The freeze–thaw cycles artificially induced in the ground by ground source heat pump systems working at sub-zero temperatures, or during underwater soil excavation, could cause unwanted permanent effects on the hydro-mechanical behaviour of the surrounding deposits. Cohesive deposits present a specific sensitivity to these processes connected to phase changes occurring in the interstitial fluid. The experiments conducted on normally consolidated cohesive soils point out that freeze–thaw cycles can cause a hysteretic increase in vertical permeability of about one order of magnitude, despite a significant concurrent reduction in void ratio. Although the induced freeze–thaw processes are quite constrained close to the borehole heat exchanger, the vertical permeability increase may lead to the risk of connecting aquifers previously separated by cohesive layers, with an overall effect that increases according to the number of probes in the array. Permeability variation appears to be due to the irreversible alteration of the microstructure provoked during the development of freeze–thaw processes. In this paper, the changes of the macroscale hydraulic conductivity are related to changes in the porous microstructural variations, detected in a wide dimensional range by means of a combination of X-ray computed micro tomography, scanning electron microscopy and mercury intrusion porosimetry.

The geothermal energy applications are undergoing a rapid development. However, there are still several challenges in the successful exploitation of geothermal energy resources. A special effort is required to characterize the thermal properties of the ground and to implement the thermal energy transfer technologies. Aim of this study is to provide original heat conductivity values for rocks and sediments in regions included in the VIGOR Project (southern Italy), to overcome the existing lack of data. Thermal properties tests were performed on several samples, both in dry and wet conditions, using thermal analyzer operating following the Modified Transient Plane Source method

The design phase of ground source heat pump systems is an extremely important one as many of the decisions made at that time can affect the system’s energy performance as well as installation and operating costs. The current study examined the interpretation of thermal response testing measurements used to evaluate the equivalent ground thermal conductivity and thus to design the system. All the measurements were taken at the same geological site located in Molinella, Bologna (Italy) where a variety of borehole heat exchangers (BHEs) had been installed and investigated within the project Cheap-GSHPs (Cheap and efficient application of reliable Ground Source Heat exchangers and Pumps) of the European Union’s Horizon 2020 research and innovation program. The measurements were initially analyzed in accordance with the common interpretation based on the first-order approximation of the solution for the infinite line source model and then by utilizing the complete solutions of both the infinite line and cylinder source models. An inverse numerical approach based on a detailed model that considers the current geometry of the BHE and the axial heat transfer as well as the effect of weather on the ground surface was also used. Study findings revealed that the best result was generally obtained using the inverse numerical interpretation.

The performance of very shallow geothermal systems (VSGs), interesting the first 2 m of depth from ground level, is strongly correlated to the kind of sediment locally available. These systems are attractive due to their low installation costs, less legal constraints, easy maintenance and possibility for technical improvements. The Improving Thermal Efficiency of horizontal ground heat exchangers Project (ITER) aims to understand how to enhance the heat transfer of the sediments surrounding the pipes and to depict the VSGs behavior in extreme thermal situations. In this regard, five helices were installed horizontally surrounded by five different backfilling materials under the same climatic conditions and tested under different operation modes. The field test monitoring concerned: (a) monthly measurement of thermal conductivity and moisture content on surface; (b) continuous recording of air and ground temperature (inside and outside each helix); (c) continuous climatological and ground volumetric water content (VWC) data acquisition. The interactions between soils, VSGs, environment and climate are presented here, focusing on the differences and similarities between the behavior of the helix and surrounding material, especially when the heat pump is running in heating mode for a very long time, forcing the ground temperature to drop below 0 °C.