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In this study, the thermal behavior of the coaxial and double U borehole heat exchangers was investigated using numerical simulations in both the long- and short-term. As a reference for borehole heat exchanger specifications, the existing coaxial and double U probes of a geothermal heat pump installed within the Horizon 2020 research project named “Cheap GSHPs” were considered. Nine years of simulations revealed that when borehole heat exchangers are subjected to a balanced thermal load, and intermittent operating modes of the ground source heat pump system are set, the coaxial pipes’ configuration provides better thermal performance due to the higher thermal capacitance of the heat-carrier fluid and the lower borehole thermal resistance. The analysis was conducted considering two different types of ground with different thermal conductivity values. As result, the more conductive ground type highlights the higher yield of the coaxial probe.

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.

Nowadays, the energy price fluctuations and the economic crisis are jeopardizing the development and diffusion of renewable technologies and sources. With the aim of both reducing the overall costs of shallow geothermal systems and improving their installation safety, an European project has took place recently, under the Horizon 2020 EU Framework Programme for Research and Innovation. The acronym of the mentioned project is Cheap-GSHPs, meaning “cheap and efficient application of reliable ground source heat exchangers and pumps”; the Cheap-GSHPs project involves 17 partners among 9 European countries such as Belgium, France, Germany, Greece, Ireland, Italy, Romania, Spain and Switzerland. In order to achieve the planned targets, a holistic approach is adopted, where all involved elements that take part of shallow geothermal activities are here integrated. In order to reduce the specific costs of geothermal installations, some newly designed geometries of heat basket-type ground source heat exchanger (GSHE) are modified drastically to receive a better performance of the geothermal installation. Within the sector of very shallow geothermal systems, these new developments are also tested on six representative demonstration sites around Europe. At the German test site in Northern Bavaria, four heat basket-type GSHEs are installed and equipped with certain monitoring systems (moisture, two different temperature sensors) and various backfilling materials of different grain size classes. The different installations will be tested for 12 months to evaluate the best combination of the newly designed heat basket-type GSHE and corresponding backfilling material mixture.