also demonstrated with the results of a wide laboratory investigation performed on some cohesive soils.