Efficiency analysis of the main components of a vertical closed-loop system in a borehole heat exchanger

Abstract

In vertical closed-loop systems, it is common to use single or double U-tube heat exchangers separated by longitudinal spacers. In addition, the helical-shaped pipe is another configuration that requires lower drilling lengths but it is less used. The aim of the present research is to study the influence of these components on the total efficiency of a borehole heat exchanger (BHE). Thus, the differences between using single/double U-tubes (with or without spacers) and helical pipes are analysed in terms of efficiency. Through different laboratory tests, a small vertical closed-loop system was simulated in order to analyse all these possible configurations. The grouting materials and the temperatures of the ground were modified at the same time in these tests. Regarding the heat exchange process between the ground and the heat carrier fluid, it must be highlighted that the best results were obtained for the helical-shaped pipe configuration. Some of the improvements offered by this heat exchanger typology with respect to the vertical configuration is that a lower drilling depth is required even it requires a larger diameter. This leads to significant economic savings in the performing drilling process. Finally, it is also worth noting the importance of using spacers in vertical U-tubes and that no improvements have been found regarding the use of single or double configuration of U-tubes. Thanks to the laboratory results derived from this study it is possible to establish the optimum behaviour pattern for the entire vertical closed-loop systems.