Integrated analytical modeling of transient heat transfer inside and outside U-tube ground heat exchanger: A new angle from composite-medium method

Ground heat exchanger (GHE) is vital to geothermal energy utilization, seasonal energy storage, etc. The transient heat transfer analytical model is still a challenge for U-tube GHE due to the asymmetric structure and complex heat interaction among fluid, grout and soil. This study proposed a novel analytical model for U-tube GHE from a new angle of composite medium method. An inspiration from heat transfer of GHE in multilayer soil helps with the new modeling. Combining with the techniques of treating non-uniform pipe wall temperature/heat flux as well as effect of ground surface flux, a new analytical model of U-tube GHE is established. Both experimental data and other simulation data are used for model verifications. The investigation on the temperature evolution in longitudinal and cross section of GHE is realized through visualization method. The analysis on grout material selection and borehole depth design revealed some new results. The spatial distribution of temperature curve in different grout materials share the same shape in steady state stage. This result can offer a useful guide for quick evaluation of effect grout material. At last, the proper time step and U-tube segmentation is suggested to ensure a fast computation and a better simulation accuracy. This study offered a new paradigm for GHE simulation which may contribute to the system optimization, design and many other types modeling as well. (C) 2020 Elsevier Ltd. All rights reserved.