Thermal Response Testing Results of Different Types of Borehole Heat Exchangers: An Analysis and Comparison of Interpretation Methods

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.