Review of simulation methods for co-axial ground heat exchangers: Simplified models, predictions, and comparisons

Abstract

The calculation of heat extraction for buried ground heat exchangers (GHEs) is a significant research topic in the geothermal energy utilization field. Due to the considerable size of computational domain of buried pipes, various simplified models and solutions have been developed to achieve efficient simulation. However, the characteristics of existing simplified methods have yet to be comprehensively evaluated and compared. This article presents critical reviews of simplified solution models for co-axial GHEs, comparisons between available experimental measurements and predictions, and comparative analysis of various prediction methods. Unlike detailed numerical methods that solve full set of governing equations including continuity, momentum, energy, and coupled turbulence transport, simplified models use a one-dimensional (axial) approach to simulate fluid flow and heat transfer of the working fluid, assuming azimuthal symmetry. Radial heat transfer is modelled applying Newton's law of cooling, Nusselt number correlations and thermal resistance from Fourier's law of conduction. Either two-dimensional or three-dimensional rock-soil models are applied mostly. This review focuses on elucidating various Nusselt number correlations and the thermal properties of working fluid and rock-soil. Additionally, the relatively small discrepancies in various simulation methods can be primarily attributed to that the validations for the accuracy of these methods predominantly rely on the short-term results of working fluid temperatures. This review aims to clearly outline the advantages and disadvantages of existing simplified model algorithms, providing a reference for the accurate prediction of heat exchange rates in buried pipes.