Abstract
The materials and the technology used to build the ground heat exchangers significantly affect the heat transfer performance of a geothermal system, in addition to the local geological and hydrogeological context. Among expense items such as the coupled heat pumps and the applied drilling technology, the heat exchangers play a key role in the shallow geothermal market. For this reason, they are usually made with plastic. Metal tubes are not widely used because of corrosion issues, which can compromise the reliability of the system over time. According to best practices, metal is an unfavorable choice if the pipes are not made of corrosion resistant alloys, such as stainless steel, but the overall performance is strongly related to the heat transfer efficiency. In this study, laser-flash technique is applied on carbon steel samples with anti-corrosion coatings and on corrosion resistant materials (stainless steel grades used for pipes), thus, allowing the comparison of their thermophysical properties. These properties are used to evaluate each solution in terms of thermal resistance. This study demonstrates that there are no particular corrosion resistant steel pipe configurations that are thermally favorable over others in a critical way.
Highlights
Shallow geothermal systems have proved reliability in efficiently supplying heating and cooling to buildings
Ground heat exchangers (GHEs) are tubes inserted into the ground, where a fluid flows inside, allowing the heat exchange between the ground and the building
Six different specimens have been measured in order to obtain the thermal diffusivity of HDPE, stainless steels (AISI 304 and AISI 316), carbon steel (S235JRH), and anti-corrosion coatings
Summary
Shallow geothermal systems have proved reliability in efficiently supplying heating and cooling to buildings. The characteristics of the properly designed drilling head and its operation significantly limit the abrasion with the subsoil Using this drilling technique, the external tube does not rotate during installation because the torque is transferred to the drill, but from an internal shaft, while the tube is mechanically decoupled by means of ball bearing elements while it is dragged into the ground. It is well known that there are other factors that could affect the heat transfer between the fluid and the ground (e.g., the viscosity of the fluid, its laminar or turbulent flow, and the presence of fins or other elements [9,10], such as changes in the diameter or the presence of incrustations on the inside wall of the pipe) They have not been considered because they do not influence the goal of this study focused on assessing and comparing the effects of anti-corrosion measures applied on the outer wall only.
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