Heat transfer performance of new efficient coaxial casing heat exchanger based on thermostatic thermal response test

Abstract

Ground-source heat pump (GSHP) systems are helpful for reducing building-sector heating and cooling energy consumption. As the core part of the GSHP system, the underground heat exchanger has a large influence on the heat transfer performance (HTP) of the system. The ground temperature variation characteristics of a new efficient coaxial casing heat exchanger (ECCHE) for heat extraction or rejection were investigated. The study was based on dynamic analysis of linear thermostatics used in the thermal response test and the HTP of the ECCHE. The influences of drilled borehole depth, circulation flow rate, and exchanger structure were considered. Results show that the HTP of the ECCHE is 1.61 (1.46) times that of the doublet U-pipe heat exchangers during heat extraction (heat rejection), and the radius thermal influence distance of the ECCHE is > 10 m. The spiral ribs of the ECCHE extended the heat transfer time between the fluid and surrounding rock, and the heat transfer in the upper part of the borehole was greater than that near the deeper part, so it is easy to produce continuous heat or cold accumulation near the borehole tops. Higher thermal conductivity rock, such as gneiss, has shown a faster recovery rate of the ground temperature after heat extraction. This work provides a theoretical basis for the optimization of the design and installation of GSHPs equipped with ECCHEs.