Simulation-Based Comparison Between the Thermal Behavior of Coaxial and Double U-Tube Borehole Heat Exchangers

Quaggiotto Quaggiotto,Psyk Psyk,Mendrinos Mendrinos,Vercruysse Vercruysse,Emmi Emmi,De Carli De Carli,Pockelé Pockelé,Zarrella Zarrella,Bernardi Bernardi,Galgaro Galgaro,Righini Righini

doi:10.3390/en12122321

Abstract

In this study, the thermal behavior of the coaxial and double U borehole heat exchangers was investigated using numerical simulations in both the long- and short-term. As a reference for borehole heat exchanger specifications, the existing coaxial and double U probes of a geothermal heat pump installed within the Horizon 2020 research project named “Cheap GSHPs” were considered. Nine years of simulations revealed that when borehole heat exchangers are subjected to a balanced thermal load, and intermittent operating modes of the ground source heat pump system are set, the coaxial pipes’ configuration provides better thermal performance due to the higher thermal capacitance of the heat-carrier fluid and the lower borehole thermal resistance. The analysis was conducted considering two different types of ground with different thermal conductivity values. As result, the more conductive ground type highlights the higher yield of the coaxial probe.

Highlights

Technological innovations in the field of ground heat exchangers (GHEs) sizing and design can help improve their thermal performance and, to reduce the length necessary to satisfy building energy needs
Several works have recently been conducted on coaxial GHEs, which consist of an inner central pipe and an annular channel in direct contact with the surrounding ground
Small-size coaxial heat exchangers (SCGHEs) with an external stainless steel tube, directly driven into the soil up to a depth of about 20 m, were analyzed through finite-element numerical simulations performed using the software package COMSOL Multiphysics (COMSOL AB, Stockholm, Sweden) to evaluate the effects of thermal short-circuiting on the performance of SCGHEs and to determine the changes in the performance of a SCGHE if the flow direction is reversed [4]

Summary

Introduction

Technological innovations in the field of ground heat exchangers (GHEs) sizing and design can help improve their thermal performance and, to reduce the length necessary to satisfy building energy needs. This reduction decreases the installation costs, which are high for vertical systems. Several works have recently been conducted on coaxial GHEs, which consist of an inner central pipe and an annular channel in direct contact with the surrounding ground. Small-size coaxial heat exchangers (SCGHEs) with an external stainless steel tube, directly driven into the soil (no borehole is made to install them) up to a depth of about 20 m, were analyzed through finite-element numerical simulations performed using the software package COMSOL Multiphysics (COMSOL AB, Stockholm, Sweden) to evaluate the effects of thermal short-circuiting on the performance of SCGHEs and to determine the changes in the performance of a SCGHE if the flow direction is reversed [4]

Methods

Results

Conclusion