Abstract
Kindergarten Grdelin in the city of Buzet, Istria, Croatia, was chosen to undergo a deep retrofit of the current thermotechnical system, as a part of the HORIZON 2020 HAPPEN project1. The existing shallow spiral heat exchanger field is insufficient to cover heating loads of the building. Therefore, additional BHEs were drilled and completed to determine optimal borehole heat exchanger type within the same geological environment. Four BHEs, either single U (1U) or double U (2U), with different geometrical setting and depth were tested: BHE-1 (50 m, 2U DN32 ribbed), BHE-2 (75 m, 2U DN40 ribbed), BHE-3 (100 m, 2U DN32 smooth) and BHE-4 (150 m, 1U DN45 ribbed). A thermal response test (TRT) was performed to obtain the ground thermal properties. Furthermore, synthetic TRT curves were calculated to describe temperature response in the case of different heat pulses. This was done to determine heat extraction rates and the capacity of each BHE type, according to EN14511 norm. It was established that the BHE-4 is the optimal design for heating and cooling purposes on the selected site due to positive impact of the geothermal gradient, higher initial borehole temperature and a positive effect of the ribbed inner wall.
Highlights
The use of shallow geothermal energy is promoted in the European Union as one of the energy sources to exploit in order to reduce the carbon footprint and increase the use of renewable energy sources (Directive (EU) 2018/2001)
Before turning on the electrical heaters, the initial borehole temperature was recorded for 20 min by circulating fluid through the borehole heat exchangers (BHE), using the thermal response test (TRT) apparatus, in order to obtain the undisturbed ground temperature (Kurevija and Vulin, 2010)
The objective of the HORIZON 2020 HAPPEN project is to demonstrate the possibilities for retrofitting energy inefficient buildings in the Mediterranean region, dealing with various solutions
Summary
The use of shallow geothermal energy is promoted in the European Union as one of the energy sources to exploit in order to reduce the carbon footprint and increase the use of renewable energy sources (Directive (EU) 2018/2001). The main findings showed that 2U design is preferred over 1U – in order to satisfy the building’s heating/cooling loads, the total drilling depth is shorter by around 15% when using 2U BHE DN25 in comparison to 1U DN32 This is due to the 2U pipes having a larger surface area where the heat transfer is taking place. Scientific merit, as part of the project objective and results, is to test and compare heat extraction rates of the different borehole geometrical settings and depth in the same geological environment. Such a procedure would show which geoexchange system design has the comparative thermodynamic advantage to be implemented in larger scale projects on the Istrian peninsula in the future. Scientific research on the drilled boreholes was carried out by the Thermal Response Test apparatus during March/April of 2019
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