New trilobular geometry using advanced materials for experimentally validated enhanced heat transfer in shallow geothermal applications

The Adapted Conductivity Trilobular (ACT) design, comprising an up-flow central pipe and three satellite downward pipes, represents a novel configuration of a borehole heat exchanger. The novelty lies not only in the geometry of the heat exchanger, but also in the use of materials perfectly adapted to the thermal use for which they have been specified. The central pipe is constructed of a composite bilayer material with very low thermal conductivity, whereas the satellite pipes are based on a novel highly conductive plastic material. The primary objective behind this design was to simplify installation, by using essentially the same drilling and grouting methods used for conventional single-U or double-U borehole heat exchangers. In response to the rising demand for more efficient and affordable ground heat exchangers to be connected to Ground Source Heat Pump (GSHP) systems, this innovation was one of the promising outcomes of the GEOCOND European project. To assess ACT performance, the Geothermal Laboratory on the campus of the Universitat Politècnica de Valencia conducted a number of thermal response tests (TRTs), the experimental results of which are collected and analyzed in this paper. Additionally, a modified version of the Composite Two Region Line Source modeling scheme is described and used to assess the experimental data. A total of four demonstration boreholes – two single-U baseline and two ACT boreholes – have been tested. The experimental investigation using the C2RLS methodology reveals that the ACT configuration has a significantly lower thermal than the typical BHE arrangement and allows a greater specific heat injection rate. Specifically, a borehole resistance drop from 0.149 (m.K)/W to 0.07 (m.K)/W allowing higher injection rates to be achieved without an unsustainable increase in ground temperature. These findings may have significant implications for the development of more efficient and cost-effective GSHP systems.