Field thermal response experiments of large diameter energy shaft: Applicability comparison between constant heat flow method and constant temperature method

Abstract

The field thermal response test is a core technique for obtaining the thermal physical parameters of underground soils. It plays a decisive role in the precise design and energy efficiency optimization of ground source heat pump systems. This study addresses the characteristics of prefabricated energy shafts, which feature large diameters, shallow burial depths, and hollow concrete columns, by overcoming the limitations of conventional buried pipe structures and innovatively designing a cylindrical concrete testing structure. This novel structure achieves comprehensive simulation of actual energy shafts across three dimensions: geometric similarity, material homology, and equivalence in heat transfer mechanisms, significantly enhancing the reliability of test results and their engineering applicability. Through the implementation of two test schemes, namely the constant heat flow method and the constant temperature method, it was revealed for the first time that the differences in soil thermal conductivity and average heat transfer coefficient between the cylindrical concrete structure and the traditional underground pipe structure were as high as 23.72 % and 288 % respectively. This strongly validates the scientific representativeness of the structure for characterizing the thermal transfer properties of energy shafts. The results indicate that the constant heat flux method is advantageous for analyzing thermal physical parameters, whereas the constant temperature method is better suited for measuring heat exchange rate per unit depth. Each method has distinct technical merits and applicable scopes. The outcomes of this research not only establish a standardized technical protocol for thermal response testing of energy shafts, substantially reducing testing costs, but also fill a critical technological gap in the thermal physical testing methodology for prefabricated energy shafts. This work holds significant practical engineering value for advancing low-carbon energy innovations in the building industry.