Adapted composite two-region line source methods for evaluation of borehole heat exchangers with advanced materials

Abstract

[EN] The results of a large number of Thermal Response Tests (TRTs) performed on four different borehole heat exchangers (BHEs) were analyzed to explore the effect of the novel grouting and plastic pipe materials developed in a previous project on their thermal performance. For this purpose, two versions of the composite two-region line source approach have been adapted for its use for evaluating BHEs in practical situations, allowing us to obtain more comprehensive and consistent information on key thermal parameters of ground and fillers. We develop a scheme that combines the composite two-region model with unsteady TRT injection data to avoid some shortcomings that have appeared in previous implementations of this approach. Our new method enables us to obtain parameters otherwise difficult to obtain, such as the diffusivities, heat capacities, and conductivities of ground and grout. In comparison with standard line source based analysis, it allows to improve and enrich TRT execution, with a reduction in the test time required, lower uncertainties of the parameters obtained, and a deeper understanding of the thermal properties of the grout, ground, and borehole heat exchanger as all estimates are correlated to actual physical, mensurable soil and ground properties, which allows independent critical revision and check against alternative estimation methods.