Influence of eccentricity on the thermal performance of pipe-in-pipe heat exchanger utilized for geothermal heating, an experimental study

Abstract

In the past few decades, pipe-in-pipe coaxial borehole heat exchangers (BHEs) have become increasingly widespread for building heating through the usage of medium-deep geothermal energy. Enhancing the heat transfer efficiency of BHEs is crucial in order to achieve higher efficiency in utilizing geothermal energy. This study introduced a novel consideration regarding the eccentric inner pipe as a technique to enhance the thermal performance of the pipe-in-pipe BHE. The impact of eccentricity varying from 0 to 0.9 on the BHE performance was experimentally explored to establish the correlation for Nusselt number. The convective heat transfer coefficients were obtained based on 480 sets of test results conducted under varied heating powers (200 – 4500 W/m2) and flow rates (0.16 – 1.00 m3/h). The results demonstrated that in comparison to the coaxial BHE, the eccentricity leads to a considerable enhancement in the heat transfer coefficient, ranging from 20 % to 35 %. The thermal performance is higher when the eccentricity is between 0.4 and 0.6, while the pressure loss can be decreased by 33 % − 54 %. The research results are expected to provide a theoretical basis to improve the geothermal energy utilization efficiency without improving the cost of BHE systems.